Afferents to the basal forebrain cholinergic cell area from pontomesencephalic--catecholamine, serotonin, and acetylcholine--neurons

Integrated contributions of basal forebrain and thalamus to neocortical activation elicited by pedunculopontine tegmental stimulation in urethane-anesthetized rats

Efferents from the pedunculopontine tegmentum (PPTg) exert widespread control over neocortical electrocorticographic (ECoG) activity and aid in maintaining high-frequency ECoG activation during waking and rapid eye movement sleep. The mechanisms and subcortical routes that allow the PPTg to influence cortical activity remain controversial. We examined the relative contributions of the thalamus and basal forebrain in ECoG activation elicited by PPTg stimulation in urethane-anesthetized rats. Stimulation (100 Hz, 2 s) of the PPTg suppressed large-amplitude, low-frequency oscillations, replacing them with high-frequency beta-gamma activity. Systemic administration of the anti-muscarinic drug scopolamine (1 mg/kg, i.p.) abolished activation elicited by PPTg stimulation, suggestive of an essential role of acetylcholine in this effect. Local infusions of lidocaine (1 microl, 1%) into the region of the cholinergic basal forebrain complex produced a strong reduction in activation elicited by PPTg stimulation. Lidocaine infusions into the reticular thalamic nucleus had no effect, but infusions into central thalamus produced a small attenuation of PPTg-evoked cortical activation. Combined basal forebrain-central thalamic infusions (1 microl/site) produced roughly additive effects, leading to a greater loss of activation than single-site infusions. These results indicate that, under the present experimental conditions, high-frequency cortical ECoG activation elicited by the PPTg involves relays in both the basal forebrain and central thalamus, with a predominant role of the basal forebrain. After concurrent central thalamic-basal forebrain inactivation, the forebrain can maintain only limited, short-lasting activation in response to PPTg stimulation. The additivity of infusion effects suggests that, rather than participating in one serial system, basal forebrain and central thalamus constitute parallel activating pathways. These findings aid in resolving previous controversies regarding the role of thalamus and basal forebrain in activation by emphasizing the importance of multiple, large-scale networks between brainstem and cortex in regulating the activation state of the mammalian neocortex.

Differential distribution of calbindin D28k and parvalbumin among functionally distinctive sets of structures in the macaque brainstem

In a study of brainstem in the cynomolgus monkey, we found that the distribution of calbindin D28K (CB) and parvalbumin (PV) is nonoverlapping among functionally distinct sets of brainstem structures. Nuclei involved in representation and regulation of the organism's internal state contain CB, whereas those involved in the representation of the external environment and the representation or execution of externally directed actions contain only PV. Moreover, our findings indicate that different nuclei known as components of the ascending reticular activating system (ARAS) contain either CB or PV or both, suggesting that this system in primates operates with both CB and PV. In line with previously reported findings, we also found that unmyelinated pathways contain only CB, whereas myelinated pathways contain PV. Distribution of CB and PV in the macaque brainstem follows a pattern comparable to, but in some instances significantly different than, the pattern previously reported in the rat. We argue that the nonoverlapping distribution of CB and PV among different structures of the brainstem might reflect underlying differences in the physiological, anatomic, and perhaps phylogenetic properties of these structures. Considering our recent findings of selective vulnerability of brainstem structures to Alzheimer's disease, the present data suggest that the majority of macaque brainstem nuclei that contain CB are vulnerable to neurofibrillary tangles in humans. By contrast, only few nuclei that contain PV exhibit pathologic changes. Some of these nuclei are affected with a high number of neuritic plaques without ever developing neurofibrillary tangles.

Muscarinic and GABAA receptors modulate acetylcholine release in feline basal forebrain

Acetylcholine (ACh) release within the basal forebrain changes significantly as a function of sleep and wakefulness, hence identifying the neurochemical modulators of basal forebrain ACh release will contribute to a mechanistic understanding of sleep cycle regulation. This study tested the hypothesis that muscarinic and gamma aminobutyric acid(A) (GABAA) receptors modulate basal forebrain ACh release. Cats were anaesthetized with halothane to hold arousal state constant and a microdialysis probe was aimed stereotaxically for the substantia innominata region of the basal forebrain. Four concentrations of the muscarinic antagonist scopolamine (0.1, 0.3, 1.0, and 10 nm) and five concentrations of the GABAA antagonist bicuculline (3, 10, 30, 100, and 300 micro m) were delivered by reverse dialysis from the same probes used to collect ACh. These results are based on 27 experiments in nine animals. Scopolamine and bicuculline each caused a concentration dependent enhancement of ACh release. Scopolamine increased ACh by 118% above control levels whereas bicuculline was more effective, causing a 287% increase in ACh release. Scopolamine was more potent (EC50 = 0.16 nm) than bicuculline (EC50 > or = 90 micro m) for increasing ACh release. The results support the hypothesis that substantia innominata ACh release is modulated by muscarinic autoreceptors and inhibited by GABAA receptors. These findings are consistent with the interpretation that inhibition of basal forebrain cholinergic neurotransmission by GABA contributes to the generation of sleep.

The caudal sublenticular region/anterior amygdaloid area is the only part of the rat forebrain and mesopontine tegmentum occupied by magnocellular cholinergic neurons that receives outputs from the central division of extended amygdala

Ascending cholinergic projections and the central nucleus of the amygdala (CeA) have both been implicated in attentional and orienting mechanisms leading to adaptive behavioral responses. In view of this, the present study was carried out to identify relevant neuroanatomical relationships in the form of projections from the CeA and a related structure, the dorsolateral divison of the bed nucleus of the stria terminalis (dlBST), to parts of the basal forebrain and mesopontine tegmentum that contain magnocellular cholinergic neurons. The CeA and dlBST are components of the 'central division of extended amygdala'. Following injections of the anterogradely transported compounds, Phaseolus vulgaris-leucoagglutinin or biotinylated dextran amine, into the CeA or dlBST, sections were processed with immunohistochemical reagents to localize the anterograde tracer and choline acetyltransferase (ChAT). The trajectories of efferent projections from CeA and dlBST were qualitatively similar. Few ChAT-immunoreactive (ir) neurons were present within the extended amygdala or regions containing the dense terminations of its efferent projections, with the striking exception of the caudal sublenticular/anterior amygdaloid region. The ChAT-ir neurons there, however, were significantly smaller and weakly ChAT-ir as compared to those located outside of the dense extended amygdaloid terminations. In the mesopontine tegmentum, the robust downstream projection from the extended amygdala was centered medial to ChAT-ir neurons of the pedunculopontine tegmental nucleus. The differentiated character of the relationships between extended amygdala and forebrain and mesopontine districts containing ChAT-ir neurons that give rise to ascending projections may have significant implications for the control of cortical and diencephalic acetylcholine release and accompanying effects on attention, vigilance and locomotor activation.

Origin of the dopaminergic innervation of the central extended amygdala and accumbens shell: a combined retrograde tracing and immunohistochemical study in the rat

The origin of the dopaminergic innervation of the central extended amygdala (EAc; i.e., the lateral bed nucleus of the stria terminalis [BSTl]-central amygdaloid nucleus [Ce] continuum) and accumbens shell (AcSh) was studied in the rat by combining retrograde transport of Fluoro-Gold (FG) with tyrosine hydroxylase (TH) immunofluorescence. Perikaryal profiles (PP) immunoreactive to FG and to both FG and TH were counted in A8-A14 dopaminergic districts. Our results suggest that dopaminergic inputs to the EAc and AcSh arise from the ventral tegmental area-A10, substantia nigra, pars compacta-A9, and retrorubral nucleus-A8 groups as well as from the dorsal raphe nucleus and periaqueductal gray substance, housing the dorsocaudal part of A10 group (A10dc). Quantitative estimates reveal that the A10dc group contains approximately half of the total number of FG/TH double-labeled PP projecting to Ce and BSTl. By using an anti-dopamine serum, DR/PAG projections to Ce were confirmed to be in part dopaminergic. In contrast, modest numbers of FG/TH double-labeled PP were seen in the A10dc group after injections in the sublenticular extended amygdala, interstitial nucleus of the posterior limb of the anterior commissure or AcSh. Ventral mesencephalic projections to the EAc display a crude mediolateral topographic organization, whereas those to the AcSh are topographically organized along a mediolateral and an inverted dorsoventral dimension. The diencephalic dopaminergic groups do not innervate the EAc or AcSh, except for the periventricular gray-A11 which sends light dopaminergic projections to Ce and BSTl. Overall, the present results provide additional details on the organization of the mesolimbic dopaminergic system that critically controls behavioral responsiveness to salient environmental stimuli.

Physiology of sleep and wakefulness as it relates to the physiology of epilepsy

This paper reviews the present knowledge about the cellular origins of vigilance states (wakefulness and slow-wave sleep) from the perspective of their involvement in the triggering of epileptic seizures. The data stem from intracellular recordings (most of them dual impalements of pairs of neurons and glia), extracellular ionic concentrations (mainly K and Ca ) and simultaneous intracortical field potentials from the cortex of cats. These data were corroborated with recordings from naturally sleeping animals and humans. It is shown that sleep is dominated by a cortically generated slow (<1 Hz) oscillation resulting from the complex interplay within networks of neurons and glia, which are modulated by the more diffuse action of extracellular currents of ions. Wakefulness is produced through the activation of brainstem and basal forebrain structures, which disrupt sleep oscillations and elicit a global change of the extraneuronal milieu, with profound modifications of glial and cerebral blood flow parameters. Paroxysmal events arising during quiet sleep evolve within the cortex from normal slow sleep oscillations. The synchronization of large cortical and eventually subcortical territories relies on the propagation of increased currents of K through the glial syncytium, which compensate for the reduced synaptic efficacy due to the depletion of extracellular Ca.

Specific contributions of the basal forebrain corticopetal cholinergic system to electroencephalographic activity and sleep/waking behaviour

The present study examined the role of the basal forebrain corticopetal cholinergic projection in the regulation of cortical electroencephalographic activity across sleep/wake states in rats. Selective lesions of this projection were effected by local intraparenchymal infusions of the immunotoxin 192 IgG-saporin. Lesions spared the septo-hippocampal cholinergic system, as well as p75-receptor-bearing noncholinergic neurons in the suprachiasmatic nucleus. Relative to sham-lesioned control animals, rats with lesions of basal forebrain cholinergic neurons displayed a significant reduction in high frequency EEG activity, characterized especially by a reduction in gamma EEG power. Lesions did not significantly alter the overall proportion of sleeping and waking states as defined behaviourally, but the attenuation of high frequency EEG activity was apparent across all stages, including REM-like periods. Results are consistent with the view that the basal forebrain corticopetal cholinergic system exerts a general activational effect on the cortical mantle. Although this system may not be essential for sleep/wake stage-switching, it does impact on the cortical states associated with those stages.

Interaction between apnea, prone sleep position and gliosis in the brainstems of victims of SIDS

Among 27,000 infants studied prospectively to characterize their sleep-wake behavior, 38 infants died suddenly and unexpectedly under 6 months of age. Of these, 26 died from sudden infant death syndrome (SIDS), 5 from congenital cardiac abnormalities, 2 from infected pulmonary dysplasia, 2 from septic shock with multi-organ failure, 1 from a prolonged seizure, 1 from prolonged neonatal hypoxemia, and 1 from meningitis and brain infarction. The frequency and duration of apneas recorded some 3-12 weeks prior to the infants' death were analyzed. The brainstem materials were collected and studied in an attempt to elucidate the relationship between sleep apnea, and prone sleep position and gliosis in some nuclei associated with cardiorespiratory characteristics, such as nucleus ambiguus in the medulla oblongata and the solitary nucleus, as well as structures associated with arousal phenomenon, such as the reticular formation, the superior central nucleus and the nucleus raphe magnus in the pons, the dorsal raphe nuclei in the midbrain and medulla oblongata, periaqueductal gray matter in midbrain, and locus ceruleus. Gliosis was estimated as the density of glial fibrillary acidic protein (GFAP)-positive reactive astrocytes. Variant-covariant analyses were carried out using the characteristics of apnea as an independent variable and sleep position and gliosis as dependent variables. A significant association was found only in the frequency of obstructive apnea and prone position (P<0.001) and gliosis in the raphe nuclei in the midbrain (P<0.001). Although prone position is a well-known risk factor for SIDS, the frequency of obstructive apnea has not been associated with the prone sleep position. The observed relation between prone sleep and the density of gliosis does not relate to epidemiological findings. Further studies are needed to investigate the unexpected statistical association.

Electroencephalographic activation by tacrine, deprenyl, and quipazine: cholinergic vs. non-cholinergic contributions

Drugs that stimulate central cholinergic transmission can induce activated, high frequency electroencephalographic (EEG) activity in rats. Monoaminergic enhancement also produces EEG activation, either by a direct stimulation of monoaminergic transmission in cortex, or a transsynaptic excitation of cholinergic projection neurons receiving excitatory monoaminergic afferents. We examined the degree of cholinergic involvement in EEG activation produced by monoaminergic and cholinergic drugs in rats. All animals were pretreated with 10 mg/kg reserpine and either 1 or 5 mg/kg scopolamine to abolish EEG activation. The acetylcholinesterase inhibitor tacrine (5-20 mg/kg) restored EEG activation in the low dose scopolamine group, but was less effective against the high scopolamine dose. The monoamine oxidase inhibitor deprenyl and the serotonergic receptor agonist quipazine restored EEG activation, an effect that was largely unaffected by different scopolamine doses. These results confirm that tacrine produces EEG activation by means of cholinergic stimulation. In contrast, activation produced by deprenyl or quipazine does not appear to be mediated by a transsynaptic excitation of cholinergic neurons and likely depends on a direct enhancement of cortical monoaminergic neurotransmission.

Crossed unilateral lesions of medial forebrain bundle and either inferior temporal or frontal cortex impair object recognition memory in Rhesus monkeys

In monkeys, section of the fornix, amygdala and anterior temporal stem results in a severe anterograde amnesia. Immunolesions of the cholinergic cells of the basal forebrain suggest that this amnesia is a result of isolating the inferior temporal cortex and medial temporal lobe from their cholinergic basal forebrain afferents. In this experiment, six monkeys were trained in a delayed match-to-sample task and then received a section of the medial forebrain bundle in one hemisphere and an ablation of either the frontal or inferior temporal cortex in the opposite hemisphere. All the animals were severely impaired in the performance of this task following this surgery, and the severity of the impairment was independent of the cortical area from which the medial forebrain bundle was disconnected. These results support a model of fronto-temporal interaction via the basal forebrain in new learning, in which midbrain sites related to reward modulate the cholinergic basal forebrain activity.

Amphetamine-stimulated cortical acetylcholine release: role of the basal forebrain

Systemic administration of amphetamine results in increases in the release of acetylcholine in the cortex. Basal forebrain mediation of this effect was examined in three experiments using microdialysis in freely-moving rats. Experiment 1 examined whether dopamine receptor activity within the basal forebrain was necessary for amphetamine-induced increase in cortical acetylcholine by examining whether intra-basalis perfusion of dopamine antagonists attenuates this increase. Systemic administration of 2.0 mg/kg amphetamine increased dopamine efflux within the basal forebrain nearly 700% above basal levels. However, the increase in cortical acetylcholine efflux following amphetamine administration was unaffected by intra-basalis perfusions of high concentrations of D1- (100 microM SCH 23390) or D2-like (100 microM sulpiride) dopamine receptor antagonists. Experiments 2 and 3 determined whether glutamatergic or GABAergic local modulation of the excitability of the basal forebrain cholinergic neurons influences the ability of systemic amphetamine to increase cortical acetylcholine efflux. In Experiment 2, perfusion of kynurenate (1.0 mM), a non-selective glutamate receptor antagonist, into the basal forebrain attenuated the increase in cortical acetylcholine produced by amphetamine. Experiment 3 revealed that positive modulation of GABAergic transmission by bilateral intra-basalis infusion of the benzodiazepine receptor agonist chlordiazepoxide (40 microg/hemisphere) also attenuated the amphetamine-stimulated increase in cortical acetylcholine efflux. These data suggest that amphetamine increases cortical acetylcholine release via a complex neuronal network rather than simply increasing basal forebrain D1 or D2 receptor activity.

Discharge profiles of juxtacellularly labeled and immunohistochemically identified GABAergic basal forebrain neurons recorded in association with the electroencephalogram in anesthetized rats

The basal forebrain ostensibly plays a dual role in the modulation of cortical activation and behavioral state. It is essential for stimulating cortical activation in association with waking (and paradoxical sleep), yet also important for attenuating cortical activation and promoting slow wave sleep. Using juxtacellular recording and labeling of neurons with Neurobiotin followed by immunohistochemical staining for glutamic acid decarboxylase (GAD), we studied the discharge properties of identified GABAergic basal forebrain neurons in relation to electroencephalographic (EEG) activity in urethane-anesthetized rats to determine the part or parts that they may play in this dual role. The GABAergic neurons displayed distinct discharge profiles in relation to somatosensory stimulation-evoked cortical activation. Whereas a significant minority increased its average discharge rate, the majority decreased its average discharge rate in association with cortical activation. Moreover, subgroups displayed distinct discharge patterns related to different cortical activities, including very regular high-frequency tonic spiking within a gamma EEG frequency range and rhythmic cluster spiking within a theta-like frequency range during cortical activation. During irregular slow EEG activity in absence of stimulation, one subgroup displayed spike bursts correlated with cortical slow oscillations. As relatively large in size and also antidromically activated from the cortex, many GABAergic neurons recorded were considered to be cortically projecting and thus capable of directly modulating cortical activity. Subgroups of GABAergic basal forebrain neurons would thus have the capacity to promote cortical activation by modulating gamma or theta activity and others to attenuate cortical activation by modulating irregular slow oscillations that normally occur during slow wave sleep.

Adenosinergic modulation of basal forebrain and preoptic/anterior hypothalamic neuronal activity in the control of behavioral state

This review describes a series of animal experiments that investigate the role of endogenous adenosine (AD) in sleep. We propose that AD is a modulator of the sleepiness associated with prolonged wakefulness. More specifically, we suggest that, during prolonged wakefulness, extracellular AD accumulates selectively in the basal forebrain (BF) and cortex and promotes the transition from wakefulness to slow wave sleep (SWS) by inhibiting cholinergic and non-cholinergic wakefulness-promoting BF neurons at the AD A1 receptor. New in vitro data are also compatible with the hypothesis that, via presynaptic inhibition of GABAergic inhibitory input, AD may disinhibit neurons in the preoptic/anterior hypothalamus (POAH) that have SWS-selective activity and Fos expression. Our in vitro recordings initially showed that endogenous AD suppressed the discharge activity of neurons in the BF cholinergic zone via the AD A1 receptor. Moreover, in identified mesopontine cholinergic neurons, AD was shown to act post-synaptically by hyperpolarizng the membrane via an inwardly rectifying potassium current and inhibition of the hyperpolarization-activated current, I(h). In vivo microdialysis in the cat has shown that AD in the BF cholinergic zone accumulates during prolonged wakefulness, and declines slowly during subsequent sleep, findings confirmed in the rat. Moreover, increasing BF AD concentrations to approximately the level as during sleep deprivation by a nucleoside transport blocker mimicked the effect of sleep deprivation on both the EEG power spectrum and behavioral state distribution: wakefulness was decreased, and there were increases in SWS and REM sleep. As predicted, microdialyis application of the specific A1 receptor antagonist cyclopentyltheophylline (CPT) in the BF produced the opposite effects on behavioral state, increasing wakefulness and decreasing SWS and REM. Combined unit recording and microdialysis studies have shown neurons selectively active in wakefulness, compared with SWS, have discharge activity suppressed by both AD and the A1-specific agonist cyclohexyladenosine (CHA), while discharge activity is increased by the A1 receptor antagonist, CPT. We next addressed the question of whether AD exerts its effects locally or globally. Adenosine accumulation during prolonged wakefulness occurred in the BF and neocortex, although, unlike in the BF, cortical AD levels declined in the 6th h of sleep deprivation and declined further during subsequent recovery sleep. Somewhat to our surprise, AD concentrations did not increase during prolonged wakefulness (6 h) even in regions important in behavioral state control, such as the POAH, dorsal raphe nucleus, and pedunculopontine tegmental nucleus, nor did it increase in the ventrolateral/ventroanterior thalamic nucleii. These data suggest the presence of brain region-specific differences in AD transporters and/or degradation that become evident with prolonged wakefulness, even though AD concentrations are higher in all brain sites sampled during the naturally occurring (and shorter duration) episodes of wakefulness as compared to sleep episodes in the freely moving and behaving cat. Might AD also produce modulation of activity of neurons that have sleep selective transcriptional (Fos) and discharge activity in the preoptic/anterior hypothalamus zone? Whole cell patch clamp recordings in the in vitro horizontal slice showed fast and likely GABAergic inhibitory post-synaptic potentials and currents that were greatly decreased by bath application of AD. Adenosine may thus disinhibit and promote expression of sleep-related neuronal activity in the POAH. In summary, a growing body of evidence supports the role of AD as a mediator of the sleepiness following prolonged wakefulness, a role in which its inhibitory actions on the BF wakefulness-promoting neurons may be especially important.

Effects of serotonergic 5-HT1A and 5-HT1B ligands on ventral pallidal neuronal activity

To clarify the role of the 5-HT system in limbic outputs, the present study compared the effects of the 5-HT1A agonist 8-OH-DPAT and the 5-HT1B agonist CP-94253 with the non-selective 5-HT agonist TFMPP on the firing rate of ventral pallidal (VP) neurons recorded in chloral hydrate-anesthetized rats. 8-OH-DPAT (0.25-256 microg/kg i.v.) dose-dependently enhanced (9/26 neurons) or suppressed (8/26) activity, and the 5-HT1A antagonist (+)WAY-100135 often attenuated these responses. TFMPP (0.011-1.453 mg/kg i.v.) dose-dependently reduced the firing rate of 7/8 VP neurons tested. In contrast, CP-94253 (0.013-12.8 mg/kg i.v.) had little or no effect. In sum, these data suggest that the 5-HT1A receptor appears to be particularly important in influencing limbic outputs mediated via the VP.

Catecholamine systems in the brain of vertebrates: new perspectives through a comparative approach

A comparative analysis of catecholaminergic systems in the brain and spinal cord of vertebrates forces to reconsider several aspects of the organization of catecholamine systems. Evidence has been provided for the existence of extensive, putatively catecholaminergic cell groups in the spinal cord, the pretectum, the habenular region, and cortical and subcortical telencephalic areas. Moreover, putatively dopamine- and noradrenaline-accumulating cells have been demonstrated in the hypothalamic periventricular organ of almost every non-mammalian vertebrate studied. In contrast with the classical idea that the evolution of catecholamine systems is marked by an increase in complexity going from anamniotes to amniotes, it is now evident that the brains of anamniotes contain catecholaminergic cell groups, of which the counterparts in amniotes have lost the capacity to produce catecholamines. Moreover, a segmental approach in studying the organization of catecholaminergic systems is advocated. Such an approach has recently led to the conclusion that the chemoarchitecture and connections of the basal ganglia of anamniote and amniote tetrapods are largely comparable. This review has also brought together data about the distribution of receptors and catecholaminergic fibers as well as data about developmental aspects. From these data it has become clear that there is a good match between catecholaminergic fibers and receptors, but, at many places, volume transmission seems to play an important role. Finally, although the available data are still limited, striking differences are observed in the spatiotemporal sequence of appearance of catecholaminergic cell groups, in particular those in the retina and olfactory bulb.

Effects of glutamate agonist versus procaine microinjections into the basal forebrain cholinergic cell area upon gamma and theta EEG activity and sleep-wake state

Serving as the ventral, extra-thalamic relay from the brainstem reticular activating system to the cerebral cortex, basal forebrain neurons, including importantly the cholinergic cells therein, are believed to play a significant role in eliciting and maintaining cortical activation during the states of waking and paradoxical sleep. The present study was undertaken in rats to examine the effects upon electroencephalogram (EEG) activity and sleep-wake state of inactivating basal forebrain neurons with microinjections of procaine versus activating them with microinjections of agonists of glutamate, which is the primary neurotransmitter of the brainstem reticular activating system. Microinjections into the basal forebrain were performed using a remotely controlled device in freely moving, naturally sleeping/waking rats during the day when they are asleep the majority of the time. Procaine produced a decrease in gamma (30-60 Hz) and theta (4-8 Hz) EEG activities, and an increase in delta (1-4 Hz) associated with a loss of paradoxical sleep, despite the persistence of slow wave sleep. alpha-Amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) and N-methyl-D-aspartate (NMDA) produced an increase in gamma and a decrease in delta, while eliciting waking. In addition, NMDA, which has been shown in vitro to induce rhythmic bursting in the cholinergic cells, significantly increased theta activity. Following the microinjections of NMDA, c-Fos protein, which has been shown to reflect neural activity, was found in numerous cholinergic, and also GABAergic (gamma-aminobutyric acid) and other non-cholinergic neurons, in the substantia innominata and magnocellular preoptic nucleus near the microinjection cannulae. These results substantiate the role of cholinergic, possibly together with other, basal forebrain neurons in cortical activation, including elicitation of gamma and theta activities that underlie cortical arousal during waking and paradoxical sleep.

Concurrent modulation of anxiety and memory

We have previously shown that the ventromedial prefrontal cortex (vmPFC) is involved in spontaneous working memory and anxiety-related behaviour in CD-1 mice. Specifically, pretrial microinjection of the kappa(1) agonist, U-69,593, in the infralimbic (IL) area of the vmPFC produced a robust anxiolytic behavioural profile in the elevated plus-maze and enhanced spontaneous working memory in the Y-maze. In the present study we sought to determine whether these effects were specific to IL kappa receptors. We hypothesized that microinjection of the kappa antagonist, norBNI, in the IL cortex would influence anxiety and spontaneous memory in an opposite direction to the effects produced by the kappa(1) agonist. In week 1, transfer-latency reference memory and anxiety were tested in the elevated plus-maze in two separate trials with an intertrial interval of 24 h. In week 2, spontaneous working memory was tested in the Y-maze followed immediately by defensive/withdrawal anxiety in the open field for one half of the animals in each group, and the other half was tested in reverse order. Pretreatment with one injection of vehicle, 1, 5 or 10 nmol/0.5 microl norBNI in the IL cortex dose-dependently reduced transfer-latencies and produced an anxiogenic behavioural profile in the first elevated plus-maze trial. Following a 24 h delay, transfer-latency reference memory was not influenced, but a robust anxiogenic behavioural profile was observed in the second no-injection anxiety trial in the elevated plus-maze relative to control animals. In week 2, the same groups of mice were again pretreated with one injection of the same doses of norBNI in the IL cortex and tested in the open field and Y-maze. NorBNI pretreatment was anxiogenic in the defensive/withdrawal anxiety test and disrupted spontaneous working memory regardless of testing order. The present results show the influence of kappa receptor modulation on anxiety induction and spontaneous working memory. These results also support the hypothesis that immediate memory processing may modulate the induction of anxiety-related behaviours.

Cortical cholinergic inputs mediating arousal, attentional processing and dreaming: differential afferent regulation of the basal forebrain by telencephalic and brainstem afferents

Basal forebrain corticopetal neurons participate in the mediation of arousal, specific attentional functions and rapid eye movement sleep-associated dreaming. Recent studies on the afferent regulation of basal forebrain neurons by telencephalic and brainstem inputs have provided the basis for hypotheses which, collectively, propose that the involvement of basal forebrain corticopetal projections in arousal, attention and dreaming can be dissociated on the basis of their regulation via major afferent projections. While the processing underlying sustained, selective and divided attention performance depends on the integrity of the telencephalic afferent regulation of basal forebrain corticopetal neurons, arousal-induced attentional processing (i.e. stimulus detection, selection and processing as a result of a novel, highly salient, aversive or incentive stimuli) is mediated via the ability of brainstem ascending noradrenergic projections to the basal forebrain to activate or "recruit" these telencephalic afferent circuits of the basal forebrain. In rapid eye movement sleep, both the basal forebrain and thalamic cortiocopetal projections are stimulated by cholinergic afferents originating mainly from the pedunculopontine and laterodorsal tegmenta in the brainstem. Rapid eye movement sleep-associated dreaming is described as a form of hyperattentional processing, mediated by increased activity of cortical cholinergic inputs and their cortical interactions with activated thalamic efferents. In this context, long-standing speculations about the similarities between dreaming and psychotic cognition are substantiated by describing the role of an over(re)active cortical cholinergic input system in either condition. Finally, while determination of the afferent regulation of basal forebrain corticopetal neurons in different behavioral/cognitive states assists in defining the general cognitive functions of cortical acetylcholine, this research requires a specification of the precise anatomical organization of basal forebrain afferents and their interactions in the basal forebrain. Furthermore, the present hypotheses remain incomplete because of the paucity of data concerning the regulation and role of basal forebrain non-cholinergic, particularly GABAergic, efferents.

Effect of tacrine on EEG slowing in the rat: enhancement by concurrent monoamine therapy

A dominant electrophysiological characteristic of Alzheimer's disease (AD) is the loss of desynchronized EEG activity and shift toward low-frequency EEG synchronization. In rats, similar EEG changes resulted from administering the anti-cholinergic scopolamine (1 mg/kg) and the monoamine depletor reserpine (10 mg/kg); amplitude increases between 0.5-20 Hz, with the delta (0.5-4 Hz) and theta (4-8 Hz) bands affected most severely. The acetylcholinesterase inhibitor tacrine, at doses between 10 and 20 mg/kg, reversed these EEG changes; co-administration of tacrine and the noradrenaline-serotonin reuptake inhibitor imipramine (10 mg/kg) enhanced tacrine's action to suppress delta activity. Co-administration of tacrine and the monoamine-oxidase inhibitor pargyline (20 mg/kg) enhanced EEG restoration by tacrine in all frequency bands between 0.5 to 20 Hz, but co-administration of the selective serotonin reuptake inhibitor fluoxetine (2 mg/kg) was ineffective. These results show that some drug therapies aimed at concurrently stimulating cholinergic and monoaminergic neurotransmission are more effective in reversing EEG slowing than cholinergic therapy alone. Significant monoaminergic deficits occur in Alzheimer's disease, in addition to the atrophy of cholinergic neurons. Thus, combined cholinergic-monoaminergic therapy may provide an enhanced restoration of cortical functioning, in addition to limiting the required treatment dose of cholinesterase inhibitors.

Stimulation of NMDA and AMPA receptors in the rat nucleus basalis of Meynert affects sleep

The nucleus basalis of Meynert (NBM), a heterogeneous area in the basal forebrain involved in the modulation of sleep and wakefulness, is rich in glutamate receptors, and glutamatergic fibers represent an important part of the input to this nucleus. With the use of unilateral infusions in the NBM, the effects of two different glutamatergic subtype agonists, namely N-methyl-D-aspartic acid (NMDA) and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) hydrobromide, on sleep and wakefulness parameters were determined in freely moving rats by means of polygraphic recordings. NMDA (5 nmol) and AMPA (0.4 nmol) induced an increase in wakefulness and an inhibition of slow-wave sleep. AMPA, but not NMDA, also caused a decrease in desynchronized sleep. These AMPA- and NMDA-mediated effects were counteracted by a pretreatment with the specific NMDA antagonist 2-amino-5-phosphonopentanoic acid (20 nmol) and the specific AMPA antagonist 6,7-dinitroquinoxaline-2,3-dione (2 nmol), respectively. The results reported here indicate that 1) the NBM activation of both NMDA and AMPA glutamate receptors exert a modulatory influence on sleep and wakefulness, and 2) AMPA, but not NMDA receptors, are involved in the modulation of desynchronized sleep, suggesting a different role for NBM NMDA and non-NMDA receptors in sleep modulation.

Effects of microdialysis application of monoamines on the EEG and behavioural states in the cat mesopontine tegmentum

The peri-locus coeruleus alpha (peri-LCalpha) of the mediodorsal pontine tegmentum contains cholinergic and non-cholinergic neurons, and is critically implicated in the regulation of both wakefulness and paradoxical sleep (PS). The peri-LCalpha receives dense monoaminergic (adrenergic, noradrenergic, serotonergic, dopaminergic and histaminergic) afferent projections, but little is known about their exact roles in the control of sleep-wake cycles. We have therefore examined the in vivo effects of microdialysis application of monoamines to the peri-LCalpha and adjacent cholinergic and non-cholinergic tegmental structures on behavioural states and the electroencephalogram (EEG) in freely moving cats. Norepinephrine, epinephrine and dopamine selectively inhibited PS and induced PS without atonia when applied to the caudal part of the peri-LCalpha, which mainly contains non-cholinergic descending neurons, whereas histamine and serotonin had no effect at this site. In the rostral part of the peri-LCalpha and the adjacent X area (nucleus tegmenti pedunculopontinus, pars compacta), which contain many ascending cholinergic neurons, norepinephrine and epinephrine suppressed PS with a significant increase in waking and a decrease in slow-wave sleep, as expressed by a marked decrease in the power of the cortical and hippocampal delta (0.5-2.5 Hz) and cortical alpha (8-14 Hz) bands, and an increase in the cortical gamma (30-60 Hz) band. At these sites, histamine had similar waking and EEG-desynchronizing effects, but never suppressed PS, while dopamine and serotonin had no effect. These findings indicate a special importance of the adrenergic, noradrenergic and dopaminergic systems in the inhibitory or permissive mechanisms of PS, and of the adrenergic, noradrenergic and histaminergic systems in the control of behavioural and EEG arousal.

Monoaminergic-cholinergic interactions in the primate basal forebrain

Anatomical studies in the rat have shown that the cholinergic cells of the nucleus basalis receive synapses from monoamine axons, but similar evidence is lacking in primates. We used single- and double-labeling immunocytochemistry to visualize monoamine axons and their relationship with the cholinergic cells of the basal forebrain of the monkey. Norepinephrine axons, labeled with dopamine-beta-hydroxylase antibodies, formed a bed of fine varicose axons that co-distributed with the cholinergic cells. Tyrosine hydroxylase-immunoreactive axons, presumed to be mainly dopaminergic, were 10-20 times more abundant than dopamine-beta-hydroxylase axons throughout the basal forebrain, except in the medial septal area, where their density was lower. Serotonin-immunoreactive axons formed a dense axon plexus throughout the basal forebrain. Double-labeling light microscopy demonstrated that each of the three types of monoamine axons formed frequent direct contacts with the cholinergic cells. Electron microscopy showed that the noradrenergic and the putative dopaminergic axons synapsed on the cholinergic cells. In the human brain, immunolabeling with antibodies to dopamine-beta-hydroxylase, tyrosine hydroxylase and tryptophan hydroxylase (for serotonin axons) showed axon densities in the nucleus basalis comparable to those of the monkey brain. The data demonstrate that all three of these monoamine systems innervate the cholinergic and possibly also the non-cholinergic cells of the nucleus basalis, and therefore affect the release of acetylcholine in the cerebral cortex.

Regulation of limbic motor seizures by GABA and glutamate transmission in nucleus tractus solitarius

PURPOSE

The nucleus of the solitary tract (NTS) is a primary site at which vagal afferents terminate. Because afferent vagal nerve stimulation has been demonstrated to have anticonvulsant effects, it is likely that changes in synaptic transmission in the NTS can regulate seizure susceptibility. We tested this hypothesis by examining the influence of gamma-aminobutyric acid (GABA) ergic and glutamatergic transmission in the NTS on seizures evoked by systemic and focal bicuculline and systemic pentylenetetrazol (PTZ) in rats.

METHODS

Muscimol (256 pmol), a GABA(A)-receptor agonist, bicuculline methiodide (177 pmol), a GABA(A)-receptor antagonist, kynurenate (634 pmol), a glutamate-receptor antagonist, or lidocaine (100 nl; 5%), a local anesthetic, was microinjected into the mediocaudal (m)NTS. Ten minutes later, seizure activity was induced by either a focal microinfusion of bicuculline methiodide (177 pmol) into the rostral piriform cortex, systemic PTZ (50 mg/kg, i.p.), or systemic bicuculline (0.35 mg/kg, i.v.).

RESULTS

Muscimol in mNTS (but not in adjacent regions of NTS) attenuated seizures in all seizure models tested, whereas bicuculline methiodide into mNTS did not alter seizure responses. Kynurenate infusions into mNTS significantly reduced the severity of seizures evoked both systemically and focally. Anticonvulsant effects also were obtained with lidocaine application into the same region of mNTS. Unilateral injections were sufficient to afford seizure protection.

CONCLUSIONS

Our results demonstrate that an increase in GABA transmission or a decrease in glutamate transmission in the rat mNTS reduces susceptibility to limbic motor seizures. This suggests that inhibition of mNTS outputs enhances seizure resistance in the forebrain and provides a potential mechanism for the seizure protection obtained with vagal stimulation.

Serotonergic input to cholinergic neurons in the substantia innominata and nucleus basalis magnocellularis in the rat

The aim of the present study was to determine, at the light microscopic level, whether the serotonergic fibers originating from the dorsal raphe nucleus (B7), median raphe nucleus (B8) and ventral tegmentum (B9) make putative synaptic contacts with cholinergic neurons of the nucleus basalis magnocellularis and substantia innominata. For this purpose, we utilized: (i) the anterograde transport of Phaseolus vulgaris leucoagglutinin combined with choline acetyltransferase immunohistochemistry; (ii) choline acetyltransferase/tryptophan hydroxylase double immunohistochemistry; and (iii) the FluoroGold retrograde tracer technique combined with tryptophan hydroxylase immunohistochemistry. Following iontophoretic injections of Phaseolus vulgaris leucoagglutinin in the dorsal raphe nucleus, labeling was observed primarily in the ventral aspects of the nucleus basalis magnocellularis and in the intermediate region of the substantia innominata. When Phaseolus vulgaris leucoagglutinin was combined with choline acetyltransferase immunohistochemistry, a close association between the Phaseolus vulgaris leucoagglutinin-positive fibers and cholinergic neurons was observed, even though the majority of the Phaseolus vulgaris leucoagglutinin-immunoreactive terminals seemed to establish contact with non-cholinergic elements. Following Phaseolus vulgaris leucoagglutinin injection in the median raphe nucleus, very few labeled fibers with no evident close contact with nucleus basalis magnocellularis and substantia innominata cholinergic neurons were observed. After tryptophan hydroxylase/choline acetyltransferase double immunohistochemistry, a plexus of serotonergic (tryptophan hydroxylase-positive) fibers in the vicinity of choline acetyltransferase-immunoreactive neurons of the substantia innominata and nucleus basalis magnocellularis was observed, and some serotonergic terminals have been shown to come into very close contact with the cholinergic cells. Most of the tryptophan hydroxylase-immunoreactive terminals seem to establish contacts with non-cholinergic cells. Following FluoroGold injection in the nucleus basalis magnocellularis and substantia innominata, the majority of retrogradely labeled neurons was observed mainly in the ventromedial cell group of the dorsal raphe nucleus. In this area, a minority of the FluoroGold-positive neurons was tryptophan hydroxylase immunoreactive. These findings show that serotonergic terminals, identified in very close association with the cholinergic neurons in the substantia innominata and nucleus basalis magnocellularis, derive primarily from the B7 serotonergic cell group of the dorsal raphe nucleus, and provide the neuroanatomical evidence for a direct functional interaction between these two neurotransmitter systems in the basal forebrain.

Rat diencephalic neurons producing melanin-concentrating hormone are influenced by ascending cholinergic projections

Innervation of diencephalic neurons producing melanin-concentrating hormone by choline acetyltransferase-containing axons was examined using double immunohistochemistry. In the rostromedial zona incerta and perifornical regions of the lateral hypothalamic area, many choline acetyltransferase-positive fibers were detected in the immediate vicinity of melanin-concentrating hormone perikarya and their proximal dendrites. Putative contact sites were less abundant in the far lateral hypothalamus, and only scattered close to the third ventricle. After injections of the retrograde tracer FluoroGold, most of these projections appeared to originate in the pedunculopontine and laterodorsal tegmental nuclei. Finally, to determine the putative effect of acetylcholine on the melanin-concentrating hormone neuron population, the cholinergic agonist carbachol was added to the medium of hypothalamic slices in culture. Using competitive reverse transcriptase-polymerase chain reaction, carbachol was found to induce a rapid increase in the melanin-concentrating hormone messenger RNA expression. This response was abolished by both atropine, a muscarinic antagonist, and hexamethonium, a nicotinic antagonist. Thus, the bulk of these results indicates that the diencephalic melanin-concentrating hormone neurons are targeted by activating ascending cholinergic projections.

Basal forebrain afferent projections modulating cortical acetylcholine, attention, and implications for neuropsychiatric disorders

Cortical acetylcholine (ACh) mediates the detection, selection, and processing of stimuli and associations, and the allocation of processing resources for these attentional functions. For example, loss of cortical cholinergic inputs impairs the performance of rats in tasks designed to assess sustained or divided attention. Intrabasalis infusions of benzodiazepine receptor (BZR) agonists block increases in cortical ACh efflux and impair attentional abilities. Studies on the regulation of cortical ACh efflux by nucleus accumbens (NAC) dopamine (DA) demonstrate that increases in cortical ACh efflux are attenuated by intra-accumbens administration of D1 and, more potently, D2 receptor antagonists. These and other data support the hypothesis that NAC DA, via GABAergic projections to the basal forebrain, controls the excitability of basal forebrain cholinergic neurons. As increases in NAC DA have been hypothesized to represent a major neuronal mediator of schizophrenia and the compulsive use of addictive drugs, the data predict that the abnormal regulation of cortical ACh release represents a crucial neuronal mechanism mediating the cognitive components of these psychopathological disorders.

The basal forebrain corticopetal system revisited

The medial septum, diagonal bands, ventral pallidum, substantia innominata, globus pallidus, and internal capsule contain a heterogeneous population of neurons, including cholinergic and noncholinergic (mostly GABA containing), corticopetal projection neurons, and interneurons. This highly complex brain region, which constitutes a significant part of the basal forebrain has been implicated in attention, motivation, learning, as well as in a number of neuropsychiatric disorders, such as Alzheimer's disease, Parkinson's disease, and schizophrenia. Part of the difficulty in understanding the functions of the basal forebrain, as well as the aberrant information-processing characteristics of these disease states lies in the fact that the organizational principles of this brain area remained largely elusive. On the basis of new anatomical data, it is proposed that a large part of the basal forebrain corticopetal system be organized into longitudinal bands. Considering the topographic organization of cortical afferents to different divisions of the prefrontal cortex and a similar topographic projection of these prefrontal areas to basal forebrain regions, it is suggested that several functionally segregated cortico-prefronto-basal forebrain-cortical circuits exist. It is envisaged that such specific "triangular" circuits could amplify selective attentional processing in posterior sensory cortical areas.

Projections of medullary and pontine noradrenergic neurons to the horizontal limb of the nucleus of diagonal band in the rat

Recent investigations in the rat have implicated a noradrenergic innervation to the horizontal nucleus of the diagonal band of Broca as a critical link in a neural circuit that conveys baroreceptor information centrally to inhibit the firing of vasopressin-secreting neurons in the hypothalamic supraoptic nucleus. In this study we used small intra-diagonal band injections of a retrograde tracer, rhodamine latex microspheres, in combination with tyrosine hydroxylase histochemistry to identify brainstem noradrenergic cells contributing to this innervation. In three cases where tracer injections were limited to the horizontal limb of the diagonal band, we observed 20-50 double-labelled neurons ipsilaterally in the dorsal part of the locus coeruleus (A6) and the caudal nucleus tractus solitarius (A2), and bilaterally in the caudal ventrolateral medulla (A1). Double-labelled neurons were also noted in the ventral tegmental area (dopaminergic A10 cell group). Although all major brainstem noradrenergic cell groups contribute fibers to the horizontal limb of the nucleus of diagonal band, data from physiological studies suggest that the noradrenergic A2 neurons in the nucleus tractus solitarius are the most likely pathway through which it receives this baroreceptor information.

The role of basal forebrain neurons in tonic and phasic activation of the cerebral cortex

The basal forebrain and in particular its cholinergic projections to the cerebral cortex have long been implicated in the maintenance of cortical activation. This review summarizes evidence supporting a close link between basal forebrain neuronal activity and the cortical electroencephalogram (EEG). The anatomy of basal forebrain projections and effects of acetylcholine on cortical and thalamic neurons are discussed along with the modulatory inputs to basal forebrain neurons. As both cholinergic and GABAergic basal forebrain neurons project to the cortex, identification of the transmitter specificity of basal forebrain neurons is critical for correlating their activity with the activity of cortical neurons and the EEG. Characteristics of the different basal forebrain neurons from in vitro and in vivo studies are summarized which might make it possible to identify different neuronal types. Recent evidence suggests that basal forebrain neurons activate the cortex not only tonically, as previously shown, but also phasically. Data on basal forebrain neuronal activity are presented, clearly showing that there are strong tonic and phasic correlations between the firing of individual basal forebrain cells and the cortical activity. Close analysis of temporal correlation indicates that changes in basal forebrain neuronal activity precede those in the cortex. While correlational, these data, together with the anatomical and pharmacological findings, suggest that the basal forebrain has an important role in regulating both the tonic and the phasic functioning of the cortex.

Parabrachial origin of calcitonin gene-related peptide-immunoreactive axons innervating Meynert's basal nucleus

Meynert's basal nucleus is innervated by calcitonin gene-related peptide (CGRP)-immunoreactive axons synapsing with cholinergic principal cells. Origin of CGRP-immunopositive axons was studied in the albino rat. Since beaded axons containing the nicotinic acetylcholine receptor (nAChR) are also present in the basal nucleus, the microstructural arrangement raises the question whether or not an interaction between CGRP and nAChR exists like in the neuromuscular junction. We found that electrolytic lesion of the parabrachial nucleus results in degeneration of CGRP-immunoreactive axons in the ipsilateral nucleus basalis and induces shrinkage of principal cholinergic neurons while the contralateral nucleus basalis remains intact. Electrolytic lesions in the thalamus, caudate-putamen, and hippocampus did not induce alterations in Meynert's basal nucleus. Disappearance of CGRP after lesions of the parabrachial nucleus does not impair presynaptic nAChR in the basal nucleus, suggesting that, unlike in the neuromuscular junction, CGRP is not involved in the maintenance of nAChR in the basal forebrain. It is concluded that the parabrachial nucleus is involved in the activation of the nucleus basalis-prefrontal cortex system, essential in gnostic and mnemonic functions.

Nucleus subputaminalis (Ayala): the still disregarded magnocellular component of the basal forebrain may be human specific and connected with the cortical speech area

The small magnocellular group located within the rostrolateral extension of the basal forebrain was named and described as the nucleus subputaminalis in the human and chimpanzee brain by Ayala. Analysis of cytoarchitectonic and cytochemical characteristics of this cell group has been largely disregarded in both classical and more current studies. We examined the nucleus subputaminalis in 33 neurologically normal subjects (ranging from 15 weeks of gestation to 71 years-of-age) by using Nissl staining, choline acetyltransferase immunohistochemistry, acetyl cholinesterase histochemistry and nerve growth factor receptor immunocytochemistry. In addition, we applied reduced nicotinamide adenine dinucleotide phosphate-diaphorase histochemistry and calbindin-D28k immunocytochemistry in three neurologically normal subjects. At the most rostrolateral levels we describe the previously poorly characterized component of the lateral (periputaminal) subdivision of the subputaminal nucleus, which may be human specific since it is not described in non-human primates. Moreover, we find the human subputaminal nucleus best developed at the anterointermediate level, which is the part of the basal nucleus that is usually much smaller or missing in monkeys. The location of subputaminal cholinergic neurons within the frontal lobe, the ascension of their fibers through the external capsule towards the inferior frontal gyrus, the larger size of the subputaminal nucleus on the left side at the most rostral and anterointermediate levels and the most protracted development among all magnocellular aggregations within the basal forebrain strongly suggest that they may be connected with the cortical speech area. These findings give rise to many hypotheses about the possible role of the subputaminal nucleus in various neurodegenerative, neurological and psychiatric disorders, particularly Alzheimer's disease and primary progressive aphasia. Therefore, future studies on the basal forebrain should more carefully investigate this part of the basal nucleus.

Endogenous nitric oxide in the rat pons promotes sleep

Pontine cholinergic structures are known to play a key role in the regulation of vigilance states associated with desynchronised EEG, i. e., wakefulness and paradoxical sleep. As the cholinergic cells of these nuclei, the pedunculopontine tegmentum (PPT) and the laterodorsal tegmentum, are enriched with nitric oxide synthase (NOS), we tested the hypothesis that nitric oxide (NO) in the pons is implicated in wake and sleep regulation. For this reason, a NOS inhibitor, a NO precursor and a NO donor were injected in the PPT of rats. Vigilance states were recorded for 6 h following the injections. Quantification of vigilance states after drug injections were compared to those obtained in control conditions. It appeared that the NO donor had a slight effect on vigilance states, but the NOS inhibitor decreased sleep and inversely the NO precursor increased sleep. These results show for the first time in the rat that a NOS inhibitor, injected directly into the PPT, is able to reduce sleep and that a NO precursor had the opposite effect. They suggest that endogenous NO production in the PPT has a somnogenic effect. The participation of endogenous NO in vigilance regulation is discussed in light of the role attributed to pontine cholinergic system in wakefulness and sleep.

Cholinergic neurons of the nucleus basalis of Meynert receive cholinergic, catecholaminergic and GABAergic synapses: an electron microscopic investigation in the monkey

An electron microscopic analysis of the nucleus basalis in the macaque monkey was carried out following the immunohistochemical labeling of choline acetyltransferase, either by itself or in conjunction with glutamate decarboxylase or tyrosine hydroxylase. Cholinergic axon varicosities were frequently encountered, and formed large, usually asymmetric, synapses on both choline acetyltransferase-immunopositive and -immunonegative dendrites of nucleus basalis neurons. Catecholaminergic (tyrosine hydroxylase-immunoreactive) axon varicosities formed synapses which in most cases were classified as asymmetric, and glutamate decarboxylase-immunoreactive (GABAergic) axons formed clearly symmetric synapses, each on to choline acetyltransferase-immunopositive or -immunonegative dendrites. These findings indicate that cholinergic cells in the nucleus basalis of the monkey, also known as Ch4 neurons, receive numerous synaptic inputs from cholinergic, catecholaminergic and GABAergic axons.

Test-retest variability of serotonin 5-HT2A receptor binding measured with positron emission tomography and [18F]altanserin in the human brain

The role of serotonin in CNS function and in many neuropsychiatric diseases (e.g., schizophrenia, affective disorders, degenerative dementias) support the development of a reliable measure of serotonin receptor binding in vivo in human subjects. To this end, the regional distribution and intrasubject test-retest variability of the binding of [18F]altanserin were measured as important steps in the further development of [18F]altanserin as a radiotracer for positron emission tomography (PET) studies of the serotonin 5-HT2A receptor. Two high specific activity [18F]altanserin PET studies were performed in normal control subjects (n = 8) on two separate days (2-16 days apart). Regional specific binding was assessed by distribution volume (DV), estimates that were derived using a conventional four compartment (4C) model, and the Logan graphical analysis method. For both analysis methods, levels of [18F]altanserin binding were highest in cortical areas, lower in the striatum and thalamus, and lowest in the cerebellum. Similar average differences of 13% or less were observed for the 4C model DV determined in regions with high receptor concentrations with greater variability in regions with low concentrations (16-20%). For all regions, the absolute value of the test-retest differences in the Logan DV values averaged 12% or less. The test-retest differences in the DV ratios (regional DV values normalized to the cerebellar DV) determined by both data analysis methods averaged less than 10%. The regional [18F]altanserin DV values using both of these methods were significantly correlated with literature-based values of the regional concentrations of 5-HT2A receptors determined by postmortem autoradiographic studies (r2 = 0.95, P < 0.001 for the 4C model and r2 = 0.96, P < 0.001 for the Logan method). Brain uptake studies in rats demonstrated that two different radiolabeled metabolites of [18F]altanserin (present at levels of 3-25% of the total radioactivity in human plasma 10-120 min postinjection) were able to penetrate the blood-brain barrier. However, neither of these radiolabeled metabolites bound specifically to the 5-HT2A receptor and did not interfere with the interpretation of regional [18F]altanserin-specific binding parameters obtained using either a conventional 4C model or the Logan graphical analysis method. In summary, these results demonstrate that the test-retest variability of [18F]altanserin-specific binding is comparable to that of other PET radiotracers and that the regional specific binding of [18F]altanserin in human brain was correlated with the known regional distribution of 5-HT2A receptors. These findings support the usefulness of [18F]altanserin as a radioligand for PET studies of 5-HT2A receptors.

The serotonin inhibition of high-voltage-activated calcium currents is relieved by action potential-like depolarizations in dissociated cholinergic nucleus basalis neurons of the guinea-pig

The aim of the present study was to investigate whether the voltage-dependent inhibition of calcium currents by serotonin 5-HT1A agonists can be alleviated (facilitated) by action potential-like depolarizations. In dissociated cholinergic basal forebrain neurons using whole-cell recordings, it is shown that a selective serotonin 5-HT1A agonist (8-OH-DPAT) predominantly blocks N-type HVA calcium current, although a minor reduction of P-type current was also observed. The inhibition may principally occur through Gi-Go subtypes of G-proteins because it was prevented by N-ethylmaleimide, a substance known to block specifically pertussis-sensitive G-proteins. The inhibitory effect of 8-OH-DPAT on calcium currents is voltage-dependent because it was alleviated by long-lasting depolarizing prepulses. Interestingly, the inhibition could also be reversed by prepulses made-up of action potential-like depolarizations that were given at a frequency of 200 Hz. This observation may have important implications during periods of high-frequency rhythmic bursts, a firing pattern that is prevalent in cholinergic basal forebrain neurons.

GABAergic and cholinergic basal forebrain and preoptic-anterior hypothalamic projections to the mediodorsal nucleus of the thalamus in the cat

The present study examined projections of GABAergic and cholinergic neurons from the basal forebrain and preoptic-anterior hypothalamus to the "intermediate" part of the mediodorsal nucleus of the thalamus. Retrograde transport from this region of the mediodorsal nucleus was investigated using horseradish peroxidase-conjugated wheatgerm agglutinin in combination with peroxidase-antiperoxidase immunohistochemical staining for glutamic acid decarboxylase and choline acetyltransferase. A relatively large number of retrogradely-labelled glutamic acid decarboxylase-positive neurons are located in the basal forebrain, amounting to more than 7% of the total population of glutamic acid decarboxylase-positive cells in this region. Moreover, retrogradely-labelled choline acetyltransferase-positive cells are interspersed among glutamic acid decarboxylase-positive neurons, accounting for about 6% of the total choline acetyltransferase-positive cell population in the basal forebrain. The glutamic acid decarboxylase-positive and choline acetyltransferase-positive retrogradely-labelled neurons are distributed throughout several regions of the basal forebrain, including the medial septum, the diagonal band of Broca, the magnocellular preoptic nucleus, the substantia innominata pars anterior, the substantia innominata pars posterior, and the globus pallidus where only a few retrogradely-labelled neurons were seen. The choline acetyltransferase-positive mediodorsal-projecting neurons are morphologically different from the choline acetyltransferase-positive neurons in the basal forebrain, suggesting that those projecting to the mediodorsal nucleus are a small proportion of the cholinergic neuronal population in the basal forebrain. In the preoptic-anterior hypothalamus, many retrogradely-labelled glutamic acid decarboxylase-positive cells were found, amounting to more than 7% of the total population of glutamic acid decarboxylase-positive cells in this region. These retrogradely-labelled glutamic acid decarboxylase-positive neurons are distributed throughout the preoptic-anterior hypothalamus in a continuous line with those in the basal forebrain, including the lateral preoptic area, the medial preoptic area, the bed nucleus of the stria terminalis, and the anterior and dorsal hypothalamic areas. The highest percentage of mediodorsal-projecting GABAergic neurons is in the anterior lateral hypothalamus where more than 25% of the total population of glutamic acid decarboxylase-positive cells project to the mediodorsal nucleus of the thalamus. Overall, of the large population of retrogradely-labelled neurons in the basal forebrain and preoptic-anterior hypothalamus, a significant proportion are glutamic acid decarboxylase-positive neurons (> 60% in the basal forebrain and > 30% in the preoptic-anterior hypothalamus), while the choline acetyltransferase-positive neurons amount to a smaller percentage of the neurons projecting to the mediodorsal nucleus (< 13% in the basal forebrain and < 2% in the preoptic-anterior hypothalamus). These results provide anatomical evidence of direct GABAergic projections from the basal forebrain and preoptic-anterior hypothalamic regions to the "intermediate" part of the mediodorsal nucleus in the cat. This GABAergic projection field could be the direct pathway by which the basal forebrain directly modulates thalamic excitability and may also be involved in mechanisms modulating electroencephalographic synchronization and sleep through the "intermediate" mediodorsal nucleus.

Pharmacology of sensory stimulation-evoked increases in frontal cortical acetylcholine release

Recent research has demonstrated that a variety of sensory stimuli can increase acetylcholine release in the frontal cortex of rats. The aim of the present experiments was to investigate the pharmacological regulation of sensory stimulation-induced increases in the activity of basal forebrain cholinergic neurons. To this end, the effects of agonists and antagonists at a variety of neurotransmitter receptors on basal and tactile stimulation-evoked increases in frontal cortical acetylcholine release were studied using in vivo brain microdialysis. Tactile stimulation, produced by gently stroking the rat's neck with a nylon brush for 20 min, significantly increased frontal cortical acetylcholine release by more than 100% above baseline. The noradrenergic alpha2 agonist clonidine (0.1 or 0.2 mg/kg) and alpha1 antagonist prazosin (1 mg/kg) failed to affect basal cortical acetylcholine release; however, both compounds significantly reduced the increases evoked by sensory stimulation. In contrast, the alpha2 antagonist yohimbine (3 mg/kg) increased basal cortical acetylcholine release, thereby preventing meaningful investigation of its effects on tactile stimulation-evoked increases. The benzodiazepine agonist diazepam (5 mg/kg) reduced, and the GABA(A) receptor antagonist picrotoxin (2 mg/kg) increased basal cortical acetylcholine release; in addition, diazepam attenuated the increases in cortical acetylcholine release evoked by tactile stimulation. While dopaminergic D1 (SCH 23390, 0.15 mg/kg) and D2 (raclopride, 1 mg/kg) receptor antagonists did not by themselves significantly influence the increases evoked by tactile stimulation, their co-administration produced a significant reduction. The opioid receptor antagonist naltrexone (1.5 mg/kg) failed to affect either basal or tactile stimulation-evoked increases in acetylcholine overflow. Finally, the non-competitive N-methyl-D-aspartate receptor antagonist, dizocilpine maleate (MK-801; 0.025 and 0.05 mg/kg) increased basal cortical acetylcholine release. These results confirm that cortically projecting cholinergic neurons are activated by sensory stimuli, and indicate that the increases in cortical acetylcholine release produced by tactile stimulation are inhibited by stimulation of alpha2 or blockade of alpha1 noradrenergic receptors, and by enhanced GABAergic transmission. In addition, simultaneous blockade of dopamine D1 and D2 receptors appears necessary to achieve a significant reduction of sensory stimulation-evoked acetylcholine release in the frontal cortex. The results are consistent with the hypothesis that cortical acetylcholine release is a component of the neurochemistry of arousal and/or attention and indicate that this is modulated by GABAergic, noradrenergic and dopaminergic systems. In contrast, endogenous opioid actions do not appear to be involved.

Distribution of catecholaminergic afferent fibres in the rat globus pallidus and their relations with cholinergic neurons

The topographical distribution of catecholaminergic nerve fibres and their anatomical relationship to cholinergic elements in the rat globus pallidus were studied. Peroxidase-antiperoxidase and two-colour immunoperoxidase staining procedures were used to demonstrate tyrosine hydroxylase (TH), dopamine beta-hydroxylase (DBH), phenylethanolamine N-methyltransferase (PNMT) and choline acetyltransferase (ChAT) immunoreactivities, combined with acetylcholinesterase (AChE) pharmacohistochemistry. TH immunoreactive nerve fibres were seen to enter the globus pallidus from the medial forebrain bundle. The greatest density of such fibres was found in the ventral region of the globus pallidus, which was also characterized by the greatest density of ChAT immunoreactive neurons. TH immunoreactive nerve fibres showed varicose arborizations and sparse boutons, which were occasionally seen in close opposition to cholinergic structures. In all regions of the globus pallidus, there were also larger, smooth TH immunoreactive nerve fibres of passage to the caudate putamen. A smaller number of DBH immunoreactive nerve fibres and terminal arborizations were found in the substantia innominata, internal capsule and in the globus pallidus bordering these structures. A few PNMT immunoreactive nerve fibres in the substantia innominata and internal capsule did not enter the globus pallidus. Electron microscopy revealed TH immunoreactive synaptic profiles in the ventromedial area of the globus pallidus corresponding to the nucleus basalis magnocellularis of Meynert (nBM). These made mainly symmetrical and only a few asymmetrical synaptic contacts with dendrites containing AChE reaction product. The results indicate that cholinergic structures in the nBM are innervated by dopaminergic fibres and terminals, with only a very small input from noradrenergic fibres.

Daily variation of muscarinic receptors in visual cortex but not suprachiasmatic nucleus of Syrian hamsters

Intraventricular administration of carbachol can induce phase shifts in wheel-running activity in rodents, which depend on circadian phase and are mediated via muscarinic cholinergic receptors in Syrian hamsters. We studied the circadian variation in binding of [3H]-N-methylscopolamine ([3H]NMS), a hydrophilic muscarinic receptor antagonist, in micropunches obtained from the anterior hypothalamus and occipital cortex of Syrian hamsters housed in a 14:10 light:dark cycle. Binding sites were characterized on cells contained within 1 mm punches (obtained from slices 300 microm thick), using a method to selectively detect cell surface (functional) receptors. Atropine sulphate was used to determine nonspecific binding. Cortex showed a significant daily rhythm in [3H]NMS binding with a peak occurring late in the light phase and a trough at lights on, while the hypothalamus showed no detectable rhythm. Following suprachiasmatic nucleus (SCN) ablation or maintenance in constant darkness, the rhythm in the cortex was abolished. These findings suggest that photic information conveyed via the SCN is responsible for the receptor binding rhythm in the cortex. Autoradiographic studies ([3H]NMS; 2 nM, 3 weeks exposure) clearly revealed both M1 and M2 subtypes of muscarinic receptors in the region of the SCN and the visual cortex.

Parvalbumin immunoreactive neurons in the rat septal complex have substantial glial coverage and receive few direct contacts from catecholaminergic terminals

Our previous studies have demonstrated that septohippocampal neurons in the rat septal complex have substantial glial coverage and have a number of synaptic associations with catecholaminergic terminals. While similar ultrastructural characteristics are observed for septal cholinergic neurons, the morphology and synaptic relations of catecholaminergic terminals with septal GABAergic neurons is largely unknown. Since the GABAergic septohippocampal neurons colocalize the calcium-binding protein, parvalbumin (PVA), the present study examined the ultrastructural relations of PVA neurons with catecholaminergic terminals in the septal complex. Single sections were dually labeled with antibodies to PVA and either tyrosine hydroxylase (TH) or dopamine-beta-hydroxylase (DBH). By light microscopy, processes with TH- and DBH- (TH/DBH) immunoreactivity were near PVA-labeled neurons. By electron microscopy, PVA-labeled perikarya had an average diameter of 14.9+/-6 microm and were ovoid or elongated. PVA-labeled perikarya (n = 124) had a large amount of astrocytic coverage (75+/-14%) and a low amount of terminal coverage (15+/-12%). PVA-labeled perikarya and dendrites mostly were contacted by terminals lacking immunoreactivity for either PVA or TH/DBH (82% of 1,663). Of the TH/DBH terminals or axons near PVA somata and dendrites, few (3% of 1,663) directly contacted them while the majority abutted adjacent glial or neuronal profiles. Some TH/DBH- and PVA-labeled terminals contacted the same dendrites; a few of these contained immunoreactivity for PVA. The results demonstrate that PVA-containing GABAergic septal neurons, like cholinergic neurons, are mostly surrounded by astrocytes and have very little terminal coverage. However, in contrast to cholinergic neurons, PVA-containing neurons are contacted primarily by non-catecholaminergic terminals suggesting that any functional interactions would be indirect. These findings further support the functional diversity of subpopulations of septohippocampal neurons.

Some cholinergic themes related to Alzheimer's disease: synaptology of the nucleus basalis, location of m2 receptors, interactions with amyloid metabolism, and perturbations of cortical plasticity

Cholinergic neurons in the nucleus basalis of Meynert (nbM) receive cholinergic, GABAergic and monoaminergic synapses. Only few of these neurons display the sort of intense m2 immunoreactivity that would be expected if they were expressing m2 as their presynaptic autoreceptor. The depletion of cortical m2 in Alzheimer's disease (AD) appears to reflect the loss of presynaptic autoreceptors located on incoming axons from the nucleus basalis of Meynert (nbM) and also the loss of postsynaptic receptors located on a novel group of nitric oxide producing interstitial neurons in the cerebral cortex. The defect of cholinergic transmission in AD may enhance the neurotoxicity of amyloid beta, leading to a vicious cycle which can potentially accelerate the pathological process. Because acetylcholine plays a critical role in regulating axonal growth and synaptic remodeling, the cholinergic loss in AD can perturb cortical plasticity so as to undermine the already fragile compensatory reserve of the aging cerebral cortex.

Pathophysiological mechanisms of genetic absence epilepsy in the rat

Generalized non-convulsive absence seizures are characterized by the occurrence of synchronous and bilateral spike and wave discharges (SWDs) on the electroencephalogram, that are concomitant with a behavioral arrest. Many similarities between rodent and human absence seizures support the use of genetic rodent models, in which spontaneous SWDs occur. This review summarizes data obtained on the neurophysiological and neurochemical mechanisms of absence seizures with special emphasis on the Genetic Absence Epilepsy Rats from Strasbourg (GAERS). EEG recordings from various brain regions and lesion experiments showed that the cortex, the reticular nucleus and the relay nuclei of the thalamus play a predominant role in the development of SWDs. Neither the cortex, nor the thalamus alone can sustain SWDs, indicating that both structures are intimely involved in the genesis of SWDs. Pharmacological data confirmed that both inhibitory and excitatory neurotransmissions are involved in the genesis and control of absence seizures. Whether the generation of SWDs is the result of an excessive cortical excitability, due to an unbalance between inhibition and excitation, or excessive thalamic oscillations, due to abnormal intrinsic neuronal properties under the control of inhibitory GABAergic mechanisms, remains controversial. The thalamo-cortical activity is regulated by several monoaminergic and cholinergic projections. An alteration of the activity of these different ascending inputs may induce a temporary inadequation of the functional state between the cortex and the thalamus and thus promote SWDs. The experimental data are discussed in view of these possible pathophysiological mechanisms.

Neuropharmacological characterization of basal forebrain cholinergic stimulated cataplexy in narcoleptic canines

Basal forebrain (BF) cholinergic regulation of cataplexy was investigated in narcoleptic canines. Specific cholinergic agonists and antagonists, and excitatory or inhibitory amino acid neurotransmitter receptor agonists, were perfused through microdialysis probes implanted bilaterally in the BF of narcoleptic canines. Cataplexy was monitored using the food-elicited cataplexy test (FECT) and recordings of electroencephalogram, electrooculogram, and electromyogram. In narcoleptic canines, carbachol and oxotremorine (10(-5)-10(-3) M), but not McN-A-343 or nicotine (10(-4)-10(-3) M), produced a dose-dependent increase in cataplexy. In addition, N-methyl-d-aspartate (10(-4)-10(-3) M) and kainic acid (10(-5)-10(-4) M) did not have any effects, while muscimol (10(-3) M) produced a weak (P < 0.10) increase in cataplexy. In control canines, carbachol (10(-5)-10(-3) M), but not oxotremorine (10(-4)-10(-3) M), produced muscle atonia after the highest concentration in one of three animals. Carbachol (10(-3) M)-induced cataplexy in narcoleptic canines was blocked by equimolar perfusion with the muscarinic antagonists atropine, gallamine, and 4-DAMP but not pirenzepine. These findings indicate that carbachol-stimulated cataplexy in the BF of narcoleptic canines is mediated by M2, and perhaps M3, muscarinic receptors. The release of acetylcholine in the BF was also examined during FECT and non-FECT behavioral stimulation in narcoleptic and control canines. A significant increase in acetylcholine release was found in both narcoleptic and control BF during FECT stimulation. In contrast, simple motor activity and feeding, approximating that which occurs during an FECT, did not affect acetylcholine release in the BF of narcoleptic canines. These findings indicate that BF acetylcholine release is enhanced during learned emotion/reward associated behaviors in canines.

Differential modulation of high-frequency gamma-electroencephalogram activity and sleep-wake state by noradrenaline and serotonin microinjections into the region of cholinergic basalis neurons

Several lines of evidence indicate that cholinergic basalis neurons play an important role in cortical activation. The present study was undertaken to determine the effect of noradrenergic and serotonergic modulation of the cholinergic neurons on cortical EEG activity and sleep-wake states. The neurotransmitters were injected into the region of the basalis neurons by remote control in freely moving, naturally sleeping-waking rats during the day when the rats are normally asleep the majority of the time. Effects were observed on behavior and EEG activity, including high-frequency gamma activity (30-60 Hz), which has been demonstrated to reflect behavioral and cortical arousal in the rat. Noradrenaline, which has been shown in previous in vitro studies to depolarize and excite the cholinergic cells, produced a dose-dependent increase in gamma-EEG activity, a decrease in delta activity, and an increase in waking. Serotonin, which has been found in previous in vitro studies to hyperpolarize the cholinergic neurons, produced a dose-dependent decrease in gamma-EEG activity with no significant change in amounts of wake or slow wave sleep. Both chemicals resulted in a dose-dependent decrease in paradoxical sleep. These results demonstrate that noradrenaline and serotonin exert differential modulatory effects on EEG activity through the basal forebrain, the one facilitating gamma activity and eliciting waking and the other diminishing gamma activity and not significantly affecting slow wave sleep. The results also confirm that the cholinergic basalis neurons play an important role in cortical activation and particularly in the high-frequency gamma activity that underlies cortical and behavioral arousal of the wake state.

Peptidergic innervation and the nicotinic acetylcholine receptor in the primate basal nucleus

Peptidergic innervation and localization of the neuronal nicotinic acetylcholine receptor (nAChR) was studied in the basal forebrain of Macaca fascicularis in order to provide microstructural proofs for the theory (Changeux et al., 1992) that calcitonin gene-related peptide (CGRP) is responsible for the maintenance of the acetylcholine receptor. Distribution and localization of five neuropeptides, namely substance P (SP), CGRP, neuropeptide Y (NPY), vasoactive intestinal polypeptide (VIP) neurotensin (NT), and the neuropeptides parvalbumin (PV) and the alpha-bungarotoxin- (alpha-BTX-) binding protein was studied by means of light- and electron microscopic pre-embedding immunocytochemistry. Immunohistochemical double staining revealed that large cholinergic principal nerve cells in the basal forebrain, corresponding to cell group Ch4 constituting Meynert's basal nucleus (BNM), and exerting intense choline acetyltransferase (ChAT) immunoreactivity, are synaptically innervated by axons displaying CGRP immunoreactivity. While SP, NPY, PV and CGRP establish dense networks in BNM, innervation by NT and VIP is sparse. Biotinylated alpha-BTX visualizes beaded axons that surround dendrites and perikarya of cholinergic principal cells. Electron microscopic organization of the neuropil in BNM is characterized by a glomerular (or rather cartridge-like) arrangement of axons surrounding dendrites of non-cholinergic principal nerve cells. At least one of the axons establishing the glomerulus (cartridge) exerts CGRP immunopositivity while alpha-BTX-immunopositive axons, presynaptic to dendrites of principal cells, are attached to the glomeruli (cartridges) from outside. As alpha-BTX-binding indicates localization of the alpha7 subunit of the neuronal nAChR, the microtopographical arrangement supports the idea that, in a manner similar to that in the neuromuscular junction, CGRP might contribute to the maintenance of nAChR also in BNM. Our results suggest that presynaptic nAChR-s are involved in the regulation of acetylcholine release from a feed-forward amplification mechanism of cholinergic principal cells of BNM.

Pharmacological characterization and differentiation of non-cholinergic nucleus basalis neurons in vitro

Using intracellular recordings in guinea pig brain slices, the pharmacology of electrophysiologically identified and immunohistochemically confirmed non-cholinergic nucleus basalis neurons was studied to determine their response to the major neurotransmitters of the subcortical afferents to this region. The cells were differentiated into three types: Type A cells (approximately 44%) were depolarized by noradrenaline (NA) and muscarine, Type B cells (approximately 23%) were depolarized by NA but hyperpolarized by muscarine, and Type C cells (approximately 15%) were hyperpolarized by both agonists. These cell types were also differentially responsive to serotonin (hyperpolarizing B, C) and histamine (depolarizing A, B). Accordingly, the non-cholinergic neurons share certain discharge properties but appear nonetheless to comprise distinct types which respond differentially to the major modulatory neurotransmitters and thus play potentially different roles in cortical modulation across the sleep-wake cycle.

A possible role for cholinergic neurons of the basal forebrain and pontomesencephalon in consciousness

Excitation at widely dispersed loci in the cerebral cortex may represent a neural correlate of consciousness. Accordingly, each unique combination of excited neurons would determine the content of a conscious moment. This conceptualization would be strengthened if we could identify what orchestrates the various combinations of excited neurons. In the present paper, cholinergic afferents to the cerebral cortex are hypothesized to enhance activity at specific cortical circuits and determine the content of a conscious moment by activating certain combinations of postsynaptic sites in select cortical modules. It is proposed that these selections are enabled by learning-related restructuring that simultaneously adjusts the cytoskeletal matrix at specific constellations of postsynaptic sites giving all a similar geometry. The underlying mechanism of conscious awareness hypothetically involves cholinergic mediation of linkages between microtubules and microtubule-associated protein-2 (MAP-2). The first reason for proposing this mechanism is that previous studies indicate cognitive-related changes in MAP-2 occur in cholinoceptive cells within discrete cortical modules. These cortical modules are found throughout the cerebral cortex, measure 1-2 mm2, and contain approximately 10(3)-10(4) cholinoceptive cells that are enriched with MAP-2. The subsectors of the hippocampus may function similarly to cortical modules. The second reason for proposing the current mechanism is that the MAP-2 rich cells throughout the cerebral cortex correspond almost exactly with the cortical cells containing muscarinic receptors. Many of these cholinoceptive, MAP-2 rich cells are large pyramidal cell types, but some are also small pyramidal cells and nonpyramidal types. The third reason for proposing the current mechanism is that cholinergic afferents are module-specific; cholinergic axons terminate wholly within individual cortical modules. The cholinergic afferents may be unique in this regard. Finally, the tapering apical dendrites of pyramidal cells are proposed as primary sites for cholinergic mediation of linkages between MAP-2 and microtubules because especially high amounts of MAP-2 are found here. Also, the possibility is raised that muscarinic actions on MAP-2 could modulate microtubular coherence and self-collapse, phenomena that have been suggested to underlie consciousness.

Modulation of cholinergic nucleus basalis neurons by acetylcholine and N-methyl-D-aspartate

Known to exert an important modulatory influence on the cerebral cortex, the cholinergic neurons of the basal forebrain are modulated in turn by neurotransmitters which may include acetylcholine released from processes of brainstem or forebrain neurons. In the present study, we examined the effect of carbachol, a non-specific cholinergic agonist, either alone or in the presence of N-methyl-D-aspartate upon electrophysiologically identified cholinergic basalis neurons in guinea-pig basal forebrain slices. Carbachol produced a direct postsynaptic hyperpolarization, accompanied by a decrease in membrane resistance. Muscarine could mimic this hyperpolarizing effect, whereas nicotine produced a direct postsynaptic membrane depolarization. The interaction of carbachol with N-methyl-D-aspartate was subsequently tested since, in a prior study, N-methyl-D-aspartate was shown to induce rhythmic bursting in cholinergic cells when they were hyperpolarized by continuous injection of outward current. Applied simultaneously with N-methyl-D-aspartate in the absence of current injection, carbachol was also found to promote rhythmic bursting in half of the cells tested. Since the bursts under these conditions were markedly longer in duration than those observed in the presence of N-methyl-D-aspartate alone, it was hypothesized that carbachol might have another action, in addition to the membrane hyperpolarization. Using dissociated cells, it was found that brief applications of carbachol could indeed diminish the slow afterhyperpolarizations that follow single spikes, short bursts or long trains of action potentials in cholinergic basalis neurons. These results indicate that, through its dual ability to hyperpolarize cholinergic neurons and to reduce their afterhyperpolarizations, acetylcholine can promote the occurrence of rhythmic bursting in the presence of N-methyl-D-aspartate. Accordingly, whether derived from brainstem or local sources, acetylcholine may facilitate rhythmic discharge in cholinergic basalis neurons which could in turn impose a rhythmic modulation upon cortical activity during particular states across the sleep-waking cycle.