Inhibition of synaptically evoked cortical acetylcholine release by intracortical glutamate: involvement of GABAergic neurons

Intracerebral microdialysis of adenosine and adenosine monophosphate - a systematic review and meta-regression analysis of baseline concentrations

Microdialysis is a method to study the extracellular space in vivo, based on the principle of diffusion. It can be used to measure various small molecules including the neuroregulator adenosine. Baseline levels of the compounds measured with microdialysis vary over studies. We systematically reviewed the literature to investigate the full range of reported baseline concentrations of adenosine and adenosine monophosphate in microdialysates. We performed a meta‐regression analysis to study the influence of flow rate, probe membrane surface area, species, brain area and anaesthesia versus freely behaving, on the adenosine concentration. Baseline adenosine concentrations in microdialysates ranged from 0.8 to 2100 nM. There was limited evidence on baseline adenosine monophosphate concentrations in microdialysates. Across studies, we found effects of flow rate and anaesthesia versus freely behaving on dialysate adenosine concentrations (p ≤ 0.001), but not of probe membrane surface, species, or brain area (p ≥ 0.14). With increasing flow rate, adenosine concentrations decreased. With anaesthesia, adenosine concentrations increased. The effect of other predictor variables on baseline adenosine concentrations, for example, post‐surgical recovery time, could not be analysed because of a lack of reported data. This study shows that meta‐regression can be used as an alternative to new animal experiments to answer research questions in the field of neurochemistry. However, current levels of reporting of primary studies are insufficient to reach the full potential of this approach; 63 out of 133 studies could not be included in the analysis because of insufficient reporting, and several potentially relevant factors had to be excluded from the analyses. The level of reporting of experimental detail needs to improve.

Effect of canagliflozin and metformin on cortical neurotransmitters in a diabetic rat model

BACKGROUND

The rapid economic development in the Arabian Gulf has resulted in lifestyle changes that have increased the prevalence of obesity and type 2 diabetes, with the greatest increases observed in Kuwait. Dyslipidemia and diabetes are risk factors for disruptions in cortical neurotransmitter homeostasis. This study investigated the effect of the antidiabetic medications canagliflozin (CAN) and metformin (MET) on the levels of cortical neurotransmitters in a diabetic rat model.

MATERIALS AND METHODS

The rats were assigned to the control (C) group, the diabetic group that did not receive treatment (D) or the diabetic group treated with either CAN (10 mg/kg) or MET (100 mg/kg) for 2 or 4 weeks. Blood and urine glucose levels and cortical acetylcholinesterase (AChE) activity were assayed, and amino acid and monoamine levels were measured using HPLC.

RESULTS

The diabetic group exhibited a significant increase in AChE activity and a decrease in monoamine and amino acid neurotransmitter levels. In the CAN group, AChE was significantly lower than that in the D and D + MET groups after 2 weeks of treatment. In addition, a significant increase in some cortical monoamines and amino acids was observed in the D + MET and D + CAN groups compared with the D group. Histopathological analysis revealed the presence of severe focal hemorrhage, neuronal degeneration, and cerebral blood vessel congestion, with gliosis in the cerebrum of rats in the D group. The CAN-treated group exhibited severe cerebral blood vessel congestion after 2 weeks of treatment and focal gliosis in the cerebrum after 4 weeks of treatment. Focal gliosis in the cerebrum of rats in the MET-treated group was observed after 2 and 4 weeks of treatment.

CONCLUSIONS

We conclude that the effect of CAN and MET on neurotransmitters is potentially mediated by their antihyperglycemic and antihyperlipidemic effects. In addition, the effects of CAN on neurotransmitters might be associated with its receptor activity, and the effect of MET on neurotransmitters might be associated with cerebral metabolism.

Optogenetic activation of septal cholinergic neurons suppresses sharp wave ripples and enhances theta oscillations in the hippocampus

Theta oscillations in the limbic system depend on the integrity of the medial septum. The different populations of medial septal neurons (cholinergic and GABAergic) are assumed to affect different aspects of theta oscillations. Using optogenetic stimulation of cholinergic neurons in ChAT-Cre mice, we investigated their effects on hippocampal local field potentials in both anesthetized and behaving mice. Cholinergic stimulation completely blocked sharp wave ripples and strongly suppressed the power of both slow oscillations (0.5-2 Hz in anesthetized, 0.5-4 Hz in behaving animals) and supratheta (6-10 Hz in anesthetized, 10-25 Hz in behaving animals) bands. The same stimulation robustly increased both the power and coherence of theta oscillations (2-6 Hz) in urethane-anesthetized mice. In behaving mice, cholinergic stimulation was less effective in the theta (4-10 Hz) band yet it also increased the ratio of theta/slow oscillation and theta coherence. The effects on gamma oscillations largely mirrored those of theta. These findings show that medial septal cholinergic activation can both enhance theta rhythm and suppress peri-theta frequency bands, allowing theta oscillations to dominate.

Signals from the brainstem sleep/wake centers regulate behavioral timing via the circadian clock

Sleep-wake cycling is controlled by the complex interplay between two brain systems, one which controls vigilance state, regulating the transition between sleep and wake, and the other circadian, which communicates time-of-day. Together, they align sleep appropriately with energetic need and the day-night cycle. Neural circuits connect brain stem sites that regulate vigilance state with the suprachiasmatic nucleus (SCN), the master circadian clock, but the function of these connections has been unknown. Coupling discrete stimulation of pontine nuclei controlling vigilance state with analytical chemical measurements of intra-SCN microdialysates in mouse, we found significant neurotransmitter release at the SCN and, concomitantly, resetting of behavioral circadian rhythms. Depending upon stimulus conditions and time-of-day, SCN acetylcholine and/or glutamate levels were augmented and generated shifts of behavioral rhythms. These results establish modes of neurochemical communication from brain regions controlling vigilance state to the central circadian clock, with behavioral consequences. They suggest a basis for dynamic integration across brain systems that regulate vigilance states, and a potential vulnerability to altered communication in sleep disorders.

γ-Aminobutyric acid type A receptor inhibition triggers a nicotinic neuroprotective mechanism

Nicotinic acetylcholine receptor (nAChR)-mediated neuroprotection has been implicated in the treatment of neurodegenerative disorders such as Alzheimer's and Parkinson's diseases and hypoxic ischemic events as well as other diseases hallmarked by excitotoxic and apoptotic neuronal death. Several modalities of nicotinic neuroprotection have been reported. However, although this process generally involves α4β2 and α7 subtypes, the underlying mechanisms are largely unknown. Interestingly, both activation and inhibition of α7 nAChRs have been reported to be neuroprotective. We have shown that inhibition of α7 nAChRs protects the function of acute hippocampal slices against excitotoxicity in an α4β2-dependent manner. Neuroprotection was assessed as the prevention of the N-methyl-D-aspartate-dependent loss of the area of population spikes (PSs) in the CA1 area of acute hippocampal slices. Our results support a model in which α7 AChRs control the release of γ-aminobutyric acid (GABA). Blocking either α7 or GABA(A) receptors reduces the inhibitory tone on cholinergic terminals, thereby promoting α4β2 activation, which in turn mediates neuroprotection. These results shed light on how α7 nAChR inhibition can be neuroprotective through a mechanism mediated by activation of α4β2 nAChRs.

nAChR agonist-induced cognition enhancement: integration of cognitive and neuronal mechanisms

The identification and characterization of drugs for the treatment of cognitive disorders has been hampered by the absence of comprehensive hypotheses. Such hypotheses consist of (a) a precisely defined cognitive operation that fundamentally underlies a range of cognitive abilities and capacities and, if impaired, contributes to the manifestation of diverse cognitive symptoms; (b) defined neuronal mechanisms proposed to mediate the cognitive operation of interest; (c) evidence indicating that the putative cognition enhancer facilitates these neuronal mechanisms; (d) and evidence indicating that the cognition enhancer facilitates cognitive performance by modulating these underlying neuronal mechanisms. The evidence on the neuronal and attentional effects of nAChR agonists, specifically agonists selective for alpha4beta2* nAChRs, has begun to support such a hypothesis. nAChR agonists facilitate the detection of signals by augmenting the transient increases in prefrontal cholinergic activity that are necessary for a signal to gain control over behavior in attentional contexts. The prefrontal microcircuitry mediating these effects include alpha4beta2* nAChRs situated on the terminals of thalamic inputs and the glutamatergic stimulation of cholinergic terminals via ionotropic glutamate receptors. Collectively, this evidence forms the basis for hypothesis-guided development and characterization of cognition enhancers.

Adenosine A(1) and A(2A) receptors in mouse prefrontal cortex modulate acetylcholine release and behavioral arousal

During prolonged intervals of wakefulness, brain adenosine levels rise within the basal forebrain and cortex. The view that adenosine promotes sleep is supported by the corollary that N-methylated xanthines such as caffeine increase brain and behavioral arousal by blocking adenosine receptors. The four subtypes of adenosine receptors are distributed heterogeneously throughout the brain, yet the neurotransmitter systems and brain regions through which adenosine receptor blockade causes arousal are incompletely understood. This study tested the hypothesis that adenosine A(1) and A(2A) receptors in the prefrontal cortex contribute to the regulation of behavioral and cortical arousal. Dependent measures included acetylcholine (ACh) release in the prefrontal cortex, cortical electroencephalographic (EEG) power, and time to waking after anesthesia. Sleep and wakefulness were also quantified after microinjecting an adenosine A(1) receptor antagonist into the prefrontal cortex. The results showed that adenosine A(1) and A(2A) receptors in the prefrontal cortex modulate cortical ACh release, behavioral arousal, EEG delta power, and sleep. Additional dual microdialysis studies revealed that ACh release in the pontine reticular formation is significantly altered by dialysis delivery of adenosine receptor agonists and antagonists to the prefrontal cortex. These data, and early brain transection studies demonstrating that the forebrain is not needed for sleep cycle generation, suggest that the prefrontal cortex modulates EEG and behavioral arousal via descending input to the pontine brainstem. The results provide novel evidence that adenosine A(1) receptors within the prefrontal cortex comprise part of a descending system that inhibits wakefulness.

Histological evidence for drug diffusion across the cerebral meninges into the underlying neocortex in rats

Transmeningeal pharmacotherapy has been proposed to treat neurological disorders with localized pathology, such as intractable focal epilepsy. As a step toward understanding the diffusion and intracortical spread of transmeningeally delivered drugs, the present study used histological methods to determine the extent to which a marker compound, N-methyl-D-aspartate (NMDA), can diffuse into the neocortex through the meninges. Rats were implanted with bilateral parietal cortical epidural cups filled with 50 mM NMDA on the right side and artificial cerebrospinal fluid (ACSF) in the contralateral side. After 24 h, the histological effects of these treatments were evaluated using cresyl violet (Nissl) staining. The epidural NMDA exposure caused neuronal loss that in most animals extended from the pial surface through layer V. The area indicated by this neuronal loss was localized to the neocortical region underlying the epidural cup. These results suggest that NMDA-like, water soluble, small molecules can diffuse through the subdural/subarachnoid space into the underlying neocortex and spread in a limited fashion, close to the meningeal penetration site.

Inactivation of prefrontal cortex abolishes cortical acetylcholine release evoked by sensory or sensory pathway stimulation in the rat

Sensory stimulation and electrical stimulation of sensory pathways evoke an increase in acetylcholine release from the corresponding cortical areas. The pathways by which such sensory information reaches the cholinergic neurons of the basal forebrain that are responsible for this release are unclear, but have been hypothesized to pass through the prefrontal cortex (PFC). This hypothesis was tested in urethane-anesthetized rats using microdialysis to collect acetylcholine from somatosensory, visual, or auditory cortex, before and after the PFC was inactivated by local microdialysis delivery of the GABA-A receptor agonist muscimol (0.2% for 10 min at 2 microl/min). Before PFC inactivation, peripheral sensory stimulation and ventral posterolateral thalamic stimulation evoked 60 and 105% increases, respectively, in acetylcholine release from somatosensory cortex. Stimulation of the lateral geniculate nucleus evoked a 57% increase in acetylcholine release from visual cortex and stimulation of the medial geniculate nucleus evoked a 72% increase from auditory cortex. Muscimol delivery to the PFC completely abolished each of these evoked increases (overall mean change from baseline = -7%). In addition, the spontaneous level of acetylcholine release in somatosensory, visual, and auditory cortices was reduced by 15-59% following PFC inactivation, suggesting that PFC activity has a tonic facilitatory influence on the basal forebrain cholinergic neurons. These experiments demonstrate that the PFC is necessary for sensory pathway evoked cortical ACh release and strongly support the proposed sensory cortex-to-PFC-to-basal forebrain circuit for each of these modalities.

The median preoptic nucleus reciprocally modulates activity of arousal-related and sleep-related neurons in the perifornical lateral hypothalamus

The perifornical-lateral hypothalamic area (PF/LH) contains neuronal groups playing an important role in control of waking and sleep. Among the brain regions that regulate behavioral states, one of the strongest sources of projections to the PF/LH is the median preoptic nucleus (MnPN) containing a sleep-active neuronal population. To evaluate the role of MnPN afferents in the control of PF/LH neuronal activity, we studied the responses of PF/LH cells to electrical stimulation or local chemical manipulation of the MnPN in freely moving rats. Single-pulse electrical stimulation evoked responses in 79% of recorded PF/LH neurons. No cells were activated antidromically. Direct and indirect transsynaptic effects depended on sleep-wake discharge pattern of PF/LH cells. The majority of arousal-related neurons, that is, cells discharging at maximal rates during active waking (AW) or during AW and rapid eye movement (REM) sleep, exhibited exclusively or initially inhibitory responses to stimulation. Sleep-related neurons, the cells with elevated discharge during non-REM and REM sleep or selectively active in REM sleep, exhibited exclusively or initially excitatory responses. Activation of the MnPN via microdialytic application of L-glutamate or bicuculline resulted in reduced discharge of arousal-related and in excitation of sleep-related PF/LH neurons. Deactivation of the MnPN with muscimol caused opposite effects. The results indicate that the MnPN contains subset(s) of neurons, which exert inhibitory control over arousal-related and excitatory control over sleep-related PF/LH neurons. We hypothesize that MnPN sleep-active neuronal group has both inhibitory and excitatory outputs that participate in the inhibitory control of arousal-promoting PF/LH mechanisms.

Multi-target strategies for the improved treatment of depressive states: Conceptual foundations and neuronal substrates, drug discovery and therapeutic application

Major depression is a debilitating and recurrent disorder with a substantial lifetime risk and a high social cost. Depressed patients generally display co-morbid symptoms, and depression frequently accompanies other serious disorders. Currently available drugs display limited efficacy and a pronounced delay to onset of action, and all provoke distressing side effects. Cloning of the human genome has fuelled expectations that symptomatic treatment may soon become more rapid and effective, and that depressive states may ultimately be "prevented" or "cured". In pursuing these objectives, in particular for genome-derived, non-monoaminergic targets, "specificity" of drug actions is often emphasized. That is, priority is afforded to agents that interact exclusively with a single site hypothesized as critically involved in the pathogenesis and/or control of depression. Certain highly selective drugs may prove effective, and they remain indispensable in the experimental (and clinical) evaluation of the significance of novel mechanisms. However, by analogy to other multifactorial disorders, "multi-target" agents may be better adapted to the improved treatment of depressive states. Support for this contention is garnered from a broad palette of observations, ranging from mechanisms of action of adjunctive drug combinations and electroconvulsive therapy to "network theory" analysis of the etiology and management of depressive states. The review also outlines opportunities to be exploited, and challenges to be addressed, in the discovery and characterization of drugs recognizing multiple targets. Finally, a diversity of multi-target strategies is proposed for the more efficacious and rapid control of core and co-morbid symptoms of depression, together with improved tolerance relative to currently available agents.

Tobacco cembranoids protect the function of acute hippocampal slices against NMDA by a mechanism mediated by alpha4beta2 nicotinic receptors

Nicotine has been reported to be neuroprotective in experimental and epidemiological studies. In addition to nicotine, tobacco and cigarette smoke contain cembranoids, which are antagonists of neuronal nicotinic receptors (nAChR). Exposure of hippocampal slices to N-methyl-D-aspartate (NMDA) decreases the population spikes (PS). This parameter has been used as a measure of excitotoxicity. Surprisingly, both nicotine and tobacco cembranoids protected against NMDA and this neuroprotection was not blocked by methyllycaconitine (MLA), an antagonist of alpha7 nAChR. On the contrary, MLA had a neuroprotective effect of its own. We examined the effect of the tobacco cembranoid (1S,2E,4R,6R,7E,11E)-cembra-2,7,11-triene-4,6-diol (4R) on the neuroprotection against NMDA. DHbetaE, a selective antagonist of alpha4beta2 nAChR, inhibited the neuroprotection by nicotine, 4R, and MLA, suggesting the involvement of alpha4beta2 nAChRs in the neuroprotection. The cell-signaling pathways underlying the neuroprotection by 4R and by nicotine are different. The activity of phosphatidylinositol-3 kinase (PI3K) was required in both cases; however, 4R required the activity of L-type calcium channels and CAM kinase, whereas nicotine required the extracellular signal regulated kinase-1,2 (ERK) and protein kinase C (PKC). In addition, 4R did not enhance total phospho-ERK-1/2 but increased the amount of total Akt/PKB phosphorylated on the activation site and of glycogen synthase kinase 3-beta phosphorylated on the inhibitory site. Total levels of phosphoenzymes are presented instead of the ratio of phospho- over total enzyme because in preliminary experiments total ERK-1/2 levels were slightly increased by 4R. In conclusion, these findings demonstrate that there are two different nicotinic neuroprotective mechanisms mediated by alpha4beta2.

Involvement of spinal GABA receptors in the regulation of intraspinal acetylcholine release

It has been shown that analgesics such as morphine, lidocaine and clonidine increase the release of spinal acetylcholine. Acetylcholine may therefore play an important role in the regulation of spinal pain threshold. Since behavioral as well as in vitro studies have shown a clear involvement of GABA (gamma-amino butyric acid) receptors in the regulation of spinal nociceptive mechanisms, the present study focused on the role of GABA receptors for spinal acetylcholine release control. GABA receptor agonists and antagonists were infused via a spinal microdialysis probe and acetylcholine release was measured. The GABA(A) receptor agonist muscimol decreased acetylcholine release and the antagonist bicuculline increased acetylcholine release. The GABA(B) receptor agonist baclofen decreased acetylcholine release whereas the antagonist saclofen did not change acetylcholine release. The results suggest that both GABA receptor subtypes have an inhibitory role on spinal dorsal horn acetylcholine release and that the GABA(A) receptors are tonically regulating acetylcholine release.

Pathogenic factors in vascular dementia and Alzheimer's disease. Multiple actions of heparin that probably are beneficial

The following areas are discussed in this review: atherogenesis; cerebrovascular factors; hypoperfusion; beta-amyloid production; beta-amyloid fibril formation; beta-sheets; metal cations; reactive oxygen species/free radicals; chronic inflammatory factors; endogenous plasma heparin; lipoprotein lipase; polyamines; protein kinase C; casein kinases; phospholipase A2; serine proteases; myeloperoxidase; cyclooxygenase 2; cysteine proteases; caspases; proprotein convertases; aspartic proteases; cyclin proteinases; thrombin; tau hyperphosphorylation; advanced glycosylation end products; activator protein 1; calcium; apolipoprotein E epsilon4; histamine; blood-brain barrier; glutamate; transglutaminase; insulin-like growth factor 1.

Modality- and region-specific acetylcholine release in the rat neocortex

The basal forebrain is the major source of acetylcholine in the neocortex, and this projection has been variously described as either diffuse or highly specific. We used in vivo microdialysis to examine this discrepancy by collecting acetylcholine release simultaneously from visual, somatosensory and prefrontal cortical areas. Urethane-anesthetized rats were presented with visual and somatosensory stimulation in counter-balanced order and acetylcholine was measured using HPLC. Evoked acetylcholine release was modality-specific, i.e. visual stimulation evoked a large (75%) increase from visual cortex and little (24%) change from the somatosensory area whereas skin stimulation had the opposite effect. No increase was apparent in prefrontal cortex with either stimulation protocol. This experiment extends early studies using cortical cups to collect acetylcholine, and is consistent with the concept of functional specificity within the cholinergic basal forebrain with respect to both its sensory inputs and projections to the neocortex. This functional specificity within the cholinergic basal forebrain might be utilized in the modulation of different cortical regions during selective attention and plasticity.

GABAA receptors inhibit acetylcholine release in cat pontine reticular formation: implications for REM sleep regulation

This study used in vivo microdialysis in cat (n=12) to test the hypothesis that gamma aminobutyric acid A (GABAA) receptors in the pontine reticular formation (PRF) inhibit acetylcholine (ACh) release. Animals were anesthetized with halothane to hold arousal state constant. Six concentrations of the GABAA receptor antagonist bicuculline (0.03, 0.1, 0.3, 1, 3, and 10 mM) were delivered to a dialysis probe in the PRF, and endogenously released ACh was collected simultaneously. Bicuculline caused a concentration dependent increase in ACh release (maximal increase=345%; EC50=1.3 mM; r2=0.997). Co-administration of the GABAA receptor agonist muscimol prevented the bicuculline-induced increase in ACh release. In a second series of experiments, the effects of bicuculline (0.1, 0.3, 1, and 3 mM) on ACh release were examined without the use of general anesthesia. States of wakefulness, rapid-eye-movement (REM) sleep, and non-REM sleep were identified polygraphically before and during dialysis delivery of bicuculline. Higher concentrations of bicuculline (1 and 3 mM) significantly increased ACh release during wakefulness (36%), completely suppressed non-REM sleep, and increased ACh release during REM sleep (143%). The finding that ACh release in the PRF is modulated by GABAA receptors is consistent with the interpretation that inhibition of GABAergic transmission in the PRF contributes to the generation of REM sleep, in part, by increasing pontine ACh release.

Serotonin receptors: their key role in drugs to treat schizophrenia

Serotonin (5-HT)-receptor-based mechanisms have been postulated to play a critical role in the action of the new generation of antipsychotic drugs (APDs) that are usually referred to as atypical APDs because of their ability to achieve an antipsychotic effect with lower rates of extrapyramidal side effects (EPS) compared to first-generation APDs such as haloperidol. Specifically, it has been proposed by Meltzer et al. [J. Pharmacol. Exp. Ther. 251 (1989) 238] that potent 5-HT2A receptor antagonism together with weak dopamine (DA) D2 receptor antagonism are the principal pharmacologic features that differentiate clozapine and other apparent atypical APDs from first-generation typical APD. This hypothesis is consistent with the atypical features of quetiapine, olanzapine, risperidone, and ziprasidone, which are the most common treatments for schizophrenia in the United States and many other countries, as well as a large number of compounds in various stages of development. Subsequent research showed that 5-HT1A agonism may be an important consequence of 5-HT2A antagonism and that substitution of 5-HT1A agonism for 5-HT2A antagonism may also produce an atypical APD drug when coupled with weak D2 antagonism. Aripiprazole, the most recently introduced atypical APD, and a D2 receptor partial agonist, may also owe some of its atypical properties to its net effect of weak D2 antagonism, 5-HT2A antagonism and 5-HT1A agonism [Eur. J. Pharmacol. 441 (2002) 137]. By contrast, the alternative "fast-off" hypothesis of Kapur and Seeman [Am. J. Psychiatry 158 (2001) 360] applies only to clozapine and quetiapine and is inconsistent with the "slow" off rate of most atypical APDs, including olanzapine, risperidone and ziprasidone. 5-HT2A and 5-HT1A receptors located on glutamatergic pyramidal neurons in the cortex and hippocampus, 5-HT2A receptors on the cell bodies of DA neurons in the ventral tegmentum and substantia nigra and GABAergic interneurons in the cortex and hippocampus, and 5-HT1A receptors in the raphe nuclei are likely to be important sites of action of the atypical APDs. At the same time, evidence has accumulated for the important modulatory role of 5-HT2C and 5-HT6 receptors for some of the effects of some of the current APDs. Thus, 5-HT has joined DA as a critical target for developing effective APDs and led to the search for novel drugs with complex pharmacology, ending the exclusive search for single-receptor targets, e.g., the D3 or D4 receptor, and drugs that are selective for them.

The thalamo-cortical auditory receptive fields: regulation by the states of vigilance, learning and the neuromodulatory systems

The goal of this review is twofold. First, it aims to describe the dynamic regulation that constantly shapes the receptive fields (RFs) and maps in the thalamo-cortical sensory systems of undrugged animals. Second, it aims to discuss several important issues that remain unresolved at the intersection between behavioral neurosciences and sensory physiology. A first section presents the RF modulations observed when an undrugged animal spontaneously shifts from waking to slow-wave sleep or to paradoxical sleep (also called REM sleep). A second section shows that, in contrast with the general changes described in the first section, behavioral training can induce selective effects which favor the stimulus that has acquired significance during learning. A third section reviews the effects triggered by two major neuromodulators of the thalamo-cortical system--acetylcholine and noradrenaline--which are traditionally involved both in the switch of vigilance states and in learning experiences. The conclusion argues that because the receptive fields and maps of an awake animal are continuously modulated from minute to minute, learning-induced sensory plasticity can be viewed as a "crystallization" of the receptive fields and maps in one of the multiple possible states. Studying the interplays between neuromodulators can help understanding the neurobiological foundations of this dynamic regulation.

Cholinergic modulation of the cortical neuronal network

Acetylcholine (ACh) is an important neurotransmitter of the CNS that binds both nicotinic and muscarinic receptors to exert its action. However, the mechanisms underlying the effects of cholinergic receptors have still not been completely elucidated. Central cholinergic neurons, mainly located in basal forebrain, send their projections to different structures including the cortex. The cortical innervation is diffuse and roughly topographic, which has prompted some authors to suspect a modulating role of ACh on the activity of the cortical network rather than a direct synaptic role. The cholinergic system is implicated in functional, behavioural and pathological states including cognitive function, nicotine addiction, Alzheimer's disease, Tourette's syndrome, epilepsies and schizophrenia. As these processes depend on the activation of glutamatergic and GABAergic systems, the cholinergic terminals must exert their effects via the modulation of excitatory and/or inhibitory neurotransmission. However, the understanding of cholinergic modulation is complex because it is the result of a mixture of positive and negative modulation, implying that there are various types, or even subtypes, of cholinergic receptors. In this review, we summarize the current knowledge on central cholinergic systems (projections and receptors) and then aim to focus on the implications for ACh in the modulation of cortical neuronal activity.

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.