The origins of cholinergic and other subcortical afferents to the thalamus in the rat

Afferent connections of the anterior thalamus in rabbits

This study was designed to determine whether axons of cholinergic dorsal tegmental neurons terminate on cells in the anterior thalamus in rabbits as in other species, and to localize projecting tegmental cells for future studies of their contributions to anterior thalamic learning-relevant neuronal activity. The distribution of retrogradely labeled neurons was examined following injections of wheat germ agglutinin horseradish peroxidase (WGA-HRP) centered in the anterior ventral (AV) thalamic nucleus. The results confirm past findings in rabbits indicating projections to anterior thalamus from the mammillary nuclei, the posterior cingulate cortex, presubiculum and postsubiculum. Demonstrated for the first time in rabbits were projections from the lateral dorsal and the pedunculopontine tegmental nuclei, locus coeruleus, dorsal raphe nucleus, Gudden's dorsal tegmental nucleus, pretectum and reticular thalamic nucleus.

Thalamocortical oscillations in the sleeping and aroused brain

Sleep is characterized by synchronized events in billions of synaptically coupled neurons in thalamocortical systems. The activation of a series of neuromodulatory transmitter systems during awakening blocks low-frequency oscillations, induces fast rhythms, and allows the brain to recover full responsiveness. Analysis of cortical and thalamic networks at many levels, from molecules to single neurons to large neuronal assemblies, with a variety of techniques, ranging from intracellular recordings in vivo and in vitro to computer simulations, is beginning to yield insights into the mechanisms of the generation, modulation, and function of brain oscillations.

Branching projections from mesopontine nuclei to the nucleus reticularis and related thalamic nuclei: a double labelling study in the rat

Branching projections from pedunculopontine and laterodorsal tegmental nuclei to different thalamic targets were studied by means of a double retrograde tracing technique. The results show a topographic distribution of mesopontine neurons projecting to different thalamic targets. In addition, the present data demonstrate that a small percentage (< or = 5%) of mesopontine neurons projecting to the intralaminar nuclei or to the rostral pole of the reticular nucleus innervate both these areas by means of branching axons. By contrast, a large number of mesopontine neurons projecting to the sensorimotor thalamic nuclei send axon collaterals to the caudal part of the reticular nucleus. The present findings support the hypothesis of an inhomogeneity of different sectors of the thalamic reticular nucleus. Thus, this nucleus can be differentiated into two functional areas, in accordance with their connections with functionally different cortical fields and thalamic districts. The possibility that these two areas of the thalamic reticular nucleus subserve different mechanisms during sleep phenomena is discussed.

Spontaneous activity in the thalamic reticular nucleus during the sleep/wake cycle of the freely-moving rat

Neurons of the somatosensory thalamic reticular nucleus (TRN) were studied by extracellular recordings through the sleep/wake cycle in the unanesthetized, freely-moving rat. All electrophysiologically-identified TRN neurons expressed rhythmic patterns of discharge that altered with shifts in sleep/wake state. During slow wave (SW) sleep, neurons displayed spike-burst discharges in long trains followed by pauses. high-frequency oscillations in auto-correlograms in the spindle-frequency range (approximately 10 Hz) reflected the rhythm of interburst intervals within the trains whereas low-frequency oscillations (0.3-0.2 Hz) displayed the rhythm of intertrain intervals. During rapid eye movement (REM) sleep, a more continuous pattern of spike-burst discharges was prominent, resulting in absence of a detectable, low-frequency rhythm but persistence of spindle-frequency firing. At the transitions between SW and REM sleep, cell discharge was more tonic than during either sleep state and lacked a dominant rhythm. During the wake (AW) state, neurons fired in a single-spike mode that also lacked rhythmicity. Unlike their pattern of discharge, TRN neurons' mean rate of discharge did not distinguish sleep/wake state. The mean discharge rates were: SW, 18.4 +/- 1.3; REM, 17.4 +/- 1.2; AW, 22.3 +/- 2.1 (Hz +/- S.E.M.). Mean discharge rate during transitions from SW to REM sleep (28.6 +/- 2.1) was significantly higher, however, than during any sleep/wake state. Compelling evidence was lacking for segregation of TRN neurons into discrete populations according to absolute discharge rate. Neurons recorded simultaneously from the same electrode discharged synchronous trains of spike-bursts and pauses during SW sleep. This phenomenon may be related to generation of EEG slow waves.(ABSTRACT TRUNCATED AT 250 WORDS)

Basal forebrain control of cortical cerebral blood flow is independent of local cortical neurons

To determine whether intrinsic cortical neurons participate in mediating increases in cortical cerebral blood flow (CBF) in response to electrical stimulation of the basal forebrain (BF), cortical CBF was assessed by laser-Doppler flowmetry in rats before and after unilaterally removing local cortical neurons with the excitotoxin ibotenic acid (IBO). On the first day of testing, CBF responses to right and left BF stimulation were nearly identical in right and left frontal cortices, corresponding to the frequency of stimulation, up to a maximum at 25 Hz (+180%). Subsequently, animals received a unilateral microinjection of IBO and a contralateral microinjection of phosphate-buffered saline (PBS) into the responsive cortical sites. After five days, responses in lesioned cortices were remarkably intact both in comparison to the contralateral PBS-injected site and to the same site tested prior to lesioning on day 1. IBO lesions of the response sites were histologically confirmed to extend through the entire depth of the frontal cortex and to encompass a large surface area (7.7 +/- 0.5 mm2). These results indicate that local cortical neurons are not critical to the mediation of increases in cortical CBF as elicited by BF stimulation. This study further supports the role of the BF as a distinct intracerebral neurogenic regulator of cortical CBF.

gamma-Hydroxybutyric acid induced spike and wave discharges in rats: relation to high-affinity [3H]gamma-hydroxybutyric acid binding sites in the thalamus and cortex

gamma-Hydroxybutyric acid is a naturally occurring compound which induces bilaterally synchronous spike and wave discharges in rats. The gamma-hydroxybutyric acid model of absence seizures simulates clinical absence seizures behaviorally as well as electrographically. The present study was undertaken in order to establish the role of the high-affinity gamma-hydroxybutyric acid binding sites in the generation of gamma-hydroxybutyric acid-induced spike and wave discharges. Spike and wave discharges induced by gamma-hydroxybutyric acid were recorded with the aid of bipolar depth electrodes implanted in discrete regions of thalamus, cortex and hippocampus. In the present study we found that ventroposterolateral, ventroposteromedial, medial and the reticular nuclei of the thalamus discharged synchronously with the cortical generation of spike and wave discharges. In the cortex, the superficial layers (I-IV) of frontoparietal cortex generated spike and wave discharges, whereas no spike and wave discharges were recorded from deeper layers (V-VI) of frontoparietal cortex. At the onset of spike and wave discharges induced by gamma-hydroxybutyric acid, a rapid but reversible upregulation of gamma-hydroxybutyric acid binding sites was observed. This increased [3H]gamma-hydroxybutyric acid binding was characterized by an increase in the number of gamma-hydroxybutyric acid sites with no significant change in their affinity for gamma-hydroxybutyric acid. Moreover, the change in [3H]gamma-hydroxybutyric acid binding was observed only in those thalamic structures and cortical layers which were found to be involved in the generation of spike and wave discharges induced by gamma-hydroxybutyric acid. The CA3 field or dorsal hippocampus possesses the highest density of [3H]gamma-hydroxybutyric acid binding sites of all brain regions. However, no significant change in [3H]gamma-hydroxybutyric acid binding was observed in this region nor was the CA3 field involved in the generation of spike and wave discharges during gamma-hydroxybutyric acid-induced absence-like seizures. These findings confirm that gamma-hydroxybutyric acid-induced absence-like seizures originate from thalamocortical pathways and that the onset of gamma-hydroxybutyric acid-induced spike and wave discharges is directly related to the regulation of gamma-hydroxybutyric acid binding sites in those regions which constitute the involved thalamocortical loop.

Mechanisms of oscillatory activity in guinea-pig nucleus reticularis thalami in vitro: a mammalian pacemaker

1. The ionic mechanisms of rhythmic burst firing and single spike, tonic discharge were investigated with extracellular and intracellular recordings of single neurones in the guinea-pig nucleus reticularis thalami (NRT) maintained as a slice in vitro. 2. Activation of cortical/thalamic afferents to NRT neurones resulted in a short latency burst of action potentials which could be followed by a rhythmic sequence of oscillatory burst firing. Intracellularly, this oscillatory activity was associated with an alternating sequence of low threshold Ca2+ spikes separated by after-hyperpolarizing potentials. Intracellular injection of short duration hyperpolarizing current pulses resulted in a similar sequence of oscillatory burst firing, suggesting that this activity is an intrinsic property of NRT cells. The frequency of rhythmic burst firing was highly voltage and temperature dependent and was between 7-12 Hz at -65 to -60 mV at 38 degrees C. In addition, at depolarized membrane potentials, oscillatory burst firing was typically followed by a prolonged tail of single spike activity. 3. Application of the Na+ channel poison tetrodotoxin blocked the generation of fast action potentials, but left intact the rhythmic sequence of low threshold Ca2+ spikes separated by after-hyperpolarizing potentials (AHPs). The reversal potential of the AHPs was -94 mV, suggesting that it was mediated by an increase in K+ conductance. Extracellular application of tetraethylammonium or apamin, or intracellular injection of Cs+ or the Ca2+ chelating agent EGTA, blocked the Ca2+ spike AHP, indicating that it is mediated by a Ca(2+)-activated K+ current. 4. Block of the AHP resulted in the marked enhancement of a slow after-depolarizing potential (ADP). The slow ADP occurred only following the generation of low threshold Ca2+ spikes. Replacement of extracellular Ca2+ with Mg2+ or Sr2+ resulted in an abolition of the slow ADP. In addition, the increase in [Mg2+]o resulted in an abolition of the low threshold Ca2+ spike. In contrast, replacement of extracellular Ca2+ with Ba2+ did not abolish the slow ADP. These results indicate that the ADP can be activated by either Ca2+ or Ba2+, but not by Mg2+ or Sr2+. 5. Replacement of extracellular Na+ with choline+ did not abolish the slow ADP, while replacement with N-methyl-D-glucamine+ did, indicating that the slow ADP can be supported by choline+, but not by N-methyl-D-glucamine+. Neither chemical affected the low threshold Ca2+ spike. These results are consistent with the slow ADP being mediated by a Ca(2+)-activated non-selective cation (CAN) current.(ABSTRACT TRUNCATED AT 400 WORDS)

Cell-specific expression of preproenkephalin intronic heteronuclear RNA in the rat forebrain

Using in situ hybridization with multiple probes to the rat preproenkephalin gene, we have identified a novel population of cells in the reticular thalamic nucleus and basal forebrain which express RNA derived from the preproenkephalin gene. These cells contain nuclear RNA from downstream of an alternate transcription start site in intron A of the preproenkephalin gene (Kilpatrick et al., Mol. Cell Biol., 10 (1990) 3717-3726), while in the same cells preproenkephalin exon 2 RNA is undetectable. The results suggest that in this population of cells, preproenkephalin gene transcription initiates from the intron A initiation site, and is regulated by an additional mechanism which results in the accumulation of nuclear preproenkephalin intron A-derived heteronuclear RNA. The anatomical distribution of these cells indicates that they may be involved in the control of cerebral cortical function.

Electrophysiology of the dorsolateral mesopontine reticular formation during Pavlovian conditioning in the rabbit

Extracellular single-unit recording methods were used to study the activity of neurons within a restricted portion of the dorsolateral mesopontine reticular formation, an area which includes the parabrachial, pedunculopontine tegmental and cuneiform nuclei. Recordings were obtained during presentations of unfamiliar and familiar sensory stimuli, during Pavlovian differential conditioning procedures that elicited conditioned bradycardia, and while stimulating the amygdaloid central nucleus to identify neurons that projected to, or received projections from, the amygdaloid central nucleus. Activity in most dorsolateral mesopontine reticular neurons was altered during sensory stimulation, and the convergence of auditory and somatic inputs onto single neurons was common. Moreover, neural responses were often of a different magnitude and/or direction to auditory stimuli that were unfamiliar vs familiar vs reinforced (paired with pinna stimulation), and many of these differentially responsive neurons were activated orthodromically by stimulation of the amygdaloid central nucleus. In contrast, neurons activated antidromically by stimulation of the amygdaloid central nucleus were relatively quiescent during all phases of the experiment. Results are discussed in relation to current hypotheses concerning the functional significance of various neuronal subpopulations within the dorsolateral mesopontine reticular formation during Pavlovian conditioning.

Distribution of choline acetyltransferase immunoreactivity in the brain of the lizard Gallotia galloti

The aim of the present study is to provide a complete description of the distribution of choline acetyltransferase (ChAT) immunoreactivity (i) in the brain of the lizard Gallotia galloti, on the basis of two different primary antisera: rat anti-ChAT and rabbit anti-chicken ChAT. Considering that the brain is a segmented structure, we have analysed our data with respect to transverse segmental domains (or neuromeres), which have been previously described by several authors in the brain of vertebrates. In the telencephalon, ChATi neurons are seen in the cortex, anterior dorsal ventricular ridge, basal ganglia, diagonal band, and bed nucleus of the stria terminalis. Further caudally, ChATi cell bodies are located in the preoptic area, hypothalamus, habenula, isthmus, and all motor efferent centers of the brainstem and spinal cord. Plexuses of ChATi fibers are observed in the areas containing cholinergic cell bodies. In addition, distinct plexuses are found in the cortex, the posterior dorsal ventricular ridge, the neuropiles of all primary visual centers of the diencephalon and mesencephalon, and several non-visual nuclei of the brainstem. The distribution of ChAT immunoreactivity in the brain of G. galloti resembles in many respects that of other vertebrates, and differences are mainly observed in the pretectum and midbrain tectum. Transverse segmental domains were identified in the brainstem and forebrain of Gallotia when the cranial nerve roots and fiber tracts were used as a reference, and most cranial motor nuclei were found to occupy the same segmental positions as have been reported in the chick.

Differential localization of NADPH-diaphorase and calbindin-D28k within the cholinergic neurons of the basal forebrain, striatum and brainstem in the rat, monkey, baboon and human

The localization of Calbindin-D28k and NADPH-diaphorase in the cholinergic neurons of the basal forebrain, striatum and brainstem was investigated in the rat, monkey, baboon and human using calbindin and choline acetyltransferase immunohistochemistry and NADPH-diaphorase histochemistry. Considerable regional and species-specific variations were observed. Double-stained sections demonstrated that NADPH-diaphorase activity occurred in as much as 20-30% of basal forebrain cholinergic neurons in the rat but in virtually none of those neurons in the monkey, baboon or human. In all of the species studied, virtually every cholinergic neuron within the pedunculopontine and laterodorsal tegmental nuclei contained NADPH-diaphorase activity, while none of the cholinergic neurons of the striatum did so. In the rat brain, calbindin immunoreactivity was not present in any of the cholinergic neurons of the basal forebrain, while in the primate brain virtually all of the basal forebrain cholinergic neurons were also calbindin-positive. None of the cholinergic neurons of the striatum, pedunculopontine nucleus or laterodorsal tegmental nucleus were found to be calbindin-positive in any of the species examined. These results demonstrate major species-specific differences in the cytochemical signatures of the basal forebrain cholinergic neurons, in contrast to the cholinergic neurons of the striatum and brainstem, which displayed little interspecies variation with respect to the markers that were used in this study. Our findings also suggest that caution must be exercised in using results from studies of rodent basal forebrain cholinergic systems to infer the role of this system in the primate brain.

Effects of cholinergic drugs on genetic absence seizures in rats

Wistar rats of a selected strain show spontaneous generalized non-convulsive seizures with bilateral synchronous spike-wave discharges on the cortical electroencephalograph (EEG). The 7 to 9 c/s spike-wave discharges occur predominantly in waking states of inactivity. The effects of cholinergic drugs on the cumulated duration of spike-wave discharges were investigated in this rat model of absence epilepsy. I.p. injections of drugs which potentiate cholinergic neurotransmission, namely the acetylcholinesterase inhibitor, physostigmine (0.1-0.5 mg/kg), the muscarinic receptor agonists, oxotremorine (0.25-1 mg/kg) and pilocarpine (0.125-2 mg/kg), and the nicotinic receptor agonist, nicotine (0.062-2 mg/kg), suppressed discharges in a dose-dependent manner and induced an arousal-like cortical EEG. The muscarinic receptor antagonist, scopolamine, increased the spike-wave discharges at doses below 0.05 mg/kg; at higher doses (0.05-1 mg/kg) it decreased discharges and induced a sleep-like EEG. The nicotinic receptor antagonist, mecamylamine (0.5-6 mg/kg), had no effect on spike-wave discharges or the EEG. These results suggest that cholinergic activity accounts for the preferential occurrence of absence seizures in states of reduced arousal.

Nerve growth factor receptor-containing cholinergic neurons of the basal forebrain project to the thalamic reticular nucleus in the rat

The origin of nerve growth factor receptor-immunoreactive (NGFr-ir) fibers innervating the thalamic reticular nucleus (Rt) was here investigated in the rat using retrograde tracers in combination with immunocytochemistry. Neurons retrogradely labeled from Rt were scattered ipsilaterally throughout the medial septal nucleus and the other cell groups of the basal forebrain, which contained NGFr-ir cells; 10-20% of these retrogradely labeled neurons were also NGFr-ir. Furthermore, a few retrogradely labeled NGFr-ir cells were detected in the basal forebrain on the contralateral side. Retrograde tracing combined with a double immunocytochemical procedure revealed that all the NGFr-ir neurons labeled from Rt also displayed immunoreactivity for choline acetyltransferase. The present results demonstrate that the NGFr-ir neurons of the basal forebrain which project to Rt are cholinergic. The possible functional implications of these findings are discussed.

Collateral axons of cholinergic pontine neurones projecting to midline, mediodorsal and parafascicular thalamic nuclei in the rat

The organization of collateral axons projecting from neurones in the pontine laterodorsal tegmental nucleus (LDTg) has been examined using combinations of retrograde neuronal tracers with immunocytochemical markers for the acetylcholine-synthesizing enzyme choline acetyltransferase (CHAT), focussing on projections to the midline, mediodorsal and parafascicular thalamic nuclei and the ventral tegmental area. 25-59% of LDTg neurones projecting to the mediodorsal nucleus provided collaterals to the midline nuclei. Virtually all (87-96%) of these double retrogradely labelled neurones appeared cholinergic. 9-18% of LDTg neurones projecting to the parafascicular nuclei also provided a collateral to the midline nuclei and 50-78% of these double retrogradely labelled neurones stained for CHAT. 26-29% of the single LDTg neurones which projected collaterals to both the mediodorsal and midline nuclei, were found to project a third collateral to the ventral tegmental area. These anatomical findings, taken together with functional evidence, suggest that cholinergic terminals arising from LDTg are involved in coordinating thalamic mechanisms of brain state control; and in regulating dopaminergic pathways, both directly and via the thalamus.

Control of 40-Hz firing of reticular thalamic cells by neurotransmitters

This study bears on the control exerted by neurotransmitters on the expression of a 40-Hz pacemaker activity observed in reticular thalamic cells. Experiments were conducted in urethane-anaesthetized rats using extracellular recordings and local applications of antagonists against the neurotransmitters involved in the modulation of reticular thalamic cells. All drugs were dissolved in a Ringer's solution (pH 7.4) and were applied in small quantities (25-150 nl) by pressure through one barrel of a micropipette assembly. Forty-Hertz firing was abolished by local application of the alpha 1 antagonist prazosin and by bilateral lesion of the locus coeruleus. Local applications of glutamate antagonists reduced the rate of discharges by 30-50% as did cortical cooling or complete transection of the internal capsule. Conversely, scopolamine exerted a permissive action on the expression of 40-Hz activities; many spontaneously bursting units started firing at 40 Hz under the influence of this muscarinic antagonist. Since reticular thalamic cells are GABAergic and synaptically coupled via axonal collaterals, we investigated how GABAergic drugs affected the regular firing of these cells. Local applications of bicuculline produced a transient increase of the firing rates while the application of GABA induced intermittent pauses on a background of regular discharges. The application of piperidine-4-sulphonic acid, a GABAA receptor agonist, produced a similar effect. The length of pauses generated by piperidine was statistically analysed. It was found that the duration of short pauses was a multiple integer of the mean interspike interval of surrounding discharges. The preservation of the period and phase of the rhythm across the pauses implies that a subthreshold oscillation was presented into the cells during the arrests of discharges. Given the mode of action of noradrenaline and acetylcholine on reticular thalamic neurons, and considering a possible metabotropic action of glutamate, the above results suggest that deactivation of a leaky K conductance is critically involved in the regular firing of these cells in urethane-anaesthetized rats. Alternatively, because reticular cells are coupled via inhibitory synapses, it is proposed that the 40-Hz firing frequency reflects, in the frequency domain, a point of equilibrium in the reticular thalamic network when the leaky K conductance is fully deactivated by the metabotropic effects of monoamines and/or excitatory amino acids.

In situ hybridization localization of choline acetyltransferase mRNA in adult rat brain and spinal cord

The cellular distribution of choline acetyltransferase (ChAT) mRNA within the adult rat central nervous system was evaluated using in situ hybridization. In forebrain, hybridization of a 35S-labeled rat ChAT cRNA densely labeled neurons in the well-characterized basal forebrain cholinergic system including the medial septal nucleus, diagonal bands of Broca, nucleus basalis of Meynert and substantia innominata, as well as in the striatum, ventral pallidum, and olfactory tubercle. A small number of lightly labeled neurons were distributed throughout neocortex, primarily in superficial layers. No cellular labeling was detected in hippocampus. In the diencephalon, dense hybridization labeled neurons in the ventral aspect of the medial habenular nucleus whereas cells in the lateral hypothalamic area and supramammillary region were more lightly labeled. Hybridization was most dense in neurons of the motor and autonomic cranial nerve nuclei including the oculomotor, Edinger-Westphal, and trochlear nuclei of the midbrain, the abducens, superior salivatory, trigeminal, facial and accessory facial nuclei of the pons, and the hypoglossal, vagus, and solitary nuclei and nucleus ambiguous of the medulla. In addition, numerous cells in the pedunculopontine and laterodorsal tegmental nuclei, the ventral nucleus of the lateral lemniscus, the medial and lateral divisions of the parabrachial nucleus, and the medial and lateral superior olive were labeled. Occasional labeled neurons were distributed in the giantocellular, intermediate, and parvocellular reticular nuclei, and the raphe magnus nucleus. In the medulla, light to moderately densely labeled cells were scattered in the nucleus of Probst's bundle, the medial vestibular nucleus, the lateral reticular nucleus, and the raphe obscurus nucleus. In spinal cord, the cRNA densely labeled motor neurons of the ventral horn, and cells in the intermediolateral column, surrounding the central canal, and in the spinal accessory nucleus. These results are in good agreement with reports of the immunohistochemical localization of ChAT and provide further evidence that cholinergic neurons are present within neocortex but not hippocampus.

Effects of some cholinergic agonists on neocortical slow wave activity in rats with basal forebrain lesions

Chronic rats, prepared with unilateral injections of kainic acid in the left basal forebrain, displayed prominent large amplitude slow wave activity in the neocortex ipsilateral to the injection. Oxotremorine and pilocarpine, given systemically following pretreatment with methyl scopolamine to block peripheral muscarinic effects, restored low voltage fast activity (LVFA) in a dose-related manner. Oxotremorine was more potent than pilocarpine. Arecoline was not consistently effective. Tetrahydroaminoacridine abolished abnormal 4-6 Hz rhythmical slow waves in the left neocortex but had little effect on large amplitude irregular slow waves. Direct-acting cholinergic agonists can restore near-normal neocortical activity after extensive cholinergic deafferentation of the neocortex.

The organization of central cholinergic systems and their functional importance in sleep-waking states

Since the demonstration some 50 years ago of the presence and synthesis of acetylcholine (ACh) in specific neuronal systems within the brain, a wealth of information concerning the organization and functional importance of central cholinergic neurons has emerged through immunohistochemical, neuroanatomical, pharmacological, biochemical and neurophysiological studies. Many of the original theses have proven valid concerning the key structural and functional position of cholinergic neurons within the central reticular core of the brain, where the basic sleep-waking cycle is determined. The two major cholinergic cell groups of this core, one within the pontomesencephalic tegmentum that projects rostrally into the non-specific thalamo-cortical relay system and the other within the basal forebrain that receives input from the brainstem reticular formation and projects in turn as the ventral, extrathalamic relay upon the cerebral cortex, are critically involved in processes of cerebral activation that accompany the states of wakefulness and paradoxical sleep. By interaction with other cell groups, including monoaminergic and GABAergic neurons, and by differential modes of firing, the cholinergic neurons may furthermore shape the responsiveness and activity of the reticular core and thalamo-cortical systems across the sleep-waking cycle.

Cholinergic innervation of the mediodorsal thalamic nucleus in the monkey: ultrastructural evidence supportive of functional diversity

The ultrastructural organization of association nuclei in the primate thalamus is largely unexplored. In the present study we have combined electron microscopy with immunocytochemistry for the acetylcholine synthesizing enzyme choline acetyltransferase (ChAT) to assess the cholinergic synaptic organization of the mediodorsal (MD) nucleus in macaque monkeys. The cholinergic innervation of the MD nucleus showed striking regional variations with the greatest density of immunoreactive axons and varicosities found within the parvicellular division. Electron microscopic examination revealed that these ChAT immunoreactive (ChAT-IR) axons were primarily small and unmyelinated. The majority of immunoreactive synaptic profiles were found within the extraglomerular neuropil (80.5%), with the remainder present in glomerular regions. Within the glomerular and extra-glomerular neuropil ChAT-IR profiles made contact with both conventional, presumably relay cell dendrites (CD), as well as with synaptic vesicle containing dendrites (SVCD) of local circuit neurons. In the glomeruli the frequency of synapses was approximately equal for CDs and SVCDs while in the extraglomerular areas 75% of the synaptic contacts were with CDs. ChAT-IR synaptic profiles had a diversity of junctional complex morphologies. Within glomeruli they made symmetric synapses with CDs and predominantly asymmetric with SVCDs. The majority of extraglomerular contacts (60%) were classified as asymmetric and these as well as the smaller number of symmetric synapses contacted both CDs and SVCDs. In accord with results of physiological studies, these anatomical data indicate that cholinergic input to thalamic nuclei influences relay cell activity both directly and indirectly via local circuit neurons.

Enhanced learning produced by injection of neurokinin substance P into the region of the nucleus basalis magnocellularis: mediation by the N-terminal sequence

The effect of unilateral injection of the neurokinin substance P (SP) and of certain N- or C-terminal SP-fragments into the region of the nucleus basalis magnocellularis (NBM) on inhibitory avoidance learning was investigated. Rats with chronically implanted cannulae were tested on a one-trial uphill avoidance task. Immediately after the training trial, rats were injected with 0.74 pmol SP or equimolar dosed SP(1-7), DIME-C7, or SP(7-11). Control groups included vehicle-injected rats and a group given an injection of SP(1-7) 5-h after the trial. When tested 24 h later, rats treated with SP or SP(1-7), but not with DIME-C7 or SP(7-11), exhibited longer step-up latencies than vehicle-treated controls. The retention latencies for rats in the SP(1-7) 5-h delay group did not differ from those of vehicle-injected animals, ruling out proactive effects of SP(1-7) on performance. The results show that SP facilitates retention of an inhibitory avoidance response when injected into the NBM. Furthermore, the amino acid sequence that encodes this effect may be located in the N-terminal part of the SP-molecule.

Cholinergic innervation of the human thalamus: dual origin and differential nuclear distribution

The cholinergic innervation of the human thalamus was studied with antibodies against the enzyme choline acetyltransferase (ChAT) and nerve growth factor receptor (NGFr). Acetylcholinesterase histochemistry was used to delineate nuclear boundaries. All thalamic nuclei displayed ChAT-positive axons and varicosities. Only the medial habenula contained ChAT-positive perikarya. Some intralaminar nuclei (central medial, central lateral, and paracentral), the reticular nucleus, midline nuclei (paraventricular and reuniens), some nuclei associated with the limbic system (anterodorsal nucleus and medially situated patches in the mediodorsal nucleus) and the lateral geniculate nucleus displayed the highest density of ChAT-positive axonal varicosities. The remaining sensory relay nuclei and the nuclei interconnected with the motor and association cortex displayed a lower level of innervation. Immunoreactivity for NGFr was observed in cholinergic neurons of the basal forebrain but not in cholinergic neurons of the upper brainstem. The contribution of basal forebrain afferents to the cholinergic innervation of the human thalamus was therefore studied with the aid of NGFr-immunoreactive axonal staining. The anterior intralaminar nuclei, the reticular nucleus, and medially situated patches in the mediodorsal nucleus displayed a substantial number of NGFr-positive varicose axons, presumably originating in the basal forebrain. Rare NGFr-positive axonal profiles were also seen in many of the other thalamic nuclei. These observations suggest that thalamic nuclei affiliated with limbic structures and with the ascending reticular activating system are likely to be under particularly intense cholinergic influence. While the vast majority of thalamic cholinergic input seems to come from the upper brainstem, the intralaminar and reticular nuclei, and especially medially situated patches within the mediodorsal nucleus also appear to receive substantial cholinergic innervation from the basal forebrain.

Topographic organization of subcortical projections to the anterior thalamic nuclei in the rat

Subcortical projections to the anterior thalamic nuclei were studied in the rat, with special reference to projections from the mammillary nuclei, by retrograde and anterograde transport of wheat germ agglutinin conjugated to horseradish peroxidase. The medial mammillary nucleus (MM) projects predominantly ipsilaterally to the entire anterior thalamic nuclei, whereas the lateral mammillary nucleus projects bilaterally to the anterodorsal nucleus (AD) of the anterior thalamic nuclei. A topographic relationship was recognized between the MM and the anterior thalamic nuclei. The dorsal region of the pars mediana of the MM projects to the interanteromedial nucleus (IAM), whereas the ventral region projects to the rostral part of the anteromedial nucleus (AM). The dorsal and the ventral regions of the pars medialis project to the dorsomedial part of the AM at its caudal and rostral levels, respectively. The dorsomedial region of the pars lateralis projects to the ventral AM. The ventrolateral region of the pars lateralis projects to the ventral part of the anteroventral nucleus (AV) in such a manner that rostral cells project rostrally and caudal cells project caudally. The pars basalis projects predominantly ipsilaterally to the dorsolateral AV and bilaterally to the AD. The rostrolateral region of the pars posterior projects to the lateral AV, whereas the medial and the caudal regions of the pars posterior project to the dorsomedial AV. The rostrodorsal part of the nucleus reticularis thalami was found to project to the anterior thalamic nuclei; cells located rostrally in this part project to the IAM and AM, whereas cells located caudodorsally project to the AV and AD. The laterodorsal tegmental nucleus projects predominantly ipsilaterally to the AV, especially to its dorsolateral part. The present study demonstrates that subdivisions of the subcortical structures are connected to the subnuclei of the anterior thalamic nuclei, with a clear-cut topography arranged in the dorsoventral and the rostrocaudal dimensions.

Afferent projections to the cholinergic pedunculopontine tegmental nucleus and adjacent midbrain extrapyramidal area in the albino rat. I. Retrograde tracing studies

The afferent connections of the pedunculopontine tegmental nucleus (PPT) and the adjacent midbrain extrapyramidal area (MEA) were examined by retrograde tracing with wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP). Major afferents to the PPT originate in the periaqueductal gray, central tegmental field, lateral hypothalamic area, dorsal raphe nucleus, superior colliculus, and pontine and medullary reticular fields. Other putative inputs originate in the paraventricular and preoptic hypothalamic nuclei, the zona incerta, nucleus of the solitary tract, central superior raphe nucleus, substantia innominata, posterior hypothalamic area, and thalamic parafascicular nucleus. The major afferent to the medially adjacent MEA originates in the lateral habenula, while other putative afferents include the perifornical and lateral hypothalamic area, periaqueductal gray, superior colliculus, pontine reticular formation, and dorsal raphe nucleus. MEA inputs from basal ganglia nuclei include moderate projections from the substantia nigra pars reticulata, entopeduncular nucleus, and a small projection from the globus pallidus, but not the subthalamic nucleus. Dense anterograde labeling was observed in the substantia nigra pars compacta, entopeduncular nucleus, subthalamic nucleus, globus pallidus, and caudate-putamen only following WGA-HRP injections involving the MEA. The results of this study demonstrate that the PPT and MEA share many potential afferents. Remarkable differences were found that support distinguishing between these two nuclei in future studies regarding the functional organization of the midbrain and pons. The results, for example, confirm our previous observations that the largely reciprocal connections between the midbrain and basal ganglia distinguish the MEA from the PPT. Afferents from the lateral habenula and contralateral superior colliculus represent extensions of more traditional basal ganglion circuitry which further delineate the MEA from the PPT. The results are discussed with respect to the important role of the midbrain and pons in behavioral state control and locomotor mechanisms.

Sources of presumptive glutamatergic/aspartatergic afferents to the mediodorsal nucleus of the thalamus in the rat

The distribution of presumptive glutamatergic and/or aspartatergic neurons retrogradely labeled following injections of 3HD-aspartate into the mediodorsal nucleus of the thalamus (MD) in the rat was compared to the distribution of neurons labeled by comparable injections of the nonspecific retrograde tracer wheat germ agglutinin conjugated horseradish peroxidase (WGA-HRP). Cells retrogradely labeled by WGA-HRP were found in the prefrontal and agranular insular cortices; in forebrain structures such as the amygdaloid complex, the piriform cortex, the ventral pallidum and the reticular nucleus of the thalamus; and in several different parts of the brainstem, such as the superior colliculus, central grey, and substantia nigra, pars reticulata. Some, but not all, of these projections are presumably glutamatergic and/or aspartatergic. The projections to MD from the prefrontal and agranular insular cortices are well labeled with 3H-D-aspartate, as are projections from the anterior cortical amygdaloid nucleus. Projections from the superior colliculus to the lateral portion of MD also label with this tracer. However, other forebrain and brainstem projections to MD are not labeled with 3H-D-aspartate, and apparently do not use glutamate or aspartate as a neurotransmitter. These include the projections from the basal and accessory basal amygdaloid nuclei, as well as possibly GABAergic projections from the ventral pallidum and the substantia nigra, pars reticulata. A small fraction of the cells in the piriform cortex that project to MD label with 3H-D-aspartate, suggesting that this projection may be heterogeneous. In other experiments, presumptive GABAergic projections to MD were studied by using 3H-GABA as a retrograde tracer. Although in these cases the thalamic reticular nucleus is well labeled, the ventral pallidum and the substantia nigra, pars reticulata are only poorly labeled. Pallidal projections to the ventromedial thalamic nucleus (VM), which are likely to be GABAergic, were also studied with this technique. After injections of 3H-GABA into VM, only a few cells in the substantia nigra, pars reticulata, or entopeduncular nucleus were labeled. This result suggests 3H-GABA has limited usefulness as a transmitter-specific retrograde tracer.

Noradrenergic innervation of the thalamic reticular nucleus: a light and electron microscopic immunohistochemical study in rats

Fluoro-ruby injections in the rat locus coeruleus result in scattered chain-like arrays of varicose anterogradely labeled axons within the thalamic reticular nucleus of rats. An abundant meshwork of axons giving rise to en passant boutons is detected immunohistochemically within this thalamic nucleus by means of an antibody to dopamine-beta-hydroxylase (DBH). The density of DBH-positive axonal boutons within the reticular nucleus neuropil is greater than that found in the relay nuclei of the dorsal thalamus (with the exception of the anterior group nuclei). Single DBH-positive axons appear to contact both proximal and distal dendrites and occasionally the somata of reticular nucleus neurons. Labeled axons are seen closely juxtaposed not only to the swollen segments of the beaded reticular neuron dendrites, but to the constricted segments as well. Electron microscopic examination of DBH-positive axon terminals within the reticular nucleus neuropil indicates that many of the axonal boutons detected light microscopically participate in asymmetric synaptic contacts. The postsynaptic densities of these synapses are thicker than those of nearby symmetric synapses, but often subtend a shorter length of the postsynaptic membrane than the densities associated with other nearby asymmetric synapses. These observations indicate that the ascending noradrenergic system, in addition to influencing the dorsal thalamus and the cerebral cortex directly, is well situated to influence signal transmission through the nuclei of the dorsal thalamus indirectly via a moderately dense terminal projection upon the thalamic reticular nucleus.

Lesions of the tegmental pedunculopontine nucleus: effects on the locomotor activity induced by morphine and amphetamine

One of the important questions in the neurobiology of motivation asks how the incentive impact of stimuli acting on the limbic system of the forebrain are ultimately translated into action and approach behavior. Bilateral ibotenic acid lesions of the tegmental pedunculopontine nucleus (TPP) (a brainstem output of the limbic system that receives neuronal input from limbic forebrain and midbrain sites identified as primary sites for psychoactive drug reward) have been shown previously to block the acquisition, but not the retention, of morphine and amphetamine conditioned place preferences in formerly drug-naive rats. These results suggest a deficit in the processing of the unconditioned rewarding effects of these drugs. The TPP projects to widespread parts of the brain and spinal cord involved in various somatomotor responses. Thus, we investigated the role of the TPP in morphine- and amphetamine-induced locomotion as assessed in an open field. We report that TPP lesions blocked the locomotor excitation, as well as the conditioned hyperactivity, produced by amphetamine. TPP lesions also blocked the conditioned increase in locomotion, but not the catalepsy, produced by morphine. TPP lesions were behaviorally specific in that the analgesic properties of morphine in a tail-flick test were not attenuated, nor did the lesions affect the locomotion induced by naloxone-precipitated withdrawal in morphine-dependent animals. We suggest that the neural circuits mediating the acute rewarding effects of drug stimuli acting at forebrain sites exit the limbic system in the TPP region of the brainstem, where motivation gains access to (or is isomorphic with) motor systems that initiate approach and exploration.

Effects of selective toxic lesions of cholinergic neurons of the laterodorsal tegmental nucleus on experimental seizures

This study determined the effects of bilateral discrete partial lesions of cholinergic neurons of the laterodorsal tegmental nucleus (LDTg) of the pontomesencephalic tegmentum on seizures induced by intravenous pentylenetetrazol (PTZ). Relatively selective lesions produced by bilateral 50 nl microinjections of 75 pmol of the cholinergic neurotoxin ethylcholine mustard aziridinium ion (AF64A) resulted in a significant reduction in the threshold of myoclonic and facial-forelimb clonic seizures but not tonic seizures when PTZ was infused 7 days later. This demonstrates that this cholinergic nucleus is a key site of subcortical seizure regulation. We propose that this control is mediated by ascending projections from the LDTg to the central medial intralaminar nucleus of the thalamus.

Projections from the nucleus accumbens to cholinergic neurons of the ventral pallidum: a correlated light and electron microscopic double-immunolabeling study in rat

A correlated light- and electron microscopic double-immunolabeling study combining choline acetyltransferase immunocytochemistry with anterograde tracing of Phaseolus vulgaris leucoagglutinin (PHA-L) revealed that axons of the nucleus accumbens terminate on cholinergic neurons of the ventral pallidum. These findings are discussed with respect to the possibility that these cholinergic neurons may be part of parallel circuits, providing feedback to the same cortical and amygdaloid areas which innervate the nucleus accumbens.

Serotonin hyperpolarizes cholinergic low-threshold burst neurons in the rat laterodorsal tegmental nucleus in vitro

Serotonergic suppression of cholinergic neuronal activity implicated in the regulation of rapid eye movement sleep and its associated phenomenon, pontogeniculooccipital waves, has long been postulated, but no direct proof has been available. In this study, intracellular and whole-cell patch-clamp recording techniques were combined with enzyme histochemistry to examine the intrinsic electrophysiological properties and response to serotonin (5-HT) of identified cholinergic rat laterodorsal tegmental nucleus neurons in vitro. Sixty-five percent of the recorded neurons demonstrated a prominent low-threshold burst, and of these, 83% were cholinergic. In current-clamp recordings 64% of the bursting cholinergic neurons tested responded to the application of 5-HT with a membrane hyperpolarization and decrease in input resistance. This effect was mimicked by application of the selective 5-HT type 1 receptor agonist carboxamidotryptamine maleate. Whole-cell patch-clamp recordings revealed that the hyperpolarizing response was mediated by an inwardly rectifying K+ current. Application of 5-HT decreased excitability and markedly modulated the discharge pattern of cholinergic bursting neurons: during a 5-HT-induced hyperpolarization these neurons exhibited no rebound burst after hyperpolarizing current input and a burst in response to depolarizing current input. In the absence of 5-HT, the relatively depolarized cholinergic bursting neurons responded to an identical hyperpolarizing current input with a burst and did not produce a burst after depolarizing current input. These data provide a cellular and molecular basis for the hypothesis that 5-HT modulates rapid eye movement sleep phenomenology by altering the firing pattern of bursting cholinergic neurons.

Fine structure of the ventral lateral nucleus (VL) of the Macaca mulatta thalamus: cell types and synaptology

Ultrastructure of the major cerebellar territory of the monkey thalamus, or VL as delineated in sagittal maps by Ilinsky and Kultas-Ilinsky (J. Comp. Neurol. 262:331-364, '87), was analyzed by using neuroanatomical tracing, immunocytochemical, and quantitative morphometric techniques. The VL nucleus contains nerve cells of two types. Multipolar neurons (PN) retrogradely labeled with wheat germ agglutinin-horseradish peroxidase (WGA-HRP) from the precentral gyrus display a tufted branching pattern of the proximal dendrites and have a range of soma areas from 200 to 1,000 microns2 (mean 535.2 microns2, SD = 159.5). Small glutamic acid decarboxylase (GAD) immunoreactive cells (LCN) exhibit sizes from 65 to 210 microns2 (mean 122.5 microns2, SD = 32.8) and remain unlabeled after cortical injections. The two cell types can be further distinguished by ultrastructural features. Unlike PN, LCN display little perikaryal cytoplasm, a small irregularly shaped nucleolus, and synaptic vesicles in proximal dendrites. The ratio of PN to LCN is 3:1. The LCN dendrites establish synaptic contacts on PN somata and all levels of dendritic arbor either singly or as a part of complex synaptic arrangements. They are also presynaptic to other LCN dendrites. Terminals known as LR type, i.e., large boutons containing round vesicles, are the most conspicuous in the neuropil. They form asymmetric contacts on somata and proximal dendrites of PN as well as on distal dendrites of LCN. The areas of these boutons range from 0.7 to 12 microns2 and the appositional length on PN dendrites ranges from 1.1 to 14 microns. All LR boutons except the largest ones become anterogradely labeled from large WGA-HRP injections in the deep cerebellar nuclei. These boutons are also encountered as part of triads and glomeruli, but very infrequently since the latter complex synaptic arrangements are rare. The most numerous axon terminals in the neuropil are the SR type, i.e., small terminals (mean area 0.42 micron2) containing round vesicles. The SR boutons become anterogradely labeled after WGA-HRP injections in the precentral gyrus. They form distinct asymmetric contacts predominantly on distal PN and LCN dendrites; however, their domain partially overlaps that of LR boutons at intermediate levels of PN dendrites. The SR boutons are components of serial synapses with LCN dendrites which, in turn, contact somata and all levels of dendritic arbors of PN. They also participate in complex arrangements that consist of sequences of LCN dendrites, serial synapses, and occasional boutons with symmetric contacts.(ABSTRACT TRUNCATED AT 400 WORDS)

Substantia nigra reticulata neurons during sleep-waking states: relation with ponto-geniculo-occipital waves

We have previously hypothesized that the spike bursts of brainstem peribrachial (PB) neurons, leading to ponto-geniculo-occipital (PGO) waves in thalamocortical systems, are triggered by phasic hyperpolarizations of sufficient magnitude or by excitatory inputs reaching a steadily hyperpolarized membrane. We have proposed that the source of these hyperpolarizing actions are substantia nigra pars reticulata (SNr) cells that project to, and exert inhibitory effects upon, PB neurons. Here we tested this hypothesis by recording antidromically identified SNr-PB cells in chronically implanted, naturally sleeping cats. A subpopulation of SNr-PB cells exhibited tonically increased firing preceding by 70-200 ms the thalamic PGO wave. These data support the hypothesis that an enhancement in SNr-cells' discharges may lead to hyperpolarization of PB neurons, with the consequence of spike bursts in one class of PGO-related PB-thalamic neurons.

Ultrastructure and synaptic organization of axon terminals from brainstem structures to the mediodorsal thalamic nucleus of the rat

The ultrastructural characteristics and synaptic organization of afferent terminals from the brainstem to the mediodorsal thalamic nucleus (MD) of the rat have been studied with the electron microscope, by means of anterograde transport of wheat germ agglutinin-horseradish peroxidase (WGA-HRP). Labeled fibers were seen predominantly in the lateral portion of MD after the injections of WGA-HRP into the substantia nigra pars reticulata (SNr), the superior colliculus (SC), and the dorsal tegmental region (DT). The boutons arising from the SC were relatively small (less than 1.5 microns in diameter), formed asymmetric synaptic contacts with small dendrites and dendritic spines, and contained round synaptic vesicles. The axon terminals from the DT were mostly large boutons (2-4.5 microns) with asymmetric synaptic specializations and round vesicles. These boutons and their postsynaptic targets formed synaptic glomeruli that were entirely or partially ensheathed by glial lamellae. The ultrastructural features are almost identical to those of boutons in the medial and central segments of MD that were previously shown to originate from the basal amygdaloid nucleus and the piriform cortex. The boutons from the SNr had a wide range in size, but the majority were medium-sized to large (1.5-4 microns). The nigral boutons established symmetric synaptic contacts with dendritic shafts and occasionally with somata, and contained pleomorphic vesicles. However, like the DT terminals, they participated in glomerular formations. The nigral terminals closely resemble previously described terminals in the medial part of MD from the ventral pallidum, except that the nigral terminals formed en passant and axosomatic synapses as well as axodendritic synapses. A combined immunohistochemistry and WGA-HRP tracing study revealed that the nigral inputs were immunoreactive for glutamic acid decarboxylase and the axon terminals from the DT were immunoreactive for choline acetyltransferase. In a separate study, the colliculothalamic fibers have been shown to take up and transport the transmitter specific tracer [3H]-D-aspartate, and are therefore putatively glutamatergic and/or aspartatergic. Taken together with this, the present results suggest that the collicular afferents are excitatory and glutamatergic and/or aspartatergic, that the inputs from the DT are also excitatory and cholinergic, while the nigral inputs are inhibitory and GABAergic.

Serotoninergic innervation of the thalamus in the primate: an immunohistochemical study

Little is known of the serotoninergic innervation of the thalamus in primates; therefore, we undertook a detailed study of the distribution of 5-hydroxytryptamine (5-HT)-immunoreactive neuronal profiles in the thalamus of the squirrel monkey (Saimiri sciureus) with a specific antibody directly raised against 5-HT. All thalamic nuclei in the squirrel monkey displayed 5-HT-immunoreactive fibers, but none contained immunopositive cell bodies. The 5-HT innervation of the thalamus derived from extrinsic fibers arising mostly from the midbrain raphe nuclei and forming the transtegmental system. Most of the fibers destined to the thalamus collected into a major bundle that swept dorsoventrally within the midbrain tegmentum and coursed beneath the thalamus along its entire caudorostral extent. Several fiber fascicles broke off from this main bundle at different levels and ascended dorsally to innervate the various thalamic nuclei. Overall, the 5-HT innervation of the thalamus in the squirrel monkey was more massive than would have been expected from earlier studies in nonprimate species. Marked differences in the regional density of innervation were noted both between the various nuclei and within single nuclei. The most densely innervated nuclei were those delineating the principal subdivisions of the thalamic mass, that is, the midline, rostral intralaminar, limitans, and reticular nuclei, where very dense fields of isolated axonal varicosities occurred. In contrast to the rostral intralaminar nuclei, which were rather uniformly innervated, the centre médian/parafascicular complex contained immunoreactive fibers and isolated varicosities distributed according to a mediolateral gradient. The habenula and the ventral anterior nucleus were among the most weakly innervated nuclei. In the latter nucleus, as well as in more densely innervated nuclei, thin varicose fibers formed numerous pericellular contacts on cell bodies and proximal dendrites of thalamic neurons. The 5-HT innervation of the lateral nuclear group as well as that of the medial and lateral geniculate nuclei ranged from very weak to dense. The mediodorsal nucleus displayed a highly heterogeneous 5-HT innervation that varied from weak in its central portion to moderate or dense in its medial and lateral borders. A moderate 5-HT innervation was observed in the anterior nuclear group. The surprisingly dense and heterogeneous 5-HT innervation of the thalamus noted in the present study suggests that serotonin may be involved in several specific functions of the thalamus in primates.

Nigropedunculopontine projection in the rat: an anterograde tracing study with phaseolus vulgaris-leucoagglutinin (PHA-L)

The termination of the substantia nigra pars reticulata efferents in the nucleus tegmenti pedunculopontinus was studied in the rat by using the anterograde tracer Phaseolus vulgaris-leucoagglutinin (PHA-L). Both large and small injections of PHA-L in various portions of the substantia nigra pars reticulata labeled varicose fibers in the ipsilateral and contralateral nucleus tegmenti pedunculopontinus, subnucleus dissipatus as well as in the ipsilateral nucleus tegmenti pedunculopontinus, subnucleus compactus. However, the bulk of the nigral fibers appeared to terminate in the medial two-thirds of the ipsilateral subnucleus dissipatus of the pedunculopontine nucleus and exhibited a discrete dorsoventral topographical pattern. The terminal plexus displayed patches of uneven density, which was partly due to the numerous fiber bundles passing through the pedunculopontine nucleus, but also to an obvious preference of nigral fibers for some cells. Electron microscopic examination confirmed that nearly all of the varicosities observed in the light microscope contained synaptic vesicles and represented either terminal boutons or boutons en passant. The labeled boutons were elongated (average length: 1.5 microns) and consistently contained a prominent group of mitochondria. The results suggest that the nigral input to the nucleus tegmenti pedunculopontinus may be directed toward specific subpopulation(s) of pedunculopontine neurons and may influence not only cells in the subnucleus dissipatus, but also in the subnucleus compactus.

Extracellular characteristics of putative cholinergic neurons in the rat laterodorsal tegmental nucleus

The extracellular electrophysiological properties of neurons in the laterodorsal tegmental nucleus (LDT), a major source of cholinergic afferents to the thalamus, were studied in chloral hydrate-anesthetized rats. A combination of antidromic activation from the thalamus and histological verification of recording sites was used to correlate the identity of extracellular recordings in the rat LDT with cholinergic neurons in that region. All neurons antidromically activated by stimulation of the anteroventral thalamus were histologically verified to be within clusters of cholinergic (NADPH-d-positive) cells in the LDT or in the adjacent nucleus locus coeruleus (LC). The thalamically projecting LDT neurons had a homogeneous neurophysiological profile consisting of long duration action potentials (mean = 2.5 ms), slow conduction velocities (mean = 0.78 m/s), and lengthy chronaxie values (mean = 0.725 ms). The appearance and axonal characteristics of these neurons resembled those of noradrenergic LC neurons, but the two populations exhibited substantially different spontaneous activity patterns and sensory responsiveness. These characteristics may be useful in the preliminary identification of putative cholinergic neurons in vivo, and thereby provide a foundation for exploring the neuropharmacology, afferent modulation, sensory responsiveness and behavioral correlates of the brainstem cholinergic system.

Cholinergic modulation of responses to glutamate in the thalamic reticular nucleus of the anesthetized rat

Neurons in the thalamic reticular nucleus (TRN) of the chloral hydrate-anesthetized rat were studied with extracellular recording and microiontophoretic application of cholinergic agents. In most cases (63%), the ejection of the agonist, carbachol, had no observable effect on spontaneous activity, and in an additional 33% of cases was observed to inhibit discharge rate. Carbachol ejections with identical current and duration parameters proved capable of antagonizing the uniformly facilitatory responses produced by glutamate ejection in these same cells. The muscarinic nature of cholinergic effects was documented by scopolamine's specific antagonism of the responses. The muscarinic antagonists, pirenzepine and AF-DX-116, both diminished the effects of carbachol. Application of muscarinic agonists, such as McN-A-343 and oxotremorine-M, yielded qualitatively the same results as carbachol, though, with current as a criterion, oxotremorine-M was slightly more and McN-A-343 much less potent than carbachol. The functional implications of cholinergic modulation of the facilitatory inputs to TRN are discussed, with particular emphasis on the role of acetylcholine and the TRN in the sleep/wake-related activity of thalamic neurons.

Pathology of Parkinson's disease. Changes other than the nigrostriatal pathway

In Parkinson's disease (PD), in addition to degeneration of the nigrostriatal dopaminergic pathway, a variety of neuronal systems are involved, causing multiple neuromediator dysfunctions that account for the complex patterns of functional deficits. Degeneration affects the dopaminergic mesocorticolimbic system, the noradrenergic locus ceruleus (oral parts) and motor vagal nucleus, the serotonergic raphe nuclei, the cholinergic nucleus basalis of Meynert, pedunculopontine nucleus pars compacta, Westphal-Edinger nucleus, and many peptidergic brainstem nuclei. Cell losses in subcortical projection nuclei range from 30 to 90% of controls; they are more severe in depressed and demented PD patients. Most of the lesions are region-specific, affecting not all neurons containing a specific transmitter or harboring Lewy bodies. In contrast to Alzheimer's disease (AD), subcortical system lesions in Parkinson's disease appear not to be related to cortical pathology, suggesting independent or concomitant degeneration. The pathogenesis of multiple-system changes contributing to chemical pathology and clinical course of Parkinson's disease are unknown.

The role of cholinergic projections from the nucleus basalis in memory

The behavioral effects of lesions of the nucleus basalis magnocellularis (NBM) are reviewed, focusing on the anatomical extent of the lesion, the involvement of neurotransmitter systems and the alterations in memory processes. Most behavioral deficits after NBM lesions can be attributed to damage to the NBM itself, although during spontaneous or pharmacologically induced recovery, other brain structures might play a role. The neurochemical deficit underlying the behavioral impairments is most likely the decrease in cholinergic functioning, since, for example, enhancement of cholinergic functioning is sufficient for behavioral improvement. However, since the lesions are not specific for cholinergic neurons, the extent to which noncholinergic damage causes behavioral deficits is still unclear. Finally, lesions of the NBM impair memory, but affect also other behavioral processes, such as discrimination and habituation. A common process underlying these various impairments could be that of insufficiently focused processing of stimuli.

Learning disturbances following excitotoxic lesion of cholinergic pedunculo-pontine nucleus in the rat

Compared to brain anterior cholinergic systems such as the septo-hippocampal and nucleus basalis-cortical pathways, posterior cholinergic groups have received little attention with respect to their involvement in learning and memory. In this study, the effect of lesion of the cholinergic pedunculo-pontine cell bodies (PPN) by the excitotoxin quisqualic acid was investigated on spontaneous locomotor activity and learning in rats. Behavioral tasks designed to test both reference memory (cross maze and water maze) or working memory (radial maze) were used. PPN lesion had no effect on initial nor on nocturnal locomotor activity in a circular corridor. The lesion disrupted learning in the water and radial mazes, but was without influence on acquisition in the cross maze. The difference in results obtained in the two tasks designed to test reference memory (cross maze and water maze) indicated that the disturbance depended on task difficulty rather than on a particular memory component. It is suggested that the PPN is involved in the sustained attention required to perform correctly in water and radial mazes. The PPN cannot therefore be considered as a uniquely extrapyramidal structure. In addition to its descending outputs, the PPN has ascending connections to the neocortex, either directly or indirectly via the thalamus, and so pathological changes in this region may be partly responsible for the cognitive disorders of aging or those observed in various neurodegenerative conditions.

A GABA immunocytochemical study of rat motor thalamus: light and electron microscopic observations

A light and electron microscopic study of GABA-immunoreactive neurons and profiles in the ventroanterior-ventrolateral and ventromedial nuclei of rat dorsal thalamus was conducted using antiserum raised against GABA. Less than 1% of the neurons in these motor-related nuclei exhibited GABA immunoreactivity, confirming previous reports that these nuclei are largely devoid of interneurons. Immunoreactive neurons in the ventral anterior-ventral lateral complex and ventromedial nucleus were bipolar or multipolar in shape, and tended to be smaller than non-immunoreactive neurons. GABA immunoreactivity in the neuropil consisted of labeled axon terminals and myelinated and unmyelinated axons, and was lower in the ventral anterior-ventral lateral complex and ventromedial nucleus than in neighboring thalamic nuclei. The density of neuropil immunolabeling was slightly higher in ventral anterior-ventral lateral complex than in ventromedial nucleus. GABA-immunoreactive axon terminals, collectively termed MP boutons for their medium size and pleomorphic vesicles (and corresponding to "F" profiles of some previous studies of thalamic ultrastructure), formed symmetric synapses and puncta adhaerentia contacts predominantly with large and medium-diameter (i.e. proximal) non-immunoreactive dendrites. Approximately 12 and 18% of boutons in the ventral anterior-ventral lateral complex and ventromedial nucleus, respectively, were GABA-immunopositive. Many of these immunoreactive profiles probably arose from GABAergic neurons in the thalamic reticular nucleus, substantia nigra pars reticulata and entopeduncular nucleus. Two types of non-immunoreactive axon terminals were distinguished based on differences in morphology and synaptic termination sites. Boutons with small ovoid profiles and round vesicles that formed prominent asymmetric synapses onto small-diameter dendrites were observed. Mitochondria were rarely observed within these boutons, which arose from thin unmyelinated axons. These boutons composed approximately 82 and 74% of boutons in the ventral anterior-ventral lateral complex and ventromedial nucleus, respectively, and were considered to arise predominantly from neurons in the cerebral cortex. In contrast, boutons with large terminals that contained round or plemorphic vesicles and formed multiple asymmetric synapses predominantly with large-diameter dendrites were also observed. Puncta adhaerentia contacts were also common. Mitochondria were numerous within large boutons with round vesicles, which arose from myelinated axons. Many of the large boutons were likely to have originated from neurons in the cerebellar nuclei. Approximately 6% of the boutons in the ventral anterior-ventral lateral complex and 8% in ventromedial nucleus were of the large type.(ABSTRACT TRUNCATED AT 400 WORDS)

Noradrenergic locus coeruleus neurons: their distant connections and their relationship to neighboring (including cholinergic and GABAergic) neurons of the central gray and reticular formation

Noradrenergic LC neurons appear to be relatively unique in the brain, being unsurpassed in the divergence and ubiquity of their projections through the central nervous system. In this regard, they share certain characteristics with peripheral noradrenaline neurons of the sympathetic nervous system. As such they would be assumed to play a very general role in modulating the activity of large populations of neurons in multiple, functionally diverse systems. Like other periventricular and reticular neurons, they have the potential to receive afferent information from multiple sources via long dendrites, upon which the majority of their inputs from brainstem and forebrain may arrive. They appear closely related to the cholinergic neurons of the laterodorsal tegmental nucleus, their neighbors that are located medial and rostral to them within the periventricular gray and that have similarly oriented and positioned long dendrites that would allow reception of similar afferent input as the LC neurons and also possibly interaction with the LC neurons. As evidenced by input to the noradrenergic cell bodies in the compact portion of the nucleus, a moderate GABAergic innervation, that may derive in part from local neurons, could have a potent influence on the activity of the cells. Periventricular GABAergic cells could also serve as intermediaries to other afferent input, from a distance, terminating in the periventricular region or from local neurons such as the cholinergic cells of the laterodorsal tegmental nucleus.