Cholinergic systems in the rat brain: III. Projections from the pontomesencephalic tegmentum to the thalamus, tectum, basal ganglia, and basal forebrain

An examination of the effects of bilateral excitotoxic lesions of the pedunculopontine tegmental nucleus on responding to sucrose reward

The effects of bilateral excitotoxic lesions of the pedunculopontine tegmental nucleus (PPTg) on sucrose intake were examined in three experiments. First, in tests of conditioned place preference using 20% sucrose as the reinforcer, it was shown that lesioned rats, regardless of whether they were food deprived or non-deprived, formed normal place preferences and showed normal amounts of locomotion. However, consumption of 20% sucrose in the pairing trials was increased in the deprived PPTg lesioned rats compared to their matched controls. A second experiment showed that sucrose consumption in the home cage was increased in both deprived and non-deprived PPTg lesioned rats, but only when the concentration of sucrose was greater than 12%: below this there were no differences in intake between the lesioned and control rats. In a third home cage experiment, it was again shown that non-deprived PPTg lesioned rats increased their consumption of 20% sucrose compared to controls. PPTg lesioned rats concomitantly reduced their intake of lab chow such that overall energy intake remained the same as that of control rats. These data are taken to suggest (i) that bilateral excitotoxic lesions of the PPTg increase consumption of sucrose selectively in conditions of high motivational excitement; (ii) that the perception of the rewarding value of 20% sucrose, as judged by place preference, is not affected by these lesions; and (iii) that PPTg lesioned rats are able to adjust their energy intake to accommodate increased sucrose loads. These data are consistent with the hypothesis that bilateral excitotoxic lesions of the PPTg do not affect energy balance regulation or judgment of the hedonic value of sucrose, but that they do affect the control of responding in the face of high levels of motivational excitement.

Microinjection of procaine into the pedunculopontine tegmental nucleus suppresses hippocampal theta rhythm in urethane-anesthetized rats

It was found that the cholinergic component of the pedunculopontine tegmental nucleus (PPN) is involved in the generation of theta rhythm in the hippocampus. However, it is still not known how important PPN is in the brainstem theta-generating system, where the nucleus reticularis pontis oralis is regarded as a primary generator. In the present experiment, performed on urethane-anesthetized rats, we studied the effect on the tail pinch-elicited hippocampal theta of unilateral inactivation of PPN by means of direct procaine microinjection. Procaine induced ipsilateral suppression of theta rhythm, manifested as desynchronization of hippocampal EEG, a shift of the fast Fourier transformation (FFT) power peak toward lower frequencies, and a reduction of FFT peak magnitude at theta band. Hippocampal field activity returned to normal (both its FFT peak frequency and magnitude) within 30 min after the injection. The results obtained indicate that PPN is critical for hippocampal theta generation but it may not be involved in encoding theta frequency.

Sensory-motor gating and cognitive control by the brainstem cholinergic system

As an essential component of ascending activating systems, cholinergic neurons with diffuse projections are supposed to be involved in the regulation of cognitive processes such as attention, consciousness, learning, and memory. As for the role of cholinergic projections from the basal forebrain nuclei to cerebral cortical regions including hippocampus, a couple of models have been proposed that acetylcholine facilitates extrinsic inputs to the cortex and inhibits intracortical processing. In this review, to explore the possibility that there exists a generalized principle on the role of cholinergic systems in the brain, we summarized the knowledge so far obtained on the action of a brainstem cholinergic nucleus, the pedunculopontine tegmental nucleus (PPTN) at their target regions. By in vitro experiments we clarified that cholinergic inputs to the intermediate layer of the superior colliculus, presumably originating from the PPTN, facilitate generation of its motor outputs for the initiation of saccades. Furthermore, cholinergic inputs may enhance excitatory responses of mesopontine dopaminergic cells, for instance to reward-related signals. In addition, we observed that PPTN neurons showed multi-modal activities in behaving monkeys; their activities were related to execution and preparation of saccades, the level of task performance, and reward. The multi-modal activities encoded in the PPTN may suggest that PPTN associates movement-related activities with those related to task performance and reward. Together with the already reported facilitatory action on the sensory processing at the visual thalamus, these observations suggest that the brainstem cholinergic system facilitates the central processes for motor command generation and extrinsic sensory processing. For our final goal of exploring the general working principle of the cholinergic systems, further studies are needed to clarify the effects of the brainstem cholinergic system on the intrinsic processing in the brain.

Determination of acetylcholine and dopamine content in thalamus and striatum after excitotoxic lesions of the pedunculopontine tegmental nucleus in rats

The pedunculopontine tegmental nucleus (PPTg) contains cholinergic neurons whose principal ascending connections are with thalamic nuclei and structures associated with the striatum. It has been hypothesized that PPTg neurons are more closely associated with the substantia nigra (and therefore striatal motor systems) than with the ventral tegmental area (and therefore limbic striatal functions). In the present experiments we have examined the hypothesis that the PPTg is similarly associated with motor nuclei in the thalamus. Rats received unilateral ibotenate lesions of PPTg and were sacrificed 1, 2, 4 or 7 days later. Discrete thalamic nuclei, and samples of caudate-putamen and nucleus accumbens, were punched out and thalamic acetylcholine (ACh) and striatal ACh and dopamine (DA) content examined. Anteroventral nucleus had decreased ACh content after PPTg lesion, but a time dependent increase was found in mediodorsal nucleus; ACh concentration was unchanged in thalamic reticular nucleus or medial geniculate. No long-term lesion-dependent changes in striatal ACh or DA content were found. The effects of PPTg lesion on thalamic ACh content are consistent with the hypothesis that it has effects on motor nuclei, but also indicate that PPTg lesions have complex and dynamic effects on thalamic ACh content.

Tremor in Parkinson's disease

Rest tremor is a common feature of Parkinson's disease, but its underlying pathophysiology remains unknown. This review hypothesizes that tremor is related to selective loss of components of the substantia nigra. The relative scarcity of tremor in related Parkinsonian conditions may indicate a dissociation associated with different pathological involvement of the substantia nigra and its connections. Connections of the subthalamic nucleus with the pallidum, modified by cortical and nigral inputs, allow for the transfer of tremorogenic activity to the thalamus. Thalamo-cortical interactions, tempered by cerebellar input, generate the final common pathway for tremor production.

The fine organization of nigro-collicular channels with additional observations of their relationships with acetylcholinesterase in the rat

The nigro-collicular pathway that links the basal ganglia to the sensorimotor layers of superior colliculus plays a crucial role in promoting orienting behaviors. This connection originating in the pars reticulata and lateralis of the substantia nigra has been shown in rat and cat to be topographically organized. In rat, a functional compartmentalization of the substantia nigra has also been shown reflecting that of the striatum. In light of this, we reinvestigated the topographical arrangement of the nigro-collicular pathway by examining the innervation of each nigral functional zone. We performed small injections of either biocytin or wheatgerm agglutinin conjugated with horseradish peroxidase restricted to identified somatic, visual and auditory nigral zones. Frontally cut sections showed that innervations provided by the three main nigral zones form a mosaic of complementary domains stratified from the stratum opticum to the ventral part of the intermediate collicular layers, with the somatic afferents sandwiched between the visual and the auditory ones. When reconstructed from semi-horizontal sections, nigral innervations organized in the form of a honeycomb-like array composed of 100 cylindrical modules covering three-quarters of the collicular surface. Such a modular architecture is reminiscent of the acetylcholinesterase lattice we previously described in rat intermediate collicular layers. In the enzyme lattice, the surroundings of the cylindrical modules are composed of a mosaic of dense and diffuse enzyme subdomains. Thus, we compared the distribution of the overall nigral projection and of its constituent channels with the acetylcholinesterase lattice. The procedure combined axonal labelling with histochemistry on single sections for acetylcholinesterase activity. The results demonstrate that the overall nigral projection overlaps the acetylcholinesterase lattice and its constituent channels converge with either the dense or the diffuse enzyme subdomains. The stereometric arrangement of the nigro-collicular pathway is suggestive of an architecture promoting the selection of collicular motor programs for different classes of orienting behavior.

The distribution and morphological characteristics of cholinergic cells in the brain of monotremes as revealed by ChAT immunohistochemistry

The present study employs choline acetyltransferase (ChAT) immunohistochemistry to identify the cholinergic neuronal population in the central nervous system of the monotremes. Two of the three extant species of monotreme were studied: the platypus (Ornithorhynchus anatinus) and the short-beaked echidna (Tachyglossus aculeatus). The distribution of cholinergic cells in the brain of these two species was virtually identical. Distinct groups of cholinergic cells were observed in the striatum, basal forebrain, habenula, pontomesencephalon, cranial nerve motor nuclei, and spinal cord. In contrast to other tetrapods studied with this technique, we failed to find evidence for cholinergic cells in the hypothalamus, the parabigeminal nucleus (or nucleus isthmus), or the cerebral cortex. The lack of hypothalamic cholinergic neurons creates a hiatus in the continuous antero-posterior aggregation of cholinergic neurons seen in other tetrapods. This hiatus might be functionally related to the phenomenology of monotreme sleep and to the ontogeny of sleep in mammals, as juvenile placental mammals exhibit a similar combination of sleep elements to that found in adult monotremes.

The urotensin II receptor is expressed in the cholinergic mesopontine tegmentum of the rat

Urotensin II (UII) is a peptide known to be a potent vasoconstrictor. The urotensin II receptor (UII-R) is expressed not only in peripheral tissues but also in the brain of rodents. As a basis for studies of UII central nervous system actions, UII-R localization in the rat brain was analyzed by in situ hybridization and by in situ binding. UII-R mRNA was found in the mesopontine tegmental area colocalizing with choline acetyltransferase. Binding sites were detected throughout the brain with the highest levels found in the pedunculopontine tegmental area, the lateral dorsal tegmental area, and the lateral septal, medial habenular, and interpeduncular nuclei. The majority of these brain nuclei are sites of axonal termination originating from the mesopontine areas, suggesting that UII-R is a presynaptic receptor. This distribution of UII-R in the cholinergic mesopontine area indicates that the UII system may be involved in sensory-motor integration and perhaps in central nervous system blood flow.

Expression of cGMP-specific phosphodiesterase 9A mRNA in the rat brain

cGMP has been implicated in the regulation of many essential functions in the brain, such as synaptic plasticity, phototransduction, olfaction, and behavioral state. Cyclic nucleotide phosphodiesterase (PDE) hydrolysis of cGMP is the major mechanism underlying the clearance of cGMP and is likely to be important in any process that depends on intracellular cGMP. PDE9A has the highest affinity for cGMP of any PDE, and here we studied the localization of this enzyme in the rat brain using in situ hybridization. PDE9A mRNA is widely distributed throughout the brain with varying regional expression. The pattern of PDE9A mRNA expression closely resembles that of soluble guanylyl cyclase (sGC) in the rat brain, suggesting a possible functional association or coupling of these two enzymes in the regulation of cGMP levels. Most of the brain areas expressing PDE9A mRNA also contain neuronal nitric oxide synthase (NOS), the enzymatic source of NO and the principal activator of sGC. PDE9A is the only cGMP-specific PDE with significant expression in the forebrain, and as such is likely to play an important role in NO-cGMP signaling.

Muscarine reduces calcium-dependent electrical activity in substantia nigra dopaminergic neurons

The effect of muscarine on Ca2+ dependent electrical activity was studied in dopamine (DA) neurons located in the substantia nigra pars compacta (SNc) in brain slices from young rats, using sharp electrodes. In most DA neurons tested, muscarine (50 microM) reduced the amplitude of spontaneous oscillatory potentials and evoked Ca2+-dependent potentials recorded in the presence of TTX. Muscarine also reduced the amplitude of the slow afterhyperpolarization (sAHP) following action potentials in most DA neurons. These data suggest that muscarine reduces Ca2+ entry in SNc DA neurons. The reduction of the amplitude of the sAHP by muscarine in DA neurons may facilitate bursting initiated by glutamatergic input by increasing the frequency at which DA neurons can fire. The reduction of the sAHP via activation of muscarinic receptors in vivo may provide a mechanism whereby cholinergic inputs to DA neurons from the tegmental peduncular pontine nucleus could modulate dopamine release at dopaminergic targets in the brain.

Activity-dependent nitric oxide concentration dynamics in the laterodorsal tegmental nucleus in vitro

The behavioral-state related firing of mesopontine cholinergic neurons of the laterodorsal tegmental nucleus appears pivotal for generating both arousal and rapid-eye-movement sleep. Since these neurons express high levels of nitric oxide synthase, we investigated whether their firing increases local extracellular nitric oxide levels. We measured nitric oxide in the laterodorsal tegmental nucleus with a selective electrochemical microprobe (35 microm diam) in brain slices. Local electrical stimulation at 10 or 100 Hz produced electrochemical responses that were attributable to nitric oxide. Stimulus trains (100 Hz; 1 s) produced biphasic increases in nitric oxide that reached a mean peak concentration of 33 +/- 2 (SE) nM at 4.8 +/- 0.4 s after train onset and decayed to a plateau concentration of 8 +/- 1 nM that lasted an average of 157 +/- 23.4 s (n = 14). These responses were inhibited by N(G)-nitro-L-arginine-methyl-ester (1 mM; 92% reduction of peak; n = 3) and depended on extracellular Ca(2+). Chemically reduced hemoglobin attenuated both the electrically evoked responses and those produced by authentic nitric oxide. Application of the precursor, L-arginine (5 mM) augmented the duration of the electrically evoked response, while tetrodotoxin (1 microM) abolished it. Analysis of the stimulus-evoked field potentials indicated that electrically evoked nitric oxide production resulted from a direct, rather than synaptic, activation of laterodorsal tegmental neurons because neither nitric oxide production nor the field potentials were blocked by ionotropic glutamate receptor inhibitors. Nevertheless, application of N-methyl-D-aspartate also increased local nitric oxide concentration by 39 +/- 14 nM (n = 8). Collectively, these data demonstrate that laterodorsal tegmental neuron activity elevates extracellular nitric oxide concentration probably via somatodendritic nitric oxide production. These data support the hypothesis that nitric oxide can function as a local paracrine signal during the states of arousal and rapid-eye-movement sleep when the firing of mesopontine cholinergic neurons are highest.

Selective deficits in attentional performance on the 5-choice serial reaction time task following pedunculopontine tegmental nucleus lesions

Sustained attention requires the integrity of basal forebrain cholinergic systems. The pedunculopontine tegmental nucleus (PPTg) has direct and indirect connections (via the thalamus) with the basal forebrain, suggesting that the PPTg may also play an important role in attentional processes. We examined this hypothesis by testing the effects of PPTg lesions in rats on performance in the 5-choice serial reaction time test. Bilateral lesions reduced accuracy, increased errors of omission, and increased the latency to correct responses. The deficits were more severe when neuronal damage was bilateral and concentrated in the posterior PPTg. Attentional demands of the task were increased by decreasing the stimulus duration, the stimulus brightness, or the inter-trial interval, and by introducing random bursts of white noise. These challenges impaired performance of all animals, but the magnitude of deficit was increased in the lesioned group. Conversely, lesion-induced deficits were partially alleviated when the attentional demands of the task were reduced. This pattern of results suggests that PPTg lesions produce a global deficit in attention, rather than a specific impairment in one process. The PPTg may control attentional processes through its direct projections to the forebrain cholinergic system or, indirectly, through activation of thalamocortical projections.

Muscarinic m1 and m2 receptor proteins in local circuit and projection neurons of the primate striatum: anatomical evidence for cholinergic modulation of glutamatergic prefronto-striatal pathways

The cellular and subcellular localization of muscarinic receptor proteins m1 and m2 was examined in the neostriatum of macaque monkeys by using light and electron microscopic immunocytochemical techniques. Double-labeling immunocytochemistry revealed m1 receptors in calbindin-D28k--positive medium spiny projection neurons. Muscarinic m1 labeling was dramatically more intense in the striatal matrix compartment in juvenile monkeys but more intense in striosomes in the adult caudate, suggesting that m1 expression undergoes a developmental age-dependent change. Ultrastructurally, m1 receptors were predominantly localized in asymmetric synapse-forming spines, indicating that these spines receive extrastriatal excitatory afferents. The association of m1-positive spines with lesion-induced degenerating prefronto-striatal axon terminals demonstrated that these afferents originate in part from the prefrontal cortex. The synaptic localization of m1 in these spines indicates a role of m1 in the modulation of excitatory neurotransmission. To a lesser extent, m1 was present in symmetric synapses, where it may also modulate inhibitory neurotransmission originating from local striatal neurons or the substantia nigra. Conversely, m2/choline acetyltransferase (ChAT) double labeling revealed that m2-positive neurons corresponded to large aspiny cholinergic interneurons and ultrastructurally, that the majority of m2 labeled axons formed symmetric synapses. The remarkable segregation of the m1 and m2 receptor proteins to projection and local circuit neurons suggests a functional segregation of m1 and m2 mediated cholinergic actions in the striatum: m1 receptors modulate extrinsic glutamatergic and monoaminergic afferents and intrinsic GABAergic afferents onto projection neurons, whereas m2 receptors regulate acetylcholine release from axons of cholinergic interneurons.

Microinjection of glutamate into the pedunculopontine tegmentum induces REM sleep and wakefulness in the rat

The aim of this study was to test the hypothesis that the cells in the brain stem pedunculopontine tegmentum (PPT) are critically involved in the normal regulation of wakefulness and rapid eye movement (REM) sleep. To test this hypothesis, one of four different doses of the excitatory amino acid L-glutamate (15, 30, 60, and 90 ng) or saline (control vehicle) was microinjected unilaterally into the PPT while the effects on wakefulness and sleep were quantified in freely moving chronically instrumented rats. All microinjections were made during wakefulness and were followed by 6 h of polygraphic recording. Microinjection of 15- ng (0.08 nmol) and 30-ng (0.16 nmol) doses of L-glutamate into the PPT increased the total amount of REM sleep. Both doses of L-glutamate increased REM sleep at the expense of slow-wave sleep (SWS) but not wakefulness. Interestingly, the 60-ng (0.32 nmol) dose of L-glutamate increased both REM sleep and wakefulness. The total increase in REM sleep after the 60-ng dose of L-glutamate was significantly less than the increase from the 30-ng dose. The 90-ng (0.48 nmol) dose of L-glutamate kept animals awake for 2-3 h by eliminating both SWS and REM sleep. These results show that the L-glutamate microinjection into the PPT can increase wakefulness and/or REM sleep depending on the dosage. These findings support the hypothesis that excitation of the PPT cells is causal to the generation of wakefulness and REM sleep in the rat. In addition, the results of this study led to the identification of the PPT dosage of L-glutamate that optimally induces wakefulness and REM sleep. The knowledge of this optimal dose will be useful in future studies investigating the second messenger systems involved in the regulation of wakefulness and REM sleep.

Muscarinic tone sustains impulse flow in the septohippocampal GABA but not cholinergic pathway: implications for learning and memory

Systemic infusions of the muscarinic cholinergic receptor antagonists atropine and scopolamine (atr/scop) produce an amnesic syndrome in humans, subhuman primates, and rodents. In humans, this syndrome may resemble early symptoms of Alzheimer's disease. Behavioral studies in rats have demonstrated that the medial septum/diagonal band of Broca (MSDB), which sends cholinergic and GABAergic projections to the hippocampus, is a critical locus in mediating the amnesic effects of atr/scop. The amnesic effects of atr/scop in the MSDB have been presumed but not proven to be caused by a decrease in hippocampal acetylcholine (ACh) release after blockade of a muscarinic tone in the MSDB. Using electrophysiological recordings and fluorescent-labeling techniques to identify living septohippocampal neurons in rat brain slices, we now report that, contrary to current belief, a blockade of the muscarinic tone in the MSDB does not decrease impulse flow in the septohippocampal cholinergic pathway; instead, it decreases impulse flow in the septohippocampal GABAergic pathway via M(3) muscarinic receptors. We also report that the muscarinic tone in the MSDB is maintained by ACh that is released locally, presumably via axon collaterals of septohippocampal cholinergic neurons. As such, cognitive deficits that occur in various neurodegenerative disorders that are associated with a loss or atrophy of septohippocampal cholinergic neurons cannot be attributed solely to a decrease in hippocampal acetylcholine release. An additional, possibly more important mechanism may be the concomitant decrease in septohippocampal GABA release and a subsequent disruption in disinhibitory mechanisms in the hippocampus. Restoration of impulse flow in the septohippocampal GABA pathway, possibly via M(3) receptor agonists, may, therefore, be critical for successful treatment of cognitive deficits associated with neurodegenerative disorders such as Alzheimer's and Parkinson's disease.

Cortical cholinergic activity is related to the novelty of the stimulus

A number of studies have related cholinergic activity to the mediation of learning and memory. However, the acetylcholine (ACh) participation has been recently implicated in the early stages of memory formation but not during retrieval. The aim of the present study is to evaluate ACh release in the insular cortex (IC) during presentation of different taste stimuli and during their re-exposition by means of the free-moving microdialysis technique. We evaluated the changes in ACh release when a novel taste, saccharin or quinine was presented to the rat and after several presentations of saccharin. Unilateral microdialysis was performed in the IC 1 h before and 1 h after the presentation of: (1) a familiar stimulus (water), (2) a novel taste (quinine), (3) another novel taste (saccharin), (4) a second presentation, (5) a third presentation, and (6) a fourth presentation of saccharin. The volume consumed by the animals was registered as a behavioral parameter. The ACh levels from the microdialysis fractions were analyzed by an HPLC-ED system. Biochemical results showed a significant increment in the cortical ACh release induced by a novel stimulus compared with the release observed during the presentation of a familiar stimulus. The ACh release observed after several presentations of the stimuli decreased to the same levels as those produced by the familiar taste, indicating an inverse relationship between familiarity and cortical ACh release. These results suggest that the cholinergic system plays an important role in the identification and characterization of different kinds of stimuli.

Chemical anatomy of striatal interneurons in normal individuals and in patients with Huntington's disease

This paper reviews the major anatomical and chemical features of the various types of interneurons in the human striatum, as detected by immunostaining procedures applied to postmortem tissue from normal individuals and patients with Huntington's disease (HD). The human striatum harbors a highly pleomorphic population of aspiny interneurons that stain for either a calcium-binding protein (calretinin, parvalbumin or calbindin D-28k), choline acetyltransferase (ChAT) or NADPH-diaphorase, or various combinations thereof. Neurons that express calretinin (CR), including multitudinous medium and a smaller number of large neurons, are by far the most abundant interneurons in the human striatum. The medium CR+ neurons do not colocalize with any of the known chemical markers of striatal neurons, except perhaps GABA, and are selectively spared in HD. Most large CR+ interneurons display ChAT immunoreactivity and also express substance P receptors. The medium and large CR+ neurons are enriched with glutamate receptor subunit GluR2 and GluR4, respectively. This difference in AMPA GluR subunit expression may account for the relative resistance of medium CR+ neurons to glutamate-mediated excitotoxicity that may be involved in HD. The various striatal chemical markers display a highly heterogeneous distribution pattern in human. In addition to the classic striosomes/matrix compartmentalization, the striosomal compartment itself is composed of a core and a peripheral region, each subdivided by distinct subsets of striatal interneurons. A proper knowledge of all these features that appear unique to humans should greatly help our understanding of the organization of the human striatum in both health and disease states.

Laterodorsal tegmental stimulation elicits dopamine efflux in the rat nucleus accumbens by activation of acetylcholine and glutamate receptors in the ventral tegmental area

Cholinergic and glutamatergic neurons in the laterodorsal tegmentum (LDT) and neighbouring mesopontine nuclei are thought to influence mesolimbic dopaminergic neuronal activity involved in goal-directed behaviours. We measured the changes in dopamine oxidation current (corresponding with dopamine efflux) in the nucleus accumbens (NAc) in response to electrical stimulation of the LDT using in vivo chronoamperometry in urethane-anaesthetized rats. LDT stimulation (35 Hz pulse trains for 60 s, 1 s intertrain interval) evoked a three-component change in dopamine efflux in the NAc: (i) an initial stimulation time-locked increase in the dopamine signal above baseline, followed by (ii) an immediate decrease below baseline, and thereafter by (iii) a prolonged increase in the dopamine signal above baseline. Intra-VTA infusion of the nicotinic receptor antagonist mecamylamine (5 microg/0.5 microL) or the ionotropic glutamate receptor antagonist kynurenate (10 microg/microL) attenuated the first LDT-elicited component. The second suppressive component was abolished by intra-LDT infusions of either the nonselective or the M2-selective muscarinic receptor antagonists scopolamine (100 microg/microL) and methoctramine (50 microg/microL), respectively. In contrast, intra-VTA infusions of scopolamine (200 microg/microL) resulted in a selective attenuation of the third facilitatory component, whereas both second and third components were abolished by systemic injections of scopolamine (5 mg/kg). These results suggest that the initial increase, subsequent decrease, and final prolonged increase in extracellular dopamine levels in the NAc are selectively mediated by LDT-elicited activation of (i) nicotinic and glutamatergic receptors in the VTA, (ii) muscarinic M2 autoreceptors on LDT cell bodies, and (iii) muscarinic receptors in the VTA, respectively.

Neural circuits and topographic organization of the basal ganglia and related regions

The present review was attempted to analyze the multiple channels of basal ganglia-thalamocortical connections, and the connections of their related nuclei. The prefrontal and motor areas consist of a number of modules, which seem to provide multiple subloops of the basal ganglia-thalamocortical connections in subhuman primates. There may be a great degree of convergence of the limbic, associative and motor loops at the level of the striatum, substantia nigra, pallidum, and the subthalamic nucleus as well as the pedunculopontine nucleus. Nigral dopaminergic neurons receive limbic input directly as well as indirectly through the striosomes in the striatum. Dopamine contributes to behavioral learning by signaling motivation and reinforcement. The pedunculopontine nucleus might be involved in behavioral state control, learning and reinforcement processes, locomotion and autonomic functions. Each subdivision of the motor areas receives a mixed and weighted transthalamic input from both the cerebellum and basal ganglia. In particular, based on the author's data, the hand/arm motor area and adjacent premotor area receive strong superficial basal ganglia-thalamocortical projections as well as the deep cerebello-thalamocortical projections. These areas, have very dense corticocotrical connections with other cortical areas, receive polymodal afferents from the parietal and temporal cortices, and integrated information, via multiple routes, from the prefrontal cortex. The author suggests that the ventrolateral part of the caudal medial pallidal segment (GPi) and the ventromedial part of the GPi are linked directly to these areas by ways of the oral part of ventral lateral nucleus (VLo) and the ventral part of the parvicellular part of ventral anterior nucleus (VApc), respectively. These connections are thought to be involved in the acquisition and coordination of motor sequences.

Organization of somatic motor inputs from the frontal lobe to the pedunculopontine tegmental nucleus in the macaque monkey

To reveal the somatotopy of the pedunculopontine tegmental nucleus that functions as a brainstem motor center, we examined the distribution patterns of corticotegmental inputs from the somatic motor areas of the frontal lobe in the macaque monkey. Based on the somatotopical map prepared by intracortical microstimulation, injections of the anterograde tracers, biotinylated dextran amine and wheat germ agglutinin-conjugated horseradish peroxidase, were made into the following motor-related areas: the primary motor cortex, the supplementary and presupplementary motor areas, the dorsal and ventral divisions of the premotor cortex, and the frontal eye field. Data obtained from the present experiments were as follows: (i) Corticotegmental inputs from orofacial, forelimb, and hindlimb representations of the primary motor cortex tended to be arranged orderly from medial to lateral in the pedunculopontine tegmental nucleus. However, the distribution areas of these inputs considerably overlapped; (ii) The major input zones from distal representations of the forelimb and hindlimb regions of the primary motor cortex were located medial to those from their proximal representations, although there was a substantial overlap between the distribution areas of distal versus proximal limb inputs; (iii) The main terminal zones from the forelimb regions of the primary motor cortex, the supplementary and presupplementary motor areas, and the dorsal and ventral divisions of the premotor cortex appeared to overlap largely in the mediolaterally middle aspect of the pedunculopontine tegmental nucleus; and (iv) Corticotegmental input from the frontal eye field was scattered over the pedunculopontine tegmental nucleus.Thus, the present results indicate that the pedunculopontine tegmental nucleus is likely to receive partly separate but essentially convergent cortical inputs not only from multiple motor-related areas representing the same body part, but also from multiple regions representing diverse body parts. This suggests that somatotopical representations are intermingled rather than segregated in the pedunculopontine tegmental nucleus.

Metabolic activity of excitatory parafascicular and pedunculopontine inputs to the subthalamic nucleus in a rat model of Parkinson's disease

Using a combination of metabolic measurement and retrograde tracing, we show that the neurons in the pedunculopontine nucleus and parafascicular nucleus of the thalamus that project to the subthalamic nucleus are hyperactive after nigrostriatal dopaminergic denervation in rats. In Parkinson's disease, the loss of dopaminergic neurons induces a cascade of functional changes in the basal ganglia circuitry including a hyperactivity of the subthalamic nucleus. This hyperactivity is thought to be due to a diminution of the inhibitory pallidal influence. However, recent studies have suggested that other cerebral structures are involved in the subthalamic neuronal hyperactivity. This study was undertaken to identify these cerebral structures. Neurons projecting to the subthalamic nucleus were identified by retrograde transport of wheat germ agglutinin conjugated to horseradish peroxidase, injected into the subthalamic nucleus of rats with 6-hydroxydopamine unilateral lesion of the substantia nigra pars compacta and sham-lesioned animals. Metabolic activity was determined in the same neurons using in situ hybridization for the first subunit of cytochrome oxidase messenger RNA, a metabolic marker, and image analysis. Horseradish peroxidase-labeled neurons were found in the globus pallidus, parafascicular and pedunculopontine nucleus and sometimes in raphe nuclei and the substantia nigra pars compacta. Measurement of metabolic activity was performed for the globus pallidus, the pedunculopontine and parafascicular nuclei. The expression level of the first subunit of cytochrome oxidase messenger RNA in neurons projecting to the subthalamic nucleus was 62% higher in parafascicular neurons and 123% higher in pedunculopontine neurons in 6-hydroxydopamine-lesioned rats, compared to sham-lesioned animals. An increase was also observed in the globus pallidus, but did not reach significance. Our results suggest that hyperactivity of subthalamic neurons could be due, at least in part, to an increase of excitatory input arising from the pedunculopontine and parafascicular nuclei. These data also suggest that the latter structures may play an important role in the physiopathology of Parkinson's disease.

Substantia nigra pars reticulata lesion induces preconvulsive behavior and changes in glutamate receptor gene expression in the rat brain

The substantia nigra pars reticulata (SNpr) has been proposed to play an important role in the control of the propagation and/or the generation of epileptic seizures. Earlier studies have shown differential effects of the lesion of the SNpr on seizure genesis that demonstrated a regional difference in the anterior and posterior parts of the SNpr in preconvulsive behavior induced by unilateral reticulata injection of dopamine (DA). This study was aimed to investigate some of the underlying mechanisms of the preconvulsive behavior elicited by unilateral SNpr DA injection by the study of changes in the gene expression of glutamate receptor subunits (GluR1, GluR2 and NMDAR1) and of changes in animal behavior following coinfusion of DA and a DA D1 antagonist SCH 23390 into the SNpr. Unilateral injection of exogenous DA into the anterior region of the SNpr induced rapid and short lasting preconvulsive behavior up to wet dog shakes stage and a significant reduction of gene expression for GluR1, GluR2 and NMDAR1 subunits in rat hippocampal subfields including CA1 through CA4 and dentate gyrus (DG) at 1 day after nigral DA injection. The effect was long lasting and persisted for at least 3 weeks. Both preconvulsive behavior and downregulation of glutamate receptor subunit genes were completely blocked by simultaneous coinfusion of DA and SCH 23390. The results suggest, for the first time, that DA D1 receptor in the SNpr may mediate the nigral-involved seizure development. Glutamate desensitization, and/or selective early neuronal damage might be responsible for the downregulation of glutamate receptor subunits by transient preconvulsive activity.

The pedunculopontine tegmental nucleus and the role of cholinergic neurons in nicotine self-administration in the rat: a correlative neuroanatomical and behavioral study

The objective of this study was to determine whether the pedunculopontine tegmental nucleus plays a role in the maintenance of nicotine self-administration, and whether the ascending cholinergic projection from this nucleus to midbrain dopamine neurons in the ventral tegmental area might be involved. Studies were done with rats trained to self-administer nicotine intravenously. Self-administration was examined before and after the pedunculopontine tegmental nucleus was lesioned with the ethylcholine mustard aziridinium ion, a selective cholinergic toxin. Lesions were assessed qualitatively and quantitatively in histological sections stained for either nicotinamide adenine dinucleotide phosphate-diaphorase histochemistry to identify cholinergic neurons, or for Nissl. Self-administration was also tested after an acute manipulation in which microinfusions of the nicotinic cholinergic antagonist dihydro-beta-erythroidine were made into the pedunculopontine tegmentum. Infusions of neurotoxin into the pedunculopontine tegmentum reduced nicotine self-administration behaviour when tested weeks later. Toxin treatment reduced the number of cholinergic neurons in the tegmentum, while largely sparing the non-cholinergic population in this area. Lesions were limited to the pedunculopontine area and did not extend to the neighboring laterodorsal tegmental nucleus or to the substantia nigra. Acute manipulation of the pedunculopontine tegmental nucleus with microinfusions of dihydro-beta-erythroidine also produced an attenuation of nicotine self-administration. Collectively these data show that the pedunculopontine tegmental nucleus is part of the neuronal circuitry mediating nicotine self-administration, and that the population of cholinergic neurons is likely a critical element.

Protection against dopaminergic nigrostriatal cell death by excitatory input ablation

The importance of enhanced glutamatergic neurotransmission in the basal ganglia and related structures has recently been highlighted in the development of Parkinson's disease. The pedunculopontine tegmental nucleus (PPN) is the major origin of excitatory, glutamatergic input to dopaminergic nigrostriatal neurons of which degeneration is well known to cause Parkinson's disease. Based on the concept that an excitatory mechanism mediated by glutamatergic neurotransmission underlies the pathogenesis of neurodegenerative disorders, we made an attempt to test the hypothesis that removal of the glutamatergic input to the nigrostriatal neurons by PPN lesions might prevent 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced parkinsonism in the macaque monkey. The PPN was lesioned unilaterally with microinjection of kainic acid, and, then, MPTP was administered systemically. In these monkeys, the degree of parkinsonian motor signs was behaviourally evaluated, and the histological changes in the dopaminergic nigrostriatal system were analysed by means of tyrosine hydroxylase immunohistochemistry. The present results revealed that nigrostriatal cell loss and parkinsonian motor deficits were largely attenuated in the MPTP-treated monkey group whose PPN had been lesioned, compared with the control, MPTP-treated monkey group with the PPN intact. This clearly indicates that the onset of MPTP neurotoxicity is suppressed or delayed by experimental ablation of the glutamatergic input to the nigrostriatal neurons. Such a protective action of excitatory input ablation against nigrostriatal cell death defines evidence that nigral excitation driven by the PPN may be implicated in the pathophysiology of Parkinson's disease.

Honeycomb-like structure of the intermediate layers of the rat superior colliculus, with additional observations in several other mammals: AChE patterning

The aim of the present study was to reinvestigate the stereometric pattern of acetylcholinesterase (AChE) activity staining in the intermediate layers of the superior colliculus in several mammalian species. A pioneering study in the cat and the monkey by Graybiel (1978) stressed the regular arrangement of AChE staining in the deep collicular layers. According to her description, made in the frontal plane, the enzyme was arranged in a mediolateral series of patches, the cores of which tended to line up in the longitudinal axis of the structure, so they formed roughly parallel bands. As exhaustive a description as possible of the AChE distribution was undertaken in the rat by compiling observations in the frontal, sagittal, and tangential planes. It emerged that AChE-positive elements are organized in the form of a conspicuous honeycomb-like network that is divided into about 100 rounded compartments, over virtually the full extent of the intermediate layers. The generality of the rat model was then tested in other rodents such as mouse and hamster and also in cat and monkey. For these species we resorted to a single tangential cutting plane, which proved to be more appropriate for disclosing such a modular arrangement. The data revealed that in all species AChE staining followed the same architectural plan and identified the striking similarity in the number of compartments that compose the various honeycomb-like lattices. In conclusion, the present findings support a unified model of the AChE arrangement within the intermediate layers of the mammalian colliculus; the model comprehensively incorporates the classical description of the patchy and stripy features of the enzyme distribution. We hypothesize here that the modular AChE arrangement might be the anatomical basis for collicular vectorial encoding of orienting movements.

Pedunculopontine tegmental nucleus lesions impair stimulus--reward learning in autoshaping and conditioned reinforcement paradigms

The role of the pedunculopontine tegmental nucleus (PPTg) in stimulus-reward learning was assessed by testing the effects of PPTg lesions on performance in visual autoshaping and conditioned reinforcement (CRf) paradigms. Rats with PPTg lesions were unable to learn an association between a conditioned stimulus (CS) and a primary reward in either paradigm. In the autoshaping experiment, PPTg-lesioned rats approached the CS+ and CS- with equal frequency, and the latencies to respond to the two stimuli did not differ. PPTg lesions also disrupted discriminated approaches to an appetitive CS in the CRf paradigm and completely abolished the acquisition of responding with CRf. These data are discussed in the context of a possible cognitive function of the PPTg, particularly in terms of lesion-induced disruptions of attentional processes that are mediated by the thalamus.

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

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

Acetylcholine-mediated modulation of striatal function

Striatal spiny neurones serve as a major anatomical locus for the relay of cortical information flow through the basal ganglia. these projection neurones also represent the main synaptic target of cholinergic interneurones, whose physiological role in striatal activity still remains largely enigmatic. The striatal cholinergic system has been implicated in the pathophysiology of movement disorders such as Parkinson's disease, but the cellular mechanisms underlying cholinergic-neurone function are still unknown. On the basis of in vitro electrophysiological evidence, obtained from a rat corticostriatal-slice preparation, we propose that endogenous ACh exerts a complex modulation of striatal synaptic transmission, which produces both short-term and long-term effects. ACh-mediated mechanisms might be of crucial importance in processing the cortical inputs to the striatum.

The effect of electrolytic thalamic lesions on the NADPH-diaphorase activity of neurons of the laterodorsal tegmental and pedunculopontine nuclei in rats

Cholinergic neurons of the mesopontine complex have extensive ascending projections to the forebrain: the laterodorsal tegmental nucleus extensively innervates the anterior thalamus, the anteroventral nucleus in particular, whereas the pedunculopontine nucleus has widespread projections to both the thalamus and extrapyramidal structures. Most of their neurons express nitric oxide synthase (NOS) activity. Following electrolytic lesions of the anteroventral thalamic nucleus, nicotinamide adenine dinucleotide phosphate-diaphorase (NADPHd) activity in neurons of the laterodorsal tegmental nucleus changed drastically. The intensity of NADPH-diaphorase staining increased in laterodorsal tegmental neurons ipsilateral to the lesion side, but decreased contralaterally. The intensity of the NADPH-diaphorase staining of neurons of the pedunculopontine nucleus, however, remained unchanged bilaterally. After partial lesions of the anteroventral thalamic nucleus a similar effect was noted. In contrast, large electrolytic lesions involving other thalamic nuclei or extrapyramidal structures did not change the number of NADPH-diaphorase neurons or their intensity of staining in the laterodorsal tegmental nuclei. These data show that electrolytic lesions of target areas can lead to an upregulation of NOS expression in the parent cell bodies, provided that there is no wide collateralization as found for the pedunculopontine nucleus.

Mesopontine cholinergic and non-cholinergic neurons in schizophrenia

Mesopontine cholinergic neurons influence midbrain dopaminergic neurons, and thalamic and cerebellar structures which have been implicated in the neuroanatomy of schizophrenia. It has been reported that there are approximately twice as many mesopontine cholinergic neurons in schizophrenics than in normals, using nicotinomide adenosine dinucleotide phosphatediaphorase histochemistry to identify the cholinergic neurons. The present study sought to replicate this finding by analysing mesopontine cholinergic neurons using an antibody against choline acetyltransferase. The mesopontine cholinergic neurons are located in the pars compacta and pars dissipata of the pedunculopontine nucleus, and in the laterodorsal tegmental nucleus. Quantitative computer imaging techniques were used to map the distribution of mesopontine cholinergic neurons. In addition, all medium-sized and large neurons in a region of interest containing the middle portion of the pedunculopontine nucleus pars compacta were counted in Nissl-stained sections. There was no difference between schizophrenic and normal brains in terms of: (i) the rostral-caudal length of the cholinergic cell complex, approximately 10 mm; (ii) the estimated total number of cholinergic neurons in the combined pedunculopontine nucleus and laterodorsal tegmental nucleus, approximately 20,000 cells unilaterally; and (iii) the combined number of cholinergic and non-cholinergic Nissl-stained neurons in the middle portion of the pedunculopontine nucleus. The present data do not support the previous observation of increased numbers of mesopontine cholinergic neurons in schizophrenia.

Activity of midbrain reticular formation and neocortex during the progression of human non-rapid eye movement sleep

To clarify the neural correlates and brain activity during the progression of human non-rapid eye movement (NREM) sleep, we examined the absolute regional cerebral blood flow (rCBF) during light and deep NREM sleep and during wakefulness in normal humans using positron emission tomography with H(2)(15)O. Relative changes in rCBF during light and deep NREM sleep in comparison to the rCBF during wakefulness were also analyzed. During light NREM sleep, the rCBF in the midbrain, in contrast to that in the pons and thalamic nuclei, did not decrease when compared to that during wakefulness, whereas rCBF decreased in the left medial frontal gyrus, left inferior frontal gyrus, and left inferior parietal gyrus of the neocortex. During deep NREM sleep, the rCBF in the midbrain tegmentum decreased, and there was a marked and bilateral decrease in the rCBF in all neocortical regions except for the perirolandic areas and the occipital lobe. There have been three groups of brain structures, each representing one type of deactivation during the progression of NREM sleep. The activity of the midbrain reticular formation is maintained during light NREM sleep and therefore represents a key distinguishing characteristic between light and deep NREM sleep. Selective deactivation of heteromodal association cortices, including those related to language, occurs with increasingly deep NREM sleep, which supports the recent theory that sleep is not a global, but it is a local process of the brain.

Characterization of the extent of pontomesencephalic cholinergic neurons' projections to the thalamus: comparison with projections to midbrain dopaminergic groups

We sought to determine whether pontomesencephalic cholinergic neurons which we have been shown previously to project to the substantia nigra and ventral tegmental area also contribute to the thalamic activation projection from the pedunculopontine and laterodorsal tegmental nuclei. Retrograde tracing, immunohistochemical localization of choline acetyltransferase and statistical methods were used to determine the full extent of the cholinergic projection from the pedunculopontine and laterodorsal tegmental nuclei to the thalamus. Progressively larger Fluoro-Gold injections in to the thalamus proportionally labeled increasing numbers of pontomesencephalic cholinergic cells both ipsi- and contralaterally in the pedunculopontine and laterodorsal tegmental nuclei. Multiple large thalamic injections left only a small fraction of the ipsilateral pontomesencephalic cholinergic group unlabeled. This small remainder did not correspond to the populations which project to the substantia nigra and ventral tegmental area, thereby indicating that substantia nigra- and ventral tegmental area-projecting cholinergic neurons must also project to the thalamus. We examined whether there existed any set of cholinergic neurons in the pedunculopontine and laterodorsal tegmental nuclei which did not innervate a thalamic target. The distribution of descending projections of the pedunculopontine and laterodorsal tegmental nuclei demonstrated that the unlabeled remainder cannot correspond to a purely descending group. We also show that substance P-positive cholinergic cells in the laterodorsal tegmental nucleus project to the thalamus. Further studies demonstrated that the small population of cholinergic cells left unlabeled from the thalamus were the smallest sized cholinergic cells, and included two groups of small, light-staining cholinergic cells located in the parabrachial area and central gray, adjacent to the main pedunculopontine and laterodorsal tegmental nuclei cholinergic groups. These small cells, in contrast to thalamic-projecting cholinergic cells, did not stain positively for reduced nicotinamide adenine dinucleotide phosphate-diaphorase. Taken together, these results indicated that all of the reduced nicotinamide adenine dinucleotide phosphate diaphorase-positive/choline acetyltransferase-positive neurons of the pedunculopontine/laterodorsal tegmental nuclei ascend to innervate some portion of the thalamus, in addition to the other targets they innervate. These findings indicate that the diverse physiological and behavioral effects attributed to the activity of pontomesencephalic cholinergic neurons should not be dissociated from their activating effects in the thalamus.

Activation of the reticulothalamic cholinergic pathway by the major metabolites of aniracetam

The aim of the study was to further investigate the effects of aniracetam, a cognition enhancer, and its metabolites on the brain cholinergic system. We measured choline acetyltransferase activity and acetylcholine release using in vivo brain microdialysis in stroke-prone spontaneously hypertensive rats (SHRSP). The enzyme activity in the pons-midbrain and hippocampus, and basal acetylcholine release in the nucleus reticularis thalami were lower in SHRSP than in age-matched Wistar Kyoto rats, indicating central cholinergic deficits in SHRSP. Repeated treatment of aniracetam (50 mg/kg p.o. x 11 for 6 days) preferentially increased the enzyme activity in the thalamus, whereas decreased it in the striatum. Among the metabolites of aniracetam, local perfusion of N-anisoyl-gamma-aminobutyric acid (GABA, 0.1 and/or 1 microM) and p-anisic acid (1 microM) into the nucleus reticularis thalami, dorsal hippocampus and prefrontal cortex of SHRSP produced a significant but delayed increase of acetylcholine release. We failed, however, to find any effect of aniracetam itself. A direct injection of N-anisoyl-GABA (1 nmol) into the pedunculopontine tegmental nucleus of SHRSP enhanced the release in the nucleus reticularis thalami. Thus, these data prove that aniracetam can facilitate central cholinergic neurotransmission via both metabolites. Based on its pharmacokinetic profile, N-anisoyl-GABA may contribute to the clinical effects of aniracetam, mainly by acting on the reticulothalamic cholinergic pathway.

Ultrastructural localization of the alpha4-subunit of the neuronal acetylcholine nicotinic receptor in the rat substantia nigra

The distribution of the alpha4-subunit of the neuronal nicotinic acetylcholine receptor (nAChR) in the rat brain was examined at light and electron microscopy levels using immunohistochemical staining. In the present study we demonstrate the specificity, in both tissue homogenates and brain sections, of a polyclonal antibody raised against the rat nAChR alpha4-subunit. The characterization of this antibody involved: (1) Western blot analysis of rat brain homogenates and membrane extracts from cells previously transfected with diverse combinations of neuronal nAChR subunits, and (2) immunohistochemistry using transfected cells and rat brain tissue. At the light microscope level, the alpha4-subunit-like-immunoreactivity (LI) was widely distributed in the rat brain and matched the distribution of the alpha4-subunit transcripts observed previously by in situ hybridization. Strong immunohistochemical labeling was detected in the mesencephalic dopaminergic nuclei. The nAChRs in this region are thought to be responsible for the modulation of dopaminergic transmission. The neurotransmitter identity of alpha4-immunolabeled neurons in the substantia nigra pars compacta (SNpc) and the ventral tegmental area was thus assessed by investigating the possible colocalization of the nAChR alpha4-subunit with tyrosine hydroxylase using confocal microscopy. The double labeling experiments unambiguously indicated that the alpha4-subunit-LI is present in dopaminergic neurons. At the electron microscope level, the neurons in the SNpc exhibited alpha4-subunit-LI in association with a minority of postsynaptic densities, suggesting that the alpha4-subunit may be a component of functional nAChRs mediating synaptic transmission between midbrain cholinergic neurons and mesencephalic dopaminergic neurons.

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

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

Quantification of cholinergic and select non-cholinergic mesopontine neuronal populations in the human brain

The pars compacta and pars dissipata of the pedunculopontine nucleus contain cholinergic cell group Ch5, and the laterodorsal tegmental nucleus contains cholinergic cell group Ch6. The pedunculopontine nucleus has been implicated in a variety of functions, including mediation of rapid eye movement sleep and in extrapyramidal motor function, although the role of cholinergic and non-cholinergic neurons is unclear. Quantitative neuroanatomical techniques were used to map the distribution of cholinergic neurons in the mesopontine nuclei of the adult human brain. In addition, the number and distribution of comparably sized non-cholinergic neurons at selected anatomical levels were compared. An antibody raised against human choline acetyltransferase was used to stain immunohistochemically the mesopontine neurons in six brains, ranging in age from 28 to 60 years. The rostrocaudal length of the Cb5/Ch6 cell complex was approximately 10 mm. The estimated total number of cells was similar for all brains, and varied by less than 7%. The estimated average number of cholinergic cells in the combined pedunculopontine and laterodorsal tegmental nuclei was approximately 20,000, with 30% of the cells in the pedunculopontine nucleus pars compacta, 57% in the pedunculopontine nucleus pars dissipata and 13% in the laterodorsal tegmental nucleus. There was no correlation between cell number and age. Within areas of mesopontine tegmentum occupied by the Ch5 cholinergic neurons, there were often more noncholinergic neurons than comparably sized cholinergic neurons. The present study provides detailed maps of the distribution and number of mesopontine cholinergic neurons in the normal human brain. Many non-cholinergic neurons are intermixed with the cholinergic pedunculopontine neurons. One region of the pedunculopontine nucleus pars dissipata containing few cholinergic neurons, located adjacent to the ventral border of the pedunculopontine nucleus pars compacta, may correspond to the midbrain-extrapyramidal area as defined previously in rodent and in non-human primate. These data will be useful for quantitative neuropathological studies concerning the role of both cholinergic and non-cholinergic mesopontine neurons in diseases proposed to affect these neurons, including Parkinson's disease, schizophrenia and progressive supranuclear palsy.

Muscarinic receptor subtypes in the lateral geniculate nucleus: a light and electron microscopic analysis

Neural activity in the dorsal lateral geniculate nucleus of the thalamus (DLG) is modulated by an ascending cholinergic projection from the brainstem. The purpose of this study was to identify and localize specific muscarinic receptors for acetylcholine in the DLG. Receptors were identified in rat and cat tissue by means of antibodies to muscarinic receptor subtypes, ml-m4. Brain sections were processed immunohistochemically and examined with light and electron microscopy. Rat DLG stained positively with antibodies to the m1, m2,and m3 receptor subtypes but not with antibodies to the m4 receptor subtype. The m1 and m3 antibodies appeared to label somata and dendrites of thalamocortical cells. The m1 immunostaining was pale, whereas m3-positive neurons exhibited denser labeling with focal concentrations of staining. Strong immunoreactivity to the m2 antibody was widespread in dendrites and somata of cells resembling geniculate interneurons. Most m2-positive synaptic contacts were classified as F2-type terminals, which are the presynaptic dendrites of interneurons. The thalamic reticular nucleus also exhibited robust m2 immunostaining. Cat DLG exhibited immunoreactivity to the m2 and m3 antibodies. The entire DLG stained darkly for the m2 receptor subtype, except for patchy label in the medial interlaminar nucleus and the ventralmost C laminae. The staining for m3 was lighter and was distributed more homogeneously across the DLG. The perigeniculate nucleus also was immunoreactive to the m2 and m3 subtype-specific antibodies. Immunoreactivity in cat to the m1 or m4 receptor antibodies was undetectable. These data provide anatomical evidence for specific muscarinic-mediated actions of acetylcholine on DLG thalamocortical cells and thalamic interneurons.

Frontal syndrome as a consequence of lesions in the pedunculopontine tegmental nucleus: a short theoretical review

In this review, it is argued that the consequence of bilateral damage to the pedunculopontine tegmental nucleus (PPTg) in experimental animals is the production of a form of frontal syndrome. Frontal syndrome is a term used to describe the behavioural consequences of damage to the frontal lobes in human patients. These behavioural changes can be classified as disinhibition of behaviour (a release of behavioural control), the production of inappropriate behaviour (which in patients can be either inappropriate actions or verbal behaviour), and the production of perseverative behaviour (the maintenance of an action beyond the point at which it should have been terminated). The psychological changes which underlie these behavioural changes are thought to involve executive functions, which include such things as the prospective planning of sequences of actions, attentional shifting and working memory. In this review, I attempt to demonstrate two things: first, that there are significant anatomical connections from frontostriatal systems to the PPTg. The motor cortex projects directly to the PPTg while the prefrontal cortex contacts it via striatal circuitry, forming clear routes by which the frontal lobes can communicate with the PPTg. Second, having established the existence of connections between frontostriatal systems and the PPTg, behavioural data are described. Experimental animals bearing bilateral lesions of the PPTg have been examined in a wide variety of tasks. Animals bearing such lesions are not impaired in basic processes of feeding, drinking, locomotion, or grooming and simple observation of lesioned rats' normal behaviour reveals no obvious gross impairment in function. However, the results of more subtle tests reveal a wide variety of deficits in various tasks. The outcome of these experiments are in many ways contradictory, but in the vast majority of cases, the changes can be described as involving disinhibition of behaviour, the release of inappropriate behaviour, and the production of perseverative behaviour. Anatomical and behavioural data support the conclusion that there are functional connections between frontal systems and the PPTg. This review also discusses what psychological processes might be served by such connections.

The organization of corticothalamic projections: reciprocity versus parity

All neocortical areas receive inputs from and project back to the thalamus. It is often said that the corticothalamic projections are organized in a way that reciprocates the spatial distribution of thalamocortical pathways. The present review examines to what extent this rule of reciprocity is actually supported by the most recent neuroanatomical data, particularly those relating to the central organization of the vibrissal sensory system in the rat. A critical survey of previous studies is made and new results are presented concerning the fine-grained organization of corticothalamic projections in this sensory system. Together, prior results and the present set of new data confirm the existence of both, reciprocal and nonreciprocal patterns of corticothalamic connectivity. This conclusion leads us to propose that the spatial organization of corticothalamic connections complies with a more fundamental rule, the rule of parity, from which reciprocity follows as a general, but not obligatory consequence. The rule of parity states that the distribution of corticothalamic projections across and within the thalamic nuclei is determined by the branching patterns of the different classes of prethalamic afferents. The anatomical, developmental and physiological consequences of this rule are discussed. The rule of parity suggests that, according to the behavioral context, both prethalamic and corticothalamic pathways may function in a feedback mode.

Recovery of cable properties through active and passive modeling of subthreshold membrane responses from laterodorsal tegmental neurons

The laterodorsal tegmental nucleus (LDT) is located in the dorsolateral pontine reticular formation. Cholinergic neurons in the LDT and the adjacent pedunculopontine tegmental nucleus (PPT) are hypothesized to play a critical role in the generation of the electroencephalographic-desynchronized states of wakefulness and rapid eye movement sleep. A quantitative analysis of the cable properties of these cells was undertaken to provide a more detailed understanding of their integrative behavior. The data used in this analysis were the morphologies of intracellularly labeled guinea pig LDT neurons and the voltage responses of these cells to somatic current injection. Initial attempts to model the membrane behavior near resting potential and in the presence of tetrodotoxin (TTX, 1 microM) as purely passive produced fits that did not capture many features of the experimental data. Moreover, the recovered values of membrane conductance or intracellular resistivity were often very far from those reported for other neurons, suggesting that a passive description of cell behavior near rest was not adequate. An active membrane model that included a subthreshold A-type K+ current and/or a hyperpolarization-activated cation current (H-current) then was used to model cell behavior. The voltage traces calculated using this model were better able to reproduce the experimental data, and the cable parameters determined using this methodology were more consistent with those reported for other cells. Additionally, the use of the active model parameter extraction methodology eliminated a problem encountered with the passive model in which parameter sets with widely varying values, sometimes spanning an order of magnitude or more, would produce effectively indistinguishable fits to the data. The use of an active model to directly fit the experimentally measured voltage responses to both long and short current pulses is a novel approach that is of general utility.

Effects of pedunculopontine tegmental nucleus lesions on responding for intravenous heroin under different schedules of reinforcement

The pedunculopontine tegmental nucleus (PPTg) is believed to play important roles in reward and learning. We examined the effect of PPTg lesions (0.5 microl of 0.1 M NMDA injected bilaterally over 10 min) on the learning of an operant response for opiate reward. In 14 adult male Long-Evans rats, bilateral lesions of the PPTg disrupted the acquisition of responding for intravenous heroin (0.1 mg/kg infused at a rate of 0.25 ml/28 sec) on a fixed ratio-1 (FR-1) schedule of reinforcement. The 12 remaining lesioned animals increased their heroin intake over the acquisition sessions but did not reach the response levels of sham-lesioned animals on the 15th and final session. The sham- and PPTg-lesioned animals that learned the FR-1 task exhibited similar patterns of responding during extinction and reacquisition sessions. When tested on a progressive ratio (PR) schedule of reinforcement, however, PPTg-lesioned animals had lower break points than sham-lesioned animals. Asymmetric lesions, which destroyed the majority of the nucleus in one hemisphere only, did not produce any behavioral deficits. Rats that were lesioned after training also did not show deficits in responding under either FR or PR schedules. These findings suggest that PPTg lesions reduce the rewarding effect of opiates but do not disrupt the ability either to learn an operant response or the response requirements of a PR schedule.

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

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

A structural basis for memory storage in mammals

It is proposed that altered dendrite length and de novo formation of new dendrite branches in cholinoceptive cells are responsible for long-term memory storage, a process enabled by the degradation of microtubule-associated protein-2. These memories are encoded as modality-specific associable representations. Accordingly, associable representations are confined to cytoarchitectonic modules of the cerebral cortex, hippocampus, and amygdala. The proposed sequence of events leading to long-term storage in cholinoceptive dendrites begins with changes in neuronal activity, then in neurotrophin release, followed by enhanced acetylcholine release, muscarinic response, calcium influx, degradation of microtubule-associated protein-2, and finally new dendrite structure. Hypothetically, each associable representation consists of altered dendrite segments from approximately 5000-15,000 cholinoceptive cells contained within one or a few module(s). Simultaneous restructuring during consolidation of long-term memory is hypothesized to result in a similar infrastructure among dendrite sets, facilitating co-activation of those dendrite sets by neurotransmitters such as acetylcholine, and conceivably enabling high energy interactions between those dendrites by phenomena such as quantum optical coherence. Based on the specific architecture proposed, it is estimated that the human telecephalon contains enough dendrites to encode 50 million associable representations in a lifetime, or put another way, to encode one new associable representation each minute. The implications that this proposal has regarding treatments for Alzheimer's disease are also discussed.

Mesolimbic component of the ascending cholinergic pathways: electrophysiological-pharmacological study

The cholinergic input from the pontomesencephalic cholinergic neurons to the diencephalic and basal forebrain structures has been implicated in a number of limbically controlled overt behaviors. The cellular mechanism by which the cholinergic terminals initiate behavioral manifestations is not clear. The objective of this study was to investigate the effects of the ascending cholinergic projection from the laterodorsal tegmental nucleus (LDT) on neuronal firing in the anterior hypothalamic-medial preoptic region (AHMP), known to be involved in agonistic behavior. Experiments were performed on urethan-anesthetized rats. Iontophoretic application of carbachol (CCh) into the vicinity of single cells in the AHMP caused a dose-dependent decrease in the mean firing rate of 83% of units and an increase in 10% of units. The inhibitory effect of CCh, but not the excitatory effect, was reversed by iontophoretic pretreatment with scopolamine. The inhibition of the firing rate was repeatable for the same dose of CCh and dose dependent. Electrical stimulation of neurons in the LDT caused a comparable, current-dependent decrease in the mean firing rate of AHMP neurons that also was reversed by pretreatment of neurons in the AHMP with scopolamine. The antagonizing effects of scopolamine were reversible with time. The same units in the AHMP that inhibited their firing to stimulation of the LDT also responded with a similar inhibition to local iontophoretic CCh. Finally, the fluorescent carbocyanine dye, 4-(4-(dihexadecylamino)styryl)-N-methylpyridinium iodide, (DiA), has been used as a retrograde axonal tracer and was injected into the recording sites immediately after the electrophysiological recordings. After 1 wk, DiA dye was found in numerous neurons in the LDT as shown by the fluorescence confocal microscopy. Results of the study suggest that LDT cholinergic neurons project and terminate in the AHMP and that their activation causes a decrease in the mean firing rate of the AHMP neurons. It is postulated that this inhibitory effect is implicated in the initiation of some of the behavioral patterns like defensive or alarm vocalization and behavioral inhibition.

Cholinergic and non-cholinergic afferents of the caudolateral parabrachial nucleus: a role in the long-term enhancement of rapid eye movement sleep

A single microinjection of the cholinergic agonist carbachol into the feline caudolateral parabrachial nucleus produces an immediate increase in state-independent ipsilateral ponto-geniculooccipital waves, followed by a long-term rapid eye movement sleep enhancement lasting 7-10 days. Using retrogradely-transported fluorescent carbachol-conjugated nanospheres and choline acetyltransferase immunohistochemistry, afferent projections to this injection site for long-term rapid eye movement sleep enhancement were mapped and quantified. Six regions in the brain stem contained retrogradely-labelled cells: the raphe nuclei, locus coeruleus, laterodorsal tegmental nucleus, pedunculopontine tegmental nucleus, parabrachial nucleus, and the pontine reticular formation. The retrogradely-labelled (rhodamine+) cells in the pontine reticular formation and pedunculopontine tegmental nucleus contributed the predominant input to the parabrachial nucleus injection site (34.3 +/- 5.3% and 28.4 +/- 5.6%, respectively), compared to the laterodorsal tegmental nucleus (5.8 +/- 3.8%), parabrachial nucleus (13.5 +/- 3.1%), raphe nuclei (12.9 +/- 2.7%), and locus coeruleus (5.1 +/- 2.4%). By comparison with findings of afferent input to the induction site for short-latency rapid eye movement sleep in the anterodorsal pontine reticular formation, the parabrachial nucleus injection site is characterized by a similar proportion of afferents, except that the raphe nuclei were found to provide more than a two-fold greater input. Retrogradely-labelled neurons quantified in these nuclear regions consisted of 21.5% double-labelled (rhodamine+/choline acetyltransferase+) cholinergic and 78.5% noncholinergic (rhodamine+/choline acetyltransferase-) cells. The pedunculopontine tegmental nucleus contributed the predominant (51.7 +/- 8.2%) cholinergic input, compared to laterodorsal tegmental nucleus (20.7 +/- 10.2%), parabrachial nucleus (23.1 +/- 7.5%), and pontine reticular formation (4.4 +/- 2.1%). A comparative analysis of the total retrogradely-labelled cells within each nuclear region which were also double-labelled showed the highest proportion in the laterodorsal tegmental nucleus (76.2 +/- 7.5%) compared to pedunculopontine tegmental nucleus (39.4 +/- 3.6%), parabrachial nucleus (37.3 +/- 2.8%), and pontine reticular formation (3.2 +/- 2.1%). These data indicate that while pedunculopontine tegmental nucleus and laterodorsal tegmental nucleus neurons exert a powerful cholinergic influence on the injection site for long-term rapid eye movement enhancement, a major component of the afferent circuitry is non-cholinergic. Since the non-cholinergic input includes contributions from the locus coeruleus and raphe nuclei, it is probable that the caudolateral parabrachial nucleus contains cholinergic and aminergic afferent systems that participate in the long-term enhancement of rapid eye movement sleep.

Distribution of the vesicular transporter for acetylcholine in the rat central nervous system

In order to develop another selective marker for cholinergic cell bodies and fibres, we have raised a highly specific polyclonal antibody against a peptide derived from the C-terminus of a recently cloned putative vesicular acetylcholine transporter. This antibody recognizes the vesicular acetylcholine transporter protein on western blots of membranes from transfected monkey fibroblast COS cells as well as from various rat brain regions but not from untransfected COS cells or rat liver. In separate mapping studies, the antibody was found to stain cell bodies and fibres in all of the regions of the nervous system known to be cholinergic, including (i) the various nuclei of the basal nuclear complex and their projections to the hippocampus, amygdala, and cerebral cortex, (ii) the caudate-putamen nucleus, accumbens nucleus, olfactory tubercle, and islands of Calleja complex, (iii) the medial habenula, (iv) the mesopontine cholinergic complex and its projections to the thalamus, extrapyramidal motor nuclei, basal forebrain, cingulate cortex, raphe and reticular nuclei, and some cranial nerve nuclei, and (v) the somatic motor and autonomic nuclei of the cranial and spinal nerves. In many of these cholinergic neurons, it is possible to detect immunoreactivity for the vesicular acetylcholine transporter in proximal portions of processes and their branches, as well as in numerous puncta in close association with them. Some of these puncta are large and surround cell bodies and processes of neurons in several regions, including the somatic motor neurons of cranial nerve nuclei in the brainstem and in the ventral horn of the spinal cord. Double immunofluorescence studies indicated that neurons positive for the vesicular acetylcholine transporter also stained for the biosynthetic enzyme of acetylcholine, choline acetyltransferase. We conclude that antibody against the C-terminus of the putative vesicular acetylcholine transporter provides another marker for cholinergic neurons that, unlike in situ hybridization procedures, labels terminals as well as cell bodies. Therefore this antibody has the potential to reveal changes in number and morphology of cholinergic cell bodies and their terminal varicosities that occur in both physiologic and pathologic conditions.

Effects of ethylcholine mustard azirinium ion (AF64A) on the choline acetyltransferase and nitric oxide synthase activities in mesopontine cholinergic neurons of the rat

The choline analogue, ethylcholine mustard azirinium ion (AF64A), has been proposed as a selective neurotoxin that produces degeneration of central cholinergic neurons. However, the mechanisms of action and the specificity or non-specificity of this toxin are still undefined. In this study, we have investigated the effects of AF64A, in comparison with kainic acid, on cholinergic neurons of the mesopontine formation (pedunculopontine and laterodorsal tegmental nuclei), a neuronal population also expressing nitric oxide synthase, the enzyme responsible for the synthesis of nitric oxide. We used choline acetyltransferase immunohistochemistry as a marker of acetylcholine activity, and nitric oxide synthase immunohistochemistry and NADPH-diaphorase histochemistry as markers of nitric oxide synthase activity. Our results show that the injection of low doses of AF64A produces: (1) an area of cavitation in the injection site of pedunculopontine tegmental nucleus (local non-specific effect), and (2) a transient decrease in choline acetyltransferase immunoreactivity in choline acetyltransferase-nitric oxide synthase neurons in both the ipsilateral laterodorsal tegmental nucleus and the perilesional area of the pedunculopontine tegmental nucleus, while their morphology and nitric oxide synthase immunoreactivity remain unaltered (post-diffusion specific effect). These findings indicate that the loss of choline-related enzymatic activity is not necessarily associated with degeneration of cholinergic neurons, and that the recovery of choline acetyltransferase immunoreactivity may arise from neurons whose activity is diminished during the first postinjection weeks. Taking into account that AF64A is a suitable tool to develop a reversible model of neurological disorders related to cholinergic deficit, further efforts should be directed toward elimination of its local non-specific effect.