An anatomical study of cholinergic innervation in rat cerebral cortex

Selective acetylcholine and dopamine lesions in neonatal rats produce distinct patterns of cortical dendritic atrophy in adulthood

Acetylcholine and dopamine afferents reach their cortical targets during periods of synaptogenesis, and are in position to influence the cytoarchitectural development of cortical neurons. To determine the effect of removing these afferents on dendritic development, we lesioned rat pups at 7 days of age with the selective immunotoxins 192 IgG-saporin, or 6-hydroxydopamine, or both. One group of rats was killed in adulthood for neurochemistry and another was prepared for morphology using Golgi-Cox staining. Changes in morphology were compared in layer V pyramidal cells from medial prefrontal cortex, which sustained the greatest dopamine depletion, and in layer II/III pyramidal cells from retrosplenial cortex, which sustained the greatest choline acetyltransferase depletion. In rats with acetylcholine lesions, layer V medial prefrontal cells had smaller apical tufts and fewer basilar dendritic branches. Both apical and basilar spine density was substantially reduced. Layer II/III retrosplenial cells also had smaller apical tufts and substantially smaller basilar dendritic trees. Apical and basilar spine density did not change. In rats with dopamine lesions, layer V medial prefrontal cells had fewer oblique apical dendrites and atrophied basilar trees. Layer II/III retrosplenial cells had fewer apical dendritic branches. In neither area were spine densities significantly different from control. Neurons from rats with combined lesions were always smaller and less complex than those from singly lesioned rats. However, these cells were simple, additive composites of the morphology produced by single lesions. These data demonstrate that ascending acetylcholine and dopamine afferents play a vital role in the development of cortical cytoarchitecture.

The acetylcholine fiber density of the neocortex is altered by isolated rearing and early methamphetamine intoxication in rodents

Alterations in the cholinergic physiology of the brain were the first to be observed when research on environmental influences on postnatal brain development began 35 years ago. Since then, the effects of isolated rearing (IR) or early pharmacological insults have been shown not only on the physiology, but also the anatomy of a variety of transmitter systems. The cholinergic fiber density, however, still remained to be assessed. We therefore used a histochemical procedure to stain cholinergic fibers in the brains of young adult gerbils reared either in groups in enriched environments or isolated in standard makrolon cages. Half of the animals from each rearing condition had received a single high dose of methamphetamine on postnatal day 14. Fiber densities were measured by computerized image analysis in the medial and orbital prefrontal cortex (PFC), dysgranular and granular insular cortex, sensorimotor cortices, and the entorhinal cortex of both hemispheres. Isolation rearing increased the cholinergic fiber densities in the prefrontal cortices of the left hemisphere and in the entorhinal cortex of the right hemisphere by about 10%, with no effect in the respective contralateral side. The early methamphetamine intoxication showed no influence in prefrontal and entorhinal cortices, but diminished the acetylcholine (ACh) innervation of the forelimb area of cortex in both hemispheres in IR gerbils and of the left hemisphere in ER gerbils, and reduced the acetylcholine innervation in the hindlimb area in both sides in both rearing groups. These results demonstrate that (a) cholinergic fiber density is differentially regulated in different cortical areas and (b) the plasticity of the cholinergic system can only be understood in the interplay with other neuromodulatory innervations.

Facilitation of cholinergic transmission by combined treatment of ondansetron with flumazenil after cortical cholinergic deafferentation

We have studied the effects of concomitant blockade of 5-HT(3) and GABA(A) receptors on acetylcholine (ACh) release in the frontal cortex of rats with a selective cholinergic lesion. Lesions were performed by microinjection of the cholinergic toxin 192 IgG-saporin into the nucleus basalis magnocellularis. Single treatment with either the 5-HT(3) receptor antagonist ondansetron, 0.1 microg/kg, or the GABA(A) receptor benzodiazepine site antagonist flumazenil, 10 mg/kg, did not affect ACh release. However, the combined ondansetron + flumazenil administration significantly increased ACh release to a similar extent as a depolarising stimulus with K(+), 100 mM, at both 7 and 30 days post-lesion. Cortical perfusion with the combined ondansetron + flumazenil treatment also increased [(3)H]ACh efflux "in vitro" 30 days after lesion, suggesting that local events within the frontal cortex may participate in the interaction of ondansetron with GABAergic neurons, modulating ACh release in situations of cholinergic hypoactivity. No differences in the expression of 5-HT(3) and GABA(A) receptors in the frontal cortex were found after the cholinergic lesion. These results suggest that a combined ondansetron + flumazenil treatment would contribute to restoring a diminished cholinergic function and may provide a basis for using this treatment in the therapy of cognitive disorders associated with degeneration of the cholinergic system.

Scopolamine-induced amnesia can be prevented by heat shock pretreatment in rats

The current study used the passive avoidance test to examine whether heat shock pretreatment has an effect on the memory impairment induced by scopolamine. Heat shock protein (HSP) 72 overexpression was detected in different brain structures in rats 16 h after heat shock treatment, but not in rats receiving no heat shock or 48 h after heat shock treatment. The step-through latency of either pre- or post-training administration of scopolamine in rats 16 h after heat shock treatment was significantly higher than those of the rats receiving no heat shock or 48 h after heat shock treatment. However, rats, 16 h after heat shock treatment and having been given scopolamine, performed no better than rats treated only with scopolamine. Hence, the present results indicate that heat shock has a protective, but not therapeutic, effect on the memory impairment induced by scopolamine by overexpression of HSP72 in rat brain.

Selective lesioning of the cholinergic septo-hippocampal pathway does not disrupt spatial short-term memory: a comparison with the effects of fimbria-fornix lesions

Rats receiving intrahippocampal injections of 192 IgG-saporin (SAP-HPC), fimbria-fornix lesions (FF), or sham control surgeries were tested in a series of delayed matching (DMTP)- and nonmatching (DNMTP)-to-position tasks. The FF group was significantly impaired on a pretrained DNMTP task relative to the control and SAP-HPC groups, which did not differ. All groups then acquired a matching-to- position rule at the same rate, and only the FF group showed a delay-dependent deficit when longer retention intervals were introduced for DMTP testing. Results demonstrate the importance of the fimbria-fornix fiber system in spatial short-term memory but suggest that the cholinergic septohippocampal component of this pathway is not required for successful delayed matching (or nonmatching)-to-position performance.

Perirhinal N-methyl-D-aspartate and muscarinic systems participate in object recognition in rats

To determine the possible involvement of N-methyl-d-aspartate (NMDA) and muscarinic activation of the perirhinal cortex in object recognition, an NMDA antagonist (d,l-2-amino-5-phosphonopentanoic acid (AP5)) and a muscarinic antagonist (scopolamine) were injected into the perirhinal cortex of rats. A high dose of AP5 (60 mM) and two doses of scopolamine (20 and 80 mM), but not a low dose of AP5 (30 mM) alone, significantly impaired discrimination between novel and familiar objects in a spontaneous object recognition task, which is one of the recognition memory tasks. These results suggest that activation of both NMDA and muscarinic receptors in the perirhinal cortex contributes to object recognition.

Effects of beta-amyloid protein on serotoninergic, noradrenergic, and cholinergic markers in neurons of the pontomesencephalic tegmentum in the rat

The effects on serotoninergic, noradrenergic and cholinergic markers on neurons of the pontomesencephalic tegmentum nuclei were studied in rats following local administration of fibrillar beta-amyloid peptide (Abeta1-40) into the left retrosplenial cortex. Focal deposition of Abeta in the retrosplenial cortex resulted in a loss of serotoninergic neurons in the dorsal and median raphe nuclei. The dorsal raphe nucleus showed a statistically significant reduction of 31.7% in the number of serotoninergic neurons and a decrease (up to 17.38%) in neuronal density in comparison with the same parameters in uninjected controls. A statistically significant reduction of 50.3%, together with a significant decrease of 53.94% in the density of serotoninergic neurons, was also observed in the median raphe nucleus as compared with control animals. Furthermore, a significant reduction of 35.07% in the number of noradrenergic neurons as well as a statistically significant decrease of 56.55% in the density of dopamine-beta-hydroxylase-immunoreactive neurons were also found in the locus coeruleus as compared with the corresponding hemisphere in uninjected controls. By contrast, a reduction of 24.37% in the number of choline acetyltransferase-positive neurons and a slight decrease (up to 22.28%) in the density of cholinergic neurons, which were not statistically significant, was observed in the laterodorsal tegmental nucleus in comparison with the same parameters in control animals. These results show that three different neurochemically defined populations of neurons in the pontomesencephalic tegmentum are affected by the neurotoxicity of Abeta in vivo and that Abeta might indirectly affect serotoninergic, noradrenergic and cholinergic innervation in the retrosplenial cortex.

Neonatal handling increases sensitivity to acute neurodegeneration in adult rats

Environmental stimuli during the perinatal period can result in persistent individual differences in neural viability and cognitive functions. Earlier studies have shown that brief daily maternal separation and/or handling of rat pups during the first weeks of life reduces stress reactivity during adulthood and attenuates neuronal loss and cognitive decline during aging. In the present study we examined whether neonatal handling also affects the sensitivity of the adult brain to an acute neurotoxic insult. Postnatally handled and nonhandled control rats were left undisturbed from weaning onwards until the age of 11 months. At this age, the animals were subjected to a neurotoxic challenge by unilateral infusion of 60 mM of the glutamate analogue N-methyl-D-aspartate (NMDA) into the nucleus basalis magnocellularis (NBM). The brains were collected to measure cholinergic cell and fiber loss. In the nonlesioned side of the brain, cholinergic cell number in the NBM and fiber density in the cortex were not different between postnatally handled and control rats. However, in the lesioned hemisphere handled animals exhibited a significantly higher loss of choline-acetyltransferase-immunoreactive and acetylcholinesterase-positive fibers in the somatosensory cortex. The present results provide evidence for an enhanced vulnerability of postnatally handled rats to acute neurodegeneration in contrast to the previously reported attenuation of spontaneous aging-related neurodegenerative processes.

Effects of Methanol Extract of Uncariae Ramulus et Uncus on Ibotenic Acid-Induced Amnesia in the Rat

In the present study, we investigated the effects of Uncariae Ramulus et Uncus (UR) on learning and memory in the Morris water maze task and the central cholinergic system of rats with excitotoxic medial septum (MS) lesion. In the water maze test, the animals were trained to find a platform in a fixed position during 6 days and then received a 60-s probe trial in which the platform was removed from the pool on the 7th day. Ibotenic lesion of the MS showed impaired performance of the maze test and severe cell losses in the septohippocampal cholinergic system (SHC), as indicated by decreased choline acetyltransferase-immunoreactivity and acetylcholinesterase-reactivity in the hippocampus. Daily administrations of UR (100 mg/kg, i.p.) for 21 consecutive days produced significant reversals of ibotenic acid-induced deficit in learning and memory. These treatments also reduced the loss of cholinergic immunoreactivity in the hippocampus induced by ibotenic acid. These results demonstrated that impairments of spatial learning and memory may be attributable to degeneration of SHC neurons and that UR ameliorated learning and memory deficits partly through neuroprotective effects on the central acetylcholine system. Our studies suggest that UR may be useful in the treatment of Alzheimer's disease.

The medial prefrontal cortex in the rat: evidence for a dorso-ventral distinction based upon functional and anatomical characteristics

The prefrontal cortex in rats can be distinguished anatomically from other frontal cortical areas both in terms of cytoarchitectonic characteristics and neural connectivity, and it can be further subdivided into subterritories on the basis of such criteria. Functionally, the prefrontal cortex of rats has been implicated in working memory, attention, response initiation and management of autonomic control and emotion. In humans, dysfunction of prefrontal cortical areas with which the medial prefrontal cortex of the rat is most likely comparable is related to psychopathology including schizophrenia, sociopathy, obsessive-compulsive disorder, depression, and drug abuse. Recent literature points to the relevance of conducting a functional analysis of prefrontal subregions and supports the idea that the area of the medial prefrontal cortex in rats is characterized by its own functional heterogeneity, which may be related to neuroanatomical and neurochemical dissociations. The present review covers recent findings with the intent of correlating these distinct functional differences in the dorso-ventral axis of the rat medial prefrontal cortex with anatomical and neurochemical patterns.

Rabbit forebrain cholinergic system: morphological characterization of nuclei and distribution of cholinergic terminals in the cerebral cortex and hippocampus

Although the rabbit brain, in particular the basal forebrain cholinergic system, has become a common model for neuropathological changes associated with Alzheimer's disease, detailed neuroanatomical studies on the morphological organization of basal forebrain cholinergic nuclei and on their output pathways are still awaited. Therefore, we performed quantitative choline acetyltransferase (ChAT) immunocytochemistry to localize major cholinergic nuclei and to determine the number of respective cholinergic neurons in the rabbit forebrain. The density of ChAT-immunoreactive terminals in layer V of distinct neocortical territories and in hippocampal subfields was also measured. Another cholinergic marker, the low-affinity neurotrophin receptor (p75(NTR)), was also employed to identify subsets of cholinergic neurons. Double-immunofluorescence labeling of ChAT and p75(NTR), calbindin D-28k (CB), parvalbumin, calretinin, neuronal nitric oxide synthase (nNOS), tyrosine hydroxylase, or substance P was used to elucidate the neuroanatomical borders of cholinergic nuclei and to analyze the neurochemical complexity of cholinergic cell populations. Cholinergic projection neurons with heterogeneous densities were found in the medial septum, vertical and horizontal diagonal bands of Broca, ventral pallidum, and magnocellular nucleus basalis (MBN)/substantia innominata (SI) complex; cholinergic interneurons were observed in the caudate nucleus, putamen, accumbens nucleus, and olfactory tubercule, whereas the globus pallidus was devoid of cholinergic nerve cells. Cholinergic interneurons were frequently present in the hippocampus and to a lesser extent in cerebral cortex. Cholinergic projection neurons, except those localized in SI, abundantly expressed p75(NTR), and a subset of cholinergic neurons in posterior MBN was immunoreactive for CB and nNOS. A strict laminar distribution pattern of cholinergic terminals was recorded both in the cerebral cortex and in CA1-CA3 and dentate gyrus of the hippocampus. In summary, the structural organization and chemoarchitecture of rabbit basal forebrain may be considered as a transition between that of rodents and that of primates.

Input-specific effects of acetylcholine on sensory and intracortical evoked responses in the "barrel cortex" in vivo

The somatosensory neocortex processes extrinsic information from the thalamus and intrinsic information from local circuits. We compared the effects of acetylcholine (Ach) on neocortical field potential responses evoked by stimulation of the whiskers and by local electrical stimulation in the upper layers of the neocortex vibrissae representation ("barrel cortex") of adult rats anesthetized with urethane. In the barrel cortex, the cholinergic system was manipulated using microdialysis by exogenous application of Ach, by increasing the endogenous levels of Ach with physostigmine and by applying specific cholinergic agonists. The results revealed that Ach selectively enhances the sensory response relative to the intracortical response. Thus, pathways in the barrel cortex are differentially regulated by cholinergic inputs.

Comparison of the cholinergic system in neocortical field Ep in cats with strong and weak cognitive abilities

Comparative analysis of the cholinergic system of the neocortex, consisting of the terminals of neurons from the magnocellular basal nuclei and intracortical neurons, in cats with strong and weak abilities to solve abstraction and generalization tasks was performed by isolating subfractions of synaptic membranes and synaptoplasm from "light" (C) and "heavy" (D) synaptosomes from associative field Ep and measuring choline acetyltransferase and acetylcholinesterase activities, protein content, and sulfhydryl group concentrations. These experiments showed that all measures were significantly lower in subfractions from C synaptosomes from cats with strong cognitive abilities. This leads to the conclusion that relatively small numbers of cholinergic synapses form in field Ep of the brains of cats with strong cognitive abilities, while their location in the C fraction demonstrates that they correspond to neurons of the magnocellular basal nuclei. The possible physiological significance of the "deficiency" of cholinergic inputs in field Ep from these nuclei as a correlate of the animal's cognitive ability is discussed. The D subfractions from able cats had significantly higher acetylcholinesterase activity, while choline acetyltransferase activity was not different; this identifies differences between groups of animals in the organization of non-cholinergic acetylcholinesterase-containing synapses in field Ep.

Protective effects of the alpha 2-adrenoceptor antagonist, dexefaroxan, against degeneration of the basalocortical cholinergic system induced by cortical devascularization in the adult rat

It has been hypothesized [Colpaert, F.C., 1994. In: Briley, M., Marien, M. (Eds.), Noradrenergic Mechanisms in Parkinson's Disease. CRC Press, Boca Raton, FL, pp. 225-254] that a deficiency in the noradrenergic system originating from the locus coeruleus is a decisive factor in the progression of central neurodegenerative disorders including Alzheimer's disease, and that treatments which boost noradrenergic transmission (e.g. via blockade of alpha(2)-adrenoceptors) could provide both symptomatic and trophic benefits against the disease. Studies in the rat in vivo demonstrating that the selective alpha(2)-adrenoceptor antagonist dexefaroxan increases acetylcholine release in the cortex, improves measures of cognitive performance and protects against excitotoxin lesions, support this concept. As a further test of the hypothesis, we investigated the effect of dexefaroxan in a rat model of unilateral cortical devascularization that induces a loss of the cortical cholinergic terminal network and a retrograde degeneration of the cholinergic projections that originate in the nucleus basalis magnocellularis. Lesioned and sham-operated rats received a 28-day subcutaneous infusion of dexefaroxan (0.63 mg/rat/day) or vehicle, delivered by osmotic minipumps implanted on the day of the cortical devascularization procedure. In lesioned rats, the dexefaroxan treatment was associated with a significantly higher number and size of vesicular acetylcholine transporter-immunoreactive boutons in comparison to the vehicle treatment; this effect was most marked within cortical layer V. Dexefaroxan also significantly reduced the atrophy of cholinergic neurons within the nucleus basalis magnocellularis. Dexefaroxan had no observable effect on any of these parameters in sham-operated cohorts. These results show that systemically administered dexefaroxan mitigates cholinergic neuronal degeneration in vivo, and provide further evidence for a therapeutic potential of the drug in neurodegenerative diseases such as Alzheimer's disease, where central cholinergic function is progressively compromised.

Galanin regulates spatial memory but not visual recognition memory or synaptic plasticity in perirhinal cortex

It has previously been shown that the neuropeptide galanin plays a role in the age-dependent regulation of hippocampal synaptic plasticity and spatial memory. Here, we further extend these studies by demonstrating that galanin knockout mice also have deficits in an object-in-place spatial memory task. In contrast however, there is no deficit in single item object recognition memory, a memory that depends on perirhinal cortex. Furthermore, in perirhinal cortex slices there are no differences in activity-dependent long-term potentiation or depotentiation, nor in muscarinic receptor-dependent long-term depression between galanin knockout mice and wild-type litter-mates. Therefore, these results suggest that galanin has a differential role in hippocampal-dependent and perirhinal cortex-dependent memory.

17beta-estradiol enhances cortical cholinergic innervation and preserves synaptic density following excitotoxic lesions to the rat nucleus basalis magnocellularis

Estradiol exerts beneficial effects on neurodegenerative disorders associated with the decline of cognitive performance. The present study was designed to further investigate the effect of 17beta-estradiol on learning and memory, and to evaluate its neuroprotective action on cholinergic cells of the nucleus basalis magnocellularis, a neural substrate of cognitive performance. Female rats were ovariectomized at an age of 6 months. Three weeks later they received injections of either a mid-physiological dose of 17beta-estradiol or vehicle (oil), every other day for 2 weeks. The effect of estradiol on cognitive performance was tested in two associative learning paradigms. In the two-way active shock avoidance task estradiol-replaced animals learned significantly faster, while in the passive shock avoidance test no differences were observed between the experimental groups. Subsequent unilateral infusion of N-methyl-D-aspartate in the nucleus basalis magnocellularis resulted in a significant loss of cholinergic neurons concomitant with the loss of their fibers invading the somatosensory cortex. Estradiol treatment did not affect the total number of choline-acetyltransferase-immunoreactive neurons and their coexpression of the p75 low-affinity neurotrophin receptor either contralateral or ipsilateral to the lesion. In contrast, cholinergic fiber densities in estradiol-treated animals were greater both in the contralateral and ipsilateral somatosensory cortices as was detected by quantitative choline-acetyltransferase and vesicular acetylcholine transporter immunocytochemistry. However, estradiol treatment did not affect the lesion-induced relative percentage loss of cholinergic fibers. A significant decline of synaptophysin immunoreactivity paralleled the cholinergic damage in the somatosensory cortex of oil-treated animals, whereas an almost complete preservation of synaptic density was determined in estradiol-treated rats. Our results indicate that estradiol treatment enhances the cortical cholinergic innervation but has no rescuing effect on cholinergic nerve cells in the basal forebrain against excitotoxic damage. Nevertheless, estradiol may restore or maintain synaptic density in the cerebral cortex following cholinergic fiber loss. This estradiol effect may outweigh the lack of cellular protection on cholinergic cells at the functional level.

Cholinergic nerve terminals establish classical synapses in the rat cerebral cortex: synaptic pattern and age-related atrophy

This study addresses the issue of whether cholinergic varicosities in the cerebral cortex establish 'classical synapses' or whether they communicate with their targets non-synaptically by 'volume transmission'. Most recent studies in the neocortex have suggested that acetylcholine acts non-synaptically, however in the present study we provide ultrastructural evidence that suggests synaptic mechanisms prevail. This conclusion is based upon our ultrastructural observations that cholinergic boutons--as revealed by immunoreactivity for the specific cholinergic market, vesicular acetylcholine transporter--establish a high percentage of classical synapses in layer V of the rat parietal cortex. Furthermore, the combination of this approach with the intracellular labeling of large pyramidal neurons on slice preparations revealed significant incidences of cholinergic contacts abutting preferentially on dendritic shafts. Finally, we have gathered information suggesting that cholinergic boutons undergo atrophy with aging which could be related to the well-known cholinergic and cognitive decline. These results illustrate that the cholinergic terminations in the neocortex establish proper synaptic connections and that they experience important age-dependent atrophy.

Evidence for target preferences by cholinergic axons originating from different subdivisions of the basal forebrain

Possible target preferences of basal forebrain cholinergic neurons were studied in organotypic slice cultures. Cholinergic neurons in slices of medial septum or substantia innominata send axons into both hippocampus and neocortex when co-cultured together. However, septal cholinergic axons course through adjacent slices of neocortex to reach and branch densely in slices of hippocampus, but septal axons seldom grow beyond adjacent hippocampal tissue to reach neocortex. In contrast, cholinergic axons from substantia innominata commonly grow through hippocampus to reach neocortex, and also grow through neocortex to reach hippocampus, with similar branching densities in each target. The greater density of septal axonal branches in hippocampus than in neocortex suggests a preference of septal axons for the hippocampal target.

NMDA and muscarinic blockade in the perirhinal cortex impairs object discrimination in rats

To determine the possible involvement of NMDA and muscarinic activation of the perirhinal cortex in object discrimination, an NMDA antagonist, D,L-2-amino-5-phosphonopentanoic acid (AP5), and a muscarinic antagonist, scopolamine (SCP) were injected into the perirhinal cortex of rats. Each drug at the higher dose (AP5 60 mM, SCP 80 mM) significantly decreased correct choices on the retention test of object discrimination. SCP, but not AP5, also significantly increased response latency, but this increase was not necessarily related to the time spent for a choice. These results suggest that activation of both NMDA and muscarinic receptors contributes to object discrimination.

Activation of muscarinic receptors induces protein synthesis-dependent long-lasting depression in the perirhinal cortex

There is strong evidence that decrements in neuronal activation in perirhinal cortex when a novel stimulus is repeated provide a neural substrate of visual recognition memory. There is also strong evidence that muscarinic acetylcholine (ACh) receptors are involved in learning and memory. However, the mechanisms underlying neuronal decrements in the perirhinal cortex and the basis of ACh involvement in learning and memory are not understood. In an in vitro preparation of rat perirhinal cortex we now demonstrate that activation of ACh receptors by carbachol (CCh) produces long-lasting depression (LLD) of synaptic transmission that is dependent on muscarinic M1 receptor activation. Crucially, the induction of this form of LLD requires neither N-methyl-D-aspartate receptor activation nor synaptic stimulation. CCh-induced LLD was not blocked by the protein kinase C inhibitors staurosporine or BIM, or by the protein phosphatase inhibitor okadaic acid. However, each of cyclopiazonic acid (an agent that depletes intracellular calcium stores) and anisomycin (an inhibitor of protein synthesis) significantly reduced the magnitude of CCh-induced LLD. These mechanisms triggered by muscarinic receptor activation could play a role in the induction and/or expression of certain forms of activity-dependent long-term depression in perirhinal cortex. An understanding of CCh-induced LLD may thus provide clues to the mechanisms underlying lasting neuronal decrements that occur in the perirhinal cortex and hence for neural substrates of visual recognition memory.

Neurons of a limited subthalamic area mediate elevations in cortical cerebral blood flow evoked by hypoxia and excitation of neurons of the rostral ventrolateral medulla

Sympathoexcitatory reticulospinal neurons of the rostral ventrolateral medulla (RVLM) are oxygen detectors excited by hypoxia to globally elevate regional cerebral blood flow (rCBF). The projection, which accounts for >50% of hypoxic cerebral vasodilation, relays through the medullary vasodilator area (MCVA). However, there are no direct cortical projections from the RVLM/MCVA, suggesting a relay that diffusely innervates cortex and possibly originates in thalamic nuclei. Systematic mapping by electrical microstimulation of the thalamus and subthalamus revealed that elevations in rCBF were elicited only from a limited area, which encompassed medial pole of zona incerta, Forel's field, and prerubral zone. Stimulation (10 sec train) at an active site increased rCBF by 25 +/- 6%. Excitation of local neurons with kainic acid mimicked effects of electrical stimulation by increasing rCBF. Stimulation of the subthalamic cerebrovasodilator area (SVA) with single pulses (0.5 msec; 80 microA) triggered cortical EEG burst-CBF wave complexes with latency 24 +/- 5 msec, which were similar in shape to complexes evoked from the MCVA. Selective bilateral lesioning of the SVA neurons (ibotenic acid, 2 microg, 200 nl) blocked the vasodilation elicited from the MCVA and attenuated hypoxic cerebrovasodilation by 52 +/- 12% (p < 0.05), whereas hypercarbic vasodilation remained preserved. Lesioning of the vasodilator site in the basal forebrain failed to modify SVA-evoked rCBF increase. We conclude that (1) excitation of intrinsic neurons of functionally restricted region of subthalamus elevates rCBF, (2) these neurons relay signals from the MCVA, which elevate rCBF in response to hypoxia, and (3) the SVA is a functionally important site conveying vasodilator signal from the medulla to the telencephalon.

Muscarinic Depression of Synaptic Transmission in the Epileptogenic GABA Withdrawal Syndrome Focus

The GABA withdrawal syndrome (GWS) is a model of local status epilepticus consecutive to the interruption of a prolonged GABA infusion into the rat somatomotor cortex. Bursting patterns in slices from GWS rats include intrinsic bursts of action potentials (APs) induced by intracellular depolarizing current injection and/or paroxysmal depolarization shifts (PDSs) induced by white matter stimulation. Possible changes in the effects of cholinergic drugs after in vivo induction of GWS were investigated on bursting cells ( n = 30) intracellularly recorded in neocortical slices. In GWS slices, acetylcholine (Ach, 200-1000 μM) or carbachol (Cch, 50 μM) applications increased the number of bursts induced by depolarizing current injection while synaptically induced PDSs were significantly diminished (by 50–60%) or even blocked independently of the cholinergic-induced depolarization. The intrinsic burst facilitation and PDS depression provoked by Ach or Cch were mimicked by methyl-acetylcholine (mAch, 100–400 μM, n = 11), were reversed by atropine application (1–50 μM, n = 3), and were not mimicked by nicotine (50–100 μM, n = 4), indicating the involvement of muscarinic receptors. In contrast, in nonbursting cells from the same epileptic area ( n = 42) or from equivalent area in control rats ( n = 24), a nonsignificant muscarinic depression of EPSPs was induced by Cch and Ach. The mAch depression of excitatory postsynaptic potential (EPSPs) was significantly lower than that seen for PDSs in GWS rats. None of the cholinergic agonists caused bursting appearance in these cells. Therefore the present study demonstrates a unique implication of muscarinic receptors in exerting opposite effects on intrinsic membrane properties and on synaptic transmission in epileptiform GWS. Muscarinic receptor mechanisms may therefore have a protective role against the development and spread of epileptiform activity from the otherwise-activated epileptic focus.

Basal forebrain glutamatergic modulation of cortical acetylcholine release

The mediation of cortical ACh release by basal forebrain glutamate receptors was studied in awake rats fitted with microdialysis probes in medial prefrontal cortex and ipsilateral basal forebrain. Repeated presentation of a stimulus consisting of exposure to darkness with the opportunity to consume a sweetened cereal resulted in a transient increase in cortical ACh efflux. This stimulated release was dependent on basal forebrain glutamate receptor activity as intrabasalis perfusion with the ionotropic glutamate receptor antagonist kynurenate (1.0 mM) markedly attenuated darkness/cereal-induced ACh release. Activation of AMPA/kainate receptors by intrabasalis perfusion of kainate (100 microM) was sufficient to increase cortical ACh efflux even under basal (nonstimulated) conditions. This effect of kainate was blocked by coperfusion with the antagonist DNQX (0.1-5.0 mM). Stimulation of NMDA receptors with intrabasalis perfusion of NMDA (50 or 200 microM) did not increase basal cortical ACh efflux. However, perfusion of NMDA in rats following exposure to the darkness/cereal stimulus resulted in a potentiation of both the magnitude and duration of stimulated cortical ACh efflux. Moreover, intrabasalis perfusion of the higher dose of NMDA resulted in a rapid increase in cortical ACh efflux even before presentation of the darkness/cereal stimulus, suggesting an anticipatory change in the excitability of basal forebrain cholinergic neurons. These data demonstrate that basal forebrain glutamate receptors contribute to the stimulation of cortical ACh efflux in response to behavioral stimuli. The specific roles of basal forebrain glutamate receptor subtypes in mediating cortical ACh release differ and depend on the level of activity of basal forebrain cholinergic neurons.

Neuropeptides in hippocampus and cortex in transgenic mice overexpressing V717F beta-amyloid precursor protein--initial observations

Immunohistochemistry was used to analyse 18- and 26-month-old transgenic mice overexpressing the human beta-amyloid precursor protein under the platelet-derived growth factor-beta promoter with regard to presence and distribution of neuropeptides. In addition, antisera/antibodies to tyrosine hydroxylase, acetylcholinesterase, amyloid peptide, glial fibrillary acidic protein and microglial marker OX42 were used. These mice have been reported to exhibit extensive amyloid plaques in the hippocampus and cortex [Masliah et al. (1996) J. Neurosci. 16, 5795-5811]. The most pronounced changes were related to neuropeptides, whereas differences between wild-type and transgenic mice were less prominent with regard to tyrosine hydroxylase and acetylcholinesterase. The main findings were of two types; (i) involvement of peptide-containing neurites in amyloid beta-peptide positive plaques, and (ii) more generalized changes in peptide levels in specific layers, neuron populations and/or subregions in the hippocampal formation and ventral cortices. In contrast, the parietal and auditory cortices were comparatively less affected. The peptide immunoreactivities most strongly involved, both in plaques and in the generalized changes, were galanin, neuropeptide Y, cholecystokinin and enkephalin. This study shows that there is considerable variation both with regard to plaque load and peptide expression even among homozygotes of the same age. The most pronounced changes, predominantly increased peptide levels, were observed in two 26-month-old homozygous mice, for example, galanin-, enkephalin- and cholecystokinin-like immunoreactivities in stratum lacunosum moleculare, and galanin, neuropeptide Y, enkephalin and dynorphin in mossy fibers. Many peptides also showed elevated levels in the ventral cortices. However, decreases were also observed. Thus, galanin-like immunoreactivity could not any longer be detected in the diffusely distributed (presumably noradrenergic) fiber network in all hippocampal and cortical layers, and dynorphin-like immunoreactivity was decreased in stratum moleculare, cholecystokinin-like immunoreactivity in mossy fibers and substance P-like immunoreactivity in fibers around granule cells. The significance of generalized peptide changes is at present unclear. For example, the increase in the mainly inhibitory peptides galanin, neuropeptide Y, enkephalin and dynorphin and the decrease in the mainly excitatory peptide cholecystokinin in mossy fibers (and of substance P fibers around granule cells) indicate a shift in balance towards inhibition of the input to the CA3 pyramidal cell layer. Moreover, it may be speculated that the increase in levels of some of the peptides represents a reaction to nerve injury with the aim to counteract, in different ways, the consequences of injury, for example by exerting trophic actions. Further studies will be needed to establish to what extent these changes are typical for Alzheimer mouse models in general or are associated with the V717F mutation and/or the platelet-derived growth factor-beta promoter.

In vivo Hebbian and basal forebrain stimulation treatment in morphologically identified auditory cortical cells

The present study concerns the interactions of local pre/postsynaptic covariance and activity of the cortically-projecting cholinergic basal forebrain, in physiological plasticity of auditory cortex. Specifically, a tone that activated presynaptic inputs to a recorded auditory cortical neuron was repeatedly paired with a combination of two stimuli: (1) local juxtacellular current that excited the recorded cell and (2) basal forebrain stimulation which desynchronized the cortical EEG. In addition, the recorded neurons were filled with biocytin for morphological examination. The hypothesis tested was that the combined treatment would cause increased potentiation of responses to the paired tone, relative to similar conditioning treatments involving either postsynaptic excitation alone or basal forebrain stimulation alone. In contrast, there was no net increase in plasticity and indeed the combined treatment appears to have decreased plasticity below that previously found for either treatment alone. Several alternate interpretations of these results are discussed.

Cholinergic innervation in adult rat cerebral cortex: a quantitative immunocytochemical description

A method for determining the length of acetylcholine (ACh) axons and number of ACh axon varicosities (terminals) in brain sections immunostained for choline acetyltransferase (ChAT) was used to estimate the areal and laminar densities of this innervation in the frontal (motor), parietal (somatosensory), and occipital (visual) cortex of adult rat. The number of ACh varicosities per length of axon (4 per 10 microm) appeared constant in the different layers and areas. The mean density of ACh axons was the highest in the frontal cortex (13.0 m/mm(3) vs. 9.9 and 11.0 m/mm(3) in the somatosensory and visual cortex, respectively), as was the mean density of ACh varicosities (5.4 x 10(6)/mm(3) vs. 3.8 and 4.6 x 10(6)/mm(3)). In all three areas, layer I displayed the highest laminar densities of ACh axons and varicosities (e.g., 13.5 m/mm(3) and 5.4 x 10(6)/mm(3) in frontal cortex). The lowest were those of layer IV in the parietal cortex (7.3 m/mm(3) and 2.9 x 10(6)/mm(3)). The lengths of ACh axons under a 1 mm(2) surface of cortex were 26.7, 19.7, and 15.3 m in the frontal, parietal, and occipital areas, respectively, for corresponding numbers of 11.1, 7.7, and 6.4 x 10(6) ACh varicosities. In the parietal cortex, this meant a total of 1.2 x 10(6) synaptic ACh varicosities under a 1 mm(2) surface, 48% of which in layer V alone, according to previous electron microscopic estimates of synaptic incidence. In keeping with the notion that the synaptic component of ACh transmission in cerebral cortex is preponderant in layer V, these quantitative data suggest a role for this innervation in the processing of cortical output as well as input. Extrapolation of particular features of this system in terms of total axon length and number of varicosities in whole cortex, length of axons and number of varicosities per cortically projecting neuron, and concentration of ACh per axon varicosity, should also help in arriving at a better definition of its roles and functional properties in cerebral cortex.

Regional and cellular distribution of protein kinase C in rat cerebellar Purkinje cells

Protein kinase C (PCK) is a family of isoforms that are implicated in subcellular signal transduction. The authors investigated the distribution of several PKC isoforms (PKC-alpha, PKC-beta, PKC-gamma, PKC-delta, and PKC-epsilon) within major cerebellar cell types as well as cerebellar projection target neurons, including Purkinje neurons, cerebellar nuclear neurons, and secondary vestibular neurons. PKC-alpha, PKC-beta, PKC-gamma, PKC-delta, and PKC-epsilon are found within the cerebellum. Of these isoforms, PKC-gamma and PKC-delta are highly expressed in Purkinje cells. PKC-gamma is expressed in all Purkinje cells, whereas the expression of PKC-delta is restricted to sagittal bands of Purkinje cells in the posterior cerebellar cortex. In the lower folia of the uvula and nodulus, Purkinje cell expression of PKC-delta is uniformly high, and the sagittal banding for PKC-delta expression is absent. Within the cerebellar nuclei, PKC-delta-immunolabeled axons terminate within the medial aspect of the caudal half of the ipsilateral interpositus nucleus. PKC delta-immunolabeled axons also terminated within the caudal medial and descending vestibular nuclei (MVN and DVN, respectively), the parasolitary nucleus (Psol), and the nucleus prepositus hypoglossi (NPH). PKC-gamma-immunolabeled axons terminated in all of the cerebellar nuclei as well as in the lateral and superior vestibular nuclei and the MVN, DVN, Psol, and NPH. The projection patterns of PKC-immunolabeled Purkinje cells were confirmed by lesion-depletion studies in which unilateral uvula-nodular lesions caused depletion of PKC-immunolabeled terminals ipsilateral to the lesion in the vestibular complex. These data identify circuitry that is unique to cerebellar-vestibular interactions.

Multiple output pathways of the basal forebrain: organization, chemical heterogeneity, and roles in vigilance

Studies over the last decade have shown that the basal forebrain (BF) consists of more than its cholinergic neurons. The BF also contains non-cholinergic neurons, including gamma-aminobutyric acid-ergic neurons which co-distribute and co-project with the cholinergic neurons. Both types of neuron project, in variable proportions, to the cerebral cortex, hippocampus, thalamus, amygdala, and olfactory bulb, whereas descending projections to the posterior hypothalamus and brainstem nuclei are predominantly non-cholinergic. Some of the cholinergic and non-cholinergic projection neurons contain neuropeptides such as galanin, nitric oxide synthase, and possibly glutamate. To understand better the function of the BF, the organization of the multiple ascending and descending projections of BF neurons is reviewed along with their neurochemical heterogeneity, and possible functions of individual pathways are discussed. It is proposed that BF neurons belong to multiple systems with distinct cognitive, motivational, emotional, motor, and regulatory functions, and that through these pathways, the BF plays a role in controlling both cognitive and non-cognitive aspects of vigilance.

Role of cholinergic and GABAergic systems in the feedback inhibition of dorsal raphe 5-HT neurons

Several observations indicate that 5-HT1A receptors found on a long neuronal feedback loop, originating from the medial prefrontal cortex, regulate 5-HT neuronal firing. In the present study, the muscarinic (M) receptor antagonists atropine and scopolamine as well as the M2 receptor antagonist AF-DX 116, but not the preferential M1 receptor antagonist pirenzepine, reduced the suppressant effect of the 5-HT1A receptor agonist 8-OH-DPAT on the spontaneous firing activity of rat dorsal raphe 5-HT neurons. Moreover, AF-64A-induced lesions of cholinergic neurons directly in the medial prefrontal cortex and after its i.c.v. injection attenuated the effect of 8-OH-DPAT. Finally, the NMDA receptor antagonist (+)MK-801 and the GABA(B) receptor antagonist SCH-50911, but not the GABA(A) receptor antagonist (-)bicuculline, dampened the latter response. The present study unveiled a key role for the cholinergic and GABAergic systems in the feedback inhibition of dorsal raphe 5-HT neurons.

Localization of amino acids, neuropeptides and cholinergic markers in neurons of the septum-diagonal band complex projecting to the retrosplenial granular cortex of the rat

Retrograde labelling was combined with immunohistochemistry to localize neurons containing choline acetyltransferase, gamma-aminobutyric acid (GABA), glutamate, leu-enkephalin, neurotensin, and substance P-like immunoreactivity in the projection pathways from the septum-diagonal band complex to the retrosplenial granular cortex in the rat. Injections of horseradish peroxidase conjugated to subunit B of cholera toxin (CT-HRP) into the retrosplenial granular cortex resulted in retrogradely labelled neurons in the ipsilateral nuclei of the diagonal band of Broca, especially in the horizonatal nucleus of the diagonal band, and small numbers of CT-HRP-labelled neurons were also found in the medial septal nucleus. In the horizontal and vertical nuclei of the diagonal band of Broca, 90-95% of CT-HRP-labelled neurons (35-45 per section) were immunoreactive for choline acetyltransferase and small numbers of retrogradely labelled neurons (2 to 4-5 per section) were also immunoreactive for GABA, glutamate, neurotensin, leu-enkephalin, or substance P. In the medial septal nucleus approximately 75-80% of the retrogradely labelled neurons (8-10 per section) were immunoreactive for choline acetyltransferase and up to 25% of the CT-HRP labelled neurons (1-3 per section) in the medial septal nucleus also displayed GABA-, glutamate-, neurotensin-, leu-enkephalin-, or substance P-immunoreactivity. These results suggest that the complexity of the neurotransmitter(s)/neuromodulator(s) of septum-diagonal band complex projections to the retrosplenial granular cortex should be taken into account when considering the mechanisms of cortical activation.

Local GABAergic modulation of acetylcholine release from the cortex of freely moving rats

Cortical perfusion with GABA agonists and antagonists modulates the spontaneous release of cortical acetylcholine and GABA in freely moving rats. Twenty-four hours after implantation of a dialysis fibre, cerebral cortex spontaneously released acetylcholine (3.8 +/- 0.2 pmol/10 min) and GABA (6.6 +/- 0.4 pmol/10 min) at a stable rate. Local administration of GABA (1 or 5 mM) or the GABAA agonist muscimol (25 or 50 microM) had no effect on the spontaneous release of acetylcholine. However, bicuculline (1-25 microM), a GABAA antagonist, added to the dialysis perfusate, elicited a concentration-dependent increase of acetylcholine release to approximately double that of control. This effect of bicuculline (25 microM) was completely prevented by coperfusion with muscimol (50 microM). Local administration of the GABAB receptor agonist baclofen (10 or 50 microM) elicited a concentration-dependent increase in spontaneous acetylcholine release with a maximal increase of about 60%. Intracortical administration of baclofen also decreased the spontaneous release of GABA. The GABAB receptor antagonist CGP 35348 (1 mM), administered alone for 20 min through the dialysis fibre, was without effect on spontaneous acetylcholine release; however, it completely blocked both the baclofen-induced increase in acetylcholine release and the decrease in GABA release. These results suggest that cortically released GABA exerts a tonic influence on cholinergic activity.

Effect of amphetamine on extracellular acetylcholine and monoamine levels in subterritories of the rat medial prefrontal cortex

The present study sought to investigate the contributions of the dorsal prelimbic/anterior cingulate and ventral prelimbic/infralimbic cortices to the reverse microdialysis of amphetamine (1, 10, 100, 500, and 1000 microM) on dialysate acetylcholine, choline, norepinephrine, and serotonin levels. The results demonstrate that basal levels of acetylcholine, choline, and serotonin were homogeneous within subregions of the medial prefrontal cortex. In contrast, dialysate norepinephrine levels were significantly higher in the anterior cingulate cortex compared with the infralimbic cortex. Reverse microdialysis of amphetamine in both subareas of the medial prefrontal cortex produced a dose-dependent increase in norepinephrine and serotonin levels; the magnitude of this effect was similar in both subterritories of the medial prefrontal cortex. Microinfusion of amphetamine increased dialysate acetylcholine levels in a dose-dependent manner only in the infralimbic cortex. Finally, amphetamine decreased choline levels in both subregions of the medial prefrontal cortex. The magnitude of this effect was larger in the anterior cingulate cortex compared with its infralimbic counterpart. Since depletions of frontal cortical acetylcholine result in severe cognitive deficits, the present data raise the possibility that the type of neural integrative processes that acetylcholine mediates depends, at least in part, on the subterritories that characterize the medial prefrontal cortex.

Choline acetyltransferase: the structure, distribution and pathologic changes in the central nervous system

Choline acetyltransferase (ChAT), the enzyme responsible for the biosynthesis of acetylcholine, is presently the most specific indicator for monitoring the functional state of cholinergic neurones in the central and peripheral nervous systems. ChAT is a single-strand globular protein. The enzyme is synthesized in the perikaryon of cholinergic neurones and transported to the nerve terminals probably by both slow and rapid axoplasmic flows. ChAT exists in at least two forms in cholinergic nerve terminals: (i) soluble; and (ii) non-ionically membrane-bound forms. Multiple mRNA species of ChAT (R-, N-and M-types) are transcribed from different promoter regions and produced by different splicing in the mouse, rat, and human. All transcripts encode the same ChAT protein in rodents, while in human M-type mRNA has the capability to generate both large and small forms of ChAT proteins and R-and N-types ChAT mRNA generate a small form, which corresponds to the rodent ChAT. The genomic structure of ChAT is unique compared with other enzymes for neurotransmitters. The first intron of the ChAT gene encompasses the open reading frame encoding another protein, vesicular acetylcholine transporter (VAChT), which is responsible for the transportation of acetylcholine from the cytoplasm into the synaptic vesicles. The expressions of ChAT and VAChT appear to be coordinately regulated by multiple regulatory elements in cholinergic neurones. Immunohistochemical and in situ hybridization studies have revealed the localization of cholinergic neurones in the central nervous system: the medial septal nucleus, the nucleus of the diagonal band of Broca, the basal nucleus of Meynert, the caudate nucleus, the putamen, the nucleus accumbens, the pedunculopontine tegmental nucleus, the laterodorsal tegmental nucleus, the medial habenular nucleus, the parabigeminal nucleus, some cranial nerve nuclei, and the anterior horn of the spinal cord. Focally distributed cholinergic neurones project fibers to many areas in the central nervous system and construct a complicated cholinergic network, playing an important role in neuropsychic activities, such as learning, memory, arousal, sleep and movement. Central cholinergic neurones are involved in several neurodegenerative diseases such as Alzheimer's disease and amyotrophic lateral sclerosis, in which disturbance of the central cholinergic system does not appear to be closely related to the etiology, but rather to the development of clinical symptoms. In addition, abnormalities of ChAT in the brain have been recently demonstrated in schizophrenia and sudden infant death syndrome.

Carbachol induces fast oscillations in the medial but not in the lateral entorhinal cortex of the isolated guinea pig brain

Fast oscillations at 25-80 Hz (gamma activity) have been proposed to play a role in attention-related mechanisms and synaptic plasticity in cortical structures. Recently, it has been demonstrated that the preservation of the entorhinal cortex is necessary to maintain gamma oscillations in the hippocampus. Because gamma activity can be reproduced in vitro by cholinergic activation, this study examined the characteristics of gamma oscillations induced by arterial perfusion or local intracortical injections of carbachol in the entorhinal cortex of the in vitro isolated guinea pig brain preparation. Shortly after carbachol administration, fast oscillatory activity at 25.2-28.2 Hz was observed in the medial but not in the lateral entorhinal cortex. Such activity was transiently associated with oscillations in the theta range that showed a variable pattern of distribution in the entorhinal cortex. No oscillatory activity was observed when carbachol was injected in the lateral entorhinal cortex. Gamma activity in the medial entorhinal cortex showed a phase reversal at 200-400 microm, had maximal amplitude at 400-500 microm depth, and was abolished by arterial perfusion of atropine (5 microM). Local carbachol application in the medial entorhinal cortex induced gamma oscillations in the hippocampus, whereas no oscillations were observed in the amygdala and in the piriform, periamygdaloid, and perirhinal cortices ipsilateral and contralateral to the carbachol injection. Hippocampal oscillations had higher frequency than the gamma activity recorded in the entorhinal cortex, suggesting the presence of independent generators in the two structures. The selective ability of the medial but not the lateral entorhinal cortex to generate gamma activity in response to cholinergic activation suggests a differential mode of signal processing in entorhinal cortex subregions.

Acetylcholine suppresses the spread of excitation in the visual cortex revealed by optical recording: possible differential effect depending on the source of input

Optical recording with a voltage-sensitive dye was performed in visual cortical slices of the rat to determine the effect of acetylcholine (ACh) on the spread of excitation. In the presence of ACh, the spread of excitation initiated by stimulation at the white matter/layer VI (WM/VI) was greatly suppressed throughout the cortex, with less suppression in the middle layers. By comparing the effect of ACh with that of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), the fraction of the synaptic component that was sensitive to ACh was evaluated. ACh suppressed approximately 40-50% (maximum 55.8%, n = 11) of the initial synaptic component in the superficial and deep layers. In the middle, however, the effect was weakest and only approximately 20-30% (minimum 20.9%, n = 11) of the initial synaptic component was suppressed. On the basis of histological analysis, the region with the weakest ACh effect extended from upper V to lower II/III. To identify the site of ACh action in terms of pre- versus postsynaptic localization, exogenous glutamate was applied. Because ACh did not suppress the excitation induced by glutamate, the site of the ACh action was indicated to be presynaptic. When layer II/III was stimulated instead of WM/VI, the suppression was uniform throughout the cortex. A muscarinic receptor antagonist, atropine, blocked the suppression by ACh. In conclusion, our results indicate the following two points. First, ACh strongly suppresses intracortical connectivity through presynaptic muscarinic receptors. Secondly, in contrast to the intracortical connection, some group(s) of fibres, possibly thalamocortical afferents that arise from white matter and terminate in the middle cortical layers are suppressed much less by ACh. While ACh has been reported to have confusingly diverse effects, e.g. direct depolarization and hyperpolarization as well as synaptic facilitation and suppression, its effect on the propagation of excitation is very clear; suppression on intracortical connection, leaving thalamocortical inputs rather intact. We postulate that cholinergic innervation enables the afferent input to have a relatively dominant effect in the cortex.

Carbachol-induced changes in excitability and [Ca2+]i signalling in projection cells of medial entorhinal cortex layers II and III

The entorhinal cortex (EC) is a major gateway for sensory information into the hippocampus and receives a cholinergic input from the forebrain. Therefore, we studied muscarinic effects on excitability and intracellular Ca2+ signalling in layer II stellate and layer III pyramidal projection neurons of the EC. In both classes of neurons, local pressure-pulse application of carbachol (1 mM) caused small, atropine-sensitive membrane depolarizations that were not accompanied by any detectable changes in [Ca2+]i. At a higher concentration (10 mM), carbachol induced a larger membrane depolarization associated with synaptic oscillations and epileptiform activity in both classes of neurons. In contrast to the intrinsic theta rhythm in stellate cells with one dominant peak frequency at approximately 7 Hz, the synaptically mediated oscillation induced by carbachol showed three characteristic peaks in the theta and gamma frequency range at approximately 11, 23 and 40 Hz. Although carbachol-induced epileptiform activity was associated with increases in intracellular free Ca2+ in both layer II and III cells, the observed [Ca2+]i accumulation was significantly larger in layer III than in layer II cells. Responses to intracellular current injections showed differences in Ca2+ accumulation in layer II and III cells at the same membrane potentials, suggesting a dominant expression of low- and high-voltage-activated Ca2+ channels in these layer II and III cells, respectively. In conclusion, we present evidence for significant differences in the [Ca2+]i regulation between layer II stellate and layer III pyramidal cells of the medial EC.

Muscarinic acetylcholine receptors in the hippocampus, neocortex and amygdala: a review of immunocytochemical localization in relation to learning and memory

Immunocytochemical mapping studies employing the extensively used monoclonal anti-muscarinic acetylcholine receptor (mAChR) antibody M35 are reviewed. We focus on three neuronal muscarinic cholinoceptive substrates, which are target regions of the cholinergic basal forebrain system intimately involved in cognitive functions: the hippocampus; neocortex; and amygdala. The distribution and neurochemistry of mAChR-immunoreactive cells as well as behaviorally induced alterations in mAChR-immunoreactivity (ir) are described in detail. M35+ neurons are viewed as cells actively engaged in neuronal functions in which the cholinergic system is typically involved. Phosphorylation and subsequent internalization of muscarinic receptors determine the immunocytochemical outcome, and hence M35 as a tool to visualize muscarinic receptors is less suitable for detection of the entire pool of mAChRs in the central nervous system (CNS). Instead, M35 is sensitive to and capable of detecting alterations in the physiological condition of muscarinic receptors. Therefore, M35 is an excellent tool to localize alterations in cellular cholinoceptivity in the CNS. M35-ir is not only determined by acetylcholine (ACh), but by any substance that changes the phosphorylation/internalization state of the mAChR. An important consequence of this proposition is that other neurotransmitters than ACh (especially glutamate) can regulate M35-ir and the cholinoceptive state of a neuron, and hence the functional properties of a neuron. One of the primary objectives of this review is to provide a synthesis of our data and literature data on mAChR-ir. We propose a hypothesis for the role of muscarinic receptors in learning and memory in terms of modulation between learning and recall states of brain areas at the postsynaptic level as studied by way of immunocytochemistry employing the monoclonal antibody M35.

Expression of the transcription factor Zif268 in the visual cortex of monocularly deprived rats: effects of nerve growth factor

Neurotrophins are known to be involved in experience-dependent plasticity of the visual cortex. Here, we have characterized in detail the effects of intraventricular nerve growth factor infusion in monocularly deprived rats by using immunostaining for the immediate-early gene product Zif268 as a marker of functional activity with cellular resolution. We have taken advantage of the rapid regulation of Zif268 by visual input to reveal the cortical units that are responsive to the deprived eye after a period of monocular deprivation. We found that responses to the deprived eye were significantly preserved in the cortex of monocularly deprived rats infused with nerve growth factor. The effects of nerve growth factor were greater for cortical cells located in deep layers and with more peripheral receptive fields. Results from Zif268 staining correlated very well with those obtained by single-cell recordings from the visual cortex. Our results demonstrate that exogenous nerve growth factor preserves the functional input from the deprived eye, enabling cortical neurons to activate immediate-early gene expression in response to stimulation of the deprived eye. Furthermore, we show that the intraventricular infusion of nerve growth factor differentially affects the ocular dominance of cells at various depths and eccentricities in the developing cortex.

Blockade of GABAA receptors facilitates induction of NMDA receptor-independent long-term potentiation

An N-methyl-D-aspartate (NMDA)-independent form of long-term potentiation (LTP), which depends on postsynaptic, voltage-dependent calcium channels (VDCCs), has been demonstrated in area CA1 of hippocampus. GABA acting at GABAA receptors limits postsynaptic depolarization during LTP induction. Blockade of GABAA receptors should therefore enhance activation of postsynaptic VDCCs and facilitate the induction of this NMDA receptor-independent, VDCC-dependent LTP. In agreement with this hypothesis, pharmacological blockade of GABAA receptors in the in vitro rat hippocampal slice increased the magnitude of LTP resulting from a normally effective, high-frequency (200 Hz) tetanic stimulation protocol. In addition, GABAA receptor blockade allowed a lower frequency (25 Hz) and normally ineffective tetanic stimulation protocol to induce this form of LTP. Intracellular recordings from CA1 pyramidal cells revealed that blocking GABAA receptors during tetanic stimulation allowed greater postsynaptic depolarization, increased the number of postsynaptic action potentials fired during the tetanization, and also increased the duration of synaptically evoked action potentials. To mimic the increased action potential firing observed when GABAA receptors were blocked, we paired 25-Hz antidromic stimulation with 25-Hz orthodromic stimulation. Paired antidromic + orthodromic 25-Hz stimulation induced NMDA receptor-independent LTP, whereas neither antidromic nor orthodromic stimulation alone induced LTP. Increased action potential firing can therefore at least partially account for the facilitation of NMDA receptor-independent LTP caused by blockade of GABAA receptors. This conclusion is consistent with prior studies demonstrating that action potentials are particularly effective stimuli for the gating of VDCCs in CA1 pyramidal cell dendrites.

Identification and transgenic analysis of a murine promoter that targets cholinergic neuron expression

Choline acetyltransferase (ChAT) is a specific phenotypic marker of cholinergic neurons. Previous reports showed that different upstream regions of the ChAT gene are necessary for cell type-specific expression of reporter genes in cholinergic cell lines. The identity of the mouse ChAT promoter region controlling the establishment, maintenance, and plasticity of the cholinergic phenotype in vivo is not known. We characterized a promoter region of the mouse ChAT gene in transgenic mice, using beta-galactosidase (LacZ) as a reporter gene. A 3,402-bp segment from the 5'-untranslated region of the mouse ChAT gene (from -3,356 to +46, +1 being the translation initiation site) was sufficient to direct the expression of LacZ to selected neurons of the nervous system; however, it did not provide complete cholinergic specificity. A larger fragment (6,417 bp, from -6,371 to +46) of this region contains the requisite regulatory elements that restrict expression of the LacZ reporter gene only in cholinergic neurons of transgenic mice. This 6.4-kb DNA fragment encompasses 633 bp of the 5'-flanking region of the mouse vesicular acetylcholine transporter (VAChT), the entire open reading frame of the VAChT gene, contained within the first intron of the ChAT gene, and sequences upstream of the start coding sequences of the ChAT gene. This promoter will allow targeting of specific gene products to cholinergic neurons to evaluate the mechanisms of diseases characterized by dysfunction of cholinergic neurons and will be valuable in design strategies to correct those disorders.

Effects of local cholinesterase inhibition on acetylcholine release assessed simultaneously in prefrontal and frontoparietal cortex

To investigate whether acetylcholine is released in a similar fashion in different regions of the cortex, in vivo microdialysis was used to measure acetylcholine efflux simultaneously in the medial prefrontal and the frontoparietal cortex, under both basal conditions and following tactile stimulation. Additionally, the effects of including two different concentrations (0.05 microM and 0.5 microM) of a cholinesterase inhibitor (neostigmine) in the perfusion fluid were assessed. Basal levels of acetylcholine (i.e. during non-stimulated sessions) were similar in medial prefrontal and frontoparietal areas. Tactile stimulation reliably increased acetylcholine efflux in a similar fashion (up to 140% increase above baseline) in both cortical areas studied. Predictably, the higher concentration of neostigmine (0.5 microM) increased basal acetylcholine efflux by about 150% from levels observed with the lower neostigmine concentration (0.05 microM), but the concentration of local neostigmine had no effect on either the magnitude or the duration of the increased acetylcholine efflux following tactile stimulation. These results suggest that the pattern of acetylcholine release may be comparable in different areas of the cortex, supporting the idea that cholinergic projections from the basal forebrain to the cortex represent a globally regulated system. Furthermore, while the inclusion of neostigmine in perfusion fluid must be taken into account when interpreting acetylcholine efflux data, it appears that concentrations of up to 0.5 microM do not interfere fundamentally with the lability of cortical acetylcholine efflux in response to behavioural stimulation.

In situ mRNA hybridization study of the distribution of choline acetyltransferase in the human brain

We examined the distribution of choline acetyltransferase (ChAT) mRNA in the brain of six autopsied individuals by in situ hybridization with 35S-labeled human ChAT riboprobes. Neurons containing hybridization signal for ChAT mRNA were observed in the nucleus of the diagonal band of Broca, the basal nucleus of Meynert, the caudate nucleus, the putamen, the pedunculopontine tegmental nucleus, the laterodorsal tegmental nucleus, the parabigeminal nucleus, the oculomotor nucleus and the trochlear nucleus. These findings were in good agreement with previous ChAT-immunohistochemical data. In contrast, labeled neurons were not observed in the medial septal and medial habenular nuclei, in which previously ChAT-immunoreactive neurons have been identified in many mammalian species, including the human. An unexpected result of the present study was the demonstration of neurons with ChAT mRNA signal in restricted areas of the human cerebral cortex.

Differential interaction between 5-HT3 receptors and GABAergic neurons inhibiting acetylcholine release in rat entorhinal cortex slices

The 5-HT3 receptor antagonists, ondansetron, MDL 72222 and granisetron (0.01-1 microM), produced a concentration-dependent increase of K+-evoked [3H]ACh efflux in slices from rat entorhinal cortex preloaded with [3H]choline. Bicuculline and flumazenil, antagonists at different sites of the GABAA receptor, also enhanced [3H]ACh efflux. While the ACh releasing effect of ondansetron was markedly potentiated, in a TTX-sensitive manner, by bicuculline, the effects of MDL 72222 and granisetron were not significantly modified. A qualitatively identical interaction was found by using flumazenil, a GABAA antagonist at the benzodiazepine recognition site, in combination with the 5-HT3 receptor antagonists. The potentiation by the GABAA antagonists of [3H]ACh efflux was also observed in a superfusion medium deficient in Cl-. The nonspecific K+-channel blockers TEA and Ba2+ also increased K+-evoked [3H]ACh efflux in this preparation but the releasing effect was not modified by bicuculline. The results support the functional interaction of ondansetron with GABAergic interneurons in the rat entorhinal cortex, GABA-independent mechanisms may however be involved in the regulation of cortical cholinergic function by other 5-HT3 receptor antagonists.

Distinctive morphological features of a subset of cortical neurons grown in the presence of basal forebrain neurons in vitro

Basal forebrain cholinergic neurons (BFCNs) provide the major subcortical source of cholinergic input to cerebral cortex and play an important role in regulating cortical activity. The present study examined the ability of BFCNs to influence neocortical neuronal growth by examining effects of the presence of BFCNs on certain cortical neurons grown under the controlled conditions of dissociated cell culture. Initial experiments demonstrated distinctive morphological features of a population of neurons (labeled with SMI-32, a monoclonal antibody to nonphosphorylated neurofilament proteins that labels pyramidal neurons in vivo) in cocultures containing basal forebrain (BF) and cortical cells. These neurons (large neurons immunoreactive for SMI-32 [SMI-32(+) neurons]) were characterized as having extensive axons, greater soma size, and more dendritic growth than did most SMI-32(+) neurons in the cultures. Staining for SMI-32 in cocultures in which the cortical neurons were labeled with a fluorescent marker before adding the BF cells indicated that virtually all large SMI-32(+) neurons were of cortical origin. Eliminating BFCNs with the selective cholinergic immunotoxin 192 IgG-saporin resulted in a >80% decrease in the number of large SMI-32(+) neurons, although causing little damage to other cells in the treated cultures; this suggests that survival or maintenance of large SMI-32(+) neurons may depend on ongoing trophic support from BFCNs. Thus, present findings suggest that BFCNs may provide powerful growth- and/or survival-enhancing signals to a subset of cortical neurons.

Role of the basal forebrain cholinergic projection in somatosensory cortical plasticity

Trimming all but two whiskers in adult rats produces a predictable change in cortical cell-evoked responses characterized by increased responsiveness to the two intact whiskers and decreased responsiveness to the trimmed whiskers. This type of synaptic plasticity in rat somatic sensory cortex, called "whisker pairing plasticity," first appears in cells above and below the layer IV barrels. These are also the cortical layers that receive the densest cholinergic inputs from the nucleus basalis. The present study assesses whether the cholinergic inputs to cortex have a role in regulating whisker pairing plasticity. To do this, cholinergic basal forebrain fibers were eliminated using an immunotoxin specific for these fibers. A monoclonal antibody to the low-affinity nerve growth factor receptor 192 IgG, conjugated to the cytotoxin saporin, was injected into cortex to eliminate cholinergic fibers in the barrel field. The immunotoxin reduces acetylcholine esterase (AChE)-positive fibers in S1 cortex by >90% by 3 wk after injection. Sham-depleted animals in which either saporin alone or saporin unconjugated to 192 IgG is injected into the cortex produces no decrease in AChE-positive fibers in cortex. Sham-depleted animals show the expected plasticity in barrel column neurons. In contrast, no plasticity develops in the ACh-depleted, 7-day whisker-paired animals. These results support the conclusion that the basal forebrain cholinergic projection to cortex is an important facilitator of synaptic plasticity in mature cortex.