Cholinergic function and intellectual decline in Alzheimer's disease

Circumscribed changes of the cerebral cortex in neuropsychiatric disorders of later life

The extent and distribution of biochemical abnormalities thought to reflect disorders of subpopulations of neurons have been determined in the cerebral cortex from brains of patients with Alzheimer-type dementia and depressive illness who died of natural causes. In dementia, loss of gray matter from areas of the parietal and temporal lobes is most obvious. In depression, these areas are not affected, but the pars opercularis and temporal pole are smaller than in controls. Results expressed per unit mass of total protein indicate selective reductions in both disorders of serotonin 2 recognition sites in all areas examined and of somatostatin content in only the temporal pole of the six areas examined. In dementia alone a selective loss was found of somatostatin content of the superior parietal lobule and of serotonin 1A sites and choline acetyltransferase activity in all areas examined. Results for depression expressed per entire area indicate additionally reduced somatostatin content and serotonin 1A sites in the pars opercularis and serotonin 1A sites in the temporal pole. These multiple analyses performed on each sample provide further support for a prominent disorder of pyramidal neurons in dementia as well as more evidence for alterations in cortical neurons in depression, either as a result of the disease itself or its treatment.

Gangliosides in the brain in adult Down's syndrome and Alzheimer's disease

Quantitative analysis of total gangliosides and of ganglioside composition by HPTLC has been carried out on the gray matter of frontal cerebral cortex of six brains from Down's syndrome (DS) adults, six age-matched controls, six Alzheimer's disease (AD) adults, and six controls matched for age with the AD brains, as well as on three DS and six control cerebellum specimens. In addition, the analyses were carried out on specimens of corpus callosum of five adult DS and five control brains. No abnormalities were found in the gangliosides of DS corpus callosum. In DS frontal cortex, the concentration of total gangliosides was reduced, and there was a decrease in the fraction of GT1b and GD1b, and an increase in those of GT1a, GD3, GM1 and GM2; the ratio of total b-series to a-series gangliosides was decreased. Very similar abnormalities were found in the gangliosides of DS cerebellum. In AD frontal cortex, by contrast, the total gangliosides and their composition were normal by comparison with age-matched controls, with the minor exception of reductions in the fractions of GQ1b and GT1L. It is concluded that abnormalities in gangliosides exist in the brain in DS that are unrelated to AD-type pathology and may reflect developmental disturbances.

The olfactory system and Alzheimer's disease

Alzheimer's disease (AD) is considered to be the number one health problem and seems to be reaching epidemic proportion in the USA. The cause of AD is not known, a reliable animal model of the disease has not been found and appropriate treatment of this dementia is wanting. The present review focuses on the possibility that a virus or exogenous toxic materials may gain access to the CNS using the olfactory mucosa as a portal of entry. Anterograde and retrograde transport of the virus/zeolites to olfactory forebrain regions, which receive primary and secondary projections from the main olfactory bulb (MOB) and which, in turn, project centrifugal axons to the MOB, may initiate cell degeneration at such loci. Pathological changes may, thus, be initially confined to projecting and intrinsic neurons localized in cortical and subcortical olfactory structures; arguments are advanced which favor the view that excitotoxic phenomena could be mainly responsible for the overall degenerative picture. Neurotoxic activity may follow infection by the virus itself, be facilitated by loss of GABAergic terminals in olfactory cortex, develop following repeated episodes of physiological long term potentiation (which unmasks NMDA receptors) or be due to excessive release, faculty re-uptake or altered glutamate receptor sensitivity. Furthermore, a reduction in central inhibitory inputs to the MOB might then result in disinhibition of mitral/tufted neurons and enhance the excitotoxic phenomena in the MOB projecting field. Within this context, and in line with recent studies, it is believed that pathology begins at cortical (mainly olfactory) regions, basal forebrain neurons being secondarily affected due to retrograde degeneration. In addition, failure to produce a critical level of neurotrophic factors by a damaged MOB and olfactory cortex, could adversely affect survival of basal cholinergic neurons which innervate both regions. Support for these hypothesis is provided, first, by recent reports on pathological findings in AD brains which seem to involve preferentially the olfactory and entorhinal cortices, the olfactory amygdala and the hippocampus, all of which receive primary or secondary projections from the MOB; secondly, by the presence of severe olfactory deficits in the early stages of the disease, mainly of a discriminatory nature, which points to a malfunction of central olfactory structures.

Septal alpha-noradrenergic antagonism in vivo blocks the testing-induced activation of septo-hippocampal cholinergic neurones and produces a concomitant deficit in working memory performance of mice

In order to test the hypothesis that alpha-noradrenergic receptors in the septum 1) play an important functional role in the mediation of trans-synaptic control of the neurones of the cholinergic septo-hippocampal pathway and 2) produce resultant modulation of working memory performance, we have investigated the effects in vivo of the acute intraseptal injection of an alpha-antagonist, phenoxybenzamine, in mice. Neurochemical analysis was performed using measures of the kinetics of sodium-dependent high-affinity choline uptake in samples of hippocampus from injected mice and their relevant controls in both quiet conditions and immediately following selective working memory testing in an 8-arm radial maze. Results show that whereas the injection of phenoxybenzamine produces no significant alteration of the activity of the cholinergic septo-hippocampal neurones in quiet conditions, the pretrial (20 min) administration of this drug almost totally abolished the usually observed increase in hippocampal cholinergic activity induced by testing. This inhibition of cholinergic activation was associated with a parallel working memory deficit. The results provide further direct support for the hypothesis that septal noradrenergic afferents via alpha-receptors mediate a phasic and net excitatory trans-synaptic influence on the cholinergic septo-hippocampal pathway during working memory testing and thereby significantly contribute to the modulation of the level of working memory performance.

Function of neurotrophic factors in the adult and aging brain and their possible use in the treatment of neurodegenerative diseases

This review summarizes the current knowledge of characterized neurotrophic factors, including nerve growth factor (NGF) which serves as paradigmatic example when studying novel molecules. Special consideration is given to the function of neurotrophic factors in the adult and aging brain. Strategies are discussed for the eventual development of pharmacological applications of these molecules in the treatment of neurodegenerative diseases.

Scopolamine treatment and fimbria-fornix lesions: mimetic effects on radial maze performance

Long-Evans female rats were "trained" in an 8-arm radial maze and subsequently tested under systemic treatment with physostigmine (0.05 mg/kg, IP), scopolamine methylbromide (MBr) and scopolamine hydrobromide (HBr; 0.5 mg/kg, IP), whose effects were compared to those of aspirative lesions of the fimbria-fornix pathways. During the predrug trials, rats with lesions showed impaired performances compared to those of intact rats. Whereas physostigmine had no significant effect in either group, scopolamine HBr impaired performances of intact rats in a manner closely parallel to all measured behavioral effects of the lesions (errors, "correct arms" and strategies). The scopolamine HBr-induced deficits were not correlated with the percentage of "spatial" strategies. Under scopolamine HBr treatment the performances of rats showing preferences for "spatial" strategies did not differ significantly from those of rats showing preferences for "orientation" strategies. These results provide further support for the involvement of cholinergic processes in working memory and suggest that scopolamine-induced central cholinergic disruption may mimic the effects of fimbria-fornix lesions in an 8-arm radial maze. They also somewhat qualify previous reports on 1) the poor sensitivity of an uninterrupted radial maze testing procedure to pharmacological treatment and 2) the abilities of rats to resist muscarinic blockade depending on the strategies they use in the maze.

Tetrahydroaminoacridine (THA) as a pharmacological probe in Alzheimer's disease (AD) and other neurodegenerative disorders

Unlike other potent enhancers of cholinergic function in the central nervous system (CNS), THA appears to sustain improved function in many moderately impaired AD patients when the Summers procedure is followed. THA has a complex pharmacology. In addition to its enhancement of cholinergic transmission a hydroxylated metabolite might chelate aluminum (A1), thereby removing multiple toxicological constraints on CNS function. This mobilized THA metabolite-A1 complex might either be re-distributed to less sensitive sites or removed from the CNS across the blood-brain barrier (BBB). Since the known presence of A1 in AD brain is not necessarily causal, a positivistic approach to research and treatment with THA and its metabolites might serve to clarify this difficult and challenging problem.

Effects of Nucleus Basalis Magnocellularis Lesions in Rats on Delayed Matching and Non-Matching to Position Tasks

The effects of quisqualic acid lesions of the nucleus basalis magnocellularis on short-term memory capacities of the rat have been investigated using the delayed matching and non-matching to position tasks. The lesions do not disrupt performance of either task by pretrained animals, but do disrupt the ability to acquire the non-matching contingency, and to reverse to the non-matching task when trained on the matching task. The unidirectional nature of the reversal deficit has been replicated. The generalized disruption of performance of either task by the muscarinic antagonist scopolamine was comparable in lesioned and control rats. The lesions were associated with extensive loss of acetylcholinesterase staining in the basal forebrain and in the neocortex, and 55% depletions of choline acetyltransferase activity in the neocortex but not in the hippocampus. These observations demonstrate that the cholinergic projection from nucleus basalis to the neocortex is not critical for normal short-term memory, but that lesions involving this system do disrupt specific types of conditional discrimination learning.

Intracerebral drug delivery in rats with lesion-induced memory deficits

Pharmacological treatments directed at increasing cortical acetylcholine activity in patients with Alzheimer's disease have largely been disappointing, perhaps because denervated areas of brain may not be exposed to adequate amounts of drug. A new method has been developed to enable localized intracerebral delivery of neurotransmitter substances using a polymeric drug delivery system. Microspheres of a polyanhydride sebacic acid copolymer were impregnated with bethanechol, an acetylcholinesterase-resistant cholinomimetic. Twenty rats received bilateral fimbria-fornix lesions, producing cholinergic denervation of the hippocampus and marked impairment in spatial memory. The animals were trained for 2 weeks to run after which they received bilateral intrahippocampal implants of saline (five rats), blank polymer (five rats), or bethanechol-impregnated polymer (10 rats). Following implantation, spatial memory was assessed by radial-maze performance testing for 40 days. Untreated lesioned rats showed persistently poor spatial memory, entering maze arms with near random frequency. Similarly, animals treated with saline and blank polymer did not improve after implantation. Rats treated with bethanechol-impregnated microspheres, however displayed significant improvement within 10 days after implantation; this improvement persisted for the duration of the experiment (p less than 0.05, Student's t-test). Histological analysis of regional acetylcholinesterase staining showed widespread loss of activity throughout the hippocampus bilaterally in all animals. The microsphere implants were visible within the hippocampus, with minimal reactive changes in surrounding brain. It is concluded that intracerebral polymeric drug delivery successfully reversed lesion-induced memory deficits, and has potential as a neurosurgical treatment method for Alzheimer's disease and other neurodegenerative disorders.

Age-related decline of acetylcholine release evoked by depolarizing stimulation

Release of endogenous acetylcholine (ACh) from the dorsal hippocampus in response to depolarizing stimulation with high-K+ infusion was examined in young and aged rats using the method of in vivo dialysis. ACh content in the dialysate was determined by high-performance liquid chromatography-electrochemical detection (HPCC-ECD). During the high-K+ stimulation, the concentration of ACh in the dialysate only slightly increased in aged rats in contrast with young rats where the ACh content during stimulation increased about 2-fold of the basal level. These results showed that ACh release evoked by depolarizing stimulation declined through aging in the hippocampus.

A cell line producing recombinant nerve growth factor evokes growth responses in intrinsic and grafted central cholinergic neurons

The rat beta nerve growth factor (NGF) gene was inserted into a mammalian expression vector and cotransfected with a plasmid conferring resistance to neomycin into mouse 3T3 fibroblasts. From this transfection a stable cell line was selected that contains several hundred copies of the rat NGF gene and produces excess levels of recombinant NGF. Such genetically modified cells were implanted into the rat brain as a probe for in vivo effects of NGF on central nervous system neurons. In a model of the cortical cholinergic deficits in Alzheimer disease, we demonstrate a marked increase in the survival of, and fiber outgrowth from, grafts of fetal basal forebrain cholinergic neurons, as well as stimulation of fiber formation by intact adult intrinsic cholinergic circuits in the cerebral cortex. Adult cholinergic interneurons in intact striatum also sprout vigorously toward implanted fibroblasts. Our results suggest that this model has implications for future treatment of neurodegenerative diseases.

Brain muscarinic receptor subtypes are differently affected in Alzheimer's disease and Parkinson's disease

The muscarinic receptor subtypes determined by displacing [3H]quinuclidinyl benzylate by carbachol showed distinct profiles in Alzheimer's disease (AD) and in Parkinson's disease (PD). A decrease in the M2-receptor count was seen in the hippocampus in AD as compared to controls, while PD patients had unaltered binding. By contrast, PD patients had increased M2-binding in the frontal and temporal cortex, whereas unchanged binding was seen in AD. In PD the total number of cortical muscarinic receptors was negatively associated with the activity of ChAT, but had a positive correlation with the degree of dementia of the patients.

Neuronal membrane depolarization and the control of cholinergic muscarinic receptors: selective effect on different neuronal cell types

1. The possibility that a long-lasting neuronal activation regulates the expression of muscarinic cholinergic receptors was studied with three cultured neuronal cell lines. 2. Continuous depolarization of a subclone of the neuroblastoma-glioma NG108-15 hybrid cells with potassium chloride increased by 45-75% the number of cholinergic muscarinic receptors, monitored with 3H-QNB, whereas a short incubation with KCl for 10 min or 6 hr had no effect. 3. The calcium channel blocker verapamil increased the effect of KCl. 4. Two cell lines, named SC9 and WC5, that originate from the rat brain, also bind 3H-QNB. They were therefore used to test whether the effect of chronic depolarization is universal. Depolarized SC9 and WC5 cells, in the presence or absence of verapamil, did not show an increased 3H-QNB binding. 5. Muscarinic receptors of both SC9 and WC5 cells have a higher affinity to pirenzepine than the M-3 receptor subtype of the neuroblastoma-glioma cells, suggesting therefore that the two rat brain cell lines possess M-1 or M-2 receptors. 6. The physiological significance of this differential role of depolarization on the expression of different muscarinic receptors is discussed in the context of their postreceptor second messengers.

Telencephalic cholinergic system of the New World monkey (Cebus apella): morphological and cytoarchitectonic assessment and analysis of the projection to the amygdala

While the cholinergic projection from the nucleus basalis to the cortical mantle has received considerable attention, a similar projection to the magnocellular basal nucleus of the amygdala has not been studied in such detail. The present study analyzed the cholinergic basal forebrain projection to the amygdala in the Cebus apella monkey by using combined tract-tracing and immunocytochemical techniques. As a foundation for this assessment, the morphological and cytoarchitectonic organization of the cholinergic telencephalic system of the New World C. apella monkey was examined by using choline acetyltransferase (ChAT) immunocytochemistry. Although there were minor differences, the telencephalic cholinergic system of Cebus monkeys is similar to that seen in Old World nonhuman primates. ChAT-immunoreactive neurons were observed throughout the Ch1-4 regions of the basal forebrain, with subdivisions of the Ch4 region similar to those previously described (Mesulam et al., '83a). Most cholinergic neurons were hyperchromic and magnocellular; however, some neurons were parvicellular. Like most species, cholinergic neurons were also observed throughout the striatum. However, unlike in rodents, cholinergic perikarya were not observed within the cortex or hippocampus. To analyze the cholinergic fiber projections from the basal forebrain to the amygdala, monkeys received an intraamygdaloid injection of the retrograde tracer horseradish peroxidase conjugated to wheat germ agglutinin. Retrogradely labeled neurons that colocalized ChAT or acetylcholinesterase (AChE) were found predominantly in the anterolateral portion of the CH4 region. Fewer double-labeled neurons were found in the anteromedial and intermediate portion of CH4 and in the CH3 region. Neurons that exhibited retrograde labeling were only occasionally discerned in the posterior portions of the CH4 region, in the medullary laminae of the globus pallidus, or lodged within the internal capsule. These data are discussed in terms of the putative role this cholinergic input might play in cognitive processing in primates.

Hemicholinium-3 impairs spatial learning and the deficit is reversed by cholinomimetics

The effects of hemicholinium-3 (HC-3) on spatial discrimination learning were studied. Rats were equipped with indwelling cannulae in the right lateral ventricle and, following recovery, were trained on a two platform spatial discrimination task in a water maze. In this task a visible escape platform remains in a fixed position in the pool during a single training session, whilst the location of an identical "float" (which affords no escape) is randomly varied. For each session the location of the fixed escape platform was changed and the rats were retrained to criterion following pretreatment either with artificial cerebrospinal fluid (CSF) or HC-3 (2.5, 5.0 micrograms/rat/ICV) 1 h before training. Each rat received every treatment according to a latin square design. The results showed that spatial learning was dose dependently impaired by HC-3, choice accuracy being reduced to chance levels by the higher dose. There was no evidence of motoric difficulty, as choice latencies were not significantly increased. Experiments were then conducted to test for reversal of the deficit using a range of psychotropic drugs. Rats were treated with CSF or HC-3 (5 micrograms/rat ICV) 60 min prior to testing and test drugs were injected 15 min before testing. Some doses of physostigmine (46-460 micrograms/kg/SC) and tetrahydroaminoacridine (THA) (2.2-10 mg/kg/SC) reversed the spatial learning deficit. The muscarinic agonists arecoline (0.046-1 mg/kg/SC), aceclidine (1-10 mg/kg/SC), oxotremorine (30-100 micrograms/kg/SC) and RS-86 (0.46, 1.0 microgram/kg/SC) were also effective. Pilocarpine (0.22-2.2 mg/kg/SC) showed marginal activity and isoarecoline (4.6-10 mg/kg/SC) was inactive. Nicotine (0.32, 1, 3.2 mg/kg/SC) and piracetam (10, 30, 100 mg/kg IP) were also inactive. The alpha 2 agonist, clonidine (46, 100 micrograms/kg SC) and the antagonist idazoxan (32, 100 micrograms/kg SC) were also inactive. Learning deficits were not reversed by haloperidol (20, 60 micrograms/kg), amphetamine (0.1, 0.46 mg/kg), the selective 5-HT1A agonist 8-OH-DPAT (30, 100 micrograms/kg) or by the benzodiazapine antagonist ZK-93426 (1, 3.2, 10 mg/kg). The results show that forebrain Ach depletion by HC-3 impairs spatial discrimination learning and these deficits are reversed by cholinesterase inhibitors and some muscarinic receptor agonists. Some degree of pharmacological selectivity is indicated by the failure of a range of other drugs to reverse the impairments.

Acetylcholinesterase stain intensity variation in the rat dentate gyrus: a quantitative description based on digital image analysis

Three-dimensional patterns of variation in the intensity of acetylcholinesterase histochemical staining and the width of stain-defined subregions were quantified for the dentate gyrus of the adult male Long-Evans rat. Matched tissue sections sampled through the central hippocampal formation of five rats were measured with a digital image analysis computer system. The width and stain intensity were determined for defined portions of the dentate gyrus related to gross acetylcholinesterase staining patterns and the known distribution of dentate afferents. Normalized values reflecting stain intensity at defined positions within this standardized sampling array were examined to investigate regional differences in acetylcholinesterase distribution along the primary dendritic axis of dentate granule neurons. The data illustrate quantitative differences in the partitioning of acetylcholinesterase as a function of intrahippocampal position. The variation is more pronounced in the septal-temporal axis than the granule cell layer crest-tip axis. Furthermore, the septal-temporal variations in acetylcholinesterase intensity demonstrate some independence according to proximal-distal location within the molecular layer. The results suggest that acetylcholinesterase distribution within the dentate gyrus may reflect local physiological characteristics of those afferent systems related to this enzyme, including but not necessarily limited to those that are specifically cholinergic.

Cholinergic modulation of frequency receptive fields in auditory cortex: I. Frequency-specific effects of muscarinic agonists

Previously we reported that acetylcholine (ACh) and acetyl-beta-methacholine (MCh) modify responses of neurons in auditory cortex to individual frequencies. The purpose of this study was to determine whether muscarinic agonists produce frequency-specific alterations or general changes in cellular responses. Frequency-specific modifications would be evident in alterations of frequency receptive fields (FRF) that differed across frequencies while general effects would be seen as changes that were more or less the same over frequencies. Responses of single neurons to designated sets of tones were recorded in the auditory cortex of chronically prepared awake cats before, during, and following ejection of ACh or MCh by iontophoresis or micropressure using multibarrel micropipettes. Frequency receptive fields were determined by presenting isointensity tones across a range of frequencies including the cell's best frequency (BF) to tone onset. FRF for "off" and "sustained (through)" responses were also determined quantitatively. The effects of ACh and MCh were predominantly frequency-specific (77%, 39/51 cells); general changes (19%, 10/51) and no effects (4%, 2/51) were less likely. Frequency-specific effects involved both facilitation and reduction of the same response component to different frequencies within the same neuron. For responses to tone onset (but not "through" and "off" responses), agonists were more likely to produce a decrease at the BF while simultaneously increasing responses to other frequencies. Agonists could increase or decrease frequency selectivity. Effects of agonists could be blocked by atropine, suggesting involvement of muscarinic receptors.

Galanin-like immunoreactivity within Ch2 neurons in the vertical limb of the diagonal band of Broca in aging and Alzheimer's disease

The neuropeptide galanin is known to inhibit the evoked release of acetylcholine in ventral hippocampus of the rat. Co-localization of this peptide with choline acetyltransferase in neurons of the cholinergic septal nuclei has been demonstrated in the rat and non-human primate. The severe deficiency of the cholinergic hippocampal projection system arising mainly from the vertical limb nucleus of the diagonal band of Broca, also referred to as Ch2 region, is a constant finding in Alzheimer's disease, a disorder which is neuropathologically characterized by the appearance of senile plaques, neurofibrillary tangles and congophilic angiopathy in neo- and archicortical structures. In the present study for the first time galanin immunoreactivity in the human Ch2 region is morphologically investigated and related to the severity of hippocampal plaques and neurofibrillary tangles in Alzheimer's disease. An inverse relationship between decreasing galanin immunoreactivity in the Ch2 region and amounts of senile plaques and neurofibrillary tangles in the hippocampus is indicated. Considering the cholinergic deficiency in Alzheimer's disease as a secondary phenomenon to primary cortical and hippocampal lesions, and realizing the inhibitory effect of galanin upon acetylcholine release in hippocampus, this preliminary study suggests that a decreased galanin immunoreactivity in Ch2 in Alzheimer's disease, reflects a possible negative feedback mechanism to a degenerating cholinergic projection system.

Nerve growth factor receptor mRNA distribution in human brain: normal levels in basal forebrain in Alzheimer's disease

Nerve growth factor (NGF) receptor mRNA was found to be widely distributed throughout the human central nervous system, with the highest levels in the basal forebrain; this suggests that NGF may function as a retrograde trophic messenger for basal forebrain magnocellular cholinergic nerve cells. The degeneration of the latter constitutes one of the main features of Alzheimer's disease and it may be responsible for some of the cognitive impairment that characterizes the disease. No evidence was obtained for an insufficient synthesis of NGF receptor mRNA in the basal forebrain in Alzheimer's disease, where NGF receptor-like immunoreactivity was confined to neuronal cell bodies. NGF could thus be therapeutically beneficial. It could be expected to induce basal forebrain cholinergic cells to hypertrophy, synthesize more choline acetyltransferase and extend neurites.

Cholinergic system and memory in the rat: effects of chronic ethanol, embryonic basal forebrain brain transplants and excitotoxic lesions of cholinergic basal forebrain projection system

Oral administration of ethanol (20% v/v) to male Sprague-Dawley rats for different periods of time up to 28 weeks resulted in profound reductions of acetylcholine content, in vitro synthesis and release of acetylcholine, choline uptake, activities of choline acetyltransferase, acetylcholinesterase and pyruvate decarboxylase, content of noradrenaline, serotonin and, to a lesser extent, dopamine throughout the brain. Changes were fully and partially reversible by a 4 weeks' ethanol-free period following a treatment of 8 and 18 weeks, respectively. They remained persistent, however, after 28 weeks of treatment. Performance in an eight arm-radial maze revealed a severe impairment in both spatial and non-spatial reference and working memory. A similar pattern of memory impairment was obtained after ibotenate lesion of the cholinergic basal forebrain projection system. In order to test whether this memory impairment depends on cholinergic deafferentation of the cortex, cholinergic-rich fetal basal forebrain cell suspensions were transplanted into cortex, hippocampus or both these sites in ethanol treated rats. Cholinergic-rich transplants, but not cholinergic-poor transplants, were effective in ameliorating impaired memory function and measures of cholinergic activity in the basal forebrain projection system. The behavioural efficacy of the basal forebrain grafts was well correlated with measures of both transplant volume and the degree to which they restored acetylcholine content at the transplant site; these transplants had no effect, however, on brain monoamine levels. The effects of the cholinergic-rich transplants into cortical and hippocampal sites were additive in their amelioration of performance in the radial maze. Similarly, ibotenate lesions of the sites of origin of the cholinergic projections to neocortex (in the region of the nucleus basalis magnocellularis) and hippocampus (the medial septal areas and nucleus of the diagonal band), respectively, were additive in their deleterious effects on maze performance. There were no qualitative differences in the susceptibility of the four different types of memory performance measured (spatial and non-spatial reference and working memory) to the effects of ethanol, ibotenate lesions of the cholinergic projection system, or cholinergic-rich brain tissue transplants. Thus, overall, the results indicate that the forebrain cholinergic system acts as a whole, without major functional differences between the projections originating in the medial septal area/diagonal band complex and the basal nucleus, and that it discharges a very general function in cognitive processes.(ABSTRACT TRUNCATED AT 400 WORDS)

Nootropic drugs and brain cholinergic mechanisms

1. This review has two aims: first, to marshal and discuss evidences demonstrating an interaction between nootropic drugs and brain cholinergic mechanisms; second, to define the relationship between the effects on cholinergic mechanisms and the cognitive process. 2. Direct or indirect evidences indicating an activation of cholinergic mechanisms exist for pyrrolidinone derivatives including piracetam, oxiracetam, aniracetam, pyroglutamic acid, tenilsetam and pramiracetam and for miscellaneous chemical structures such as vinpocetine, naloxone, ebiratide and phosphatidylserine. All these drugs prevent or revert scopolamine-induced disruption of several learning and memory paradigms in animal and man. 3. Some of the pyrrolidinone derivatives also prevent amnesia associated with inhibition of acetylcholine synthesis brought about by hemicholinium. Oxiracetam prevents the decrease in brain acetylcholine and amnesia caused by electroconvulsive shock. Oxiracetam, aniracetam and pyroglutamic acid prevent brain acetylcholine decrease and amnesia induced by scopolamine. Comparable bell-shaped dose-effect relationships result for both actions. Phosphatidylserine restores acetylcholine synthesis and conditioned responses in aging rats. 4. The mechanisms through which the action on cholinergic systems might take place, including stimulation of the high affinity choline uptake, are discussed. The information available are not yet sufficient to define at which steps of the cognitive process the action on cholinergic system plays a role and which are the influences of the changes in cholinergic function on other neurochemical mechanisms of learning and memory.

Localization of nerve growth factor receptors in the normal human brain and in Alzheimer's disease

NGF receptors were visualized in human brain sections with an immunohistochemical procedure using a monoclonal antibody. This method results in the selective visualization of a population of neurons in the medial septal nucleus, the nucleus of the diagonal band of Broca, and the nucleus basalis of Meynert. Several lines of evidence indicate that this neuronal population is identical to the cholinergic neurons of the basal forebrain. NGF receptor immunohistochemistry therefore represents a sensitive and reliable procedure to selectively visualize forebrain cholinergic neurons for post-mortem analysis. NGF receptors were found to be expressed during the entire life span. However, the intracellular staining intensity was reduced in normal aging, suggesting the tentative conclusion that NGF receptor synthesis may decline in the aged brain. In Alzheimer's disease, the number of NGF receptor-positive cells was decreased. The morphological characteristics of surviving neurons were similar to immuno-positive neurons visualized in normal aged brains.

Nerve growth factor receptor is associated with cholinergic neurons of the basal forebrain but not the pontomesencephalon

Sequential immunohistochemical demonstration of nerve growth factor receptor and cholinergic acetyltransferase on the same tissue section in the rat revealed that approximately 92% of all cholinergic neurons in the basal forebrain possessed that receptor. Only 0.9% of the neurons demonstrating nerve growth factor receptor in the basal nuclear complex lacked the cholinergic synthetic enzyme, and a similarly small percentage of cholinergic cells, 7.1%, were choline acetyltransferase-positive but nerve growth factor receptor-negative. Affiliation of nerve growth factor receptor with structural entities morphologically indistinguishable from those demonstrating choline acetyltransferase on separate but corresponding tissue sections was also observed in the telencephalic fiber tracts and terminal fields of basal forebrain cholinergic neurons, including cholinergic puncta in the reticular nucleus of the thalamus. Nerve growth factor receptor was not found in association with choline acetyltransferase-positive somata of the pedunculopontine and laterodorsal tegmental nuclei, however, nor were fibers immunoreactive for nerve growth factor receptor observed originating from those cell bodies. These results suggest that nerve growth factor receptor, which is probably synthesized in cholinergic basal forebrain somata and transported throughout their dendritic and axonal arbors, has a physiologic role in those cells in the adult nervous system. This does not appear to be the case for phenotypically similar neurons of the pontomesencephalotegmental cholinergic complex.

Strength of scopolamine-induced amnesia as a function of time between training and testing

Variations in the strength of scopolamine-induced amnesia as a function of age of the habit were studied in Swiss Webster mice. Animals were trained in an active avoidance task to a criterion of 9/10 avoidances and immediately following training injected with scopolamine hydrochloride (1.0 mg/kg) or saline. Retention of the avoidance learning was evaluated by testing different groups of animals 1, 3, 7, 10, 14, and 28 days following training. The retention test consisted of five trials in which the CS but not the UCS was presented. Results indicated that saline-treated mice exhibited near-perfect retention up to 14 days post-training with forgetting beginning to be apparent at 28 days. Scopolamine treatment produced strong amnesia in animals tested 1 and 3 days post-training but normal retention in animals tested 7 and 10 days after learning. The amnesia abruptly reappeared at 14 days after which time it remained stable. The marked similarity of the scopolamine retention curve to changes in the strength of memory of discrimination learning in undertrained rats reported by Deutsch suggested that scopolamine resulted in the storage of a weak memory of the avoidance response. To explore this idea further we trained mice to a criterion (4/5) which would result in a weak avoidance response and tested different groups 1, 3, 10, 14, and 28 days following learning. Results showed that strength of the memory of avoidance learning increased up to 10 days and then decreased abruptly at 14 days thus replicating the general shape of the retention curve produced by injecting scopolamine following strong training. These data suggest that scopolamine disrupts processes essential for the formation of durable memories.

Differential hippocampal and cortical cholinergic activation during the acquisition, retention, reversal and extinction of a spatial discrimination in an 8-arm radial maze by mice

Possible differentiation of the intervention of cholinergic septohippocampal and magnocellular forebrain (NBM) projections to cortex during learning and memory processes has been investigated directly using mice. High-affinity choline uptake velocities in the hippocampus and cortex were analyzed, in parallel, at various periods during the acquisition, over 8 days, as were the subsequent retention, reversal and extinction of a spatial discrimination in an 8-arm radial maze. Initial acquisition induced an immediate (30 s) and long-lasting (approx. 3 h) increase in mean hippocampal (+33%) and cortical (+23%) cholinergic activities. The time course of this activation was structure-dependent and correlations of hippocampal-cortical cholinergic activities showed large and consistent alterations as a function of time after training. Cholinergic activation in both brain regions was observed immediately following each daily training session with amplitudes which did not vary significantly in spite of a progressive daily increment in performance. Following acquisition mice were tested for retention, reversal and extinction: 30 s following the retention session, cholinergic activation was observed in both cortex and hippocampus, with magnitudes similar to those observed at the end of acquisition. However, in the reversal and extinction groups, a treatment-dependent attenuation of cholinergic activation was observed which was accompanied by a significant loss of correlation of cholinergic activity between these two brain regions. The results are discussed in relation to the concepts of reference and working memory and also to novelty, stress, arousal and frustrative non-reward. The data constitute direct experimental evidence for a differential involvement of cholinergic septohippocampal and NBM-cortical projections in learning and memory processes.

New perspectives in basal forebrain organization of special relevance for neuropsychiatric disorders: the striatopallidal, amygdaloid, and corticopetal components of substantia innominata

The basal forebrain is critically involved in functions representing the highest levels of integration. Only recently has a relatively clear anatomical picture of this important area begun to emerge. The territory that has generally been referred to as the "substantia innominata" appears to be composed of portions of three recognizable forebrain structures: the ventral striatopallidal system, the extended amygdala and the magnocellular corticopetal system. (1) Rostrally, the striatopallidal system reaches ventrally to the base of the brain. (2) Caudal to the ventral extension of the striatopallidal system elements of the centromedial amygdala and bed nucleus of the stria terminalis are merged so that these two areas together with this subpallidal corridor form a large forebrain unit that might be described as an "extended amygdala". (3) Large cholinergic and non-cholinergic corticopetal neurons form a more or less continuous aggregate that is interwoven with the striatopallidal and extended amygdala systems in basal forebrain. Consideration of morphological and connectional characteristics of basal forebrain suggests that the corticopetal cell groups, together with magnocellular elements of the striatum, serve similar functional roles for the striatopallidal system, the extended amygdala, and the septal-diagonal band complex. Specifically, the output of medium spiny neurons in striatum, extended amygdala, and lateral septum are directed toward somewhat larger sparsely or moderately spiny neurons with radiating dendrites which in turn project to diencephalon and brainstem or provide either local feedback (e.g. in striatum) or distal feedback to cortex. The functional implications of this parallel processing of descending forebrain afferents are discussed.

Age-related disruption of classical conditioning: a model systems approach to memory disorders

The model systems approach to the neurobiology of memory involves studying a well characterized learned response in a relatively simple and well controlled preparation. The best characterized mammalian model system is classical conditioning of the rabbit's eyeblink response. Using this preparation, significant progress has been made toward understanding the neurobiological systems and mechanisms involved in elaboration of the conditioned response. Using a well characterized model system such as classical eyeblink conditioning, it should be possible to both characterize the changes in learning and memory that accompany aging and to investigate their neural substrates. Our strategy for using the conditioned eyeblink preparation for studying age-related memory deficits is four-fold and includes investigating conditioning deficits in: (1) humans across the life span, (2) rabbits across the life span, (3) Alzheimer's disease patients, and (4) rabbits with aluminum-induced neurofibrillary degeneration. In this paper, we present exemplary data from each of these lines of research. If similar deficits occur in each of these groups, it may be possible to begin to form hypotheses about the neurobiology of age-related memory disorders.

Use of tissue culture models to study neuronal regulatory trophic and toxic factors in the aged brain

Dementia is believed to result from the loss of selective neurons within the brain, but approaches for systematic study of that degenerative process are hampered by the complexity of the neuronal milieu. Tissue culture models provide a means to reduce dramatically the variables inherent in the study of neuronal plasticity. Three levels of complexity can be described: cellular and molecular diversity; primary and secondary interconnections; and finally, the dynamics influenced by age. The following review discusses the advantages and disadvantages of tissue culture models for the detailed study of neuronal trophic and toxic factors. Our selection of factors is broadened to include ions, intermediate metabolites, antioxidants, steroids, neuropeptides, gangliosides, metals, neurotransmitters, brain extracts, and protein molecules. Most of these factors have been shown to be altered in the aged brain, to have a significant effect on cultured neurons, or both. This multilevel analysis provides the reader with an overview of the events regulating neuronal survival, differentiation and death. An understanding of these basic questions is necessary to sequence the molecular events resulting in neuronal death.

Rat beta-nerve growth factor sequence and site of synthesis in the adult hippocampus

A rat beta-nerve growth factor (NGF) genomic sequence encoding the entire 3' exon of preproNGF was cloned, and its nucleotide sequence was determined. Rat NGF shows very high homology with other known NGFs in both the prepropeptide and the 3' untranslated regions. The presumptive signal sequence, the cysteine residues important for tertiary structure, possible glycosylation sites, and dibasic amino acids required for proteolytic cleavage to mature NGF are conserved across species. Comparison of the hydrophobicity plots and amino acid sequences revealed an evolutionary divergent domain on the external surface of NGF, which may account for the poor immunologic crossreactivities of the various NGFs. In situ hybridization with a rat-specific oligodeoxynucleotide indicated high levels of NGF mRNA synthesis in both hippocampal granule and pyramidal cell layers. These results are consistent with one role for NGF in the CNS as a neuronally released, retrogradely transported neurotrophic factor for basal forebrain cholinergic neurons.

Scopolamine in rats impairs acquisition but not retention in a 14-unit T-maze

To follow up a previous report noting that scopolamine impaired acquisition performance of young rats in a shock-motivated 14-unit T-maze, the present study assessed the effects of muscarinic antagonism on retention aspects of the same task. The broader objective was to further the investigation of possible defects in cholinergic neurotransmission that might underlie the age-related impairments previously observed in this task. Young (3-month) male F-344 rats were given preliminary training to criterion in one-way active avoidance in a straight runway. Then on the first day of complex maze training, each rat received 5 acquisition (AQ) trials followed by a second 10-trial retention (RET) session conducted the following day. Subjects were assigned to one of eight groups receiving an intraperitoneal injection of either scopolamine hydrochloride (1.0 mg/kg) or saline as follows: (a) 30 min prior to training on the first day (PRE-AQ); (b) 30 min prior to training on both the first and second day (PRE-AQ-RET); (c) immediately after completing the trial on the first day (POST-AQ); (d) 30 min prior to testing on the second day (PRE-RET). Dependent measures included errors, alternation errors, run time, number of shocks, and total shock received. On the first day of maze training, all performance measures except for alternation errors were significantly higher for the two acquisition groups (PRE-AQ and PRE-AQ-RET) compared to all other groups which did not differ significantly.(ABSTRACT TRUNCATED AT 250 WORDS)

Alleviation of scopolamine amnesia by different retrieval enhancing treatments

Mice were trained in a one-way active avoidance task to a criterion of 9/10 avoidances. Immediately following training they were injected with scopolamine hydrochloride (1 mg/kg SC) or with saline. Retention was assessed 3 days after training by 5 test trials on which the UCS was not present. Thirty min prior to the test, groups were injected with different doses of arecoline, d-amphetamine sulphate or with saline. Other scopolamine-treated mice were exposed to the CS or the UCS 24 hr prior to the test. The scopolamine-induced amnesia was attenuated by both 0.5 and 1.0 mg/kg arecoline and by 2.0 mg/kg d-amphetamine. Retention was also improved by exposure to the CS and the UCS. These data show that scopolamine amnesia can be alleviated by treatments which activate retrieval processes.

High density of benzodiazepine binding sites in the substantia innominata of the rat

In order to study the neuronal basis of the pharmacological interactions between benzodiazepine receptor ligands and cortical cholinergic turnover, we examined the regional distribution of specific benzodiazepine binding sites using in vitro autoradiography. In the basal forebrain, the substantia innominata contained a high density of [3H]lormetazepam (LMZ) binding sites (Bmax = 277 fmol/mg tissue; Kd = 0.55 nM). The label could be displaced by diazepam (IC50 = 100 nM), the benzodiazepine receptor antagonist beta-carboline ZK 93426 (45 nM) and the partial inverse agonist beta-carboline FG 7142 (540 nM). It is hypothesized that the amnesic effects of benzodiazepine receptor agonists are exerted through benzodiazepine receptors which are situated on cholinergic neurons in the substantia innominata and are involved in a tonic inhibition of cortical acetylcholine release. The benzodiazepine receptor antagonist ZK 93426 may exert its nootropic effects via benzodiazepine receptors in the substantia innominata and, consequently, by disinhibiting cortical acetylcholine release.

GABAergic basal forebrain neurons project to the neocortex: the localization of glutamic acid decarboxylase and choline acetyltransferase in feline corticopetal neurons

Our objective was to determine whether GABAergic and cholinergic basal forebrain neurons project to the neocortex. The retrograde connectivity marker wheat germ agglutinin lectin-bound horseradish peroxidase was injected into the neocortex of adult cats. Histo- and immunohistochemical methods were combined to label sequentially connectivity and transmitter markers (glutamic acid decarboxylase; choline acetyltransferase) in forebrain neurons. The labels of each marker were identified by correlative light and electron microscopy. Two principal types of doubly labeled neurons were demonstrated. The connectivity marker was colocalized with glutamic acid decarboxylase or choline acetyltransferase. The neurons were located in the basal forebrain. Their ultrastructural, cellular, and regional organization supported 2 conclusions. (1) GABAergic basal forebrain neurons project to the neocortex. This is important new morphological evidence for the origin of inhibitory neocortical afferents from a subcortical brain site. (2) The GABAergic and cholinergic basal forebrain neurons projecting to the neocortex exhibit remarkable structural similarities. The transmitter diversity of these intertwined neocortical afferents may be significant for the pathology and treatment of human neurological disorders such as Alzheimer's disease.

Combined cholinergic and serotonergic denervation of the forebrain produces severe deficits in a spatial learning task in the rat

The purpose of the present experiments was to study the effects of a combined cholinergic and serotonergic denervation of the rat forebrain on spatial learning using the Morris water maze task. Experiment 1 compared the acute effects of a radiofrequency lesion of the septum, an intraventricular 5,7-dihydroxytryptamine (5,7-DHT) lesion, and a combined septal plus 5,7-DHT lesion. Although the 5,7-DHT lesion alone did not produce any significant deficits in the water maze task, the lesion greatly potentiated the learning impairments produced by the septal lesion. Thus, the rats with both lesions combined showed severe difficulties in finding the platform and they did not develop any place navigational search strategy. This effect was not dependent on any effect on swimming ability or locomotor activity. The long-term effects of the combined septal and 5,7-DHT lesion was investigated in experiment 2, where the rats were tested in the water maze both 5 and 24-25 weeks after surgery. In this experiment, the rats showed the same severe deficits in spatial learning in both tests, showing that the impairments remain for long periods and after extended training. The results show that a combination of a cholinergic and a serotonergic denervation of the rat forebrain produces pronounced impairments in spatial learning in the Morris water maze task, and that this effect is long-lasting. This indicates that the recently proposed serotonergic deficit in patients with Alzheimer's disease may contribute importantly to the cognitive disabilities in these patients.

Distribution and some projections of cholinergic neurons in the brain of the common marmoset, Callithrix jacchus

The distribution of choline acetyltransferase-immunoreactive (ChAT-IR) neurons was studied in the brain of the common marmoset by using immunohistochemistry. ChAT-IR neurons were found in the medial septal nucleus, vertical and horizontal limb nuclei of the diagonal band, the nucleus basalis of Meynert, pedunculopontine nucleus and laterodorsal tegmental nucleus, and also in the striatum, habenula, and brainstem cranial nerve motor nuclei. The organization of ChAT-IR neurons in the basal forebrain, midbrain, and pons is consistent with the Ch1-Ch6 nomenclature introduced by Mesulam et al. ('83). The combination of the retrograde transport of HRP-WGA with ChAT immunohistochemistry revealed the distribution of neurons in the Ch4 cell group projecting to the dorsolateral prefrontal cortex. The activity of ChAT was highest in limbic cortical structures, such as the hippocampus, and lowest in association areas of the neocortex. Lesions at various loci in the basal forebrain resulted in differential patterns of ChAT loss in the cortex, which suggests some degree of topographical organization of Ch4 projections to the cortical mantle.

Unilateral infusion of GABA and saline into the nucleus basalis of rats: 1. Effects on motor function and brain morphology

Motor behavior was investigated in rats following acute and chronic gamma-aminobutyric acid (GABA) microinfusions into the nucleus basalis. For acute treatment, the rats received GABA (100 micrograms in 1 microliter), then saline, or these solutions in the reverse order, into the nucleus basalis contralateral to their preferred turning direction in a radial maze. For chronic treatment, half the rats received saline (1 microliter/h for 4 days), and than GABA (100 micrograms/microliters/h) for the same period of time ('saline-first' group). In the other half, this sequence was reversed ('GABA-first' group). Acute microinjections of GABA decreased turning towards the non-injected side; chronic treatment enhanced this effect by reversing the preferred turning direction. Return to initial turning direction was observed after acute GABA-injection in both experimental groups, but only in the 'saline-first' group after chronic treatment. The 'GABA-first' group showed gliosis in and around the nucleus basalis area and a reduction of cortical acetyl-cholinesterase-positivity which were significantly greater than in the 'saline-first' group. This, chronic saline pretreatment is associated with diminished neurotoxicity of chronic GABA infusion and with a reversibility of the drug-induced behavioral effects.

Chronic treatment with choline or scopolamine indicates the presence of muscarinic cholinergic receptor plasticity in the frontal cortex of young but not of aged mice

Chronic treatment (2 weeks) with either scopolamine (4 mg/kg, once daily p.o.) or choline (200 mg/kg, once daily p.o.) resulted in a pronounced muscarinic cholinergic receptor up- or down-regulation in the frontal cortex of young (4 weeks) but not of aged (18 months) female mice. It is speculated that a similar age-related decline of muscarinic receptor plasticity might contribute to the profound dysfunction of cholinergic neurotransmission in Alzheimer's disease.

Radial-arm maze deficits produced by colchicine administered into the area of the nucleus basalis are ameliorated by cholinergic agents

Rats were given bilateral injections of colchicine into the area of the nucleus basalis. Colchicine produced dose-dependent alterations in the acquisition of a food-reinforced working-memory task. Colchicine-induced deficits in maze performance were attenuated by cholinergic agents, including physostigmine, RS-86 (2-ethyl-8-methyl-2,8-diazospiro-(4,5)-decan-1,3-dione-hydro bromide) and nicotine. Naloxone and vasopressin did not affect radial-arm maze performance of colchicine-treated rats. Subsequent neurochemical analysis showed that colchicine decreased choline acetyltransferase (ChAT) activity and levels of norepinephrine, dopamine, 3,4-dihydroxyphenylacetic acid, serotonin and 5-hydroxyindoleacetic acid in the neocortex. However, ChAT activity and other neurochemical measures were not altered in the hippocampus or corpus striatum. Histological assessment indicated damage limited to the injection in the area of the nucleus basalis and enlarged cerebrolateral ventricles. These data suggest the possible utility of the colchicine model in the study of cognitive deficits associated with neurodegenerative diseases.

Piracetam elevates muscarinic cholinergic receptor density in the frontal cortex of aged but not of young mice

Chronic treatment (2 weeks) with piracetam (500 mg/kg, once daily PO) elevated m-cholinoceptor density in the frontal cortex of aged (18 months) female mice by about 30-40%, but had no effect on m-cholinoceptor density in the frontal cortex of young (4 weeks) mice. The effect of piracetam on m-cholinoceptor density as determined by the specific binding of tritiated QNB was not affected by concomitant daily treatment with either choline (200 mg/kg) or scopolamine (4 mg/kg). It is concluded that the effect of piracetam on m-cholinoceptor density could explain the positive effects which have been reported for combinations of cholinergic precursor treatment with piracetam on memory and other cognitive functions in aged experimental animals and patients and could also represent part of the possible mechanism of action of piracetam alone.

Muscarinic agonists modulate spontaneous and evoked unit discharge in auditory cortex of cat

The present experiments studied the effects of cholinergic agonists and antagonists on the spontaneous and acoustic-evoked discharge of auditory cortical neurons and examined whether these effects were mediated by muscarinic cholinergic receptors. A primary focus of this report is the analysis of specific effects of these agents on the spontaneous and tone-evoked discharge and on different temporal components of the evoked discharge. Single neurons were recorded in the auditory cortex of chronically prepared, awake cats with multibarrel micropipette electrodes. The responses to acoustic stimuli were obtained before, during, and following continuous ejection of cholinergic agonist or antagonists by micropressure. The mean rate of discharge of the neurons was analyzed quantitatively for spontaneous discharge and for different peaks of the tone-evoked PSTH corresponding to tone "on," "through," and "off" responses. Acetylcholine (ACh) and acetyl-beta-methacholine (MCh) produced significant effects on spontaneous activity in 72% and 68% of neurons tested, respectively. Tone-evoked responses were effected in 92% and 82% of cells tested, respectively. The ability of these agonists to modify spontaneous or evoked activity was dose-dependent. Agonist effects on spontaneous and evoked activity were often different in the same cell; however, effects on spontaneous activity did predict effects on "through" responses. The most common effect of ACh or MCh on evoked activity was facilitation of the tone "on" response. For neurons with multicomponent discharge patterns in response to tones, the agonists had nonuniform effects on different response components. However, the effects of ACh on the "on" and "off" responses covaried. Hence cholinergic agonists produce heterogeneous, selective effects on different components of the responses of auditory cortical neurons rather than simple increases or decreases in discharge level. The effects of cholinergic agonists were modified in the presence of atropine. The effects of MCh were blocked by atropine in a higher proportion of cases than those of ACh.

Effects of scopolamine on performance of rats in a delayed-response radial maze task

The effects of the cholinergic blocker scopolamine on performance of rats in an 8-arm radial maze were studied. In Experiment 1, rats received injections of scopolamine-HBr (0.2 mg/kg, IP) or saline, 20 min before a trial. The drug impaired performance only when midtrial delays were introduced (10 sec, 5, 20, 60 or 120 min), during which the animals were removed from the apparatus, and to a similar extent at all delays. Injection of scopolamine directly after choice 4 in a 20 min delayed trial affected performance at a dose of 0.4 mg/kg, but not at 0.2 mg/kg. In Experiment 2, the rats were kept in the apparatus during the delay. There was a significant effect of scopolamine at 0.1 and at 0.2 mg/kg, initially irrespective of the length of the delay (10 sec, 2.5 or 5 min). After considerable training, administration of scopolamine (0.2 mg/kg) had no significant effect at the 10 sec delay but did impair performance after longer (5 or 10 min) midtrial intervals. These results suggest that scopolamine has differential effects, depending on dose and degree of training, and that an effect on memory storage may be one of them.

Beta-carbolines as tools in memory research: animal data and speculations

Benzodiazepines induce in animals, as in humans, almost exclusively anterograde amnesia. The mechanism of this effect is still unsettled; however, explanations like state dependency which may be based on sedative or emotional properties of benzodiazepines are usually favoured in contrast to an interpretation in terms of true amnesia. It is proposed that by the use of beta-carbolines with agonist, partial agonist, antagonist and partial inverse agonist properties, the nature of the amnesia induced by benzodiazepine receptor agonists may be characterised. From a series of experiments it is concluded that the major reason for benzodiazepine-induced amnesia might be an impaired ability to filter interfering stimuli; that is, an attentional deficit. Since the antagonist beta-carbolines may play a key role in providing evidence as to the GABAergic involvement in cognitive processes, the pharmacological profile of ZK 93426 is presented. The results of the interaction of beta-carbolines with scopolamine will provide a basis on which to speculate on the GABAergic control of cholinergic neurotransmission and its therapeutic implications.