Near-total loss of 'learning' and 'memory' as a result of combined cholinergic and serotonergic blockade in the rat

Cognition and serotonin in Parkinson's disease

Cognitive impairment affects up to 80% of patients with Parkinson's disease (PD) and is associated with poor quality of life. PD cognitive dysfunction includes poor working memory, impairments in executive function and difficulty in set-shifting. The pathophysiology underlying cognitive impairment in PD is still poorly understood, but there is evidence to support involvements of the cholinergic, dopaminergic, and noradrenergic systems. Only rivastigmine, an acetyl- and butyrylcholinesterase inhibitor, is efficacious for the treatment of PD dementia, which limits management of cognitive impairment in PD. Whereas the role of the serotonergic system in PD cognition is less understood, through its interactions with other neurotransmitters systems, namely, the cholinergic system, it may be implicated in cognitive processes. In this chapter, we provide an overview of the pharmacological, clinical and pathological evidence that implicates the serotonergic system in mediating cognition in PD.

Alterations and interactions of subcortical modulatory systems in Alzheimer's disease

The pathogenesis of Alzheimer's disease (AD) is not fully understood. Here we summarize current knowledge on the involvement of the serotonergic, noradrenergic, dopaminergic, cholinergic, and opioid systems in AD, emphasizing the importance of interactions between the serotonergic and the other subcortical modulatory systems during the progression of AD. In physiological conditions, all neurotransmitter systems function in concert and are interdependent at both the neuroanatomical and molecular levels. Through their early involvement in AD, cognitive and behavioral abilities that rely on their interactions also become disrupted. Considering that serotonin (5HT) regulates the release of noradrenaline (NA), dopamine (DA) and acetylcholine (ACh), any alteration in 5HT levels leads to disturbance of NA, DA, and ACh homeostasis in the brain. One of the earliest pathological changes during the prodromal phase of AD is a decrease of serotonergic transmission throughout the brain, with serotonergic receptors being also affected. Additionally, serotonergic and noradrenergic as well as serotonergic and dopaminergic nuclei are reciprocally interconnected. As the serotonergic dorsal raphe nucleus (DRN) is affected by pathological changes early in AD, and the noradrenergic locus coeruleus (LC) and dopaminergic ventral tegmental area (VTA) exhibit AD-related pathological changes, their connectivity also becomes altered in AD. Such disrupted interactions among neurotransmitter systems in AD can be used in the development of multi-target drugs. Some of the potential AD therapeutics (such as ASS234, RS67333, tropisetron) target multiple neurotransmitter systems to achieve the best possible improvement of cognitive and behavioral deficits observed in AD. Here, we review how serotonergic system interacts with other subcortical modulatory systems (noradrenergic, dopaminergic, cholinergic, and opioid systems) during AD.

Monoaminergic neuropathology in Alzheimer's disease

None of the proposed mechanisms of Alzheimer's disease (AD) fully explains the distribution patterns of the neuropathological changes at the cellular and regional levels, and their clinical correlates. One aspect of this problem lies in the complex genetic, epigenetic, and environmental landscape of AD: early-onset AD is often familial with autosomal dominant inheritance, while the vast majority of AD cases are late-onset, with the ε4 variant of the gene encoding apolipoprotein E (APOE) known to confer a 5-20 fold increased risk with partial penetrance. Mechanisms by which genetic variants and environmental factors influence the development of AD pathological changes, especially neurofibrillary degeneration, are not yet known. Here we review current knowledge of the involvement of the monoaminergic systems in AD. The changes in the serotonergic, noradrenergic, dopaminergic, histaminergic, and melatonergic systems in AD are briefly described. We also summarize the possibilities for monoamine-based treatment in AD. Besides neuropathologic AD criteria that include the noradrenergic locus coeruleus (LC), special emphasis is given to the serotonergic dorsal raphe nucleus (DRN). Both of these brainstem nuclei are among the first to be affected by tau protein abnormalities in the course of sporadic AD, causing behavioral and cognitive symptoms of variable severity. The possibility that most of the tangle-bearing neurons of the LC and DRN may release amyloid β as well as soluble monomeric or oligomeric tau protein trans-synaptically by their diffuse projections to the cerebral cortex emphasizes their selective vulnerability and warrants further investigations of the monoaminergic systems in AD.

New perspectives on the auditory cortex: learning and memory

Primary ("early") sensory cortices have been viewed as stimulus analyzers devoid of function in learning, memory, and cognition. However, studies combining sensory neurophysiology and learning protocols have revealed that associative learning systematically modifies the encoding of stimulus dimensions in the primary auditory cortex (A1) to accentuate behaviorally important sounds. This "representational plasticity" (RP) is manifest at different levels. The sensitivity and selectivity of signal tones increase near threshold, tuning above threshold shifts toward the frequency of acoustic signals, and their area of representation can increase within the tonotopic map of A1. The magnitude of area gain encodes the level of behavioral stimulus importance and serves as a substrate of memory strength. RP has the same characteristics as behavioral memory: it is associative, specific, develops rapidly, consolidates, and can last indefinitely. Pairing tone with stimulation of the cholinergic nucleus basalis induces RP and implants specific behavioral memory, while directly increasing the representational area of a tone in A1 produces matching behavioral memory. Thus, RP satisfies key criteria for serving as a substrate of auditory memory. The findings suggest a basis for posttraumatic stress disorder in abnormally augmented cortical representations and emphasize the need for a new model of the cerebral cortex.

Neuroanatomical organization of the cholinergic system in the central nervous system of a basal actinopterygian fish, the senegal bichir Polypterus senegalus

Polypterid bony fishes are believed to be basal to other living ray-finned fishes, and their brain organization is therefore critical in providing information as to primitive neural characters that existed in the earliest ray-finned fishes. The cholinergic system has been characterized in more advanced ray-finned fishes, but not in polypterids. In order to establish which cholinergic neural centers characterized the earliest ray-finned fishes, the distribution of choline acetyltransferase (ChAT) is described in Polypterus and compared with the distribution of this molecule in other ray-finned fishes. Cell groups immunoreactive for ChAT were observed in the hypothalamus, the habenula, the optic tectum, the isthmus, the cranial motor nuclei, and the spinal motor column. Cholinergic fibers were observed in both the telencephalic pallium and the subpallium, in the thalamus and pretectum, in the optic tectum and torus semicircularis, in the mesencephalic tegmentum, in the cerebellar crest, in the solitary nucleus, and in the dorsal column nuclei. Comparison of the data within a segmental neuromeric context indicates that the cholinergic system in polypterid fishes is generally similar to that in other ray-finned fishes, but cholinergic-positive neurons in the pallium and subpallium, and in the thalamus and cerebellum, of teleosts appear to have evolved following the separation of polypterids and other ray-finned fishes.

Memory disorders associated with consumption of drugs: updating through a case/noncase study in the French PharmacoVigilance Database

AIMS

To investigate putative associations of reports of memory disorders and suspected drugs.

METHODS

We used the case/noncase method in the French PharmacoVigilance Database (FPVD). Cases were reports of memory loss in the FPVD between January 2000 and December 2009. Noncases were all other reports during the same period. To assess the association between memory impairment and drug intake, we calculated an odds ratio with its 95% confidence interval.

RESULTS

Among the 188,284 adverse drug reactions recorded, we identified 519 cases of memory loss. The sex ratio was 0.6 and the median age was 54 years (range 4-93). The maximal number of cases occurred between 40-49 and 50-59 years. Evolution was favourable in 63% of the cases. We found significant odds ratios for benzodiazepines (alprazolam, bromazepam, prazepam, clonazepam etc.), benzodiazepine-like hypnotics (zolpidem and zopiclone), antidepressants (fluoxetine, paroxetine and venlafaxine), analgesics (morphine, nefopam and tramadol), anticonvulsants (topiramate, pregabalin, levetiracetam etc.), antipsychotics (aripiprazole and lithium) and other drugs, such as trihexyphenidyl, ciclosporin and isotretinoin.

CONCLUSIONS

Our study confirmed an association between memory disorders and some drugs, such as benzodiazepines and anticonvulsants. However, other drugs, such as benzodiazepine-like hypnotics, newer anticonvulsants, serotonin reuptake inhibitor antidepressants, isotretinoin and ciclosporin were significantly associated with memory disorders, although this was not described or poorly described in the literature. Taking account of the limits of this study in the FPVD (under-reporting, notoriety bias etc.), the case/noncase method allows assessment and detection of associations between exposure to drugs and a specific adverse drug reaction, such as memory disorders, and could thus generate signals and orientate us to further prospective studies to confirm such associations.

The neuropharmacology of implicit learning

Two decades of pharmacologic research on the human capacity to implicitly acquire knowledge as well as cognitive skills and procedures have yielded surprisingly few conclusive insights. We review the empirical literature of the neuropharmacology of implicit learning. We evaluate the findings in the context of relevant computational models related to neurotransmittors such as dopamine, serotonin, acetylcholine and noradrenalin. These include models for reinforcement learning, sequence production, and categorization. We conclude, based on the reviewed literature, that one can predict improved implicit acquisition by moderately elevated dopamine levels and impaired implicit acquisition by moderately decreased dopamine levels. These effects are most prominent in the dorsal striatum. This is supported by a range of behavioral tasks in the empirical literature. Similar predictions can be made for serotonin, although there is yet a lack of support in the literature for serotonin involvement in classical implicit learning tasks. There is currently a lack of evidence for a role of the noradrenergic and cholinergic systems in implicit and related forms of learning. GABA modulators, including benzodiazepines, seem to affect implicit learning in a complex manner and further research is needed. Finally, we identify allosteric AMPA receptors modulators as a potentially interesting target for future investigation of the neuropharmacology of procedural and implicit learning.

Antidepressants are a rational complementary therapy for the treatment of Alzheimer's disease

There is a high prevalence rate (30-50%) of Alzheimer's disease (AD) and depression comorbidity. Depression can be a risk factor for the development of AD or it can be developed secondary to the neurodegenerative process. There are numerous documented diagnosis and treatment challenges for the patients who suffer comorbidity between these two diseases. Meta analysis studies have provided evidence for the safety and efficacy of antidepressants in treatment of depression in AD patients. Preclinical and clinical studies show the positive role of chronic administration of selective serotonin reuptake inhibitor (SSRI) antidepressants in hindering the progression of the AD and improving patient performance. A number of clinical studies suggest a beneficial role of combinatorial therapies that pair antidepressants with FDA approved AD drugs. Preclinical studies also demonstrate a favorable effect of natural antidepressants for AD patients. Based on the preclinical studies there are a number of plausible antidepressants effects that may modulate the progression of AD. These effects include an increase in neurogenesis, improvement in learning and memory, elevation in the levels of neurotrophic factors and pCREB and a reduction of amyloid peptide burden. Based on this preclinical and clinical evidence, antidepressants represent a rational complimentary strategy for the treatment of AD patients with depression comorbidity.

Nuclear organization of cholinergic, putative catecholaminergic and serotonergic nuclei in the brain of the eastern rock elephant shrew, Elephantulus myurus

The organization of the nuclear subdivisions of the cholinergic, putative catecholaminergic and serotonergic systems of the brain of the elephant shrew (Elephantulus myurus) were determined following immunohistochemistry for choline acetyltransferase, tyrosine hydroxylase and serotonin, respectively. This was done in order to determine if differences in the nuclear organization of these systems in comparison to other mammals were evident and how any noted differences may relate to specialized behaviours of the elephant shrew. The elephant shrew belongs to the order Macroscelidea, and forms part of the Afrotherian mammalian cohort. In general, the organization of the nuclei of these systems resembled that described in other mammalian species. The cholinergic system showed many features in common with that seen in the rock hyrax, rodents and primates; however, specific differences include: (1) cholinergic neurons were observed in the superior and inferior colliculi, as well as the cochlear nuclei; (2) cholinergic neurons were not observed in the anterior nuclei of the dorsal thalamus as seen in the rock hyrax; and (3) cholinergic parvocellular nerve cells forming subdivisions of the laterodorsal and pedunculopontine tegmental nuclei were not observed at the midbrain/pons interface as seen in the rock hyrax. The organization of the putative catecholaminergic system was very similar to that seen in the rock hyrax and rodents except for the lack of the rodent specific C3 nucleus, the dorsal division of the anterior hypothalamic group (A15d) and the compact division of the locus coeruleus (A6c). The nuclear organization of the serotonergic system was identical to that seen in all eutherian mammals studied to date. The additional cholinergic neurons found in the cochlear nucleus and colliculi may relate to a specific acoustic signalling system observed in elephant shrews expressed when the animals are under stress or detect a predator. These neurons may then function to increase attention to this type of acoustic signal termed foot drumming.

Cholinergic modulation of the cerebral metabolic response to citalopram in Alzheimer's disease

Pre-clinical and human neuropharmacological evidence suggests a role of cholinergic modulation of monoamines as a pathophysiological and therapeutic mechanism in Alzheimer's disease. The present study measured the effects of treatment with the cholinesterase inhibitor and nicotinic receptor modulator, galantamine, on the cerebral metabolic response to the selective serotonin reuptake inhibitor, citalopram. Seven probable Alzheimer's disease patients and seven demographically comparable controls underwent two positron emission tomography (PET) glucose metabolism scans, after administration of a saline placebo infusion (Day 1) and after citalopram (40 mg, IV, Day 2). The scan protocol was repeated in the Alzheimer's disease patients 2 months after titration to a 24 mg galantamine dose. At baseline, cerebral glucose metabolism was reduced in Alzheimer's disease patients relative to controls in right middle temporal, left posterior cingulate and parietal cortices (precuneus and inferior parietal lobule), as expected. Both groups demonstrated acute decreases in cerebral glucose metabolism after citalopram to a greater extent in the Alzheimer's disease patients. In the patients, relative to the controls, citalopram decreased glucose metabolism to a greater extent in middle frontal gyrus (bilaterally), left middle temporal gyrus and right posterior cingulate prior to treatment. Galantamine treatment alone increased metabolism in the right precuneus, right inferior parietal lobule and right middle occipital gyrus. In contrast, during galantamine treatment, citalopram increased metabolism in the right middle frontal gyrus, right post-central gyrus, right superior and middle temporal gyrus and right cerebellum. The combined cerebral metabolic effects of galantamine and citalopram suggest, consistent with preclinical data, a synergistic interaction of cholinergic and serotonergic systems.

Safety pharmacology assessment of central nervous system function in juvenile and adult rats: effects of pharmacological reference compounds

INTRODUCTION

Recent EU/US pediatric legislation and FDA/EMEA guidelines recognize the potential differences in safety profiles of drugs in adults versus young patients. Hence safety studies are recommended to investigate key functional domains of e.g. the developing CNS.

METHODS

Selected psychoactive stimulants (caffeine, d-amphetamine, scopolamine) and depressants (baclofen, diazepam, haloperidol, chlorpromazine, imipramine, morphine) were characterized upon single administration with regard to behavioural parameters, locomotor activity, body temperature, pro-/anti-convulsive activity (pentylenetetrazole, PTZ), and nocifensive responses (hotplate) in neonatal (2 weeks), juvenile (4 weeks) and adult rats (8-9 weeks).

RESULTS

In vehicle-treated rats, behavioural patterns matured with age, locomotor activity and handling-induced rise in body temperature were enhanced, whereas PTZ convulsion threshold dose and nocifensive response latency decreased. Single test compound treatment elicited behavioural effects characteristic for psychoactive drugs with stimulating and depressing properties regardless of age. However, incidence of certain behaviours, and magnitude of effects on locomotor activity and body temperature varied with age and became generally more pronounced in adult rats. Pro-/anti-convulsive effects and delayed nocifensive responses did not differ between juvenile and adult rats.

CONCLUSION

CNS effects of selected psychoactive reference compounds were qualitatively similar, but quantitatively different in neonatal, juvenile and adult rats.

Combined but not individual administration of beta-adrenergic and serotonergic antagonists impairs water maze acquisition in the rat

This study examined the effects of serotonergic depletion and beta-adrenergic antagonism on performance in both visible platform and hidden platform versions of the water maze task. Male Long-Evans rats received systemic injections of p-chlorophenylalanine (500 mg/kg x 2) to deplete serotonin, or propranolol (20 or 40 mg/kg) to antagonize beta-adrenergic receptors. Some rats received treatments in combination. To separate strategies learning from spatial learning, half of the rats underwent Morris' water maze strategies pretraining before drug administration and spatial training. Individual depletion of serotonin or antagonism of beta-adrenergic receptors caused few or no impairments in either naive or pretrained rats in either version of the task. In contrast, combined depletion of serotonin and antagonism of beta-adrenergic receptors impaired naive rats in the visible platform task and impaired both naive and strategies-pretrained rats in the hidden platform task, and also caused sensorimotor impairments. This is the first finding of a 'global' water maze task/sensorimotor impairment with combined administration of two agents that, at the high doses that were given individually, produced few or no impairments. The data imply that (1) serotonergic and beta-adrenergic systems may interact in a manner that is important for adaptive behavior; (2) impairments in these systems found in Alzheimer patients may be important for their cognitive and behavioral impairments; and (3) the approach used here can model aspects of the cognitive and behavioral impairments in Alzheimer disease.

Antidepressant drugs and memory: insights from animal studies

This is a selective review of the literature concerning the effects of antidepressant drugs on animal memory, which was performed with the aid of the PubMed database. Monoamine oxidase inhibitors tend to either have no effect on memory or result in its improvement. Studies with cyclic antidepressants have reported no effect or, more often, memory impairments. Pre-training administration of selective serotonin reuptake inhibitors (SSRIs) has been shown to have either no effect on memory or undermine it (with some isolated exceptions, in which improvements have been recorded), while post-training administration of SSRIs has been demonstrated to improve memory or have no effect. A small group formed by the remaining antidepressants has been shown to improve memory, with the exception of trazodone, which impairs memory. These findings are discussed in the light of knowledge regarding the actions of antidepressants on several neurotransmission systems. The possibility that the effects of antidepressants on memory are the core of the therapeutic effects of these drugs is also considered.

Elicited hippocampal theta rhythm: a screen for anxiolytic and procognitive drugs through changes in hippocampal function?

Hippocampal damage produces cognitive deficits similar to dementia and changes in emotional and motivated reactions similar to anxiolytic drugs. The gross electrical activity of the hippocampus contains a marked 'theta rhythm'. This is a relatively high voltage sinusoidal waveform, resulting from synchronous phasic firing of cells, variation in which correlates with behavioural state. Like the hippocampus, theta has been linked to both cognitive and emotional functions. Critically, it has recently been shown that restoration of theta-like rhythmicity can restore lost cognitive function. We review the effects of systemic administration of drugs on hippocampal theta elicited by stimulation of the reticular formation. We conclude that reductions in the frequency of reticular-elicited theta provide what is currently the best in-vivo means of detecting antianxiety drugs. We also suggest that increases in the power of reticular-elicited theta could detect drugs useful in the treatment of disorders, such as dementia, that involve memory loss. We argue that these functionally distinct effects should be seen as indirect and that each results from a change in a single form of cognitive-emotional processing that particularly involves the hippocampus.

Effect of selective cholinergic denervation on the serotonergic system: implications for learning and memory

The cholinergic system has been widely implicated in cognitive processes and cholinergic loss is a classical hallmark in Alzheimer disease. Increasing evidence supports a role of the serotonergic system in cognition, possibly through a modulation of cholinergic activity. We compared selective cholinergic denervation by administration of the immunotoxin 192 IgG-saporin in the nucleus basalis of Meynert (NBM) with intracerebroventricular (ICV) lesions of the basal forebrain in male rats 7 days after lesioning. NBM lesions induced significant changes in cholinergic markers in the frontal cortex, whereas ICV lesions produced significant decreases in cholinergic markers both in the frontal cortex and hippocampus. Only ICV lesions lead to memory impairments in passive avoidance and Morris water maze tasks. Both models lead to reductions of serotonin levels in the frontal cortex. Similar changes in 5-hydroxytriptophan levels were observed, suggesting a downregulation of the rate-limiting enzyme for the synthesis of serotonin along with the cholinergic deficit. Neither 5-HT1A nor 5-HT1B receptors seem to mediate this process. These data imply that the serotonergic system in the frontal cortex can compensate for diminished cholinergic function and support the investigation of the serotonergic system as a therapeutic target to treat Alzheimer disease.

Evidence for the involvement of central serotonergic mechanisms in cholinergic tremor induced by tacrine in Balb/c mice

Tacrine is a potent and reversible inhibitor of acetylcholinesterase (AChE) in the brain. It produces tremor in animals, which is believed to be due to an increase in the brain acetylcholine level following AChE inhibition. The present study was undertaken to investigate the involvement, if any, of biogenic amines in the genesis of this motor dysfunction. Administration of tacrine (10-20 mg/kg, i.p.) produced dose- and time-dependent tremor in Balb/c mice. While in vivo inhibition of striatal AChE activity was observed only for the highest dose of tacrine, a dose-dependent increase in striatal choline acetyltransferase activity was obtained. Serotonin (5-HT) levels, as assayed following a sensitive HPLC-electrochemical procedure, were significantly increased in nucleus caudatus putamen, nucleus accumbens, substantia nigra, nucleus raphe dorsalis, olivary nucleus and the cerebellum. However, dopamine or norepinephrine levels remained unaltered in these areas of the brain. In animals treated with p-chlorophenylalanine, a specific tryptophan hydroxylase inhibitor and 5-HT depletor, tacrine failed to elevate the levels of 5-HT in the brain regions, and significantly attenuated tremor response to the drug. Tacrine-induced tremor was also significantly (83%) attenuated by 5-HT(2A/2C) receptor antagonist mianserin (5 mg/kg, i.p.), but methysergide (5 mg/kg, i.v.) could block tacrine-induced tremor only by 20%. Atropine (5 mg/kg, i.p.) antagonized tacrine-induced tremor by about 53%, but a combination of atropine and mianserin completely blocked the tremor response. These results indicate that the cholinergic tremor produced by tacrine in Balb/c mice is mediated via central serotonergic mechanisms, and stimulation of 5-HT(2A/2C) receptors plays a pivotal role in this motor dysfunction.

Effects of selegiline alone or with donepezil on memory impairment in rats

Selegiline, a monoamine oxidase-B inhibitor, is reported to improve memory and learning in dementia of Alzheimer's type. However, only a few studies have reported its use in animal models. Here, we evaluated the effects of selegiline only or its combined use with donepezil, a selective acetylcholinesterase inhibitor on memory impairment, using a Morris water maze. Selegiline dose-dependently attenuated ethylcholine aziridinium ion-induced memory impairment. Co-administration of selegiline and donepezil, at doses that do not exert efficacy individually, significantly ameliorated scopolamine+p-chlorophenylalanine-induced memory deficits. These results suggest that selegiline improves memory impairment mediated by the cholinergic system, and provide evidence of the usefulness of co-treatment with selegiline and donepezil for treating spatial deficits in dementia.

Effects of MDMA administration on scopolamine-induced disruptions of learning and performance in rats

Functional deficits following short-course high-dose administration of 3,4-methylenedioxymethamphetamine (MDMA) have been difficult to characterize despite evidence indicating that MDMA is neurotoxic in several species. Therefore, the present research used rats trained to respond under a complex behavioral procedure (i.e., a multiple schedule of repeated acquisition and performance of response chains), pharmacological challenge with scopolamine and neurotransmitter assays to examine the effects of MDMA neurotoxicity on learning. Prior to MDMA administration, 0.032-0.32 mg/kg of scopolamine produced dose-dependent rate-decreasing and error-increasing effects in both components of the multiple schedule. Administration of 10 mg/kg of MDMA twice per day for 4 days also produced rate-decreasing and error-increasing effects on these days, but responding returned to baseline levels several days after the final injection. In contrast to the recovery of responding, this regimen of MDMA in untrained rats significantly reduced levels of both serotonin and its major metabolite, 5-hydroxyindoleacetic acid (5-HIAA), for 13-14 days. Furthermore, the rate-decreasing and error-increasing effects of scopolamine were significantly attenuated after MDMA treatment. These results indicate that certain complex operant behaviors rapidly recover from the effects of short-course high-dose MDMA administration, despite the reduced levels of serotonin in the central nervous system (CNS), and that this MDMA-induced loss of serotonin may affect cholinergic transmission.

Cholinergic-serotonergic imbalance contributes to cognitive and behavioral symptoms in Alzheimer's disease

Neuropsychiatric symptoms seen in Alzheimer's disease (AD) are not simply a consequence of neurodegeneration, but probably result from differential neurotransmitter alterations, which some patients are more at risk of than others. Therefore, the hypothesis of this study is that an imbalance between the cholinergic and serotonergic systems is related to cognitive symptoms and psychological syndromes of dementia (BPSD) in patients with AD. Cholinergic and serotonergic functions were assessed in post-mortem frontal and temporal cortex from 22 AD patients who had been prospectively assessed with the Mini-Mental State examination (MMSE) for cognitive impairment and with the Present Behavioral Examination (PBE) for BPSD including aggressive behavior, overactivity, depression and psychosis. Not only cholinergic deficits, but also the cholinacetyltransferase/serotonin ratio significantly correlated with final MMSE score both in frontal and temporal cortex. In addition, decreases in cholinergic function correlated with the aggressive behavior factor, supporting a dual role for the cholinergic system in cognitive and non-cognitive disturbances associated to AD. The serotonergic system showed a significant correlation with overactivity and psychosis. The ratio of serotonin to acetylcholinesterase levels was also correlated with the psychotic factor at least in women. It is concluded that an imbalance between cholinergic-serotonergic systems may be responsible for the cognitive impairment associated to AD. Moreover, the major findings of this study are the relationships between neurochemical markers of both cholinergic and serotonergic systems and non-cognitive behavioral disturbances in patients with dementia.

Combined beta-adrenergic and cholinergic antagonism produces behavioral and cognitive impairments in the water maze: implications for Alzheimer disease and pharmacotherapy with beta-adrenergic antagonists

This study examined the effects of beta-adrenergic and muscarinic blockade on spatial learning and strategy use in the water maze. Male Long-Evans rats received systemic injections of propranolol (PRO; 10 or 20 mg/kg) or scopolamine (SCO; 0.3 or 1.0 mg/kg) either singly or in combination. To separate strategies learning from spatial learning approximately half of the rats underwent water maze strategies pretraining prior to drug administration and spatial training. PRO did not impair performance in any group. SCO impaired naive but not pretrained rats. PRO and SCO given together in high doses impaired all aspects of behavior in both naive and pretrained rats, and caused sensorimotor disturbances in some groups. PRO (10 mg/kg) and SCO (0.3 mg/kg) together caused a specific spatial reversal learning impairment in pretrained rats without causing strategies impairments or sensorimotor disturbances. Nadolol administered with SCO failed to produce the same impairments as PRO, suggesting that PRO produced its effects by acting on central nervous system sites. These results point to a greater than additive impairing effect of PRO and SCO on adaptive behavior, and a specific role for beta-adrenergic and cholinergic systems working in conjunction in spatial learning. They also suggest that some of the behavioral and cognitive impairments seen in Alzheimer patients or patients receiving pharmacotherapy with beta-adrenergic antagonists in which cholinergic activity is also compromised may result from the combined impairment of beta-adrenergic and cholinergic systems.

Cholinergic and serotonergic neocortical projection lesions given singly or in combination cause only mild impairments on tests of skilled movement in rats: evaluation of a model of dementia

The cholinergic (ACh) projections of the nucleus basalis and the serotonergic (5-HT) projections of the raphe nuclei to the neocortex are required for the normal function of the neocortex. Nevertheless, damage to either system alone has little effect on the behavior of rats, but conjoint damage to both systems is reported to produce dementia to the point that animals are described as being unable to engage in intelligent behavior. Because rats with bilateral damage to both systems are so severely impaired, they are not useful for chronic studies. The objective of the present research was to determine whether unilateral depletions produce a functional impairment. Rats received unilateral neurotoxic lesions to either the nucleus basalis (quisqualic acid), or the medial forebrain bundle (5,7-dihydroxytryptamine), or both, which reduced neocortical levels of ACh (55%) and 5-HT (63%). The rats then received a battery of tests sensitive to unilateral neocortical injury. The 5-HT lesion produced no quantitative or qualitative deficits on reaching for food, walking across a horizontal ladder, forelimb placement in a cylinder, sensory detection of adhesive paper applied to the wrists, or forelimb inhibition during swimming. The ACh lesion produced mild qualitative deficits in reaching. Combined lesions produced mild deficits in skilled reaching, ladder walking, and sensory detection. In contrast to the mild impairments produced by the lesions, pharmacological blockade of either ACh with atropine or 5-HT with methiothepin mesylate systemically blocked skilled motor behavior as assessed by skilled reaching. The results are discussed in relation to the problems associated with the development of a unilateral model of dementia.

Neural Transplantation in Animal Models of Dementia

Neural transplantation provides a powerful novel technique for investigating the neurobiological basis and potential strategies for repair of a variety of neurodegenerative conditions. The present review considers applications of this technique to dementia. After a general introduction (section 1), attempts to replace damaged neural systems by transplantation are considered in the context of distinct animal models of dementia. These include grafting into aged animals (section 2), into animals with neurotransmitter-selective lesions of subcortical nuclei, in particular involving basal forebrain cholinergic systems (section 3), and into animals with non-specific lesions of neocortical and hippocampal systems (section 4). The next section considers the alternative use of grafts as a source of growth/trophic factors to inhibit degeneration and promote regeneration in the aged brain (section 5). Finally, a number of recent studies have employed transplanted tissues to model and study the neurodegenerative processes associated with ageing and Alzheimer's disease taking place within the transplant itself (section 6). It is concluded (section 7) that although neural transplantation does not offer any immediate prospect of therapeutic repair in clinical dementia, the technique does offer a powerful neurobiological tool for studying the neuropathological processes involved in both spontaneous degeneration and specific diseases of ageing. New understandings derived from neural transplantation may be expected to lead to rational development of novel strategies to inhibit the neurodegenerative process and to promote regeneration in the aged brain.

5,7-DHT-induced hippocampal 5-HT depletion attenuates behavioural deficits produced by 192 IgG-saporin lesions of septal cholinergic neurons in the rat

Adult Long-Evans male rats sustained injections of 5,7-dihydroxytryptamine into the fimbria-fornix (2.5 microg/side) and the cingular bundle (1.5 microg/side) and/or to intraseptal injections of 192 IgG-saporin (0.4 microg/side) in order to deprive the hippocampus of its serotonergic and cholinergic innervations, respectively. Sham-operated rats were used as controls. The rats were tested for locomotor activity (postoperative days 18, 42 and 65), spontaneous T-maze alternation (days 20-29), beam-walking sensorimotor (days 34-38), water maze (days 53-64) and radial maze (days 80-133) performances. The cholinergic lesions, which decreased the hippocampal concentration of ACh by about 65%, induced nocturnal hyperlocomotion, reduced T-maze alternation, impaired reference-memory in the water maze and working-memory in the radial maze, but had no effect on beam-walking scores and working-memory in the water maze. The serotonergic lesions, which decreased the serotonergic innervation of the hippocampus by about 55%, failed to induce any behavioural deficit. In the group of rats given combined lesions, all deficits produced by the cholinergic lesions were observed, but the nocturnal hyperlocomotion and the working-memory deficits in the radial maze were attenuated significantly. These results suggest that attenuation of the serotonergic tone in the hippocampus may compensate for some dysfunctions subsequent to the loss of cholinergic hippocampal inputs. This observation is in close concordance with data showing that a reduction of the serotonergic tone, by pharmacological activation of somatodendritic 5-HT(1A) receptors on raphe neurons, attenuates the cognitive disturbances produced by the intrahippocampal infusion of the antimuscarinic drug, scopolamine. This work has been presented previously [Serotonin Club/Brain Research Bulletin conference, Serotonin: From Molecule to the Clinic (satellite to the Society for Neuroscience Meeting, New Orleans, USA, November 2-3, 2000)].

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

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

Effects of cyclazocine and scopolamine on swim-to-platform performance in rats

DL-Cyclazocine (0.5-2.0 mg/kg, i.p.) produced no impairment in rats' acquisition and retention of the behavior of swimming to a large visible platform in a water tank. However, cyclazocine produced a significant enhancement or potentiation of the impairment in swim-to-platform behavior produced by scopolamine. Since cyclazocine has previously been shown to abolish serotonin-dependent electrocortical activation (enabling it, in combination with central muscarinic blockade, to block all cortical activation), the results lend further support to the hypothesis that blockade of electrocortical activation produces dementia rather than sleep or coma as was previously believed.

Hyperactivity and intact hippocampus-dependent learning in mice lacking the M1 muscarinic acetylcholine receptor

Members of the muscarinic acetylcholine receptor family are thought to play key roles in the regulation of a large number of important functions of the CNS. However, the precise roles of the individual muscarinic receptor subtypes in modulating these processes are not well understood at present, primarily because of the lack of ligands with sufficient receptor subtype selectivity. To investigate the behavioral significance of the M(1) muscarinic receptor (M(1)R), which is abundantly expressed in the forebrain, we subjected M(1) receptor-deficient mice (M(1)R(-/-) mice) to a battery of behavioral tests. M(1)R(-/-) mice showed no significant impairments in neurological reflexes, motor coordination, pain sensitivity, and prepulse inhibition. Strikingly, however, M(1)R(-/-) mice consistently exhibited a pronounced increase in locomotor activity in various tests, including open field, elevated plus maze, and light/dark transition tests. Moreover, M(1)R(-/-) mice showed reduced immobilization in the Porsolt forced swim test and reduced levels of freezing after inescapable footshocks, suggesting that M(1)R(-/-) mice are hyperactive under stressful conditions as well. An increased number of social contacts was observed in a social interaction test. Surprisingly, M(1)R(-/-) mice displayed no significant cognitive impairments in the Morris water maze and in contextual fear conditioning. M(1)R(-/-) mice showed slight performance deficits in auditory-cued fear conditioning and in an eight-arm radial maze, most likely because of the hyperactivity phenotype displayed by the M(1)R(-/-) mice. Our results indicate that M(1) muscarinic receptors play an important role in the regulation of locomotor activity but appear to be less critical for cognitive processes, as generally assumed.

Crossed unilateral lesions of the medial forebrain bundle and either inferior temporal or frontal cortex impair object-reward association learning in Rhesus monkeys

In an accompanying paper we showed that combined transection of the fornix, amygdala and temporal stem in monkeys produced dense amnesia, including an impairment in visual object-reward association learning. We proposed that this combined surgical section had its effect by isolating temporal cortex from the ascending projections of the basal forebrain and midbrain structures. To test this hypothesis, in the present experiment we disconnected the inferior temporal cortex from these basal forebrain and midbrain structures, while sparing cortical white matter, by crossed unilateral lesions of the medial forebrain bundle in one hemisphere and inferior temporal cortex in the opposite hemisphere. The aim of the medial forebrain bundle lesion was to section axons of cells, both those that project to the cortex via the medial forebrain bundle, and those which control the activity of these same structures. A single unilateral lesion alone had no effect on the ability to learn and remember visual object-reward associations, but the crossed unilateral lesions produced an impairment in this task which was equal in severity to the impairment seen earlier after bilateral section of the fornix, amygdala and temporal stem. The impairment was not an effect of interrupting fibres to the cortex from the ventromedial hypothalamus, or of unilateral sensory neglect. This supports the hypothesis that these midbrain and basal forebrain afferents to the inferior temporal cortex are important for new visual learning. Furthermore, an impairment of equal severity was demonstrated in a separate group of animals that received crossed unilateral lesions of the medial forebrain bundle in one hemisphere and of the frontal cortex in the opposite hemisphere. We propose that the frontal cortex acts to modulate basal forebrain activity which in turn reinforces object representations in the inferior temporal cortex during learning.

Deficits in allothetic and idiothetic spatial behavior in rats with posterior cingulate cortex lesions

The cingulate cortex plays a central role in bridging neocortical and limbic structures involved in allothetic navigation, a form of navigation requiring the use of external cues. Animals can also navigate using idiothetic cues, which are cues generated by self-movement, but there have been no definitive tests of whether cingulate cortex also plays a role in idiothetic navigation. Rats with anterior cingulate (medial frontal) and posterior cingulate cortex (retrosplenial) suction ablations were trained to search for large food pellets on an open table, and the accuracy with which they returned home with the food was measured. In the idiothetic task they searched for food from a novel starting location under infrared light, and with surface olfactory cues displaced. The rats also received two tests of allothetic navigation. They were tested on a matching-to-place task in which they foraged for food from a number of successively presented new locations under normal room light, and they were trained to locate a hidden platform in a swimming pool (Morris place task). The group with posterior cingulate cortex lesions was severely impaired on all of the navigation tasks whereas the group with anterior cingulate cortex lesions displayed no deficit on the idiothetic task and only moderate deficits on the other tasks. The results demonstrate a role for posterior cingulate region in idiothetic navigation.

Individual and combined manipulation of muscarinic, NMDA, and benzodiazepine receptor activity in the water maze task: implications for a rat model of Alzheimer dementia

Recent evidence indicates that Alzheimer disease typically involves different degrees of impairment in a variety of neurotransmitter systems, behaviors, and cognitive abilities in different patients. To investigate the relations between neurotransmitter system, behavioral, and cognitive impairments in an animal model of Alzheimer disease we studied spatial learning in a Morris water maze in male Long-Evans rats given neurochemical agents that targeted muscarinic cholinergic, NMDA, or benzodiazepine systems. Naive rats given a single agent or a combination of agents were severely impaired in place responding and had behavioral strategy impairments. Rats made familiar with the required water maze behavioral strategies by non-spatial pretraining performed as well as controls if given a single agent. Non-spatially pretrained rats with manipulation of both muscarinic cholinergic and NMDA or muscarinic cholinergic and benzodiazepine systems had a specific place response impairment but no behavioral strategy impairments. The results suggest that impairment of both muscarinic cholinergic and NMDA, or muscarinic cholinergic and benzodiazepine systems may model some aspects of human Alzheimer disease (impairments in navigation in familiar environments), but not other aspects of this disorder (global dementia leading to general loss of adaptive behavior). Previous research suggests that impairment of both muscarinic cholinergic and serotonergic systems may provide a better model of global dementia. The water maze testing and detailed behavioral analysis techniques used here appear to provide a means of investigating the contributions of various combinations of neurotransmitter system impairments to an animal model of Alzheimer disease.

Are neocortical gamma waves related to consciousness?

Previous research has shown that neocortical gamma waves (approximately 30-80 Hz) are continuously present during low voltage fast neocortical activity (LVFA) occurring during waking or active sleep. Gamma waves occur in a burst-suppression pattern in association with large amplitude slow waves during quiet sleep or anesthesia. The present experiments show that continuous gamma activity is also present in rats during LVFA occurring during surgical anesthesia (with ether, isoflurane or urethane) and that a burst-suppression pattern of gamma activity occurs during large amplitude slow waves occurring in the waking state either spontaneously in undrugged rats or as a result of treatment with parachlorophenylalanine and scopolamine. The amplitude of gamma activity occurring during anesthesia is variable but is often greater than it is in the normal waking state. It is concluded that the pattern of neocortical gamma wave activity is strongly related to the presence or absence of large amplitude slow waves but is quite independent of the state of behavioral arousal. Whether or not gamma wave activity is related to subjective awareness is a very difficult question which cannot be answered with certainty at the present time.

Behavioral effects of anti-muscarinic, anti-serotonergic, and anti-NMDA treatments: hippocampal and neocortical slow wave electrophysiology predict the effects on grooming in the rat

Previous research has shown that hippocampal and neocortical activation accompanies the postural changes occurring during self-grooming in rats but is absent or reduced during the stereotyped components of grooming, including head-washing and licking or biting. Since electrocortical activation is dependent on ascending cholinergic and serotonergic projections, we hypothesized that central muscarinic and serotonergic blockade would disrupt grooming by degrading cerebral control of changes in posture. Consistent with this, we find that systemic injections of scopolamine: (a) markedly reduce the occurrence of adaptive changes in posture during grooming; (b) reduce the probability of transitions from head-washing to body grooming; (c) reduce both the probability and duration of sequences of body grooming; and (d) do not affect the duration of head-washing or the probability of transitions from washing the snout to washing over the top of the head. Destruction of central serotonergic neurons with intracerebral injections of 5,7-dihydroxytryptamine increases the tendency of scopolamine to shorten the duration and increase the number of separate sequences of grooming. Systemic injections of a NMDA antagonist (CGS 19755) also impair grooming behavior. The data show that blockade of muscarinic and glutamatergic transmission impairs instinctive behavior as well as learned behavior and that the behavioral effects of muscarinic and serotonergic blockade are consistent with data obtained from the study of cortical slow wave electrophysiology.

Serotonergic input to cholinergic neurons in the substantia innominata and nucleus basalis magnocellularis in the rat

The aim of the present study was to determine, at the light microscopic level, whether the serotonergic fibers originating from the dorsal raphe nucleus (B7), median raphe nucleus (B8) and ventral tegmentum (B9) make putative synaptic contacts with cholinergic neurons of the nucleus basalis magnocellularis and substantia innominata. For this purpose, we utilized: (i) the anterograde transport of Phaseolus vulgaris leucoagglutinin combined with choline acetyltransferase immunohistochemistry; (ii) choline acetyltransferase/tryptophan hydroxylase double immunohistochemistry; and (iii) the FluoroGold retrograde tracer technique combined with tryptophan hydroxylase immunohistochemistry. Following iontophoretic injections of Phaseolus vulgaris leucoagglutinin in the dorsal raphe nucleus, labeling was observed primarily in the ventral aspects of the nucleus basalis magnocellularis and in the intermediate region of the substantia innominata. When Phaseolus vulgaris leucoagglutinin was combined with choline acetyltransferase immunohistochemistry, a close association between the Phaseolus vulgaris leucoagglutinin-positive fibers and cholinergic neurons was observed, even though the majority of the Phaseolus vulgaris leucoagglutinin-immunoreactive terminals seemed to establish contact with non-cholinergic elements. Following Phaseolus vulgaris leucoagglutinin injection in the median raphe nucleus, very few labeled fibers with no evident close contact with nucleus basalis magnocellularis and substantia innominata cholinergic neurons were observed. After tryptophan hydroxylase/choline acetyltransferase double immunohistochemistry, a plexus of serotonergic (tryptophan hydroxylase-positive) fibers in the vicinity of choline acetyltransferase-immunoreactive neurons of the substantia innominata and nucleus basalis magnocellularis was observed, and some serotonergic terminals have been shown to come into very close contact with the cholinergic cells. Most of the tryptophan hydroxylase-immunoreactive terminals seem to establish contacts with non-cholinergic cells. Following FluoroGold injection in the nucleus basalis magnocellularis and substantia innominata, the majority of retrogradely labeled neurons was observed mainly in the ventromedial cell group of the dorsal raphe nucleus. In this area, a minority of the FluoroGold-positive neurons was tryptophan hydroxylase immunoreactive. These findings show that serotonergic terminals, identified in very close association with the cholinergic neurons in the substantia innominata and nucleus basalis magnocellularis, derive primarily from the B7 serotonergic cell group of the dorsal raphe nucleus, and provide the neuroanatomical evidence for a direct functional interaction between these two neurotransmitter systems in the basal forebrain.

Serotonin and acetylcholine release response in the rat hippocampus during a spatial memory task

By using in vivo microdialysis we monitored the extracellular levels of acetylcholine and serotonin in the hippocampus of rats performing a spatial memory task. After rats were trained for 10 consecutive days to master a food-reinforced radial-arm maze task, they were implanted with a microdialysis probe in the dorsal hippocampus. On day 12, rats were tested in the maze and acetylcholine and serotonin outputs were monitored before the test, during the waiting phase and while performing the trials. In trained, food-rewarded rats, hippocampal acetylcholine levels increased during the waiting period (181 +/- 90 of baseline) and further increased during the radial-maze performance to 236 +/- 13% of baseline values, while serotonin levels did not change during the waiting period but increased to 142 +/- 3% during the maze performance. To discriminate whether the increase of acetylcholine and serotonin levels during the testing was associated with memory performance or with food consumption, we monitored hippocampal acetylcholine and serotonin release in rats that were trained, but not food rewarded, or in rats that were not trained, but rewarded only on the test day. In the trained, non-rewarded group, acetylcholine release increased during the waiting phase to 168 +/- 6%, but did not increase further during the task performance. In contrast, no change in serotonin release was observed in this group in any phase of the test. In rats which were not trained, but food rewarded, acetylcholine increased only during the maze period (150 +/- 5%). Serotonin increased gradually and become significant at the end of the trials. (130 +/- 3%). While both neurotransmitters could be implicated in feeding behaviour, only activation of cholinergic neurotransmission appears to be associated with memory function. Our results support the following hypotheses: (i) hippocampal acetylcholine could be involved in attentional and cognitive functions underlying motivational processes; (ii) serotonin could be implicated in non-cognitive processes (i.e. in the control of motor and feeding behaviour). Since serotonin and acetylcholine neurotransmission is simultaneously activated during the spatial memory task, this suggests that these neurotransmitter systems regulate behavioural and cognitive functions.

Rats show unimpaired learning within minutes after recovery from single bolus propofol anesthesia

PURPOSE

To examine the learning ability of rats shortly after recovery from a bolus dose of propofol by assessing learning on a swim-to-platform task. Also, muscarinic blockade was used as a pharmacological test of whether learning shortly after propofol anesthesia resembles normal learning.

METHODS

Propofol anesthetized rats (15-20 mg x kg(-1) i.v.) were trained on a swim-to-platform task five to seven minutes after recovering from surgical anesthesia and tested two to three hours later In addition, the muscarinic antagonist scopolamine hydrobromide (5 mg x kg(-1) s.c.) was given to a subgroup of rats before testing. During 10 trials, the number of times a given rat took 10 sec or longer to locate and climb onto a visible platform was tabulated and counted as errors.

RESULTS

When trained shortly after recovery from the anesthetic, propofol anesthetized rats made 3.2 +/- 0.4 compared with 1.0 +/- 0.1 errors in controls (P < 0.0001). Two to three hours later both groups performed equally well. Rats trained after propofol anesthesia and given scopolamine before testing made 0.7 +/- 0.5 errors and performed as well as normal controls, 1.2 +/- 0.2 errors when subjected to the same procedures without propofol anesthesia, and better than scopolamine-treated untrained rats, 5.5 +/- 0.7 errors, (P < 0.05).

CONCLUSION

Training five to seven minutes after recovery from propofol anesthesia resulted in normal retention of the swim- to-platform task. It also produced the same resistance to the disruptive effects of scopolamine as did training in rats that were not anesthetized. Thus, the ability to learn recovers rapidly after propofol anesthesia induced by a single intravenous bolus dose.

Pre- or post-training injection of buspirone impaired retention in the inhibitory avoidance task: involvement of amygdala 5-HT1A receptors

The present study investigated the effect of buspirone on memory formation in an aversive learning task. Male Wistar rats were trained on the inhibitory avoidance task and tested for retention 1 day after training. They received peripheral or intra-amygdala administration of buspirone or other 5-HT1A drugs either before or after training. Results indicated that pretraining systemic injections of buspirone caused a dose-dependent retention deficit; 5. 0 mg/kg had a marked effect and 1.0 mg/kg had no effect. Post-training injections of the drug caused a time-dependent retention deficit, which was not due to a state-dependent effect on retrieval. When training in the inhibitory avoidance task was divided into a context-training phase and a shock-training phase, buspirone impaired retention only when administered in the shock-training phase, suggesting that the drug influenced memory processing of affective events. Further results indicated that post-training intra-amygdala infusion of buspirone or the 5-HT1A agonist 8-hydroxy-di-n-propylaminotetralin (8-OH-DPAT) caused a time-dependent and dose-dependent retention deficit. Post-training intra-amygdala infusion of the 5-HT1A antagonist WAY100635 (N-(2-(4-(2-methoxyphenyl)-1-piperazinyl)-N-(2-pyridyl) cyclohexane carboxamine maleate) attenuated the memory-impairing effects of buspirone. These findings suggest that buspirone may modulate memory storage processes in the inhibitory avoidance task through an action on amygdaloid 5-HT1A receptors.

Effect of Choto-san, a kampo medicine, on impairment of passive avoidance performance in mice

In mice with procephalic ischaemia loading, disrupted passive avoidance retention performance was dose-dependently improved by Choto-san, but this effect was antagonized by NAN-190, a serotonin1A receptor antagonist. In mice with decreased intracerebral serotonin concentration, Choto-san prevented disturbance in acquiring a passive avoidance response after scopolamine administration, but did not influence the decreased serotonin concentration. These results suggested that Choto-san showed the anti-amnestic effect based on the stimulation of serotonin1A receptors.

The influence of neonatal nutrition on behavioral development: a critical appraisal

Specific nutrients appear to modify the metabolism of neurotransmitters, which are endogenous regulators of neurogenesis, neural migration, and synaptogenesis during both embryonic and early postnatal life. This has led to the question of whether, by affecting neurotransmission, malnutrition during the early neonatal period affects behavioral development. The literature based on animal models suggests that nutrient deficiencies during early life influence neurotransmission and, in some instances, also affect behavioral outcomes. A clear answer to the question, however, remains elusive. This can be attributed to the complexity of the process of brain development, where changes at a cellular level may not necessarily translate into changes at a behavioral level. Future investigations in this important area of research should work toward refinement of the design of behavioral experiments so that these studies can contribute to the understanding of the putative mechanisms involved.

Systemic administration of atipamezole, a selective antagonist of alpha-2 adrenoceptors, facilitates behavioural activity but does not influence short-term or long-term memory in trimethyltin-intoxicated and control rats

The present study used trimethyltin (TMT)-intoxicated rats as a model for the behavioural syndrome seen after neuronal damage to the limbic system. Behavioural assessments indicated increased locomotor activity and reduced number of groomings in an open-arena task in TMT-intoxicated (6.6 mg/kg as a free base) rats, as has been found previously. A novel finding was the severe deficit in swimming to a visible platform in the water maze task, with reduced swimming speed at the beginning of the training period. During the reacquisition phase of a radial arm maze task, TMT-intoxicated rats made more short-term and long-term memory errors, and their behavioural activity was increased in comparison with controls. The administration of atipamezole (300 micrograms/kg), a selective antagonist of alpha 2-adrenoceptors, enhanced locomotor activity compared to saline-treated rats, but these effects did not differ between the TMT group and their controls. Atipamezole did not enhance short-term or long-term memory in either TMT or control groups. Taken together, the present data indicate that TMT intoxication is a model for global dementia rather than for a specific loss of relational memory. Previous studies on the neurochemical effects of TMT and the alleviation or prevention of neurotoxicity of TMT are reviewed.

A structural basis for memory storage in mammals

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

The role of the dorsal raphe-serotonergic system and cholinergic receptors in the modulation of working memory

The present study investigated the role of the dorsal raphe-serotonergic system and its interaction with muscarinic or nicotinic receptors in the modulation of working memory and motor activity by assessing the effects of serotonin lesion with pCA and cholinergic receptor blockade on the performance of rats in a working memory (delayed non-matching to position, DNMTP) task. The pCA lesion did not impair the choice accuracy or motor activity of rats in the DNMTP-task. The lower dose of scopolamine (0.075 mg/kg) impaired percent correct responses already at the shortest delay which is not indicative of a working memory impairment per se. Scopolamine also disrupted motor activity markedly. The effects of scopolamine 0.075 mg/kg on the choice accuracy were aggravated by pCA treatment. Furthermore, the effects of N-methylscopolamine (0.150 mg/kg) were comparable with scopolamine. The higher dose of mecamylamine (3.0 mg/kg) also interfered with motor activity and it decreased the choice accuracy. The performance disruption induced by mecamylamine was not as severe as that seen with scopolamine. Mecamylamine did not reveal any interaction with the serotonergic lesion. Hexamethonium slightly decreased the percent correct responses, while not interfering with motor activity of rats. The present results suggest that: (i) lesion of serotonergic fibers with pCA does not significantly impair the choice accuracy or interfere with motor activity of rats; (ii) the blockade of cholinergic receptors does not impair working memory per se, but disrupts motor activity, and (iii) pCA lesion of serotonergic fibers aggravates the non-mnemonic choice accuracy impairment induced by central muscarinic blockade, while not interacting with the cholinolytics in modulation of motor activity.

Stimulation of 5-HT1A receptors in the dorsal raphe reverses the impairment of spatial learning caused by intrahippocampal scopolamine in rats

This study investigated the effect of stimulating 5-HT1A receptors in the dorsal raphe on the impairment of learning caused by 4 microg/microL scopolamine injected in the CA1 region of the dorsal hippocampus in rats performing a two-platform spatial discrimination task. At 1 (but not 0.2) microg/0.5 microL administered in the dorsal raphe on each acquisition training day 5 min before bilateral intrahippocampal injection of 4 microg/microL scopolamine, 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT), a 5-HT1A receptor agonist, had no effect on choice accuracy and latency or errors of omission but completely antagonized the impairment of choice accuracy by intrahippocampal scopolamine. Administered into the dorsal raphe at 0.2 and 1 microg/0.5 microL, WAY 100635, a 5-HT1A receptor antagonist, had no effect on rats' performance or on the impairment caused by intrahippocampal scopolamine but dose-dependently antagonized the effect of 1 microg/0.5 microL 8-OH-DPAT on the scopolamine-induced deficit. The results show that stimulation of presynaptic 5-HT1A receptors in the dorsal raphe reverses the deficit caused by intrahippocampal scopolamine, probably by facilitating the transfer of facilitatory information from the entorhinal cortex to the hippocampus. Together with a previous study showing that blockade of postsynaptic hippocampal 5-HT1A receptors antagonized the effect of intrahippocampal scopolamine in the two-platform spatial discrimination task (Carli et al., 1995b), the results suggest that drugs with presynaptic stimulatory and postsynaptic blocking actions on 5-HT1A receptors, such as partial agonists at these receptors, may be useful in the symptomatic treatment of human memory disturbances associated with loss of cholinergic innervation to the hippocampus.

The effect of nonspatial water maze pretraining in rats subjected to serotonin depletion and muscarinic receptor antagonism: a detailed behavioural assessment of spatial performance

A detailed behavioural analysis of water maze spatial performance in the rat was utilized to determine the effect of single and combined administration of p-chlorophenylalanine (PCPA; 1000 mg/kg, i.p.), an inhibitor of serotonin biosynthesis, and scopolamine hydrobromide (SCO; 1.0 mg/kg, i.p.), a muscarinic receptor antagonist. In some groups a water maze pretraining regimen known as non-spatial pretraining (NSP) was used to familiarize the animals with the general requirements of the task before spatial training was begun. The results showed that: (a) depletion of serotonin with PCPA had no effect on water maze performance and produced no sensorimotor disturbances; (b) antagonism of muscarinic receptors produced impairments in spatial and sensorimotor function in naive rats but neither effect was observed in rats first given NSP; (c) combined disruption of muscarinic and serotonergic function produced a severe deficit in spatial performance that was only partially alleviated by NSP; and (d) there was an association between poor maze acquisition scores and a high incidence of sensorimotor dysfunction. In addition to the water maze task the rats were also assessed for motoric performance on a beam walking test. The role of cholinergic and serotonergic systems in learning and memory is discussed.

Prenatal nicotine exposure affects the development of the central serotonergic system as well as the dopaminergic system in rat offspring: involvement of route of drug administrations

The present study was undertaken to examine the effects of prenatal nicotine exposure by two different routes of drug administration, injection and infusion, on the development of monoaminergic systems and open field behavior in the neonatal and juvenile rat. The nicotine administration to pregnant Sprague-Dawley rats was carried out by subcutaneous injection (3 mg/kg twice daily) or infusion via implanted osmotic minipumps (6 mg/kg/day) from gestational day 4 (GD4) until GD20. At postnatal day 7 (PD7), 15 and 22, the contents of the neurotransmitters and their metabolites including noradrenaline (NA), dopamine (DA), dihydroxyphenylacetic acid (DOPAC), homovanilic acid (HVA), serotonin (5-HT) and 5-hydroxy-3-indolacetic acid (5-HIAA) were measured in the midbrain+pons - medulla (M + P - M), forebrain and cerebellum. Prenatal nicotine exposure caused a persistent reduction of DA turnover in the forebrain at PD15 and PD22. In addition, the 5-HT system was also affected by prenatal nicotine, and reductions of 5-HT turnover in the M + P - M at PD15 and in the forebrain and the cerebellum at PD22 were found. Although there was no effect of prenatal nicotine on NE contents, the involvement of this system remains uncertain since we measured only NE contents without metabolites. In the present study, we also found significant route-related changes in the contents of the monoamines and metabolites in the NA, DA and 5-HT systems in all brain regions in rat offspring besides the effects of prenatal nicotine. In addition, the difference in administration route reflected the results of the open field test and the number of ambulations in the injection-group was less than that in the infusion-groups with no relation to nicotine administration. Therefore, such effects of "prenatal stress" accompanied by drug administration are not negligible in considering the risk assessment of prenatal nicotine exposure.

The decrease of serotonin release induced by a tryptophan-free amino acid diet does not affect spatial and passive avoidance learning

We assessed whether consumption of a diet lacking in tryptophan (TRP) resulted in alteration in learning and memory performance and hippocampal 5-HT release in rats. Two hours after the acute administration of TRP-free (T) and balanced (B) diet rats were trained in a one-trial passive avoidance task. The two groups of rats showed no significant difference in retention latencies. Two other groups of rats, fed with the above diets during the acquisition of a radial-arm maze task, showed no difference in baseline performance. The acute ingestion of the T diet produced a significant and long lasting decrease of hippocampal and cortical 5-HT release in rats when compared to the B diet, while the 12th day of the T diet, 5-HT was not detectable in the dialysate. These data indicate that the diminished brain release of 5-HT induced by a T diet is not sufficient to impair cognitive processes.

Strain-dependent effects of D2 dopaminergic and muscarinic-cholinergic agonists and antagonists on memory consolidation processes in mice

The interaction between muscarinic-cholinergic and dopaminergic systems in the modulation of memory storage of Y-maze discrimination (YMD) task was examined in C57BL/6 and DBA/2 strains of mice. In C57BL/6 mice, post-training systemic (i.p.) administration of the D2-agonist quinpirole facilitated retention and the D2-antagonist (-)-sulpiride impaired retention. Opposite effects were observed in DBA/2 strain. The facilitating or impairing effects of quinpirole and (-)-sulpiride were blocked by simultaneous post-training administration of muscarinic-cholinergic agonists and antagonists. The memory enhancing effects of the cholinergic agonist oxotremorine were not blocked by simultaneous administration of sulpiride in C57BL/6 mice or quinpirole in DBA/2 mice. Furthermore, the memory impairing effects of the cholinergic antagonist atropine were not blocked by simultaneous administration of quinpirole in C57BL/6 mice or sulpiride in DBA/2 mice. These findings indicate that the effects of D2-receptor agonists and antagonists on retention of YMD task are strain-dependent and mediated through muscarinic-cholinergic mechanisms.