The role of interactions between the cholinergic system and other neuromodulatory systems in learning and memory

Chronic propranolol induces deficits in retention but not acquisition performance in the water maze in mice

Agents that alter adrenergic receptors, such as "beta-blockers," also alter memory storage. However, reports suggest that beta-adrenergic receptor antagonists, such as propranolol, have conflicting behavioral effects with acute vs chronic dosing. This study was designed to evaluate the effects of chronic propranolol on retention for a spatial learning task. Adult male ICR mice were given daily injections of propranolol (2, 4, 8, or 12 mg/kg ip) or 0. 9% NaCl for 15 days prior to, and during, trials in a Morris water maze. Mice received five massed acquisition (escape) trials in each of three daily sessions, followed by a single 60-s probe trial on the fourth day. The location of the submerged platform was constant for each animal over acquisition trials, but varied across animals; starting position varied across trials. A 5 (dose) x 3 (trial blocks) mixed factorial ANOVA for escape time yielded a significant trial blocks effect only (p <.001), showing performance improving over sessions. Time spent in the target quadrant on the probe trial was shorter under all doses of propranolol when compared to vehicle group (all p <.001), indicating poorer retention of prior platform location. This effect, however, was not dose-related. Swim speed was not significantly affected by propranolol. These data demonstrate that chronic dosing with propranolol can impair retention of spatial learning, which cannot be attributed to reduced arousal or motor function.

Traumatic brain injury as a risk factor for Alzheimer's disease: a review

Accumulating epidemiological evidence implicates traumatic brain injury as a pathogenic agent in the development of Alzheimer's disease (AD). Considering the increase in the prevalence of both traumatic brain injury and AD in recent times, the possibility that brain trauma may provoke the early development of AD has important implications for health service planning, preventative efforts, and medico-legal compensation settlements. This paper evaluates the plausibility of the proposed link between traumatic brain injury and AD, largely by way of exploring a theoretical perspective advanced by Satz (1993) and considering recent contributions from the epidemiological, neuropathological, and biochemical literature that are pertinent to this issue. The literature reviewed provides sufficient support and empirical vindication to give credence to the proposed association between these two neuropsychological entities at the statistical, theoretical, and biological level.

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

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

In vivo neurochemical effects of the acetylcholinesterase inhibitor ENA713 in rat hippocampus

Oral ENA713 (0.5, 1.5 and 4.5 mg/kg), an acetylcholinesterase inhibitor (AChEI), dose-dependently enhanced extracellular acetylcholine concentrations in the hippocampus of freely moving rats. This effect was paralleled by changes in both noradrenergic and dopaminergic transmission. In particular, ENA713 significantly decreased noradrenaline concentrations, whereas it significantly increased homovanillic acid levels, without affecting dopamine concentrations. Neither serotonin nor gamma-aminobutyric acid levels were modified by ENA713. These findings extend the neurochemical profile of ENA713 and suggest that it could be useful for the treatment of Alzheimer-type dementia which is associated with multiple neurotransmitter abnormalities in the brain.

Synthesis of new N-(4-pyridyl)-1-aminopyrazoles and their muscarinic and adrenergic properties

The synthesis of new N-(4-pyridyl)-1-aminopyrazoles is described. Their binding properties were tested for muscarinic and other neurotransmitter receptors, together with their acetylcholinesterase inhibitory activity. The series derived from 3,5-dimethyl-1H-pyrazole showed moderate activities in both muscarinic and adrenergic receptor binding tests.

Immunohistochemical and neurochemical correlates of learning deficits in aged rats

This study examined whether cholinergic and monoaminergic dysfunctions in the brain could be related to spatial learning capabilities in 26-month-old, as compared to three-month-old, Long-Evans female rats. Performances were evaluated in the water maze task and used to constitute subgroups with a cluster analysis statistical procedure. In the first experiment (histological approach), the first cluster contained young rats and aged unimpaired rats, the second one aged rats with moderate impairment and the third one aged rats with severe impairment. Aged rats showed a reduced number of choline acetyltransferase- and p75(NTR)-positive neurons in the nucleus basalis magnocellularis, and choline acetyltransferase-positive neurons in the striatum. In the second experiment (neurochemical approach), the three clusters comprised young rats, aged rats with moderate impairment and aged rats with severe impairment. Alterations related to aging consisted of reduced concentration of acetylcholine, norepinephrine and serotonin in the striatum, serotonin in the occipital cortex, dopamine and norepinephrine in the dorsal hippocampus, and norepinephrine in the ventral hippocampus. In the first experiment, there were significant correlations between water maze performance and the number of; (i) choline acetyltransferase- and p75(NTR)-positive neurons in the nucleus basalis magnocellularis; (ii) choline acetyltransferase-positive neurons in the striatum and; (iii) p75(NTR)-positive neurons in the medial septum. In the second experiment, water maze performance was correlated with the concentration of; (i) acetylcholine and serotonin in the striatum; (ii) serotonin and norepinephrine in the dorsal hippocampus; (iii) norepinephrine in the frontoparietal cortex and; (iv) with other functional markers such as the 5-hydroxyindoleacetic acid/serotonin ratio in the striatum, 3,4-dihydroxyphenylacetic acid/dopamine ratio in the dorsal hippocampus, 5-hydroxyindoleacetic acid/serotonin and homovanillic acid/dopamine ratios in the frontoparietal cortex, and 3,4-dihydroxyphenylacetic acid/dopamine ratio in the occipital cortex. The results indicate that cognitive deficits related to aging might involve concomitant alterations of various neurochemical systems in several brain regions such as the striatum, the hippocampus or the cortex. It also seems that these alterations occur in a complex way which, in addition to the loss of cholinergic neurons in the basal forebrain, affects dopaminergic, noradrenergic and serotonergic processes.

Septohippocampal pathway as a site for the memory-impairing effects of ethanol

Ethanol affects behavior by interacting with synaptic sites at many levels of the nervous system. However, it targets most readily and at the lowest concentrations those sites mediating higher cognitive functions such as attention and memory. The memory-impairing effects of ethanol are thought to involve the hippocampus, a structure particularly vulnerable to the effects ethanol at low concentrations and early in the rising phase of the blood ethanol concentration curve. One of the early, low-dose effects of ethanol is an interruption of the normal physiological regulation of the hippocampus by the ascending septohippocampal pathway originating in the medial septal area (MSA). Ethanol enhances GABAergic transmission in the MSA, thereby reducing the regularity and vigor with which rhythmically bursting neurons of the MSA drive the hippocampal theta rhythm. Disruption of septohippocampal activity also has consequences on the response of the hippocampus to cortical inputs. Ethanol produces a loss of hippocampal responsivity that reduces the ability of the hippocampus to encode and retrieve relevant stimulus information necessary for accurate memory. This paper examines the behavioral and neural evidence for hippocampal vulnerability to ethanol and explores the hypothesis that these effects are due to ethanol disrupting septohippocampal modulation of the hippocampus, resulting in impairments of memory.

Role of serotonin in memory impairment

As a result of its presence in various structures of the central nervous system serotonin (5-HT) plays a role in a great variety of behaviours such as food intake, activity rythms, sexual behaviour and emotional states. Despite this lack of functional specialization, the serotonergic system plays a significant role in learning and memory, in particular by interacting with the cholinergic, glutamatergic, dopaminergic or GABAergic systems. Its action is mediated via specific receptors located in crucial brain structures involved in these functions, primarily the septo-hippocampal complex and the nucleus basalis magnocellularis (NBM)-frontal cortex. Converging evidence suggests that the administration of 5-HT2A/2C or 5-HT4 receptor agonists or 5-HT1A or 5-HT3 and 5-HT1B receptor antagonists prevents memory impairment and facilitates learning in situations involving a high cognitive demand. In contrast, antagonists for 5-HT2A/2C and 5-HT4, or agonists for 5-HT1A or 5-HT3 and 5-HT1B generally have opposite effects. A better understanding of the role played by these and other serotonin receptor subtypes in learning and memory is likely to result from the recent availability of highly specific ligands, such as 5-HT1A, 5-HT1B, 5-HT2A receptor antagonists, and new molecular tools, such as gene knock-out mice, especially inducible mice in which a specific genetic alteration can be restricted both temporally and anatomically.

Effect of R-(-)-alpha-methylhistamine and thioperamide on in vivo release of norepinephrine in the rat hippocampus

1. The modifications of hippocampal release of norepinephrine following the administration of R-(-)-alpha-methylhistamine and thioperamide, respectively agonist and antagonist of histamine H3 receptors, were assessed in freely moving rats by microdialysis. 2. Both the systemic (2 mg/kg i.p.) and local (100 microM via the probe) administration of thioperamide caused no modifications of basal release, indicating that the histaminergic system is not tonically involved in regulating the hippocampal noradrenergic activity. 3. R-(-)-alpha-methylhistamine (1 and 100 microM) produced a slight, short-lasting and dose-dependent reduction of norepinephrine release antagonized by local perfusion (100 microM) and prevented by systemic administration of thioperamide 2 mg/kg. 4. The results seem to indicate that the modulation of norepinephrine release through presynaptic H3-receptors in the rat hippocampus plays a minor role in the memory-enhancing effects of thioperamide.

Activation of 5-HT2A receptors impairs response control of rats in a five-choice serial reaction time task

The present experiments investigated the effects of agents acting at serotonin (5-HT)-2 receptors on the performance of rats in a choice serial reaction time (5-CSRT) task in order to examine the role of 5-HT2 receptors in the modulation of attention and response control. The results indicate that DOI, [(+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane hydrochloride; 0.05, 0.1 and 0.2 mg/kg, subcutaneously], a 5-HT(2A/2C) agonist, slightly impaired the choice accuracy of the well performing rats and markedly increased their premature responding. DOI (0.05 and 0.1 mg/kg) had no effect on the latency to collect earned food pellets or to respond correctly, indicating that these lower doses of DOI did not reduce motivation for the food reward in this task. The selective effect of a low dose of DOI (0.1 mg/kg) on premature responding was completely blocked by ketanserin (0.2 mg/kg), a 5-HT2A antagonist, and ritanserin (0.3 mg/kg), a 5-HT(2A/2C) antagonist, but only partially blocked by a high dose of SER082 (1.0 mg/kg), a 5-HT2C antagonist. In contrast to DOI, mCPP, [1-(3-phenyl)piperazine; 0.05 and 0.15 mg/kg], a 5-HT2C agonist, had no effect on choice accuracy or premature responding, but it reduced behavioral activity and/or arousal as indicated by the decreased number of trials completed and increased the probability of omissions. SER082 (1.0 mg/kg) blocked the effects of mCPP on performance. These data suggest that the overactivation of 5-HT2A receptors impairs response control in a 5-CSRT task, whereas the overactivation of 5-HT2C receptors can affect behavioral activity and/or arousal state of the animals for this food rewarded task.

The effects of a specific alpha(2)-adrenoceptor antagonist, atipamezole, on cognitive performance and brain neurochemistry in aged Fisher 344 rats

The present experiments investigated the effects of a specific and potent alpha(2)-adrenoceptor antagonist, atipamezole, on cognitive performance and neurochemistry in aged rats. Aged control Fisher 344 rats, which had lower activities of choline acetyltransferase in the frontal cortex, were impaired in the acquisition of the linear arm maze task both in terms of repetition errors and their behavioural activity (the speed of arm visits), and they needed longer time to complete this task as compared to adult control rats. Atipamezole treatment (0.3 mg/kg) facilitated the acquisition of this task in the aged rats as they committed fewer errors and completed the task more quickly than saline-treated aged control rats. A separate experiment indicated that atipamezole enhanced the turnover of noradrenaline both in the adult and aged rats, but this effect was more pronounced in the aged rats. Furthermore, atipamezole enhanced significantly the turnover of serotonin and dopamine only in the aged rats when analysed in the whole brain samples. As alpha(2)-adrenoceptor antagonists are known to alleviate akinesia in the experimental models of Parkinson's disease, the present results could be especially relevant for the development of palliative treatment for demented Parkinsonian patients.

Combined lesions of cholinergic and serotonergic neurons in the rat brain using 192 IgG-saporin and 5,7-dihydroxytryptamine: neurochemical and behavioural characterization

This study assessed behavioural and neurochemical effects of i.c.v. injections of both the cholinergic toxin 192 IgG-saporin (2 microgram) and the serotonergic toxin 5,7-dihydroxytryptamine (5,7-DHT; 150 microgram) in Long-Evans female rats. Dependent behavioural variables were locomotor activity, forced T-maze alternation, beam walking, Morris water-maze (working and reference memory) and radial-maze performances. After killing by microwave irradiation, the concentrations of acetylcholine, monoamines and 5-hydroxyindoleacetic acid (5-HIAA) were measured in the hippocampus, frontoparietal cortex and striatum. 192 IgG-saporin reduced the concentration of acetylcholine by approximately 40% in the frontoparietal cortex and hippocampus, but had no effect in the striatum. 5,7-DHT lesions reduced the concentration of serotonin by 60% in the frontoparietal cortex and 80% in the hippocampus and striatum. Noradrenaline was unchanged in all structures except the ventral hippocampus where it was slightly increased in rats given 192 IgG-saporin. Cholinergic lesions induced severe motor deficits but had no other effect. Serotonergic lesions produced diurnal and nocturnal hyperactivity but had no other effect. Rats with combined lesions were more active than those with only serotonergic lesions, showed motor dysfunctions similar to those found in rats with cholinergic lesions alone, and exhibited impaired performances in the T-maze alternation test, the water-maze working memory test and the radial-maze. Taken together and although cholinergic lesions were not maximal, these data show that 192 IgG-saporin and 5,7-DHT lesions can be combined to selectively damage cholinergic and serotonergic neurons, and confirm that cholinergic-serotonergic interactions play an important role in some aspects of memory, particularly in spatial working memory.

Effects of acupuncture at pai-hui on the deficit of memory storage in rats

In this study, we investigated the effects of Pai-Hui by acupuncture on cycloheximide (CXM)-induced impairment of the passive avoidance response in rats. Acupuncture at Pai-Hui (Go-20) treated 15 min before or immediately after training trial for 15 min significantly attenuated CXM-induced impairment of passive avoidance response in rats, but did not have the same effect 30 and 60 min before or 30 min after the training trial or before the retention trial. Acupuncture at Pai-Hui 15 min before the training trial for 15, 30 and 60 min significantly attenuated CXM-induced impairment of passive avoidance response in rats, and its efficacy paralleled the acupuncture duration. Furthermore, acupuncture at Pai-Hui did not attenuate scopolamine (SCOP)-induced impairment of passive avoidance response, but was slightly inhibited by SCOP at 0.3 mg/kg. Second, acupuncture at Pai-Hui attenuated p-chloroamphetamine (PCA)-induced impairment of passive avoidance response and was significantly antagonized by PCA at 1 mg/kg. These results suggest that acupuncture at Pai-Hui mainly affects the memory storage process and has preventive and immediate therapeutic effects on CXM-induced impairment of passive avoidance response. Its efficacy paralleled the acupuncture duration. The preventive effect of acupuncture at Pai-Hui on CXM-induced impairment is significantly reduced by serotonergic 5-HT releaser, and slightly by cholinergic manipulations.

The growth hormone axis and cognition: empirical results and integrated theory derived from giant transgenic mice

Sleep is required for the consolidation of memory for complex tasks, and elements of the growth-hormone (GH) axis may regulate sleep. The GH axis also up-regulates protein synthesis, which is required for memory consolidation. Transgenic rat GH mice (TRGHM) express plasma GH at levels 100-300 times normal and sleep 3.4 h longer (30%) than their normal siblings. Consequently, we hypothesized that they might show superior ability to learn a complex task (8-choice radial maze); 47% of the TRGHM learned the task before any normal mice. All 17 TRGHM learned the task, but 33% of the 18 normal mice learned little. TRGHM learned the task significantly faster than normal mice (p < 0.05) and made half as many errors in doing so, even when the normal nonlearners were excluded from the analysis. Whereas normal mice expressed a linear learning curve, TRGHM showed exponentially declining error rates. The contribution of the GH axis to cognition is conspicuously sparse in literature syntheses of knowledge concerning neuroendocrine mechanisms of learning and memory. This paper synthesizes the crucial role of major components of the GH axis in brain functioning into a holistic framework, integrating learning, sleep, free radicals, aging, and neurodegenerative diseases. TRGHM show both enhanced learning in youth and accelerated aging. Thus, they may provide a powerful new probe for use in gaining an understanding of aspects of central nervous system functioning, which is highly relevant to human health.

Raloxifene and estradiol benzoate both fully restore hippocampal choline acetyltransferase activity in ovariectomized rats

Selective estrogen receptor modulators (SERMs) demonstrate tissue-specific estrogen receptor (ER) agonist or antagonist properties. Raloxifene, a prototypical SERM, has ER agonist properties in bone and on cholesterol metabolism but full antagonist properties in the uterus and breast. To characterize the ER agonist/antagonist profile of raloxifene in the brain, we have examined its effect on the activity of a known estrogen-responsive gene product, choline acetyltransferase (ChAT), in the hippocampus and other brain regions of 6-month-old ovariectomized (OVX) Sprague-Dawley rats. Three weeks post-ovariectomy, animals received estradiol benzoate (EB, 0.03 mg or 0.3 mg kg(-1) day(-1) for 3 or 10 days); raloxifene HCl (3.0 mg kg(-1) day(-1) for 3 or 10 days), tamoxifen (3.0 mg kg(-1) day(-1) for 10 days) or vehicle (20% CDX). As previously reported, ChAT activity decreased by approximately 20%-50% in the hippocampus of OVX compared with SHAM-operated control rats with no change in ChAT activity observed in the hypothalamus. Raloxifene or EB reversed the OVX-induced decrease in ChAT activity in the hippocampus but did not change ChAT activity in the hypothalamus. Animals that received combined EB (0.03 mg/kg) plus raloxifene (1 mg/kg) or tamoxifen alone (3.0 or 10 mg/kg) also showed increased hippocampal ChAT activity. Raloxifene failed to increase uterine weight and blocked the estrogen-induced increase in uterine weight, while another SERM, tamoxifen, increased uterine weight. These data demonstrate that raloxifene has estrogen-like properties on hippocampal ChAT activity in vivo, and suggest that benzothiophene SERMs may exert estrogen-like beneficial effects on cholinergic neurotransmission in brain without producing peripheral stimulation of breast or uterine tissue.

Involvement of hippocampal synaptic plasticity in age-related memory decline

This article examines the functional significance of Ca(2+)-dependent synaptic plasticity in relation to compromised memory function during aging. Research characterizing an age-related decline in memory for tasks that require proper hippocampal function is summarized. It is concluded that aged animals possess the mechanisms necessary for memory formation, and memory deficits, including rapid forgetting, result from more subtle changes in memory processes for memory storage or maintenance. A review of experimental studies concerning changes in hippocampal neural plasticity over the course of aging indicates that, during aging, there is a shift in mechanisms that regulate the thresholds for synaptic modification, including Ca(2+) channel function and subsequent Ca(2+)-dependent processes. The results, combined with theoretical considerations concerning synaptic modification thresholds, provide the basis for a model of age-related changes in hippocampal synaptic function. The model is employed as a foundation for interpretation of studies examining therapeutic intervention in age-related memory decline. The possible role of altered synaptic plasticity thresholds in learning and memory deficits suggests that treatments that modify synaptic plasticity may prove fruitful for the development of early therapeutic interventions in age-related neurodegenerative diseases.

Fetal alcohol-exposed rats exhibit differential response to cholinergic drugs on a delay-dependent memory task

Fetal alcohol exposure in human and rodents produces a number of cognitive deficits including impairments in learning and memory. Recent evidence in our laboratory has shown that fetal alcohol-exposed (FAE) rats respond differently to systemic administration of cholinergic drugs when tested for vigilance and locomotor activity. The present study examined the effects of muscarinic and nicotinic agonists and antagonists on memory performance in a delayed alternation task. Subjects were male offspring of Sprague-Dawley rats fed a 35% ethanol-derived caloric diet, pair-fed with sucrose, or chow-fed with lab chow during the last 2 weeks of gestation. Rats (3 months old) were food-deprived prior to training in the T-maze. Rats were first trained in the alternation task at no delay for five sessions. Rats were then trained at longer delays (20, 60, 180 s) until all groups showed similar performance for two consecutive sessions. Each animal was then tested following systemic injections of the cholinergic antagonists scopolamine and mecamylamine (60-s delay) and the cholinergic agonists pilocarpine and nicotine (180-s delay). Rats received saline injections on alternate days of testing. The results revealed that FAE rats exhibited no impairments in alternation performance at the no delay and 20-s delay, but showed impairments on both the 60- and 180-s delays during the initial sessions. However, with additional training, FAE rats showed performance similar to that of control groups at these delays. Following both pilocarpine and nicotine injections, control groups, but not the FAE group, showed significant memory enhancement in the alternation task. Following scopolamine injections, the FAE rats showed a significant impairment, while control groups showed a nonsignificant decrease in performance. All three groups showed impairments in the alternation task following administration of mecamylamine compared to saline treatment. These findings suggest that alterations in the cholinergic system in FAE rats may underlie some of the cognitive deficits observed with prenatal alcohol exposure.

Acute effect of KA-672, a putative cognitive enhancer, on neurotransmitter receptor binding in mouse brain

7-Methoxy-6-[3-[4-(2-methoxyphenyl)piperazin-1-yl]propoxy]-3,4-dim ethyl-2H-1-benzopyran-2-one hydro-chloride (KA-672), structurally related to naturally occurring coumarins, has been described as a potential drug for enhancing cognitive functions. However, a detailed characterization of the pharmacological profile of KA-672 in vivo is still lacking. Quantitative neurotransmitter receptor autoradiography was used as a tool to screen for KA-672-induced changes in a number of transmitter receptors including cholinergic, noradrenergic, glutamatergic, GABAergic, and serotonergic subtypes throughout the brain. Two hours following treatment of mice with 1 mg/kg KA-672 per os, slight increases of nicotinic and M1-muscarinic cholinergic receptor binding, of alpha2-and beta-adrenoceptor as well as 5-HT1A receptors in various cerebral cortical regions were observed, whereas 5-HT2A binding sites were strikingly increased throughout the brain following KA-672 treatment. In contrast, (+/-)-alphaamino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor binding was significantly decreased in some cortical regions after drug treatment. No effects of KA-672 treatment on N-methyl-D-aspartate, kainate, GABA(A) and benzodiazepine receptor as well as M2-muscarinic cholinergic and high-affinity choline uptake binding were observed. As interactions between the cholinergic, noradrenergic and serotonergic neurotransmission have been stressed to play important roles in realizing learning and memory events, the cognition-enhancing effects of KA-672 may be due to this complex in vivo pharmacological profile of KA-672.

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

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

Stimulation of 5-HT1A receptors by systemic or medial septum injection induces anxiogenic-like effects and facilitates acquisition of a spatial discrimination task in mice

1. In the present study, the authors addressed the issue of the possible modulation of both emotional and learning processes by the stimulation of 5-HT1A receptors. In this respect, we have carried out two series of experiments: the first series examined the effects of systemic injections of 8-OH-DPAT, a 5-HT1A receptor agonist, successively on a model of anxiety and on a learning task; secondly the effects of selective infusions into the medial septum were studied in the same experimental design. 2. Mice were tested in an elevated plus-maze before being submitted to a spatial discrimination task in an 8-arm radial maze. The 8-OH-DPAT (1 mg/kg) administered intraperitoneally 30 minutes prior to testing, induced anxiogenic-like effects in the plus-maze and improved the acquisition of the spatial discrimination. 3. Moreover, a regression analysis showed that the index of anxiety measured in the elevated plus-maze was positively correlated with the performance level reached at the forth day of training in the spatial discrimination task. The intraseptal infusion of the drug (1 microgram) demonstrated the same pattern of results, although the effects were less pronounced. Again a correlation between the index of anxiety and acquisition performance was obtained. 4. These results suggest that anxiogenic-like effects induced by selective stimulation of 5-HT1A receptors have a positive influence on the acquisition of a memory task. As systemic injections appeared to be more effective than intra-septal infusions, these effects might be mediated by both pre- and postsynaptic 5-HT1A receptors.

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.

Cognitive performance and biochemical markers in septum, hippocampus and striatum of rats after an i.c.v. injection of streptozotocin: a correlation analysis

In the present study we evaluated the effects of an intracerebroventricular injection of streptozotocin on cognitive behavior and biochemical markers in the brain of middle-aged Wistar rats. Intracerebroventricular injected streptozotocin has previously been reported to decrease the central metabolism of glucose. We found that streptozotocin-treated rats showed an impaired cognitive performance in the delayed non-matching to position task and the Morris water escape task. Glial fibrillary acidic protein, an indicator of reactive astroglial changes, was measured in three different (soluble, Triton X-100 soluble and crude cytoskeletal) protein fractions and its content in the fractions of the septum, hippocampus and striatum of streptozotocin-treated rats was increased. Furthermore, the glial fibrillary acidic protein response of each protein fraction to streptozotocin treatment appeared to be differently regulated. In streptozotocin-treated rats the choline acetyltransferase activity was decreased in the hippocampus only, which was correlated with the hippocampal glial fibrillary acidic protein contents of all three hippocampal protein fractions, thus suggesting that the cholinergic deficit is a consequence of direct damage to the hippocampus. The cognitive deficits in both tasks were related to the increased glial fibrillary acidic protein contents, especially of the soluble and cytoskeletal fraction, and the decreased choline acetyltransferase activity in the hippocampus. Taken together, these findings indicate that it is important to take into account which protein fraction has been used for measuring the glial fibrillary acidic protein response to a stressor. Furthermore, intracerebroventricular injected streptozotocin may provide a relevant model for studying neurodegenerative changes due to a metabolic insufficiency and testing neuroprotective effects of substances.

Short-term memory processes in delayed visual differentiation in rhesus macaques after bilateral removal of field 7 of the parietal cortex

Monkeys (Macaca mulatta) with preliminary removal of field 7 of the lower parietal cortex and previously trained to differentiate images differing in shape, color, size, orientation, and spatial relationships were used to study the processes involved in short-term storage of different types of information required for a delayed (by 0-8 sec) visual differentiation task and the effects on these processes of the antioxidant Oxymetacil. Significant differences were found in comparison with intact animals. Removal of field 7 sharply worsened short-term storage processes during visual differentiation of different types of images, including those differing in terms of properties such as color, geometrical shape, and the spatial relationships between image elements. There were significant reductions in the level of correct responses for all delay periods with significant increases in the motor reaction time, indicating a sharp reduction in the duration of short-term information storage, which suggests that the monkeys' short-term memory mechanisms were disrupted. Oxymetacil had a correcting effect only in relation to stimuli differing in terms of color and shape, but had no effect at all on the short-term storage of spatial information. It is suggested that these data suggest that field 7 has at least two functions. These are, firstly, a role in processes underlying the evaluation, differentiation, and storage of spatial information depending on visual-vestibular interactions, and secondly, a role in the mechanisms underlying the attention system, which is disrupted by removal of field 7 and restored by treatment with the antioxidant when there is no need to differentiate spatial information, a process which depends on assessment of the body image and egocentric orientation based on visual-vestibular interactions.

Improving cognitive function after brain injury: the use of exercise and virtual reality

OBJECTIVE

To assess the impact of exercise and virtual reality (VR) on the cognitive rehabilitation of persons with traumatic brain injury (TBI).

DESIGN

Before-after trial assessed cognitive function after a 4-week intervention program. A random allocation crossover assessed changes in reaction and movement times after a single bout of VR exercise and a no-exercise control condition.

SETTING

Brain injury rehabilitation unit in Edinburgh, Scotland.

PATIENTS

(1) Four-week intervention: a consecutive sample of 13 suitable TBI adults were compared to control populations (n > 25) of previous TBI patients of similar age, severity, and time postinjury. (2) Single-bout intervention: a consecutive sample of 13 suitable adults with moderate TBI, 6.29 to 202.86 weeks postinjury.

INTERVENTION

Nonimmersive VR exercise.

MAIN OUTCOME MEASURES

(1) Tests of attention, information processing, learning, and memory. (2) Reaction and movement times.

RESULTS

After the 4-week intervention patients performed significantly better than controls on the digit symbol (p < .01). verbal (p < .01), and visual learning tasks (p < .05). Significant improvements in reaction times (p < .01) and movement times (p < .05) were gained following a single bout of VR exercise.

CONCLUSION

Exercising in a virtual environment offers the potential for significant gains in cognitive function.

Combined action of thioperamide plus scopolamine, diphenhydramine, or methysergide on memory in mice

The aim of the present experiments was to test the role played by the interaction of the selective H3 receptor antagonist, thioperamide, with the cholinergic, histaminergic, and serotonergic systems in modifying memory. The behavioral tests used (open-field and passive-avoidance repetition) were selected on the basis of the action displayed by thioperamide in these behavioral situations. Posttrial administration of thioperamide (5 mg/kg) resulted in an improvement in memory consolidation, as tested in the repetition of the open-field test, but repeated posttrial administration of thioperamide (2 or 5 mg/kg) had no effect in the repetition of passive avoidance test. Scopolamine (2 mg/kg) caused a deterioration in the memory processes in both tests: this effect was blocked by 2 mg/kg of thioperamide, which was itself ineffective in the test. These results may suggest that both the improvement in memory due to thioperamide and its antagonism of the amnestic effects of scopolamine are determined by activation of central cholinergic systems, due to thioperamide inhibition of H3 heteroreceptors. Diphenhydramine (2 or 10 mg/kg) was itself ineffective in the tests, but counteracted the memory improvement caused by thioperamide in the repetition of the open-field test. The effect of diphenhydramine is discussed in terms of interactions between histaminergic and cholinergic systems. Methysergide counteracted the effect of thioperamide in the open-field test only at a high dosage (50 mg/kg). The possible implication of serotonergic systems on the effects of the methysergide-thioperamide interaction in the memory process is discussed.

Behavioral and neuroanatomical consequences of chronic ethanol intake and withdrawal

We have examined if long-term (13 months) alcohol consumption and the same treatment followed by a 6-week withdrawal period cause different neuropathological changes in rats. Spatial reference and working memory of alcohol-consuming and withdrawn rats were evaluated by comparison of their performance with age-matched controls in the Morris water maze. In the reference memory task we did not observe significant cognitive deficits in rats continuously exposed to ethanol, whereas withdrawn animals showed an obvious impairment of their overall performance. The reference memory deficit in withdrawn rats was evident in the spatial probe trial; these animals required significantly longer swimming distances to approach the former position of the platform when compared with controls and alcohol-consuming animals. In contrast, working memory was not significantly altered in either experimental group. Stereological methods were applied to compare the neurodegenerative changes produced by alcohol intake and withdrawal in the hippocampal formation. In the alcohol-consuming animals there was a significant cell loss in CA1 (18%) and CA3 (19%) hippocampal regions. Moreover, in withdrawn rats there was a further decay in the total number of pyramidal neurons, which amounted to 15% relative to nonwithdrawn animals. In the granular layer of the dentate gyrus there was a trend in the same direction, but it did not reach significance. Thus, our findings indicate that withdrawn rats are cognitively impaired relative to animals submitted to continuous alcohol consumption and to age-matched controls, which fits the morphological data showing that withdrawal aggravates ethanol-induced degenerative processes in the hippocampal formation.

Effects of nucleotides on learning and memory in a Morris water maze test in normal and basal forebrain-lesioned rats

The effects of nucleotides on learning and memory were studied in normal and basal forebrain-lesioned rats using a Morris water maze test. Chronic oral administration of a nucleotide mixture (500 mg/kg), containing an equal weight of the disodium salts of adenosine 5'-monophosphate, guanosine 5'-monophosphate, inosine 5'-monophosphate, cytidine 5'-monophosphate, and uridine 5'-monophosphate facilitated learning acquisition in normal rats. In basal forebrain-lesioned rats, administration of the nucleotide mixture showed a tendency to improve learning acquisition and memory retrieval. In the biochemical studies, no significant changes were observed in brain choline and acetylcholine levels by treatment with the nucleotide mixture at the doses tested in both normal and basal forebrain-lesioned rats. The nucleotides did not affect the monoaminergic systems in normal rats, but did cause some changes in these systems in basal forebrain-lesioned rats. The present studies indicate that nucleotides ameliorate learning and memory processes in normal rats, but not in basal forebrain-lesioned rats, and they also modulate the activity of the central monoaminergic systems under certain conditions.

In situ hybridization analysis of preprotachykinin-A mRNA levels in young and old rats

The present study examined the effects of ageing on preprotachykinin-A (PPT-A) mRNA levels in discrete regions of the rat brain. Semiquantitative analysis of silver grains revealed a 16% statistically significant decrease in PPT-A mRNA in the shell of the nucleus accumbens (AcbSh), a 27.6% statistically significant lower level of PPT-A mRNA in the olfactory tubercle (Tu), a 19.2% and 31. 5% statistically significant decrease in PPT-A mRNA in the dorsal and ventral caudate-putamen (d-CPu) (v-CPu), respectively, a 30% statistically significant lower expression of PPT-A mRNA in the bed nucleus of the stria terminalis (BNST), a 33.7% statistically significant decrease in PPT-A mRNA in the habenula (Hb) and a 30% statistically significant decrease of PPT-A mRNA levels in the postero-dorsal part of the medial amygdala (MePD). No changes in PPT-A mRNA levels were found in the nucleus accumbens, core (AcbC), in the islands of Calleja (Icj), and in the medial preoptic area (mPOA). These results show that ageing of the central nervous system (CNS) is associated with widespread changes in tachykinin gene expression, suggesting that alteration in the tachykinergic system may have implications in the physio-pathology of the elderly.

Specific characteristics of cholinergic mechanisms of short-term memory in monkeys for different types of visual information: the effects of amizil

Experiments on rhesus macaques were used to study the relationship between the characteristics of delayed visual differentiation and stimulus properties in conditions of pharmacological treatment with the m-cholinoreceptor blocker amizil, with the aim of identifying how modification of cholinergic structures affects different types of information. Disturbances to short-term memory for all stimuli consisted of reductions in the duration of retention and increases in motor reaction times, but occurred at different doses of the blocker: amizil at a dose of 0.3 mg/kg significantly decreased the retention duration for information relating to spatial relationships. Delayed discrimination of shape, contrast, and size worsened after treatment with amizil at a dose of 0.45-0.50 mg/kg, while decreases in the duration of short-term storage of information relating to color started after amizil doses of 0.6-0.8 mg/kg. It is suggested that the short-term memory system includes a set of neurophysiological mechanisms in which the cholinergic structures are organized differently and whose specific properties result in differences in the characteristics of short-term storage of different types of visual information.

Sensitivity to cholinergic drug treatments of aged rats with variable degrees of spatial memory impairment

As a first step, the present experiment aimed at characterizing learning and memory capabilities, as well as some motor and sensorimotor faculties, in aged (24-26.5 months) Long-Evans female rats. As a second step, a psychopharmacological approach was undertaken in order to examine the sensitivity of aged rats to muscarinic blockade and to cholinomimetic treatments. Young adult (3-5.5 months) and aged rats were tested for beam-walking performance, locomotor activity in the home cage and an open field, and spatial learning/memory performance in a water maze and a radial maze. Spontaneous alternation rates were assessed in a T-maze. Statistical analysis discriminated between aged rats showing moderate impairment (AMI) and those showing severe impairment (ASI) in the water maze test. Beside their different degrees of impairment in the water maze, AMI and ASI rats were similarly (no significant difference) impaired in beam-walking capabilities, home cage activity and radial maze performance. In the spontaneous alternation task aged rats were not impaired and, in the open-field test, AMI rats were hypoactive, but not as much as ASI rats. Neither of the cognitive deficits was correlated with a locomotor or a sensorimotor variable, or with the body weight. When tested in the radial maze, a low dose of scopolamine (0.1 mg/kg i.p.) produced memory impairments which were significant in AMI and ASI rats, but not in young rats. Combined injections of scopolamine and physostigmine (0.05 and 0.1 mg/kg) or tacrine (THA, 3 mg/kg) showed physostigmine (0.1 mg/kg) to compensate for the scopolamine-induced impairments only in AMI rats. whereas THA was efficient in both AMI and ASI rats. The results indicate: (i) that rats with different degrees of spatial memory impairment in the water maze are similarly hypersensitive to muscarinic blockade when tested in a radial maze test; and (ii) that under the influence of a dose of scopolamine which is subamnesic in young rats, aged rats respond to anticholinesterase treatments according to the level of performance achieved in the water maze: moderately impaired rats are sensitive to both physostigmine and THA, whereas more severely impaired rats are sensitive only to THA.

The neurosteroid pregnenolone sulfate infused into the nucleus basalis increases both acetylcholine release in the frontal cortex or amygdala and spatial memory

The effects of an infusion (5 ng) of the neurosteroid pregnenolone sulfate into the nucleus basalis magnocellularis on acetylcholine release in the frontoparietal cortex and basolateral amygdala were evaluated during the 130 min post-injection in male Sprague-Dawley rats using in vivo microdialysis coupled "on line" with high performance liquid chromatography detection. One week later, the same animals were tested for spatial memory after another infusion of pregnenolone sulfate (5 ng) into the nucleus basalis. Results show that pregnenolone sulfate enhanced acetylcholine release by more than 50% of baseline concentrations in the two structures relative to a control injection. The duration of this effect was longer in cortex (130 min) than in amygdala (30 min). Furthermore, pregnenolone sulfate improved memory performance in a task based upon spatial recognition of a familiar environment. A significant positive correlation (r=0.49) was found between the recognition score in the spatial memory test and the levels of acetylcholine release in the frontoparietal cortex but not in the basolateral amygdala. Therefore, our results suggest that the nucleus basalis magnocellularis-cortical pathway could be in part responsible for the promnesic effect of pregnenolone sulfate. This neurosteroid acts as a negative modulator of the GABA(A) receptor complex and positively modulates the N-methyl-D-aspartate receptor, possibly resulting in a global stimulatory effect on central cholinergic neurotransmission.

Estrogen, the ovary, and neutotransmitters: factors associated with aging

Our studies in the C57BL/6J mouse have been designed to examine the interactions of aging and the ovary, and their mutual effects on neuroendocrine function. In the pituitary, ovarian status and not age determines responsiveness to gonadotropin hormone releasing hormone (GnRH), but estrogen (E2) is an important mediator in CNS changes, and removal of the ovary (OVX) is deleterious to the neuroendocrine hypothalamus. OVX for just six days in young animals results in synaptic loss between noradrenergic terminals and gonadotropin hormone releasing hormone (GnRH) neurons. Long-term OVX, hypothesized to protect against neuroendocrine aging, fails to guard against any studied age-related changes. Some age-related changes occur as early as midlife. Although neuron number remains constant at middle age, opiatergic neurons undergo significant functional changes by producing opiate antagonist peptides. This change appears to be caused by alterations in the prohormone convertases, which cleave propeptide to peptide. Altered peptides may trigger the loss of reproductive capacity. The midlife shift in opiate peptide production is a component of natural developmental processes that begin in the neonate and continue through old age. In the cholinergic system, E2 mediates numbers of cholinergic receptors, cholinergic neurons, and cholinergic-modulated memory systems in both young and old animals. Regardless of age, ovarian steroids, if present at physiologic levels, are beneficial to the neuroendocrine CNS, and long-term deprivation from ovarian-produced factors is deleterious in the systems we have examined. Our studies have shown that deprivation from ovarian steroid hormones in the female appears to be a major factor in the health of the CNS and in events associated with aging.

The specific dopamine uptake inhibitor GBR 12783 improves learning of inhibitory avoidance and increases hippocampal acetylcholine release

The specific dopamine uptake inhibitor, GBR 12783 was tested on the retention performance of a one-trial passive avoidance test. For a moderate electric shock intensity, GBR 12783 (10 mg/kg), injected before acquisition session, improved retention performance. Scopolamine (0.125-0.5 mg/kg) completely blocked the promnesic effect of GBR 12783. Moreover, GBR 12783 increased hippocampal acetylcholine release in vivo. These data suggest that the promnesic effect of GBR 12783 is mediated by an increase in the septo-hippocampal cholinergic transmission.

The acetylcholine, GABA, glutamate triangle in the rat forebrain

The present overview demonstrates that stress, fear, novelty, and learning processes are associated with arousal and increases of extracellular levels of cortical and hippocampal ACh, independently of increases of motor activity. Forebrain cholinergic systems appears to be regulated by GABAergic and glutamatergic inputs. However, several other neurotransmitter systems play a role.

Effects of GABAergic drugs on physostigmine-induced improvement in memory acquisition of passive avoidance learning in mice

1. The effect of gamma-aminobutyric acid (GABA) receptor agonists and antagonists on acquisition of a step-down passive avoidance learning in mice was measured in the presence and absence of physostigmine. 2. Intraperitoneal injection of different doses of the anticholinesterase drug physostigmine (0.1-0.3 mg/kg) increased acquisition in mice dose dependently. The maximum response was obtained with 0.3 mg/kg of the drug. Higher doses of the drug impaired acquisition of the learned response. To show the effect of the GABAergic system on acquisition, GABAA receptor agonists and antagonists were challenged against 0.2 mg/kg of physostigmine. 3. Administration of the GABAA receptor agonist muscimol but not the GABAB receptor agonist baclofen decreased the acquisition of the learned task. However, the improvement induced by physostigmine (0.2 mg/kg) was decreased by both muscimol and baclofen. A combination of both agonists caused a higher inhibitory effect on the physostigmine response. 4. Pretreatment of animals with the higher doses of GABAA receptor antagonists bicuculline and picrotoxin but not the GABAB receptor antagonist phaclofen impaired learning. Both the GABAA and GABAB receptor antagonists reduced the learning improvement induced by physostigmine. The inhibitory effects of the GABAA and GABAB receptor antagonists are lost when combined together. 5. Bicuculline, picrotoxin or phaclofen increased the impairment of learning induced by muscimol, whereas a combination of either of the antagonists with baclofen did not alter the learning. The GABAA antagonists reduced the inhibitory effect of muscimol, whereas a higher dose of phaclofen increased the inhibition of the physostigmine response induced by muscimol and baclofen on physostigmine-induced learning improvement. 6. Phaclofen decreased but a higher dose of bicuculline increased the baclofen-induced inhibition of physostigmine effect. 7. It is concluded that both GABAA and GABAB activation inhibit improvement of acquisition induced by physostigmine.

Involvement of medial septal glutamate and GABAA receptors in behaviour-induced acetylcholine release in the hippocampus: a dual probe microdialysis study

In the present study, the role of medial septal receptors in behaviour-induced increase in acetylcholine (ACh) release in hippocampus was investigated using dual-probe microdialysis in combination with a simple behavioural procedure. gamma-Aminobutyric acid (GABA) and glutamate receptor agonists and antagonists were administered by retrograde dialysis into the medial septal area, while ACh was measured in the ventral hippocampus. Rats were behaviourally activated by immobilization or handling, but only handling was performed during drug-treatment. The GABAA receptor agonist muscimol did not affect ACh release, but blocked the handling-induced increase in ACh release completely. In addition, muscimol administration induced behavioural activity. Administration of the GABAA receptor antagonist bicuculline increased ACh release more than 2-fold. However, handling-induced increase in ACh release, expressed as percentage of drug-induced release, was similar to that of controls. Administration of the glutamate receptor agonists N-methyl-D-aspartate (NMDA) and (S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) in the septal area increased hippocampal ACh levels, but reduced the handling-induced increase in ACh release. The NMDA antagonist, 3-((R)-2-carboxypiperazin-4-yl) (CPP) increased ACh levels moderately, and reduced handling-induced increase in ACh release. However, similarly to muscimol, CPP administration induced behavioural activity. The AMPA/kainate receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX) did not affect behaviour or basal ACh levels, but attenuated the handling-evoked ACh release. We conclude that the handling-induced increase in hippocampal ACh levels is mediated via stimulation of septal non-NMDA, and possibly NMDA receptors, whereas GABAA receptors are probably not involved. The feasibility of the experimental design is further discussed.

Is there a rationale for the use of acetylcholinesterase inhibitors in the therapy of Alzheimer's disease?

Since the 1980s, the cholinergic hypothesis of the pathogenesis of Alzheimer's disease has proven to be a strong stimulus to pharmacological strategies aimed at correcting the cognitive deficit by manipulating cholinergic neurotransmission. Among these strategies, the one based on acetylcholinesterase inhibition is currently the most extensively developed for the therapy of Alzheimer's disease. The inhibitors' mechanisms of action are complex, including changes in the release of acetylcholine, and modulation of acetylcholine receptors. Various clinical trials of various inhibitors have shown that, on the whole, their effects were modest and, in the case of some drugs, were associated with frequent adverse reactions. Among the conceivable reasons for the limited efficacy of these drugs, those related to the pharmacological target deserve particular attention. This review, therefore, focuses on the complex nature of the acetylcholine system, the alterations of acetylcholinesterase and muscarinic receptor signal transduction in Alzheimer's disease, and the involvement of other neurotransmitters.

KA 672-HCl, a neuronal activator against dementia: tolerability, safety, and preliminary pharmacokinetics after single and multiple oral doses in healthy male and female volunteers

In vitro and animal experiments have characterized KA 672-HCl as a potent functional antagonist of excitatory amino acid-induced convulsions and mortality. In receptor-binding studies, the compound displayed high affinities to several serotoninergic, adrenergic, and dopaminergic receptors and to the sigma receptor. The potential for short- and long-term toxicity of KA 672-HCl in rats and dogs was found to be low. Double-blind, randomized, placebo-controlled studies were undertaken in healthy volunteers ranging from 52 to 74 years of age to determine tolerability, safety, and preliminary pharmacokinetics of single and repeated doses in humans. Single doses up to 40 mg were well tolerated, with no difference in effect from placebo. At 60 mg, approximately half of the volunteers experienced a moderate drug-related orthostatic syndrome. After repeated doses of 10 or 20 mg KA 672-HCl for 14 days only minor adverse events of mild intensity were reported with no clear relation to dose or a clinically relevant difference from placebo. A mild decrease in semisupine and standing blood pressure 4 hours after administration was observed in the 20 mg group with no occurrence of orthostasis. Linear pharmacokinetics were observed after repeated doses. However, this was not the case after single-dose administration, as generally higher plasma concentrations were observed after the 20-mg dose than would have been predicted from the 10-mg data. The mean terminal phase half-life after the 20 mg dose was 11.1 hours and 13.7 hours after repeated and single doses, respectively. The safety and tolerability data support a continuation of therapeutic trials. KA 672-HCl is currently entering phase II development.

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.

Present imperfect: a critical review of animal models of the mnemonic impairments in Alzheimer's disease

This paper reviews the current literature on animal models of the memory impairments of Alzheimer's disease (AD). The authors suggest that modeling of the mnemonic deficits in AD be limited to the amnesia observed early in the course of the disease, to eliminate the influence of impairments in non-mnemonic processes. Tasks should be chosen for their specificity and selectivity to the behavioral phenomena observed in early-stage AD and not for their relevance to hypothetical mnemonic processes. Tasks that manipulate the delay between learning and remembering are better able to differentiate Alzheimer patients from persons with other disorders, and better able to differentiate effects of manipulations in animals. The most commonly used manipulations that attempt to model the amnesia of AD are reviewed within these constraints. The authors conclude that of the models examined, lesions of the medial septal nucleus produce behavioral deficits that are most similar to the mnemonic impairments in the earliest stage of AD. However, the parallel is not definitive and more work is needed to clarify the relationship between neurobiology and behavior in AD.

Galanin/GMAP- and NPY-like immunoreactivities in locus coeruleus and noradrenergic nerve terminals in the hippocampal formation and cortex with notes on the galanin-R1 and -R2 receptors

By using immunofluorescence methodology, extensive galanin (GAL) and GAL message-associated peptide (GMAP)-positive terminal networks were observed in the hippocampal formation. The majority of the GAL/GMAP fibers were dopamine beta-hydroxylase- (DBH) positive, that is, they were noradrenergic. This finding was established with GAL/GMAP-DBH double-staining and with 6-hydroxy-dopamine treatment, which totally abolished all fibers in which GAL/GMAP and DBH coexisted. Also, reserpine treatment caused a marked depletion of GAL. No evidence for GAL/GMAP coexistence with 5-hydroxytryptamine was obtained. In the ventral hippocampus, GAL/GMAP-, DBH-negative fibers were seen in the stratum oriens, the anterior stratum radiatum, along the granule cell layer and in the strata oriens and alveus. In the locus coeruleus (LC), around 80% of the GMAP-positive neurons contained neuropeptide tyrosine (NPY), and about 40% of the NPY-positive neurons expressed GMAP. GAL-R1 receptor mRNA was expressed in Barrington's nucleus (close to the LC), but was not detected in the hippocampal formation/dorsal cortical areas. GAL-R2 receptor mRNA was found in the granule cell layer in the dentate gyrus. The present results show that most, but not all, immunohistochemically detectable GAL/GMAP in the hippocampal formation/dorsal cortex is present in noradrenergic nerve terminals originating in the LC, which has a robust GAL/GMAP synthesis. The functional role of GAL may be related to noradrenaline, possibly by a presynaptic action. However, the presence of GAL in other systems and of GAL-R2 receptor mRNA in granule cells also indicates other targets.

Septal lesions impair the acquisition of a cued place navigation task: attentional or memory deficit?

These experiments were designed to analyze how medial septal lesions reducing the cholinergic innervation in the hippocampus might affect place learning. Rats with quisqualic lesions of the medial septal area (MS) were trained in a water maze and on a homing table where the escape position was located at a spatially fixed position and further indicated by a salient cue suspended above it. The lesioned rats were significantly impaired in reaching the cued escape platform during training. In addition rats, did not show any discrimination of the training sector during a probe trial in which no platform or cue was present. This impairment remained significant during further training in the absence of the cue. When the cued escape platform was located at an unpredictable spatial location, the MS-lesioned rats showed no deficit and spent more time under the cue than control rats during the probe trial. On the homing board, with a salient object in close proximity to the escape hole, the MS rats showed no deficit in escape latencies, although a significant reduction in spatial memory was observed. However, this was overcome by additional training in the absence of the cue. Under these conditions, rats with septal lesions were prone to develop a pure guidance strategy, whereas normal rats combined a guidance strategy with a memory of the escape position relative to more distant landmarks. The presence of a salient cue appeared to decrease attention to environmental landmarks, thus reducing spatial memory. These data confirm the general hypothesis that MS lesions reduce the capacity to rely on a representation of the relation between several landmarks with different salience.

Antagonism by abecarnil of enhanced acetylcholine release in the rat brain during anticipation but not consumption of food

Changes in the extracellular concentration of acetylcholine (ACh) were evaluated in the prefrontal cortex and hippocampus of freely moving rats habituated for 35 days to consume their daily meal during a fixed 2-h period. During the 40 min immediately before presentation, ACh output increased by 49 and 55% in the prefrontal cortex and hippocampus, respectively. ACh release increased further during the first 40 min of consumption phase in the prefrontal cortex (+220%) and hippocampus (175%). Administration of abecarnil (0.1 mg/kg, IP) 40 min before food presentation prevented the increase in ACh output in both brain regions during the anticipatory phase. In contrast, although abecarnil reduced the ACh content achieved during the consummatory phase, it did not prevent the increase in ACh release in the prefrontal cortex or hippocampus induced by food intake. Finally, the binding of [35S]TPBS to cerebral cortex, hippocampus, or septum of rats killed 20 min before food presentation was significantly higher than the values for animals killed 2 h after food presentation. These results suggest that during ingestive behavior ACh release is regulated by at least two independent mechanisms: one, associated with the anticipatory phase, that is sensitive to the activation of GABA(A) receptors. and a second, associated with the consummatory phase, that is insensitive to abecarnil.

Neurochemical effects of consumption of Great Lakes salmon by rats

This study, part of a larger project to determine the health consequences of both perinatal and adult exposure to contaminated salmon from the Great Lakes, determined the neurochemical effects of exposure of rats to chow adulterated with lyophilized salmon fillets. Concentrations of biogenic amines, their metabolites, and choline acetyltransferase (ChAT) were determined in the frontal cortex (FC), nucleus accumbens, caudate nucleus (CN), hippocampus (HC), and substantia nigra (SN) of adult rats who had been exposed, both perinatally and as adults, to standard rat chow adulterated with either 5 or 20% (w/w) lyophilized fillets from either Lake Huron (LH) or Lake Ontario (LO) salmon. Dopamine (DA) concentrations in the FC were significantly decreased following exposure to both 20% fish diets. CN DA concentrations were significantly reduced in rats exposed to all diets, while SN DA was decreased only in the LO20-fed animals. SN norepinephrine concentrations were reduced in all groups except for the LO5-fed rats. 3,4-Dihydroxyphenylacetic acid (DOPAC) concentrations in the FC were significantly increased in the LH20 and LO5 groups, while CN DOPAC concentrations were reduced in LH20, LO5, and LO20 animals. 5-Hydroxyindoleacetic acid concentrations were reduced in the FC and CN of all animals exposed to diets adulterated with Great Lakes salmon. ChAT concentrations were unaffected in rats exposed to any of the adulterated diets. The significant reductions in DA, particularly in the FC and CN, suggest that either fish-borne contaminants or consumption of fish, per se, may affect behaviors that require inhibition of normal responding. We conclude that consumption of contaminated fish from the Great Lakes may result in sufficient reductions in biogenic amine function to result in significant deficits in important behavioral functions in the rat and, by inference, in the perinatally exposed human.

GALR1 galanin receptor mRNA is co-expressed by galanin neurons but not cholinergic neurons in the rat basal forebrain

The neuropeptide galanin (GAL) has been proposed to be an inhibitory modulator of cholinergic transmission in the hippocampus and may impair memory by directly affecting the activity of basal forebrain (BF) cholinergic neurons. Alternatively, GAL may act indirectly and modulate the activity of other neurotransmitter systems which, in turn, influence cholinergic transmission. We have used double in situ hybridization histochemistry to evaluate the co-expression of the GAL receptor subtype, GALR1, within cholinergic neurons in the medial septum/diagonal band of adult male rats. In alternate brain sections, we assessed the co-expression of GALR1 mRNA within another forebrain cell group implicated in memory functions, the neurons of the bed nucleus of the stria terminalis (BNST) and medial amygdala (AMe) which co-express vasopressin (VP) and GAL and project to septo-hippocampus. Despite the abundance of GALR1 mRNA-expressing neurons in the cholinergic BF, we found no evidence for the co-expression of this receptor subtype within cholinergic neurons in the medial septum/diagonal band. In contrast, we detected an extensive co-expression (95%) of GALR1 mRNA within extrahypothalamic VP/GAL neurons. These results do not support the idea that GAL, acting via the GALR1 receptor, directly impairs BF cholinergic neurons but suggest, instead, that non-cholinergic neurons in the BF may play a role in mediating the inhibitory actions of GAL on cholinergic function. However, our findings provide anatomical evidence that GAL could directly modulate the activity and/or secretion pattern of extrahypothalmic VP/GAL neurons into septo-hippocampal regions.

Rolipram and its optical isomers, phosphodiesterase 4 inhibitors, attenuated the scopolamine-induced impairments of learning and memory in rats

We investigated the effects of (+/-)-rolipram, a phosphodiesterase (PDE) 4 inhibitor, and its isomers on scopolamine-induced deficits of learning and memory in rats using an 8-arm radial maze task and a passive avoidance task. 1) In the 8-arm radial maze task, (+/-)-rolipram (0.02-0.2 mg/kg, p.o.), (-)-rolipram (0.01-0.02 and 0.2-0.5 mg/kg, p.o.) and (+)-rolipram (20-50 mg/kg, p.o.) attenuated the scopolamine-induced deficits of spatial cognition. As for the minimum effective dose of each drug, (-)-rolipram was 2 and 2000 times as potent as (+/-)-rolipram and (+)-rolipram, respectively. (-)-Rolipram produced a biphasic dose-response and (+/-)-rolipram produced a broad dose-response. 2) (+/-)-Rolipram and its isomers also attenuated the scopolamine-induced deficits in the passive avoidance response. Also for the minimum effective dose, (-)-rolipram (0.01-0.02 mg/kg) was 2 and 200 times as potent as (+/-)-rolipram (0.02-0.1 mg/kg) and (+)-rolipram (2mg/kg). 3) The behaviorally effective doses of (+/-)-rolipram and its isomers also enhanced the oxotremorine-induced tremors in mice. Comparing these racemic isomers, (-)- and (+/-)-rolipram have more potent effects than (+)-rolipram on scopolamine-induced deficits in the 8-arm radial maze task and passive avoidance task. Especially (+/-)-rolipram has a wide dose range in these behavioral study. These results suggest that the ameliorating effects of rolipram might result from the indirect potentiation of various transmitters including cholinergic and noradrenergic systems by an increase in cAMP with the inhibition of PDE4.

Repeated administration of tacrine to normal rats: effects on cholinergic, glutamatergic, and GABAergic receptor subtypes in rat brain using receptor autoradiography

Tacrine, a potent acetylcholinesterase inhibitor, has been reported to improve cognitive function in patients with Alzheimer's disease. The present investigation was conducted to elucidate in vivo any interaction between tacrine-induced cortical cholinergic hyperactivity and glutamatergic and GABAergic neurotransmission, which might influence the therapeutic potential of tacrine. Seven days after a daily dosage of 10 mg/kg tacrine i.p. quantitative receptor autoradiography was performed in coronal sections throughout the brain. Repeated administration of tacrine resulted in decreased binding to high-affinity choline uptake, nicotinic and M2-muscarinic acetylcholine receptor sites in a number of cortical regions, while reductions in M1-muscarinic receptor binding were restricted to the cingulate and entorhinal cortex as well as caudate-putamen. Moreover, tacrine injections decreased cortical AMPA receptor binding throughout the brain, while NMDA, kainate, and GABAA receptor binding remained unchanged. Tacrine administration alters cortical AMPA receptor binding in the opposite direction to that observed in patients with Alzheimer's disease, suggesting that tacrine may exert a reversal in up/down-regulation of cortical glutamate receptor subtypes in Alzheimer patients. However, the drug-induced reductions in cortical high-affinity choline uptake sites as well as in nicotinic and in muscarinic acetylcholine receptor binding might partially counteract the cognition-enhancing effects of tacrine produced by acetylcholinesterase inhibition.