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

Gonadal steroids maintain 24 h acetylcholine release in the hippocampus: organizational and activational effects in behaving rats

Extracellular acetylcholine (ACh) levels in the dorsal hippocampus increases during learning or exploration, exhibiting a sex-specific 24 h release profile. To examine the activational effect of gonadal steroid hormones on the sex-specific ACh levels and its correlation with spontaneous locomotor activity, we observed these parameters simultaneously for 24 h. Gonadectomy severely attenuated the ACh levels, whereas the testosterone replacement in gonadectomized males or 17beta-estradiol replacement in gonadectomized females successfully restored the levels. 17beta-Estradiol-priming in gonadectomized males could not restore the ACh levels, and testosterone replacement in gonadectomized females failed to raise ACh levels to those seen in testosterone-primed gonadectomized males, revealing a sex-specific activational effect. Spontaneous locomotor activity was not changed in males by gonadectomy or the replacement of gonadal steroids, but 17beta-estradiol enhanced the activity in gonadectomized females. Gonadectomy severely reduced the correlation between ACh release and activity levels, but the testosterone replacement in gonadectomized males or 17beta-estradiol replacement in gonadectomized females successfully restored it. To further analyze the sex-specific effect of gonadal steroids, we examined the organizational effect of gonadal steroids on the ACh release in female rats. Neonatal testosterone or 17beta-estradiol treatment not only increased the ACh levels but also altered them to resemble male-specific ACh release properties without affecting levels of spontaneous locomotor activity. We conclude that the activational effects of gonadal steroids maintaining the ACh levels and the high correlation with spontaneous locomotor activity are sex-specific, and that the organizational effects of gonadal steroids suggest estrogen receptor-mediated masculinization of the septo-hippocampal cholinergic system.

Activational and organisational effects of gonadal steroids on sex-specific acetylcholine release in the dorsal hippocampus

Acetylcholine (ACh) release in the dorsal hippocampus increases during stress, exploration or learning, exhibiting sex-specific 24-h release profile. We review the role of gonadal steroids on the ACh release in the dorsal hippocampus. In our studies, we found that male rats showed higher extracellular ACh levels than females, but gonadectomy decreased ACh levels in both sexes of rats and subsequently eliminated the sex difference. To examine the sex difference under comparable gonadal steroid levels, we implanted steroid capsules after gonadectomy. Oestradiol supplementation maintained circulating oestradiol to the levels in proestrous female rats, whereas testosterone capsules maintained circulating testosterone to the levels similar to intact male rats. Under comparable gonadal steroids levels, ACh levels were sex-specific. Testosterone replacement in orchidectomised rats clearly restored ACh levels, which were greater than ovariectomised testosterone-primed rats. Similarly, oestradiol replacement in ovariectomised rats successfully restored ACh levels, which were higher than orchidectomised oestradiol-primed rats. These results suggest sex-specific activational effects of gonadal steroids on ACh release. To further examine the organisational effect, female pups were neonatally treated with oil, testosterone, oestradiol, or dihydrotestosterone. These rats were bilaterally ovariectomised and a testosterone capsule was implanted at postnatal week 8. Neonatal treatment of either testosterone or oestradiol clearly increased ACh levels, whereas neonatal dihydrotestosterone treatment failed to change levels. These results suggest that: (i) gonadal steroids maintain the sex-specific ACh release in the dorsal hippocampus and (ii) neonatal activation of oestrogen receptors is sufficient to mediate masculinisation of the septo-hippocampal cholinergic system.

MK801- and scopolamine-induced amnesias are reversed by an Amazonian herbal locally used as a "brain tonic"

RATIONALE

Traditional remedies prepared from Ptychopetalum olacoides (PO) are used throughout the Amazon to alleviate age-related conditions. These formulas are mainly used by elders, and alleged effects may be related to the anticholinesterase properties identified in a standardized ethanol extract of this species [P. olacoides standardized ethanol extract (POEE)].

OBJECTIVES

To further characterize the potential of this extract for developing drugs useful to treat cognitive deficits, the effects of POEE on scopolamine (scop)- and MK801-induced amnesias (acquisition, consolidation, and retrieval) in mice were investigated.

RESULTS

Scop (3.0 mg/kg, ip) significantly impaired memory (all three phases) in the step-down inhibitory avoidance protocol. As expected, MK801 (0.1 mg/kg, ip) was amnesic regarding acquisition and consolidation, but not retrieval. POEE (100 mg/kg, ip) reversed the scop-induced impairment in all three phases of long-term and short memories, whereas only the memory consolidation deficit was reversed with MK801-induced amnesia.

CONCLUSIONS

This study complements previously reported promnesic properties of this plant extract and suggests that POEE may be further developed for treating conditions associated with cognitive deficits, especially those linked with cholinergic malfunction.

Anxiolytic-like effects of substance P administration into the dorsal, but not ventral, hippocampus and its influence on serotonin

Substance P (SP) is known to be involved in processes related to learning and memory, fear, anxiety and stress. SP and NK1 receptors are localized in the hippocampus, a brain structure involved in learning and memory as well as emotional processes. As there is evidence for differential functions of the ventral (VH) and dorsal (DH) hippocampus in a variety of behaviors, we here evaluated the effects of injections of SP into the VH and DH in rats submitted to the elevated plus-maze (EPM) and open field (OF) tests. The results obtained showed that infusions of 100 and 1000 ng of SP into the DH, but not VH, increased open arm activity in the EPM and in the central zone of the OF, indicative of anxiolytic-like action. These effects were observed in the absence of significant changes in general motor activity. In an additional experiment to examine whether these effects of SP are mediated by local serotoninergic mechanisms, extracellular concentrations of this monoamine were assessed by use of in vivo microdialysis. Infusions of SP into the DH did not influence the extracellular concentration of serotonin. These data indicate that neurokinins in the DH, but not VH, are involved in mechanisms associated with anxiety and that the mediation of SP in anxiety-related behaviors is independent of local serotonergic mechanisms.

Serotonin depletion in the dorsal and ventral hippocampus: effects on locomotor hyperactivity, prepulse inhibition and learning and memory

We present an overview of our studies on the differential role of serotonergic projections from the median raphe nucleus (MRN) and dorsal raphe nucleus (DRN) in behavioural animal models with relevance to schizophrenia. Stereotaxic microinjection of the serotonin neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) into the MRN or one of its main projections regions, the dorsal hippocampus, induced a marked enhancement of phencyclidine-induced locomotor hyperactivity and a disruption of prepulse inhibition (PPI) in rats. There was no enhancement of locomotor hyperactivity induced by amphetamine or MK-801 or after 5,7-DHT lesions of the DRN or ventral hippocampus. Rats with dorsal hippocampus lesions did not show significant changes in the Y-maze test for short-term spatial memory, the Morris water maze for long-term spatial memory, or in the T-maze delayed alternation test for working memory. These chronic lesion studies suggest a modulatory influence of serotonergic projections from the MRN to the dorsal hippocampus on phencyclidine effects and prepulse inhibition, but not on different forms of learning and memory. The results provide new insight into the role of serotonin in the dorsal hippocampus in aspects of schizophrenia.

Stress and re-stress increases conditioned taste aversion learning in rats: possible frontal cortical and hippocampal muscarinic receptor involvement

Symptoms of posttraumatic stress disorder are often precipitated by sensory cues in the form of visual, auditory, olfactory and gustatory "flashbacks" resulting in enhanced fear-memory consolidation and the characteristic symptoms of re-experiencing, avoidance and hyper-arousal. Single prolonged stress with and without re-stress have been used to explore the neurobiology of this disorder, particularly with respect to contextual conditioning and spatial memory impairment. However, less work has been done regarding associative sensory-related memories linked to aversive events. Although growing evidence supports a role for cholinergic pathways in stress, this has not been studied in the above animal models. We studied the effects of single prolonged stress with and without re-stress on conditioned taste aversion learning in rats, together with differential analysis of frontal cortical and hippocampal [3H]-quinuclidinyl benzylate ([3H]-QNB) muscarinic receptor binding. Single prolonged stress with and without re-stress both enhanced associative sensory aversion learning 7 days after stressor-taste pairing, although re-stress did not strengthen this response. Increased cortical and hippocampal muscarinic receptor density (Bmax) was found 7 days after single prolonged stress with re-stress, although receptor affinity remained unaltered. Frontal cortical and hippocampal muscarinic receptor changes may thus underlie conditioned taste aversion learning in rats exposed to stress and re-stress. These data suggest that it may be useful to study the role of cholinergic pathways in mediating associative memory in psychiatric disorders such as posttraumatic stress disorder.

Positive modulation of cognition and mood in the healthy elderly volunteer following the administration of Centella asiatica

AIMS OF THIS STUDY

Centella asiatica has a reputation to restore decline cognitive function in traditional medicine and in animal model. However, little evidence regarding the efficacy of Centella asiatica from systematized trials is available. Therefore, the present randomized, placebo-controlled, double-blind study investigated the effect of Centella asiatica on cognitive function of healthy elderly volunteer.

MATERIALS AND METHODS

Twenty-eight healthy elderly participants received the plant extract at various doses ranging 250, 500 and 750 mg once daily for 2 months. Cognitive performance was assessed using the computerized test battery and event-related potential whereas mood was assessed using Bond-Lader visual analogue scales prior to the trial and after single, 1 and 2 months after treatment.

RESULTS

The results showed that the high dose of the plant extract enhanced working memory and increased N100 component amplitude of event-related potential. Improvements of self-rated mood were also found following the Centella asiatica treatment.

CONCLUSION

Therefore, the present findings suggest the potential of Centella asiatica to attenuate the age-related decline in cognitive function and mood disorder in the healthy elderly. However, the precise mechanism(s) underlying these effects still require further investigation.

Gonadal steroid hormones maintain the stress-induced acetylcholine release in the hippocampus: simultaneous measurements of the extracellular acetylcholine and serum corticosterone levels in the same subjects

To examine the role of gonadal steroid hormones in the stress responses of acetylcholine (ACh) levels in the hippocampus and serum corticosterone levels, we observed these parameters simultaneously in intact, gonadectomized, or gonadectomized steroid-primed rats. In both sexes of rats, neither gonadectomy nor the replacement of gonadal steroid hormone affected the baseline levels of ACh. However, gonadectomy severely attenuated the stress response of ACh, whereas the replacement of corresponding gonadal hormone successfully restored the response to intact levels. The gonadal hormones affected the serum corticosterone levels in a different manner; the testosterone replacement in orchidectomized rats suppressed the baseline and the stress response of corticosterone levels, whereas the 17beta-estradiol replacement in ovariectomized rats increased the levels. We further found that letrozole or flutamide administration in intact male rats attenuated the stress response of ACh. In addition, flutamide treatment increased the baseline levels of corticosterone, whereas letrozole treatment attenuated the stress response of corticosterone. Moreover, we found a low positive correlation between the ACh levels and corticosterone levels, depending on the presence of gonadal steroid hormone. We conclude that: 1) gonadal steroid hormones maintain the stress response of ACh levels in the hippocampus, 2) the gonadal steroid hormone independently regulates the stress response of ACh in the hippocampus and serum corticosterone, and 3) the sex-specific action of gonadal hormone on the cholinergic stress response may suggest a neonatal sexual differentiation of the septohippocampal cholinergic system in rats.

Blunted hippocampal, but not striatal, acetylcholine efflux parallels learning impairment in diencephalic-lesioned rats

A rodent model of diencephalic amnesia, pyrithiamine-induced thiamine deficiency (PTD), was used to investigate the dynamic role of hippocampal and striatal acetylcholine (ACh) efflux across acquisition of a nonmatching-to-position (NMTP) T-maze task. Changes in ACh efflux were measured in rats at different time points in the acquisition curve of the task (early=day 1, middle=day 5, and late=day 10). Overall, the control group had higher accuracy scores than the PTD group in the latter sessions of NMTP training. During the three microdialysis sampling points, all animals displayed significant increases in ACh efflux in both hippocampus and striatum, while performing the task. However, on day 10, the PTD group showed a significant behavioral impairment that paralleled their blunted hippocampal--but not striatal--ACh efflux during maze training. The results support selective diencephalic-hippocampal dysfunction in the PTD model. This diencephalic-hippocampal interaction appears to be critical for successful episodic and spatial learning/memory.

Comparative brain cholinesterase-inhibiting activity of Glycyrrhiza glabra, Myristica fragrans, ascorbic acid, and metrifonate in mice

The central cholinergic pathways play a prominent role in the learning and memory processes. Acetylcholinesterase is an enzyme that inactivates acetylcholine. The present study was undertaken to estimate the acetylcholinesterase- inhibiting activity of extracts of Glycyrrhiza glabra, Myristica fragrans seeds, and ascorbic acid and compare these values with a standard acetylcholinesterase-inhibiting drug, metrifonate. Aqueous extract of G. glabra (150 mg/kg p.o. for 7 successive days), n-hexane extract of M. fragrans seeds (5 mg/kg p.o. for 3 successive days), ascorbic acid (60 mg/kg i.p. for 3 successive days), and metrifonate (50 mg/kg i.p.) were administered to young male Swiss albino mice. Acetylcholinesterase enzyme was estimated in brains of mice. G. glabra, M. fragrans, ascorbic acid, and metrifonate significantly decreased acetylcholinesterase activity as compared with their respective vehicle-treated control groups.

Effect of long-lasting serotonin depletion on environmental enrichment-induced neurogenesis in adult rat hippocampus and spatial learning

The dentate gyrus of the hippocampal formation produces new neurons throughout adulthood in mammalian species. Several experimental statuses and factors regulating to neurogenesis have been identified in the adult dentate gyrus. For example, exposure to an enriched environment enhances neurogenesis in the dentate gyrus and improves hippocampus-dependent spatial learning. Furthermore, serotonin is known to influence adult neurogenesis, and learning and memory. However, the effects of long-lasting depletion of serotonin over the developing period on neurogenesis have not been investigated. Thus, we examined the influence of long-lasting serotonin depletion on environmental enrichment-induced neurogenesis and spatial memory performance. As reported previously, environmental enrichment significantly increased new neurons in the dentate gyrus. However, there was no improvement of the spatial learning test in adult rats in standard and in environmental enrichment housings. Intracisternal administration of the serotonergic neurotoxin, 5,7-dihydroxytryptamine, on postnatal day 3 apparently reduced serotonin content in the adult hippocampus without regeneration. This experimental depletion of serotonin in the hippocampus of rats housed in an enriched environment had no effect on spatial memory performance, but produced significant decreases in the number of bromodeoxyuridine-labeled new cells in the dentate gyrus. These findings indicate that newly generated cells stimulated by environmental enrichment are not critical for improvements in hippocampus-dependent learning. Furthermore, numbers of bromodeoxyuridine-labeled cells in the dentate gyrus of 5,7-dihydroxytryptamine-injected rats did not differ between 1 day and 4 weeks after bromodeoxyuridine injection. These data suggest that survival of newly generated dentate gyrus cells remains relatively constant under long-lasting serotonin depletion.

Aging is accompanied by a subfield-specific reduction of serotonergic fibers in the tree shrew hippocampal formation

The hippocampal formation is a crucial structure for learning and memory, and serotonin together with other neurotransmitters is essential in these processes. Although the effects of aging on various neurotransmitter systems in the hippocampus have been extensively investigated, it is not entirely clear whether or how the hippocampal serotonergic innervation changes during aging. Rat studies, which have mostly focused on aging-related changes in the dentate gyrus, have implied a loss of hippocampal serotonergic fibers. We used the tree shrew (Tupaia belangeri), an intermediate between insectivores and primates, as a model of aging. We applied immunocytochemistry with an antibody against serotonin to assess serotonergic fiber densities in the various hippocampal subfields of adult (0.9-1.3 years) and old (5-7 years) tree shrews. Our results have revealed a reduction of serotonergic fiber densities in the stratum radiatum of CA1 and CA3, and in the stratum oriens of CA3. A partial depletion of serotonin in the hippocampal formation, as can be expected from our current observations, will probably have an impact on the functioning of hippocampal principal neurons. Our findings also indicate that the rat and the tree shrew hippocampal serotonergic innervation show some variations that seem to be differentially affected during aging.

Altered dendritic arborization of amygdala neurons in young adult rats orally intubated with Clitorea ternatea aqueous root extract

Young adult (60 day old) Wistar rats of either sex were orally intubated with 50 mg/kg body weight and 100 mg/kg body weight of aqueous root extract of Clitoria ternatea (CTR) for 30 days, along with age-matched saline controls. These rats were then subjected to passive avoidance tests and the results from these studies showed a significant increase in passive avoidance learning and retention. Subsequent to the passive avoidance tests, these rats were killed by decapitation. The amygdala was processed for Golgi staining and the stained neurons were traced using a camera lucida and analysed. The results showed a significant increase in dendritic intersections, branching points and dendritic processes arising from the soma of amygdaloid neurons in CTR treated rats especially in the 100 mg/kg group of rats, compared with age-matched saline controls. This improved dendritic arborization of amygdaloid neurons correlates with the increased passive avoidance learning and memory in the CTR treated rats as reported earlier. The results suggest that Clitoria ternatea aqueous root extract enhances memory by increasing the functional growth of neurons of the amygdala.

Muscarinic acetylcholine receptor activation enhances hippocampal neuron excitability and potentiates synaptically evoked Ca(2+) signals via phosphatidylinositol 4,5-bisphosphate depletion

Using single cell Ca(2+) imaging and whole cell current clamp recordings, this study aimed to identify the signal transduction mechanisms involved in mACh receptor-mediated, enhanced synaptic signaling in primary cultures of hippocampal neurons. Activation of M(1) mACh receptors produced a 2.48 +/- 0.26-fold enhancement of Ca(2+) transients arising from spontaneous synaptic activity in hippocampal neurons. Combined imaging of spontaneous Ca(2+) signals with inositol 1,4,5-trisphosphate (IP(3)) production in single neurons demonstrated that the methacholine (MCh)-mediated enhancement required activated G(q/11)alpha subunits and phospholipase C activity but did not require measurable increases in IP(3). Electrophysiological studies demonstrated that MCh treatment depolarized neurons from -64 +/- 3 to -45 +/- 3 mV and increased action potential generation. Depletion of plasma membrane phosphatidylinositol 4,5-bisphosphate (PIP(2)) enhanced neuronal excitability and prolonged the action of MCh. These studies suggest that, in addition to producing the second messengers IP(3) and diacylglycerol, mACh receptor activation may directly utilize PIP(2) hydrolysis to regulate neuronal excitability.

Feeding with powdered diet after weaning increases visuospatial ability in association with increases in the expression of N-methyl-d-aspartate receptors in the hippocampus of female rats

We determined whether feeding with powdered diet improved the visuospatial ability in female rats by checking the expression of N-methyl-D-aspartate receptor (NMDAR) subunit 1 (NR1) mRNA in the hippocampus. In rats fed standard pelleted diet, males performed better than females in a radial 8-arm maze task as we reported previously. We found that the expression of NR1 mRNA, which may be the key mediator in visuospatial ability in the hippocampus, was also higher in males than in females. However, in rats fed powdered diet, no sex difference was seen in the radial 8-arm maze task and the expression of NR1 mRNA in the hippocampus, since feeding with powdered diet improved the visuospatial ability with increases in the expression of NR1 mRNA in the hippocampus in females. We suggest that the sex difference in visuospatial ability is at least in part due to feeding conditions.

Fluoxetine disrupts the integration of anxiety and aversive memories

Anxiety disorders may result from an overexpression of aversive memories. Evidence suggests that the hippocampal cholinergic system could be the point of convergence of anxiety and memory. We propose that clinically effective anxiolytics may exert their effect by interfering with this integration mechanism. To assess anxiety and aversive memory, we used the shock-probe burying test. A reduction in anxiety in this test is indicated by decreased burying, whereas impaired cognition is reflected by an increased number of probe-contacts and/or reduced retention latency. Both an aversive stimulus and the memory of that stimulus significantly increased hippocampal acetylcholine (ACh) levels (Experiment 1). In fact, the memory of the event seemed to be more important than the event itself since the aversive memory induced a greater increase in hippocampal ACh. Injections (i.p.) of fluoxetine (Prozac) reduced burying behavior, while not affecting probe contacts or retention latency (Experiment 2). Although injections of fluoxetine did not affect basal hippocampal ACh efflux (Experiment 3), fluoxetine abolished the increase in ACh induced by the aversive stimulus and the memory of that stimulus (Experiment 4), emphasizing the significance of aversive memories in anxiety disorders. These actions may be mediated by a decrease in the event-related enhancement in cholinergic neurotransmission through M1 cholinergic receptors (Experiment 5). Therefore, anxiety disorders may stem from an unopposed formation of aversive memories and clinically effective anxiolytics hinder the association between emotional and cognitive processing. This reduces the emotional impact of aversive memories, thereby opposing consequent anxiety.

Feeding with powdered diet after weaning affects sex difference in acetylcholine release in the hippocampus in rats

We have reported in the past that female rats fed a powdered diet showed better spatial learning and memory functions than female rats a fed pelleted diet. In the present study, we examined the effects of feeding with powdered diet on acetylcholine release in the hippocampus in both sexes of rats. After weaning (3 weeks of age), rats were fed either standard pelleted diet or powdered diet, and after maturation (9-12 weeks of age), they were used in an in vivo microdialysis study, in which no eserine (a cholinesterase inhibitor) was added to the perfusate. The dialysate was collected from the dorsal hippocampus at 20-min intervals under freely moving conditions for more than 24 h. Acetylcholine in the dialysate was measured by high performance liquid chromatography. As we reported previously, the acetylcholine release showed a clear daily rhythm in both sexes, and males showed significantly greater acetylcholine release in the hippocampus than females in rats fed pelleted diet. Conversely, in rats fed powdered diet, no sex difference in the acetylcholine release was observed, since feeding with powdered diet significantly increased the acetylcholine release only in females. To further examine the number of cholinergic neurons in the medial septum and horizontal limb of the diagonal band of Broca, immunocytochemistry for choline acetyltransferase was performed in both sexes of rats fed either standard pelleted diet or powdered diet. However, neither sex nor feeding conditions affect the number of choline acetyltransferase immunoreactive cells in the areas. These results suggest that powdered diet after weaning enhances spontaneous acetylcholine release in the hippocampus in female rats without changes in the number of cholinergic neurons in the areas. It is possible that this effect of feeding contributes to improve the performance in spatial learning and memory functions in female rats fed powdered diet.

Inhibition of acetylcholine-induced activation of extracellular regulated protein kinase prevents the encoding of an inhibitory avoidance response in the rat

It has been demonstrated that the forebrain cholinergic system and the extracellular regulated kinase signal transduction pathway are involved in the mechanisms of learning, encoding, and storage of information. We investigated the involvement of the cholinergic and glutamatergic systems projecting to the medial prefrontal cortex and ventral hippocampus and of the extracellular regulated kinase signal transduction pathway in the acquisition and recall of the step-down inhibitory avoidance response in the rat, a relatively simple behavioral test acquired in a one-trial session. To this aim we studied by microdialysis the release of acetylcholine and glutamate, and by immunohistochemistry the activation of extracellular regulated kinase during acquisition, encoding and recall of the behavior. Cholinergic, but not glutamatergic, neurons projecting to the medial prefrontal cortex and ventral hippocampus were activated during acquisition of the task, as shown by increase in cortical and hippocampal acetylcholine release. Released acetylcholine in turn activated extracellular regulated kinase in neurons located in the target structures, since the muscarinic receptor antagonist scopolamine blocked extracellular regulated kinase activation. Both increased acetylcholine release and extracellular regulated kinase activation were necessary for memory formation, as administration of scopolamine and of extracellular regulated kinase inhibitors was followed by blockade of extracellular regulated kinase activation and amnesia. Our data indicate that a critical function of the learning-associated increase in acetylcholine release is to promote the activation of the extracellular regulated kinase signal transduction pathway and help understanding the role of these systems in the encoding of an inhibitory avoidance memory.

Genetic deletion and pharmacological blockade of CB1 receptors modulates anxiety in the shock-probe burying test

Cannabinoids affect various behavioral processes, including emotion, learning and memory, which may be specifically regulated through the CB1 receptors. The exact role CB1 receptors play in anxiety remains unclear. Both genetic and pharmacological blockade of CB1 receptors have produced inconsistent effects on anxiety. However, these studies examined passive avoidance as an index of anxiety. In the present study, both active and passive avoidance were examined using the shock-probe burying test while CB1 receptors were blocked genetically or pharmacologically. In the shock-probe burying test, anxiety is reflected by increased burying (increased active avoidance) and increased freezing (increased passive avoidance). In addition, probe-contacts may reflect cognitive performance and/or passive avoidance. As there have been few studies examining mouse behavior in the shock-probe burying test, experiment 1 was designed to pharmacologically validate this model in mice. Our results indicated that administration (i.p.) of chlordiazepoxide (4 mg/kg) or FG7412 (5 mg/kg) decreased and increased burying behavior, respectively, without affecting freezing or the number of probe contacts. Experiments 2 and 3 showed that both CB1 knockout mice and mice injected (i.p.) with 3 or 10 mg/kg, but not 1 mg/kg, of the CB1 receptor antagonist SR141716A had lower burying scores, fewer contacts with the probe and similar freezing times compared with wild-type mice and mice injected with vehicle (experiments 2 and 3). Collectively, these results suggest that CB1 receptor blockade reduces some, but not all, aspects of anxiety. The decrease in probe contacts induced by CB1 receptor blockade may be due to enhanced cognition.

Septohippocampal acetylcholine: involved in but not necessary for learning and memory?

The neurotransmitter acetylcholine (ACh) has been accorded an important role in supporting learning and memory processes in the hippocampus. Cholinergic activity in the hippocampus is correlated with memory, and restoration of ACh in the hippocampus after disruption of the septohippocampal pathway is sufficient to rescue memory. However, selective ablation of cholinergic septohippocampal projections is largely without effect on hippocampal-dependent learning and memory processes. We consider the evidence underlying each of these statements, and the contradictions they pose for understanding the functional role of hippocampal ACh in memory. We suggest that although hippocampal ACh is involved in memory in the intact brain, it is not necessary for many aspects of hippocampal memory function.

Changes in acetylcholine extracellular levels during cognitive processes

Measuring the changes in neurotransmitter extracellular levels in discrete brain areas is considered a tool for identifying the neuronal systems involved in specific behavioral responses or cognitive processes. Acetylcholine (ACh) is the first neurotransmitter whose diffusion from the central nervous system was investigated and whose extracellular levels variations were correlated to changes in neuronal activity. This was done initially by means of the cup technique and then by the microdialysis technique. The latter, notwithstanding some technical limitations, makes it possible to detect variations in extracellular levels of ACh in unrestrained, behaving animals. This review summarizes and discusses the results obtained investigating the changes in ACh release during performance of operant tasks, exposition to novel stimuli, locomotor activity, and the performance of spatial memory tasks, working memory, and place preference memory tasks. Activation of the forebrain cholinergic system has been demonstrated in many tasks and conditions in which the environment requires the animal to analyze novel stimuli that may represent a threat or offer a reward. The sustained cholinergic activation, demonstrated by high levels of extracellular ACh observed during the behavioral paradigms, indicates that many behaviors occur within or require the facilitation provided by the cholinergic system to the operation of pertinent neuronal pathways.

Association of learning and memory impairments with changes in the septohippocampal cholinergic system in rats with kaolin-induced hydrocephalus

OBJECTIVE

The septohippocampal cholinergic (SHC) system plays an important role in the maintenance of normal memory and learning. However, the fact that memory and learning impairments under hydrocephalic conditions are directly related to the SHC system is less well known. We investigated the relationships between pathological changes in SHC neurons and impairments in memory and learning among hydrocephalic rats.

METHODS

Rats with kaolin-induced hydrocephalus were prepared with injections of kaolin suspension into the cisterna magna. Learning and memory performance was assessed with the passive avoidance and Morris water maze tests. Ventricular sizes were measured for the lateral and third ventricles. Acetylcholinesterase and choline acetyltransferase immunostaining was performed to investigate degenerative changes in cholinergic neurons in the medial septum and hippocampus.

RESULTS

Hydrocephalic rats demonstrated significant learning and memory impairments in the passive avoidance and Morris water maze tests. Decreased hesitation times in the passive avoidance test and markedly increased acquisition times and decreased retention times in the Morris water maze test indicated learning and memory dysfunction among the hydrocephalic rats. The numbers of cholinergic neurons in the medial septum and hippocampus were decreased in the hydrocephalic rats. The decreases in choline acetyltransferase and acetylcholinesterase immunoreactivity were significantly correlated with enlargement of the ventricles.

CONCLUSION

Impairment of spatial memory and learning may be attributable to degeneration of SHC neurons. These results suggest that learning and memory impairments in rats with kaolin-induced hydrocephalus are associated with the dysfunction of the SHC system induced by ventricular dilation.

Switching memory systems during learning: changes in patterns of brain acetylcholine release in the hippocampus and striatum in rats

This experiment measured acetylcholine (ACh) release simultaneously in the hippocampus and striatum while rats were trained in a cross maze. Consistent with past findings, rats initially showed learning on the basis of place (i.e., turning to the correct position relative to the room), but after extensive training, rats shifted to learning on the basis of response (i.e., turning to the right/left to find the food). Profiles of ACh release in the hippocampus and striatum were markedly different during training. In the hippocampus, ACh release increased by approximately 60% at the onset of training and remained at that level of release throughout training, even after the rats began to show learning on the basis of turning rather than place. In the striatum, increases in ACh release occurred later, reaching asymptotic increases of 30-40%, coincident with a transition from expressing place learning to expressing response learning. These findings suggest that the hippocampal and striatal systems both participate in learning in this task, but in a manner characterized by differential activation of the neural systems. The hippocampal system is apparently engaged first before the striatum is activated and, to the extent the hippocampus is important for place learning, promotes the use of a place solution to the maze. Later in training, although the hippocampus remains activated, the striatum is also activated in a manner that may enable the use of a response strategy to solve the maze. These findings may offer a neurobiological marker of a transition during skill learning from declarative to procedural learning.

Rats living in small cages respond to restraint stress with adrenocortical corticosterone release but not with hippocampal acetylcholine release

We previously reported that the restriction of environmental space attenuated the hippocampal acetylcholine release and impaired spatial learning function. To examine the effect of the restriction of environmental space on the stress response of the hippocampal acetylcholine release, an in vivo microdialysis study was performed in male rats after 4 days of housing in a large cylindrical cage (diameter=35 cm) or a small cylindrical cage (diameter=19 cm). Significant stress response of the hippocampal acetylcholine release was observed in rats in the large cages (N=5), but it was not observed in rats in the small cages (N=5). The corticosterone concentration in serum was significantly increased by the restraint stress in both groups of rats. Although cage size does not influence stress-induced secretion of corticosterone, rats housed in a small cage exhibit lower levels of stress-induced ACh release than rats living in a large cage.

Vitamin A deficiency produces spatial learning and memory impairment in rats

Vitamin A and its derivatives (retinoids) play important roles in many physiological processes. The recent finding of high levels of cellular retinol-binding protein type 1 immunoreactivity, cellular retinoic acid-binding protein type 1 immunoreactivity and the presence of nuclear retinoid receptors in the central nervous system of adult rodents suggests that retinoids may carry out important roles in the adult brain. In consideration of the role of the hippocampus in spatial learning and memory we evaluated the effect of vitamin A deprivation in adult rats on these functions. Following 12 weeks of vitamin A-free diet, rats were trained to acquire a radial-arm maze task. Results show that this diet induced a severe deficit in the spatial learning and memory task. The cognitive impairment was fully restored when vitamin A was replaced in the diet. We also found a significant decrease in hippocampal acetylcholine release induced by scopolamine, assessed using microdialysis technique, and a reduction in the size of hippocampal nuclei of CA1 region in vitamin-deficient rats, compared to rats fed with a vitamin A-sufficient diet. These results demonstrate that vitamin A has a critical role in the learning and memory processes linked to a proper hippocampal functioning.

Hippocampal brain amines in methotrexate-induced learning and memory deficit

Intrathecal methotrexate in children with leukemia is known to cause seizures, dementia, leukoencephalopathy, and cognitive dysfunction after long-term treatment. To investigate the cognitive dysfunction, male Wistar rats were given multiple intracerebroventricular injections of methotrexate. Its effect on behaviour was tested in the two-compartment conditioned avoidance task and dark-bright arena test. Levels of brain amines in the hippocampal region of the brain were estimated by HPLC. The qualitative and quantitative histopathological changes in the different regions of the hippocampus were studied by cresyl violet staining. Multiple injections (1 or 2 mg/kg) produced convulsions and learning and memory impairment but did not induce anxiolytic activity. They also reduced concentrations of all three brain amines (norepinephrine, dopamine, and serotonin) and the serotonin metabolite 5-hydroxyindoleacetic acid. The CA4 region of the hippocampus was severely affected by intraventricular methotrexate. Disruption of brain monoamines has been proposed as a cause of brain dysfunction from this chemotherapy, and that disruption may in turn involve cytotoxic effects of methotrexate on brain tissue. The outcomes of this study may have therapeutic implications in the management of cancer conditions, particularly in childhood lymphoblastic leukemia.

Dorsal and ventral hippocampal cholinergic systems modulate anxiety in the plus-maze and shock-probe tests

There is emerging evidence that increased acetylcholine levels in brain reduce anxiety. More specifically there is evidence that some of these anxiolytic effects of acetylcholine are modulated by the hippocampus. In the present study we examined the roles of the cholinergic systems in the dorsal and ventral hippocampus in two animal models of anxiety: the elevated plus-maze and the shock-probe burying tests. We found that microinfusions (10 microg/0.5 microl) of the acetylcholinesterase inhibitor physostigmine in either the dorsal or the ventral hippocampus increased rats' open arm exploration in the plus-maze test, and decreased burying behavior in the shock-probe test. Interestingly, infusions in the ventral, but not the dorsal hippocampus also increased the number of contacts rats made with the shock-probe. Overall, the results suggest that cholinergic stimulation in the dorsal and ventral hippocampus modulate anxiety, but that only the ventral hippocampal cholinergic system is involved in the passive avoidance of painful stimuli.

Differences in paired-pulse facilitation and long-term potentiation between dorsal and ventral CA1 regions in anesthetized rats

To clarify hippocampal regional differences in synaptic plasticity, paired-pulse facilitation (PPF, a form of short-term plasticity), long-term potentiation (LTP, a form of long-term plasticity), and their interactions were studied in the dorsal and ventral hippocampal CA1 regions of anesthetized rats. Baseline PPF and post-LTP PPF experiments were conducted at interstimulus intervals (ISIs) of 20-320 ms. A general protocol (100 Hz, 1 s) and a stronger protocol (250-Hz pulse series) were applied for LTP induction. PPF were observed in both regions; however, the degree was lower and the range of ISIs was narrower in the ventral region compared with the dorsal region. The degree of ventral LTP was lower than that of the dorsal LTP. The interaction between PPF and LTP was observed in both regions (PPF change correlated inversely with degree of baseline PPF). However, this was also different in each region. Dorsal PPF increased or decreased; in contrast, ventral PPF of short ISIs after LTP only decreased. These regional differences in short-term and long-term synaptic plasticity may explain a consequence of different afferent inputs and information processing.

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)].

Nuclear calcium signaling evoked by cholinergic stimulation in hippocampal CA1 pyramidal neurons

The cholinergic system is thought to play an important role in hippocampal-dependent learning and memory. However, the mechanism of action of the cholinergic system in these actions in not well understood. Here we examined the effect of muscarinic receptor stimulation in hippocampal CA1 pyramidal neurons using whole-cell recordings in acute brain slices coupled with high-speed imaging of intracellular calcium. Activation of muscarinic acetylcholine receptors by synaptic stimulation of cholinergic afferents or application of muscarinic agonist in CA1 pyramidal neurons evoked a focal rise in free calcium in the apical dendrite that propagated as a wave into the soma and invaded the nucleus. The calcium rise to a single action potential was reduced during muscarinic stimulation. Conversely, the calcium rise during trains of action potentials was enhanced during muscarinic stimulation. The enhancement of free intracellular calcium was most pronounced in the soma and nuclear regions. In many cases, the calcium rise was distinguished by a clear inflection in the rising phase of the calcium transient, indicative of a regenerative response. Both calcium waves and the amplification of action potential-induced calcium transients were blocked the emptying of intracellular calcium stores or by antagonism of inositol 1,4,5-trisphosphate receptors with heparin or caffeine. Ryanodine receptors were not essential for the calcium waves or enhancement of calcium responses. Because rises in nuclear calcium are known to initiate the transcription of novel genes, we suggest that these actions of cholinergic stimulation may underlie its effects on learning and memory.

Unaltered radial maze performance and brain acetylcholine of the endothelial nitric oxide synthase knockout mouse

Proceeding from previous findings of a beneficial effect of endothelial nitric oxide synthase (eNOS) gene inactivation on negatively reinforced water maze performance, we asked whether this improvement in place learning capacities also holds for a positively reinforced radial maze task. Unlike its beneficial effects on the water maze task, eNOS gene inactivation did not facilitate radial maze performance. The acquisition performance over the days of place learning did not differ between eNOS knockout (eNOS-/-) and wild-type mice (eNOS+/+). eNOS-/- mice displayed a slight and eNOS+/+ mice a more severe working memory deficit in the place learning version of the radial maze compared to the genetic background C57BL/6 strain. Possible differential effects of eNOS inactivation, related to differences in reinforcement contingencies between the Morris water maze and radial maze tasks, behavioral strategy requirements, or to different emotional and physiological concomitants inherent in the two tasks are discussed. These task-unique characteristics might be differentially affected by the reported anxiogenic and hypertensional effects of eNOS gene inactivation. Post-mortem determination of acetylcholine concentrations in diverse brain structures revealed that acetylcholine and choline contents were not different between eNOS-/- and eNOS+/+ mice, but were increased in eNOS+/+ mice compared to C57BL/6 mice in the frontal cortex. Our findings demonstrate that phenotyping of learning and memory capacities should not rely on one learning task only, but should include tasks employing both negative and positive reinforcement contingencies in order to allow valid statements regarding differences in learning capacities between rodent strains.

Memory-related acetylcholine efflux from rat prefrontal cortex and hippocampus: a microdialysis study

To investigate the relationship between the prefrontal and hippocampal acetylcholine (ACh) systems and working memory, an in vivo microdialysis study was conducted. A group of rats was trained to perform a working memory task, delayed alternation, in an operant chamber for food reinforcement. The rats had to choose one of two response levers in an alternative manner in each trial, with a certain interval between trials. They had to remember which lever they chose in the previous trial without the assistance of external cues. Another group was trained to perform a reference memory task, cued alternation, in which the behavioral sequence was identical, but an external cue was provided. After stable behavior was established, a dialysis probe was implanted into the prefrontal cortex or the hippocampus of each rat. The extracellular concentration of ACh in the dialysates from the prefrontal cortex increased during performance of the delayed alternation task, while the hippocampal ACh showed a more distinct increase during performance of the cued alternation task. These results suggest that the prefrontal ACh is mainly related to working memory, whereas the hippocampal ACh is mainly related to reference memory.

Effects of rat ventral and dorsal hippocampus temporal inactivation on delayed alternation task

To determine the involvement of the hippocampal regions in a operant-type delayed alternation task of short delay or long delay, microinjections of muscimol into the hippocampus were used for temporal inactivation during the behavioral test in each rat. Dorsal hippocampal inactivation impaired the correct ratio of long delay. Ventral hippocampal inactivation showed no changes in the correct ratio, however, it increased the tendency of perseveration in long delay. These findings suggest hippocampus has regional differentiation in delayed alternation task.

A possible emerging role of phytochemicals in improving age-related neurological dysfunctions: a multiplicity of effects

It is rare to see a day pass in which we are not told through some popular medium that the population is becoming older. Along with this information comes the "new" revelation that as we enter the next millennium there will be increases in age-associated diseases (e.g., cancer, cardiovascular disease) including the most devastating of these, which involve the nervous system (e.g., Alzheimer's disease [AD] and Parkinson's disease [PD]). It is estimated that within the next 50 years approximately 30% of the population will be aged 65 years or older. Of those between 75 and 84 years of age, 6 million will exhibit some form of AD symptoms, and of those older than 85 years, over 12 million will have some form of dementia associated with AD. What appears more ominous is that many cognitive changes occur even in the absence of specific age-related neurodegenerative diseases. Common components thought to contribute to the manifestation of these disorders and normal age-related declines in brain performance are increased susceptibility to long-term effects of oxidative stress (OS) and inflammatory insults. Unless some means is found to reduce these age-related decrements in neuronal function, health care costs will continue to rise exponentially. Thus, it is extremely important to explore methods to retard or reverse age-related neuronal deficits as well as their subsequent, behavioral manifestations. Fortunately, the growth of knowledge in the biochemistry of cell viability has opened new avenues of research focused at identifying new therapeutic agents that could potentially disrupt the perpetual cycle of events involved in the decrements associated with these detrimental processes. In this regard, a new role in which certain dietary components may play important roles in alleviating certain disorders are beginning to receive increased attention, in particular those involving phytochemicals found in fruits and vegetables.

Cognitive effects of nicotine

Nicotine and other nicotinic agonists have been found to improve performance on attention and memory tasks. Clinical studies using nicotine skin patches have demonstrated the efficacy of nicotine in treating cognitive impairments associated with Alzheimer's disease, schizophrenia, and attention-deficit/hyperactivity disorder. Experimental animal studies have demonstrated the persistence of nicotine-induced working memory improvement with chronic exposure, in addition to the efficacy of a variety of nicotinic agonists. Mechanistic studies have found that alpha7 and alpha4beta2 nicotinic receptors in the hippocampus are critical for nicotinic involvement in cognitive function. Clinical and experimental animal studies provide mutually supporting information for the development of novel nicotinic therapies for cognitive dysfunction.

Impairment of maze learning in rats by restricting environmental space

We previously reported that the restriction of environmental space attenuates spontaneous locomotor activity and hippocampal acetylcholine release. To examine the effect of the restriction of environmental space on spatial learning function, male rats were individually housed in a cylindrical large cage (diameter=35 cm) or small cage (diameter=19 cm) for 5 days. Eight-arm radial maze performance was examined to evaluate spatial learning and memory functions. The task was performed once a day between 21:00 and 22:00 h in the dark phase. Although all rats learned and performed the task, those in the small cage had lower scores and took more trial time than those in the large cage. These results suggest that the restriction of environmental space impairs spatial learning in the dark phase in rats.

Application of in vivo microdialysis to the study of cholinergic systems

The application of in vivo microdialysis to the study of acetylcholine (ACh) release has contributed greatly to our understanding of cholinergic brain systems. This article reviews standard experimental procedures for dialysis probe selection and implantation, perfusion parameters, neurochemical detection, and data analysis as they relate to microdialysis assessments of cholinergic function. Particular attention is focused on the unique methodological considerations that arise when in vivo microdialysis is dedicated expressly to the recovery and measurement of ACh as opposed to other neurotransmitters. Limitations of the microdialysis technique are discussed, as well as methodological adaptations that may prove useful in overcoming these limitations. This is followed by an overview of recent studies in which the application of in vivo microdialysis has been used to characterize the basic pharmacology and physiology of cholinergic neurons. Finally, the usefulness of the microdialysis approach for testing hypotheses regarding the cholinergic systems' involvement in cognitive processes is examined. It can be concluded that, in addition to being a versatile and practical method for studying the neurochemistry of cholinergic brain systems, in vivo microdialysis represents a valuable tool in our efforts to better comprehend ACh's underlying role in a variety of behavioral processes.

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.