Effect of chronic stress on cholinergic transmission in rat hippocampus

Septohippocampal cholinergic system at the intersection of stress and cognition: Current trends and translational implications

Deficits in hippocampus-dependent memory processes are common across psychiatric and neurodegenerative disorders such as depression, anxiety and Alzheimer's disease. Moreover, stress is a major environmental risk factor for these pathologies and it exerts detrimental effects on hippocampal functioning via the activation of hypothalamic-pituitary-adrenal (HPA) axis. The medial septum cholinergic neurons extensively innervate the hippocampus. Although, the cholinergic septohippocampal pathway (SHP) has long been implicated in learning and memory, its involvement in mediating the adaptive and maladaptive impact of stress on mnemonic processes remains less clear. Here, we discuss current research highlighting the contributions of cholinergic SHP in modulating memory encoding, consolidation and retrieval. Then, we present evidence supporting the view that neurobiological interactions between HPA axis stress response and cholinergic signalling impact hippocampal computations. Finally, we critically discuss potential challenges and opportunities to target cholinergic SHP as a therapeutic strategy to improve cognitive impairments in stress-related disorders. We argue that such efforts should consider recent conceptualisations on the dynamic nature of cholinergic signalling in modulating distinct subcomponents of memory and its interactions with cellular substrates that regulate the adaptive stress response.

Dose-related effects of ciproxifan on brain tissue in rats with cerebral ischemia-reperfusion

PURPOSE

Cerebral ischemia is the result of decreased or interrupted blood flow to the brain. It is the third leading cause of death after cardiovascular disease and cancer. Cerebral ischemia is reversible or irreversible in neurons in the affected area, and subsequent free radical damage can be exacerbated if reperfusion occurs. Ciproxifan is used to study the involvement of histaminergic neurons in different phases such as wakefulness and cognition. We wanted to find out whether ciproxifan has a protective effect on the brain of rats with cerebral ischemia-reperfusion injury.

MATERIALS AND METHODS

A total of 64 adult rats (32 male and 32 female) were used for the experiment. Eight cages were formed with randomly selected rats. No substance was administered to the rats in Group 1 and no surgical procedure was performed. The cerebral ischemia-reperfusion model (clamping of the left common carotid artery for 15 min followed by reperfusion for 24 h) was applied to rats in Group 2, Group 3, and Group 4 after 7 days/single dose of saline and ciproxifan (10 mg/kg, 30 mg/kg). After that, the activitymeter, forced swim test (FST), and Morris water maze (MWM) were performed on all animals.

RESULTS

Rats treated with ciproxifan exhibit neurons and glial cells with histologic structures similar to those of the control group, and interestingly, these differences became more pronounced with increasing dose. Rats administered ciproxifan improved motor coordination, decreased total distance behavior, and improved learning ability. However, when the groups were compared by sex, no significant difference was found in the parameters.

CONCLUSION

Thus, we could conclude that ciproxifan has a protective effect on the brain to a certain extent, regardless of the dose.

Repurposing Cholinesterase Inhibitors as Antidepressants? Dose and Stress-Sensitivity May Be Critical to Opening Possibilities

When stress becomes chronic it can trigger lasting brain and behavioral changes including Major Depressive Disorder (MDD). There is conflicting evidence regarding whether acetylcholinesterase inhibitors (AChEIs) may have antidepressant properties. In a recent publication, we demonstrated a strong dose-dependency of the effect of AChEIs on antidepressant-related behavior in the mouse forced swim test: whereas the AChEI donepezil indeed promotes depression-like behavior at a high dose, it has antidepressant-like properties at lower doses in the same experiment. Our data therefore suggest a Janus-faced dose-response curve for donepezil in depression-related behavior. In this review, we investigate the mood-related properties of AChEIs in greater detail, focusing on both human and rodent studies. In fact, while there have been many studies showing pro-depressant activity by AChEIs and this is a major concept in the field, a variety of other studies in both humans and rodents show antidepressant effects. Our study was one of the first to systematically vary dose to include very low concentrations while measuring behavioral effects, potentially explaining the apparent disparate findings in the field. The possibility of antidepressant roles for AChEIs in rodents may provide hope for new depression treatments. Importantly, MDD is a psychosocial stress-linked disorder, and in rodents, stress is a major experimental manipulation for studying depression mechanisms, so an important future direction will be to determine the extent to which these depression-related effects are stress-sensitive. In sum, gaining a greater understanding of the potentially therapeutic mood-related effects of low dose AChEIs, both in rodent models and in human subjects, should be a prioritized topic in ongoing translational research.

Effect of acupuncture on cognitive task performance of college students: a pilot study

OBJECTIVES

Reduction in cognitive function of college students affects their academic recitation. Acupuncture is widely practiced for health promotion. Objective of this study was to evaluate the effect of acupuncture on cognitive task performance in college students.

METHODS

Sixty students aged 18-25 years were randomly allocated into acupuncture group (AG) (n=30) and control group (CG) (n=30). The AG underwent 20 min of acupuncture/day, while the CG underwent their normal routine for 10 days. Assessments were performed before and after the intervention.

RESULTS

Between-group analysis showed a significant increase in the six-letter cancellation test (SLCT) score in AG compared with CG. Within-group analysis showed a significant increase in the scores of all tests (i.e. SLCT, forward and backward Digit span test [DST]) in AG, while a significant increase in backward DST was observed in CG.

CONCLUSION

Acupuncture has a beneficial effect on improving the cognitive function of college students.

Enduring effects of muscarinic receptor activation on adult hippocampal neurogenesis, microRNA expression and behaviour

The cholinergic system is one of the most important neurotransmitter systems in the brain with key roles in autonomic control and the regulation of cognitive and emotional responses. However, the precise mechanism by which the cholinergic system influences behaviour is unclear. Adult hippocampal neurogenesis (AHN) is a potential mediator in this context based on evidence, which has identified this process as putative mechanism of antidepressant action. More recently, post-transcriptional regulation by microRNAs is another candidate mechanism based on its involvement in the regulation of AHN and neurotransmission. Taking into account this background, we evaluated the behavioural effects of a non-convulsant dose of pilocarpine - a non-selective muscarinic receptor (mAChR) agonist - in adult Wistar rats. Furthermore, we quantified the expression of different microRNAs implicated in the regulation of AHN. Our results suggests that pilocarpine treatment increases AHN in the granular cell layer but also induced ectopic neurogenesis. Pilocarpine treatment reduced immobility time in forced swimming test but did not affect fear conditioning response, sucrose preference or novelty supressed feeding behaviour. In addition, treatment with pilocarpine down-regulated the expression of 6 microRNAs implicated in the regulation of neurotrophin signalling and axon guidance pathways. Therefore, we suggest that the low-dose stimulation of the cholinergic system is sufficient to alter AHN of rats through post-transcriptional mechanisms, which might contribute to long-lasting behavioural effects.

Tricyclic antidepressants inhibit hippocampal α7* and α9α10 nicotinic acetylcholine receptors by different mechanisms

The activity of tricyclic antidepressants (TCAs) at α7 and α9α10 nicotinic acetylcholine receptors (AChRs) as well as at hippocampal α7-containing (i.e., α7*) AChRs is determined by using Ca²⁺ influx and electrophysiological recordings. To determine the inhibitory mechanisms, additional functional tests and molecular docking experiments are performed. The results established that TCAs (a) inhibit Ca²⁺ influx in GH3-α7 cells with the following potency (IC₅₀ in μM) rank: amitriptyline (2.7 ± 0.3) > doxepin (5.9 ± 1.1) ∼ imipramine (6.6 ± 1.0). Interestingly, imipramine inhibits hippocampal α7* AChRs (42.2 ± 8.5 μM) in a noncompetitive and voltage-dependent manner, whereas it inhibits α9α10 AChRs (0.53 ± 0.05 μM) in a competitive and voltage-independent manner, and (b) inhibit [³H]imipramine binding to resting α7 AChRs with the following affinity rank (IC₅₀ in μM): imipramine (1.6 ± 0.2) > amitriptyline (2.4 ± 0.3) > doxepin (4.9 ± 0.6), whereas imipramine's affinity was no significantly different to that for the desensitized state. The molecular docking and functional results support the notion that imipramine noncompetitively inhibits α7 AChRs by interacting with two overlapping luminal sites, whereas it competitively inhibits α9α10 AChRs by interacting with the orthosteric sites. Collectively our data indicate that TCAs inhibit α7, α9α10, and hippocampal α7* AChRs at clinically relevant concentrations and by different mechanisms of action.

Behavioural effects of PNU-282987 and stress in an animal model of Alzheimer's disease

BACKGROUND

Cholinergic deficits play an important role in both cognitive and behavioural alterations in Alzheimer's disease. This study was aimed at evaluating the possible therapeutic role of PNU-282987 (PNU), an α7 nicotinic cholinergic receptor agonist, and the possible effects of stress in precipitating the onset of behavioural deficits in animals with susceptibility to Alzheimer's disease.

METHODS

B6C3-Tg mice with susceptibility to Alzheimer's disease and wild-type mice either with or without restraint stress received 0- or 1-mg/kg PNU. At 12 months old, mice were evaluated for activity levels, anxiety-like levels, and spatial learning and memory.

RESULTS

Data did not show the effects of PNU on activity and anxiety-like behaviour. No effect of PNU on acquisition of a spatial learning task was detected, but a reversal of stress effects on retention in the Morris water maze was observed in transgenic mice.

CONCLUSIONS

Further studies are needed in order to better understand the role of α7 nicotinic cholinergic receptor agonists in motor activity, anxiety, and spatial learning and memory and to develop more accurate pharmacological treatment of psychopathological diseases.

Ciproxifan differentially modifies cognitive impairment evoked by chronic stress and chronic corticosterone administration in rats

Despite the development of neuroscience and spectacular discoveries, the clear functions and the role of histamine are still not fully understood, especially in the context of the negative impact of prolonged stress exposure on the cognition. The purpose of this study was to evaluate the participation of hypercortisolemia in the detrimental effect of stress on cognitive function and their preclusion by affecting the histaminergic system with ciproxifan. Specifically, we attempted to characterize the preventive action of a single dose of ciproxifan (3mg/kg, i.p.) against an impairment caused by chronic restraint stress as well as parallel exogenous corticosterone (equivalent to that seen in chronically stressed rats), and show differences in the interaction on reference and working memories tested in both aversive (Morris water maze - MWM) and appetitive (Barnes maze-BM) incentives. We found that administration of ciproxifan potently prevented equally deleterious effects of chronic restraint stress (p<0.01) as well as prolonged administration of corticosterone (p<0.01), especially in the tests, which themselves generate high levels of stress. As it turns out, test provided in the less stressful conditions (BM) showed that administration of the H3 receptor antagonist to naïve rats resulted in even memory impairment (p<0.01, in some aspects of reference memory). These data support the idea that modulation of H3 receptors represents a novel and viable therapeutic strategy in the treatment but rather not for prevention of stress-evoked cognitive impairments. Even a single dose abolishes the effect of prolonged exposure to stress or steroids.

Single dose of H3 receptor antagonist--ciproxifan--abolishes negative effects of chronic stress on cognitive processes in rats

RATIONALE

The role of histamine neurons in stress evoked cognitive impairments remains unclear. Previous research has indicated that the blockade of H(3)-type histamine receptors may improve attention and memory in naïve rodents.

OBJECTIVES

The purpose of this study was to determine if ciproxifan, (cyclopropyl-(4-(3-1H-imidazol-4-yl) propyloxy) phenyl) ketone, an H(3) receptor antagonist, could alleviate cognitive deficits observed in chronically stressed rats.

METHODS

Specifically, we attempted to characterize the preventive action of single dose of ciproxifan (3 mg/kg, i.p.) against an impairment caused by chronic restraint stress (2 h daily for 21 days) on recognition memory tested in an object recognition task and on the long-term memory tested in a passive avoidance test.

RESULTS

We found that administration of ciproxifan potently prevented deleterious effects of chronic restraint stress, when administered prior to learning, or immediately after learning, or before retrieval on both the recognition (p<0.001) and the passive avoidance behavior (p<0.001).

CONCLUSIONS

These data support the idea that modulation of H(3) receptors represents a novel and viable therapeutic strategy in the treatment of stress evoked cognitive impairments.

Krill-Derived Phosphatidylserine Improves TMT-Induced Memory Impairment in the Rat

The present study examined the effects of krill-derived phosphatidylserine (Krill-PS) on the learning and memory function and the neural activity in rats with trimethyltin (TMT)-induced memory deficits. The rats were administered vehicle (medium-chain triglyceride: MCT) or Krill-PS (50, 100 mg/kg, p.o.) daily for 21 days. The cognitive improving efficacy of Krill-PS in TMT-induced amnesic rats was investigated by assessing the Morris water maze test and by performing choline acetyltransferase (ChAT), acetylcholinesterase (AChE) and cAMP responsive element binding protein (CREB) immunohistochemistry. The rats with TMT injection showed impaired learning and memory of the tasks and treatment with Krill-PS produced a significant improvement of the escape latency to find the platform in the Morris water maze at the 2^nd and 4^th day compared to that of the MCT group (p<0.05). In the retention test, the Krill-PS+MCT groups showed increased time spent around the platform compared to that of the MCT group. Consistent with the behavioral data, Krill-PS 50+MCT group significantly alleviated the loss of acetylcholinergic neurons in the hippocampus and medial septum compared to that of the MCT group. Treatment with Krill-PS significantly increased the CREB positive neurons in the hippocampal CA1 area as compared to that of the MCT group. These results suggest that Krill-PS may be useful for improving the cognitive function via regulation of cholinergic marker enzyme activity and neural activity.

Effects of an alpha7 nicotinic receptor agonist and stress on spatial memory in an animal model of Alzheimer's disease

The aim of the present study was to test the effects of PNU-282987 on spatial learning and memory and hippocampal neurogenesis in both intact and chronically stressed transgenic mice. Transgenic mice with susceptibility to Alzheimer's disease (AD) under immobilization stress and not-stressed animals receiving 0 and 1 mg/kg of PNU-282987 (PNU) were evaluated in a water maze task. The effects of PNU and stress on proliferation of new cells in the hippocampus of these animals were also assessed. The latency to escape the platform was significantly higher in transgenic stressed mice compared to those in the wild stressed group, as well as in transgenic animals without PNU compared to control wild group. On retention of the task, differences emerged on stressed wild animals, PNU wild group, and stressed wild mice receiving PNU. However, no significant differences were detected on new cell proliferation. The results of the present study did not show any impact of stress in acquisition of a spatial task both in wild and transgenic mice. No clear effects of PNU on acquisition of a spatial task in transgenic mice with susceptibility to AD were detected. Although PNU and stress effects were detected on retention of the task in wild animals, no changes were noted in transgenic mice.

Chronic Treatment with Squid Phosphatidylserine Activates Glucose Uptake and Ameliorates TMT-Induced Cognitive Deficit in Rats via Activation of Cholinergic Systems

The present study examined the effects of squid phosphatidylserine (Squid-PS) on the learning and memory function and the neural activity in rats with TMT-induced memory deficits. The rats were administered saline or squid derived Squid-PS (Squid-PS 50 mg kg(-1), p.o.) daily for 21 days. The cognitive improving efficacy of Squid-PS on the amnesic rats, which was induced by TMT, was investigated by assessing the passive avoidance task and by performing choline acetyltransferase (ChAT) and acetylcholinesterase (AchE) immunohistochemistry. 18F-Fluorodeoxyglucose and performed a positron emission tomography (PET) scan was also performed. In the passive avoidance test, the control group which were injected with TMT showed a markedly lower latency time than the non-treated normal group (P < 0.05). However, treatment of Squid-PS significantly recovered the impairment of memory compared to the control group (P < 0.05). Consistent with the behavioral data, Squid-PS significantly alleviated the loss of ChAT immunoreactive neurons in the hippocampal CA3 compared to that of the control group (P < 0.01). Also, Squid-PS significantly increased the AchE positive neurons in the hippocampal CA1 and CA3. In the PET analysis, Squid-PS treatment increased the glucose uptake more than twofold in the frontal lobe and the hippocampus (P < 0.05, resp.). These results suggest that Squid-PS may be useful for improving the cognitive function via regulation of cholinergic enzyme activity and neural activity.

Chronic variable stress alters inflammatory and cholinergic parameters in hippocampus of rats

In the present study we investigated the effect of chronic variable stress (CVS) on some parameters of the immune system, including levels of cytokines [interleukin 1β (IL-1 β), interleukin 6 (IL-6), tumor necrosis factor α (TNF- α)] and chemokine CCL2 (MCP-1) in the hippocampus of rats. Acetylcholinesterase activity was also evaluated. Sixty-day old Wistar rats were submitted to different mild stressors for 40 days. After the last stress section, the cytokines and MCP-1 were determined by immunoassay and acetylcholinesterase activity by colorimetric method. Results showed that chronic stress significantly increased the levels of IL-1β, IL-6 and TNF-α, but did not alter the levels of MCP-1. In addition, acetylcholinesterase activity was increased in the hippocampus of rats subjected to CVS. These findings suggest that inflammation and cholinergic dysfunction may be, at least in part, important contributors to the neurological dysfunction observed in some depressed patients.

The effects of acupuncture (PC6) on chronic mild stress-induced memory loss

A previous study reported that the PC6 acupuncture point can alleviate chronic mild stress (CMS)-induced anxiety [17]. Following the previous study, this study examined the effects of the PC6 acupuncture point on CMS-induced memory loss. The memory storage and acetylcholinesterase (AchE) activity in the hippocampus were measured, respectively, using a passive avoidance test (PAT) and AchE immunohistochemistry. In the PAT (retention test), the CMS group showed a markedly lower latency time than the control (post (72h): P<0.01, post (96h): P<0.05, post (120h): P<0.001). However, acupuncture at PC6 significantly recovered the impairment of memory compared to the CMS group (post (120h): P<0.001). Exposure to CMS also significantly decreased the AchE activity in the hippocampus compared to the control rats. Acupuncture stimulation at the PC6 point on the pericardium channels (3min), but not at other points (TE5), produced memory improvements and an increase in AchE reactivity in the hippocampus compared to the CMS group. These results show that the acupuncture point is effective in restoring the CMS-related biochemical and behavioral impairments, such as learning and memory.

The involvement of cholinergic and noradrenergic systems in behavioral recovery following oxotremorine treatment to chronically stressed rats

Chronic stress in rats has been shown to impair learning and memory, and precipitate several affective disorders like depression and anxiety. The mechanisms involved in these stress-induced disorders and the possible reversal are poorly understood, thus limiting the number of drugs available for their treatment. Our earlier studies suggest cholinergic dysfunction as the underlying cause in the behavioral deficits following stress. Muscarinic cholinergic agonist, oxotremorine is demonstrated to have a beneficial effect in reversing brain injury-induced behavioral dysfunction. In this study, we have evaluated the effect of oxotremorine treatment on chronic restraint stress-induced cognitive deficits. Rats were subjected to restraint stress (6 h/day) for 21 days followed by oxotremorine treatment for 10 days. Spatial learning and memory was assessed in a partially baited eight-arm radial maze task. Stressed rats exhibited impairment in performance, with decreased percentage of correct choices and an increase in the number of reference memory errors (RMEs). Oxotremorine treatment (0.1 or 0.2 mg/kg, i.p.) to stressed rats resulted in a significant increase in the percent correct choices and a decrease in the number of RMEs compared with stress as well as the stress+vehicle-treated groups. In the retention test, oxotremorine treated rats committed less RMEs compared with the stress group. Chronic restraint stress decreased acetylcholinesterase (AChE) activity in the hippocampus, frontal cortex and septum, which was reversed by both the doses of oxotremorine. Further, oxotremorine treatment also restored the norepinephrine levels in the hippocampus and frontal cortex. Thus, this study demonstrates the potential of cholinergic muscarinic agonists and the involvement of both cholinergic and noradrenergic systems in the reversal of stress-induced learning and memory deficits.

Effect of a serotonin depleting regimen of 3,4-methylenedioxymethamphetamine (MDMA) on the subsequent stimulation of acetylcholine release in the rat prefrontal cortex

The amphetamine analog 3,4-methylenedioxymethamphetamine (MDMA) is considered to be selectively neurotoxic to serotonergic nerve terminals. Although the long term effects of MDMA on serotonin (5-HT) terminals have been well studied, other potential neurochemical consequences associated with MDMA-induced 5-HT depletion have been less well investigated. In view of the cognitive impairments in human MDMA abusers and the role of acetylcholine (ACh) in learning and memory, it was of interest to determine the influence of a 5-HT depleting regimen of MDMA on subsequent stimulation of ACh release in the prefrontal cortex (PFC). Male rats received vehicle or MDMA (10 mg/kg, i.p. every 2 h for four injections) and underwent in vivo microdialysis 7 days later to assess the subsequent drug- (e.g., MDMA, 5-HT1A agonist) or stress- (e.g., tail pinch, presence of an intruder rat) induced stimulation of ACh release. The increase in the extracellular concentration of ACh in the PFC produced by MDMA (10 mg/kg, i.p.) was significantly less in rats previously exposed to the neurotoxic regimen of MDMA than that in control animals. In contrast, there was no difference in the magnitude of the stimulation of cortical ACh release elicited by the 5-HT1A agonist, 8-hydroxy-2-(di-n-propyl-amino)tetralin (8-OH-DPAT, 0.3mg/kg, s.c.), tail pinch (30 min) or the presence of an intruder rat (40 min) between control animals and animals previously exposed to a neurotoxic regimen of MDMA. These results suggest that although MDMA-induced 5-HT depletion diminishes subsequent MDMA-induced ACh release, there is little impact on cortical ACh release elicited by the stress of pain or the novelty of an environmental intruder.

Psychobiological mechanisms of socioeconomic differences in health

The association between low socioeconomic status and poor health is well established. Empirical studies suggest that psychosocial factors are important mediators for these effects, and that the effects are mediated by psychobiological mechanisms related to stress physiology. The objective of this paper is to explore these psychobiological mechanisms. Psychobiological responses to environmental challenges depend on acquired expectancies (learning) of the relations between responses and stimuli. The stress response occurs whenever an individual is faced with a challenge. It is an essential element in the total adaptive system of the body, and necessary for adaptation, performance and survival. However, a period of recovery is necessary to rebalance and to manage new demands. Individuals with low social status report more environmental challenges and less psychosocial resources. This may lead to vicious circles of learning to expect negative outcomes, loss of coping ability, strain, hopelessness and chronic stress. This type of learning may interfere with the recovery processes, leading to sustained psychobiological activation and loss of dynamic capacity to respond to new challenges. Psychobiological responses and health effects in humans and animals depend on combinations of demands and expected outcomes (coping, control). In studies of humans with chronic psychosocial stress, and low SES, cortisol baseline levels were raised, and the cortisol response to acute stress attenuated. Low job control was associated with insufficient recovery of catecholamines and cortisol, and a range of negative health effects. Biological effects of choice of lifestyle, which also depends on the acquired outcome expectancies, reinforce these direct psychobiological effects on health. The paper concludes that sustained activation and loss of capacity to respond to a novel stressor could be a cause of the higher risk of illness and disease found among people with lower SES.

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.

Impaired and spared cholinergic functions in the hippocampus after lesions of the medial septum/vertical limb of the diagonal band with 192 IgG-saporin

To lesion the cholinergic input to the hippocampus, rats received injections of 192 IgG-saporin into the medial septum/vertical limb of the diagonal band (MS/VDB). The lesions produced near-total loss of choline acetyltransferase (ChAT)-positive neurons in the MS/VDB. The loss was accompanied, however, by only partial decreases (to 40% of control levels) in acetylcholine (ACh) release in the hippocampus. Moreover, ACh release in the hippocampus increased when lesioned and control rats were tested on a spontaneous alternation task, indicating that there was significant residual cholinergic function in the hippocampus. The lesions were sufficient to impair spontaneous alternation scores. However, this impairment could be reversed by either systemic or intra-hippocampal injections of the indirect cholinergic agonist, physostigmine, providing additional evidence of residual and effective cholinergic functions in the hippocampus of lesioned rats. Moreover, systemic injections of physostigmine at doses that produced mild tremors in control rats led to more severe tremors in the lesioned rats, suggesting upregulation of cholinergic mechanisms after saporin lesions, likely in brain areas other than the hippocampus. Thus, these findings provide evidence for decreases in cholinergic input to the hippocampus accompanied by deficits on a spontaneous alternation tasks. The findings also provide evidence for considerable residual cholinergic input to the hippocampus after saporin lesions of the MS/VDB. Together, the results suggest that 192 IgG-saporin lesions of the MS/VDB, using methods often employed, do not fully remove septohippocampal cholinergic input to the hippocampus but are nonetheless sufficient to produce impairments on a task impaired by hippocampal lesions.

Morphine state-dependent learning: interactions with alpha2-adrenoceptors and acute stress

The interactions of -adrenoceptors and acute restraint stress with morphine state-dependent memory of passive avoidance were examined in mice. Memory acquired following pre-training morphine administration (5 mg/kg, i.p.) was dose- and time-dependently retrieved by pre-test morphine; this effect was reversible by yohimbine (1 mg/kg). Pre-test clonidine (0.005-0.1 mg/kg) was also effective in restoring morphine-induced memory. Pre-training clonidine (2 mg/kg) induced an amnestic effect that was restorable by pre-test clonidine or morphine; this effect was also blocked by yohimbine. Acute pre-training stress for 2 h induced an amnestic effect that was reversible by pre-test morphine (1 and 5 mg/kg) or clonidine (0.01 and 0.1 mg/kg). Finally, acute pre-test stress could restore the impairment of memory induced by pre-training morphine. The data are suggestive of a functional interaction between -opioid, -adrenergic receptors and stress in modulating state-dependent learning and memory.