Modulation of memory by post-training epinephrine: involvement of cholinergic mechanisms

Extensive evidence indicates that memory storage can be modulated by peripheral epinephrine as well as by drugs affecting the muscarinic cholinergic system. Low doses of epinephrine (Epi) facilitate memory while high doses induce amnesia. Retention is enhanced by post-training administration of cholinergic muscarinic agonists and impaired by antagonists. The present experiments examined the interaction of peripheral Epi and cholinergic drugs in memory modulation. Male CFW mice (60 days old) were trained on an inhibitory avoidance response (IA) or a Y-maze discrimination response (YMD), injected (IP) immediately post-training and tested 24 h later. In the Y-maze task, retention was assessed by discrimination reversal training. Findings obtained in the two tasks were comparable. Epi (IA: 3.0 g/kg; YMD: 30.0 g/kg) potentiated the memory-enhancing effect of low doses of oxotremorine (Otm) (IA: 16; YMD: 5.0 g/kg) and physostigmine (Phy) (6.8 and 22.0 g/kg for both IA and YMD). The memory-facilitating effect of Otm (50.0 g/kg) was not blocked by an amnestic dose of Epi (IAT: 0.3 mg/kg; YMD: 1.0 mg/kg). Retention was not affected when these amnestic doses of Epi were administered together with a memory-facilitating dose of Phy (68.0 g/kg). In contrast, atropine (IA: 10.0 mg/kg; YMD: 3.0 mg/kg) completely blocked the facilitatory effect of Epi (IA: 10.0 g/kg; YMD: 300 g/kg). These findings indicate that, when administered in low doses, Epi interacts with Otm and Phy. However, cholinergic drugs can block both the memory-enhancing and the memory-impairing effects of Epi. The findings suggest that Epi may modulate memory through cholinergic systems.     

Involvement of the cholinergic system of CA1 on harmane-induced amnesia in the step-down passive avoidance test

β-carboline alkaloids such as harmane (HA) are naturally present in the human food chain. They are derived from the plant Peganum harmala and have many cognitive effects. In the present study, effects of the nicotinic system of the dorsal hippocampus (CA1) on HA-induced amnesia and exploratory behaviors were examined. One-trial step-down and hole-board paradigms were used to assess memory retention and exploratory behaviors in adult male mice. Pre-training (15?mg/kg) but not pre-testing intraperitoneal (i.p.) administration of HA decreased memory formation but did not alter exploratory behaviors. Moreover, pre-testing administration of nicotine (0.5?μg/mouse, intra-CA1) decreased memory retrieval, but induced anxiogenic-like behaviors. On the other hand, pre-test intra-CA1 injection of ineffective doses of nicotine (0.1 and 0.25?μg/mouse) fully reversed HA-induced impairment of memory after pre-training injection of HA (15?mg/kg, i.p.) which did not alter exploratory behaviors. Furthermore, pre-testing administration of mecamylamine (0.5, 1 and 2?μg/mouse, intra-CA1) did not alter memory retrieval but fully reversed HA-induced impairment of memory after pre-training injection of HA (15?mg/kg, i.p.) which had no effect on exploratory behaviors. In conclusion, the present findings suggest the involvement of the nicotinic cholinergic system in the HA-induced impairment of memory formation.     

NMDA receptor antagonists antagonize the facilitatory effects of post-training intra-basolateral amygdala NMDA and physostigmine on passive avoidance learning

In the present study, the effects of post-training intra-basolateral amygdala (BLA) injection of an N-methyl-d-aspartate (NMDA) receptor agonist and competitive or noncompetitive antagonists, on memory retention of passive avoidance learning was measured in the presence and absence of physostigmine in rats. Intra-BLA administration of lower doses of NMDA (10(-5) and 10(-4) microg/rat) did not affect memory retention, although higher doses of the drug (10(-3), 10(-2) and 10(-1) microg/rat) increased memory retention. The greatest response was obtained with 10(-1) microg/rat of the drug. The different doses of the competitive NMDA receptor antagonist DL-AP5 (1, 3.2 and 10 microg/rat) and noncompetitive NMDA receptor antagonist MK-801 (0.5, 1 and 2 microg/rat) decreased memory retention in rats dose dependently. Both competitive and noncompetitive NMDA receptor antagonists reduced the effect of NMDA (10(-2) microg/rat). In another series of experiments, intra-BLA injection of physostigmine (2, 3 and 4 microg/rat) improved memory retention. Post-training co-administration of lower doses of NMDA (10(-5) and 10(-4) microg/rat) and physostigmine (1 microg/rat), doses which were ineffective when given alone, significantly improved the retention latency. The competitive and noncompetitive NMDA receptor antagonists, DL-AP5 and MK-801, decreased the effect of physostigmine (2 microg/rat). Atropine decreased memory retention by itself and potentiated the response to DL-AP5 and MK-801. It may be concluded that amygdalar NMDA receptor mechanisms interact with cholinergic systems in the modulation of memory.     

Impairment of active avoidance by the noradrenergic neurotoxin,DSP4: attenuation by post-training epinephrine

Male Sprague-Dawley rats were treated with the selective noradrenergic neurotoxin, DSP4 prior to behavioral assessment. They were trained in an inhibitory avoidance and a one-way active avoidance task and given post-training treatment with epinephrine (EPI, 0.1 mg/kg, SC) or physiological saline. Performance on these tests was assessed at time points after treatment with DSP4 when (1) both central NE and sympathetic catecholamines were depleted and when (2) sympathetic catecholamines had returned to control levels and central NE remained depleted. Activity was also assessed at two time points after DSP4 treatment. DSP4 treatment had no effect upon inhibitory avoidance retention but impaired one-way active avoidance shuttle performance at both time points following DSP4 treatment. There was a transient depression of spontaneous activity which may indicate a deficit of behavioral initiation during the early phase after DSP4 treatment when the sympathetic catecholamine levels were depleted. This finding suggests that the peripheral sympathetic system may support some aspect of behavioral initiation. Epinephrine (0.1 mg/kg SC) administered after active avoidance training ameliorated the active avoidance retention performance deficit seen 4 days after DSP4 treatment. Post-training EPI did not significantly affect active avoidance retention performance when animals were trained and tested after peripheral sympathetic recovery (approximately 2 weeks after DSP4 treatment). These findings suggest that peripheral adrenergic activation plays a role in the EPI facilitation of retention performance in this task. As the facilitatory effects of peripherally circulating epinephrine were expressed in the presence of a severely depleted dorsal noradrenergic bundle system, they do not appear to depend upon the stimulation of NE released from this system. The present data, however, do not exclude the possibility that the modulation by peripheral EPI may depend upon its stimulation of NE release at other loci of central noradrenergic innervation.     

On the interaction of drugs with the cholinergic nervous system

Tolerance to physostigmine salicylate was induced in mice using various schedules of s.c. injections. The rate and degree of tolerance development were assessed by comparing the ED₅₀ values (equipotent doses) and by comparing the peak effects induced by a constant dose. These were measured on four systemic responses induced by the drug — hypothermia, tremor, salivation, and the effects measured in the rotarod test. The degree of tolerance development was found to be dose-dependent with a maximal achievable tolerance for every dose. The tolerance development to the four systemic effects differed in time course: tolerance to the hypothermia was induced even with daily injections, while tolerance to the salivation and rotarod effects could be detected only when the drug was given every 4 h. No tolerance developed to the tremor with any of the schedules and doses used. The maximal achievable tolerance degree and the pattern of changes of the duration were different for each systemic effect. The tolerance was found to be reversible, with different rates of recovery, for the different effects. The tolerance state could not be correlated with changes in the pattern of brain acetylcholinesterase (AcChE) inhibition by physostigmine in vivo or with changes in the rate of the enzyme's spontaneous reactivation.Scopolamine. HBr given 10 min before physostigmine prevented tolerance development. In addition, cross-tolerance to various muscarinic agonists and cholinesterase inhibitors was found in the physostigmine-tolerant mice. The correlation between these results and our previous findings concerning possible biochemical adaptations is presented and discussed.     

On the interaction of drugs with the cholinergic nervous system

A symmetrical cross-tolerance was found between two phencyclidine derivatives-phencyclidine and cyclohexamine-and also between two cholinergic drugs-physostigmine and oxotremorine. On the other hand, mice rendered tolerant to the phencyclidine derivatives showed cross-tolerance to these cholinergic drugs, but no cross-tolerance was observed in the opposite direction.The applicability of such experiments to the elucidation of neurochemical interactions of centrally acting drugs is discussed.     

On the interaction of drugs with the cholinergic nervous system

Phencyclidine [1(1-phenylcylohexyl) piperidine] and cyclohexamine [1(1-phenylcyclohexyl) ethylamine) were used as model psychotropic drugs to study the phenomenon of tolerance in mice. The behavioral effects of these drugs were measured by forced motor activity using the rotarod test. Tolerance develops progressively with chronic treatment at a rate and to a degree that are dose-dependent. The optimal conditions for tolerance induction are s.c. administration with 4-h intervals. The process of tolerance development is expressed in concomitant changes in five indices chosen for its quantification: ED50 values, duration, duration-dose dependency, critical falling time, and body weight. All these changes were found to be totally reversible, with no carry-over between two consecutive tolerance cycles. It was established that cyclohexamine is a better tolerance-inducer than phencylidine, although the nature of the tolerance developed for both drugs is qualitatively similar.     

Loss of cholinergic neurons in the rat neocortex produces deficits in passive avoidance learning

Bilateral kainic acid lesions of the ventral globus pallidus produced a significant and selective cortical decrease in choline acetyltransferase activity in the rat brain. When lesioned and control subjects were compared on performance of a step-through passive avoidance task, lesioned rats showed a marked retention deficit 24 hr after the initial training trial. This experimentally-induced memory deficit associated with a cortical cholinergic neuronal loss resembles the deficits in senile dementia of the Alzheimer type and may provide a useful animal model for studying the disease.     

Attenuation of morphine withdrawal signs by muscimol in the locus coeruleus of rats

This study examined the effects of intra-locus coeruleus injection of a gamma-amino-butyric acid type A (GABAA) receptor agonist on naloxone-induced withdrawal signs of morphine-dependent rats. Twenty different withdrawal signs were assessed. The total withdrawal score was calculated and used as an index of withdrawal intensity. The gamma-amino-butyric acid agonist and antagonists were injected 15 and 30 min before naloxone injection, respectively. Muscimol, a GABAA agonist (25, 50, and 100 ng/site), decreased the total withdrawal score in a dose-independent manner, whereas bicuculline (0.367, 3.67, and 36.7 ng/site), a GABAA antagonist, and CGP35348 (48.6 ng/site), a gamma-amino-butyric acid type B antagonist, alone had no effects. On the other hand, muscimol effects were reversed by CGP35348 (48.6 ng/site) but not by bicuculline (36.7 ng/site). It may be concluded that the effects of muscimol were antagonized by gamma-amino-butyric acid type B but not by GABAA receptor antagonists. The mechanism involved seems to be intricate and needs further study.     

Neonatal cholinergic lesions and development of exploration upon administration of the GABAa receptor agonist muscimol in preweaning rats

Neonatal rats were administered 192 IgG-saporin (192 IgG-Sap), a selective cholinergic immunotoxin, on postnatal day (PND) 7. Behavioural responsiveness to muscimol, a GABAa receptor agonist, was then assessed using locomotor activity and object exploration tests on PND 18. In Experiment 1, 192 IgG-Sap-lesioned and control rats were injected with the GABAa agonist, muscimol, on PND 18 and tested in a standard open field test. Muscimol reduced rearing responses in both control and 192 IgG-Sap-lesioned animals whereas reduced wall-rearing responses occurred in control animals only. 192 IgG-Sap also reduced rearing and wall-rearing responses. In Experiment 2, muscimol effects were evaluated on PND 18 in a spatial open field test in which object exploration in addition to locomotion and rearing responses was assessed. Neonatal cholinergic lesion per se increased locomotion during object exploration while decreasing time spent exploring objects. Depressant effects of muscimol on object exploration were also evident. As a whole, these data provide evidence for (i) basal forebrain (BF) cholinergic control on locomotor activity and object exploration and (ii) GABAa-mediated regulation of selective behavioural patterns associated with locomotion and exploration in weaning rats. Neonatal cholinergic lesions, however, do not appear to alter reactivity to GABAergic agonists in juvenile rats.     

Involvement of the amygdala in the effect of antidepressants on the passive avoidance deficit in bulbectomised rats

In the present study the role of the amygdala is demonstrated in the effect of antidepressant drugs on passive avoidance learning in bulbectomised rats. Imipramine, amitriptyline, or fluoxetine injected bilaterally (2×10 g) into the medial part of the amygdala improves passive avoidance learning by bulbectomised rats. Systemic pretreatment with metergoline attenuates the effect of local imipramine or fluoxetine injection. Microinjection of serotonin but not noradrenaline into the amygdala improves passive avoidance learning by bulbectomised rats. The relevance of the amygdala in the behavioral effects of antidepressants in animals is discussed.     

A synthetic retinoid Am80 (tamibarotene) rescues the memory deficit caused by scopolamine in a passive avoidance paradigm

Memory deficit in rats treated with scopolamine was rescued by several synthetic retinoids, RAR-ligands (Am80, Am555S, Tp80) and an RXR-ligand (HX630). These results may have implications for the treatment of Alzheimer's disease, age-related dementia, Parkinson's disease, and other neurological disorders.     

Effects of antimuscarinic cholinergic drugs injected systemically or into the hippocampo-entorhinal area upon passive avoidance learning in young rats

Passive avoidance learning was significantly impaired by atropine (5 mg/kg, IP) or scopolamine (0.5 mg/kg), but not by methyl-atropine (5 mg/kg) or methyl-scopolamine (0.5 mg/kg), from postnatal day 15 on. In contrast, an improvement was observed, not significant at 11 days and significant at 13 days, probably due to nonspecific effects. Retention of the response increased from 6 h at 13 days, to 24 h at 17 days. In treated rats, retention was abolished at 13 and 15 days, and impaired at 17 and 20 days. Acquisition of the response was also significantly impaired by bilateral injections of atropine (1, 5, and 20 g) into the posteroventral hippocampo-entorhinal (VHE) area, from day 15 on. Concomitantly, extinction was accelerated. At 14 days, atropine had no influence. At 13 days, a facilitatory action was observed, with better acquisition and greater resistance to extinction, possibly linked to affective changes. The results conform that central muscarinic cholinergic mechanisms are involved in passive avoidance learning from postnatal day 15 on, and demonstrate that some pathways of this system are located in the VHE area, become efficient at 15 days, and improve markedly between 17 and 18 days.     

Sex differences in passive avoidance depend on the integrity of the central serotonergic system

Effects of the neurotoxin para-chloroamphetamine (PCA) on sex differences in passive avoidance were studied. Seven days prior to passive avoidance training and testing, male and female rats were injected with PCA (5 mg/kg) or physiological saline (SAL). Treatment effects on brain monoamines levels were evaluated in brains collected shortly after the passive avoidance test. Compared to SAL-treated control groups PCA severely reduced both serotonin (5-HT) and 5-hydroxyindole-acetic acid (5-HIAA) in the frontal cortex of males and females. Levels of dopamine (DA) and homovanillic acid (HVA) in the frontal cortex were not affected. These data are indicative of a strong and selective depression of the central 5-HT activity. PCA- and SAL-treated male and female rats were trained and tested in a two-compartment step-through passive avoidance apparatus. Sex differences in passive avoidance were clearly observed in the SAL-treated control groups; a higher number of males did not enter either compartment within the maximum test duration. After PCA treatment sex differences in passive avoidance were abolished, mainly resulting from an increase in the number of PCA-males reentering. Irrespective of sex or treatment subjects seldom failed to choose the nonshock compartment when entering during the passive avoidance test, indicating that disturbance of memory or learning cannot explain for the present results. Rather, the data are discussed in terms of a sex-specific role of central 5-HT in punishment-induced behavioral suppression.     

Haloperidol-induced disruption of conditioned avoidance responding: Attenuation by prior training or by anticholinergic drugs

Rats injected daily with haloperidol (0.15 mg/kg) failed to acquire a one-way active avoidance response over a 9 day period (10 traisl/day). When these animals were subsequently tested without haloperidol, on the first drug-free day they performed as well as animals given saline throughout the training period and significantly better than naive saline-treated animals on the first day of training. The performance of rats were trained for two days before receiving haloperidol was only partly blocked by the drug, while animals trained for 9 days before drug administration were immune to the disruptive effects. Three anticholinergic (muscarinic) drugs, atropine (10 mg/kg), scopolamine (1 mg/kg) and benztropine (2 mg/kg) significantly reversed the effect of haloperidol on the acquisition of the avoidance response. These results are similar to those observed after bilateral destruction of the dipaminergic nigroneostriatal projection and support the view that this system is critically involved in the acquisition of learned instrumental responses. The nature of the avoidance deficit produced by these treatments is discussed with reference on the possibility that they selectively block the initiation of voluntary motor responses. According to this hypothesis, the failure of these treatments to disrupt escape responding may be due to the fact that the unconditioned stimulus generates reflexive motor responses (flinch, jump, etc.) which are sufficient to begin the motoric sequences that cannot be initiated voluntarily in response to the conditioned stimulus.     

Influence of intracerebroventricular administration of cannabinergic drugs on morphine state-dependent memory in the step-down passive avoidance test

The effects of cannabinergic drugs on morphine state-dependent memory of passive avoidance task were examined in mice. Pre-training (0.25, 0.5 and 5 mg/kg) and post-training (5 mg/kg) administration of morphine impaired memory retrieval on the test day. Impairment of memory retrieval by morphine (5 mg/kg) on the test day was reversed by pre-test administration of the same dose of the opioid. The pre-test intracerebroventricular administration of the cannabinoid CB1/CB2 receptor agonist (WIN55,212-2) (0.75 and 1 microg/mouse) not only mimicked the effect of pre-test morphine treatment, but also increased this action of the opioid. Furthermore, the pre-test intracerebroventricular administration of CB1 receptor antagonist (AM251) (20 and 100 ng/mouse) prevented the restoration of memory by morphine. Pre-training administration of WIN55,212-2 (1 microg/mouse) led to state-dependent learning with impaired memory retrieval on the test day as well, which was reversed by pre-test administration of the drug (0.5, 0.75 and 1 microg/mouse) or morphine (1 and 5 mg/kg). Restoration of impairment induced by WIN55,212-2 was decreased by both the opioid receptor antagonists, naloxone (0.01 microg/mouse) and AM251 (20 and 100 ng/mouse). In conclusion, the improvement of memory retrieval by morphine treatment on the test day seems to be induced, at least in part, by the cannabinoid CB1 receptors.     

Involvement of the cholinergic neuronal system and benzodiazepine receptors in alcohol-induced amnesia.

We investigated the involvement of the GABAergic and cholinergic neuronal systems and benzodiazepine (BZP) receptors in ethanol-induced amnesia using a passive avoidance task. Pretraining administration of ethanol impaired the passive avoidance response. The BZP agonist chlordiazepoxide potentiated the amnesia, while the GABA antagonists bicuculline and picrotoxin failed to affect it. The acetylcholine esterase inhibitor physostigmine partially attenuated the ethanol-induced amnesia. These results suggest that ethanol-induced amnesia is related to BZP receptors and a dysfunction of the cholinergic neuronal system.     

Biochemical evidence of adrenergic interaction with cholinergic function in the central nervous system of the rat

The effect of centrally administered norepinephrine (NE) into the lateral ventricle on choline acetylase activity (ChAc), endogenous levels of NE, dopamine (DA) and serotonin (5HT) was studied in different regions of the rat brain. In normal rats, the highest activity of choline acetylase was found in the cerebral cortex, followed by the brain stem, the diencephalon, the hypothalamus and least in the cerebellum. Chronic administration of NE significantly increased the choline acetylase activity in all regions studied, whereas both the acute and the in vitro studies showed no significant change except in the brain stem. Serotonin level was increased in the cerebellum, but decreased in the diencephalon and the brain stem. There was no significant alteration in the level of NE in all areas studied except in the hypothalamus where there was an increase in the mean concentration. Tissue level of DA showed a significant increase in the cerebral cortex and the hypothalamus. Behaviourally, there was a significant increase in food intake on the first day of treatment with no significant change in water intake. The data suggests that NE may be involved in the regulation of acetylcholine synthesis. The significance of interaction between different monoamines and acetylcholine may be important in the study of drug tolerance phenomena.This work was supported by USPHS grant MH-12383; Dr. Ho is a recipient of a PMA Foundation grant award in morphology-pharmacology.     

The effects of chlorpromazine on one-trial passive avoidance learning in mice: Further examination of pre- and post-learning administration

Chlorpromazine in doses of 0.5 mg/kg was administered to mice 0.5, 2, or 10 min after a one-trial passive avoidance learning experience. The drug produced effects on the magnitude and rate of extinction of the learned response dependent upon the injection, time, confirming results contained in an earlier report. In a second experiment with doses of 2.0 mg/kg, the effects of further injecion times were investigated. The drug had no effect when given 240 min before learning, but produced maximal blocking of response acquisition when given 120 and 8 min before learning. Drug injections 6 and 3 min before learning were suggested as having actions on post-learning memory traces. A distinction was noted between the effects of drug injections 1 and 1.5 min after learning and this was related to an effect on a rapidly decaying short-term memory trace. Chlorpromazine had no effect when given 20 min after learning.