Facilitation of inhibitory avoidance by hypertonic saline is reversed by a vasopressin and a nicotinic antagonist

Hypertonic saline (1 ml of 0.25, 0.50, and 1.00 M NaCl, ip) facilitated retention of a one-trial, step-through inhibitory avoidance task when injected into male Swiss mice 10 min after training, as indicated by retention performance 48 h later. A similar result was obtained after a subcutaneous injection of lysine vasopressin (LVP, 0.03 microgram/kg). Neither hypertonic saline nor LVP modified latencies to step-through of mice that had not received a footshock during training. The enhancement of retention produced both by hypertonic saline and by LVP was prevented by the vasopressin receptor antagonist AAVP (0.01 microgram/kg, sc) given after training, but 10 min before the treatments. The effect of hypertonic saline was also prevented by the central acting cholinergic nicotinic receptor antagonist mecamylamine (5 mg/kg, sc). On the contrary, neither hexamethonium (5 mg/kg, sc), a peripheral acting nicotinic receptor blocker, nor atropine (0.5 mg/kg, sc) or methylatropine (0.5 mg/kg, sc), two anticholinergic drugs which are known to act on cholinergic muscarinic receptors, prevented the effect of post-training hypertonic saline. These results suggest that a peripheral osmotic stimulus, probably through an endogenous release of vasopressin, may be behaviorally significant, and are consistent with the view that vasopressin may modulate the activity of central cholinergic nicotinic mechanisms which are critical for the behavioral change observed.     

Mecamylamine prevents the enhancement of retention induced by lysine vasopressin in mice

Lysine vasopressin (0.03 microgram/kg, sc) enhanced retention of a one-trial, step-through inhibitory avoidance task when injected into male Swiss mice immediately post-training, as indicated by retention performance 48 h later. A low dose of the vasopressin antagonist, AAVP (0.01 microgram/kg, sc), did not significantly affect retention, whereas a higher dose (0.03 microgram/kg, sc) impaired retention. Neither lysine vasopressin nor AAVP modified latencies to step-through of mice that had not received a footshock during training. The simultaneous injection of AAVP (0.01 microgram/kg, sc) prevented the enhancement of retention induced by lysine vasopressin. The influence of lysine vasopressin on retention was antagonized by the simultaneous administration of mecamylamine (5 mg/kg, sc) but not by hexamethonium (5 mg/kg, sc), atropine (0.5 mg/kg, sc), or methylatropine (0.5 mg/kg, sc). A modulatory role of vasopressin on the activity of central cholinergic nicotinic mechanisms which participate in memory formation is suggested.     

Vasopressin modulates the activity of nicotinic cholinergic mechanisms during memory retrieval in mice

Lysine vasopressin (0.03 micrograms/kg, sc) enhanced retention test performance on a one-trial step-through inhibitory avoidance task when injected into male Swiss mice 20 min before the retention test. Tests were done 48 h following training. A low dose of the vasopressin antagonist AAVP (0.01 microgram/kg, sc, 20 min prior to testing) did not significantly affect retention test performance, whereas a higher dose (0.03 microgram/kg, sc) impaired it. Neither lysine vasopressin nor AAVP when given prior to testing modified latencies to step-through of mice that had not received a footshock during training. The simultaneous administration of AAVP (0.01 microgram/kg, sc) prevented the enhancement of retention test performance induced by lysine vasopressin. The influence of lysine vasopressin on retention test performance was antagonized by the simultaneous administration of mecamylamine (5 mg/kg, sc) but not by hexamethonium (5 mg/kg, sc), atropine (0.5 mg/kg, sc), or methylatropine (0.5 mg/kg, sc). A modulatory role of vasopressin on the activity of central cholinergic nicotinic mechanisms which probably operate at the time of testing is suggested.     

Involvement of central cholinergic mechanisms in the effects of oxytocin and an oxytocin receptor antagonist on retention performance in mice

Oxytocin (OT, 0.10 microg/kg, sc) impaired retention of a one-trial step-through inhibitory avoidance task when injected into male Swiss mice 10 min after training, as indicated by retention performance 48 h later. In contrast, the immediate post-training administration of the putative oxytocin receptor antagonist d(CH(2))(5)[Tyr(Me)(2), Thr(4), Thy-NH(9)(2)] OVT (AOT, 0.30 microg/kg, sc) significantly enhanced retention performance. Neither OT nor AOT affected response latencies in mice not given footshock on the training trial, and neither the impairing effects of OT nor the enhancing effects of AOT were seen when the training-treatment interval was 180 min, suggesting that both treatments influenced memory storage. The effects of OT (0.10 microg/kg, sc) on retention were prevented by AOT (0.03 microg/kg, sc) given immediately after training, but 10 min prior to OT treatment. The central acting anticholinesterase physostigmine (35, 70, or 150 microg/kg, i.p.), but not its quaternary analogue neostigmine (150 microg/kg, i.p.), reversed the impairment of retention performance induced by OT, whereas low subeffective doses of the centrally active muscarinic cholinergic antagonist atropine (0.5 mg/kg, i.p.) or the central acting nicotinic cholinergic antagonist mecamylamine (5 mg/kg, i.p.), but not methylatropine (0.5 mg/kg, i.p.) or hexamethonium (5 mg/kg, i.p.) prevented the enhancement of retention performance caused by AOT. We suggest that oxytocin negatively modulates the activity of central cholinergic mechanisms during the posttraining period that follows an aversively motivated learning experience, leading to an impairment of retention performance of the inhibitory avoidance response.     

Glucose effects on mecamylamine-induced memory deficits and decreases in locomotor activity in mice.

Peripheral glucose administration attenuates the effects of muscarinic cholinergic antagonists on several measures, including spontaneous alternation, inhibitory avoidance, and locomotor activity. The present study examined glucose interactions with mecamylamine, a nicotinic cholinergic antagonist, on these measures. Mecamylamine (5 mg/kg, sc) significantly impaired spontaneous alternation performance. Glucose (100 mg/kg, ip) administered with mecamylamine attenuated the impairment. Treatment with hexamethonium (5 and 10 mg/kg, sc), a peripheral nicotinic blocker, did not impair performance. Pretraining treatment with mecamylamine, but not hexamethonium, significantly reduced later retention latencies on inhibitory avoidance tests. Glucose, administered with mecamylamine prior to training, significantly attenuated the impaired test performance. Mecamylamine, but not hexamethonium, significantly decreased locomotor activity. In contrast to the attenuating effects of glucose on the other measures above, glucose administered with mecamylamine potentiated the decreased locomotor activity. These findings demonstrate that glucose influences the behavioral effects of a nicotinic cholinergic antagonist in a manner generally similar to that of muscarinic cholinergic antagonists, and supports previous evidence that circulating glucose interacts with central cholinergic functions.     

Memory-modulatory effects of centrally acting noradrenergic drugs: possible involvement of brain cholinergic mechanisms.

Post-training administration of the centrally acting muscarinic agonist oxotremorine (50.0 microgram/kg, ip) facilitated 48-hr retention, in mice, of a one-trial step-through inhibitory avoidance response. Oxotremorine-induced memory facilitation was not prevented by the simultaneous post-training administration of the central beta-adrenoceptor antagonist propranolol (2.0 mg/kg, ip). In contrast, post-training administration of atropine (0.5 mg/kg, ip), but not methylatropine (0.5 mg/kg, ip), completely prevented the facilitatory effects of the central beta-adrenoceptor agonist clenbuterol (30.0 micrograms/kg, ip) on retention. Low subeffective doses of clenbuterol (3.0 micrograms/kg, ip) and oxotremorine (6.25 or 12.5 micrograms/kg, ip) potentiated their effects and facilitated retention when given simultaneously immediately post-training. These results suggest that clenbuterol may induce memory facilitation through an increase of the release of acetylcholine in the brain. Post-training administration of a high dose of clenbuterol (1.0 mg/kg, ip) significantly impaired retention. Clenbuterol (1.0 mg/kg, ip)-induced impairment of retention was completely prevented by simultaneous post-training administration of oxotremorine (6.25, 12.5, or 50.0 micrograms/kg, ip). The centrally acting anticholinesterase physostigmine (21.5 or 68.0 micrograms/kg, ip) partially prevented clenbuterol-induced impairment of memory. The peripherally acting anticholinesterase neostigmine (68.0 micrograms/kg, ip) modified neither retention nor the amnestic effects of clenbuterol. Considered together, these findings are consistent with the view that brain muscarinic cholinergic mechanisms are involved in both the facilitatory and impairing effect of post-training clenbuterol on the modulation of memory storage.     

Effects of Posttraining Administration of Glucose on Retention of a Habituation Response in Mice: Participation of a Central Cholinergic Mechanism

Male Swiss mice were allowed to explore a novel environment, provided by an open-field activity chamber, for 10 min. The procedure was repeated twice with a 24-h interval. The difference in the exploratory activity between the first (training) and the second (testing) exposures to the chamber was taken as an index of retention of this habituation task. Posttraining intraperitoneal administration of glucose (10–300 mg/kg) enhanced retention in a dose-related manner, although only the dose of 30 mg/kg of glucose produced significant effects. Thus, the dose–response curve adopted an inverted U-shaped form. Glucose (30 mg/kg) given to untrained mice did not modify their exploratory performance when recorded 24 h later. The effects of glucose on retention were time-dependent, suggesting an action on memory storage. The memory-improving actions of glucose were prevented by the simultaneous administration of both the central acting muscarinic cholinergic antagonist atropine (0.5 mg/kg) and by the central acting nicotinic cholinergic antagonist mecamylamine (5 mg/kg). In contrast, neither methylatropine (0.5 mg/kg), a peripherally acting muscarinic receptor blocker, nor hexamethonium (5 mg/kg), a peripherally acting nicotinic receptor blocker, prevented the effects of glucose on retention. Low subeffective doses of glucose (10 mg/kg) and the central anticholinesterase physostigmine (35 μg/kg), but not neostigmine (35 μg/kg), given together, act synergistically and facilitated retention. We suggest that glucose modulates memory storage of one form of learning elicited by stimuli repeatedly presented without reinforcement, probably through an enhancement of brain acetylcholine synthesis and/or its release.     

Enhancement of the Post-training Cholinergic Tone Antagonizes the Impairment of Retention Induced by a Nitric Oxide Synthase Inhibitor in Mice

The present experiments examined the role of the central cholinergic system in the memory impairment induced by post-training administration of a nitric oxide synthase (NOS) inhibitor in mice. Male Swiss mice received a one-trial inhibitory avoidance training (0.8 mA, 50 Hz, 1-s footshock) followed immediately by an ip injection of the NOS inhibitor -N^G-nitroarginine methyl ester ( -NAME; 100 mg/kg). Retention (cut-off time, 300 s) was tested 48 h after training. The administration of -NAME results in memory impairment for the inhibitory avoidance task. The effects of -NAME (100 mg/kg, ip) on retention were reversed in a dose-related manner by the centrally acting anticholinesterase physostigmine (35, 70, or 150 μg/kg, sc) administered 30 min after the NOS inhibitor. Further, -NAME (100 mg/kg, ip)-induced memory impairment was completely antagonized by the centrally acting muscarinic cholinergic agonist oxotremorine (OTM; 25, 50, or 100 μg/kg, sc) when given 30 min after -NAME. The peripherally acting anticholinesterase neostigmine (150 μg/kg, sc) did not modify the memory-impairing effects of -NAME. These findings suggest that the memory impairment following post-training administration of a NOS inhibitor is mediated, at least in part, by a reduction of the activity of central muscarinic cholinergic mechanisms and are consistent with our previous view that nitric oxide may be involved in post-training neural processes underlying the storage of newly acquired information.     

Effects of the nicotinic receptor blocker mecamylamine on radial-arm maze performance in rats

Lesions of cholinergic neurons have been found by many investigators to impair choice accuracy in the radial arm maze. Because muscarinic receptor blockers, such as scopolamine, have also repeatedly been found to impair choice accuracy in the radial-arm maze, it has generally been thought that the critical effect of cholinergic lesions is the deafferentation of muscarinic receptors. The possible involvement of nicotinic receptors in the cholinergic bases of cognitive performance in the radial-arm maze has not been as well investigated. The present study examined the effects of the blockade of nicotinic receptors on performance of female Sprague-Dawley rats in the radial-arm maze. Acute administration of the the nicotinic receptor blocker, mecamylamine (10 mg/kg) was found to significantly impair radial-arm maze choice accuracy. This dose also caused a significant increase in response latency in the maze. The effect on choice behavior but not locomotor speed seemed to be due to the central effects of mecamylamine, because administration of the peripheral nicotine receptor blocker, hexamethonium (20 mg/kg), did not impair choice accuracy, even though it did increase response latency to a similar degree as the 10-mg/kg dose of mecamylamine. Lower doses of mecamylamine (2.5 and 5 mg/kg) did not impair choice accuracy. These results indicate that central nicotinic as well as muscarinic cholinergic receptors are involved with cognitive functioning.     

Diazepam prevents post-training drug effects related to state dependency, but not post-training memory facilitation by epinephrine

Rats were trained and tested in a step-down inhibitory avoidance task (0.3-mA footshock). Training-test interval was 6 h. In Experiment 1, animals received, 1 h before training, an ip injection of vehicle or diazepam (2.0 mg/kg) and, 30 s after training and/or 30 min prior to testing, ip saline, epinephrine (6.25 micrograms/kg or 125.0 micrograms/kg), naloxone (0.5 mg/kg), or beta-endorphin (1 micrograms/kg). In the vehicle-pretreated animals, post-training epinephrine (6.25 micrograms/kg) and naloxone enhanced, and post-training beta-endorphin and epinephrine (125.0 micrograms/kg) reduced, retention test performance; and pretest beta-endorphin and epinephrine (125.0 micrograms/kg) reversed the latter effect and enhanced retention on their own. Diazepam lowered memory scores on its own and prevented all other drug effects with the exception of post-training facilitation by epinephrine (6.25 micrograms/kg). In previous papers it was shown that post-training facilitation by epinephrine is due to an influence on storage processes, whereas all the other drug effects described above result from the post-training establishment of state dependency to either beta-endorphin or epinephrine, and therefore to a process involving further acquisition and storage. The present findings suggest that diazepam selectively hindered the acquisition and/or storage processes involved in state dependency. This conclusion is strengthened by the findings from Experiment 2, which showed, using a classic 2 x 2 design, that diazepam itself did not induce state dependency but, rather, depressed acquisition and/or storage of the avoidance task.     

Nicotine enhances contextual fear conditioning in C57BL/6J mice at 1 and 7 days post-training

Nicotine has been demonstrated to enhance learning processes. The present experiments extend these results to examine the effects of nicotine on acquisition and consolidation of contextual and cued fear conditioning, and the duration of nicotine's enhancement of conditioned fear. C57BL/6 mice were trained with two pairings of an auditory CS and a foot shock US. Multiple doses of nicotine were given before or immediately after training and on testing day (0.0, 0.050, 0.125, 0.250, and 0.375 mg/kg, i.p). Freezing to both the context and auditory CS was measured 24h after training and again 1 week after training. Mice did not receive nicotine for the 1-week retest. Nicotine (0.125 and 0.250 mg/kg) given on both training and testing days enhanced freezing to the context at 24h. In addition, elevated freezing to the context was seen 1 week post-training in mice previously treated with 0.125 and 0.250 mg/kg nicotine. Thus, nicotine-treated mice did show elevated levels of freezing when retested 1 week later, even though no nicotine was administered at the 1-week retest. Mice that received nicotine on training day or testing day only and mice that received nicotine with mecamylamine, a nicotinic receptor antagonist, were not different from saline-treated mice. In addition, post-training administration of nicotine did not enhance fear conditioning. The present results indicate that nicotine enhancement of contextual fear conditioning depends on administration of nicotine on training and test days but results in a long-lasting enhancement of memories of contextual fear conditioning that remains in the absence of nicotine.     

Effects of Posttraining Administration of Insulin on Retention of a Habituation Response in Mice: Participation of a Central Cholinergic Mechanism

Male Swiss mice were allowed to explore a novel environment, provided by an open-field activity chamber for a 10-min period. The procedure was repeated twice within a 24-h interval. The difference in the exploratory activity between the first (training) and the second exposure (testing) to the chamber was taken as an index of retention of this habituation task. Posttraining intraperitoneal administration of insulin (8, 20, or 80 IU/kg) impaired retention in a dose-related manner, although only the dose of 20 IU/kg of insulin produced significant effects. Thus, the dose–response curve adopted a U-shaped form. Insulin (20 IU/kg) given to untrained mice did not modify their exploratory performance when recorded 24 h later. The effects of insulin on retention were time dependent, suggesting an action on memory storage. An ineffective dose (8 IU/kg) of insulin given together with an ineffective dose of a central acting muscarinic cholinergic antagonist atropine (0.5 mg/kg) or with a central acting nicotinic cholinergic antagonist mecamylamine (5 mg/kg) interacted to impair retention. In contrast, neither methylatropine (0.5 mg/kg), a peripherally acting muscarinic receptor blocker, nor hexamethonium (5 mg/kg), a peripherally acting nicotinic receptor blocker, interacted with the subeffective dose of insulin on retention. The impairing effects of insulin (20 IU/kg) on retention were reversed by the simultaneous administration of physostigmine (70 μg/kg) but not neostigmine (70 μg/kg). We suggest that insulin impairs memory storage of one form of learning elicited by stimuli repeatedly presented without reinforcement, probably through a decrement of brain acetylcholine synthesis.     

Oxotremorine attenuates retrograde amnesia induced by post-training administration of the GABAergic agonists muscimol and baclofen.

These experiments examined the involvement of cholinergic influences in the effects of GABAergic drugs on 24-h retention of an inhibitory avoidance response by mice. A first set of experiments confirmed previous findings indicating that post-training injections (ip) of the GABAergic agonists muscimol (1.0 and 2.0 mg/kg) and baclofen (10.0 and 20.0 mg/kg) impaired retention, as well as previous findings indicating that injections of the cholinergic agonist oxotremorine (5.0 and 10.0 micrograms/kg) enhanced retention. The findings of a second set of experiments indicated that the memory-impairing effects of muscimol and baclofen were attenuated by concurrent injections of a low, and otherwise ineffective, dose of oxotremorine (2.5 micrograms/kg). These findings are interpreted as suggesting that GABAergic drugs affect memory storage through influences on cholinergic systems.     

Nicotinic acetylcholine receptors of the ventral tegmental area are involved in mediating morphine-state-dependent learning

In the present study, the possible role of nicotinic acetylcholine (nACh) receptors of the ventral tegmental area (VTA) on morphine-state-dependent learning was studied in adult male Wistar rats. As a model of memory, a step-through type passive avoidance task was used. All animals were bilaterally implanted with chronic cannulae in the VTA, trained using a 1 mA foot shock, and tested 24 h after training to measure step-through latency. Post-training subcutaneous (s.c.) injection of morphine (0.5–5 mg/kg) dose-dependently reduced the step-through latency, showing morphine-induced amnesia. Amnesia induced by post-training morphine was significantly reversed by pre-test administration of morphine (2.5–5 mg/kg, s.c.) and induced morphine-state-dependent learning. Pre-test injection of nicotine (0.25–1 μg/rat) into the VTA plus an ineffective dose of morphine (0.5 mg/kg) significantly restored the memory retrieval. It should be noted that pre-test intra-VTA injection of the same doses of nicotine (0.25–1 μg/rat) alone cannot affect memory retention. Furthermore, pre-test intra-VTA injection of the nicotinic acetylcholine receptor antagonist, mecamylamine (1–3 μg/rat) 5 min before the administration of morphine (5 mg/kg, s.c.) dose-dependently inhibited morphine-state-dependent learning. Pre-test injection of the higher dose of mecamylamine (3 μg/rat) into the VTA by itself decreased the step-through latency and induced amnesia. On the other hand, mecamylamine (0.5 and 1 μg/rat, intra-VTA) reversed the effect of nicotine on morphine response. The results indicate that nACh receptors in the VTA participate in the modulation of morphine-induced recovery of memory, on the test day.     

The impairment of retention induced by pentylenetetrazol in mice may be mediated by a release of opioid peptides in the brain

Pentylenetetrazol (PTZ, 45 mg/kg, ip) impaired retention of a one-trial step-through inhibitory avoidance task when injected into male Swiss mice 10 min after training, as indicated by retention performance 48 h later. The amnestic effect of PTZ was prevented by naltrexone (0.01 or 0.10 mg/kg, ip) administered after training, but prior to PTZ-treatment. On the contrary, neither naltrexone methyl bromide (0.01, 0.10, or 10.0 mg/kg, ip), a quaternarium analog of naltrexone, nor MR2266 (0.01 or 0.10 mg/kg, ip), a putative kappa opiate receptor antagonist, modified the behavioral effects of PTZ. On the other hand, the body seizures produced by PTZ were unaffected by any of the three opiate receptor antagonists that were given before the convulsant. Taken together, these results suggest that the effects of PTZ on retention are mediated, at least in part, by opioid peptides of central origin, and rules out a possible participation of opioid peptides derived from prodynorphin-precursor molecule. Administration of beta-endorphin (0.01 or 0.10 microgram/kg, ip) 10 min prior to testing attenuate the retrograde amnesia caused by PTZ. The effect of beta-endorphin was prevented by the simultaneous administration of naltrexone (0.10 mg/kg, ip) prior to testing. Naltrexone has no effect of its own upon retrieval. These results suggest that the impairment of retention induced by PTZ is probably due, at least in part, to a release of opioid peptides in the brain during the post-training period. PTZ given after training do not affect consolidation or memory storage, as mice thus treated may retrieve the learned information when they are submitted to an appropriate neurohumoral and/or hormonal state in the test session, that is, beta-endorphin injection. Therefore, the action of PTZ would be primarily at the level of the mechanism that make stored information available for late retrieval.     

Pharmacological evidence of a central effect of naltrexone, morphine, and beta-endorphin and a peripheral effect of met- and leu-enkephalin on retention of an inhibitory response in mice

Immediate post-training administration of the central acting opioid receptor antagonist naltrexone (0.01-1.00 mg/kg) facilitated 48-h retention of a one-trial inhibitory avoidance task. An inverted-U dose-response curve was obtained. In this dose range naltrexone did not significantly affect response latencies of mice not given a footshock during the training. However, higher doses of naltrexone (3.0 and 10.0 mg/kg) increased latencies of both shocked and unshocked mice. The peripheral-acting opioid receptor blocker, naltrexone methyl bromide (MR 2263) (0.01-10.00 mg/kg), did not significantly influence retention latencies of either shocked or unshocked mice. Further, MR 2263 (0.1, 1.0, or 10.0 mg/kg) did not block the retention impairment produced by concurrently administered morphine (3.0 mg/kg) or beta-endorphin (0.1 microgram/kg). These findings indicate that the effect of these agonists on memory are not due to a peripheral influence. However, MR 2263 does prevent the memory-impairing effect of both metenkephalin (1.0 microgram/kg) and leu-enkephalin (0.3 microgram/kg) on retention. Those results suggest that enkephalins affect retention through influences initiated peripherally. Thus, different sites and mechanisms of action for beta-endorphin and the enkephalins are proposed.     

N-Methyl-D-aspartate receptors in the ventral tegmental area are involved in retrieval of inhibitory avoidance memory by nicotine

The interaction of opiate, cholinergic, glutamatergic and (possibly) dopaminergic inputs in the ventral tegmental area (VTA) influencing a learned behavior is certainly a topic of great interest. In the present study, the effect of intra-VTA administration of N-methyl-d-aspartate (NMDA) receptor agents on nicotine's effect in morphine state-dependent learning was investigated. An inhibitory avoidance (IA) task was used for memory assessment in male Wistar rats. Subcutaneous (s.c.) administration of morphine (5 and 7.5mg/kg) immediately after training decreased IA response on the test day, which was reinstated by pre-test administration of the same doses of the opioid; this is known as state-dependency. Moreover, pre-test administration of nicotine (0.2, 0.4 and 0.6 mg/kg, s.c.) also reversed the decrease in IA response because of post-training morphine (5mg/kg). Here, we also show that when infused into the VTA before testing, NMDA (0.01 and 0.1 microg/rat) reverse the post-training morphine effect on memory. In addition, the sub-effective doses of NMDA (0.0001 and 0.001 microg/rat) in combination with a low dose of nicotine (0.1mg/kg) which had no effects by themselves, synergistically improved retrieval of IA memory on the test day. In contrast, pre-test administration of a competitive NMDA receptor antagonist D-AP5 (0.5, 1 and 2 microg/rat) which had no effect alone prevented the nicotine reversal of morphine effect on memory. Our data indicate that NMDA receptors in the VTA are involved in the reversing effect of nicotine on morphine induced state-dependency.     

The Impairment of Retention Induced by Insulin in Mice May Be Mediated by a Reduction in Central Cholinergic Activity

Immediate posttraining intraperitoneal injection of nonconvulsive doses of insulin (2-20 IU/kg) significantly impaired retention of male Swiss mice tested 24 h after training in a one-trial step-through inhibitory avoidance task. The dose-response curve showed a U-shaped form. However, of the doses tested, only 8 IU/kg was effective. Insulin did not affect response latencies in mice not given the footshock on the training trial, indicating that the actions of insulin on retention performance were not due to nonspecific proactive effects on response latencies. The impairing effects of insulin (8 IU/kg) on retention were time-dependent, which suggests that insulin impaired memory storage. The simultaneous administration of glucose (10-1000 mg/kg) antagonized, in a dose-related manner, the actions of insulin (8 IU/kg) on retention, suggesting that the hormone may have produced a hypoglycemic response leading to a decrease in CNS glucose availability with a subsequent memory impairment. Low subeffective doses of atropine (0.5 mg/kg) or mecamylamine (5 mg/kg), but not methylatropine (0.5 mg/kg) or hexamethonium (5 mg/kg), given immediately after training but 10 min before an ineffective dose of insulin (4 IU/kg), interacted with and impaired retention. The central anticholinesterase physostigmine (35 or 70 μg/kg), but not its quaternary analog neostigmine (35 or 70 μg/kg), prevented the memory impairment induced by insulin (8 IU/kg). Considered together, these findings are consistent with the view that a decrease in the CNS glucose availability impairs the synthesis and/or release of acetylcholine in brain regions critically involved in memory storage.     

Benzodiazepine receptor ligand influences on acquisition: suggestion of an endogenous modulatory mechanism mediated by benzodiazepine receptors

In rats, pretraining ip administration of the central benzodiazepine receptor antagonist, flumazenil (5.0 mg/kg), or of the inverse agonist, n-butyl-beta-carboline-3-carboxylate (BCCB) (0.2 or 0.5 mg/kg), facilitated retention of a step-down inhibitory avoidance task; the central agonists, clonazepam and diazepam (0.4 or 1.0 mg/kg), had an opposite effect, and the peripheral agonist, 4'-chlordiazepam (1.25 or 6.25 mg/kg), was without effect. Pre- but not post-training flumazenil (2.0 mg/kg) blocked the effect of BCCB (0.5 mg/kg), clonazepam (1.0 mg/kg), or diazepam (1.0 mg/kg) given also pretraining. The post-training administration of all of these drugs had no effect on retention of the avoidance task. Flumazenil (5.0 mg/kg) and BCCB (0.5 mg/kg), given before training, enhanced retention test performance of habituation to a buzzer but not of habituation to an open field. In the three tasks studied, none of the drugs used had any appreciable effect on training session parameters. These results suggest that there is an endogenous mechanism mediated by benzodiazepine agonists, sensitive to inverse agonists, that normally down-regulates acquisition of certain behaviors; this mechanism becomes activated only when the tasks involve or occur with a certain degree of stress or anxiety (i.e., inhibitory avoidance or habituation to the buzzer) and not in less stressful or anxiogenic tasks (i.e., habituation to an open field).     

Post-training memory facilitation blocks extinction but not retroactive interference

Rats were trained in a step-down inhibitory avoidance task using a 0.2-mA footshock and tested 6 h later. Exposure to a session of extinction (animals placed on the box where they had been trained and left to explore it freely for 100.0 s without footshocks) or to a series of 10 tones presented in another box, in the dark, 2 h after training, hindered retention test performance. The immediate post-training ip administration of epinephrine (5.0 micrograms/kg), ACTH-(1-24) (0.2 microgram/kg), or lysine-vasopressin (10.0 micrograms/kg) facilitated retention test performance and cancelled the effect of extinction, but not the retroactive interference caused by exposure to the tones. These results support the concept that post-training facilitation induced by the hormones is due to a strengthening of the memory trace left by the avoidance task, whereas the retroactive interference caused by the tones occurs independently from that process and is more likely due to the incorporation of postevent information.