NO synthesis inhibition decreases cortical ACh release and impairs retention of a conditioned response.

We investigated in rats the effect N(G)-nitro-L-arginine methyl ester (L-NAME) on retention of a passive avoidance response, and cortical ACh release monitored using the microdialysis technique. Post-training administration of L-NAME impaired 24 h retention of a passive avoidance and decreased cortical ACh release. Both effects of L-NAME were reversed by L-Arg. These results suggest that nitric oxide is involved in retention of the passive avoidance response through the modulation of the forebrain cholinergic system.

Glucose plus choline improve passive avoidance behaviour and increase hippocampal acetylcholine release in mice.

The present study tests the effects of glucose and choline, the biosynthetic precursors of acetylcholine, on passive avoidance behaviour and hippocampal acetylcholine release measured by microdialysis in awake mice. Glucose (10 and 30mg/kg) or choline chloride (6-60mg/kg), given by i.p. injection immediately after training, dose-dependently enhanced retention in an inhibitory avoidance task. Combinations of low doses of glucose (10mg/kg) and choline chloride (20mg/kg) which alone were submaximally effective significantly increased retention latencies in a synergistic manner, an effect which was sensitive to atropine (0.5mg/kg). This beneficial effect vanished when higher doses of glucose or choline were combined. Basal hippocampal acetylcholine release in mice habituated to their environment was not affected by administration of glucose and choline. However, when hippocampal acetylcholine release was stimulated either by infusion of scopolamine (0.3microM) or by transferring the mice into a novel environment, the combination of glucose plus choline further increased acetylcholine release to a significant extent. We conclude that low doses of glucose and choline act synergistically to improve memory storage, an effect which is due to facilitation of acetylcholine release. This finding reinforces the view that central cholinergic functions are influenced under certain conditions by dietary intake of precursors.

Adenosine and memory storage: effect of A(1) and A(2) receptor antagonists.

RATIONALE: Caffeine is a non-selective A(1)/A(2 )adenosine receptor antagonist which is known to improve cognitive performance in humans. This effect of caffeine has been attributed to its antagonism of adenosine receptors. OBJECTIVE: The present study was devised to identify the role of A(1 )and A(2A) adenosine receptors in the facilitation of memory consolidation in mice performing a passive avoidance task. METHODS: Adult albino Swiss male mice were used. The mice were trained in a step-through inhibitory avoidance task in which they were punished by a foot-shock (0.4 mA, 5 Hz, for 3 s) delivered through the grid floor. Caffeine (0.1, 0.3, 1.0 and 3.0 mg/kg), SCH 58261 (0.1, 0.3, 1.0 and 3.0 mg/kg) and DPCPX (0.1, 0.3, 1.0 and 3.0 mg/kg) were injected IP immediately or 180 min after training. The retention test was performed 24 h after training. RESULTS: Caffeine and the selective A(2A) adenosine receptor antagonist SCH 58261 facilitated retention when administered immediately after training, but not when administered 180 min later. The dose response was a bell-shaped curve. Conversely, post-training administration of the selective A(1) adenosine receptor antagonist DPCPX did not affect retention. Caffeine and SCH 58261 had no effect in mice not given the foot-shock on the training trial, a finding indicating that the drug's effect on retention was specific. CONCLUSIONS: These results suggest that A(2A) but not A(1) adenosine receptors are involved in memory retention and consolidation.

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 microg/kg) but not neostigmine (70 microg/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.

Phlorizin, a competitive inhibitor of glucose transport, facilitates memory storage in mice.

Posttraining intraperitoneal administration of phlorizin (3.0-300.0 microg/kg), a competitive inhibitor of glucose transport from blood to brain, facilitated 48-h retention, in male Swiss mice, of a one-trial step-through inhibitory avoidance task. The dose-response curve was an inverted-U shape. Phlorizin did not increase the retention latencies of mice that had not received a foot shock during training. The effects of phlorizin (30.0 microg/kg) on retention were time dependent, and the administration of phlorizin (30.0 microg/kg) 5 or 10 min prior to the retention test did not affect the retention performance of mice given posttraining injections of saline or phlorizin (30.0 microg/kg). These findings indicate that phlorizin influenced memory storage, but not memory retrieval. Finally, the simultaneous administration of phlorizin (3. 0-300.0 microg/kg, ip) antagonized, in a dose-related manner, the memory impairment induced by insulin (8 IU/kg, ip). Taken together, the results show that phlorizin enhance retention acting as a "glucose-like substance" although the mechanism(s) of this enhancement is unknown.

AF-DX 116, a presynaptic muscarinic receptor antagonist, potentiates the effects of glucose and reverses the effects of insulin on memory.

Male Swiss mice were tested 24 h after training in a one-trial step-through inhibitory avoidance task. Low subeffective doses of d-(+)-glucose (10 mg/kg, ip), but not its stereoisomer l-(-)-glucose (30 mg/kg,ip), administered immediately after training, and AF-DX 116 (0.3 mg/kg,ip), a presynaptic muscarinic receptor antagonist, given 10 min after training, interact to improve retention. Insulin (8 IU/kg, ip) impaired retention when injected immediately after training, and the effects were reversed, in a dose-related manner, by AF-DX 116 (0.3, 1.0, or 3.0 mg/kg, ip) administered 10 min following insulin. Since AF-DX 116 possibly blocks autoreceptors mediating the inhibition of acetylcholine release from cholinergic nerve terminals, the present data support the view that changes in the central nervous system glucose availability, subsequent to modification of circulating glucose levels, influence the activity of central cholinergic mechanisms involved in memory storage of an inhibitory avoidance response in mice.

Effects of a single administration of oxytocin or vasopressin and their interactions with two selective receptor antagonists on memory storage in mice.

The neuropeptides arginine vasopressin (AVP) and oxytocin (OT) have been thought to play a significant role in behavioral regulation in general and in learning and memory in particular. Experimental evidence suggests that AVP improves, and OT impairs, learning and memory. The present paper investigates the posttraining effects of OT and of an OT receptor antagonist, and their interaction, on memory storage in mice. Additional studies were conducted to determine the specificity of the interaction between OT and its receptors. Male Swiss mice were tested 48 h after training in a one-trial step-through inhibitory avoidance task. Immediate posttraining subcutaneous injection of OT (0.01, 0.03, 0.10, 0.30, and 1.00 microg/kg) impaired retention performance. The dose-response curve showed a U-shaped form, with a significant impairment seen at doses of 0.10 and 0.30 microg/kg of OT. In contrast, the immediate posttraining administration of the putative oxytocin receptor antagonist d(CH2)5[Tyr(Me)2, Thr4, Thy-NH(9)2]OVT (AOT, 0.03, 0.10, 0.30, and 1.00 microg/kg) significantly enhanced retention performance. The dose-response curve was an inverted "U" in this range of doses. However, of the doses tested, only 0.30 microg/kg was effective. Neither OT nor AOT affected response latencies in mice not given the footshock on the training trial, indicating that the actions of both treatments on retention performance were not due to nonspecific proactive effects on response latencies. Neither the imparing effects of OT (0.10 microg/kg) nor the enhancing effects of AOT (0.30 microg/kg) were seen when the training-treatment interval was 180 min, suggesting that both treatments influenced the storage of recently acquired information. The effects of OT (0.10 microg/kg) on retention were prevented by AOT (0.03 microg/kg) administered immediately after training, but 10 min prior to oxytocin treatment. This dose of antagonist did not affect retention by itself, either under the standard experimental conditions or in mice trained with a lower level of footshock. On the contrary, OT (0.10 microg/kg) impaired retention in mice pretreated with the V1a vasopressin receptor antagonist d(CH2)5[Tyr(Me)2]AVP (0.01 microg/kg), which, however, was able to prevent the enhancement of retention induced by posttraining administration of AVP (0.03 microg/kg). Finally, the effects of AVP (0.03 microg/kg) on retention were not prevented by AOT (0.03 microg/kg). Considered together, these findings suggest that the impairment of retention of an inhibitory avoidance response in mice induced by posttraining oxytocin is probably due to an interaction of the neuropeptide with specific receptors.

Acetylcholine release from the frontal cortex during exploratory activity.

The activation of the cortical cholinergic system was investigated in 3- and 25-month-old male Wistar rats, by measuring by transversal microdialysis the changes in cortical extracellular acetylcholine (ACh) levels during the performance of simple spontaneous tasks involving exploratory activity and working memory. Two days after implantation of the microdialysis probe in the frontal cortex, object recognition was investigated by either moving the rats from the home cage to the arena containing the objects or keeping the rats in the arena and introducing the objects. Spontaneous alternation was investigated in a Y runway. Young rats discriminated between familiar and novel objects and alternated in the Y runway, while aged rats were unable to discriminate. Whenever rats were moved from the home cage to the arena, ACh release increased (+70-80%) during the exploratory activity. Handling per se had no effect on extracellular ACh levels. When young rats were left in the arena, introduction of the objects caused some exploratory activity and object recognition but no increase in ACh release. ACh release increased by about 300% during spontaneous alternation. In aging rats basal extracellular ACh levels and their increase after placement in the arena were less than half that in young rats. Our work demonstrates that a novel environment activates the cortical cholinergic system, which presumably is associated with arousal mechanisms and selective attentional functions. It also demonstrates that in aging rats the cortical cholinergic hypofunction is associated with a loss of non-spatial working memory.

Selective muscarinic antagonists differentially affect in vivo acetylcholine release and memory performances of young and aged rats.

Brain acetylcholine release and memory performance were investigated in young (three- to six-months) and old (20- to 24-months) rats. Acetylcholine release was measured in vivo in the cortex and hippocampus of freely-moving animals, under basal conditions and in the presence of the following muscarinic antagonists: scopolamine, (+/-)-5,11-dihydro-11-[[(2-[2-[(dipropylamino) methyl]-1-piperidinyl]ethyl) amino] carbonyl]-6H-pyrido(2,3-b)(1,4)-benzodiazepine-6-one (AFDX 384) and pirenzepine. The amount of acetylcholine released from the cortex and hippocampus of old rats was significantly reduced. In the presence of scopolamine and AFDX 384 but not of pirenzepine, the acetylcholine release was significantly higher in the old than the young rats, suggesting that changes in presynaptic M2/M4 muscarinic receptor function occur with ageing in the two brain regions. Cognitive capacities were evaluated using two different behavioural tasks: object recognition and passive avoidance response. Old rats were unable to discriminate between familiar and novel objects and had impaired performance in the passive avoidance test. AFDX 384 restored the performance in both tests. Furthermore, in young rats AFDX 384 reversed the impairment of both object recognition and passive avoidance response induced by scopolamine. The effect of AFDX 384 on acetylcholine release and behaviour in the old rats offers further support to a relationship between the age-related cholinergic hypofunction and cognitive impairment and indicates the blockade of presynaptic muscarinic receptors as a possible selective target for therapeutic strategies aimed at improving age-associated memory deficits.

The enhancement of retention performance induced by picrotoxin in mice may be mediated through a release of endogenous vasopressin.

Male Swiss mice were tested 48h after training in a one-trial step-through inhibitory avoidance task. Immediately post-training i.p. injection of the GABA antagonist picrotoxin (0.3-3.0mg/kg), at nonconvulsive doses, induced a dose-dependent modification of retention performance. The lower doses of picrotoxin (0.1-1.0mg/kg) enhanced retention, whereas the highest dose (3.0mg/kg) impaired retention. Picrotoxin did not affect response latencies in mice not given the footshock on the training trial, indicating that the actions of picrotoxin on retention performance were not due to nonspecific proactive effects on response latencies. The enhancing effects of picrotoxin (1.0mg/kg) on retention were time-dependent, which suggests that picrotoxin enhanced storage of recently acquired information. The enhancement of retention induced by picrotoxin (1.0mg/kg) was prevented by the vasopressin receptor antagonist, AAVP (0.01µg/kg, s.c.) administered immediately after training, but prior to picrotoxin treatment. This dose of AAVP did not affect retention by itself, either under the standard experimental conditions, or in mice trained with a high footshock. Low subeffective doses of picrotoxin (0.1mg/kg, s.c.) administered immediately after training, and hypertonic saline (1ml of 0.5M NaCl, i.p.), given 10min after training, interacted to improve retention. Considered together, these findings suggest that the better retention performance induced by post-training administration of picrotoxin could result, at least in part, from an endogenous release of vasopressin.

A nitric oxide synthase inhibitor impairs memory storage in mice.

Posttraining administration of the L-enantiomer of the competitive inhibitor of nitric oxide synthase, NG-nitro-L-arginine methyl ester (L-NAME, 3-100 mg/kg, ip), impaired 48-h retention of a one-trial step-through inhibitory shock-avoidance task in male Swiss mice. The effects were dose-dependent and were not observed when the D-enantiomer (D-NAME, 3-100 mg/kg, ip) was injected instead of L-NAME. Retention latencies of mice that had not received a footshock during training were not affected by L-NAME. The memory impairment produced by L-NAME was time-dependent, suggesting an action on memory storage. The effects of L-NAME on memory were overcome by the injection of L-(but not D-)arginine (300 mg/kg, ip) along with the inhibitor. Considered together, these findings suggest that the L-arginine/nitric oxide pathway may be involved in memory storage of an inhibitory avoidance response in mice.

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 L-NG-nitroarginine methyl ester (L-NAME; 100 mg/kg). Retention (cut-off time, 300 s) was tested 48 h after training. The administration of L-NAME results in memory impairment for the inhibitory avoidance task. The effects of L-NAME (100 mg/kg, ip) on retention were reversed in a dose-related manner by the centrally acting anticholinesterase physostigmine (35, 70, or 150 microg/kg, sc) administered 30 min after the NOS inhibitor. Further, L-NAME (100 mg/kg, ip)-induced memory impairment was completely antagonized by the centrally acting muscarinic cholinergic agonist oxotremorine (OTM; 25, 50, or 100 microg/kg, sc) when given 30 min after L-NAME. The peripherally acting anticholinesterase neostigmine (150 microg/kg, sc) did not modify the memory-impairing effects of L-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 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 microg/kg), but not neostigmine (35 microg/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.

The post-training memory enhancement induced by physostigmine and oxotremorine in mice is not state-dependent.

Immediate post-training subcutaneous administration of either the centrally acting anticholinesterase physostigmine (35, 70, or 150 mu g/kg) or the centrally acting muscarinic cholinergic agonist oxotremorine (OTM; 25, 50, or 100 mu g/kg) significantly enhanced retention of male Swiss mice tested 48 h after training in a one-trial step-through inhibitory avoidance task (0.8 mA, 50 Hz, 1 s footshock). Neither physostigmine nor OTM affected latencies to step through in mice not given the footshock on the training trial, suggesting that the effects of both cholinomimetics on retention performance were not due to nonspecific actions on response test latencies. The peripherally acting anticholinesterase neostigmine (35, 70, or 150 mu g/kg) did not significantly influence retention latencies of either shocked or unshocked mice. The influences of physostigmine (150 mu g/kg) or OTM (100 mu g/kg) 30 min prior to the retention test did not affect the retention performance of mice given post-training injections of either saline, physostigmine (150 mu g/kg), or OTM (100 mu g/kg). Considered together, these findings indicate that the memory-enhancing effects of post-training administration of physostigmine or OTM are not state-dependent and are consistent with the view that the behavioral effects of the cholinomimetics drugs are mediated through an interaction with the neural processes underlying the storage of acquired information.

Memory modulation by post-training glucose or insulin remains evident at long retention intervals.

Immediate posttraining intraperitoneal injection of alpha-D[+]-glucose (30 mg/kg) facilitated, whereas a nonconvulsive dose of insulin (8 IU/kg) impaired, 24-h retention, in male Swiss mice, of a one-trial step-through inhibitory avoidance task. When mice were trained and received immediate post-training glucose or insulin injections and were tested for retention either 1 week or 1 month later, at each retention interval performance was comparable to that found with a 24-h retention interval. Thus, memory modulation by post-training administration of either glucose or insulin remain evident at long retention intervals.

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 micrograms/kg), but not its quaternary analog neostigmine (35 or 70 micrograms/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.

Memory-improving actions of glucose: involvement of a central cholinergic muscarinic mechanism.

Post-training intraperitoneal administration of alpha-D[+]-glucose (10-300 mg/kg) facilitated 24-h retention, in male Swiss mice, of a one-trial step-through inhibitory avoidance task. The dose-response curve was an inverted U. Glucose did not increase the retention latencies of mice that had not received a footshock during training. The effect of glucose (30 mg/kg, ip) on retention was time-dependent, which suggests that the drug facilitated memory storage. The memory facilitation induced by glucose (30 mg/kg, ip) was prevented by atropine (0.5 mg/kg, ip) administered after training, but 10 min prior to glucose treatment. In contrast, neither methylatropine (0.5 mg/kg, ip), a peripherally acting muscarinic receptor blocker, nor mecamylamine (5 mg/kg, ip) or hexamethonium (5 mg/kg, ip), two cholinergic nicotinic receptor antagonists, prevented the effects of post-training glucose on retention. Low subeffective doses of the central acting anticholinesterase physostigmine (35 micrograms/kg, ip), administered immediately after training, and glucose (10 mg/kg, ip), given 10 min after training, acted synergistically to improve retention. The effects of glucose (10 mg/kg, ip) were not influenced by the peripherally acting anticholinesterase neostigmine (35 micrograms/kg, ip). Considered together, these findings suggest that the memory facilitation induced by post-training administration of glucose could result from an enhancement of brain acetylcholine synthesis and/or its release that, in turn, might modulate the activity of muscarinic cholinergic mechanisms that are critically involved in memory storage.

Glucose enhancement of memory is not state-dependent.

Immediate post-training intraperitoneal administration of alpha-D[+]-glucose (10-300 mg/kg) significantly enhanced retention of male Swiss mice tested 24 h after training in an inhibitory avoidance task. The dose-response curve was an inverted U in this range of dose. However, of the doses tested, only 30 mg/kg was effective. Glucose did not affect response latencies in mice not given the footshock on the training trial, suggesting that the actions of glucose on retention performance were not due to nonspecific effects on response latencies. The influence of glucose (30 mg/kg) was time-dependent, which suggests that glucose facilitated memory consolidation processes. Administration of glucose (30 mg/kg) 2 or 10 min prior to the retention test did not affect the retention performance of mice given post-training injections of either saline or glucose (30 mg/kg). These findings indicate that the memory-enhancing effects of post-training administration of glucose are not state-dependent and are consistent with the view that the behavioral effects of glucose are mediated through an interaction with the neural or neurohumoral processes underlying the storage of acquired information.

Facilitation of memory storage by the acetylcholine M2 muscarinic receptor antagonist AF-DX 116.

Post-training administration of the acetylcholine muscarinic M2 presynaptic receptor antagonist AF-DX 116 (0.1-10.0 mg/kg, ip), facilitated 48 h retention, in male Swiss mice, of a one-trial step-through inhibitory avoidance task. The dose-response curve was an inverted U. AF-DX 116 did not increase the retention latencies of mice that had not received a footshock during training. The influence of AF-DX 116 (1 mg/kg, ip) on retention was time-dependent, which suggests that the drug facilitated memory storage. The memory facilitation induced by AF-DX 116 (1 mg/kg, ip) was prevented by atropine (0.5 mg/kg, ip) administered after training, but 10 min prior to AF-DX 116 treatment. In contrast, neither methylatropine (0.5 mg/kg, ip), a peripherally acting muscarinic receptor blocker, nor mecamylamine (5 mg/kg, ip) or hexamethonium (5 mg/kg, ip), two cholinergic nicotinic receptor antagonists, prevented the effects of post-training AF-DX 116 on retention. Low subeffective doses of the central acting anticholinesterase physostigmine (35 micrograms/kg, ip), administered immediately after training, and AF-DX 116 (0.1 mg/kg, ip), given 10 min after training, acted synergistically to improve retention. The effects of AF-DX 116 (0.1 mg/kg, ip) were not influenced by the peripherally acting anticholinesterase neostigmine (35 micrograms/kg, ip). Considered together, these findings suggest that the activation of a muscarinic cholinergic presynaptic inhibitory mechanism, probably by increasing brain acetylcholine release, may modulate the activity of post-training processes involved in memory storage.