The Effects of Intra-amygdala Infusion of the AMPA Receptor Antagonist CNQX on Retention Performance Following Aversive Training

The aim of these experiments was to determine whether impaired retention performance in aversively motivated tasks, induced by blockade of amygdala AMPA receptors, is due to influences on mechanisms underlying memory retrieval or to other influences on performance. Rats received either footshock escape training (1 or 10 trials), or no foot shock, in a two-compartment straight alley and bilateral intra-amygdala infusions of the AMPA receptor antagonist CNQX (0.5 μg) were subsequently administered prior to inhibitory avoidance retention testing 8 days later. The CNQX impaired, but did not block, inhibitory avoidance retention performance as indicated by the initial latencies to enter the shock compartment. The animals were then retained in the alley until they remained in the starting compartment for 100 consecutive s and entries into the shock compartment were recorded as errors. In both the controls and CNQX-treated groups, increases in amount of original training resulted in fewer errors, indicating memory for the escape training. Furthermore, regardless of the amount of original training (i.e., 0, 1, or 10 trials), CNQX-treated groups made more errors. Other experiments examined intra-amygdala CNQX effects on reactivity to footshock, locomotor activity, and anxiety. CNQX decreased reactivity to footshock, blocked shock-induced decreases in locomotor activity, and had an anxiolytic effect in an elevated plus maze comparable to that induced by midazolam (0.5 μg). These findings suggest that intra-amygdala infusions of CNQX prior to retention testing affect inhibitory avoidance retention performance following aversive training by altering locomotor activity, reducing sensitivity to footshock, and reducing anxiety. The implications of these findings for hypotheses concerning amygdala function in aversively motivated learning and memory is discussed.     

Modulation of memory consolidation by the basolateral amygdala or nucleus accumbens shell requires concurrent dopamine receptor activation in both brain regions

Previous findings indicate that the basolateral amygdala (BLA) and the nucleus accumbens (NAc) interact in influencing memory consolidation. The current study investigated whether this interaction requires concurrent dopamine (DA) receptor activation in both brain regions. Unilateral, right-side cannulae were implanted into the BLA and the ipsilateral NAc shell or core in male Sprague-Dawley rats ( approximately 300 g). One week later, the rats were trained on an inhibitory avoidance (IA) task and, 48 h later, they were tested for retention. Drugs were infused into the BLA and NAc shell or core immediately after training. Post-training intra-BLA infusions of DA enhanced retention, as assessed by latencies to enter the shock compartment on the retention test. Infusions of the general DA receptor antagonist cis-Flupenthixol (Flu) into the NAc shell (but not the core) blocked the memory enhancement induced by the BLA infusions of DA. In the reverse experiment, post-training intra-NAc shell infusions of DA enhanced retention and Flu infusions into the BLA blocked the enhancement. These findings indicate that BLA modulation of memory consolidation requires concurrent DA receptor activation in the NAc shell but not the core. Similarly, NAc shell modulation of memory consolidation requires concurrent DA receptor activation in the BLA. Together with previous findings, these results suggest that the dopaminergic innervation of the BLA and NAc shell is critically involved in the modulation of memory consolidation.     

Phthalic acid amygdalopetal lesion of the nucleus basalis magnocellularis induces reversible memory deficits in rats

The basolateral amygdala (BLA) is extensively implicated in emotional learning and memory. The current study investigated the contribution of cholinergic afferents to the BLA from the nucleus basalis magnocellularis in influencing aversive learning and memory. Sprague-Dawley rats were given permanent unilateral phthalic acid (300 ng) lesions of the nucleus basalis magnocellularis and were chronically implanted with cannulas aimed at the ipsilateral BLA. Lesioned rats showed a pronounced inhibitory avoidance task retention deficit that was attenuated by acute posttraining infusions of the muscarinic cholinergic agonist oxotremorine (4 ng) or the indirect agonist physostigmine (1 microg) into the BLA. Continuous multiple-trial inhibitory avoidance training and testing revealed that lesioned rats have a mild acquisition deficit, requiring approximately 1 additional shock to reach the criterion, and a pronounced consolidation deficit as indicated by a shorter latency to enter the shock compartment on the retention test. Because lesioned rats did not differ from sham-operated controls in performance on a spatial water maze task or in shock sensitivity, it is not likely that the memory impairments produced by the phthalic acid lesions are due to any general sensory or motor deficits. These findings suggest that the dense cholinergic projection from the nucleus basalis magnocellularis to the BLA is involved in both the acquisition and the consolidation of the aversive inhibitory avoidance task.     

Involvement of the basolateral amygdala in muscarinic cholinergic modulation of extinction memory consolidation

Previous studies have reported that drugs affecting neuromodulatory systems within the basolateral amygdala (BLA), including drugs affecting muscarinic cholinergic receptors, modulate the consolidation of many kinds of training, including contextual fear conditioning (CFC). The present experiments investigated the involvement of muscarinic cholinergic influences within the BLA in modulating the consolidation of CFC extinction memory. Male Sprague Dawley rats implanted with unilateral cannula aimed at the BLA were trained on a CFC task, using footshock stimulation, and 24 and 48 h later were given extinction training by replacing them in the apparatus without footshock. Following each extinction session they received intra-BLA infusions of the cholinergic agonist oxotremorine (10 ng). Immediate post-extinction BLA infusions significantly enhanced extinction but infusions administered 180 min after extinction training did not influence extinction. Thus the oxotremorine effects were time-dependent and not attributable to non-specific effects on retention performance. These findings provide evidence that, as previously found with original CFC learning, cholinergic activation within the BLA modulates the consolidation of CFC extinction.     

Intra-basolateral amygdala infusions of AP-5 impair or enhance retention of inhibitory avoidance depending on training conditions

Previous evidence has suggested that N-methyl-D-aspartate receptors (NMDARs) in the basolateral amygdala (BLA) are critically involved in the acquisition of aversively based learning tasks. However, the role of NMDARs in the BLA in the consolidation of memory of aversive training has not been well elucidated. In the present study, the NMDAR antagonist AP-5 (1 or 3 microg) was infused into the BLA of male Sprague-Dawley rats immediately before, immediately after, or 6h after training on an inhibitory avoidance task with either a high footshock (HFS; only high dose of AP-5 given) or a low footshock (LFS; both doses of AP-5 given). The 48 h retention of animals given AP-5 (3 microg) immediately before or after HFS training was significantly impaired compared to that of vehicle-controls. In contrast, the retention of rats given AP-5 (3 microg) immediately after LFS training was significantly enhanced compared to that of vehicle-controls. AP-5 (3 microg) infusions administered 6h after training with either an HFS or LFS did not affect retention. These findings suggest that the NMDARs in the BLA are involved in both the acquisition and consolidation of aversive memory. In addition, the AP-5-induced enhancement of memory obtained with LFS training suggests that NMDARs in the BLA are involved in other mechanisms influencing synaptic transmission, in addition to their well-established role in neuroplasticity.     

Distinct contributions of the basolateral amygdala and the medial prefrontal cortex to learning and relearning extinction of context conditioned fear

We studied the roles of the basolateral amygdala (BLA) and the medial prefrontal cortex (mPFC) in learning and relearning to inhibit context conditioned fear (freezing) in extinction. In Experiment 1, pre-extinction BLA infusion of the NMDA receptor (NMDAr) antagonist, ifenprodil, impaired the development and retention of inhibition but post-extinction infusion spared retention. Pre-extinction infusion of the GABA(A) agonist, muscimol, depressed freezing and impaired retention as did post-extinction infusion. In Experiment 2, pre-extinction mPFC infusion of ifenprodil spared the development of inhibition whereas muscimol depressed freezing. Both impaired retention when infused pre- or post-extinction. Thus, the development of inhibition involves NMDAr activation in the BLA, whereas its consolidation involves both NMDAr activation in the mPFC and NMDAr-independent mechanisms in the BLA. In Experiment 3, BLA infusion of ifenprodil impaired relearning and retention of inhibition when infused before but did not impair retention when infused after re-extinction. BLA infusion of muscimol depressed freezing but did not impair retention when infused before or after re-extinction. In Experiment 4, mPFC infusion of ifenprodil impaired relearning when infused before re-extinction, whereas muscimol depressed responses. Both drugs impaired retention when infused into the mPFC before or after re-extinction. Thus, relearning to inhibit fear responses involves NMDAr activation in both the BLA and mPFC and consolidation of the inhibitory memory involves NMDAr activation in the mPFC. However, relearning and consolidation occur in the absence of neuronal activity within the BLA. We propose that NMDAr in the mPFC supports relearning inhibition when the BLA is inactivated.     

Epinephrine modulates long-term retention of an aversively motivated discrimination

These experiments examined the effects of post-training epinephrine (Epi) on retention of an aversively motivated discrimination task. Male CFW mice were trained to escape from footshock by entering one of two alleys of a Y-maze. On a 24-h retention test (six trials) the correct alley was reversed. The findings of Experiment 1 indicate that errors on the discrimination reversal varied directly with number of trials (criterion of 0, 3, or 6 successive correct choices) on the original training. These findings indicate that errors on discrimination reversal training provide a sensitive index of retention of the original training. In Experiment 2, mice were trained to a criterion of three successive correct choices and were given post-training injections of saline or Epi (0.1, 0.3, or 1.0 mg/kg ip). On a 24-h discrimination reversal test mice given the low doses of Epi made more errors than did saline controls while mice given the high dose made fewer errors. In Experiment 3, mice trained as in Exp 2 received post-training saline or Epi (0.3 or 1.0 mg/kg) 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. The findings provide additional evidence that post-training Epi produces long-lasting dose-dependent modulating effects on memory storage.     

Intra-amygdala muscimol injections impair freezing and place avoidance in aversive contextual conditioning

Rats were trained by shocking them in a closed compartment. When subsequently tested in the same closed compartment with no shock, normal rats showed an increased tendency to freeze. They also showed an increased tendency to actively avoid the compartment when given access to an adjacent neutral compartment for the first time. Amygdala inactivation with bilateral muscimol injections before training attenuated freezing and eliminated avoidance during the test. Rats trained in a normal state and given intra-amygdala muscimol injections before the test did not freeze or avoid the shock-paired compartment. This pattern of effects suggests that amygdala inactivation during training impaired acquisition of a conditioned response (CR) due either to inactivation of a neural substrate essential for its storage or to elimination of a memory modulation effect that facilitates its storage in some other brain region(s). The elimination of both freezing and active avoidance by amygdala inactivation during testing suggests that neither of these behaviors is the CR. The possibility that the CR is a set of internal responses that produces both freezing and avoidance as well as other behavioral effects is discussed.     

Enhanced inhibitory avoidance learning produced by post-trial injections of substance P into the basal forebrain

The effects of injections of the neuropeptide substance P or the GABA agonist muscimol on performance of a step-down inhibitory avoidance task were examined. Immediately after the training trial, rats with chronically implanted cannulas were injected with 100 or 10 ng of substance P or 500 or 50 ng of muscimol into the region of the nucleus basalis magnocellularis. Control groups included vehicle-injected rats, a sham-operated group, a substance P 5-h delay group, and a substance P no-footshock group. Rats injected with 100 ng of substance P exhibited longer step-down latencies when tested 24 h later than did vehicle-injected rats. The retention latencies for rats in the substance P 5-h delay group did not differ from those of vehicle-injected animals, indicating that proactive effects on performance were not responsible for the effect. In contrast to injections of SP, injections of 500 or 50 ng of muscimol disrupted performance. However, in the absence of a delayed-injection control group, proactive effects cannot be ruled out.     

Lasting increases in basolateral amygdala activity after emotional arousal: implications for facilitated consolidation of emotional memories

Manipulations that reduce or enhance the activity of basolateral amygdala (BLA) neurons in the minutes to hours after training have been shown to respectively impair or facilitate retention on the inhibitory avoidance task. Although this suggests that BLA activity is altered after emotional arousal, such changes have not been directly demonstrated. To test this, we devised a feline analog of the inhibitory avoidance task and recorded BLA unit activity before and after a single inescapable footshock. Single-unit recordings revealed that the firing rate of many BLA neurons gradually increased after the footshock, peaking 30-50 min post-shock and then subsiding to baseline levels 2 h later. During this period of increased activity, the discharges of simultaneously recorded BLA cells were more synchronized than before the shock. Although it was known that pairing innocuous (conditioned stimulus, CS) and noxious stimuli modifies the responsiveness of BLA neurons to the CS, our results constitute the first demonstration that emotional arousal produces lasting increases in the spontaneous firing rates of BLA neurons. We propose that these changes in BLA activity may promote Hebbian interactions between coincident but spatially distributed activity patterns in BLA targets, facilitating the consolidation of emotional memories.     

Effects of post-training bicuculline and muscimol on retention: lack of state dependency

Immediate post-training intraperitoneal injections of the GABA antagonist bicuculline (0.25 or 0.5 mg/kg) or of the GABA agonist muscimol (1.0 or 2.0 mg/kg) improved and impaired, respectively, retention of CD1 mice tested 24 h after training in a one-trial inhibitory avoidance task. Administration of bicuculline or muscimol prior to the retention test did not modify retention latencies of mice that had received either saline or the same drug immediately after training. These findings indicate that the effects of post-training administration of bicuculline and muscimol on retention are not state dependent and, thus, argue against a general state-dependency interpretation of the effects of post-training treatments affecting retention. The findings are consistent with previous evidence indicating that GABAergic drugs affect retention through influences on memory storage processes.     

Involvement of the rostral anterior cingulate cortex in consolidation of inhibitory avoidance memory: interaction with the basolateral amygdala

Previous findings suggest that the rostral anterior cingulate cortex (rACC) is involved in memory for emotionally arousing training. There is also extensive evidence that the basolateral amygdala (BLA) modulates the consolidation of emotional arousing training experiences via interactions with other brain regions. The present experiments examined the effects of posttraining intra-rACC infusions of the cholinergic agonist oxotremorine (OXO) on inhibitory avoidance (IA) retention and investigated whether the BLA and rACC interact in enabling OXO effects on memory. In the first experiment, male Sprague-Dawley rats were implanted with bilateral cannulae above the rACC and given immediate posttraining OXO infusions. OXO (0.5 or 3 ng) induced significant enhancement of retention performance on a 48-h test. In the second experiment, unilateral posttraining OXO infusions (0.5, 3.0 or 10 ng) enhanced retention when infused into rACC, but not caudal ACC, consistent with previous evidence that ACC is composed of functionally distinct regions. A third experiment investigated the effects of posttraining intra-rACC OXO infusions (0.5 or 10 ng) in rats with bilateral sham or NMDA-induced lesions of the BLA. The BLA lesions did not impair IA retention, but blocked the enhancement induced by posttraining intra-rACC OXO infusions. Lastly, unilateral NMDA lesions of rACC blocked the enhancement of IA retention induced by posttraining ipsilateral OXO infusions into the BLA. These findings support the hypothesis that the rACC is involved in modulating the storage of emotional events and provide additional evidence that the BLA modulates memory consolidation through interactions with efferent brain regions, including the cortex.     

The enhancing effects of hippocampal infusions of glucose are not restricted to spatial working memory

Extensive evidence shows that hippocampal infusions of glucose enhance spontaneous alternation (SA) performance or reverse deficits in this task. The current experiments determined whether the enhancing effects of hippocampal infusions of glucose are restricted to spatial working memory. Specifically we tested whether hippocampal infusions of glucose would reverse deficits in an emotional reference memory task (continuous multiple trial inhibitory avoidance [CMIA]) produced by septal infusions of the gamma-aminobutyric acid agonist muscimol. Male Sprague-Dawley rats were given septal infusions of vehicle or muscimol (0.15 nmol: SA; 5 nmol: CMIA) combined with hippocampal infusions of vehicle or glucose (50 nmol) 15 min prior to assessing SA or CMIA training. CMIA retention was tested 48 h later. Muscimol infusions decreased percent alternation scores and avoidance retention latencies. Importantly, hippocampal infusions of glucose reversed the deficits produced by the septal muscimol infusions on both tasks. These findings show for the first time that hippocampal glucose infusions also influence emotional memory, indicating that the enhancing effects of glucose generalize to memory tasks that vary in motivational and cognitive demand.     

Enhancement of extinction memory consolidation: the role of the noradrenergic and GABAergic systems within the basolateral amygdala

Evidence from previous studies indicates that the noradrenergic and GABAergic influences within the basolateral amygdala (BLA) modulate the consolidation of memory for fear conditioning. The present experiments investigated whether the same modulatory influences are involved in regulating the extinction of fear-based learning. To investigate this issue, male Sprague Dawley rats implanted with unilateral or bilateral cannula aimed at the BLA were trained on a contextual fear conditioning (CFC) task and 24 and 48 h later were given extinction training. Immediately following each extinction session they received intra-BLA infusions of the GABAergic antagonist bicuculline (50 ng), the beta-adrenocepter antagonist propranolol (500 ng), bicuculline with propranolol, norepinephrine (NE) (0.3, 1.0, and 3.0 microg), the GABAergic agonist muscimol (125 ng), NE with muscimol or a control solution. To investigate the involvement of the dorsal hippocampus (DH) as a possible target of BLA activation during extinction, other animals were given infusions of muscimol (500 ng) via an ipsilateral cannula implanted in the DH. Bilateral BLA infusions of bicuculline significantly enhanced extinction, as did infusions into the right, but not left BLA. Propranolol infused into the right BLA together with bicuculline blocked the bicuculline-induced memory enhancement. Norepinephrine infused into the right BLA also enhanced extinction, and this effect was not blocked by co-infusions of muscimol. Additionally, muscimol infused into the DH did not attenuate the memory enhancing effects of norepinephrine infused into the BLA. These findings provide evidence that, as with original CFC learning, noradrenergic activation within the BLA modulates the consolidation of CFC extinction. The findings also suggest that the BLA influence on extinction is not mediated by an interaction with the dorsal hippocampus.     

Posttraining inactivation of excitatory afferent input to the locus coeruleus impairs retention in an inhibitory avoidance learning task

These experiments examined whether the nucleus paragigantocellularis (PGi) contributes to memory storage processing via its ascending excitatory influence on locus coeruleus (LC) neuronal activity. Activation of the LC leads to memory enhancement and also results in a widespread release of norepinephrine in target structures, such as the amygdala and hippocampus. Infusion of norepinephrine into either structure also improves memory for several types of learned responses. Thus, the capacity for norepinephrine to modulate memory within limbic structures may be contingent upon the functional connections between PGi and the LC. To examine this hypothesis, male Sprague-Dawley rats were implanted with cannula aimed above PGi (Experiments 1 and 2) or 1.5 mm dorsal or medial to PGi (Experiment 3). Immediately following inhibitory avoidance training (0.45 mA, 0. 5 s), phosphate-buffered saline, lidocaine (Experiment 1), or 12.5 or 25 nmol/0.5 microl of the GABA agonist muscimol (Experiment 2) was infused into PGi. On a retention test given 48 h later, the latency to reenter the footshock compartment was significantly shorter for subjects given either lidocaine or 12.5 or 25.0 nmol of muscimol compared to controls. In Experiment 3, infusion of lidocaine or muscimol into areas 1.5 mm dorsal or medial to PGi did not significantly alter retention, indicating that the memory impairment observed in Experiments 1 and 2 was site specific and not due to the spread of drug to cell groups surrounding PGi. These findings suggest that PGi may serve a vital function in relaying biologically relevant information to forebrain structures involved in memory via its excitatory influence on the LC.     

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.     

Phlorizin,a Competitive Inhibitor of Glucose Transport,Facilitates Memory Storage in Mice

Posttraining intraperitoneal administration of phlorizin (3.0–300.0 μg/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 μg/kg) on retention were time dependent, and the administration of phlorizin (30.0 μg/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 μg/kg). These findings indicate that phlorizin influenced memory storage, but not memory retrieval. Finally, the simultaneous administration of phlorizin (3.0–300.0 μg/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.     

The central nucleus of the amygdala is essential for acquiring and expressing conditional fear after overtraining

The basolateral complex of the amygdala (BLA) is critical for the acquisition and expression of Pavlovian fear conditioning in rats. Nonetheless, rats with neurotoxic BLA lesions can acquire conditional fear after overtraining (75 trials). The capacity of rats with BLA lesions to acquire fear memory may be mediated by the central nucleus of the amygdala (CEA). To examine this issue, we examined the influence of neurotoxic CEA lesions or reversible inactivation of the CEA on the acquisition and expression of conditional freezing after overtraining in rats. Rats with pretraining CEA lesions (whether alone or in combination with BLA lesions) did not acquire conditional freezing to either the conditioning context or an auditory conditional stimulus after extensive overtraining. Similarly, post-training lesions of the CEA or BLA prevented the expression of overtrained fear. Lastly, muscimol infusions into the CEA prevented both the acquisition and the expression of overtrained fear, demonstrating that the effects of CEA lesions are not likely due to the destruction of en passant axons. These results suggest that the CEA is essential for conditional freezing after Pavlovian fear conditioning. Moreover, overtraining may engage a compensatory fear conditioning circuit involving the CEA in animals with damage to the BLA.     

Orphanin FQ/nociceptin interacts with the basolateral amygdala noradrenergic system in memory consolidation

Extensive evidence indicates that the basolateral complex of the amygdala (BLA) mediates hormonal and neurotransmitter effects on the consolidation of emotionally influenced memory and that such modulatory influences involve noradrenergic activation of the BLA. As the BLA also expresses a high density of receptors for orphanin FQ/nociceptin (OFQ/N), an opioid-like peptide with anxiolytic and amnestic properties, the present experiments investigated whether the BLA is involved in mediating OFQ/N effects on memory consolidation and whether such effects require noradrenergic activity. OFQ/N (0.01-100 pmol in 0.2 microL) administered bilaterally into the BLA of male Sprague-Dawley rats immediately after aversively motivated inhibitory avoidance training induced dose-dependent impairment on a 48-h retention trial. The beta(1)-adrenoceptor antagonist atenolol (2.0 nmol) administered concurrently into the BLA potentiated the dose-response effects of OFQ/N. In contrast, immediate post-training infusions of the peptidergic OFQ/N receptor antagonist [Nphe(1)]nociceptin(1-13)NH(2) (1-100 pmol in 0.2 microL) into the BLA enhanced 48-h retention of inhibitory avoidance training, an effect that was blocked by coadministration of atenolol. Delayed infusions of OFQ/N or [Nphe(1)]nociceptin(1-13)NH(2) into the BLA administered either 6 or 3 h after training, respectively, or immediate post-training infusions of OFQ/N into the adjacent central amygdala did not significantly alter retention performance. These findings indicate that endogenously released OFQ/N interacts with noradrenergic activity within the BLA in modulating memory consolidation.     

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.