Memory-enhancing effects of posttraining naloxone: involvement of beta-noradrenergic influences in the amygdaloid complex

Rats (220-250 g) were bilaterally implanted with cannulae in the amygdala, trained on an inhibitory avoidance response and two weeks later, on a Y-maze discrimination response. Immediately following the training on each task, they were injected intraperitoneally (i.p.) or intra-amygdally. Retention was tested one week after training for each task. Retention of the Y-maze task was assessed by discrimination reversal training. Naloxone administered i.p. (3.0 mg/kg) significantly facilitated retention of both tasks in unoperated control rats as well as in rats implanted bilaterally with amygdala cannulae. The memory-enhancing effect of naloxone i.p. was blocked by propranolol (0.3 or 1.0 microgram) injected in the amygdala, but not when this beta-noradrenergic antagonist was injected (0.3 micrograms) into either the caudate or the cortex dorsal to the amygdala. Further, intra-amygdala injections of the beta 1-adrenoceptor blocker atenolol (0.3 or 1.0 microgram) and the beta 2-adrenoceptor blocker zinterol (0.3 or 1.0 microgram), in doses which were ineffective when administered alone, blocked naloxone-induced (3.0 mg/kg, i.p.) memory facilitation. In contrast, posttraining intra-amygdala administration (1.0 micrograms) of the alpha-antagonists prazosin (alpha 1) or yohimbine (alpha 2) did not attenuate the memory-enhancing effects of systemically administered naloxone. These findings support the view that naloxone-induced enhancement of memory is mediated by the activation of beta- but not alpha-noradrenergic receptors located within the amygdaloid complex.     

Memory enhancement with intra-amygdala post-training naloxone is blocked by concurrent administration of propranolol

Sprague-Dawley rats were first trained on an inhibitory avoidance task (IA) and then, two weeks later, on a Y-maze discrimination task (YMD). Bilateral intra-amygdala injections were given through implanted cannulae immediately post-training. Retention was evaluated one week following training on each task. Naloxone (0.1, 0.3 or 1.0 microgram) facilitated retention performance in both tasks. The most effective doses were 0.1 microgram for the IA task and 0.3 microgram for the YMD task. Since naloxone (0.1 microgram) did not affect retention when administered via cannulae implanted in either the caudate-putamen or cortex dorsal to the amygdala, the effects of intra-amygdala naloxone is not due to diffusion of the drug to these brain regions. Intra-amygdala injections of the beta 1,2-adrenoceptor blocker propranolol (0.3 microgram) blocked the memory enhancing effects of intra-amygdala naloxone (IA: 0.1 microgram; YMD: 0.3 microgram) administered concurrently immediately post-training. We interpret these findings as indicating that the enhancing effects of intra-amygdala naloxone are mediated by the activation of beta-noradrenergic receptors within the amygdala. Such effects are presumably due to blocking of inhibitory effects of opioid peptides on the release of norepinephrine.     

Involvement of amygdala pathways in the influence of post-training intra-amygdala norepinephrine and peripheral epinephrine on memory storage

These experiments examined the role of two major amygdala afferent-efferent pathways--the stria terminalis (ST) and the ventral amygdalofugal pathway (VAF)--in mediating the effects, on memory storage, of post-training intra-amygdala injections of norepinephrine (NE) and subcutaneous (s.c.) injections of epinephrine (E). Rats with either ST lesions or VAF transections and sham-operated rats were trained on a one-trial step-through inhibitory avoidance task and immediately after training received intra-amygdala injections of NE or a buffer solution. Other groups of VAF-transected animals received post-training s.c. injections of E or saline. ST lesions blocked the memory-enhancing effect of intra-amygdala injections of a low dose of NE (0.2 microgram) as well as the amnestic effect of a high dose of NE (5.0 microgram). In contrast, VAF transections did not block the memory-enhancing effect of NE (0.2 microgram). However, VAF transections attenuated the memory-enhancing effect of s.c. injections of E: the effective dose of E was shifted from 0.1 to 0.5 mg/kg. These findings, considered together with previous evidence that ST lesions block the memory-enhancing effect of peripheral E injections, suggest that the VAF is involved in mediating the central influence of peripheral E on amygdala functioning, while the ST is involved in mediating amygdala influences on memory storage elsewhere in the brain.     

Influence on memory of posttraining or pre-test injections of ACTH, vasopressin, epinephrine, and beta-endorphin, and their interaction with naloxone

The intraperitoneal (i.p.) injection of ACTH 1-24 (0.2 microgram/kg), lysine--vasopressin (10.0 micrograms/kg) or epinephrine HCl (5.0 micrograms/kg) shortly after training or prior to testing caused memory facilitation of a step-down inhibitory avoidance task in rats, acquired with low intensity training footshocks (0.3 mA, 60 Hz). Naloxone HCl (0.4 mg/kg) potentiated their posttraining effect, but antagonized their pre-test effect. Naloxone on its own caused retrograde memory facilitation but had no effect on the test session. Posttraining human beta-endorphin (1.0 microgram/kg) was amnestic, and its pre-test administration enhanced retention. Both effects were naloxone-reversible. Neither the pre-test facilitation caused by beta-endorphin nor those caused by any of the other drugs (which are possible releasers of endogenous beta-endorphin) were observed in animals in which the influence of endogenous opioids was prevented at the posttraining period by the administration of naloxone. These results are compatible with, and considerably strengthen, the previously advanced hypothesis that learning of this task, and possibly others, depends on a state induced by beta-endorphin after training, and that it would normally be dissociated because this peptide is normally not released during test sessions. In addition, the posttraining facilitation caused by ACTH, vasopressin, and epinephrine stands out as an effect separate from, and in fact normally hindered by, posttraining beta-endorphin release.     

Norepinephrine attenuation of amnesia produced by diethyldithiocarbamate

Rats given 680 mg/kh diethyldithiocarbamate, approximately one half hour before training in an inhibitory avoidance task, had impaired retention performance when tested one week after training. Intracerebroventricular or subcutaneous injections of norepinephrine administered shortly after training attenuated the disruptive effects of DDC on retention performance. The effect depended upon the footshock intensity used during training. NE(0.01 microgram) administered centrally attenuated the DDC induced retention deficit when animals were trained with a high (2 mA) but not a low footshock (0.5 mA). The effect of peripherally administered NE also varied with intensity of footshock. The lowest dose of subcutaneously administered NE (5 microgram/kg) was effective in attenuating DDC induced retention deficits only when animals were trained with higher footshock. Higher doses of NE (50 microgram/kg, 500 microgram/kg) were more effective when animals were trained with lower footshock.     

Effects of intra-amygdala injections of NMDA receptor antagonists on acquisition and retention of inhibitory avoidance

These experiments examined the effects of intra-amygdala injections of NMDA receptor antagonists on the acquisition and retention of inhibitory avoidance. In Expt. I, rats received bilateral intra-amygdala injections of the NMDA antagonists D,L-AP5 (1-10 micrograms), D-AP5 (0.03-1 micrograms), CPP (0.125 or 0.375 microgram), or MK-801 (0.2 or 0.5 microgram) prior to training in a continuous multiple-trial inhibitory avoidance (CMIA) task. Acquisition of the task was not significantly affected by any of the drug injections. In contrast, all three competitive antagonists, D,L-AP5, D-AP5 and CPP, produced dose-dependent impairment of 48 h retention performance. Although the MK-801 injections did not significantly impair retention performance, the retention scores of the 0.5 microgram MK-801 group were bimodally distributed, indicating retention impairment in a subgroup of the animals given that dose. Intra-amygdala injections of 3 or 10 micrograms D,L-AP5 did not affect footshock sensitivity (Expt. II) or locomotor activity (Expt. III) and their retention-impairing effects were not due to induction of state dependency (Expt. IV). The retention-impairing effects of intra-amygdala injections of NMDA antagonists were not due to diffusion of the drugs dorsally: injections of 1 microgram D-AP5 into the striatal area directly above the amygdala impaired acquisition but not retention performance (Expt. V). The retention-impairing effects of 1 microgram D-AP5 or 0.5 microgram MK-801 were attenuated by giving additional training to the animals shortly after receiving intra-amygdala injections (Expt. VI). The implications of these findings for hypotheses concerning amygdala function in learning and memory are discussed.     

Pre- or post-training injection of buspirone impaired retention in the inhibitory avoidance task: involvement of amygdala 5-HT1A receptors

The present study investigated the effect of buspirone on memory formation in an aversive learning task. Male Wistar rats were trained on the inhibitory avoidance task and tested for retention 1 day after training. They received peripheral or intra-amygdala administration of buspirone or other 5-HT1A drugs either before or after training. Results indicated that pretraining systemic injections of buspirone caused a dose-dependent retention deficit; 5. 0 mg/kg had a marked effect and 1.0 mg/kg had no effect. Post-training injections of the drug caused a time-dependent retention deficit, which was not due to a state-dependent effect on retrieval. When training in the inhibitory avoidance task was divided into a context-training phase and a shock-training phase, buspirone impaired retention only when administered in the shock-training phase, suggesting that the drug influenced memory processing of affective events. Further results indicated that post-training intra-amygdala infusion of buspirone or the 5-HT1A agonist 8-hydroxy-di-n-propylaminotetralin (8-OH-DPAT) caused a time-dependent and dose-dependent retention deficit. Post-training intra-amygdala infusion of the 5-HT1A antagonist WAY100635 (N-(2-(4-(2-methoxyphenyl)-1-piperazinyl)-N-(2-pyridyl) cyclohexane carboxamine maleate) attenuated the memory-impairing effects of buspirone. These findings suggest that buspirone may modulate memory storage processes in the inhibitory avoidance task through an action on amygdaloid 5-HT1A receptors.     

Amygdala and dorsal hippocampus lesions block the effects of GABAergic drugs on memory storage

These experiments examined the effects of posttraining systemic administration of the GABAergic agonist muscimol and the GABAergic antagonist bicuculline on retention in mice with bilateral lesions of the amygdala, dorsal hippocampus or caudate nucleus. Unoperated male CD1 mice and mice with either sham lesions or electrolytically induced lesions of these 3 brain regions were trained in a one-trial inhibitory avoidance task and, immediately after training, received i.p. injections of either muscimol, (1.0, 2.0 or 3.0 mg/kg), bicuculline, (0.25, 0.5 or 1.0 mg/kg), or control solutions. Retention was tested 24 h after training. Lesions of the 3 brain regions produced comparable impairment of retention. In the unoperated controls and sham controls muscimol and bicuculline produced dose-dependent impairment and enhancement, respectively, of retention. The drug effects on retention were blocked by lesions of the amygdala and hippocampus, but were not blocked by lesions of the caudate nucleus. These findings are consistent with other recent evidence suggesting that the amygdala and hippocampus are involved in mediating posttraining neuromodulatory influences on memory storage.     

Evidence that cholecystokinin-enhanced retention is mediated by changes in opioid activity in the amygdala

Mice, partially trained to avoid footshock in a T-maze, showed enhanced retention relative to vehicle-injected mice when treated peripherally with arecoline, D-amphetamine, cholecystokinin octapeptide (CCK-8), epinephrine or naloxone. Both intra-amygdaloid and intraventricular injections of beta-endorphin resulted in amnesia. D-amphetamine and arecoline blocked the amnestic effect of beta-endorphin administered into the amygdala but it required higher doses for CCK-8, epinephrine and naloxone to block the amnestic effect of beta-endorphin. The effects of CCK-8, epinephrine and naloxone showed a differential ability to block amnesia induced by beta-endorphin intraventricularly with epinephrine and naloxone preventing amnesia but CCK-8 not improving retention. This data suggests that the memory enhancement produced by peripherally administered CCK-8 involves the amygdala and that both CCK-8 and epinephrine interact with opioid amnestic mechanisms within the amygdala to alter memory processing.     

Propranolol corrects the abnormal catecholamine response to light during migraine

The catecholamine (CA) response to light before and after propranolol therapy was studied in 25 migrainous subjects. Before therapy an abnormal CA response to light consisting of a rise in epinephrine excretion and a depression in norepinephrine (NE) excretion was noticed in migrainous patients. After propranolol administration (60 mg daily for 10 days) the post-photic augmentation in epinephrine excretion and the post-photic depression in NE excretion no longer occurred. As epinephrine discharge may be pathogenetic for migraine attack, the favorable effect of propranolol in migraine therapy could be due to its ability to prevent the epinephrine release induced by light or other stimuli.     

Lesions of the stria terminalis attenuate the enhancing effect of post-training epinephrine on retention of an inhibitory avoidance response

This study investigated the effect of the stria terminalis (ST) lesions on the retention facilitation produced by post-training epinephrine. Rats with sham or bilateral ST lesions were trained on a one-trial step-through inhibitory avoidance task and, immediately after training, received saline or one of 3 doses (0.01, 0.1 or 1.0 mg/kg, s.c.) of epinephrine. In the rats with sham ST lesions, all 3 doses of epinephrine significantly enhanced retention performance. Lesions of the ST did not affect retention significantly. However, the lesions attenuated the facilitating effect of epinephrine on retention. These findings suggest that the integrity of the ST, which connects the amygdala and other brain structures, is essential for the effect of epinephrine on memory processes. The possible involvement of the brain structures processing visceral information in the memory modulatory effect of epinephrine is discussed.     

Vasoactive agonists prevent erythrocyte extravasation in thrombocytopenic hamsters

The mediating action of selected vasoactive amines and their respective antagonists on vascular fragility, visible as cutaneous petechiae, was assayed with thrombocytopenic (TCP) hamsters. Serotonin (5-HT), norepinephrine (NE), epinephrine, dopamine and isoproterenol administered IP reduced petechiae significantly within 10 min; phenylephrine had no effect. Of the natural amines, 5-HT and NE were most effective in reducing petechial sensitivity to values obtained with untreated, normal animals; hence these two amines only were tested pharmacologically. Pretreatment of TCP animals with Ketanserin or propranolol, administered IP or IV, abolished any petechial inhibitory action of 5-HT and NE respectively; pretreatment with phenoxybenzamine reduced significantly the NE inhibition of petechiae, but to a lesser degree than propranolol. In contrast, atenolol, prazosin and yohimbine had no significant effect. Ketanserin abolished the action of NE, but adrenoceptor blockers had no effect on 5-HT-treated TCP hamsters. The results suggest that 5-HT and NE inhibition of petechiae may be receptor-mediated and that there may be receptor interaction. This was supported by the observation that non-additive subthreshold doses of 5-HT and NE, which individually did not prevent petechial formation in TCP hamsters, when combined totally inhibited petechiae. The theorized importance of endogenous 5-HT and NE to maintain postcapillary venule junctional integrity (site of petechial hemorrhaging) was also demonstrated by treating normal hamsters with drugs known to block or antagonize either 5-HT or NE uptake. In every instance petechial sensitivity rapidly occurred, and the loss of microvascular integrity in Ketanserin-treated hamsters mimicked quantitatively the petechial sensitivity observed with TCP animals.     

Endogenous benzodiazepine modulation of memory processes

The immediate posttraining administration of the GABA antagonist, bicuculline, or of the Cl-channel blockers, picrotoxin or Ro 5-4864, enhances memory. These drugs are effective when injected into the amygdaloid nucleus. Intraamygdala muscimol has an opposite effect. All this suggests that memory is modulated at the posttraining period by GABA-A receptors. The pre-, but not posttraining systemic administration of benzodiazepines hinders, and that of inverse agonists, or of the benzodiazepine antagonist, flumazenil enhances retention of diverse tasks. Flumazenil, at doses lower than those that cause an enhancement, antagonizes the effect of benzodiazepine agonists and inverse agonists. This suggests that memory is modulated during acquisition by endogenous benzodiazepine receptor ligands: possibly the diazepam that was recently discovered in brain. Pretraining intraamygdala muscimol administration depresses memory, at doses several times higher than those that are effective posttraining. Pretraining Ro 5-4864 has no effect. This suggests that the release of endogenous benzodiazepines during training may modulate a GABA-A receptor complex, possibly in the amygdala, making it more sensitive to muscimol or Ro 5-4864 in the immediate posttraining period.     

The memory-modulatory effects of glucocorticoids depend on an intact stria terminalis

This study examined the effects of stria terminalis (ST) lesions on glucocorticoid-induced modulation of memory formation for inhibitory avoidance training and spatial learning in a water maze. Systemic (s.c.) posttraining injections of the glucocorticoid receptor agonist dexamethasone (0.3 or 1.0 mg/kg) enhanced memory for inhibitory avoidance training in rats with sham ST lesions. Removal of the adrenal glands (adrenalectomy; ADX) significantly impaired spatial memory in a water maze, and immediate posttraining injections of dexamethasone (0.3 mg/kg) attenuated the memory impairment. Bilateral lesions of the ST did not significantly affect retention of these two tasks. However, ST lesions did block the effects of short-term ADX and dexamethasone administration on memory for both tasks. These results are similar to those of previous experiments examining the effects of lesions of the basolateral nucleus of the amygdala on the glucocorticoid-induced modulation of memory for both tasks. These findings suggest that the integrity of the ST, which connects the amygdala with other brain structures, is essential for the modulating effects of glucocorticoids on memory storage.     

Tetrahydroaminoacridine improves passive avoidance retention defects induced by aging and medial septal lesion but not by fimbria-fornix lesion.

The present study examines whether tetrahydroaminoacridine (THA) can improve the deterioration in passive avoidance (PA) retention performance induced by medial septal (MS) and fimbria-fornix (FF) lesions in young rats or by aging. Retention of young MS-lesioned rats was improved by pretraining injection of THA at 3 mg/kg, but not by THA at 1 mg/kg or by either of the posttraining doses of THA (1 and 3 mg/kg). Pretraining injections of THA at 1 or 3 mg/kg had no effect on the PA retention performance of FF-lesioned rats. Age-induced PA failure was alleviated by pretraining administration of THA at 1 and 3 mg/kg. Posttraining injections of THA (1 or 3 mg/kg) had no effect on PA retention performance of aged rats. These results demonstrate that 1) THA may improve hippocampal cholinergic denervation-induced functional deficits and 2) some of the age-related PA deficits may be due to a cholinergic deficit and can be reversed with THA.     

Discriminative stimulus effects of morphine: central versus peripheral training

While it is well known that rats can discriminate a peripheral injection of morphine from a saline injection, to our knowledge no one has trained rats to discriminate a direct brain-site injection of morphine from saline. In the present series of studies, one group of rats was trained to discriminate morphine (0.3 microgram) from saline injected into the perifornical area of the hypothalamus (PFA), a process that took rats about 37 sessions to learn. A dose response generalization curve for PFA-injected morphine (0.01, 0.03, 0.1, and 0.17 microgram) was generated in which the two highest doses of morphine generalized to the morphine-appropriate training stimulus. Intraperitoneal (i.p.) injection of 3 mg/kg, but not 1 mg/kg morphine, resulted in morphine-appropriate responding in the PFA morphine-trained rats. A second group of rats was trained to discriminate i.p. injections of 3 mg/kg morphine from injections of saline. A dose-response generalization test for i.p.-injected morphine (0.3, 0.56, 1.0, and 1.7 mg/kg) was conducted in which the 0.17 mg/kg dose of morphine generalized to the morphine-appropriate training stimulus. Generalization tests using PFA-injected morphine doses (0.17, 0.56, 1.0, and 3.0 microgram) failed to result in morphine-appropriate responding in the i.p. morphine-trained rats. Naloxone administered into the PFA (50 microgram) or the periphery (3 mg/kg, i.p.) blocked morphine discrimination in the PFA-trained rats. However, when naloxone was injected into the PFA (50 microgram) together with i.p. morphine (3 mg/kg) in animals trained using i.p. injections, the antagonist failed to block morphine-appropriate responding. Thus, while peripheral injection of morphine generalized to the discriminative stimulus effects of morphine produced under PFA-injection training, the opposite effects were not noted.     

Effect of ACTH,epinephrine, β-endorphin,naloxone, and of the combination of naloxone or β-endorphin with ACTH or epinephrine on memory consolidation

The effect on retention of the post-training intraperitoneal administration of ACTH_1–24 (0.2 or 2.0 μg/kg), epinephrine HC1 (5.0 or 50.0 μg/kg), human β-endorphin (0.1 or 1.0 μg/kg), naloxone (0.4 mg/kg), and of the combination of naloxine or β-endorphin with ACTH or epinephrine was studied in two different but closely related step-down inhibitory avoidance tasks in rats: task 1 (5 cm high 25 × 25 cm platform; 0.5 mA continuous footshock) and task 2 (7 × 25 cm platform, 0.3 mA discontinuous footshock). In task 1, saline control animals showed good retention in a test session carried out 24 hr later; β-endorphin, ACTH and epinephrine caused amnesia; β-endorphin potentiated the amnesic effect of ACTH and epinephrine; and naloxone caused memory facilitation and reversed the amnesic effect of ACTH and epinephrine. In task 2, control animals showed poor retention; β-endorphin caused amnesia at the dose of 0.1 but not 1.0 μg/kg; the other three drugs caused memory facilitation; naloxone potentiated the facilitatory effect of ACTH and epinephrine; and β-endorphin reversed it and transformed it into a deep amnesia.These findings suggest that an opioid-mediated amnesic mechanism modulates the effect of ACTH and epinephrine on memory consolidation, either by dampening that effect when training parameters tend to make it facilitatory, or by enhancing it when training conditions tend to make it amnesic. On the basis of these and previous data it seems likely that the amnesic effect of ACTH and epinephrine could be mediated by endogenous β-endorphin release.     

Bicuculline administered into the amygdala after training blocks benzodiazepine-induced amnesia

Male Sprague-Dawley rats were injected (i.p.) with either midazolam (MDZ 2.0 mg/kg) or vehiele (1.0 ml/kg) 10 min before they were trained on a multiple-trial inhibitory avoidance task. Immediately following the training, bicuculline methiodide (BMI: 2.0, 5.6, 56.0 or 197.0 pmol/0.5 μl) or vehicle (0.5 μl) was infused bilaterally into the amygdala. On a 48 h retention test the performance of the MDZ-treated animals was significantly poorer than that of controls. The retention of MDZ-treated animals given intra-amygdala injections of the lowest dose of BMI (2.0 pmol) was comparable to that of controls, whereas higher doses of BMI impaired retention. The present results are consistent with other findings indicating that the amygdala mediates the amnestic effects of benzodiazepines on aversive learning. Furthermore, these data suggest that benzodiazepines impair memory by disrupting post-training processes underlying memory consilidation.     

Amygdala lesions block the amnestic effects of diazepam

This experiment examined the effects of pre-training systemic injections of the benzodiazepine (BZ) diazepam (DZP) on learning and retention of an inhibitory avoidance response in rats with bilateral lesions of the amygdaloid complex (AC) induced by intra-amygdala injections of the excitotoxin N-methyl-D-aspartic acid (NMDA). Unoperated, sham-operated and AC-lesioned rats received i.p. injections of DZP (1.0 or 2.0 mg/kg) or vehicle 30 min prior to training in a continuous multiple-trial inhibitory avoidance task. Retention was tested 48 h later. The acquisition and retention of the AC-lesioned rats were impaired, relative to that of the unoperated and sham controls. In the unoperated and sham controls, DZP impaired retention but did not affect acquisition. In contrast, in animals with AC lesions, DZP did not affect either acquisition or retention. These findings suggest that the amnestic effects of DZP are mediated, at least in part, through influences involving the AC.     

Involvement of the amygdaloid complex in neuromodulatory influences on memory storage

Neuromodulatory systems activated by training experiences appear to play a role in influencing memory storage processes. The research summarized in this paper examined the effects, on memory, of posttraining administration of treatments affecting adrenergic, opioid peptidergic and GABAergic systems. When administered after training, drugs affecting these systems all produce dose- and time-dependent effects on memory storage. The drug effects on memory are blocked by lesions of the amygdaloid complex as well as lesions of the stria terminalis, a major amygdala pathway. The effects of drugs affecting these neuromodulatory systems are also blocked by injections of beta-adrenergic antagonists administered to the amygdaloid complex. Thus, the findings suggest that the neuromodulatory systems affect memory storage through influences involving the activation of beta-adrenergic receptors within the amygdala. These findings are consistent with the view that the amygdala is involved in regulating the storage of memory in other brain regions.