Dose-dependent modulation of memory by the enkephalin analog FK 33-824 in C57BL/6 mice

Peripheral modulation of learning and memory: enkephalins as a model system

Extensive research on the effects of enkephalins on conditioning is reviewed and used as the basis for a model of peripheral modulation of learning and memory. An overall theme emphasized throughout our discussion is that these peptides can influence the strength with which a memory is acquired and stored by acting outside the blood-brain barrier. This assertion is supported by research on the behavioral effects of systemically administered enkephalins and opioid antagonists, the rapid hydrolysis of circulating enkephalins in vivo, and the limited ability of these peptides to penetrate the blood-brain barrier. A consideration of the extensive distribution of enkephalins throughout peripheral autonomic systems leads to the proposal that enkephalins may act to modulate learning and memory by altering peripheral autonomic function; autonomic afferents may then communicate with the memory trace in the CNS through a central modulatory pathway outlined herein. Evidence that some stressful experiences may lead to increases in circulating enkephalins also is discussed. The sites of action of these circulating enkephalins may involve peripheral autonomic sites, or additionally may involve the circumventricular organs. As a further regulatory mechanism, circulating enkephalin levels may be controlled by experience-dependent alterations of the activity of enzyme systems that participate in their breakdown. Finally, it is emphasized that the mechanisms of enkephalin action postulated herein may be applicable to the actions of other peripheral hormones, peptides, and neurotransmitters that participate in the modulation of learning and memory storage processes.

Psychopharmacology of memory modulation: evidence for multiple interaction among neurotransmitters and hormones

Experimental results are reviewed which indicate that memory storage can be altered by a number of post-training treatments that affect different hormones and neurotransmitters. Moreover, evidence was reported which suggests that the action of treatments effective on memory processes involves interactions among different systems, consistently with the complexity of brain systems. In the last decade, inbred strains have been exploited to investigate the role of neurotransmitter and hormone systems in learning and memory, leading to behavioural and neurochemical correlations based on strain differences that provide unique information on the biological systems underlying behaviour. Research carried out on the inbred strains of mice C57BL/6 (C57) and DBA/2 (DBA), demonstrates that the genetic makeup plays an important role in modulating response to drug administration. Thus, recent results have shown that in C57 mice, similarly to what occurs in outbred strains of mice or in rats, GABAergic agonists impair memory and antagonists improve it, whilst the opposite is evident in the DBA strain. By contrast, post-training administration of selective D1 or D2 agonists impairs and post-training administration of selective antagonists improves retention in DBA mice, whilst these agents have opposite effects in the C57 strain. Dose- and strain-dependent effects are evident also following post-training corticosterone as well as opioid agonists and antagonists administration. On the other side, these two strains react similarly to oxotremorine (improvement) and to atropine (impairment) administration, DBA mice being more responsive to the effects of both drugs than C57 mice. Data on the interactions between agents acting upon different neurotransmitter and/or hormonal systems in these strains indicate strain-dependent synergistic or antagonistic interactions among some of these systems, pointing to inbred strains of mice as an important methodological tool in the study of neural and hormonal factors involved in emotion and in its effects on cognition. In particular, these studies have been carried out on inbred strains of mice from which recombinant inbred (RI) strains are available that have recently been proposed as a choice experimental method in psychopharmacogenetics.

Behavioral effects of morphine in mice: role of experimental housing

Behavioral effects of morphine were assessed in isolated-timid Swiss mice, and were compared with those observed following morphine administration in nonaggressive-grouped subjects. For this purpose saline- and morphine- (0.5, 1.0, 2.5, and 5.0 mg/kg, IP) injected isolated-timid and nonaggressive-grouped mice interacting with a social partner were observed during a 4-min test. Three main points emerged from the results: a) in basal conditions, compared with social mice, in timid mice the offensive ambivalent behaviors were significantly less pronounced, while the defensive ambivalent behaviors (and all flight behaviors) were significantly more evident; b) 2.5 mg/kg of morphine increased offensive and decreased defensive ambivalent behaviors in timid mice; c) in social mice morphine (2.5 mg/kg) treatment increased defensive ambivalent behaviors and time spent in crouch. The results, which show that the behavioral effects of morphine depend on the state of the individual, are interpreted on the basis of the antiemotional properties of this opiate.

Effects of naloxone and naltrexone on memory consolidation in CD1 mice: involvement of GABAergic mechanisms

The involvement of GABAergic mechanisms in the effects exerted by the opioid antagonists naloxone and naltrexone on memory consolidation was investigated in CD1 mice tested in a one-trial inhibitory avoidance task. In a first group of experiments posttraining administration of naloxone (2.0 and 4.0 but not 1.0 mg/kg) and naltrexone (0.5 and 1.0 but not 0.25 mg/kg), as well as those of the GABA-antagonists picrotoxin (0.5 and 1.0 but not 0.25 mg/kg) and bicuculline (0.25 and 0.5 but not 0.1 mg/kg) enhanced, whereas those of the GABA-agonist muscimol (1.0 and 2.0 but not 0.5 mg/kg) impaired retention on a 24-hr test. In a second group of experiments, picrotoxin, or bicuculline, administration enhanced, while muscimol treatment attenuated the effects of naloxone and naltrexone on retention. The results suggest that naloxone and naltrexone may influence memory consolidation in CD1 mice by interacting with the GABAergic system.

Subjects' skill and biphasic effects of met-enkephalin upon the acquisition and retention of a perceptual motor task

The effects of met-enkephalin on the acquisition and retention over 10 days of a perceptual motor skill were investigated. Mice were randomly assigned to either one of two experimental groups (1-mg or 3-mg injections per 1-kg of body weight of met-enkephalin) or one of two control groups (dH2O injections). During the acquisition phase of the study, they were separated into "slow" and "fast" learners on the basis of their skill in negotiating a water maze. The results indicated an inhibitory effect of met-enkephalin in the 3-mg condition in the retention phase of the experiment. Subjects' skill was not implicated as a critical factor in retention of this simple task.

Enkephalins and learning and memory: a review of evidence for a site of action outside the blood-brain barrier

A series of studies indicate that enkephalins exert dramatic influences on learning and memory in rats and mice, when studied with conditioning tasks that are both negatively and positively motivated. Pharmacological analysis of these enkephalin actions on conditioning suggests that the [leu]enkephalin acts through a delta opioid receptor which is located outside the blood-brain barrier. Control studies indicate that enkephalins do not simply affect the performance of a conditioned response through actions on shock sensitivity or locomotor activity. Characterization of the peripheral enkephalin mechanism that affects behavior suggests an action through an enzymatic system that controls the concentrations of enkephalin present at its receptors in the periphery. This enzymatic mechanism is sensitive to experience, since its activity changes following conditioning, which suggests that it may be a regulatory mechanism for behavior.

Effects of ethanol on passive avoidance behavior in the mouse: involvement of GABAergic mechanisms

A passive avoidance methodology was used to test the effect of ethanol, and its interference with GABAergic mechanisms, on memory in male CD1 mice. Retention performance was reduced in a dose-related manner, by ethanol and by muscimol, a GABA agonist, while it was increased by the GABA antagonists picrotoxin and bicuculline. These effects were evident when treatments were carried out immediately, but not 120 min, after training, suggesting that they were due to a specific action of the drugs on the time-dependent memory consolidation process. The ethanol-induced reduction of retention performance was enhanced by muscimol and decreased by picrotoxin and bicuculline administrations. Taken together the results confirm the involvement of a GABAergic mechanism in memory consolidation and demonstrate that it underlies the negative effect of ethanol on passive avoidance behavior in the mouse.

Regulation of feeding behaviour in monosodium glutamate (MSG) treated mice

Regulation of food intake was studied in mice treated neonatally with MSG. A diurnal pattern of food intake with a significantly greater nocturnal intake was observed. In response to restricted food availability (4 hours/day). MSG mice increased intake progressively although less efficiently than control mice. The opioid kappa receptor agonist ketocyclazocine enhanced food intake. It is concluded that in MSG treated mice opioid kappa receptor mechanisms involved with regulation of food intake are basically intact.

Passive avoidance behavior in mice: interaction between age and genotype

BALB/c (BALB) and C57BL/6 (C57) mice were tested in a passive avoidance apparatus at different ages (6, 12 and 24 months). Results showed a clear-cut retention decrease only in 24 month old BALB mice in comparison with younger animals. No significant age effect was evident in C57 mice.

Naloxone influences retention behaviour depending on the degree of novelty inherent to the training situation

The effects of immediate posttraining subcutaneous administration of naloxone (0.25, 1 or 5 mg/kg) on retention behaviour of rats trained in an inhibitory avoidance task, subjected or not to familiarization with the training situation prior to the training trial (pretraining) have been investigated. Naloxone did not influence performance of pretrained rats not subjected to footshock at training. The drug did not significantly modify retention latencies of pretrained rats subjected to a weak footshock. However, administration of naloxone facilitated retention behaviour of non-pretrained rats subjected to a weak footshock. Likewise, naloxone significantly increases retention latencies of pretrained rats subjected to a high footshock at the training trial. These data indicate that naloxone influences retention behaviour depending on the degree of novelty linked to the training situation: a facilitatory effect of the drug is observed when the training trial becomes associated with a clear novel situation for the animals (high footshock in pretrained rats or a weak footshock in non-pretrained animals).

Prenatal exposure to morphine in mice: enhanced responsiveness to morphine and stress

Some behavioral effects of prenatal morphine administration were studied in CD1 mice. Two sets of experiments were carried out. In a first set, which was performed during development: (a) measures of postnatal reflexes revealed only a light deficit in tests involving motor control, (b) activity measures showed a significant reduction of spontaneous activity which was evident only in the course of the first postnatal days. In a second set of experiments, in which adult mice were tested for activity, analgesia and passive avoidance learning: (a) no difference was observed, in baseline conditions, between the performances of the mice prenatally exposed to saline and those preexposed to morphine, (b) as compared with controls, enhanced responsiveness to morphine administration (for the activity and passive avoidance measures), and to morphine and stress (for the analgesic measures) were found.

Naltrexone-reversible effects of flunitrazepam on locomotor activity and passive avoidance behaviour in mice

Some behavioural effects of flunitrazepam were investigated in two sets of experiments in C57BL/6 mice. In the first set, flunitrazepam administration (0.025 or 0.05 mg/kg) enhanced the locomotor activity of mice. In the second set, memory impairments were observed following posttraining (immediately or 30 min, but not 60 min) treatment with the drug (0.05 mg/kg). All effects were antagonized by a per se ineffective dose of naltrexone (0.5 mg/kg for the activity, 0.1 mg/kg for the memory experiments), suggesting the involvement of opioid receptors. The results are discussed on the basis of recent evidence suggesting a link between benzodiazepine and opiate mechanisms of action.

Morphine and memory in DBA/2 mice: effects of stress and of prior experience

In a first set of experiments, immediately post-training morphine (1.0 or 2.5 but not 0.5 mg/kg) treatment, or immobilization stress (30 or 60 but not 15 min) impaired memory processes of non-pretrained DBA/2 mice tested in a passive avoidance box. The effects were naloxone-reversible and time-dependent (they were absent in mice injected with morphine, or immobilized, starting 120 min after training). No effect was evident in no-footshock groups injected with morphine (2.5 mg/kg) or immobilized (60 min), thus showing lack of proactive influence of the treatments on performance. In a second set of experiments, in which pretrained animals were used, both morphine and immobilization stress were less effective in disrupting memory processes of mice. In both sets of experiments a per se ineffective stress enhanced the effects of morphine. A number of possible hypotheses concerning the results obtained are examined. In particular the possible role of emotional factors in the effects of morphine on memory is discussed.

Endogenous opiates: 1982

This article is the fifth installment in an annual series of reviews of successive year's research dealing with the endogenous opiate peptides. Due to the continuing massive increase in the number of studies in this field, it has become impossible to continue comprehensive reviews of all aspects of this work. As a result we have decided that beginning this year the coverage will be abbreviated to emphasize non-analgesic and behavioral work. The specific areas discussed include stress, tolerance and dependence, consummatory responses, alcohol consumption, schizophrenia and emotional disorders, learning and memory, cardiovascular responses, respiratory effects, thermoregulatory effects, neurological deficits and other disorders, activity, and other, miscellaneous behaviors. As in previous years, we have attempted a relatively comprehensive review of the subjects covered only for the previous year and have not made an attempt to evaluate their contributions relative to those of past years.

Strain-dependent modulation of memory by stress in mice

The effects of immobilization stress were investigated in Swiss Webster (Swiss), DBA/2 (DBA), and C57BL/6 (C57) mice, tested in a passive avoidance situation. Retention performance was impaired in Swiss and DBA mice, and improved in C57 mice, immobilized immediately, but not 2 hr, after training. These effects lasted for less than 7 days in DBA and Swiss mice, while they were still present, in the C57 strain, 14 days after training. The naloxone antagonism of the effects observed was also demonstrated. The results are discussed in terms of the possible role of endogenous opioids, stress hormones, and genetic makeup in the stress-induced modulation of memory processes in the mouse.