Effect of high doses of naloxone on shuttle avoidance acquisition in rats

Quaternary naloxone enhances acquisition of a discriminated Y-maze escape and a one-way active avoidance task in mice

Methylnaloxonium, a quaternary form of naloxone that does not readily cross the blood brain barrier, enhanced acquisition of a Y-maze shock escape and a one-way active avoidance response in mice in doses that did not affect shock-induced locomotor activity in an open field. Since methylnaloxonium is a opioid antagonist, it is likely that the observed enhancement of learning is due to the drug preventing endogenous opioid peptides from reaching their receptors. The results therefore suggest that manipulation of peripheral opioid systems alters the strength of a conditioned response.

Place avoidance learning and stress-induced analgesia in the attacked mouse: role of endogenous opioids

In this study, mechanisms of pain inhibition (tail-flick test) and memory (place avoidance paradigm) were investigated in attacked, DBA/2 and C57BL/6, mice. During training, exposure of test animals to 10 or 30 bites by an aggressive, isolated ICR mouse situated in the dark half of a bright/dark conditioning box induced a significantly higher social conflict analgesia in DBA than in C57 mice. Naltrexone (0.5 and 2.0 mg/kg) reduced this response in DBA mice that received 30, but not 10, bites and was ineffective in C57 mice. This points to different, opioid versus naltrexone-insensitive nonopioid, analgesic mechanisms. During place choice testing in the same box 24 h later, DBA mice that had received 30, but not 10, bites showed a significant, naltrexone-reversible, avoidance of the attack place. No place avoidance learning was observed in C57 mice. The data provided unequivocal evidence that place avoidance learning was a result of associative conditioning, in that neither pairing nor social conflict per se significantly changed the preference for the dark side seen in experimentally naive DBA mice. Antagonism of place avoidance conditioning was observed regardless of whether testing was carried out in the drugged or undrugged state, excluding possible state-dependent effects as an explanation for the naltrexone-induced impairment. Individual correlational analysis in saline-injected, attacked DBA mice revealed a negative relationship between the analgesic state immediately after training and the avoidance of attack place during testing. In summary, the results suggest strain-dependent analgesic and learning mechanisms and indicate that endogenous opioids released in attacked DBA mice support pain inhibition and modulate the memorization of attack place by their analgesic effects, as well as by mechanisms independent of pain inhibitory systems.

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.

Antagonism of endogenous opioids modulates memory processing

The studies reported here demonstrate that opioid antagonism enhances memory in two classes of animals viz. Aves and Mammalia. In mice, immediate posttraining administration of naloxone produces a time-dependent improvement in retention tested one week later. This effect is stereospecific. As naloxone was approximately 1000-fold more potent when administered intracerebroventricularly compared to subcutaneously, it appears that it produces its effect within the central nervous system. Pretest administration of naloxone, at a dose that failed to alter acquisition, also improved test performance, suggesting that naloxone also improved recall. Similar improvement in retention was demonstrated with the more potent opioid antagonist, nalmefene, at a 500-fold lower dose. The dose response to naloxone in both the mouse and the chick and to nalmefene in the mouse had the characteristics of an inverted U, with high doses either being ineffective or suppressing memory retention. In mice, naloxone demonstrated anti-amnestic properties against both anisomycin, a protein synthesis inhibitor, and scopolamine, an acetylcholine receptor blocker. Administration of beta-funaltrexamine (B-FNA) 72 h prior to training did not alter acquisition but did enhance retention. In studies where the mu-opioid receptor was blocked with B-FNA, naloxone was unable to enhance retention. B-FNA failed to impair the memory enhancing properties of arecoline, fluoxetine or clonidine. This demonstrates specificity of the B-FNA ability to prevent naloxone from enhancing memory and suggests that the opioid antagonist effects on memory are mediated by the mu-receptor.

Effect of morphine and naloxone on shock avoidance learning in headless cockroaches (Periplaneta americana L.)

Systemic administration of high doses of morphine (56 micrograms morphine/g body weight) or of naloxone (54 micrograms naloxone/g body weight) results in a significant improvement of shock avoidance behavior in headless cockroaches. In both cases the learning parameter stimulation time (time during which an animal receives shocks) is significantly decreased. The behavioral parameters, stimulation rate (activity) and mean stimulation duration (shock responsiveness), as the two factors of the stimulation time considered individually, do not significantly change with morphine. Only the responses of both parameters together give the significant decrease of stimulation time (improvement of shock avoidance behavior) as mentioned above. However, the administration of naloxone causes a significant shortening of the mean stimulation duration indicating that these animals avoid the shocks by learning to lift their legs more quickly out of electrified saline in order to terminate the shocks (escape learning). Combined administration of both drugs together also causes a significant improvement of the leg-lifting response even at lower doses (morphine: 0.56 microgram plus naloxone: 0.54 microgram/g; morphine: 0.0056 microgram/g plus naloxone: 0.0054 microgram/g). Although the various effective doses of these drugs administered in combination have similar effects on the stimulation time, they affect the behavioral parameters in different ways. These different actions on activity and shock responsiveness as well as the efficacy of doses smaller than those known from vertebrates are discussed as behavioral evidence of opiate receptors in the cockroach.

The effect of MSH/ACTH 4-10 on delayed response performance and post-test locomotor activity in rats

Delayed response performance was measured in male, Long-Evans rats 1 hr after IP administration of various doses of MSH/ACTH 4-10 or control in a Hunter delayed reaction apparatus. Additional treatments consisting of naloxone 500 micrograms/kg (IP) and naloxone 500 micrograms/kg in conjunction with MSH/ACTH 4-10 95 micrograms/kg were also administered. Directly after delayed response performance was assessed, gross locomotor activity was determined. MSH/ACTH 4-10, at a dose of 95 micrograms/kg, significantly enhanced retention of a visual stimulus, while MSH/ACTH 4-10, at doses of 195 and 285 micrograms/kg, significantly impaired delayed response performance. Naloxone treatment resulted in significantly impaired delayed response performance when compared to control. However, naloxone plus MSH/ACTH 4-10 treatment failed to produce a significant difference from control in the delayed response performance paradigm. In post-test locomotor activity determination, an apparent dose-response existed for MSH/ACTH 4-10 with the two highest doses (190 and 285 micrograms/kg) resulting in significantly increased locomotor activity. The observed delayed response performance data support theories implicating MSH/ACTH peptides in attentional processes involving visual stimuli. The fact that large doses of MSH/ACTH 4-10 disrupt delayed response performance while increasing post-test activity suggest that an optimum level of effect caused by the MSH/ACTH peptide exists in this paradigm.

Differential effect of naloxone on food and self-stimulation rewarded acquisition of a behavioral response pattern

The involvement of endogenous opioids in self-stimulation reward was investigated by repeated administration of the opioid antagonist naloxone to rats during acquisition of a behavioral response pattern that was rewarded with electrical (self-)stimulation of the ventral tegmental area. A control experiment was performed using food deprived rats in which a comparable response pattern was rewarded with food pellets. The response patterns consisted of gradually decreasing amounts of reward per response, which could be reset to maximal reward by another response. It was found that naloxone disrupted the acquisition of the stimulation rewarded response pattern, while it did not influence the food rewarded behavior. It is suggested that endorphin systems are actively involved in the acquisition of self-stimulation reward procedures, and that this involvement may be specific for self-stimulation reward.

Endogenous opiates: 1983

This paper is the sixth in an annual series of reviews of research involving the endogenous opiates, each installment being restricted to work published during the previous year. Although the early articles in the series attempted to be comprehensive and cover the complete range of research with the opiate peptides, in the last two years we have limited our coverage to non-analgesic and behavioral work due to the enormous number of articles published in the field. The specific areas discussed here include stress, tolerance and dependence, consummatory responses, other gastrointestinal functions, interactions with alcohol, mental illness, learning and memory, cardiovascular responses, respiratory effects, thermoregulation, neurological disorders, activity, and miscellaneous other topics. As in previous years, we have attempted to present a relatively complete review of the subjects covered only for the previous year and generally have not tried to evaluate their contributions relative to those of past years.