Leu-enkephalin actions on avoidance conditioning are mediated by a peripheral opioid mechanism

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.

Roles of delta and mu opioid receptors in mediating the effects of enkephalins on avoidance conditioning

The effects on one-way active avoidance conditioning of pre-training, systemic administration of the selective mu-receptor agonist [D-Ala2,N-Me-Phe4, Gly-ol]enkephalin (DAGO), and the selective mu-receptor antagonist (D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP), were determined in Swiss-Webster mice. A low dose of DAGO (0.92 micrograms/kg) moderately enhanced avoidance acquisition, whereas a 100 micrograms/kg dose of CTOP more dramatically impaired acquisition. However, the avoidance-enhancing dose of DAGO significantly increased locomotor activity as measured in a separate group of mice in the avoidance chamber, and the avoidance-impairing dose of CTOP significantly decreased activity. Under these same training conditions, earlier studies (Schulteis et al. 1988; Schulteis and Martinez 1990) demonstrated that enkephalins impaired avoidance learning, and selective delta-receptor antagonists such as ICI 174,864 enhanced learning; in contrast to the present study, both of these effects were dissociated from performance effects such as alterations in locomotor activity. Taken together, the results suggested that the effects of enkephalins were mediated by the delta-, but not mu-, class of opioid receptor.

Behavior of rats during chronic activation and blockade of the neostriatal opiate system

The effects of daily microinjections (MI), over of three weeks, bilaterally into the rostral striatum, of morphine, promedol, native leu-enkephalin and its synthetic tetrapeptide analogs were studied in experiments on rats. Naloxone was used as an antagonist. An active avoidance conditioned reflex was developed preliminary in a shuttle box. A decrease in the accuracy of the realization and an increase in the latent period of the reflex were observed after the first MI of morphine and enkephalins. The effect of the most stable aminated ornithine-containing tetrapeptides proved to be the strongest. A search stereotypy and increased motoric activity were recorded in the rats during the development of the chronic effects of the activators of the opiate system. A clear correlation was not found between the motor and conditioned reflex shifts. The blockade of the opiate receptors with naloxone did not lead to substantial changes in behavior. The data obtained confirm the current hypothesis regarding the important role of the enkephalinergic system of the neostriatum in the regulation of complex forms of behavior and its close functional association with the dopaminergic system.

Enkephalins, brain and immunity: modulation of immune responses by methionine-enkephalin injected into the cerebral cavity

There is a large number of interactions at molecular and cellular levels between the nervous system and the immune system. It has been demonstrated that the opioid neuropentapeptide methionine-enkephalin (Met-Enk) is involved in humoral and cell-mediated immune reactions. Met-Enk injected peripherally produces a dual and dose-dependent immunomodulatory effect: high doses suppress, whereas low doses potentiate the immune reactivity. The present mini-review concerns the immunological activity of Met-Enk after its administration into the lateral ventricles of the rat brain, and describes the extraordinary capacity of centrally applied Met-Enk to regulate/modulate the immune function. This survey is composed of sections dealing with (a) the role of opioid peptides in the central nervous system (CNS); (b) the activity of opioid peptides in the immune system; (c) the application of Met-Enk into the cerebral cavity; (d) the influence of centrally administered Met-Enk on nonspecific local inflammatory reaction; (e) the effect of Met-Enk injected intracerebroventricularly (i.c.v.) on specific delayed hypersensitivity skin reaction, experimental allergic encephalomyelitis, anaphylactic shock, plaque-forming cell response, and hemagglutinin production; (f) the central antagonizing action of quaternary naltrexone, an opioid antagonist that does not cross the brain-blood barrier, on Met-Enk-induced immunomodulation; (g) the alteration of immune responsiveness by i.c.v. injection of enkephalinase-degrading enzymes; (h) the participation of the brain-blood/blood-brain barrier in the CNS-immune system interaction; and (i) the role of opioid receptors in immunological activity of Met-Enk. A hypothesis has been advanced for the reaction of Met-Enk and opioid receptor sitting on the cell membrane. This concept suggests that the constellation of chemical residues of enkephalin and receptor in the microenvironment determines the binding between the opioid partners. The plurality of conformational structures of enkephalins and receptors makes possible their involvement in a variety of processes which occur in different physiological systems, including the nervous system and the immune system, and intercommunications between the two systems.

Uptake and metabolism of [3H]-Leu-enkephalin following either its intraperitoneal or subcutaneous administration to mice

The uptake and metabolism of 30 micrograms/kg [3H]-Leu-enkephalin ([3H]-LE) following either intraperitoneal (IP) or subcutaneous (SC) administration to Swiss Webster mice was examined. Uptake of [3H] was rapid, with peak levels of radioactivity in plasma observed at 5 or 10 min following IP or SC peptide injection, respectively. The majority (80-99% +/- 0.8) of plasma radioactivity at all postinjection plasma collection time points was in the form of tyrosine-containing enkephalin metabolites, indicating a substantial and rapid in vivo hydrolysis rate for exogenously administered LE. Leu-enkephalin is metabolized in vivo faster than previously reported in vitro in mouse plasma. However, despite this extensive hydrolysis, levels of intact LE remaining in plasma following its systemic administration are within or above endogenous LE plasma levels.

Chronic opioids impair acquisition of both radial maze and Y-maze choice escape

Chronic morphine impaired acquisition of two dissimilar behavioral tasks. In the radial maze, the performance of saline-treated and morphine-treated groups diverged with the latter failing to improve despite extensive training. In contrast, rats treated with naltrexone became skilled in the procedure 2-4 times as rapidly as saline controls. Withdrawal of treatment significantly improved performance of morphine-treated rats, with no change for rats treated with saline or naltrexone. When a second group of rats was extensively trained prior to instituting chronic morphine treatment, performance scores were not affected, suggesting that morphine does not impair spatial working memory despite subjective evidence of other gross behavioral effects, such as ataxia. In the Y-maze choice escape task, acquisition of a response strategy was significantly impaired in rats that had been previously treated with morphine for 17-21 days, despite clear indications that morphine-treated rats were sensitive to the aversive stimulus.

Only tyrosine-containing metabolites of [Leu]enkephalin impair active avoidance conditioning in mice

The effects of the enkephalin metabolites, Tyr, des-Tyr-[Leu]enkephalin (GGFL), and Tyr-Gly-Gly (YGG), on acquisition of an active avoidance task following their IP administration to mice were determined. Neither free Tyr (3.9-390.0 micrograms/kg) nor GGFL (7.1-710.0 micrograms/kg) altered acquisition of the avoidance response. In contrast, 53, but not 16 micrograms/kg, of YGG significantly impaired response acquisition. A 390.0, but not 39.0 micrograms/kg, dose of Tyr decreased locomotor activity levels measured in an open field. Together with previous findings that the enkephalin metabolites Tyr-Gly and Tyr-Gly-Gly-Phe also impair avoidance acquisition, these data indicate that the dipeptide Tyr-Gly is the minimum sequence needed to intefere with acquisition of an active avoidance response. Because the various enkephalin metabolites do not bind to opioid receptors, it is likely that their effects on avoidance acquisition represent a separate class of pharmacological agents whose effects are mediated by a nonopioid receptor mechanism. These results are important to the interpretation of behavioral studies involving peripheral administration of the opioid peptide, [Leu]enkephalin (LE).

Administration of leu-enkephalin impairs the acquisition of preference for ethanol

The effects of subcutaneous administration of leu-enkephalin (LEU-E) (10, 100 and 300 micrograms/kg) and LEU-E (100 micrograms/kg) plus naloxone (2.5 mg/kg) on ethanol preference and fluid intake have been investigated in rats. Under our procedural conditions, rats develop ethanol preference through forced ethanol drinking (conditioning session). Preconditioning administration of LEU-E induced a reduction of later ethanol preference. Post-conditioning administration of LEU-E (10 and 100 micrograms/kg) also attenuated the development of ethanol preference. NX antagonized the effects of LEU-E on ethanol preference and fluid consumption in the two experimental procedures used, indicating an involvement of opioid receptors in the LEU-E-induced impairment of the acquisition of ethanol preference.

DPen2-[DPen5]enkephalin, a delta opioid receptor-selective analog of [Leu]enkephalin, impairs avoidance learning in an automated shelf-jump task in rats

Both [Leu]enkephalin and DPen2-[DPen5]enkephalin, a delta opioid receptor selective analog of [Leu]enkephalin, impaired acquisition of an automated shelf-jump response in rats. A similar level of impairment was produced by equimolar doses of the two enkephalins. As is seen for [Leu]enkephalin when tested in a one-way active avoidance task, the dose-response function for the impairment produced by DPen2-[DPen5]enkephalin in the automated shelf-jump task is U-shaped. These results, together with our previous findings that DPen2-[DPen5]enkephalin and [Leu]enkephalin both impair acquisition of a one-way active avoidance response in mice, and that [Leu]enkephalin impairs acquisition of that same response in rats, support our suggestion that delta opioid receptors are implicated in the effects of [Leu]enkephalin on conditioning. In addition, these results indicate that the involvement of delta opioid receptors in acquisition impairment extends to two species of rodents and to two different avoidance conditioning tasks.

Peripheral versus intracerebroventricular administration of quaternary naltrexone and the enhancement of Pavlovian conditioning

When rats are placed in a context with mild electric shock (1 mA/0.75 s), the environmental cues alone can provoke an immobile crouching behavior termed freezing. Freezing response is a Pavlovian conditional response provoked by stimuli that come to be associated with shock. Previous research has shown that peripheral injection of opioid antagonists can enhance this response. Two experiments were conducted to determine if peripheral and/or central opioid mechanisms are involved in this enhancement of freezing by employing quaternary naltrexone (QNTX), an opioid antagonist which does not readily penetrate the 'blood-brain barrier'. QNTX (5 and 10 micrograms/rat) administered i.c.v. prior to shock significantly enhanced freezing 24 h later, whereas i.p. injected QNTX, at doses as high as 20 mg/kg, had no effect. These results suggest that the enhancement of conditional freezing produced by QNTX is mediated by central, not peripheral, opioid mechanisms.

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.

Endogenous opiates: 1986

This is the ninth installment of our annual review of research involving the endogenous opiate peptides. It is restricted to the non-analgesic and behavioral studies of the opiate peptides published in 1986. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal, renal, and hepatic processes; mental illness; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; activity; sex, pregnancy, and development; and some other behaviors.

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.

Modification of place preference conditioning in mice by systemically administered [Leu]enkephalin

[Leu]enkephalin (300 micrograms/kg, i.p.) induced either a positive or negative conditioned place preference response in mice depending on whether the animals were trained against or towards their initial preference. Induction of a positive preference (300 micrograms/kg) was partially blocked by simultaneous addition of methylnaloxonium (10 mg/kg, i.p.), an opioid antagonist that does not readily cross the blood-brain barrier; methylnaloxonium alone (3 or 10 mg/kg) had no effect on the place preference response. The results indicate that [Leu]enkephalin treatment reverses the initial preference of the animal regardless of training, and that some aspect of the [Leu]enkephalin effect on place preference conditioning is mediated by peripheral opioid receptors. These findings challenge the notion that place preference conditioning is a simple measure of opioid reward.