Differential effects on active avoidance performance and locomotor activity of two major enkephalin metabolites, tyr-gly-gly and des-tyr-[leu]enkephalin

Two Metabolites of [Leu]enkephalin, Tyr-Gly and Tyr-Gly-Gly-Phe, Impair Acquisition of an Active Avoidance Response in Mice

Two tyrosine-containing enkephalin metabolites, Tyr-Gly and Tyr-Gly-Gly-Phe, were found to impair acquisition of a one-way active avoidance response when administered intraperitoneally to mice. Prior studies demonstrated that the enkephalin metabolite Tyr-Gly-Gly also impairs avoidance acquisition, but that the tetrapeptide Gly-Gly-Phe-Leu is without similar effects. Since [leu]enkephalin also impairs acquisition of the avoidance response, these data are consistent with the suggestion that enkephalin metabolites containing an N-terminal tyrosine may be the active principals for the behavioral effects produced by [leu]enkephalin. Alternatively, these metabolites may represent a separate class of pharmacological agents that shares some behavioral effects with the enkephalins, but that produces effects on learning through a different 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.

The effects of methionine-enkephalin and its related metabolites upon the duration of the dorsal immobility response in rats

The effects of SC injections of methionine-enkephalin (Met1-5-Enk) and its N-terminal and C-terminal fragments upon the duration of the dorsal immobility response (DIR) over a 60-min time course were investigated. Experiment 1 analyzed the effects of various dosages (0.00-100.0 micrograms/kg) on DIR resulting in a potentiation of the duration in a dose-time course function. The effects of various fragments of Met1-5-Enk (10.0 micrograms/kg) from the N-terminal in Experiment 2 and from the C-terminal in Experiment 3 on the DIR resulted in the potentiation of the duration with the Met2-5-Enk and Met1-3-Enk fragments. All other fragments were not significant. The results were discussed in reference to the processing and metabolism of Met1-5-Enk.

[Leu]enkephalin and its metabolite, Tyr-Gly-Gly, impair active avoidance retention

The current study examined the effects of [leu]enkephalin and its metabolite, Tyr-Gly-Gly, given immediately posttraining on active avoidance performance measured 24 h later. Initial experiments revealed that, in comparison to zero or one training trials, providing mice with two training trials significantly increased active avoidance performance measured 24 h later; this enabled us to examine the effects on retention of peptides administered immediately after the two training trials. It was found that Tyr-Gly-Gly (16 and 53 micrograms/kg) and [leu]enkephalin (30 and 100 micrograms/kg) administered in this fashion both significantly impaired retention; the dose-response functions for both peptides were U-shaped. Since the effects of enkephalins are most likely mediated by opioid delta-receptors, and Tyr-Gly-Gly has little or no activity at opioid receptors, the effects of the parent peptide(s) and metabolite are presumably pharmacologically distinct.

Enkephalin hydrolysis by mouse plasma in vitro

Hydrolysis of [Leu]- and [Met]enkephalin was determined in samples of pooled whole mouse plasma in vitro by using HPLC-ECD to measure accumulation of Tyr-containing metabolites. More Tyr-Gly-Gly accumulated from [Met]enkephalin than from [Leu]enkephalin hydrolysis, and [Met]enkephalin's half-life in mouse plasma was approximately half that of [Leu]enkephalin. Comparisons of metabolite formation in the presence versus the absence of inhibitors with high selectivity for various peptidases demonstrated that a bestatin-sensitive aminopeptidase, presumably aminopeptidase M, as well as enkephalinase and angiotensin converting enzyme, participate in the hydrolysis of enkephalin in mouse plasma.

Further characterization of the in vitro hydrolysis of [Leu]- and [Met]enkephalin in rat plasma: HPLC-ECD measurement of substrate and metabolite concentrations

Hydrolysis of [Leu]- and [Met]enkephalin was determined in whole rat plasma in vitro by using HPLC-ECD to measure Tyr, Tyr-Gly and Tyr-Gly-Gly formation. Although [Leu]- and [Met]enkephalin did not differ in Tyr or Tyr-Gly accumulation, the amount of Tyr-Gly-Gly resulting from [Met]enkephalin hydrolysis was greater than that resulting from [Leu]enkephalin hydrolysis, and [Met]enkephalin's half-life in plasma was slightly shorter than that of [Leu]enkephalin. By comparing metabolite formation in the presence and absence of peptidase inhibitors with high selectivity for their respective enzymes, these studies demonstrated that aminopeptidase M and angiotensin converting enzyme are the major peptidases that hydrolyze enkephalins in rat plasma.

Hydrolysis of [Leu]enkephalin by chick brain in vitro

Using high-performance liquid chromatography with electrochemical detection to measure substrate disappearance and metabolite accumulation following addition of [Leu]enkephalin to samples prepared from chick brain in vitro, the following were found: 1. [Leu]enkephalin hydrolysis by whole forebrain homogenates is almost solely attributable to aminopeptidase MII activity. 2. [Leu]enkephalin hydrolysis by whole forebrain P2 membrane fractions is attributable to both aminopeptidase MII and dipeptidyl carboxypeptidase activity. 3. Differences are apparent in both [Leu]enkephalin disappearance and Tyr-Gly-Gly accumulation in P2 membrane fractions, but not in homogenate fractions, prepared from several regions of the chick brain.

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).

Hydrolysis and protection from hydrolysis of enkephalins in human plasma

Seven groups of enkephalin-degrading enzymes and three groups of inhibitors active on these enzymes were separated from human plasma. The activity of the enzymes in hydrolyzing enkephalins and of the inhibitors in protecting enkephalins from proteolysis was measured. Results obtained with the endogenous inhibitors were compared to those relative to synthetic inhibitors. Data obtained indicate that all enkephalin-degrading enzymes found in plasma are significantly inhibited by the endogenous substances present in this tissue. The inhibition of the different classes of plasma enzymes by two of the three groups of endogenous substances is quite uniform, while one group of inhibitors appears specific to dipeptidylpeptidases. Results obtained are discussed in terms of the functional role of the inhibitory substances and of the possible pharmacological implication of their presence in human plasma.

Plasma uptake and in vivo metabolism of [Leu]enkephalin following its intraperitoneal administration to rats

To understand better how [Leu]enkephalin (LE) acts to modulate learning and memory in rats, the plasma uptake, disappearance, and metabolism of LE were investigated following its intraperitoneal administration. Concentrations of [3H]-LE and its radioactive metabolites were determined by thin layer chromatography in plasma samples withdrawn from rats at various times after injection of peptide. As measured in rats receiving an IP injection of a dose of LE (3 micrograms/kg) that impairs active avoidance conditioning, the LE was very rapidly metabolized, with greater than 95% of plasma [3H] in the form of metabolites by 1 min after injection. Despite this rapid metabolism, low but measurable quantities of intact LE were detectable in plasma at all sampling times. Consistent with a greater potency of D-Ala2-[D-Leu5]enkephalin (DADLE) than of LE in modulating avoidance conditioning, DADLE was less rapidly metabolized than was LE following its IP administration. The metabolism of DADLE and LE in vivo was more rapid than it was in plasma in vitro, suggesting a role for membrane bound enzymes in the metabolism of IP-administered enkephalins. The data demonstrate that, despite a rapid hydrolysis of LE in vivo, sufficient LE is present in plasma following IP administration of a behaviorally active dose to support a role of circulating intact LE in the modulation of avoidance conditioning.

Effect of leucine enkephalin on ethanol produced behavioral depression in the mouse

The effect of leucine enkephalin (LNK) on spontaneous locomotor activity was studied in mice of both sexes. The effect of a depressant dose of ethanol (ET) on LNK-mediated response on motility was also studied. In addition, the determination of specific activities of the hepatic enzymes primarily involved in the metabolism of ET and acetaldehyde was studied. Lactate dehydrogenase (LDH) isoenzymes were also assayed in both plasma and heart tissues. Intraperitoneal injection of LNK, 100 mg/kg, exerted behavioral depression in the male but not female mouse compared to controls. This effect was apparent for the initial 30 min posttreatment. Injection of two smaller doses of LNK, which were devoid of effect on mouse motility, prior to a depressant dose of ET counteracted ET-caused depression of motility only in the female mouse. This became apparent 30 min postdrug injection and lasted for 60 min thereafter. The LNK and ET treatment inhibited only male mouse liver aldehyde dehydrogenase in both cytoplasmic and mitochondrial preparations concomitant with reduction of plasma but not heart LDH1 isoenzyme from corresponding controls. The results suggest that LNK and ET affect different systems involved in behavioral depression tested and show a potential for LNK in antagonizing the ET effect studied. The data also indicate a sex-dependent effect of LNK on motility and of its interaction with ethanol on the enzymes studied.