D-Pen2-[D-Pen5]enkephalin impairs acquisition and enhances retention of a one-way active avoidance response in rats

Peptides and hormesis

Biology is replete with examples of hormesis, the term introduced and developed by Calabrese. The corresponding concept in the field of peptide research has been characterized as the inverted U-shaped dose-response relationship. The articles by Calabrese in this issue summarize the notable progress occurring in the past three decades. In contrast to the skepticism encountered when we introduced this concept for peptides in the early 1970s, hormesis is now becoming recognized as characteristic of many actions of these small proteins. Calabrese is performing a considerable service by his strong advocacy and promotion of the concept to a more general readership. Hopefully, hormesis will be routinely considered in the design of research projects and the discovery of pharmaceutical agents.

Opioid receptors modulate recovery from consummatory successive negative contrast

Three experiments explored the role of the opioid system in consummatory successive negative contrast. In Experiment 1, rats treated with the nonspecific opioid-receptor antagonist naloxone (2mg/kg) exhibited increased suppression after a shift from 32% to 6% sucrose solution (32-->6), relative to 6-->6 unshifted controls. A similar but shorter effect was observed with the delta-opioid receptor antagonist naltrindole (1mg/kg). In Experiment 2, naloxone increased suppression after a more conventional 32-->4 sucrose shift. In Experiment 3, rats classified as expressing slow recovery from contrast (after a 32-->4 sucrose downshift) were more sensitive to naloxone in an activity test than fast-recovery rats. Whereas it was previously known that contrast was reduced by the extrinsic administration of opioid agonists, the effects reported here with antagonists provide the first evidence that the opioid system is intrinsically engaged by situations involving surprising reward loss.

Selective effects of the delta-opioid receptor agonist DPDPE on consummatory successive negative contrast

Two experiments explored the role of the opioid system in a situation involving a surprising reduction in reward magnitude: consummatory successive negative contrast. Rats received access to 32% sucrose solution (preshift Trials 1-10) followed by 4% solution (postshift Trials 11-15). Independent groups received an injection of either the vehicle or the delta-receptor agonist [D-Ala2-,N-Me-Phe4,Gly-ol] enkephalin (DPDPE; 24 microg/kg). DPDPE attenuated the contrast effect when injected before Trial 11 but not when injected before Trial 12. An additional experiment showed that the attenuating effect of partial reinforcement on the recovery from contrast was reduced by DPDPE injections administered before nonreinforced preshift trials.

Effects of peptides on animal and human behavior: a review of studies published in the first twenty years of the journal Peptides

This review catalogs effects of peptides on various aspects of animal and human behavior as published in the journal Peptides in its first twenty years. Topics covered include: activity levels, addiction behavior, ingestive behaviors, learning and memory-based behaviors, nociceptive behaviors, social and sexual behavior, and stereotyped and other behaviors. There are separate tables for these behaviors and a short introduction for each section.

Dopamine and opioid regulation of the memory retrieval recovery in mice

The reactivation effects of the delta-opioid receptor blockade and D2 dopamine receptor activation on the detention-induced memory deficit in mice were investigated, in order to study possible interactions between opioid and dopamine systems in memory retrieval. Animals were trained in a one-trial passive-avoidance task. Pretesting treatment with ICI 174,864 (1, 3 or 5 mg/kg, i.p.) or quinpirole (0.5, 1 or 2 mg/kg, i.p.) facilitated retrieval of memory trace in saline-pretreated mice. Pretraining injection of the dopamine autoreceptor agonist, (+)-3PPP (2 mg/kg), having no effect alone in learning, prevented the ability of ICI 174,864 to produce the memory-enhancing effect. It is suggested that the normal functioning of the dopamine system was critical for the facilitation of retrieval by delta-antagonist. Quinpirole-induced reactivátion of memory retrieval was enhanced by pretreatment with Leu-enkephalin (0.2 mg/kg), inducing increased retention. We discuss these results in the context of an important interactions between D2 dopamine and delta-opioid receptors.

Effects of the delta-opioid agonist, [D-Pen2,D-Pen5]-enkephalin, on fetal lamb EEG

Opiates are known to exert biphasic effects on level of arousal, with excitation at low doses and depression at higher doses. It has been suggested that this dual excitatory and depressant actions of opiates may be mediated by different receptor subtypes. We have previously shown that activation of mu 1-opioid receptors evoked EEG activation in the fetal lamb. The purpose of the present study was to quantitate the effects of DPDPE, a highly selective delta-opioid agonist, on fetal EEG. When infused ICV (4.6-154 nmol/h), DPDPE elicited dose-dependent activation of fetal EEG, with a reduction in power distribution in the delta (1-4 Hz) band, and an increase in the beta (15-32 Hz) band. This activation was reflected by an increase in the spectral edge frequency. This EEG activation was greatly attenuated at DPDPE doses greater than 154 nmol/h, resulting in a U-shaped dose-response curve. The EEG activation was completely blocked by naloxone or naltrindole (delta antagonist), but not by naloxonazine (mu 1 antagonist). These results indicate that the activation of delta-opioid receptors will evoke EEG activation in the fetal lamb.

CNS effects of peptides: a cross-listing of peptides and their central actions published in the journal Peptides, 1986-1993

The centrally mediated effects of peptides as published in the journal Peptides from 1986 to 1993 are tabulated in two ways. In one table, the peptides are listed alphabetically. In another table, the effects are arranged alphabetically. Most of the effects observed after administration of peptides are grouped, wherever possible, into categories such as cardiovascular and gastrointestinal. The species used in most cases has been rats; where other animals were used, the species is noted. The route of administration of peptides and source of information also are included in the tables, with a complete listing provided at the end. Many peptides have been shown to exert a large number of centrally mediated effects.

Endogenous opiates: 1992

This paper is the fifteenth installment of our annual review of research concerning the opiate system. It includes papers published during 1992 involving the behavioral, non-analgesic, effects of the endogenous opiate peptides. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal and renal function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunological responses; and other behaviors.

Tyr-D-Arg-Phe-beta-Ala-NH2, a novel dermorphin analog, impairs memory consolidation in mice

Tyr-D-Arg-Phe-beta-Ala-NH2 (TAPA), a novel dermorphin analog with high selectivity and affinity for mu-opioid receptors, was administered intracerebroventricularly to mice before or immediately after training in a one-trial step-down type passive avoidance task. The pre- and post-training administration of TAPA (0.3 and/or 3 ng) impaired retention performance 24 h after training. In particular, the post-training administration of TAPA was much more effective because a lower dose (0.3 ng) of TAPA exclusively inhibited retention performance. The amnesic effects of TAPA were reversed by the mu-selective opioid antagonist beta-funaltrexamine (5 micrograms, i.c.v.). In addition, TAPA (0.3 and 3 ng) had no effects on nociceptive responses in a tail-flick test or on behavioral responses to electric shock during training. These results suggest that activation of mu-opioid receptors impairs passive avoidance learning, resulting in a dysfunction of memory consolidation, without affecting other behavioral responses.