A comparison of the ability of opioid peptides and opiates to affect active avoidance conditioning in rats

Kappa opioid receptor activity modulates memory for peck-avoidance training in the 2-day-old chick

To examine the role of kappa opioid receptors in memory formation, 2-day-old chicks were injected intracerebrally with either the endogenous opioid peptide dynorphin(1-13), the highly kappa selective agonist U-50,488 or the kappa selective antagonist nor-binaltorphimine (nor-BNI), given one-trial peck-avoidance training, and tested 24 h later. Dynorphin(1-13) impaired memory in a dose dependent manner at 24 h test. Injection of U-50,488 caused a biphasic dose-dependent effect on memory; low doses caused a trend toward enhanced memory and high doses caused significant impairment. Conversely, injection of low doses of nor-BNI caused a trend toward memory impairment, and higher doses caused significant memory enhancement. The results indicate that memory formation for one-trial peck-avoidance training may be modulated by kappa opioid receptor activity.

Identification of the opioid receptors involved in passive-avoidance learning in the day-old chick during the second wave of neuronal activity

Long-term memory formation for passive-avoidance learning in the day-old chick is known to have two distinct time windows of protein synthesis (F.M. Freeman, S.P.R. Rose, A.B. Scholey, 1995. Two time windows of anisomycin-induced amnesia for passive-avoidance training in the day-old chick. Neurobiol. Learn. Mem. 63, 291-295). The lobus parolfactorius (LPO) is thought to be an important site for the second wave of protein synthesis which occurs 4-5 h after training. Birds received bilateral intracranial injections of agonists and antagonists for the mu-, delta-, kappa-opioid receptors and the opioid receptor-like (ORL(1)) receptor directly into the LPO at 5 h post-training and were tested for recall 24 h later. Also, 100 microM beta-funaltrexamine (beta-FAN), a mu-opioid receptor antagonist, significantly impaired memory formation (P<0.01). The delta-opioid receptor was also involved in memory formation at this time-point since antagonism of this receptor by 1 mM ICI-174,864 caused amnesia (P<0.01) which was reversed by the agonist, DPLPE. The kappa-opioid receptor appeared not to be involved during the second phase of neuronal activity since neither stimulation by dynorphin nor inhibition by nor-BIN caused amnesia for the task. The ORL(1) receptor agonist orphanin FQ also had no effect suggesting that this receptor was not involved at this 5-h time-point. Cytosolic and mitochondrial protein synthesis has been shown to be important in passive-avoidance learning in the day-old chick. Both chloramphenicol (CAP) and anisomycin (ANI), inhibitors of mitochondrial and cytosolic protein synthesis, respectively, caused disruption when injected 5 h post-training into the LPO (P<0.05). Endomorphin-2 (Endo-2), a mu-opioid receptor agonist, reversed both the ANI- and CAP-sensitivity. However, DPLPE, a delta-opioid receptor agonist, only reversed the effect due to CAP. Possible mechanisms for these effects are discussed.

Reduced cerebrospinal fluid dynorphin A1-8 in Alzheimer's disease

Cerebrospinal fluid (CSF) measures of dynorphin A were compared in three groups. Alzheimer patients (n = 9), elderly depressives (n = 9), and age-matched normal controls (n = 9). The Alzheimer patients revealed a 40% decrease in CSF dynorphin compared with controls (36 +/- 15 versus 60 +/- 21 pg/ml, p less than 0.05). In contrast, other peptide measures [Neuropeptide Y (NPY), vasoactive intestinal peptide (VIP), and galanin] remained unchanged across groups. This finding was further supported when an additional 20 Alzheimer patients with similar clinical backgrounds also showed reduced CSF dynorphin (37 +/- 13 pg/ml). CSF dynorphin did not correlate significantly with clinical variables or other CSF measures of monoamine metabolites [i.e., 3-methoxy-4-hydroxyphenylglycol (MHPG), 5-hydroxyindoleacetic acid (5-HIAA), and homovanillic acid (HVA)]. Given the previous report of increased kappa binding of Alzheimer brains at autopsy, the authors speculate about a possible up-regulation of opiate receptors in Alzheimer's disease and suggest ways to test this hypothesis in vivo.

The opioid system in neurologic and psychiatric disorders and in their experimental models

Evidence from experimental and clinical studies suggests the involvement of the endogenous opioid system in several neurologic and psychiatric disorders (Alzheimer's, Huntington's and Parkinson's diseases, drug-induced movement disorders, Gilles de la Tourette syndrome, stroke, ischemia, brain and spinal cord injury, epilepsy, schizophrenia and affective disorders). However, its involvement is rather a secondary one, perhaps being a severe consequence of a primary, nonopioid disturbance. Thus, treatment of an opioidergic manifestation of a disorder of nonopioidergic origin is necessarily symptomatic and targets only the restoration of the opioid system; such treatment may be beneficial in ameliorating the clinical symptoms of the disorder.

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.

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.

Selective changes in mu, delta and kappa opioid receptor binding in certain limbic regions of the brain in Alzheimer's disease patients

Total opioid binding and levels of the three major types of opioid binding sites were measured in homogenates of various limbic structures from post-mortem brains of Alzheimer's disease patients and age-matched control individuals. The most consistent finding in Alzheimer's disease brains was an increase in kappa binding in all 6 areas of the limbic system examined, with the putamen and caudate regions showing significant increases of 114% and 53%, respectively. In addition, the Alzheimer's disease putamen showed a significantly higher level of total binding (85% increase). The amygdala of Alzheimer's disease patients exhibited significantly lower levels of mu and delta binding (41% and 55% decrease, respectively). Total binding and binding to mu and delta receptors in frontal cortex, caudate and hippocampus of Alzheimer's disease brains was indistinguishable from levels seen in these brain areas from control individuals.

Behavioral effects of centrally administered dynorphin and [D-ala2-D-leu] enkephalin (DADLE) in rats

Dynorphin and [D-ala2-D-leu]enkephalin (DADLE) were administered directly into the cerebrolateral ventricles of rats and effects on various indices of sensorimotor function and retention of a passive avoidance task were measured. Dynorphin markedly suppressed exploratory motor activity and decreased responsiveness to an acoustic stimulus. Although increases in latency to respond to a noxious thermal stimulus were seen in rats after dynorphin, these changes were always associated with alterations in motor capacity. Injection of dynorphin immediately after a passive avoidance training trial had no significant effect on retention 1 week later. The physiological effects of DADLE were clearly different than those of dynorphin. DADLE produced a biphasic decrease followed by an increase in motor activity and an increased acoustic startle reactivity. DADLE had no effect on reactivity to a noxious thermal stimulus. Posttrial administration of DADLE significantly impaired retention of a step-through passive avoidance task 1 week after training. These data indicate different neurobiological roles for kappa and delta opiate receptors in the central nervous system.

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

Leu-enkephalin (100 micrograms/kg, i.p.) administered to mice 5 min before training in a one way active avoidance task significantly reduced the number of avoidances observed in the peptide treated animals. This impairing action of Leu-enkephalin was partially attenuated by methylnaloxonium (naloxonium), a quarternary form of naloxone with a limited ability to penetrate the blood brain barrier. Passive immunization (i.v.) of mice with a Leu-enkephalin antiserum 4 hrs before training produced an effect on avoidance conditioning that was the opposite to that observed with Leu-enkephalin alone. That is, passive immunization increased the number of avoidances observed in the treated mice. The results suggest that Leu-enkephalin actions on avoidance conditioning are mediated by a peripheral opioid mechanism, that leu-enkephalin may have a primary site of action outside the blood brain barrier, and that peripheral Leu-enkephalin systems may normally operate to influence conditioned avoidance behavior.

Central versus peripheral actions of Leu-enkephalin on acquisition of a one-way active avoidance response in rats

A comparison was carried out of the effectiveness of intracerebroventricular or intraperitoneal administration of Leu-enkephalin to affect acquisition of a one-way active avoidance response, locomotor activity or shock sensitivity. Leu-enkephalin impaired acquisition of the avoidance response only when administered peripherally. In contrast, either central or peripheral administration of Leu-enkephalin increased locomotor activity in the open field, and Leu-enkephalin also produced analgesia when administered directly into the brain. The pattern of results suggests that Leu-enkephalin acts at a peripheral site to influence avoidance conditioning.

Enkephalin actions on avoidance conditioning may be related to adrenal medullary function

The present series of studies was undertaken to better characterize the mechanisms mediating enkephalin actions on avoidance conditioning. Both Leu-and Met-enkephalin administered i.p. in amounts of 1.0 or 10.0 micrograms/kg 5 min before training in an active avoidance task significantly impaired acquisition of the response. Adrenal medullectomy, which removes an endogenous store of enkephalin-like peptides, abolished the effect on avoidance conditioning of 10.0 micrograms/kg of exogenously administered Met- or Leu-enkephalin. Increasing the dose of Leu-enkephalin by 10- or 100-fold restored its behavioral activity in adrenal demedullated rats. However, increasing (10-100-fold) or decreasing (10-fold) the dose of Met-enkephalin did not restore its activity in adrenal demedullated animals. This finding suggests that the impairing actions of the two enkephalins are not produced in the same way, but it is consistent with the interpretation that the adrenal medulla may play a role in mediating the actions of Met- and Leu-enkephalin on active avoidance conditioning.

Endogenous opiates: 1981

This paper is the fourth of an annual series reviewing the research concerning the endogenous opiate peptides. This installment covers only work published during 1981 and attempts to provide a comprehensive, but not exhaustive, survey of the area. Previous papers in the series have dealt with research done before 1981. Topics concerning endogenous opiates reviewed here include a delineation of their receptors, their distribution, their precursors and degradation, behavioral effects resulting from their administration, their possible involvement in physiological responses, and their interactions with other peptides and hormones. Due to the burgeoning literature in this field, the comprehensive nature of this review in the future will be limited to considerations of behavioral phenomena related to the endogenous opiates.

Effect of morphine, ACTH, epinephrine, Met-, Leu- and des-Tyr-Met-enkephalin on beta-endorphin-like immunoreactivity of rat brain

Morphine (1,0 mg/kg), ACTH1-24 (10.0 micrograms/kg), epinephrine (12.0 micrograms/kg), Met-enkephalin (2.0 and 5.0 micrograms/kg), Leu-enkephalin (2.0 micrograms/kg) and des-Tyr-Met-enkephalin (2.0 micrograms/kg) all produced marked reductions of beta-endorphin-like immunoreactivity in the rat diencephalon. At a dose of 0.4 mg/kg, naloxone had no effect of its own and was unable to reverse the depleting effect of the other substances. The depletion of beta-endorphin-like immunoreactivity caused by the various treatments is attributable to release and subsequent degradation of beta-endorphin and/or of its precursors. The various behavioral effects of morphine, ACTH, epinephrine and the enkephalins may be explained by the release of endogenous beta-endorphin.