Endogenous opiates: 1982

Changes in β-endorphin level in the cingulate cortex in rats after peripheral loperamide and methylnaloxone administration at rest and during emotional stress

β-Endorphin content in the extracellular space of rat cingulate cortex was measured using intravital microdialysis followed by ELISA. Intragastric administration of μ-opioid ligands loperamide and methylnaloxone not crossing the blood-brain barrier produced different effects on β-endorphin level: loperamide reduced and methylnaloxone significantly increased the release of β-endorphin into the extracellular space of rat cingulate cortex. Emotional stress caused by immobilization resulted in slight increase in β-endorphin level in the cingulate cortex. Peripheral administration of loperamide (but not methylnaloxone) significantly increased the release of the neuropeptide during stress. These findings support our hypothesis of reciprocal interaction between the central and peripheral compartments of the endogenous opioid system and provide explanations for the anti-stress effects of loperamide.

The Tyr-MIF-1 family of peptides

A review of research on the Tyr-MIF-1 family of peptides is presented with emphasis on Tyr-MIF-1 and its structure, passage through the blood-brain barrier, and both opiate antagonist and agonist properties. Family members MIF-1, Tyr-W-MIF-1 and Tyr-K-MIF-1 are also included.

Intravenous beta-endorphin: behavioral and physiological effects in conscious monkeys

Beta-endorphin (0.7 and 2.8 mg/kg) and morphine (0.15 and 0.60 mg/kg) were administered intravenously to rhesus monkeys responding on an operant schedule. Beta-endorphin injections resulted in dose-dependent effects which included marked, but relatively brief disruptions in behavioral responding, decreases in systolic blood pressure, and more protracted increases in heart rate. Morphine injections were followed by much longer duration decreases in response rates and systolic blood pressure, and an irregular but largely deceleratory heart rate response. On a molar basis, beta-endorphin was approximately twice as potent as morphine. It was concluded that intravenously administered beta-endorphin exerts behavioral and physiological effects in the unanesthetized primate.

Dual 24-hour feeding response to 2DG in rats: daytime increase and nighttime decrease

Thirty-six rats were injected IP with 2DG (0, 250, or 500 mg/kg) at 7-day intervals, once at light onset (7 a.m.) and once at dark onset (7 p.m.), and postinjection food intake was monitored for 24 hours. Five hundred mg/kg 2DG caused food intake to rise above control levels during the first 6 hours of daylight, regardless of whether the injection had occurred that morning or the previous evening, whereas intake during the first 6 hours of darkness was consistently below control levels. In a second study, 24 rats were injected first at 7 a.m. (500 mg/kg 2DG or saline), and 7 days later at 7 p.m. (opposite drug), and food was withheld 12 hours until the light:dark period had changed. For 12 hours after food was returned, 2DG again decreased nighttime food intake (Injection 1) and increased daytime intake (Injection 2). 2DG's dual long-term effects cannot be accounted for either by malaise or by an initial action that later is compensated by its opposite. Rather, 2DG (500 mg/kg) appears to exert two independent, opposite alimentary effects which persist 18-24 hours and which change direction with phase changes in the light:dark cycle.

Effects of opioid peptides and naloxone on nervous tissue in culture

It was shown that opioid peptides stimulate nervous tissue growth in culture in the rat, which manifests itself in augmented outgrowth of neurites from explants and in an increase in the number of glial and fibroblast-like cells in the growth zone. The effects of opioid peptides ([Leu]- and [Met]-enkephalins, beta- and gamma-endorphins and some synthetic analogues of [Leu]-enkephalin) on the growth of organotypic cultures of rat sympathetic and dorsal root ganglia and spinal cord were investigated. Neurite outgrowth, cell composition, and size of the growth zone as well as the dynamics of its formation were estimated. Changes in the survival of neurons in dorsal root ganglion cultures were determined. The experiments were performed with living cultures as well as with fixed preparations. In experiments with sympathetic ganglia, it was demonstrated that a significant growth-promoting effect is exerted by peptides taken at concentrations of 10(-8) M to 10(-14) M. Naloxone does not eliminate the effects of peptides, but stimulates the growth at 10(-5) M to 10(-7) M. Studies with spinal cord revealed that naloxone (10(-6) M) enhances the response to [Leu]-enkephalin (10(-9) M). The survival of dorsal root ganglion neurons under the influence of a [leu]-enkephalin analog (10(-9) M) exceeds control values by approximately two to four times. Thus, opioid peptides were shown to exert a strong growth-promoting effect on nervous tissue in culture. This effect is dual: in neurons the peptides stimulate the outgrowth of neurites and their survival, while in glial cells they change the rate of their migration and, probably, their proliferation. It is suggested that opioid peptides, besides their already established functions, may play a role in the development and regeneration of nervous tissue.

Central nervous system effects of peptides, 1980-1985: a cross-listing of peptides and their central actions from the first six years of the journal Peptides

A tabular synopsis is presented for articles concerned with the effects of peptides on the central nervous system that appeared in the journal Peptides from 1980-1985. A table arranged alphabetically by peptide and one arranged by effects, both listing routes of injection, species, direction of change, and qualifying notes, provides easy cross-referencing of peptides and their effects. Over 80 peptides and over 135 effects are listed. The list of peptides includes, but is not limited to: ACTH, angiotensin, bombesin, bradykinin, calcitonin, casomorphin, CCK, ceruletide, CGRP, CRF, dermorphin, DSIP, dynorphin, endorphins, enkephalins, GRF, gastrin, LHRH, litorin, metkephamid, MIF-l, motilin, MSH, NPY, NT, oxytocin, ranatensin, sauvagine, substances P and K, somatostatin, TRH, VIP, vasopressin, and vasotocin. The list of effects includes, but is not limited to: aggression, alcohol, analgesia, attention, avoidance, behavior, cardiovascular regulation, catalepsy, conditioned behavior, convulsions, dopamine binding and metabolism, discrimination, drinking, EEG, exploration, feeding, fever, gastric secretion, GI motility, grooming, learning, locomotor behavior, mating, memory, neuronal activity, open field, operant behavior, rearing, respiration, satiety, scratching, seizure, sleep, stereotypy, temperature, thermoregulation and tolerance.

Morphine distribution following infusion into lateral ventricle during hibernation and euthermia

The intracerebral pattern of diffusion of 3H-morphine was studied autoradiographically following continuous infusion (4, 9, and 18 hr; 1 microliter/hr) into the lateral ventricle during hibernation and euthermia (i.e., not hibernating) in ground squirrels (Citellus lateralis). Morphine diffusion into the parenchyma during both states was extensive, resulting in increased autoradiographic optical density of 34 structures examined. The zone of radiolabeled tissue was primarily ipsilateral, and it expanded with increasing duration of infusion. Diffusion into contralateral regions was more evident in hibernation, although the total area of radiolabeled tissue was not significantly greater than that of euthermic animals. The average optical densities of autoradiographs from hibernating brains were significantly greater than those from euthermic animals, suggesting greater accumulation of labeled material during hibernation. These data suggest that neuroactive compounds released into the ventricular space can achieve widespread distribution within the brain during hibernation (in which all physiological parameters are profoundly depressed) as well as during euthermia. Thus, the apparent lack of development of physical dependence to morphine during hibernation is not due to a restricted distribution of morphine in the hibernating brain.

Non-opiate analgesia following stressful acoustic stimulation

The change in the nociceptive reactions of rats was characterized after stressful acoustic (115 dB) stimulation. Acoustic loading for five minutes resulted in considerable analgesia in the hot-plate test, whereas a significant analgesic response was not observed in the tail-flick test. The analgesic reaction after acoustic stimulation was resistant to naloxone pretreatment and was also found in morphine-tolerant rats, but the acute thermoregulatory and analgesic effects of morphine were greatly potentiated by simultaneous acoustic loading. Substance P or cholecystokinin treatment likewise failed to prevent the analgesic effect of auditory stimulation. No tolerance developed to the analgesic effect on repeated stressing. Diltiazem, a slow calcium channel blocker, facilitated the analgesia. The data suggest a stress-induced analgesia with obviously non-opiate properties, although an indirect involvement of opiate effects could not be excluded.

Electroencephalographic and autonomic effects of centrally administered dermorphin in rabbits

Dermorphin, administered into the third ventricle of conscious rabbits, induces an increase of the total power density spectrum of the cortex and a decrease of the total power of the hippocampus. The electrocortical pattern is similar to that found with other opiates and reported as specific of mu agonists. Simultaneously the peptide causes respiratory depression, bradycardia and hypothermia. Naloxone (0.5 mg/kg IV) quickly and completely inverts all these effects. The activity of serotoninergic, Gabaergic, catecholaminergic and cholinergic systems does not seem to be required for these dermorphin actions. Thus, the hypothesis that dermorphin acts directly in modifying cerebral electrical activity is put forward.

Influence of morphine and cyclazocine on the cortical epileptic foci in rabbits

The effects of morphine, cyclazocine and naloxone on penicillin- and strychnine-induced epileptic foci were studied in rabbits. The intracortical injection of penicillin (75, 150 and 300 units) elicited isolated spikes followed by repeated ictal events. The application of strychnine (0.062 and 0.125%) over the cortical surface of one side induced appearance of ipsilateral spiking spreading to the contralateral cortex. Administration of morphine (0.25-0.75 mg/kg i.v.) or cyclazocine (0.05-3.0 mg/kg i.v.) inhibited the occurrence or the duration of the EEG and motor manifestations induced by penicillin (75 and 150 units) and strychnine (0.062 and 0.125%), while it did not influence the effect of 300 units of penicillin. High doses of morphine (up to 10 mg/kg i.v.) failed to affect the epileptic responses to penicillin and strychnine and at the same time significantly reduced the pO2 in arterial blood. Naloxone per se potentiated the effects of the lower doses of penicillin and strychnine. Only at very high doses (20 mg/kg i.v.) displayed a weak antagonism towards the anticonvulsant effect of the two opiates. A full antagonism is only observed towards the effect of cyclazocine (2 mg/kg i.v.) administered after penicillin. Present data provide additional evidence of the heterogeneity of regulation by opioids of convulsive phenomena. One can hypothesize that the anticonvulsant effect of the two opiate agonists is mediated by naloxone-insensitive opiate receptors, while the proconvulsant-convulsant effect of naloxone might be related to an inhibition of GABA and glycine-mediated transmission.