Effects of conditioned fear and environmental novelty on plasma beta-endorphin in the rat

Neonatal MSG reduces hypothalamic DA, beta-endorphin, and delays weight gain in genetically obese (A viable yellow/alpha) mice

Neonatal treatment with monosodium glutamate (MSG) decreases proopiomelanocortin (POMC) peptides and results in obesity. The yellow mouse is a model of obesity induced by the viable yellow (Avy) gene at the agouti locus on Chromosome 2, which results in overproduction of a POMC receptor antagonist. Thus we hypothesized that MSG, when imposed on the genetically susceptible model, would alter the development of obesity. Both yellow obese (Avy) and black lean (alpha/alpha) males were injected on Postnatal Days 1, 3, 5, 7, and 9 with 2.0 mg/g body weight MSG or saline SC. Their food intake, growth parameters, and neurochemical status were examined. Paradoxically, MSG interacted with the yellow phenotype to delay the rapid rate of weight gain characteristic of this model (p < 0.05). Food intake was decreased (p < 0.05) in both phenotypes treated with MSG, as was hypothalamic content of dopamine (p < 0.05) and of the POMC peptide, beta-endorphin (p < 0.001). The yellow obese phenotype was more sensitive than the black lean phenotype to the neurochemical effect of early postnatal MSG administration. Recent reports suggest the agouti locus protein is an antagonist of the receptor for another POMC peptide, melanocyte-stimulating hormone (MSH). Therefore, the balance of functional activity between various POMC peptides appears to be an important factor in the development of both acquired and genetic obesity.

Perinatal exposure to morphine affects adult sexual behavior of the male golden hamster

Duromorph, a long-acting form of morphine, was administered to pregnant golden hamsters and/or their pups over the last 4 days of pregnancy and/or the first 4 days after birth. As adults, offspring were gonadectomized, primed with estrogen and progesterone, and tested for their ability to display feminine sexual behavior when placed with a stud male. They were then given testosterone over a 4-week period and tested for their ability to display masculine sexual behavior in the presence of a receptive female. Perinatal morphine exposure had little effect on the females' ability to display either feminine or masculine sexual behavior. In contrast, feminine sexual behavior was significantly enhanced in males exposed to morphine over the perinatal period. This suggests that exposure to opiates during the critical period of sexual differentiation may prevent the defeminization process in this species.

Exposure to mosquitoes, Aedes togoi (Theo.), induces and augments opioid-mediated analgesia in mice

Mosquitoes and other biting flies are natural aversive stimuli commonly encountered by wild and domestic animals and by humans. We observed that male mice exposed for 30 min to a low density of female mosquitoes (Aedes togoi) displayed significant increases in nociceptive responses which were indicative of the induction of analgesia. This analgesia was blocked by the prototypic opiate antagonist naloxone (1.0 mg/kg). Exposure to a novel stimulus had no significant effect on nociception, whereas 30 min of restraint stress induced a naloxone-reversible analgesia qualitatively similar to that observed after exposure to mosquitoes. Confinement in a small chamber for 30 min also had a significant analgesic effect whose amplitude and duration were markedly potentiated by concurrent exposure to mosquitoes. These results show that exposure to mosquitoes, and likely other biting flies, both induces an opioid-mediated analgesia and augments the analgesic effects of other stressful stimuli.

Social conflict-induced changes in nociception and beta-endorphin-like immunoreactivity in pituitary and discrete brain areas of C57BL/6 and DBA/2 mice

The present study characterizes the time course of social conflict analgesia and its reversibility by opioid antagonist drugs in the C57BL/6 and DBA/2 inbred strains of mice and examines the relationship between alterations in brain and pituitary levels of beta-endorphin-like immunoreactivity (beta-ELIR) and the antinociception elicited by social stress. Data revealed statistically significant strain differences in regard to beta-ELIR in control animals. The pituitary content of beta-ELIR was higher in DBA/2, while the values in the periaqueductal grey (PAG) and in the amygdala were higher in C57BL/6 mice. No interstrain differences were found in the hypothalamus. Exposure to 50 attack bites resulted in a 6-fold higher analgesia in DBA/2 mice and in a strain-independent fall of beta-ELIR in pituitary (approximately 27%) and PAG (23%). PAG but not pituitary beta-ELIR levels in C57BL/6 mice correlated positively with the increase in tail-flick latency after attack. Mere confrontation with a non-aggressive opponent failed to induce analgesia and was associated in C57BL/6 mice with a significant reduction in the beta-ELIR content of both the pituitary and the PAG. The data are discussed in terms of genotype-dependent sensitivity of the beta-endorphin system to stress and its relation to analgesia.

Adrenocortical, beta-endorphin and behavioral responses to graded stressors in differentially reared rats

Isolation rearing has long been suspected to alter hormonal and behavioral responses to stress. Two experiments were conducted to test the hypothesis that isolates are more timid or fearful than socially reared rats when exposed to novel test environments. In both, isolate response to 3 graded stressors was compared to that of socially-reared rats. In the first experiment, animals were handled, shocked or not treated prior to testing to produce three levels of conditioned fear. They were then tested on four paradigms previously shown sensitive to conditioned fear: open field activity, emergence latency, auditory startle, and latency to accept food from the experimenter. In the second experiment, rats were given a 0-, 5- or 20-min forced swim, then sacrificed for analysis of plasma corticosterone and pituitary and hypothalamic beta-endorphin. It was found that isolates showed little evidence of enhanced behavioral timidity, although rearing effects were seen on all 4 behavioral measures. Plasma corticosterone levels increased in a graded fashion over the course of the forced swim, but there was no effect of rearing conditions. While there were no effects of rearing or stress on hypothalamic beta-endorphin, pituitary beta-endorphin content was lower in females than in males, and isolate males had lower pituitary endorphin than social males. In summary, these experiments provide no evidence that isolation rearing produces a primary, global increase in fearfulness, but identify several behavioral and hormonal differences associated with differential housing in rats.

Novelty-induced opioid analgesia in deer mice (Peromyscus maniculatus): sex and population differences

Exposure to a new environment elicited significant, naloxone (1.0 mg/kg) reversible analgesic responses in three different populations of deer mice; Peromyscus maniculatus artemisiae from the mainland, and P. m. angustus and P. m. triangularis from small marine islands. In all cases male deer mice displayed significantly greater levels of analgesia than females. In addition, the levels of analgesia were significantly greater in the insular than in the mainland populations. These results indicate that there are substantial sex and population differences in the novelty-induced analgesia displayed by natural and laboratory populations of deer mice.

Tyr-MIF-1 attenuates antinociceptive responses induced by three models of stress-analgesia

Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH2), a biologically active brain peptide, has previously been shown to antagonize the analgesia induced by morphine. In this report experiments are described in which mice were tested on the hot-plate in three models of antinociception - shock, novel environment, and warm-water swim - after the administration of various doses of Tyr-MIF-1 without any exogenous opiates. The peptide reduced the antinociception produced by all three methods of inducing endogenous antinociception. These results add further support for the existence of peptides like Tyr-MIF-1 that act as opiate antagonists.

Shuttlebox avoidance in rhesus monkeys: effects on plasma cortisol and beta-endorphin

Groups of monkeys either extensively pretrained to avoid shocks in a shuttlebox or with minimal prior experience were compared for plasma cortisol and beta-endorphin levels immediately following: (1) an exposure to the box with no shock, (2) the box providing repeated inescapable shocks or (3) a re-exposure to the box, again with no shock presentation. Mere exposure to the unfamiliar box elevated plasma cortisol just as much as exposure + shock did when inexperienced monkeys were tested. However, animals with a history of previously successful shock avoidance showed smaller elevations when exposed to the box alone, than they did when inescapable shock was received. Plasma beta-endorphin levels following shuttlebox exposure showed only a sporadic pattern of elevations in either inexperienced or pretrained monkeys. However, levels of beta-endorphin as determined under control conditions in the home cage were lower in pretrained animals, as were plasma levels of cortisol. The results indicate that behavioral factors may effect plasma cortisol and beta-endorphin following both acute and chronic shuttlebox experience.