Stress and morphine mediated changes in pituitary-adrenal axis in guinea-pigs

Relation of glucocorticosteroids and testosterone to the annual cycle of free-living degus in semiarid central Chile

We investigated seasonal patterns of plasma glucocorticosteroids (GCs) in both sexes and testosterone (T) in males in relation to the annual cycle in central Chile of a natural population of the degu (Octodon degus), a caviomorph rodent. We wanted to find out which GCs are present in degus, whether their seasonal variation suggests suppressive or synergistic interrelationships with T, and whether seasonal variation in GC levels indicates a relationship with energy mobilization and demands of reproduction. Degus mated in late autumn, and female body mass increased in pregnancy and remained high during lactation and throughout spring. Over the subsequent period of summer drought both sexes declined to a minimal body mass before the next mating season. Cortisol appears to be the principal GC in degus. In fact cortisol levels were so high that the extremely low levels of corticosterone measured were probably largely due to the cross-reactivity of our corticosterone antiserum with cortisol. Titers of cortisol in females exceeded 1000 ng/ml at lactation in the spring of 2 years; cortisol declined greatly following lactation and during the summer and reached its lowest mean level of about 500 ng/ml at mating. Males were more difficult to capture than females and thus our sampling was limited, but male cortisol levels were similar to those of females during the times of year when we measured them. Male T levels remained within a low range all year, but at mating, when mean T was highest (0.16 ng/ml) and when most males had detectable T, degus showed their lowest cortisol levels. The minimal cortisol level of males during mating represents a possible suppressive effect of T, as described in other mammals. At the time of their spring emergence, 60% of juvenile males had detectable T levels comparable to those of adults, suggesting important organizational effects of T at that time in their maturation. Peak cortisol titers in both sexes were associated with lactation in females, when energy mobilization, production, and body mass were at their greatest.

Gender influences the core temperature response to a simulated open field in adult guinea pigs

The induction of psychological stress is often accompanied by a transient increase in core temperature, commonly referred to as stress induced hyperthermia. Although stress-induced hyperthermia occurs when rats, mice, and pigs are exposed to a novel stimulus (e.g., a simulated open field, restraint, etc.), whether or not it occurs in guinea pigs has not been investigated. The present experiments were therefore carried out to investigate the thermoregulatory responses of both male (n = 7) and female (n = 7) adult guinea pigs when they were exposed to a simulated open field. Unexpectedly, neither the male nor female guinea pigs developed stress-induced hyperthermia. To the contrary, female but not male guinea pigs significantly decreased their core temperature during an open field experiment. The mechanism of the gender-specific thermoregulatory response of the adult guinea pig to psychological stress is presently unknown.

Food restriction-mediated adrenal influences on antigen-induced bronchoconstriction and airway eosinophil influx in the guinea pig

The aim of this study was to measure the effects of food restriction on antigen-induced bronchoconstriction and inflammatory cell influx in guinea pigs and to determine the role of plasma cortisol and catecholamine concentrations. Ovalbumin (OA; 0.3 mg/kg, i.v.) was administered to OA-sensitized, anesthetized guinea pigs which had been allowed free access to food or had been food restricted for 18 h prior to OA challenge. In addition to higher plasma levels of epinephrine (30% increase) and cortisol (33% increase), fasted guinea pigs had significantly lower (60% decreased) maximal bronchoconstrictor responses to OA than nonfasted, sensitized litter mates. Additionally, groups of fasted or fed animals were subdivided into two additional treatment groups: (1) saline-pretreated or (2) polyethylene glycol 400 (PEG)-pretreated (1 ml/kg, p.o., 1 h prior to antigen challenge). In saline-treated, fasted animals, bronchoconstrictor responses to antigen were significantly diminished (67% decreased) and epinephrine and cortisol levels were increased (64 and 34%, respectively) compared to the corresponding fed group. In both fasted and fed groups, the PEG-treated guinea pigs had higher plasma epinephrine and cortisol levels than animals which received saline, but no significant differences were detected within the PEG-treated group. Plasma norepinephrine concentrations were lower in all fasted groups. In a separate model in conscious guinea pigs, there were no differences in aerosol OA-induced bronchoconstriction and eosinophil influx between fasted and fed groups. However, compared to the saline pretreatment group, PEG administration reduced the antigen-induced bronchoconstriction and eosinophilia in both fed and fasted guinea pigs. We speculate that the reduced responsiveness to antigen in fasted versus fed animals may result from food-restriction-induced, stress-related release of epinephrine and cortisol from the adrenal glands, thereby suppressing mast cell degranulation or reducing responsiveness to spasmogenic and chemotactic mediators. In addition, the results suggest that oral dosing with 100% PEG may enhance this phenomenon.

Influence of acute and chronic morphine or stadol on the secretion of adrenocorticotrophin and its hypothalamic releasing hormone in the rat

The effects of acute and chronic treatment with morphine and stadol on the functional activity of the hypothalamo-pituitary-adrenocortical (HPA) system in the rat were studied by investigating their effects on the secretion of adrenocorticotrophin (ACTH) by the pituitary gland and corticotrophin-releasing hormone (CRH) by the hypothalamus. The acute injection of morphine or stadol (3.5 mg/100 g body weight i.p.) caused a rise at 5 and 25 min followed by a fall at 90 and 120 min in the concentrations of ACTH in the plasma and adenohypophysis and in hypothalamic CRH content. It appears that, in the rat, the response of HPA system to acute morphine or stadol administration could change depending upon the time of courses. In addition, chronic morphine or stadol (0.5 mg/100 g body weight i.p. daily) administration for a period of 7 days have little effect on plasma and adenohypophysis ACTH concentrations and hypothalamic CRH content. This may indicate that drug tolerance might have developed. Conversely, repeated daily doses of morphine or stadol (2 mg/ 100 g body weight i.p.) for 7 days cause a significant lowering of plasma and pituitary ACTH concentrations and hypothalamic CRH content. These data suggest that the effect of both drugs is dose related. Overall, the present results are consistent with an increased release of pro-opiomelanocortin-derived peptides after acute morphine or stadol treatment for a short-term, and with a decreased release of these peptides in chronic treatment. However, the results indicate that morphine and stadol change HPA activity by acting on specific receptors in the hypothalamus and raise the possibility that opioid peptides and their receptors are physiologically important in the control of the secretion of CRH.