Albert Ratner

Habituation of the prolactin response in rats to psychological stress

It is well known that stress is a stimulant for prolactin release. However, relatively few studies have investigated the role of psychological factors in prolactin secretion, and investigators have typically used one-time exposure and a single collection period in their studies. In our studies, attempts were made to carefully characterize the prolactin response to different psychological stressors by serially sampling blood from an indwelling cannula and to determine if repeated exposure to the stressor leads to habituation of the prolactin response. Exposure of the male rats to different novel situations such as being placed in a new cage, being placed on a platform in water, or being handled resulted in increased prolactin levels. As the rats habituated behaviorally to repeated exposure to similar situations, the prolactin response also attenuated. These findings show that psychological factors do play a role in influencing prolactin secretion and are consistent with the idea that as the psychological stress imposed by a stimulus becomes habituated, the prolactin response to that stimulus also becomes habituated.

Monosodium glutamate: lack of effects on brain and reproductive function in rats.

Monosodium glutamate was injected subcutaneously in infant rats of both sexes. The lateral preoptic and arcuate nuclei and median eminence were examined by light and electron microscopy for possible monosodium glutamate effects. As adults, treated animals showed no adverse monosodium glutamate effects on the reproductive system and neural morphology.

Stimulation of luteinizing hormone release in vitro by dibutyryl-cyclic-AMP and theophylline

Abstract Dibutyryl-cyclic-AMP (1.5 × 10−3M) and theophylline (11.0 × 10−3M) each caused an increase in luteinizing hormone (LH) release by rat anterior pituitaries in vitro . When dibutyryl-cyclic-AMP and theophylline were added concomitantly, the response was synergistic. With respect to other adenine nucleotides, AMP (2.0 × 10−3M) showed some stimulatory ability, while ADP and ATP had no effect upon LH release. These data indicate that the release of LH in vitro is stimulated by cyclic AMP.

Effect of corticosterone on the prolactin response to psychological and physical stress in rats.

Both corticosterone and prolactin (PRL) levels increase in response to stress. In these studies we examined the effect of corticosterone on the PRL response to both physical (footshock) and psychological (novel environment) stress. Three groups of rats were used: sham adrenalectomized (SHAM), adrenalectomized (ADX), and adrenalectomized with corticosterone replacement (ADX+CORT). The corticosterone-treated animals received 80 micrograms corticosterone/ml drinking water. Blood samples were drawn via an indwelling cannula and PRL values determined using radioimmunoassay. ADX rats showed a consistently greater PRL response to being placed on a platform above water (novel environment) or when receiving intermittant footshock than did ADX+CORT rats. The PRL response of the latter group was similar to that of the SHAM animals. These findings indicate that corticosterone levels of an animal can significantly attenuate the magnitude of the PRL response to both physical and psychological stress. These findings further emphasize that the PRL response to stress is dependent not only upon the immediate action of the stressor, but also the prior stress history of the animal.

Dissociation Between LH Release and Pituitary Cyclic Nucleotide Accumulation in Response to Synthetic LH-Releasing Hormone in vivo

In order to clarify the role of cyclic nucleotides in the mediation of LH-releasing hormone (LH-RH) action on LH release, the effect of LH-RH on LH release and pituitary cyclic AMP and cyclic GMP accumulation was studied in vivo. Pituitary cyclic AMP levels were increased 10 min after the intraveneous injection of 2 mg dibutyryl cyclic AMP (db-cyclic AMP) or 25 mg aminophylline. However, serum LH levels were not altered at 10, 30 or 60 min after the administration of either agent. Synthetic LH-RH (100 ng/rat) increased serum LH levels as measured 10 min after injection, but no effect on either pituitary cyclic AMP or cyclic GMP was seen 1, 5 or 10 min after LH-RH administration. These in vivo results are contradictory to the second messenger hypothesis for either cyclic AMP or cyclic GMP in the mediation of LH-RH-induced LH released and suggest that further work must be done in order to confirm or reject this hypothesis.

The effect of two different types of stress on locus coeruleus alpha-2 receptor binding

The central noradrenergic neurons of the locus coeruleus (LC) are important in regulating an animals response to a stressor. Changes in alpha-2 autoreceptors on LC neurons during stress could alter the functional state of the LC. We investigated the changes in binding of LC alpha-2 receptors immediately following 1 h and 4 h of two stressors: novelty + cold and prone restraint. We measured the specific binding of the alpha-2 receptor antagonist [3H]idazoxan in the LC of rats using in vitro autoradiography. Specific binding was increased after 1 h of novelty + cold exposure but decreased after 4 h, and after both 1 and 4 h of prone restraint. The increase after 1 h of stress may be due to the ability of LC neurons to regulate their own activity because this would produce a dampening of the high LC activity produced by the stressor. However, extended time and/or intensity of a stressor may overwhelm this initial response and produce an agonist-induced decrease in receptor binding.

Prolactin and corticosterone response to repeated footshock stress in male rats

The prolactin (PRL) and corticosterone (CORT) responses to footshock stress were measured in rats after the first, fifth, and 57th exposure to the stress procedure. No reduction in the PRL or CORT responses was seen after repeated application of the footshock stress and the animals showed similar behavioral responses throughout. These results indicate that the hormonal and behavioral responses to footshock stress are not attenuated after an animal has been repeatedly exposed to the stress.

Effect of endogenous LH secretion on ovarian cyclic AMP and cyclic GMP levels in the rat

Abstract Injection of LH (2 and 10 μg) into proestrus rats increased ovarian cyclic AMP levels and concomitantly decreased the levels of cyclic GMP. When injected into diestrus rats, cyclic AMP increases were even greater, whereas cyclic GMP levels were not significantly different from controls receiving saline injections. Ovarian cyclic nucleotide levels were also examined on different days of the cycle. On the afternoon of proestrus (1700 h), the time when circulating levels of LH are at their maximum, the concentration of cyclic AMP showed a moderate but insignificant increase. At the same time, cyclic GMP levels were significantly decreased. An inverse relation between cyclic AMP and cyclic GMP levels was seen on each day of the cycle. When rats were injected with pentobarbital (35 mg/kg) on the afternoon of proestrus (1300 h) to block the LH surge, the expected increases in ovarian cyclic AMP and decreases in cyclic GMP were effectively blocked. These results indicate that ovarian cyclic AMP and cyclic GMP levels are regulated by circulating LH. The apparent differences in direction of nucleotide response to LH, suggest divergent roles for the nucleotides in ovarian function.

Hypothalamic Unit Activity in the Cat: Effects of Estrogen and Vaginal Stimulation

Summary Unit activity in the anterior and posterior hypothalamus was studied during stimulation of the vaginal cervix in anestrus, ovariectomized, and estrogen-treated ovariectomized cats. The results were as follows: (i) cells in the anterior and posterior hypothalamus were influenced by stimulation of the vaginal cervix; (ii) there was a greater tendency for increased firing rates in the anterior as compared to the posterior hypothalamus; (iii) estrogen treatment did not change the responsiveness of cells in the anterior hypothalamus; (iv) estrogen treatment was associated with a general depression of unit activity in the posterior hypothalamus. These findings suggest that the posterior hypothalamus may play a role in the regulation of ovulation and expression of sexual behavior in the cat.

EFFECT OF RESTRAINT STRESS ON PROLACTIN AND CORTICOSTERONE LEVELS IN STREPTOZOTOCIN-INDUCED DIABETIC RATS

Changes in neuroendocrine function have been shown to occur in diabetic animals. The aim of the present study was to examine both the prolactin (PRL) and corticosterone (CORT) responses to a short period of restraint stress after the animals had been made diabetic for six weeks. The streptozotocin - induced diabetic rats had resting CORT levels which were significantly higher than the control animals. Acute restraint significantly increased CORT levels in both the control and diabetic rats. The CORT levels after stress were higher in the diabetic rats. However, the magnitude of the response (percent increase) was less in these animals. The resting PRL levels were not significantly different in the diabetic and control animals. The PRL levels significantly increased in both the control and diabetic rats when they were exposed to the restraint stress. The PRL levels after stress were significantly less in the diabetic rats, indicating a blunted PRL stress response. These results indicate that the diabetic state can affect an animals PRL and CORT response to a new acute stress.