S. Taleisnik

Role of Catecholamines in the Control of Melanocyte-Stimulating Hormone Secretion in Rats

Suppression of the adrenergic system by injection of reserpine (0.2 mg/kg for 2 days) into male rats resulted in a decrease of the pituitary MSH content. The same effect was observed 30, 60, or 120 mi

Release of Melanocyte-Stimulating Hormone by Several Stimuli through the Activation of a 5-Hydroxytryptamine-Mediated Inhibitory Neuronal Mechanism

The acute depletion of the melanocyte-stimulating hormone (MSH) concentration in the pituitary gland of rats was used as the end point for evaluating the effect of different stimuli on MSH secretion.

Topographic Distribution of the Melanocyte-Stimulating Hormone-Releasing Factor in Rat Hypothalamus.

Summary The topographic distribution of MSH-RF in the hypothalamus was studied in the rat. MSH-RF was determined by measuring the capacity of hypothalamic extracts to deplete pituitary MSH concentration in recipient animals. The activity of extracts prepared from sections of hypothalamus taken in 3 different planes showed the highest concentration of the releasing factor in the area of the paraventricular nucleus. The median eminence was the second most important active zone. Extracts from neural lobe of the hypophysis were also active though to a lesser degree than those from hypothalamus and their effect did not depend on either oxy-tocin or vasopressin. The possibility that MSH-RF could be produced by neurons having their soma in the paraventricular nucleus and axons ending in the median eminence and neural lobe of the hypophysis is discussed.

Mechanisms that Determine the Changes in Pituitary MSH Activity During Pseudopregnancy Induced by Vaginal Stimulation in the Rat

Pituitary MSH content as measured by an in vitro bioassay was found to decrease 1 h after vaginal stimulation in rats at estrus. The values remained low up to the 6th day, but regained the pre-stimulation levels on the 10th day. The early decrease in pituitary MSH was not prevented if the animals were ovariectomized 2 h before the vaginal stimulation, but failed to occur in chronic ovariectomized rats MSH-release-inhibiting factor (MSH-R-IF) was found to increase in stalk median eminence (SME) extracts 2 h after vaginal stimulation in rats in the first day of diestrus, suggesting that the induction of pseudopregnancy (PSP) blocks the release of this agent. Animals ovariectomized 2 h after vaginal stimulation showed normal values of pituitary MSH two or six days later, indicating that the ovaries are necessary for maintaining the low content of the hormone during PSP. Administration of 2 mg progesterone daily to the spayed rats restored normal values. MSH-R-IF was absent in SME extracts from rats at day six of PSP or in ovariectomized animals treated with progesterone. It is concluded that there exist two stages in PSP induced by vaginal stimulation. During the first one (neurogenic phase), the stimulus promotes a low pituitary MSH content, probably by blocking the release of MSH-R-IF. During the second stage (hormone-neural phase), progesterone secreted by the ovaries assures continuance of the phenomenon, probably by blocking MSH-R-IF synthesis.

Estrogen Influence on the Hypothalamic Enzymes Involved in the Formation of Melanocyte-Stimulating Hormone Release-Inhibiting Factor (MSH-R-IF)

Summary The activity of the enzyme which yielded MSH-R-IF upon incubation of hypothalamic extracts with oxytocin was found to be estrogen sensitive. It was absent in ovariectomized rats and increased after estrogen treatment. A dose of 10 μg EB induced a 10-fold increase in the hypothalamic activity of the enzyme. Cycloheximide and actinomycin D inhibited the effect of estrogen suggesting that estrogen promotes the formation of new enzyme.