Antagonistic effect on melanocyte-stimulating hormone release of two neural tissue extracts

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Abstract
Rat hypothalamic extracts injected intravenously into rats or toads induced a depletion of pituitary melanocyte-stimulating hormone (MSH) activity. No effect was observed by injecting toad hypothalamic extracts, which, on the contrary, prevented the decrease in pituitary MSH iduced by rat hypothalamic extracts when both materials were injected together. This effect was not obtained with neural tissue extract, nor with synthetic arginine-vasopressin, arginine-vasotocin, or oxytocin. The inhibitory activity of toad hypothalamic extracts was linearly related to the logarithm of the dose. The active agent was thermostable and was inactivated when incubated with trypsin. Toad hypothalamic extracts also inhibited postnursing fall in pituitary MSH activity. Since toad hypothalamic extracts did not affect the release of ACTH nor of LH it is inferred that they specifically inhibited the release of MSH from the pituitary. Toad neurol lobe extracts evoked a decrease of pituitary MSH activity, and this effect was also inhibited by toad hypothalamic extracts. It is concluded that in toads a dual neurohumoral factor controls the release of pituitary MSH.

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