Formation of A Melanocyte-Stimulating Hormone-Release Inhibiting Factor By Hypothalamic Extracts from Rats

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Abstract
Median eminence extracts from male rats induced a drop in pituitary melanocyte-stimulating hormone (MSH) when injected into recipient animals because they evoked the release of MSH. This effect was lost if the extracts were incubated for 2.5 h. Incubation was shown to produce a factor (IF) which inhibited the release of MSH. Two agents are responsible for this reaction, one which is thermolabile and non-diffusible and the other heat-stable and diffusible. The non-diffusible fraction is associated with microsomes. Both agents are present in hypothalamic extracts but not in cerebral cortex extracts. Oxytocin and lysine-vasopressin can act as the heat-stable agent and form IF when incubated with the non-diffusible fraction. This system has a pH optimum between 7 and 8 and the amount of IF formed increases both with time and with the amount of non-diffusible fraction. It is suggested that IF is formed by an enzyme present in hypothalamic neurosecretory neurones from a precursor molecule.