[Melanotropic activity in the hypophysis and blood of Wistar rats in early postnatal development]

The activity of melanotropins in pituitary homogenates and blood of Wistar rats obtained from the nursery Stolbovaia (Russian Academy of Medical Sciences) was studied using the method of biological testing (Hogben, Slome, 1931; Golichenkov, 1980). In order to elucidate, to what extent the normal status of melanotropic activity is retained in albino rats, two experimental series were performed. Melanotropic activity was determined in (1) intact rats and (2) in rats receiving subcutaneous injections of parachlorophenylalanine, a specific inhibitor of serotonin synthesis. Melanotropic activity of Wistar rats hypophysis during the studied period of development was similar to that of normal pigmented rats. However, the peak of melanotropic activity of blood which is characteristically observed in pigmented rats on day 3 of postnatal development is absent in the case of Wistar rats. The experimentally induced decrease in the serotonin level did not result in the increased melanotropic activity of blood. These data suggest that certain structures of hypophysis responsible for MSH secretion can be disturbed in Wistar rats. A short-term peak of blood MSH in rats during the first week after birth is known to provide for maturation of dopaminergic neurons located in the arcuate nucleus of the hypothalamus. Consequently, the absence of such peak in Wistar rats should result in neuroendocrine disturbances such as inadequate functioning of dopaminergic neurons responsible for melanotropin secretion by hypophysis beginning from the second week after birth (Lichtensteiger, Schlumpf, 1986).