Hypothalamic metabolism of neurotransmitters (serotonin, norepinephrine, dopamine) and NPY, and gonadal and adrenal activities, during the early postnatal period in the rat.

It is noteworthy that in the rat the early postnatal life is marked by an activation of both the corticostimulating function of the adenohypophysis in neonates of both sexes and of the gonadostimulating function mainly in males. In order to specify if such neuroendocrine variations are temporally correlated with changes in the hypothalamic metabolism of neurotransmitters, the hypothalamic metabolism of serotonin (5 HT), norepinephrine (NE), and dopamine (DA) and the hypothalamic content of neuropeptide Y (NPY) have been investigated in newborn rats of both sexes, delivered at term by cesarean section, as well as changes in the activity of both the hypothalamo-pituitary adrenal axis (HPA) and the hypothalamo-pituitary gonadal axis (HPG). Experimental data suggested that 1) in males a rise in hypothalamic metabolism of 5 HT, NE and DA occurs during the first two hours after delivery, whereas in females, only the metabolism of NE increases. Moreover, the postnatal metabolism of NE was higher in females than in littermate males; 2) NPY content of the hypothalamus, which was at birth significantly higher in males than in females, dropped in the former but not in the latter; 3) in newborn males, an early surge of plasma testosterone occurs, suggesting postnatal activation of the HPG axis; on the other hand, in females, a late and slight increase in plasma estradiol is observed; 4) in early postnatal life, a sex-independent rise in plasma ACTH and adrenal and plasma corticosterone levels suggest a comparable activation of the HPA axis in newborns of both sexes. In conclusion, the early postnatal activation of the corticostimulating function in neonates of both sexes and that of the gonadostimulating function, mainly in males, could be temporally correlated with a rise in the hypothalamic metabolism of two neurotransmitters, 5 HT and NE, and of NPY content. According to our data, a sex-dependent metabolism of neurotransmitters in the hypothalamus is already apparent in early postnatal life.