The Sexual Differentiation of the Rat Hypothalamus: Organizational and Activational Effects of Sex Steroids

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The ventromedial nucleus of the hypothalamus (VMN) is associated with estrogen-dependent neural regulation of autonomic, neuroendocrine and behavioral functions. VMN cells with axons to the midbrain central gray (CG) have been demonstrated as one of the neural substrates participating in these regulatory processes. The aim of the present series of experiments was to identify and characterize estrogen-sensitive VMN neurons with axons projecting to the CG. Antidromic action potentials were evoked in VMN cells following CG stimulation in the female and male rats, which received various endocrine treatments. Estrogen treatment of adults significantly decreased the antidromic activation threshold and shortened the refractory period in female rats or neonatally castrated male rats, but not in males or females exposed neonatally to testosterone. In the female rats which received no neonatal treatment, estrogen-sensitive neurons were localized in the rostral VMN, had large soma with axonal pro … More jections to the CG via the zona incerta or the supraoptic commissure.A similar type of sexual dimorphism in the estrogen sensitivity existed in the hypothalamic paraventricular nucleus (PVN). Extracellular antidromic potentials were recorded from the PVN of the male and female rat following electrical stimulation of the neurohypophysis. Effects of estrogen-treatment in the castrated animals were investigated after classifying the antidromically identified cells into tonically or phasically-firing, or silent groups according to their patterns of spontaneous discharge. Estrogen significantly decreased the antidromic activation threshold and shortened the refractory period as well as the antidromic spike latency in the tonically-firing cells in the female but not those in the male.Hypothalamic tissue slices containing the PVN were obtained from the ovariectomized female rat. The slices were perfused by aerated artificial cerebrospinal fluid. Acute effects of estrogen were examined on the firing rate as well as on the modulatory responses induced by biogenic amines. Among 51 cells tested, addition of estrogen to the perfusate caused immediate activation in 7 and inhibition in 23 cells, respectively. In a group of estrogen-insensitive cells, simultaneous administration of estrogen with noradrenaline or dopamine resulted in augmented responses to these substances. These data indicated potential estrogen action at the neuronal membrane, to control the activity of specific groups of the hypothalamic cells. Less