Progesterone-Induced Changes in Hypothalamic Luteinizing Hormone-Releasing Hormone and Catecholamines: Differential Effects of Pentobarbital*

Abstract

The effects of pentobarbital (Pnt) treatment on the progesterone (P)-induced afternoon increase in the medial basal hypothalamic (MBH) LHRH and serum LH and FSH levels in ovariectomized estradiol benzoate-primed rats were studied. Pnt injection before P blocked the afternoon rise in serum gonadotropins but failed to alter the increase in the MBH LHRH levels. Moreover, when Pnt¹ was injected 150 min after P, the MBH LHRH content continued to rise to levels 25–37% above those seen in control rats. Analyses of LHRH concentrations in discrete hypothalamic nuclei revealed that the Pntinduced accumulation was confined mainly to the median eminence, with a small increase in the suprachiasmatic nuclei region. P administration increased the MBH norepinephrine activity and concurrently decreased dopamine activity. Pnt was ineffective in suppressing the MBH LHRH response in these rats, but drastically reduced norepinephrine and accelerated dopamine turnovers in the MBH. These studies show 1) no definve cause and effect relationship of the increments in MBH LHRH either with LH release (or LHRH release) or with changes in hypothalamic catecholamines induced by P treatment, and 2) that the striking rise in the MBH LHRH levels in estradiol benzoateprimed rats may represent formation of new immunoreactive LHRH predominantly in the median eminence region.