Effect of norepinephrine synthesis inhibitors and a dopamine agonist on hypothalamic LH-RH serum gonadotrophin and prolactin levels in gonadal steroid treated rats.

Abstract

ABSTRACT The relationship between the medial basal hypothalamus (MBH) LH-RH activity and LH release was studied following progesterone (P) treatment of oestrogen-primed ovariectomized rats (day 0). Following P administration at 10.00 h (day 2) serum LH levels increased rapidly after 13.00 h to peak levels attained at 15.00 h and maintained until 18.00 h. Coincident with the onset of augmented release and peak serum LH concentrations at 15.00 h there was a significant enhancement in the MBH LH-RH activity. Thereafter, the MBH LH-RH stores promptly fell and remained at morning low levels through the rest of the LH surge period. P treatment also stimulated release of FSH and prolactin in the afternoon. Administration of norepinephrine (NE) synthesis inhibitors, diethyldithiocarbamate (DDC) and U-14,624 before P blocked the afternoon increments in serum gonadotrophins and the MBH LH-RH levels; prolactin release was also suppressed in DDC treated rats. In contrast, lergotrile mesylate (dopamine agonist) treatment prior to P administration suppressed only the afternoon increase in prolactin release. These studies show that (1) P can stimulate MBH LH-RH activity in oestrogen-primed rats and these effects are transmitted to the LH-RH peptidergic neurons via NE synapses in the preoptic area and (2) a common central NE system may mediate the stimulatory feedback effects of P on gonadotrophin and prolactin release.