EFFECT OF NOREPINEPHRINE SYNTHESIS INHIBITORS AND A DOPAMINE AGONIST ON HYPOTHALAMIC LH-RH, SERUM GONADOTROPHIN AND PROLACTIN LEVELS IN GONADAL STEROID TREATED RATS

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Abstract
The relationship between the medial basal hypothalamus (MBH) LH-RH [luliberin] activity and LH [lutropin] release was studied following progesterone (P) treatment of estrogen-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 [follitropin] and prolactin in the afternoon. Administration of norepinephrine (NE) synthesis inhibitors, diethyldithiocarbamate (DDC) and U-14,624 [1-phenyl-3-(2-thiazolyl)-2-Thiourea] before P blocked the afternoon increments in serum gonadotropins and the MBH LH-RH levels; prolactin release was also suppressed in DDC treated rats. Lergotrile mesylate (dopamine agonist) treatment prior to P administration suppressed only the afternoon increase in prolactin release. Apparently P can stimulate MGH LH-RH activity in estrogen-primed rats and these effects are transmitted to the LH-RH peptidergic neurons via NE synapses in the preoptic area and a common central NE system may mediate the stimulatory feedback of P on gonadotropin and prolactin release.

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