Electrical Stimulation of the Arcuate Nucleus in Ovariectomized Rats Inhibits Episodic Luteinizing Hormone (LH) Release but Excites LH Release After Estrogen Priming12

Abstract

The effect of electrical stimulation of the arcuate nucleus of the hypothalamus (ARH) on blood levels of luteinizing hormone (LH) was studied in ovariectomized (ovx) rats. Unanesthetized, unrestrained rats were bled continuously (30 or 50 .mu.l whole blood/4-6 min) through indwelling right atrial cannulae for 1 1/2-2 h prior to the onset of stimulation. The bleeding was continued for the following 3 h, during which electrical stimulation was applied for two 60 min periods separated by a 60 min non-stimulation period. Whole blood was analyzed for LH by radioimmunoassay. Electrical stimulation of the ARH in ovx rats inhibited the episodic LH release characteristic of these animals. The inhibition was rapid in onset, beginning within minutes, and lasted for most if not all of the stimulation period or longer. Blood LH levels returned to control values prior to the 2nd stimulation period during which the inhibitory response was again seen. Stimulation in other hypothalamic areas outside the ARH was generally ineffective, although repeatable inhibition of episodic LH release did occur in 2 of 3 rats stimulated in the periventricular nucleus. In a 2nd study ovx rats were primed with 5 .mu.g estradiol benzoate/100 g body wt/day for 2 days prior to stimulation. In these rats stimulation of the ARH or median eminence produced increases in LH release, whereas stimulation in other hypothalamic areas outside the ARH was ineffective. The parameters of electrical stimulation that produced increases in LH release were the same as those that caused the inhibition of episodic LH release. Electrical stimulation of the ARH inhibited LH release in ovx rats, but increases LH release if the animals were primed with estrogen.