Temporal Patterns of LH Release Following Graded Preoptic Electrochemical Stimulation in Proestrous Rats1

Abstract

The relationship between the extent of preoptic area (POA) tissue stimulated electrochemically and the temporal patterns of LH release was studied in proestrous rats. In all animals, sequential carotid blood samples were taken and LH determined by radioimmunoassay. In unanesthetized proestrous rats peak plasma LH concentrations exceeded 400 ng/ml whereas baseline values of only 5–10 ng/ml were obtained in Nembutal—blocked animals. In Nembutalized unilaterally POA stimulated rats, threshold current for complete ovulation was 20 μA/60 sec (lesion 300–450 μ diameter) which resulted in peaks of 44 ng/ml. Currents of 40 and 60,μA/60 sec (lesion 400–650, 650–800 μ) significantly increased LH release to peak plasma values of 70 and 110 ng/ml. Increasing the current to 100 μA/ 60 sec unilaterally resulted in a significant increase in the LH peak (164 ng/ml); the same current delivered bilaterally produced a plasma LH peak twice that of the unilateral group (326 ng/ml) and was comparable to the LH release in normal, untreated proestrous rats. In all groups LH did not deviate significantly from baseline until 30 min after stimulation, peaks appeared at 80-120 min and concentrations approached baseline by 200 min. In order to study POA influences on hypothalamic structures relative to the 30–min delay in plasma LH elevation, the POA of other rats were stimulated (60 μA/60 sec) and then surgically separated from the hypothalamus. Cuts placed in the anterior hypothalamic area 5 but not 15 min after stimulation blocked ovulation. Our results support the hypothesis of a quantitative relationship between the amount of POA activated and the amount of LH released by the adenohypophysis. Further, POA influences on hypothalamic structures need be exerted only up to 15 min to induce ovulation. (Endocrinology92: 755, 1973)