Comparative effectiveness of preoptic and tuberal stimulation for luteinizing hormone release and ovulation in two strains of rats.

Proestrous rats under pentobarbital anesthesia during the 'critical period' were electrically stimulated in either the medial preoptic area (mPOA) or the anterior basal tuber. In Osborne-Mendel (O-M) and Charles River CD rats, comparisons were made of the ovulatory responses to trains of matched biphasic pulse pairs of different microamperage, 30 sec on and 30 sec off for 45 min. In CD rats, comparisons were made of the relative efficiencies of stimulation through a coaxial platinum electrode near the midline and a pair of platinum electrodes spaced bilaterally 2 mm apart across either the mPOA or the tuber. Other comparisons, with either type of electrode in the basal tuber, were made between 4-day and 5-day cyclic CD rats with respect both to the ovulatory responses and to the serum concentrations of LH 90 min after the start of the 45-min stimulus. In both O-M and CD rats, tuberal stimulation was more effective for ovulation than mPOA stimulation. CD rats were much less responsive to mPOA stimulation (spaced electrodes) than O-M rats, but the responses of both strains to tuberal stimulation were essentially alike. In 4-day cyclic CD rats, the ovulatory responses to mPOA stimulation with the coaxial electrode were much superior to those from spaced-electrode stimulation. In contrast, tuberal stimulation with either type of electrode gave equivalent ovulation frequencies and equivalent tubal ovum numbers in both 4-day and 5-day cyclic rats. However, the serum LH concentrations disclosed somewhat better responses to stimulation with the coaxial electrode (coaxial/spaced=1.57). The advantage of stimulation with the coaxial electrode may be due to high current density or, at least in part, to its more medial location. The results are compatible with the concept of preoptic-tuberal neuronal system, diffuse rostrally and convergent upon the medial basal tuber, controlling the ovulatory surge of LH.