Modulation of the proestrous surge of luteinizing hormone by electrochemical stimulation of the amygdala and hippocampus in the unanesthetized rat.

The roles played by the amygdala and hippocampus in controlling the release of pituitary luteinizing hormone (LH) were studied in the freely moving rat. Monopolar stainless steel electrodes were implanted into the corticomedial (CM) amygdala, basolateral (BL) amygdala and dorsal hippocampus of female rats. When the animal had recovered from surgery and shown two consecutive 4-day estrous cycles, a chronic atrial cannula was introduced during the afternoon of diestrus II. On the following day (proestrus) electrochemical stimulation (ECS) was applied (20--50 micronA anodal DC 120 sec) bilaterally to the amygdala or hippocampus and blood samples were taken every 90 min from 12.00 to 21.00 h for radioimmunoassay (RIA) of LH. Next day, uterine tubes were examined for ova as evidence of ovulation. ECS of the amygdala exerted two divergent influences on LH release. Stimulation of the BL amygdala at 13.45 h, just before the critical period (14.00--16.00 h), was effective in delaying and reducing the LH surge, whereas ECS of the CM amygdala at 12.00 h resulted in an early synchronization in the timing of the LH curves. All of the rats in both groups ovulated, in contrast to the results of applying ECS to the dorsal hippocampus; there the LH surge and ovulation were completely blocked in 7 out of 9 rats. Thus, in the freely moving rat, the hippocampus can exert a potent inhibitory influence on LH release whereas the amygdala plays a modulatory role in the process.