Effect of electrical stimulation of the dorsomedial hypothalamic nucleus on pulsatile LH release in ovariectomized rats.

The present experiments were carried out to examine the influence of electrical stimulation of the dorsomedial hypothalamic nucleus (DMH) on pulsatile LH release in ovariectomized rats, and the possible involvement of serotonin (5HT) in mediating any observed effects. Unanesthetized animals were bled continuously through jugular vein cannulae for an initial 1 1/2 h control period. Rats were then stimulated for two 1-hour periods, separated by a 45-min nonstimulation period. DMH stimulation suppressed pulsatile LH release and decreased mean blood LH levels during both stimulation periods in control rats as well as animals in which 5HT synthesis was inhibited by p-chlorophenylalanine or 5HT receptors were blocked by metergoline. The decrease in mean blood LH levels in all groups was due solely to an increase in the LH interpulse interval. LH pulses that did occur during the stimulation-induced suppression did not show a decreased LH pulse amplitude. Rather, in control and metergoline-treated animals this parameter increased during stimulation, suggesting a buildup of readily releasable LHRH and/or pituitary LH. Lastly, estradiol benzoate suppressed pulsatile LH secretion but did not reverse the inhibitory LH response to DMH stimulation. These experiments indicate that the DMH, because of neurons originating within this nucleus and/or fibers passing through, may be a brain region that influences pulsatile LH release in a suppressive manner. This effect is exerted solely on the periodicity of the process and not through a 5HT-mediated mechanism.