Hormonal and chemical modulation of ventromedial hypothalamic neurons responsive to vaginocervical stimulation

Neurons of the ventromedial hypothalamus (VMH) in female rats were electrophysiologically recorded via multibarrelled glass micropipettes and tested for responsiveness to vaginocervical stimulation as well as to a variety of other peripheral stimuli. A small percentage of VMH neurons were found to be specifically responsive to vaginocervical stimulation (type I response) in the ovariectomized animal. Priming the rats with estrogen (E) and progesterone (P) significantly increased the percentage of neurons responding specifically to vaginocervical stimulation but had no effect on the percentage of nonspecifically responding (type II) and nonresponding (type III) neurons. Pharmacological testing of all three types of VMH neurons was accomplished by iontophoretic application of luteinizing hormone releasing hormone (LHRH), prolactin (PRL), and dopamine (DA) to the cell membrane. PRL excited the majority of neurons tested and DA inhibited the majority of tested units regardless of the hormonal condition of the animal or the response elicited by vaginocervical and peripheral stimulation, LHRH, however, produced changes in firing rate which were related to the type of response evoked by vaginocervical stimulation. In E-P-primed animals, neurons which were specifically or nonspecifically affected by vaginocervical stimulation responded to LHRH in a similar manner whereas a different LHRH response profile was obtained in those neurons which were not effected by vaginocervical stimulation. The results indicate that an afferent pathway from the vaginocervix to the VMH exists, that E-P priming in some manner increases the probability that a VMH neuron will respond specifically to vaginocervical probing, and that the effect of LHRH is different in neurons responding to vaginocervical stimulation than in nonresponding neurons.