Alterations in unit activity of hypothalamic ventromedial nuclei by stimuli which affect gonadotropic hormone secretion

Abstract

The preoptic area (POA) appears to be closely related to the ventromedial-arcuate nuclei (VMN) of the hypothalamus in the control of gonadotropin secretion. The present work attempts to analyze the alterations produced in VMN unit activity after electrical excitation of the POA or after peripheral stimulation by pain, cold, and vaginal cervix probing. Cortical EEG and VMN unit activity were recorded simultaneously in all experiments. Stimulation of POA increased the firing rate of 44% of VMN cells while 44% were inhibited and 12% remained unresponsive. Cold and pain predominantly accelerated whereas vaginal cervix probing predominantly inhibited VMN units. In contrast, these peripheral stimuli primarily accelerated thalamic neuron firing. Also, the discharge patterns of VMN cells were frequently altered by electrical POA or peripheral sensory stimuli. Cortical EEG activity was positively correlated with about 50% of VMN unit activity; in the remainder, cell firing rates were either independent of or opposite to EEG changes. While this study does not offer specific proof that the response of VMN cells to electrical stimulation of vaginal cervix probing is directly related to gonadotropin secretion, the data suggest this conclusion as one possibility. In addition, the high degree of independence of VMN unit activity with respect to EEG changes after electrical stimulation or vaginal cervix probing suggests a certain degree of specificity of such VMN units regardless of more general CNS responses.