Joint and separate effects of estrogen and progesterone on responses of midbrain neurons to lordosis-controlling somatic stimuli in the female golden Syrian hamster

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Abstract
Single midbrain neurons were examined for effects of ovarian hormone administration on responsiveness to an array of lordosis-controlling types of somatosensory stimuli in anesthetized, ovariectomized Syrian hamsters. Neuronal responses were recorded under four different hormonal treatment conditions: 1) estradiol benzoate (EB) followed by progesterone (P), 2) EB alone, 3) P alone, or 4) no hormone administration. Only the hamsters receiving both EB and P showed lordosis in a mating test immediately prior to preparation for recording. The joint administration of EB and P strongly facilitated unit responsiveness to lordosis-eliciting (e.g., lumbosacral tactile) forms of stimulation. The incidence of units showing sustained changes in firing of at least +/- 30% in response to these stimuli was highest in animals having received EB and P (69%) and lowest in those given P alone (37%), with the occurrence of responsive units significantly different across the four hormonal conditions. The magnitude of the median unit response to lordosis-trigger stimuli in hamsters given EB and P was significantly higher than unit responses in hamsters receiving either P alone or no hormone. Bilateral shoulders stimulation, a weak stimulus for lordosis elicitation, produced the most responses in units from animals given EB and P (42%) and the fewest in hamsters given EB alone (12%), with a significant difference in responsiveness across the hormonal conditions. The incidence of units responding exclusively to facial stimulation, which is strongly antagonistic to lordosis in behaving animals, was greatest in hamsters injected with P alone (22%) and least for cells from the animals given both EB and P (6%). The difference in unit responsiveness to this stimulus across the four hormone conditions was significant. Analysis of unit-response patterns by the multivariate technique of discriminant analysis revealed that neuronal responses to most stimuli, especially flanks, back, shoulders, and face, were differentially affected by the hormone treatments such that these response patterns could be used to identify the four hormonal conditions from which the neurons were sampled. In addition, the hormonal treatments were found to have influenced the incidence of accelerative or decelerative unit responses to somatic stimuli. Discriminant analysis of unit responses as a function of location of the cells within the midbrain showed that neurons in the tectum and central gray had response patterns highly distinguishable from each other as well as from those of cells in the central and ventromedial tegmental regions.(ABSTRACT TRUNCATED AT 400 WORDS)