Convergence of nociceptive information in the forebrain of female rats: Reproductive organ response variations with stage of estrus

Neurons in the preoptic area (POA) of the hypothalamus and the bed nucleus of stria terminalis (BST) play an important role in the neuroendocrine control of the reproductive cycle, mating behaviors and nociception. Single unit extracellular recordings were performed in the POA and BST region of 20 urethane anesthetized female rats during either the proestrus (elevated levels of estrogen/progesterone) or metestrus (low circulating hormones) stage of the estrous cycle. A total of 118 neurons in the POA and 65 neurons in the BST responded to the search stimuli, bilateral electrical stimulation of the viscerocutaneous branch of the pelvic nerve and/or sensory branch of the pudendal nerve (i.e., dorsal nerve of clitoris). Most of the neurons responding to the electrical search stimuli received a high degree of somatovisceral convergence, including inputs from the abdominal branches of the vagus, cervix, vagina, colon and skin territories on the perineum and trunk. Mean neuronal response thresholds for vaginal and cervical stimulation but not colon distention were significantly higher for animals tested during proestrus. Also, there was a shift in POA and BST neuronal responsiveness towards more inhibition and less excitation during proestrus for a variety of somatovisceral inputs. These data demonstrate that the changes in hormonal status affect the properties of POA and BST neurons, which likely relates not only to the functional importance of these inputs for reproductive behaviors but also for nociceptive processing as well.