OBJECTIVE: The physiologic roles of nonphasic firing oxytocin neurons in the paraventricular nucleus of female rats throughout pregnancy and parturition was studied from the viewpoint of changes in the membrane activities. STUDY DESIGN: Thin colonal hypothalamic slices containing the paraventricular nucleus were prepared. Immunohistochemical studies of oxytocin in the paraventricular nucleus were performed to determine the localization of oxytocin-containing neurons. Extracellular single neural activities were recorded with a glass micropipette from the oxytocin-dense area, and physiologic concentrations of oxytocin were applied. RESULTS: Nonphasic firing activity was recorded from 61 neurons in virgin female, pregnant, delivering and lactating rats. After bath application of < 5.0 x 10(-11) mol/L of oxytocin, firing rates of 12 of 17 paraventricular neurons in virgin rats decreased and only one increased, whereas the remaining four showed no response to oxytocin. In pregnancy at day 15 all four neurons decreased in activity, whereas at day 21 rats 20 of 26 neurons decreased, two increased, and four showed no response. On the contrary, in delivering rats seven of eight neurons exhibited excitatory responses, and only one showed no response. This excitation reversed to inhibition again after the lactating period ended. CONCLUSION: Negative feedback by oxytocin in virgin and pregnant rats might reverse to positive feedback in delivering and lactating animals as a result of changes in hormonal conditions. This reverse might be closely related to the initiation of delivery.
OBJECTIVE: The physiologic roles of nonphasic firing oxytocin neurons in the paraventricular nucleus of female rats throughout pregnancy and parturition was studied from the viewpoint of changes in the membrane activities. STUDY DESIGN: Thin colonal hypothalamic slices containing the paraventricular nucleus were prepared. Immunohistochemical studies of oxytocin in the paraventricular nucleus were performed to determine the localization of oxytocin-containing neurons. Extracellular single neural activities were recorded with a glass micropipette from the oxytocin-dense area, and physiologic concentrations of oxytocin were applied. RESULTS: Nonphasic firing activity was recorded from 61 neurons in virgin female, pregnant, delivering and lactating rats. After bath application of < 5.0 x 10(-11) mol/L of oxytocin, firing rates of 12 of 17 paraventricular neurons in virgin rats decreased and only one increased, whereas the remaining four showed no response to oxytocin. In pregnancy at day 15 all four neurons decreased in activity, whereas at day 21 rats 20 of 26 neurons decreased, two increased, and four showed no response. On the contrary, in delivering rats seven of eight neurons exhibited excitatory responses, and only one showed no response. This excitation reversed to inhibition again after the lactating period ended. CONCLUSION: Negative feedback by oxytocin in virgin and pregnant rats might reverse to positive feedback in delivering and lactating animals as a result of changes in hormonal conditions. This reverse might be closely related to the initiation of delivery.