Electrophysiological studies on the neural networks among estrogen and progesterone effective brain areas on lordosis behavior of the rat.

The present study was performed in an attempt to elucidate the possible neural networks and their functional modification by progesterone among estrogen and progesterone effective brain areas for lordosis behavior of the rat. Single and multiple unit activities were recorded in the medial preoptic area (MPO), dorsomedial thalamic nucleus (MD), interpeduncular nucleus (IP) and ventral part of midbrain reticular formation (MRF), and single or train pulse stimulation was applied to the MPI, MD, IP, MRF, caudate-putamen (CP), habenular nucleus (HB), dorsal hippocampus (HPC), medial and lateral septum (m- and 1-SEPT) and lateral amygdala (1-AMYG). Unit activity was recorded under urethane anesthesia in ovariectomized rats pretreated with estrogen or with estrogen and progesterone. MPO units showed facilitatory responses to stimulation in the MRF, HB and 1-SEPT, and inhibitory responses to stimulation in the MD, HPC, m-SEPT and 1-AMYG in ovariectomized estrogen-primed rats. CP stimulation inhibited the MD unit activity. Stimulation in the IP had no effect on the MPO or MD units, while the IP units were affected by stimulation in the MPO, MD, CP and HB. MRF unit was accelerated by the MD and CP and inhibited by the MPO stimulation. In the ovariectomized, estrogen and progesterone-treated rat, the MPO unit responses to the MRF and MD stimulation, and the MD unit responses to the MRF and CP stimulation were different from those in the rat with only estrogen treatment. In addition, in the ovariectomized estrogen-primed rat, local application of progesterone to the MD prevented the inhibitory effect of the MD on the MPO unit. Progesterone, when given to the stimulated brain area, accelerated the MPO unit which was activated by the MRF stimulation, and inhibited the MD neuron which was inhibited by the CP stimulation. Multiple unit recordings showed similar results. Moreover, multi-unit activity in the HB and IP was elevated following a systemic progesterone application. On the basis of these results, the possible neural networks in the brain which participate in the lordosis control mechanisms are discussed.