Abstract
Lordosis, a typical feminine sexual behavior in rats, is seldom observed in the males even if large doses of estrogen are administered to them. The lateral septum (LS) plays an inhibitory role in lordosis expression and is involved in the sex difference in sexual behavioral patterns. In an attempt to clarify the lordosis-inhibiting neurons in the LS and the projecting site of those neurons, a retrograde tracer (Fluoro-Gold, FG) was injected into the midbrain central gray (MCG) in male rats with or without a transection of the ventral output fibers of the LS. Also, males were treated with estrogen, and then were tests for lordosis. In males without the transection, many FG-labeled neurons were located in the intermediate part of the LS (LSi), and lordotic activity was very low. Males with the transection had few FG-labeled neuron, and showed considerable levels of lordosis. Compared with males, females had greater numbers of FG-labeled neurons in the LSi after FG injection into the MCG. These results suggest that the sexually dimorphic LSi-MCG connection is involved in the inhibition of lordosis and responsible for the sex difference in the regulation of lordosis. The LSi-MCG connection is sexually differentiated during the neonatal period, because neonatal estrogen treatment to females decreased both lordotic activity and the density of the LSi-MCG connection to the level of males. During the postnatal development period, the number of apoptotic cells was greater in the LSi of males than females. Apoptotic cell death may contribute to the organization of the sexually dimorphic LSi-MCG connection. [Jpn J Physiol 54 Suppl:S41 (2004)]
