Characterization of projections from a sexually dimorphic motor nucleus in the spinal cord of adult green anoles.

Male green anoles possess two copulatory organs (hemipenes), which are independently controlled by bilateral muscles: the transversus penis (TPN) and retractor penis magnus (RPM). Adult females do not possess hemipenes or either of the two related muscles. Motoneurons projecting to the TPN lie in spinal segments trunk 17 and sacral 1 (T17-S1). Overall, motoneurons in this region are larger and more numerous in males than females. The present studies were designed to determine 1) whether motoneurons projecting to the RPM are located in the same sexually dimorphic nucleus, 2) other targets of T17-S1 motoneurons, and 3) the approximate proportion of motoneurons projecting to each muscle. In Study 1, unilateral injection of the retrograde tracer Fast Blue (FB) into RPMs and simultaneous unilateral injection of either Cholera Toxin-fluorescein (CT-FITC) or Diamidino Yellow into TPNs revealed that RPM and TPN motoneurons are indeed interdigitated in T17-S1. In Study 2, FB was used to characterize other targets of this nucleus in both males and females. In adult males, projections to four muscles accounted for 96% of the T17-S1 motoneurons: the TPN, RPM, caudifemoralis (CF), and cloacal sphincter (SC). In adult females, projections to the CF and SC comprised 70% of this nucleus. These data demonstrate that the T17-S1 nucleus is a mixed spinal nucleus that has projections to muscles present in both sexes, as well as those present only in males and specialized for male copulatory behavior.