Effects of castration on volumes of the preoptic nucleus and the amygdala and on immunoreactivity of LH-RH fibers in the brain of the toad, Bufo japonicus.

We examined the influence of castration on the volumes of sexually dimorphic nuclei, the amygdala pars medialis (Am) and lateralis (Al), and the anterior part of the preoptic nucleus, and on the immunoreactivity of luteinizing hormone-releasing hormone (LH-RH) in brains of Japanese toads captured in spring and autumn. Animals were castrated (GnX) and half were implanted with testosterone (T) (GnX + T) and then killed and dissected after 1 month. Compared with sham-operated (Sham) toads, plasma androgen levels in autumn toads were significantly decreased by the castration, and those in both spring and autumn GnX toads were significantly elevated by the T implantation. The volume of Am in autumn toads was significantly reduced by GnX. Although not significant, changes in the volumes of the other nuclei, except for Al in spring toads, showed the following tendency 30 days after the operation: GnX + T greater than Sham greater than GnX. GnX did not alter LH-RH immunoreactivity in the median eminence. However, dense immunoreactive LH-RH fibers were found in the mesencephalic tegmental region in spring GnX toads but not in the other operation groups in both spring and autumn. LH-RH immunoreactivity was not altered in autumn toads. In spring GnX toads, thumb pads degenerated and evoked release calling was infrequent. These results suggest that (i) the volumes of sexually dimorphic nuclei, especially Am in the autumn toad, are androgen-dependent, and (ii) castration can modulate activity of the extrahypothalamic LH-RH-ergic projection in toad brain.