Effects of testosterone on the synaptology of the medial preoptic nucleus of male Japanese quail

The medial preoptic nucleus (POM) of male Japanese quail is a sexually dimo rphic testosterone-dependent structure that plays a key role in the activat ion of male sexual behavior. Both the total volume of the nucleus and the s ize of the dorsolateral neurons are decreased in gonadectomized males, Immu nocytochemical studies have revealed a complex pattern of innervation: immu nopositive fibers for several neuropeptides and neurotransmitters have been detected in the POM; some of them (e.g. vasotocin-immunoreactive fibers) a re sexually dimorphic and testosterone-dependent. To understand the anatomi cal bases of these testosterone-dependent neurochemical changes, we perform ed an ultrastructural study of the POM neuropil in intact sexually mature, gonadectomized, or testosterone-treated gonadectomized males. A complex syn aptic organization of the POM neuropil was observed in intact male quail re flecting the heterogeneity of the neurotransmitters and neuropeptides prese nt in this nucleus. Changes in this organization were observed after the en docrine manipulations. The number of axosomatic synapses per cell body decr eased after gonadectomy and was restored to the level observed in the intac t group after the administration of testosterone, By contrast, no significa nt change was observed in the density of axodendritic and axospinal synapse s after hormonal manipulations which suggests that the total number of syna pses in the nucleus should be affected by testosterone (constant density in a changing total volume). The cross-sectional area of synaptic boutons was also decreased by castration and restored to intact level by testosterone, The action of testosterone on the activation of male copulatory behavior i n gonadectomized birds is hence paralleled by an extensive rearrangement of neuropil in the POM, (C) 1999 Elsevier Science Inc.