Molecular basis of synaptic plasticity

雄性大鼠球海绵体脊束核(SNB)的运动神经元支配附着在阴茎上的会阴横纹肌、球海绵体肌和提肛肌。它们在交配行为中起着重要作用，而交配行为对循环中雄激素水平的变化很敏感。雄激素在雄激素敏感的SNB运动神经元的突触连接重组中也起着至关重要的作用。为阐明雄激素对突触可塑性的分子机制，用免疫组织化学方法研究了雄激素对成年雄性大鼠黑质运动神经元突触轴突终末谷氨酸、γ-氨基丁酸(GABA)、酪氨酸羟化酶、脑啡肽和P物质等神经递质或神经调节剂表达的影响。发现酪氨酸羟化酶、脑啡肽或P物质免疫反应轴突终末与正常雄性大鼠SNB运动神经元发生突触接触，但这些终末的发生率很低。相反，检测到一些与SNB运动神经元发生突触接触的GABA免疫反应轴突终末。去势去掉雄激素可显著减少GABA免疫反应轴突终末的数量，而睾丸激素治疗可阻止这种改变。这些结果表明，雄激素调节GABA免疫反应轴突终末与SNB运动神经元的突触发生，并可能为激素诱导SNB运动神经元突触可塑性的分子机制提供证据。

Motoneurons of the spinal nucleus of the bulbocavernosus (SNB) of male rats innervate the perineal striated muscles bulbocavernosus and levator ani that attach to the penis. They have an important role in copulatory behavior, which is sensitive to alterations in circulating levels of androgen. Androgen is also known to play a crucial role in reorganizing synaptic connections in androgen-sensitive SNB motoneurons. To clarify molecular mechanisms of androgen for synaptic plasticity, we examined androgenic regulation of the expression of several kinds of neurotransmitters or neuromodulators such as glutamic acid, gamma-aminobutyric acid (GABA), tyrosine hydroxylase, enkephalin and substance P in the axon terminals making synaptic contact with SNB motoneurons of adult male rats by immunohistochemical analysis. Tyrosine hydroxylase-, enkephalin- or substance P-immunoreactive axon terminals were found to make synaptic contact with SNB motoneurons of intact male rats although the incidence of these terminals was very low. In contrast, a number of GABA-immunoreactive axon terminals making synaptic contact with SNB motoneurons were detected. Removal of androgen by castration significantly reduced the number of GABA-immunoreactive axon terminals, whereas the change was prevented by testosterone treatment. These results suggest that androgen regulates the incidence of GABA-immunoreactive axon terminals that synapse onto SNB motoneurons and may provide evidence for the molecular mechanisms of hormonally-induced synaptic plasticity in the SNB motoneuron.