Ayre E A, Pang S F
Department of Physiology, University of Hong Kong.
Biol Signals. 1994 Mar-Apr;3(2):71-84. doi: 10.1159/000109528.
Through the synthesis and secretion of the hormone, melatonin, the pineal has been assigned the role of synchronizing a reproductive response to appropriate environmental conditions. Theoretical melatonin target sites may occur at several levels of the hypothalamic-pituitary-gonadal hierarchy, including a direct action on the gonads. The availability of a biologically active radioligand, 2-[125I]iodomelatonin, has provided the opportunity to examine the possible direct melatonin action on the gonads. 2-[125I]Iodomelatonin binding sites were identified in the testes and ovaries of chickens, ducks and quail but were not measurable in mammalian gonads, with the exception of tree shrew testes. The avian gonadal 2-[125I]iodomelatonin binding sites were stable, saturable, reversible, specific and of high affinity. 2-[125I]Iodomelatonin appeared to label a single class of binding sites as evidenced by the linearity of Rosenthal analysis of the specific binding data, the Hill coefficients close to unity and the monophasic competition curves. The high affinity on the gonadal 2-[125I]iodomelatonin binding sites, characterized by apparent equilibrium dissociation constants in the low picomolar range, was in accordance with circulating levels of melatonin suggesting that they may be physiologically relevant. Autoradiography indicated that these 2-[125I]iodomelatonin binding sites were widely distributed throughout the testes but localized in ovarian follicles in the birds studied. Specific inhibition of testicular 2-[125I]iodomelatonin binding by a guanine nucleotide analog has provided evidence that the 2-[125I]iodomelatonin binding sites in chicken testes may be coupled to a guanine nucleotide binding protein-effector system, thus promoting the idea that testicular 2-[125I]iodomelatonin binding sites may mediate a cascade of intracellular events. Although no circadian rhythm in the density or affinity of 2-[125I]iodomelatonin binding in chicken ovaries was found, there was a decrease in 2-[125I]iodomelatonin binding affinity at middark in chicken testes with no change in the number of testicular 2-[125I]iodomelatonin binding sites. The present evidence is in line with the hypothesis of a direct melatonin action on the gonads and further investigations on the above problem will be rewarding.
通过激素褪黑素的合成与分泌，松果体被赋予了使生殖反应与适宜环境条件同步的作用。理论上，褪黑素的靶位点可能存在于下丘脑 - 垂体 - 性腺轴系的多个水平，包括对性腺的直接作用。具有生物活性的放射性配体2 - [125I]碘褪黑素的可得性，为研究褪黑素对性腺可能的直接作用提供了机会。在鸡、鸭和鹌鹑的睾丸及卵巢中鉴定出了2 - [125I]碘褪黑素结合位点，但除树鼩睾丸外，在哺乳动物性腺中无法检测到。禽类性腺中的2 - [125I]碘褪黑素结合位点具有稳定性、饱和性、可逆性、特异性且亲和力高。2 - [125I]碘褪黑素似乎标记了一类单一的结合位点，这由特异性结合数据的罗森塔尔分析的线性、接近1的希尔系数以及单相竞争曲线所证明。性腺中2 - [125I]碘褪黑素结合位点的高亲和力，其表观平衡解离常数处于低皮摩尔范围，与褪黑素的循环水平相符，表明它们可能具有生理相关性。放射自显影显示，这些2 - [125I]碘褪黑素结合位点在整个睾丸中广泛分布，但在所研究的鸟类卵巢卵泡中定位。鸟嘌呤核苷酸类似物对睾丸中2 - [125I]碘褪黑素结合的特异性抑制提供了证据，表明鸡睾丸中的2 - [125I]碘褪黑素结合位点可能与鸟嘌呤核苷酸结合蛋白 - 效应器系统偶联，从而支持了睾丸中2 - [125I]碘褪黑素结合位点可能介导一系列细胞内事件的观点。虽然在鸡卵巢中未发现2 - [125I]碘褪黑素结合密度或亲和力的昼夜节律，但在鸡睾丸中，2 - [125I]碘褪黑素结合亲和力在午夜时降低，而睾丸中2 - [125I]碘褪黑素结合位点的数量没有变化。目前的证据符合褪黑素对性腺有直接作用的假说，对上述问题的进一步研究将是有意义的。
