Stuart A E, Morgan J R, Mekeel H E, Kempter E, Callaway J C
Department of Physiology, University of North Carolina at Chapel Hill 27599-7545, USA.
J Neurosci. 1996 May 15;16(10):3178-88. doi: 10.1523/JNEUROSCI.16-10-03178.1996.
The synapses made by many arthropod photoreceptors are disinhibitory and use histamine as their transmitter. Because decreases and not increases in the cleft concentration of transmitter constitute the important event at these synapses, a transporter to clear the cleft of histamine would seem particularly crucial to signal transfer. We report here that 3H-histamine is taken up selectively into barnacle photoreceptors by a Na+-dependent mechanism, presumably a transporter. Using light microscopic autoradiography, we observe heavy label over axons and presynaptic terminals of these neurons when they are stimulated during uptake. The radioactivity taken up was identified as 3H-histamine by thin layer chromatography; no metabolites were detected, even after 5 hr. Radiolabeled 5-hydroxytryptamine and GABA are not taken up by the photoreceptor. 3H-histamine uptake into photoreceptors is decreased markedly by an excess of unlabeled histamine and by chlorpromazine and phenoxybenzamine. Unexpectedly for uptake dependent on the NA+ gradient, photoreceptor terminals label more intensely in the light (when depolarized) than in the dark (when hyperpolarized). Glia label more strongly than photoreceptors in dark-incubated preparations. The presence of presynaptic uptake strengthens the evidence that histamine is the neurotransmitter of arthropod photoreceptors and provides a mechanism by which this synapse could recycle transmitter, control its steady-state cleft concentration, and clear it from the cleft in response to decreases in its release from the photoreceptors.
许多节肢动物光感受器形成的突触具有去抑制作用，并以组胺作为神经递质。由于在这些突触处，递质在突触间隙浓度的降低而非升高才是重要事件，因此一种清除突触间隙组胺的转运体对于信号传递似乎尤为关键。我们在此报告，³H-组胺通过一种依赖钠离子的机制被选择性地摄取到藤壶光感受器中，推测这是一种转运体。利用光学显微镜放射自显影技术，我们观察到在摄取过程中受到刺激时，这些神经元的轴突和突触前终末会出现大量标记。通过薄层色谱法鉴定，摄取的放射性物质为³H-组胺；即使在5小时后也未检测到代谢产物。放射性标记的5-羟色胺和γ-氨基丁酸不会被光感受器摄取。过量的未标记组胺、氯丙嗪和酚苄明会显著降低³H-组胺进入光感受器的摄取量。对于依赖钠离子梯度的摄取而言，出人意料的是，光感受器终末在光照下（去极化时）比在黑暗中（超极化时）标记更强。在黑暗孵育的标本中，神经胶质细胞的标记比光感受器更强。突触前摄取的存在强化了组胺是节肢动物光感受器神经递质的证据，并提供了一种机制，通过该机制这种突触可以回收递质、控制其稳态突触间隙浓度，并在其从光感受器释放减少时将其从突触间隙清除。
