Guo Xiaochuan, Lester Robin A J
Department of Neurobiology, McKnight Brain Institute, University of Alabama at Birmingham, 1825 University Boulevard, Birmingham AL 35294-2182, USA.
J Neurophysiol. 2007 Jan;97(1):83-92. doi: 10.1152/jn.01046.2005. Epub 2006 Oct 18.
The fraction of inward current carried by Ca(2+) (FCa(2+)) through nicotinic acetylcholine receptors (nAChRs) on acutely isolated rat medial habenula (MHb) neurons was calculated from experiments that simultaneously monitored agonist-induced membrane currents and intracellular [Ca(2+)], measured with patch-clamp and indo-1 fluorescence, respectively. In physiological concentrations of extracellular Ca(2+) (2 mM) at -50 mV, the percentage of current carried by Ca(2+) was determined to be roughly 3-4%, which is in close agreement with measurements from other heteromeric nicotinic receptors expressed in peripheral tissue. Among factors that may have affected this measurement, such as Ca(2+) influx through voltage-gated Ca(2+) channels, the concentration of intracellular Ca(2+) buffer, and Ca(2+) sequestration and release from intracellular stores, only Ca(2+) uptake by mitochondria was shown to confound the analysis. Furthermore, we find that because of the high density of nAChRs on MHb cells, low concentrations of ACh (10 microM) and its hydrolysis product, choline (1 mM), can significantly elevate intracellular Ca(2+). Moreover, during persistent activation of nAChRs, the level of intracellular Ca(2+) is proportional to its extracellular concentration in the physiological range. Together, these findings support the suggestion that nAChRs may be capable of sensing low concentrations of diffusely released neurotransmitter and, in addition, transfer information about ongoing local synaptic activity by changes in extracellular Ca(2+).
通过分别使用膜片钳和indo-1荧光同时监测激动剂诱导的膜电流和细胞内[Ca(2+)]，计算了急性分离的大鼠内侧缰核（MHb）神经元上通过烟碱型乙酰胆碱受体（nAChRs）的内向电流中Ca(2+)所携带的部分（FCa(2+)）。在-50 mV的细胞外Ca(2+)生理浓度（2 mM）下，Ca(2+)所携带电流的百分比被确定约为3-4%，这与在外周组织中表达的其他异聚烟碱型受体的测量结果非常一致。在可能影响该测量的因素中，如通过电压门控Ca(2+)通道的Ca(2+)内流、细胞内Ca(2+)缓冲剂的浓度以及细胞内储存库的Ca(2+)螯合和释放，只有线粒体对Ca(2+)的摄取被证明会干扰分析。此外，我们发现由于MHb细胞上nAChRs的高密度，低浓度的乙酰胆碱（10 microM）及其水解产物胆碱（1 mM）可显著提高细胞内Ca(2+)。而且，在nAChRs持续激活期间，细胞内Ca(2+)水平在生理范围内与其细胞外浓度成正比。这些发现共同支持了以下观点：nAChRs可能能够感知低浓度的扩散释放神经递质，此外，还能通过细胞外Ca(2+)的变化传递有关正在进行的局部突触活动的信息。
