Bajic D, Proudfit H K, Van Bockstaele E J
Department of Pharmacology, University of Illinois at Chicago, Chicago, Illinois 60612, USA.
J Comp Neurol. 2000 Nov 27;427(4):649-62. doi: 10.1002/1096-9861(20001127)427:4<649::aid-cne11>3.0.co;2-m.
Previous reports using light microscopy have provided anatomical evidence that neurons in the ventrolateral periaqueductal gray (PAG) innervate the medial pericoerulear dendrites of noradrenergic neurons in the nucleus locus coeruleus (LC). The present study used anterograde tracing and electron microscopic analysis to provide more definitive evidence that neurons in the ventrolateral PAG form synapses with the somata or dendrites of noradrenergic LC neurons. Deposits of either biotinylated dextran amine or Phaseolus vulgaris leucoagglutinin into the rat ventrolateral PAG labeled a moderate to high number of axons in the region of the medial pericoerulear region and Barrington's nucleus, but a relatively low number were labeled in the nuclear core of the LC. Ultrastructural analysis of anterogradely labeled terminals at the levels of the rostral (n = 233) and caudal (n = 272) subdivisions of the LC indicated that approximately 20% of these form synapses with tyrosine hydroxylase-immunoreactive dendrites; most of these were located in the medial pericoerulear region. In rostral sections, about 12% of these were symmetric synapses, 9% were asymmetric synapses, and 79% were membrane appositions without clear synaptic specializations. In caudal sections, about 30% were symmetric synapses, 11% were asymmetric synapses, and 59% were appositions. In both rostral and caudal sections, 60% of the anterogradely labeled terminals formed synapses with noncatecholamine dendrites, and 20% formed axoaxonic synapses. These results provide direct evidence for monosynaptic projections from neurons in the ventrolateral PAG to the extranuclear dendrites of noradrenergic LC neurons. This monosynaptic pathway may mediate in part the analgesia, reduced responsiveness to external stimuli, and decreased excitability of somatic motoneurons produced by stimulation of neurons in the ventrolateral PAG.
以往使用光学显微镜的报告提供了解剖学证据，表明腹外侧导水管周围灰质（PAG）中的神经元支配蓝斑核（LC）中去甲肾上腺素能神经元的蓝斑周内侧树突。本研究采用顺行示踪和电子显微镜分析，以提供更确凿的证据，证明腹外侧PAG中的神经元与去甲肾上腺素能LC神经元的胞体或树突形成突触。将生物素化葡聚糖胺或菜豆白细胞凝集素注入大鼠腹外侧PAG后，在内侧蓝斑周区域和巴林顿核区域标记了中等数量至高数量的轴突，但在LC的核芯中标记的数量相对较少。对LC头端（n = 233）和尾端（n = 272）亚区水平的顺行标记终末进行超微结构分析表明，其中约20%与酪氨酸羟化酶免疫反应性树突形成突触；其中大部分位于内侧蓝斑周区域。在头端切片中，约12%为对称突触，9%为不对称突触，79%为无明显突触特化的膜并列。在尾端切片中，约30%为对称突触，11%为不对称突触，59%为并列。在头端和尾端切片中，60%的顺行标记终末与非儿茶酚胺能树突形成突触，20%形成轴-轴突触。这些结果为从腹外侧PAG中的神经元到去甲肾上腺素能LC神经元核外树突的单突触投射提供了直接证据。这条单突触通路可能部分介导了腹外侧PAG神经元刺激所产生的镇痛作用、对外界刺激反应性降低以及躯体运动神经元兴奋性降低。
