Van Bockstaele E J, Cestari D M, Pickel V M
Department of Neurology and Neuroscience, Cornell University Medical College, New York, NY 10021.
Brain Res. 1994 Jun 6;647(2):307-22. doi: 10.1016/0006-8993(94)91330-7.
Microinfusion of serotonin (5-hydroxytryptamine; 5-HT) into the ventral tegmental area enhances the release of dopamine in the nucleus accumbens, a major target of midbrain dopamine neurons. We examined the synaptic basis for 5-HT modulation of neurons in the ventral tegmental area which either (i) project to the nucleus accumbens or (ii) contain the catecholamine synthesizing enzyme tyrosine hydroxylase, a marker of dopamine neurons in this brain region. In the first study, immunoperoxidase labeling of 5-HT in the ventral tegmental area was combined with retrograde transport of gold particles following unilateral injections of the tracer into the nucleus accumbens of adult rats. The gold particles had been previously coupled to wheat germ agglutinin conjugated to inactive horseradish peroxidase. Gold particles were enlarged for visualization using a silver enhancement procedure. By brightfield microscopy, retrogradely labeled neurons contained black punctate granules within their perikarya and proximal processes. The labeled cells were scattered ipsilateral to the injection within the paranigral and parabrachial subdivisions of the ventral tegmental area. Both regions also contained 5-HT immunoreactive varicosities. By electron microscopy, irrespective of the ventral tegmental subdivision, 5-HT labeling was seen primarily in unmyelinated axons and axon terminals. The terminals contained small, clear and large dense core vesicles and ranged from 0.3 micron to 1.4 microns in cross-sectional diameter. 22% (n = 250) of the axon terminals containing 5-HT immunoreactivity formed synaptic contacts with neurons containing the retrograde label. Of these 5-HT terminals, 16% formed asymmetric type contacts and 6% formed symmetric junctions on the retrogradely labeled neurons. The remaining 5-HT terminals were either apposed to (but lacked recognized synapses on) perikarya and large dendrites containing the retrogradely transported protein-gold tracer or contacted unlabeled neurons. In the second set of experiments combining immunoperoxidase of 5-HT and immunogold silver for tyrosine hydroxylase, 32% (n = 250) of the 5-HT-labeled terminals formed synaptic junctions with perikarya or dendrites containing tyrosine hydroxylase immunoreactivity. Of these 5-HT terminals, 23% formed asymmetric type junctions. The remainder were either symmetric or lacked recognized membrane densities. The prominence of asymmetric junctions formed by 5-HT-labeled terminals on neurons projecting to the nucleus accumbens and those containing tyrosine hydroxylase in the ventral tegmental area suggests a cellular basis for serotonergic excitation of mesoaccumbens dopamine neurons. Additionally, the multiplicity of junctions formed by 5-HT terminals on targets with or without retrograde labeling or tyrosine hydroxylase immunoreactivity is consistent with known diverse physiological actions of 5-HT in the tegmental area.
向腹侧被盖区微量注射5-羟色胺（血清素；5-HT）可增强伏隔核中多巴胺的释放，伏隔核是中脑多巴胺能神经元的主要靶区。我们研究了5-HT对腹侧被盖区神经元调节的突触基础，这些神经元要么（i）投射到伏隔核，要么（ii）含有儿茶酚胺合成酶酪氨酸羟化酶，该酶是该脑区多巴胺能神经元的标志物。在第一项研究中，将成年大鼠伏隔核单侧注射示踪剂后，腹侧被盖区5-HT的免疫过氧化物酶标记与金颗粒的逆行运输相结合。金颗粒先前已与缀合有失活辣根过氧化物酶的麦胚凝集素偶联。使用银增强程序放大金颗粒以进行可视化。通过明场显微镜观察，逆行标记的神经元在其胞体和近端突起内含有黑色点状颗粒。标记细胞散在于腹侧被盖区的黑质旁和臂旁亚区内注射同侧。这两个区域也含有5-HT免疫反应性曲张体。通过电子显微镜观察，无论腹侧被盖区的亚区如何，5-HT标记主要见于无髓轴突和轴突终末。这些终末含有小的、清亮的和大的致密核心囊泡，横径范围为0.3微米至1.4微米。22%（n = 250）含有5-HT免疫反应性的轴突终末与含有逆行标记的神经元形成突触联系。在这些5-HT终末中，16%形成不对称型突触，6%在逆行标记的神经元上形成对称型突触。其余的5-HT终末要么与含有逆行转运的蛋白金示踪剂的胞体和大树突相邻（但在其上缺乏公认的突触），要么与未标记的神经元接触。在第二组将5-HT的免疫过氧化物酶与酪氨酸羟化酶的免疫金银染色相结合的实验中，32%（n = 250）的5-HT标记终末与含有酪氨酸羟化酶免疫反应性的胞体或树突形成突触连接。在这些5-HT终末中，23%形成不对称型突触。其余的要么是对称型突触，要么缺乏公认的膜致密物。5-HT标记终末在投射到伏隔核的神经元和腹侧被盖区含有酪氨酸羟化酶的神经元上形成的不对称突触的突出表明了中脑边缘多巴胺能神经元5-羟色胺能兴奋的细胞基础。此外，5-HT终末在有或无逆行标记或酪氨酸羟化酶免疫反应性的靶标上形成的多种突触与5-HT在被盖区已知的多种生理作用一致。
