Oxytocinergic input to the sympathetic preganglionic neurons in the rat. A light and electron microscopic study using immunocytochemical technique.

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A combination of retrograde cell body labeling and immunohistochemistry was employed to elucidate how oxytocinergic fibers make contact with sympathetic preganglionic neurons (SPNs) in the rat spinal cord from T_1 to T_4. SPNs were labeled retrogradely using cholera toxin subunit B (CTb) or horseradish peroxidase conjugated CTb. Oxytocin immunoreactive (ir) fibers were found in the intermediate zone including the sympathetic preganglionic subnuclei. In the central autonomic nucleus and the intercalated nucleus, brown-stained oxytocin-ir varicosities or terminals were frequently observed to stud black-stained dendrites of SPNs. Electron microscopical observations showed that oxytocin-ir terminals form synapses with dendrites or soma of the sympathetic preganglionic neurons. The terminals contained numerous small clear round vesicles and a few large cored vesicles. These results clearly show that a large part of SPNs is innervated by oxytocin containing fibers. The origin of these fibers is discussed, and it is concluded that they are probably descending fibers from the paraventricular nucleus of the hypothalamus.