The colocalization of immunoreactivity to nitric oxide synthase (NOS), vasoactive intestinal polypeptide (VIP) and tyrosine hydroxylase (TH) in the superior cervical ganglion (SCG) was investigated in the quail. In this bird, a substantial amount of NOS-immunoreactive (IR) cells were consistently found in the SCG without colchicine treatment or nerve ligation. The finding worthy of pointing out was that three-fourths of these NOS-IR cells were positive for TH. VIP-IR cells appeared with markedly low frequency than NOS-IR cells. They were generally small in size and often located in the ganglion peripheral. There were no VIP-IR cells positive for TH or negative for NOS: VIP immunoreactivity always appears in NOS-IR cells negative for TH. Thus, the results of the present study clearly showed the existence of two distinct subpopulations of postganglionic NOS-IR neurons (one is catecholaminergic and negative for VIP, and the other is non-catecholaminergic and positive for VIP). This suggests that nitric oxide (NO) and possibly VIP act as postganglionic neurotransmitters or neuromodulators in the quail SCG. The predominant appearance of the former category of NOS-IR cells must be considered in relation to some specific NO-induced controlling mechanisms of SCG neurons.
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