Coexistence of NADPH-diaphorase and tyrosine hydroxylase in the mesencephalic catecholaminergic system of the Japanese quail.

Previous studies have shown the presence of a large number of tyrosine hydroxylase immunoreactive neurons and nicotinamide adenine dinucleotide phosphate (NADPH)-diaphorase positive elements within the mesencephalic and pontine regions of the Japanese quail. In the present study histochemical and immunohistochemical procedures reveal that cells expressing at least one of these two neurochemical markers coexist throughout a large part of the substantia nigra and of the area ventralis of Tsai. Also about 40% of the neurons in these two regions that contain immunoreactive tyrosine hydroxylase also exhibit NADPH-diaphorase activity. This is not a general property of the quail catecholaminergic system: in the locus coeruleus (the main noradrenergic group) there is a complete separation between these two neuronal populations. The number of neurons expressing either neurochemical marker is not different between males and females in any of the regions that have been investigated. NADPH-diaphorase is known to be an indicator of the enzyme nitric oxide synthase; these results therefore suggest that nitric oxide may play an important role in the regulation of the activity of a significant part of the avian mesencephalic dopaminergic system.