Innervation of NADPH diaphorase‐containing neurons correlated with acetylcholinesterase, tyrosine hydroxylase, and neuropeptides in the pigeon cloaca

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Abstract
The motility of the avian cloaca is under neural control, but little is known about the neural network that accomplishes this function. This present study was designed to determine the distribution of nitric oxide‐synthesising neurons in the pigeon cloaca by enzyme histochemistry for reduced nicotinamide adenine dinucleotide phosphate‐diaphorase (NADPH‐d). NADPH‐d‐positive staining was seen in the neurons and fibres in the cloaca. The highest density of nerve fibres was noted in the coprodeum and the lowest in the proctodeum. In the coprodeum, NADPH‐d neurons were found singly, formed small groups of 2–10 neurons, or were seen in plexuses in the muscle layer, lamina propria, or around the arterioles. Several NADPH‐d‐positive neurons were also observed in the ganglia of the cloaca. NADPH‐d fibres ran in the muscle layer, lamina muscularis mucosae and lamina propria, or surrounded blood vessels. The distribution pattern of acetylcholinesterase (AChE)‐stained neurons and fibres in the cloaca was similar to that of NADPH‐d. Double staining for NADPH‐d and AChE showed colocalisation of the 2 enzymes in many neurons of the cloaca. Tyrosine hydroxylase (TH)‐immunoreactive nerve fibres originating outside the cloaca were also noted. In the urodeum and proctodeum, neurons or fibres positive for NADPH‐d, AChE or TH were scattered in the lamina propria. Nerve fibres immunoreactive for calcitonin‐gene related peptide, galanin, methionine‐enkephalin, substance P, and vasoactive intestinal peptide were found sparsely in the cloaca. Our results demonstrate that nitrergic neurons constitute a subpopulation which is closely associated with the cholinergic system in the pigeon cloaca.