The Distribution of Nitric Oxide-Synthesizing Neurons and Soluble Guanylate Cyclase in Relation to Dopaminergic Systems in the Pigeon Brain.

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Nitric oxide (NO) is a diffusible neuromodulator that acts as a retrograde messenger in vertebrates, regulating synaptic plasticity and interacting with dopaminergic systems. Despite its established functions in mammals, little is known about the anatomical distribution and potential roles of NO in birds. Using histochemical and immunohistochemical techniques, we mapped NO-synthesizing neurons-identified by NADPH-diaphorase (NADPH-d) and neuronal nitric oxide synthase (nNOS)-and analyzed their relationship with soluble guanylate cyclase (sGC) and tyrosine hydroxylase (TH)-positive dopaminergic pathways in the pigeon brain. We found NADPH-d labeling throughout forebrain, midbrain, and hindbrain regions. Within dopaminergic midbrain structures, the locus coeruleus showed strong colocalization with nNOS, and moderate overlap was observed in the ventral tegmental area, substantia grisea centralis, and substantia nigra. In the nidopallium caudolaterale (NCL), a prefrontal-like associative area, sGC-positive neurons were contacted by both TH- and NADPH-d-positive fibers. Our results highlight an anatomical link between NO and dopaminergic pathways in pigeons, suggesting that mechanisms of memory flexibility known from other species may also operate in birds. In particular, our findings support the potential for NO-dopamine interactions in avian species, reminiscent of recently described mechanisms in Drosophila melanogaster. In this invertebrate model, a subset of dopaminergic neurons was shown to co-release NO, targeting sGC-expressing neurons to modulate memory flexibility, a circuit-level organization not yet described in vertebrates.