Multi-transmitter characteristics and functional specialization of oxytocin neuron subpopulations in zebrafish.

Teleost oxytocin (OXT) neurons are much fewer in number than those reported in mammals, suggesting that individual neurons may support a broader range of functions. This phenomenon raises the possibility that co-expression of multiple neurotransmitters allows teleost OXT neurons to support various functions. To test this possibility, we systematically examined neurotransmitter characteristics and activity patterns of OXT neuron subtypes in adult zebrafish brain. We found that OXT neurons are distributed across the parvocellular preoptic nucleus (anterior, PPa; posterior, PPp) and the periventricular posterior tuberculum (TPp), segregating by soma size into parvocellular (≤11.62 μm) and magnocellular (>11.62 μm) subtypes. The number of the parvocellular OXT neurons follows an anterior-to-posterior enrichment gradient, whereas magnocellular neurons display an inverse posterior-to-anterior gradient. The OXT neurons co-express glutamatergic, GABAergic and cholinergic neuron markers and show partial co-localization with vasotocin (AVT), while lacking dopaminergic and serotonergic synthesis markers. We also quantified activity-dependent p-S6 expression during mating. p-S6 increased in parvocellular OXT neurons of the PPa and PPp, but not in magnocellular neurons of these regions or in either neuronal type in the TPp. Together, these findings suggest that zebrafish OXT neurons may coordinate behavioral and endocrine functions through multi-transmitter expression and partial AVT co-expression. These results further indicate that anatomically distinct OXT neuron subtypes differ in their functional recruitment, with parvocellular neurons in the PPa and PPp possibly involved in mating-related interactions.