Sex and temporal variations of the vasotocin neuronal system in the damselfish brain.

The neuropeptide vasotocin (VT) is an important regulator of reproduction and social behaviors, and hypothesized to function as a neuromodulator of sensory and motor processing. In adult fishes, VT is primarily produced in three different cell groups (parvocellular, magnocellular, and gigantocellular) within preoptic nuclei, but little is known about sex and seasonal variations of these somata and their relationship to sensory and motor processing. I used immunocytochemistry to (1) test for sex and seasonal variations in VT-immunoreactive (-ir) somata number, size, and fiber densities in the brain of a soniferous damselfish, and (2) test the hypothesis that VT-ir axons project to and vary seasonally in sensory and motor regions of the brain. Sex differences in somata number and size were restricted to parvocellular neurons, while seasonal variations were found within parvocellular and gigantocellular, but not magnocellular neurons. Both males and females had more gigantocellular neurons during peak spawning compared to other times. VT-ir fibers were most abundant in sensory and motor processing regions of the auditory-mechanosensory torus semicircularis (TS), facial lobe, and vagal motor nucleus (VMN), while sparse innervation was found to the tectum and hindbrain auditory and mechanosensory nuclei. VT-ir fiber densities in the TS and VMN were higher during peak spawning, and correlated with gigantocellular (TS, VMN) and parvocellular (TS) somata number. These results provide neuroanatomical support for a relationship between temporal changes in specific VT somata and projections to some sensory and motor processing regions in the damselfish brain that may influence complex communicative and social behaviors.