Sexually-dimorphic expression of tyrosine hydroxylase immunoreactivity in the brain of a vocal teleost fish (Porichthys notatus).

Vocal communication has emerged as a powerful model for the study of neural mechanisms of social behavior. Modulatory neurochemicals postulated to play a central role in social behavior, related to motivation, arousal, incentive and reward, include the catecholamines, particularly dopamine and noradrenaline. Many questions remain regarding the functional mechanisms by which these modulators interact with sensory and motor systems. Here, we begin to address these questions in a model system for vocal and social behavior, the plainfin midshipman fish (Porichthys notatus). We mapped the distribution of immunoreactivity for the catecholamine-synthesizing enzyme tyrosine hydroxylase (TH) in the midshipman brain. The general pattern of TH(+) cell groups in midshipman appears to be highly conserved with other teleost fish, with a few exceptions, including the apparent absence of pretectal catecholamine cells. Many components of the midshipman vocal and auditory systems were innervated by TH(+) fibers and terminals, including portions of the subpallial area ventralis, the preoptic complex, and the anterior hypothalamus, the midbrain periaqueductal gray and torus semicircularis, several hindbrain auditory nuclei, and parts of the hindbrain vocal pattern generator. These areas thus represent potential sites for catecholamine modulation of vocal and/or auditory behavior. To begin to test functionally whether catecholamines modulate vocal social behaviors, we hypothesized that male and female midshipman, which are sexually dimorphic in both their vocal-motor repertoires and in their responses to hearing conspecific vocalizations, should exhibit sexually dimorphic expression of TH immunoreactivity in their vocal and/or auditory systems. We used quantitative immunohistochemical techniques to test this hypothesis across a number of brain areas. We found significantly higher levels of TH expression in male midshipman relative to females in the TH cell population in the paraventricular organ of the diencephalon and in the TH-innervated torus semicircularis, the main teleost midbrain auditory structure. The torus semicircularis has been implicated in sexually dimorphic behavioral responses to conspecific vocalizations. Our data thus support the general idea that catecholamines modulate vocal and auditory processing in midshipman, and the specific hypothesis that they shape sexually dimorphic auditory responses in the auditory midbrain.