Distribution of tyrosine hydroxylase-immunoreactive neurons in the brain of the viviparous fish Gambusia affinis.

Tyrosine hydroxylase (TH) is the common precursor enzyme involved in the biosynthetic pathway of the catecholaminergic neurotransmitters, dopamine and norepinephrine. In this investigation, the neuroanatomical distribution of TH-immunoreactivity was studied in the brain of the female mosquitofish Gambusia affinis. Numerous intensely stained TH-immunoreactive (ir) neurons were scattered in the olfactory bulb with their fibres extending towards the medial olfactory tract, whereas few telencephalic TH-ir cells with distinct fibres were observed in the dorsal nucleus of area ventralis telencephali and the posterior nucleus of area ventralis telencephali regions. Large TH-ir cell populations were seen in the suprachiasmatic nucleus and the nucleus dorsomedialis thalami regions of the diencephalon. Distinct TH-ir cells with long fibres were found at the preoptic area and the nucleus preopticus pars magnocellularis as well as the nucleus preopticus pars parvocellularis regions. Numerous intensely stained TH-ir cells were observed in the paraventricular organ and the nucleus posterior tuberis regions, whereas moderately stained cells were present in the nucleus of recessus lateralis medialis. Several TH-ir neurons were detected in medial and lateral subdivisions of the nucleus lateralis tuberis. Furthermore, the projections of the TH-ir fibres were seen in the proximal pars distalis region of the pituitary gland, where gonadotropin-secreting cells are located, suggesting the communication between TH cells and gonadotrope cells. In the rostral spinal cord, dense aggregations of the TH-ir fibres were noticed. Overall, the widespread distribution of the TH-ir neurons throughout the brain and their fibres in the spinal cord and the pituitary gland suggests diverse roles for the catecholaminergic neurons in various physiological functions including reproduction in the mosquitofish.