Distribution of Tryptophan Hydroxylase Immunoreactivity in the Spiral Ganglion Neurons of Mouse Cochlea

Serotonin, also known as 5-hydroxytryptamine, is an important neurotransmitter and neuromodulator in the nervous system. Spiral ganglion neurons (SGNs) are the first station of the peripheral afferent auditory nervous system. We have previously demonstrated positive serotonin immunoreactivity in mouse SGNs. Whether serotonin in the SGNs is absorbed from the circumstances, or is synthesized by the neurons themselves is unknown. The purpose of this study was to find out the source of serotonin in the SGNs. Tryptophan hydroxylase is the rate-limiting enzyme in the process of serotonin biosynthesis. We detected tryptophan hydroxylase in the mouse SGNs using immunohistochemistry. The positive immunostaining of tryptophan hydroxylase was observed in the cytoplasm of SGNs. The cytoplasm of the SGNs was not get stained in the negative control group. Our results suggest that SGNs can synthesize serotonin themselves, and they may serve as an additional source of serotonin in the cochlea. The SGNs are serotonergic neurons. The receptors of serotonin on the membrane of SGNs interact with serotonin as auto-receptors. Serotonin in these neurons plays its physiological role in an autocrine/paracrine way.