Monoamine innervation of vocal and auditory circuits in the plainfin midshipman fish.

Abstract

Monoamines, which include catecholamines (e.g., dopamine and noradrenaline) and serotonin, are important regulators of reward, motivation and reproductive-related behaviors across vertebrates. Studies in tetrapods have demonstrated that monoamines can also serve a modulatory function for vocalization and audition. Antibodies directed against serotonin (5-HT) and tyrosine hydroxylase (TH) (the rate limiting enzyme in catecholamine synthesis) were employed to investigate monoamine innervation of known central vocal and auditory nuclei and the sensory epithelium of the saccule, the main endorgan of hearing, in the plainfin midshipman fish, Porichthys notatus. Both 5-HT and TH-immunoreactive (-ir) terminals are abundant in hindbrain (dorsal descending octaval nucleus), midbrain (torus semicircularis), thalamic (central posterior nucleus), and ventral telencephalic auditory nuclei. In addition, very robust TH-ir is found in the octavolateralis efferent nucleus, and prominent TH-ir projections travel along the eighth nerve and terminate within the hair cell layer of the saccular epithelium. Aside from the midbrain periaqueductal gray, TH-ir is much more pronounced than 5-HT-ir throughout the descending vocal motor system, particularly in the tuberal hypothalamus and the hindbrain-spinal vocal motor nucleus, which directly innervates vocal musculature on the swimbladder. These data provide anatomical evidence that monoamines are modulators of vocal and auditory-driven behaviors in fishes.