Dopamine D1Aa and D2a Receptor Expression in the Auditory System of a Vocal Fish.

Although dopamine receptors (DARs) have been identified in various vertebrate neural circuits, their expression in the central auditory system remains poorly characterized. Reproductive-state changes in catecholamine innervation of central auditory nuclei in the plainfin midshipman fish (Porichthys notatus) highlight a unique, ethologically relevant role for catecholamines, including dopamine, in modulating auditory function to enhance reproductive success. Dopamine's effects on these systems are mediated via its receptors; thus, the goal of the present study is to characterize excitatory dopamine D1Aa and inhibitory dopamine D2a receptor transcript expression throughout the central auditory system of the plainfin midshipman. Fluorescence in situ hybridization-immunohistochemistry (FISH-IHC) revealed robust D1Aa and D2a expression in forebrain auditory-recipient centers that receive catecholaminergic input: postcommissural and ventral nuclei of the ventral telencephalon, anterior tuberal nucleus, central posterior nucleus of the thalamus, and parvocellular preoptic nuclei. Large dopamine neurons in the periventricular posterior tuberculum, which are, in part, responsible for the reproductive-state changes in central and peripheral catecholamine innervation only express D2a, whereas large noradrenergic neurons in the locus coeruleus express both D1Aa and D2a. The midbrain torus semicircularis, periaqueductal gray, hindbrain octavolateralis efferent nucleus, and descending/secondary octaval nuclei also express both receptor types. D2a expression predominates over D1Aa, and we identify a subpopulation of cells throughout the auditory system that co-express both receptors. The robust distribution of inhibitory and excitatory DAR expression in the central auditory system, coupled with co-expression in a subset of cells, provides strong neuroanatomical evidence of dopamine's complex role in modulating auditory sensitivity and processing.