Dopamine receptor D2 expression in serotonergic neurons: sex differences and role in behavioral modulation

Abstract

Serotonergic neurons modulate diverse functions. Evidence suggests heterogeneity across this neuronal system, with cell subsets specialized to regulate distinct biological processes. One such subpopulation shown to modulate social behavior in mice is distinguished by expression of the Drd2 gene, encoding the dopamine D2 receptor, and the Pet1 (alias Fev) gene, encoding a master regulator of serotonergic neuronal differentiation. These serotonergic neurons, referred to as Drd2-Pet1 neurons, are inhibited cell-autonomously by DRD2 agonism in slice, and, when constitutively silenced in male mice, affect levels of defensive, aggressive, and exploratory behaviors. While electrophysiological recordings from brain slices demonstrate inhibition via DRD2 signaling in these neurons, the requirement for DRD2 for specific behavioral outcomes is unknown. To test this, I generated and characterized mice, referred to as Drd2Pet-CKO, that were null for Drd2 selectively in serotonergic neurons. Analyses of Drd2Pet-CKO mice showed highly specific behavioral abnormalities. Females, but not males, exhibited attenuated acoustic startle responses – a defensive, protective reflex. Drd2Pet-CKO males, but not females, showed increased social dominance. Assays probing auditory brainstem responses, locomotion, cognition, and anxiety- and depression-like behaviors were indistinguishable between Drd2Pet-CKO mice and controls, regardless of sex. Sex-specific differences in Drd2-Pet1 neurons were found at the cellular and molecular levels. These included increased Gad2 transcript abundance (important for GABA synthesis) and increased action potential duration in male Drd2-Pet1 neurons, relative to female. Proportional distribution of axonal collaterals also differed between sexes. Exploring such differences in Drd2Pet-CKO cells was constrained by limitations in visualizing mutant cells. Nonetheless, we developed an in situ mRNA hybridization protocol capable of detecting knock-out Drd2 transcripts, thus identifying Drd2Pet-CKO cells. With this method, we found Gad2 expression in a greater percentage of mutant cells in males as compared to females. In summary, I show the importance of Drd2 expression in a subtype of serotonin-producing neurons for normal auditory processing in females and social behavior in males. These differences could be influenced by the identified sex-specific molecular and cellular features distinguishing male versus female Drd2-Pet1 cells. This work is expected to have translational relevance as related behaviors in humans too show sex differences and modulation by serotonin.