KCTD8 and KCTD12 Facilitate Axonal Expression of GABA_B Receptors in Habenula Cholinergic Neurons

GABA_B receptors in habenula cholinergic neurons mediate strong presynaptic excitation and control aversive memory expression. K⁺ channel tetramerization domain (KCTD) proteins are key interacting partners of GABA_B receptors; it remains unclear whether and how KCTDs contribute to GABA_B excitatory signaling. Here, we show that KCTD8 and KCTD12 in these neurons facilitate the GABA_B receptors expression in axonal terminals and contribute to presynaptic excitation by GABA_B receptors. Genetically knocking out KCTD8/12/16 or KCTD8/12, but not other combinations of the three KCTD isoforms, substantially reduced GABA_B receptors-mediated potentiation of glutamate release and presynaptic Ca²⁺ entry in response to axonal stimulation, whereas they had no effect on GABA_B-mediated inhibition in the somata of cholinergic neurons within the habenulo-interpeduncular pathway in mice of either sex. The physiological phenotypes were associated with a significant decrease in the GABA_B expression within the axonal terminals but not the somata. Overexpressing either KCTD8 or KCTD12 in the KCTD8/12/16 triple knock-out mice reversed the changes in axonal GABA_B expression and presynaptic excitation. In mice lacking the KCTDs, aversion-predicting cues produced stronger neuronal activation in the interpeduncular nucleus, and the infusion of GABA_B agonist in this nucleus produced a weaker effect on fear extinction. Collectively, our results reveal isoform-specific roles of KCTD proteins in enriching the axonal expression of GABA_B receptors, facilitating their presynaptic signaling, and modulating aversion-related memory processes.SIGNIFICANCE STATEMENT GABA_B receptors represent the principal inhibitory neurotransmitter receptor, but they mediate strong presynaptic excitation in the habenulo-interpeduncular pathway and modulate aversion memory expression. KCTD proteins are integral constituents of GABA_B receptors. By analyzing the physiological, neuroanatomical, and behavioral phenotypes of multiple KCTD knock-out mouse lines, we show that KCTD8 and KCTD12 facilitate the axonal expression and hence presynaptic excitation of GABA_B receptors in habenula cholinergic neurons and control cued-aversion memory formation and expression in the habenulo-interpeduncular pathway. These results expand the physiological and behavioral functions of KCTDs in modulating the brain neural circuits.