A Developmental Switch in Cholinergic Mechanisms of Modulation in the Medial Nucleus of the Trapezoid Body

The medial nucleus of the trapezoid body (MNTB) has been intensively investigated as a primary source of inhibition in brainstem auditory circuitry. MNTB-derived inhibition plays a critical role in the computation of sound location, as temporal features of sounds are precisely conveyed through the calyx of Held/MNTB synapse. In adult gerbils, cholinergic signaling influences sound-evoked responses of MNTB neurons via nicotinic acetylcholine receptors (nAChRs) (Zhang et al., 2021) establishing a modulatory role for cholinergic input to this nucleus. However, the cellular mechanisms through which acetylcholine (ACh) mediates this modulation in the MNTB remain obscure. To investigate these mechanisms, we used whole-cell current and voltage-clamp recordings to examine cholinergic physiology in MNTB neurons from Mongolian gerbils(Merionas unguiculatis)of both sexes. Membrane excitability was assessed in brain slices, in pre- (p9-13) and post-hearing onset (p18-20) MNTB neurons during bath application of agonists and antagonists of nicotinic (nAChRs) and muscarinic receptors (mAChRs). Muscarinic activation induced a potent increase in excitability most prominently prior to hearing onset with nAChR modulation emerging at later time points. Pharmacological manipulations further demonstrated that the voltage gated K+channel KCNQ (Kv7) is the downstream effector of mAChR activation that impacts excitability early in development. Cholinergic modulation of Kv7 reduces outward K+conductance and depolarizes resting membrane potential. Immunolabeling revealed expression of Kv7 channels as well as mAChRs containing M1 and M3 subunits. Together, our results suggest that mAChR modulation is prominent but transient in the developing MNTB and that cholinergic modulation functions to shape auditory circuit development.Significance statementThis study is the first to examine downstream cellular mechanisms that underlie modulatory effects of acetylcholine (ACh) in MNTB neurons. The MNTB is a primary source of inhibition in the superior olive and features the calyx of Held, an intensively studied giant synapse that plays a pivotal role in precise encoding of acoustic cues. Recently, we discovered that ACh modulates MNTB responses in adult gerbils through nicotinic receptors. Here, we demonstrate that ACh has potent effects on membrane excitability prior to hearing onset primarily via muscarinic receptors and describe the expression of two muscarinic receptor subtypes. Our results suggest that developmentally transient cholinergic modulation of a voltage-gated K+conductance is poised to influence circuit development during the peri-hearing onset period.