Figure 2.

GABA_BRs expressed in fibers innervating cochlear hair cells. (A) Schematic of afferent (purple) and efferent (blue) innervation of inner (IHC) and outer hair cells (OHC) in the immature cochlea showing the known localization of the GABA_BR (green). Receptors have been found in type I and II spiral ganglion (SG) neurons and their afferent terminals on the IHC and OHC, where they are thought to regulate glutamate-evoked responses, and in the terminals of the medial olivocochlear bundle (MOC), which originates around the medial superior olive (MSO). The presence of GABA_BRs in fibers of the lateral olivocochlear bundle (LOC) arising from the lateral superior olive (LSO) was not detected. (B) In the adult cochlea, GABA_BRs disappear from the terminals of efferent fibers, along with efferent innervation of IHC by MOC fibers. (C) Schematic representation of the cholinergic synapse formed by MOC fibers on the IHC somata. At this synapse, GABA_BRs control the secretion of acetylcholine (ACh, brown dots), which is released together with GABA (blue dots) from the MOC terminal. GABA is thought to activate presynaptic GABA_B1a/2 autoreceptors, which regulate acetylcholine release by inhibiting presynaptic VGCC channels. Acetylcholine binds to nicotinic receptors (nACh) on the IHC, which elicit postsynaptic Ca²⁺ transients and K⁺ currents via Ca²⁺-dependent K⁺ channels (KCNN2) (see section 3 for further details).