iGABASnFR data from: Co-release of GABA and ACh from medial olivocochlear neurons as a fine regulatory mechanism of cochlear efferent inhibition

During development, inner hair cells (IHCs) in the mammalian cochlea are
unresponsive to acoustic stimuli but instead exhibit spontaneous activity.
During this same period, neurons originating from the medial olivocochlear
complex (MOC) transiently innervate IHCs, regulating their firing pattern
which is crucial for the correct development of the auditory pathway.
Although the MOC-IHC is a cholinergic synapse, previous evidence indicates
the widespread presence of gamma-aminobutyric acid (GABA) signaling
markers, including presynaptic GABAB receptors (GABABR). In this study, we
explore the source of GABA by optogenetically activating either
cholinergic or GABAergic fibers. The optogenetic stimulation of MOC
terminals from GAD;ChR2-eYFP and ChAT;ChR2-eYFP mice evoked synaptic
currents in IHCs that were blocked by α-bungarotoxin. This suggests that
GABAergic fibers release ACh and activate α9α10 nicotinic acetylcholine
receptors (nAChRs). Additionally, MOC cholinergic fibers release not only
ACh but also GABA, as the effect of GABA on ACh response amplitude was
prevented by applying the GABAB-R blocker (CGP 36216). Using optical
neurotransmitter detection and calcium imaging techniques, we examined the
extent of GABAergic modulation at the single synapse level. Our findings
suggest heterogeneity in GABA modulation, as only 15 out of 31 recorded
synaptic sites were modulated by applying the GABABR specific antagonist,
CGP (100-200 µM). In conclusion, we provide compelling evidence that GABA
and ACh are co-released from at least a subset of MOC terminals. In this
circuit, GABA functions as a negative feedback mechanism, locally
regulating the extent of cholinergic inhibition at certain efferent-IHC
synapses during an immature stage.