GABA regulates the release of ACh at the transient olivocochlear efferent-inner hair cell synapse through presynaptic GABAB receptors.

CAROLINA WEDEMEYER; JAVIER ZORRILLA DE SAN MARTÍN; ANA VANESSA TORBIDONI; ANA BELÉN ELGOYHEN; ELEONORA KATZ

Lisboa

Workshop; Inner Ear Biology Workshop; 2011

Before the onset of hearing, inner hair cells (IHCs) of the mammalian cochlea are transiently innervated by medial olivocohlear (MOC) efferent fibers. Although acetylcholine (ACh) is the main neurotransmitter, γ-aminobutiric acid (GABA) is also present at MOC synaptic terminals. To study the role of the Gabaergic system at the MOC-IHC synapse, we evaluated the effects of compounds selective for GABAB receptors on the quantal content of transmitter release. Postsynaptic currents evoked by electrically stimulating the efferent fibers were recorded in voltage-clamped IHCs from isolated mouse organs of Corti at postnatal days 9 to 11. The quantal content of evoked release was increased by the GABAB antagonist CGP35348 (65±19%, n=6 p<0.05) and decreased by the agonist baclofen (43.07±8%, n=8, p<0.001). These results, together with the lack of effect of these two drugs on spontaneous synaptic current amplitude, suggest that GABA exerts a negative control on synaptic transmission through presynaptic GABAB receptors. To confirm this hypothesis, we studied the effects of baclofen on GABAB knock-out (KO) mice. Functional GABAB receptors are formed by the GABAB2 subunit with either the GABAB1a or the GABAB1b subunit. We compared the effects of baclofen in GABAB1a-1b, GABAB1a and GABAB1b KO mice. Application of 1uM baclofen, caused a significant reduction in the quantal content of evoked release in both wild-type (32.3±6.9 %, n=5, p<0.05) and GABAB1b KO mice (26.7±7.7 %, n=4, p<0.05). However, baclofen did not affect this parameter in GABAB1a-1b (p>0.05, n=3) or GABAB1a KO mice (p>0.05, n=3). Our results show that ACh release at the MOC-IHC synapse is negatively regulated by GABA acting on presynaptic GABAB1a receptors.