Inhibitory Synapses onto Medial Olivocochlear Neurons in Ventral Nuclei of the Trapezoid Body are Glycinergic

Authors

• L Torres Cadenas
• C Weisz

Abstract

Synaptic inputs onto medial olivocochlear (MOC) neurons in the ventral nuclei of the trapezoid body (VNTB) in the auditory brainstem are poorly understood. Excitatory, glutamatergic inputs originate in the cochlear nucleus, but inhibitory synaptic inputs have not been demonstrated. We performed whole-cell voltage-clamp electrophysiological recordings in labeled MOC neurons in brainstem slices from P12-P23 ChAT-cre x tdTomato mice. Some spontaneous post-synaptic currents (PSC) were blocked with CNQX. The reversal potential of remaining spontaneous PSCs was about -30 mV, which corresponds with the Cl ¯ reversal potential, indicating inhibitory synaptic currents. Strychnine application further reduced PSCs, indicating glycinergic, inhibitory inputs. To determine the source of synaptic inputs to MOC neurons, PSCs were evoked from subsets of pre-synaptic axons. To accomplish this, a current pulse was applied via a micropipette placed near axons, including those of the MNTB, to stimulate neurotransmitter release while PSCs were measured in the MOC neurons. The ‘convergence ratio’ (CR) (maximum evoked PSC divided by minimum evoked PSC) was calculated to determine the number of axons synapsing onto each MOC neuron. The CR ranged from 1.4 to 23.6, indicating that MOC neurons receive inputs from a widely varying number of inhibitory axons. Evoked PSCs in MOC neurons using ‘paired pulse’ stimulation at inter-stimulus intervals (ISI) from 10-500 ms suggests that there is a facilitation of synaptic inputs at ISIs from 50-200 ms. Future work will confirm the source of inhibitory synaptic inputs to the MOC neurons.

Scientific Focus Area: Neuroscience

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