Synaptic inhibition plays a fundamental role in the neural computation of the interaural-level difference (ILD), an important cue for the localization of high frequency sound. Here, we studied the inhibitory synaptic currents in the chicken posterior portion of the dorsal nucleus of the lateral lemniscus (LLDp), the first binaural level difference encoder of the avian auditory pathway. Using whole-cell recordings in brain slices, we provide the first evidence confirming a monosynaptic inhibition driven by direct electrical and chemical stimulation of the contralateral LLDp, establishing the reciprocal inhibitory connection between the two LLDp, a long-standing assumption in the field. This inhibition was largely mediated by GABAA receptors, however functional glycine receptors were also identified. The reversal potential (Erev) for the Cl− channels measured with gramicidin perforated patch recordings was hyperpolarizing (−88 mV), corresponding to a low intracellular Cl− concentration (5.2 mM). Pharmacological manipulations of KCC2 (outwardly Cl− transporter) activity demonstrate that LLDp neurons can maintain a low intracellular Cl− concentration under a high Cl− load, allowing for the maintenance of hyperpolarizing inhibition. We further demonstrate that hyperpolarizing inhibition was more effective at regulating cellular excitability than depolarizing inhibition in LLDp neurons.
Significance Statement: Sensory processing performed by distinct neural circuits requires proper synaptic inhibitory inputs. Properties of synaptic inhibition, such as transmitter types and ionic mechanisms of synaptic responses, vary among different neural circuits. Here, we provide the first physiological evidence demonstrating direct inhibitory connections between two avian auditory brainstem nuclei that encode information for sound localization using interaural level difference as a cue. We have characterized the physiological and pharmacological properties of this synaptic inhibition, and demonstrate the role of effective hyperpolarizing inhibition in the circuit.
The authors declare no competing financial interests.
HHS | NIH | National Institute on Deafness and Other Communication Disorders (NIDCD) [F31DC015707].