THE ROLE OF GABAERGIC INPUTS FOR COINCIDENCE DETECTION IN THE NEURONSOF NUCLEUS LAMINARIS OF THE CHICK

1. Synaptic inputs to nucleus laminaris (NL) neurones were studied in a brainstem slice preparation of chick embryos (E15-20) using the whol e-cell patch clamp technique. NL neurones are third order auditory neu rones and are proposed to behave as coincidence detectors concerned vi ith interaural timing discrimination. 2. Under voltage clamp condition s, electrical stimuli applied to either ventral or dorsal dendritic la yers evoked EPSCs. These fast currents decayed with a time constant of 1.1 ms near the resting potential, reversed close to 0 mV, and were b locked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 20 mu M) or 6,7- dinitro-quinoxaline-2,3-dione (DNQX, 20 mu M). Coincident or near coin cident stimulation of the ventral and the dorsal dendritic layers incr eased the probability of action potential generation (response probabi lity). 3. Ln the presence of CNQX (40 mu M) other postsynaptic current s (PSCs) were observed, which reversed close to the equilibrium potent ial for chloride (E-Cl), and were reversibly blocked by bicuculline (2 0 mu M) and, therefore, were mediated by GABA, receptors. Spontaneous GABAergic PSCs were inward going near the resting membrane potential i mmediately after starting whole-cell recording with a low Cl- (5 mM, E -Cl = -90 mV) pipette medium, but became outward-going with time. This indicates that GABAergic inputs may generate depolarizing potentials in intact NL neurones. 4. Local GABA (10 mu M) application reduced bot h the EPSP and EPSC amplitude and shortened the EPSP decay time consta nt (from 5.3 to 1.2 ms), while the EPSC decay time constant wets not a ffected (from 1.3 to 1.2 ms). These GABA effects were mostly due to th e shunting conductance of the postsynaptic GABA, receptors. 5. Depolar izing current injections combined with electrical stimuli to a unilate ral axon bundle simulated bilateral synaptic inputs. Response probabil ity increased with decreased interstimulus intervals, while local GABA (10 mu M) application to the soma narrowed the time dependence of the response probability.6. These results suggest that GABAergic inputs t o NL neurones may serve to improve coincidence detection of the bilate ral excitatory inputs through an increase in membrane conductance.