PATHWAY-SPECIFIC VARIANTS OF AMPA RECEPTORS AND THEIR CONTRIBUTION TONEURONAL SIGNALING

Neurons of the nucleus magnocellularis (nMAG) of the chick express AMP A a-amino-3-hydroxy-5-methyl-4-isoxazole-propionate) receptors display ing unusually rapid kinetics of desensitization (Raman and Trussell, 1 992a). To investigate whether fast AMPA receptors are present in other auditory neurons, we compared the properties of AMPA receptors in aud itory as well as nonauditory cells. These included neurons of nMAG, th e nucleus angularis, the nucleus laminaris, the cochlear ganglion, the Purkinje cell layer of the cerebellum, the ventral horn of the spinal cord, and the brainstem nucleus of the glossopharyngeal nerve (ncIX). Rapid application of glutamate to voltage-clamped outside-out membran e patches indicated that AMPA receptors in the four types of auditory cells had significantly faster kinetics of desensitization than did th e three types of nonauditory neurons. Channel kinetics in auditory (nM AG) and nonauditory (ncIX) cells were also compared by means of spectr al analysis and the time course of current deactivation upon removal o f glutamate. Both techniques revealed a burst duration for nMAG channe ls of 400-500 mu sec at room temperature, two to three times shorter t han for ncIX cells. Single channels in nMAG had a burst duration of, 5 50 mu sec and an open time of approximate to 150 mu sec. Miniature exc itatory postsynaptic currents of brainstem auditory neurons in slices were also brief, with decay constants of 150-250 mu sec at 29-32 degre es C. We demonstrated that the fast kinetics of this AMPA receptor are physiologically important, since cyclothiazide, which reduces desensi tization and prolongs synaptic currents, doubled the relative refracto ry period of orthodromic spikes in nMAG cells in brain slices. We conc lude that auditory brainstem neurons express a specialized subtype of AMPA receptors. This ''fast'' AMPA receptor may be useful in transmitt ing signals necessary for sound localization.