1. Field potentials were evoked from rat olfactory cortex slices in vitro at room temperature by lateral olfactory tract stimulation. At very slow stimulation rates (less than 0-1 Hz) a delayed negative wave (late N-wave) was found to follow the first negative wave (N-wave). This late N-wave started 30 msec after the stimulus and lasted for 50-150 msec. 2. Low Ca2+/high Mg2+ medium abolished the late N-wave more rapidly than the N-wave, suggesting a possible multisynaptic origin. 3. The GABA antagonist bicuculline (10(-6) M) abolished the late N-wave without affecting the N-wave. 4. During the late N-wave, both the tract action potential and the N-wave to a second stimulus were reduced. This was attributed, at least in part, to collision with antidromic action potentials which could be detected during the late N-wave. 5. These findings are discussed in terms of a possible presynaptic depolarizing action of a GABA-like transmitter giving rise to presynaptic inhibition and the late N-wave.
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