Physiological characterization of layer III non-pyramidal neurons in piriform (olfactory) cortex of rat

We performed whole-cell recordings of layer III non-pyramidal neurons in th e piriform cortex of Sprague-Dawley rats. For comparison purposes, recordin gs were made from deep pyramidal cells, which are also present in layer III . These two cell types could be distinguished both anatomically and physiol ogically. Anatomically, the layer III non-pyramidal neuron displayed smooth beady dendrites, while deep pyramidal cells showed thicker dendrites with spines. The dendrites of the layer III non-pyramidal neuron also tended to be restricted to layer III while deep pyramidal cells had long, apical dend rites that spanned layers I and II. Although the resting membrane potential s of both cell types were very similar, significant differences were noted in other physiological measures. Layer III non-pyramidal neurons typically displayed higher input resistances, faster time constants, smaller spike am plitudes, shorter spike widths, and higher spike thresholds. In addition, l ayer III non-pyramidal neurons were able to spike at much higher rates when stimulated with the same level of threshold normalized current injection. The most dramatic differences in physiology were seen in the pattern of spi king in response to increasing levels of positive constant current pulses. Layer III non-pyramidal neurons showed qualitatively different responses at low and high levels of stimulation. At low levels, spikes occurred with lo ng latency and the firing frequency increased throughout the duration of th e current pulse, At high levels, non-pyramidal neurons started spiking with short latency, followed by a decrease in firing frequency, which in turn w as followed by an increase in firing frequency. Deep pyramidal neurons diff ered dramatically from this pattern, displaying a qualitatively similar res ponse at all levels of current injection. This response was characterized b y short latency spikes and spike adaptation for the duration of the current pulse. (C) 2000 Elsevier Science B.V. All rights reserved.