Deep neurons in piriform cortex. II. Membrane properties that underlie unusual synaptic responses.

1. Membrane properties of deep pyramidal and multipolar cells in layer III of the rat piriform cortex and multipolar cells in the underlying endopiriform nucleus (layer IV) were studied in a slice preparation with the primary goal of elucidating the origin of the unusual synaptic responses described in the companion paper. 2. Micropipettes containing either Lucifer yellow (LY) for combined morphological-physiological analysis or potassium acetate (KAc) were used for the analysis. Comparison of membrane properties of pyramidal cells measured with these two electrolytes revealed significant differences. With LY and other Li+ salts, resting membrane potentials were more depolarized, input resistances higher, spike amplitudes lower, and spike durations longer. 3. As measured with KAc-containing electrodes, membrane properties of deep pyramidal and multipolar cells were similar to each other, but differed from those of superficial pyramidal cells. Resting membrane potentials were more depolarized, thresholds lower, input resistances higher, membrane time constants slower, and spikes smaller and slower. 4. In response to depolarizing current pulses, both deep pyramidal and multipolar cells exhibited an initial depolarizing peak of graded amplitude that fell to a steady state within 150 ms. Current-voltage (I-V) relationships displayed a large increase in slope resistance during the depolarizing peak, but were relatively linear in the depolarizing direction at steady state. In cells with relatively hyperpolarized resting membrane potentials, threshold for the depolarizing peak could be -65 mV or below. Based on a reduction by steady depolarization, reduction by Co2+ and potentiation by Ba2+, it is postulated that the peak is generated in part by a low threshold inactivating Ca2+ current. A partial blockage of this peak by tetrodotoxin (TTX) suggests that a Na+ current also contributes. 5. In response to hyperpolarizing current pulses, especially at depolarized membrane potentials, there was usually a sag from an initial maximum and a depolarizing rebound after current offset in both deep pyramidal and multipolar cells. Based on the dependence on membrane potential (Vm), insensitivity to TTX and blockage by carbamylcholine chloride (carbachol), it is postulated that an M-current contributes to the sag and rebound. 6. The depolarizing rebound that followed offset of hyperpolarizing current pulses could trigger a Ba2+-potentiated local response that resembled the depolarizing peak triggered by depolarizing current, suggesting that the postulated low-threshold inactivating Ca2+ current contributes to its generation.(ABSTRACT TRUNCATED AT 400 WORDS)