1. Physiological and morphological Properties of layer V nonpyramidal and pyramidal cells in isolated slices of frontal cortex from young rats ( 16-22 days postnatal) were studied by whole-cell, current-clamp recording of visualized cell bodies coupled with intracellular staining by biocytin at 26-27-degrees-C. 2. Plotting of spike width at half amplitude against input resistance revealed two physiological categories of nonpyramidal cells. One class (n = 29) had input resistances lower than 400 MOMEGA and spike widths at half amplitude shorter than 0.8 ms; the other (n = 22) had input resistances higher than 400 MOMEGA and spike widths longer than 0.8 ms. According to their spike firing characteristics, the former are called fast-spiking (FS) cells, and the latter low-threshold spike (LTS) cells. 3. Resting potentials were more negative in FS cells than in LTS cells. Membrane time constants in LTS cells were four times larger than those of FS cells. Afterhyperpolarizations (AHPs) following action potentials consisted of a single component in FS cells, but two components with early and late peaks were observed in LTS cells. AHPs of FS cells had faster time-to-peak and larger amplitude than the early component of the AHPs of LTS cells. 4. Low-threshold spikes induced by depolarizing current pulses were observed at hyperpolarized potentials in LTS cells, but not in FS cells. The low-threshold spikes in LTS cells could be activated at hyperpolarized potentials by synaptic potentials. 5. Spike trains elicited by depolarizing current pulses in FS cells showed almost no spike-frequency adaptation, whereas those in LTS cells showed adaptation. 6. Excitatory postsynaptic potentials (EPSPs) of both groups of nonpyramidal cells contained N-methyl-D-aspartate (NMDA) receptor-mediated components. A combination of stimulation-induced EPSPs with depolarization caused repetitive firing in FS cells that was abolished by NMDA receptor blockers. Repetitive firing was not observed in LTS cells under these conditions. 7. The somal size of the two classes of nonpyramidal cells was similar. FS cells were all multipolar in shape, whereas LTS cells included both multipolar and bitufted types. The dendrites of some FS cells extended up into layers II/III, but there-were also other FS cells with their dendrites restricted in layer V. Dendrites of LTS cells were mostly restricted to layer V. Dendrites of FS cells were mostly smooth, but those of LTS cells possessed a modest but consistent population of spines. 8. FS cells innervated more densely in areas adjacent to the somata, and their axonal branches ran more horizontally either at the same depth as or above their somata. LTS cells extended axons more vertically up to layer I and in some cases also to deeper layers. 9. Pyramidal cells in layer V were divided into two groups by their somatic size and input resistance. One group [high input-resistance (HR) pyramidal cells] had somal diameters < 16 mum and input resistances >400 MOMEGA. The other [low input-resistance (LR) pyramidal cells] had somata > 17 mum and input resistance <400 MOMEGA. Time constants of HR cells were three times greater than those of LR cells. Characteristics of action potentials and AHPs were similar in the two groups. Both showed spike-frequency adaptation and could not be repetitively fired by synaptic activation with depolarization. 10. The two groups of pyramidal cells showed different dendritic morphologies. Both had branches arising from their apical dendrites in layers I and V. LR cells, however, had dendritic branches in layers II/III, but HR cells did not. LR cells had more basal dendritic branches. Dendritic fields of HR cells were wider in layer V and smaller in layers I and II/III than those of LR cells. Dendritic fields in layer V of LR cells were similar in horizontal width to those in layers I and II/III. 11. In comparison among nonpyramidal and pyramidal cells. spike widths were shorter in nonpyramidal cells than in pyramidal cells and shorter in FS cells than in LTS cells. Resting potentials of FS cells were more negative than those of LTS cells. Input resistances of FS and LR cells were lower than those of LTS and HR cells. FS cells had time constants smaller than LR cells, and LR cells had smaller time constants than LTS and HR cells. AHPs following fast spikes had a single component in FS cells and two components in LTS cells and in pyramidal cells. Time-to-peak of late AHPs of LTS cells were shorter than those of pyramidal cells. FS cells showed almost no spike-frequency adaptation and fired repetitively by synaptic activation with depolarization. Low-threshold spikes were most prominent in LTS cells. 12. These results show that nonpyramidal and pyramidal cells in layer V are functionally heterogeneous and can be divided into at least two distinct groups. These suggest layer- and column-specific differentiation of neocortical intrinsic neurons with different firing modes.
