HETEROGENEITY OF RAT CORTICOSPINAL NEURONS

In order to examine the degree of diversity within a population of cor tical projection neurons, rat corticospinal cells were retrogradely la beled in vivo by injecting rhodamine-tagged microspheres into the cerv ical spinal cord, and subsequently studied electrophysiologically and anatomically in neocortical slices maintained in vitro, by use of stan dard current clamp techniques and a double-labeling protocol (Tseng et al., J. Neurosci. Meth. 37.121-131, 1991). Three different subgroups were distinguished on the basis of their spiking behavior: (1) Adaptin g cells had a marked fast (50 ms) and slow phase (200 ms) of spike fre quency adaptation; (2) regular spiking (RS) cells had only a period of fast adaptation; (3) some regular spiking neurons had prominent depol arizing afterpotentials (DAPs) and could generate bursts of spikes, of ten in repetitive fashion (RS(DAP) cells). Subgroups of RS(DAP) cells had different patterns of burst responses to depolarizing current puls es, suggesting differences in the types and/or sites of underlying ion ic conductances. Adapting cells had a slightly higher membrane input r esistance and more prominent slow hyperpolarizing afterpotentials than RS and RS(DAP) neurons; however, the activation of presumed anomalous rectifier current by intracellular hyperpolarizations was less promin ent in adapting neurons. Orthodromic stimulation in. layer I evoked pr esumed excitatory and inhibitory postsynaptic potentials (EPSPs and IP SPs) in all three types of cells, but prominent short-latency IPSPs we re found in a higher percentage of adapting neurons. The morphology of electrophysiologically characterized corticospinal neurons was studie d following intracellular injection of biocytin. All three spiking typ es were typical layer V pyramids with apical dendrites reaching layer 1, basal dendrites in infragranular layers, and deep-directed axons th at had a moderate density of local collaterals in lower cortical layer s. The profuseness of dendrites, examined by Sholl's analysis of two-d imensional, camera lucida-reconstructed neurons was comparable in the three neuronal subgroups, although a smaller somatic area and more sle nder apical dendritic trunk were found in adapting neurons. Our result s suggest that corticospinal cells in rats are a heterogeneous populat ion of projection neurons with respect to their spiking behavior, memb rane properties, synaptic connections, and, to a lesser extent, their morphology. This diversity revealed in vitro adds new complexity to th e classification of corticospinal neurons. (C) 1993 Wiley-Liss, Inc.