Heterogeneity of rat corticospinal neurons

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Abstract
In order to examine the degree of diversity within a population of cortical projection neurons, rat corticospinal cells were retrogradely labeled in vivo by injocting rhodamine-tagged microspheres into the cervical spinal cord, and subsequently studied electrophysiologically and anatomically in neocortical slices maintained in vitro, by use of Standard current clamp techniques and a double-labeling protocol (Tseng et al, J. Neurosci. Meth. 37′. 121–131, 1991). Three different subgcroups were distinguished on the basis of their spiking behavior: (l) Adapting cells had a marked fast (50 ms) and slow phase (200 ms) of spike frequency adaptation; (2) regulr spiking (RS) cells had only a period offast adaptation; (3) some regulr spiking neurons had prominent depolarizing afterpotentials (DAPs) and could generate bursts of Spikes, often in repetitive fashion (RSoAp cells). Subgroups of RSc.Ap cells had different patterns of burst responses to depolarizing current pulses, suggesting differences in the types and/or sites ofunderlying ionic conduetances. Adapting cells had a slightly higher membrane input resistance and more prominent slow hyperpolarizing afterpotentials than RS and RSDAP neurons; however, the activation of presumed anomalous rectifier current by intracellular hyperpolarizations was less prominent in adapting neurons. Orthodrornic Stimulation in layer l evoked presumed excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs) in all three types of cells, but prominent short-latency IPSPs were found in a higher percentage of adapting neurons. The morphology of electrophysiologically characterized corticospinal neurons was studied following intracellular injection of biocytin. All three spiking types were typical layer V pyramids with apical dendrites reaching layer l, basal dendrites in infragranular layers, and deep-directed axons that had a moderate density oflocal collaterals in lower cortical layers. The profuseness of dendrites, examined by Sholl's analysis of two-dimensional, camera lucidareconstructed neurons was comparable in the three neuronal subgroups, although a smaller somatic area and more siender apical dendritic trunk were found in adapting neurons. Our results suggest that corticospinal cells in rats are a heterogeneous population of projection neurons with respect to their spiking behavior, membrane properties, synaptic connections, and, to a lesser extent, their morphology. This diversity revealed in vitro adds new complexity to the classification of corticospinal neurons.