Differential expression of hyperpolarization-activated currents reveals distinct classes of visual cortical projection neurons

Combining in vivo retrograde labeling and in vitro electrophysiological recording techniques, we examined the distributions, densities, and biophysical properties of hyperpolarization-activated inward currents in two types of isolated, identified visual cortical projection neurons, superior colliculus-projecting (SCP) and callosal-projecting (CP) cells. In SCP cells, two kinetically distinct time-dependent hyperpolarization-activated inward current components are present. We have termed these I(h,f) and I(h,s) to denote the fast and slow components, respectively, of I(h) activation. In CP cells, in contrast, I(h,f) and I(h,s) are differentially expressed. In 59% of the CP cells examined, for example, both I(h,f) and I(h,s) were present. The properties of the currents are indistinguishable from those recorded from SCP cells, although both I(h,f) and I(h,s) are expressed at significantly lower densities in this subset of CP cells (as compared to the current densities in SCP cells). Of the remaining 41% of the CP cells studied, 26% were found to express only I(h,s), and 12% of the cells expressed neither I(h,f) nor I(h,s). Taken together, these results reveal that the electrical properties of CP visual cortical neurons are considerably more heterogeneous than those of SCP cells. The differential expression of I(h,f) and I(h,s) is expected to influence the integrated responses of different types of cortical projection neurons to excitatory and inhibitory synaptic inputs.