Reversed somatodendritic I(h) gradient in a class of rat hippocampal neurons with pyramidal morphology

In CA1 and neocortical pyramidal neurons, I(h) is present primarily in the dendrites. We asked if all neurons of a pyramidal morphology have a similar density of I(h). We characterized a novel class of hippocampal neurons with pyramidal morphology found in the stratum radiatum, which we termed the 'pyramidal-like principal' (PLP) neuron. Morphological similarities to pyramidal neurons were verified by filling the neurons with biocytin. PLPs did not stain for markers associated with interneurons, and projected to both the septum and olfactory bulb. By using cell-attached patch-clamp recordings, we found that these neurons expressed a high density of I(h) in the soma that declined to a lower density in the dendrites, a pattern that is reversed compared to pyramidal neurons. The voltage-dependent activation and activation time constants of I(h) in the PLPs were similar to pyramidal neurons. Whole-cell patch-clamp recordings from the soma and dendrites of PLP neurons showed no significant differences in input resistance and local temporal summation between the two locations. Blockade of I(h) by ZD7288 increased the input resistance and temporal summation of simulated EPSPs, as in pyramidal neurons. When NMDA receptors were blocked, temporal summation at the soma of distal synaptic potentials was similar to that seen with current injections at the soma, suggesting a 'normalization' of temporal summation similar to that observed in pyramidal neurons. Thus, we have characterized a principal neuronal subtype in the hippocampus with a similar morphology but reversed I(h) somatodendritic gradient to that previously observed in CA1 hippocampal and neocortical pyramidal neurons.