Functional connectivity between the thalamus and postsubiculum: analysis of evoked responses elicited by stimulation of the laterodorsal thalamic nucleus in anesthetized rats.
Intracellular recordings and Neurobiotin-injection were used to examine the electrophysiology and morphology of presubicular and parasubicular cortical neurons in horizontal slices from rat brains. Evoked responses were obtained by stimulation of subicular and entorhinal cortices. Stellate cells were recorded in layers II and V of presubiculum and parasubiculum. Superficial layer cells had spiny dendrites that were found to reach layer I. Deep layer cells had sparsely spiny dendrites or dendrites without spines that did not reach past layer IV. Pyramidal cells were recorded in layers III and V of presubiculum and layers II and V of parasubiculum. Superficial layer cells had spiny dendrites that were found to reach layer I. Deep layer cells had sparsely spiny dendrites or dendrites without spines that could reach layer II. Electrophysiologically, stellate and pyramidal cells were similar to one another, regardless of cell layer, exhibiting repetitive single spiking in response to depolarizing current injection. No cells were found to burst in response to current injection. While there were subtle electrophysiological differences among the cell types, stellate cells were more similar to pyramidal cells from the same or adjacent layers than to other stellate cells from more distant layers. Similarly, pyramidal cells were electrophysiologically more similar to nearby stellate cells than to other distant pyramidal cells. Cells of all layers responded to subicular stimulation with a short latency (< 9 ms), excitatory postsynaptic potential. Superficial layer cells responded at short (< 9 ms), longer (10-20 ms) and very long latencies (> 20 ms) to stimulation of superficial layers of medial entorhinal cortex. Deep layer cells responded at short latencies (< 9 ms) to stimulation of deep layers of medial entorhinal cortex. Many cells responded to both subicular and entorhinal inputs. Both pyramidal and stellate cells in the deep layer of pre/parasubiculum could exhibit population bursting behavior in response to stimulation of subiculum or entorhinal cortex. The results define the cellular morphology and basic electrophysiology of presubicular and parasubicular neurons of the rat brain as a step toward understanding the physiology of the retrohippocampal cortices.