Antidromic and orthodromic responses by subicular neurons in rat brain slices

The subiculum forms part of the region of transition between hippocampus and entorhinal cortex and is one of the primary output structures of the hippocampal formation. Intracellular recordings from subicular bursting and non-bursting cell types and field potential recordings were taken in horizontal slices from rat brains. The inputs and outputs of the two cell types were studied for the purpose of reinforcing or refuting the dichotomy proposed on the basis of membrane properties. Some bursting cells were antidromically activated by stimuli applied to the superficial or deep layers of presubiculum, but never by stimuli applied to deep layers of medial entorhinal cortex (dMEC). Some non-bursting subicular neurons were antidromically activated by stimuli applied to dMEC, but never by stimuli applied to presubiculum. Antidromic population events in subiculum were single spikes when deep MEC was stimulated, but were bursts when presubiculum was stimulated, even in the presence of glutamate receptor antagonists. Population bursts consist of 2 or more population spikes with peak to peak intervals of approximately 5 ms. That population bursts occur in slices where excitatory transmission is blocked suggests that such population bursts reflect coincident bursts by individual neurons. Short-latency (< 5 ms) excitatory postsynaptic potentials (EPSPs) were evoked in both subicular cell types in response to single entorhinal, presubicular and CA1 stimuli. Long-latency (> 10 ms) EPSPs were seen in both cell types in response to presubicular, but not entorhinal or CA1 stimulation. Bursting cells responded to brief trains of orthodromic stimuli (2-10 pulses, 5-10 ms interstimulus interval) with a burst of action potentials even when the cell was previously depolarized out of bursting range by current injection. Non-bursting cells responded to brief trains of orthodromic stimuli with repetitive firing (< or = 1 spike/stimulus) at all holding potentials. Spike intervals could reach those seen in bursts by bursting cells. It is concluded that: (1) the distinction between bursting and non-bursting subicular neurons is a dichotomy and cells do not change their identity when activated antidromically or orthodromically; (2) the outputs of the two cell types may be different: bursting cells projected to presubiculum and non-bursting cells projected to entorhinal cortex; and (3) non-bursting cells can, when repetitively stimulated, fire repetitive spikes with interspike intervals in the range of intervals seen in bursts.