ANTIDROMIC AND ORTHODROMIC RESPONSES BY SUBICULAR NEURONS IN RAT-BRAIN SLICES

The subiculum forms part of the region of transition between hippocamp us and entorhinal cortex and is one of the primary output structures o f the hippocampal formation. Intracellular recordings from subicular b ursting and non-bursting cell types and field potential recordings wer e taken in horizontal slices from rat brains. The inputs and outputs o f the two cell types were studied for the purpose of reinforcing or re futing the dichotomy proposed on the basis of membrane properties. Som e bursting cells were antidromically activated by stimuli applied to t he superficial or deep layers of presubiculum, but never by stimuli ap plied to deep layers of medial entorhinal cortex (dMEC). Some non-burs ting subicular neurons were antidromically activated by stimuli applie d to dMEC, but never by stimuli applied to presubiculum. Antidromic po pulation events in subiculum were single spikes when deep MEC was stim ulated, 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 similar to 5 ms. That population bursts occur in slices where excitatory trans mission is blocked suggests that such population bursts reflect coinci dent bursts by individual neurons. Short-latency (<5 ms) excitatory po stsynaptic 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 pre subicular, but not entorhinal or CA1 stimulation. Bursting cells respo nded to brief trains of orthodromic stimuli (2-10 pulses, 5-10 ms inte rstimulus interval) with a burst of action potentials even when the ce ll was previously depolarized out of bursting range by current injecti on. Non-bursting cells responded to brief trains of orthodromic stimul i with repetitive firing (less than or equal to 1 spike/stimulus) at a ll holding potentials. Spike intervals could reach those seen in burst s 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 orthodro mically; (2) the outputs of the two cell types may be different: burst ing cells projected to presubiculum and non-bursting cells projected t o entorhinal cortex; and (3) non-bursting cells can, when repetitively stimulated, fire repetitive spices with interspike intervals in the r ange of intervals seen in bursts. (C) 1997 Elsevier Science B.V.