REPETITIVE FIRING AND OSCILLATORY ACTIVITY OF PYRAMIDAL-LIKE BURSTINGNEURONS IN THE RAT SUBICULUM

Electrophysiological characterization of neurons within the rat subicu lum was carried out with intracellular recordings in an in vitro slice preparation. Subicular neurons responded to threshold pulses of depol arizing current delivered at a resting membrane potential (RMP) of -65 .7+/-5.8 mV (mean+/-SD, n=85) with an initial burst of three to five f ast action potentials that rode on a depolarizing envelope and was ter minated by an afterhyperpolarization (burst AHP) (duration 113+/-35 ms ; peak amplitude 2.7+/-0.6 mV, n=10). Tonic firing replaced the bursti ng mode at membrane potential less negative than -55 mV. Suprathreshol d depolarizing pulses evoked at RMP both an initial burst and successi ve tonic firing. Intracellular staining with biocytin showed morpholog ical features typical of pyramidal cells (n=8). The relationship betwe en frequency of repetitive firing and injected current (f-I) revealed that the burst firing frequency (250-300 Hz) was only slightly influen ced by the amount of injected current. By contrast, the f-I curve of t he tonic firing phase depended upon current intensity: it displayed an initial segment that increased at first linearly and then turned into a plateau for both the early and the late inter-spike intervals. The frequency of the tonic firing declined only slightly with time, thus s uggesting a lack of adaptation. During tonic firing, each single actio n potential was followed by a fast AHP and a depolarizing afterpotenti al. Termination of repetitive firing was followed by an AHP (spike-tra in AHP; duration 223+/-101 ms, peak amplitude 5.6+/-2.4 mV, n=17). Fas t spike-train and burst AHPs were reduced by bath application of the C a2+-channel blockers Co2+ (2 mM) and Cd2+ (1 mM) (n=8), thus suggestin g the participation of Ca2+ dependent K+ conductances in these AHPs. S ubicular bursting neurons generated persistent, subthreshold voltage o scillations at 5.3+/-1 Hz (n=20) during steady depolarization positive to -60 mV; at values positive to -55 mV, the oscillatory activity cou ld trigger clusters of single action potentials with a periodicity of 0.9-2 Hz. Oscillations were not prevented by application of excitatory amino acid receptor and GABAA receptor antagonists (n=5), Ca2+-channe l blockers (n=5), or Cs+ (3 mM; n=4), but were abolished by the Na+-ch annel blocker tetrodotoxin (1 mu M; n=6). Our findings demonstrate tha t pyramidal-like subicular neurons generate both bursting and non-adap ting tonic firing, depending upon their membrane potential. These neur ons also display oscillatory activity in the range of theta frequency that depends on the activation of a voltage-gated Na+ conductance. The se electrophysiological proper ties may play a role in the process of signals arising from the hippocampal formation before being funnelled towards other limbic structures.