Potentials evoked by alvear tract in hippocampal CA1 region of rats. I. Topographical projection, component analysis, and correlation with unit activities

The field potentials and unit activities evoked by the alvear tract (AT) in CA1 region of the dorsal hippocampus of rats were studied under sodium pentobarbital anesthesia. 2. The localized activity evoked anterior to an AT stimulus began as a compound action potential, followed by a slower negative wave, and ended in a long-lasting, slow positive wave. Observed with a 64-electrode recording array, topographical projections of the AT in CA1 were seen as parallel strips inclined at an angle of 5-30 degrees medially from the sagittal plane. 3. Three overlapping components in the averaged evoked potentials (AEPs) were distinguished. The first event (component I) was a brief compound antidromic action potential of pyramidal cells. The second field event (component II) reversed from surface negative to deep positive at 200 micrometer from the ventricular surface, increased rapidly with stimulus intensity, potentiated with double shocks, and followed stimulus frequency up to 50/s. The third component was long lasting (up to 200 ms), surface positive and ventral negative (turnover at 150 micron below the pyramidal layer), followed stimulus frequency up to about 10/s, and saturated at a low stimulus intensity (about 3 x threshold). 4. In some preparations, another fast negative peak of about 2 ms duration was found to follow the axon compound action potential on the hippocampal surface and appeared to propagate from the pyramidal layer to the ventricular surface. It was probably of nonsynaptic origin, perhaps due to the centrifugal basal dendritic spikes of the pyramidal cells. 5. Single units were recorded in CA1. Antidromic units were identified by their firing at a fixed latency (1.5 ms) and ability to follow high stimulus frequencies. Units firing at about 2.7 ms latency possessed characteristics of monosynaptic excitation. Under light anesthesia, many of the latter units also showed a late, prolonged suppression of background firing. Tentative interneuronal types fired with peak latencies of 4-5 ms or showed prolonged increase in firing rate. 6. From the correlation with unit post-stimulus time histograms, AEP component II was inferred to be the extracellular, monosynaptic, excitatory postsynaptic potentials, and component III the di- or polysynaptic inhibitory postsynaptic potentials. These postsynaptic potentials were generated by the pyramidal cells and interneurons.