Neuronal transmission through hippocampal pathways dependent on behavior

Abstract

1. In chronically prepared, freely moving rats, electrical stimulation was applied to the angular bundle, and responses were recorded extracellularly at a variety of sites in the ipsilateral hippocampal formation. At each recording site the stimulus-response relationship was tested during four different behavioral states. These were slow-wave sleep (SWS), REM sleep ( REM), and also two waking behaviors consisting of the still, alert condition (labeled SAL), and voluntary movement (AW theta). 2. Two varieties of evoked responses were recorded: those due to the synchronous firing of neuronal action potentials (EAPs) and those produced by excitatory synaptic activity (ESPs). The overall pattern of monosynaptic, di-, and trisynaptic responses found was similar in the rat to that found by Andersen et al. (3-5) in cat and rabbit. 3. When the trisynaptic EAP was recorded in CA1, the threshold was similar during all four behavioral states. However, suprathreshold stimuli evoked a greater response during SWS than during the other three states. The trisynaptic ESP was also greater during SWS. 4. Disynaptically, EAPs were recorded in CA3. These were greater in magnitude during SWS than during SAL, but were intermediate in mean amplitude during AWtheta and REM. Response variability was much greater during AWtheta and REM. 5. The monosynaptic EAP recorded in the upper blade of the dentate gyrus (DG) exhibited the same behaviorally correlated properties found disynaptically in CA3. 6. The monosynaptic ESP recorded in the DG, in contrast to the EAP, was greater in magnitude during SAL than during SWS. 7. The primary afferent volley was also recorded at high gain in the DG. The amplitude of this was found to be dependent solely on stimulus intensity and not on behavioral state. 8. The results are interpreted as suggesting that the granule cell membranes in the DG are relatively hyperpolarized during SAL compared with SWS as the result of either tonic excitatory bombardment occurring during SWS or tonic inhibitory bombardment during SAL.