Relationship between positive sharp wave bursts and unitary discharges in the cat hippocampus during slow wave sleep

In the cat hippocampus bursts of positive sharp waves (PSWs) appeared sporadically almost exclusively during slow wave sleep. The PSW burst was most often found in cell-rich areas in the CA1 and subiculum, and its occurrence was almost synchronized in different regions. An individual burst was usually composed of 3-5 PSWs of about 10 msec duration and showed a considerable fluctuation in amplitude. It was occasionally followed by a negative-going deflection of large amplitude and long duration (post-PSW negativity). The amplitude of PSWs and post-PSW negativity in the CA1 was high in the area giving a large sized-evoked response after stimulation of the contralateral CA3. The spike discharge rate during the burst was two or three times higher than that during the period just preceding the burst, but the discharge never occurred in the positive phase of the PSWs. During the initial part of the post-PSW negativity the high firing probability was maintained. Even when the PSW burst was not followed by a detectable post-PSW negativity, the firing probability during the period corresponding to the post-PSW negativity was still significantly higher than the pre-PSW period. It was suggested that the PSW bursts and post-PSW negativity were triggered off in cell-rich areas by diffuse excitatory inputs impinging possibly upon the hippocampal pyramidal cells and subicular principal cells. The rhythmic PSWs may be post-synaptic inhibitory potentials produced on the somata of those cells after activation of recurrent interneuronal circuits.