SHARP WAVE-ASSOCIATED HIGH-FREQUENCY OSCILLATION (200-HZ) IN THE INTACT HIPPOCAMPUS - NETWORK AND INTRACELLULAR MECHANISMS

Sharp wave bursts, induced by a cooperative discharge of CA3 pyramidal cells, are the most synchronous physiological pattern in the hippocam pus. In conjunction with sharp wave bursts, CA1 pyramidal cells displa y a high-frequency (200 Hz) network oscillation (ripple), In the prese nt study extracellular field and unit activity was recorded simultaneo usly from 16 closely spaces sites in the awake rat and the intracellul ar activity of CA1 pyramidal cells during the network oscillation was studied under anesthesia, Current source density analysis of the high- frequency oscillation revealed circumscribed sinks and sources in the vicinity of the pyramidal layer, Single pyramidal cells discharged at a low frequency but were phase locked to the negative peak of the loca lly derived field oscillation, Approximately 10% of the simultaneously recorded pyramidal cells fired during a given oscillatory event. Puta tive interneurons increased their discharge rates during the field rip ples severalfold and often maintained a 200 Hz frequency during the os cillatory event, Under urethane and ketamine anesthesia the frequency of ripples was slower(100-120 Hz) than in the awake rat(180-200 Hz), H alothane anesthesia prevented the occurrence of high-frequency field o scillations in the CA1 region. Both the amplitude(1-4 mV) and phase of the intracellular ripple, but not its frequency, were voltage depende nt, The amplitude of intracellular ripple was smallest between -70 and -80 mV, The phase of intracellular oscillation relative to the extrac ellular ripple reversed when the membrane was hyperpolarized more than -80 mV, A histologically verified CA1 basket cell increased its firin g rate during the network oscillation and discharged at the frequency of the extracellular ripple. These findings indicate that the intracel lularly recorded fast oscillatory rhythm is not solely dependent on me mbrane currents intrinsic to the CA1 pyramidal cells but it is a netwo rk driven phenomenon dependent upon the participation of inhibitory in terneurons. We hypothesize that fast field oscillation (200 Hz) in the CA1 region reflects summed IPSPs in pyramidal cells as a result of hi gh-frequency barrage of interneurons. The sharp wave associated synchr onous discharge of pyramidal cells in the millisecond range can exert a powerful influence on retrohippocampal targets and may facilitate th e transfer of transiently stored memory traces from the hippocampus to the entorhinal cortex.