MODULATION OF ENSEMBLE FIRING PATTERNS IN RAT VENTRAL STRIATUM IN CLOSE ASSOCIATION WITH HIPPOCAMPAL RIPPLES/SHARP WAVES.

C. M. A. Pennartz*, E. Lee, J. Verheul, P. Lipa, C. A. Barnes and B. L. McNaughton.

Netherlands Institute for Brain Research and University of Amsterdam, Amsterdam, the Netherlands; Arizona Research Laboratories, Dept. Neural Systems, Memory and Aging, Tucson, AZ 85724.

Communication between the hippocampal formation (HPF) and ventral striatum (VS) is thought to be essential for the transfer of contextual information to the emotional motor system. Although the HPF is known to send a strong projection to the ventral striatum, it is unclear how these two structures interact during effortful cognitive processing and off-line information processing such as during sleep. Specifically, ripples/sharp waves in the hippocampus, characterized by transient ~200 Hz oscillations, have been associated with spontaneous reactivation of ensemble firing patterns established during prior behavioral experience. Here we examine to what extent hippocampal ripple activity influences information processing in the VS.
Three male Fischer 344 rats were trained on a T-maze task. Multiple tetrodes were implanted unilaterally in VS and EEG electrodes were positioned in stratum pyramidale and radiatum of area CA1. Each rat was subjected to a protocol consisting of a first sleep phase, a phase of reward-searching behavior on the maze and a second sleep phase. Spike trains from 14-20 well-isolated VS units were recorded in parallel during all 3 phases. Filtered hippocampal EEG traces were used for off-line automatic detection of ripples. Across 21 sessions (340 unit recordings, analyzed during post-behavioral sleep), a subset of recorded ensembles showed significant firing rate modulation by ripples (ANOVA: P < 0.05; mean + sem: 4.1 + 0.3 cells per session; fractional size: 25.4 + 2.1 %), mostly excitatory but sometimes inhibitory or mixed excitatory-inhibitory. Thus, hippocampal ripples are closely associated in time with significant transient changes of firing rate in a subpopulation of VS neurons.

Supported by HFSP Grant RGP0127/2001-B and PHS grant MH46823 and MH01565.