THE BEHAVIOR OF MOSSY CELLS OF THE RAT DENTATE GYRUS DURING THETA-OSCILLATIONS IN-VIVO

Intracellular current clamp recordings were obtained from mossy cells (n = 6, identified by intracellular injection of biocytin) of the dors al dentate gyrus from rats under ketamine-xylazine anesthesia. During electroencephalographic theta rhythm (4-6 Hz), recorded with a macroel ectrode placed in the contralateral dorsal hippocampus near the fissur e, mossy cells displayed intracellular membrane potential oscillations at low frequencies (4-6 Hz) which appeared to be phase locked to the electroencephalographic theta rhythm. The frequency of the intracellul ar theta rhythm was independent of the membrane potential. However, th e phase difference between the intracellular and the electroencephalog raphic theta rhythms as well as the amplitude of the intracellular the ta oscillations were voltage-dependent. These findings are consistent with the hypothesis that rhythmic GABA(A) receptor-mediated inhibitory postsynaptic potentials contribute to the genesis of the intracellula r theta rhythm. Indeed, mossy cells displayed an early, fast inhibitor y postsynaptic potential in response to electrical stimulation of the entorhinal cortex, which most likely represents a GABA(A) receptor-med iated event, indicating that mossy cells possess functional GABA(A) re ceptors. At the resting membrane potential, mossy cells did not fire a t each cycle of the electroencephalographic theta rhythm but fired onl y rarely (< 1 Hz). However, when they did fire they did so preferentia lly in phase with the peak positivity of the electroencephalographic t heta rhythm. Reconstruction of two mossy cells with axonal projections to the inner molecular layer showed that the spatial ''tent of the in fluence such weakly discharging mossy cells may have on other dentate gyrus neurons during theta oscillations can be several millimeters in the septotemporal direction. In conclusion, these findings show that m ossy cells of the rat hilus during ketamine-xylazine anesthesia partic ipate in theta oscillations of the hippocampal formation, during which their low-frequency firing may contribute to the phase-locking of a l arge number of spatially distributed postsynaptic neurons with postsyn aptic sites in the inner molecular layer of the dentate gyrus.