Abstracts

Rationale: Perisomatic inhibitory basket cells in the hippocampus play a critical role in gating network excitability. We previously investigated the cell-type specific control of the neuropeptide cholecystokinin (CCK), and have found that CCK can selectively and strongly enhance the activity of parvalbumin-expressing (PV+) interneurons. (Földy et al. 2007) Here, we investigate the signal transduction pathway for the CCK-B receptors at PV+ interneurons. Methods: We prepared 350um acute hippocampal slices from 16-21 day old Sprague-Dawley rats, and performed whole-cell patch clamp recordings on PV+ interneurons in the CA1 region. Identity of all interneurons was confirmed by immunocytochemistry. Results: Bath application of 500nM CCK-8S invoked an inward current of about -95pA in PV+ interneurons. Blockers of potassium and sodium channels had no significant effect on this current. However, removing calcium from the recording medium increased the magnitude of this holding current, while replacing sodium with choline chloride abolished it. Taken together, these data are consistent with the properties of a channel nonselectively permeable to cations in which calcium flows more slowly than sodium due ion size. One family of such channels is the TRP channels. Indeed, SKF-96365 and flufenamic acid, widely used TRP channel inhibitors, significantly reduced the CCK-induced current, further indicating the involvement of these channels. Exploration into the intracellular signalling pathway shows that blocking PLC, Src-kinase or ER calcium pumps has no significant effect in the CCK-induced response. But NEM (N-ethylmaleimide) an inhibitor of the Gi-protein family (Gi, Go), or intracellular application of a Gi/o/t/z antibody into the recording pipette significantly abolishes the CCK-induced change in holding current. In addition, single-cell RT-PCR confirms the expression of TRPC channels specifically in the parvalbumin-positive interneurons. Conclusions: These data show that the CCK-induced current depolarizing the parvalbumin-positive interneurons is mediated by the coupling of CCK-B receptors to TRP channels. The results here strongly implicate cell-specific intracellular pathways involved in the control of perisomatic inhibition by CCK in the rat hippocampus. Supported by NIH (NS #38580 to I.S. and NS #33213 to M.S.) and the George E. Hewitt Foundation for Medical Research (to JS)