Abstracts

Rationale: The synchrony and output of the principal cell population in the CA1 region is controlled by specialized perisomatically-targeting inhibitory neurons called the basket cells. Recently we have shown that cholecystokinin (CCK), through CCK2 receptors, powerfully influences the activity of the parvalbumin-expressing (PV+) basket cells (Földy et al., 2007). To elucidate how CCK exerts its control over perisomatic inhibition, we seek to characterize the CCK signaling pathway in the PV+ basket cells. Methods: We prepared 350um acute hippocampal slices from juvenile (P16-21) rats, and performed whole-cell patch clamp recordings on parvalbumin-expressing basket cells from the CA1 region. Identity of all recorded cells was confirmed with immunocytochemistry and DAB staining to visualize the axonal morphology. Results: Application of 500nM CCK induced an inward current of -91.4 ± 17.4 pA in the PV+ basket cells. While blockers of voltage-gated sodium and potassium channels had no effect on the CCK-induced current, ion subsitution experiments suggested a role for the TRP channels. Indeed, blockers of TRP channels were able to significantly reduce the CCK-induced current, and the current reversal potential was near -21mV, indicative of a nonspecific cation conductance. RT-PCR also confirms expression of the TRPC channels in the PV+ basket cells. To elucidate the signaling link between CCK2 receptor activation to TRP channel activation, we looked for a possible role for intracellular calcium stores. We show that while PLC and IP3 receptors are not involved, ryanodine receptors and intracellular calcium stores play a critical role in mediating the CCK-induced current. In addition, calcium imaging experiments confirmed a rise in intracellular calcium in the PV+ basket cells after CCK application. Interestingly, we have found that the control of CCK is specific to perisomatic inhibition, as the dendritically-targeting PV+ bistratified cells show no significant depolarizing current after CCK application. Conclusions: Our results reveal a novel signaling pathway for CCK specifically in the PV+ basket cells. We show that here, CCK signals through a less canonical G-protein signaling pathway, involving ryanodine receptors and intracellular calcium stores, and ultimately opening TRP channels to depolarize the PV+ basket cells. As these basket cells exert strong control over network activity in the hippocampus, it is of critical importance to gain a detailed understanding of the factors that control perisomatic inhibition. A breakdown in this finely-tuned control of basket cells by CCK could have severe consequences on hippocampal excitability. Studies are underway to explore whether any alterations in this fine-tuned modulation of basket cells by CCK may play a role in development of the persistent hyperexcitability seen after post-traumatic brain injury. This project was supported by the NIH (NS #38580 to I.S. and NS #33213 to M.S.), the George E. Hewitt Foundation for Medical Research (to J.S.)), and the American Epilepsy Society (to S.L.)