Abstract

N-(4-Acetyl-1-piperazinyl)-p-fluorobenzamide monohydrate (FK960), a novel antidementia drug, has been demonstrated to ameliorate memory deficits in various experimental models of dementia. This drug selectively increases somatostatin release from hippocampal slices and augments long-term potentiation (LTP) in the CA3 area of the hippocampus. In the present study, the effects of FK960 on voltage-activated Ca²⁺channels were investigated in acutely isolated rat hippocampal neurons, using whole-cell patch-clamp technique to clarify the cellular mode of action of FK960. Application of somatostatin significantly reduced Ca²⁺currents via G protein-coupled signaling pathways. This inhibitory effect was significantly abolished by FK960 when applied in combination. In contrast, FK960 showed only modest inhibition on the reduction in Ca²⁺currents produced by baclofen, an agonist of GABA_Breceptor. Intracellular application of the protein kinase inhibitor H-7 did not alter somatostatin-induced inhibition and had no significant effect on blockade by FK960. In addition, application of FK960 alone produced modest but apparent increases in Ca²⁺currents without significant changes in the activation kinetics of the channels. The dose-response relationship on calcium current enhancement was bell-shaped with a maximum effect at 0.1 μM FK960, the same concentration as that for increasing on somatostatin release and CA3-LTP. These results show that FK960 reverses G protein-dependent inhibition of Ca²⁺currents by somatostatin in hippocampal neurons. Enhancement of Ca²⁺currents by FK960 may be due to its modulatory actions on Ca²⁺channels, rather than removal of G protein-inhibited tonic currents. Together, these mechanisms may be involved in the selective effects of FK960 on somatostatin release, excitatory transmission, and synaptic plasticity in the hippocampus.