Enhancement of hippocampal pyramidal cell excitability by the novel selective slow-afterhyperpolarization channel blocker 3-(triphenylmethylaminomethyl)pyridine (UCL2077)

The slow afterhyperpolarization (sAHP) in hippocampal neurons has been implicated in learning and memory. However, its precise role in cell excitability and central nervous system function has not been explicitly tested for 2 reasons: 1) there are, at present, no selective inhibitors that effectively reduce the underlying current in vivo or in intact in vitro tissue preparations, and 2) although it is known that a small conductance K+ channel that activates after a rise in [Ca2(+)](i) underlies the sAHP, the exact molecular identity remains unknown. We show that 3-(triphenylmethylaminomethyl) pyridine (UCL2077), a novel compound, suppressed the sAHP present in hippocampal neurons in culture (IC50 = 0.5 mu M) and in the slice preparation (IC50 approximate to 10 mu M). UCL2077 was selective, having minimal effects on Ca2+ channels, action potentials, input resistance and the medium afterhyperpolarization. UCL2077 also had little effect on heterologously expressed small conductance Ca2+-activated K+ SK) channels. Moreover, UCL2077 and apamin, a selective SK channel blocker, affected spike firing in hippocampal neurons in different ways. These results provide further evidence that SK channels are unlikely to underlie the sAHP. This study also demonstrates that UCL2077, the most potent, selective sAHP blocker described so far, is a useful pharmacological tool for exploring the role of sAHP channels in the regulation of cell excitability in intact tissue preparations and, potentially, in vivo.