Substance P inhibits the M‐current in bullfrog sympathetic neurones

Substance P (SP, 2.5–10 μm) was applied by rapid bath perfusion to bullfrog lumbar sympathetic neurones in vitro, voltage‐clamped through a single micro‐electrode. In unclamped cells, SP produced a depolarization accompanied by an increase in apparent input resistance. Under voltage‐clamp a voltage‐dependent inward current was induced by SP, during which the time‐dependent relaxations induced by square voltage commands were inhibited. It is concluded that SP inhibits the M‐current (IM), a species of voltage‐dependent K+‐current, and that IM‐inhibition was the primary cause of the inward current and membrane depolarization in the cells tested.

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