ω‐Conotoxin Inhibits the Acute Activation of Tyrosine Hydroxylase and the Stimulation of Norepinephrine Release by Potassium Depolarization of Sympathetic Nerve Endings

Abstract

Increased Ca²⁺ influx serves as a signal that initiates multiple biochemical and physiological events in neurons following depolarization. The most widely studied of these phenomena is the release of neurotransmitters. In sympathetic neurons, depolarization also increases the rate of synthesis of the transmitter norepinephrine (NE), via an activation of the enzyme tyrosine hydroxylase (TH), and this effect also seems to involve Ca²⁺ entry. We have examined whether the mechanism of Ca²⁺ entry relevant to TH activation is via voltage‐sensitive Ca²⁺ channels and, if so, whether the type of Ca²⁺ channel involved is the same as that involved in the stimulation of NE release. We have investigated the isolated rat iris, allowing us to examine transmitter biosynthesis and release in sympathetic nerve terminals in the absence of sympathetic cell bodies and dendrites. Potassium depolarization produced a three‐ to fivefold increase in TH activity and an ∼ 100‐fold increase in NE release. Both effects were dependent on Ca²⁺ being present in the extracellular medium, and both were inhibited by ω‐conotoxin (1μM), which inhibits N‐type voltage‐sensitive Ca²⁺ channels. In contrast, the di‐hydropyridine nimodipine (1–3 μM), which blocks L‐type Ca²⁺ channels, had no effect on either measure. These data support the hypothesis that increases in NE biosynthesis and release in sympathetic nerve terminals during periods of depolarization are both initiated by an influx of Ca²⁺ through voltage‐sensitive Ca²⁺ channels and that a similar type of Ca²⁺ channel is involved in both processes.