Local anesthetic action of phentolamine on insect mechanoreceptors

Summary

• 1. The effect of phentolamine on the response properties of insect mechanoreceptors and on the conduction in their axons was examined using electrophysiological techniques.

• 2. Phentolamine blocked conduction of action potentials along axons, an effect which exhibited 3 characteristics typical of local anesthetics: the effect was frequency-dependent, reversible and varied for nerves with different diameters.

• 3. The concentration of phentolamine required to block axonal conduction (1–2×10⁻³ M) was significantly higher than that required to abolish the response of receptors to mechanical stimulation (3–5×10⁻⁴ M).

• 4. All mechanoreceptors that were examined in Locusta migratoria and Periplaneta americana were inactivated by phentolamine (Table 1). The type I receptors (chordotonal, campaniform and hair sensilla) were inactivated within 5–15 min following phentolamine application. The only type II receptor examined (forewing stretch-receptor) underwent a phase of repetitive discharge before being inactivated.

• 5. Tolazoline and metoclopramide inactivated, like phentolamine, mechanoreceptors at lower concentrations than necessary to block axonal conduction. However, yohimbine and chlorpromazine inactivated mechanoreceptors and blocked axonal conduction at similar concentrations.

• 6. These findings suggest that phentolamine affects sense-organ specific ionic processes that are more sensitive to the drug than the ionic processes along the axons.