Phentolamine selectively affects the fast sodium component of sensory adaptation in an insect mechanoreceptor.

Phentolamine and related compounds have several different actions on nervous tissues in vertebrates and invertebrates, including a local anesthetic effect. However, recent work suggests that phentolamine can interfere with sensory transduction in insect mechanoreceptors at significantly lower concentrations than are required for conduction block. We tested the actions of phentolamine on sensory transduction and encoding in an insect mechanoreceptor, the cockroach tactile spine neuron and found that 500 microM phentolamine increased the action potential threshold by 50%. The passive membrane properties of the neuron were not affected, but one component of dynamic threshold change was strongly and selectively reduced. This component has previously been attributed to slowly inactivating sodium channels in the action potential initiating region, suggesting that these channels are the most phentolamine-sensitive sites.