Substance P receptor activation and desensitization as monitored by M current inhibition

Abstract

Substance P (SP) is a neuropeptide that plays a variety of roles in the human body, including inflammation, nociception, and smooth muscle regulation. In the central and peripheral nervous systems, SP indirectly acts as an excitatory neurotransmitter by inhibiting the M-current, (IM), a time- and voltage-dependent outward potassium current. Activation of IM produces an outward (hyperpolarizing) current which acts to resist neuronal depolarization and action potential production. By inhibiting IM, SP has the ability to modulate neuronal excitability. Since activation of the SP receptor results in IM inhibition, IM amplitude was utilized in the present study as an indirect measure for SP receptor activation and desensitization. Whole-cell voltage-clamp recordings were used to monitor IM in acutely isolated bullfrog sympathetic ganglia neurons. The cell was voltage-clamped at a holding potential of -30 mV, where IM is largely activated. Steps to -60 mV deactivated IM, producing a slow, exponential decrease in current amplitude, followed by an exponential increase in current (reactivation) upon stepping back to -30 mV. Single exponential curve fits were utilized to measure IM amplitude during the reactivation phase. SP application (1 μM) initially produced a rapid and significant inhibition in IM amplitude. However, with continuous SP application, IM amplitude significantly recovered, returning to the control amplitude. This desensitization, defined as a decrease in responsiveness following prolonged or repeated exposure to an agonist, suggests a similar desensitizing loss of functioning by the SP receptor during SP application. The findings presented here support previous studies showing that SP application initially produces inhibition of IM, but continued drug application results in desensitization and loss of IM inhibition.