Effects of acetylcholine and cyclic GMP on input resistance of cortical neurons in awake cats.

The effects on input resistance (Rm) of extracellular iontophoresis of acetylcholine (ACh) and of intracellular iontophoresis of cyclic GMP (cGMP) were studied in neurons of the coronal-pericruciate cortex of awake cats. Control studies were also conducted including iontophoresis of saline extracellularly and 5'-GMP intracellularly. (1) Substantial increases in Rm occurred in approximately half the neurons given ACh or cGMP. (2) In the absence of associated repetitive spike discharge induced by intracellular injection of depolarizing current pulses during iontophoretic applications, the increases in Rm were transient occurring in less than 30 sec and lasting 4--5 min. (3) With associated current-induced spike discharge, the increases in Rm persisted for as long as the neurons could be held -- up to 1.5 h maximally. (4) Rm was not increased if saline was substituted for ACh, if 5'-GMP was substituted for cGMP, or if the neuron was only discharged repeatedly. (5) The magnitude and time course of both transient and persistent increases in Rm were comparable between cells given ACh or cGMP and whether action potentials and resting potentials were greater than or equal to 40 mV (average 47 mV)* or were less.