Responses of morphologically identified cortical neurons to intracellularly injected cyclic AMP

Intracellular injections of cyclic adenosine monophosphate (cAMP) and horseradish peroxidase (HRP) were made in neurons of the motor cortex of awake cats. Eighty-six percent of injected cells responded to cAMP and HRP with a rapid decrease in input resistance. The decreases in input resistance occurred immediately after injection and began to return toward baseline 2 to 3 min later. The decreases were significantly greater than the small decreases in input resistance normally seen in uninjected cells held for 2 min or more after penetration and exceeded comparably small decreases in input resistance seen after control injections of 5'-AMP plus HRP. Pyramidal cells of layer V were identified as responding to cAMP with a decreased input resistance. A spiny stellate cell of layer III and a pyramidal cell of layer VI were also identified that showed similar responses. Increased rates of discharge were also observed after penetration with electrodes containing cAMP, but significant changes in input resistance were not found in association with the increased rates of discharge. After pressure injection of cAMP, the rates of discharge decreased toward more normative values. Our findings indicate that cAMP has an effect on cortical neurons similar to that found in some types of invertebrate (molluscan) neurons and dissimilar to the effect of cyclic guanosine monophosphate.