Catecholamine effects on frog dorsal root terminals

Dopamine, norepinephrine and epinephrine applied to the isolated superfused frog spinal cord had complex effects on the terminals of primary afferent fibers. The most cosistent finding was a slow hyperpolarization of terminals with lower concentrations (10 μM or lower), but depolarization either following or admixed with the hyperpolarization were seen. These were particularly prominent when the catecholamines were applied in high concentrations or for prolonged periods of time. A part of the response of afferent terminals appears to be indirect since the potential changes were reduced following exposure of the cord to tetrodotoxin, Mn 2+, or mephenesin. The hyperpolarizations were augmented by imipramine, a known inhibitor of catecholamine uptake. These observations are consistent with a role of catecholamines in the processing of sensory input in the spinal cord.