Dopamine modulation of two delayed rectifier potassium currents in a small neural network

Delayed rectifier potassium currents [ I (K(V))] generate sustained, noninactivating outward currents with characteristic fast rates of activation and deactivation and play important roles in shaping spike frequency. The pyloric motor network in the stomatogastric ganglion of the spiny lobster, Panulirus interruptus, is made up of one interneuron and 13 motor neurons of five different classes. Dopamine ( DA) increases the firing frequencies of the anterior burster ( AB), pyloric ( PY), lateral pyloric ( LP), and inferior cardiac ( IC) neurons and decreases the firing frequencies of the pyloric dilator ( PD) and ventricular dilator ( VD) neurons. In all six types of pyloric neurons, I-K(V) is small with respect to other K (+) currents. It is made up of at least two TEA- sensitive components that show differential sensitivity to 4- aminopyridine and quinidine, and have differing thresholds of activation. One saturable component is activated at potentials above - 25 mV, whereas the second component appears at more depolarized voltages and does not saturate at voltage steps up to + 45 mV. The magnitude of the components varies among cell types but also shows considerable variation within a single type. A subset of PY neurons shows a marked enhancement in spike frequency with DA; DA evokes a pronounced reversible increase in I-K(V) conductance of <= 30% in the PY neurons studied, and on average significantly increases both components of I-K(V). The AB neuron also shows a reversible 20% increase in the steady state I-K(V). DA had no effect on I-K(V) in PD, LP, VD, and IC neurons. The physiological roles of these currents and their modulation by DA are discussed.