Dopamine modulation of two subthreshold currents produces phase shifts in activity of an identified motoneuron

1. The lateral pyloric (LP) neuron is a component of the 14-neuron pyloric central pattern generator in the stomatogastric ganglion of the spiny lobster, Panulirus interruptus. In the pyloric rhythm, this neuron fires rhythmic bursts of action potentials whose phasing depends on the pattern of synaptic inhibition from other network neurons and on the intrinsic postinhibitory rebound properties of the LP cell itself. Bath-applied dopamine excites the LP cell and causes its activity to be phase advanced in the pyloric motor pattern. At least part of this modulatory effect is due to dopaminergic modulation of the intrinsic rate of postinhibitory rebound in the LP cell. 2. The LP neuron was isolated from all detectable synaptic input. We measured the rate of recovery after 1-s hyperpolarizing current injections of varying amplitudes, quantifying the latency to the first spike following the hyperpolarizing prepulse and the interval between the first and second action potentials. Dopamine reduced both the first spike latency and the first interspike interval (ISI) in the isolated LP neuron. During the hyperpolarizating presteps, the LP cell showed a slow depolarizing sag voltage that was enhanced by dopamine. 3. We used voltage clamp to analyze dopamine modulation of subthreshold ionic currents whose activity is affected by hyperpolarizing prepulses. Dopamine modulated the transient potassium current I(A) by reducing its maximal conductance and shifting its voltage dependence for activation and inactivation to more depolarized voltages. This outward current is normally transiently activated after hyperpolarization of the LP cell, and delays the rate of postinhibitory rebound: by reducing I(A), dopamine thus accelerates the rate of rebound of the LP neuron. 4. Dopamine also modulated the hyperpolarization-activated in ward current I(h) by shifting its voltage dependence for activation 20 mV in the depolarizing direction and accelerating its rate of activation. This enhanced inward current helps accelerate the rate of rebound in the LP cell after inhibition. 5. The relative roles of I(h) and I(A) in determining the first spike latency and first ISI were explored using pharmacological blockers of I(h) (Cs⁺) and I(A) [4-aminopyridine (4-AP)]. Blockade of I(h) prolonged the first spike latency and first ISI, but only slightly reduced the net effect of dopamine. In the continued presence of Cs⁺, blockade of I(A) with 4-AP greatly shortened the first spike latency and first ISI. Under conditions where both I(h) and I(A) were blocked, dopamine had no additional effect on the LP cell. 6. We used the dynamic clamp technique to further study the relative roles of I(A) and I(h) modulation in dopamine's phase advance of the LP cell. We blocked the endogenous I(h) with Cs⁺ and replaced it with a simulated current generated by a computer model of I(h). The neuron with simulated I(h) gave curves relating the hyperpolarizing prepulse amplitude to first spike latency that were the same as in the untreated cell. Changing the computer parameters of the simulated I(h) to those induced by dopamine without changing I(A) caused only a slight reduction in first spike latency, which was ~20% of the total reduction caused by dopamine in an untreated cell. Bath application of dopamine in the presence of Cs⁺ and simulated I(h) (with control parameters) allowed us to determine the effect of altering I(A) but not I(h): this caused a significant reduction in first spike latency, but it was still only ~70% of the effect of dopamine in the untreated cell. Finally, in the continued presence of dopamine, changing the parameters of the simulated I(h) to those observed with dopamine reduced the first spike latency to that seen with dopamine in the untreated cell. 7. We generated a mathematical model of the lobster LP neuron, based on the model of Buchholtz et al. for the crab LP neuron. This model generated a curve of hyperpolarizing prepulse amplitude to first spike latency similar to that seen in normal LP neurons. Alteration of the parameters of I(A) and I(h) to those observed in dopamine caused a significant reduction in first spike latency. Alteration of the I(h) parameters alone had only a small effect, whereas alteration of the I(A) parameters alone had a much larger effect that was only slightly smaller than altering both currents together.