Dopamine modulates two potassium currents and inhibits the intrinsic firing properties of an identified motor neuron in a central pattern generator network.

The two pyloric dilator (PD) neurons are components [along with the anterior burster (AB) neuron] of the pacemaker group of the pyloric network in the stomatogastric ganglion of the spiny lobster Panulirus interruptus. Dopamine (DA) modifies the motor pattern generated by the pyloric network, in part by exciting or inhibiting different neurons. DA inhibits the PD neuron by hyperpolarizing it and reducing its rate of firing action potentials, which leads to a phase delay of PD relative to the electrically coupled AB and a reduction in the pyloric cycle frequency. In synaptically isolated PD neurons, DA slows the rate of recovery to spike after hyperpolarization. The latency from a hyperpolarizing prestep to the first action potential is increased, and the action potential frequency as well as the total number of action potentials are decreased. When a brief (1 s) puff of DA is applied to a synaptically isolated, voltage-clamped PD neuron, a small voltage-dependent outward current is evoked, accompanied by an increase in membrane conductance. These responses are occluded by the combined presence of the potassium channel blockers 4-aminopyridine and tetraethylammonium. In voltage-clamped PD neurons, DA enhances the maximal conductance of a voltage-sensitive transient potassium current (IA) and shifts its Vact to more negative potentials without affecting its Vinact. This enlarges the "window current" between the voltage activation and inactivation curves, increasing the tonically active IA near the resting potential and causing the cell to hyperpolarize. Thus DA's effect is to enhance both the transient and resting K+ currents by modulating the same channels. In addition, DA enhances the amplitude of a calcium-dependent potassium current (IO(Ca)), but has no effect on a sustained potassium current (IK(V)). These results suggest that DA hyperpolarizes and phase delays the activity of the PD neurons at least in part by modulating their intrinsic postinhibitory recovery properties. This modulation appears to be mediated in part by an increase of IA and IO(Ca). IA appears to be a common target of DA action in the pyloric network, but it can be enhanced or decreased in different ways by DA in different neurons.

Quantitative single-cell-reverse transcription-PCR demonstrates that A-current magnitude varies as a linear function of shal gene expression in identified stomatogastric neurons.

Different Shaker family alpha-subunit genes generate distinct voltage-dependent K+ currents when expressed in heterologous expression systems. Thus it generally is believed that diverse neuronal K+ current phenotypes arise, in part, from differences in Shaker family gene expression among neurons. It is difficult to evaluate the extent to which differential Shaker family gene expression contributes to endogenous K+ current diversity, because the specific Shaker family gene or genes responsible for a given K+ current are still unknown for nearly all adult neurons. In this paper we explore the role of differential Shaker family gene expression in creating transient K+ current (IA) diversity in the 14-neuron pyloric network of the spiny lobster, Panulirus interruptus. We used two-electrode voltage clamp to characterize the somatic IA in each of the six different cell types of the pyloric network. The size, voltage-dependent properties, and kinetic properties of the somatic IA vary significantly among pyloric neurons such that the somatic IA is unique in each pyloric cell type. Comparing these currents with the IAs obtained from oocytes injected with Panulirus shaker and shal cRNA (lobster Ishaker and lobster Ishal, respectively) reveals that the pyloric cell IAs more closely resemble lobster Ishal than lobster Ishaker. Using a novel, quantitative single-cell-reverse transcription-PCR method to count the number of shal transcripts in individual identified pyloric neurons, we found that the size of the somatic IA varies linearly with the number of endogenous shal transcripts. These data suggest that the shal gene contributes substantially to the peak somatic IA in all neurons of the pyloric network.

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 pre-steps, 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 IA 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 IA, dopamine thus accelerates the rate of rebound of the LP neuron. 4. Dopamine also modulated the hyperpolarization-activated inward current Ih 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 Ih and IA in determining the first spike latency and first ISI were explored using pharmacological blockers of Ih (Cs+) and IA [4-aminopyridine (4-AP)]. Blockade of Ih 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 IA with 4-AP greatly shortened the first spike latency and first ISI. Under conditions where both Ih and IA 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 IA and Ih modulation in dopamine's phase advance of the LP cell. We blocked the endogenous Ih with Cs+ and replaced it with a simulated current generated by a computer model of Ih. The neuron with simulated Ih 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 Ih to those induced by dopamine without changing IA caused only a slight reduction in first spike latency, which was approximately 20% of the total reduction caused by dopamine in an untreated cell. Bath application of dopamine in the presence of Cs+ and simulated Ih (with control parameters) allowed us to determine the effect of altering IA but not Ih: this caused a significant reduction in first spike latency, but it was still only approximately 70% of the effect of dopamine in the untreated cell. Finally, in the continued presence of dopamine, changing the parameters of the simulated Ih 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.