The phases at which network neurons fire in rhythmic motor outputs are critically important for the proper generation of motor behaviors. The pyloric network in the crustacean stomatogastric ganglion generates a rhythmic motor output wherein neuronal phase relationships are remarkably invariant across individuals and throughout lifetimes. The mechanisms for maintaining these robust phase relationships over the long-term are not well described. Here we show that tonic nanomolar dopamine (DA) acts at type 1 DA receptors (D1Rs) to enable an activity-dependent mechanism that can contribute to phase maintenance in the lateral pyloric (LP) neuron. The LP displays continuous rhythmic bursting. The activity-dependent mechanism was triggered by a prolonged decrease in LP burst duration, and it generated a persistent increase in the maximal conductance (G max ) of the LP hyperpolarization-activated current (I h ), but only in the presence of steady-state DA. Interestingly, micromolar DA produces an LP phase advance accompanied by a decrease in LP burst duration that abolishes normal LP network function. During a 1 h application of micromolar DA, LP phase recovered over tens of minutes because, the activity-dependent mechanism enabled by steady-state DA was triggered by the micromolar DA-induced decrease in LP burst duration. Presumably, this mechanism restored normal LP network function. These data suggest steady-state DA may enable homeostatic mechanisms that maintain motor network output during protracted neuromodulation. This DA-enabled, activitydependent mechanism to preserve phase may be broadly relevant, as diminished dopaminergic tone has recently been shown to reduce I h in rhythmically active neurons in the mammalian brain.

Dopamine (DA) modifies the motor pattern generated by the pyloric network in the stomatogastric ganglion (STG) of the spiny lobster, Panulirus interruptus, by directly acting on each of the circuit neurons. The 14 pyloric neurons fall into six cell types, and DA actions are cell type specific. The transient potassium current mediated by shal channels (I(A)) is a common target of DA modulation in most cell types. DA shifts the voltage dependence of I(A) in opposing directions in pyloric dilator (PD) versus lateral pyloric (LP) neurons. The mechanism(s) underpinning cell-type specific DA modulation of I(A) is unknown. DA receptors (DARs) can be classified as type 1 (D1R) or type 2 (D2R). D1Rs and D2Rs are known to increase and decrease intracellular cAMP concentrations, respectively. We hypothesized that the opposing DA effects on PD and LP I(A) were due to differences in DAR expression patterns. In the present study, we found that LP expressed somatodendritic D1Rs that were concentrated near synapses but did not express D2Rs. Consistently, DA modulation of LP I(A) was mediated by a Gs-adenylyl cyclase-cAMP-protein kinase A pathway. Additionally, we defined antagonists for lobster D1Rs (flupenthixol) and D2Rs (metoclopramide) in a heterologous expression system and showed that DA modulation of LP I(A) was blocked by flupenthixol but not by metoclopramide. We previously showed that PD neurons express D2Rs, but not D1Rs, thus supporting the idea that cell specific effects of DA on I(A) are due to differences in receptor expression.

Neuromodulatory effects can vary with their mode of transmission. Phasic release produces local and transient increases in dopamine (DA) up to micromolar concentrations. Additionally, since DA is released from open synapses and reuptake mechanisms are not nearby, tonic nanomolar DA exists in the extracellular space. Do phasic and tonic transmissions similarly regulate voltage dependent ionic conductances in a given neuron? It was previously shown that DA could immediately alter the transient potassium current (IA) of identified neurons in the stomatogastric ganglion (STG) of the spiny lobster, Panulirus interruptus. Here we show that DA can also persistently alter IA, and that DA’s immediate and persistent effects oppose one another. The lateral pyloric neuron (LP) exclusively expresses type 1 DA receptors (D1Rs). Micromolar DA produces immediate depolarizing shifts in the voltage dependence of LP IA, whereas tonic nanomolar DA produces a persistent increase in LP IA maximal conductance (Gmax) through a translation dependent mechanism involving target of rapamycin (TOR). The pyloric dilator neuron (PD) exclusively expresses type 2 DA receptors (D2Rs). Micromolar DA produces an immediate hyperpolarizing shift in PD IA voltage dependence of activation, whereas tonic DA persistently decreases PD IA Gmax through a translation dependent mechanism not involving TOR. The persistent effects on IA Gmax do not depend on LP or PD activity. These data suggest a role for tonic modulators in the regulation of voltage gated ion channel number; and furthermore, that dopaminergic systems may be organized to limit the amount of change they can impose on a circuit.

Neuromodulators alter network output and have the potential to destabilize a circuit. The mechanisms maintaining stability in the face of neuromodulation are not well described. Using the pyloric network in the crustacean stomatogastric nervous system, we show that dopamine (DA) does not simply alter circuit output, but activates a closed loop in which DA-induced alterations in circuit output consequently drive a change in an ionic conductance to preserve a conductance ratio and its activity correlate. DA acted at low affinity type 1 receptors (D1Rs) to induce an immediate modulatory decrease in the transient potassium current (IA) of a pyloric neuron. This, in turn, advanced the activity phase of that component neuron, which disrupted its network function and thereby destabilized the circuit. DA simultaneously acted at high affinity D1Rs on the same neuron to confer activity-dependence upon the hyperpolarization activated current (Ih) such that the DA-induced changes in activity subsequently reduced Ih. This DA-enabled, activity-dependent, intrinsic plasticity exactly compensated for the modulatory decrease in IA to restore the IA:Ih ratio and neuronal activity phase, thereby closing an open loop created by the modulator. Activation of closed loops to preserve conductance ratios may represent a fundamental operating principle neuromodulatory systems use to ensure stability in their target networks.

Long-term intrinsic and synaptic plasticity must be coordinated to ensure stability and flexibility in neuronal circuits. Coordination might be achieved through shared transduction components. Dopamine (DA) is a well-established participant in many forms of long-term synaptic plasticity. Recent work indicates that DA is also involved in both activity-dependent and -independent forms of long-term intrinsic plasticity. We previously examined DA-enabled long-term intrinsic plasticity in a single identified neuron. The lateral pyloric (LP) neuron is a component of the pyloric network in the crustacean stomatogastric nervous system (STNS). LP expresses type 1 DA receptors (D1Rs). A 1 h bath application of 5 nM DA followed by washout produced a significant increase in the maximal conductance (Gmax) of the LP transient potassium current (IA) that peaked ~4 h after the start of DA application; furthermore, if a change in neuronal activity accompanied the DA application, then a persistent increase in the LP hyperpolarization activated current (Ih) was also observed. Here, we repeated these experiments with pharmacological and peptide inhibitors to determine the cellular processes and signaling proteins involved. We discovered that the persistent, DA-induced activity-independent (IA) and activity-dependent (Ih) changes in ionic conductances depended upon many of the same elements that enable long-term synaptic plasticity, including: the D1R-protein kinase A (PKA) axis, RNA polymerase II transcription, RNA interference (RNAi), and mechanistic target of rapamycin (mTOR)-dependent translation. We interpret the data to mean that increasing the tonic DA concentration enhances expression of a microRNA(s) (miRs), resulting in increased cap-dependent translation of an unidentified protein(s).