Hormonal modulation of two coordinated rhythmic motor patterns.

Neuromodulation is well known to provide plasticity in pattern generating circuits, but few details are available concerning modulation of motor pattern coordination. We are using the crustacean stomatogastric nervous system to examine how co-expressed rhythms are modulated to regulate frequency and maintain coordination. The system produces two related motor patterns, the gastric mill rhythm that regulates protraction and retraction of the teeth and the pyloric rhythm that filters food. These rhythms have different frequencies and are controlled by distinct mechanisms, but each circuit influences the rhythm frequency of the other via identified synaptic pathways. A projection neuron, MCN1, activates distinct versions of the rhythms, and we show that hormonal dopamine concentrations modulate the MCN1 elicited rhythm frequencies. Gastric mill circuit interactions with the pyloric circuit lead to changes in pyloric rhythm frequency that depend on gastric mill rhythm phase. Dopamine increases pyloric frequency during the gastric mill rhythm retraction phase. Higher gastric mill rhythm frequencies are associated with higher pyloric rhythm frequencies during retraction. However, dopamine slows the gastric mill rhythm frequency despite the increase in pyloric frequency. Dopamine reduces pyloric circuit influences on the gastric mill rhythm and upregulates activity in a gastric mill neuron, DG. Strengthened DG activity slows the gastric mill rhythm frequency and effectively reduces pyloric circuit influences, thus changing the frequency relationship between the rhythms. Overall dopamine shifts dependence of frequency regulation from intercircuit interactions to increased reliance on intracircuit mechanisms.

Divergent co-transmitter actions underlie motor pattern activation by a modulatory projection neuron.

Co-transmission is a common means of neuronal communication, but its consequences for neuronal signaling within a defined neuronal circuit remain unknown in most systems. We are addressing this issue in the crab stomatogastric nervous system by characterizing how the identified modulatory commissural neuron (MCN)1 uses its co-transmitters to activate the gastric mill (chewing) rhythm in the stomatogastric ganglion (STG). MCN1 contains gamma-aminobutyric acid (GABA) plus the peptides proctolin and Cancer borealis tachykinin-related peptide Ia (CabTRP Ia), which it co-releases during the retractor phase of the gastric mill rhythm to influence both retractor and protractor neurons. By focally applying each MCN1 co-transmitter and pharmacologically manipulating each co-transmitter action during MCN1 stimulation, we found that MCN1 has divergent co-transmitter actions on the gastric mill central pattern generator (CPG), which includes the neurons lateral gastric (LG) and interneuron 1 (Int1), plus the STG terminals of MCN1 (MCN1(STG)). MCN1 used only CabTRP Ia to influence LG, while it used only GABA to influence Int1 and the contralateral MCN1(STG). These MCN1 actions caused a slow excitation of LG, a fast excitation of Int1 and a fast inhibition of MCN1(STG). MCN1-released proctolin had no direct influence on the gastric mill CPG, although it likely indirectly regulates this CPG via its influence on the pyloric rhythm. MCN1 appeared to have no ionotropic actions on the gastric mill follower motor neurons, but it did use proctolin and/or CabTRP Ia to excite them. Thus, a modulatory projection neuron can elicit rhythmic motor activity by using distinct co-transmitters, with different time courses of action, to simultaneously influence different CPG neurons.

Intercircuit control via rhythmic regulation of projection neuron activity.

Synaptic feedback from rhythmically active neuronal circuits commonly causes their descending inputs to exhibit the rhythmic activity pattern generated by that circuit. In most cases, however, the function of this rhythmic feedback is unknown. In fact, generally these inputs can still activate the target circuit when driven in a tonic activity pattern. We are using the crab stomatogastric nervous system (STNS) to test the hypothesis that the neuronal circuit-mediated rhythmic activity pattern in projection neurons contributes to intercircuit regulation. The crab STNS contains an identified projection neuron, modulatory commissural neuron 1 (MCN1), whose tonic stimulation activates and modulates the gastric mill (chewing) and pyloric (filtering of chewed food) motor circuits in the stomatogastric ganglion (STG). During tonic stimulation of MCN1, the pyloric circuit regulates both gastric mill cycle frequency and gastropyloric coordination via a direct synapse onto a gastric mill neuron in the STG. However, when MCN1 is spontaneously active, it has a pyloric-timed activity pattern attributable to synaptic input from the pyloric circuit. This pyloric-timed activity in MCN1 provides the pyloric circuit with a second pathway for regulating the gastric mill rhythm. At these times, the direct STG synapse from the pyloric circuit to the gastric mill circuit is not necessary for pyloric regulation of the gastric mill rhythm. However, in the intact system, these two pathways play complementary roles in this intercircuit regulation. Thus, one role for rhythmicity in modulatory projection neurons is to enable them to mediate the interactions between distinct but related neuronal circuits.

Extracellular peptidase activity tunes motor pattern modulation.

We are examining how extracellular peptidase activity sculpts the peptidergic actions of modulatory projection neurons on rhythmically active neuronal circuits, using the pyloric circuit in the stomatogastric ganglion (STG) of the crab Cancer borealis. Neurally released peptides can diffuse long distances to bind to their receptors. Hence, different neurons releasing the same neuropeptide into the same neuropil may reach the same receptor complement. However, extracellular peptidases can limit neuropeptide diffusion and terminate its actions. Distinct versions of the pyloric rhythm are elicited by selective activation of different projection neurons, including those with overlapping sets of cotransmitters. Two of these projection neurons, modulatory commissural neuron 1 (MCN1) and the modulatory proctolin neuron (MPN), contain the neuropeptide proctolin plus GABA. MCN1 also contains Cancer borealis tachykinin-related peptide Ia (CabTRP Ia). CabTRP Ia is not fully responsible for the distinct actions of MCN1 and MPN. Because there is aminopeptidase activity in the STG that terminates proctolin actions, we tested the hypothesis that the differences in the actions of MCN1 and MPN that are not mediated by CabTRP Ia result from the differential actions of aminopeptidase activity on proctolin released from these two projection neurons. We found that the pyloric circuit response to these two projection neurons becomes more similar when this aminopeptidase activity is blocked. This result supports the hypothesis that extracellular peptidase activity enables different projection neurons to use the same neuropeptide transmitter for eliciting distinct outputs from the same neuronal circuit.