In vivoEffects of Temperature on the Heart and Pyloric Rhythms in the Crab,Cancer borealis

Summary StatementTemperature elevation increases the frequency of the heart and pyloric rhythms of the crab,Cancer borealis, but the heart rhythm has a higher critical temperature than the pyloric rhythm.AbstractThe heart and pyloric rhythms of crustaceans have been studied separately and extensively over many years. Local and hormonal neuromodulation and sensory inputs onto these central pattern generating circuits play a significant role in the animals’ responses to perturbations, but are usually lost or removed duringin vitrostudies. To examine simultaneously thein vivomotor output of the heart and pyloric rhythms, we used photoplethysmography (PPG). In the population measured (n = 49), the heart rhythm frequencies ranged from 0.3–2.3 Hz. The pyloric rhythms varied from 0.2–1.6 Hz. During multiple hour-long recordings, many animals held at control temperature showed strong inhibitory bouts in which the heart decreased in frequency or become quiescent and the pyloric rhythm also decreased in frequency. Many animals show significant coherence in frequency between the rhythms at the frequency of the heart rhythm. We measured the simultaneous responses of the rhythms to temperature ramps by heating or cooling the saline bath while recording both the heart and pyloric muscle movements. Q10s, critical temperatures (temperatures at which function is compromised), and changes in frequency were calculated for each of the rhythms tested. The heart rhythm was more robust to high temperature than the pyloric rhythm.

Circuit robustness to temperature perturbation is altered by neuromodulators

SUMMARYIn the ocean, the crab, Cancer borealis, is subject to daily and seasonal temperature changes. Previous work, done in the presence of descending modulatory inputs, had shown that the pyloric rhythm of the crab increases in frequency as temperature increases, but maintains its characteristic phase relationships until it “crashes” at extreme high temperatures. To study the interaction between neuromodulators and temperature perturbations, we studied the effects of temperature on preparations from which the descending modulatory inputs were removed. Under these conditions the pyloric rhythm was destabilized. We then studied the effects of temperature on preparations in the presence of oxotremorine, proctolin, and serotonin. Oxotremorine and proctolin enhanced the robustness of the pyloric rhythm, while serotonin made the rhythm less robust. These experiments reveal considerable animal-to-animal diversity in their crash stability, consistent with the interpretation that cryptic differences in many cell and network parameters are revealed by extreme perturbations.

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.

Amine Modulation of Ih in a Small Neural Network

We studied the functional role and modulation of the hyperpolarization-activated inward current ( Ih) in the pyloric network of the lobster stomatogastric ganglion. In isolated neurons, Ih is a small current with a hyperpolarized voltage of half-activation ( VAct) and a slow time constant of activation (τAct). Bath application of dopamine (DA), octopamine (OCT), or serotonin (5HT) modified Ih in selected synaptically isolated pyloric neurons. DA significantly enhanced Ih in the anterior burster (AB) neuron by depolarizing its VAct, accelerating its τAct, and enhancing its maximal conductance ( gmax). DA more weakly enhanced Ih in the pyloric constrictor (PY) and ventricular dilator (VD) neurons. OCT weakly depolarized VAct and accelerated τAct in the VD and inferior cardiac (IC) neurons. 5HT depolarized VAct in the IC neuron. Under control conditions with intact modulatory inputs from other ganglia, the pyloric rhythm cycles strongly at about 1–2 Hz. Bath application of the Ih blocker cesium (Cs+) caused a mean increase in the period of 8%, although this effect was highly variable. When Cs+ was applied to an isolated ganglion where the pyloric rhythm had been activated only by DA, the cycle period was consistently increased by 13.5%, with no other strong changes in rhythm parameters. These results suggest that Ih regulates the pyloric rhythm by accelerating AB pacemaker frequency, but that this effect can vary with the modulatory conditions.

Red Pigment Concentrating Hormone Strongly Enhances the Strength of the Feedback to the Pyloric Rhythm Oscillator But Has Little Effect on Pyloric Rhythm Period

The neuropeptide, red pigment concentrating hormone (RPCH), strengthened the inhibitory synapse from the lateral pyloric (LP) neuron to the pyloric dilator (PD) neurons in the pyloric network of the stomatogastric ganglion (STG) of the lobster, Homarus americanus. RPCH produced several-fold increases in the amplitude of both action potential–mediated and non–impulse-mediated transmission that persisted for as long as the peptide remained present. Because the LP to PD synapse is the only feedback to the pacemaker kernel of the pyloric network, which consists of the electrically coupled two PD neurons and the anterior burster (AB) neuron, it might have been expected that strengthening the LP to PD synapse would increase the period of the pyloric rhythm. However, the period of the pyloric rhythm increased only transiently in RPCH, and a transient increase in cycle period was observed even when the LP neuron was hyperpolarized. Phase response curves were measured using the dynamic clamp to create artificial inhibitory inputs of variable strength and duration to the PD neurons. Synaptic conductance values seen in normal saline were ineffective at changing the pyloric period throughout the pyloric cycle. Conductances similar to those seen in 10−6 M RPCH also did not evoke phase resets at phases when the LP neuron is typically active. Thus the dramatic effects of RPCH on synaptic strength have little role in modulation of the period of the pyloric rhythm under normal operating conditions but may help to stabilize the rhythm when the cycle period is too slow or too fast.

Long-Lasting Activation of Rhythmic Neuronal Activity by a Novel Mechanosensory System in the Crustacean Stomatogastric Nervous System

Sensory neurons enable neural circuits to generate behaviors appropriate for the current environmental situation. Here, we characterize the actions of a population (about 60) of bilaterally symmetric bipolar neurons identified within the inner wall of the cardiac gutter, a foregut structure in the crab Cancer borealis. These neurons, called the ventral cardiac neurons (VCNs), project their axons through the crab stomatogastric nervous system to influence neural circuits associated with feeding. Brief pressure application to the cardiac gutter transiently modulated the filtering motor pattern (pyloric rhythm) generated by the pyloric circuit within the stomatogastric ganglion (STG). This modulation included an increased speed of the pyloric rhythm and a concomitant decrease in the activity of the lateral pyloric neuron. Furthermore, 2 min of rhythmic pressure application to the cardiac gutter elicited a chewing motor pattern (gastric mill rhythm) generated by the gastric mill circuit in the STG that persisted for ≤30 min. These sensory actions on the pyloric and gastric mill circuits were mimicked by either ventral cardiac nerve or dorsal posterior esophageal nerve stimulation. VCN actions on the STG circuits required the activation of projection neurons in the commissural ganglia. A subset of the VCN actions on these projection neurons appeared to be direct and cholinergic. We propose that the VCN neurons are mechanoreceptors that are activated when food stored in the foregut applies an outward force, leading to the long-lasting activation of projection neurons required to initiate chewing and modify the filtering of chewed food.