Voltage-clamp analysis of membrane currents and excitation-contraction coupling in a crustacean muscle

Synaptic Dynamics Convey Differential Sensitivity to Input Pattern Changes in Two Muscles Innervated by the Same Motor Neurons

Postsynaptic responses depend on input patterns as well as short-term synaptic plasticity, summation, and postsynaptic membrane properties, but the interactions of those dynamics with realistic input patterns are not well understood. We recorded the responses of the two pyloric dilator (PD) muscles, cpv2a and cpv2b, that are innervated by and receive identical periodic bursting input from the same two motor neurons in the lobster Homarus americanus. Cpv2a and cpv2b showed quantitative differences in membrane nonlinearities and synaptic summation. At a short timescale, responses in both muscles were dominated by facilitation, albeit with different frequency and time dependence. Realistic burst stimulations revealed more substantial differences. Across bursts, cpv2a showed transient depression, whereas cpv2b showed transient facilitation. Steady-state responses to bursting input also differed substantially. Neither muscle had a monotonic dependence on frequency, but cpv2b showed particularly pronounced bandpass filtering. Cpv2a was sensitive to changes in both burst frequency and intra-burst spike frequency, whereas, despite its much slower responses, cpv2b was largely insensitive to changes in burst frequency. Cpv2a was sensitive to both burst duration and number of spikes per burst, whereas cpv2b was sensitive only to the former parameter. Neither muscle showed consistent sensitivity to changes in the overall spike interval structure, but cpv2b was surprisingly sensitive to changes in the first intervals in each burst, a parameter known to be regulated by dopamine (DA) modulation of spike propagation of the presynaptic axon. These findings highlight how seemingly minor circuit output changes mediated by neuromodulation could be read out differentially at the two synapses.

Saxitoxin-producing Raphidiopsis raciborskii (cyanobacteria) inhibits swimming and physiological parameters in Daphnia similis

In this study we tested the effects of a neurotoxic strain of the cyanobacterium Raphidiopsis raciborskii (CYRF-01) on the swimming activity and physiological parameters of Daphnia similis such as movements of the antennae, thoracic limbs, post-abdominal claw and heart rate. An acute assay was performed to test the effect on swimming activity, exposing newborns (<24 h) to different concentrations of live cells and observing the number of immobilized animals over a period of 48 h. For testing the effects on physiological parameters adult females (10-15 days) were exposed in a flow-through system and recorded with a digital camera. Results showed rapid effect of the strain CYRF on all parameters. Animals started to be immobilized in the first 30 min exposure and showed complete paralysis after 2 h in 500 μg L^-1 and after 24 h in the other concentrations. Physiological parameters accompanied the same response pattern with effects starting after 30 min and some recovery at the end of 6 h exposure. Antennae stopped moving after 2-3 h at 250-500 μg L^-1, explaining the paralysis of the swimming activity in Daphnia. Thoracic limbs movements were significantly inhibited after 30 min in all concentrations, staying at lower levels than control through the experiment. Post-abdominal claw movement were completely ceased after 30 min and remained stopped until the end of the trial. Heart rate showed a tendency to decrease abruptly in the first 30 min exposure in all concentrations, but showed significant lower values than control only at 500 μg L^-1, between 3 and 4 h exposure, and a recovery at the end of 6 h. In conclusion, results show that neurotoxic cyanobacteria can impose severe constrains on the physiology of daphniids, which can have consequences to the oxygen uptake, swimming and feeding behavior and to the overall fitness of those organisms.

Regulation of the crab heartbeat by FMRFamide-like peptides: multiple interacting effects on center and periphery

We are studying the functional "logic" of neuromodulatory actions in a simple central pattern generator (CPG)-effector system, the heart of the blue crab Callinectes sapidus. The rhythmic contractions of this heart are neurogenic, driven by rhythmic motor patterns generated by the cardiac ganglion (CG). Here we used anatomical and physiological methods to examine the sources and actions on the system of the FMRFamide-like peptides (FLPs) TNRNFLRFamide (F(1)), SDRNFLRFamide (F(2)), and GYNRSFLRFamide, an authentic Callinectes FLP. Immunohistochemical localization revealed a plexus of FLP-immunoreactive fibers in the pericardial organs (POs), from which modulators are released to reach the heart as circulating neurohormones. Combined backfill and immunohistochemical experiments indicated that the FLPs in the POs originated in the CNS, from large neurosecretory cells in the B cluster of the first thoracic neuromere. In physiological experiments, we examined the actions of the FLPs on the intact working heart, on the semi-intact heart in which we could record the motor patterns as well as the muscle contractions, on the isolated CG, and in a preparation developed to assess direct actions on the muscle with controlled patterns of motor neuron spikes. The FLPs had strong positive chronotropic and inotropic effects. Dissection of these effects suggested that they were produced through at least two primary actions of the FLPs exerted both on the heart muscle and on the CG. These primary actions elicited numerous secondary consequences mediated by the feedforward and feedback interactions that integrate the activity of the complete, coupled CPG-effector system.

The neuropeptide proctolin potentiates contractions and reduces cGMP concentration via a PKC-dependent pathway

As in many other arthropods, the neuropeptide proctolin enhances contractures of muscles in the crustacean isopod Idotea emarginata. The enhancement of high K+-induced contractures by proctolin (1 micromol l-1) was mimicked upon application of the protein kinase C (PKC) activator phorbol-12-myristate 1-acetate (PMA) and was inhibited by the PKC inhibitor bisindolylmaleimide (BIM-1). The potentiation was not inhibited by H89, a protein kinase A (PKA) inhibitor. Proctolin did not change the intracellular concentration of 3',5'-cyclic adenosine monophosphate (cAMP) whereas it significantly reduced the intracellular concentration of 3',5'-cyclic guanosine monophosphate (cGMP). The reduction of cGMP was not observed in the presence of the PKC inhibitor BIM-1. 8-Bromo-cGMP, a membrane-permeable cGMP analogue, reduced the potentiating effect of proctolin on muscle contracture. We thus conclude that proctolin in the studied crustacean muscle fibres induces an activation of PKC, which leads to a reduction of the cGMP concentration and, consequently, to the potentiation of muscle contracture.

Mitochondrial distribution and glycogen dynamics suggest diffusion constraints in muscle fibers of the blue crab, Callinectes sapidus

The scaling of mitochondrial distribution, citrate synthase activity, and post-contractile glycogen recovery was examined in muscle fibers of the blue crab, Callinectes sapidus. The fast-twitch muscle fibers of C. sapidus can reach extremely large dimensions, which may impose constraints on aerobic metabolic processes. However, muscle cells from small crabs are not giant, meaning that during development muscle fibers cross and greatly exceed the surface area to volume (SAV) and diffusion threshold that is adhered to by the cells of most organisms. Cell diameters in the smallest size class were approximately 100 microm, while the largest size class had cell diameters in excess of 500 microm. In the smallest cells, the fractional area of subsarcolemmal and intermyofibrillar mitochondria was similar. However, in the largest cells, mitochondria were almost exclusively subsarcolemmal. Total fractional area of mitochondria was highest in the largest cells due to a proliferation of subsarcolemmal mitochondria. In contrast, citrate synthase activity decreased as cell size increased. Following burst contractile activity, glycogen concentrations decreased significantly and remained depressed for several hours in muscle comprised of giant cells, consistent with previous findings that anaerobic glycogenolysis fuels certain components of post-contractile recovery. However, in muscle composed of the smallest muscle cells, glycogen levels did not decrease significantly following burst activity. While normal scaling of aerobic metabolism would predict a slower aerobic recovery in larger animals, the present results suggest that cellular organization, SAV, and intracellular diffusion distances also impose constraints on aerobic processes in C. sapidus.

Localization of a FMRFamide-related peptide in efferent neurons and analysis of neuromuscular effects of DRNFLRFamide (DF2) in the crustacean Idotea emarginata

In the ventral nerve cord of the isopod Idotea emarginata, FMRFamide-immunoreactive efferent neurons are confined to pereion ganglion 5 where a single pair of these neurons was identified. Each neuron projects an axon into the ipsilateral ventral and dorsal lateral nerves, which run through the entire animal. The immunoreactive axons form numerous varicosities on the ventral flexor and dorsal extensor muscle fibres, and in the pericardial organs. To analyse the neuromuscular effects of a FMRFamide, we used the DRNFLRFamide (DF2). DF2 acted both pre- and postsynaptically. On the presynaptic side, DF2 increased transmitter release from neuromuscular endings. Postsynaptically, DF2 depolarized muscle fibres by approximately 10 mV. This effect was not observed in leg muscles of a crab. The depolarization required Ca2+, was blocked by substituting Ca2+ with Co2+, but not affected by nifedipine or amiloride. In Idotea, DF2 also potentiated evoked extensor muscle contractions. The amplitude of high K+ contractures was increased in a dose dependent manner with an EC50 value of 40 nm. In current-clamped fibres, DF2 strongly potentiated contractions evoked by current pulses exceeding excitation-contraction threshold. In voltage-clamped fibres, the inward current through l-type Ca2+ channels was increased by the peptide. The observed physiological effects together with the localization of FMRFamide-immunoreactive efferent neurons suggest a role for this type of peptidergic modulation for the neuromuscular performance in Idotea. The pre- and postsynaptic effects of DF2 act synergistically and, in vivo, all should increase the efficacy of motor input to muscles resulting in potentiation of contractions.