Characterization of crustacean cardioactive peptide-like immunoreactivity in the horseshoe crab, Limulus polyphemus

Limulus and heart rhythm

Great interest in the comparative physiology of hearts and their functions in Animalia has emerged with classic papers on Limulus polyphemus and mollusks. The recurrent cardiac activity-heart rate-is the most important physiological parameter and when present the kardia (Greek) is vital to the development of entire organs of the organisms in the animal kingdom. Extensive studies devoted to the regulation of cardiac rhythm in invertebrates have revealed that the basics of heart physiology are comparable to mammals. The hearts of invertebrates also beat spontaneously and are supplied with regulatory nerves: either excitatory or inhibitory or both. The distinct nerves and the source of excitation/inhibition at the level of single neurons are described for many invertebrate genera. The vertebrates and a majority of invertebrates have myogenic hearts, whereas the horseshoe crab L. polyphemus and a few other animals have a neurogenic cardiac rhythm. Nevertheless, the myogenic nature of heartbeat is precursor, because the contraction of native and stem-cell-derived cardiomyocytes does occur in the absence of any neural elements. Even in L. polyphemus, the heart rhythm is myogenic at embryonic stages.

Regulation of the crab heartbeat by crustacean cardioactive peptide (CCAP): central and peripheral actions

In regulating neurophysiological systems, neuromodulators exert multiple actions at multiple sites in such a way as to control the activity in an integrated manner. We are studying how this happens 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). In this study, we used anatomical and physiological methods to examine the sources and actions on the system of crustacean cardioactive peptide (CCAP). Immunohistochemical localization revealed a plexus of CCAP-immunoreactive fibers in the pericardial organs (POs), neurohemal structures from which blood-borne neurohormones reach the heart. Combined backfill and immunohistochemical experiments indicated that the CCAP in the POs originated from a large contralateral neuron in each thoracic neuromere. In physiological experiments, we examined the actions of exogenous CCAP on the intact working heart, on the semi-intact heart in which we could record the motor patterns as well as the muscle contractions, and on the isolated CG. CCAP had strong positive inotropic and chronotropic effects. Dissection of these effects in terms of dose dependency, time course, and the preparation type in which they occurred suggested that they were produced by the interaction of three primary actions of CCAP exerted both on the heart muscle and on the CG. We conclude that CCAP released from the POs as a neurohormone regulates the crab heart by multiple actions on both the central and peripheral components of this model CPG-effector system.

The control of anterior foregut motility during a larval molt of the moth Manduca sexta involves the modulation of presynaptic activity

In the moth, Manduca sexta, anterior foregut motility is modulated during the larval–larval molts in order to control the timing of molting fluid (MF) ingestion. MF is the enzymatic mixture that destroys the outer cuticle so that it can be shed at the end of the molt. The onset of the larval–larval molt is characterized by a dramatic decline in the amplitude of the anterior foregut contractions so that MF is not prematurely ingested. As the end of the molt approaches, the robust contractions of the anterior foregut return and the MF is ingested, enabling the larva to free itself from its old cuticle. In the present study we examine possible mechanisms involved in modulating anterior foregut motility during a larval–larval molt. Our results reveal that the release of a blood-borne factor plays a role in the decline in anterior foregut peristaltic activity during the molt. This blood-borne factor reduces the efficacy of the presynaptic endings of the motorneurons, resulting in a reduction in the amplitude of the excitatory junctional potential (EJP) recorded from the anterior foregut musculature. We also present evidence that crustacean cardioactive peptide (CCAP) targets the motorneuron terminals and its actions are sufficient to trigger the dramatic increase in EJP amplitude and anterior foregut contractions. Finally, the surgical ablation of the subesophageal ganglion, which has been previously described to be a source of CCAP neurons and the CCAP projections to the anterior foregut region, blocks both the increase in anterior foregut motility and the ingestion of MF that normally occur at the end of a larval–larval molt.

Modulation of oscillator interactions in the crab stomatogastric ganglion by crustacean cardioactive peptide

The modulation of the pyloric rhythm of the stomatogastric ganglion of the crab, Cancer borealis, by crustacean cardioactive peptide (CCAP) is described. CCAP activated pyloric rhythms in most silent preparations, and altered the phase relationships of pyloric motor neuron firing in all preparations. In CCAP, the pyloric rhythms were characterized by long lateral pyloric (LP) neuron bursts of action potentials. The threshold for CCAP action was approximately 10(-10) M, with increasing effects at higher CCAP concentrations. The changes in motor pattern evoked by CCAP produced significant changes in LP-innervated muscle movement. These movements were additionally potentiated by CCAP applications to isolated nerve-muscle preparations. Thus, enhanced motor neuron firing and increase of the gain of the neuromuscular junctions are likely to operate coordinately in response to hormonally released CCAP. High CCAP concentrations sometimes resulted in modification of the normal 1:1 alternation between the pyloric dilator (PD) and LP neurons to patterns of 2:1, 3:1, or 4:1 alternation. CCAP seems to activate slow intrinsic oscillations in the LP neuron, as well as enhance faster oscillations in the pacemaker group of PD/anterior burster (AB) neurons. Simulations of fast and slow oscillators with reciprocal inhibitory coupling suggest mechanisms that could account for the mode switch from 1:1 alternation to multiple PD bursts alternating with one LP neuron burst.