Myomodulin

The myomodulin-related neuropeptides: characterization of a gene encoding a family of peptide cotransmitters in Aplysia

The myomodulin-related peptides comprise a family of cotransmitters that modulate neuromuscular signaling in the feeding system of Aplysia. In this study, cDNA clones encoding a myomodulin precursor polypeptide were isolated and characterized. This precursor contains seven different myomodulin-related peptides, one of which, myomodulin A, is present in 10 contiguous copies. The sequence of a myomodulin genomic clone indicates that all of these myomodulin-related peptides are encoded on a single exon. The myomodulin gene is expressed in a tissue-specific manner and myomodulin mRNA is localized to specific neurons in the Aplysia CNS. The presence of multiple related neuropeptides can greatly increase the range and precision of signaling at synapses where they act as modulator cotransmitters.

Myomodulin increases Ih and inhibits the NA/K pump to modulate bursting in leech heart interneurons

In the medicinal leech, a rhythmically active 14-interneuron network composes the central pattern generator for heartbeat. In two segmental ganglia, bilateral pairs of reciprocally inhibitory heart interneurons (oscillator interneurons) produce a rhythm of alternating bursts of action potentials that paces activity in the pattern-generating network. The neuropeptide myomodulin decreases the period of this bursting and increases the intraburst spike frequency when applied to isolated ganglia containing these oscillator interneurons. Myomodulin also decreases period, increases spike frequency, and increases the robustness of endogenous bursting in synaptically isolated (with bicuculline) oscillator interneurons. In voltage-clamp experiments using hyperpolarizing ramps, we identify an increase in membrane conductance elicited by myomodulin with the properties of a hyperpolarization-activated current. Voltage steps confirm that myomodulin indeed increases the maximum conductance of the hyperpolarization-activated current I(h). In similar experiments using Cs(+) to block I(h), we demonstrate that myomodulin also causes a steady offset in the ramp current that is not associated with an increase in conductance. This current offset is blocked by ouabain, indicating that myomodulin inhibits the Na/K pump. In current-clamp experiments, when I(h) is blocked with Cs(+), myomodulin decreases period and increases spike frequency of alternating bursting in synaptically connected oscillator interneurons, suggesting that inhibiting the Na/K pump modulates these burst characteristics. These observations indicate that myomodulin decreases period and increases spike frequency of endogenous bursting in synaptically isolated oscillator heart interneurons and alternating bursting of reciprocally inhibitory pairs of interneurons, at least in part, by increasing I(h) and by decreasing the Na/K pump.

Neuropeptides myomodulin, small cardioactive peptide, and buccalin in the central nervous system of Lymnaea stagnalis: purification, immunoreactivity, and artifacts

The neuropeptides myomodulin, small cardioactive peptide (SCP), and buccalin are widely distributed in the phylum Mollusca and have important physiological functions. Here, we describe the detailed distribution of each class of peptide in the central nervous system (CNS) of the snail Lymnaea stagnalis by the use of immunocytochemical techniques combined with dye-marking of electrophysiologically identified neurons. We report the isolation and structural characterization of a Lymnaea myomodulin, PMSMLRLamide, identical to myomodulin A of Aplysia californica. Myomodulin immunoreactivity was localized in all 11 ganglia, in their connectives, and in peripheral nerves. In many cases, myomodulin immunoreactivity appeared localized in neuronal clusters expressing FMRFamide-like peptides, but also in a large number of additional neurons. Double-labelling experiments demonstrated myomodulin immunoreactivity in the visceral white interneuron, involved in regulation of cardiorespiration. SCP-like immunoreactivity also appeared in all ganglia, and double-labelling experiments revealed that in many locations it was specifically associated with clusters expressing distinct exons of the FMRFamide gene that are differentially expressed in the CNS. Characterization of the two types of SCP-antisera used in this study, however, suggested that they cross-reacted with both FMRFamide and N-terminally extended FMRFamide-like peptides. Selective preadsorption with these cross-reacting peptides resulted in elimination of the widespread staining and retention of bona fide SCP immunoreactivity in the buccal and pedal ganglia only. Buccalin immunoreactivity was limited to the buccal and pedal ganglia. It did not coincide with the distribution of either myomodulin or SCP. Most immunoreactive clusters were found in the pedal ganglia.

Myomodulin: a bioactive neuropeptide present in an identified cholinergic buccal motor neuron of Aplysia

When Aplysia are initially exposed to food stimuli, their biting responses show progressive increases in speed and strength. The accessory radula closer (ARC) buccal muscles have been used to study this phenomenon, and it has been shown that changes in ARC muscle contraction are partially due to activity of a serotonergic neuron that modulates this muscle, by both a direct action and an action on two ARC motor neurons (B15 and B16). The motor neurons use acetylcholine as their excitatory transmitter, but they also contain bioactive peptides that can potentiate muscle contractions when they are exogenously applied. Motor neuron B15 contains the structurally related small cardioactive peptides A and B, whereas motor neuron B16 contains a different peptide--termed myomodulin. In the present study we determined the full amino acid sequence of myomodulin. Myomodulin is present in the ARC muscle, and exogenous application of the peptide potentiates ARC muscle contractions in a manner similar to the potentiation by small cardioactive peptides A and B. The structure of myomodulin, however, bears little resemblance to the small cardioactive peptides. Thus it appears that ARC muscle contractions may be regulated by at least three distinct classes of neuromodulators: serotonin, the small cardioactive peptides, and myomodulin.

Identification and characterization of a myomodulin-like peptide in the leech

A novel myomodulin-like peptide, GMGALRLamide, has been purified and sequenced from extracts of 1000 medicinal leech nerve cords. Synthetic leech myomodulin-like peptide blocked the specific staining pattern of leech ganglia by the antiserum against Aplysia myomodulin A PMGMLRLamide. Moreover, the synthetic leech myomodulin-like peptide GMGALRLamide showed identical neuronal modulation effect on the giant leech Retzius cell compare to that by the synthetic Aplysia myomodulin A PMGMLRLamide.