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Peptidergic Modulation in the Lobster Cardiac Neuromuscular System: A Transcriptomic Analysis of Peptides and Peptide Receptors in Cardiac Ganglion and Muscle

Central pattern generator (CPG)‐effector systems can produce flexible rhythmic motor output via modulation by both intrinsically‐ and extrinsically‐derived neuropeptides. In the lobster, Homarus americanus, the cardiac neuromuscular system, which consists of the cardiac ganglion (CG) and cardiac muscle (CM), controls the rhythmic motor output of the heart. This is a well‐studied, but simple, CPG‐effector system that has been used extensively for investigating peptidergic control of rhythmic behavior. Here, transcriptomes were generated for the two components that comprise the cardiac neuromuscular system, the cardiac ganglion (CG) and cardiac muscle (CM). Transcripts encoding precursor proteins for approximately ten peptide families, including allatostatin‐C (AST‐C), diuretic hormone 31 (DH31), diuretic hormone 44, myosuppressin, and proctolin, were identified from the CG transcriptome, suggesting that these peptide groups may be produced by neurons in the CG. Transcripts encoding receptors for many of the peptides identified from the CG, including those for all four of the above‐mentioned peptide families, were also identified from the CG and/or CM transcriptomes, suggesting that these peptides serve as intrinsic modulators of the cardiac neuromuscular system. To investigate hormonal modulation in the cardiac neuromuscular system, the transcriptomes were mined for the receptors of peptides known to be present in the sinus gland and/or pericardial organs, two neuroendocrine organs. Receptors for seven peptide families were identified, with some receptors restricted either to the CG (crustacean cardioactive peptide, red pigment concentrating hormone, and tachykinin‐related peptide) or CM (pigment dispersing hormone), and others found in both tissues (AST‐C, myosuppressin, and proctolin). These data suggest that some peptides may have different sites of action within the cardiac neuromuscular system. Immunohistochemistry confirmed that DH31 is produced by the motor neurons of the CG, and suggested that AST‐C is present in the CG. Immunohistochemical confirmation of the other putative intrinsic CG modulators is ongoing.