Mechanisms Underlying Differential Responses to the Neuropeptide Allatostatin‐C (AST‐C) in the Cardiac Ganglion of the Lobster, Homarus americanus

In order to respond to environmental stimuli, animals need flexibility in the neural circuitry that controls behavioral patterns. Central pattern generators (CPGs), which control rhythmic behaviors, like breathing and walking, tend to have relatively fixed circuitry, and achieve flexibility primarily through peptide modulation. While many of the mechanisms that underlie the function of peptide neuromodulators are known, little is understood about the variability in response to peptide modulators between individuals tested at different times. For example, in the lobster, Homarus americanus , modulation by neuropeptides is thought to differ during different stages of the molt cycle, a physiologically demanding process that requires altered behavior, including responses that allow the animal to function with an increased blood volume. One such neuromodulator is allatostatin‐C (AST‐C). AST‐C modulates the output of the cardiac ganglion (CG), a well‐studied CPG in the heart of the lobster that drives rhythmic heart contractions. AST‐C, in particular, causes a consistent decrease in the contraction frequency of the heart; however, the peptide causes either increases or decreases in contraction amplitude among different individuals. There are numerous hypotheses that could explain this differential response. The hypothesis currently being addressed suggests that the mechanism responsible for the variability in response is a differential distribution of at least two types of AST‐C receptors expressed in individual lobsters. Bioinformatic analysis of RNA isolated from pooled neural tissue was used to identify precursor transcripts for both peptides and receptors present in the lobster. Using a well‐vetted system, the AST‐C peptide and two full‐length and one partial‐length AST‐C receptor transcripts were identified. The existence of two full‐length receptors supports the hypothesis that receptor expression could be varied among individuals with different responses to the AST‐C peptide. Furthermore, previous research suggested a correlation between AST‐C response and molt stage, which could act as the physiological trigger that catalyzes the change in receptor expression. To further explore this possibility, additional transcriptomes tagged with response to AST‐C and molt stage are currently being constructed; these will be compared to reference transcriptomes to identify what factors may be changing in individual lobsters. Future bioinformatic assessment of these transcriptomes will confirm present findings and elucidate the role peptidergic signaling plays in producing variability in patterned behaviors, as well as the physiological factors that necessitate such variability. Support or Funding Information The American Physiological Society Undergraduate Summer Research Fellowship, Doherty Fund of Bowdoin College, Maine Space Grant Consortium MERITS program, NSF IOS‐1354567 and IOS‐1353023, NIH P20GM0103423 from NIGMS