Date: 2019-05-01

Creator: Casey Breslow

Access: Open access

Modulation in neural systems is important for regulating physiology and behavior (Wright et al., 2010). Peptides, hormones, and amines are common neural modulators, acting on many neural systems across species. One group of neural networks that can be regulated are central pattern generators (CPGs), which generate rhythmic neural patterns, which drive behaviors (Marder and Bucher, 2001). Octopamine, and its precursor tyramine, are two amines that have been found to regulate (CPGs) across species (Cooke, 2002; Fussnecker et al., 2006). One role of octopamine in the decapod neurogenic heart is regulating the frequency and the duration of heart beats. However, the precise site of octopamine modulation within the cardiac system is not yet known (Kurumoto and Ebara, 1991). One possible site of action is the cardiac ganglion (CG), the CPG in decapod hearts. The transcripts for the enzymes required to synthesize octopamine from tyramine have been identified and localized in the CG (Christie et al., 2018). This would suggest that octopamine is produced in the CG, where it could have a direct action on those neurons, or it could be released peripherally. We have found individual variation in the response to octopamine and its precursor tyramine, and significant effects of frequency and contraction amplitude in the whole heart.

Date: 2019-05-01

Creator: Matthew Maguire

Access: Access restricted to the Bowdoin Community

Date: 2020-01-01

Creator: Grace Bukowski-Thall

Access: Open access

Central pattern generators (CPGs) are neural networks that generate the rhythmic outputs that control behaviors such as locomotion, respiration, and chewing. The stomatogastric nervous system (STNS), which contains the CPGs that control foregut movement, and the cardiac ganglion (CG), which is a CPG that controls heartbeat, are two commonly studied systems in decapod crustaceans. Neuromodulators are locally or hormonally released neuropeptides and amines that change the output patterns of CPGs like the STNS and CG to allow behavioral flexibility. We have hypothesized that neuromodulation provides a substrate for the evolution of behavioral flexibility, and as a result, systems exhibiting more behavioral flexibility are modulated to a greater degree. To examine this hypothesis, we evaluated the extent to which the STNS and the CG are modulated in the majoid crab species Chionoecetes opilio, Libinia emarginata, and Pugettia producta. C. opilio and L. emarginata are opportunistic feeders, whereas P. producta has a highly specialized kelp diet. We predicted that opportunistic feeding crabs that chew and process a wide variety of food types would exhibit greater STNS neuromodulatory capacity than those with a specialized diet. The STNS of L. emarginata and C. opilio responded to the seven endogenous neuromodulators oxotremorine, dopamine, CabTrp Ia, CCAP, myosuppressin, proctolin, and RPCH, whereas the STNS of P. producta only responded to proctolin, oxotremorine, myosuppressin, RPCH (25% of the time), variably to dopamine, and not at all to CabTrp and CCAP. Because P. producta, L. emarginata, and C. opilio all belong to the Majoidea superfamily, their primary distinctions are their feeding habits. For this reason, we further predicted that there would be no relationship between diet and modulatory capacity in the cardiac ganglion (CG) of the neurogenic heart. This would suggest that a lack of STNS modulatory capacity in P. producta relative to L. emarginata and C. opilio is specific to evolved foregut function. Whole-heart recordings from P. producta indicated that, unlike the STNS, the CG responds to CabTrp and CCAP. P. producta hearts also responded to oxotremorine and inconsistently to dopamine and proctolin. The CG of C. opilio was modulated by CabTrp, CCAP, dopamine, proctolin, myosuppressin, and oxotremorine, but not RPCH. The CG of L. emarginata responded to CCAP, and inconsistently to CabTrp, dopamine, and proctolin, but not to myosuppressin, RPCH, and surprisingly oxotremorine. Although cardiac responses were not identical between species, opportunistic and specialist feeders responded more similarly to the modulators tested in the heart than in the STNS. Notably, P. producta responded to each modulator in a similar manner to C. opilio and/or L. emarginata. However, L. emarginata’s surprising lack of cardiac response to oxotremorine suggests that phylogenetic closeness may not control for differences in CG and STNS function between species. Nevertheless, sample sizes of all three species were quite small, and individual differences lead to inconsistencies in the data. As a result, sample size must be enlarged to draw firm conclusions.

Date: 2020-01-01

Creator: William Allen

Access: Open access

Central pattern generators (CPGs) are neural circuits whose component neurons possess intrinsic properties and synaptic connections that allow them to generate rhythmic motor outputs in the absence of descending inputs. The cardiac ganglion (CG) is a nine-cell CPG located in the American lobster, Homarus americanus. Stretch of the myocardium feeds back to the CG through mechano-sensitive dendrites and is thought to play a role in maintaining regularity in the beating pattern of the heart. The novel peptide AMGSEFLamide has been observed to induce irregular beating patterns when applied at high concentrations. This study investigated the interaction between stretch-related feedback and AMGSEFLamide modulation in generating irregular beating patterns in the whole heart of Homarus americanus. It was hypothesized that greater longitudinal stretch of the heart would result in greater regularity in the instantaneous beat frequency, based on previous findings that stretch-sensitive dendrites play a role in the regulation of the heartbeat. Furthermore, it was predicted that the elimination of stretch feedback via deafferentation of the heart would augment the irregularity induced by AMGSEFLamide. Data showed significantly increased irregularity in beating in response to 10-6 M AMGSEFLamide application. Longitudinal stretch did not reliably alter baseline variability in frequency, nor did it influence the modulatory effect of AMGSEFLamide. Deafferentation did not significantly alter baseline irregularity. Deafferented preparations did exhibit a trend of responding to AMGSEFLamide with a greater percent increase in irregularity compared to when afferents were intact, suggesting a potential role of stretch-stabilization in response to modulatory perturbations in the Homarus heart.

Date: 2020-01-01

Creator: Jacob Salman Kazmi

Access: Open access

Neuromodulation may be a substrate for the evolution of behavioral diversity. The extent to which a central pattern generator is modulated could serve as a mechanism that enables variability in motor output dependent on an organism’s need for behavioral flexibility. The pyloric circuit, a central pattern generator in the crustacean stomatogastric nervous system (STNS), stimulates contractions of foregut muscles in digestion. Since neuromodulation enables variation in the movements of pyloric muscles, more diverse feeding patterns should be correlated with a higher degree of STNS neuromodulation. Previous data have shown that Cancer borealis, an opportunistic feeder, is sensitive to a wider array of neuromodulators than Pugettia producta, a specialist feeder. The observed difference in modulatory capacity may be coincidental since these species are separated by phylogeny. We predict that the difference in modulatory capacity is a product of a differential need for variety in foregut muscle movements. This study examined two members of the same superfamily as P. producta, the opportunistically feeding snow crab (Chionoecetes opilio) and portly spider crab (Libinia emarginata). Using extracellular recording methods, the responses of isolated STNS preparations to various neuromodulators were measured. Initial qualitative results indicate that the STNS of C. opilio is sensitive to all of these neuromodulators. Additionally, previous data on the neuromodulatory capacity of L. emarginata was supported through similar electrophysiological analysis of the isolated STNS. As a first step in determining the mechanism of differential sensitivity between species, tissue-specific transcriptomes were generated and mined for neuromodulators.