Co-modulation of the Pyloric Circuit in the Stomatogastric Nervous System of the Cancer Borealis

Date: 2025-01-01

Creator: Margaret Broaddus

Access: Open access

ABSTRACT CHAPTER I: All nervous systems are influenced by circulating hormones, which can modulate neural circuits to produce different outputs from the same set of neurons. Invertebrate models, particularly crustaceans, serve as excellent models for studying neuromodulation because they contain neural circuits that continue to generate fictive activity when dissected out of the animal. The stomatogastric nervous system (STNS) of the Jonah Crab (Cancer borealis) has long been used to study neuromodulation due to its well-characterized circuits. Even in such a compact neural network, little is known about how these circuits are modulated, and this remains a question in all animals, particularly in humans. Here we investigated the modulation of the pyloric circuit by applying bulk hemolymph to the dissected STNS preparation. The hemolymph contains all of the circulating modulators, some of which have known effects on the pyloric rhythm (though many are still unknown). Interestingly, when hemolymph is applied to the isolated STNS, the pyloric rhythm is suppressed. This is surprising given that in vivo the STNS is continually exposed to hemolymph (the STG is situated within an artery, and thus, exposed to circulating hemolymph) and the pyloric rhythm is constitutively active. Therefore, I hypothesized that there are synaptically released neurotransmitters that excite the pyloric rhythm. To test this hypothesis, we applied three different excitatory modulators – proctolin, serotonin, and oxotremorine – separately in the presence of hemolymph. I found that proctolin and oxotremorine restore the pyloric rhythm in the presence of hemolymph. However, serotonin did not consistently overcome the inhibition of hemolymph. ABSTRACT CHAPTER II: A plethora of work has begun to identify how endogenous neural and hormonal modulators interact to influence the pyloric network. Here we examined the modulation of the stomatogastric nervous system (STNS) via two excitatory endogenous modulators CabTRP Ia and corazonin. CabTRP Ia and corazonin both excite the pyloric rhythm, but in distinct ways. Preliminary data by Nusbaum and Christie from 2003 suggested that an initial corazonin application gated a stronger response to subsequent CabTRP Ia when compared the inverse application of these neuromodulators. We sought to validate this gating phenomenon, but found no significant difference between the effects of the first and second applications of CabTRP Ia. Given that these animals are wild caught and surviving in a changing oceanic environment, it is possible that this modulatory effect in the Jonah Crab has changed over the last few decades due to environmentally driven shifts in receptor expression and channel conductances.