References: Calabrese, R. 1998. Cellular, synaptic, network, and modulatory mechanisms involved in rhythm generation. Curr Opin Neurobiol 8: 710-717. Dennison, B. 2008. Regulation of central pattern generators by the ganglionic sheath in the American lobster, Homarus americanus. Dickinson P., Mecsas C., Marder E. 1990. Neuropeptide fusion of two motor-pattern generator circuits. Nature 344: 155-158. Fernlund, P. 1974. Synthesis of the red-pigment-concentrating hormone of the shrimp, Pandalus borealis. Biochim Biophysics Acta 371: 312-322. Selverston, A.I. 1974. Structural and functional basis of motor pattern generation in stomatogastric ganglion of lobster. American Zoologist 74: 957-972. Skiebe, P. 2001. Neuropeptides are ubiquitous chemical mediators: using the stomatogastric nervous system as a model system. J Exp Biol 204: 2035-2048. Thirumalai, V. and Marder, E. 2002. Colocalized neuropeptides activate a central pattern generator by acting on different circuit targets. J Neurosci 22: 1874-1882.

Permeability of the stomatogastric ganglion sheath to RPCH in the stomatogastric nervous system of Homarus americanus

Laura Keller, 2015

Central pattern generators are mechanisms that function to produce and regulate the fixed

outputs of a nervous system that generate rhythmic movement. In order to respond to differing environment conditions, neuromodulators, which can alter membrane properties or synaptic connections, enable pattern generators to elicit a variety of rhythmic patterns. These neuromodulators can either be released locally within the nervous system or released hormonally, circulating in the hemolymph of the lobster. In Homarus americanus, the American lobster, the stomatogastric ganglion (STG) is one of the four ganglia in the stomatogastric nervous system that generate these rhythmic patterns; all of these ganglia are encased in connective tissue sheaths. It has been found that this sheath can serve as a differential barrier to certain hormonal modulators of the lobster, as the sheath is only permeable to some modulators. To the discover the extent to which this sheath can serve as a differential barrier, known modulators need to be tested when the sheath is present or absent, and by comparing the modulatory effects of the given modulator, it can be ascertained whether the modulator could serve as a hormonal modulator or only be released within the nervous system, and the threshold level in which significant effects are elicited can be determined.

In my research, I am looking at red pigment-concentrating hormone (RPCH)—a hydrophobic compound that has been found to excite neuronal activity in many crustaceans—and the threshold level at which RPCH can serve as an activator when the STG is sheathed and desheathed. First, I compared the activity levels of several neurons involved in two different rhythmic stomatogastric patterns, LG in the gastric mill pattern and LP and PD in the pyloric pattern, via extracellular recordings on various motor nerves in the stomatogastric nervous system. In higher concentrations, like 10-6 M, there are similar levels of excitation whether the STG is sheathed or not, suggesting that the sheath is permeable to RPCH and thus RPCH can serve as a hormonal modulator. However, RPCH is most likely found in lower concentrations naturally in the hemolymph of lobsters, so I looked at 10-7, 10-8, and 10-9 M concentrations. I found the same effects repeated in the 10-7 M concentration and preliminarily in the 10-8 M concentration; however, there was not a noticeable activation of either the gastric mill or pyloric patterns at the 10-9 M concentration in the desheathed condition, which suggests that the threshold for RPCH is between a 10-8 and 10-9 M concentration. Faculty Mentor: Patsy Dickinson Funded by the Howard Hughes Medical Institute

References: Calabrese, R. 1998. Cellular, synaptic, network, and modulatory mechanisms involved in rhythm generation. Curr Opin Neurobiol 8: 710-717. Dennison, B. 2008. Regulation of central pattern generators by the ganglionic sheath in the American lobster, Homarus americanus. Dickinson P., Mecsas C., Marder E. 1990. Neuropeptide fusion of two motor-pattern generator circuits. Nature 344: 155-158. Fernlund, P. 1974. Synthesis of the red-pigment-concentrating hormone of the shrimp, Pandalus borealis. Biochim Biophysics Acta 371: 312-322. Selverston, A.I. 1974. Structural and functional basis of motor pattern generation in stomatogastric ganglion of lobster. American Zoologist 74: 957-972. Skiebe, P. 2001. Neuropeptides are ubiquitous chemical mediators: using the stomatogastric nervous system as a model system. J Exp Biol 204: 2035-2048. Thirumalai, V. and Marder, E. 2002. Colocalized neuropeptides activate a central pattern generator by acting on different circuit targets. J Neurosci 22: 1874-1882.

Figure 1.