Institution: Department of Biology, University of Kentucky, Lexington, KY, USA
Abstract: Many types of gram-negative bacteria are responsible for serious infections, such as septicemia. Lipopolysaccharides (LPS), the endotoxins released from these bacteria, are responsible for inducing the immune response of organisms such as crustaceans, who have well-conserved Toll- like receptors. Little is known about the direct impact LPS has on primary sensory neurons apart from this immune reaction. Previous studies have demonstrated that motor neurons increase both spontaneous and evoked firing frequencies with LPS, but differences have been observed across species. Here, the effects of LPS from two strains of gram-negative bacteria (Serratia marcescensand Pseudomonas aeruginosa) on firing frequency of primary sensory proprioceptors in the crab propodite-dactylopodite (PD) organ and crayfish muscle receptor organ (MRO) is examined. These sensory organs correlate to mammalian proprioception, as the MRO is analogous to the mammalian muscle spindle, and the PD organ allows for the separation of motor nerve function from sensory neuronal transduction. The neuronal function of the two model organisms was studied through the stretch-activation of rapidly-adapting and slowly-adapting sensory neurons. Results indicated that there is no statistically significant impact on sensory transduction through the application of LPS; however, in the crab PD organ, the application of LPS from both strains decreased the nerve activity except when the LPS from both bacteria was applied together. In the crayfish MRO, there usually was an increase in nerve activity. In saline controls, there was also an increase in firing of the neurons in both preparations, but this also was not statistically significant. Interestingly, the MRO muscle fibers often contracted upon the addition of LPS, perhaps indicating that the known impact LPS has on motor nerve function is partially responsible for the results obtained.
Keywords: Sensory, proprioception; crustacean, endotoxin