The C. elegans defecation cycle is characterized by the sequential contraction of the posterior body wall muscles, anterior body wall muscles and enteric muscles. This motor program occurs every fifty seconds and requires the coordinated activity of three distinct tissues - the intestine, the muscles and neurons. Laser ablation of candidate motor neurons and analysis of synaptic transmission mutants suggest that the periodicity and execution of the first step of the defecation motor program, the posterior body contraction, is controlled by a non neuronal cell type. In vivo calcium imaging studies revealed a correlation of intestinal calcium spiking and the initiation of the posterior body contraction (Dal Santo et al., 1999). Because alteration of intestinal inositol 1,4,5-trisphosphate receptor (IP3R) function by mutation or overexpression can stop, slow, or speed the clock's rhythm, the IP3R intracellular calcium release channel is likely to directly regulate the intestinal clock through cyclic releases of calcium (Dal Santo et al., 1999). To determine how cyclic calcium spiking within the intestine leads to posterior body wall contraction, mutants that specifically eliminate the posterior body wall contraction (pbos) were identified. Two of these mutants, pbo-4 and pbo-5 have been cloned and characterized. pbo-4 encodes a protein homologous to Na+/H+ exchangers (NHEs) and is expressed only in the posterior intestine. Activation of mammalian NHEs can be regulated by calcium. The identification of a potential calmodulin binding site as well as consensus calmodulin dependent kinase II phosphorylation sites in pbo-4 further suggests calcium levels may modulate PBO-4's activity. pbo-5 encodes a novel, ligand-gated ion channel that is distantly related to acetylcholine receptors. PBO-5 is expressed in the posterior body wall muscles, adjacent to the PBO-4 expressing cells. One possible model is that PBO-4 releases protons from the intestine in response to calcium spikes. PBO-5 could recognize this signal and depolarize the muscles to initiate the posterior body contraction. We are testing this model by characterizing the electrophysiological properties of PBO-5 in Xenopus oocytes. In addition, we recently identified a closely related acetylcholine receptor, F11C7.1, that is also expressed in the posterior body wall muscles. Therefore, the electrophysiological properties of an F11C7.1 and PBO-5 oligomer are being analyzed. Finally, two additional pbo alleles are being mapped. One of these alleles, ox10, maps to the right arm of V, between unc-51 and rol-9. Dal Santo, P., Logan, M. A., Chisholm, A. D, and Jorgensen, E. M. (1999). The inositol trisphosphate receptor regulates a 50-second behavioral rhythm in C. elegans, Cell 98, 757-67.