The Journal of Neurophysiology Vol. 81 No. 4 April 1999, pp. 1818-1826
Copyright ©1999 by the American Physiological Society

FMRFamide-Activated Ca²⁺ Channels in Lymnaea Heart Cells Are Modulated by "SEEPLY," a Neuropeptide Encoded on the Same Gene

 ¹Ottawa-Carleton Institute of Biology, Carleton University, Ottawa, Ontario K1S 5B6, Canada; and  ²Sussex Centre for Neuroscience, School of Biological Sciences, University of Sussex, Falmer, Brighton, East Sussex BN1 9QG, United Kingdom

Brezden, B. L., M. S. Yeoman, D. R. Gardner, and P. R. Benjamin. FMRFamide-activated Ca²⁺ channels in Lymnaea heart cells are modulated by "SEEPLY," a neuropeptide encoded on the same gene. The cell-attached, patch-clamp technique was used to investigate the modulatory role of the neuropeptide SEQPDVDDYLRDVVLQSEEPLY ("SEEPLY") on FMRFamide-activated Ca²⁺ channels in isolated Lymnaea heart ventricular cells. Both SEEPLY and FMRFamide are encoded on the same neuropeptide gene and are coexpressed in a pair of excitatory motor neurons that innervate the heart. FMRFamide applied alone was capable of significantly increasing the P_(open) time of a Ca²⁺ channel in isolated heart muscle cells. However, SEEPLY applied alone did not significantly alter the basal level of Ca²⁺ channel activity in the same cells. Repeated applications of FMRFamide (15 s every min) resulted in a progressive reduction in the number of Ca²⁺ channel openings and the overall P_(open) time of the channel. The fifth successive 15-s application of FMRFamide failed to cause the Ca²⁺ channels to open in the majority of cells tested. When FMRFamide and SEEPLY were repeatedly applied together (2-min applications every 4 min) the FMRFamide-activated Ca²⁺ channels continued to respond after the fifth application of the two peptides. Indeed channel activity was seen to continue after repeated 2-min applications of FMRFamide and SEEPLY for as long as the patch lasted (60 min). As well as preventing the loss of response to FMRFamide, SEEPLY was also capable of both up- and down-regulating the response of the Ca²⁺ channel to FMRFamide. The direction of the response depended on the P_(open) time of the channel before the application of SEEPLY. When the P_(open) time for the FMRFamide-activated channel was initially 0.004 ± 0.002 (means ± SE), subsequent perfusion with a mixture of FMRFamide and SEEPLY produced a statistically significant increase in Ca²⁺ channel activity (13 cells). In two cells where no channel activity was observed in response to an initial application of FMRFamide, superfusing the heart cells with a mixture of FMRFamide and SEEPLY induced openings of the Ca²⁺ channel. When the P_(open) time of FMRFamide-induced Ca²⁺ channel openings was 0.058 ± 0.017 the subsequent application of a mixture of SEEPLY and FMRFamide caused a statistically significant decrease in Ca²⁺ channel activity (8 cells). As up- and down-regulation of FMRFamide-activated Ca²⁺ channel openings by SEEPLY were observed in the same cells (8 cells), this suggested that corelease of the two peptides might act together to regulate the level of Ca²⁺ channel activity within a defined range.