FMRF-amide-like substances in the leech. II. Bioactivity on the heartbeat system.

In the preceding paper (Kuhlman, J. R., C. Li, and R. L. Calabrese (1985) J. Neurosci. 5: 2301-2309) FMRF-amide-like immunoreactivity was localized to a specific set of neurons in the leech. Three types of these neurons are involved in controlling the animal's heartbeat: HE motor neurons and HA modulatory neurons which directly innervate the hearts, and the swim-initiating interneurons (cells 204) which can accelerate the heartbeat central pattern generator. Application of synthetic FMRF-amide had effects on the hearts and the heartbeat central pattern generator that mimicked the actions of the HA and cell 204 neurons. Bath application of FMRF-amide (10(-7) to 10(-6) M) to the hearts activated their myogenic rhythm and increased their beat tension, thus mimicking the effects of activity in HA cells. Bath application of lower concentrations of FMRF-amide (10(-9) to 10(-8) M) to the isolated central nervous system dramatically accelerated the central motor program for heartbeat, thus mimicking the effects of activity in cell 204. These observations suggest that an FMRF-amide-like substance may be used as a chemical signal by HA and cell 204 neurons. The role of the FMRF-amide-like substance contained in HE motor neurons remains unclear, but it may be released along with the HE cell's neuromuscular transmitter, acetylcholine.