REGULATION OF HEARTBEAT IN LYMNAEA BY MOTONEURONS CONTAINING FMRFAMIDE-LIKE PEPTIDES

1. Intracellular stimulation of single neurons in the Lymnaea CNS was carried out to identify heart motoneurons. 2. Two of the identified motneurons, the E heart excitor (E(he)) cells were shown to contain Phe-Met-Arg-Phe-NH2 (FMRFamide)-like peptides by immunocytochemical staining of dyemarked neurons and by radioimmunoassay (RIA) applied to extracts of single dissected cells. 3. Bursts of spikes in the E(he) cells increased heart rate, beat amplitude and muscle tonus. This response was mimicked by perfusion of exogenous FMRFamide at low concentration (10-6 to 5 x 10-8 M) through the interior of the intact heart. 4. Application of selective antagonists to 5-hydroxytryptamine (5-HT) and dopamine failed to block E(he) cardiac effects. 5. Detailed evidence that the E(he) cells were heart motoneurons was obtained. 1) Anatomic mapping using the dye Lucifer yellow showed E(he) cells had perpheral projections restricted mainly to the intestinal nerve, the only nerve known to innervate the heart. 2) Perfusion of the CNS with a saline containing Co2+ blocked central chemical synapses but did not affect activity of E(he) cells on the heart. 3) Simultaneous intracellular recordings from E(he) cells and auricle muscle fibers showed unitary excitatory junction potentials following with constant latency from spikes in E(he) cells. 6. The present study elucidates the role of FMRFamide in cardioregulation and provides the first evidence that it acts as an excitatory neurotransmitter on the snail heart.