Co-ordination of firing activity of neurosecretory cells with cardiac activity in the silkmoth Bombyx mori

Electrocardiograms and electrical action potentials of cerebral neurosecretory cells producing bombyxin (an insulin-related neuropeptide) were simultaneously recorded from male pupae of the silkmoth Bombyx mori. A pupa showed alternations in the flow of haemolymph due to a rhythmic heartbeat reversal: a train of retrograde heartbeat with a slow pulse rate followed a train of anterograde heartbeat with a higher pulse rate. Intervals of heartbeat reversals changed throughout the pupal period. At any stage of the pupal period, firing activity of a population of bombyxin-producing (BP) cells rapidly declined after the start of anterograde heartbeat and an inactive state of cells continued during an anterograde heartbeat period. Analyses of ultradian bursting rhythmicity of a single BP cell revealed that a bursting phase of the cell often delayed at a time when the anterograde cardiac activity occurred at the preceding inter-burst period of firing rhythm. The results support the postulation that firing (secretory) activity of an insect neurosecretory cell system may be co-ordinated with circulation of haemolymph for rapid and pulsatile delivery of the peptides released to target organs.