Postingestive chemosensation and feeding by leeches

Distinct chemical cues acting on chemosensory structures on the dorsal lip of bloodsucking leeches activate the entire complement of appetitive and in gestive feeding behaviors. However, it is not known whether the maintenance of ingestion depends on continuous stimulation of these peripheral chemose nsors. Leeches of the species Hirudo medicinalis and Macrobdella decora wer e fed for 2 min on an artificial blood solution containing 150 mM NaCl/1 mM arginine before switching the feeding solution to various experimental mix tures. Leeches did not start to feed on, but continued to ingest solutions in which equiosmolar KCI or lysine substituted for NaCl or arginine, respec tively, until sated. In contrast, they rejected water and dropped off the f eeding apparatus within 30 s of the exchange. Leeches also detached from th e feeding tube when quinine, denatonium, or caffeine were added to the NaCl /arginine solution during an ongoing feeding bout. The duration of ingestio n following the switch correlated inversely with the concentration of the d rugs (0.1-10 mM). Superfusion of the dorsal lip with high concentrations of the bitter chemicals, while feeding was in progress, had no effect on the duration of ingestion. However, injections of the bitter substances directl y into the gut, during a feeding bout, abruptly stopped ingestion. The resu lts suggest that while leeches continue to sample their food once ingestion has begun, secondary chemosensory mechanisms situated downstream from the dorsal lip may be involved in the maintenance of ingestion and the rapid po stingestive rejection of noxious foods. (C) 1999 Elsevier Science Inc.