Peptide-Induced Emesis in Dogs

Systemic administration of several but not all neuropeptides induces emesis in dogs. The threshold dose for leucine-enkephalin was 0.05 mgkg, for angiotensin II was 0.20 mgkg, and for neurotensin was 0.05 mgkg. These values compare to a threshold of 0.005 mgkg for apomorphine. Emesis was also observed after systemic administration of gastrin, vasoactive intestinal polypeptide VIP, substance P, vasopressin, oxytocin, bombesin, methionine enkephalin, and thyrotropin-releasing hormone TRH. Other peptides, including somatostatin, cholecystokinin, secretin, bradykinin, carosine, and leutinizing hormone- releasing hormone LHRH, were not emetic. Receptors for leucine-enkephalin and angiotensin II show receptor desensitization, such that a second systemic administration 5 min after the first is ineffective. Application of the other peptide or apomorhine is effective at 5 min, however. These results indicate that there are distinct receptors for angiotensin II and leucine-enkephalin, presumably located on neurons of the chemosensory trigger zone in the area postrema. The effects of chlorpromazine, domperidone, nalozone, and saralasin were tested on emetic sensitivity to apomorphine, leucine-enkephalin, and angiotensin II.