Reduction of vagal pressor reflexes by neurohypophyseal peptides and related compounds

The effects of intracisternal injections of [Lys8]vasopressin and [Arg8]vasopressin (25, 50, 100, 200 mU/kg) and related compounds oxytocin (25, 50, 100, 200 mU/kg), felypressin (25, 50, 100, 200 mU/kg) and vasotocin (100, 200, 400, 800 ng/kg) on the acute neurogenic pressor responses to afferent vagal stimulation (5, 10, 20 and 30 Hz) were studied in urethane-anaesthetized dogs. [Arg8]vasopressin and [Lys8]vasopressin (50, 100, 200 mU/kg) elicited a significant decrease in the pressor responses. The depressor effects of oxytocin (50, 100, 200 mU/kg) were less marked and felypressin or vasotocin remained inactive. These results suggest that vasopressin, and oxytocin, containing neurons may be modulators involved in central cardiovascular regulation. Since these cardiovascular reflexes are related to an increase in sympathetic tone, the present work suggests that neurohypophyseal peptides could play an inhibitory role in the regulation of blood pressure through a central inhibition of sympathetic tone. In term of structure-activity relationships the present work suggests that under our experimental conditions the sequence 1-5 of amino acids in vasopressin is important for the depressor action.