Maturational effect of enkephalin on respiratory control in newborn rabbits.

The respiratory responses to systemic infusion of the opioid peptide, [D-Ala2, D-Leu5]enkephalin (ENK) were determined in 39 unanesthetized tracheotomized rabbits (age range 1-20 days). At all ages, ventilation (VE), measured in a body plethysmograph, was depressed after ENK infusion in association with a decrease in CO2 elimination (VCO2) and body temperature. The degree of VE depression varied inversely with increasing age and was directly related to changes in mean inspiratory flow (i.e., VT/TI) while the ratio of inspiratory to total breath duration (TI/TT) was unaltered, except in rabbits under about 1 wk of age. Maturational differences in the VE response to ENK were related to age-dependent variation in the stability of the central inspiratory activity, which was manifested as periodic breathing with apnea in rabbits under about 5 days of age. Since the initial inspiratory volume-time profile was little affected by ENK and vagal afferent influence on respiration was not diminished, the depression in VE could be explained by an inhibition of the central inspiratory "off-switch" threshold and delay in central inspiratory "on-switching." All effects of ENK were reversed by the opiate antagonist, naloxone.