Ventilatory effects of naloxone and morphine in the developing opossum.

To determine whether endogenous opiates might tonically suppress ventilation in the immature mammal, young opossums between the 15th and 50th day of development were challenged with the opiate antagonist, naloxone, while ventilation was assessed using pressure plethysmorgraphy. The youngest animals (to about 25 days of age) typically showed an increase in breathing, but this was accompanied by somatic motor activity (assessed using electromyography) resembling behavioral arousal. In older animals a behavioral response was not always obtained; but in the absence of a behavioral effect, stimulation of breathing was not typically observed. When animals were tested with the opiate agonist morphine, respiratory depression was observed. Again, the most potent effects were obtained in the youngest animals. These data suggest that activation of opiate receptors can result in depression of breathing in young opossums; however, the stimulation of breathing by an opiate antagonist is best interpreted in terms of effects on wakefulness, rather than removal of tonic inhibition from respiratory neurons.