A stable, parenterally active analogue of methionine-enkephalin, D-met2-Pro5-enkephalinamide, was injected into the right atrium of decerebrate male Wistar rats weighing 400-450g. The analogue produced apnea, bradycardia and hypotension within 1-2 seconds. These effects lasted for 10-30 seconds. Apnea was followed by a 10-15 minute period of rapid and shallow breathing. These affects were abolished by bilateral vagotomy and naloxone (1 mg/kg, right atrium). Direct nerve recordings showed that the enkephalin analogue produced cessation of phrenic nerve (cephalic-end) activity. Concomitantly, pulmonary J-receptors were activated in thin vagal (caudal-end) strands. These results indicate that enkephalins stimulate pulmonary J-receptors and this in turn results in reflex bradycardia and fast and shallow breathing.
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