Differential respiratory patterns induced by opioids applied to the ventral medullary and dorsal pontine surfaces of cats.

The purpose of the present study was to make a functional dissection of the respiratory action of opioids, by their restricted application to the ventral surface of the medulla oblongata and to the rostro-dorsal surface of the pons in cats. The effects were compared to those induced after intracerebroventricular (i.c.v.) injection. Two mu-agonists, morphine and D-Ala2-Me-Phe4-Met(O)ol5-enkephalin (FK-33824), and the delta-agonist D-Ala2-D-Leu5-enkephalin (DADLE) were used. When applied to the ventral medullary surface, the opioids selectively depressed the generating mechanisms for tidal volume and the response to CO2, whereas the frequency was increased. The application to the rostral dorsal surface of the pons was followed by a selective depression of the respiratory frequency. By intracerebroventricular administration, the opioids depressed both the tidal volume and frequency generating mechanisms. The effects were always reversed by naloxone. The pontine structures were more sensitive to the action of the opioids than were the medullary centres. These findings suggest that the opioids can interact with different populations of respiratory neurones and that the respiratory output differs depending on the group of neurones selectively affected and the function they subserve in regulating respiratory activity.