Effects of opiates and methionine-enkephalin on pontine and bulbar respiratory neurones of the cat.

The effects of microelectrophoretically applied opiate agonists and the antagonist naloxone have been investigated on extracellularly recorded neurones in pontine and bulbar respiratory centers of the cat. Morphine, levorphanol and methionine-enkephalin depressed the spontaneous discharge of respiration related units and the firing induced in these cells by phoretically applied L-glutamate. The rhythmic pattern of these neurones was modified, in such a way that basal activity was hardly affected while peak frequency was markedly reduced. These effects are mediated via stereospecific opiate receptors, since they were antagonized by naloxone and not mimicked by the D+ enantiomer of levorphanol, dextrophan. The few excitations observed following opiate agonist application were not antagonizable by naloxone. These data may provide a basis for the explanation of the depressant effect on central respiratory rhythm of systemically injected opiates which have been obtained in control experiments. The results are interpreted in terms of opiates lowering excitatory synaptic efficacy.