EFFECTS OF OPIATE-LIKE PEPTIDES, MORPHINE, AND NALOXONE IN THE PHOTOSENSITIVE BABOON, PAPIO-PAPIO

The effects of intracerebroventricular (i.c.v.) or systemic injections of Met- or Leu-enkephalin, β-endorphin, FK 33.824 (D-Ala2, MePhe4, Met(O5)-ol-enkephalin) and of morphine and naloxone have been studied in baboons, Papio papio, which spontaneously show photically induced epileptic responses. Animals were chronically implanted with epidural or deep recording electrodes and a cannula in one lateral ventricle, and tested whilst seated in a primate chair. In some animals the natural syndrome was enhanced by the prior administration of dl-allylglycine, 100-200 mg/kg, i.v. Met- or Leu-enkephalin, 1-10 mg, i.c.v., did not lead to any manifest focal or generalized seizure discharges. Nor did it lead to any consistent enhancement or reduction of photically induced myoclonic responses (as tested 5-10 min after injection). β-Endorphin, 0.1-0.5 mg, i.c.v., did not enhance or impair photically induced myoclonic responses. FK 33.824, 0.1-0.5 mg, i.c.v., depressed respiration and slowed EEG background rhythms for 9-15 h. This was associated with a loss of myoclonic responses to photic stimulation. These effects were reversed for 20-40 min following the injection of naloxone, 1 mg/kg i.m. A depression of respiration and a slowing of EEG rhythms was seen beginning 5-20 min after FK 33.824, 2 or 4 mg/kg, i.v. The higher dose also abolished photically induced myoclonic responses. Naloxone, 1 mg/kg, definitively reversed these effects. Morphine, 5-10 mg i.c.v., tended to increase the latency to onset of generalized myoclonus during photic stimulation. Myoclonic responses were delayed or diminished after morphine, 5 mg/kg, i.m. Naloxone, 1-2 mg/kg i.m., reversed this effect. Naloxone, 0.2-5.0 mg/kg i.m., alone, did not significantly modify photically induced myoclonus, either in animals of low or high initial responsiveness, or in those pretreated with allylglycine. © 1979.