Interaction between nicotine and endogenous opioid mechanisms in the unanesthetized dog

Nicotine produced a distinct reproducible syndrome in the conscious dog when injected intravenously or intracerebroventricularly. Intravenously administered nicotine (40 micrograms/kg/min for 20 minutes) increased cardiac and respiratory rates and produced analgesia, miosis, hypothermia, behavioral restlessness and emesis. When microinjected into the third cerebral ventricle, nicotine (100-200 micrograms) similarly increased cardiac and respiratory rates and pupillary diameter; and produced behavioral restlessness, emesis, erratic analgesia and maintained wakefulness and a desynchronized EEG. Microinjection of nicotine (5-25 micrograms) into the periaqueductal gray failed to alter any of the parameters studied. Intravenous pretreatment with the opioid antagonist naltrexone (2 mg/kg) influenced the action of intravenous nicotine on certain physiological systems. While naltrexone alone produced a significant degree of tachycardia, miosis, and analgesia, it potentiated the tachypnea and antagonized the miotic response evoked by nicotine. Methionine-enkephalin was detected in perfusates obtained from the lateral cerebral ventricles of conscious dogs. Nicotine produced a non-significant decrease in enkephalin levels. These observations suggest that there are interactions between endogenous opioid and nicotinic processes. However, they are complex and may differ from one functional system to another.