Addictive agents and intracranial stimulation (ICS): Naloxone blocks morphine’s acceleration of pressing for ICS

Ten rats were fixed with chronically indwelling electrodes for intracranial stimulation (ICS) of the lateral hypothalamus. The rats were then trained to press a lever for the self administration of ICS at two current intensities; one intensity produced a low rate of pressing and the other a high rate of pressing for ICS. Subsequent to obtaining stable press rates for each intensity of ICS, five rats received daily injections of morphine (10 mg/kg) 4 h prior to testing and five rats were injected with saline. After a significant acceleration of pressing for ICS was demonstrated by the morphine group, both groups were injected with naloxone (1 mg/kg) 15 min prior to testing. No effect was shown for the placebo group, while naloxone blocked morphine’s potentiation of pressing for ICS. The blocking effect persisted for 3 days after the single dose of naloxone despite the continued administration of morphine (10 mg/kg/day), which had previously accelerated pressing for ICS.