Using antagonists to assess neurochemical coding of a drug's ability to establish a conditioned place preference

Rats were given morphine as an agent of putative conditioning to establish a place preference. Doses of 4 and 8 mg/kg of morphine did establish reliable conditioned place preferences (CPP's). Other rats were given one of the doses of morphine and one of a number of antagonists in procedures designed to assess which antagonists would specifically block morphine's ability to establish a CPP indicative of positivity. Doses of naloxone and larger doses of naltrexone but not smaller ones did antagonize morphine's effects. A dose of the benzodiazepine antagonist Ro 15-1788 did not attenuate morphine's effects. It was concluded that morphine's positivity is dependent upon actions by way of receptors sensitive to naloxone and naltrexone, but that morphine's positivity is less sensitive to naltrexone's effects than morphine's analgesia.