THE ANTINOCICEPTIVE AND MOTIVATIONAL EFFECTS OF INTRANIGRAL INJECTIONOF OPIOID AGONISTS

The antinociceptive potency of morphine and the morphine metabolite mo rphine-6-glucuronide (M6G) was examined after injection into the subst antia nigra and periaqueductal gray (PAG) of rats. Both drugs produced antinociception in both sites. The antinociceptive potency of M6G was significantly greater than morphine in the nigra. There was no differ ence in the antinociceptive potency of M6G in the nigra and FAG. M6G a nd other opioids were also examined for. motivational effects after in tranigral injection. A high dose of intranigral morphine (10.0 nmol) p roduced a conditioned place preference. No significant motivational ef fects were produced by 1.0 nmol of M6G, D-Ala(2), N-Me-Phe(4), Gly(5)- ol-enkephalin (DAGO), D-Pen(2),D-Pen(5)-enkephalin (DPDPE), or U-50,48 8H. It is concluded that the substantia nigra plays an important role in opioid antinociception. The role of the nigra in opioid reward is q uestionable.