A comparison of the antinociceptive and behavioral effects of intrathecally administered opiates, alpha-2-adrenergic agonists, and local anesthetics in mice and rats.

This study was undertaken to compare the antinociceptive and behavioral effects of intrathecally administered opiates, alpha-2-adrenergic agonists, and local anesthetics injected by lumbar puncture in the mouse and rat. Antinociception was determined by observing the response to a clamp applied to the tail (Haffner test) of the mouse and by the rat tail-flick test; log dose-response curves for antinociception were generated for each drug in each test. Motor coordination and other behavioral effects were also observed. Morphine and fentanyl (mu-opiate agonists) as well as ethylketocyclazocine (EKC) and U50488H (kappa-opiate agonists), together with buprenorphine (partial mu-opiate agonist) and the alpha-2-adrenergic agonist clonidine, all produced antinociception in both species without causing significant behavioral or motor dysfunctions at antinociceptive doses. Xylazine (also an alpha-2-adrenergic agonist), ketamine, procaine, and lidocaine inhibited responses but only at doses that also produced motor impairment or paralysis. Nalbuphine (mixed opiate agonist-antagonist) was without any effect in both species. These data suggest that the mu- and kappa-opiate agonists and clonidine are the preferred agents for producing antinociception without compromising motor function.