MORPHINE AND METHADONE-INDUCED ANTINOCICEPTION IN RATS PERMANENTLY DEPLETED OF BRAIN DOPAMINE

  • 1 January 1977
    • journal article
    • research article
    • Vol. 229  (2) , 287-300
Abstract
The role of brain norepinephrine (NE) and/or dopamine (DA) in the production of the antinociceptive effects of morphine and methadone was studied. Rats were permanently depleted of brain Ne and/or DA via 6-hydroxydopamine (6-OHDA) treatments at 2 wk of age. Six to 8 wk later, the antinociceptive activity of morphine and methadone was determined in control, NE and DA-depleted, and DA-depelted rats by tail-flick methods. Both dose-response and time-duration studies were conducted. Apparently DA was important to morphine, but not to methadone-induced antinociception. This relationship appeared contingent on the level of DA-depletion. An antagonism of morphine''s effects was observed when DA levels were down by at least 85% whereas a facilitation of morphine''s effect occurred when DA levels were 50% or less. DA-related behaviors may be contingent upon a balance between excitatory and inhibitory DA neurons pools, and antinociception may (in relation to DA) also be controlled via a balance between this system (DA) and NE systems. The potency of morphine''s antinociceptive effects appears contingent upon a balance between 1 or more of these neuron pathways.