title: Morphine and methadone-induced antinociception in rats permanently depleted of brain dopamine.
abstract: The purpose of the present study was to evaluate the role of brain norepinephrine (NE) and/or dopamine (DA) in the production of the antinociceptive effects of morphine and methadone. Rats were permanently depleted of brain NE and/or DA via 6-hydroxydopamine (6-OHDA) treatments at two weeks of age. Six to eight weeks later, the antinociceptive activity of morphine and methadone was determined in control, NE and DA-depleted, and DA-depleted rats by tail-flick methods. Both dose-response and time-duration studies were conducted. The data obtained suggested that DA was important to morphine, but not to methadone-induced antinociception. However, this relationship appeared contingent upon 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. These findings were discussed in relation to more recent concepts suggesting that: 1) DA-related behaviors are contingent upon a balance between excitatory and inhibitory DA neuron pools, and 2) antinociception may (in relation to DA) also be controlled via a balance between this system (DA) and NE systems. Thus, it was suggested that the potency of morphine's antinociceptive effects appears contingent upon a balance between one or more of these neuron pathways.
mesh_terms: D000700:Analgesics; D000818:Animals; D001921:Brain / Q000502:physiology*; D004298:Dopamine / Q000502:physiology*; D005260:Female; D006892:Hydroxydopamines / Q000494:pharmacology; D008691:Methadone / Q000494:pharmacology*; D009020:Morphine / Q000494:pharmacology*; D009638:Norepinephrine / Q000032:analysis; D011247:Pregnancy; D051381:Rats
chemical_list: D000700:Analgesics; D006892:Hydroxydopamines; D009020:Morphine; D008691:Methadone; D004298:Dopamine; D009638:Norepinephrine