Noradrenergic modulation of the discriminative-stimulus effects of methamphetamine in rats.
BACKGROUND
Neurochemical and clinical studies indicate involvement of noradrenergic (NE) neurotransmitter system in the actions of methamphetamine.


OBJECTIVE
The present study investigated NE involvement in the discriminative-stimulus effects of methamphetamine.


METHODS
In Sprague-Dawley rats trained to discriminate 1.0 mg/kg methamphetamine, IP, from saline under a fixed-ratio schedule of food presentation, effects of various NE agonists, antagonists and uptake inhibitors were tested.


RESULTS
Desipramine (3.0-18.0 mg/kg) and nisoxetine (5.6-30.0 mg/kg), two selective NE-uptake inhibitors, did not significantly generalize to methamphetamine when administered alone, but 5.6 mg/kg desipramine and 10.0 mg/kg nisoxetine significantly shifted the methamphetamine dose-response curve to the left. The beta NE agonist, isoproterenol (0.56-3.0 mg/kg), and antagonist, propranolol (1.0-18.0 mg/kg), neither generalized to methamphetamine when given alone nor altered the discriminative-stimulus effects of methamphetamine when administered in combination. The alpha- NE agonist methoxamine (1.0-5.6 mg/kg) failed to generalize to the methamphetamine training stimulus. When given in combination with methamphetamine, the alpha-1 NE antagonist, prazosin (1.0 mg/kg), shifted the methamphetamine dose-response curve somewhat to the right and partially blocked the discriminative-stimulus effects of the 1.0 mg/kg training dose of methamphetamine, but these changes were not significant or dose-related, with further increases in prazosin dose (1.8-10.0 mg/kg) either producing similar or smaller changes. The alpha-2 NE agonist, clonidine, partially generalized to methamphetamine at doses of 0.1-0.18 mg/kg and increased drug-appropriate responding at lower doses of methamphetamine, but it partially blocked the discriminative-stimulus effects of higher 0.56-1.0 mg/kg doses of methamphetamine over the same dose range. The alpha-2 NE antagonist, yohimbine, also partially generalized to methamphetamine and blocked the discriminative-stimulus effects of the 1.0 mg/kg training dose of methamphetamine at doses of 5.6-10.0 mg/kg. A lower 3.0 mg/kg dose of yohimbine increased methamphetamine-appropriate responding when given together with low 0.1-0.3 mg/kg doses of methamphetamine.


CONCLUSIONS
The present data suggest that the NE system plays a modulatory role in the discriminative-stimulus effects of methamphetamine. These effects appear to be mediated through NE uptake sites and alpha-2 receptors, with limited involvement of alpha- receptors and beta receptors.
Munzar
P
P
Preclinical Pharmacology Laboratory, National Institutes of Health, National Institute on Drug Abuse, Intramural Research Program, Baltimore, MD 21224, USA.
Goldberg
S R
SR
D018663:Adrenergic Agents / Q000494:pharmacology*; D000818:Animals; D004193:Discrimination Learning / Q000187:drug effects*; D008297:Male; D008694:Methamphetamine / Q000494:pharmacology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D018340:Receptors, Adrenergic, alpha-1 / Q000187:drug effects; D018341:Receptors, Adrenergic, alpha-2 / Q000187:drug effects
D018663:Adrenergic Agents; D018340:Receptors, Adrenergic, alpha-1; D018341:Receptors, Adrenergic, alpha-2; D008694:Methamphetamine