The dopamine D3/2 agonist 7-OH-DPAT attenuates the development of morphine tolerance but not physical dependence in rats.
BACKGROUND
The dopamine (DA) D3/2 agonist 7-OH-DPAT attenuates the acute antinociceptive, discriminative stimulus, locomotor activating, and reinforcing effects of mu agonists (for example, morphine).


OBJECTIVE
To examine the ability of 7-OH-DPAT to modulate the development of morphine tolerance and physical dependence in the rat.


METHODS
Morphine antinociception was assessed using a warm water tail-withdrawal procedure before and following chronic treatment with morphine (15 mg/kg)/7-OH-DPAT (0.3-3.0 mg/kg). Physical dependence was assessed following naloxone-precipitated (1.0 mg/kg) withdrawal in rats treated chronically with morphine (15 and 7.5 mg/kg)/7-OH-DPAT (1.0-10 mg/kg).


RESULTS
7-OH-DPAT attenuated the antinociceptive effects of morphine in both morphine naive and tolerant rats. Additionally, morphine tolerance was attenuated by the coadministration of 7-OH-DPAT in a dose- and time-dependent manner. The magnitude of the attenuation obtained when morphine and 7-OH-DPAT were administered at the same time was similar to that obtained when administration of these drugs was separated by 6 h, indicating that 7-OH-DPAT did not alter morphine pharmacokinetics. In rats rendered tolerant to morphine, the subsequent coadministration of morphine/7-OH-DPAT failed to reverse morphine tolerance, but did attenuate its further development. The level of physical dependence (number and frequency of withdrawal signs) was greater in rats treated with 15 than 7.5 mg/kg morphine. Under both treatment conditions, physical dependence was not altered by 7-OH-DPAT. In morphine-dependent (15 mg/kg) rats, 7-OH-DPAT (3.0 and 10 mg/kg) failed to precipitate withdrawal.


CONCLUSIONS
The D3/2 agonist 7-OH-DPAT can attenuate the antinociceptive effects of morphine in both acute and chronic preparations as well as the development of morphine tolerance. 7-OH-DPAT does not, however, alter morphine physical dependence.
Cook
C D
CD
Department of Psychology, University of North Carolina at Chapel Hill, 27599-3270, USA.
Barrett
A C
AC
Syvanthong
C
C
Picker
M J
MJ
D000818:Animals; D018491:Dopamine Agonists / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D004361:Drug Tolerance; D008297:Male; D009020:Morphine / Q000494:pharmacology*; D009021:Morphine Dependence / Q000523:psychology*; D009043:Motor Activity / Q000187:drug effects; D009270:Naloxone / Q000494:pharmacology; D009292:Narcotic Antagonists / Q000494:pharmacology; D009294:Narcotics / Q000494:pharmacology*; D051381:Rats; D020318:Rats, Long-Evans; D017448:Receptors, Dopamine D2 / Q000819:agonists*; D050637:Receptors, Dopamine D3; D013375:Substance Withdrawal Syndrome / Q000523:psychology; D013764:Tetrahydronaphthalenes / Q000494:pharmacology*; D013997:Time Factors
D018491:Dopamine Agonists; C493595:Drd3 protein, rat; D009292:Narcotic Antagonists; D009294:Narcotics; D017448:Receptors, Dopamine D2; D050637:Receptors, Dopamine D3; D013764:Tetrahydronaphthalenes; D009270:Naloxone; D009020:Morphine; C031349:7-hydroxy-2-N,N-dipropylaminotetralin