TY - JOUR
T1 - Tolerance of hypothalamic β-endorphin neurons to μ-opioid receptor activation after chronic morphine
AU - Zhang, Ge
AU - Lagrange, Andre H.
AU - Rønnekleiv, Oline K.
AU - Kelly, Martin J.
N1 - Copyright:
Copyright 2007 Elsevier B.V., All rights reserved.
PY - 1996/4
Y1 - 1996/4
N2 - The μ-opioid receptor is an autoreceptor on hypothalamic β-endorphin neurons that when activated inhibits cell firing via increasing an inwardly rectifying potassium conductance. The membrane hyperpolarization to DAMGO ([D-Ala2, N-Me-Phe4, Gly-ol5]-enkephalin) in β-endorphin and other arcuate (ARC) neurons was investigated in hypothalamic slices from control and morphine-treated, ovariectomized guinea pigs. Chronic morphine treatment caused both a decreased potency (EC50: 220 ± 10 nM vs. 64 ± 3 nM in controls) and a decreased efficacy (V(max): -7.1 ± 1.1 mV vs. -10.7 ± 0.6 mV in controls) of DAMGO in a population of ARC neurons including β- endorphin neurons. In another population of ARC neurons from morphine- treated animals, DAMGO was less potent (EC50: 110 ± 4 nM) than in controls (EC50: 64 ± 3 nM), but there was not a significant change in the efficacy of DAMGO. Twenty percent of ARC neurons did not exhibit any signs of tolerance. The density of μ-opioid receptors labeled with the antagonist radioligand [3H]diprenorphine was found to be significantly decreased in the ARC and surrounding mediobasal hypothalamus after morphine treatment (B(max): 217 ± 9 vs. 276 ± 16 fmol/mg protein in controls), which is consistent with the altered response in β-endorphin neurons. In summary, chronic morphine treatment decreases μ-opioid receptor density and the functional coupling of μ-opioid receptors to K+ channels in ARC neurons. This expression of morphine tolerance by β-endorphin (ARC) neurons may serve as a homeostatic mechanism to maintain opioid control of a variety of systems ranging from reproduction to motivation and reward.
AB - The μ-opioid receptor is an autoreceptor on hypothalamic β-endorphin neurons that when activated inhibits cell firing via increasing an inwardly rectifying potassium conductance. The membrane hyperpolarization to DAMGO ([D-Ala2, N-Me-Phe4, Gly-ol5]-enkephalin) in β-endorphin and other arcuate (ARC) neurons was investigated in hypothalamic slices from control and morphine-treated, ovariectomized guinea pigs. Chronic morphine treatment caused both a decreased potency (EC50: 220 ± 10 nM vs. 64 ± 3 nM in controls) and a decreased efficacy (V(max): -7.1 ± 1.1 mV vs. -10.7 ± 0.6 mV in controls) of DAMGO in a population of ARC neurons including β- endorphin neurons. In another population of ARC neurons from morphine- treated animals, DAMGO was less potent (EC50: 110 ± 4 nM) than in controls (EC50: 64 ± 3 nM), but there was not a significant change in the efficacy of DAMGO. Twenty percent of ARC neurons did not exhibit any signs of tolerance. The density of μ-opioid receptors labeled with the antagonist radioligand [3H]diprenorphine was found to be significantly decreased in the ARC and surrounding mediobasal hypothalamus after morphine treatment (B(max): 217 ± 9 vs. 276 ± 16 fmol/mg protein in controls), which is consistent with the altered response in β-endorphin neurons. In summary, chronic morphine treatment decreases μ-opioid receptor density and the functional coupling of μ-opioid receptors to K+ channels in ARC neurons. This expression of morphine tolerance by β-endorphin (ARC) neurons may serve as a homeostatic mechanism to maintain opioid control of a variety of systems ranging from reproduction to motivation and reward.
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M3 - Article
C2 - 8613967
AN - SCOPUS:0030437250
SN - 0022-3565
VL - 277
SP - 551
EP - 558
JO - Journal of Pharmacology and Experimental Therapeutics
JF - Journal of Pharmacology and Experimental Therapeutics
IS - 1
ER -