Table 1

Affinity (K i) values and inhibition of adenylyl cyclase activity by morphine and DAMGO in control GH3MOR cells and cells chronically treated with morphine or DAMGO . The affinity of morphine or DAMGO for μ-opioid receptors in GH3MOR cell membranes was determined by their displacement of 2 nM [3H]diprenorphine. The ability of μ-opioid agonists to reduce 10 μM forskolin-stimulated cAMP accumulation was assessed in transfected GH3 cells as described under Experimental Procedures. Data are presented as the percentage of control cAMP levels (i.e., no opioid). Data points represent the mean ± S.E.M. for three or four experiments performed in triplicate.

Acute DrugChronic Pre-treatmentBindingKiAdenylyl Cyclase
IC50IMAX
nM nM %
MorphineNone 7.20  ± 1.601-a 1-b 81.0  ± 1.301-c 57.0  ± 2.301-c
DAMGONone1.00  ± 0.601-a  18.6  ± 4.501-e 1-f 76.2  ± 1.661-e 1-f
DAMGOMorphineN.T.34.9  ± 22.11-f 34.0  ± 2.571-d 1-f
DAMGODAMGON.T.  325  ± 99.11-d 1-e 19.8  ± 3.501-d 1-e
  • 1-a  Values obtained from Piros et al., 1995.

  • 1-b  Significantly different from corresponding (Acute) DAMGO/(Chronic) None value, p < 0.05, (unpaired Student's t test).

  • 1-c  Significantly different from corresponding (Acute) DAMGO/(Chronic) None value, p < 0.01 (unpaired Student's t test).

  • 1-d  Significantly different from corresponding (Acute) DAMGO/(Chronic) None value, p < 0.05 (Tukey's test).

  • 1-e  Significantly different from corresponding (Acute) DAMGO/(Chronic) Morphine value, p< 0.05 (Tukey's test).

  • 1-f  Significantly different from corresponding (Acute) DAMGO/(Chronic) DAMGO value, p < 0.05 (Tukey's test).

  • N.T., not tested.