Ligand-Specific Regulation of the Endogenous Mu-Opioid Receptor by Chronic Treatment with Mu-Opioid Peptide Agonists

Table 1

Changes in DAMGO-stimulated [³⁵S]GTP𝛾S binding induced by in vivo chronic opioid peptide treatments in rat brain subcellular fractions.

Treatment	𝐸max (% over basal)				EC50 (nM)
	SPM		MI		SPM		MI
	Control	Treated	Control	Treated	Control	Treated	Control	Treated
ACHC-EM2	95±5	106±5	85±9	109±2	44±6	80±8^∗	100±18	179±25^∗
DAMGO	112±12	130±14	60±3	72±9	87±9	107±14	101±14	86±2
DAMCK	112±12	100±9	60±3	61±17	87±9	126±22	101±14	200±42^∗

ACHC-EM2, DAMGO, and DAMCK were chronically administered to rats as described in Methods. Control animals received CSF. Subcellular fractionation of brain homogenates to obtain synaptic plasma membrane (SPM) and microsomal (MI) fractions was performed. Full concentration curves of DAMGO, consisting of 5-6 concentrations between 10⁻⁸--10⁻⁴ M, were measured in [³⁵S]GTP𝛾S binding assay. The parameters shown were obtained from nonlinear regression analysis using Graph Pad Prism 4 considering a sigmoidal dose response curve for DAMGO. Results shown are as % stimulation of [³⁵S]GTPγS binding over basal values (i.e., binding in the absence of DAMGO). Data are mean ± S.E.M. of 3–6 independent experiments each performed in triplicate. Significant difference between the appropriate values in control and treated membrane fractions was determined by the Student’s 𝑡-test and set as ^∗𝑃<0.05.