Long-term adaptation to high doses of morphine causes desensitization of mu-OR- and delta-OR-stimulated G-protein response in forebrain cortex but does not decrease the amount of G-protein alpha subunits.
BACKGROUND
The functional activity of trimeric guanine-nucleotide-binding proteins (G-proteins) represents an essential step in linking and regulation of the opioid receptor (mu-, delta- and kappa-OR)-initiated signaling pathways. Theoretical basis and/or molecular mechanism(s) of opioid tolerance and addiction proceeding in the central nervous system were not studied in the forebrain cortex of mammals with respect to quantitative analysis of opioid-stimulated trimeric G-protein activity.


METHODS
G-protein activity was measured in PercollR-purified plasma membranes (PM) isolated from the frontal brain cortex of control and morphine-treated rats by both high-affinity [32P]GTPase and [35S]GTPgammaS binding assays. Exposition to morphine was performed by intra-muscular application of this drug. Control animals were injected with sterile PBS.


RESULTS
Both mu-OR (DAMGO)- and delta-OR (DADLE)-responses were clearly desensitized in PM isolated from morphine-treated rats; kappa-OR (U-69593)- and baclofen (GABAB-R)-stimulated [35S]GTPgammaS binding was unchanged, indicating the specificity of the morphine effect. Under such conditions, the amount of G-protein alpha subunits was unchanged. The order of efficacy DADLE>DAMGO>U-69593 was the same in control and morphine-treated PM. Behavioral tests indicated that morphine-treated animals were fully drug-dependent and developed tolerance to subsequent drug addition.


CONCLUSIONS
Prolonged exposure of rats to high doses of morphine results in decrease of the over-all output of OR-stimulated G-protein activity in the forebrain cortex but does not decrease the amount of these regulatory proteins. These data support the view that the mechanism of the long-term adaptation to high doses of morphine is primarily based on desensitization of OR-response preferentially oriented to mu-OR and delta-OR.
Bourova
Lenka
L
Laboratory of Membrane Receptors, Institute of Physiology, Academy of Sciences of the Czech Republic, Prague, Czech Republic. bourova@biomed.cas.cz
Vosahlikova
Miroslava
M
Kagan
Dmytro
D
Dlouha
Katerina
K
Novotny
Jiri
J
Svoboda
Petr
P
D000222:Adaptation, Physiological / Q000187:drug effects*; D000818:Animals; D001418:Baclofen / Q000494:pharmacology; D001522:Behavior, Animal / Q000187:drug effects; D002462:Cell Membrane / Q000187:drug effects / Q000201:enzymology; D002540:Cerebral Cortex / Q000187:drug effects / Q000378:metabolism*; D004305:Dose-Response Relationship, Drug; D004361:Drug Tolerance; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)- / Q000494:pharmacology; D044385:GTP-Binding Protein alpha Subunits / Q000378:metabolism*; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000378:metabolism; D006153:Guanosine Diphosphate / Q000378:metabolism; D008297:Male; D009020:Morphine / Q000008:administration & dosage* / Q000494:pharmacology*; D051381:Rats; D017208:Rats, Wistar; D017465:Receptors, Opioid, delta / Q000819:agonists / Q000378:metabolism*; D017450:Receptors, Opioid, mu / Q000819:agonists / Q000378:metabolism*; D013375:Substance Withdrawal Syndrome / Q000503:physiopathology; D013997:Time Factors
D044385:GTP-Binding Protein alpha Subunits; D017465:Receptors, Opioid, delta; D017450:Receptors, Opioid, mu; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; D006153:Guanosine Diphosphate; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D009020:Morphine; D001418:Baclofen