Effects of chronic morphine treatment on opioid-induced inhibition and facilitation of acetylcholine release in guinea-pig thalamic slices.
The effect of chronic morphine treatment on acetylcholine (ACh) release from guinea-pig thalamic slices and on [3H]-dihydro-morphine binding to the brain of normal, tolerant and abstinent guinea-pigs was studied. Morphine (30 microM) inhibited the electrically-evoked ACh release to the same extent in normal and tolerant slices. This effect was antagonized by naloxone. Morphine (30 microM) in the presence of naloxone (10 microM) facilitated electrically-evoked release of ACh. This effect displayed tachyphylaxis in normal slices and was absent in the brain taken from tolerant animals. The reduction of [Ca++] in the medium increased the facilitatory response in normal slices and the inhibitory response in normal and tolerant tissue. The high and low affinity binding sites to [3H]-dihydro-morphine were the same in the thalami, caudate nuclei and cortices of normal, tolerant and abstinent animals. It is concluded that the cholinergic structures of the guinea-pig thalamus are unlikely to be involved in morphine tolerance. In fact, the facilitation appears to be an ancillary phenomenon which quickly displays tachyphylaxis in normal tissue while the inhibition of ACh release remains unchanged.
D000109:Acetylcholine / Q000378:metabolism*; D000818:Animals; D002118:Calcium / Q000502:physiology; D004558:Electric Stimulation; D004723:Endorphins / Q000494:pharmacology*; D005260:Female; D006168:Guinea Pigs; D066298:In Vitro Techniques; D008297:Male; D009020:Morphine / Q000494:pharmacology*; D009270:Naloxone / Q000494:pharmacology; D013618:Tachyphylaxis / Q000187:drug effects; D013788:Thalamus / Q000187:drug effects / Q000378:metabolism* / Q000502:physiology
D004723:Endorphins; D009270:Naloxone; D009020:Morphine; D000109:Acetylcholine; D002118:Calcium