Effect 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 μM) inhibited the electrically-evoked ACh release to the same extent in normal and tolerant slices. This effect was antagonized by naloxone. Morphine (30 μM) in the presence of naloxone (10 μM) 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 ...