Agonist-induced desensitization of muscarinic acetylcholine receptor in rat brain

Intact brain cell aggregates were dissociated from adult rat brains without cerebellum using a sieving technique. This preparation was used to elucidate the binding characteristics of agonist to muscarinic acetylcholine receptors (mAchR) in brain. Incubation of cells with carbamylcholine (carbachol) was shown agonist-induced receptor down-regulation depending on the concentration of agonist, not depending on the incubation time. This effect of carbachol was due to a reduction in the maximal binding capacity (B _max ) to the mAchR without decreasing the affinity of the remaining receptors in incubation at 37 °C but was not apparent in incubation at 15 °C. In addition, it was abolished when the receptors were blocked by atropine. The decline in (³H)N-methylscopolamine((³H)NMS) binding induced by agonist was reflected as a significant reduction in the receptor density with no change in receptor affinity, suggesting that 'tru' receptor down-regulation takes place. Moreover, when the receptors were labeled with the lipophilic antagonist (³)NMS, the magnitude of the observed receptor down-regulation was significantly lower in case of the former than the latter. This suggests that exposure of intact brain cells to muscarinic agonists might induce a slight degree of accumulation of receptors in intracellular sites before the receptors are actually degraded.