Effects of repeated administration of tetrahydroaminoacridine (THA) on muscarinic receptor subtypes in the rat brain.

The effects of a chronic treatment (21 days) with the acetylcholinesterase (AChE) inhibitor tetrahydroaminoacridine (THA) on muscarinic receptors subtypes were investigated at various times after the last administration of the drug, in various brain areas including cortex, striatum, hippocampus and cerebellum. Forty eight hours after the end of chronic THA treatment, the number of muscarinic receptors, labelled with [(3)H]NMS, was significantly lowered in the cortex and the striatum but not in the hippocampus or cerebellum. High affinity pirenzepine binding sites (M(1) receptors), directly assayed using [(3)H]pirenzepine saturation assays or estimated by pirenzepine [(3)H]NMS competition, were lowered only in the cortex and in the striatum of THA-treated rats. In contrast, the number of low affinity pirenzepine sites (M(2) receptors), was not significantly modified. At shorter wash-out period (18 h), the density of M(1) receptors decreased by 26, 46 and 52% in the hippocampus, cerebral cortex and striatum, respectively. In all cases, K(d) values remained unchanged suggesting that the loss of M(1) sites was not due to a modification of radioligand affinity for the receptors. Although THA displayed a micromolar affinity for M(1) and M(2) receptors in vitro, this AChE inhibitor did not interfere with the receptor assays since no trace of residual free THA was detected in rat brain at 48 h post-treatment. These results suggest that chronic treatment with THA produced a selective down-regulation of M(1) receptors; they also indicate that these receptors may be regulated differently in cortical, striatal, hippocampal or cerebellar regions.