Effects of serotonergic denervation on the density and plasticity of brain muscarinic receptors in the rat.

This study investigated whether serotonergic lesion may affect density, sensitivity, and plasticity of muscarinic receptors in hippocampus and cerebral cortex. Intracerebroventricular injection of 5,7-dihydroxytryptamine (5,7-DHT) in rats produced a 90% reduction in cortical and hippocampal 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) contents. In these brain areas, the 5,7-DHT lesion did not affect the overall density of muscarinic receptors or those of M1 and non-M1 muscarinic receptor subtypes as assayed using [3H]N-methylscopolamine ([3H]NMS), [3H]pirenzepine, and [3H]NMS in the presence of pirenzepine, respectively. In addition, the binding of the muscarinic agonist [3H]oxotremorine-M (OXO-M), taken as an indirect index of coupling efficiency of non-M1 receptors with G-proteins, did not change significantly in cortex and hippocampus of 5,7-DHT-lesioned rats. Similarly, carbachol-induced accumulation of [3H]inositol phosphates (InPs) in hippocampal miniprisms showed no significant differences between tissues from 5,7-DHT-lesioned and sham-operated rats. In sham-operated rats, an intraperitoneal (i.p.) injection of scopolamine (10 mg/kg once daily) during 21 days caused an increased density of [3H]NMS binding sites in cortex (+20%) and hippocampus (+26%). This up-regulation was restricted to non-M1 receptors subtypes. In 5,7-DHT-lesioned rats, chronic scopolamine failed to modify significantly the density of cortical or hippocampal M1 or non-M1 receptors. These results suggest 1) that 5-HT denervation did not affect the density and sensitivity of muscarinic receptors and 2) that the ability of cortical and hippocampal non-M1 receptors to up-regulate following repeated injection of scopolamine requires the integrity of 5-HT neurons terminating in these brain structures.