Possible role of protein kinase C in regulation of 5-hydroxytryptamine 2A receptors in rat brain

This study was undertaken to evaluate the effect of acute in vivo treatment with 1-(2,5-dimethoxy-4-iodo-phenyl)2-aminopropane (DOI), a selective 5-hydroxytryptamine 2A/2C (5HT2A/2c) receptor agonist, on the kinetic parameters of [3H]ketanserin binding to 5HT2A receptors and on the protein kinase C (PKC) activity in subcellular fractions of rat brain tissue. A single injection of DOI (10 mg/kg i.p.) downregulated (by 50%) 5HT2A receptor density in a cortical synaptosomal preparation assayed 24 h later. This effect was dose dependent, since a single injection of 5 mg/kg of DOI reduced the Bmax of [3H]ketanserin binding by 23% (without a change in Kd) and a single 1 mg/kg dose of DOI was without effect. Repeated doses of DOI (10 mg/kg for 3 days) further downregulated (by 63%) 5HT2A sites in cortical synaptosomes. A similar degree (50%) of downregulation of 5 HT2A receptors by DOI (10 mg/kg) was seen in p-chloroamphetamine (PCA) lesioned rats, suggesting that the site of action of DOI in downregulation of 5HT2A receptors in rat brain is postsynaptic. An increase (by 38%) of PKC activity in the particulate fraction of the cortical synaptosomal preparation following a single injection of DOI (10 mg/kg) paralleled the decrease in 5HT2A receptor density, suggesting that 5HT2A sites may be downregulated as a result of phosphorylation of the receptor by activation of PKC after receptor stimulation with agonist. This possibility is further supported by the observation that three consecutive daily injections of DOI resulted in a significant decrease (by 19%) in cytosolic PKC activity and an increase (by 24%) of PKC activity in the particulate fraction. A single injection of DOI also induced a translocation of PKC activity from the cytosolic to the membrane fraction in PCA-lesioned rats. The present investigation has shown that downregulation of 5HT2A receptors in rat cerebral cortex by in vivo DOI treatment is accompanied by translocation of PKC activity from the cytosolic to the membrane fraction.Key words: protein kinase C, 5-hydroxytryptamine 2A/2C (5HT2A/2C) receptor, 1-(2,5-dimethoxy-4-iodo-phenyl)2-aminopropane, p-chloroamphetamine, rat cerebrocortical synaptosomes