The role of myo-inositol in G-protein coupled receptor-mediated sub-cellular transduction mechanisms in neuronal cell lines

Abstract

myo-lnositol (ml), a simple polyol isomer of glucose, is an important osmolyte in the brain and a precursor of the phosphatidyl inositol metabolic pathway. It is known to facilitate various cellular events such as membrane trafficking and regulation of cell death and survival. In addition, high oral doses of ml have been reported to be effective in the treatment of various psychiatric disorders such as depression, panic disorders and obsessive compulsive disorder. ml is also a natural component of the human diet and is obtained from food sources like fruits and whole wheat grains. There are no serious side effects documented relating to the use of even high doses of ml. Changes in the pharmacological management of depression is necessary to develop drugs with less side effects, greater tolerability and better patient compliance and these criteria makes ml a potential attractive drug for treatment of these disorders. The current study aimed to investigate the mechanism of action of ml at molecular level, specifically by investigating a possible modulatory role of ml on 5-HT2A receptor (5-HT2A-R) function and number, as expressed in transfected human neuroblastoma (5-HT2A-SH-SY5Y) cells). The effect of ml was compared to the effects of two prototype antidepressants, namely fluoxetine (a selective serotonin re-uptake inhibitor) and imipramine (a tricyclic antidepressant). To investigate the possible effect of ml, fluoxetine and imipramine on 5-HT2A-R function and expression, 5-HT2A-SH-SY5Y cells were pre-treated with different concentrations of these respective substances. Thereafter, functional studies, radio-ligand binding studies and intracellular [3H]-ml uptake studies were performed. Membranes were also prepared and [35S]-GTPyS binding studies were performed. Receptor function was measured by second messenger [3H]-IPx (Inositolphosphates) accumulation and [35S]-GTPyS binding to Gaq, protein. Relative receptor number was determined from appropriate radio-ligand binding experiments and total [3H]-ml uptake into cells was measured directly from cell lysates. The current study shows that ml may exert its therapeutic effect in depression and related anxiety disorders in part by decreasing 5HT,-R function and specifically by decreasing the receptor signalling capacity through Gq proteins. ml pre-treatments cause a decrease in [3H]-Ipx production and a decrease in [3H]-ml uptake without any significant effect on 5-HT2A-R binding. Fluoxetine pre-treatment also significantly decreased [3H]-IPx production and [3H] -ml uptake without any significant effect on 5-HT2& radio-ligand binding. However, imipramine pre-treatment significantly increased receptor function. These results may explain why ml was found to be effective exclusively in selective serotonin reuptake inhibitor sensitive disorders. In previous studies from our laboratory conducted on mAChR function it was found that ml, fluoxetine and imipramine pre-treatments all reduce mAChR function and that the attenuating effect of ml on mAChRs is partially dependent on the phosphoinositide (PI) metabolic pathway. Taken together, the current data suggests that ml may exert its antidepressant action via down regulation of 5-HT2A-R signalling and by attenuating cholinergic hypersensitivity. Further detailed studies are, however, necessary to resolve the full mechanism of action of ml at subcellular level.