Activation of cerebellar granule cells GABA(A) receptors by guanidinoacetate.

The extracellular concentration of guanidinoacetate (GAA) in the brain increases in guanidino acetate methyl transferase (GAMT) deficiency, an inherited disorder. We tested whether the levels which this substance can reach in the brain in GAMT deficiency are able to activate GABA(A) receptors in key cerebellar neurons such as the cerebellar granules. GAA in fact activates these receptors in rat cerebellar granules in culture although at quite high concentrations, in the millimolar range. However, these millimolar GAA levels are not reached extracellularly in the brain in GAMT deficiency. In addition, GAA does not act as a partial agonist on granules' GABA(A) receptors. This appears to deny an effect by this molecule on cerebellar function in the disease via interference with granule cells' GABA(A) receptors. Study of partial blockage by furosemide of chloride currents activated by GABA and GAA in granule cells allowed us to distinguish two populations of GABA(A) receptors presumably involved in granule cells' tonic inhibition. One is devoid of alpha6 subunit and another one contains it. The latter when activated by GABA has a decay kinetics much slower than the former. GAA does not distinguish between these two populations. In any case, the very high extracellular GAA concentrations able to activate them are not likely to be reached in GAMT deficiency.