Mercury interaction with the GABA(A) receptor modulates the benzodiazepine binding site in primary cultures of mouse cerebellar granule cells

Mercury compounds are neurotoxic compounds with a great specificity for cerebellar granule cells. The interaction of mercury compounds with proteins in the central nervous system may underlie some of their effects on neurotransmission. In this work we study the interaction of mercuric chloride (HgCl2) and methylmercury (MeHg) with the GABAA receptor in primary cultures of cerebellar granule cells. Both compounds increased, dose dependently, the binding of flunitrazepam to the benzodiazepine recognition site. EC50 values for this effect were 3.56 and 15.24 μM for HgCl2 and MeHg, respectively, after 30 min exposure of intact cultured cerebellar granule cells. The increase of flunitrazepam binding by mercury compounds was completely inhibited by the GABAA receptor antagonists bicuculline and picrotoxinin, and by the organochlorine pesticide α-endosulfan. It was also partially inhibited by the anion transporter blocker DIDS, however this effect could be due to a possible chelation of mercury by DIDS. Intracellular events, like intracellular calcium, kinase activation/inactivation or antioxidant conditions did not affect flunitrazepam binding or its increase induced by mercury compounds. The sulfhydryl alkylating agent N-ethylmaleimide mimicked the effect of mercury compounds on flunitrazepam binding suggesting a common mechanism. We conclude that mercury compounds interact with the GABAA receptor by the way of alkylation of SH groups of cysteinyl residues found in GABAA receptor subunit sequences.
This research was supported by Grant number 00/1094 and 01/1318 from FIS (Spanish Fondo de Investigaciones Sanitarias de la Seguridad Social) and Grant numbers 1999SGR00210 and 1999SGR00212 from CIRIT (Generalitat de Catalunya, Spain). E. Fonfrı́a is recipient of a fellowship from the Institut d'Investigacions Biomèdiques August Pi i Sunyer (IDIBAPS).