Induction of giant depolarizing potentials by zinc in area CA1 of the rat hippocampus does not result from block of GABAB receptors.

The possibility that zinc (Zn2+) induces giant depolarizing potentials (GDPs) by blocking pre- and postsynaptic gamma-aminobutyric acidB (GABAB) receptors in area CA1 of rat hippocampal slices was investigated. Monosynaptic GABAA receptor-mediated fast and GABAB receptor-mediated late inhibitory postsynaptic potentials (IPSPs) were evoked in the presence of the excitatory amino acid (EAA) receptor antagonists 6,7-dinitroquinoxaline-2,3-dione (DNQX) and D,L-amino-5-phosphonovalerate (APV). Addition of Zn2+ (0.3 mM) resulted in the appearance of long-lasting GDPs which obscured monosynaptic late IPSPs. The GABAA receptor antagonist bicuculline methiodide (BMI; 30 microM) blocked fast monosynaptic IPSPs and GDPs, revealing a monosynaptic late IPSP that was prolonged in the presence of Zn2+ and blocked by the GABAB receptor antagonist CGP 35,348 (100 microM). The selective GABAB receptor agonist baclofen (10 microM) depressed monosynaptic IPSPs and population excitatory postsynaptic potentials (pEPSPs) by acting at presynaptic GABAB receptors. Depression of synaptic potentials by baclofen was unaffected by Zn2+. These results suggest that induction of GDPs in area CA1 does not result from an action of Zn2+ at GABAB receptors. We suggest instead that Zn2+ induces GDPs by inducing synchronized discharge of GABAergic interneurons.