P.3.005 The aversive effect of a glutamate metabotropic receptor agonist injected into the dorsal periaqueductal grey may partially depend on glutamate release

Abstract

The excitatory and inhibitory actions of metabotropic glutamate receptor (mGluR) agonists were investigated in acutely dissociated rat hippocampal CA3 pyramidal neurons, using the conventional whole-cell and nystatin-perforated patch recording configurations under the voltage-clamp condition. With the conventional whole-cell recording, glutamate (Glu) and quisqualic acid (QA) induced only ionotropic inward currents accompanied by increased membrane conductance at a holding potential (VH) of -45 mV. The response was reversibly blocked in the presence of D-2-amino-5-phosphonopentanoic acid (d-AP5) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), the antagonists ofN-methyl-d-aspartate (NMDA) receptor and non-NMDA receptor, respectively. With nystatin-perforated patch recording, mGlu responses insensitive to bothd-AP5 and CNQX were observed. Fifty-five % of the cells responded by a slow inward current accompanied by conductance decrease (ImGlui) at aVH of −44 mV. One % of the neurons showed an outward current with conductance increase (ImGluo), and 34% of the neurons showedImGluo followed byImGlui. The onset ofImGluo occurred approximately 900 ms after the response to 30 mM K+. The time to peak ofImGluo were 32- to 79-times longer than those of ionotropic responses.ImGlui appeared at lower concentrations than ionotropic Glu responses, whereasImGluo appeared at similar concentrations as ionotropic responses. The rank order of affinity was QA > Glu > (±)-1-aminocyclopentane-trans-1,3-dicar☐ylic acid (tACPD) for bothImGlui andImGluo. Half-maximal effective concentrations (EC50) and the threshold concentrations for the three agonists were four- to tenfold lower forImGlui than forImGluo. The current-voltage relationship showed that the reversal potentials ofImGlui andImGluo shifted 55 and 59 mV, respectively, for a tenfold change in extracellular K+ concentration, indicating that K+ is the charge carrier of both mGlu responses. DuringImGlui, both the leakage current and muscarine-sensitive voltage-dependent K+ current (M current) were suppressed.ImGluo induced by 10−4 M tACPD was abolished by 3 · 10−7 M charybdotoxin and 10−6 M ryanodine. These results show that there are two components of mGlu responses in CA3 pyramidal neurons and thatImGlui andImGluo show different pharmacological properties.