Presynaptic nicotinic receptors evoke endogenous glutamate and aspartate release from hippocampal synaptosomes by way of distinct coupling mechanisms

The present work aimed to investigate whether and through which mechanisms selective a7 and a4b2 nicotinic receptor (nAChR) agonists stimulate endogenous glutamate (GLU) and aspartate (ASP) release in rat hippocampus. Rat hippocampal synaptosomes were purified on Percoll gradients and superfused in vitro to study endogenous GLU and ASP release. The synaptosomes were superfused with selective a7 and a4b2 nAChR agonists and antagonists. The excitatory amino acid (EAA) content of the samples of superfusate was determined by HPLC after pre-column derivatization and separation on a chromatographic column coupled with fluorimetric detection. Choline (Ch), a selective a7 receptor agonist, elicited a significant release of both GLU and ASP which was blocked by the a7 receptor antagonist methyllycaconitine (MLA), but was unaltered by the a4b2 receptor antagonist dihydro-b-erythroidine (DHbE). The stimulant effect of Ch was strongly reduced in a Ca2+-free medium, was not inhibited by Cd2+ and tetrodotoxin (TTX), but was antagonized by dantrolene, xestospongin C and thapsigargin. 5-Iodo-A-85380 dihydrochloride (5IA85380), a selective a4b2 receptor agonist, elicited EAA release in a DHbE-sensitive, MLA-insensitive fashion. The 5IA85380-evoked release was dependent on extracellular Ca2+, blocked by Cd2+ and TTX, but unaffected by dantrolene. Our study shows for the first time that rat hippocampal synaptosomes possess a7 and a4b2 nAChR subtypes, which can enhance the release of endogenous GLU and ASP via two distinct mechanisms of action. These results extend our knowledge of the nicotinic modulation of excitatory synaptic transmission in the hippocampus.