Effects of glutamatergic agonists and antagonists on membrane potential and intracellular Na+ activity of leech glial and nerve cells

Abstract

The membrane potential of neuropile glial cells and Retzius neurones in the central nervous system of the leech Hirudo medicinalis was measured using electrolyte-filled single-barreled microelectrodes. Intracellular Na+ activity (aNai) was recorded with Na(+)-sensitive double-barreled microelectrodes. Bath-application of kainate, quisqualate and L-glutamate elicited concentration-dependent membrane depolarizations in both cell types as demonstrated by dose-response curves. The competitive quinoxalinedione antagonists 6,7-dinitroquinoxaline-2,3-dione (DNQX) or 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) to the non-NMDA glutamate receptor inhibited the membrane depolarizations in neuropile glial cells completely, but in Retzius neurones only partially. These results confirm that leech neuropile glial cells have a kainate- and quisqualate-preferring non-NMDA glutamate receptor similar to that in the Retzius neurones. The initial decrease in aNai in neuropile glial cells in kainate- or quisqualate-containing solutions and the afterhyperpolarization in these glial cells and the Retzius neurones following the removal of both glutamate antagonists, were blocked in the presence of the cardiac glycoside ouabain (10(-4) M). In saline solutions containing 42.5 mM Li+ instead of Na+ the afterhyperpolarizations were blocked in neuropile glial cells and Retzius neurones. We conclude that the initial aNai changes and the afterhyperpolarization could be due to the stimulation of the electrogenic Na+/K+ pump in the glial and neuronal membranes.