Modulation of mEPSCs in Olfactory Bulb Mitral Cells by Metabotropic Glutamate Receptors

• Published in: Journal of neurophysiology Vol. 78; no. 3; pp. 1468 - 1475
• Main Authors: Schoppa, N. E, Westbrook, G. L
• Format: Journal Article
• Language: English
• Published: United States Am Phys Soc 01-09-1997
• Subjects: Adenylyl Cyclases - metabolism; alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid - pharmacology; Animals; Animals, Newborn; Axons - drug effects; Axons - physiology; Cadmium - pharmacology; Calcium Channels - drug effects; Calcium Channels - physiology; Cells, Cultured; Cycloleucine - analogs & derivatives; Cycloleucine - pharmacology; Electric Stimulation; Excitatory Amino Acid Agonists - pharmacology; Membrane Potentials - drug effects; Membrane Potentials - physiology; Neurons - drug effects; Neurons - physiology; Neuroprotective Agents - pharmacology; Olfactory Bulb - cytology; Olfactory Bulb - drug effects; Olfactory Bulb - physiology; Patch-Clamp Techniques; Rats; Receptors, Metabotropic Glutamate - antagonists & inhibitors; Receptors, Metabotropic Glutamate - physiology; Synaptic Transmission - drug effects; Synaptic Transmission - physiology; Tetrodotoxin - pharmacology
• ISSN: 0022-3077; 1522-1598
• DOI: 10.1152/jn.1997.78.3.1468

N. E. Schoppa and
G. L. Westbrook
Vollum Institute for Advanced Biomedical Research, Oregon Health Sciences University, Portland, Oregon 97201
Schoppa, N. E. and G. L. Westbrook. Modulation of mEPSCs in olfactory bulb mitral cells by metabotropic glutamate receptors. J. Neurophysiol. 78: 1468-1475, 1997. Olfactory bulb mitral cells express group I (mGluR1), group II (mGluR2), and group III (mGluR7 and mGluR8) metabotropic glutamate receptors. We examined the role of these mGluRs on excitatory synaptic transmission in cultured mitral cells with the use of whole cell patch-clamp recordings. The effects of group-selective mGluR agonists and antagonists were tested on -amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-receptor-mediated miniature excitatory postsynaptic currents (mEPSCs). (1S,3R)-1-amino-cyclopentane-1,3-dicarboxylate (ACPD) or the group-I-selective agonist 3,5-dihydroxyphenylglycine evoked an inward current accompanied by a decrease in membrane conductance, consistent with the previously described closure of potassium channels by group I agonists. The increased cellular excitability was accompanied by an increase in mEPSC frequency in some cells. When calcium entry was blocked by cadmium, ACPD or the group-II-selective agonist 2-(2,3-dicarboxycyclopropyl)-glycine reduced the mEPSC frequency. L -2-amino-4-phosphonobutyric acid ( L -AP4), a group-III-selective agonist, caused a similar decrease. The concentration-dependence of L -AP4-mediated inhibition was most consistent with activation of mGluR8. We investigated two possible effector mechanisms for the group III presynaptic receptor. Bath application of forskolin or 3-isobutyl-1-methylxantine had no effect on mEPSC frequency. Increasing calcium influx by raising extracellular K + caused a large increase in the mEPSC frequency but did not enhance L -AP4-mediated inhibition. Thus inhibition of mEPSCs involves a mechanism downstream of calcium entry and appears to be independent of adenosine 3 ,5 -cyclic monophosphate. Our results indicate that both group II and III receptors can inhibit glutamate release at mitral cell terminals. Although group II/III receptors had a similar effect on mEPSCs, differences in location on nerve terminals and in glutamate sensitivity suggest that each mGluR may have discrete actions on mitral cell activity.