L-GLUTAMATE CONDITIONALLY MODULATES THE K-CELLS( CURRENT OF MULLER GLIAL)

L-Glutamate inhibits the K+ conductance that dominates the electrical behavior of a Muller glial cell. The effect of glutamate is enhanced b y simultaneous exposure to dopamine. L-Glutamate acts at a metabotropi c receptor that controls the K+ conductance through two pathways. A ra pid pathway produces a partial inhibition in less than 2 s. Thereafter , a slow pathway progressively inhibits the conductance with a half-ti me of minutes. Pathways initiated by L-glutamate and dopamine appear t o converge on and stimulate adenylyl cyclase. A subsequent step is the activation of a cAMP-dependent protein kinase, PKA. The local overflo w of L-glutamate from active synapses may functionally remove K+ chann els from nearby glial membranes. A uniform rise in extracellular L-glu tamate concentration, as might occur during pathological conditions, s hould suppress a glial cell's K+ conductance and allow other voltage-d ependent processes to be influenced by depolarization.