EXTRACELLULAR CA2-NERVOUS-SYSTEM( CHANGES DURING TRANSMITTER APPLICATION IN THE LEECH CENTRAL)

Changes in extracellular Ca2+ concentration ([Ca2+](e)) evoked by tran smitters and transmitter agonists, respectively, and by elevation of b ath K+ concentration were recorded in isolated segmental ganglia of th e leech Hirudo medicinalis using Ca2+-selective microelectrodes. A 1-m in bath application of kainate (10 mu M), glutamate (1 mM), aspartate (1 mM), or carbachol (200 mu M) decreased [Ca2+](e) by up to 1 mM, whe reas the inhibitory transmitters gamma-amino butyric acid (GABA, 100 m u M) and serotonin (5-HT, 100 mu M) did not change [Ca2+](e). The ampl itude of the kainate-induced changes in [Ca2+](e) increased with repet itive applications, and changes were blocked by 6-cyano-7-dinitroquino zaline-2,3-dione (CNQX). Elevation of bath K+ concentration from 4 to 40 mM led to a Ni2+-sensitive decrease in [Ca2+](e) by 0.9 mM. Our res ults suggest that excitatory transmission in the leech central nervous system might be accompanied by substantial decreases in [Ca2+](e).