Meeting Abstract

P1.57  Saturday, Jan. 4 15:30  Alteration in synaptic transmission by CO2: glutamate insensitivity COOPER, R.L.*; MAJEED, Z.R.; SANTIN, J.M.; HARTZLER, L.K.; Univ of KY; Univ of KY; Univ. of Salahaddin, Iraq; Wright State Univ; Wright State Univ RLCOOP1@email.uky.edu

Glutamate receptors at the Drosophila and crayfish neuromuscular junctions (NMJ) decrease their sensitivity to glutamate upon exposure to saline containing CO2. The indirect effect of CO2 is potentially by acidification of the muscle cytoplasm given that CO2 rapidly diffuses across membranes and reduces intracellular pH. Given that saline containing CO2 also results in a pH drop, saline with lowered pH to the same level as with CO2 exposure was examined. Synaptic transmission at the NMJ and response to glutamate is reduced with lowered pH extracellularly but does not result in complete decreased sensitivity to glutamate. We examined the drop in intracellular pH and the time domain in association with sensitivity to evoked synaptic transmission. NMJs exposed to saline equilibrated with 75% CO2 results in a cessation of transmission within 1-2mins. Upon exchanging the high CO2 saline with room air equilibrated saline transmission resumes in 1-2mins and completely recovers by 3-5mins. These responses are common to both crayfish and Drosophila NMJs. However, monitoring intracellular pH with pH-sensitive fluorescent dyes pyranine (injection, crayfish) or BCECF-AM (incubation, Drosophila), intracellular pH rapidly drops with exposure to CO2 but remains low for several minutes upon washing away the CO2 containing saline. The pH recovers with a very slow rise over 20 to 60mins in these muscles. Thus, the proton exchange is slow in these cells at room temperature. Also, synaptic responses return prior to intracellular pH recovering. The rapid synaptic changes suggest that the block in synaptic transmission may be due to molecular CO2 itself and/or extracellular acidification. We speculate that molecular CO2 may be blocking the inotropic glutamate receptor directly within the pore or in the binding domain for glutamate to partially account for the effects