Ionic currents were studied in Xenopus laevis oocytes using the patch-clamp and the whole-cell voltage-clamp techniques. 2. Single-channel currents were recorded from the cell-attached patches in oocytes injected with rat brain mRNA when glutamate was applied locally outside the patch. The single-channel conductance was 3.66 pS, and the extrapolated equilibrium potential was -23.0 mV, indicating that the channels were chloride selective. 3. Single-channel currents with similar characteristics were observed in cell-attached patches in native oocytes in response to injection of inositol 1,4,5-trisphosphate (IP3) or Ca2+. 4. Whole-cell currents were evoked by glutamate in oocytes injected with rat brain mRNA. They usually showed an oscillatory component, and reversed direction at about the chloride equilibrium potential. Injection of IP3 or Ca2+ into a native oocyte evoked a transient whole-cell current. The reversal potential was near the chloride equilibrium potential, and it changed from negative to positive in low-chloride solution. 5. The results suggest that the glutamate receptors are not directly coupled with the endogenous chloride channels but indirectly activate these via the messenger system IP3-Ca2+.
