DIRECT DEMONSTRATION OF FUNCTIONAL DISCONNECTION BY ANOXIA OF INHIBITORY INTERNEURONS FROM EXCITATORY INPUTS IN RAT HIPPOCAMPUS

1. We studied the effects of anoxia on excitatory and inhibitory posts ynaptic currents (EPSCs and IPSCs) evoked by electrical stimulation in the stratum radiatum in concomitantly recorded pyramidal cells and in terneurons of the CA1 region of rat hippocampal slices. We used the bl ind whole cell patch-clamp technique, and anoxia was induced by switch ing perfusion of the slice from oxygenated artificial cerebral spinal fluid (ACSF) to ACSF saturated with 95% N-2-5% CO2 for 4-6 min. 2. As in pyramidal neurons, anoxia induced in interneurons outward currents, during and shortly after the anoxic episode. Both currents were, howe ver, significantly larger in interneurons than in pyramidal neurons. 3 . EPSCs are more rapidly depressed by anoxia in interneurons than in s imultaneously recorded pyramidal cells. 4. In pyramidal neurons, polys ynaptic IPSCs (pIPSCs) evoked by conventional distant stimulation (>1 mm) are more sensitive to anoxia then EPSCs. In contrast, in interneur ons, anoxia blocks with a similar latency EPSCs and polysynaptic IPSCs . 5. To determine whether this block of pIPSCs in pyramidal cells is d ue to a shift in driving force or a change in conductance, we examined the current voltage (I/V) relationships. The block by anoxia of pIPSC s is due to a reduction of IPSC conductance (>98%) that occlude other events including the shift of IPSCs on reversal potential (E(Cl)). In contrast, monosynaptic IPSCs (mIPSCs) evoked by close stimulation (<0. 5 mm) in the presence of glutamate-receptors antagonists 6-cyano-7-nit roquinoxaline-2,3-dione (CNQX. 20 mu M) and 2-amino-5-phosphonovaleric acid (APV, 50 mu M) were only reduced by anoxia because of a positive shift of Eo and smaller change in the conductance. 6. These data prov ide direct evidence that inhibitory interneurons are functionally disc onnected from excitatory inputs by anoxia, and this disconnection is a major reason for the failure of inhibition in pyramidal cells. Modifi cation of Cl- gradient in pyramidal cells during anoxia has a minor co ntribution to the anoxia-induced depression of polysynaptic inhibitory pathway, in contrast to the monosynaptic one.