The effects of metabolic stress on glutamate receptor-mediated depolarizations in the in vitro rat hippocampal slice

Abstract

A grease-gap preparation for the in vitro rat hippocampal slice has been used to record field excitatory postsynaptic potentials (fEPSPs), extracellular d.c. potential and depolarizations in response to glutamate receptor agonists before, during and after hypoxic/ischaemic episodes in the CA1 region. Synaptic transmission was depressed by hypoxia in a temperature-dependent manner ( t 1 2 at 28 ° C, 1.9 ± 0.2 min; t 1 2 at 36 ° C, 1.0 ± 0.1 min ) but was unaffected by the absence of d-glucose during hypoxia (ischaemia) at 28 °C. The reappearance of the fEPSP during hypoxic/ischaemic episodes was a prelude to severe disruptions of synaptic transmission if control conditions were not reinstated within 1 min of the secondary depression of the fEPSP. For a 10 min episode of hypoxia, recovery of synaptic transmission at 28 °C (96 ± 1.5% of control) was significantly better than recovery following either hypoxia at 36 °C or ischaemia at 28 °C (41 ± 17.2% and 55 ± 21% of control, respectively). Chart recordings of the d.c. potential during hypoxia revealed a predominate (67% of all episodes) triphasic sequence of events (i, hyperpolarization; ii, depolarization; iii, post-hypoxic hyperpolarization on reoxygenation). Depolarizing responses to N-methyl-D-aspartate (NMDA, 20–40 μM; in 1 mM extracellular Mg 2+), α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA, 2–10 μM) and l-glutamate ( l-Glu, 2–5 mM) could be elicited at times when fEPSPs were completely depressed and up to 20 min into a hypoxic episode, the latest time-point examined. This implies, as others have suggested, that the hypoxic depression of excitatory synaptic transmission is presynaptic in origin. The application of AMPA or NMDA during the hypoxic depression of the fEPSP occasionally resulted in a short-lasting (12–45 min) potentiation (117–143% of control) of the fEPSP on return to normoxia. Furthermore, in other slices, which were exposed to severe metabolic stress, synaptic transmission was depressed to a significantly greater extent than AMPA depolarizations (mean depression; 76 ± 5% and 28 ± 8%, respectively).