Intracerebral microdialysis combined with recording of extracellular field potential: a novel method for investigation of depolarizing drugs in vivo.

1. The purpose of this study was to examine whether depolarizations evoked by excitatory amino acids can be recorded quantitatively, in vivo, with a microelectrode incorporated within a microdialysis probe. 2. Microdialysis probes incorporating a chlorided silver wire were implanted in the striatum of anaesthetized rats and perfused with artificial cerebrospinal fluid (ACSF). Increasing concentrations of excitatory amino acids were applied for 2 min via the microdialysis probe, and the extracellular direct current (d.c.) potential was recorded between the microdialysis electrode and a reference electrode placed under the scalp. 3. N-methyl-D-aspartate (NMDA, 25-500 microM), alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA, 5-1000 microM), kainate (5-500 microM), and glutamate (0.25-100 mM) evoked concentration-dependent depolarizations with maxima ranging from 7 to 10 mV, i.e. 3 to 10 times larger than those recorded from brain slices in vitro. Depolarizations evoked by glutamate receptor agonists applied by microdialysis shared several features with those recorded from brain slices. The most characteristic were: steep onset and recovery of NMDA and glutamate responses; marked post-depolarization hyperpolarization with NMDA; and very slow recovery after kainate application. At high concentrations (500 microM), NMDA occasionally initiated spreading depression. The relative potency of glutamate and NMDA was of the same order of magnitude to that obtained with the cortical wedge and hippocampal slices, glutamate being 100 to 400 times less potent than NMDA. 4. Two consecutive series of NMDA-stimuli within the same procedure evoked comparable depolarizations, indicating that reliable quantitative analysis of drug action can be performed, with each animal serving as its own control. This is relevant to the study of drugs acting on glutamate receptors especially antagonists. The remarkable inter-animal reproducibility is also a valuable feature.5. Pretreatment with dizocilpine maleate (MK-801, 2mgkg'1, i.p.) reduced by 65% the responses evoked by NMDA (500 fM). The non-NMDA antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX,100 1M) applied via the microdialysis probe reduced by around 78% the responses to AMPA and kainate (250 micro M). The fact that drugs, especially antagonists, can be administered either systemically, or directly through the dialysis probe to by-pass the blood-brain barrier or avoid peripheral effects, is especially relevant for neuropharmacological studies.6. Intracerebral microdialysis combined with in vivo recording of extracellular field potential is a novel and valuable method for the quantitative analysis of the action of drugs acting on glutamate receptors.This method should prove especially useful for comparing the sensitivity of specific brain structures to selective glutamate receptor agonists under normal conditions and when the neuronal micro environment is altered. It should also be useful for investigating the action of other depolarizing agents, such as veratridine, and their antagonists.