Spreading depression (SD) is a propagating transient suppression of electrical activity, associated with cellular depolarization, which probably underlies the migraine aura and may contribute to neuronal damage in focal ischaemia. The purpose of this study was to examine whether L-701,324 (7-chloro-4-hydroxy-3-(3-phenoxy)phenyl-2-(1H)-quinolone), a high affinity antagonist at the glycine site of the N-methyl-D-aspartate (NMDA) receptor complex, inhibits the initiation and propagation of K(+)-induced SD in the rat cerebral cortex in vivo. 2. Microdialysis probes incorporating a recording electrode were implanted in the cerebral cortex of anaesthetized rats and perfused with artificial cerebrospinal fluid (ACSF). Five episodes of repetitive SD were elicited by switching to a medium containing 130 mM K+ for 20 min, each separated by 40 min of recovery (i.e. perfusion with normal ACSF). The brief negative shifts of the extracellular direct current (d.c.) potential, characteristic of SD elicitation, were recorded with the microdialysis electrode and a reference electrode placed under the scalp. Propagation of SD was examined using glass capillary electrodes inserted about 3 mm posterior to the microdialysis electrode. L-701,324 (5 or 10 mg kg-1) or its vehicle were administered i.v. 10 min after the end of the second K(+)-stimulus. The effects of L-701,324 were compared to those of dizocilpine (MK-801; 1 mg kg-1 i.v.), a NMDA-channel blocker known to potently block SD elicitation. 3. Potassium-induced SD initiation was inhibited by 10 mg kg-1 (but not by 5 mg kg-1) of L-701,324. Thirty minutes after administration of 10 mg kg-1 L-701,324, the cumulative area of SD peaks elicited during 20 min was 15.3 +/- 2.1 mV min, versus 23.2 +/- 1.1 mV min in animals which received only the drug vehicle (P < 0.02; n = 6). The delay between application of 130 mM K+ and occurrence of the first SD was also significantly increased. It was approximately doubled in animals treated with 10 mg kg-1 of L-701,324. 4. SD propagation was more sensitive than SD elicitation to L-701,324, as both 5 and 10 mg kg-1 produced an effective inhibition. Even at the lower dose of 5 mg kg-1, L-701,324 completely blocked the propagation of SD elicited 30 min after drug administration. This differential sensitivity of SD elicitation and propagation is not specific to L-701,324 since it was previously observed with other drugs. At doses effective against SD, L-701,324 did not produce any marked alterations of the electroencephalogram. 5. L-701,324 (10 mg kg-1) and MK-801 (1 mg kg-1) had identical effects on the d.c. potential when administered during the recovery which followed the second K+ stimulus. Both drugs produced a positive shift of around 4.5 mV within 10 min of i.v. drug administration, indicating rapid drug penetration into the CNS. Paradoxically, L-701,324 (10 mg kg-1) was markedly less effective than MK-801 (1 mg kg-1) in blocking SD, since this dose of MK-801 was sufficient virtually to abolish SD initiation and completely block its propagation. The higher potency of MK-801 against SD may reflect its use-dependency, i.e. binding of MK-801 and channel blockade are enhanced when the NMDA-receptor ionophore is open. 6. Taken together, these data demonstrate that L-701,324 has an inhibitory effect on both SD initiation and propagation. This action may be beneficial in focal ischaemia, and possibly also against migraine, especially as this drug was shown to be active when administered orally.
