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1 Evidence has recently suggested that NMDA receptors may play a role in the aetiology and possible treatment of depression and that weak noncompetitive NMDA receptor antagonists such as amantadine can synergize with conventional antidepressants in a model of the illness. 2 To try to obtain a neurochemical rationale for these findings, we have studied the effects of acute and chronic administration of amantadine or the related drug budipine on cortical release of 5-hydroxytryptamine (5-HT) following the antidepressants reboxitine (REB), paroxetine (PAROX) and clomipramine (CLOM) in freely moving rats by using microdialysis. ) all failed to significantly alter extracellular 5-HT in the cortex. However, when either amantadine or budipine was administered 30 min prior to any of the three antidepressants, a significant rise in 5-HT was observed. 4 For chronic studies, the effects of the drugs were studied at 4, 7, 14 and 21 days. Amantadine and budipine did not significantly alter extracellular 5-HT at any time point. The three antidepressant drugs all elicited a gradual increase in 5-HT, which became significant after 14 days and tended to plateau thereafter. When either amantadine (20 mg kg À1 ) or budipine (5 mg kg À1 ) was coadministered with any of the three antidepressants, two differences were seen compared with the effects of the antidepressants alone. Firstly, the time required for significant increases in cortical 5-HT was reduced with elevated levels now being observed by 7 days. Secondly, the absolute magnitude of the increase in extracellular 5-HT was markedly greater in these rats from day 7 until the end of the experiment. 5 If, as is widely considered, an increase in extracellular 5-HT represents a critical step in the mechanism of action of antidepressants, these data suggest that combined treatment with clinically tolerated NMDA antagonists such as amantadine could reduce the delay in therapeutic onset of antidepressants as well as possibly enhance their efficacy.

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