Antidepressant action and circadian rhythm changes

192 the activation of presynaptic receptors on glutamate and aspartate nerve terminals, whereas the neuro-excitatory effects may involve direct interaction with post- synaptic receptor sites 14. The recent availa- bility of methyltetrahydrofolate labelled to high specific radioactivity may allow a direct examination of the specificity of folate binding to membrane preparations and resolve the conflicting claims for this vitamin in relation to kainate action. Direct membrane effects are not the only sites at which folates interact with aspects of neurotransmission. Their role in 1-C metabolism results in folates also being involved in the metabolism of the inhibitory neurotransmitter glycine, in the synthesis of tetrahydro-fl-carboline alkaloids x5 and in regulating the supply of S-adenosyl methionine for methylation reactions TM. In this context, the reported anti-depressant action of S-adenosyl methionine 17 merits attention. Whether this relates in any way to the depression observed in folic acid defi- ciency awaits investigation. The proposed role of folates at synapses has fluctuated considerably over the last decade. It appears to he at a peak at the moment. Only time will tell whether it is to remain there. ANTHONY J TURNER