The effect of single oral doses of zopiclone on nocturnal melatonin secretion in healthy male volunteers

1. The effects of single oral doses of zopiclone and temazepam were investi gated in eight healthy male volunteers using a single blind, placebo contro lled cross over study. Doses of zopiclone were 7.5 and 15mg while the dose of temazepam was 20mg. 2. Each dose was separated by at least a one-week washout period. For each subject the dim light melatonin onset (DLMO) was determined on a screening night and the drugs were administered at the time of the DLMO. 3. Melatonin concentrations were determined by radioimmunoassay from plasma samples collected throughout the night. Both temazepam and zopiclone tende d to reduce the amount of melatonin secreted, as determined by the area und er the plasma concentration time curve. 4. The differences from placebo were not statistically significant (F (3.31 )=1.07. P>0.1). Similarly a repeated measures analysis of variance on the p lasma concentration-time curves did not show any statistically significant differences between drugs and placebo (F (3.28)=1.15, P>0.1). 5. There was no evidence from this study of a phase shifting effect of the drugs used. The reasons for the lack of effect on melatonin may be due to t he differences in potency of the interaction of these drugs with the GABA-b enzodiazepine-chloride ion channel.