Diurnal Variation in the Effects of Alcohol and in the Brain 5‐Hydroxytryptamine Metabolism in Mice

: Adult male mice show diurnal variation in their performance in the tilted‐plane test and in their open field behaviour. Maximum sliding angle and minimum open field ambulation values were found at 04.00 hrs, minimum sliding angle and maximum open field ambulation values at 20.00 hrs. Diurnal variation was also found in the effects of ethyl alcohol. Its effect on the sliding angle values was greatest at 20.00 hrs and at 12.00 hrs, and on the open field ambulation at 16.00 hrs. The effects of alcohol were thus more pronounced when the activity level of the animals was high. No clear diurnal variation was found in the brain levels of 5‐hydroxytryptamine (5‐HT) and 5‐hydroxyindole acetic acid (5‐HIAA). The brain level of 5‐HIAA showed a trend to a negative correlation (r8 = −0.43) with the open field ambulation scores recorded at the same time of the day. The results indicate that the adrenergic and/or serotonergic brain mechanisms may be important in mediating the effects of alcohol.