LACK OF CIRCADIAN VARIATION IN THE SENSITIVITY OF RAT TERMINAL 5-HT1BAUTORECEPTORS

The sensitivity of terminal 5-HT1B autoreceptors in the cerebral corte x and hippocampus to both agonist and antagonist was determined at fou r different time points in the light:dark cycle of the rat to evaluate whether changes in their responsiveness underlie the circadian rhythm in 5-hydroxytryptamine (5-HT) release. No significant circadian diffe rences were evident in the apparent pIC50 values calculated for 5-HT t o inhibit K+-evoked tritium efflux or in the apparent pA2 values calcu lated for methiothepin to antagonize the effect of 5-HT, at the differ ent time points, in either brain region. These findings suggest that t he sensitivity of terminal 5-HT1B autoreceptors in rat cerebral cortex and hippocampus does not change in a circadian pattern, during the li ght: dark cycle, and argues against such a change influencing the circ adian variation in 5-HT release.