Guinea pig terminal 5-HT1D autoreceptors do not display a circadian variation in their responsiveness to serotonin

Many of the components involved in the synthesis and release of serotonin (5-HT) display a circadian variation in their activity. Autoreceptors located on nerve terminals were recently suggested to underlie some of these circadian variations. The aim of this study was to examine whether terminal 5-HT1D autoreceptors in the cerebral cortex of the guinea pig exhibit a circadian variation in their responsiveness. The responsiveness of these autoreceptors was assessed by the ability of exogenously applied 5-HT to inhibit the potassium-evoked release of [3H]5-HT from slices of guinea pig cortex. Identical experiments were conducted at four different, equally spaced time points during the light:dark cycle of the guinea pig. The results presented here demonstrate that terminal 5-HT1D autoreceptors do not exhibit a circadian variation in their responsiveness. Therefore, terminal 5-HT1D autoreceptors bear similarity to terminal 5-HT1B autoreceptors identified in rat brain in being devoid of a significant rhythm in their responsiveness.