GUINEA-PIG TERMINAL 5-HT(1D) AUTORECEPTORS DO NOT DISPLAY A CIRCADIANVARIATION IN THEIR RESPONSIVENESS TO SEROTONIN

Many of the components involved in the synthesis and release of seroto nin (5-HT) display a circadian variation in their activity. Autorecept ors located on nerve terminals were recently suggested to underlie som e of these circadian variations. The aim of this study was to examine whether terminal 5-HT1D autoreceptors in the cerebral cortex of the gu inea pig exhibit a circadian variation in their responsiveness. The re sponsiveness of these autoreceptors was assessed by the ability of exo genously applied 5-HT to inhibit the potassium-evoked release of [H-3] 5-HT from slices of guinea pig cortex. Identical experiments were cond ucted at four different, equally spaced time points during the light:d ark cycle of the guinea pig. The results presented here demonstrate th at 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 i n being devoid of a significant rhythm in their responsiveness.