LACK OF CIRCADIAN VARIATION IN THE RESPONSIVENESS OF ALPHA(2)-HETERORECEPTORS REGULATING SEROTONIN RELEASE

We have previously shown that the circadian variation in 5-HT release is not the consequence of a variation in the activity of terminal 5-HT 1B autoreceptors. However, recently identified alpha(2)-adrenoceptors located on 5-HT nerve terminals may be important in regulating the rel ease of 5-HT from serotonergic neurones. The sensitivity of hippocampa l alpha(2)-heteroreceptors to both agonist and antagonist was determin ed at different time points in the light:dark cycle of the rat. No sig nificant circadian differences were evident in either the apparent pD( 2) values calculated for noradrenaline to inhibit potassium-evoked tri tium efflux or in the apparent pA(2) values calculated for phentolamin e to antagonize the effect of noradrenaline. The corollary of the lack of a circadian rhythm in sensitivity to the alpha(2)-heteroreceptor i s that this receptor population will accurately reflect any circadian variation in noradrenaline release and in the available concentration of noradrenaline at the receptor sites.