CANNABINOID CB1 RECEPTOR-MEDIATED INHIBITION OF NORADRENALINE RELEASEIN THE HUMAN AND GUINEA-PIG HIPPOCAMPUS

We examined the question of whether cannabinoid receptors modulating n oradrenaline release are detectable in the brain of humans and experim ental animals. For this purpose, hippocampal slices from humans, guine a-pigs, rats and mice and cerebellar, cerebrocortical and hypothalamic slices from guinea-pigs were incubated with [H-3]noradrenaline and th en superfused. Tritium overflow was evoked either electrically (0.3 or 1 Hz) or by introduction of Ca2+ ions (1.3 mu M) into Ca2+-free, K+-r ich medium (25 mu M) containing tetrodotoxin 1 mu M. Furthermore, the cAMP accumulation stimulated by forskolin 10 mu M was determined in gu inea-pig hippocampal membranes. We used the following drugs: the canna binoid receptor agonists ]-trans-4-(3-hydroxypropyl)cyclohexanol(CP-55 ,940) and ,3-de]-1,4-benzoxazinyl]-(1-naphthalenyl)methanone (WIN 55,2 12-2), the inactive S(-)-enantiomer of the latter (WIN 55,212-3) and t he CB1 receptor antagonist N-piperidino-5-(4-chlorophenyl)-1-(2 ,4-dic hlorophenyl)-4-methyl-3-pyrazole-carboxamide (SR 141716). The electric ally evoked tritium overflow from guinea-pig hippocampal slices was re duced by WIN 55,212-2 (pIC(30%) 6.5) but not affected by WIN 55,212-3 up to 10 mu M The concentration-response curve of WIN 55,212-2 was shi fted to the right by SR 141716 (0.032 mu M) (apparent pA(2) 8.2), whic h by itself did not affect the evoked overflow. WIN 55,212-2 1 mu M al so inhibited the Ca2+-evoked tritium overflow in guinea-pig hippocampa l slices and the electrically evoked overflow in guinea-pig cerebellar , cerebrocortical and hypothalamic slices as well as in human hippocam pal slices but not in rat and mouse hippocampal slices. SR 141716 (0.3 2 mu M) markedly attenuated the WIN 55,212-2-induced inhibition in gui nea-pig and human brain slices. SR 141716 0.32 mu M by itself increase d the electrically evoked tritium overflow in guinea-pig hippocampal s lices but failed to do so in slices from the other brain regions of th e guinea-pig and in human hippocampal slices. The cAMP accumulation st imulated by forskolin was reduced by CP-55,940 and WIN 55,212-2. The c oncentration-response curve of CP 55,940 was shifted to the right by S R 141716 (0.1 mu M; apparent pA(2) 8.3), which by itself did not affec t cAMP accumulation. In conclusion, cannabinoid receptors of the CB1 s ubtype occur in the human hippocampus, where they may contribute to th e psychotropic effects of cannabis, and in the guinea-pig hippocampus, cerebellum, cerebral cortex and hypothalamus. The CB1 receptor in the guinea-pig hippocampus is located presynaptically, is activated by en dogenous cannabinoids and may be negatively coupled to adenylyl cyclas e.