EVIDENCE FOR PRESYNAPTIC LOCATION OF INHIBITORY 5-HT1D-BETA-LIKE AUTORECEPTORS IN THE GUINEA-PIG BRAIN CORTEX

The effects of 5-hydroxytryptamine (5-HT) re receptor agonists and ant agonists on tritium overflow evoked by high K+ were determined in supe rfused synaptosomes and slices, preincubated with [H-3]5-HT, from guin ea-pig brain cortex. In addition, we estimated the potencies of 5-HT r eceptor ligands in inhibiting specific [3H]5-HT binding (in the presen ce of 8-hydroxy-2(di-n-propylamino)tetralin and mesulergine to prevent binding to 5-HT1A and 5-HT2C sites) to guinea-pig cortical synaptosom es and membranes. 5-HT receptor agonists inhibited the K+-evoked triti um overflow from synaptosomes and slices. In synaptosomes the rank ord er of potencies was 2-[5-[3-(4-methylsulphonylamino) benzyl-1,2,4-oxad iazol-5-yl]-1H-indole-3-yl] ethylamine (L-694,247) >5-carboxamidotrypt amine (5-CT) > oxymetazoline (in the presence of idazoxan) greater tha n or equal to 5-HT > sumatriptan greater than or equal to ethoxy-3(1,2 ,3,6-tetrahydropyridin-4-yl)-1H-indole (RU 24969). The potencies of th e agonists in inhibiting tritium overflow from slices correlated with those in synaptosomes, suggesting that the same site of action is invo lved in both preparations. In synaptosomes the nonselective antagonist at cloned human 5-HT1D alpha and 5-HT1D beta receptors, methiothepin, shifted the concentration-response curve for 5-CT to the right (appar ent pA(2): 7.87). In contrast, ketanserin at a concentration which sho uld block the 5-HT1D alpha, but not the 5-HT1D beta, receptor did not alter the inhibitory effect of 5-CT on tritium overflow. In cortical s ynaptosomes and membranes, [H-3]5-HT bound to a single site with high affinity. In competition experiments, 5-HT receptor agonists and antag onists inhibited specific [H-3]5-HT binding. In synaptosomes the rank order was L-694,247 > methiothepin >5-CT >5-methoxytryptamine >5-HT gr eater than or equal to sumatriptan greater than or equal to oxymetazol ine > RU 24969 > ketanserin > ritanserin. A very similar rank order wa s obtained in cerebral cortical membranes. The potencies of the 5-HT r eceptor agonists in inhibiting tritium overflow from synaptosomes and slices correlated with their potencies in inhibiting [H-3]5-HT binding to synaptosomes and membranes In conclusion, the 5-HT receptors media ting inhibition of 5-HT release in the guinea-pig cortex are located o n the serotoninergic axon terminals and, hence, represent presynaptic inhibitory autoreceptors. The [H-3]5-HT binding sites in cerebral cort ical synaptosomes and membranes exhibit the pharmacological properties of 5-HT1D receptors. The correlation between the functional responses and the binding data confirms the 5-HT1D character of the presynaptic 5-HT autoreceptors. According to the results of the interaction exper iment of ketanserin and methiothepin with 5-CT on 5-HT release, the pr esynaptic 5-HT autoreceptors can be subclassified as 5-HT1D beta-like.