L-364,718 AND L-365,260, 2 CCK ANTAGONISTS, HAVE NO AFFINITY FOR CENTRAL BENZODIAZEPINE BINDING-SITES IN CHICKENS

It has recently been demonstrated that L-365,260, a CCK-B antagonist i n mammals, causes an increase in food intake in chickens. In contrast, L-364,718, a CCK-A antagonist in mammals, shows this effect only at v ery high dose levels. It has been shown that L-365,260 has very low af finity for chicken CCK receptors. Thus, the mechanism of action of L-3 65,260 remains unknown. As L365,260 is a benzodiazepine derivative, on e may hypothesize that it would be acting on benzodiazepine binding si tes. The aims of this work were to establish the existence of benzodia zepine binding sites in the chicken brain, and to check the possibilit y that L-365,260 was acting on these receptors, determining the affini ty of L-364,718 and L-365,260 for them. We have found specific binding for tritiated flunitrazepam (a benzodiazepine agonist) ([H-3]-flunitr azepam) in chicken brain membranes. A single binding site was detected with a K-d of 3.58+/-0.97 nM and a B-max of 451.6+/-23.3 fmol/mg prot ein. L-365,260 and L-364,718 exhibited very low affinity for these bin ding sites (K-i=1.17x10(-6)+/-0.16x10(-6) M and K-i >10(-5) M, respect ively). Thus, these results demonstrate that the increase in food inta ke caused by L-365,260 in the chicken is not due to a direct action on benzodiazepine receptors. Other possible explanations for its effect are discussed.