Stimulation of δ‐Opioid Receptors Reduces the In Vivo Binding of the Cholecystokinin (CCK)‐B‐Selective Agonist [ 3 H]pBC 264: Evidence for a Physiological Regulation of CCKergic Systems by Endogenous Enkephalins

Cholecystokinin (CCK) and enkephalins appear to be colocalized in several brain structures, and a physiological interaction between these peptides has been suggested by a large number of pharmacological studies. In this work we have shown, by in vivo binding experiments, that the endogenous enkephalins, protected from degrading enzymes by mixed inhibitors such as kelatorphan and N‐[(R,S)‐2‐benzyl‐3‐[(S)2‐amino‐4‐methylthiobutyldithio]‐1‐oxopropyl]‐L‐phenylalanine benzyl ester (RB 101), a systemically active prodrug, modulate CCK release in mouse brain, leading to an overall increase in the extracellular levels of CCK. This was quantified by measuring the effects of both inhibitors on the in vivo binding of [ 3 H]propionyl‐Tyr(SO 3 H)‐gNle‐mGly‐Trp‐( N ‐Me)Nle‐Asp‐Phe‐NH 2 ([ 3 H]‐pBC 264), a selective and highly potent CCK‐B agonist. Thus, intracerebroventricular injection of kelatorphan produced a dose‐dependent inhibition of the in vivo binding of [ 3 H]pBC 264 with a maximal effect (40%) at 50 nmol. A similar response was observed after intravenous injection of RB 101 (40 mg/kg). The specific binding of [ 3 H]pBC 264 was also inhibited (25%) by intravenous injection of the selective δ‐opioid agonist H‐Tyr‐D‐Cys(StBu)‐Gly‐Phe‐Leu‐Thr(OtBu)‐OH (BUBUC; 2 mg/kg) but not by the μ‐agonist H‐Tyr‐D‐Ala‐Gly‐( N ‐Me)Phe‐Gly‐ol (5 mg/kg), suggesting a preferential involvement of δ‐opioid receptors in the modulation of CCK release. This was confirmed by using the selective δ‐opioid antagonist naltrindole, which prevented the inhibitory effects of BUBUC and of enkephalin‐degrading enzyme inhibitors on [ 3 H]pBC 264 binding. These results are discussed in terms of homeostatic regulation of the nociceptive threshold to opioids by endogenous CCK.