SLIDE-BINDING CHARACTERIZATION AND AUTORADIOGRAPHIC LOCALIZATION OF DELTA-OPIOID RECEPTORS IN RAT AND MOUSE BRAINS WITH THE TETRAPEPTIDE ANTAGONIST [H-3]TIPP

Slide-binding and autoradiographic studies were performed on cryostat sections from brains of adult Sprague-Dawley rats and BALE C mice to d escribe the binding characteristics of the tetrapeptide [H-3]TIPP, an antagonist with high specificity and affinity for the delta opioid rec eptors. Steady-state binding of [H-3]TIPP to cryostat sections of brai n paste was reached in 120-180 min of incubation. Specific [H-3]TIPP b inding resulted in maximal numbers of binding sites (B-max) of 15.59 a nd 23.91 fmol/mg protein, and dissociation constants (K-d) of 0.46 and 0.85 nM for rat and mouse brain paste sections, respectively. TIPP di splayed the highest affinity for delta opioid receptors in inhibiting specific [H-3]TIPP binding, with IC50 values of 0.82 nM and 0.14 nM in rat and mouse brain sections, respectively. While DPDPE was also effe ctive in displacing the specific binding of [H-3]TIPP (IC50 = 3.18 +/- 0.53 nM and 0.63 +/- 0.42 nM in rat and mouse brain paste sections, r espectively), other subclass-selective or nonopioid ligands were much less effective, or ineffective. Autoradiographic localization of [H-3] TIPP binding revealed the characteristic distribution of delta opioid receptors in both species. In consequence of its antagonistic nature, and of its unnatural amino acid residue, which makes this ligand more resistant to biodegradation, [H-3]TIPP is a superior ligand for evalua tion of the binding characteristics and autoradiogaphic distribution o f the delta opioid receptors.