Characterization of type 2 opiate receptors

Differential coupling of mu-competitive and mu-noncompetitive delta opiate receptors to guanine nucleotide binding proteins in rat brain membranes

The effects of fentanyl isothiocyanate (FIT) and pertussis toxin on the binding of [3H]D-Ala2, D-Leu5-enkephalin ([3H]DADLE) to rat brain membranes were compared. The site of action of pertussis toxin was confirmed by the labeling of a 41,000 dalton protein in the presence of [alpha-32P]NAD. Both reagents produced inhibition of [3H]DADLE binding when binding was assayed in 10 mM Tris-HCl buffer alone. FIT inhibited binding 91% whereas pertussis toxin treatment resulted in 27% inhibition. However, when binding was assayed in 10 mM Tris-HCl containing SMG (100 mM NaCl, 3 mM manganese acetate, and 2 microM guanosine triphosphate), inhibition due to both reagents was attenuated markedly: 66% for FIT and 5% for toxin. In addition, both reagents markedly potentiated enhancement of binding by SMG. Thus, the effects of FIT and pertussis toxin on [3H]DADLE binding were qualitatively similar. These results suggest that FIT and pertussis toxin affect binding of [3H]DADLE to the same population of delta receptors. This was further supported by the observation that treatment of membranes with FIT prior to pertussis toxin treatment blocked the effect of toxin on [3H]DADLE binding. FIT selectively eliminates the SMG-insensitive, mu-competitive [3H]DADLE binding site [Rothman et al., Neuropeptides 4, 201 (1984); Rothman et al., Molec. Pharmac. 27, 399 (1985)]. These results indicate that this site is coupled to G protein substrates for pertussis toxin and that it mediates the inhibitory effects of delta ligands on adenylate cyclase. The FIT-insensitive, SMG-sensitive mu-noncompetitive [3H]DADLE site appears not to be coupled to G protein substrates for pertussis toxin and may mediate some other biochemical effects of delta ligands.

Quantitative autoradiography of [3H]CTOP binding to mu opioid receptors in rat brain

[3H]H-D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 ([3H]CTOP), a potent and highly selective mu opioid antagonist, was used to localize the mu receptors in rat brain by light microscopic autoradiography. Radioligand binding studies with [3H]CTOP using slide-mounted tissue sections of rat brain produced a Kd value of 1.1 nM with a Bmax value of 79.1 fmol/mg protein. Mu opioid agonists and antagonists inhibited [3H]CTOP binding with high affinity (IC50 values of 0.2-2.4 nM), while the delta agonist DPDPE, delta antagonist ICI 174,864, and kappa agonist U 69, 593 were very weak inhibitors of [3H]CTOP binding (IC50 values of 234-3631 nM). Light microscopic autoradiography of [3H]CTOP binding sites revealed regions of high density (nucleus of the solitary tract, clusters in the caudate-putamen, interpeduncular nucleus, superior and inferior colliculus, subiculum, substantia nigra zona reticulata, medial geniculate, locus coeruleus and dorsal motor nucleus of the vagus) and regions of moderate labeling (areas outside of clusters in the caudate-putamen, cingulate cortex, claustrum and nucleus accumbens). The cerebral cortex (parietal) showed a low density of [3H]CTOP binding.

Strategies in neuropeptide receptor binding research

Strategies and general approaches used in neuropeptide receptor binding assays are described. Special attention is given to the nature of the ligand, its physical and chemical stability and the demonstration of an appropriate ligand selectivity pattern. Examples are given to illustrate critical aspects of neuropeptide receptor binding assays. Strong correlation between binding and bioassay data is also stressed.

Evaluation of the interactions of mu and delta selective ligands with [3H]D-Ala2-D-Leu5-enkephalin binding to mouse brain membranes

The interactions of putative mu and delta selective ligands with [3H]D-ala2-D-leu5 enkephalin (DADLE) binding to mouse brain membranes were investigated. Computerized curve fitting of displacement curves performed at three different concentrations of 3H-DADLE indicated that a one site competitive model was sufficient to explain the interactions of leu-enkephalin (LE) and D-ser2-thr6-leucine enkephalin with 3H-DADLE binding. Similar experiments with morphine and morphiceptin were unique in that the multiple displacement curves crossed over one another. A two-site competitive model was required to adequately describe the interactions of these mu selective ligands with 3H-DADLE. This two-site model was one in which the inhibitor had higher affinity for the site labeled with lower affinity by 3H-DADLE. However, this two site model did not correctly predict the interaction of LE with 3H-DADLE in the presence of morphiceptin. These data indicate that: 1) putative mu and delta selective ligands do not bind to a common high affinity site; 2) mu selective ligands are not simple mixed inhibitors of a single site labeled by 3H-DADLE; and 3) competitive binding models may not explain the interaction of mu ligands with 3H-DADLE binding.

Opiate receptor localization in rat cerebral cortex

The differential distributions of [3H]naloxone-labeled and [3H]D-Ala-D-Leu-enkephalin-labeled opiate receptors in rat cerebral cortex were localized autoradiographically and quantified by grain counting and computerized densitometry. In addition, receptor distributions were compared to terminal patterns of thalamocortical projections labeled by axoplasmic transport of [3H]amino acids. Opiate receptors labeled with [3H]naloxone in a mu ligand selectivity pattern show striking laminar heterogeneity and are densest in limbic cortical areas, intermediate in the motor cortex, and fewest in the primary sensory areas. By contrast, opiate receptors labeled with [3H]D-Ala2-D-Leu5-enkephalin in a delta ligand selectivity pattern are much more homogeneously distributed across both regions and laminae within regions. Mu receptors in most cortical areas have density peaks in layers I and VI and each peak shows a density gradient that is sloped within the layer so that the highest densities are at the most superficial and the deepest portions of cortex. In addition, there is an intermediate peak whose laminar position varies depending on the area in which it is found. In rostral agranular cortex, including limbic and motor areas, the [3H]naloxone binding peaks are in layers I, III, and VI. In primary somatosensory cortex, the intermediate peak is in layer Va and in most of remaining homotypical cortex it is in layer IV. Some areas have only bilaminar labeling, in superficial and deep layers; these include portions of the sulcal and retrosplenial cortices. Piriform and entorhinal cortices have dense [3H]naloxone binding only in the deepest layer and show a descending gradient of density toward the superficial layer. The positions of the mu receptor peaks were compared with termination patterns of projections originating in the thalamus. Close correspondence was found between receptor binding in the prelimbic, primary somatosensory, and entorhinal areas and projection terminations arising from the thalamic mediodorsal, posterior, and central medial nuclei, respectively. Although regional variations in [3H]D-Ala2-D-Leu5-enkephalin-labeled receptor density are uncommon, a gradual decrease in the number of sites along the dorsomedial wall of the cortex from anterior cingulate to caudal retrosplenial limbic cortex can be observed. Laminar variations in binding density are small as well; higher concentrations of the peptide binding sites are usually found in the deep cortical layers. These findings emphasize aspects of opiate receptor architecture which may be relevant to identifying cortical "opiatergic" neurocircuitry and raise the possibility of opiate modulation of thalamocortical transmission.

Autoradiographic distribution of mu and delta opiate receptors in rat brain using highly selective ligands

We have characterized the autoradiographic distribution of mu and delta opiate receptors in rat brain using [3H]-Tyr-D-Ala-Gly-NMe-Phe-Gly-ol (DAGO) and [3H]D-Thr2, Thr6leu-enkephalin (DTLET), two highly selective ligands for mu and delta opiate receptors, respectively. Both ligands label an apparent single class of sites. Ligand selectivity shows that [3H]DAGO and [3H]DTLET are more selective ligands for their respective receptors than [3H]dihydromorphine (DHM) and [3H][D-Ala2,D-Leu5]enkephalin (DADLE). Autoradiographically, mu opiate receptors are highly discretely distributed in certain areas. High densities are found in "patches" in caudate-putamen, nucleus accumbens, cingulate cortex, habenula, various thalamic nuclei, amygdala, superior colliculus, and interpeduncular nucleus. Delta opiate receptors are mostly concentrated in the external plexiform layers of the olfactory bulb, caudate-putamen, olfactory tubercule, claustrum, and deep layers (layers V and VI) of the cortex. Since [3H]DAGO and [3H]DTLET are more selective ligands for their respective opiate receptor subtypes, they should be better ligands to characterize the fine distribution of mu and delta opiate receptors.

Visualization and solubilization of rat brain opiate receptors with a "kappa" ligand selectivity pattern

1. Specific binding of [3H]ethylketocyclazocine (EkappaC), a prototype kappa-opiate agonist, to slide-mounted rat striatal sections is increased in the presence of 100 mM NaCl at 4 degrees C. 2. Under similar incubation conditions, binding of mu and delta prototype opiates is reduced to almost undetectable levels. 3. Correlation (P less than 0.01) of the ligand selectivity pattern of [3H]EKC displacement with the potencies of various opiate drugs in inhibiting the contractions of the rabbit vas deferens, a kappa-opiate receptor bioassay, suggests that the binding site under study represents the pharmacologically relevant kappa-opiate receptor. 4. Visualization of these kappa-opiate receptors with tritium-sensitive film reveals a striking, highly discrete brain distribution pattern (e.g., striatal patches, habenular stripe) which is similar to that of [3H]dihydromorphine and [3H]naloxone. 5. Soluble [3H]EKC binding sites obtained from rat membranes also possess a kappa-like ligand selectivity pattern, with bremazocine being a potent displacer while mu and delta ligands are almost inactive. 6. A possible explanation of these data is that the "kappa"-opiate binding site in rat brain is one transitional state of an opiate receptor capable of assuming distinct conformations with characteristic ligand selectivity patterns. Other possibilities such as pre and post-synaptic locations should also be considered.