Rapid agonist-induced loss of 125I-beta-endorphin opioid receptor sites in NG108-15, but not SK-N-SH neuroblastoma cells

The delta opioid receptor tool box

In recent years, the delta opioid receptor has attracted increasing interest as a target for the treatment of chronic pain and emotional disorders. Due to their therapeutic potential, numerous tools have been developed to study the delta opioid receptor from both a molecular and a functional perspective. This review summarizes the most commonly available tools, with an emphasis on their use and limitations. Here, we describe (1) the cell-based assays used to study the delta opioid receptor. (2) The features of several delta opioid receptor ligands, including peptide and non-peptide drugs. (3) The existing approaches to detect delta opioid receptors in fixed tissue, and debates that surround these techniques. (4) Behavioral assays used to study the in vivo effects of delta opioid receptor agonists; including locomotor stimulation and convulsions that are induced by some ligands, but not others. (5) The characterization of genetically modified mice used specifically to study the delta opioid receptor. Overall, this review aims to provide a guideline for the use of these tools with the final goal of increasing our understanding of delta opioid receptor physiology.

Receptor-induced internalization of selective peptidic mu and delta opioid ligands

The binding and internalization of radioiodinated and fluorescent mu and delta opioid peptides in mammalian cells were quantitatively studied by biochemical techniques and directly visualized by confocal microscopy. The labeled peptides were prepared by inserting either a 125I-Bolton-Hunter group or a fluorescent probe into the C-terminal part of 5-aminopentylamide derivatives of deltorphin-I and [Lys7]dermorphin. The purified derivatives kept most of their specificity and selectivity toward delta and mu opioid receptors, respectively. Biochemical and confocal microscopy data showed that both mu and delta opioid peptides were internalized in mammalian cells transfected with the corresponding opioid receptor according to a receptor-mediated mechanism. The internalization process was time- and temperature-dependent and was completely blocked by the endocytosis inhibitor phenylarsine oxyde. Internalization of both delta and mu ligands occurred from a single large cap at one pole of the cell, indicating that polymerization of ligand-receptor complexes preceeded internalization. Finally, green and red fluorescent analogues of deltorphin-I and [Lys7]dermorphin, respectively, were found to internalize through partly distinct endocytic pathways in cells co-transfected with mu and delta receptors, suggesting that each of these receptors interacts with distinct proteins mediating intracellular sorting and trafficking.

Effects of steroids on the brain opioid system

The experiments reported here add further evidence in support of the view that sex steroids may influence the binding characteristics of brain opioid receptors. In particular, it has been shown that: (a) the number of mu-opioid receptors varies in the hypothalamus of regularly cycling female rats according to the different phases of the estrous cycle, which are characterized by fluctuations of circulating levels of sex steroids; (b) the number of mu-opioid receptors decreases in the hypothalamus and in the corpus striatum when ovariectomized rats are submitted to treatments with estradiol and progesterone able to induce a "positive" feedback effect on LH release. A treatment with estrogen alone able to induce a "negative" feedback effect on LH release brings about an increase of the number of mu-opioid receptors in the thalamus and in the hippocampus; (c) in addition to the mu-receptors, receptors of the delta type may also be involved in the control of gonadotropin secretion; recent results here presented indicate that a line of immortalized hypothalamic cells (GT1 cells), which synthesize and secrete LHRH, present delta opioid receptors on their membranes; these are apparently involved in the control of LHRH release from these cells.

Co-localization of mu and delta opioid receptors on SK-N-SH cells detected by fluorescence microscopy using labeled anti-idiotypic antibodies

Selective fluorescence labeling of opioid receptor subclasses on SK-N-SH cultured cells has been accomplished using labeled polyclonal anti-idiotypic antibodies along with subclass-selective opioid agonists (DPDPE, delta-selective; DAMGO, mu-selective) as blocking reagents. Labeling of the cells was examined using conventional fluorescence microscopy. Co-localization of mu- and delta-opioid receptors on SK-N-SH cells has been studied by double labeling fluorescence experiments. In agreement with our own, and other workers', previous observations on NG108-15 cells, a subpopulation of viable cells in asynchronous cultures are labeled. Among those SK-N-SH cells that are labeled, both subclasses of receptors are seen. On the basis of sequential blocking experiments we interpret our combined results to be consistent with a model where mu- and delta- binding sites reside on different subunits of a multimeric complex.