Comparison between δ‐opioid receptor functional response and autoradiographic labeling in rat brain and spinal cord

Abstract

The distribution of delta-opioid receptors (DORs) in the rat central nervous system has been previously characterized by radioligand binding and immunohistochemistry. However, the functional neuroanatomy of DORs has not been mapped in any detail; this is potentially important, because these receptors appear to be primarily cytosolic. Opioid receptors can couple to G(i/o) G proteins, a process that is detected by agonist-stimulated [35S]guanylyl-5'-O-(gamma-thio)-triphosphate ([35S]GTPgammaS) binding. The purpose of this study was therefore to determine the distribution of functional DORs, as assessed by [35S]GTPgammaS autoradiographic labeling in response to the DOR agonist deltorphin II. For comparison, adjacent sections were labeled with [125I]deltorphin II or the DOR antagonist [125I]AR-M100613. In all three assays, mu-opioid receptors were blocked pharmacologically. The distributions of [125I]deltorphin II and [125I]AR-M100613 were highly correlated but not identical. Deltorphin II increased [35S]GTPgammaS binding in a concentration-dependent and naltrindole-sensitive manner. The regional [35S]GTPgammaS response to deltorphin II was only moderately predicted by agonist or antagonist radioligand binding (r = 0.67 and 0.50, respectively). [35S]GTPgammaS responses to deltorphin II were strongest in the extended striatum (caudate putamen, nucleus accumbens, olfactory tubercle) and cerebral cortex. In contrast, some areas reported to mediate DOR analgesia (brainstem, spinal cord) possessed a much lower [35S]GTPgammaS response. These findings demonstrate the existence of a partial mismatch between DOR radioligand binding and [35S]GTPgammaS response. This divergence possibly reflects regional heterogeneity in G-protein receptor coupling, or in the subcellular localization of DOR.

Molecular Pharmacology of δ-Opioid Receptors

“…Moreover, immunocytochemistry analysis revealed that the majority of Kir3.1 and Kir3.2 subunits were present in nonpeptidergic small size afferents (Chung et al, 2014), which also express DOPr (Bardoni et al, 2014). The postsynaptic distribution of Kir3.1/3.2 subunits in lamina IIo of the spinal dorsal horn (Marker et al, 2005) is similarly consistent with DOPr expression, as visualized by autoradiography (Pradhan and Clarke, 2005) and in DOPr-GFP-expressing mice ). In keeping with this distribution, analgesia by intrathecal administration of deltorphin II was reduced in tertiapin Q-treated mice and in mice lacking either Kir3.1 or Kir3.2 subunits (Marker et al, 2005).…”

Mechanisms Responsible for the Enhanced Antinociceptive Effects of μ-Opioid Receptor Agonists in the Rostral Ventromedial Medulla of Male Rats with Persistent Inflammatory Pain

“…However, significant stimulation was obtained in the striatum, which served as a positive control. Pradhan and Clarke (2005) similarly identified only negligible binding of [ 125 I]DELT, which has higher specific activity, and virtually no stimulation of [ 3 H]GTP␥S binding by DELT in the RVM of naive rats. Unfortunately, methods offering greater anatomical resolution proved unworkable in the RVM.…”

Preclinical Pharmacology of AZD2327: A Highly Selective Agonist of the δ-Opioid Receptor

“…It is interesting to speculate as to why differences among agonists can be observed. It is known, for example, that different agonists can differentially traffic the receptor (Pradhan and Clarke, 2005;Pradhan et al, 2009Pradhan et al, , 2010 and that different agonists can signal differentially, as suggested by the appearance of pharmacologic subtypes of the receptor. It is possible that these and other mechanisms can contribute to agonists' differences.…”

“…All of the compounds shown have been described as full ␦-opioid receptor agonists but nevertheless can have some important differences in their pharmacologies. For example, SNC-80 (4-[(R)-[(2S,5R)-4-allyl-2,5-dimethyl-piperazin-1-yl]-(3-methoxyphenyl)methyl]-N,N-diethyl-benzamide) has been shown to reliably stimulate locomotor activity and to produce convulsions in a number of preclinical species (Broom et al, 2002a;Jutkiewicz et al, 2005), whereas there have been no reports of either seizure activity or alteration in locomotor activity with AR-M100390 (N,N-diethyl-4-[phenyl(piperidin-4-ylidene)methyl]benzamide) (Pradhan and Clarke, 2005;Smagin et al, 2008;T. Hudzik, unpublished observations), which may represent the opposite end of the spectrum of activity.…”