Opioid receptors regulate neuronal activity by both pre- and postsynaptic mechanisms. We recently reported that the cloned delta- opioid receptor (DOR1) is primarily targeted to axons, suggesting a presynaptic role. In the present study we have studied the distribution and targeting of another opioid receptor, the mu-opioid receptor (MOR1), by raising anti-peptide antisera to the C-terminal peptide of MOR1. The specificity of the antisera was determined by analysis of transfected cells, Western blots, and immunoisolation studies. Immunohistochemistry showed that MOR1 immunoreactivity was enriched in many brain areas including cerebral cortex, striatum, hippocampus, locus coeruleus, and the superficial laminae of the dorsal horn. Moreover, MOR1-expressing neurons seem to target this receptor preferentially to their somatodendritic domain as determined by double- labeling experiments with MAP2. However, discrete populations of neurons target MOR1 to their axons, including some primary afferent neurons that express DOR1. In many regions enkephalin-containing axons were complementary to MOR1, suggesting by their proximity that enkephalins may be physiologically relevant ligands for this receptor. Thus, these results provide a morphological basis for understanding pre- and postsynaptic functions mediated by MOR1.

Cloning of multiple opioid receptors has presented opportunities to investigate the mechanisms of multiple opioid receptor signaling and the regulation of these signals. The subsequent identification of receptor gene structures has also provided opportunities to study the regulation of receptor gene expression and to manipulate the concentration of the gene products in vivo. Thus, in the current review, we examine recent advances in the delineation basis for the multiple opioid receptor signaling, and their regulation at multiple levels. We discuss the use of receptor knockout animals to investigate the function and the pharmacology of these multiple opioid receptors. The reasons and basis for the multiple opioid receptor are addressed.

SUMMARY Why do opiates make human beings itch ? Spinal opioid-induced itch, a prevalent side effect of pain management, has been considered to occur as a result of pain inhibition. We report that morphine-induced scratching (MIS) is abolished in mice lacking either gastrin-releasing peptide receptor (GRPR) or the μ opioid receptor (MOR). Using exon-specific knockdown, we identified the MOR1D isoform as essential for MIS, whereas MOR1 is important for morphine-induced analgesia (MIA) with no cross activity present. MOR1D and GRPR form constitutive heterodimers in the spinal cord and relay itch information upon morphine activation. Morphine induces internalization of both GRPR and MOR1D, whereas GRP induces that of GRPR but not MOR1D, when co-expressed. Moreover, GRP-induced scratching (GIS) is independent of MOR activation. These results suggest a unidirectional cross-activation of GRPR signaling by MOR1D via heterodimerization, and that opioid-induced itch is an active process concomitant with but independent of opioid analgesia.

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We have recently developed antisera which recognize epitopes of the cloned delta-opioid receptor (DOR; Dado et al., 1993). In the present report we have further characterized these antisera, and raised additional antisera in rats. We used these antisera to determine the distribution of DOR-like immunoreactivity (-Ll) in rat spinal cord and brainstem in relation to serotoninergic, noradrenergic, and enkephalinergic neurons. We found DOR-Ll in fibers and varicosities distributed throughout the spinal cord gray matter, with highest densities in the superficial dorsal horn, in autonomic regions, around the central canal as well as in the ventral horn motor nuclei. In the brainstem a dense innervation of DOR-immunoreactive (-IR) fibers was found in several nuclei such as spinal trigeminal nuclei, midline raphe nuclei, parabrachial nuclei, periaqueductal gray matter (PAG), interpeduncular nucleus, ans substantia nigra. A group of DOR-positive cells was seen in the laterodorsal tegmental nucleus. In addition, a few DOR-IR cell bodies were demonstrated in the parabrachial nuclei, interpeduncular nucleus, PAG, and superior and inferior colliculi as well as around the central canal in the spinal cord. All DOR-positive cells showed a punctuate staining pattern within the cytoplasm of the cell body and in primary dendrites. No plasma membrane staining of cells or dendrites could be demonstrated using the DOR antisera. Double-labeling experiments for DOR and 5-hydroxytryptamine (5HT, serotonin) revealed that some 5HT-IR neurons in the raphe complex were surrounded by DOR-IR fibers. In the spinal cord a high degree of coexistence was found between DOR and 5HT in nerve fibers and varicosities in the neuropil around the motoneurons and in lamina V of the dorsal horn. In autonomic regions of the spinal cord, a low degree of colocalization was seen between DOR and 5HT; in the superficial dorsal horn no coexistence was found. Tyrosine hydroxylase (TH)-positive neurons in the brainstem (in the A5 area, locus coeruleus, and A7 area) were apposed by DOR-positive fibers. However, no coexistence could be seen between DOR and TH in any part of the spinal cord. A close relation, but no coexistence, was observed between DOR- and enkephalin (ENK)-IR fibers in the spinal cord ventral horn; in the intermediolateral nucleus a low degree of colocalization was observed. Thus, a delta-opioid receptor may affect the activity of descending serotoninergic and noradrenergic neurons by means of modulating the release of neurotransmitters from afferents to these neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

The cellular location of extracellular signal-regulated kinases (ERKs) activated by a G protein-coupled receptor was shown to be dependent on the pathway that mediated their activation. In general, fast activation of ERKs (2 min) mediated by G proteins resulted in the nuclear translocation of phosphorylated ERKs, whereas a slower activation of ERKs (10 min) mediated by ␤-arrestins resulted in the cytosolic retention of the phosphorylated ERKs. However, we observed distinct differences from this established ERKs cellular itinerary with the -opioid receptor-activated ERKs. Agonists such as morphine and methadone activated ERKs via the protein kinase C-dependent pathway but not the ␤-arrestin-dependent pathway. The activated ERKs did not translocate into the nucleus, but phosphorylated 90-kDa ribosomal S6 kinase and induced the activity of transcription factor cAMP response element-binding protein. In contrast, agonists such as etorphine and fentanyl activated ERKs in a ␤-arrestin-dependent manner. The phosphorylated ERKs translocated into the nucleus, resulting in increases in Elk-1 activity and GRK2 and ␤-arrestin2 transcriptions. Thus, the cellular location of phosphorylated ERKs and subsequent activities on gene transcriptions are dictated by the agonist used to activate the receptor and the subsequent signaling pathway involved.

Opioids can evoke analgesia by inhibiting neuronal targets in either the brain or spinal cord, and multiple presynaptic and postsynaptic inhibitory mechanisms have been implicated. The relative significance of presynaptic and postsynaptic inhibition to opioid analgesia is essentially unknown, as are the identities and relevant locations of effectors mediating opioid actions. Here, we examined the distribution of G-protein-gated potassium (GIRK) channels in the mouse spinal cord and measured their contribution to the analgesia evoked by spinal administration of opioid receptor-selective agonists. We found that the GIRK channel subunits GIRK1 and GIRK2 were concentrated in the outer layer of the substantia gelatinosa of the dorsal horn. GIRK1 and GIRK2 were found almost exclusively in postsynaptic membranes of putative excitatory synapses, and a significant degree of overlap with the -opioid receptor was observed. Although most GIRK subunit labeling was perisynaptic or extrasynaptic, GIRK2 was found occasionally within the synaptic specialization.

The pharmacological effect of morphine as a painkiller is mediated mainly via the mu opioid receptor (MOR) and is dependent on the number of MORs in the cell surface membrane. While several studies have reported that the MOR gene is regulated by various cis-and trans-acting factors, many questions remain unanswered regarding in vivo regulation. The present study shows that epigenetic silencing and activation of the MOR gene are achieved through coordinated regulation at both the histone and DNA levels. In P19 mouse embryonal carcinoma cells, expression of the MOR was greatly increased after neuronal differentiation. MOR expression could also be induced by a demethylating agent (5-aza-2-deoxycytidine) or histone deacetylase inhibitors in the P19 cells, suggesting involvement of DNA methylation and histone deacetylation for MOR gene silencing. Analysis of CpG DNA methylation revealed that the proximal promoter region was unmethylated in differentiated cells compared to its hypermethylation in undifferentiated cells. In contrast, the methylation of other regions was not changed in either cell type. Similar methylation patterns were observed in the mouse brain. In vitro methylation of the MOR promoters suppressed promoter activity in the reporter assay. Upon differentiation, the in vivo interaction of MeCP2 was reduced in the MOR promoter region, coincident with histone modifications that are relevant to active transcription. When MeCP2 was disrupted using MeCP2 small interfering RNA, the endogenous MOR gene was increased. These data suggest that DNA methylation is closely linked to the MeCP2-mediated chromatin structure of the MOR gene. Here, we propose that an epigenetic mechanism consisting of DNA methylation and chromatin modification underlies the cell stage-specific mechanism of MOR gene expression.Opioids exert their pharmacological and physiological effects through binding to their endogenous receptors. Three types of opioid receptors, mu (), delta (␦), and kappa (), all belonging to the G-protein-coupled receptor superfamily, have been cloned. Upon agonist binding, these receptors couple to G proteins and affect several signal transduction pathways thought to mediate a broad range of functions and pharmacological effects of endogenous and exogenous opioids (51). Previous studies suggested that the opioid receptor (MOR) plays a key role in mediating the major clinical effects of analgesics, such as morphine, as well as the development of tolerance and physical dependence upon prolonged administration (39). MOR is mainly expressed in the central nervous system, with densities varying greatly in different regions, which can display different functional roles (55). During mouse embryonic development, the MOR message was specifically observed as early as embryonic day 8.5 (E8.5) using the reverse transcription (RT)-PCR method (44). In contrast, MOR transcripts were detected only beginning at E12 using the radioligand binding method (70) and at E10.5 by in situ hybridization (85). Transcript levels gradually incr...