Δ-opiate Receptor Research Articles

Involvement of central opiate receptors in modulation of centrally administered oxytocin-induced antinociception.

Objective(s):Oxytocin is involved in modulation of many brain-mediated functions. In the present study, we investigated the central effects of oxytocin and its receptor antagonist, atosiban on inflammatory pain. The contribution of opiate receptors was explored using non-selective and selective antagonists. Materials and Methods:The fourth ventricle of the brain of anesthetized rats was implanted with a guide cannula. Inflammatory pain in the orofacial region was induced by subcutaneous (SC) injection of formalin into the vibrissa pad, and time duration of face rubbing behavior was measured for 45 min. Results:A typical biphasic pain was observed after formalin injection. This biphasic pain behavior was attenuated by intra-fourth ventricle administration of oxytocin (12.5, 50, and 200 ng/rat). Central prior administration of 400 ng/rat atosiban (an oxytocin receptor antagonist), naloxone (a non-selective opiate receptor antagonist), naloxonazine (a selective µ-opiate receptor antagonist), and nor-binaltorphimine (a selective κ-opiate receptor antagonist), but not naltrindole (a δ-opiate receptor antagonist), prevented oxytocin-induced (200 ng/rat) antinociception. Except for naltrindole, other antagonists increased pain intensity when used alone. Above-mentioned drugs did not alter locomotor activity. Conclusion:Oxytocin, as a neuropeptide neurotransmitter, may be involved in the supraspinal modulation of inflammatory pain through µ- and κ-, but not δ-opiate receptors.

Opiate Antagonist Prevents μ- and δ-Opiate Receptor Dimerization to Facilitate Ability of Agonist to Control Ethanol-altered Natural Killer Cell Functions and Mammary Tumor Growth

In the natural killer (NK) cells, δ-opiate receptor (DOR) and μ-opioid receptor (MOR) interact in a feedback manner to regulate cytolytic function with an unknown mechanism. Using RNK16 cells, a rat NK cell line, we show that MOR and DOR monomer and dimer proteins existed in these cells and that chronic treatment with a receptor antagonist reduced protein levels of the targeted receptor but increased levels of opposing receptor monomer and homodimer. The opposing receptor-enhancing effects of MOR and DOR antagonists were abolished following receptor gene knockdown by siRNA. Ethanol treatment increased MOR and DOR heterodimers while it decreased the cellular levels of MOR and DOR monomers and homodimers. The opioid receptor homodimerization was associated with an increased receptor binding, and heterodimerization was associated with a decreased receptor binding and the production of cytotoxic factors. Similarly, in vivo, opioid receptor dimerization, ligand binding of receptors, and cell function in immune cells were promoted by chronic treatment with an opiate antagonist but suppressed by chronic ethanol feeding. Additionally, a combined treatment of an MOR antagonist and a DOR agonist was able to reverse the immune suppressive effect of ethanol and reduce the growth and progression of mammary tumors in rats. These data identify a role of receptor dimerization in the mechanism of DOR and MOR feedback interaction in NK cells, and they further elucidate the potential for the use of a combined opioid antagonist and agonist therapy for the treatment of immune incompetence and cancer and alcohol-related diseases.

Design and synthesis of new potential ligands of μ- and δ-opiate receptors in the series of decahydro-4a-hydroxy-6-isoquinolone

A new approach has been proposed for simplification of the morphine molecule in order to obtain new potential μ- and δ-opiate receptor ligands. The reaction of 1,2,5-trimethyl-4-piperidone and 2,2-di-methyl-4-tetrahydropyrone with substituted benzalacetones under Robinson ring fusion conditions gave decahydroisoquinoline and decahydroisochromene derivatives. A thorough conformational analysis of these products was carried out. The effect of the products on the functional activity of opiate receptors on the mouse vas deferens isolated organ (MVD) model was studied. Prior screening showed that 8-(p-dimethylaminophenyl)-4a-hydroxy-2,3,8a-trimethyldecahydro-6-isoquinolone displays opiate activity and has agonist properties.

A red clover ( Trifolium pratense) phase II clinical extract possesses opiate activity

Trifolium pratense (TP) is one of the most common herbs for the relief of menopausal symptoms. Little is known about its mechanisms of action. In this study, we investigated the affinity of TP at the μ- and δ-opiate receptors. We found that a clinically used TP extract bound to the μ-opiate receptor with a high affinity ( K i = 9.7 ± 1.6 μg/ml). The same extract was also found to have affinity at the δ-opiate receptor with K i of 15.9 ± 2.4 μg/ml. These results for the first time suggest a potential new mechanism of action of TP at the opiate receptors. Given the essential role of the opioid system in regulating temperature, mood, and hormonal levels and actions, this may explain in part the beneficial effect of TP in alleviating menopausal symptoms.

Opiate Drugs and δ-Receptor–Mediated Myocardial Protection

Background —Hypothermic myocardial arrest is necessary to complete most cardiac surgery, which limits the success of such operations. Similarly, cold, inhospitable environments limit the survival of warm-blooded animals. Animals have successfully adapted to this challenge through hibernation. Hibernation is an energy-conserving state, now known to be governed by cyclical variation in endogenous opiate compounds. It may also be induced in nonhibernators via hibernating animal serum factors or δ-opiate peptides. Furthermore, hibernation-induction triggers extend organ preservation in many models. This study examined whether opiate drugs with an affinity for the δ-opiate receptor confer similar protection. Methods and Results —Isolated hearts harvested from New Zealand White rabbits were treated with either cardioplegia alone or δ-opiate drugs (fentanyl, morphine, buprenorphine, pentazocine) followed by 2 hours of 34°C ischemia. Hearts were then reperfused, and functional and metabolic indices of treated groups were compared with untreated controls. Isovolumic developed pressure, coronary flow, and oxygen consumption were compared as a percent of preischemia versus 45 minutes after reflow. Developed pressure and oxygen consumption were better preserved in the morphine, buprenorphine, and pentazocine groups when compared with cardioplegia alone. Conclusions —Drugs with δ-opiate activity confer myocardial protection, which is additive to cardioplegia. Use of δ-opiate drugs in this context may have important clinical implications.

The Effects of Intravenous Naltrindole and Naltrindole 5′-Isothiocyanate on Sufentanil-Induced Respiratory Depression and Antinociception in Rats

Although the interactions between the μ- and the δ-opiate receptor subtypes are well documented with regard to supraspinal analgesia, less is known about the mutual interactions on respiratory depression. To clarify the functional interactions between both opiate receptor subtypes with regard to antinociception and respiratory depression, male Wistar rats were intravenously injected with 2.5 μg/kg of the μ-opiate agonist sufentanil and subsequently intravenously challenged with the delta antagonist naltrindole (NTI) or naltrindole 5′-isothiocyanate (5′-NTII), a δ-2 antagonist. Antinociception was measured by means of the tail-flick latency, and respiratory depression was evaluated by means of analysis of PaCO 2, PaO 2, and oxygen saturation. To quantify the antagonistic properties of NTI and 5′-NTII, mean areas under the curve were calculated for groups treated with sufentanil, control vehicle, and sufentanil plus a dose of the antagonists. NTI, but not 5′-NTII, antagonized the sufentanil-induced antinociception at 10 mg/kg NTI. Below this dose the effects were inconsistent. The sufentanil-induced hypercapnia and hypoxia were diminished with 10 mg/kg NTI or 5′-NTII. These data indicate that NTI antagonizes the sufentanil-induced antinociception and respiratory depression in rats. A dissociation between the antinociception and respiratory depression following intravenous sufentanil could be obtained with 10 mg/kg 5′-NTII pointing to different regulatory effects of opiate δ receptor subtypes on μ-opiate agonist-induced behavioral effects.

Correction of electrical instability of the heart with opiate receptor ligands

Peripheral injections of the μ-opiate receptor agonist DALDA,k-opiate receptor agonist spiradoline, and δ-opiate receptor blocker DuP734 significantly increased the ventricular fibrillation threshold in animals with modeled postinfarction cardiosclerosis or stress-induced damage to the heart.

Opiate receptors modulate estrogen-induced cholecystokinin and tachykinin but not enkephalin messenger RNA levels in the limbic system and hypothalamus

Cholecystokinin, substance P and methionine enkephalin all regulate the display of reproductive behaviour. Their expression is exquisitely regulated by estrogen in the limbic–hypothalamic circuit, a circuit that regulates the display of estrogen-sensitive female reproductive behaviour. Relatively little is known, however, about the interaction of endogenous opioid peptides with cholecystokinin and substance P in the limbic–hypothalamic circuit. Opiates antagonize the release of cholecystokinin and substance P in the hypothalamus and periaqueductal gray and stimulate cholecystokinin messenger RNA levels in the amygdala. To determine the effect of endogenous opioid input on estrogen-induced cholecystokinin, enkephalin and substance P expression, in situ hybridization histochemistry was used to examine estrogen-induced messenger RNA levels of these neuropeptides in specific nuclei of the limbic system and hypothalamus in the presence of opiate receptor antagonists. Estrogen treatment of ovariectomized rats significantly elevated cholecystokinin messenger RNA levels in the central portion of the medial preoptic nucleus, the encapsulated portion of the bed nucleus of the stria terminalis and the posterodorsal medial amygdala, as well as increased preproenkephalin and preprotachykinin messenger RNA levels in the ventromedial hypothalamic nucleus and the posterodorsal medial amygdala. The universal opiate receptor antagonist naltrexone and the δ-opiate receptor antagonist naltrindole each potentiated the estrogen-induced increase and elevated cholecystokinin messenger RNA levels an additional 1.9- to 2.8-fold depending on the nucleus examined, but had no effect on the estrogen-induced expression of either preproenkephalin or preprotachykinin messenger RNA. β-Funaltrexamine, a μ-opiate receptor antagonist, had no effect on the medial preoptic or medial amygdaloid cholecystokinin messenger RNA levels or on the estrogen-induced expression of preproenkephalin messenger RNA but did cause a decrease in estrogen-induced cholecystokinin messenger RNA levels in the bed nucleus of the stria terminalis and a decrease in the preprotachykinin messenger RNA levels in the ventromedial hypothalamic nucleus. These results indicate that endogenous opioids, acting on the δ-opiate receptor within nuclei of the limbic–hypothalamic circuit, restrain the estrogen-induced increase of cholecystokinin messenger RNA expression. Activation of the μ-opiate receptor, however, may facilitate cholecystokinin messenger RNA expression in the bed nucleus of the stria terminalis and preprotachykinin messenger RNA expression in the ventromedial hypothalamic nucleus. Thus, endogenous opioid peptides may act in a site- and receptor-specific manner to modulate estrogen-induced neuropeptide levels in the limbic system and hypothalamus.

Reversal of spinal cord non-opiate analgesia by conditioned anti-analgesia in the rat

The spinal cord contains endogenous substances (such as cholecystokinin, FMRFamide, etc.) that can block the analgesic effects of opiates. Anti-opiate actions have been most commonly studied by exogenous administration of receptor agonists and receptor antagonists of these substances. However, we have recently demonstrated that anti-analgesia can be brought under environmental control through Pavlovian conditioning. Whereas analgesia can be conditioned to signals for danger, anti-analgesia can be conditioned to signals for safety. Using this paradigm, we have previously demonstrated that conditioned anti-analgesia can reverse a variety of opiate analgesic states, including those produced by conditioned danger signals, systemic morphine, and intrathecal μ- and δ-opiate receptor agonists. These data raise the question of the generality of anti-analgesia actions. The present series of experiments examined the ability of conditioned anti-analgesia to affect non-opiate analgesic states induced by spinal delivery of GABA A, GABA B, 5HT 2+5HT 1, and 5HT 3 receptor agonists. While conditioned anti-analgesia had no effect on GABA A or 5HT 2+5HT 1 non-opiate analgesias, conditioned anti-analgesia completely blocked GABA B and 5HT 3 non-opiate analgesias. These findings clearly demonstrate that conditioned anti-analgesia can powerfully modulate non-opiate as well as opiate analgesias and bring into question whether putative anti-opiate neuroactive substances may have broader actions than previously suggested.

Estrogen Modulation of Opioid and Cholecystokinin Systems in the Limbic-Hypothalamic Circuit

The display of lordosis behavior has been correlated with the estrogen-induced expression of cholecystokinin (CCK) and enkephalin within the limbic–hypothalamic circuit. These neuropeptides have opposing effects on lordosis; for example, in the medial preoptic nucleus, CCK facilitates and opiates inhibit lordosis. Antisense oligodeoxynucleotide blockade of receptor expression indicated that CCK modulates lordosis in the medial preoptic nucleus through the CCKA-receptor. Sequence-specific antibodies directed against δ- and μ-opiate receptor proteins labeled fibers in the medial preoptic nucleus. Estrogen treatment of ovariectomized rats or etorphine (a nonselective opiate agonist) treatment altered the appearance of the immunoreactivity from a diffuse pattern to one of distinctly stained μ-opiate receptor immunoreactive cells and varicose fibers in the medial preoptic nucleus. Such a pattern of staining reflects an internalization of μ-opiate receptors following agonist stimulation. This type of internalization has been used as an indication of synaptic activity. The distribution of receptor internalization surrounds the distribution of CCK cells in the medial preoptic nucleus, suggesting that endogenous opioid peptides may modulate estrogen-induced CCK mRNA expression. Interestingly, nonselective and δ-opiate receptor selective antagonists potentiated the estrogen-induced CCK mRNA expression in the medial preoptic nucleus. Together, these results suggest that endogenous opioid peptides may modulate the estrogenic upregulation of CCK mRNA expression and demonstrate an important level of regulation of gene expression in which synaptic activity modifies hormonal input.

On the involvement of endogenous μ- and δ-opiate receptor agonists in the antiarrhythmic effect of adaptation

Rats adapted to stress showed a decreased severity and incidence of cardiac arrhythmias induced by epinephrine, and these effects of adaptation were abolished by naloxone. It is suggested that stress adaptation mitigates arrhythmia by activating the endogenous opioid system and stimulating the μ-opiate receptors.

The contribution of dopamine autoreceptors to the reactivating effect of opioid antagonists

The dopamine-opioid interaction is analyzed in amnesia and forgetfulness with the use of passive avoidance conditioning in experiments on mice. Naloxone or ICI174.864 administration restored the conditioned response in both learning situations against the background of saline. Pretreatment with (+)3PPP eliminated the reactivating effects of μ- and δ-opiate receptor blockade in the case of amnesia. The activation of dopamine autoreceptors in forgetfulness disrupted the memory restoration induced by ICI174.864, but not that by naloxone. The findings attest that reactivation of an amnestied memory trace by opioid receptor blockade depends on dopaminergic system functioning, whereas in the case of forgetfulness dopamine-opioid interactions are probably determined by the functional heterogeneity of the μ- and δ-opiate receptors and diminished contribution of the dopamine system to the process.

Opiatergic mechanisms of the cardiotropic effect of acute cooling

Changes in myocardial contractility after an acute cold exposure following intracerebroventricular administration of opiate receptor agonists were studied in rat hearts isolated after Langendorff. Cold exposures were carried out individually for each animal in chambers at −10°C for 4 h. Thirty min before being exposed to cold the animals were administered in a brain ventricle 10 μl of μ- or δ-opiate receptor agonists (DAGO or DADLE, respectively). Isolation and perfusion of the hearts were performed directly after the cold exposure was over. The mechanism of reduction of myocardial contractility and coronary flow induced by an acute cold exposure is believed to include stimulation of μ-opiate receptors as one of its main components, and the effect of intracerebral hypertension on hemodynamic parameters is partially mediated through activation of δ-opiate receptors.

Topographical distribution and synaptic localization of δ-opiate receptor immunoreactivity in rat prefrontal cortex

Topographical distribution and synaptic localization of δ-opiate receptor immunoreactivity in rat prefrontal cortex

Effects of intracerebroventricular administration of opiate receptor antagonists on the suppressed pulsatile LH release during acute fasting in ovariectomized estradiol-treated rats

The involvement of specific opiate receptors in the suppression of LH release during acute fasting in ovariectomized estradiol-treated rats was examined by intracerebroventricular (i.c.v.) administration of opiate receptor antagonists that exert a specificity directed mainly, although not absolutely, towards the δ-, κ- or μ-opiate receptors. Fasting for 48 h significantly decreased mean plasma LH levels in estradiol-treated animals by increasing sensitivity to the negative feedback effect of estradiol. Injecting i.c.v. the μ-opiate receptor antagonist naloxone (10 or 100 nmol in 2 μl of saline) blocked the inhibitory effect of fasting on pulsatile LH release and reinstated LH pulses. On the other hand, i.c.v. administration of the same dosages of a δ-opiate receptor antagonist ICI 174,864 or a κ-opiate receptor antagonist WIN 44441-3 did not have any effect. These results suggest that the increased sensitivity of the LH-releasing mechanism to the negative feedback effect of estradiol during fasting involves the endogenous opioids mainly through the selective activation of the μ-opiate receptors.

Stereoselectivity and subtype of the opiate receptor involved in stress-induced hypertension

Naloxone enantiomorphs were given intracerebroventricularly (i.c.v.) to rats socially deprived for a brief period of time (7–14 days) in order to ascertain the mediation of central opiate receptors in the reversal effect of opiate antagonists on the high systolic blood pressure induced by this type of stress. While the active enantiomorph ((−)-naloxone, 20 nmol per rat) lowered the elevated blood pressure, the (+)-enantiomorph (which shows a 10 000-fold lower affinity for opiate receptors) had no effect. Additionally, the antihypertensive effect induced by i.c.v. administration of an antagonist of the μ-opiate receptor (β-funaltrexamine, 20 nmol per rat), but not of the δ-opiate receptor (ICI 174, 864, 15 nmol per rat) pointed to the involvement of μ-opiate receptors as the endogenous component of the hypertensive response of rats to stress.

Monoclonal antibodies against the S2-serotonin receptor from rat brain that cross-react with dopamine and opiate receptors

Balb/c mice were immunized with rat striatal integral membrane proteins. After hybridization of splenocytes with myeloma cells, hybridoma lines secreting antibodies against serotonin, dopamine and opiate receptors were detected by inhibition of ligand binding to brain membrane preparations. Antibodies from two positive lines, Mab/a9 and Mab/a18, were able to inhibit ligand binding to the S2-serotonin (Kd range: 10–100 nM), the μ-opiate (Kd range: 0.4–3 μM) and the δ-opiate receptors (Kd range: 0.7–1.1 μM), while Mab/a9 was also found to inhibit ligand binding to the D2/D4-dopamine receptor (Kd ∼ 50 nM). An apparent molecular mass of 60 kDa could be ascribed to the δ-opiate receptor and apparent molecular masses of 29 and 36 kDa to the μ-opiate receptor by ligand elution from immuno-precipitates.

Chronic exposure to opiate agonists increases proenkephalin biosynthesis in NG108 cells

The neuroblastoma-glioma NG108 cell line has been shown to contain both a δ-opiate receptor and enkephalin peptides. In this paper, the presence of authentic proenkephalin mRNA and proenkephalin-derived peptides are demonstrated. Growth of the cells in the presence of etorphine for 5–7 days resulted in a 3-fold increase of proenkephalin mRNA, which was accompanied by comparable increases in proenkephalin peptides and free enkephalin. The effect was mimicked by either morphine or [ d-Ala 2, d-Met 5]enkephalinamide, and was blocked by naloxone. The EC 50 for the effect of etorphine was 10 −9 M. The cyclic AMP content of cells grown for 5 days in the presence of etorphine was the same as that of control cells. Forskolin treatment also increased the proenkephalin mRNA content of the cells: the effect was not additive with that of etorphine, suggesting that the effect of opiate agonists was not occurring through their inhibition of adenylate cyclase. The results suggest that proenkephalin synthesis in NG108 cells can be regulated by two different mechanisms, one involving cyclic AMP while the other, regulated by the opiate receptor, is yet to be determined.

Synaptosomal free [Ca 2+] is reduced by clonidine and dynorphin A-(1–13) and increased by idazoxan

Intrasynaptosomal free calcium concentrations measured in rat cortical synaptosomes using the calcium sensitive fluorescent dye quin 2, were found to be 0.107–0.122 μM (n = 33). Stimulation of α 2-adrenoceptors or κ-opiate receptors with clonidine (0.1–100 μM) or dynorphin A fragment (1–13) (0.2–5.0 μM) respectively, produced a dose dependent reduction in the intrasynaptosomal free calcium concentration. The α 2-adrenoceptor mediated reduction produced by clonidine (100 μM) was antagonised by the selective α 2-antagonist idazoxan (RX781094) (200 μM). Dynorphin A-(1–13) (1 μM) caused a reduction which was antagonised by a high (20 μM) but not low (2.0 μM) concentration of naloxone. We conclude that unlike the μ- and δ-opiate receptor, stimulation of α 2-adrenoceptors or κ-opiate receptors elicits changes in intrasynaptosomal free calcium concentration which are independent of the voltage sensitive calcium channel. This does not preclude the possibility that the α 2-adrenoceptor and κ-opiate receptor also have an effect at the voltage sensitive calcium channel.

Splenic lymphocyte production of an endorphin during endotoxic shock

Endogenous opioids have been reported to elicit some of the pathophysiologic responses to endotoxic shock by binding to the δ-opiate receptor. We have previously reported the production of immunoreactive (ir)-endorphin by B lymphocytes treated with bacterial lipopolysaccharide (LPS). We postulated that this lymphocyte-derived ir-endorphin may be an extrapituitary source of the endogenous opioid component associated with the pathophysiology of endotoxic shock. To test this hypothesis, we chose to study the LPS-sensitive ( C3HeB FeJ ) and -resistant ( C3H HeJ ) inbred mouse model. We treated these mice with intraperitoneal injections of LPS or B-lymphocyte-derived ir-endorphin. The LPS-sensitive mice presented with a severe hypothermic and pathophysiologic response pattern when treated with LPS or with ir-endorphin. The LPS-resistant mice, which were unresponsive to the LPS, however, presented with the typical hypothermic and pathophysiologic responses to the ir-endorphin. Immunofluorescence on the splenic leukocytes in the LPS-treated mice showed significant ir-endorphin present only in the LPS-sensitive mice at a time point preceding onset of the pathophysiologic response pattern. Taken together, this evidence strongly suggests a role for B-lymphocyte-derived ir-endorphin in the pathophysiology of endotoxic shock. The implications of immune system regulation of neuroendocrine function are discussed.