Functional analysis of cloned opioid receptors in transfected cell lines

Mechanisms of μ-opioid receptor inhibition of NMDA receptor-induced substance P release in the rat spinal cord

The interaction between NMDA receptors and μ-opioid receptors in primary afferent terminals was studied by using NMDA to induce substance P release, measured as neurokinin 1 receptor internalization. In rat spinal cord slices, the μ-opioid receptor agonists morphine, DAMGO and endomorphin-2 inhibited NMDA-induced substance P release, whereas the antagonist CTAP right-shifted the concentration response of DAMGO. In vivo, substance P release induced by intrathecal NMDA after priming with BDNF was inhibited by DAMGO. ω-Conotoxins MVIIC and GVIA inhibited about half of the NMDA-induced substance P release, showing that it was partially mediated by the opening of voltage-gated calcium (Cav) channels. In contrast, DAMGO or ω-conotoxins did not inhibit capsaicin-induced substance P release. In cultured DRG neurons, DAMGO but not ω-conotoxin inhibited NMDA-induced increases in intracellular calcium, indicating that μ-opioid receptors can inhibit NMDA receptor function by mechanisms other than inactivation of Cav channels. Moreover, DAMGO decreased the ω-conotoxin-insensitive component of the substance P release. Potent inhibition by ifenprodil showed that these NMDA receptors have the NR2B subunit. Activators of adenylyl cyclase and protein kinase A (PKA) induced substance P release and this was decreased by the NMDA receptor blocker MK-801 and by DAMGO. Conversely, inhibitors of adenylyl cyclase and PKA, but not of protein kinase C, decreased NMDA-induced substance P release. Hence, these NMDA receptors are positively modulated by the adenylyl cyclase-PKA pathway, which is inhibited by μ-opioid receptors. In conclusion, μ-opioid receptors inhibit NMDA receptor-induced substance P release through Cav channel inactivation and adenylyl cyclase inhibition.

Endomorphin-2 Decreases Excitatory Synaptic Transmission in the Spinal Ventral Horn of the Rat

Motor impairment is one of the serious side-effects of morphine, which is an exogenous agonist of the μ-opioid receptor (MOR) as well as a widely used analgesic drug in clinical practice for chronic pain treatment. Endomorphins (EMs, including EM-1 and EM-2), the most effective and specific endogenous agonists of the MOR, exert more potent analgesia in acute and neuropathic pain than other opiates, such as morphine. Although EMs had fewer side-effects comparing to other opiates, motor impairment was still one unwanted reaction which limited its clinical application. In order to prevent and treat the motor impairment, it is critical to reveal the neural mechanisms underlying such locomotion disorder. The purpose of the present study was to reveal the neural mechanisms underlying the effects of EM-2 on the activity of motoneurons in the spinal ventral horn. First, we examine the distribution of EM-2-immunoreactive (IR) primary afferent fibers and their synaptic connections with the motoneurons innervating the skeletal muscles of the lower limb revealed by sciatic nerve retrograde tracing. The results showed that EM-2-IR fibers and terminals were sparsely observed in lamina IX and they formed symmetric synaptic connections with the motoneurons within lamina IX of the spinal ventral horn. Then, whole-cell patch-clamp technique was used to observe the effects of EM-2 on the spontaneous excitatory postsynaptic current (sEPSC) of motoneurons in lamina IX. The results showed that EM-2 could decrease both the frequency and amplitude of the sEPSC of the motoneurons in lamina IX, which was reversed by the MOR antagonist CTOP. These results indicate that EM-2-IR fibers originated from primary afferent fibers form symmetric synaptic connections with motoneurons innervating skeletal muscles of the lower limbs in lamina IX of the spinal ventral horn and EM-2 might exert inhibitory effects on the activities of these motoneurons through both presynaptic and postsynaptic mechanisms.

Generation of a KOR-Cre knockin mouse strain to study cells involved in kappa opioid signaling

The kappa opioid receptor (KOR) has numerous important roles in the nervous system including the modulation of mood, reward, pain, and itch. In addition, KOR is expressed in many non-neuronal tissues. However, the specific cell types that express KOR are poorly characterized. Here, we report the development of a KOR-Cre knockin allele, which provides genetic access to cells that express KOR. In this mouse, Cre recombinase (Cre) replaces the initial coding sequence of the Opkr1 gene (encoding the kappa opioid receptor). We demonstrate that the KOR-Cre allele mediates recombination by embryonic day 14.5 (E14.5). Within the brain, KOR-Cre shows expression in numerous areas including the cerebral cortex, nucleus accumbens and striatum. In addition, this allele is expressed in epithelium and throughout many regions of the body including the heart, lung, and liver. Finally, we reveal that KOR-Cre mediates recombination of a subset of bipolar and amacrine cells in the retina. Thus, the KOR-Cre mouse line is a valuable new tool for conditional gene manipulation to enable the study of KOR.

Regulation of nonsmall-cell lung cancer stem cell like cells by neurotransmitters and opioid peptides

Nonsmall-cell lung cancer (NSCLC) is the leading type of lung cancer and has a poor prognosis. We have shown that chronic stress promoted NSCLC xenografts in mice via stress neurotransmitter-activated cAMP signaling downstream of beta-adrenergic receptors and incidental beta-blocker therapy was reported to improve clinical outcomes in NSCLC patients. These findings suggest that psychological stress promotes NSCLC whereas pharmacologically or psychologically induced decreases in cAMP may inhibit NSCLC. Cancer stem cells are thought to drive the development, progression and resistance to therapy of NSCLC. However, their potential regulation by stress neurotransmitters has not been investigated. In the current study, epinephrine increased the number of cancer stem cell like cells (CSCs) from three NSCLC cell lines in spheroid formation assays while enhancing intracellular cAMP and the stem cell markers sonic hedgehog (SHH), aldehyde dehydrogenase-1 (ALDH-1) and Gli1, effects reversed by GABA or dynorphin B via Gαi -mediated inhibition of cAMP formation. The growth of NSCLC xenografts in a mouse model of stress reduction was significantly reduced as compared with mice maintained under standard conditions. Stress reduction reduced serum levels of corticosterone, norepinephrine and epinephrine while the inhibitory neurotransmitter γ-aminobutyric acid (GABA) and opioid peptides increased. Stress reduction significantly reduced cAMP, VEGF, p-ERK, p-AKT, p-CREB, p-SRc, SHH, ALDH-1 and Gli1 in xenograft tissues whereas cleaved caspase-3 and p53 were induced. We conclude that stress neurotransmitters activate CSCs in NSCLC via multiple cAMP-mediated pathways and that pharmacologically or psychologically induced decreases in cAMP signaling may improve clinical outcomes in NSCLC patients.

Physiology, signaling, and pharmacology of opioid receptors and their ligands in the gastrointestinal tract: current concepts and future perspectives

Opioid receptors are widely distributed in the human body and are crucially involved in numerous physiological processes. These include pain signaling in the central and the peripheral nervous system, reproduction, growth, respiration, and immunological response. Opioid receptors additionally play a major role in the gastrointestinal (GI) tract in physiological and pathophysiological conditions. This review discusses the physiology and pharmacology of the opioid system in the GI tract. We additionally focus on GI disorders and malfunctions, where pathophysiology involves the endogenous opioid system, such as opioid-induced bowel dysfunction, opioid-induced constipation or abdominal pain. Based on recent reports in the field of pharmacology and medicinal chemistry, we will also discuss the opportunities of targeting the opioid system, suggesting future treatment options for functional disorders and inflammatory states of the GI tract.

Morphine as a Potential Oxidative Stress-Causing Agent

Morphine exhibits important pharmacological effects for which it has been used in medical practice for quite a long time. However, it has a high addictive potential and can be abused. Long-term use of this drug can be connected with some pathological consequences including neurotoxicity and neuronal dysfunction, hepatotoxicity, kidney dysfunction, oxidative stress and apoptosis. Therefore, most studies examining the impact of morphine have been aimed at determining the effects induced by chronic morphine exposure in the brain, liver, cardiovascular system and macrophages. It appears that different tissues may respond to morphine diversely and are distinctly susceptible to oxidative stress and subsequent oxidative damage of biomolecules. Importantly, production of reactive oxygen/nitrogen species induced by morphine, which have been observed under different experimental conditions, can contribute to some pathological processes, degenerative diseases and organ dysfunctions occurring in morphine abusers or morphine-treated patients. This review attempts to provide insights into the possible relationship between morphine actions and oxidative stress.

Development of κ opioid receptor antagonists

κ opioid receptors (KORs) belong to the G-protein-coupled class of receptors (GPCRs). They are activated by the endogenous opioid peptide dynorphin (DYN) and expressed at particularly high levels within brain areas implicated in modulation of motivation, emotion, and cognitive function. Chronic activation of KORs in animal models has maladaptive effects including increases in behaviors that reflect depression, the propensity to engage in drug-seeking behavior, and drug craving. The fact that KOR activation has such a profound influence on behaviors often triggered by stress has led to interest in selective KOR antagonists as potential therapeutic agents. This Perspective provides a description of preclinical research conducted in the development of several different classes of selective KOR antagonists, a summary of the clinical studies conducted thus far, and recommendations for the type of work needed in the future to determine if these agents would be useful as pharmacotherapies for neuropsychiatric illness.

EGFR dependent subcellular communication was responsible for morphine mediated AC superactivation

Compensatory adenylyl cyclase (AC) superactivation has been postulated to be responsible for the development of morphine tolerance and dependence, the underlying mechanism was demonstrated to comprise c-Src-dependent upregulation of AC5 within the lipid rafts. In the present study, we demonstrated that chronic morphine treatment sensitized EGFR signaling by augmenting EGFR phosphorylation and translocation into ER, which was essential for CRT-MOR tethering within the lipid rafts and AC5 superactivation. Intriguingly, synaptic clustering of CRT-MOR was dependent on EGFR phosphorylation and presumed to implicate in alignment and organization of synaptic compartments. Taken together, our data raised the possibility that an adaptive change in MOR and EGFR signal systems might establish CRT related subcellular communication, the signaling network within brain synaptic zone was proposed to implicate in morphine tolerance and dependence.

Inhibition of sensory afferents activation and visceral pain by a brominated algal diterpene

BACKGROUND

In the search of new therapeutic options for the treatment of pain, isolation, and testing of secondary metabolites from plant extracts has raised significant attention. We have investigated the effects of the brominated diterpene O(11) 15- cyclo-14-bromo-14,15-dihydrorogiol-3,11-diol (that we have named VLC5), extracted from the Mediterranean red algae Laurencia glandulifera.

METHODS

The pure extract was tested on primary afferent calcium signals induced by high concentration of KCl, transcient receptor potential vanilloid (TRPV)1 (capsaicin) or TRPV4 agonists, histamine, or protease-activated receptor-2 (PAR(2) ) agonist. It was also tested in mice in a model of mustard oil-induced colonic hypersensitivity.

KEY RESULTS

VLC5 was inhibited PAR(2) agonist or histamine-induced calcium mobilization in mouse primary afferents, but did not modify calcium signals induced by high concentrations of KCl, TRPV1 or TRPV4 agonists. The effect of VLC5 on histamine-induced calcium signal in primary afferent was inhibited by pertussis toxin pretreatment and was dependent on the activation of mu- or kappa-opioid receptor agonists, as it was inhibited by selective antagonists of those two receptors, but not by selective antagonist of the delta-opioid receptor. Intraperitoneal treatment of mice with VLC5 (10 mg kg(-1)) significantly reduced visceral pain behaviors induced by the intracolonic administration of mustard oil, in an opioid receptor-dependent manner.

CONCLUSIONS & INFERENCES

We have demonstrated significant analgesic properties for the algal metabolite VLC5, which is able to signal directly to primary afferents, through a mechanism dependent on the activation of opioid receptors. This identifies a new natural compound capable of activating peripheral opioidergic systems, exerting analgesic properties.

The mu opioid receptor modulates neurotransmission in the rat spinal ventral horn

BACKGROUND

Opioids inhibit excitatory neurotransmission and produce antinociception through μ opioid receptors (MORs). Although MORs are expressed in the spinal ventral horn, their functions and effects are largely unknown. Therefore, we examined the neuromodulatory effects of μ opioids in spinal lamina IX neurons at the cellular level.

METHODS

The effects of the selective μ agonist [D-Ala(2),-N-Me-Phe(4), Gly(5)-ol]enkephalin (DAMGO) on synaptic transmission were examined in spinal lamina IX neurons of neonatal rats using the whole-cell patch-clamp technique.

RESULTS

DAMGO produced outward currents in 56% of the lamina IX neurons recorded, with a 50% effective concentration of 0.1 μM. Analysis of the current-voltage relationship revealed a reversal potential of approximately -86 mV. These currents were not blocked by tetrodotoxin but were inhibited by Ba(2+) or a selective μ antagonist. Moreover, the currents were suppressed by the addition of Cs(+) and tetraethylammonium or guanosine 5'-[β-thio]diphosphate trilithium salt to the pipette solution. In addition, DAMGO decreased the frequency of spontaneous excitatory and inhibitory postsynaptic currents, and these effects were unaltered by treatment with tetrodotoxin.

CONCLUSION

Our results suggest that DAMGO hyperpolarizes spinal lamina IX neurons by G protein-mediated activation of K(+) channels after activation of MORs. Furthermore, activation of MORs on presynaptic terminals reduces both excitatory and inhibitory transmitter release. Although traditionally opioids are not thought to affect motor function, the present study documents neuromodulatory effects of μ opioids in spinal lamina IX neurons, suggesting that MORs can influence motor activity.

A unique role of RGS9-2 in the striatum as a positive or negative regulator of opiate analgesia

The signaling molecule RGS9-2 is a potent modulator of G-protein-coupled receptor function in striatum. Our earlier work revealed a critical role for RGS9-2 in the actions of the μ-opioid receptor (MOR) agonist morphine. In this study, we demonstrate that RGS9-2 may act as a positive or negative modulator of MOR-mediated behavioral responses in mice depending on the agonist administered. Paralleling these findings we use coimmunoprecipitation assays to show that the signaling complexes formed between RGS9-2 and Gα subunits in striatum are determined by the MOR agonist, and we identify RGS9-2 containing complexes associated with analgesic tolerance. In striatum, MOR activation promotes the formation of complexes between RGS9-2 and several Gα subunits, but morphine uniquely promotes an association between RGS9-2 and Gαi3. In contrast, RGS9-2/Gαq complexes assemble after acute application of several MOR agonists but not after morphine application. Repeated morphine administration leads to the formation of distinct complexes, which contain RGS9-2, Gβ5, and Gαq. Finally, we use simple pharmacological manipulations to disrupt RGS9-2 complexes formed during repeated MOR activation to delay the development of analgesic tolerance to morphine. Our data provide a better understanding of the brain-region-specific signaling events associated with opiate analgesia and tolerance and point to pharmacological approaches that can be readily tested for improving chronic analgesic responsiveness.

Development of a cyclic adenosine monophosphate assay for Gi-coupled G protein-coupled receptors by utilizing the endogenous calcitonin activity in Chinese hamster ovary cells

Activation of G(i)-coupled G protein-coupled receptor (GPCRs) by their ligands leads to inhibition of adenylyl cyclase (AC) and reduction of cyclic adenosine monophosphate (cAMP) levels in cells. The traditional cAMP assay for G(i)-coupled GPCRs commonly uses forskolin, a nonspecific AC activator, to increase the basal cAMP level in cells to create an assay window for ligand detection. However, there is still a need to develop a nonforskolin-based cAMP assay because of the challenges inherent in titrating the concentration of forskolin to achieve a reliable assay window, along with issues related to the cAMP-independent effects of forskolin. Herein, we describe such an assay by utilizing the endogenous activity of the calcitonin receptor in Chinese hamster ovary (CHO) cells. The calcitonin receptor is a G(s)-coupled GPCR that, when activated by calcitonin, leads to the stimulation of AC and increases cAMP in cells. Thus, we use calcitonin, instead of forskolin, to increase the basal cAMP level in CHO cells to achieve an assay window. We demonstrated that calcitonin peptides robustly increased cAMP accumulation in several CHO cell lines stably expressing well-known G(i)-coupled GPCRs, such as the Dopamine D2 receptor, the Opioid μ receptor, or the Cannabinoid receptor-1. Agonists of these G(i)-coupled GPCRs attenuated calcitonin-induced cAMP production in their receptor stable cell lines. On the other hand, antagonists and/or inverse agonists blocked the effects of their agonists on calcitonin-induced cAMP production. This calcitonin-based cAMP assay has been demonstrated to be sensitive and robust and exhibited acceptable assay windows (signal/noise ratio) and, thus, can be applied to screen for agonists and antagonists/inverse agonists of G(i)-coupled GPCRs in high-throughput screening formats.

Ionic storm in hypoxic/ischemic stress: can opioid receptors subside it?

Neurons in the mammalian central nervous system are extremely vulnerable to oxygen deprivation and blood supply insufficiency. Indeed, hypoxic/ischemic stress triggers multiple pathophysiological changes in the brain, forming the basis of hypoxic/ischemic encephalopathy. One of the initial and crucial events induced by hypoxia/ischemia is the disruption of ionic homeostasis characterized by enhanced K(+) efflux and Na(+)-, Ca(2+)- and Cl(-)-influx, which causes neuronal injury or even death. Recent data from our laboratory and those of others have shown that activation of opioid receptors, particularly delta-opioid receptors (DOR), is neuroprotective against hypoxic/ischemic insult. This protective mechanism may be one of the key factors that determine neuronal survival under hypoxic/ischemic condition. An important aspect of the DOR-mediated neuroprotection is its action against hypoxic/ischemic disruption of ionic homeostasis. Specially, DOR signal inhibits Na(+) influx through the membrane and reduces the increase in intracellular Ca(2+), thus decreasing the excessive leakage of intracellular K(+). Such protection is dependent on a PKC-dependent and PKA-independent signaling pathway. Furthermore, our novel exploration shows that DOR attenuates hypoxic/ischemic disruption of ionic homeostasis through the inhibitory regulation of Na(+) channels. In this review, we will first update current information regarding the process and features of hypoxic/ischemic disruption of ionic homeostasis and then discuss the opioid-mediated regulation of ionic homeostasis, especially in hypoxic/ischemic condition, and the underlying mechanisms.

Long-acting kappa opioid antagonists disrupt receptor signaling and produce noncompetitive effects by activating c-Jun N-terminal kinase

Norbinaltorphimine (NorBNI), guanidinonaltrindole, and atrans-(3R,4R)-dimethyl-4-(3-hydroxyphenyl) piperidine (JDTic) are selective kappa opioid receptor (KOR) antagonists having very long durations of action in vivo despite binding non-covalently in vitro and having only moderately high affinities. Consistent with this, we found that antagonist treatment significantly reduced the subsequent analgesic response of mice to the KOR agonist U50,488 in the tail-withdrawal assay for 14-21 days. Receptor protection assays were designed to distinguish between possible explanations for this anomalous effect, and we found that mice pretreated with the readily reversible opioid antagonists naloxone or buprenorphine before norBNI responded strongly in the tail-flick analgesia assay to a subsequent challenge with U50,488 1 week later. Protection by a rapidly cleared reagent indicates that norBNI did not persist at the site of action. In vitro binding of [(3)H]U69,593 to KOR showed that K(d) and Bmax values were not significantly affected by prior in vivo norBNI exposure, indicating that the agonist binding site was intact. Consistent with the concept that the long-lasting effects might be caused by a functional disruption of KOR signaling, both norBNI and JDTic were found to stimulate c-Jun N-terminal kinase (JNK) phosphorylation in HEK293 cells expressing KOR-GFP but not in untransfected cells. Similarly, norBNI increased phospho-JNK in both the striatum and spinal cord in wild type mice but not in KOR knock-out mice. Pretreatment of mice with the JNK inhibitor SP600125 before norBNI attenuated the long acting antagonism. Together, these results suggest that the long duration KOR antagonists disrupt KOR signaling by activating JNK.

Phosphorylation: A molecular switch in opioid tolerance

Protein phosphorylation is a key posttranslational modification mechanism controlling the conformation and activity of many proteins. Increasing evidence has implicated an essential role of phosphorylation by several major protein kinases in promoting and maintaining opioid tolerance. We review some of the most recent studies on protein kinase C (PKC), cyclic AMP dependent protein kinase A (PKA), calcium/calmodulin-dependent protein kinase II (CaMKII), protein kinase G (PKG), and G protein receptor kinase (GRK). These kinases act as the molecular switches to modulate opioid tolerance. Pharmacological interventions at one or more of the protein kinases and phosphatases may provide valuable strategies to improve opioid analgesia by attenuating tolerance to these drugs.

Kappa opioid receptor activation of p38 MAPK is GRK3- and arrestin-dependent in neurons and astrocytes

AtT-20 cells expressing the wild-type kappa opioid receptor (KOR) increased phospho-p38 MAPK following treatment with the kappa agonist U50,488. The increase was blocked by the kappa antagonist norbinaltorphimine and not evident in untransfected cells. In contrast, U50,488 treatment of AtT-20 cells expressing KOR having alanine substituted for serine-369 (KSA) did not increase phospho-p38. Phosphorylation of serine 369 in the KOR carboxyl terminus by G-protein receptor kinase 3 (GRK3) was previously shown to be required for receptor desensitization, and the results suggest that p38 MAPK activation by KOR may require arrestin recruitment. This hypothesis was tested by transfecting arrestin3-(R170E), a dominant positive form of arrestin that does not require receptor phosphorylation for activation. AtT-20 cells expressing both KSA and arrestin3-(R170E) responded to U50,488 treatment with an increase in phospho-p38 consistent with the hypothesis. Primary cultured astrocytes (glial fibrillary acidic protein-positive) and neurons (gamma-aminobutyric acid-positive) isolated from mouse striata also responded to U50,488 by increasing phospho-p38 immunolabeling. p38 activation was not evident in either striatal astrocytes or neurons isolated from KOR knock-out mice or GRK3 knock-out mice. Astrocytes pretreated with small interfering RNA for arrestin3 were also unable to activate p38 in response to U50,488 treatment. Furthermore, in striatal neurons, the kappa-mediated phospho-p38 labeling was colocalized with arrestin3. These findings suggest that KOR may activate p38 MAPK in brain by a GRK3 and arrestin-dependent mechanism.

Pharmacological and functional biochemical properties of d-Ala2-d-Nle5-enkephalin-Arg-Phe

Tyr-D-Ala-Gly-Phe-D-Nle-Arg-Phe (DADN) a synthetic analogue of the endogenous Met-enkephalin-Arg-Phe (Tyr-Gly-Gly-Phe-Met-Arg-Phe; MERF), was investigated in radioligand binding assays, [(35)S]GTPgammaS stimulation experiments as well as in in vivo algesiometric tests. Binding properties of [(3)H]DADN were measured in crude membrane fractions of rat spinal cord tissues and in homogenates of Chinese hamster ovary (CHO) cells selectively expressing delta-, kappa-or micro-opioid receptors. The highest affinity for [(3)H]DADN binding was observed in membranes from CHO cells transfected with micro-opioid receptors confirming the micro-selectivity of the peptide. Unlabeled DADN was also investigated in functional biochemical experiments by measuring opioid receptor-mediated G-protein activation in rat brain membrane fractions. The peptide stimulated the activity of the regulatory G-proteins in a concentration dependent manner, and the stimulation was efficiently inhibited in the presence of micro-receptor specific antagonist ligands further supporting the selectivity profile of DADN. Intrathecally administered DADN produced a dose-related, naloxone-reversible antinociception in rat hot water tail-flick tests. Among the selective opioid antagonists tested, the delta-selective naltrindole (NTI) and the kappa-specific norbinaltorphimine (norBNI) showed only slight blocking effects compared with naloxone. The results obtained in the in vitro agonist-stimulated [(35)S]GTPgammaS binding assays are in good agreement with the opioid agonist effect seen in the in vivo pain test.

Expression and distribution of μ opioid receptors in the inner ear of the rat

Opioid peptides have demonstrated modulatory effects on the vestibular afferent discharge and are putative vestibular efferent neuromodulators. The distribution of their receptors in the mammalian vestibular epithelia is not known. We used reverse transcriptase-polymerase chain reaction (RT-PCR), in situ hybridization, Western blots and immunohistochemistry to study the expression of mu opioid receptor (MOR) in the Scarpa's ganglia and cristae ampullares of rats. MOR transcript was only detected in the somata of the vestibular afferent neurons. MOR-like immunoreactivity was observed in the somata of vestibular afferents and in nerve terminals in the cristae ampullares epithelia both in the center and peripheral regions. Double labeling of cristae sections with the MOR1 antibody in combination with antibodies against calretinin (a marker for vestibular afferents terminating in calices) and peripherin (a marker for afferents terminating in boutons), respectively showed that MOR1 immunoreactivity was in calyx, dimorphic and bouton vestibular afferents. MOR immunoreactivity was not detected in vestibular efferent fibers identified with choline acetyltransferase immunohistochemistry. These results indicate that MOR may mediate effects of vestibular efferents on afferents.

Opioid receptor activation attenuates nicotinic enhancement of spontaneous GABA release in lateral spiriform nucleus of the chick

We examined the effects of opioids on the nicotinic enhancement of spontaneous GABA release from presynaptic terminals in the lateral spiriform nucleus (SpL) of the chick. Whole cell recordings from SpL neurons in brain slices were used to monitor spontaneous GABA release. Nicotine (1 microM) produced an 8-fold increase in the frequency of GABA events without changing their amplitude, consistent with an increase of GABA release from presynaptic terminals. L-enkephalin (1 microM) blocked these effects of nicotine on presynaptic GABA release, and the opioid antagonist naloxone (100 nM) antagonized the actions of L-enkephalin. The selective mu agonist DAMGO (300 nM) also attenuated the nicotine-mediated enhancement of GABA release, and the mu selective antagonist CTOP (1 microM) blocked the actions of DAMGO. In contrast, the kappa opioid agonist U50488 (3 microM) and the delta opioid agonist DPDPE (1 microM) had no effect. The results demonstrate that presynaptic release of GABA in the SpL can be regulated by both nicotinic agonists and mu opioids. While mu opioids have little effect on GABA release by themselves, they are able to block the marked enhancement of GABA release normally produced by nicotine. Since both cholinergic and enkephalinergic nerves are present in the SpL, the interactions of these two neurotransmitter systems may serve to precisely regulate GABA release in this brain region.

Characterization of opioid receptors that modulate nociceptive neurotransmission in the trigeminocervical complex

1. Opioid agonists have been used for many years to treat all forms of headache, including migraine. We sought to characterize opioid receptors involved in craniovascular nociceptive pathways by in vivo microiontophoresis of micro -receptor agonists and antagonists onto neurons in the trigeminocervical complex of the cat. 2. Cats were anaesthetized with alpha-chloralose 60 mg kg(-1), i.p. and 20 mg kg(-1), i.v. supplements after induction and surgical preparation using halothane. Units were identified in the trigeminocervical complex responding to supramaximal electrical stimulation of the superior sagittal sinus, and extracellular recordings of activity made. 3. Seven- or nine-barrelled glass micropipettes incorporating tungsten recording electrodes in their centre barrels were used for microiontophoresis of test substances onto cell bodies. 4. Superior sagittal sinus (SSS)-linked cells whose firing was evoked by microiontophoretic application of L-glutamate (n=8 cells) were reversibly inhibited by microiontophoresis of H(2)N-Tyr-D-Ala-Gly-N-Me-Phe-Gly-ol (DAMGO) (n=12), a selective micro -receptor agonist, in a dose dependent manner, but not by control ejection of sodium or chloride ions from a barrel containing saline. 5. The inhibition by DAMGO of SSS-linked neurons activated with L-glutamate could be antagonized by microiontophoresis of selective micro -receptor antagonists D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH(2) (CTOP) or D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH(2) (CTAP), or both, in all cells tested (n=4 and 6, respectively). 6. Local iontophoresis of DAMGO during stimulation of the superior sagittal sinus resulted in a reduction in SSS-evoked activity. This effect was substantially reversed 10 min after cessation of iontophoresis. The effect of DAMGO was markedly inhibited by co-iontophoresis of CTAP. 7. Thus, we found that micro -receptors modulate nociceptive input to the trigeminocervical complex. Characterizing the sub-types of opioid receptors that influence trigeminovascular nociceptive transmission is an important component to understanding the pharmacology of this synapse, which is pivotal in primary neurovascular headache.

Functional properties of Cav1.3 (alpha1D) L-type Ca2+ channel splice variants expressed by rat brain and neuroendocrine GH3 cells

Ca(2+) enters pituitary and pancreatic neuroendocrine cells through dihydropyridine-sensitive channels triggering hormone release. Inhibitory metabotropic receptors reduce Ca(2+) entry through activation of pertussis toxin-sensitive G proteins leading to activation of K(+) channels and voltage-sensitive inhibition of L-type channel activity. Despite the cloning and functional expression of several Ca(2+) channels, those involved in regulating hormone release remain unknown. Using reverse transcription-polymerase chain reaction we identified mRNAs encoding three alpha(1) (alpha(1A), alpha(1C), and alpha(1D)), four beta, and one alpha(2)-delta subunit in rat pituitary GH(3) cells; alpha(1B) and alpha(1S) transcripts were absent. GH(3) cells express multiple alternatively spliced alpha(1D) mRNAs. Many of the alpha(1D) transcript variants encode "short" alpha(1D) (alpha(1D-S)) subunits, which have a QXXER amino acid sequence at their C termini, a motif found in all other alpha(1) subunits that couple to opioid receptors. The other splice variants identified terminate with a longer C terminus that lacks the QXXER motif (alpha(1D-L)). We cloned and expressed the predominant alpha(1D-S) transcript variants in rat brain and GH(3) cells and their alpha(lD-L) counterpart in GH(3) cells. Unlike alpha(1A) channels, alpha(1D) channels exhibited current-voltage relationships similar to those of native GH(3) cell Ca(2+) channels, but lacked voltage-dependent G protein coupling. Our data demonstrate that alternatively spliced alpha(1D) transcripts form functional Ca(2+) channels that exhibit voltage-dependent, G protein-independent facilitation. Furthermore, the QXXER motif, located on the C terminus of alpha(1D-S) subunit, is not sufficient to confer sensitivity to inhibitory G proteins.

Cannabinoid and kappa opioid receptors reduce potassium K current via activation of G(s) proteins in cultured hippocampal neurons

The current study showed that potassium K current (I(K)), which is evoked at depolarizing potentials between -30 and +40 mV in cultured hippocampal neurons, was significantly reduced by exposure to the CB1 cannabinoid receptor agonist WIN 55,212-2 (WIN-2). WIN-2 (20-40 nM) produced an average 45% decrease in I(K) amplitude across all voltage steps, which was prevented by SR141716A, the CB1 receptor antagonist. The cannabinoid receptor has previously been shown to be G(i/o) protein-linked to several cellular processes; however, the decrease in I(K) was unaffected by modulators of G(i/o) proteins and agents that alter levels of protein kinase A. In contrast, CB1 receptor-mediated or direct activation of G(s) proteins with cholera toxin (CTX) produced the same decrease in I(K) amplitude as WIN-2, and the latter was blocked in CTX-treated cells. G(s) protein inhibition via GDPbetaS also eliminated the effects of WIN-2 on I(K). Consistent with this outcome, activation of protein kinase C (PKC) by arachidonic acid produced similar effects to WIN-2 and CTX. Kappa opioid receptor agonists, which also reduce I(K) amplitude via G(s) proteins, were compared with WIN-2 actions on I(K.) The kappa receptor agonist U50,488 reduced I(K) amplitude in the same manner as WIN-2, while the kappa receptor antagonist, nor-binaltorphimine, actually increased I(K) amplitude and significantly reduced the effect of co-administered WIN-2. The results indicate that CB1 and kappa receptor activation is additive with respect to I(K) amplitude, suggesting that CB1 and kappa receptors share a common G(s) protein signaling pathway involving PKC.

Cloned delta-opioid receptors in GH(3) cells inhibit spontaneous Ca(2+) oscillations and prolactin release through K(IR) channel activation

Opioid receptors can couple to K(+) and Ca(2+) channels, adenylyl cyclase, and phosphatidyl inositol turnover. Any of these actions may be important in the regulation of neurotransmitter and hormone release from excitable cells. GH(3) cells exhibit spontaneous oscillations of intracellular Ca(2+) concentration ([Ca(2+)](i)) and prolactin release. Activation of cloned delta-opioid receptors stably expressed in GH(3) cells inhibits both spontaneous Ca(2+) signaling and basal prolactin release. The objective of this study was to examine a possible role for K(+) channels in these processes using the patch-clamp technique, fluorescence imaging, and a sensitive ELISA for prolactin. The selective delta receptor agonist [D-Pen(2), D-Pen(2)]enkephalin (DPDPE) inhibited [Ca(2+)](i) oscillations in GH(3) cells expressing both mu and delta receptors (GH(3)MORDOR cells) but had no effect on control GH(3) cells or cells expressing mu receptors alone (GH(3)MOR cells). The inhibition of [Ca(2+)](i) oscillations by DPDPE was unaffected by thapsigargin pretreatment, suggesting that this effect is independent of inositol 1,4,5-triphosphate-sensitive Ca(2+) stores. DPDPE caused a concentration-dependent inhibition of prolactin release from GH(3)MORDOR cells with an IC(50) of 4 nM. DPDPE increased inward K(+) current recorded from GH(3)MORDOR cells but had no significant effect on K(+) currents recorded from control GH(3) cells or GH(3)MOR cells. The mu receptor agonist morphine also had no effect on currents recorded from control cells but activated inward K(+) currents recorded from GH(3)MOR and GH(3)MORDOR cells. Somatostatin activated inward currents recorded from all three cell lines. The DPDPE-sensitive K(+) current was inwardly rectifying and was inhibited by Ba(2+) but not TEA. DPDPE had no effect on delayed rectifier-, Ca(2+)-, and voltage-activated or A-type K(+) currents, recorded from GH(3)MORDOR cells. Ba(2+) attenuated the inhibition of [Ca(2+)](i) and prolactin release by DPDPE, whereas TEA had no effect, consistent with an involvement of K(IR) channels in these actions of the opioid.

mu Opioid receptor: role for the amino terminus as a determinant of ligand binding affinity

The importance of the amino-terminal domain of the mu opioid receptor (MOR) as a component of the high affinity ligand-binding pocket was evaluated. A deletion mutant lacking 64 amino acids from the amino-terminus of MOR (DeltaN64) was constructed and expressed in HEK 293 cells. The affinities of bremazocine and cyclazocine were similar for the truncated and full-length MORs. Affinities of the mu receptor antagonist, naloxone, and the mu receptor agonist, morphine, were decreased 3.5-fold and 6-fold, respectively, for the truncated receptor relative to the wild-type MOR. Similarly, the affinities of the opioid peptide agonists, DAMGO (Tyr-D-Ala-Gly-MePhe-Gly-ol), beta-endorphin, and DADL (Tyr-D-Ala-Gly-Phe-D-Leu), for the DeltaN64 receptor were decreased from 3- to 8-fold as a result of the deletion. In contrast, the affinities of the alkaloid agonists, methadone and fentanyl, and the peptide agonists, endomorphin 1 and endomorphin 2, for the truncated receptor relative to MOR were reduced dramatically by 20- to 60-fold. MOR is glycosylated when expressed in HEK 293 cells; however, analysis of N-glycosidase F-treated membranes indicated that N-glycan chains within the amino-terminal domain of MOR do not contribute significantly to ligand affinities. These results indicate that amino acid residues within the amino-terminal domain of MOR play a crucial role in the composition of the binding pocket for a select group of agonists.

Immunoprecipitation of high-affinity, guanine nucleotide-sensitive, solubilized mu-opioid receptors from rat brain: coimmunoprecipitation of the G proteins G(alpha o), G(alpha i1), and G(alpha i3)

Antibodies directed against the C-terminal and the N-terminal regions of the mu-opioid receptor were generated to identify the G proteins that coimmunoprecipitate with the mu receptor. Two fusion proteins were constructed: One contained the 50 C-terminal amino acids of the mu receptor, and the other contained 61 amino acids near the N terminus of the receptor. Antisera directed against both fusion proteins were capable of immunoprecipitating approximately 70% of solubilized rat brain mu receptors as determined by [3H][D-Ala2,N-Me-Phe4,Gly-ol5]-enkephalin ([3H]DAMGO) saturation binding. The material immunoprecipitated with both of the antisera was recognized as a broad band with a molecular mass between 60 and 75 kDa when screened in a western blot. Guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) had an EC50 of 0.4 nM in diminishing [3H]DAMGO binding to the immunoprecipitated pellet. The ratio of G proteins to mu receptors in the immunoprecipitated material was 1:1. When the material immunoprecipitated with affinity-purified antibody was screened for the presence of G protein a subunits, it was determined that G(alpha)o, G(alpha)i1, G(alpha)i3, and to a lesser extent G(alpha)i2, but not G(alpha)s or G(alpha)q11, were coimmunoprecipitated with the mu receptor. Inclusion of GTPgammaS during the immunoprecipitation process abolished the coimmunoprecipitation of G proteins.

Role for G protein-coupled receptor kinase in agonist-specific regulation of mu-opioid receptor responsiveness

The G protein-coupled mu-opioid receptor (mu OR) mediates the physiological effects of endogenous opioid peptides as well as the structurally distinct opioid alkaloids morphine and etorphine. An intriguing feature of mu OR signaling is the differential receptor trafficking and desensitization properties following activation by distinct agonists, which have been proposed as possible mechanisms related to opioid tolerance. Here we report that the ability of distinct opioid agonists to differentially regulate mu OR internalization and desensitization is related to their ability to promote G protein-coupled receptor kinase (GRK)-dependent phosphorylation of the mu OR. Although both etorphine and morphine effectively activate the mu OR, only etorphine elicits robust mu OR phosphorylation followed by plasma membrane translocation of beta-arrestin and dynamin-dependent receptor internalization. In contrast, corresponding to its inability to cause mu OR internalization, morphine is unable to either elicit mu OR phosphorylation or stimulate beta-arrestin translocation. However, upon the overexpression of GRK2, morphine gains the capacity to induce mu OR phosphorylation, accompanied by the rescue of beta-arrestin translocation and receptor sequestration. Moreover, overexpression of GRK2 also leads to an attenuation of morphine-mediated inhibition of adenylyl cyclase. These findings point to the existence of marked differences in the ability of different opioid agonists to promote mu OR phosphorylation by GRK. These differences may provide the molecular basis underlying the different analgesic properties of opioid agonists and contribute to the distinct ability of various opioids to induce drug tolerance.

Interaction of agonist peptide [3H]Tyr-D-Ala-Phe-Phe-NH2 with mu-opioid receptor in rat brain and CHO-mu/1 cell line

Opioid receptor binding properties of [3H]Tyr-D-Ala-Phe-Phe-NH2 (TAPP) were characterized in rat brain and Chinese hamster ovary (CHO) cells expressing the rat mu-receptor. In rat brain, [3H]TAPP labeled a single class of opioid sites with a dissociation constant (Kd) of 0.31 nM and maximal number of binding sites (Bmax) of 119 fmol/mg protein. In CHO-mu/1 cell membranes, the Kd and Bmax values were 0.78 nM and 1806 fmol/mg protein, respectively. Binding to rat brain was demonstrated to be pharmacologically identical to that obtained with CHO-mu/1 cell membranes and modulated by Na+ ions and guanine nucleotides. The high affinity and selectivity of [3H]TAPP together with its low non-specific binding make this radioligand a useful tool for labeling the native and cloned mu-opioid receptor.