Potentiated opioid analgesia in norepinephrine transporter knock-out mice

The involvement of alpha 2A-adrenoceptors in morphine analgesia, tolerance and withdrawal in mice

Alpha(2)-adrenoceptor agonists potentiate opioid analgesia and alleviate opioid withdrawal. The effects of two alpha(2)-adrenoceptor agonists, clonidine (2 mg/kg) and dexmedetomidine (20 and 100 microg/kg), and the alpha(1)-adrenoceptor antagonist prazosin (0.5 mg/kg) were tested on morphine analgesia, tolerance, and withdrawal in wild-type and alpha(2A)-adrenoceptor knock-out (KO) mice. Analgesia and tolerance were assessed with the tail-flick test. Withdrawal was precipitated with naloxone. Prazosin potentiated morphine analgesia equally in both genotypes. Clonidine and dexmedetomidine had no analgesic effects in alpha(2A)-adrenoceptor KO mice, but morphine analgesia and tolerance were similar in both genotypes. Alpha(2A)-Adrenoceptor KO mice exhibited 70% fewer naloxone-precipitated jumps than wild-type mice; weight loss was similar in both genotypes. The alpha(2)-adrenoceptor agonists reduced opioid withdrawal signs only in wild-type mice. We conclude that alpha(2A)-adrenoceptors are not directly involved in morphine analgesia and tolerance, and not critical for potentiation of morphine analgesia by prazosin, but that alpha(2A)-adrenoceptors modulate the expression of opioid withdrawal signs in mice.

Pharmacogenetics and Human Molecular Genetics of Opiate and Cocaine Addictions and Their Treatments

Opiate and cocaine addictions are major social and medical problems that impose a significant burden on society. Despite the size and scope of these problems, there are few effective treatments for these addictions. Methadone maintenance is an effective and most widely used treatment for opiate addiction, allowing normalization of many physiological abnormalities caused by chronic use of short-acting opiates. There are no pharmacological treatments for cocaine addiction. Epidemiological, linkage, and association studies have demonstrated a significant contribution of genetic factors to the addictive diseases. This article reviews the molecular genetics and pharmacogenetics of opiate and cocaine addictions, focusing primarily on genes of the opioid and monoaminergic systems that have been associated with or have evidence for linkage to opiate or cocaine addiction. This evidence has been marshalled either through identification of variant alleles that lead to functional alterations of gene products, altered gene expression, or findings of linkage or association studies. Studies of polymorphisms in the mu opioid receptor gene, which encodes the receptor target of some endogenous opioids, heroin, morphine, and synthetic opioids, have contributed substantially to knowledge of genetic influences on opiate and cocaine addiction. Other genes of the endogenous opioid and monoaminergic systems, particularly genes encoding dopamine beta-hydroxylase, and the dopamine, serotonin, and norepinephrine transporters have also been implicated. Variants in genes encoding proteins involved in metabolism or biotransformation of drugs of abuse and also of treatment agents are reviewed.

Spinal noradrenaline transporter inhibition by reboxetine and Xen2174 reduces tactile hypersensitivity after surgery in rats

Spinal noradrenaline (NA) released in response to noxious stimuli may play an important role in suppression of nociceptive transmission. Here, we investigated the efficacy of a competitive NA transporter inhibitor (reboxetine) and a noncompetitive NA transporter inhibitor peptide, Xen2174, isolated from the Pacific cone snail, to treat tactile hypersensitivity following paw incisional surgery. Male Sprague-Dawley rats were anesthetized, an incision of the plantar aspect of the hind paw was performed, and withdrawal threshold to von Frey filaments near the surgical site determined. Reboxetine (0.5-5 microg) and Xen2174 (0.3-100 microg) increased withdrawal threshold when injected 24h after paw incision, with a peak effect at 15-60 min, for Xen2174, an ED50 value of 0.64 microg. Administration of Xen2174 (3-30 microg) 15 min before incision also reduced hypersensitivity in a dose-dependent manner. Withdrawal threshold after the single 30 microg dose was greater than vehicle control even at 2, 3, and 5 days after incision. Doses <or=30 microg did not alter spontaneous behavior. The anti-hypersensitivity effect of 10 microg of Xen2174 was totally blocked by the alpha2-adrenoceptor antagonist, idazoxan, and partially blocked by the muscarinic antagonist, atropine. These data suggest that selective NA transporter inhibition suppresses post-incisional hypersensitivity through a different mechanism from that of neuropathic pain, since we previously reported that reversal of hypersensitivity by intrathecal clonidine, an alpha2-adrenoceptor agonist, following spinal nerve ligation is completely blocked by intrathecal atropine. Finally, these data suggest that intrathecal administration of Xen2174 at the time of spinal anesthesia might produce postoperative analgesia in humans.

Gap junctions in developing neocortex: a review

Several independent lines of evidence indicate that gap junctional coupling is widespread and functionally important in early cortical development. The extensive expression of many connexins, the clusters of coupled neuroblasts or neurons revealed by tracer injections or the functional coactivation of neurons shown by calcium imaging can be seen as pieces of the same puzzle. At the same time, there is a lack of electrophysiological experiments at early developmental stages that directly demonstrate coupling between pairs of cells. The function of this gap junctional coupling is still mysterious, though it may be involved in controlling neurogenesis, differentiation or the formation of columnar microcircuits.

Opioid receptors

Opioid receptors belong to the large superfamily of seven transmembrane-spanning (7TM) G protein-coupled receptors (GPCRs). As a class, GPCRs are of fundamental physiological importance mediating the actions of the majority of known neurotransmitters and hormones. Opioid receptors are particularly intriguing members of this receptor family. They are activated both by endogenously produced opioid peptides and by exogenously administered opiate compounds, some of which are not only among the most effective analgesics known but also highly addictive drugs of abuse. A fundamental question in addiction biology is why exogenous opioid drugs, such as morphine and heroin, have a high liability for inducing tolerance, dependence, and addiction. This review focuses on many aspects of opioid receptors with the aim of gaining a greater insight into mechanisms of opioid tolerance and dependence.

Comparative study of the effects of desipramine and reboxetine on locus coeruleus neurons in rat brain slices

Several studies have suggested that the locus coeruleus may play an important role in the pathophysiology of depression. The aim of this study was to characterize, using single-unit extracellular recordings, the in vitro effects of the noradrenaline reuptake inhibitors desipramine and reboxetine, on locus coeruleus neurons from control rats and from those chronically treated with desipramine. Bath application of desipramine (1-100 microM) and reboxetine (0.1-10 microM) decreased the firing rate of locus coeruleus neurons in a concentration-dependent manner and the alpha(2)-adrenoceptor antagonist RX 821002 (10 microM) reversed these effects. In addition, reserpine (5 mg/kg, 3 h before the experiment) almost completely blocked the inhibitory effect of desipramine. Both drugs (1 microM desipramine and 0.1 microM reboxetine) potentiated the inhibitory effect of noradrenaline (10 microM). A 7-day treatment with desipramine (3 mg/kg/12 h, i.p.) caused a decrease in sensitivity to the alpha(2)-adrenoceptor agonist bromoxidine (EC(50) increased by 3.3-fold), but not to noradrenaline or reboxetine. In contrast, this treatment potentiated the inhibitory effect of desipramine with respect to control. Moreover, 14-day treatment with desipramine (3 mg/kg/12 h, i.p.) or reboxetine (10 mg/kg/12 h, i.p.) also potentiated the in vitro effect of desipramine without modifying the in vitro effect of reboxetine. These results show that desipramine and reboxetine modulate the activity of locus coeruleus neurons by noradrenaline acting on alpha(2)-adrenoceptors, and reveal that alpha(2)-adrenoceptor-independent mechanisms may also underlie the action of noradrenaline uptake inhibitors.

Regulated Internalization and Phosphorylation of the Native Norepinephrine Transporter in Response to Phorbol Esters

The effects of norepinephrine in the brain and periphery are terminated primarily by active reuptake of the catecholamine via cocaine- and amphetamine-sensitive norepinephrine transporters (NETs). Activation of protein kinase C (PKC) down-regulates NET by sequestering it from the plasma membrane, although the underlying mechanism is not yet known. Previously, we showed robust expression of endogenous NETs in rat placental trophoblasts (Jayanthi, L. D., Vargas, G., and DeFelice, L. J. (2002) Br. J. Pharmacol. 135, 1927-1934). Here we report a significant reduction in native NET function and surface expression in these cells following phorbol ester (beta-PMA) treatment. The beta-PMA-mediated down-regulation of NET occurs by a rapid sequestration of NETs from the plasma membrane and is calcium-independent. Reversible biotinylation experiments revealed a significant enhancement of NET endocytosis following beta-PMA treatment. Chemical treatments and expression of dominant negative mutants of dynamin 1 and 2 failed to prevent the beta-PMA effect, suggesting a clathrin-independent pathway. In contrast, treatment with the cholesterol-disrupting agent filipin, which blocks caveolae/lipid raft-mediated internalization, completely blocked the beta-PMA-mediated NET sequestration. Discontinuous sucrose density gradient centrifugation revealed NET in the lipid raft fractions. Following beta-PMA treatment, there was reduced NET levels in the lipid raft fractions suggesting that cholesterol-rich lipid rafts mediate PKC-triggered NET internalization. Metabolic labeling and immunoprecipitation studies revealed that NET phosphorylation is stimulated severalfold by PKC activation and protein phosphatase 1/2A inhibition. Together, these findings demonstrate for the first time that in trophoblasts (i) PKC activation regulates NET function and surface expression by an enhanced internalization process that is lipid raft-mediated and (ii) PKC and protein phosphatase(s) modulation regulates NET phosphorylation.

No evidence of linkage or association between the norepinephrine transporter (NET) gene MnlI polymorphism and adult ADHD

Attention deficit hyperactivity disorder (ADHD) is a prevalent psychiatric condition in children and follow-up studies have indicated that one to two thirds of patients continue to suffer from ADHD during late adolescence and adulthood. The mechanism of action of the new selective noradrenergic reuptake inhibitor, atomoxetine, efficacious in the treatment of ADHD symptoms, suggests that the norepinephrine transporter (NET) may be involved in ADHD. The aim of this study was to investigate for the presence of linkage disequilibrium between the MnlI RFLP in the NET gene and adult ADHD in a sample of nuclear families. The MnlI polymorphism was typed in 128 trios and analyzed using the transmission disequilibrium test (TDT). There was no preferential transmission of either allele (chi(2) = 0.209, df = 1, P = 0.647).

Antinociceptive effects of tricyclic antidepressants and their noradrenergic metabolites

This study evaluates the antinociceptive effect of several tricyclic antidepressants in four nociceptive tests which employ either thermal (hot plate and tail flick tests) or chemical (formalin and acetic acid tests) stimuli. Forced swimming test was also performed as a model of depression and an activity test was also performed. Mixed antidepressants in current clinical use: amitriptyline, imipramine and clorimipramine and their respective main secondary metabolites which preferentially inhibit noradrenaline reuptake: nortriptyline, desipramine and desmethylclorimipramine, were tested (2.5-20 mg/kg, i.p.) in mice. The results show a stronger antinociceptive effect in chemical tests induced by all the drugs, compared with thermal tests. The doses needed to produce antinociception were lower than those inducing an antidepressive effect, both effects being mutually independent. The overall results show that preferentially noradrenergic tricyclics induced an antinociceptive effect comparable with that of mixed tricyclics, indicating that noradrenaline reuptake plays an important role in tricyclic-induced antinociception.

Monoamine transporters: from genes to behavior

Modulation of fast neurotransmission by monoamines is critically involved in numerous physiological functions and pathological conditions. Plasma membrane monoamine transporters provide one of the most efficient mechanisms controlling functional extracellular monoamine concentrations. These transporters for dopamine (DAT), serotonin (SERT), and norepinephrine (NET), which are expressed selectively on the corresponding neurons, are established targets of many psychostimulants, antidepressants, and neurotoxins. Recently, genetic animal models with targeted disruption of these transporters have become available. These mice have provided opportunities to investigate the functional importance of transporters in homeostatic control of monoaminergic transmission and to evaluate, in an in vivo model system, their roles in physiology and pathology. The use of these mice as test subjects has been helpful in resolving several important issues on specificity and mechanisms of action of certain pharmacological agents. In the present review, we summarize recent advances in understanding the physiology and pharmacology of monoamine transporters gained in mice with targeted genetic deletion of DAT, SERT, and NET.

Endogenous opioid analgesia in hemorrhagic shock

BACKGROUND

Hemorrhagic shock produces an immediate activation of the autonomic nervous system and endogenous opioid pathways. Our studies have demonstrated that endogenous opioid activation aggravates the hemodynamic and inflammatory responses to shock. However, it is unclear whether endogenous opioid activation is triggered by noxious stimuli and furthermore whether it produces analgesia.

METHODS

Experiments were conducted in chronically catheterized, conscious, unrestrained, nonheparinized, male, Sprague-Dawley rats subjected to fixed pressure hemorrhage. Blood samples were obtained for determinations of circulating beta-endorphin and substance P. Analgesia was measured using the tail-flick response to a noxious stimulus before and during hemorrhage. The contribution of sensory neurons to eliciting the neuroendocrine, opioid, and inflammatory responses to hemorrhage was investigated in capsaicin-treated animals.

RESULTS

Hemorrhagic shock produced marked naltrexone-sensitive analgesia without significant modulation of substance P. Peripheral sensory denervation did not alter the hemodynamic, neuroendocrine, or inflammatory responses to shock.

CONCLUSION

Endogenous opioid activation during shock produces analgesia. Sensory neuron activation appears to have limited effect on shock-induced hemodynamic and proinflammatory responses. Furthermore, these results suggest that the activation of neuroendocrine and opioid pathways during shock is not likely to be a response to noxious stimuli.

Endogenous opioids support the spinal inhibitory action of an alpha 2-adrenoceptor agonist in the decerebrated, spinalised rabbit

The present study examined the possible contribution of endogenous opioids to inhibition of spinal reflexes by an alpha(2)-adrenoceptor agonist. In rabbits decerebrated and spinalised under halothane/nitrous oxide anaesthesia, the selective alpha(2)-adrenoceptor agonist dexmedetomidine (3-30 microg intrathecal) induced significant decreases in short- and long-latency reflex responses evoked in medial gastrocnemius (MG) motoneurones by stimulation of the sural nerve. After recovery from dexmedetomidine, the mu-opioid receptor antagonist beta-funaltrexamine (beta-FNA; 100 microg intrathecal) significantly enhanced short-latency but not long-latency MG reflex responses. After beta-FNA, inhibition of all reflexes by dexmedetomidine was significantly weaker than in the control state, whereas the cardiovascular actions of dexmedetomidine were unaffected. These data confirm that activation of spinal alpha(2)-adrenoceptors depresses MG reflexes evoked by all groups of sural nerve afferent fibres, and shows that endogenous opioid tone supports the inhibitory action of alpha(2) agonists, possibly by a synergistic interaction in the spinal cord.

Genetic animal models of anxiety

The focus of this review is on progress achieved in identifying specific genes conferring risk for anxiety disorders through the use of genetic animal models. We discuss gene-finding studies as well as those manipulating a candidate gene. Both human and animal studies thus far support the genetic complexity of anxiety. Clinical manifestations of these diseases are likely related to multiple genes. While different anxiety disorders and anxiety-related traits all appear to be genetically influenced, it has been difficult to ascertain genetic influences in common. Mouse studies have provisionally mapped several loci harboring genes that affect anxiety-related behavior. The growing array of mutant mice is providing valuable information about how genes and environment interact to affect anxious behavior via multiple neuropharmacological pathways. Classical genetic methods such as artificial selection of rodents for high or low anxiety are being employed. Expression array technologies have as yet not been employed, but can be expected to implicate novel candidates and neurobiological pathways.

Influence of prazosin and clonidine on morphine analgesia, tolerance and withdrawal in mice

Rapid development of tolerance and dependence limits the usefulness of morphine in long-term treatment. We examined the effects of clonidine (alpha(2)-adrenoceptor agonist) and prazosin (alpha(1)-adrenoceptor antagonist) on morphine analgesia, tolerance and withdrawal. Morphine tolerance was induced using a 3-day cumulative twice-daily dosing regimen with s.c. doses up to 120 mg/kg. Tolerance was assessed on day 4, as loss of the antinociceptive effect of a test dose of morphine (5 mg/kg). After 10 h, morphine withdrawal was precipitated with naloxone (1 mg/kg). Prazosin had no analgesic effect alone but dose-dependently potentiated morphine analgesia in morphine-naive mice. Another alpha(1)-adrenoceptor antagonist, corynanthine, had similar effects. Prazosin also increased the analgesic potency of the morphine test dose in morphine-tolerant mice. Naloxone-precipitated vertical jumping was not affected, but weight loss was reduced by prazosin. Acutely administered clonidine potentiated morphine analgesia and alleviated opioid withdrawal signs, as expected. We conclude that in addition to the already established involvement of alpha(2)-adrenoceptors in opioid actions, also alpha(1)-adrenoceptors have significant modulatory role in opioid analgesia and withdrawal.

Autonomic control after blockade of the norepinephrine transporter: a model of orthostatic intolerance

Orthostatic intolerance is a debilitating syndrome characterized by tachycardia on assumption of upright posture. The norepinephrine (NE) transporter (NET) has been implicated in a genetic form of the disorder. We assessed the combined central and peripheral effects of pharmacological NET blockade on cardiovascular regulation and baroreflex sensitivity in rats. NE reuptake was blocked chronically in female Sprague-Dawley rats by the NET antagonist desipramine (DMI). Treated animals demonstrated an elevated supine heart rate, reduced tyramine responsiveness, and a reduced plasma ratio of the intraneuronal NE metabolite dihydroxyphenylglycol relative to NE, all of which are consistent with observations in human NET deficiency. Spectral analysis revealed a dramatic decrease in low-frequency spectral power after DMI that was consistent with decreased sympathetic outflow. Stimulation of the baroreflex with the vasodilator nitroprusside revealed an attenuated tachycardia in DMI-treated animals. This indicated that the DMI-induced sympathoinhibitory effects of increased NE in the brain stem predominates over the functional elevation of NE stimulation of peripheral targets. Thus attenuated baroreflex function and reduced sympathetic outflow may contribute to the orthostatic intolerance of severe NET deficiency.

Differential mechanisms of morphine antinociceptive tolerance revealed in (beta)arrestin-2 knock-out mice

Morphine induces antinociception by activating mu opioid receptors (muORs) in spinal and supraspinal regions of the CNS. (Beta)arrestin-2 (beta)arr2), a G-protein-coupled receptor-regulating protein, regulates the muOR in vivo. We have shown previously that mice lacking (beta)arr2 experience enhanced morphine-induced analgesia and do not become tolerant to morphine as determined in the hot-plate test, a paradigm that primarily assesses supraspinal pain responsiveness. To determine the general applicability of the (beta)arr2-muOR interaction in other neuronal systems, we have, in the present study, tested (beta)arr2 knock-out ((beta)arr2-KO) mice using the warm water tail-immersion paradigm, which primarily assesses spinal reflexes to painful thermal stimuli. In this test, the (beta)arr2-KO mice have greater basal nociceptive thresholds and markedly enhanced sensitivity to morphine. Interestingly, however, after a delayed onset, they do ultimately develop morphine tolerance, although to a lesser degree than the wild-type (WT) controls. In the (beta)arr2-KO but not WT mice, morphine tolerance can be completely reversed with a low dose of the classical protein kinase C (PKC) inhibitor chelerythrine. These findings provide in vivo evidence that the muOR is differentially regulated in diverse regions of the CNS. Furthermore, although (beta)arr2 appears to be the most prominent and proximal determinant of muOR desensitization and morphine tolerance, in the absence of this mechanism, the contributions of a PKC-dependent regulatory system become readily apparent.

General anesthetic actions on norepinephrine, dopamine, and gamma-aminobutyric acid transporters in stably transfected cells

The effects of general anesthetics on neurotransmitter uptake by plasma membrane transporters are controversial. We analyzed the effects of representative volatile and IV general anesthetics on recombinant transporters for norepinephrine (human NET), dopamine (rat DAT), or gamma-aminobutyric acid (rat GAT-1) stably expressed in a porcine kidney cell line (LLC-PK(1)). This approach avoids complicating factors associated with neuronal preparations, such as the involvement of multiple transporters and the indirect effects of membrane potential. At clinical concentrations, human NET was inhibited only by halothane (50% inhibitory concentration [IC(50)] = 0.54 mM), rat DAT was sensitive to both halothane and isoflurane (IC(50) = 0.60 and 0.64 mM, respectively), and rat GAT-1 was insensitive to both volatile anesthetics. Human NET was inhibited in a dose-dependent fashion by propofol (IC(50) = 41 micro M), ketamine (IC(50) = 150 micro M), and etomidate (IC(50) > 200 micro M), but not by pentobarbital. Only propofol inhibited NET at a clinically relevant concentration (5 micro M). Rat DAT was inhibited in a dose-dependent fashion by propofol (IC(50) = 120 micro M), etomidate (IC(50) = 100 micro M), and ketamine (IC(50) = 210 micro M), but not by pentobarbital. None of these anesthetics was predicted to inhibit DAT at concentrations that produce anesthesia. Propofol inhibited rat GAT-1, but only at the largest concentration tested. General anesthetics have drug- and subtype-selective actions on neurotransmitter transporters. We conclude that effects on catecholamine, but not gamma-aminobutyric acid, transporters may contribute to secondary synaptic actions of certain anesthetics but are unlikely to be essential to their anesthetic properties.

IMPLICATIONS

Previous studies have implicated neurotransmitter transporters as targets for general anesthetic effects on synaptic transmission. Recombinant transporters for norepinephrine and dopamine were sensitive to certain volatile and IV anesthetics, whereas gamma-aminobutyric acid transporters were insensitive. These anesthetic- and neurotransmitter-specific effects may underlie some of the secondary effects of general anesthetics.

Intravenous cocaine induced-activity and behavioural sensitization in norepinephrine-, but not dopamine-transporter knockout mice

Previously, it was reported that both norepinephrine transporter (NET) and dopamine transporter (DAT) knockout (KO) mice were sensitive to the reinforcing effects of cocaine. However, assessing the locomotor-stimulant effects of cocaine in these subjects has proven difficult due to significant differences in their baseline activity compared to wild-type controls. The present studies were designed to clarify the role of NET and DAT in the stimulant effects of acute and repeated cocaine utilizing these knockout mice, and thereby assess the role of these substrates in the locomotor stimulant effects of cocaine. Mice were habituated to the test environment for sufficient time to ensure equal baselines at the time of cocaine administration. Mice then received cocaine (3-25 mg/kg) intravenously according to a within-session cumulative dose-response design. Cocaine dosing was repeated at 48-h intervals for four sessions to assess behavioural sensitization. NET-KO mice exhibited a reduced response to acute cocaine administration compared to wild-type (WT) controls. However, comparable sensitization developed in NET-KO and WT mice. The DAT-KO and DAT-heterozygote (HT) mice displayed no locomotor activation following either acute or repeated cocaine administration. These data suggest a role for the NET in the acute response to cocaine, but no involvement in sensitization to cocaine. In contrast, DAT appears to be necessary for both the acute locomotor response to cocaine and the subsequent development of sensitization. In addition to existing data concerning the reinforcing effects of cocaine in DAT-KO mice, these data suggest a dissociation between the reinforcing and locomotor stimulant effects of cocaine.

Monoamine transporter pharmacology and mutant mice

Monoamine transporters, such as the dopamine transporter, 5-HT transporter and noradrenaline transporter, in the plasma membrane provide effective control over the intensity of monoamine-mediated signaling by recapturing neurotransmitters released by presynaptic neurons. These proteins represent established targets for several psychotropic drugs, including psychostimulants and antidepressants; however, important issues regarding the selectivity and mechanisms of action of these drugs remain unresolved. Although monoamine transporter knockout mice have profound changes in neurotransmission, they provide useful in vivo models to analyze the effects of psychotropic drugs. In this review, we summarize recent insights into the pharmacology of psychotropic drugs using mice in which the genes encoding these transporters have been deleted.

Genetic manipulation of noradrenergic neurons

The neurotransmitter norepinephrine has been the focus of intense investigation for nearly a century. With advances in technology come novel approaches for testing hypotheses about the physiological roles of norepinephrine and the genes involved in norepinephrine (NE) biosynthesis, metabolism, and noradrenergic signaling. Homologous recombination techniques, which generate mice deficient in specific gene products, aid the integrated physiologist and pharmacologist in the evaluation of protein function. Mouse models lacking proteins involved in NE biosynthesis or metabolism provide tools to expand the knowledge previously gleaned from pharmacologic studies. Removal of the biosynthetic enzymes tyrosine hydroxylase and dopamine-beta-hydroxylase yield animals deficient in norepinephrine and have been used to further examine the role of NE in diverse physiologic roles. Complete removal of the vesicular monoamine transporter has demonstrated that mobilizing neurotransmitters to vesicles is required for animal survival. Lastly, the generation of animals in which the ability to remove NE from the synapse is impaired (norepinephrine transporter deficiency and extraneuronal monoamine transporter deficiency) and in which the enzymes responsible for the metabolism of NE have been removed (catechol-O-methyltransferase and monoamine oxidase) has facilitated the study of the long-term physiological consequences of altered NE homeostasis.

Descending control of pain

Upon receipt in the dorsal horn (DH) of the spinal cord, nociceptive (pain-signalling) information from the viscera, skin and other organs is subject to extensive processing by a diversity of mechanisms, certain of which enhance, and certain of which inhibit, its transfer to higher centres. In this regard, a network of descending pathways projecting from cerebral structures to the DH plays a complex and crucial role. Specific centrifugal pathways either suppress (descending inhibition) or potentiate (descending facilitation) passage of nociceptive messages to the brain. Engagement of descending inhibition by the opioid analgesic, morphine, fulfils an important role in its pain-relieving properties, while induction of analgesia by the adrenergic agonist, clonidine, reflects actions at alpha(2)-adrenoceptors (alpha(2)-ARs) in the DH normally recruited by descending pathways. However, opioids and adrenergic agents exploit but a tiny fraction of the vast panoply of mechanisms now known to be involved in the induction and/or expression of descending controls. For example, no drug interfering with descending facilitation is currently available for clinical use. The present review focuses on: (1) the organisation of descending pathways and their pathophysiological significance; (2) the role of individual transmitters and specific receptor types in the modulation and expression of mechanisms of descending inhibition and facilitation and (3) the advantages and limitations of established and innovative analgesic strategies which act by manipulation of descending controls. Knowledge of descending pathways has increased exponentially in recent years, so this is an opportune moment to survey their operation and therapeutic relevance to the improved management of pain.

Tramadol inhibits norepinephrine transporter function at desipramine-binding sites in cultured bovine adrenal medullary cells

Tramadol is a widely used analgesic, but its mode of action is not well understood. To study the effects of tramadol on norepinephrine transporter (NET) function, we assayed the effect of tramadol on [3H]-norepinephrine ([3H]-NE) uptake and [3H]-desipramine binding to plasma membranes isolated from bovine adrenal medulla. We then characterized [14C]-tramadol binding in cultured bovine adrenal medullary cells. Tramadol inhibited the desipramine-sensitive uptake of [3H]-NE by the cells in a concentration-dependent manner (50% inhibitory concentration = 21.5 +/- 6.0 microM). Saturation analysis revealed that tramadol increased the apparent Michaelis constant of [3H]-NE uptake without changing the maximal velocity, indicating that inhibition occurred via competition for the NET (inhibition constant, K(i) = 13.7 microM). Tramadol inhibited the specific binding of [3H]-desipramine to plasma membranes. Scatchard analysis of [3H]-desipramine binding revealed that tramadol increased the apparent dissociation constant (K(d)) for binding without altering maximal binding, indicating competitive inhibition (K(i) = 11.2 microM). The binding of [14C]-tramadol to the cells was specific and saturable, with a K(d) of 18.1 +/- 2.4 microM. These findings indicate that tramadol competitively inhibits NET function at desipramine-binding sites.

IMPLICATIONS

Tramadol competitively inhibits norepinephrine transporter function at desipramine-binding sites in the adrenal medullary cells and probably the noradrenergic neurons of the descending inhibitory system.

The NK1 receptor mediates both the hyperalgesia and the resistance to morphine in mice lacking noradrenaline

Noradrenaline (NA), a key neurotransmitter of the endogenous pain inhibitory system, acutely inhibits nociceptive transmission (including that mediated by substance P), potentiates opioid analgesia, and underlies part of the antinociceptive effects of the widely prescribed tricyclic antidepressants. Lesions of noradrenergic neurons, however, result in either normal or reduced pain behavior and variable changes in morphine antinociception, undermining the proposed association between noradrenaline (NA) deficiency and chronic pain (hyperalgesia). We used mice lacking the gene coding for dopamine beta-hydroxylase, the enzyme responsible for synthesis of NA from dopamine, to reexamine the consequences of a lack of NA on pain behavior. Here, we show that absence of NA in the central nervous system results in a substance P-mediated chronic hyperalgesia (decreased nociceptive threshold) to thermal, but not mechanical, stimuli and decreased efficacy of morphine. Contrary to studies that show substance P-mediated hyperalgesia requires intense stimuli, we found that even a mild stimulus is sufficient to evoke substance P-dependent hyperalgesia in the NA-deficient mice. Restoring central NA normalized both the nociceptive threshold and morphine efficacy, which is consistent with a tonic inhibitory effect of NA on nociceptive transmission. Unexpectedly, however, antagonists to the substance P receptor (the NK1 receptor) could achieve the same effect as NA replacement. We conclude that when unopposed by NA, substance P acting at the NK1 receptor causes chronic thermal hyperalgesia, and that the reduced opioid efficacy associated with a lack of NA is due to increased NK1-receptor stimulation.

Physiological genomics of antidepressant targets: keeping the periphery in mind

The plasma membrane transporters that clear extracellular serotonin (5-HT) and norepinephrine (NE), serotonin transporters (SERTs) and NE transporters (NETs), have received considerable attention over the past four decades because of their roles in amine neurotransmitter inactivation. In addition, they interact with many centrally active drugs, including multiple classes of antidepressants such as the serotonin-selective reuptake inhibitors, typified by fluoxetine (Prozac), and the more recently developed norepinephrine-selective transporter antagonists, such as reboxetine. The therapeutic utility of these agents supports biogenic amine theories of affective disorders and raises the question as to whether SERT and NET exhibit a functional genetic variation that could influence risk for behavioral disorders. Although evidence exists that a promoter polymorphism in SERT may influence behavioral states, this contention is not without complexity and its mechanism of action remains poorly understood. The identification of coding variants of NETs and SERTs would offer important opportunities to connect genotype to phenotype. However, given the limited frequency of transporter coding variations evident to date in general population surveys or in psychiatric genetic studies, the identification of informative functional variants of transporters will likely require refined phenotypes. In this regard, NET and SERT play critical roles in cardiovascular and gastrointestinal physiology, respectively. This perspective reviews recent human and mouse studies that suggest how peripheral autonomic phenotypes, linked to genetic disruption of NET and SERT function, can aid in the phenotypic segregation needed for advanced theories of biogenic amine dysfunction and pharmacogenetics.