A greater role for the norepinephrine transporter than the serotonin transporter in murine nociception

Duloxetine and Amitriptyline Reduce Neuropathic Pain by Inhibiting Primary Sensory Input to Spinal Dorsal Horn Neurons via α1- and α2-Adrenergic Receptors

Antidepressants, such as duloxetine and amitriptyline, are effective for treating patients with chronic neuropathic pain. Inhibiting norepinephrine and serotonin transporters at presynaptic terminals raises extracellular concentrations of norepinephrine. The α1- and α2-adrenergic receptor agonists inhibit glutamatergic input from primary afferent nerves to the spinal dorsal horn. However, the contribution of spinal α1- and α2-adrenergic receptors to the analgesic effect of antidepressants and associated synaptic plasticity remains uncertain. In this study, we showed that systemic administration of duloxetine or amitriptyline acutely reduced tactile allodynia and mechanical and thermal hyperalgesia caused by spinal nerve ligation in rats. In contrast, duloxetine or amitriptyline had no effect on nociception in sham rats. Blocking α1-adrenergic receptors with WB-4101 or α2-adrenergic receptors with yohimbine at the spinal level diminished the analgesic effect of systemically administered duloxetine and amitriptyline. Furthermore, intrathecal injection of duloxetine or amitriptyline similarly attenuated pain hypersensitivity in nerve-injured rats; the analgesic effect was abolished by intrathecal pretreatment with both WB-4101 and yohimbine. In addition, whole-cell patch-clamp recordings in spinal cord slices showed that duloxetine or amitriptyline rapidly inhibited dorsal root-evoked excitatory postsynaptic currents in dorsal horn neurons in nerve-injured rats but had no such effect in sham rats. The inhibitory effect of duloxetine and amitriptyline was abolished by the WB-4101 and yohimbine combination. Therefore, antidepressants attenuate neuropathic pain predominantly by inhibiting primary afferent input to the spinal cord via activating both α1- and α2-adrenergic receptors. This information helps the design of new strategies to improve the treatment of neuropathic pain.

Chemotherapy-Induced Peripheral Neuropathy: Epidemiology, Pathomechanisms and Treatment

PURPOSE

This review provides an update on the current clinical, epidemiological and pathophysiological evidence alongside the diagnostic, prevention and treatment approach to chemotherapy-induced peripheral neuropathy (CIPN).

FINDINGS

The incidence of cancer and long-term survival after treatment is increasing. CIPN affects sensory, motor and autonomic nerves and is one of the most common adverse events caused by chemotherapeutic agents, which in severe cases leads to dose reduction or treatment cessation, with increased mortality. The primary classes of chemotherapeutic agents associated with CIPN are platinum-based drugs, taxanes, vinca alkaloids, bortezomib and thalidomide. Platinum agents are the most neurotoxic, with oxaliplatin causing the highest prevalence of CIPN. CIPN can progress from acute to chronic, may deteriorate even after treatment cessation (a phenomenon known as coasting) or only partially attenuate. Different chemotherapeutic agents share both similarities and key differences in pathophysiology and clinical presentation. The diagnosis of CIPN relies heavily on identifying symptoms, with limited objective diagnostic approaches targeting the class of affected nerve fibres. Studies have consistently failed to identify at-risk cohorts, and there are no proven strategies or interventions to prevent or limit the development of CIPN. Furthermore, multiple treatments developed to relieve symptoms and to modify the underlying disease in CIPN have failed.

IMPLICATIONS

The increasing prevalence of CIPN demands an objective approach to identify at-risk patients in order to prevent or limit progression and effectively alleviate the symptoms associated with CIPN. An evidence base for novel targets and both pharmacological and non-pharmacological treatments is beginning to emerge and has been recognised recently in publications by the American Society of Clinical Oncology and analgesic trial design expert groups such as ACTTION.

Effects of repeated treatment with monoamine-transporter-inhibitor antidepressants on pain-related depression of intracranial self-stimulation in rats

RATIONALE

Synaptic neurotransmission with dopamine (DA), norepinephrine (NE), and serotonin (5-HT) is terminated primarily by reuptake into presynaptic terminals via the DA, NE, and 5-HT transporters (DAT/NET/SERT, respectively). Monoamine transporter inhibitors constitute one class of drugs used to treat both depression and pain, and therapeutic effects by these compounds often require repeated treatment for days or weeks.

OBJECTIVES

The present study compared antinociceptive effects produced by repeated treatment with monoamine transporter inhibitors in a preclinical assay of pain-related depression of positively reinforced operant responding.

METHODS

Adult Sprague-Dawley rats equipped with microelectrodes targeting a brain-reward area responded for pulses of electrical brain stimulation in an intracranial self-stimulation (ICSS) procedure. Intraperitoneal injection of dilute lactic acid served as a noxious stimulus that repeatedly depressed ICSS and also produced weight loss during 7 days of repeated acid administration.

RESULTS

Acid-induced depression of both ICSS and body weight were completely blocked by repeated pretreatment with the nonsteroidal anti-inflammatory drug ketorolac. The DAT-selective inhibitor bupropion also fully blocked acid-induced ICSS depression and weight loss throughout all 7 days of treatment. The NET-selective inhibitor nortriptyline and the SERT-selective inhibitor citalopram were generally less effective, but both drugs blocked acid-induced ICSS depression by the end of the 7-day treatment. Acid-induced depression of ICSS and body weight were not blocked by the kappa opioid receptor (KOR) agonist U69593 or the KOR antagonist norbinaltorphimine.

CONCLUSIONS

These results support effectiveness of bupropion to alleviate signs of pain-related behavioral depression in rats and further suggest that nortriptyline and citalopram produce significant but less reliable effects.

Prevention and Treatment for Chemotherapy-Induced Peripheral Neuropathy: Therapies Based on CIPN Mechanisms

BACKGROUND

Chemotherapy-induced peripheral neuropathy (CIPN) is a progressive, enduring, and often irreversible adverse effect of many antineoplastic agents, among which sensory abnormities are common and the most suffering issues. The pathogenesis of CIPN has not been completely understood, and strategies for CIPN prevention and treatment are still open problems for medicine.

OBJECTIVES

The objective of this paper is to review the mechanism-based therapies against sensory abnormities in CIPN.

METHODS

This is a literature review to describe the uncovered mechanisms underlying CIPN and to provide a summary of mechanism-based therapies for CIPN based on the evidence from both animal and clinical studies.

RESULTS

An abundance of compounds has been developed to prevent or treat CIPN by blocking ion channels, targeting inflammatory cytokines and combating oxidative stress. Agents such as glutathione, mangafodipir and duloxetine are expected to be effective for CIPN intervention, while Ca/Mg infusion and venlafaxine, tricyclic antidepressants, and gabapentin display limited efficacy for preventing and alleviating CIPN. And the utilization of erythropoietin, menthol and amifostine needs to be cautious regarding to their side effects.

CONCLUSIONS

Multiple drugs have been used and studied for decades, their effect against CIPN are still controversial according to different antineoplastic agents due to the diverse manifestations among different antineoplastic agents and complex drug-drug interactions. In addition, novel therapies or drugs that have proven to be effective in animals require further investigation, and it will take time to confirm their efficacy and safety.

5-HT3 receptor signaling in serotonin transporter-knockout rats: a female sex-specific animal model of visceral hypersensitivity

The irritable bowel syndrome (IBS) is a functional gastrointestinal motor and visceral sensation disorder that is more common in women than men. Female serotonin transporter (SERT)-gene knockout (KO) rats exhibit hypersensitivity to colorectal balloon distention (CRD) that mimics colonic hypersensitivity occurring in female IBS patients. Alosetron (5-HT₃ receptor antagonist) is used to treat diarrhea-predominant IBS in female patients. Other 5-HT₃ receptor antagonists are ineffective at treating IBS symptoms. The visceromotor response (VMR) to CRD in SERT-KO and wild-type (WT) rats was measured following subcutaneous (sc), intracerobroventricular (icv), or intrathecal (it) treatment with 5-HT₃ receptor antagonists and an agonist. Alosetron (sc) and granisetron (antagonists) caused a paradoxical increase in the VMR to CRD in SERT-KO female rats. Alosetron (sc) increased the VMR to CRD in WT male rats. Alosetron (it) increased the VMR to CRD in SERT-KO female rats only, and the 5-HT₃ receptor agonist SR-52772 increased the VMR to CRD in SERT-KO male rats. Depletion of spinal 5-HT using 5,7-dihydroxytryptamine prevented the increase in VMR to CRD in SERT-KO female and male rats treated it with alosetron and SR-52772, respectively. Alosetron (icv) did not affect the VMR to CRD in WT or KO female rats, but it increased the VMR in male SERT-KO but not WT male rats. These data suggest that 5-HT₃ receptor signaling at the dorsal spinal cord mediates visceral hypersensitivity in female SERT-KO rats. Such differences could facilitate development of sex-specific drug treatments for visceral pain. NEW & NOTEWORTHY We studied a model of female sex-specific visceral hypersensitivity using rats that had a loss of function of the serotonin transporter (SERT) caused by gene truncation. Female SERT-KO rats exhibited visceral hypersensitivity in response to colorectal balloon distention. We found that increased 5-HT signaling at dorsal spine 5-HT₃ receptors was responsible for visceral hypersensitivity in female but not male SERT-KO rats.

Tetrabenazine, a vesicular monoamine transporter-2 inhibitor, attenuates morphine-induced hyperlocomotion in mice through alteration of dopamine and 5-hydroxytryptamine turnover in the cerebral cortex

A single administration with morphine (30 mg/kg, i.p.) induced long-lasting hyperlocomotion in male ICR mice. Pretreatment of mice with a benzoquinolizine derivative tetrabenazine (TBZ; a reversible vesicular monoamine transporter-2 inhibitor) (1 mg/kg, i.p.) for 30 min significantly attenuated the hyperlocomotion induced by morphine, as compared with vehicle (saline)-pretreated mice. No significant change in locomotion was observed in mice pretreated with TBZ (1 mg/kg) alone. Mice treated with TBZ (1 mg/kg) showed an increase in immobility time in a tail suspension test, as compared with saline-treated mice. Pretreatment with TBZ (1 mg/kg) had no effect on morphine (1-30 mg/kg)-induced antinociception. TBZ at a dose of 1 mg/kg inhibited dopamine turnover (the ratio of 3,4-dihydroxyphenylacetic acid/dopamine) and 5-hydroxytryptamine turnover (the ratio of 5-hydroxyindoleacetic acid/5-hydroxytryptamine) in the cerebral cortex of mice challenged with morphine, as compared with saline-pretreated mice challenged with morphine. No stereotypic behavior was observed in mice treated with morphine (30 mg/kg) in combination with TBZ (1 mg/kg), so the reduction in observed locomotion did not result from induction of stereotypical behavior. Moreover, TBZ (1 and 2 mg/kg) pretreatment had no effect on stereotyped behaviors observed in mice challenged with 10 mg/kg methamphetamine. These data support the potential antagonistic actions of TBZ on some opiate actions, and encourage further exploration of potential effects on morphine reinforcement.

Tramadol's Inhibitory Effects on Sexual Behavior: Pharmacological Studies in Serotonin Transporter Knockout Rats

Tramadol is an effective pharmacological intervention in human premature ejaculation (PE). To investigate whether the inhibitory action of tramadol is primarily caused by its selective serotonin reuptake inhibitory (SSRI) effects we tested the dose–response effects of tramadol on sexual behavior in serotonin transporter wild type (SERT^+/+), heterozygous (SERT^+/-), and knockout (SERT^-/-) rats. To investigate whether other mechanisms contribute to the inhibitory effects, WAY100,635, a 5-HT_1A receptor antagonist and naloxone, a μ-opioid receptor antagonist, were tested on sexual behavior together with tramadol. Tramadol dose-dependently decreases sexual activity in all genotypes. In all studies, SERT^+/- rats did not respond differently from SERT^+/+ rats. WAY100,635 did not affect sexual activity in SERT^+/+, but dose-dependently reduced sexual activity in SERT^-/- rats. WAY100,635 (0.3 mg/kg) combined with tramadol (20 mg/kg) significantly reduced sexual activity in SERT^+/+ and even stronger in SERT^-/- rats. Naloxone did not affect sexual behavior consistently in SERT^+/+ rats, while in SERT^-/- rats all doses reduced ejaculation frequency mildly. Combining naloxone (20 mg/kg) and tramadol (20 mg/kg) decreased ejaculation frequencies in both genotypes. Interestingly, combining tramadol (20 mg/kg), WAY100,635 (0.3 mg/kg) and naloxone (20 mg/kg) led to complete elimination of all sexual activity in both SERT^+/+ and SERT^-/- rats. These findings suggest that the inhibitory effects of tramadol on male sexual behavior in SERT^+/+ rats is mainly, if not exclusively, due to SERT inhibition, with an important role for 5-HT_1A receptors, although influence of other systems (e.g., noradrenergic) cannot be excluded. As SSRIs exert their sexual inhibition after chronic administration, tramadol may be therapeutically attractive as “on demand” therapy for PE.

Opioid-induced inhibition of the human 5-HT and noradrenaline transporters in vitro: link to clinical reports of serotonin syndrome

Background and Purpose

Opioids may inhibit the 5‐HT transporter (SERT) and the noradrenaline transporter (NET). NET inhibition may contribute to analgesia, and SERT inhibition or interactions with 5‐HT receptors may cause serotonergic toxicity. However, the effects of different opioids on the human SERT, NET and 5‐HT receptors have not been sufficiently studied.

Experimental Approach

We determined the potencies of different opioids to inhibit the SERT and NET in vitro using human transporter‐transfected HEK293 cells. We also tested binding affinities at 5‐HT_1A, 5‐HT_2A and 5‐HT_2C receptors. Additionally, we assessed clinical cases of the serotonin syndrome associated with each opioid reported by PubMed and a World Health Organization database.

Key Results

Dextromethorphan, l(R)‐methadone, racemic methadone, pethidine, tramadol and tapentadol inhibited the SERT at or close to observed drug plasma or estimated brain concentrations in patients. Tapentadol was the most potent NET inhibitor. Pethidine, tramadol, l(R)‐methadone, racemic methadone, dextromethorphan and O‐desmethyltramadol also inhibited the NET. 6‐Monoacetylmorphine, buprenorphine, codeine, dihydrocodeine, heroin, hydrocodone, hydromorphone, morphine, oxycodone and oxymorphone did not inhibit the SERT or NET. Fentanyl interacted with 5‐HT_1A receptors and methadone, pethidine and fentanyl with 5‐HT_2A receptors, in the low micromolar range. Opioids most frequently associated with the serotonin syndrome are tramadol, fentanyl, tapentadol, oxycodone, methadone and dextromethorphan.

Conclusions and Implications

Some synthetic opioids interact with the SERT and NET at potentially clinically relevant concentrations. SERT inhibition by tramadol, tapentadol, methadone, dextromethorphan and pethidine may contribute to the serotonin syndrome. Direct effects on 5‐HT_1A and/or 5‐HT_2A receptors could be involved with methadone and pethidine.

Prior fear conditioning does not impede enhanced active avoidance in serotonin transporter knockout rats

Stressors can be actively or passively coped with, and adequate adaption of the coping response to environmental conditions can reduce their potential deleterious effects. One major factor influencing stress coping behaviour is serotonin transporter (5-HTT) availability. Abolishment of 5-HTT is known to impair fear extinction but facilitates acquisition of signalled active avoidance (AA), a behavioural task in which an animal learns to avoid an aversive stimulus that is predicted by a cue. Flexibility in adapting coping behaviour to the nature of the stressor shapes resilience to stress-related disorders. Therefore, we investigated the relation between 5-HTT expression and ability to adapt a learned coping response to changing environmental conditions. To this end, we first established and consolidated a cue-conditioned passive fear response in 5-HTT^-/- and wildtype rats. Next, we used the conditioned stimulus (CS) to signal oncoming shocks during signalled AA training in 5-HTT^-/- and wildtype rats to study their capability to acquire an active coping response to the CS following fear conditioning. Finally, we investigated the behavioural response to the CS in a novel environment and measured freezing, exploration and self-grooming, behaviours reflective of stress coping strategy. We found that fear conditioned and sham conditioned 5-HTT^-/- animals acquired the signalled AA response faster than wildtypes, while prior conditioning briefly delayed AA learning similarly in both genotypes. Subsequent exposure to the CS in the novel context reduced freezing and increased locomotion in 5-HTT^-/- compared to wildtype rats. This indicates that improved AA performance in 5-HTT^-/- rats resulted in a weaker residual passive fear response to the CS in a novel context. Fear conditioning prior to AA training did not affect freezing upon re-encountering the CS, although it did reduce locomotion in 5-HTT^-/- rats. We conclude that independent of 5-HTT signalling, prior fear conditioning does not greatly impair the acquisition of subsequent active coping behaviour when the situation allows for it. Abolishment of 5-HTT results in a more active coping style in case of novelty-induced fear and upon CS encounter in a novel context after AA learning.

Dezocine exhibits antihypersensitivity activities in neuropathy through spinal μ-opioid receptor activation and norepinephrine reuptake inhibition

Dezocine is the number one opioid painkiller prescribed and sold in China, occupying 44% of the nation’s opioid analgesics market today and far ahead of the gold-standard morphine. We discovered the mechanisms underlying dezocine antihypersensitivity activity and assessed their implications to antihypersensitivity tolerance. Dezocine, given subcutaneously in spinal nerve-ligated neuropathic rats, time- and dose-dependently produced mechanical antiallodynia and thermal antihyperalgesia, significantly increased ipsilateral spinal norepinephrine and serotonin levels, and induced less antiallodynic tolerance than morphine. Its mechanical antiallodynia was partially (40% or 60%) and completely (100%) attenuated by spinal μ-opioid receptor (MOR) antagonism or norepinephrine depletion/α₂-adrenoceptor antagonism and combined antagonism of MORs and α₂-adenoceptors, respectively. In contrast, antagonism of spinal κ-opioid receptors (KORs) and δ-opioid receptors (DORs) or depletion of spinal serotonin did not significantly alter dezocine antiallodynia. In addition, dezocine-delayed antiallodynic tolerance was accelerated by spinal norepinephrine depletion/α₂-adenoceptor antagonism. Thus dezocine produces antihypersensitivity activity through spinal MOR activation and norepinephrine reuptake inhibition (NRI), but apparently not through spinal KOR and DOR activation, serotonin reuptake inhibition or other mechanisms. Our findings reclassify dezocine as the first analgesic of the recently proposed MOR-NRI, and reveal its potential as an alternative to as well as concurrent use with morphine in treating pain.

Preclinical to clinical translation of CNS transporter occupancy of TD-9855, a novel norepinephrine and serotonin reuptake inhibitor

BACKGROUND

Monoamine reuptake inhibitors exhibit unique clinical profiles that reflect distinct engagement of the central nervous system (CNS) transporters.

METHODS

We used a translational strategy, including rodent pharmacokinetic/pharmacodynamic modeling and positron emission tomography (PET) imaging in humans, to establish the transporter profile of TD-9855, a novel norepinephrine and serotonin reuptake inhibitor.

RESULTS

TD-9855 was a potent inhibitor of norepinephrine (NE) and serotonin 5-HT uptake in vitro with an inhibitory selectivity of 4- to 10-fold for NE at human and rat transporters. TD-9855 engaged norepinephrine transporters (NET) and serotonin transporters (SERT) in rat spinal cord, with a plasma EC50 of 11.7 ng/mL and 50.8 ng/mL, respectively, consistent with modest selectivity for NET in vivo. Accounting for species differences in protein binding, the projected human NET and SERT plasma EC50 values were 5.5 ng/mL and 23.9 ng/mL, respectively. A single-dose, open-label PET study (4-20mg TD-9855, oral) was conducted in eight healthy males using the radiotracers [(11)C]-3-amino-4- [2-[(di(methyl)amino)methyl]phenyl]sulfanylbenzonitrile for SERT and [(11)C]-(S,S)-methylreboxetine for NET. The long pharmacokinetic half-life (30-40 h) of TD-9855 allowed for sequential assessment of SERT and NET occupancy in the same subject. The plasma EC50 for NET was estimated to be 1.21 ng/mL, and at doses of greater than 4 mg the projected steady-state NET occupancy is high (>75%). After a single oral dose of 20mg, SERT occupancy was 25 (±8)% at a plasma level of 6.35 ng/mL.

CONCLUSIONS

These data establish the CNS penetration and transporter profile of TD-9855 and inform the selection of potential doses for future clinical evaluation.

The mu-opioid receptor agonist/noradrenaline reuptake inhibition (MOR-NRI) concept in analgesia: the case of tapentadol

Tapentadol is a novel, centrally-acting analgesic drug, with an analgesic efficacy comparable to that of strong opioids such as oxycodone and morphine. Its high efficacy has been demonstrated in a range of animal models of acute and chronic, nociceptive, inflammatory, and neuropathic pain as well as in clinical studies with moderate to severe pain arising from a number of different etiologies. At the same time, a favorable gastrointestinal tolerability has been demonstrated in rodents and humans, and advantages over morphine regarding tolerance development and physical dependence were shown in animal studies. Furthermore, a low level of abuse and diversion is beginning to emerge from first post-marketing data. Tapentadol acts as a μ-opioid receptor (MOR) agonist and noradrenaline reuptake inhibitor (NRI). Both mechanisms of action have been shown to contribute to the analgesic activity of tapentadol and to produce analgesia in a synergistic manner, such that relatively moderate activity at the two target sites (MOR and noradrenaline reuptake transporter) is sufficient to produce strong analgesic effects. It has been suggested that tapentadol is the first representative of a proposed new class of analgesics, MOR-NRI. This review presents the evidence that has led to this suggestion, and outlines how the pharmacology of tapentadol can explain its broad analgesic activity profile and high analgesic potency as well as its favorable tolerability.

Impaired noradrenaline homeostasis in rats with painful diabetic neuropathy as a target of duloxetine analgesia

BACKGROUND

Painful diabetic neuropathy (PDN) is a serious complication of diabetes mellitus that affects a large number of patients in many countries. The molecular mechanisms underlying the exaggerated nociception in PDN have not been established. Recently, duloxetine (DLX), a serotonin and noradrenaline re-uptake inhibitor, has been recommended as one of the first-line treatments of PDN in the United States Food and Drug Administration, the European Medicines Agency and the Japanese Guideline for the Pharmacologic Management of Neuropathic pain. Because selective serotonin re-uptake inhibitors show limited analgesic effects in PDN, we examined whether the potent analgesic effect of DLX contributes toward improving the pathologically aberrant noradrenaline homeostasis in diabetic models.

RESULTS

In streptozotocin (STZ) (50 mg/kg, i.v.)-induced diabetic rats that exhibited robust mechanical allodynia and thermal hyperalgesia, DLX (10 mg/kg, i.p.) significantly and markedly increased the nociceptive threshold. The analgesic effect of DLX was nullified by the prior administration of N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4) (50 mg/kg, i.p.), which drastically eliminated dopamine-beta-hydroxylase- and norepinephrine transporter-immunopositive fibers in the lumbar spinal dorsal horn and significantly reduced the noradrenaline content in the lumbar spinal cord. The treatment with DSP-4 alone markedly lowered the nociceptive threshold in vehicle-treated non-diabetic rats; however, this pro-nociceptive effect was occluded in STZ-treated diabetic rats. Furthermore, STZ-treated rats exhibited a higher amount of dopamine-beta-hydroxylase- and norepinephrine transporter-immunopositive fibers in the dorsal horn and noradrenaline content in the spinal cord compared to vehicle-treated rats.

CONCLUSIONS

Impaired noradrenaline-mediated regulation of the spinal nociceptive network might underlie exaggerated nociception in PDN. DLX might exert its analgesic effect by selective enhancement of noradrenergic signals, thus counteracting this situation.

Norepinephrine transporter heterozygous knockout mice exhibit altered transport and behavior

The norepinephrine (NE) transporter (NET) regulates synaptic NE availability for noradrenergic signaling in the brain and sympathetic nervous system. Although genetic variation leading to a loss of NET expression has been implicated in psychiatric and cardiovascular disorders, complete NET deficiency has not been found in people, limiting the utility of NET knockout mice as a model for genetically driven NET dysfunction. Here, we investigate NET expression in NET heterozygous knockout male mice (NET(+/-) ), demonstrating that they display an approximately 50% reduction in NET protein levels. Surprisingly, these mice display no significant deficit in NET activity assessed in hippocampal and cortical synaptosomes. We found that this compensation in NET activity was due to enhanced activity of surface-resident transporters, as opposed to surface recruitment of NET protein or compensation through other transport mechanisms, including serotonin, dopamine or organic cation transporters. We hypothesize that loss of NET protein in the NET(+/-) mouse establishes an activated state of existing surface NET proteins. The NET(+/-) mice exhibit increased anxiety in the open field and light-dark box and display deficits in reversal learning in the Morris water maze. These data suggest that recovery of near basal activity in NET(+/-) mice appears to be insufficient to limit anxiety responses or support cognitive performance that might involve noradrenergic neurotransmission. The NET(+/-) mice represent a unique model to study the loss and resultant compensatory changes in NET that may be relevant to behavior and physiology in human NET deficiency disorders.

Neuropathic and inflammatory pain are modulated by tuberoinfundibular peptide of 39 residues

Nociceptive information is modulated by a large number of endogenous signaling agents that change over the course of recovery from injury. This plasticity makes understanding regulatory mechanisms involved in descending inhibition of pain scientifically and clinically important. Neurons that synthesize the neuropeptide TIP39 project to many areas that modulate nociceptive information. These areas are enriched in its receptor, the parathyroid hormone 2 receptor (PTH2R). We previously found that TIP39 affects several acute nociceptive responses, leading us to now investigate its potential role in chronic pain. Following nerve injury, both PTH2R and TIP39 knockout mice developed less tactile and thermal hypersensitivity than controls and returned to baseline sensory thresholds faster. Effects of hindpaw inflammatory injury were similarly decreased in knockout mice. Blockade of α-2 adrenergic receptors increased the tactile and thermal sensitivity of apparently recovered knockout mice, returning it to levels of neuropathic controls. Mice with locus coeruleus (LC) area injection of lentivirus encoding a secreted PTH2R antagonist had a rapid, α-2 reversible, apparent recovery from neuropathic injury similar to the knockout mice. Ablation of LC area glutamatergic neurons led to local PTH2R-ir loss, and barley lectin was transferred from local glutamatergic neurons to GABA interneurons that surround the LC. These results suggest that TIP39 signaling modulates sensory thresholds via effects on glutamatergic transmission to brainstem GABAergic interneurons that innervate noradrenergic neurons. TIP39's normal role may be to inhibit release of hypoalgesic amounts of norepinephrine during chronic pain. The neuropeptide may help maintain central sensitization, which could serve to enhance guarding behavior.

Altered reward circuitry in the norepinephrine transporter knockout mouse

Synaptic levels of the monoamine neurotransmitters dopamine, serotonin, and norepinephrine are modulated by their respective plasma membrane transporters, albeit with a few exceptions. Monoamine transporters remove monoamines from the synaptic cleft and thus influence the degree and duration of signaling. Abnormal concentrations of these neuronal transmitters are implicated in a number of neurological and psychiatric disorders, including addiction, depression, and attention deficit/hyperactivity disorder. This work concentrates on the norepinephrine transporter (NET), using a battery of in vivo magnetic resonance imaging techniques and histological correlates to probe the effects of genetic deletion of the norepinephrine transporter on brain metabolism, anatomy and functional connectivity. MRS recorded in the striatum of NET knockout mice indicated a lower concentration of NAA that correlates with histological observations of subtle dysmorphisms in the striatum and internal capsule. As with DAT and SERT knockout mice, we detected minimal structural alterations in NET knockout mice by tensor-based morphometric analysis. In contrast, longitudinal imaging after stereotaxic prefrontal cortical injection of manganese, an established neuronal circuitry tracer, revealed that the reward circuit in the NET knockout mouse is biased toward anterior portions of the brain. This is similar to previous results observed for the dopamine transporter (DAT) knockout mouse, but dissimilar from work with serotonin transporter (SERT) knockout mice where Mn²⁺ tracings extended to more posterior structures than in wildtype animals. These observations correlate with behavioral studies indicating that SERT knockout mice display anxiety-like phenotypes, while NET knockouts and to a lesser extent DAT knockout mice display antidepressant-like phenotypic features. Thus, the mainly anterior activity detected with manganese-enhanced MRI in the DAT and NET knockout mice is likely indicative of more robust connectivity in the frontal portion of the reward circuit of the DAT and NET knockout mice compared to the SERT knockout mice.

Effects of monoamine reuptake inhibitors in assays of acute pain-stimulated and pain-depressed behavior in rats

Pain is associated with stimulation of some behaviors (eg, withdrawal reflexes) but depression of many other behaviors (eg, feeding, locomotion, positively reinforced operant behavior). Drugs that block reuptake of serotonin, norepinephrine, and/or dopamine are widely used to treat depression, and they have also emerged as useful drugs for treatment of pain. This study compared effects of selective and mixed-action inhibitors of serotonin, norepinephrine, and/or dopamine reuptake in assays of acute pain-stimulated and pain-depressed behavior. Intraperitoneal injection of dilute acid served as a noxious stimulus to stimulate a writhing response or depress intracranial self-stimulation (ICSS) in Sprague Dawley rats. Selective reuptake inhibitors of serotonin (citalopram, clomipramine) and norepinephrine (nisoxetine, nortriptyline) and a mixed-action reuptake inhibitor of serotonin and norepinephrine (milnacipran) blocked acid-stimulated writhing but failed to block acid-induced depression of ICSS. Selective dopamine reuptake inhibitors (RTI-113 [3ß-(4-chlorophenyl)tropane-2ß-carboxylic acid phenyl ester hydrochloride], bupropion) and a triple reuptake inhibitor of dopamine, serotonin, and norepinephrine (RTI-112 [3ß-(3-methyl-4-chlorophenyl)tropane-2ß-carboxylic acid methyl ester hydrochloride]) blocked both acid-stimulated writhing and acid-induced depression of ICSS, although these drugs also produced an abuse-related facilitation of ICSS in the absence of the noxious stimulus. These results support further consideration of dopamine reuptake inhibitors as candidate analgesics, although abuse liability remains a concern.

PERSPECTIVE

Monoamine reuptake inhibitors are used to treat depression and some forms of pain. This study examined effects of monoamine reuptake inhibitors in a preclinical assay of pain-related behavioral depression. The results support further consideration of dopamine reuptake inhibitors as candidate analgesics under selected circumstances, although abuse liability remains a concern.

Endogenous opiates and behavior: 2011

This paper is the thirty-fourth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2011 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration (Section 16); and immunological responses (Section 17).

Straub tail reaction in mice treated with σ(1) receptor antagonist in combination with methamphetamine

Straub tail reaction (STR) was observed in male ddY mice after simultaneous administration with BMY 14802 (a non-specific σ receptor antagonist) and methamphetamine (METH). The intensity and duration of STR depended on the dose of BMY 14802. The tail reaction was inhibited completely by (+)-SKF 10,047 (a putative σ(1) receptor agonist) and partially by PB 28 (a putative σ(2) receptor agonist). The STR was mimicked in mice treated with BD 1047 (a putative σ(1) receptor antagonist), but not SM-21, a putative σ(2) receptor antagonist, in combination with METH. STR evoked with BD 1047 plus METH was inhibited by (+)-SKF 10,047. STR induced by BMY 14802 and METH was abolished by naloxone (a relatively non-selective opioid receptor antagonist) or U-50,488H (a selective κ-agonist), suggesting that the STR may be mediated by activation of opioid receptor system.

Noradrenergic reuptake inhibition in the treatment of pain

INTRODUCTION

Noradrenergic reuptake inhibitors can be effective analgesics, finding application in a wide variety of clinical pain settings. Due to a shift toward noradrenergic-mediated pain pathways following nerve injury, they are particularly well suited to the treatment of neuropathic pain. This phenotypic shift makes neuropathic pain difficult to control with opioids alone; some noradrenergic reuptake inhibitors have demonstrated synergy with opioids. Agents currently in early clinical trials are discussed and include both novel delivery of old drugs and the development of new drugs.

AREAS COVERED

This review was limited to noradrenergic reuptake inhibitors and analgesia. Literature search included the terms adrenergic, noradrenergic, reuptake, inhibitors, analgesia, NET, norepinephrine transporter, and pain using Medline, Google scholar, Web of Knowledge, www.clinicaltrials.gov, and Pharmaprojects (Informa UK Ltd. 2012).

EXPERT OPINION

Topical drug delivery and the use of combinations of agents both topically and systemically are under active investigation. The intrathecal delivery of noradrenergic reuptake inhibitors, allowing delivery directly to the central nervous system thus limiting systemic exposure, represents an exciting avenue of investigation. Gaps in current knowledge have complicated the development of prophylactic therapies for susceptible individuals or preemptive intervention. Disease-modifying agents and selective inhibitors would facilitate these treatment strategies.