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

The Evolving Role of Animal Models in the Discovery and Development of Novel Treatments for Psychiatric Disorders

Historically, animal models have been routinely used in the characterization of novel chemical entities (NCEs) for various psychiatric disorders. Animal models have been essential in the in vivo validation of novel drug targets, establishment of lead compound pharmacokinetic to pharmacodynamic relationships, optimization of lead compounds through preclinical candidate selection, and development of translational measures of target occupancy and functional target engagement. Yet, with decades of multiple NCE failures in Phase II and III efficacy trials for different psychiatric disorders, the utility and value of animal models in the drug discovery process have come under intense scrutiny along with the widespread withdrawal of the pharmaceutical industry from psychiatric drug discovery. More recently, the development and utilization of animal models for the discovery of psychiatric NCEs has undergone a dynamic evolution with the application of the Research Domain Criteria (RDoC) framework for better design of preclinical to clinical translational studies combined with innovative genetic, neural circuitry-based, and automated testing technologies. In this chapter, the authors will discuss this evolving role of animal models for improving the different stages of the discovery and development in the identification of next generation treatments for psychiatric disorders.

Theory of visual attention (TVA) applied to rats performing the 5-choice serial reaction time task: differential effects of dopaminergic and noradrenergic manipulations

RATIONALE

Attention is compromised in many psychiatric disorders, including attention-deficit/hyperactivity disorder (ADHD). While dopamine and noradrenaline systems have been implicated in ADHD, their exact role in attentional processing is yet unknown.

OBJECTIVES

We applied the theory of visual attention (TVA) model, adapted from human research, to the rat 5-choice serial reaction time task (5CSRTT) to investigate catecholaminergic modulation of visual attentional processing in healthy subjects of high- and low-attention phenotypes.

METHODS

Rats trained on the standard 5CSRTT and tested with variable stimulus durations were treated systemically with noradrenergic and/or dopaminergic agents (atomoxetine, methylphenidate, amphetamine, phenylephrine and atipamezole). TVA modelling was applied to estimate visual processing speed for correct and incorrect visual perceptual categorisations, independent of motor reaction times, as measures of attentional capacity.

RESULTS

Atomoxetine and phenylephrine decreased response frequencies, including premature responses, increased omissions and slowed responding. In contrast, methylphenidate, amphetamine and atipamezole sped up responding and increased premature responses. Visual processing speed was also affected differentially. Atomoxetine and phenylephrine slowed, whereas methylphenidate and atipamezole sped up, visual processing, both for correct and incorrect categorisations. Amphetamine selectively improved visual processing for correct, though not incorrect, responses in high-attention rats only, possibly reflecting improved attention.

CONCLUSIONS

These data indicate that the application of TVA to the 5CSRTT provides an enhanced sensitivity to capturing attentional effects. Unexpectedly, we found overall slowing effects, including impaired visual processing, following drugs either increasing extracellular noradrenaline (atomoxetine) or activating the α1-adrenoceptor (phenylephrine), while also ameliorating premature responses (impulsivity). In contrast, amphetamine had potential pro-attentional effects by enhancing visual processing, probably due to central dopamine upregulation.

Alterations of monoamine neurotransmitters, HPA-axis hormones, and inflammation cytokines in reserpine-induced hyperalgesia and depression comorbidity rat model

BACKGROUND

Pain and depression often occur simultaneously, but the mechanism of this condition is still unclear.

METHODS

The aim of this study was to examine the alterations of monoamine neurotransmitters, hypothalamic-pituitary-adrenal (HPA) axis hormones, and inflammation cytokines in hyperalgesia and depression comorbidities. The reserpine-induced "Sprague Dawley" (SD) rat models were used, and the concentrations of monoamine neurotransmitters serotonin (5-HT), norepinephrine (NE), dopamine (DA), and their metabolic products 5-hydroxyindoleacetic acid (5-HIAA), Homovanillic acid (HVA), 3,4-Dihydroxyphenylacetic acid (DOPAC) in raphe nucleus region were tested by High Performance Liquid Chromatography (HPLC). Serum levels of Adrenocorticotropic Hormone (ACTH), Cortisol (CORT), and inflammatory cytokines interleukin (IL)-1β, IL-6, tumor necrosis factor (TNF)-α, interferon (IFN)-γ, IL-4, IL-10 were assessed by enzyme linked immunosorbent assay.

RESULTS

Repeated reserpine injection induced hyperalgesia and depressive behaviors with decreased sucrose preference and horizontal movement distance, and increased immobility time in forced swimming test. The concentrations of 5-HT and NE in raphe nucleus, and ACTH and CORT in serum were elevated in the model group. And the model group showed increases in serum IL-1β and IL-6, and decrease in serum IL-10.

CONCLUSION

More research in these areas is needed to understand the pathogenesis of the disease, so as to find more and better therapeutic targets.

Lack of effect of the nociceptin opioid peptide agonist Ro 64-6198 on pain-depressed behavior and heroin choice in rats

RATIONALE AND OBJECTIVE

One objective of the National Institutes of Health Helping to End Addiction Long-term (HEAL) initiative is to accelerate research on safer and more effective medications for both pain and opioid use disorder. Ligands that activate the nociceptin opioid peptide receptor (NOP) constitute one class of candidate drugs for both applications. The present preclinical study determined the effectiveness of the NOP agonist Ro 64-6198 to produce antinociception in a pain-depressed behavior procedure and attenuate opioid self-administration in a heroin-vs-food choice procedure.

METHODS

In Experiment 1, Adult Sprague-Dawley rats were equipped with microelectrodes and trained to respond for electrical brain stimulation in an intracranial self-stimulation (ICSS) procedure. The potency, time course, and receptor mechanism of effects produced by R0 64-6198 alone (0.32-3.2 mg/kg) on ICSS were examined, followed by evaluation of 0.32-1.0 mg/kg Ro 64-6198 effectiveness to block lactic acid-induced depression of ICSS. In Experiment 2, rats self-administered heroin under a heroin-vs-food choice procedure during a regimen of repeated, daily intraperitoneal administration of vehicle or Ro 64-6198 (1-3.2 mg/kg/day).

RESULTS

Ro 64-6198 produced dose- and time-dependent ICSS depression that was blocked by the selective NOP antagonist SB612111 but not by naltrexone. Ro 64-6198 failed to block acid-induced depression of ICSS. Repeated Ro 64-6198 pretreatment also failed to attenuate heroin-vs-food choice up to doses that significantly decreased operant behavior.

CONCLUSIONS

These results do not support the utility of Ro 64-6198 as a stand-alone medication for either acute pain or opioid use disorder.

Thymus persicus (Ronniger ex Rech. f.) Jalas alleviates nociceptive and neuropathic pain behavior in mice: Multiple mechanisms of action

ETHNOPHARMACOLOGICAL RELEVANCE

Thymus persicus (Roniger ex Reach F.) is an Iranian endemic medicinal plant of which essential oil and various products have numerous food and pharmaceutical applications (headache and fever treatments).

OBJECTIVE

This modern research included Swiss mice to investigate the anti-nociceptive and anti-neuropathic effects of Thymus persicus aerial parts essential oil (TPEO).

MATERIALS AND METHODS

To determine TPEO's anti-nociceptive function in the formalin-induced paw licking (FML), researchers looked at the L-arginine/NO/cGMP/KATP channel signaling pathway as well as multiple receptors as with serotonin, morphine, dopamine, and peroxisome proliferator-activated receptors. The CVC or cervical spinal cord contusion exemplar has also been used to induce neuropathic pain.

RESULTS

TPEO (50, 100, and 150 mg/kg) relative to control mice in the phase-II of FML provided strong antinociception (p < 0.05, p < 0.01, p < 0.001, respectively). Furthermore, methylene blue, glibenclamide, Nω-nitro-L-arginine methyl ester, naloxonazine, nor-binaltorphimine, prazosin, yohimbine, and ondansetron pre-treating restored the TPEO anti-nociceptive activity in the FML (phase-II) exemplar (p < 0.05). In phase-II of the FML exemplar, carvacrol (one of the active components of TPEO) also greatly reduced pain (p < 0.001). Likewise, in CVC mice, TPEO altered mechanical allodynia and hyperalgesia.

CONCLUSION

It was attained magnificently that TPEO could exerts antinociceptive effects through the involvement of L-arginine/NO/cGMP/KATP signaling pathway, adrenergic, opioid, and serotonin receptors. Moreover, it is demonstrate that anti-neuropathic activity of TPEO may be mediated by inflammatory function.

A Strategy to Prioritize Emerging Drugs of Abuse for Analysis: Abuse Liability Testing Using Intracranial Self-Stimulation (ICSS) in Rats and Validation with α-Pyrrolidinohexanophenone (α-PHP)

Novel psychoactive substances (NPS) threaten public health and safety while also straining the limited resources of forensic laboratories. To efficiently allocate the finite resources available, we propose a new strategy for prioritizing NPS with abuse liability testing using a preclinical behavioral procedure in rats known as intracranial self-stimulation (ICSS). To validate this assay, the recently-scheduled synthetic cathinone α-PHP was compared to cocaine, a mechanistically similar drug of abuse, as a positive control and saline as a negative control. Male Sprague-Dawley rats (n=6) were implanted with electrodes targeting the medial forebrain bundle and trained to respond by lever-press for electrical brain stimulation. The rats were tested with doses of 0.32, 1.0, and 3.2 mg/kg α-PHP as well as 10 mg/kg of cocaine and saline administered by intraperitoneal injection. Neither saline nor 0.32 mg/kg α-PHP altered ICSS response rates compared to baseline levels of responding; however, doses of 1.0 and 3.2 mg/kg α-PHP and 10 mg/kg cocaine facilitated ICSS responding. This ICSS profile suggests that α-PHP has high abuse potential, with a rapid onset of effects and a long duration of action, and supports the decision to schedule this compound. This study demonstrates the ability of ICSS to distinguish between compounds of low and high potential for abuse. A strategy is proposed here to screen NPS using ICSS and classify emerging drugs into four priority categories for further analysis.

Behavioral Battery for Testing Candidate Analgesics in Mice. I. Validation with Positive and Negative Controls

This study evaluated a battery of pain-stimulated, pain-depressed, and pain-independent behaviors for preclinical pharmacological assessment of candidate analgesics in mice. Intraperitoneal injection of dilute lactic acid (IP acid) served as an acute visceral noxious stimulus to produce four pain-related behaviors in male and female ICR mice: stimulation of 1) stretching, 2) facial grimace, 3) depression of rearing, and 4) depression of nesting. Additionally, nesting and locomotion in the absence of the noxious stimulus were used to assess pain-independent drug effects. These six behaviors were used to compare effects of two mechanistically distinct but clinically effective positive controls (ketoprofen and oxycodone) and two negative controls that are not clinically approved as analgesics but produce either general motor depression (diazepam) or motor stimulation (amphetamine). We predicted that analgesics would alleviate all IP acid effects at doses that did not alter pain-independent behaviors, whereas negative controls would not. Consistent with this prediction, ketoprofen (0.1-32 mg/kg) produced the expected analgesic profile, whereas oxycodone (0.32-3.2 mg/kg) alleviated all IP acid effects except depression of rearing at doses lower than those that altered pain-independent behaviors. For the negative controls, diazepam (1-10 mg/kg) failed to block IP acid-induced depression of either rearing or nesting and only decreased IP acid-stimulated behaviors at doses that also decreased pain-independent behaviors. Amphetamine (0.32-3.2 mg/kg) alleviated all IP acid effects but only at doses that also stimulated locomotion. These results support utility of this model as a framework to evaluate candidate-analgesic effects in a battery of complementary pain-stimulated, pain-depressed, and pain-independent behavioral endpoints. SIGNIFICANCE STATEMENT: Preclinical assays of pain and analgesia often yield false-positive effects with candidate analgesics. This study used two positive-control analgesics (ketoprofen, oxycodone) and two active negative controls (diazepam, amphetamine) to validate a strategy for distinguishing analgesics from nonanalgesics by profiling drug effects in a battery of complementary pain-stimulated, pain-depressed, and pain-independent behaviors in male and female mice.

Preclinical assessment of tramadol abuse potential: Effects of acute and repeated tramadol on intracranial self-stimulation in rats

BACKGROUND

Tramadol is a widely used analgesic that activates mu-opioid receptors (MOR) and inhibits serotonin and norepinephrine transporters. This mixed pharmacology may limit both its own abuse potential and its modulation of abuse potential of other MOR agonists.

AIMS

This study used an intracranial self-stimulation (ICSS) procedure to compare abuse-related effects produced by acute or repeated treatment with tramadol or morphine in rats. Abuse potential in ICSS procedures is indicated by a drug-induced increase (or 'facilitation') of ICSS responding.

METHODS

Adult male Sprague-Dawley rats were implanted with electrodes targeting the medial forebrain bundle and trained to respond on a lever for pulses of electrical brain stimulation. Tramadol effects were evaluated after acute administration (3.2-32 mg/kg) in the absence or presence of the opioid antagonist naltrexone, the CYP2D6 hepatic-enzyme inhibitor quinine or a combination of both. Additionally, both tramadol and morphine were also tested before and after repeated tramadol (32 mg/kg/day for six days) or repeated morphine (3.2 mg/kg/day for six days).

RESULTS

Acute tramadol produced primarily ICSS rate-decreasing effects that were antagonised by naltrexone but not by quinine or naltrexone + quinine. Tramadol also produced little or no ICSS facilitation after repeated tramadol or repeated morphine, and repeated tramadol did not enhance ICSS facilitation by morphine. By contrast, morphine-induced ICSS facilitation was enhanced by repeated morphine treatment.

CONCLUSIONS

These results suggest that tramadol has lower abuse potential than other abused MOR agonists and that repeated tramadol exposure produces relatively little enhancement of abuse potential of other MOR agonists.

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.

Amitifadine, a triple reuptake inhibitor, reduces self-administration of the opiate remifentanil in rats

RATIONALE

A variety of neural systems are involved in drug addiction, and some of these systems are shared across different addictive drugs. We have found several different types of drug treatments that successfully reduce nicotine self-administration.

OBJECTIVES

The current set of studies is the first in a series to determine if drug treatments that have been found to significantly reduce nicotine self-administration would reduce opiate self-administration.

METHODS

Amitifadine, a triple reuptake inhibitor of dopamine, norepinephrine, and serotonin, was assessed in female Sprague-Dawley rats to determine whether it significantly reduces remifentanil self-administration with either acute or chronic treatment.

RESULTS

Acutely, amitifadine doses of 5, 10, and 20 mg/kg each significantly reduced remifentanil self-administration. In a chronic study, repeated treatment with 10 mg/kg of amitifadine continued to reduce remifentanil self-administration, even after the cessation of treatment. However, amitifadine was not found to attenuate the rise in remifentanil self-administration with continued access. This study and our earlier one showed that the 10 mg/kg amitifadine dose did not significantly affect food motivated responding. Amitifadine did not attenuate remifentanil-induced antinociception as measured on the hot plate test but extended and maintained antinociceptive effects.

CONCLUSIONS

These studies show the promise of amitifadine as a treatment for countering opiate self-administration for adjunctive use with opioids for analgesia. Further studies are needed to determine the possible efficacy of amitifadine for combating opiate addiction or preventing it in humans during adjunctive use with opioids for chronic pain.

Characterization of Amphetamine, Methylphenidate, Nicotine, and Atomoxetine on Measures of Attention, Impulsive Action, and Motivation in the Rat: Implications for Translational Research

Amphetamine (AMP), methylphenidate (MPH), and atomoxetine (ATX) are approved treatments for ADHD, and together with nicotine (NIC), represent pharmacological agents widely studied on cognitive domains including attention and impulsive action in humans. These agents thus represent opportunities for clinical observation to be reinvestigated in the preclinical setting, i.e., reverse translation. The present study investigated each drug in male, Long Evans rats trained to perform either (1) the five-choice serial reaction time task (5-CSRTT), (2) Go/NoGo task, or (3) a progressive ratio (PR) task, for the purpose of studying each drug on attention, impulsive action and motivation. Specific challenges were adopted in the 5-CSRTT designed to tax attention and impulsivity, i.e., high frequency of stimulus presentation (sITI), variable reduction in stimulus duration (sSD), and extended delay to stimulus presentation (10-s ITI). Initially, performance of a large (> 80) cohort of rats in each task variant was conducted to examine performance stability over repeated challenge sessions, and to identify subgroups of "high" and "low" attentive rats (sITI and sSD schedules), and "high" and "low" impulsives (10-s ITI). Using an adaptive sequential study design, the effects of AMP, MPH, ATX, and NIC were examined and contrasting profiles noted across the tests. Both AMP (0.03-0.3 mg/kg) and MPH (1-6 mg/kg) improved attentional performance in the sITI but not sSD or 10-s ITI condition, NIC (0.05-0.2 mg/kg) improved accuracy across all conditions. ATX (0.1-1 mg/kg) detrimentally affected performance in the sITI and sSD condition, notably in "high" performers. In tests of impulsive action, ATX reduced premature responses notably in the 10-s ITI condition, and also reduced false alarms in Go/NoGo. Both AMP and NIC increased premature responses in all task variants, although AMP reduced false alarms highlighting differences between these two measures of impulsive action. The effect of MPH was mixed and appeared baseline dependent. ATX reduced break point for food reinforcement suggesting a detrimental effect on motivation for primary reward. Taken together these studies highlight differences between AMP, MPH, and ATX which may translate to their clinical profiles. NIC had the most reliable effect on attentional accuracy, whereas ATX was reliably effective against all tests of impulsive action.

The search for translational pain outcomes to refine analgesic development: Where did we come from and where are we going?

Pain measures traditionally used in rodents record mere reflexes evoked by sensory stimuli; the results thus may not fully reflect the human pain phenotype. Alterations in physical and emotional functioning, pain-depressed behaviors and facial pain expressions were recently proposed as additional pain outcomes to provide a more accurate measure of clinical pain in rodents, and hence to potentially enhance analgesic drug development. We aimed to review how preclinical pain assessment has evolved since the development of the tail flick test in 1941, with a particular focus on a critical analysis of some nonstandard pain outcomes, and a consideration of how sex differences may affect the performance of these pain surrogates. We tracked original research articles in Medline for the following periods: 1973-1977, 1983-1987, 1993-1997, 2003-2007, and 2014-2018. We identified 606 research articles about alternative surrogate pain measures, 473 of which were published between 2014 and 2018. This indicates that preclinical pain assessment is moving toward the use of these measures, which may soon become standard procedures in preclinical pain laboratories.

Effects of the α2/α3-subtype-selective GABAA receptor positive allosteric modulator KRM-II-81 on pain-depressed behavior in rats: comparison with ketorolac and diazepam

This study examined effects of the α2/α3-subtype-selective GABAA receptor positive allosteric modulator KRM-II-81 in an assay of pain-related behavioral depression. Adult, male Sprague-Dawley rats responded for electrical brain stimulation in a frequency-rate intracranial self-stimulation (ICSS) procedure. Intraperitoneal injection of 1.8% lactic acid served as an acute noxious stimulus to depress ICSS. Effects of KRM-II-81 were evaluated in the absence and presence of the acid noxious stimulus. The NSAID ketorolac and the benzodiazepine diazepam were tested as comparators. Neither ketorolac nor KRM-II-81 altered ICSS in the absence of the acid noxious stimulus; however, diazepam produced facilitation consistent with its abuse liability. Ketorolac blocked acid-induced depression of ICSS, and effects of 1.0 mg/kg ketorolac lasted for at least 5 h. KRM-II-81 (1.0 mg/kg) produced significant antinociception after 30 min that dissipated by 60 min. Diazepam also attenuated acid-depressed ICSS, but only at doses that facilitated ICSS when administered alone. The lack of ketorolac or KRM-II-81 effects on ICSS in the absence of the acid noxious stimulus suggests low abuse liability for both compounds. The effectiveness of ketorolac to block acid-induced ICSS depression agrees with clinical analgesic efficacy of ketorolac. KRM-II-81 produced significant but less consistent and shorter-acting antinociception than ketorolac.

Attenuated dopamine receptor signaling in nucleus accumbens core in a rat model of chemically-induced neuropathy

Neuropathy is major source of chronic pain that can be caused by mechanically or chemically induced nerve injury. Intraplantar formalin injection produces local necrosis over a two-week period and has been used to model neuropathy in rats. To determine whether neuropathy alters dopamine (DA) receptor responsiveness in mesolimbic brain regions, we examined dopamine D₁-like and D₂-like receptor (D_1/2R) signaling and expression in male rats 14 days after bilateral intraplantar formalin injections into both rear paws. D₂R-mediated G-protein activation and expression of the D₂R long, but not short, isoform were reduced in nucleus accumbens (NAc) core, but not in NAc shell, caudate-putamen or ventral tegmental area of formalin- compared to saline-treated rats. In addition, D₁R-stimulated adenylyl cyclase activity was also reduced in NAc core, but not in NAc shell or prefrontal cortex, of formalin-treated rats, whereas D₁R expression was unaffected. Other proteins involved in dopamine neurotransmission, including dopamine uptake transporter and tyrosine hydroxylase, were unaffected by formalin treatment. In behavioral tests, the potency of a D₂R agonist to suppress intracranial self-stimulation (ICSS) was decreased in formalin-treated rats, whereas D₁R agonist effects were not altered. The combination of reduced D₂R expression and signaling in NAc core with reduced suppression of ICSS responding by a D₂R agonist suggest a reduction in D₂ autoreceptor function. Altogether, these results indicate that intraplantar formalin produces attenuation of highly specific DA receptor signaling processes in NAc core of male rats and suggest the development of a neuropathy-induced allostatic state in both pre- and post-synaptic DA receptor function.

Core Outcome Measures in Preclinical Assessment of Candidate Analgesics

All preclinical procedures for analgesic drug discovery involve two components: 1) a "pain stimulus" (the principal independent variable), which is delivered to an experimental subject with the intention of producing a pain state; and 2) a "pain behavior" (the principal dependent variable), which is measured as evidence of that pain state. Candidate analgesics are then evaluated for their effectiveness to reduce the pain behavior, and results are used to prioritize drugs for advancement to clinical testing. This review describes a taxonomy of preclinical procedures organized into an "antinociception matrix" by reference to their types of pain stimulus (noxious, inflammatory, neuropathic, disease related) and pain behavior (unconditioned, classically conditioned, operant conditioned). Particular emphasis is devoted to pain behaviors and the behavioral principals that govern their expression, pharmacological modulation, and preclinical-to-clinical translation. Strengths and weaknesses are compared and contrasted for procedures using each type of behavioral outcome measure, and the following four recommendations are offered to promote strategic use of these procedures for preclinical-to-clinical analgesic drug testing. First, attend to the degree of homology between preclinical and clinical outcome measures, and use preclinical procedures with behavioral outcome measures homologous to clinically relevant outcomes in humans. Second, use combinations of preclinical procedures with complementary strengths and weaknesses to optimize both sensitivity and selectivity of preclinical testing. Third, take advantage of failed clinical translation to identify drugs that can be back-translated preclinically as active negative controls. Finally, increase precision of procedure labels by indicating both the pain stimulus and the pain behavior in naming preclinical procedures.

Effects of repeated treatment with methcathinone, mephedrone, and fenfluramine on intracranial self-stimulation in rats

RATIONALE

Synthetic cathinones constitute a class of abused drugs that can act at dopamine, norepinephrine, and serotonin transporters (DAT, NET, and SERT, respectively). Intracranial self-stimulation (ICSS) is a preclinical procedure that can be used to evaluate abuse potential of drugs, and prior studies have indicated that abuse-related ICSS effects of monoamine-transporter substrates, including some synthetic cathinones, are positively correlated with drug selectivity for DAT vs. SERT. Abuse potential of drugs can also be influenced by regimens of repeated drug exposure, but the role of repeated exposure on abuse-related ICSS effects of synthetic cathinones has not been examined.

OBJECTIVES

This study used ICSS to evaluate effects of repeated treatment with the DAT>SERT substrate methcathinone, the DAT<SERT substrate fenfluramine, and the DAT≈SERT substrate mephedrone.

METHODS

Male Sprague-Dawley rats were trained in a frequency-rate ICSS procedure, and different groups were used to evaluate effects of methcathinone, mephedrone, and fenfluramine before, during, and after regimens of repeated treatment with the designated drug.

RESULTS

Before repeated treatment, methcathinone produced dose-dependent and abuse-related ICSS facilitation, fenfluramine produced dose-dependent ICSS depression, and mephedrone produced mixed effects that included both facilitation and depression. Chronic treatment produced no change in effects of methcathinone, but complete tolerance to effects of fenfluramine. For mephedrone, chronic treatment produced partial tolerance to ICSS depression and enhanced expression of ICSS facilitation.

CONCLUSIONS

Repeated exposure to mixed-action DAT≈SERT substrates such as mephedrone can result in increased abuse potential due to sustained expression of DAT-mediated abuse-related effects and tolerance to SERT-mediated abuse-limiting effects.

Effects of monoamine uptake inhibitors on pain-related depression of nesting in mice

Pain is a significant public health problem, and assessment of pain-related impairment of behavior is a key clinical indicator and treatment target. Similar to opioids and NSAIDs, dopamine (DA) transporter inhibitors block pain-related depression of intracranial self-stimulation (ICSS) in rats. The primary goal of the present study was to determine if the effects of monoamine uptake inhibitors on pain-related depression of ICSS in rats extend to an assay of pain-related depression of nesting in mice. We hypothesized that the DA transporter-selective uptake inhibitor bupropion would block depression of nesting behavior produced by intraperitoneal injection of lactic acid, whereas selective serotonin transporter-selective citalopram, norepinephrine transporter-selective nisoxetine, and the mixed action selective serotonin transporter/norepinephrine transporter inhibitor milnacipran would be ineffective. Effects of the NSAID ketoprofen were also obtained to facilitate interpretation of the effects of the monoamine uptake inhibitors. Consistent with previous findings, ketoprofen blocked pain-related depression of nesting. In contrast, none of the monoamine uptake inhibitors blocked pain-related depression of nesting, although they all blocked pain-related stimulation of stretching. Unlike findings from studies of pain-related depression of ICSS, these results do not support consideration of DA uptake inhibitors for treatment of pain-related depression of behavior.

Interactions between Cocaine and the Putative Allosteric Dopamine Transporter Ligand SRI-31142

Drugs that inhibit the dopamine (DA) transporter (DAT) include both therapeutic agents and abused drugs. Recent studies identified a novel series of putative allosteric DAT inhibitors, but the in vivo effects of these compounds are unknown. This study examined the abuse-related behavioral and neurochemical effects produced in rats by SRI-31142 [2-(7-methylimidazo[1,2-a]pyridin-6-yl)-N-(2-phenyl-2-(pyridin-4-yl)ethyl)quinazolin-4-amine], one compound from this series. In behavioral studies, intracranial self-stimulation (ICSS) was used to compare the effects produced by SRI-31142, the abused and nonselective DAT inhibitor cocaine, and the selective DAT inhibitor GBR-12935 [1-[2-(diphenylmethoxy)ethyl]-4-(3-phenylpropyl)piperazine]. In neurochemical studies, in vivo microdialysis was used to compare the effects of SRI-31142 and cocaine on levels of DA and serotonin in nucleus accumbens (NAc). The effects of SRI-31142 in combination with cocaine were also examined in both procedures. In contrast to cocaine and GBR-12935, SRI-31142 failed to produce abuse-related increases in ICSS or NAc DA; instead, SRI-31142 only decreased ICSS and NAc DA at a dose that was also sufficient to block cocaine-induced increases in ICSS and NAc DA. Pharmacokinetic studies suggested low but adequate brain penetration of SRI-31142, in vitro binding studies failed to identify likely non-DAT targets, and in vitro functional assays failed to confirm DA uptake inhibition in an assay of DAT-mediated fluorescent signals in live cells. These results indicate that SRI-31142 does not produce cocaine-like abuse-related effects in rats. SRI-31142 may have utility to block cocaine effects and may warrant further study as a candidate pharmacotherapy; however, the role of DAT in mediating these effects is unclear, and side effects may be a limiting factor.

Neuroinflammation and Central Sensitization in Chronic and Widespread Pain

Chronic pain is maintained in part by central sensitization, a phenomenon of synaptic plasticity, and increased neuronal responsiveness in central pain pathways after painful insults. Accumulating evidence suggests that central sensitization is also driven by neuroinflammation in the peripheral and central nervous system. A characteristic feature of neuroinflammation is the activation of glial cells, such as microglia and astrocytes, in the spinal cord and brain, leading to the release of proinflammatory cytokines and chemokines. Recent studies suggest that central cytokines and chemokines are powerful neuromodulators and play a sufficient role in inducing hyperalgesia and allodynia after central nervous system administration. Sustained increase of cytokines and chemokines in the central nervous system also promotes chronic widespread pain that affects multiple body sites. Thus, neuroinflammation drives widespread chronic pain via central sensitization. We also discuss sex-dependent glial/immune signaling in chronic pain and new therapeutic approaches that control neuroinflammation for the resolution of chronic pain.

Amphetamine maintenance differentially modulates effects of cocaine, methylenedioxypyrovalerone (MDPV), and methamphetamine on intracranial self-stimulation and nucleus accumbens dopamine in rats

Amphetamine maintenance is effective clinically to reduce the consumption of the monoamine uptake inhibitor cocaine but not of the monoamine releaser methamphetamine, and its effectiveness in treating the abuse of other psychostimulants is not known. The mechanisms for differential amphetamine-maintenance effectiveness to treat different types of psychostimulant abuse are also not known. Accordingly, the present study compared the effects of amphetamine maintenance on abuse-related behavioral and neurochemical effects of cocaine, methamphetamine, and the "bath salts" constituent 3,4-methylenedioxypyrovalerone (MDPV) in rats. In behavioral studies, rats were trained to lever press for electrical brain stimulation in an intracranial self-stimulation (ICSS) procedure. In neurochemical studies, nucleus accumbens (NAc) levels of dopamine (DA) and serotonin (5-HT) were monitored by in vivo microdialysis. Cocaine, methamphetamine, and MDPV each produced dose-dependent ICSS facilitation and increases in NAc DA; cocaine and methamphetamine also increased NAc 5-HT. Amphetamine maintenance (0.32 mg/kg/h × 7 days) produced (1) sustained increases in basal ICSS and NAc DA with no change in NAc 5-HT, (2) blockade of cocaine but not methamphetamine effects on ICSS and NAc DA, and (3) no blockade of cocaine- or methamphetamine-induced increases in NAc 5-HT. Amphetamine maintenance blocked the increases in NAc DA produced by the selective DA uptake inhibitor MDPV, but it did not block MDPV-induced ICSS facilitation. These results show different effects of amphetamine maintenance on behavioral and neurochemical effects of different psychostimulants. The selective effectiveness of amphetamine maintenance to treat cocaine abuse may reflect attenuation of cocaine-induced increases in NAc DA while preserving cocaine-induced increases in NAc 5-HT.

Oral modafinil facilitates intracranial self-stimulation in rats: comparison with methylphenidate

Modafinil is a low-potency inhibitor of dopamine transporters (DAT) approved clinically to promote wakefulness. In most procedures used for abuse-liability assessment, modafinil produces effects similar to those of abused DAT inhibitors such as cocaine and methylphenidate, although modafinil often shows lower effectiveness. However, modafinil has failed to maintain drug self-administration or produce conditioned place preferences in rats. The low potency and poor solubility of modafinil complicate its delivery by parenteral routes of administration commonly used in rats, and this may contribute toward negative results. This study evaluated the effects of orally administered modafinil in rats using an assay of intracranial self-stimulation (ICSS) that has been used to examine the effects of other DAT inhibitors. Adult male Sprague-Dawley rats equipped with electrodes in the medial forebrain bundle responded for pulses of brain stimulation that varied across a range of frequencies (158-56 Hz) during daily behavioral sessions. Modafinil (20-600 mg/kg, orally) and methylphenidate (1.0-10 mg/kg, intraperitoneally; 3.2-32 mg/kg, orally) produced dose-dependent and time-dependent facilitation of ICSS, an effect produced by abused DAT inhibitors and other classes of abused drugs. These results are in agreement with other evidence for stimulant-like abuse liability of modafinil and show the sensitivity of ICSS to orally administered drug.

Dissociable effects of the kappa opioid receptor agonist nalfurafine on pain/itch-stimulated and pain/itch-depressed behaviors in male rats

RATIONALE

Nalfurafine is a G protein signaling-biased kappa opioid receptor (KOR) agonist approved in Japan for second-line treatment of uremic pruritus. Neither nalfurafine nor any other KOR agonist is currently approved anywhere for treatment of pain, but recent evidence suggests that G protein signaling-biased KOR agonists may have promise as candidate analgesics/antipruritics with reduced side effects compared to nonbiased or ß-arrestin-signaling-biased KOR agonists.

OBJECTIVES

This study compared nalfurafine effects in rats using assays of pain-stimulated and pain-depressed behavior used previously to evaluate other candidate analgesics. Nalfurafine effects were also examined in complementary assays of itch-stimulated and itch-depressed behavior.

METHODS

Intraperitoneal lactic acid (IP acid) and intradermal serotonin (ID 5HT) served as noxious and pruritic stimuli, respectively, in male Sprague Dawley rats to stimulate stretching (IP acid) or scratching (ID 5HT) or to depress positively reinforced operant responding in an assay of intracranial self-stimulation (ICSS; both stimuli).

RESULTS

Nalfurafine was equipotent to decrease IP acid-stimulated stretching and ID 5HT-stimulated scratching; however, doses of nalfurafine that decreased these pain/itch-stimulated behaviors also decreased control ICSS performance. Moreover, nalfurafine failed to alleviate either IP acid- or ID 5HT-induced depression of ICSS.

CONCLUSIONS

These results suggest that nalfurafine-induced decreases in pain/itch-stimulated behaviors may reflect nonselective decreases in motivated behavior rather than analgesia or antipruritus against the noxious and pruritic stimuli used here. This conclusion agrees with the absence of clinical data for nalfurafine analgesia and the weak clinical data for nalfurafine antipruritus. Nalfurafine bias for G protein signaling may not be sufficient for clinically safe and reliable analgesia or antipruritus.

Pharmacological modulation of neuropathic pain-related depression of behavior: effects of morphine, ketoprofen, bupropion and [INCREMENT]9-tetrahydrocannabinol on formalin-induced depression of intracranial self-stimulation in rats

Neuropathic pain is often associated with behavioral depression. Intraplantar formalin produces sustained, neuropathy-associated depression of intracranial self-stimulation (ICSS) in rats. This study evaluated pharmacological modulation of formalin-induced ICSS depression. Rats with intracranial electrodes targeting the medial forebrain bundle responded for electrical brain stimulation in an ICSS procedure. Bilateral intraplantar formalin administration depressed ICSS for 14 days. Morphine (0.32-3.2 mg/kg), ketoprofen (0.1-10 mg/kg), bupropion (3.2-32 mg/kg), and [INCREMENT]9-tetrahydrocannabinol (THC; 0.32-3.2 mg/kg) were evaluated for their effectiveness to reverse formalin-induced depression of ICSS. Drug effects on formalin-induced mechanical allodynia were evaluated for comparison. Morphine and bupropion reversed both formalin-induced ICSS depression and mechanical allodynia, and effects on ICSS were sustained during repeated treatment. Ketoprofen failed to reverse either formalin effect. THC blocked mechanical allodynia, but decreased ICSS in control rats and exacerbated formalin-induced depression of ICSS. The failure of ketoprofen to alter formalin effects suggests that formalin effects result from neuropathy rather than inflammation. The effectiveness of morphine and bupropion to reverse formalin effects agrees with other evidence that these drugs block pain-depressed behavior in rats and relieve neuropathic pain in humans. The effects of THC suggest general behavioral suppression and do not support the use of THC to treat neuropathic pain.

N-Alkylated Analogs of 4-Methylamphetamine (4-MA) Differentially Affect Monoamine Transporters and Abuse Liability

Clandestine chemists synthesize novel stimulant drugs by exploiting structural templates known to target monoamine transporters for dopamine, norepinephrine, and serotonin (DAT, NET, and SERT, respectively). 4-Methylamphetamine (4-MA) is an emerging drug of abuse that interacts with transporters, but limited structure-activity data are available for its analogs. Here we employed uptake and release assays in rat brain synaptosomes, voltage-clamp current measurements in cells expressing transporters, and calcium flux assays in cells coexpressing transporters and calcium channels to study the effects of increasing N-alkyl chain length of 4-MA on interactions at DAT, NET, and SERT. In addition, we performed intracranial self-stimulation in rats to understand how the chemical modifications affect abuse liability. All 4-MA analogs inhibited uptake at DAT, NET, and SERT, but lengthening the amine substituent from methyl to ethyl, propyl, and butyl produced a stepwise decrease in potency. N-methyl 4-MA was an efficacious substrate-type releaser at DAT that evoked an inward depolarizing current and calcium influx, whereas other analogs did not exhibit these effects. N-methyl and N-ethyl 4-MA were substrates at NET, whereas N-propyl and N-butyl 4-MA were not. All analogs acted as SERT substrates, though N-butyl 4-MA had very weak effects. Intracranial self-stimulation in rats showed that elongating the N-alkyl chain decreased abuse-related effects in vivo that appeared to parallel reductions in DAT activity. Overall, converging lines of evidence show that lengthening the N-alkyl substituent of 4-MA reduces potency to inhibit transporters, eliminates substrate activity at DAT and NET, and decreases abuse liability of the compounds.

Pharmacotherapy in smoking cessation: Corticotropin Releasing Factor receptors as emerging intervention targets

Smoking represents perhaps the single most important health risk factor and a global contributor to mortality that can unquestionably be prevented. Smoking is responsible for many diseases, including various types of cancer, chronic obstructive pulmonary disease, coronary heart disease, peripheral vascular disease and peptic ulcer, while it adversely affects fetal formation and development. Since smoking habit duration is a critical factor for mortality, the goal of treatment should be its timely cessation and relapse prevention. Drug intervention therapy is an important ally in smoking cessation. Significant positive steps have been achieved in the last few years in the development of supportive compounds. In the present review, we analyze reports studying the role of Corticotropin Releasing Factor (CRF), the principle neuroendocrine mediator of the stress response and its two receptors (CRF1 and CRF2) in the withdrawal phase as well as in the abstinence from nicotine use. Although still in pre-clinical evaluation, therapeutic implications of these data were investigated in order to highlight potential pharmaceutical interventions.

Relief of Pain-Depressed Behavior in Rats by Activation of D1-Like Dopamine Receptors

Clinically significant pain often includes a decrease in both behavior and mesolimbic dopamine signaling. Indirect and/or direct dopamine receptor agonists may alleviate pain-related behavioral depression. To test this hypothesis, the present study compared effects of indirect and direct dopamine agonists in a preclinical assay of pain-depressed operant responding. Male Sprague-Dawley rats with chronic indwelling microelectrodes in the medial forebrain bundle were trained in an intracranial self-stimulation (ICSS) procedure to press a lever for pulses of electrical brain stimulation. Intraperitoneal injection of dilute lactic acid served as an acute noxious stimulus to depress ICSS. Intraperitoneal lactic acid-induced depression of ICSS was dose-dependently blocked by the dopamine transporter inhibitor methylphenidate and the D1-selective agonist SKF82958, but not by the D2/3-selective agonists quinpirole, pramipexole, or sumanirole. The antinociceptive effects of methylphenidate and SKF82958 were blocked by the D1-selective antagonist SCH39166. Acid-induced stimulation of a stretching response was evaluated in separate groups of rats, but all agonists decreased acid-stimulated stretching, and antagonism experiments were inconclusive due to direct effects of the antagonists when administered alone. Taken together, these results suggest that D1-receptor stimulation is both sufficient to block acid-induced depression of ICSS and necessary for methylphenidate antinociception in this procedure. Conversely, D2/3-receptor stimulation is not sufficient to relieve pain-depressed behavior. These results support the hypothesis that pain-related depression of dopamine D1 receptor signaling contributes to pain-related depression of behavior in rats. Additionally, these results support further consideration of indirect dopamine agonists and direct D1 receptor agonists as candidate treatments for pain-related behavioral depression.

Sex differences in abuse-related neurochemical and behavioral effects of 3,4-methylenedioxymethamphetamine (MDMA) in rats

3,4-Methylenedioxymethamphetamine (MDMA) is a substrate for dopamine (DA), norepinephrine and serotonin (5HT) transporters that produces greater pharmacological effects on certain endpoints in females than males in both clinical and rodent preclinical studies. To evaluate potential for sex differences in abuse-related MDMA effects, the present study compared MDMA effects on intracranial self-stimulation (ICSS) and on in vivo microdialysis measurements of DA or 5HT in the nucleus accumbens (NAc) in female and male Sprague-Dawley rats. For ICSS studies, electrodes were implanted in the medial forebrain bundle and rats trained to press for electrical stimulation over a range of frequencies (56-158Hz, 0.05 log increments) under a fixed-ratio 1 schedule, and the potency (0.32-3.2mg/kg, 10min pretreatment) and time course (3.2. mg/kg, 10-180min pretreatment) of MDMA effects were determined. For in vivo microdialysis, rats were implanted with bilateral guide cannulae targeting the NAc, and the time course of MDMA effects (1.0-3.2mg/kg, 0-180min) on DA and 5HT was determined. MDMA produced qualitatively similar effects in both sexes on ICSS (both increases in low ICSS rates maintained by low brain-stimulation frequencies and decreases in high ICSS rates maintained by high brain-stimulation frequencies) and microdialysis (increases in both DA and 5HT). The duration and peak levels of both abuse-related ICSS facilitation and increases in NAc DA were longer in females. MDMA was also more potent to increase 5HT in females. These results provide evidence for heightened sensitivity of females to abuse-related behavioral and neurochemical effects of MDMA in rats.

Opposing effects of dopamine D1- and D2-like agonists on intracranial self-stimulation in male rats

Dopamine acts through dopamine Type I receptors (comprising D1 and D5 subtypes) and dopamine Type II receptors (comprising D2, D3, and D4 subtypes). Intracranial self-stimulation (ICSS) is 1 experimental procedure that can be used to evaluate abuse-related effects of drugs targeting dopamine receptors. This study evaluated effects of dopamine receptor ligands on ICSS in rats using experimental procedures that have been used previously to examine abused indirect dopamine agonists such as cocaine and amphetamine. Male Sprague-Dawley rats responded under a fixed-ratio 1 schedule for electrical stimulation of the medial forebrain bundle, and frequency of stimulation varied from 56-158 Hz in 0.05 log increments during each experimental session. Drug potency and time course were determined for the D1 ligands A77636, SKF82958, SKF38393, fenoldopam, and SCH39166 and the D2/3 ligands sumanirole, apomorphine, quinpirole, PD128907, pramipexole, aripiprazole, eticlopride, and PG01037. The high-efficacy D1 agonists A77636 and SKF82958 produced dose-dependent, time-dependent, and abuse-related facilitation of ICSS. Lower efficacy D1 ligands and all D2/3 ligands failed to facilitate ICSS at any dose or pretreatment time. A mixture of SKF82958 and quinpirole produced a mixture of effects produced by each drug alone. Quinpirole also failed to facilitate ICSS after regimens of repeated treatment with either quinpirole or cocaine. These studies provide more evidence for divergent effects of dopamine D1- and D2-family agonists on ICSS procedure in rats and suggest that ICSS may be a useful complement to other approaches for preclinical abuse potential assessment, in part because of the reproducibility of results. (PsycINFO Database Record

Effects of the noncompetitive N-methyl-d-aspartate receptor antagonists ketamine and MK-801 on pain-stimulated and pain-depressed behaviour in rats

BACKGROUND

Pain is a significant public health concern, and current pharmacological treatments have problematic side effects and limited effectiveness. N-methyl-d-aspartate (NMDA) glutamate receptor antagonists have emerged as one class of candidate treatments for pain because of the significant contribution of glutamate signalling in nociceptive processing.

METHODS

This study compared effects of the NMDA receptor antagonists ketamine and MK-801 in assays of pain-stimulated and pain-depressed behaviour in rats. The nonsteroidal anti-inflammatory drug ketoprofen was examined for comparison as a positive control. Intraperitoneal injection of dilute acid served as an acute visceral noxious stimulus to stimulate a stretching response or depress intracranial self-stimulation (ICSS) in male Sprague-Dawley rats.

RESULTS

Ketamine (1.0-10.0 mg/kg) blocked acid-stimulated stretching but failed to block acid-induced depression of ICSS, whereas MK-801 (0.01-0.1 mg/kg) blocked both acid-stimulated stretching and acid-induced depression of ICSS. These doses of ketamine and MK-801 did not alter control ICSS in the absence of the noxious stimulus; however, higher doses of ketamine (10 mg/kg) and MK-801 (0.32 mg/kg) depressed all behaviour. Ketoprofen (1.0 mg/kg) blocked both acid-induced stimulation of stretching and depression of ICSS without altering control ICSS.

CONCLUSION

These results support further consideration of NMDA receptor antagonists as analgesics; however, some NMDA receptor antagonists are more efficacious at attenuating pain-depressed behaviours.

WHAT DOES THIS STUDY ADD

NMDA receptor antagonists produce dissociable effects on pain-depressed behaviour. Provides evidence that pain-depressed behaviours should be considered and evaluated when determining the antinociceptive effects of NMDA receptor antagonists.

Effects of nicotinic acetylcholine receptor agonists in assays of acute pain-stimulated and pain-depressed behaviors in rats

Agonists at nicotinic acetylcholine receptors (nAChRs) constitute one drug class being evaluated as candidate analgesics. Previous preclinical studies have implicated α4β2 and α7 nAChRs as potential mediators of the antinociceptive effects of (–)-nicotine hydrogen tartrate (nicotine) and other nAChR agonists; however, these studies have relied exclusively on measures of pain-stimulated behavior, which can be defined as behaviors that increase in frequency, rate, or intensity after presentation of a noxious stimulus. Pain is also associated with depression of many behaviors, and drug effects can differ in assays of pain-stimulated versus pain-depressed behavior. Accordingly, this study compared the effects of nicotine, the selective α4/6β2 agonist 5-(123I)iodo-3-[2(S)-2-azetidinylmethoxy]pyridine (5-I-A-85380), and the selective α7 agonist N-(3R)-1-azabicyclo(2.2.2)oct-3-yl-4-chlorobenzamide in assays of pain-stimulated and pain-depressed behavior in male Sprague-Dawley rats. Intraperitoneal injection of dilute lactic acid served as an acute noxious stimulus to either stimulate a stretching response or depress the operant responding, which is maintained by electrical brain stimulation in an intracranial self-stimulation (ICSS) procedure. Nicotine produced a dose-dependent, time-dependent, and mecamylamine-reversible blockade of both acid-stimulated stretching and acid-induced depression of ICSS. 5-I-A-85380 also blocked both acid-stimulated stretching and acid-induced depression of ICSS, whereas N-(3R)-1-azabicyclo(2.2.2)oct-3-yl-4-chlorobenzamide produced no effect in either procedure. Both nicotine and 5-I-A-85380 were ≥10-fold more potent in blocking the acid-induced depression of ICSS than in blocking the acid-induced stimulation of stretching. These results suggest that stimulation of α4β2 and/or α6β2 nAChRs may be especially effective to alleviate the signs of pain-related behavioral depression in rats; however, nonselective behavioral effects may contribute to apparent antinociception.

Prevalence of pain-related single nucleotide polymorphisms in patients of African origin with sickle cell disease

BACKGROUND

Prospective pain genetics research is hindered by a lack of data on the prevalence of polymorphisms in pain-relevant genes for patients with sickle cell disease (SCD). For African-Americans in general, limited information is available in public databases.

METHODS

We prioritized and examined the genotype and allele frequencies of 115 SNPs from 49 candidate pain genes in 199 adult African-Americans and pediatric patients of African origin with SCD. Analyses were performed and compared with available data from public databases.

RESULTS

Genotype and allele frequencies of a number of SNPs were found to be different between our cohort and those from the databases and between adult and pediatric subjects.

CONCLUSION

As pain therapy is inadequate in a significant percentage of patients with SCD, candidate pain genetic studies may aid in designing precision pain medicine. We provide prevalence data as a reference for prospective genetic studies in this population.

Effects of the triple monoamine uptake inhibitor amitifadine on pain-related depression of behavior and mesolimbic dopamine release in rats

Pain-related depression of behavior and mood is a key therapeutic target in the treatment of pain. Clinical evidence suggests a role for decreased dopamine (DA) signaling in pain-related depression of behavior and mood. Similarly, in rats, intraperitoneal injection of dilute lactic acid (IP acid) serves as a chemical noxious stimulus to produce analgesic-reversible decreases in both (1) extracellular DA levels in nucleus accumbens (NAc) and (2) intracranial self-stimulation (ICSS), an operant behavior reliant on NAc DA. Intraperitonial acid-induced depression of ICSS is blocked by DA transporter (DAT) inhibitors, but clinical viability of selective DAT inhibitors as analgesics is limited by abuse potential. Drugs that produce combined inhibition of both DA and serotonin transporters may retain efficacy to block pain-related behavioral depression with reduced abuse liability. Amitifadine is a "triple uptake inhibitor" that inhibits DAT with approximately 5- to 10-fold weaker potency than it inhibits serotonin and norepinephrine transporters. This study compared amitifadine effects on IP acid-induced depression of NAc DA and ICSS and IP acid-stimulated stretching in male Sprague-Dawley rats. Amitifadine blocked IP acid-induced depression of both NAc DA and ICSS and IP acid-stimulated stretching. In the absence of the noxious stimulus, amitifadine increased NAc levels of both DA and serotonin, and behaviorally, amitifadine produced significant but weak abuse-related ICSS facilitation. Moreover, amitifadine was more potent to block IP acid-induced depression of ICSS than to facilitate control ICSS. These results support consideration of amitifadine and related monoamine uptake inhibitors as candidate analgesics for treatment of pain-related behavioral depression.

Effects of acute and sustained pain manipulations on performance in a visual-signal detection task of attention in rats

Preclinical Research Patients with pain often display cognitive impairment including deficits in attention. The visual-signal detection task (VSDT) is a behavioral procedure for assessment of attention in rodents. Male Sprague Dawley rats were trained in a VSDT and tested with three different noxious stimuli: (i) intraperitoneal injection of lactic acid; (ii) intraplantar injection of formalin; and (iii) intraplantar injection of complete Freund's adjuvant (CFA). The muscarinic acetylcholine receptor antagonist, scopolamine was also tested as a positive control. Scopolamine (0.01-1.0 mg/kg) dose dependently reduced accuracy and increased response latencies during completed trials with higher scopolamine doses increasing omissions. Lactic acid (0.56-5.6% ip) also increased response latencies and omissions, although it failed to alter measures of response accuracy. Formalin produced a transient decrease in accuracy while also increasing both response latency and omissions. CFA failed to alter VSDT performance. Although VSDT effects were transient for formalin and absent for CFA, both treatments produced mechanical allodynia and paw edema for up to 7 days. These results support the potential for noxious stimuli to produce a pain-related disruption of attention in rats. However, relatively strong noxious stimulation appears necessary to disrupt performance in this version of the VSDT.

Stereoselective Actions of Methylenedioxypyrovalerone (MDPV) To Inhibit Dopamine and Norepinephrine Transporters and Facilitate Intracranial Self-Stimulation in Rats

The designer stimulant methylenedioxypyrovalerone (MDPV) is a potent reuptake inhibitor at transporters for dopamine (DAT) and norepinephrine (NET) that produces a constellation of abuse-related behavioral effects. MDPV possesses a chiral center, and the abused formulation of the drug is a racemic mixture, but no data are available on the pharmacology of its isomers. Here, the individual optical isomers of MDPV were prepared and examined with respect to their neurochemical actions on neurotransmitter reuptake and behavioral effects in an assay of intracranial self-stimulation (ICSS) in rats. In assays of DAT uptake inhibition, S(+)MDPV (EC50 = 2.13 nM) was more potent than either (±)MDPV (EC50 = 4.85 nM) or R(-)MDPV (EC50 = 382.80 nM); the three drugs were less potent at NET uptake inhibition, with the same rank order of potency. Neither racemic MDPV nor its optical isomers inhibited the reuptake of serotonin at concentrations up to 10 μM. S(+)MDPV produced an abuse-related and dose-dependent facilitation of ICSS, and the potency of S(+)MDPV (significant facilitation at doses ≥ 0.1 mg/kg) was greater than that of the racemate (significant facilitation at doses ≥ 0.32 mg/kg). R(-)MDPV failed to alter ICSS at doses up to 100 times greater than the lowest effective dose of S(+)MDPV. The results indicate that abuse-related neurochemical and behavioral effects of racemic MDPV reside primarily with its S(+) isomer.

Effects of the fatty acid amide hydrolase inhibitor URB597 on pain-stimulated and pain-depressed behavior in rats

Cannabinoid receptor (CBR) agonists produce antinociception in conventional preclinical assays of pain-stimulated behavior but are not effective in preclinical assays of pain-depressed behavior. Fatty acid amide hydrolase (FAAH) inhibitors increase physiological levels of the endocannabinoid anandamide, which may confer improved efficacy and safety relative to direct CBR agonists. To further evaluate FAAH inhibitors as candidate analgesics, this study assessed the effects of the FAAH inhibitor URB597 in assays of acute pain-stimulated and pain-depressed behavior in male Sprague-Dawley rats. Intraperitoneal injection of dilute lactic acid served as a noxious stimulus to stimulate a stretching response or depress positively reinforced operant behavior (intracranial self-stimulation), and URB597 was tested 1 and 4 h after administration. Consistent with FAAH inhibitor effects in other assays of pain-stimulated behavior, URB597 (1-10 mg/kg intraperitoneally) produced dose-related and CB1R-mediated decreases in acid-stimulated stretching. Conversely, in the assay of acid-depressed intracranial self-stimulation, URB597 produced a delayed, partial and non-CBR-mediated antinociceptive effect. The antinociceptive dose of URB597 (10 mg/kg) increased plasma and brain anandamide levels. These results suggest that URB597 produces antinociception in these models of 'pain stimulated' and 'pain depressed' behavior, but with different rates of onset and differential involvement of CBRs.

Effects of μ-opioid receptor agonists in assays of acute pain-stimulated and pain-depressed behavior in male rats: role of μ-agonist efficacy and noxious stimulus intensity

Pain is associated with stimulation of some behaviors and depression of others, and μ-opioid receptor agonists are among the most widely used analgesics. This study used parallel assays of pain-stimulated and pain-depressed behavior in male Sprague-Dawley rats to compare antinociception profiles for six μ-agonists that varied in efficacy at μ-opioid receptors (from highest to lowest: methadone, fentanyl, morphine, hydrocodone, buprenorphine, and nalbuphine). Intraperitoneal injection of diluted lactic acid served as an acute noxious stimulus to either stimulate stretching or depress operant responding maintained by electrical stimulation in an intracranial self-stimulation (ICSS). All μ-agonists blocked both stimulation of stretching and depression of ICSS produced by 1.8% lactic acid. The high-efficacy agonists methadone and fentanyl were more potent at blocking acid-induced depression of ICSS than acid-stimulated stretching, whereas lower-efficacy agonists displayed similar potency across assays. All μ-agonists except morphine also facilitated ICSS in the absence of the noxious stimulus at doses similar to those that blocked acid-induced depression of ICSS. The potency of the low-efficacy μ-agonist nalbuphine, but not the high-efficacy μ-agonist methadone, to block acid-induced depression of ICSS was significantly reduced by increasing the intensity of the noxious stimulus to 5.6% acid. These results demonstrate sensitivity of acid-induced depression of ICSS to a range of clinically effective μ-opioid analgesics and reveal distinctions between opioids based on efficacy at the μ-receptor. These results also support the use of parallel assays of pain-stimulated and -depressed behaviors to evaluate analgesic efficacy of candidate drugs.

Effects of the neuropeptide S receptor antagonist RTI-118 on abuse-related facilitation of intracranial self-stimulation produced by cocaine and methylenedioxypyrovalerone (MDPV) in rats

Neuropeptide S (NPS) is a neurotransmitter that activates the NPS receptor to modulate biological functions including anxiety-like behaviors, feeding, and drug reinforcement. RTI-118 is a novel NPS receptor antagonist that decreased cocaine self-administration in rats at doses that had little or no effect on food-maintained responding. To build on these previous findings, this study examined effects of RTI-118 on cocaine-induced facilitation of intracranial self-stimulation (ICSS) in rats. To provide a context for data interpretation, effects of RTI-118 were compared to effects of the kappa opioid receptor agonist U69,593, because the kappa opioid receptor is another peptide neurotransmitter receptor reported to modulate abuse-related cocaine effects. RTI-118 effects were also examined on ICSS facilitation produced by methylenedioxypyrovalerone (MDPV), a novel designer drug of abuse with some cocaine-like effects. Male Sprague-Dawley rats (n=12) with electrodes targeting the medial forebrain bundle responded under a fixed-ratio 1 schedule for range of brain stimulation frequencies. Under control conditions, brain stimulation maintained a frequency-dependent increase in ICSS rates. Cocaine (1.0-10mg/kg) and MDPV (3.2mg/kg) facilitated ICSS. RTI-118 (3.2-32mg/kg) alone produced little effect on ICSS but dose dependently blocked cocaine-induced ICSS facilitation. U69,593 (0.25-0.5mg/kg) also attenuated cocaine effects, but blockade of cocaine effects was incomplete even at a U69,593 dose that alone depressed ICSS. RTI-118 (32mg/kg) failed to block MDPV-induced ICSS facilitation. These results support further consideration of NPS receptor antagonists as candidate treatments for cocaine abuse and provide evidence for differential effects of a candidate treatment on abuse-related effects of cocaine and MDPV.

Intracranial self-stimulation to evaluate abuse potential of drugs

Intracranial self-stimulation (ICSS) is a behavioral procedure in which operant responding is maintained by pulses of electrical brain stimulation. In research to study abuse-related drug effects, ICSS relies on electrode placements that target the medial forebrain bundle at the level of the lateral hypothalamus, and experimental sessions manipulate frequency or amplitude of stimulation to engender a wide range of baseline response rates or response probabilities. Under these conditions, drug-induced increases in low rates/probabilities of responding maintained by low frequencies/amplitudes of stimulation are interpreted as an abuse-related effect. Conversely, drug-induced decreases in high rates/probabilities of responding maintained by high frequencies/amplitudes of stimulation can be interpreted as an abuse-limiting effect. Overall abuse potential can be inferred from the relative expression of abuse-related and abuse-limiting effects. The sensitivity and selectivity of ICSS to detect abuse potential of many classes of abused drugs is similar to the sensitivity and selectivity of drug self-administration procedures. Moreover, similar to progressive-ratio drug self-administration procedures, ICSS data can be used to rank the relative abuse potential of different drugs. Strengths of ICSS in comparison with drug self-administration include 1) potential for simultaneous evaluation of both abuse-related and abuse-limiting effects, 2) flexibility for use with various routes of drug administration or drug vehicles, 3) utility for studies in drug-naive subjects as well as in subjects with controlled levels of prior drug exposure, and 4) utility for studies of drug time course. Taken together, these considerations suggest that ICSS can make significant contributions to the practice of abuse potential testing.

Dissociable effects of the noncompetitive NMDA receptor antagonists ketamine and MK-801 on intracranial self-stimulation in rats

RATIONALE

The noncompetitive NMDA antagonist ketamine produces rapid antidepressant effects in treatment-resistant patients suffering from major depressive and bipolar disorders. However, abuse liability is a concern.

OBJECTIVES

This study examined abuse-related effects of ketamine using intracranial self-stimulation (ICSS) in rats. The higher-affinity NMDA antagonist MK-801 and the monoamine reuptake inhibitor cocaine were examined for comparison.

METHODS

Male Sprague Dawley rats were implanted with electrodes targeting the medial forebrain bundle and trained to respond to brain stimulation under a frequency-rate ICSS procedure. The first experiment compared the potency and time course of ketamine (3.2-10.0 mg/kg) and MK-801 (0.032-0.32 mg/kg). The second experiment examined effects of repeated dosing with ketamine (3.2-20.0 mg/kg/day) and acute cocaine (10.0 mg/kg).

RESULTS

Following acute administration, ketamine (3.2-10 mg/kg) produced only dose- and time-dependent depressions of ICSS and failed to produce an abuse-related facilitation of ICSS at any dose or pretreatment time. In contrast, MK-801 (0.032-0.32 mg/kg) produced a mixed profile of rate-increasing and rate-decreasing effects; ICSS facilitation was especially prominent at an intermediate dose of 0.18 mg/kg. Repeated dosing with ketamine produced dose-dependent tolerance to the rate-decreasing effects of ketamine (10.0 and 18.0 mg/kg) but failed to unmask expression of ICSS facilitation. Termination of ketamine treatment failed to produce withdrawal-associated decreases in ICSS. As reported previously, 10.0 mg/kg cocaine facilitated ICSS.

CONCLUSIONS

The dissociable effects of ketamine and MK-801 suggest differences in the pharmacology of these nominally similar NMDA antagonists. Failure of ketamine to facilitate ICSS contrasts with other evidence for the abuse liability of ketamine.

Abuse-related effects of dual dopamine/serotonin releasers with varying potency to release norepinephrine in male rats and rhesus monkeys

d-Amphetamine selectively promotes release of both dopamine (DA) and norepinephrine (NE) versus serotonin (5HT), and chronic d-amphetamine treatment decreases cocaine-taking behavior in rats, nonhuman primates, and humans. However, abuse liability limits the clinical utility of amphetamine maintenance for treating cocaine abuse. One strategy to improve safety and efficacy of monoamine releasers as candidate anticocaine medications has been to develop dual DA/5HT releasers like 1-napthyl-2-aminopropane (PAL-287), but the pharmacology of this class of compounds has not been extensively examined. In particular, PAL-287 has similar potencies to release DA, 5HT, and NE, and the role of manipulating NE release potency on abuse-related or anticocaine effects of dual DA/5HT releasers is not known. To address this issue, the present study compared effects of four novel DA/5HT releasers that varied >800-fold in their selectivities to release DA/5HT versus NE: [1-(5-chloro-1H-indol-3-yl)propan-2-amine (PAL-542), 1-(5-fluoro-1H-indol-3-yl)propan-2-amine (PAL-544), 1-(1H-indol-5-yl)propan-2-amine (PAL-571), and (R)-1-(1H-indol-1-yl)propain-2-amine (PAL-569). Abuse-related effects of all four compounds were evaluated in assays of intracranial self-stimulation (ICSS) in rats and cocaine discrimination in rats and monkeys, and none of the compounds reliably facilitated ICSS or substituted for cocaine. Anticocaine effects of the compound with highest selectivity to release DA/5HT versus NE (PAL-542) were tested in an assay of cocaine versus food choice in rhesus monkeys, and PAL-542 failed to reduce cocaine choice. These results suggests that potency to release NE has minimal influence on abuse liability of dual DA/5HT releasers, and reducing relative potency to release NE versus DA/5HT does not improve anticocaine efficacy.

Dopamine signaling in C. elegans is mediated in part by HLH-17-dependent regulation of extracellular dopamine levels

In Caenorhabditis elegans, the dopamine transporter DAT-1 regulates synaptic dopamine (DA) signaling by controlling extracellular DA levels. In dat-1(ok157) animals, DA is not taken back up presynaptically but instead reaches extrasynpatic sites, where it activates the dopamine receptor DOP-3 on choligeneric motor neurons and causes animals to become paralyzed in water. This phenotype is called swimming-induced paralysis (SWIP) and is dependent on dat-1 and dop-3. Upstream regulators of dat-1 and dop-3 have yet to be described in C. elegans. In our previous studies, we defined a role for HLH-17 during dopamine response through its regulation of the dopamine receptors. Here we continue our characterization of the effects of HLH-17 on dopamine signaling. Our results suggest that HLH-17 acts downstream of dopamine synthesis to regulate the expression of dop-3 and dat-1. First, we show that hlh-17 animals display a SWIP phenotype that is consistent with its regulation of dop-3 and dat-1. Second, we show that this behavior is enhanced by treatment with the dopamine reuptake inhibitor, bupropion, in both hlh-17 and dat-1 animals, a result suggesting that SWIP behavior is regulated via a mechanism that is both dependent on and independent of DAT-1. Third, and finally, we show that although the SWIP phenotype of hlh-17 animals is unresponsive to the dopamine agonist, reserpine, and to the antidepressant, fluoxetine, hlh-17 animals are not defective in acetylcholine signaling. Taken together, our work suggests that HLH-17 is required to maintain normal levels of dopamine in the synaptic cleft through its regulation of dop-3 and dat-1.

Acute systemic infusion of bupropion decrease formalin induced pain behavior in rat

Background

The chronic pain can disturb physical, psychological, and social performances. Analgesic agents are widely used but some antidepressants (ADs) showed analgesia also. Bupropion is using for smoke cessation but it can change morphine withdrawal signs such as pain. This study tested the acute systemic effect of bupropion on formalin induced pain behavior in rats.

Methods

Wistar male healthy rats were divided into 7 groups (control, sham, and 5 treated groups with 10, 30, 90, 120, and 200 mg/kg of bupropion, i.p.). The bupropion injected 3 hours prior to formalin induced pain behavior. Formalin (50 µl, 2.5%) was injected subcutaneously in dorsal region of right hindpaw in all animals. Nociceptive signs were observed continuously on-line and off-line each minute. Common pain scoring was used for pain assessment.

Results

The analysis of data by one-way ANOVA showed that bupropion can reduce pain scores in the second phase but not in first phase. Bupropion decreased the licking/biting duration significantly in first and second phase of formalin test.

Conclusions

The results showed that bupropion has analgesic effects at systemic application. The change of second phase of the pain behavior was significant and it revealed that central mechanisms involve in bupropion analgesia.

Expression and treatment of pain-related behavioral depression

Pain is often associated with clinically relevant depression of behavior and mood, and relief of pain-related depression is a common goal of treatment in both human and veterinary medicine. In the development of pharmacological compounds to treat pain and related depression, preclinical studies may be used to evaluate the analgesic potential of new drugs. Such studies require reliable, accurate assays of pain-related behavioral depression in animals. Intracranial self-stimulation (ICSS) is a type of operant conditioning procedure that produces stable baseline behavioral response rates. The author reviews recent research on the use of ICSS to evaluate the expression and pharmacological modulation of pain-related behavioral depression in rats. Results suggest that assays of pain-depressed behavior using ICSS may serve as a useful new tool to improve the translation of preclinical findings to clinical results in analgesic drug development.

Abuse-related and abuse-limiting effects of methcathinone and the synthetic "bath salts" cathinone analogs methylenedioxypyrovalerone (MDPV), methylone and mephedrone on intracranial self-stimulation in rats

RATIONALE

Abuse of synthetic cathinones, popularized as "bath salts," has increased dramatically in the USA since their debut in 2010. Preclinical behavioral studies may clarify determinants of the abuse-related effects produced by these compounds.

OBJECTIVES

This study examined behavioral effects of (±)-methcathinone, (±)-3,4-methylenedioxypyrovalerone (MDPV), (±)-3,4-methylenedioxymethcathinone (methylone), and (±)-4-methylmethcathinone (mephedrone) in rats using intracranial self-stimulation (ICSS).

METHODS

Male Sprague-Dawley rats (n = 18) with electrodes targeting the medial forebrain bundle responded for multiple frequencies of brain stimulation and were tested in two phases. First, dose-effect curves for methcathinone (0.1-1.0 mg/kg), MDPV (0.32-3.2 mg/kg), methylone (1.0-10 mg/kg), and mephedrone (1.0-10 mg/kg) were determined. Second, time courses were determined for effects produced by the highest dose of each compound.

RESULTS

Methcathinone produced dose- and time-dependent facilitation of ICSS. MDPV, methylone, and mephedrone produced dose- and time-dependent increases in low rates of ICSS maintained by low brain stimulation frequencies, but also produced abuse-limiting depression of high ICSS rates maintained by high brain stimulation frequencies. Efficacies to facilitate ICSS were methcathinone ≥ MDPV ≥ methylone > mephedrone. Methcathinone was the most potent compound, and MDPV was the longest acting compound.

CONCLUSIONS

All compounds facilitated ICSS at some doses and pretreatment times, which is consistent with abuse liability for each of these compounds. However, efficacies of compounds to facilitate ICSS varied, with methcathinone displaying the highest efficacy and mephedrone displaying the lowest efficacy to facilitate ICSS.

The Effect of Acute Intra Locus Coeruleus (LC) Microinfusion of Bupropion on Formalin-Induced Pain Behavior in Rat

INTRODUCTION

Inflammatory pain is a common sign of chronic diseases. Some brain regions such as locus coeruleus (LC) of the brainstem nor-epinephrine (NE) system have a key role in The mechanisms of the pain modulation and dependence. Bupropion synthesized as an antidepressant, but it is using for smoke cessation. It can change morphine withdrawal signs such as pain related behaviors. This study tested the acute effect of intra-LC microinfusion of bupropion on the formalin-induced pain behavior in rats.

METHODS

Wistar male rats were divided into 6 groups (control-naïve, control-operated, shamoperated, and 3 treated groups with 10(-2), 10(-3), 10(-4) mol/µl intra-LC of bupropion). The injection guide cannulae were implanted into LC nuclei bilaterally by stereotaxic coordinated surgery under sterile condition. The sham group received normal saline as drug vehicle but control groups had no intra-LC injections. Formalin (50 µl, 2.5%) was injected subcutaneously in plantar region of the right hindpaw in all animals (30 min after drug administration in treated animals). Nociceptive signs were observed continuously and registered on-line each minute. Common pain scoring was used for pain assessment.

RESULTS

The analysis of data by one-way ANOVA showed that bupropion can reduce pain behavior scores significantly. Bupropion reduced total pain score in the phase 01 (60%) and phase 02 (52%) of maximal behavior compared to the sham group, dose dependently and significantly. The pain scores of controls and sham groups had no significant difference.

DISCUSSION

The results showed that bupropion has analgesic effects on LC neurons and can alter the neurochemical involvement of LC in pain process. Bupropion has different and significant effect on early and late phases of formalin test.

An intricate relationship between pain and depression: clinical correlates, coactivation factors and therapeutic targets

INTRODUCTION

An apparent clinical relationship between pain and depression has long been recognized, which makes an enormous impact on the individual health care. At present, the practical implication of such overlapping symptomatology between pain and depression is not clear, but the prevalence estimates for depression are substantially inflated among patients with chronic pain and vice versa. This interaction has been labeled as the depression-pain syndrome or depression-pain dyad.

AREAS COVERED

This article discusses the neurobiological substrates and neuroanatomical pathways involved in pain-depression dyad along with newer therapeutic targets.

EXPERT OPINION

Several key themes emerged from our review of the relationship between depression and pain. First, the diagnosis of depression in pain or vice versa is particularly challenging, and the development of better diagnostic framework that involves both pain and depression is particularly required. Secondly, the entwined relationship between pain and depression supports the possibility of common coactivating factors that results in their neurophysiological overlap. A broad understanding of the role played by the central nervous system (CNS) in the processing of pain and depression may eventually lead to the introduction of triple reuptake inhibitors, agomelatine, vilazodone and ketamine with novel mechanism of action, hence appear to be of promising potential for pain with depression.