Optogenetic and pharmacological interventions link hypocretin neurons to impulsivity in mice.

Neurons in the lateral hypothalamus expressing the neuropeptide Hypocretin, also known as orexin, are known critical modulators of arousal stability. However, their role in the different components of the arousal construct such as attention and decision making is poorly understood. Here we study Hypocretin neuronal circuit dynamics during stop action impulsivity in a Go/NoGo task in mice. We show that Hypocretin neuronal activity correlates with anticipation of reward. We then assessed the causal role of Hypocretin neuronal activity using optogenetics in a Go/NoGo task. We show that stimulation of Hypocretin neurons during the cue period dramatically increases the number of premature responses. These effects are mimicked by amphetamine, reduced by atomoxetine, a norepinephrine uptake inhibitor, and blocked by a Hypocretin receptor 1 selective antagonist. We conclude that Hypocretin neurons have a key role in the integration of salient stimuli during wakefulness to produce appropriate and timely responses to rewarding and aversive cues.

Dissociation of impulsive traits by subthalamic metabotropic glutamate receptor 4.

Behavioral strategies require gating of premature responses to optimize outcomes. Several brain areas control impulsive actions, but the neuronal basis of natural variation in impulsivity between individuals remains largely unknown. Here, by combining a Go/No-Go behavioral assay with resting-state (rs) functional MRI in mice, we identified the subthalamic nucleus (STN), a known gate for motor control in the basal ganglia, as a major hotspot for trait impulsivity. In vivo recorded STN neural activity encoded impulsive action as a separable state from basic motor control, characterized by decoupled STN/substantia nigra pars reticulata (SNr) mesoscale networks. Optogenetic modulation of STN activity bidirectionally controlled impulsive behavior. Pharmacological and genetic manipulations showed that these impulsive actions are modulated by metabotropic glutamate receptor 4 (mGlu4) function in STN and its coupling to SNr in a behavioral trait-dependent manner, and independently of general motor function. In conclusion, STN circuitry multiplexes motor control and trait impulsivity, which are molecularly dissociated by mGlu4. This provides a potential mechanism for the genetic modulation of impulsive behavior, a clinically relevant predictor for developing psychiatric disorders associated with impulsivity.

Gait analysis as a robust pain behavioural endpoint in the chronic phase of the monoiodoacetate-induced knee joint pain in the rat.

The monoiodoacetate-induced rat model of osteoarthritis knee pain is widely used. However, there are between-study differences in the pain behavioural endpoints assessed and in the dose of intraarticular monoiodoacetate administered. This study evaluated the robustness of gait analysis as a pain behavioural endpoint in the chronic phase of this model, in comparison with mechanical hyperalgesia in the injected (ipsilateral) joint and development of mechanical allodynia in the ipsilateral hind paws. Groups of Sprague-Dawley rats received a single intraarticular injection of monoiodoacetate at 0.5, 1, 2 or 3 mg or vehicle (saline) into the left (ipsilateral) knee joint. An additional group of rats were not injected (naïve group). The pain behavioural methods used were gait analysis, measurement of pressure algometry thresholds in the ipsilateral knee joints, and assessment of mechanical allodynia in the ipsilateral hind paws using von Frey filaments. These pain behavioural endpoints were assessed premonoiodoacetate injection and for up to 42-days postmonoiodoacetate injection in a blinded manner. Body weights were also assessed as a measure of general health. Good general health was maintained as all rats gained weight at a similar rate for the 42-day study period. In the chronic phase of the model (days 9-42), intraarticular monoiodoacetate at 3 mg evoked robust alterations in multiple gait parameters as well as persistent mechanical allodynia in the ipsilateral hind paws. For the chronic phase of the monoiodoacetate-induced rat model of osteoarthritis knee pain, gait analysis, such as mechanical allodynia in the ipsilateral hind paws, is a robust pain behavioural measure.

Pharmacological characterization of the chronic phase of the monoiodoacetate-induced rat model of osteoarthritis pain in the knee joint.

For patients with osteoarthritis (OA) of the knee, pain is the most debilitating symptom. Although it has been proposed that the chronic phase of the monoiodoacetate (MIA)-induced rodent model of knee joint pain may be superior to other chronic or acute OA models for assessing the analgesic efficacy of novel molecules, relatively few pharmacological studies have been conducted in the chronic phase of this model. Hence, this study was designed to use pharmacological methods to characterize the chronic phase of the MIA-induced rat model of knee joint OA pain. Rats received a single intraarticular injection of MIA at 2.5 mg or vehicle (saline) into the left (ipsilateral) knee joint. Pain behaviour was assessed by measuring paw withdrawal thresholds (PWTs) in the hindpaws pre-MIA injection and twice-weekly until study completion on day 42. Mechanical allodynia was fully developed in the ipsilateral hindpaws (PWTs ≤6 g) from day 7 and it persisted until day 42. MIA-injected rats with PWTs ≤6 g in the ipsilateral hindpaws received single doses of one of four clinically available drugs that represent four distinct pharmacological classes, viz gabapentin, amitriptyline, meloxicam and morphine, according to a 'washout' protocol with at least 48 hours between successive doses. Gabapentin evoked dose-dependent anti-allodynia as did morphine whereas amitriptyline and meloxicam were inactive. Our findings are aligned with clinical data showing that gabapentin and morphine alleviated OA pain in the knee. The lack of efficacy of amitriptyline is consistent with the loss of descending diffuse noxious inhibitory controls reported by others in this model.

Assessment of the Anti-Allodynic and Anti-Hyperalgesic Efficacy of a Glycine Transporter 2 Inhibitor Relative to Pregabalin, Duloxetine and Indomethacin in a Rat Model of Cisplatin-Induced Peripheral Neuropathy.

Cisplatin, which is a chemotherapy drug listed on the World Health Organisation's List of Essential Medicines, commonly induces dose-limiting side effects including chemotherapy-induced peripheral neuropathy (CIPN) that has a major negative impact on quality of life in cancer survivors. Although adjuvant drugs including anticonvulsants and antidepressants are used for the relief of CIPN, analgesia is often unsatisfactory. Herein, we used a rat model of CIPN (cisplatin) to assess the effect of a glycine transporter 2 (GlyT2) inhibitor, relative to pregabalin, duloxetine, indomethacin and vehicle. Male Sprague-Dawley rats with cisplatin-induced mechanical allodynia and mechanical hyperalgesia in the bilateral hindpaws received oral bolus doses of the GlyT2 inhibitor (3-30 mg/kg), pregabalin (3-100 mg/kg), duloxetine (3-100 mg/kg), indomethacin (1-10 mg/kg) or vehicle. The GlyT2 inhibitor alleviated both mechanical allodynia and hyperalgesia in the bilateral hindpaws at a dose of 10 mg/kg, but not at higher or lower doses. Pregabalin and indomethacin induced dose-dependent relief of mechanical allodynia but duloxetine lacked efficacy. Pregabalin and duloxetine alleviated mechanical hyperalgesia in the bilateral hindpaws while indomethacin lacked efficacy. The mechanism underpinning pain relief induced by the GlyT2 inhibitor at 10 mg/kg is likely due to increased glycinergic inhibition in the lumbar spinal cord, although the bell-shaped dose-response curve warrants further translational considerations.

Control of impulsivity by Gi-protein signalling in layer-5 pyramidal neurons of the anterior cingulate cortex.

Pathological impulsivity is a debilitating symptom of multiple psychiatric diseases with few effective treatment options. To identify druggable receptors with anti-impulsive action we developed a systematic target discovery approach combining behavioural chemogenetics and gene expression analysis. Spatially restricted inhibition of three subdivisions of the prefrontal cortex of mice revealed that the anterior cingulate cortex (ACC) regulates premature responding, a form of motor impulsivity. Probing three G-protein cascades with designer receptors, we found that the activation of Gi-signalling in layer-5 pyramidal cells (L5-PCs) of the ACC strongly, reproducibly, and selectively decreased challenge-induced impulsivity. Differential gene expression analysis across murine ACC cell-types and 402 GPCRs revealed that - among Gi-coupled receptor-encoding genes - Grm2 is the most selectively expressed in L5-PCs while alternative targets were scarce. Validating our approach, we confirmed that mGluR2 activation reduced premature responding. These results suggest Gi-coupled receptors in ACC L5-PCs as therapeutic targets for impulse control disorders.

Characterisation of a rat model of mechanical low back pain at an advanced stage using immunohistochemical methods.

Chronic low back pain (LBP) has high prevalence in the adult population which is associated with enormous disability. Hence, our aim was to further characterise our LBP rat model by using immunohistological and immunohistochemical methods at an advanced stage (day 49) of the model. Male Sprague-Dawley rats were anaesthetised and their lumbar L4/L5 and L5/L6 intervertebral discs (IVDs) were punctured (0.5 mm outer diameter, 2 mm-deep) 10 times per disc. Sham-rats underwent similar surgery, but no discs were punctured. For LBP- but not sham-rats, noxious pressure hyperalgesia was fully developed in the lumbar axial deep tissues on day 21 post-surgery, which was maintained until at least day 49. In the lumbar (L4-L6) dorsal root ganglia (DRGs), somatostatin (SRIF) and the somatostatin receptor type 4 (SST4 receptor) were co-localised with substance P and IB4, markers of small diameter unmyelinated peptidergic and non-peptidergic C-fibres respectively as well as with NF200, a marker of medium to large diameter neurons. On day 49, there was increased expression of SRIF but not the somatostatin receptor type 4 (SST4 receptor) in the lumbar DRGs and the spinal dorsal horns. There were increased DRG expression levels of the putative pro-nociceptive mediators: phosphorylated p38 (pp38) mitogen-activated protein kinase (MAPK) and phosphorylated p44/p42 MAPK (pp44/pp42 MAPK) as well as pp38 MAPK expression levels in the lumbar spinal cord. Taken together, the increased expression of SRIF in the lumbar DRGs and spinal cord and its co-localisation with nociceptive fibres in DRG sections suggest a potential role of SRIF in modulating chronic mechanical LBP.

Assessment of the anti-hyperalgesic efficacy of J-2156, relative to clinically available analgesic/adjuvant agents in a rat model of mild to moderate chronic mechanical low back pain (LBP).

Chronic mechanical low back pain (cLBP) is a leading cause of disability and a major socio-economic burden internationally. The lifetime prevalence of non-specific LBP is approximately 84%, with the prevalence of cLBP at about 23%. Clinically available analgesic/adjuvant medications often provide inadequate pain relief in patients experiencing cLBP. Hence, the urgency for discovery of effective and better tolerated medications. Fourteen days after the induction of five shallow annular punctures (5X) in the lumbar intervertebral discs at L4/L5 and L5/L6 in male Sprague-Dawley rats, mechanical hyperalgesia was fully developed in the lumbar axial deep tissues at L4/L5 (primary) and L1 (secondary). Importantly, mechanical allodynia in the hindpaws was absent. From day 28, we assessed the face validity of our mild to moderate LBP-5X rat model using four clinically available analgesic/adjuvant drugs, namely gabapentin, morphine, meloxicam and amitriptyline relative to vehicle. Additionally, the anti-hyperalgesic effects of J-2156, a highly selective small molecule somatostatin type 4 receptor agonist was assessed. Single i.p. bolus doses of gabapentin and meloxicam at the highest doses tested (100 and 30 mg/kg, respectively) alleviated secondary hyperalgesia (L1) but not primary hyperalgesia (L4/5). Morphine at 1 mg/kg alleviated both primary and secondary hyperalgesia in these tissues, whereas amitriptyline at the doses tested, lacked efficacy. These findings attest to the face validity of our model. J-2156 at 10 and 30 mg/kg alleviated secondary hyperalgesia in the lumbar axial deep tissues at L1 with a non-significant trend for relief of primary hyperalgesia in the corresponding tissues at L4/L5 in these animals.

Assessment of the anti-allodynic efficacy of a glycine transporter 2 inhibitor relative to pregabalin and duloxetine in a rat model of prostate cancer-induced bone pain.

BACKGROUND: The pathobiology of prostate cancer-induced bone pain (PCIBP) is underpinned by both inflammatory and neuropathic components. Here, we used a rat model of PCIBP to assess the analgesic efficacy of a glycine transporter 2 (GlyT2) inhibitor (N-(6-((1,3-dihydroxypropan-2-yl)amino)-2-(dimethylamino)pyridin-3-yl)-3,5-dimethoxy-4-(4-(trifluoromethyl)phenoxy) benzamide) relative to two clinically available adjuvant drugs that are recommended for the relief of neuropathic pain, viz, pregabalin and duloxetine. METHODS: PCIBP was induced in male Wistar Han rats following intra-tibial injection (ITI) of rat prostate cancer (AT3B) cells into the left tibia. Sham-rats received an ITI of heat-killed AT3B cells. PCIBP rats with fully developed mechanical allodynia in the ipsilateral hindpaws as assessed using von Frey filaments, received single oral (p.o.) bolus doses of the GlyT2 inhibitor (3-30 mg/kg), pregabalin (3-100 mg/kg), duloxetine (3-100 mg/kg), or vehicle. Baseline paw withdrawal thresholds (PWTs) were determined in the ipsilateral (injured side) and contralateral hindpaws immediately prior to dosing and at scheduled times for 3 h post dosing in individual animals. RESULTS: Single oral bolus doses of the GlyT2 inhibitor (3-30 mg/kg) evoked partial pain relief at the doses tested in the ipsilateral hindpaws of PCIBP rats without any discernible behavioural side effects. By contrast, single oral bolus doses of pregabalin at 10-100 mg/kg evoked dose-dependent and complete alleviation of mechanical allodynia. By comparison, single oral bolus doses of duloxetine at doses up to 100 mg/kg lacked efficacy. CONCLUSION: Oral administration of this GlyT2 inhibitor evoked partial pain relief in PCIBP rats and did not evoke central nervous system side effects in contrast to GlyT2 inhibitors reported by others.

Diminished Myoinositol in Ventromedial Prefrontal Cortex Modulates the Endophenotype of Impulsivity.

Maladaptive impulsivity manifests in a variety of disorders, including attention-deficit hyperactivity disorder (ADHD), depression, and substance use disorder. However, the etiological mechanisms of impulsivity remain poorly understood. In the present study, we used in-vivo proton magnetic resonance spectroscopy (1H-MRS) to investigate neurometabolite content in the prefrontal cortex (PFC) and striatum of rats exhibiting low- versus high-impulsive (LI, HI) behavior on a visual attentional task. We validated our 1H-MRS findings using regionally resolved ex-vivo mass spectroscopy, transcriptomics, and site-directed RNA interference in the ventromedial PFC. We report a significant reduction in myoinositol levels in the PFC but not the striatum of HI rats compared with LI rats. Reduced myoinositol content was localized to the infralimbic (IL) cortex, where significant reductions in transcript levels of key proteins involved in the synthesis and recycling of myoinositol (IMPase1) were also present. Knockdown of IMPase1in the IL cortex increased impulsivity in nonimpulsive rats when the demand on inhibitory response control was increased. We conclude that diminished myoinositol levels in ventromedial PFC causally mediate a specific form of impulsivity linked to vulnerability for stimulant addiction in rodents. Myoinositol and related signaling substrates may thus offer novel opportunities for treating neuropsychiatric disorders comorbid with impulsive symptomology.

Minimum Information in In Vivo Research.

Data quality, reproducibility and reliability are a matter of concern in many scientific fields including biomedical research. Robust, reproducible data and scientific rigour form the foundation on which future studies are built and determine the pace of knowledge gain and the time needed to develop new and innovative drugs that provide benefit to patients. Critical to the attainment of this is the precise and transparent reporting of data. In the current chapter, we will describe literature highlighting factors that constitute the minimum information that is needed to be included in the reporting of in vivo research. The main part of the chapter will focus on the minimum information that is essential for reporting in a scientific publication. In addition, we will present a table distinguishing information necessary to be recorded in a laboratory notebook or another form of internal protocols versus information that should be reported in a paper. We will use examples from the behavioural literature, in vivo studies where the use of anaesthetics and analgesics are used and finally ex vivo studies including histological evaluations and biochemical assays.

Transcriptomic characterisation of the optimised rat model of Walker 256 breast cancer cell-induced bone pain.

In patients with breast cancer, metastases of cancer cells to the axial skeleton may cause excruciating pain, particularly in the advanced stages. The current drug treatments available to alleviate this debilitating pain condition often lack efficacy and/or produce undesirable side effects. Preclinical animal models of cancer-induced bone pain are key to studying the mechanisms that cause this pain and for the success of drug discovery programs. In a previous study conducted in our laboratory, we validated and characterised the rat model of Walker 256 cell-induced bone pain, which displayed several key resemblances to the human pain condition. However, gene level changes that occur in the pathophysiology of cancer-induced bone pain in this preclinical model are unknown. Hence, in this study, we performed the transcriptomic characterisation of the Walker 256 cell line cultured in vitro to predict the molecular genetic profile of this cell line. We also performed transcriptomic characterisation of the Walker 256 cell-induced bone pain model in rats using the lumbar spinal cord and lumbar dorsal root ganglia tissues. Here we show that the Walker 256 cell line resembles the basal-B molecular subtype of human breast cancer cell lines. We also identify several genes that may underpin the progression of pain hypersensitivities in this condition, however, this needs further confirmatory studies. These transcriptomic insights have the potential to direct future studies aimed at identifying various mechanisms underpinning pain hypersensitivities in this model that may also assist in discovery of novel pain therapeutics for breast cancer-induced bone pain.

J-2156, a somatostatin receptor type 4 agonist, alleviates mechanical hyperalgesia in a rat model of chronic low back pain.

Chronic low back pain (LBP) ranks among the most common reasons for patient visits to healthcare providers. Drug treatments often provide only partial pain relief and are associated with considerable side-effects. J-2156 [(1'S,2S)-4amino-N-(1'-carbamoyl-2'-phenylethyl)-2-(4"-methyl-1"-naphthalenesulfonylamino)butanamide] is an agonist that binds with nanomolar affinity to the rat and human somatostatin receptor type 4 (SST4 receptor). Hence, our aim was to assess the efficacy of J-2156 for relief of chronic mechanical LBP in a rat model. Male Sprague Dawley rats were anaesthetised and their lumbar L4/L5 and L5/L6 intervertebral discs (IVDs) were punctured (0.5 mm outer diameter, 2 mm-deep) 10 times per disc. Sham-rats underwent similar surgery, but without disc puncture. For LBP-rats, noxious pressure hyperalgesia developed in the lumbar axial deep tissues from day 7 to day 21 post-surgery, which was maintained until study completion. Importantly, mechanical hyperalgesia did not develop in the lumbar axial deep tissues of sham-rats. In LBP-rats, single intraperitoneal (i.p.) injection of J-2156 (3, 10, 30 mg kg-1) alleviated primary and secondary hyperalgesia in the lumbar axial deep tissues at L4/L5 and L1, respectively. This was accompanied by a reduction in the otherwise augmented lumbar (L4-L6) dorsal root ganglia expression levels of the pro-nociceptive mediators: phosphorylated p38 (pp38) mitogen-activated protein kinase (MAPK) and phosphorylated p44/p42 MAPK and a reduction in pp38 MAPK in the lumbar enlargement of the spinal cord. The SST4 receptor is worthy of further investigation as a target for discovery of novel analgesics for the relief of chronic LBP.

Establishment and characterisation of a stavudine (d4T)-induced rat model of antiretroviral toxic neuropathy (ATN) using behavioural and pharmacological methods.

Human immuno-deficiency virus (HIV) associated sensory neuropathy (SN) is a frequent complication of HIV infection. It is extremely difficult to alleviate and hence the quality of life of affected individuals is severely and adversely impacted. Stavudine (d4T) is an antiretroviral drug that was widely used globally prior to 2010 and that is still used today in resource-limited settings. Its low cost and relatively good efficacy when included in antiretroviral dosing regimens means that there is a large population of patients with d4T-induced antiretroviral toxic neuropathy (ATN). As there are no FDA approved drugs for alleviating ATN, it is important to establish rodent models to probe the pathobiology and to identify potentially efficacious new drug treatments. In the model establishment phase, d4T administered intravenously at a cumulative dose of 375 mg/kg in male Wistar Han rats evoked temporal development of sustained mechanical allodynia in the hindpaws from day 10 to day 30 after initiation of d4T treatment. As this d4T dosing regimen was also well tolerated, it was used for ATN model induction for subsequent pharmacological profiling. Both gabapentin at 30-100 mg/kg and morphine at 0.3-2 mg/kg given subcutaneously produced dose-dependent relief of mechanical allodynia with estimated ED50's of 19 mg/kg and 0.4 mg/kg, respectively. In contrast, intraperitoneal administration of meloxicam or amitriptyline up to 30 mg/kg and 7 mg/kg, respectively, lacked efficacy. Our rat model of ATN is suitable for investigation of the pathophysiology of d4T-induced SN as well as for profiling novel molecules from analgesic drug discovery programs.

The Somatostatin Receptor-4 Agonist J-2156 Alleviates Mechanical Hypersensitivity in a Rat Model of Breast Cancer Induced Bone Pain.

In the majority of patients with breast cancer in the advanced stages, skeletal metastases are common, which may cause excruciating pain. Currently available drug treatments for relief of breast cancer-induced bone pain (BCIBP) include non-steroidal anti-inflammatory drugs and strong opioid analgesics along with inhibitors of osteoclast activity such as bisphosphonates and monoclonal antibodies such as denosumab. However, these medications often lack efficacy and/or they may produce serious dose-limiting side effects. In the present study, we show that J-2156, a somatostatin receptor type 4 (SST4 receptor) selective agonist, reverses pain-like behaviors in a rat model of BCIBP induced by unilateral intra-tibial injection of Walker 256 breast cancer cells. Following intraperitoneal administration, the ED50 of J-2156 for the relief of mechanical allodynia and mechanical hyperalgesia in the ipsilateral hindpaws was 3.7 and 8.0 mg/kg, respectively. Importantly, the vast majority of somatosensory neurons in the dorsal root ganglia including small diameter C-fibers and medium-large diameter fibers, that play a crucial role in cancer pain hypersensitivities, expressed the SST4 receptor. J-2156 mediated pain relief in BCIBP-rats was confirmed by observations of a reduction in the levels of phosphorylated extracellular signal-regulated kinase (pERK), a protein essential for central sensitization and persistent pain, in the spinal dorsal horn. Our results demonstrate the potential of the SST4 receptor as a pharmacological target for relief of BCIBP and we anticipate the present work to be a starting point for further mechanism-based studies.

Comparative analgesic efficacy of pregabalin administered according to either a prevention protocol or an intervention protocol in rats with cisplatin-induced peripheral neuropathy.

Chemotherapy-induced peripheral neuropathy (CIPN) is a type of peripheral neuropathic pain that may be dose-limiting in patients administered potentially curative cancer chemotherapy dosing regimens. In cancer survivors, persistent CIPN adversely affects patient quality of life and so adjuvant drugs (anticonvulsants eg pregabalin or antidepressants eg amitriptyline) are recommended for the relief of CIPN. However, most studies in rodent models of CIPN involve administration of single bolus doses of adjuvant drugs to assess pain-relieving efficacy. Hence this study was designed to assess the efficacy of pregabalin administered to CIPN-rats according to either a prevention or an intervention protocol. Groups of male Sprague-Dawley rats received four single intraperitoneal bolus doses of cisplatin at 3 mg/kg at once-weekly intervals to induce CIPN. For the prevention protocol, oral pregabalin (or vehicle) was administered to CIPN-rats once-daily for 21 consecutive days from day 0 to day 20 inclusive. For the intervention protocol, oral pregabalin was administered once-daily for 21 consecutive days from day 28 to day 48, inclusive. Mechanical allodynia and mechanical hyperalgesia in the bilateral hindpaws were assessed just prior to each dose of cisplatin and at least once weekly until study completion (day 27, prevention protocol; or day 48, intervention protocol). Mechanical allodynia and mechanical hyperalgesia were also determined at the time of peak effect at about 2 hours post pregabalin/vehicle administration, once weekly until study completion. For the prevention protocol in CIPN-rats, pregabalin alleviated mechanical hyperalgesia but not mechanical allodynia. For the intervention protocol, pregabalin alleviated both mechanical allodynia and mechanical hyperalgesia in the hindpaws.

The research domain criteria framework in drug discovery for neuropsychiatric diseases: focus on negative valence.

Drug discovery, particularly in the field of central nervous system, has had very limited success in the last few decades. A likely contributor is the poor translation between preclinical and clinical phases. The Research Domain Criteria of the National Institutes of Mental Health is a framework which aims to identify new ways of classifying mental illnesses that are based on observable behaviour and neurobiological measures, and to provide a guiding and evolving framework to improve the translation from preclinical to clinical research. At the core of the Research Domain Criteria approach is the assumption that the dimensional constructs described can be assessed across different units of analysis, thus enabling a more precise quantitative understanding of their neurobiological underpinnings, increasing the likelihood of identifying new and effective therapeutic approaches. In the present review, we discuss how the Research Domain Criteria can be applied to drug discovery with the domain Negative Valence, construct Potential Threat ('Anxiety') as an example. We will discuss the evidence supporting the utility of the Research Domain Criteria approach and evaluate how close we are to achieving a common thread of translational research from gene to self-report.

Establishment and Characterization of a Novel Rat Model of Mechanical Low Back Pain Using Behavioral, Pharmacologic and Histologic Methods.

Chronic low back pain (LBP), the leading cause of disability globally, is notoriously difficult to treat. Most rodent models of LBP mimic lumbar radicular pain rather than mechanical LBP. Here, we describe establishment of a new rat model of mechanical LBP that is devoid of a neuropathic component. Groups of adult male Sprague Dawley rats were anesthetized and their lumbar L4/L5 and L5/L6 intervertebral disks (IVDs) were punctured (0.5 mm outer diameter, 2mm-deep) 5 (LPB-5X), or 10 (LBP-10X) times per disk. Sham-rats underwent similar surgery, but without disk puncture. Baseline noxious pressure hyperalgesia of lumbar axial deep tissues, mechanical allodynia in the hindpaws and gait were assessed prior to surgery and once-weekly until study completion on day 49. The model was also characterized using pharmacologic and histologic methods. Good animal health was maintained for ≥ 49 days post-surgery. For LBP- but not sham-rats, there was temporal development of noxious pressure hyperalgesia in lumbar axial deep tissues at days 14-49 post-surgery. Importantly, there were no between-group differences in von Frey paw withdrawal thresholds or gait parameters until study completion. On day 49, significant histologic changes were observed in the L4/L5 and L5/L6 IVDs for LBP-10X rats, but not sham-rats. In LBP-10X rats, single bolus doses of morphine produced dose-dependent relief of primary and secondary mechanical hyperalgesia in lumbar axial deep tissues at L4/L5 and L1, respectively. In conclusion, our new rat model has considerable potential for providing novel insight on the pathobiology of mechanical LBP and for analgesic efficacy assessment of novel compounds.

Selective and interactive effects of D2 receptor antagonism and positive allosteric mGluR4 modulation on waiting impulsivity.

BACKGROUND: Metabotropic glutamate receptor 4 (mGluR4) and dopamine D2 receptors are specifically expressed within the indirect pathway neurons of the striato-pallidal-subthalamic pathway. This unique expression profile suggests that mGluR4 and D2 receptors may play a cooperative role in the regulation and inhibitory control of behaviour. We investigated this possibility by testing the effects of a functionally-characterised positive allosteric mGluR4 modulator, 4-((E)-styryl)-pyrimidin-2-ylamine (Cpd11), both alone and in combination with the D2 receptor antagonist eticlopride, on two distinct forms of impulsivity. METHODS: Rats were trained on the five-choice serial reaction time task (5-CSRTT) of sustained visual attention and segregated according to low, mid, and high levels of motor impulsivity (LI, MI and HI, respectively), with unscreened rats used as an additional control group. A separate group of rats was trained on a delay discounting task (DDT) to assess choice impulsivity. RESULTS: Systemic administration of Cpd11 dose-dependently increased motor impulsivity and impaired attentional accuracy on the 5-CSRTT in all groups tested. Eticlopride selectively attenuated the increase in impulsivity induced by Cpd11, but not the accompanying attentional impairment, at doses that had no significant effect on behavioural performance when administered alone. Cpd11 also decreased choice impulsivity on the DDT (i.e. increased preference for the large, delayed reward) and decreased locomotor activity. CONCLUSIONS: These findings demonstrate that mGluR4s, in conjunction with D2 receptors, affect motor- and choice-based measures of impulsivity, and therefore may be novel targets to modulate impulsive behaviour associated with a number of neuropsychiatric syndromes.

Comparison of Burrowing and Stimuli-Evoked Pain Behaviors as End-Points in Rat Models of Inflammatory Pain and Peripheral Neuropathic Pain.

Establishment and validation of ethologically-relevant, non-evoked behavioral end-points as surrogate measures of spontaneous pain in rodent pain models has been proposed as a means to improve preclinical to clinical research translation in the pain field. Here, we compared the utility of burrowing behavior with hypersensitivity to applied mechanical stimuli for pain assessment in rat models of chronic inflammatory and peripheral neuropathic pain. Briefly, groups of male Sprague-Dawley rats were habituated to the burrowing environment and trained over a 5-day period. Rats that burrowed ≤ 450 g of gravel on any 2 days of the individual training phase were excluded from the study. The remaining rats received either a unilateral intraplantar injection of Freund's complete adjuvant (FCA) or saline, or underwent unilateral chronic constriction injury (CCI) of the sciatic nerve- or sham-surgery. Baseline burrowing behavior and evoked pain behaviors were assessed prior to model induction, and twice-weekly until study completion on day 14. For FCA- and CCI-rats, but not the corresponding groups of sham-rats, evoked mechanical hypersensitivity developed in a temporal manner in the ipsilateral hindpaws. Although burrowing behavior also decreased in a temporal manner for both FCA-and CCI- rats, there was considerable inter-animal variability. By contrast, mechanical hyperalgesia and mechanical allodynia in the ipsilateral hindpaws of FCA- and CCI-rats respectively, exhibited minimal inter-animal variability. Our data collectively show that burrowing behavior is altered in rodent models of chronic inflammatory pain and peripheral neuropathic pain. However, large group sizes are needed to ensure studies are adequately powered due to considerable inter-animal variability.