Altered nociception in mice with genetically induced hypoglutamatergic tone

Midazolam, methamphetamine, morphine and nicotine intake in high‐drinking‐in‐the‐dark mice

The high-drinking-in-the-dark (HDID) lines of mice were selectively bred for achieving high blood alcohol levels in the drinking-in-the-dark (DID) task and have served as a unique genetic risk model for binge-like alcohol intake. However, little is known about their willingness to consume other addictive drugs. Here, we examined (a) whether the HDID-1 and HDID-2 lines of mice would voluntarily consume midazolam, methamphetamine, morphine and nicotine in a DID test and (b) whether the HDID lines differ from their founders, heterogeneous stock/Northport (HS/NPT), in consumption levels of these drugs at the concentrations tested. Separate groups of HDID-1, HDID-2 and HS/NPT mice were given 4 days of access to each drug, using the single-bottle, limited-access DID paradigm. Male and female mice of both HDID lines consumed all four offered drugs. We observed no genotype differences in 40 μg/ml methamphetamine intake, but significant differences in nicotine, midazolam and morphine intake. Both HDID lines drank significantly more (150 μg/ml) midazolam than their founders, providing strong support for a shared genetic contribution to binge ethanol and midazolam intake. HDID-2 mice, but not HDID-1 mice, consumed more morphine (700 μg/ml) and more nicotine across a range of concentrations than HS/NPT mice. These results demonstrate that the HDID mice can be utilized for tests of voluntary drug consumption other than ethanol and highlight potentially important differences between HDID lines in risk for elevated drug intake.

SGK1.1 isoform is involved in nociceptive modulation, offering a protective effect against noxious cold stimulus in a sexually dimorphic manner

The serum and glucocorticoid-regulated kinase 1 (SGK1) is a widely expressed protein in the Central Nervous System (CNS), involved in regulating the activity of a wide variety of ion channels and transporters and physiological functions, such as neuronal excitability. SGK1.1 is a neuronal splice isoform of SGK1, expressed exclusively in the CNS, distributed in brain and cerebellum, that decreases neuronal excitability via up-regulation of M-current, linked to Kv7.2/3 potassium channels. Strategies to maintain increased SGK1.1 activity could be helpful in decreasing neuronal hyperexcitability, as occurs in neuropathic pain. Transgenic mice overexpressing SGK1.1 (B6.Tg.sgk1) offer a particularly relevant opportunity to assess the physiological involvement of this protein in nociception. Behavior and physiological nociception were evaluated in male and female B6.Tg.sgk1 and wild-type mice (B6.WT), characterizing nociceptive thresholds to different nociceptive stimuli (thermal, chemical and mechanical), as well as the electrophysiological properties of cutaneous sensory Aδ-fibres isolated from the saphenous nerve. The acute antinociceptive effect of morphine was also evaluated. Compared with B6.WT animals, male and female B6.Tg.sgk1 mice showed increased spontaneous locomotor activity. Regarding nociception, there were no differences between transgenic and wild-type mice in heat, chemical and mechanical thresholds, but interestingly, male B6.Tg.sgk1 mice were less sensitive to cold stimulus; B6.Tg.sgk1 animals showed lower sensitivity to morphine. Electrophysiological properties of cutaneous primary afferent fibres were maintained. This is the first demonstration that the SGK1.1 isoform is involved in nociceptive modulation, offering a protective effect against noxious cold stimulus in a sexually dimorphic manner. B6.Tg.sgk1 mice offer a particularly relevant opportunity to further analyze the involvement of this protein in nociception, and studies in models of chronic, neuropathic pain are warranted.

Antinociceptive and Anti-Inflammatory Effects of Recombinant Crotamine in Mouse Models of Pain

Crotamine, a toxin found in the venom of the South American rattlesnake Crotalus durissus terrificus, has been reported to have antinociceptive effects. We purified recombinant crotamine expressed in Escherichia coli and investigated its antinociceptive and anti-inflammatory effects using the hot-plate test, acetic-acid-induced writhing method, and formalin test in mice. Recombinant crotamine was administered intraperitoneally (0.04–1.2 mg kg⁻¹) or intraplantarly (0.9–7.5 μg 10 μL⁻¹) before the tests. The paw volume was measured with a plethysmometer. To evaluate the antagonistic and anti-inflammatory effects of naloxone, subcutaneous naloxone (4 mg kg⁻¹) or intraplantar naloxone (5 μg 10 μL⁻¹) was administered before recombinant crotamine. For tumor necrosis factor (TNF)-α assays, blood was drawn 3 h after formalin injection and measured using enzyme-linked immunosorbent assay. Intraperitoneal and intraplantar recombinant crotamine had antinociceptive and anti-inflammatory effects, neither of which were affected by pre-treatment with naloxone. The mean serum TNF-α levels were significantly lower in the intraperitoneal recombinant crotamine (0.4 and 1.2 mg kg⁻¹) or intraplantar (2.5 and 7.5 μg 10 μL⁻¹) recombinant crotamine groups than in the saline group and were not affected by naloxone pre-treatment. In conclusion, recombinant crotamine possesses significant antinociceptive and anti-inflammatory effects that do not appear to be related to the opioid receptor. The antinociceptive and anti-inflammatory effects of intraperitoneal or intraplantar recombinant crotamine are related to TNF-α.

Preserved Thermal Pain in 3xTg-AD Mice With Increased Sensory-Discriminative Pain Sensitivity in Females but Affective-Emotional Dimension in Males as Early Sex-Specific AD-Phenotype Biomarkers

The increase of the aging population, where quite chronic comorbid conditions are associated with pain, draws growing interest across its investigation and the underlying nociceptive mechanisms. Burn injuries associated problems might be of relevance in the older adult’s daily life, but in people with dementia, exposure to high temperatures and heat sources poses a significantly increased risk of burns. In this brief report, the hind paws and tail pain withdrawal reflexes and the emotional responses to thermal nociception in 3xTg-AD mice were characterized for the first time in the plantar test and compared to their non-transgenic (NTg) counterparts. We studied a cohort of male and female 3xTg-AD mice at asymptomatic (2 months), early (6 months), middle (9 months), and advanced (12 and 15 months) stages of the disease and as compared to sex- and age-matched NTg control mice with normal aging. At 20 and 40W intensities, the sensorial-discriminative thresholds eliciting the withdrawal responses were preserved from asymptomatic to advanced stages of the disease compared to NTg counterparts. Moreover, 3xTg-AD females consistently showed a greater sensory-discriminative sensitivity already at premorbid ages, whereas increased emotionality was shown in males. False-negative results were found in “blind to sex and age” analysis, warning about the need to study sexes independently. The current results and previous report in cold thermal stimulation provide two paradigms unveiling sex-specific early AD-phenotype nociceptive biomarkers to study the mechanistic underpinnings of sex-, age- and AD-disease-dependent thermal pain sensitivity.

GLS1 Mutant Mice Display Moderate Alterations of Hippocampal Glutamatergic Neurotransmission Associated with Specific Adaptive Behavioral Changes

Significant alterations in glutamatergic neurotransmission have been reported in major depressive disorder (MDD) that could underlie psychiatric traits. Studies were mainly interested in synaptic dysfunction in the prefrontal cortex, a key structure involved in depressive-like behavior, however hippocampus has been shown to be important in MDD. As cognitive deficits such as hippocampus-memory process were observed in MDD, we investigated in a mild hypoglutamatergic model behaviors related to depression and memory, synaptic transmission parameters and glutamatergic state specifically in the hippocampus. We thus characterized these phenotypes in adult male mice partially depleted in glutaminase type 1 or GLS1 (GLS1 HET), the enzyme responsible for glutamate synthesis in neurons, that we previously characterized as displaying moderate lower levels of glutamate in brain. We showed that GLS1 mutant mice display AMPA-R-mediated response deficits after prolonged repetitive stimulation with electrophysiological recording and inability to sustain glutamate release by microdialysis experiments with no consequences on behavioral spatial learning performances. However, their ability to escape from unpleasant but repeated escapable condition was attenuated whereas they were more immobile in the unescapable situation in the FST during re-test. These results show that GLS1 mutant mice display moderate impairments of hippocampal glutamatergic neurotransmission and moderate changes in adaptive behaviors that have been shown to participate to the development of depressive-like state.

Endogenous Opiates and Behavior: 2015

This paper is the thirty-eighth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2015 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia, stress and social status, tolerance and dependence, learning and memory, eating and drinking, drug abuse and alcohol, sexual activity and hormones, pregnancy, development and endocrinology, mental illness and mood, seizures and neurologic disorders, electrical-related activity and neurophysiology, general activity and locomotion, gastrointestinal, renal and hepatic functions, cardiovascular responses, respiration and thermoregulation, and immunological responses.

Opioid system mediated anti-nociceptive effect of agomelatine in mice

AIMS

This study was planned to examine the antinociceptive efficacy of agomelatine against acute mechanical, thermal, and chemical nociceptive stimuli, as well as to determine the opioid receptor subtypes mediating these effects.

MAIN METHODS

Tail-clip, hot-plate, and acetic acid-induced writhing tests were performed to evaluate anti-nociceptive effect. Besides, possible effect of agomelatine on the motor coordination of animals was assessed with a Rota-rod test.

KEY FINDINGS

Agomelatine (40mg/kg and 60mg/kg) significantly prolonged the reaction time of mice in both the tail-clip and hot-plate tests, suggesting the antinociceptive activity is related to both spinal and supraspinal mechanisms. This drug also reduced the number of writhing behaviors indicating the presence of a peripherally mediated antinociceptive effect. Rota-rod testing displayed no notable effect on the motor activity of the animal supporting the conclusion that the observed antinociceptive effect is specific. The agomelatine-induced antinociceptive activity abrogated following pretreatment with naloxone (a non-selective opioid receptor antagonist, 5.48mg/kg, i.p.), which suggested the participation of opioid mechanisms to the antinociception. The possible contribution of μ, δ and ҡ subtypes of opioid receptors to the anti-nociceptive effect were evaluated using naloxonazine (7mg/kg, s.c.), naltrindole (0.99mg/kg, i.p.), and nor-binaltorphimine (1.03mg/kg, i.p.), respectively. Pretreatments using these antagonists abolished the antinociceptive activity of agomelatine in all of the nociceptive test paradigms used, which pointed out that μ, δ, and ҡ opioid receptors participated to the action of agomelatine on pain.

SIGNIFICANCE

These results demonstrated the therapeutic potential of agomelatine in the treatment of pain disorders.

VGLUTs and Glutamate Synthesis-Focus on DRG Neurons and Pain

The amino acid glutamate is the principal excitatory transmitter in the nervous system, including in sensory neurons that convey pain sensation from the periphery to the brain. It is now well established that a family of membrane proteins, termed vesicular glutamate transporters (VGLUTs), serve a critical function in these neurons: they incorporate glutamate into synaptic vesicles. VGLUTs have a central role both under normal neurotransmission and pathological conditions, such as neuropathic or inflammatory pain. In the present short review, we will address VGLUTs in the context of primary afferent neurons. We will focus on the role of VGLUTs in pain triggered by noxious stimuli, peripheral nerve injury, and tissue inflammation, as mostly explored in transgenic mice. The possible interplay between glutamate biosynthesis and VGLUT-dependent packaging in synaptic vesicles, and its potential impact in various pain states will be presented.