Genetic differences in response properties of rostral ventromedial medulla neurons to the μ-opioid receptor agonist DAMGO in mouse inbred strains

Fentanyl-induced acute and conditioned behaviors in two inbred mouse lines: Potential role for Glyoxalase

An increase in opioid-overdose deaths was evident before the COVID-19 pandemic, and has escalated since its onset. Fentanyl, a highly potent synthetic opioid, is the primary driver of these recent trends. The current study used two inbred mouse strains, C57BL/6 J and A/J, to investigate the genetics of behavioral responses to fentanyl. Mice were tested for conditioned place preference and fentanyl-induced locomotor activity. C57BL/6J mice formed a conditioned place preference to fentanyl injections and fentanyl increased their activity. Neither effect was noted in A/J mice. We conducted RNA-sequencing on the nucleus accumbens of mice used for fentanyl-induced locomotor activity. Surprisingly, we noted few differentially expressed genes using treatment as the main factor. However many genes differed between strains. We validated differences in two genes: suppressor APC domain containing 1 (Sapcd1) and Glyoxalase 1 (Glo1), with quantitative PCR on RNA from the nucleus accumbens and prefrontal cortex (). In both regions A/J mice had significantly higher expression of both genes than did C57BL/6 J. In prefrontal cortex, fentanyl treatment decreased Glo1 mRNA. Glyoxalase 1 catalyzes the detoxification of reactive alpha-oxoaldehydes such as glyoxal and methylglyoxal, is associated with anxiety and activity levels, and its inhibition reduces alcohol intake. We suggest that future studies assess the ability of Glo1 and related metabolites to modify opioid intake.

Fentanyl effects on breath generation in C57BL/6J and A/J mouse strains

We examined the effect of fentanyl on chemoresponsiveness in mouse strains divergent in the expression of spontaneous and post-hypoxic pauses. Frequency and tidal volume were recorded with plethysmography in A/J and C57BL/6J (B6) male mice. Mice selected at random received an intraperitoneal (IP) injection of either saline, low dose fentanyl (LDF = 0.04 mg/kg), or high dose fentanyl (HDF = 0.4 mg/kg) under hypoxia (8% O2) or hyperoxia (100%O2). LDF produced a decrease in frequency during hypoxia in B6, but not A/J, mice. HDF significantly decreased frequency and tidal volume in both strains under hypoxia and hyperoxia (p<0.01); naloxone, an opioid antagonist, reversed this response. The acute administration of fentanyl at any dose did not promote apneas in strains of mice exhibiting regular or irregular respiratory patterns. However, higher doses depressed respiratory frequency in both strains. The B6 mice responded with a depressive response to hypoxia that did not recover with reoxygenation, but did recover with time or naloxone.

Diabetic neuropathic pain: Physiopathology and treatment

Diabetic neuropathy is a common complication of both type 1 and type 2 diabetes, which affects over 90% of the diabetic patients. Although pain is one of the main symptoms of diabetic neuropathy, its pathophysiological mechanisms are not yet fully known. It is widely accepted that the toxic effects of hyperglycemia play an important role in the development of this complication, but several other hypotheses have been postulated. The management of diabetic neuropathic pain consists basically in excluding other causes of painful peripheral neuropathy, improving glycemic control as a prophylactic therapy and using medications to alleviate pain. First line drugs for pain relief include anticonvulsants, such as pregabalin and gabapentin and antidepressants, especially those that act to inhibit the reuptake of serotonin and noradrenaline. In addition, there is experimental and clinical evidence that opioids can be helpful in pain control, mainly if associated with first line drugs. Other agents, including for topical application, such as capsaicin cream and lidocaine patches, have also been proposed to be useful as adjuvants in the control of diabetic neuropathic pain, but the clinical evidence is insufficient to support their use. In conclusion, a better understanding of the mechanisms underlying diabetic neuropathic pain will contribute to the search of new therapies, but also to the improvement of the guidelines to optimize pain control with the drugs currently available.

Endogenous opiates and behavior: 2012

This paper is the thirty-fifth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2012 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Pronociception from the dorsomedial nucleus of the hypothalamus is mediated by the rostral ventromedial medulla in healthy controls but is absent in arthritic animals

The dorsomedial nucleus of the hypothalamus (DMH) has been proposed to participate in stress-induced hyperalgesia through facilitation of pronociceptive cells in the rostroventromedial medulla (RVM). We hypothesized that the DMH participates in hyperalgesia induced by arthritis. The DMH was pharmacologically manipulated while assessing heat-evoked nociceptive behavior or the discharge rates of pronociceptive RVM ON- and antinociceptive RVM OFF-like cells in NAIVE, SHAM and monoarthritic (ARTH) animals. In NAIVE and SHAM animals, the changes in nociceptive behavior induced by activation of the DMH by glutamate and inhibition by lidocaine were in line with earlier evidence indicating that the DMH has a nociceptive facilitating role. However, in ARTH animals, neither activation nor inhibition of the DMH influenced pain-like behavior evoked by stimulation of an uninflamed skin region (paw and tail). In accordance with these behavioral results, activation or inhibition of the DMH induced pronociceptive changes in the discharge rates of RVM cells in NAIVE and SHAM animals, which suggests that the DMH has a pronociceptive role mediated by the RVM in normal animals. However, in ARTH animals, both glutamate and lidocaine in the DMH failed to influence either pain-like behavior or noxious stimulation-evoked responses of RVM cells, while blocking the DMH increased spontaneous activity in the pronociceptive RVM ON cells. Our data indicate that the DMH participates in descending facilitation of cutaneous nociception in healthy controls, but it is not engaged in the regulation of cutaneous nociception in monoarthritic animals, while a minor role in tonic suppression of nociception in arthritis cannot be discarded.

Pronociceptive changes in the activity of rostroventromedial medulla (RVM) pain modulatory cells in the streptozotocin-diabetic rat

Neuropathic pain is one of the most frequent complications of diabetes. The increased neuronal activity of primary afferents and spinal cord neurons in streptozotocin (STZ)-diabetic rats increases the recruitment of the nociceptive ascending pathways, which may affect the activity of pain control circuits in the brain. This study aimed to characterize the electrophysiological responses of neurons of the rostroventromedial medulla (RVM), a key brainstem area involved in descending modulation of nociceptive neurotransmission at the spinal cord, in STZ-diabetic rats. Spontaneous and noxious-evoked activity of ON-like cells (pain facilitatory cells) and OFF-like cells (pain inhibitory cells) in the RVM were analyzed by single cell extracellular electrophysiological recordings in STZ-diabetic rats with behavioral signs of diabetic neuropathic pain 4 weeks after diabetes induction and in age-matched non-diabetic controls (CTRL). The electrophysiological analysis revealed an increase in the spontaneous activity of RVM pronociceptive ON-like cells in STZ-diabetic rats when compared to CTRL. On the contrary, the number of active antinociceptive OFF-like cells was significantly lower in the STZ-diabetic rats and their spontaneous activity was decreased when compared with CTRL. Overall, the changes in the activity of RVM pain modulatory cells in STZ-diabetic rats point to enhancement of descending pain facilitation. Based on similar results obtained at the RVM in traumatic neuropathic pain models, the changes in the electrophysiological responses of RVM in STZ-diabetic rats may account for exacerbated pain-like behaviors in diabetic neuropathy.