Differential sensitivity of mice bred for stress-induced analgesia to morphine and ACEA-1011 in the formalin test

Mechanisms Underlining Inflammatory Pain Sensitivity in Mice Selected for High and Low Stress-Induced Analgesia-The Role of Endocannabinoids and Microglia

In this work we strived to determine whether endocannabinoid system activity could account for the differences in acute inflammatory pain sensitivity in mouse lines selected for high (HA) and low (LA) swim-stress-induced analgesia (SSIA). Mice received intraplantar injections of 5% formalin and the intensity of nocifensive behaviours was scored. To assess the contribution of the endocannabinoid system, mice were intraperitoneally (i.p.) injected with rimonabant (0.3–3 mg/kg) prior to formalin. Minocycline (45 and 100 mg/kg, i.p.) was administered to investigate microglial activation. The possible involvement of the endogenous opioid system was investigated with naloxone (1 mg/kg, i.p.). Cannabinoid receptor types 1 and 2 (Cnr1, Cnr2) and opioid receptor subtype (Oprm1, Oprd1, Oprk1) mRNA levels were quantified by qPCR in the structures of the central nociceptive circuit. Levels of anandamide (AEA) and 2-arachidonoylglycerol (2-AG) were measured by liquid chromatography coupled with the mass spectrometry method (LC-MS/MS). In the interphase, higher pain thresholds in the HA mice correlated with increased spinal anandamide and 2-AG release and higher Cnr1 transcription. Downregulation of Oprd1 and Oprm1 mRNA was noted in HA and LA mice, respectively, however no differences in naloxone sensitivity were observed in either line. As opposed to the LA mice, inflammatory pain sensitivity in the HA mice in the tonic phase was attributed to enhanced microglial activation, as evidenced by enhanced Aif1 and Il-1β mRNA levels. To conclude, Cnr1 inhibitory signaling is one mechanism responsible for decreased pain sensitivity in HA mice in the interphase, while increased microglial activation corresponds to decreased pain thresholds in the tonic inflammatory phase.

Loss of Brain-Derived Neurotrophic Factor (BDNF) Resulting From Congenital- Or Mild Traumatic Brain Injury-Induced Blood-Brain Barrier Disruption Correlates With Depressive-Like Behaviour

Brain-derived neurotrophic factor (BDNF) plays an important role in processes associated with neuroplasticity and neuroprotection. Evidence suggests that decreased BDNF levels in the central nervous system (CNS) represent a mechanism underlying the development of mood disorders. We hypothesize that both congenital and traumatic brain injury (mTBI)-induced blood-brain barrier (BBB) breakdown are responsible for brain BDNF depletion that contributes to the development of depressive-like symptoms. We employed a mouse model of innate differences in BBB integrity with high (HA) and low (LA) permeability. Depressive-like behaviours were determined under chronic mild stress (CMS) conditions or following mTBI using the tail suspension test (TST). Microvascular leakage of the BBB was evaluated using the Evans Blue Dye (EBD) extravasation method. BDNF concentrations in the brain and plasma were measured using the ELISA. Control HA mice with congenitally high BBB permeability showed exacerbated depressive-like behaviours compared with LA mice. In LA mice, with normal BBB function, mTBI, but not CMS, facilitated depressive-like behaviours, which correlated with enhanced BDNF efflux from the brain. In addition, mTBI triggered upregulation of the Bdnf gene in LA mice to compensate for BDNF loss. No alterations in BDNF levels were observed in mTBI and CMS-exposed HA mice. Moreover, CMS did not induce BBB damage or affect depressive-like behaviours in HA mice despite downregulating Bdnf gene expression. To conclude, BDNF efflux through the mTBI-disrupted BBB is strongly linked to the development of depressive-like behaviours, while the depressive phenotype in mice with congenital BBB dysfunction is independent of BDNF leakage.

Bidirectional selection for high and low stress-induced analgesia affects G-protein activity

Mice selected for high (HA) and low (LA) swim stress-induced analgesia (SSIA) are a unique model for studying the genetic background of this phenomenon. HA and LA miceshow substantial differences in the magnitude of the antinociceptive response to stress and when treated with exogenous opioids. However, the direct cause underplaying this distinctive feature has not yet been identified. The current study was designed to investigate the possibility that disturbances in G-protein signaling could explain the divergent response to opioid agonists. Supraspinal and spinal opioid sensitivity was assessed in vivo with intraperitoneal morphine and subsequent thermal stimulus exposure. The level of opioid receptor-mediated G-protein activation was investigated by means of DAMGO and morphine-stimulated [³⁵S]GTPγS assay in the brain and spinal cord homogenates from HA and LA mice. Morphine (3-249 μmol/kg, i.p) was over 6 - and 3 - times more potent in HA than LA mice in the hot plate and tail-flick assays, respectively. Additionally, HA mice showed elevated β - endorphin levels in the brain. Enhanced efficacy of agonist-stimulated [³⁵S]GTPγS binding was detected in opioid receptor-rich limbic regions of HA mice like the hypothalamus and hippocampus. Increased G-protein activity also emerged in the thalamus, periaqueductal gray matter and prefrontal cortex. In conclusion, the magnitude of the antinociceptive response to opioids in HA and LA mice is correlated with alterations in G-protein activation in brain regions responsible for integration and descending modulation of nociceptive information as well as at sites governing the emotional response to stressful stimuli.

Selection for stress-induced analgesia affects the mouse hippocampal transcriptome

Stress responsiveness, including pain sensitivity and stress-induced analgesia (SIA), depends on genotype and, partially, is mediated by hippocampus. The present study examined differences in constitutive gene expression in hippocampus in lines of mice bred for high (HA) and low (LA) swim SIA. Between the lines, we found 1.5-fold or greater differences in expression of 205 genes in the hippocampus in nonstressed animals. The identity of these genes indicates that selective breeding for swim SIA affected many aspects of hippocampal neurons physiology, including metabolism, structural changes, and cellular signaling. Genes involved in calcium signaling pathway, including Slc8a1, Slc8a2, Prkcc, and Ptk2b, were upregulated in LA mice. In HA mice, robust upregulation of genes coding some transcription factors (Klf5) or receptors for neurotensin (Ntsr2) and GABA (Gabard) suggests the genetic basis for a novel mechanism of the non-opioid type of SIA in HA animals. Additional groups of differentially expressed genes represented functional networks involved in carbohydrate metabolism, gene expression regulation, and molecular transport. Our data indicate that selection for a single and very specific stress response trait, swim SIA, alters hippocampal gene expression. The results suggest that individual stress responsiveness may be associated with characteristics of the constitutive hippocampal transcriptome.

Stress-induced analgesia

For over 30 years, scientists have been investigating the phenomenon of pain suppression upon exposure to unconditioned or conditioned stressful stimuli, commonly known as stress-induced analgesia. These studies have revealed that individual sensitivity to stress-induced analgesia can vary greatly and that this sensitivity is coupled to many different phenotypes including the degree of opioid sensitivity and startle response. Furthermore, stress-induced analgesia is influenced by age, gender, and prior experience to stressful, painful, or other environmental stimuli. Stress-induced analgesia is mediated by activation of the descending inhibitory pain pathway. Pharmacological and neurochemical studies have demonstrated involvement of a large number of neurotransmitters and neuropeptides. In particular, there are key roles for the endogenous opioid, monoamine, cannabinoid, gamma-aminobutyric acid and glutamate systems. The study of stress-induced analgesia has enhanced our understanding of the fundamental physiology of pain and stress and can be a useful approach for uncovering new therapeutic targets for the treatment of pain and stress-related disorders.