Oligogenic determination of morphine analgesic magnitude: a genetic analysis of selectively bred mouse lines

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.

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.

Antigen-dependent release of IFN-gamma by cytotoxic T cells up-regulates Fas on target cells and facilitates exocytosis-independent specific target cell lysis

Effector cytolytic T (Tc) lymphocytes, deficient in the exocytosis-mediated pathway of target cell lysis, induce Fas on target cells and, in turn, delayed cell death and apoptosis via the Fas ligand-Fas interaction. The induction of Fas can be blocked by anti- IFN-gamma Abs. This Fas up-regulation on initially Fas-negative target cells is not mediated by TCR-MHC/peptide signaling per se, but by secreted IFN-gamma from Tc cells after Ag engagement. The Fas up-regulation by Tc cells can be mimicked by treatment of target cells with rIFN-gamma. Tc cells from IFN-gamma knockout mice do not induce Fas expression on target cells. Tc cell-mediated Fas expression on third party, bystander, target cells does not enhance their susceptibility to lysis by these nominal effector cells. The results are discussed as to the possible relevance of the phenomenon in efficiency and regulation of the Tc cell response to infections by viruses.

Genetic contributions to addiction

Even the most extreme environmentalists along the nature-nurture continuum in psychology now acknowledge that genes often contribute to individual differences in behavior. Behavioral traits are complex, reflecting the aggregate effects of many genes. These genetic effects are interactive, inter se and with the environments in which they are expressed. Human studies of addictive behaviors have clearly implicated both environmental and genetic influences. This review selects drug dependence as a paradigmatic addiction, and further, concentrates on the extensive literature with genetic animal models. Both traditional studies with inbred strains and selected lines and studies exploiting the new molecularly based technologies of the genomics era are discussed. Future directions for further contribution of animal models studies to our understanding of the brain dysregulations characteristic of addictions are identified.

Murine models of inflammatory, neuropathic and cancer pain each generates a unique set of neurochemical changes in the spinal cord and sensory neurons

The aim of this investigation was to determine whether murine models of inflammatory, neuropathic and cancer pain are each characterized by a unique set of neurochemical changes in the spinal cord and sensory neurons. All models were generated in C3H/HeJ mice and hyperalgesia and allodynia behaviorally characterized. A variety of neurochemical markers that have been implicated in the generation and maintenance of chronic pain were then examined in spinal cord and primary afferent neurons.Three days after injection of complete Freund's adjuvant into the hindpaw (a model of persistent inflammatory pain) increases in substance P, calcitonin gene-related peptide, protein kinase C gamma, and substance P receptor were observed in the spinal cord. Following sciatic nerve transection or L5 spinal nerve ligation (a model of persistent neuropathic pain) significant decreases in substance P and calcitonin gene-related peptide and increases in galanin and neuropeptide Y were observed in both primary afferent neurons and the spinal cord. In contrast, in a model of cancer pain induced by injection of osteolytic sarcoma cells into the femur, there were no detectable changes in any of these markers in either primary afferent neurons or the spinal cord. However, in this cancer-pain model, changes including massive astrocyte hypertrophy without neuronal loss, increase in the neuronal expression of c-Fos, and increase in the number of dynorphin-immunoreactive neurons were observed in the spinal cord, ipsilateral to the limb with cancer. These results indicate that a unique set of neurochemical changes occur with inflammatory, neuropathic and cancer pain in C3H/HeJ mice and further suggest that cancer induces a unique persistent pain state. Determining whether these neurochemical changes are involved in the generation and maintenance of each type of persistent pain may provide insight into the mechanisms that underlie each of these pain states.

The impact of genomics on mammalian neurobiology

The benefit of genomics lies in the speeding up of research efforts in other fields of biology, including neurobiology. Through accelerated progress in positional cloning and genetic mapping, genomics has forced us to confront at a much faster pace the difficult problem of defining gene function. Elucidation of the function of identified disease genes and other genes expressed in the Central nervous system has to await conceptual developments in other fields.

Mu and delta opioid receptor analgesia, binding density, and mRNA levels in mice selectively bred for high and low analgesia

The present study examined mu and delta opioid analgesia, receptor binding, and receptor mRNA levels in lines of mice from two selective breeding projects of relevance to opioid analgesia. Large differences were observed in the analgesic potency of [d-Ala2, NMPhe4, Gly-ol]enkephalin (DAMGO), [d-Pen2,5]enkephalin (DPDPE), and [d-Ala2]deltorphin II (DELT), selective mu, delta1, and delta2 opioid receptor agonists, respectively, in mice selectively bred for high (HA) and low (LA) swim stress-induced analgesia (SIA). HAR and LAR mice, selectively bred for high and low levorphanol analgesia, respectively, display equally large differences in their analgesic sensitivity to DAMGO, modest differences in sensitivity to DPDPE, and no differences in sensitivity to DELT. These sizable genotypic differences in analgesic potency were accompanied by HA/LA and HAR/LAR differences in whole-brain homogenate [3H]DPDPE and/or [3H]DELT, but paradoxically not [3H]DAMGO, binding. Solution hybridization of mRNA extracts encoding mu (MOR-1) or delta (DOR-1) opioid receptors indicated some regional differences in gene expression between high and low lines. Surprisingly, differences in these in vitro markers were often in the direction of LAR>HAR. The present data indicate that selection for either SSIA or levorphanol analgesia produces differential effects on mu and delta opioid analgesia that are accompanied by alterations on in vitro assays, the significance of which remains to be determined. The data are discussed with regard to the utility of in vitro biological markers and genetic models of analgesia.

Acute morphine dependence in mice selectively-bred for high and low analgesia

Acute morphine dependence was compared in mice selectively-bred for high (HA) and low (LA) swim stress-induced analgesia and high (HAR) and low (LAR) levorphanol analgesia by counting the number of naloxone-precipitated jumps. Whereas LAR mice displayed greater acute morphine dependence than HAR mice, HA and LA mice did not differ. No genotypic differences were observed in non-dependent mice, discounting possible differences in basal naloxone sensitivity and/or opioid peptide levels. Thus, the two selection projects, while both producing lines exhibiting highly divergent sensitivity to morphine analgesia, have not had analogous effects on all opioid measures, supporting the notion of independent genetic mediation of opioid analgesia and dependence. Further, these data suggest that analgesic sensitivity may not predict sensitivity to morphine dependence.

The genetics of pain and pain inhibition

The present review summarizes the current state of knowledge about the genetics of pain-related phenomena and illustrates the scope and power of genetic approaches to the study of pain. We focus on work performed in our laboratories in Jastrzebiec, Poland; Portland, OR; and Los Angeles, which we feel demonstrates the continuing usefulness of classical genetic approaches, especially when used in combination with newly available molecular genetic techniques.