Opiate-induced changes in brain adenosine levels and narcotic drug responses
Introduction
Efforts to improve the care of patients with chronic pain conditions have led to a marked increase in the use of opioid medications. Unfortunately, prescription opioid analgesics are more commonly misused than all other illicit drugs combined, including marijuana [reviewed in (Dodrill et al., 2011)]. New approaches and a re-evaluation of our current understanding of the mechanisms involved in narcotic drug addiction are urgently needed. To develop new strategies for addressing this public health problem, we have been analyzing a murine model of opiate dependence. Mice can be made physically dependent upon morphine, and inbred strains dramatically differ in the extent to which they manifest various features of narcotic drug addiction, which resemble those observed in humans (Liang et al., 2006a, Liang et al., 2006b, Chu et al., 2009). By analyzing these inter-strain differences, we identified four genes affecting opioid responses (Liang et al., 2006a, Liang et al., 2006b, Smith et al., 2008), including the Htr3a/5-HT3 serotonin receptor (Chu et al., 2009). We also demonstrated that administration of a commonly used 5-HT3 antagonist (ondansetron) reduced narcotic drug withdrawal symptoms in mice and in normal human subjects (Chu et al., 2009, Liang et al., 2011).
In addition to genetic data, metabolomic analysis can reveal a great deal about the physiological state of a tissue. We have very little information about metabolomic changes mediating neurobehavioral responses or diseases. It is likely that clinically important opiate responses could be mediated (at least in part) by metabolomic changes that are induced by opiates. However, the extreme differences in physicochemical properties make it impossible to accurately measure changes in all cellular metabolites with a single analytic method. Therefore, we coupled a recently developed Dansyl [5-(dimethylamino)-1-napthalene sulfonamide] derivatization method (Guo and Li, 2009) with LC/MS analysis to analyze changes in a large number of metabolites in brainstem after opiate administration. Dansylation increases metabolite detection sensitivity by 10–1000-fold, and improves metabolite retention and separation on reversed phase columns. It enables changes in many metabolites that have primary or secondary amino or other groups, to be evaluated in an unbiased fashion. This semi-targeted method was used to characterize opiate-induced metabolomic changes in a brain region that is critical for opiate responses in two inbred mouse strains, which exhibit extreme differences in the extent of physical dependence developing after opiate administration. Metabolomic changes in the brainstem were analyzed, since this region has been shown to regulate narcotic drug dependence (Gulati and Bhargava, 1989, Costall et al., 1990, Tao et al., 1998).
Section snippets
Animal studies
All experiments were performed according to protocols that were approved by the Institutional Animal Care and Use Committee at the Veterans Affairs Palo Alto Healthcare System. Male C57BL/6J and 129/SvlmJ mice strains (7–8 weeks old) were obtained from Jackson Laboratories (Bar Harbor, MA) and kept in our facility for a minimum of 1 week prior to initiation of the experiments. Mice with genetically engineered alterations in adenosine kinase (Adk) expression (which are referred to as Adk-tg and
Results
C57BL/6J mice become morphine-dependent after 4 days of administration of increasing doses of morphine; the morphine-dependent mice develop signs of withdrawal within 18 h of their last morphine dose, and naloxone administration rapidly induces substantial withdrawal symptoms (Chu et al., 2009). To characterize opiate-induced metabolomic changes, brainstem tissue was prepared from C57BL/6J mice placed into 4 treatment groups (n = 8 per group) (Fig. 1A): (1) Dependence: tissue was harvested 1 h after
Discussion
This study demonstrates that semi-targeted metabolomic profiling data can provide important insight into neurobehavioral responses. It also provides the first demonstration that systemic opiate administration decreases adenosine abundance in brainstem. The decrease in narcotic drug withdrawal behavior in mice with genetically engineered changes in brainstem adenosine levels or after administration of pharmacologic agents acting on adenosine receptors demonstrates the importance of this
Acknowledgements
G.P. was partially supported by funding from a transformative RO1 award (1R01DK090992) provided by the NIDDK.
References (46)
- et al.
Effects of (−)-N6-(R-phenylisopropyl)-adenosine (PIA) and caffeine on nociception and morphine-induced analgesia, tolerance and dependence in mice
Eur J Pharmacol
(1985) Adenosine kinase, epilepsy and stroke: mechanisms and therapies
Trends Pharmacol Sci
(2006)- et al.
Adenosine mediation of presynaptic feedback inhibition of glutamate release
Neuron
(2005) - et al.
Sites of action of ondansetron to inhibit withdrawal from drugs of abuse
Pharmacol Biochem Behav
(1990) - et al.
Brain and spinal cord 5-HT2 receptors of morphine-tolerant-dependent and -abstinent rats
Eur J Pharmacol
(1989) - et al.
Modulation of adenosine concentration by opioid receptor agonists in rat striatum
Eur J Pharmacol
(2000) - et al.
Adenosine kinase inhibitors attenuate opiate withdrawal via adenosine receptor activation
Eur J Pharmacol
(1998) - et al.
Alterations of adenosine A1 receptors in morphine dependence
Brain Res
(1994) - et al.
Subjective sleep–wake parameters in treatment-seeking opiate addicts
Drug Alcohol Depend
(1997) - et al.
Effect of adenosine receptor agonists and antagonists on the expression of opiate withdrawal in rats
Pharmacol Biochem Behav
(1997)