Morphine treatment induced calcitonin gene-related peptide and substance P increases in cultured dorsal root ganglion neurons.
The mechanism of spinal tolerance to the analgesic effects of opiates is unclear at present. We have reported previously that calcitonin gene-related peptide-like immunoreactivity was significantly increased in primary afferents of the spinal dorsal horn during the development of morphine tolerance, suggesting that changes in the level of pain-related neuropeptides in dorsal root ganglion neurons may be involved [Menard D. P. et al. (1996) J. Neurosci. 16, 2342-2351]. In this study, we investigated if in vitro treatment with morphine can mimic the in vivo findings and induce increases in calcitonin gene-related peptide-like immunostaining in cultured dorsal root ganglion neurons from young (three-month-old) and middle-aged (10-month-old) adult rats. Following a repetitive exposure to morphine sulfate (1, 5, 10 microM) for six days, the number of calcitonin gene-related peptide- and substance P-immunoreactive neurons in cultured dorsal root ganglia from three- and 10-month-old rats was significantly increased. A lower concentration (0.5 microM) of morphine induced these increases only in dorsal root ganglion neurons from middle-aged rats. Morphine treatment was also found to increase the number of calcitonin gene-related peptide-immunoreactive neurons possessing multiple, long branches (i.e. with at least one branch >0.5mm). This apparent increase in the number of calcitonin gene-related peptide- and substance P-immunoreactive neurons observed following morphine treatment was blocked by naloxone, an opiate antagonist, indicating the involvement of genuine opioid receptors. No significant change in the number of neuropeptide Y- or galanin-immunoreactive neurons in cultured dorsal root ganglia was detected following any of these treatments. These data suggest that repeated exposure to morphine rather selectively increases calcitonin gene-related peptide- and substance P-like immunoreactivity in cultured dorsal root ganglion neurons. Moreover, the sensitivity to morphine-induced changes is greater in cultured dorsal root ganglion neurons from 10- compared to three-month-old rats. Hence, cultured dorsal root ganglion neurons can provide a model to investigate the cellular and molecular mechanisms underlying alterations in neuropeptide levels following repeated exposure to opiates and their relevance to the development of opioid tolerance.
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Douglas Hospital Research Center, McGill University, 6875 Boul. LaSalle, Verdun, Quebec, H4H 1R3, Montreal, Canada.
Zheng
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Kar
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Quirion
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D000367:Age Factors; D000818:Animals; D015740:Calcitonin Gene-Related Peptide / Q000032:analysis / Q000378:metabolism*; D002478:Cells, Cultured; D019004:Galanin / Q000032:analysis; D005727:Ganglia, Spinal / Q000166:cytology* / Q000378:metabolism; D007150:Immunohistochemistry; D008297:Male; D009020:Morphine / Q000494:pharmacology*; D009270:Naloxone / Q000494:pharmacology; D009292:Narcotic Antagonists / Q000494:pharmacology; D009294:Narcotics / Q000494:pharmacology*; D009475:Neurons, Afferent / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism; D009478:Neuropeptide Y / Q000032:analysis; D051381:Rats; D017207:Rats, Sprague-Dawley; D013373:Substance P / Q000032:analysis / Q000378:metabolism*
D009292:Narcotic Antagonists; D009294:Narcotics; D009478:Neuropeptide Y; D013373:Substance P; D009270:Naloxone; D009020:Morphine; D019004:Galanin; D015740:Calcitonin Gene-Related Peptide