213 118 118 4 4 I. Nylander M. Vlaskovska L. Terenius Department of Clinical Neuroscience, Drug Dependence Research Section Karolinska Institute S-171 76 Stockholm Sweden Abstract The effect of morphine tolerance and withdrawal on prodynorphin peptides was studied in relevant brain areas and in the pituitary gland of male Sprague-Dawley rats, and compared with effects on the proenkephalin-derived peptide Met-enkephalin. After 8 days of morphine injections (twice daily), dynorphin A and B levels increased in the nucleus accumbens and dynorphin A levels increased also in the striatum. Morphine treatment increased striatal Met-enkephalin. Leu-enkephalinArg 6 levels were reduced in the ventral tegmental area (VTA). Morphine-treated rats had very low Leu-enkephalinArg 6 levels in the hippocampus as compared to saline control rats. Comparison of the relative amounts of dynorphin peptides and the shorter prodynorphin-derived peptides, Leu-enkephalinArg 6 and Leu-enkephalin, revealed a relative increase in dynorphin peptides versus shorter fragments in the nucleus accumbens, VTA and hippocampus. Morphine-tolerant rats had lower levels of dynorphin A in both lobes of the pituitary gland, whereas hypothalamic dynorphin levels were unaffected by morphine. Leu-enkephalinArg 6 levels were reduced in the hypothalamus, but not changed in the pituitary gland. Naloxone-precipitated withdrawal accentuated the increase in dynorphin A and B levels in the accumbens and dynorphin A levels in the striatum, while inducing an increase in enkephalin levels in the accumbens and Met-enkephalin in the VTA. In the hippocampus, Leu-enkephalinArg 6 levels remained low in the withdrawal state. The low dynorphin levels in the anterior part of the pituitary gland were reversed by naloxone, whereas the low dynorphin A levels in the neurointer-mediate lobe were even lower in the withdrawal state. In conclusion, morphine tolerance and withdrawal affected prodynorphin-derived peptides in areas related to central reward mechanisms, and in the pituitary gland. The dynorphin peptides and the LeuenkephalinArg 6 fragment were not affected similarly, indicating an effect also on metabolic interconversion.
Psychopharmacology – Springer Journals
Published: Apr 1, 1995
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