title: Systemic morphine-induced release of serotonin in the rostroventral medulla is not mimicked by morphine microinjection into the periaqueductal gray.
abstract: We used in vivo microdialysis in awake rats to test the hypothesis that intravenous morphine increases serotonin (5-HT) release within the rostral ventromedial medulla (RVM). We also injected morphine into various sites along the rostrocaudal extent of the periaqueductal gray (PAG), and examined the extent of its diffusion to the RVM. Intravenous morphine (3.0 mg/kg) produced thermal antinociception and increased RVM dialysate 5-HT, 5-hydroxyindole acetic acid (5-HIAA), and homovanillic acid (HVA) in a naloxone-reversible manner. As neither PAG microinjection of morphine (5 micro g/0.5 micro L) nor RVM administration of fentanyl or d-Ala(2),NMePhe(4),Gly-ol(5)]enkephalin (DAMGO) increased RVM 5-HT, we were unable to determine the precise site of action of morphine. Surprisingly, peak morphine levels in the RVM were higher after microinjection into the caudal PAG as compared to either intravenous injection or microinjection into more rostral sites within the PAG. Naloxone-precipitated withdrawal in morphine-tolerant rats not only increased extracellular 5-HT in the RVM, but also dopamine (DA) and HVA. We conclude that substantial amounts of morphine diffuse from the PAG to the RVM, and speculate that opioid receptor interactions at multiple brain sites mediate the analgesic effects of PAG morphine. Further studies will be required to elucidate the contribution of 5-HT and DA release in the RVM to opioid analgesia and opioid withdrawal.
authors.affiliation: Department of Pharmacology SL83, Health Sciences Center, Tulane University School of Medicine, 1430 Tulane Avenue, New Orleans, LA 70118, USA. taylorb@tulane.edu
authors.forename: Bradley K
authors.initials: BK
authors.lastname: Taylor
authors.forename: Allan I
authors.initials: AI
authors.lastname: Basbaum
mesh_terms: D000701:Analgesics, Opioid / Q000009:adverse effects / Q000494:pharmacology; D000818:Animals; D004333:Drug Administration Routes; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)- / Q000494:pharmacology; D005283:Fentanyl / Q000494:pharmacology; D006719:Homovanillic Acid / Q000032:analysis / Q000378:metabolism; D006897:Hydroxyindoleacetic Acid / Q000032:analysis / Q000378:metabolism; D008297:Male; D008526:Medulla Oblongata / Q000187:drug effects* / Q000378:metabolism*; D017551:Microdialysis; D008845:Microinjections; D009020:Morphine / Q000009:adverse effects / Q000494:pharmacology*; D009270:Naloxone / Q000494:pharmacology; D009292:Narcotic Antagonists / Q000494:pharmacology; D010147:Pain Measurement / Q000187:drug effects; D010487:Periaqueductal Gray / Q000187:drug effects*; D051381:Rats; D017207:Rats, Sprague-Dawley; D012701:Serotonin / Q000032:analysis / Q000378:metabolism*; D013375:Substance Withdrawal Syndrome; D014851:Wakefulness
chemical_list: D000701:Analgesics, Opioid; D009292:Narcotic Antagonists; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; D012701:Serotonin; D009270:Naloxone; D006897:Hydroxyindoleacetic Acid; D009020:Morphine; D005283:Fentanyl; D006719:Homovanillic Acid