The rostroventromedial medulla is not involved in alpha 2-adrenoceptor-mediated antinociception in the rat.
The aim of the current study was to investigate the role of the rostroventromedial medulla (RVM) in alpha 2-adrenoceptor-mediated antinociception. Medetomidine or clonidine, selective alpha 2-adrenoceptor agonists were microinjected into the RVM in unanesthetized rats with a chronic guide cannula. The antinociceptive effects were evaluated using the tail-flick and hot-plate tests. For comparison, medetomidine was microinjected into the cerebellum or the periaqueductal gray (PAG). To study the role of medullospinal pathways, the tail-flick latencies were also measured in spinalized rats. The reversal of the antinociception induced by intracerebral microinjections of medetomidine was attempted by s.c. atipamezole, a selective alpha 2-adrenoceptor antagonist. The reversal of the antinociception induced by systemic administration of medetomidine was attempted by microinjections of 5% lidocaine or atipamezole into the RVM. When administered into the RVM, medetomidine produced a dose-dependent (1-30 micrograms) antinociception in the tail-flick and hot-plate tests, which antinociceptive effect was completely reversed by atipamezole (1 mg/kg, s.c.). Also clonidine produced a dose-dependent (3-30 micrograms) antinociception following microinjection into the RVM. Microinjections of medetomidine into the cerebellum or the PAG produced an identical dose-response curve in the tail-flick test as that obtained following microinjection into the RVM. In spinalized rats the antinociceptive effect (tail-flick test) induced by medetomidine microinjected into the RVM was not less effective than in intact rats. Lidocaine (5%) or atipamezole (5 micrograms) microinjected into the RVM did not attenuate the antinociception induced by systemically administered medetomidine (100 micrograms/kg, s.c.).(ABSTRACT TRUNCATED AT 250 WORDS)
Hämäläinen
M M
MM
Department of Physiology, University of Helsinki, Finland.
Pertovaara
A
A
D000704:Analysis of Variance; D000818:Animals; D002531:Cerebellum / Q000187:drug effects / Q000502:physiology; D003000:Clonidine / Q000008:administration & dosage / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D006358:Hot Temperature; D007093:Imidazoles / Q000008:administration & dosage / Q000494:pharmacology; D008012:Lidocaine / Q000008:administration & dosage / Q000494:pharmacology; D008297:Male; D020926:Medetomidine; D008526:Medulla Oblongata / Q000187:drug effects / Q000502:physiology* / Q000503:physiopathology; D008845:Microinjections; D010146:Pain / Q000503:physiopathology*; D010487:Periaqueductal Gray / Q000187:drug effects / Q000502:physiology; D051381:Rats; D017208:Rats, Wistar; D018341:Receptors, Adrenergic, alpha-2 / Q000187:drug effects / Q000502:physiology*; D012018:Reflex / Q000187:drug effects; D012881:Skin Temperature / Q000187:drug effects; D013116:Spinal Cord / Q000502:physiology; D013997:Time Factors
D007093:Imidazoles; D018341:Receptors, Adrenergic, alpha-2; C050701:atipamezole; D008012:Lidocaine; D003000:Clonidine; D020926:Medetomidine