title: Can the alpha 2-adrenoceptor agonist-mediated suppression of nocifensive reflex responses be due to action on motoneurons or peripheral nociceptors?
abstract: To exclude the possibility that the suppression of nocifensive reflex responses induced by alpha 2-adrenergic agents is due to action on alpha-motoneurons or peripheral nociceptors, we studied the effect of medetomidine, an alpha 2-adrenoceptor agonist, on the monosynaptic reflex and on the primary afferent nociceptor-mediated antidromic vasodilator response in rats. Additionally, the effect on the dorsal root potential, an index of a transient excitability change in the central terminals of primary afferent fibers, was determined. Medetomidine was applied systemically at doses (100 and 300 micrograms/kg) which have proven strongly antinociceptive in previous studies. The amplitudes of a submaximal monosynaptic reflex volley or a submaximal dorsal root potential were not changed by medetomidine. Medetomidine induced a decrease of cutaneous blood flow but did not abolish the vasodilatatory response to antidromic stimulation of the sciatic nerve at C-fiber intensity as determined by the laser Doppler flow method. The results indicate that the alpha 2-adrenoceptor-mediated suppression of nocifensive reflex responses is not caused by a decreased excitability of motoneurons or peripheral nociceptors. An alpha 2-adrenoceptor agonist does not modulate the transient stimulus-evoked change in the excitability of central terminals of primary afferent fibers.
authors.affiliation: Department of Physiology, University of Helsinki, Finland.
authors.forename: M M
authors.initials: MM
authors.lastname: Hämäläinen
authors.forename: E
authors.initials: E
authors.lastname: Jyväsjärvi
authors.forename: A
authors.initials: A
authors.lastname: Pertovaara
mesh_terms: D058647:Adrenergic alpha-2 Receptor Agonists; D058669:Adrenergic alpha-2 Receptor Antagonists; D000818:Animals; D007093:Imidazoles / Q000494:pharmacology; D008297:Male; D020926:Medetomidine; D009046:Motor Neurons / Q000187:drug effects*; D009619:Nociceptors / Q000187:drug effects*; D051381:Rats; D017207:Rats, Sprague-Dawley; D017208:Rats, Wistar; D012018:Reflex / Q000187:drug effects*; D013997:Time Factors
chemical_list: D058647:Adrenergic alpha-2 Receptor Agonists; D058669:Adrenergic alpha-2 Receptor Antagonists; D007093:Imidazoles; C050701:atipamezole; D020926:Medetomidine