title: Pharmacology of intrathecal adrenergic agonists: cardiovascular and nociceptive reflexes in halothane-anesthetized rats.
abstract: The effects of intrathecally administered adrenergic agonists (alpha-1-methoxamine, alpha-2-dexmedetomidine, clonidine, and ST-91, beta-isoproterenol) on nociceptive (tail-flick reflex) and cardiovascular changes (blood pressure and heart rate) evoked by immersing the tail in 53 degrees C water were examined in rats anesthetized with halothane (0.75%) and in which intrathecal catheters had been chronically implanted. Administration of intrathecal alpha-2, but not alpha-1 or beta agonists, produced a dose-dependent block of the tail-flick and evoked cardiovascular responses with the order of activity being as follows: dexmedetomidine greater than clonidine greater than ST-91 much greater than methoxamine greater than or equal to isoproterenol. These effects were readily reversed by the alpha-2 antagonist idazoxan. Intravenously administered dexmedetomidine at a dose that is active when given intrathecally (0.33 micrograms) was without effect on either the tail-flick or the evoked cardiovascular responses. Without drugs, the halothane MAC was 1.23 +/- 0.07%. In the presence of intrathecally administered dexmedetomidine (0.33 micrograms), the MAC was significantly reduced to 0.9 +/- 0.09%. The intrathecal administration of alpha-2 agonists resulted in a rapid decrease in resting blood pressure and heart rate with the magnitude of hypotension being as follows: dexmedetomidine greater than clonidine greater than ST-91. These data suggest a potent spinal alpha-2-receptor-mediated modulation of somatomotor and autonomic responses to pain.
authors.affiliation: Department of Anesthesiology, University of California, San Diego, La Jolla 92093.
authors.forename: H
authors.initials: H
authors.lastname: Nagasaka
authors.forename: T L
authors.initials: TL
authors.lastname: Yaksh
mesh_terms: D000322:Adrenergic Agonists / Q000008:administration & dosage / Q000494:pharmacology*; D000769:Anesthesia, Inhalation; D000818:Animals; D003000:Clonidine / Q000008:administration & dosage / Q000494:pharmacology; D006221:Halothane; D006439:Hemodynamics / Q000187:drug effects*; D007093:Imidazoles / Q000008:administration & dosage / Q000494:pharmacology; D007278:Injections, Spinal; D007545:Isoproterenol / Q000008:administration & dosage / Q000494:pharmacology; D008297:Male; D020926:Medetomidine; D008729:Methoxamine / Q000008:administration & dosage / Q000494:pharmacology; D009619:Nociceptors / Q000187:drug effects* / Q000502:physiology; D051381:Rats; D011919:Rats, Inbred Strains; D012018:Reflex / Q000187:drug effects* / Q000502:physiology
chemical_list: D000322:Adrenergic Agonists; D007093:Imidazoles; D008729:Methoxamine; D007545:Isoproterenol; D003000:Clonidine; D020926:Medetomidine; D006221:Halothane