Spinal carbonic anhydrase contributes to nociceptive reflex enhancement by midazolam, pentobarbital, and propofol.
BACKGROUND
Systemic administration of acetazolamide blocks nociceptive hyperreflexia induced by pentobarbital. The authors assessed the effect of intrathecal carbonic anhydrase inhibitors (CAIs) on nociceptive reflex enhancement by pentobarbital, propofol, and midazolam.


METHODS
Twenty-seven rats with chronic indwelling subarachnoid catheters were studied. Nociceptive paw reflex latency (PWL) for paw withdrawal from radiant heat was measured in forelimbs and hind limbs. Measurements were obtained under control conditions, 15 min after lumbar intrathecal injection of 10 microl artificial cerebrospinal fluid containing the CAIs acetazolamide or ethoxyzolamide, and during the 55 min after intraperitoneal injection of three sedative drugs: 30 mg/kg pentobarbital, 50 mg/kg propofol, or 1.9 mg/kg midazolam.


RESULTS
Control values of PWL averaged 10.9 +/- 1.5 s in the forelimbs and 11.1 +/- 1.6 s in the hind limbs (P = 0.18). Intrathecal injection of 50 microm ethoxyzolamide reduced PWL by 8% and 4% in the forelimbs and hind limbs, respectively (P = 0.01); all other CAI injections had no effect on PWL. Following anesthetic injection, PWL in the forelimbs was reduced by approximately 35-40% of control values; in the hind limbs, CAI treatment decreased the PWL reduction to 8-16% for pentobarbital (P < 0.001), 30-32% for propofol (P < 0.02), and 9-16% for midazolam (P < 0.001). The hind limb reduction of hyperreflexia by CAI was less for propofol than for midazolam or pentobarbital (P < 0.002).


CONCLUSIONS
Spinal carbonic anhydrase contributes to nociceptive hyperreflexia induced by pentobarbital and midazolam and to a lesser extent with propofol. These findings are consistent with a role for carbonic anhydrase in nociceptive signal enhancement by these drugs.
Wang
Bing
B
Department of Anesthesiology, University of Calgary, Calgary, Alberta, Canada.
Samanani
Naaznin
N
Roth
Sheldon H
SH
Archer
David P
DP
D000086:Acetazolamide / Q000494:pharmacology; D000818:Animals; D002257:Carbonic Anhydrase Inhibitors / Q000008:administration & dosage / Q000494:pharmacology; D002256:Carbonic Anhydrases / Q000502:physiology*; D005016:Ethoxzolamide / Q000494:pharmacology; D006993:Hypnotics and Sedatives / Q000494:pharmacology*; D007278:Injections, Spinal; D008297:Male; D008874:Midazolam / Q000494:pharmacology*; D010146:Pain / Q000139:chemically induced / Q000503:physiopathology*; D010147:Pain Measurement / Q000187:drug effects; D010424:Pentobarbital / Q000494:pharmacology*; D015742:Propofol / Q000494:pharmacology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D011930:Reaction Time; D012018:Reflex / Q000187:drug effects*; D013116:Spinal Cord / Q000201:enzymology*
D002257:Carbonic Anhydrase Inhibitors; D006993:Hypnotics and Sedatives; D002256:Carbonic Anhydrases; D010424:Pentobarbital; D000086:Acetazolamide; D008874:Midazolam; D015742:Propofol; D005016:Ethoxzolamide