Sedative but not analgesic alpha2 agonist tolerance is blocked by NMDA receptor and nitric oxide synthase inhibitors.
BACKGROUND
Studies show that the sedative and analgesic effects of alpha2 adrenergic agonists decrease over time, which is a form of synaptic plasticity referred to as tolerance. Because both the N-methyl-D-aspartate (NMDA) receptor complex and nitric oxide synthase are pivotal for some forms of synaptic plasticity, their role in tolerance to the hypnotic and analgesic effects of alpha2 agonists was investigated.


METHODS
After institutional approval, rats were made tolerant to the hypnotic or analgesic action of an alpha2 agonist, dexmedetomidine. The hypnotic response to dexmedetomidine was assessed by the duration of loss of righting reflex, and the analgesic response to dexmedetomidine was assessed by the tail-flick assay. In separate cohorts, either the NMDA receptors or nitric oxide synthase was antagonized by coadministration of MK-801, ketamine, or NO2-arginine, respectively, during induction of tolerance. In a separate series of experiments, after tolerance was induced, the hypnotic and analgesic responses to dexmedetomidine were assessed in the presence of acutely administered MK-801 or NO2-arginine.


RESULTS
Induction of tolerance to the hypnotic effect of dexmedetomidine is blocked by coadministration of MK-801, ketamine, and NO2-arginine. However, after tolerance developed, acute administration of MK-801, ketamine, or NO2-arginine did not prevent the expression of tolerance. Coadministration of MK-801 or NO2-arginine neither prevents the development nor reverses the expression of tolerance to the analgesic action of dexmedetomidine.


CONCLUSIONS
The underlying processes responsible for the development of tolerance to the hypnotic and analgesic actions of systemically administered alpha2 agonists were different, with only the sedative tolerance involving the NMDA receptor and nitric oxide synthase system.
Davies
M F
MF
Department of Anesthesia, Stanford University, Palo Alto, California, USA.
Reid
K
K
Guo
T Z
TZ
Agashe
G S
GS
Amin
Y K
YK
Maze
M
M
D058647:Adrenergic alpha-2 Receptor Agonists; D000316:Adrenergic alpha-Agonists / Q000494:pharmacology*; D000700:Analgesics / Q000494:pharmacology*; D000818:Animals; D020927:Dexmedetomidine / Q000494:pharmacology; D016291:Dizocilpine Maleate / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D004361:Drug Tolerance; D004791:Enzyme Inhibitors / Q000494:pharmacology*; D018691:Excitatory Amino Acid Antagonists / Q000494:pharmacology; D006993:Hypnotics and Sedatives / Q000494:pharmacology*; D007649:Ketamine / Q000494:pharmacology; D008297:Male; D019001:Nitric Oxide Synthase / Q000037:antagonists & inhibitors*; D019335:Nitroarginine / Q000494:pharmacology; D010147:Pain Measurement / Q000187:drug effects; D004856:Postural Balance / Q000187:drug effects; D051381:Rats; D017207:Rats, Sprague-Dawley; D016194:Receptors, N-Methyl-D-Aspartate / Q000037:antagonists & inhibitors*; D012018:Reflex / Q000187:drug effects
D058647:Adrenergic alpha-2 Receptor Agonists; D000316:Adrenergic alpha-Agonists; D000700:Analgesics; D004791:Enzyme Inhibitors; D018691:Excitatory Amino Acid Antagonists; D006993:Hypnotics and Sedatives; D016194:Receptors, N-Methyl-D-Aspartate; D019335:Nitroarginine; D020927:Dexmedetomidine; D007649:Ketamine; D016291:Dizocilpine Maleate; D019001:Nitric Oxide Synthase