Anesthetic potency (MAC) is independent of forebrain structures in the rat.
BACKGROUND
The ability of general anesthetics to suppress somatomotor responses to surgical incision and other noxious stimuli is of particular clinical relevance. When the blockade is due to inhaled agents, this effect can be quantified as the minimum alveolar concentration (MAC), i.e., that concentration that blocks movement evoked by a noxious stimulus (ED50).


METHODS
To identify the neural structures that subtend this somatomotor response, we anesthetized 14 rats with isoflurane in oxygen and performed bilateral parietal-temporal craniotomies. In each rat, MAC was repeatedly tested using tail-clamping and Dixon's up-down concentration technique. After determination of baseline MAC, seven rats underwent aspiration decerebration, after which MAC was repeatedly measured.


RESULTS
In the control group (N = 7), MAC (mean +/- SD) remained constant at 1.30 +/- 0.25% for more than 6 h. In the seven rats that underwent aspiration decerebration, baseline MAC was 1.26 +/- 0.14%. These seven rats with histologically validated precollicular decerebration demonstrated no change in MAC relative to control rats, as much as 11 h after decerebration (P = 0.14).


CONCLUSIONS
These findings suggest that the anesthetic-induced unresponsiveness to noxious stimuli measured by MAC testing does not depend on cortical or forebrain structures in the rat.
Rampil
I J
IJ
Department of Anesthesia, University of California, San Francisco 94143-0648.
Mason
P
P
Singh
H
H
D000280:Administration, Inhalation; D000768:Anesthesia, General; D000777:Anesthetics / Q000493:pharmacokinetics / Q000494:pharmacology*; D000818:Animals; D003655:Decerebrate State / Q000503:physiopathology; D004305:Dose-Response Relationship, Drug; D004569:Electroencephalography; D007530:Isoflurane; D008297:Male; D009043:Motor Activity / Q000187:drug effects; D009619:Nociceptors / Q000187:drug effects; D016548:Prosencephalon / Q000166:cytology / Q000187:drug effects / Q000502:physiology*; D011650:Pulmonary Alveoli / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley; D013687:Telencephalon / Q000166:cytology / Q000187:drug effects / Q000502:physiology
D000777:Anesthetics; D007530:Isoflurane