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Anesthetic potency (MAC) is independent of forebrain structures in the rat
1Department of Anesthesia, University of California, San Francisco 94143-0648.
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.
Insights
General anesthetics block movement responses to pain, measured by minimum alveolar concentration (MAC). This study found MAC is not dependent on forebrain structures in rats, suggesting anesthesia
Area of Science:
- Neuroscience
- Anesthesiology
- Pharmacology
Background:
- General anesthetics suppress somatomotor responses to noxious stimuli, a key clinical effect.
- Minimum alveolar concentration (MAC) quantifies inhaled anesthetic potency by blocking movement responses (ED50).
Purpose of the Study:
- To identify the specific neural structures responsible for anesthetic-induced somatomotor blockade.
- To investigate the role of forebrain structures in mediating the minimum alveolar concentration (MAC) in rats.
Main Methods:
- Rats were anesthetized with isoflurane and MAC was determined using tail-clamping and Dixon's up-down method.
- Bilateral parietal-temporal craniotomies were performed, followed by aspiration decerebration in a subset of rats.
- MAC was repeatedly measured in control and decerebrated rats to assess the impact of forebrain removal.
Main Results:
- Control rats maintained a stable MAC of 1.30 +/- 0.25% for over 6 hours.
- Rats with precollicular decerebration showed no significant change in MAC compared to controls (1.26 +/- 0.14%).
- This lack of change persisted for up to 11 hours post-decerebration (P = 0.14).
Conclusions:
- Anesthetic-induced unresponsiveness to noxious stimuli, as measured by MAC, does not rely on cortical or forebrain structures.
- These findings localize the primary neural substrates for MAC to brain regions caudal to the precollicular transection.