Effects of two benzodiazepine inverse agonists, RO 15-4513 and FG 7142, on recovery from pentobarbital and halothane anesthesia in the rat

Flumazenil improves cognitive and neuromotor emergence and attenuates shivering after halothane-, enflurane- and isoflurane-based anesthesia

PURPOSE

To conduct a randomized, placebo-controlled, double-blinded, clinical experiment testing the hypothesis that flumazenil, a benzodiazepine antagonist, may affect recovery from halothane-, enflurane- and isoflurane-based anesthesia.

METHOD

Patients who underwent surgery under N(2)O/O(2) plus halothane (n=100), enflurane (n=100) or isoflurane (n=70) anesthesia were administered flumazenil 1 mg or placebo upon emergence from anesthesia, and their postanesthesia vital signs, vigilance, neurological recovery, shivering, amnesia reversal, and general subjective feeling were assessed.

RESULTS

A ten-point vigilance score showed better recovery of flumazenil-treated patients compared to those who received placebo (60-min after halothane anesthesia: 9.9 +/- 0.1 vs 9.5 +/- 0.2, P <0.01; after enflurane: 10 +/- 0 vs 9.4 +/- 0.2, P <0.01; after isoflurane: 10.0 +/- 0 vs 9.3 +/- 0.1, P <0.01). Halothane- and enflurane-flumazenil-treated patients (but not isoflurane) reached a better neurological score (2.97 +/- 0.05 or 3 +/- 0) compared to placebo (2.8 +/- 0.4 or 2.6 +/- 0.4, P <0.01), respectively. Reversal of amnesia was superior in the flumazenil group at 60 min and at 24 hr postsurgery, and more flumazenil patients rated recovery as "pleasant". Flumazenil patients shivered less than placebo patients despite their lower core temperature (at 30 min: halothane: 11% vs 28%, P <0.05; enflurane: 11% vs 30%, P <0.05; isoflurane: 17% for both groups).

CONCLUSION

Flumazenil improves recovery of high cortical and neuromotor functions following halothane, enflurane and isoflurane anesthesia, reduces shivering and improves the overall quality of emergence, including patients' subjective feeling.

A force plate system for measuring low-magnitude reaction forces in small laboratory animals

We present a force plate system which measures low-magnitude vertical reaction forces generated by small laboratory animals. The force plate mechanical design minimizes radiated transverse waves, acoustic reverberation, and standing waves caused by impacts on the force plate surface. A secondary force plate and PC-based software algorithm minimize floor vibrational artifact. The force plate was used to measure function of rats during two tests: forelimb/hindlimb hopping reaction and surface righting reaction. In control rats, forelimb hopping rate exceeded hindlimb hopping rate during 16 weeks of repeated testing. Subchronic intraperitoneal (i.p.) dosing of 10 mg/kg/day acrylamide produced a selective impairment of hindlimb hopping. In contrast, single doses of haloperidol (1-5 mg/kg, i.p.) slowed the righting reaction and produced a relatively selective impairment of forelimb hopping. The force plate system presents new opportunities for performing quantitative neurological assessments of small laboratory animals when previously such tests had been performed subjectively and qualitatively.

Chronic pentobarbital administration alters gamma-aminobutyric acidA receptor alpha 6-subunit mRNA levels and diazepam-insensitive [3H]Ro15-4513 binding

In order to study the chronic effects of pentobarbital, a positive GABAA receptor modulator, on the inverse agonist binding of the benzodiazepine site, binding of [3H]Ro15-4513 and levels of GABAA receptor alpha 6-subunit mRNA were investigated in the brains of pentobarbital-tolerant/dependent animals, using receptor autoradiography and in situ hybridization histochemistry in consecutive brain sections. Pentobarbital was administered to rats either 60 mg/kg, i.p., once, for acute treatment, or 300 micrograms/10 microliters/h i.c.v. continuously for 6 days via osmotic minipumps to render rats tolerant to pentobarbital. Rats assigned to the dependent group were sacrificed 24 h after discontinuance of pentobarbital infusion, while those assigned to the tolerant group were sacrificed at the end of infusion. The alpha 6 subunit mRNA was increased in the tolerant group only. Diazepam-insensitive [3H]Ro15-4513 binding was increased in the cerebellar granule layer of pentobarbital-tolerant and -dependent rats. No alterations in these parameters were observed in acutely treated animals. These data suggest that chronic pentobarbital treatment induced expression of alpha 6-subunit mRNA. This was in contrast to alpha 1- and gamma 2-subunit mRNA, which in tolerant animals are unchanged, but for which withdrawal triggers a surge in levels. Because the alpha 6-subunit is a major component of the diazepam-insensitive [3H]Ro15-4513 binding site, the increased diazepam-insensitive [3H]Ro15-4513 binding implied de novo synthesis of the receptor subunit protein.

Isoflurane anesthesia is stereoselective

The anesthetic effects of the (+) and (-) isomers of isoflurane were determined in mice. Like the clinically important racemate, both isomers produced dose-dependent increases in anesthetic sleep time. A statistically significant (P less than 0.005, ANOVA) difference in the potencies of these isomers ((+) greater than (-)) was observed. To our knowledge, this is the first demonstration of a stereospecific action of a volatile anesthetic in vivo.

Effects of lorazepam tolerance and withdrawal on GABAA receptor operated chloride channels in mice selected for differences in ethanol withdrawal severity

Withdrawal seizure prone (WSP) and withdrawal seizure resistant (WSR) mice were treated with 5 mg/kg lorazepam for 7 days via implanted osmotic mini pumps. Following chronic drug treatment, brains were assayed for GABA-mediated chloride flux (GABA-Cl-). Under control (drug naive) conditions, brain membranes prepared from WSP and WSR lines did not differ in flunitrazepam or ethanol stimulation of GABA-mediated 36Cl- uptake, but the WSP lines were more sensitive to inhibition of 36Cl- flux by the inverse agonist, FG-7142. Membranes from lorazepam tolerant WSP and WSR mice were resistant to flunitrazepam- and ethanol-stimulation of GABA-Cl-. Withdrawal from chronic treatment, by an acute injection with the benzodiazepine antagonist RO15-1788, returned flunitrazepam stimulation of GABA-Cl- to near control levels in WSR membranes but not in WSP membranes and restored ethanol modulation of the channel to control levels in both lines. Inhibition of chloride flux by the benzodiazepine partial inverse agonist, FG-7142 was greater in membranes from WSP mice compared with WSR mice. Tolerance to lorazepam increased sensitivity of the WSR membranes to FG-7142 without altering the response in the WSP line. Again, withdrawal restored the Cl- flux response to FG-7142 back to near control levels. Lorazepam tolerance lowered [3H]-flunitrazepam binding affinity slightly only in the WSR strain with no change in binding number. Withdrawal from chronic lorazepam treatment produced no significant change in binding affinity or number. The initial genotypic differences in benzodiazepine inverse agonist sensitivity, may be related to the selection for withdrawal seizure severity. Chronic administration of lorazepam reduces the coupling between the benzodiazepine agonist site and the chloride channel and concomitantly increases coupling between the channel and the inverse agonist site, while withdrawal resets the receptor coupling back to control response levels. However, for the WSP line, this drug environment dependent shift in channel coupling bias appears to be deficient compared with the WSR line.