Relationship between gamma-hydroxybutyrate plasma concentrations and its electroencephalographic effects in the rat.

In view of the potential interest in an objective parameter for the depth of coma in intoxications with the recreational drug gamma-hydroxybutyrate (GHB), we have studied the relationship between the plasma concentrations and the electroencephalographic (EEG) changes induced by GHB in the rat. Fifteen rats randomly received either 150 (n = 3), 200 (n = 6) or 300 mg kg(-1) (n = 6) GHB over 5 min, followed by a supramaximal dose of 450 mg kg(-1) over 5 min at the end of the experiment. Plasma concentrations were determined with HPLC. The EEG was continuously recorded and the amplitude in the 15.5-30 Hz frequency band was quantified using aperiodic analysis. The plasma concentration-time profiles were fitted to a two-compartment model with Michaelis-Menten elimination. The pharmacokinetic parameters Vmax, Km and the apparent volume of distribution (Vd) proved to be independent of the dose and the mean pooled values were Vmax 2068 +/- 140 microg min(-1) kg(-1), Km 58 +/- 16 microg mL(-1) and Vd 476 +/- 12 mL kg(-1). The EEG amplitude in the 15.5-30 Hz frequency band displayed a monophasic inhibition and the effect-plasma concentration curve showed hysteresis. This hysteresis between EEG effect and plasma concentrations was minimized by simultaneous calculation of hypothetical effect-site concentrations and fitting the effect vs effect-site concentration curve to a sigmoid inhibitory Emax model. The descriptors of this Emax model (Emax, EC50, k(e,0), gamma and E0) were independent of the dose with an equilibration half-life t1/2k(e,0) of 5.6 +/- 0.3 min (mean value of the pooled results of the 5-min treatment groups). To investigate the origin of this hysteresis, a dose of 600 mg kg(-1) GHB was infused over either 45 or 60 min each in three animals. The hysteresis was much less pronounced with 45 min than with 5 min and was absent with 60-min infusions. This indicated that the hysteresis was due to a distribution delay between the central compartment and the effect site. This study showed that the concentration-effect relationship of GHB could be characterized in individual rats using aperiodic analysis in the 15.5-30 Hz frequency band.

Influence of naloxone on the increased sensitivity to propofol during hypovolemia in the rat.

OBJECTIVE: Hypovolemia has been shown to decrease the dose requirement for propofol. This increased effect has been explained partially by an increased end organ sensitivity to the anesthetic effect of propofol. We used the opioid blocking agent naloxone to test the hypothesis that endogenous opioids may be involved in this increased sensitivity. SUBJECTS: Thirty-two chronically instrumented rats were assigned randomly to either the hypovolemia (n = 16) or the control (n = 16) group. INTERVENTIONS: After pretreatment of each rat in the two groups with either intravenous saline (n = 8) or naloxone (3 mg/kg; n = 8), an intravenous infusion of propofol (150 mg x kg(-1) x hr(-1)) was given until 5 secs of electrical suppression of the electroencephalographic signal was observed. Return of righting reflex was used to assess depth of anesthesia, and the propofol blood concentration was determined simultaneously with high-performance liquid chromatography. MEASUREMENTS AND MAIN RESULTS: The mean propofol blood concentrations at the return of righting reflex were significantly lower in the hypovolemic animals compared with the controls within both naloxone-treated (2.1 +/- 0.2 microg/mL vs. 2.9 +/- 0.2 microg/mL; p < .01) and saline-treated (2.2 +/- 0.1 vs. 3.0 +/- 0.2 microg/mL; p < .01) rats. The mean concentrations were not different between the saline- and naloxone-treated rats either within the hypovolemic group or within the control group. CONCLUSIONS: The results of our study indicate that it is unlikely that the increased end organ sensitivity to propofol during hypovolemia is mediated by endogenous opioids, because it was not reversed by naloxone.

Development and validation of a high-performance liquid chromatographic method for the determination of gamma-hydroxybutyric acid in rat plasma.

A method for the determination of gamma-hydroxybutyric acid (GHB) in rat plasma was developed using solid-phase extraction (SPE) and high-performance liquid chromatography (HPLC) with UV detection. GHB was isolated from plasma using strong anion-exchange SPE columns. The chromatographic separation was performed on a C18 Aqua column. The lower limit of quantification was 10 microg/ml using 60 microl of plasma. The linearity of the calibration curves was satisfactory as indicated by correlation coefficients of >0.990. The within-day and between-day precision were <10% (n=24), the accuracy was nearly 101%. Plasma concentrations in rats after GHB infusion determined by HPLC-UV were compared with the corresponding concentrations determined with a validated gas chromatographic-mass spectrometric method by orthogonal distance regression. A good correlation was observed and a t-test indicated no significant differences from 0 and 1 for the intercept and slope, respectively.