Application of serial sampling of cerebrospinal fluid in pharmacodynamic studies with a drug active in the CNS: heptabarbital concentrations at onset and offset of loss of righting reflex in rats.

As cerebrospinal fluid (CSF) possesses unique characteristics in order to explore concentration-pharmacological response relationships of drugs active in the CNS, the practicability of serial sampling of CSF was tested in a study with heptabarbital. Concentrations in CSF and plasma were measured simultaneously in individual rats during and after an intravenous infusion for 30 min. At the end of the infusion, the distribution equilibrium was attained with a CSF/plasma concentration ratio of 0.38, roughly equal to the fraction unbound to protein. When concentrations in blood and CSF were determined at the onset and offset of loss of righting reflex concentrations in blood were significantly greater at onset (146 +/- 19 mg/l) than at offset (108 +/- 16 mg/l, n = 6), whereas concentrations in CSF were identical (39 +/- 5 and 38 +/- 5 mg/l, respectively). This confirmed the earlier observation that the CSF is pharmacokinetically indistinguishable from the site of action. When the duration of the loss of righting reflex was varied, concentrations of heptabarbital in CSF at onset and offset were similar, independent of the duration of the loss of righting reflex (1-5 hr). These findings demonstrate the absence of the development of acute tolerance and confirmed that no (inter)active metabolites interfered with the pharmacological response. In a total number of 26 rats the concentrations in CSF at onset and offset of loss of the righting reflex were compared. The interindividual variation was 13-15% and the intra-individual variation was only 4-6%. The results demonstrate the usefulness of serial sampling of CSF in pharmacodynamic studies with centrally acting drugs.