Study on brain interstitial fluid distribution and blood-brain barrier transport of baclofen in rats by microdialysis

This study was performed to examine the distribution in the brain interstitial fluid (ISF) and the blood-brain barrier (BBB) transport of baclofen in rats by a microdialysis technique.

Following an i.v. bolus administration and/or the constant i.v. infusion of baclofen to the microdialysis cannula-bearing anesthetized rats, the concentrations of baclofen in the hippocampal ISF, whole brain tissue, cerebrospinal fluid (CSF), and plasma were determined by high-performance liquid chromatography (HPLC). Data were kinetically analyzed to estimate the transport parameters, i.e., the influx clearance (CLin) from plasma to brain and the efflux rate constant (keff) from brain to plasma, and the steady-state volume of distribution in the brain (Vd).

The concentrations of baclofen in ISF, whole brain tissue, and CSF at the pseudo-steady state were almost 30-fold lower than the plasma unbound concentration, suggesting the restricted distribution of baclofen in the brain. The estimated values of CLin and keff were 0.00157 +/- 0.00076 ml/min/g of brain and 0.0872 +/- 0.0252 min-1, respectively. The efflux clearance (CLout) calculated by multiplying keff by Vd (0.816 +/- 0.559 ml/g of brain) was 0.0712 +/- 0.0529 ml/min/g of brain, and it was significantly 40-fold greater than the CLin value and fully greater than the convective flow in ISF. Furthermore, no significant concentration gradient was observed between ISF and CSF. These results suggest that the CLout value mainly reflects the efflux clearance through the BBB. Additionally, the hippocampal ISF/plasma concentration ratio of baclofen was markedly increased by both systemic administration of probenecid and its direct instillation into ISF.

The restricted distribution of baclofen in the brain ISF may be ascribed to the efficient efflux from the brain through the BBB which is regulated possibly by a probenecid-sensitive organic anion transport system.