Further evidence that the blood/brain barrier impedes paraquat entry into the brain

The distribution of the non-selective herbicide paraquat was examined in the brain following subcutaneous administration of 20 mg kg-1 paraquat ion containing [14C]paraquat to male adult rats in order to determine whether paraquat crosses the blood/brain barrier. Following administration, [14C]paraquat reached a maximal concentration in the brain (0.05% of administered dose) within the first hour and then rapidly disappeared from the brain. However, 24 h after administration of the herbicide, about 13% of the maximal recorded concentration of paraquat remained in the brain (1.6 nmol g-1 wet weight) and could not be removed by intracardiac perfusion. Using measurements of [14C]paraquat in dissected brain regions and using quantitative autoradiography we demonstrated an asymmetrical distribution in and around the brain at 30 min (maximal concentration) and 24 h after administration. Most of the paraquat was associated with five structures, two of which, the pineal gland and linings of the cerebral ventricles lie outside the blood/brain barrier whilst the remaining three brain areas, the anterior portion of the olfactory bulb, hypothalamus and area postrema do not have a blood/brain barrier. Overall, the distribution of [14C]paraquat in the brain 24 h after systemic administration was highly correlated to the blood volume. These data indicate that any remaining paraquat in the brain 24 h after systemic administration is associated with elements of the cerebro-circulatory system, such as the endothelial cells that make up the capillary network and that there is a limited entry of paraquat into brain regions without a blood/brain barrier.(ABSTRACT TRUNCATED AT 250 WORDS)