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 admin istration 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 maxi mal 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 concentra tion of paraquat remained in the brain (1.6 nmol g^-1 wet weight) and could not be removed by intracardiac perfu sion. 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 sys tem, such as the endothelial cells that make up the capil lary network and that there is a limited entry of paraquat into brain regions without a blood/brain barrier. No [¹⁴C]paraquat was detected in regions where there has been demonstrated pathology in brains from humans with Parkinson's disease. Finally, we could find no evidence for paraquat-induced neuronal cell necrosis 24 or 48 h after systemic administration. Overall it may be concluded that systemically administered paraquat does not pose a direct major neurotoxicological risk in the majority of brain regions which have a functional blood/brain barrier since paraquat can be excluded from the brain by this barrier.