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.

The synthetic beta-triketones are a novel family of chemicals, developed as herbicides that have activity on grass and broadleaf weeds and are selective in corn. Toxicological evaluation of a number of these chemicals has established that they interfere with rat hepatic tyrosine catabolism in vivo by inhibiting the enzyme 4-hydroxyphenylpyruvate dioxygenase (HPPD). This paper describes the kinetics of inhibition of rat hepatic HPPD in vitro by the representative beta-triketone 2-[2-nitro-4-(trifluoromethyl)benzoyl]-4,4,6,6- tetramethylcyclohexane-1,3,5-trione (1). A marked inhibition of rat hepatic HPPD was observed when 1 was incubated with the enzyme for 3 min at 37 degrees C prior to the initiation of the enzyme reaction by the addition of substrate. In this system, a concentration of 200 nM 1 resulted in a > 90% loss of HPPD activity, and an apparent IC50 was established at approximately 50 nM. The rate constant for the inactivation of HPPD by 1 was (1.5 +/- 0.2) x 10(-5) s-1 nM-1 as determined by progress curve data of oxygen consumed by HPPD with time. This inhibition is reversible in that the enzyme-inhibitor complex slowly dissociates, with approximately 5.5 +/- 0.6% of the enzyme activity being recovered by 6 h at 25 degrees C (t1/2, 25 degrees C, estimated at 101 +/- 14 h). In short, our studies establish 1 to be a tight-binding inhibitor of rat hepatic HPPD in vitro. This inhibition is characterized by the rapid inactivation of HPPD by the formation of an enzyme-inhibitor complex that dissociates extremely slowly with recovery of enzyme activity.

Experiments were performed to determine the extent of paraquat entry into the brain of neonatal and elderly rats, as compared with adult rats, which may be dependent on the efficacy of the blood-brain barrier. A single, median lethal dose (20 mg/kg s.c.) of paraquat containing [14C]paraquat was administered to neonatal (10 day old), adult (3 month old) and elderly (18 month old) rats. In contrast to the adult and elderly rats where paraquat levels fell over the 24 h post-dosing period to negligible levels, paraquat concentrations in neonatal brains did not decrease with time between 0.5 and 24 h following dosing. The distribution of [14C]paraquat was measured in selective brain regions using quantitative autoradiogra phy in all three age groups of rats, 30 min and 24 h following dosing. Autoradiography demonstrated that brain paraquat distributions were similar in the rat age groups. Most of the paraquat was confined to regions outside the blood-brain barrier and to brain regions that lack a complete blood-brain barrier e.g. dorsal hypotha lamus, area postrema and the anterior olfactory bulb. Between 0.5 h and 24 h following dosing, paraquat concentrations in deeper brain structures, some distance away from the sites of entry, began to slowly increase in all the rat age groups. By 24 h following dosing, a majority of brain regions examined using quantitative autoradiogra phy revealed significantly higher paraquat concentrations in neonatal brains as compared to brain regions of adult and elderly rats. Despite increased paraquat entry into neonatal brain, we could find no evidence for paraquat- induced neuronal cell damage following a detailed histopathological examination of perfused-fixed brains. In conclusion, impaired blood-brain barrier integrity in neonatal brain thus permitting more paraquat to enter than in adult brain, did not result in neuronal damage.

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The objectives of the studies described were to assess the ultrastructural neuropathology, blood-brain barrier (BBB) integrity and calcium status of the cerebellum of rats following a single dose of 750 mg.kg-1 L-2-chloropropionic acid (L-2-CPA). The first indications of intoxication appeared at 36 h when condensation of many granule cells associated with Purkinje cell degeneration and marked astroglial swelling were observed. Some electron-lucent granule cells were also noted lying amongst these condensed forms. Condensed granule cells had swollen, electron-lucent mitochondria, dilated Golgi apparatus and nuclear crenation. Occasionally, areas of the granule cell necrosis were also present at this time. Granule cell condensation probably represents a preliminary and irreversible stage in an excitotoxic process that leads to necrosis. At 48 and 72 h, most granule cells were necrotic, and occasionally, extravasation of both erythrocytes and leucocytes into the expanded extravascular space was observed. Evaluation of the BBB by ultrastructural cytochemical visualisation of horseradish peroxidase injected i.v. 2 min before killing by perfusion fixation showed substantial leakage. At 36 h post-dose, ultrastructural calcium localisation using oxalate/pyroantimonate precipitation demonstrated a substantial increase in calcium pyroantimonate precipitate in mitochondria and other membranous cytoplasmic organelles (especially the Golgi apparatus) in condensed granule cells, but with little in their nuclei. However, their immediate neighbours (of ostensibly normal ultrastructural appearances) contained greater amounts of intranuclear precipitate. Swollen astroglial cells (especially the Bergmann glia) contained considerable quantities of precipitate. A possible excitotoxic mechanism via L-2-CPA-induced NMDA receptor agonism leading to overwhelming calcium influx and disruption of cellular calcium homeostasis is proposed.