A novel nigrostriatal dopaminergic disorder in sheep affected by Tribulus terrestris staggers

An investigation was carried out into the pathogenesis of a unique locomotory disorder of sheep. Thirty sheep which had exhibited clinical signs for more than 15 months were examined for the presence of muscle atrophy, limb paresis, and spontaneous rotational behaviour. A single large dose of levodopa was administered to 12 normal and 12 affected sheep, and the drug-related effects of sedation and excitation were monitored continuously for three hours. The striatum was removed from eight normal and eight affected sheep and assayed for dopamine (DA) and 3,4-dihydroxyphenylacetic acid (DOPAC). The findings of asymmetrical atrophy of pelvic limb extensor muscles, asymmetrical paresis of the limbs, a left:right dominance ratio of 50:50, spontaneous rotational behaviour, decreased response to levodopa loading, and decreased levels of DA (19 per cent reduction) and DOPAC (30 per cent reduction) in the striatum of affected sheep, were interpreted as indicative of a primary nigrostriatal dopaminergic disorder. It is suggested that a striatal presynaptic receptor malfunction has occurred.