Opioid circuitry and the aetiology of equine oral stereotypy

Stereotypic behaviour is generally attributed to a dysfunction of the basal ganglia, although confusion exists as to whether altered function stems from the dorsal (Caudatus and putamen) or ventral regions (nucleus accumbens). In rodents, imbalanced basal ganglia opioid physiology leads to enhanced efferent neural transmission from only the dorsal striatum to cortex. However, in horses performing oral stereotypy (crib-biting), neural alterations in dopamine receptor density have been recorded in both dorsal and ventral striatal regions (McBride and Hemmings, 2005) suggesting that both projections may be altered in the stereotypy phenotype. Indeed, if stereotypies are considered to stem from highly motivated appetitive behaviours, it seems probable that ventral elements of the basal ganglia ‘reward’ circuitry should also be involved If this is the case, then stereotypy performance has potentially rewarding consequences and could thus be employed as a coping strategy. To further address the issue of dorsal versus ventral striatum involvement in the equine oral stereotypy phenotype, comparisons of opioid receptor physiology between crib-biters and control horses were performed.