Behavioural correlates of the equine stereotypy phenotype

Abstract

Stereotypic behaviour is characterised as repetitive, topographically invariant, rigid behaviour patterns, often displayed by captive, domestic and laboratory animals. The cause of these behaviours is thought primarily to be related to impoverished or species-atypical living environments, but recent research has outlined a neural phenotype, relating to basal ganglia dopamine physiology, which is common to several species. One species displaying the neural and associated behavioural phenotypes is the Horse. The purpose of this thesis was to explore, through behavioural assays, the behavioural phenotypes associated with endogenous basal ganglia dysfunction as a neural feature of equine oral stereotypy. In the first study, the behavioural effects of one aspect of the neural phenotype – downregualtion of dopamine transmission in the nigrostriatal pathways – were examined. Animals with the medial aspect of the dorsal striatum lesioned have been shown to exhibit learning differences in spatial navigation procedures. Employing two dry-maze procedures, stereotypic and control horses were required to locate food in different locations. No specific differences between the groups were found, but the stereotypic group were found to be less exploratory and tended to adopt ‘habitual’ response patterns. In the next studies, the behavioural effects of a further feature of the neural phenotype – upregulation of mesoaccumbens dopamine transmission – were examined. Chronic amphetamine exposure leads to a similar neural phenotype, and this offered an interesting hypothesis in the context of the endogenous change reported in stereotypic horses. A Pavlovian to Instrumental Transfer task was initially employed, in which horses were first trained in a Pavolvian procedure, and subsequently trained to perform an instrumental response, reinforced by the same food as in the Pavlovian phase. In the transfer phase, the Pavlovian conditioned stimulus was introduced in the context of the instrumental responding. It was predicted that response rates would increase during the transfer phase; however, the results were inconclusive. In the third study, further behavioural effects of increased mesoaccumbens dopamine were tested by employing an instrumental choice procedure. Stereotypic horses and controls were trained to choose between two mutually-exclusive schedules of reinforcement, one associated with a short delay to gain food and one with a longer delay. Chronic amphetamine exposure leads to a decrease in sensitivity to delay, and it was demonstrated that stereotypic horses showed similar patterns, failing to choose the shorter schedule. Finally, increased dopamine transmission is known to enhance the rate at which learning shifts from planned-action to habit, and this was examined in stereotypic horses using a ‘place-response’ preparation. Evidence was found to support this hypothesis in two variations of this procedure. The latter findings suggested an imbalance of the constituent cells that form the striatum: the striosome and matrix components. Taking this in conjunction with the other findings, this imbalance may hold the key to identifying the aetiology of stereotypic behaviour