Behavioural probes of Basal ganglia function to further investigate the neurobiology of equine stereotypic behaviour

Abstract

The horse (Equus caballus) provides a useful model where the study of repetitious behaviour is concerned as they perform three distinct stereotypies including one oral (crib-biting) and two locomotor equivalents (weaving and box-walking). Whilst several preliminary investigations have been performed into the neuro-aetiology of crib-biting, no studies to date have sought to elucidate the brain mechanisms underlying locomotor stereotypy in this species. As such, the primary aim of this investigation was to probe the neural basis of locomotor stereotypy (weaving) and extend current knowledge with regards to the crib-biting response. In this regard, behavioural probes have proved useful in identifying altered striatal functioning in a number of species without the use of invasive methods. Consequently spontaneous blink rate (SBR), behavioural initiation and an extinction-devaluation paradigm were conducted on a sample of crib-biting (n=8), weaving (n=8) and control (n=8) horses to investigate striatal output patterns. Crib-biting horses demonstrated significantly lower SBR when compared to the control (p<0.05) and the weaving (p<0.01) animals. Behaviour initiation was significantly increased for the crib-biting (p<0.01) and the weaving (p<0.05) horses when compared to control equivalents. During the extinction paradigm, the control horses required significantly more trials to reach learning criterion when compared to both crib-biting (p<0.001) and weaving (p<0.001) animals. The crib-biting horses performed significantly more operant responses during extinction 1 and extinction 2 compared to weaving (p<0.001 and p<0.01 respectively) and control horses (p<0.001 and p<0.001 respectively). The crib-biting sample conducted significantly more operant responses during extinction 1 when compared to extinction 2 (p<0.005), though no difference was observed for the control or weaving group. Finally, crib-biting horses required significantly more trials to reach total extinction criterion when compared to their control (p<0.001) and weaving (p<0.01) equivalents. This data suggests that there is an initial acceleration of ventral-dorsal activity within the striatum of crib-biting horses. However, the significant reduction of operant responses during extinction 2 compared to extinction 1 is indicative of a return to action outcome monitoring in the final stages of the extinction experiment. It is possible that this reduction of operant responses in extinction 2 is resultant of dopamine receptor saturation following devaluation. As the number of operant responses during extinction 2 is significantly higher for the crib-biting horses, the crib-biting horse is therefore responding habitually during extinction 2 in response to the conditioned stimulus as motivation in terms of reward acquisition has ceased. On the other hand the weaving horses did not transit towards stimulus-response learning at any stage of the extinction paradigm. Rather the weaving horse data suggests enhanced motivation as a result of increased phasic dopamine release highlighted by significantly reduced trials to attain learning criterion compared to control animals.

Date of Award	2014
Original language	English
Awarding Institution	Coventry University Royal Agricultural University
Supervisor	Andrew Hemmings (Supervisor)