Social behaviour in sheep relates to behaviour and neurotransmitter responses to nociceptive stimuli

Sheep in the field display differences in social behaviour. These differences allow a division into three social groups with distinct behavioural occurrences and frequencies. The behavioural and neurotransmission responses of each of these groups to aversive stimuli were compared. Behavioural responses were seen to both forelimb electric shocks and thermal heating of the nose in all groups. These responses changed with stimulus repetition in a group-dependent manner. Microdialysis probe studies of neurotransmitter release in the somatosensory cortex indicated neurotransmitter responses to stimuli in all animals that varied with both animal group and stimulus repetition. Group 1 animals, aggressive and socially active, showed increases in gamma amino-4-butyric acid (GABA) with initial stimulus presentation; this increased with stimulus repetition. Behavioural responses to the stimuli decreased with repetition and nonstimulus-related behaviours, during the course of the experiment, increased. Both of these appeared dependent upon GABA. Group 2 animals, moderately aggressive and socially active, released opioid-like peptides (OLP) upon initial exposure to stimuli but, with repetition, switched to using GABA. Group 3 animals, nonaggressive and socially inactive, released OLP with initial and repeat stimuli. In groups 2 and 3, both GABA and OLP appear to reduce stimulus-related behaviour, but OLP appeared to also reduce nonstimulus-related behaviour and GABA increased these. Changes were independent of animal liveweight. Glutamate was released in response to stimuli in all 3 groups and, with repetition, fell in groups 1 and 2 but increased in group 3. An animal's social behaviour and status may predict its response to a stimulus.