Techniques for comparison of thermal and mechanical nociceptive stimuli in the sheep

The assessment of analgesia in conscious unrestrained farm animals presents particular problems. In order to provide a reliable nociceptive stimulus, two devices have been constructed, one that provides a threshold reading for a ramped thermal stimulus of the pinna of the ear and another that measures a threshold reading for a pressure stimulus to the foreleg. Both devices allow the rapid termination of the stimulus once a reaction has occurred and allow repeated readings without causing tissue damage. Examples of the analgesic action of alpha 2-adrenoceptor agonists and opioids measured by using the two pieces of apparatus are shown.