Experimental behaviour testing: pain.

Behavioural testing is a widely used method in pain research in animals, particularly in rodents. With a wide range of tests available, it is easy to tailor experiments to examine specific brain or spinal cord areas, such as the dorsal horn. This editorial outlines some of the more commonly used tests and gives examples of how they can be used to further our knowledge of neurological processes in disease conditions such as neuropathic pain or to assess the efficacy of therapeutic strategies. Pain sensation starts in receptors in the skin and viscera which project up to the spinal cord. The majority of sensory inputs to the spinal cord terminate in the dorsal horn, where afferents carrying the signal from the periphery converge onto interneurones or projection neurones. Some inhibitory or excitatory modulation can occur in the spinal cord via the interneurons before the signal continues to the brain. Sometimes, before the signal reaches the brain, efferent fibres from the spinal cord also project to the muscles surrounding the initial stimulation to elicit a reflex, such as withdrawal to protect the body from the noxious stimulus. The various pathways involved in carrying and modulating pain sensations from periphery to the brain involve complex circuitry and the majority of neurotransmitter types found in the higher centres of the central nervous system are present in the dorsal horn. In the spinal cord, the majority of interneurons either release glutamate or GABA (often in conjunction with glycine). Glutamate is an excitatory neurotransmitter, so activates the next cell in the pathway, while GABA and glycine are inhibitory, so reduce the effect of the next neurone along. Using these two neurotransmitters, incoming signals can be modulated, that is, strengthened or dampened down, before reaching the brain. The hierarchical organization of the nervous system, highlighting the sites of action of the more commonly used pain models and nociceptive tests, are illustrated in Figure 1.