Intrathecal injection of capsaicin can be used as a behavioural nociceptive test in mice.

Intrathecal injections in mice of substance P (SP), capsaicin and kainic acid elicited a behavioural response similar to that caused by noxious stimulation. Although the agents have different mechanisms of action, they showed a similar response, consisting of biting, scratching and licking the caudal parts of the body. Activation of adenosine receptors in the spinal cord with 5-N-ethylcarboxamide adenosine (NECA) had no effect on the response to substance P or to kainic acid, but reduced the response to capsaicin. It is concluded that different substances, injected intrathecally, can be used to differentiate between pre- and postsynaptic induced nociceptive behaviour, and that this assay may be valuable in assessing central antinociceptive effects of various drugs, especially those affecting systems modulated by substance P.