Effects of cysteamine on pain behaviour and on somatostatin- and substance P-like immunoreactivity in the substantia gelatinosa of the rat.

Immunohistochemical studies on rats showed cysteamine to deplete immunoreactive somatostatin, but not substance P, in the substantia gelatinosa of the spinal trigeminal nucleus and spinal cord. The effect of cysteamine treatment on chemical pain was investigated using capsaicin as the pain-producing stimulus. Thermal pain was assessed with a hot plate test after cysteamine treatment. It was found that cysteamine did not alter the hind paw lick latency in the hot plate test or the number of forepaw wipes after application of a drop of capsaicin into the cornea, compared to normal animals. In capsaicin-treated animals a significantly lower number of forepaw wipes were seen after corneal capsaicin application. However, in cysteamine-treated animals slower wiping was observed compared to untreated and capsaicin-treated animals. This suggests that somatostatin may be of importance for the modulation of nociceptive information, although it is not a major pain transmitter.