The present study demonstrated that 5-HT 1 A receptor mRNA was expressed with moderate level in the NRM neurons. Most of 5-HT 1 A receptor mRNA positive cells were 5-HT neurons, suggesting the majority of 5-HT 1 A receptor in the NRM might be autoreceptors. Eight hours after carrageenan inflammation, the expression of 5-HT 1 A receptor mRNA in the NRM neurons, especially in the 5-HT neurons, was significantly increased. These results suggest that synthesis of 5-HT 1 A receptors, including 5-HT 1 A autoreceptors, is increased in the NRM during peripheral inflammation.