Heterogenous effects of serotonin in the dorsal horn of rat: the involvement of 5-HT1 receptor subtypes

The effect of ionophoretically applied serotonin (5-HT) was tested on cutaneous sensory responses of multireceptive dorsal horn neurones in the anaesthetized rat. Three types of 5-HT action were discerned: selective inhibition of nociceptive responses (10/18 cells), non-selective inhibition of responses to both noxious and innocous stimuli as well as to excitatory amino acids (4/18 cells) and non-selective excitation of evoked responses (1/18 cells). A few cells (3/18) were unaffected by 5-HT. The use of agonists, shown to discriminate between subtypes of 5-HT₁ receptor revealed that a 5-HT_1A receptor agonist mimicked the non-selective effects of 5-HT, whereas a 5-HT_1B receptor agonist mimicked the selective antinociptive effects of 5-HT. A 5-HT₂ receptor agonist, in contrast, was without effect. Both the selective and the non-selective effects were reversed by a 5-HT₁ receptor antagonist, but not a 5-HT₂ antagonist.