Selective serotonin1A/1B agonists differentially affect spinal nociceptive reflexes

The purpose of the present experiments was to determine whether serotonin-1A (5-HT1A) and serotonin-1B (5-HT1B) binding sites, recently characterized in the spinal cord of the rat, mediate differential effects of 5-HT on spinal nociceptive processing. Several days after spinal transection at T10, rats were injected intraperitoneally at 20 min intervals, with increasing doses (0, 0.1, 0.4, 2.0, 9.0 mg/kg) of either a 5-HT1A selective agonist (8-OH-DPAT, buspirone) or a 5-HT1B agonist (mCPP, TFMPP). Nociceptive sensitivity was determined by quantifying, in cm2, changes from baseline in the receptive field areas of three spinal nociceptive withdrawal reflexes after noxious (greater than 400 mmHg) levels of mechanical stimulation. The 5-HT1A agonist 8-OH-DPAT and buspirone, significantly increased in a dose-dependent manner the receptive field areas of the three reflexes, with the following log ED50 values (nmol/kg): ventroflexion reflex--buspirone (2.75), 8-OH-DPAT (2.70); dorsiflexion reflex--buspirone (2.91), 8-OH-DPAT (2.67); lateral flexion reflex--buspirone (3.51), 8-OH-DPAT (2.77). The hypersensitivity of the reflexes after pretreatment with buspirone was effectively blocked by the 5-HT1A selective antagonist spiperone, at all doses (0.001, 0.01, 0.1 and 1.0 mg/kg) tested. The 5-HT1B selective agonists mCPP and TFMPP significantly decreased the receptive field are of the ventroflexion reflex (log ED50 values: mCPP, 3.79 nmol/kg; TFMPP, 3.61 nmol/kg) with no significant effect on the dorsiflexion or lateral flexion reflexes.