Fig. 4.
5-HT1D receptors are not endogenously active after spinal cord transection. A: mean monosynaptic stretch reflex (MSR) in S4 ventral roots in response to dorsal root stimulation (as in Fig. 1), plotted over time in a chronic spinal rat. The 5-HT1D antagonist GR127935 (5 µM) by itself does not increase the MSR, indicating no endogenous 5-HT activating this receptor. The slight decrease in MSR is likely due to the weak partial agonist action of GR127935. B: in contrast, GR127935 reduced the inhibition of the MSR by zolmitriptan (3 nM). C: at high doses, SB224289 (30 µM) is an inverse agonist at the 5-HT1D receptor but does not increase the MSR, indicating that there is no constitutive 5-HT1D receptor activity after spinal cord injury. n = 12 per condition, with similar results for each (12/12).