Sumatriptan can inhibit trigeminal afferents by an exclusively neural mechanism

Mechanical distortion of the human cranial venous sinuses is painful as is cranial venous sinus distension during migraine. Sumatriptan, the serotonin (5HT)IB/D-like receptor agonist, is highly effective in relieving migraine headache and part of its action may be due to constriction of cranial dural blood vessels. Using immunohistochemical detection of the immediate early gene Fos, we have mapped the spatial pattern of neural activation in the caudal medulla and the upper cervical spinal cord (C1, C2 and C3) in cats following either electrical or mechanical stimulation of the superior sagittal sinus. Fourteen cats were anaesthetized with alpha-chloralose and prepared for physiological monitoring of blood pressure, heart rate, rectal temperature and expired CO2. Electrical stimulation evoked significant increases in the (median) numbers of Fos-positive cells in laminae I and IIo of the superficial dorsal horn of C1, C2 and C3 cervical spinal cord (88, 92 and 18 cells, respectively) and of the trigeminal nucleus caudalis (TNC) (81 cells). Mechanical stimulation revealed a similar pattern of neural activation but with reduced intensity in laminae I and IIo of the TNC (38 cells) and of C1 and C2 (32 and 31 cells, respectively). The temporalis muscle was stimulated mechanically in the control group and the numbers and distribution of Fos-positive cells were no different from those in non-stimulated controls. Treatment with sumatriptan reduced the numbers of Fos-positive cells found in laminae I and IIo of the TNC and C2 (6, 13 cells and 9 cells, respectively) after mechanical stimulation. These data suggest that the neural effect of sumatriptan alone is sufficient for significant attenuation of transmission in the trigeminal system. The fact that sumatriptan can inhibit trigeminal activation without its vascular effects suggests that drugs without a significant activity on blood vessels may be effective in the treatment of migraine.