To further define the sensory projections to the circle of Willis, we measured concentrations of immunoreactive substance P in pial arteries of cats following either bilateral removal of the C|_ 3 dorsal root ganglia (six cats) or bilateral removal of the trigeminal ganglia (three cats). Removal of the dorsal root ganglia decreased concentrations of the tachykinin substance P in the vertebral artery and the basilar artery and its branches by 72% and 50-66%, respectively. Bilateral removal of the trigeminal ganglia decreased substance P concentrations in all forebrain vessels including the rostral basilar artery, although only concentrations in the anterior cerebral artery were significantly lower than those in unilaterally lesioned cats (/><0.01). Hence, the vertebrobasilar artery and its tributaries are invested by substance P-containing fibers originating from the upper cervical dorsal root ganglia, and the anterior cerebral artery is innervated by both trigeminal ganglia. T he pattern of headaches that develops following mechanical or electrical stimulation of the large cerebral arteries comprising the circle of Willis or due to diseases involving these vessels were elucidated in humans by the studies of Ray and Wolff, 1 McNaughton, 2 Penfield, 3 Fay/ and Fisher. 5 To determine the origin of fibers that mediate this pain, sensory projections to the circle of Willis have been traced histochemically 6 or determined by measuring the effects of trigeminal lesions on the concentration of immunoreactive peptides found in pial arteries 7 or by measuring lesioninduced changes in the density of immunoreactive perivascular axons. 8 Previous data indicate that unilateral lesions decrease concentrations of substance P (SP) in the rostral circle by only 30-50% 2-3 weeks after lesioning and do not alter SP concentration within the vertebrobasilar system, except in the superior cerebellar artery (SCA). 7 The source of the remaining SP has not been determined. Hence, our study was undertaken to determine the extent to which major pial arteries within the circle of Received July 14, 1988; accepted September 23, 1988. Willis receive SP-containing projections from the upper cervical dorsal root ganglia (DRGs) or from both trigeminal ganglia (TGs).
Materials and MethodsCats weighing 2.5-4.0 kg were anesthetized with halothane and placed in the prone position. An incision in the midline was made extending from the external occipital protuberance to the spinous process of C 4 . The muscles were separated, and the laminae of C]_ 3 were exposed. The C, DRG was located within the foramen of the C, lamina, lying just caudal to the vertebral artery (VA) and was dissected after unroofing the foramen, taking special care to preserve the integrity of the adjacent VA. The C 2 DRG, located outside the intervertebral foramen, was dissected and removed. The C 3 DRG was excised following a small laminectomy in the caudal portion of the C 2 lamina. Postoperatively, the cats demonstrated a loss of position sense and muscle strength in the nec...