SPINAL AFFERENTS TO FUNCTIONALLY DISTINCT PERIAQUEDUCTAL GRAY COLUMNSIN THE RAT - AN ANTEROGRADE AND RETROGRADE TRACING STUDY

The segmental and laminar organization of spinal projections to the fu nctionally distinct ventrolateral (vlPAG) and lateral periaqueductal g ray (lPAG) columns was examined by using retrograde and anterograde tr acing techniques. It was found 1) that spinal input to both vlPAG and lPAG columns arose predominantly from neurons in the upper cervical (C 1-4) and sacral spinal cord; 2) that there was a topographical separat ion of vl-PAG projecting and lPAG-projecting neurons within the upper cervical spinal cord; but 3) that below spinal segment C4, vlPAG-proje cting and lPAG-projecting spinal neurons were similarly distributed, p redominantly within contralateral lamina I, the nucleus of the dorsola teral fasciculus (the lateral spinal nucleus) and the lateral (reticul ar) part of lamina V. Consistent with the retrograde results, the grea test density of anterograde label, within both the vlPAG and lPAG, was found after tracer injections made either in the superficial or deep dorsal horn of the upper cervical spinal cord. Tracer injections made within the thoraco-lumbar spinal cord revealed that the vlPAG column r eceived a convergent input from both the superficial and deep dorsal h orn. However, thoraco-lumbar input to the lPAG was found to arise uniq uely from the superficial dorsal horn; whereas the deep dorsal horn wa s found to innervate the ''juxta-aqueductal'' FAG region rather than p rojecting to the IPAG. These findings suggest that similar to spino-pa rabrachial projections, spinal projections to the lPAG (and juxta-aque ductal FAG) are topographically organised, with distinct subgroups of spinal neurons projecting to specific lPAG or juxta-aqueductal FAG sub regions. In contrast, the vlPAG receives a convergent spinal input whi ch arises from the superficial and deep dorsal horn of cervical, thora cic, lumbar, and sacral spinal segments. (C) 1997 Wiley-Liss, Inc.