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Spinal afferents to functionally distinct periaqueductal gray columns in the rat: An anterograde and retrograde tracing study

The segmental and laminar organization of spinal projections to the functionally distinct ventrolateral (vlPAG) and lateral periaqueductal gray (lPAG) columns was examined by using retrograde and anterograde tracing techniques. It was found 1) that spinal input to both vlPAG and lPAG columns arose predominantly from neurons in the upper cervical (C1–4) and sacral spinal cord; 2) that there was a topographical separation of vl‐PAG projecting and lPAG‐projecting neurons within the upper cervical spinal cord; but 3) that below spinal segment C4, vlPAG‐projecting and lPAG‐projecting spinal neurons were similarly distributed, predominantly within contralateral lamina I, the nucleus of the dorsolateral fasciculus (the lateral spinal nucleus) and the lateral (reticular) part of lamina V. Consistent with the retrograde results, the greatest 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 received a convergent input from both the superficial and deep dorsal horn. However, thoraco‐lumbar input to the lPAG was found to arise uniquely from the superficial dorsal horn; whereas the deep dorsal horn was found to innervate the “juxta‐aqueductal” PAG region rather than projecting to the IPAG.