Three times as many lamina I neurons project to the periaqueductal gray than to the thalamus: a retrograde tracing study in the cat

The number and distribution of lamina I neurons projecting to the periaqueductal gray (PAG) were examined by a retrograde tracing study in the cat. WGA-HRP injections in the intermediate and caudal FAG resulted in as much as 1600 labeled lamina I neurons throughout the length of the spinal cord, counted in a 1:4 series of sections. The lamina I-PAG projection was predominantly contralateral and most labeled lamina I neurons were found in the enlargements. Comparing these results with the number of lamina I-thalamic neurons leads to the conclusion that in the cat about three times as many lamina I neurons project to the FAG than to the thalamus. Considering this, one can conclude that the spino-PAG system is a virtually neglected area in pain research. (C) 1998 Elsevier Science Ireland Ltd. All rights reserved.