Cortically projecting cells in the periaqueductal gray matter of the rat. A retrograde fluorescent tracer study

The topographical organization of the afferent input from the periaqueductal gray matter (PAG) to the cerebral cortex has been assessed in rats by retrograde transport of the fluorescent tracers Fast blue (FB) and Diamidino yellow (DY). The olfactory, medial frontal (infralimbic, prelimbic and anterior cingulate cortices), lateral frontal (motor), parietal, temporal, occipital and insular cortices were explored by placing two fluorescent tracers into two different cortical regions. The PAG contained the largest number of labeled neurons in medial frontal cortex injections, followed by olfactory and lateral frontal cortices. Fewer retrogradely labeled cells were seen after injections in parietal, temporal occipital and insular cortices. All labeled cells were exclusively located in the medial and lateroventral divisions of the PAG (PAGm and PAGlv). The longitudinal extent of the labeling in PAGm was more extensive than in PAGlv. The labeled neurons in the medial frontal cortex group extended through most of the PAG, while in the remaining groups it was restricted to the caudal one-third of the PAG. Neurons with projections to two different cortical regions were only a small fraction of the total population of labeled cells. Our data indicate that the medial frontal cortex is the most important recipient of a direct PAG input, followed by the lateral frontal cortex. Parietal, temporal, occipital and insular cortices receive only a minor projection. It is concluded that the PAG sends direct projections over the majority of the cortical mantle. Therefore, the possibility arises that the cerebral cortex receives a direct influence from the brainstem without a thalamic relay.