Laminar organization of spinal dorsal horn neurones activated by C‐ vs. A‐heat nociceptors and their descending control from the periaqueductal grey in the rat

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Abstract
The periaqueductal grey can differentially control A‐ vs. C‐nociceptor‐evoked spinal reflexes and deep spinal dorsal horn neuronal responses. However, little is known about the control of A‐ vs. C‐fibre inputs to lamina I and the lateral spinal nucleus, and how this correlates with the control of deeper laminae. To address this, the laminar distributions of neurones expressing Fos‐like immunoreactivity were determined following preferential activation of A‐ or C‐heat nociceptors, using fast or slow rates of skin heating, respectively, in the absence or presence of descending control evoked from the periaqueductal grey. In lamina I, numbers of Fos‐positive neurones following both fast and slow rates of skin heating were reduced significantly following activation in the ventrolateral and dorsolateral/lateral periaqueductal grey. In contrast, in the deep dorsal horn (laminae III–VI), activation in both the ventrolateral and dorsolateral/lateral periaqueductal grey significantly reduced the numbers of Fos‐positive neurones evoked by C‐ but not A‐nociceptor stimulation. C‐ but not A‐heat nociceptor activation evoked Fos bilaterally in the lateral spinal nucleus. Stimulation in the ventrolateral but not the dorsolateral/lateral periaqueductal grey significantly increased the numbers of Fos‐positive neurones evoked by A‐ and C‐nociceptor stimulation bilaterally in the lateral spinal nucleus. These data have demonstrated differences in the descending control of the superficial vs. the deep dorsal horn and lateral spinal nucleus with respect to the processing of A‐ and C‐fibre‐evoked events. The data are discussed in relation to the roles of A‐ and C‐nociceptors in acute and chronic pain.