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

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 Fospositive 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.

The Periaqueductal Gray Orchestrates Sensory and Motor Circuits at Multiple Levels of the Neuraxis

“…Three injections of DLH (n ϭ 7) or saline (n ϭ 7) were delivered at 10 min intervals. The animals were then maintained under anesthesia for a further 2 h, timed from the second of the three injections, to allow for expression of Fos protein in supraspinal structures (Koutsikou et al, 2007).…”

“…In five animals, a pair of stimulating needle electrodes (25 G) was inserted subcutaneously between the Achilles tendon and the distal tibial nerve of the left hindlimb (Gozariu et al, 1998;Koutsikou et al, 2014). Constant current 50 s square wave pulses were delivered at 3 s intervals.…”

“…Glass micropipettes were advanced into the caudal vlPAG under stereotaxic guidance (Paxinos and Watson, 2005). Micropipettes were filled with 50 mM the excitatory amino acid DL-homocysteic acid (DLH; Sigma-Aldrich) mixed with pontamine sky blue dye to mark the injection sites (McMullan andLumb, 2006a, 2006b;Koutsikou et al, 2007). Pressure injections of DLH (60 -80 nl) typically evoked decreases in mean arterial pressure.…”

“…The injection sites were identified by the location of the dye spread and pipette track, with reference to a stereotaxic atlas (Paxinos and Watson, 2005). Staining for FLI in the cerebellum was performed using previously described methods (Koutsikou et al, 2007).…”

“…We recently reported that activation of the vlPAG causes an increase in ␣-motoneuronal excitability, which is thought to support freezing behavior (Koutsikou et al, 2014). Defense behaviors also require that an animal's response is not perturbed from essential motor activity, as would be caused by salient sensory information modifying activity in supraspinal motor systems, leading to changes in behavior.…”