Evidence for the involvement of the spinoparabrachial pathway, but not the spinothalamic tract or post-synaptic dorsal column, in acute bone nociception

We have previously reported that acute noxious mechanical stimulation of bone activates neurons throughout the dorsal horn of the lumbar spinal cord, and argued that the spinal mechanisms that mediate bone nociception are different to those that mediate cutaneous and visceral nociception. In the present study, we provide evidence that the ascending spinal pathways that mediate acute bone nociception also differ to those that mediate acute cutaneous and visceral nociception. Injections of a retrograde tracer (Fluorogold) were made into the thalamus, gracile nucleus or lateral parabrachial nucleus to identify spinothalamic, post-synaptic dorsal column or spinoparabrachial projection neurons respectively (n = 4 in each group). Spinal dorsal horn neurons activated by acute noxious mechanical stimulation of bone (bone drilling) were identified in these animals using Fos immunohistochemistry. Fluorogold and Fos-like immunoreactivity was not colocalized in any dorsal horn neurons projecting to the thalamus or gracile nucleus. In contrast, a total of 12.2 ± 1.1% (mean ± S.E.M.) of the spinoparabrachial projection neurons contained Fos-like immunoreactive nuclei following bone drilling and this was significantly greater than the percentage (3.4 ± 0.5%) in animals of a sham surgery group (n = 4) that were not exposed to bone drilling (Mann–Whitney; p < 0.05). These data provide evidence for the involvement of the spinoparabrachial pathway, but not the spinothalamic or post-synaptic dorsal column pathways, in the relay of information regarding acute noxious mechanical stimuli applied to bone, and suggest that spinal pathways that mediate acute bone nociception may be different to those that mediate acute nociception of cutaneous and visceral origin.