Involvement of Brn3a-positive spinal dorsal horn neurons in the transmission of visceral pain in inflammatory bowel disease model mice

Spinal dorsal horn plays crucial roles in the transmission and processing of somatosensory information. Although spinal neural circuits which process several distinct types of cutaneous sensation have been extensively studied, those responsible for visceral pain transmission remain poorly understood. In the present study, we analyzed the dextran sodium sulfate (DSS)-induced inflammatory bowel disease (IBD) model mice to characterize the spinal dorsal horn neurons involved in visceral pain transmission. DSS-treated mice exhibited increased abdominal licking behavior, suggestive of experiencing visceral pain. Immunostaining of c-fos, a marker indicating neuronal activity, demonstrated that numerous c-fos-positive cells were found bilaterally in the lumbosacral spinal dorsal horn, and their distribution was particularly abundant in the shallow dorsal horn. Neurochemical characterization of these neurons revealed that the percentage of the POU transcription factor Brn3a-positive neurons among the c-fos-positive neurons in the shallow dorsal horn was 30-40% in DSS-treated mice, which was significantly higher than that in the somatic pain model mice. We further demonstrated by neuronal tracing that within the shallow dorsal horn, Brn3a-positive neurons are represented more highly in spino-solitary projection neurons than in spino-parabrachial projection ones. These results raised the possibility that Brn3a-positive spinal dorsal horn neurons make a large contribution to visceral pain transmission, and part of which was mediated through spino-solitary pathway.

Kappa opioid receptors in the central amygdala modulate spinal nociceptive processing through an action on amygdala CRF neurons

The amygdala plays an important role in the emotional-affective aspects of behaviors and pain, but can also modulate sensory aspect of pain (“nociception”), likely through coupling to descending modulatory systems. Here we explored the functional coupling of the amygdala to spinal nociception. We found that pharmacological activation of neurons in the central nucleus of the amygdala (CeA) increased the activity of spinal dorsal horn neurons; and this effect was blocked by optogenetic silencing of corticotropin releasing factor (CRF) positive CeA neurons. A kappa opioid receptor (KOR) agonist (U-69,593) was administered into the CeA by microdialysis. KOR was targeted because of their role in averse-affective behaviors through actions in limbic brain regions. Extracellular single-unit recordings were made of CeA neurons or spinal dorsal horn neurons in anesthetized transgenic Crh-Cre rats. Neurons responded more strongly to noxious than innocuous stimuli. U-69,593 increased the responses of CeA and spinal neurons to innocuous and noxious mechanical stimulation of peripheral tissues. The facilitatory effect of the agonist was blocked by optical silencing of CRF-CeA neurons though light activation of halorhodopsin expressed in these neurons by viral-vector. The CRF system in the amygdala has been implicated in aversiveness and pain modulation. The results suggest that the amygdala can modulate spinal nociceptive processing in a positive direction through CRF-CeA neurons and that KOR activation in the amygdala (CeA) has pro-nociceptive effects.