Raver, C. | Search bioRxiv

bioRxiv2020

An amygdalo-parabrachial pathway regulates pain perception and chronic pain

The parabrachial (PB) complex mediates both ascending nociceptive signaling and descending pain modulatory information in the affective/emotional pain pathway. We have recently reported that chronic pain is associated with amplified activity of PB neurons in a rat model of neuropathic pain. Here we demonstrate that similar activity amplification occurs in mice, and that this is related to suppressed inhibition to PB neurons from the central nucleus of the amygdala (CeA). Animals with pain after chronic constriction injury of the infraorbital nerve (CCI-Pain) displayed higher spontaneous and evoked activity in PB neurons, and a dramatic increase in after-discharges--responses that far outlast the stimulus--compared to controls. PB neurons in CCI-Pain animals showed a reduction in inhibitory, GABAergic inputs. We show that--in both rats and mice--PB contains few GABAergic neurons, and that most of its GABAergic inputs arise from CeA. These CeA GABA neurons express dynorphin, somatostatin and/or corticotropin releasing hormone. We find that the efficacy of this CeA-LPB pathway is suppressed in chronic pain. Further, optogenetically stimulating this pathway suppresses acute pain, and inhibiting it, in naive animals, evokes pain behaviors. These findings demonstrate that the CeA-LPB pathway is critically involved in pain regulation, and in the pathogenesis of chronic pain. Significance StatementWe describe a novel pathway, consisting of inhibition by dynorphin, somatostatin and corticotropin-releasing hormone expressing neurons in the central nucleus of the amygdala that project to the parabrachial nucleus (PB). We show that this pathway regulates the activity of pain-related neurons in PB, and that, in chronic pain, this inhibitory pathway is suppressed, and that this suppression is causally related to pain perception. We propose that this amygdalo-parabrachial pathway is a key regulator of both chronic and acute pain, and a novel target for pain relief.

neuroscience↗

bioRxiv2020

Cortical 6-9 Hz Oscillation are a Reliable Biomarker of Persistent Pain in Rats

Neural biomarkers of chronic pain offer a potential tool for improving the speed of diagnosis and delivery of treatment for this devastating disease. Here, we tested the hypothesis that pain states are associated with distinct changed in cortical brain waves. We induced neuropathic orofacial pain in female rats by chronic constriction injury of the infraorbital nerve (CCI-ION). In most animals, this resulted in lasting reductions in mechanical sensitivity thresholds, and in lasting increases in facial grimace scores. We recorded electrocortigraphy (ECoG) signals over the neocortex of these rats, before and after CCI-ION, and analyzed these signals with a novel, spectral modelling approach. Consistent with our hypothesis, power in the 6-9 Hz bandwidth of the ECoG was differentially modulated in animals displaying signs of chronic pain. Specifically, development of mechanical hypersensitivity correlated with a decrease in 6-9 Hz power. Furthermore, we show that changes in the power of this oscillation after injury, obtained at the individual animal level, provide a more sensitive marker of pain presence than do traditional between animal comparisons of post-injury oscillatory power. Identification of animals demonstrating chronic-pain behaviors was more accurate when estimates of post-injury oscillatory power were compared against each animals own pre-injury baseline than when compared against post-injury power estimates from animals not developing chronic pain. These results highlight the need for establishing individual-specific, "pain-free" baselines from which oscillation disturbances can be measured and which may constitute a reliable, low-cost approach not only for diagnosing chronic pain, but also for identifying individuals likely to transition from acute to chronic pain.

neuroscience↗