Animal Studies

Currently, cancer pain is viewed as a process orchestrated by the release of pronociceptive molecules and the invasion of neural structures, referred to as perineural invasion (PNI). Cancer pain resulting from PNI is well-documented, but the mechanisms leading to peripheral sensitization because of tumor growth are not fully known.

Microglia activation following peripheral nerve injury has been shown to contribute to central sensitization of the spinal cord for the development of neuropathic pain. In a recent study, we reported that the amount of nerve damage does not necessarily correlate with chronic pain development. Here we compared the response of spinal microglia, using immunohistochemistry as a surrogate of microglial activation, in mice with two different types of crush injury of the sciatic nerve. We confirmed that incomplete crush of the sciatic nerve (partial crush injury, PCI) resulted in tactile hypersensitivity after the recovery of sensory function (15 days after surgery), whereas the hypersensitivity was not observed after the complete crush (full crush injury, FCI). We observed that immunoreactivity for Iba-1, a microglial marker, was greater in the ipsilateral dorsal horn of lumbar (L4) spinal cord of mice 2 days after FCI compared to PCI, positively correlating with the intensity of crush injury. Ipsilateral Iba-1 reactivity was comparable between injuries at 7 days with a significant increase compared to the contralateral side. By day 15 after injury, ipsilateral Iba-1 immunoreactivity was much reduced compared to day 7 and was not different between the groups. Our results suggest that the magnitude of the early microgliosis is dependent on injury severity, but does not necessarily correlate with the long-term development of chronic pain-like hypersensitivity after peripheral nerve injury.

Neuropathic pain following peripheral nerve injury (PNI) is linked to neuroinflammation in the spinal cord marked by astrocyte activation and upregulation of interleukin 6 (IL6 chemokine (C-C motif) ligand 2 (CCL2) and chemokine (C-X-C motif) ligand 1 (CXCL1), with inhibition of each individually being beneficial in pain models.

Convergent evolution provides insights into the selective drivers underlying evolutionary change. Snake venoms, with a direct genetic basis and clearly defined functional phenotype, provide a model system for exploring the repeated evolution of adaptations. While snakes use venom primarily for predation, and venom composition often reflects diet specificity, three lineages of cobras have independently evolved the ability to spit venom at adversaries. Using gene, protein, and functional analyses, we show that the three spitting lineages possess venoms characterized by an up-regulation of phospholipase A (PLA) toxins, which potentiate the action of preexisting venom cytotoxins to activate mammalian sensory neurons and cause enhanced pain. These repeated independent changes provide a fascinating example of convergent evolution across multiple phenotypic levels driven by selection for defense.

Naked mole-rats () have adaptations within their pain pathway that are beneficial to survival in large colonies within poorly ventilated burrow systems, with lower O and higher CO ambient levels than ground-level environments. These adaptations ultimately lead to a partial disruption of the C-fiber pain pathway, which enables naked mole-rats to not feel pain from the acidosis associated with CO accumulation. One hallmark of this disruption is that naked mole-rats do not express neuropeptides, such as Substance P and calcitonin gene-related peptide in their cutaneous C-fibers, effectively making the peptidergic pain pathway hypofunctional. One C-fiber pathway that remains unstudied in the naked mole-rat is the non-peptidergic, purinergic pathway, despite this being a key pathway for inflammatory pain. The current study aimed to establish the functionality of the purinergic pathway in naked mole-rats and the effectiveness of cannabinoids in attenuating pain through this pathway. Cannabinoids can manage chronic inflammatory pain in both humans and mouse models, and studies suggest a major downstream role for the purinergic receptor, P2X3, in this treatment. Here we used Ca-imaging of cultured dorsal root ganglion neurons and behavioral testing to demonstrate that the P2X3 pathway is functional in naked mole-rats. Additionally, formalin-induced inflammatory pain was reduced by the cannabinoid receptor agonist, WIN55 (inflammatory, but not acute phase) and the P2X3 receptor antagonist A-317491 (acute and inflammatory phases). This study establishes that the purinergic C-fiber pathway is present and functional in naked mole-rats and that cannabinoid-mediated analgesia occurs in this species.

Understanding the neurobiological underpinnings of emotion relies on objective readouts of the emotional state of an individual, which remains a major challenge especially in animal models. We found that mice exhibit stereotyped facial expressions in response to emotionally salient events, as well as upon targeted manipulations in emotion-relevant neuronal circuits. Facial expressions were classified into distinct categories using machine learning and reflected the changing intrinsic value of the same sensory stimulus encountered under different homeostatic or affective conditions. Facial expressions revealed emotion features such as intensity, valence, and persistence. Two-photon imaging uncovered insular cortical neuron activity that correlated with specific facial expressions and may encode distinct emotions. Facial expressions thus provide a means to infer emotion states and their neuronal correlates in mice.