Identification of Spinal Inhibitory Interneurons Required for Attenuating Effect of Duloxetine on Neuropathic Allodynia-like Signs in Rats

Amitriptyline and duloxetine attenuate activities of superficial dorsal horn neurons in a rat reserpine-induced fibromyalgia model

Fibromyalgia (FM) is an intractable disease with a chief complaint of chronic widespread pain. Amitriptyline (AMI) and duloxetine (DLX), which are antidepressant drugs, have been reported to ameliorate pain in patients with FM and pain-related behaviors in several rodent models of FM. However, the mechanisms of action of AMI and DLX are not yet fully understood. Here, we examined the effects of these drugs on the responsiveness of superficial dorsal horn (SDH) neurons in the spinal cord, using a rat FM model developed by injecting a biogenic amine depleter (reserpine). Extracellular recordings of SDH neurons in vivo demonstrated that bath application of AMI and DLX at concentrations of 0.1-1.0 mM on the dorsal surface of the spinal cord markedly suppressed spontaneous discharge and von Frey filament-evoked mechanical firing in SDH neurons. The suppression induced by the drugs was noted in a concentration-dependent manner and the suppressive effects resolved after washing the spinal cord surface. These results show that SDH neurons are the site of action for AMI and DLX in a rat reserpine-induced FM model. Spinal mechanisms may underlie the therapeutic effects of these drugs in patients with FM.

Spinal Nmur2-positive Neurons Play a Crucial Role in Mechanical Itch

The dorsal spinal cord is crucial for the transmission and modulation of multiple somatosensory modalities, such as itch, pain, and touch. Despite being essential for the well-being and survival of an individual, itch and pain, in their chronic forms, have increasingly been recognized as clinical problems. Although considerable progress has been made in our understanding of the neurochemical processing of nociceptive and chemical itch sensations, the neural substrate that is crucial for mechanical itch processing is still unclear. Here, using genetic and functional manipulation, we identified a population of spinal neurons expressing neuromedin U receptor 2 (Nmur2+) as critical elements for mechanical itch. We found that spinal Nmur2+ neurons are predominantly excitatory neurons, and are enriched in the superficial laminae of the dorsal horn. Pharmacogenetic activation of cervical spinal Nmur2+ neurons evoked scratching behavior. Conversely, the ablation of these neurons using a caspase-3-based method decreased von Frey filament-induced scratching behavior without affecting responses to other somatosensory modalities. Similarly, suppressing the excitability of cervical spinal Nmur2+ neurons via the overexpression of functional Kir2.1 potassium channels reduced scratching in response to innocuous mechanical stimuli, but not to pruritogen application. At the lumbar level, pharmacogenetic activation of these neurons evoked licking and lifting behaviors. However, ablating these neurons did not affect the behavior associated with acute pain. Thus, these results revealed the crucial role of spinal Nmur2+ neurons in mechanical itch. Our study provides important insights into the neural basis of mechanical itch, paving the way for developing novel therapies for chronic itch. PERSPECTIVE: Excitatory Nmur2+ neurons in the superficial dorsal spinal cord are essential for mechanical but not chemical itch information processing. These spinal Nmur2+ neurons represent a potential cellular target for future therapeutic interventions against chronic itch. Spinal and supraspinal Nmur2+ neurons may play different roles in pain signal processing.