Animals and humans display two types of responses to noxious stimuli. The first includes reflexive-defensive responses to prevent or limit injury. A well-known example is the quick withdrawal of one’s hand touching a hot object. When the first-line response fails to prevent tissue damage (e.g., a finger is burnt), the resulting pain invokes a second-line coping response, such as licking the injured area to soothe suffering. However, the underlying neural circuits driving these two strings of behaviors remain poorly understood. Here we show that in mice, spinal neurons marked by coexpression of Tુ Cre and Lbx1 Flpo , called Tac1 Lbx1 , drive pain-related coping responses. Ablation of Tac1 Lbx1 neurons led to loss of persistent licking and conditioned aversion evoked by stimuli that produce sustained pain in humans, including skin pinching and burn injury, without affecting all tested reflexive-defensive reactions. This selective indifference to sustained pain resembles the phenotype seen in humans with lesions of medial thalamic nuclei 1 – 3 . Consistently, spinal Tac1 lineage neurons are connected to medial thalamic nuclei, via direct projections and indirect routes through the superior lateral parabrachial nuclei. Furthermore, the anatomical and functional segregation observed at the spinal levels is applied to primary sensory neurons. For example, in response to noxious mechanical stimuli, Mrgprd + and TRPV1 + nociceptors are required to elicit reflexive and coping responses, respectively. Our studies therefore reveal a fundamental subdivision within the cutaneous somatosensory system. The implications for translational success from preclinical pain studies will be discussed.

Acid-sensing ion channel 3 (ASIC3) is involved in acid nociception, but its possible role in neurosensory mechanotransduction is disputed. We report here the generation of Asic3-knockout/eGFPf-knockin mice and subsequent characterization of heterogeneous expression of ASIC3 in the dorsal root ganglion (DRG). ASIC3 is expressed in parvalbumin (Pv+) proprioceptor axons innervating muscle spindles. We further generate a floxed allele of Asic3 (Asic3f/f) and probe the role of ASIC3 in mechanotransduction in neurite-bearing Pv+ DRG neurons through localized elastic matrix movements and electrophysiology. Targeted knockout of Asic3 disrupts spindle afferent sensitivity to dynamic stimuli and impairs mechanotransduction in Pv+ DRG neurons because of substrate deformation-induced neurite stretching, but not to direct neurite indentation. In behavioural tasks, global knockout (Asic3−/−) and Pv-Cre::Asic3f/f mice produce similar deficits in grid and balance beam walking tasks. We conclude that, at least in mouse, ASIC3 is a molecular determinant contributing to dynamic mechanosensitivity in proprioceptors.

Acid-sensing ion channels (ASICs) are important acid sensors involved in neural modulation in the central nervous system and pain-associated tissue acidosis in the peripheral system. Among ASIC subtypes, ASIC1b is the most selectively expressed in peripheral sensory neurons. However, the role of ASIC1b is still elusive in terms of its functions and expression profile. In this study, we probed the role of ASIC1b in acid-induced muscle pain in Asic1b-knockout (Asic1b–/–) and Asic1b-Cre transgenic (Asic1bCre) mice. We tested the effect of ASIC1b knockout in a mouse model of fibromyalgia induced by dual intramuscular acid injections. In this model, a unilateral acid injection to the gastrocnemius muscle induced transient bilateral hyperalgesia in wild-type (Asic1b+/+) but not Asic1b–/– mice; a second acid injection, spaced 1 or 5 days apart, to the same muscle induced chronic hyperalgesia lasting for 4 weeks in Asic1b+/+ mice, but the duration of hyperalgesia was significantly shortened in Asic1b–/– mice. Mambalgin-1, an ASIC1b-containing channel inhibitor that was mixed with acid saline at the first injection, dose-dependently blocked the acid-induced transient and chronic hyperalgesia in Asic1b+/+ mice. In contrast, psalmotoxin 1 (PcTx1), an ASIC1a-selective antagonist, had no effect on acid-induced transient or chronic hyperalgesia. We used whole-cell patch clamp recording to study the properties of acid-induced currents in ASIC1b-expressing dorsal root ganglia (DRG) neurons from Asic1bCre-TdTomato reporter mice. Medium- to large-sized ASIC1b-expressing DRG neurons mainly exhibited an amiloride-sensitive ASIC-like biphasic current (IASIC) in response to acid stimulation, whereas small- to medium-sized ASIC1b-expressing DRG neurons predominantly exhibited an amiloride-insensitive sustained current. Specifically, mambalgin-1 selectively inhibited the IASIC in most ASIC1b-expressing DRG neurons. However, PcTx1 or APETx2 (an ASIC3-selective antagonist) had only a mild inhibitory effect on IASIC in about half of the ASIC1b-expressing DRG neurons. In situ hybridization revealed that ASIC1b-positive DRG neurons co-expressed highly with ASIC1a and ASIC2a mRNA and partially with ASIC3 and ASIC2b. Thus, ASIC1b might form a wide variety of heteromeric channels. ASIC1b-containing heteromeric channels might be promising targets for the therapeutic treatment of acid-induced chronic muscle pain.