A Probabilistic Model for Estimating the Depth and Threshold Temperature of C-fiber Nociceptors.

The subjective experience of thermal pain follows the detection and encoding of noxious stimuli by primary afferent neurons called nociceptors. However, nociceptor morphology has been hard to access and the mechanisms of signal transduction remain unresolved. In order to understand how heat transducers in nociceptors are activated in vivo, it is important to estimate the temperatures that directly activate the skin-embedded nociceptor membrane. Hence, the nociceptor's temperature threshold must be estimated, which in turn will depend on the depth at which transduction happens in the skin. Since the temperature at the receptor cannot be accessed experimentally, such an estimation can currently only be achieved through modeling. However, the current state-of-the-art model to estimate temperature at the receptor suffers from the fact that it cannot account for the natural stochastic variability of neuronal responses. We improve this model using a probabilistic approach which accounts for uncertainties and potential noise in system. Using a data set of 24 C-fibers recorded in vitro, we show that, even without detailed knowledge of the bio-thermal properties of the system, the probabilistic model that we propose here is capable of providing estimates of threshold and depth in cases where the classical method fails.

Piezo2 is the major transducer of mechanical forces for touch sensation in mice.

The sense of touch provides critical information about our physical environment by transforming mechanical energy into electrical signals. It is postulated that mechanically activated cation channels initiate touch sensation, but the identity of these molecules in mammals has been elusive. Piezo2 is a rapidly adapting, mechanically activated ion channel expressed in a subset of sensory neurons of the dorsal root ganglion and in cutaneous mechanoreceptors known as Merkel-cell-neurite complexes. It has been demonstrated that Merkel cells have a role in vertebrate mechanosensation using Piezo2, particularly in shaping the type of current sent by the innervating sensory neuron; however, major aspects of touch sensation remain intact without Merkel cell activity. Here we show that mice lacking Piezo2 in both adult sensory neurons and Merkel cells exhibit a profound loss of touch sensation. We precisely localize Piezo2 to the peripheral endings of a broad range of low-threshold mechanoreceptors that innervate both hairy and glabrous skin. Most rapidly adapting, mechanically activated currents in dorsal root ganglion neuronal cultures are absent in Piezo2 conditional knockout mice, and ex vivo skin nerve preparation studies show that the mechanosensitivity of low-threshold mechanoreceptors strongly depends on Piezo2. This cellular phenotype correlates with an unprecedented behavioural phenotype: an almost complete deficit in light-touch sensation in multiple behavioural assays, without affecting other somatosensory functions. Our results highlight that a single ion channel that displays rapidly adapting, mechanically activated currents in vitro is responsible for the mechanosensitivity of most low-threshold mechanoreceptor subtypes involved in innocuous touch sensation. Notably, we find that touch and pain sensation are separable, suggesting that as-yet-unknown mechanically activated ion channel(s) must account for noxious (painful) mechanosensation.

Stomatin-domain protein interactions with acid-sensing ion channels modulate nociceptor mechanosensitivity.

Acid-sensing ion channels (ASICs) and their interaction partners of the stomatin family have all been implicated in sensory transduction. Single gene deletion of asic3, asic2, stomatin, or stoml3 all result in deficits in the mechanosensitivity of distinct cutaneous afferents in the mouse. Here, we generated asic3(-/-):stomatin(-/-), asic3(-/-):stoml3(-/-) and asic2(-/-):stomatin(-/-) double mutant mice to characterize the functional consequences of stomatin-ASIC protein interactions on sensory afferent mechanosensitivity. The absence of ASIC3 led to a clear increase in mechanosensitivity in rapidly adapting mechanoreceptors (RAMs) and a decrease in the mechanosensitivity in both Aδ- and C-fibre nociceptors. The increased mechanosensitivity of RAMs could be accounted for by a loss of adaptation which could be mimicked by local application of APETx2 a toxin that specifically blocks ASIC3. There is a substantial loss of mechanosensitivity in stoml3(-/-) mice in which ∼35% of the myelinated fibres lack a mechanosensitive receptive field and this phenotype was found to be identical in asic3(-/-):stoml3(-/-) mutant mice. However, Aδ-nociceptors showed much reduced mechanosensitivity in asic3(-/-):stoml3(-/-) mutant mice compared to asic3(-/)(-) controls. Interestingly, in asic2(-/-):stomatin(-/-) mutant mice many Aδ-nociceptors completely lost their mechanosensitivity which was not observed in asic2(-/-) or stomatin(-/-) mice. Examination of stomatin(-/-):stoml3(-/-) mutant mice indicated that a stomatin/STOML3 interaction is unlikely to account for the greater Aδ-nociceptor deficits in double mutant mice. A key finding from these studies is that the loss of stomatin or STOML3 in asic3(-/-) or asic2(-/-) mutant mice markedly exacerbates deficits in the mechanosensitivity of nociceptors without affecting mechanoreceptor function.