Free nerve endings are key structures in sensory transduction of noxious stimuli. In spite of this, little is known about their functional organization. Transient receptor potential (TRP) channels have emerged as key molecular identities in the sensory transduction of pain-producing stimuli, yet the vast majority of our knowledge about sensory TRP channel function is limited to data obtained from in vitro models which do not necessarily reflect physiological conditions. In recent years, the development of novel optical methods such as genetically encoded calcium indicators and photo-modulation of ion channel activity by pharmacological tools has provided an invaluable opportunity to directly assess nociceptive TRP channel function at the nerve terminal.

The role of TRPA1 in the thermosensitivity of the corneal cold thermoreceptor nerve endings was studied in young and aged mice. The contribution of the TRPA1-dependent activity to basal tearing and thermally-evoked blink was also explored. The corneal cold thermoreceptors’ activity was recorded extracellularly in young (5-month-old) and aged (18-month-old) C57BL/6WT (WT) and TRPA1^−/− knockout (TRPA1-KO) mice at basal temperature (34 °C) and during cooling (15 °C) and heating (45 °C) ramps. The blink response to cold and heat stimulation of the ocular surface and the basal tearing rate were also measured in young animals using orbicularis oculi muscle electromyography (OOemg) and phenol red threads, respectively. The background activity at 34 °C and the cooling- and heating-evoked responses of the cold thermoreceptors were similar in WT and TRPA1-KO animals, no matter the age. Similar to the aged WT mice, in the young and aged TRPA1-KO mice, most of the cold thermoreceptors presented low frequency background activity, a low cooling threshold, and a sluggish response to heating. The amplitude and duration of the OOemg signals correlated with the magnitude of the induced thermal change in the WT but not in the TRPA1-KO mice. The basal tearing was similar in the TRPA1-KO and WT mice. The electrophysiological data suggest that the TRPA1-dependent nerve activity, which declines with age, contributes to detecting the warming of the ocular surface and also to integrating the thermally-evoked reflex blink.

The human hand has a refined mechanical sensitivity, allowing it to play crucial roles in tactile exploration and object manipulation. Despite its fundamental and clinical relevance, a comprehensive characterization of mechanical sensitivity across the human palm is still lacking. Here, we mapped the spatial distribution of innocuous and noxious mechanical sensitivity across the palmar surface of the human hand. We examined 66 hands from 33 healthy adults, dividing the palm into 27 areas, in each of which we measured the mechanical detection threshold, the mechanical pain threshold and the pain intensity evoked by a standard 300 g pinprick stimulus. We found distal areas (i.e., fingertips) to exhibit higher tactile sensitivity than proximal areas (i.e., the wrist). Notably, the sensitivity to innocuous and noxious mechanical stimuli were inversely correlated across areas, such that areas with higher tactile sensitivity displayed higher pain thresholds. In addition, the dominant hand was less sensitive than the non-dominant one, and women displayed higher sensitivity than men. Together, this work provides the first detailed spatial characterization of mechanical sensitivity across the human hand and introduces a systematic methodology for its assessment. These findings set the stage for future studies of the neurophysiological mechanisms of touch and pain in the human hand and for clinical research into pathological conditions involving the altered hand sensitivity.

Background and Purpose: The functional organization of corneal cold nerve endings, critical structures in maintaining the ocular surface, remains poorly understood. Here, the photoisomerizable small‐molecule diethylamine‐azobenzene‐quaternary ammonium (DENAQ) was used to photomodulate activity of cold‐sensing nerve terminals in control and chronic tear‐deficient corneas. Furthermore, DENAQ was used for in vivo photochemical regulation of the thermally induced blink reflex. Experimental Approach: Extracellular nerve terminal impulse activity was recorded on cold terminals in excised corneas of naïve and tear‐deficient guinea pigs pre‐incubated with DENAQ. Pulses of light at a wavelength of 460 nm were delivered to the perfused corneas. The thermally induced blink reflex was assessed using orbicularis oculi electromyography in anaesthetised rats after topical administration of DENAQ to the eye under blue light and darkness conditions. Key Results: Exposure to blue light robustly reduced spontaneous activity of both naïve and tear‐deficient cold nerve terminals pre‐incubated with DENAQ, while cold‐evoked responses remained unaffected. Pre‐incubation of excised corneas with DENAQ, along with pharmacological P2X receptor antagonists, prevented the DENAQ‐mediated photoreduction of the cold nerve terminal spontaneous activity. In addition, blue light increased cold‐evoked reflex blink in eyes pre‐treated with DENAQ. Conclusion and Implications: DENAQ enters guinea pig cold sensory nerve endings primarily through P2X channels in excised corneas. Subsequently, DENAQ decreases the spontaneous nerve terminal impulse activity upon light irradiation by modulating voltage‐gated potassium (K _V ) channel activity. Furthermore, the cold‐evoked blink reflex is modulated by light in DENAQ‐treated eyes. Chemical photoswitches like DENAQ might be potential new treatments for ocular discomfort and pain in dry eye disease.

Axonal regeneration represents a pivotal aspect of the adult peripheral nervous system (PNS). When an injury occurs, peripheral axons are able to regenerate and reestablish connections with their original targets. Although several precision techniques for severing axons in various non-mammalian model organisms have allowed to study the dynamics of regeneration after injury, similar axonal injury models have yet to be fully developed in mammals. In this study, we introduced a novel experimental method for laser-induced axotomy of single corneal sensory subbasal nerve fibers. This method enables the in vivo monitoring and quantification of axonal degeneration and regeneration morphological dynamics in adult mice. Results revealed that the degeneration of the distal stump of the subbasal fiber was delayed in mice lacking the protein SARM1, which promotes degenerative process after injury. Meanwhile, the proximal stump maintained the regenerative dynamics of the subbasal fiber, but regeneration of its nerve terminals was impaired. The present study introduces a valuable model for the in vivo study of the morphological plasticity dynamics of peripheral axons after injury in genetically tractable adult mammals.