Electrical microstimulation can establish causal links between the activity of groups of neurons and perceptual and cognitive functions [1][2][3][4][5][6] . However, the number and identities of neurons microstimulated, as well as the number of action potentials evoked, are difficult to ascertain 7,8 . To address these issues we introduced the light-gated algal channel channelrhodopsin-2 (ChR2) 9 specifically into a small fraction of layer 2/3 neurons of the mouse primary somatosensory cortex. ChR2 photostimulation in vivo reliably generated stimulus-locked action potentials 10-13 at frequencies up to 50 Hz. Naïve mice readily learned to detect brief trains of action potentials (5 light pulses, 1ms, 20 Hz). After training, mice could detect a photostimulus firing a single action potential in approximately 300 neurons. Even fewer neurons (approximately 60) were required for longer stimuli (5 action potentials, 250 ms). Our results show that perceptual decisions and learning can be driven by extremely brief epochs of cortical activity in a sparse subset of supragranular cortical pyramidal neurons.We used in utero electroporation 14 to introduce ChR2 fused to a green fluorescent protein (ChR2-GFP 15 ) together with a red fluorescent cytosolic marker 15 (RFP) into neocortical pyramidal neurons (Fig. 1a, Methods). In the adult brain ChR2-GFP expression was restricted to pyramidal cells in layers 2/3 (> 99.4 %), mainly in the barrel cortex (Fig 1a, 2a). In vivo two-photon imaging and retrospective immunohistology revealed that ChR2-GFP was localized to the neuronal plasma membrane. ChR2-GFP was expressed in about half (48.9 ± 5.3 %, n = 10, 5 mice; see Methods) of red fluorescent layer 2/3 neurons (Supplementary Movie 1). ChR2-GFP invaded the soma, dendrites, and axons (Fig. 1b, 1c). ChR2-GFP expression was stable for at least 8 months and did not seem to perturb neuronal morphology (Fig. 1a-c, Methods).We next characterized the responses of ChR2-GFP-expressing neurons to photostimulation in anesthetized mice. To sample from the entire population of ChR2-GFP-expressing neurons, unbiased by ChR2-GFP expression level, we recorded from red fluorescent neurons using two-photon targeted loose-patch recordings 16 (Fig. 1c, d). Photostimuli consisted of light pulses, produced by a blue miniature light emitting diode (LED; 470 nm), centered on the recording window (Fig. 1d). At maximum light intensities (I max = 11.6 mW/mm 2 at the surface of the brain, centered on the diode; 1-10 ms duration) about half (51 %) of the patched red neurons (n = 39/77, 8 mice) responded reliably to single photostimuli with at AUTOR CONTRIBUTIONS DH and KS designed the experiments. DH performed the behavioral and in vivo physiological experiments. LP, DH and KS performed the brain slice measurements. NG performed histology. SR, TH, ZM, KS provided advice and equipment. DH and KS wrote the paper. All authors discussed the results and commented on the manuscript.

Tissue injury and inflammation markedly alter touch perception, making normally innocuous sensations become intensely painful. Although this sensory distortion, known as tactile allodynia, is one of the most common types of pain, the mechanism by which gentle mechanical stimulation becomes unpleasant remains enigmatic. The stretch-gated ion channel PIEZO2 has been shown to mediate light touch, vibration detection, and proprioception. However, the role of this ion channel in nociception and pain has not been resolved. Here, we examined the importance of Piezo2 in the cellular representation of mechanosensation using in vivo imaging in mice. Piezo2-knockout neurons were completely insensitive to gentle dynamic touch but still responded robustly to noxious pinch. During inflammation and after injury, Piezo2 remained essential for detection of gentle mechanical stimuli. We hypothesized that loss of PIEZO2 might eliminate tactile allodynia in humans. Our results show that individuals with loss-of-function mutations in PIEZO2 completely failed to develop sensitization and painful reactions to touch after skin inflammation. These findings provide insight into the basis for tactile allodynia, identify the PIEZO2 mechanoreceptor as an essential mediator of touch under inflammatory conditions, and suggest that this ion channel might be targeted for treating tactile allodynia.

Henry Miller stated that “to relieve a full bladder is one of the great human joys”. Urination is critically important in health, and ailments of the lower urinary tract (LUT) cause extensive pathological burden. Nevertheless, we take urination for granted, and in-depth mechanistic insight is lacking. We have witnessed advances in understanding the central circuitry in the brain that facilitates urination 1 – 3 . Beyond central control, micturition reflexes that govern urination are all initiated by peripheral mechanical stimuli such as bladder stretch and urethral flow 4 . Surprisingly, the mechanotransduction molecules and the cell types that function as the primary stretch and pressure detectors in the urinary tract are mostly unknown. We find that the mechanosensitive ion channel PIEZO2 is expressed in lower urinary tract tissues, where it is required for low-threshold bladder stretch sensing and urethral micturition reflexes. We show that PIEZO2 acts as a sensor in both the bladder urothelium and innervating sensory neurons. Importantly, both humans and mice lacking functional PIEZO2 have impaired bladder control, and humans report deficient bladder-filling sensation. This study pinpoints PIEZO2 as a key mechanosensor in urinary function. These findings enable future work that will unlock how urothelial cells and sensory neurons interact to control urination.