Fast conducting myelinated high threshold mechanoreceptors (AHTMR) are largely thought to transmit acute nociception from the periphery. However, their roles in normal withdrawal and in nerve injury induced hyperalgesia are less well accepted. Modulation of this subpopulation of peripheral neurons would help define their roles in withdrawal behaviors. The optically active proton pump, ArchT, was placed in an AAV8 viral vector with the CAG promoter and was administered by intrathecal injection resulting in expression in myelinated neurons. Optical inhibition of peripheral neurons at the soma and transcutaneously was possible in the neurons expressing ArchT, but not in neurons from control animals. Receptive field characteristics and electrophysiology determined that inhibition was neuronal subtype specific with only AHTMR neurons being inhibited. One week following nerve injury the AHTMR are hyperexcitable, but can still be inhibited at the soma and transcutaneously. Withdrawal thresholds to mechanical stimuli in normal and in hyperalgesic nerve injured animals were also increased by transcutaneous light to the affected hindpaw. This suggests that AHTMR neurons play a role not only in threshold related withdrawal behavior in the normal animal, but also in sensitized states after nerve injury. This is the first time this subpopulation of neurons has been reversibly modulated to test their contribution to withdrawal related behaviors before and after nerve injury. This technique may prove useful to define the role of selective neuronal populations in different pain states.

Boada MD, Woodbury CJ. Physiological properties of mouse skin sensory neurons recorded intracellularly in vivo: temperature effects on somal membrane properties. J Neurophysiol 98: 668 -680, 2007. First published May 30, 2007 doi:10.1152/jn.00264.2007. Recent combined analyses of the structural, functional, and molecular attributes of individual skin sensory neurons using semi-intact in vitro preparations from mice have provided a wealth of novel insights into nociceptor biology. How these findings translate to more natural conditions nevertheless remains unresolved. Toward this end, intracellular recordings were obtained from 362 physiologically identified dorsal root ganglion (DRG) neurons in a new in vivo mouse preparation developed for combined structure/function analyses of individual skin sensory neurons. Recordings were conducted at thoracic levels in adult decorticate mice for comparison with in vitro findings from the same trunk region. In all, 270 neurons were recorded at DRG temperatures tightly regulated at normal core values to establish a baseline and 137 skin sensory neurons were included in detailed analyses of somal properties for comparisons with similar data obtained under reduced temperatures mirroring in vitro conditions. Recovery of Neurobiotin-labeled central projections was crucial for verifying perceived afferent identity of certain neurons. Further, profound temperature dependency was seen across diverse physiological properties. Indeed, the broad, inflected somal spikes normally viewed as diagnostic of myelinated nociceptors were found to be a product of reduced temperatures and were not present at normal core values. Moreover, greater complexity was observed peripherally in the mechanical and thermal sensitivity profile of nociceptive and nonnociceptive populations than that seen under in vitro conditions. This novel in vivo preparation therefore holds considerable promise for future analyses of nociceptor function and plasticity in normal and transgenic models of pain mechanisms.

Boada MD, Gutierrez S, Aschenbrenner CA, Houle TT, Hayashida K, Ririe DG, Eisenach JC. Nerve injury induces a new profile of tactile and mechanical nociceptor input from undamaged peripheral afferents. J Neurophysiol 113: 100 -109, 2015. First published October 1, 2014 doi:10.1152/jn.00506.2014.-Chronic pain after nerve injury is often accompanied by hypersensitivity to mechanical stimuli, yet whether this reflects altered input, altered processing, or both remains unclear. Spinal nerve ligation or transection results in hypersensitivity to mechanical stimuli in skin innervated by adjacent dorsal root ganglia, but no previous study has quantified the changes in receptive field properties of these neurons in vivo. To address this, we recorded intracellularly from L 4 dorsal root ganglion neurons of anesthetized young adult rats, 1 wk after L 5 partial spinal nerve ligation (pSNL) or sham surgery. One week after pSNL, hindpaw mechanical withdrawal threshold in awake, freely behaving animals was decreased in the L 4 distribution on the nerve-injured side compared with sham controls. Electrophysiology revealed that highthreshold mechanoreceptive cells of A-fiber conduction velocity in L 4 were sensitized, with a seven-fold reduction in mechanical threshold, a seven-fold increase in receptive field area, and doubling of maximum instantaneous frequency in response to peripheral stimuli, accompanied by reductions in after-hyperpolarization amplitude and duration. Only a reduction in mechanical threshold (minimum von Frey hair producing neuronal activity) was observed in C-fiber conduction velocity high-threshold mechanoreceptive cells. In contrast, low-threshold mechanoreceptive cells were desensitized, with a 13-fold increase in mechanical threshold, a 60% reduction in receptive field area, and a 40% reduction in instantaneous frequency to stimulation. No spontaneous activity was observed in L 4 ganglia, and the likelihood of recording from neurons without a mechanical receptive field was increased after pSNL. These data suggest massively altered input from undamaged sensory afferents innervating areas of hypersensitivity after nerve injury, with reduced tactile and increased nociceptive afferent response. These findings differ importantly from previous preclinical studies, but are consistent with clinical findings in most patients with chronic neuropathic pain. chronic pain model; in vivo electrophysiology; sensory neurons; spinal nerve ligation IN HUMANS, NEUROPATHIC PAIN includes both spontaneous and evoked pain, often accompanied by hypersensitivity to normal nociceptive stimuli (hyperalgesia) and normally nonnociceptive stimuli (allodynia). This pathological condition is also sometimes accompanied by dysesthesias and paresthesias (von Hehn et al. 2012), suggesting that both tactile and nociceptive information channels and/or processing are disrupted.

Peripheral neuropathy presents as a combination of positive (e.g., pain) and negative symptoms (e.g., sensory loss), which vary across disease conditions [23; 76]. As pathophysiological mechanisms of pain cannot be examined in patients, an examination of pain phenotypes helps provide clues to the underlying mechanisms [41; 76]. A subset of oral cancer patients exhibits symptoms suggestive of peripheral neuropathy such as intractable pain, function impairment, numbness, and formication [5; 44]. As diagnosis of nerve injury requires anatomical examination [34] that is challenging in patients, we seek to gain mechanistic insight by examining pain phenotypes in patients.Neuropathic pain caused by peripheral disorders is predominantly driven by primary afferent neurons including both myelinated A-and unmyelinated C-fibers [20; 23]. Altered electrophysiological properties, along with neurochemical and anatomical changes in sensory neurons, contribute to different pain phenotypes following nerve injury [20; 23; 41]. The impact of PNI on electrophysiological and neuroanatomical properties of primary afferent neurons remains unknown.We hypothesize that peripheral nerve injury and hypersensitive primary afferent neurons contribute to PNI-induced pain in oral cancer. We compared pain in oral cancer patients with and without PNI evaluated with a validated oral cancer pain questionnaire [47] that quantifies two prominent features of neuropathic pain -spontaneous pain (i.e., pain felt without obvious external stimuli) and mechanical allodynia (i.e., pain resulting from a nonpainful stimulus) [41; 50; 58]. We produced a model of PNI-induced pain and probed the peripheral mechanisms using in vivo intracellular recordings and electron microscopic analysis of nerve ultrastructure. Methods PatientsA pre-existing database composed of a cohort of 69 oral cancer patients referred to Bluestone Center for Clinical Research and Oral Cancer Center at New York University College of Dentistry from 2010-2017 was used for data analysis. All procedures involving human participants were performed in accordance with the ethical standards of the institutional and/or national research committee (NYU IRB #10-01261) and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards. All patients provided written informed consent prior to inclusion in the study. The database contains information for each patient including age, sex, racial/ethnic identity (white, Hispanic, Africa-American, Asian), drinking/smoking history (previous, never, current), and pathological information including the presence (yes or no) of PNI, lymphovascular invasion, extracapsular spread, nodal status, tumor staging (T1-4), and tumor thickness/depth of invasion. Nodal status was classified as node positive (N+) or negative (N0) by pathological examination of the neck dissection specimen following surgery. Anatomical location of the oral cancer (Supplemental Figure 1) includes tongue (55.1%), mandibular gingiva (23.2%), maxillary gingiva (7.25%),...

Sensory afferents in skin encode and convey thermal and mechanical conditions, including those that threaten tissue damage. A small proportion of skin, the glabrous skin of the distal extremities, is specialized to explore the environment in fine detail. Aside from increased innervation density, little is known regarding properties of mechanosensory afferents to glabrous skin in younger animals that explain the exquisite precision and high contrast in rapidly sampling physical structures, including those that threaten injury. To assess this, we obtained intact neuronal intracellular recordings in vivo from 115 mechanosensitive afferent neurons from lumbar and thoracic dorsal root ganglia in juvenile rats. Two characteristics were unique to glabrous skin: a threefold higher proportion of fast-conducting to slow-conducting afferents that were high-threshold mechanosensitive nociceptors compared with hairy skin and a twofold faster conduction velocity of fast-conducting nociceptors compared with hairy skin. Additionally differences were found in mechanical thresholds between glabrous skin and hairy skin for each fiber type. These differences reflect and help explain the rapid response of skin specialized to explore the physical environment. Additionally, these results highlight potential limitations of using passive electrical properties and conduction velocity alone to characterize primary afferents without knowledge of the skin type they innervated.