Mechanical and cold polymodality coexist in tactile peripheral afferents, and it's not mediated by TRPM8.

In the mammalian somatosensory system, polymodality is defined as the competence of some neurons to respond to multiple forms of energy (e.g., mechanical and thermal). This ability is thought to be an exclusive property of nociceptive neurons (polymodal C-fiber nociceptors) and one of the pillars of nociceptive peripheral plasticity. The current study uncovered a completely different neuronal sub-population with polymodal capabilities on the opposite mechanical modality spectrum (tactile). We have observed that several tactile afferents (1/5) can respond to cold in non-nociceptive ranges. These cells' mechanical thresholds and electrical properties are similar to any low-threshold mechano-receptors (LT), conducting in a broad range of velocities (Aδ to Aß), lacking CGRP and TRPM8 receptors. Due to its density, cold-response range, speed, and response to injury (or lack thereof), we speculate on its role in controlling reflexive behaviors (wound liking and rubbing) and modulation of nociceptive spinal cord integration. Further studies are required to understand the mechanisms behind this neuron's polymodality, central architecture, and impact on pain perception.

Effects of systemic oxytocin administration on ultraviolet B-induced nociceptive hypersensitivity and tactile hyposensitivity in mice.

Ultraviolet B (UVB) radiation induces cutaneous inflammation, leading to thermal and mechanical hypersensitivity. Here, we examine the mechanical properties and profile of tactile and nociceptive peripheral afferents functionally disrupted by this injury and the role of oxytocin (OXT) as a modulator of this disruption. We recorded intracellularly from L4 afferents innervating the irradiated area (5.1 J/cm2) in 4-6 old week male mice (C57BL/6J) after administering OXT intraperitoneally, 6 mg/Kg. The distribution of recorded neurons was shifted by UVB radiation to a pattern observed after acute and chronic injuries and reduced mechanical thresholds of A and C- high threshold mechanoreceptors while reducing tactile sensitivity. UVB radiation did not change somatic membrane electrical properties or fiber conduction velocity. OXT systemic administration rapidly reversed these peripheral changes toward normal in both low and high-threshold mechanoreceptors and shifted recorded neuron distribution toward normal. OXT and V1aR receptors were present on the terminals of myelinated and unmyelinated afferents innervating the skin. We conclude that UVB radiation, similar to local tissue surgical injury, cancer metastasis, and peripheral nerve injury, alters the distribution of low and high threshold mechanoreceptors afferents and sensitizes nociceptors while desensitizing tactile units. Acute systemic OXT administration partially returns all of those effects to normal.

Seeding of breast cancer cell line (MDA-MB-231LUC+) to the mandible induces overexpression of substance P and CGRP throughout the trigeminal ganglion and widespread peripheral sensory neuropathy throughout all three of its divisions.

Some types of cancer are commonly associated with intense pain even at the early stages of the disease. The mandible is particularly vulnerable to metastasis from breast cancer, and this process has been studied using a bioluminescent human breast cancer cell line (MDA-MB-231LUC+). Using this cell line and anatomic and neurophysiologic methods in the trigeminal ganglion (TG), we examined the impact of cancer seeding in the mandible on behavioral evidence of hypersensitivity and on trigeminal sensory neurons. Growth of cancer cells seeded to the mandible after arterial injection of the breast cancer cell line in Foxn1 animals (allogeneic model) induced behavioral hypersensitivity to mechanical stimulation of the whisker pad and desensitization of tactile and sensitization of nociceptive mechanically sensitive afferents. These changes were not restricted to the site of metastasis but extended to sensory afferents in all three divisions of the TG, accompanied by widespread overexpression of substance P and CGRP in neurons through the ganglion. Subcutaneous injection of supernatant from the MDA-MB-231LUC+ cell culture in normal animals mimicked some of the changes in mechanically responsive afferents observed with mandibular metastasis. We conclude that released products from these cancer cells in the mandible are critical for the development of cancer-induced pain and that the overall response of the system greatly surpasses these local effects, consistent with the widespread distribution of pain in patients. The mechanisms of neuronal plasticity likely occur in the TG itself and are not restricted to afferents exposed to the metastatic cancer microenvironment.

Peripheral nerve injury and sensitization underlie pain associated with oral cancer perineural invasion.

Cancer invading into nerves, termed perineural invasion (PNI), is associated with pain. Here, we show that oral cancer patients with PNI report greater spontaneous pain and mechanical allodynia compared with patients without PNI, suggesting that unique mechanisms drive PNI-induced pain. We studied the impact of PNI on peripheral nerve physiology and anatomy using a murine sciatic nerve PNI model. Mice with PNI exhibited spontaneous nociception and mechanical allodynia. Perineural invasion induced afterdischarge in A high-threshold mechanoreceptors (HTMRs), mechanical sensitization (ie, decreased mechanical thresholds) in both A and C HTMRs, and mechanical desensitization in low-threshold mechanoreceptors. Perineural invasion resulted in nerve damage, including axon loss, myelin damage, and axon degeneration. Electrophysiological evidence of nerve injury included decreased conduction velocity, and increased percentage of both mechanically insensitive and electrically unexcitable neurons. We conclude that PNI-induced pain is driven by nerve injury and peripheral sensitization in HTMRs.

Recovery from nerve injury induced behavioral hypersensitivity in rats parallels resolution of abnormal primary sensory afferent signaling.

Pain and hypersensitivity months after peripheral injury reflect abnormal input from peripheral afferents likely in conjunction with central sensitization. We hypothesize that peripheral changes occur in defined sensory afferents and resolve as behavioral response to injury resolves. Male Sprague-Dawley rats underwent sham or partial L5 spinal nerve ligation, and paw withdrawal threshold (PWT) was sequentially measured during recovery. At 2, 4, 8, and 12 weeks after injury, randomized animals underwent electrophysiologic assessment of L4 fast-conducting high- and low-threshold mechanoreceptors, and individual neuronal mechanical thresholds (MTs) were contrasted with PWTs in the same animals. Paw withdrawal thresholds decreased after injury and resolved over time (P < 0.001). Similarly, MTs of fast-conducting high-threshold mechanoreceptors decreased after injury and resolved over time (P < 0.001). By contrast, MTs of low-threshold mechanoreceptors increased after injury and resolved over time (P < 0.001). Distributions of recordings from each afferent subtype were perturbed after injury, and this too resolved over time. After resolution of behavioral changes, several electrical abnormalities persisted in both neuronal subtypes. These data extend previous findings that mechanically sensitive nociceptors are sensitized, whereas tactile, largely Aß afferents are desensitized after nerve injury by showing that the time course of resolution of these changes mirrors that of behavioral hypersensitivity in a surgical injury including neural damage. These data support a role of abnormal peripheral input, from both nociceptor and tactile afferents, during recovery from peripheral injury and underscore the potential importance of both classes of afferents as potential targets for pain treatment.

Nociceptive input after peripheral nerve injury results in cognitive impairment and alterations in primary afferent physiology in rats.

Pain alters cognitive performance through centrally mediated effects in the brain. In this study, we hypothesized that persistent activation of peripheral nociceptors after injury would lead to the development of a chronic pain state that impairs attention-related behavior and results in changes in peripheral neuron phenotypes. Attentional performance was measured in rats using the 5-choice serial reaction time titration variant to determine the initial impact of partial L5 spinal nerve ligation and the effect of persistent nociceptor activation on the resolution of injury. The changes in peripheral neuronal sensibilities and phenotypes were determined in sensory afferents using electrophysiologic signatures and receptive field properties from dorsal root ganglion recordings. Partial spinal nerve injury impaired attentional performance, and this was further impaired in a graded fashion by nociceptive input through an engineered surface. Impairment in attention persisted for only up to 4 days initially, followed by a second phase 7 to 10 weeks after injury in animals exposed to nociceptive input. In animals with prolonged impairment in behavior, the mechanonociceptors displayed a persistent hypersensitivity marked by decreased threshold, increased activity to a given stimulus, and spontaneous activity. Nerve injury disrupts attentional performance acutely and is worsened with peripheral mechanonociceptor activation. Acute impairment resolves, but persistent nociceptive activation produces re-emergence of impairment in the attention-related task associated with electrophysiological abnormalities in peripheral nociceptors. This is consistent with the development of a chronic pain state marked by cognitive impairment and related to persistently abnormal peripheral input.

Tachykinins modulate nociceptive responsiveness and sensitization: In vivo electrical characterization of primary sensory neurons in tachykinin knockout (Tac1 KO) mice.

Since the failure of specific substance P antagonists to induce analgesia, the role of tachykinins in the development of neuropathic pain states has been discounted. This conclusion was reached without studies on the role of tachykinins in normal patterns of primary afferents response and sensitization or the consequences of their absence on the modulation of primary mechanonociceptive afferents after injury. Nociceptive afferents from animals lacking tachykinins (Tac1 knockout) showed a disrupted pattern of activation to tonic suprathreshold mechanical stimulation. These nociceptors failed to encode the duration and magnitude of natural pronociceptive stimuli or to develop mechanical sensitization as consequence of this stimulation. Moreover, paw edema, hypersensitivity, and weight bearing were also reduced in Tac1 knockout mice 24 h after paw incision surgery. At this time, nociceptive afferents from these animals did not show the normal sensitization to mechanical stimulation or altered membrane electrical hyperexcitability as observed in wild-type animals. These changes occurred despite a similar increase in calcitonin gene-related peptide immunoreactivity in sensory neurons in Tac1 knockout and normal mice. Based on these observations, we conclude that tachykinins are critical modulators of primary nociceptive afferents, with a preeminent role in the electrical control of their excitability with sustained activation or injury.

Peripheral oxytocin restores light touch and nociceptor sensory afferents towards normal after nerve injury.

Oxytocin reduces primary sensory afferent excitability and produces analgesia in part through a peripheral mechanism, yet its actions on physiologically characterized, mechanically sensitive afferents in normal and neuropathic conditions are unknown. We recorded intracellularly from L4 dorsal root ganglion neurons characterized as low-threshold mechanoreceptors (LTMRs) or high-threshold mechanoreceptors (HTMRs) in female rats 1 week after L5 partial spinal nerve injury or sham control (n = 24 rats/group) before, during, and after ganglionic perfusion with oxytocin, 1 nM. Nerve injury desensitized and hyperpolarized LTMRs (membrane potential [Em] was -63 ± 1.8 mV in sham vs -76 ± 1.4 mV in nerve injury; P < 0.001), and sensitized HTMRs without affecting Em. In nerve-injured rats, oxytocin depolarized LTMRs towards normal (Em = -69 ± 1.9 mV) and, in 6 of 21 neurons, resulted in spontaneous action potentials. By contrast, oxytocin hyperpolarized HTMRs (Em = -68 ± 2.7 mV before vs -80 ± 3.2 mV during oxytocin exposure; P < 0.01). These effects were reversed after removal of oxytocin, and oxytocin had minimal effects in neurons from sham surgery animals. Sensory afferent neurons immunopositive for the vasopressin 1a receptor were larger (34 ± 6.3 µm, range 16-57 µm) than immunonegative neurons (26 ± 3.4 µm, range 15-43 µm; P < 0.005). These data replicate findings that neuropathic injury desensitizes LTMRs while sensitizing HTMRs and show rapid and divergent oxytocin effects on these afferent subtypes towards normal, potentially rebalancing input to the central nervous system. Vasopressin 1a receptors are present on medium to large diameter afferent neurons and could represent oxytocin's target.

Incisional Nociceptive Input Impairs Attention-related Behavior and Is Associated with Reduced Neuronal Activity in the Prefrontal Cortex in Rats.

WHAT WE ALREADY KNOW ABOUT THIS TOPIC: WHAT THIS ARTICLE TELLS US THAT IS NEW: BACKGROUND:: Cognitive capacity may be reduced from inflammation, surgery, anesthesia, and pain. In this study, we hypothesized that incision-induced nociceptive input impairs attentional performance and alters neuronal activity in the prefrontal cortex. METHODS: Attentional performance was measured in rats by using the titration variant of the 5-choice serial reaction time to determine the effect of surgical incision and anesthesia in a visual attention task. Neuronal activity (single spike and local field potentials) was measured in the medial prefrontal cortex in animals during the task. RESULTS: Incision significantly impaired attention postoperatively (area under curve of median cue duration-time 97.2 ± 56.8 [n = 9] vs. anesthesia control 25.5 ± 14.5 s-days [n = 9], P = 0.002; effect size, η = 0.456). Morphine (1 mg/kg) reduced impairment after incision (area under curve of median cue duration-time 31.6 ± 36.7 [n = 11] vs. saline 110 ± 64.7 s-days [n = 10], P < 0.001; η = 0.378). Incision also decreased cell activity (n = 24; 1.48 ± 0.58 vs. control, 2.93 ± 2.02 bursts/min; P = 0.002; η = 0.098) and local field potentials (n = 28; η = 0.111) in the medial prefrontal cortex. CONCLUSIONS: These results show that acute postoperative nociceptive input from incision reduces attention-related task performance and decreases neuronal activity in the medial prefrontal cortex. Decreased neuronal activity suggests nociceptive input is more than just a distraction because neuronal activity increases during audiovisual distraction with similar behavioral impairment. This suggests that nociceptive input and the medial prefrontal cortex may contribute to attentional impairment and mild cognitive dysfunction postoperatively. In this regard, pain may affect postoperative recovery and return to normal activities through attentional impairment by contributing to lapses in concentration for routine and complex tasks.

Neuropeptide-induced modulation of carcinogenesis in a metastatic breast cancer cell line (MDA-MB-231LUC+).

BACKGROUND: Metastatic cancer to bone is well-known to produce extreme pain. It has been suggested that the magnitude of this perceived pain is associated with disease progression and poor prognosis. These data suggest a potential cross-talk between cancer cells and nociceptors that contribute not only to pain, but also to cancer aggressiveness although the underlying mechanisms are yet to be stablished. METHODS: The in vitro dose dependent effect of neuropeptides (NPs) (substance P [SP], calcitonin gene-related peptide and neurokinin A [NKA]) and/or its combination, on the migration and invasion of MDA-MB-231LUC+ were assessed by wound healing and collagen-based cell invasion assays, respectively. The effect of NPs on the expression of its receptors (SP [NK1] and neurokinin A receptors [NK2], CALCRL and RAMP1) and kininogen (high-molecular-weight kininogen) release to the cell culture supernatant of MDA-MB-231LUC+, were measured using western-blot analysis and an ELISA assay, respectively. Statistical significance was tested using one-way ANOVA, repeated measures ANOVA, or the paired t-test. Post-hoc testing was performed with correction for multiple comparisons as appropriate. RESULTS: Our data show that NPs strongly modify the chemokinetic capabilities of a cellular line commonly used as a model of metastatic cancer to bone (MDA-MB-231LUC+) and increased the expression of their receptors (NK1R, NK2R, RAMP1, and CALCRL) on these cells. Finally, we demonstrate that NPs also trigger the acute release of HMWK (Bradykinin precursor) by MDA-MB-231LUC+, a molecule with both tumorigenic and pro-nociceptive activity. CONCLUSIONS: Based on these observations we conclude that NPs exposure modulates this breast cancer cellular line aggressiveness by increasing its ability to migrate and invade new tissues. Furthermore, these results also support the pro nociceptive and cancer promoter role of the peripheral nervous system, during the initial stages of the disease.

Post-discharge hyperpolarization is an endogenous modulatory factor limiting input from fast-conducting nociceptors (AHTMRs).

Peripheral somatosensory neurons are frequently exposed to mechanical forces. Strong stimuli result in neuronal activation of high-threshold mechanosensory afferent neurons, even in the absence of tissue damage. Among these neurons, fast-conducting nociceptors (A-fiber high-threshold mechanoreceptors (AHTMRs)) are normally resistant to sustained activation, transiently encoding the mechanical stimulus intensity but not its full duration. This rapidly adapting response seems to depend on changes in the electrical excitability of the membrane of these afferent neurons during sustained stimulation, a restraint mechanism that disappears following sensitization. Here, we examine the mechanism by which strong peripheral activation of mechanoreceptors elicits this control process in the absence of tissue injury and temporally silences afferent neurons despite ongoing stimulation. To study this, mechanoreceptors in Sprague-Dawley rats were accessed at the soma in the dorsal root ganglia from T11 and L4/L5. Neuronal classification was performed using receptive field characteristics and passive and active electrical properties. Sustained mechanical nociceptive stimulation in the absence of tissue damage of AHTMRs induces a rapid membrane hyperpolarization and a period of reduced responsiveness to the stimuli. Moreover, this phenomenon appears to be unique to this subset of afferent neurons and is absent in slow-conducting C-mechanonociceptors (C-fiber high-threshold mechanoreceptors) and rapidly adapting fast-conducting low-threshold mechanoreceptors. Furthermore, this mechanism for rapid adaptation and reducing ongoing input is ablated by repeated strong stimuli and in sensitized AHTMRs after chronic neuropathic injury. Further studies to understand the underling molecular mechanisms behind this phenomenon and their modulation during the development of pathological conditions may provide new targets to control nociceptive hyperexcitability and chronic pain.

Audiovisual Distraction Increases Prefrontal Cortical Neuronal Activity and Impairs Attentional Performance in the Rat.

Involvement of attentional processes is generally evidenced by disruption of behavior in the presence of distracting stimuli. The medial prefrontal cortex (mPFC) seems to play a role in fine-tuning activity during attentional tasks. A novel titration variant of the 5-choice serial reaction time task (5-choice serial reaction time titration variant [5CTV]) that adjusts task difficulty based on subject performance was used to evaluate the effects of audiovisual distraction (DSTR) on performance and mPFC single spike activity and local field potential (LFP). Attention was impaired in the 5CTV from DSTR, and mPFC spike activity was increased, whereas LFP was reduced. The increased spike activity in the mPFC in conjunction with DSTR suggests that conflicting attentional demands may contribute to the reduced task performance. As both hypo- and hyperactivation of the mPFC may contribute to attentional disruption, further studies using the 5CTV are needed to understand mPFC activity changes in real time during disruption of performance by other types of behavioral or neurobiological manipulations.

Nerve injury induced activation of fast-conducting high threshold mechanoreceptors predicts non-reflexive pain related behavior.

The role of specific subsets of peripheral nerves in pain related behavior remains unclear. To better understand the contribution of differential activation of fast-conducting, high-threshold mechanoreceptor (AHTMR) input, we hypothesized that neuronal activation would be distinct with nerve injury, and that nociceptive input would predictt behavior in the freely exploring animal. A series of surfaces was used to deliver mechanical input to the hindpaws of rats upon voluntary movement and exploration. Neuronal activation increased as apex surface decreased (0.2, 0.6, 1.0 and 1.5mm) using in vivo recording in L4 DRG neurons, and this relationship was enhanced following partial ligation of L5 (pSNL). In behaving animals, apex size was correlated to time spent on each surface following pSNL, but not with sham. Morphine normalized the discriminatory behavior following pSNL. These data indicate that noxious mechanical activation of AHTMR upon normal movement predicts behavior using paradigms that do not rely on reflexive withdrawal responses suggesting that AHTMR activation and central nervous system input contribute to higher order pain behavior after nerve injury beyond the immediate early pain input long attributed to these neurons.

Mechanical sensibility of nociceptive and non-nociceptive fast-conducting afferents is modulated by skin temperature.

The ability to distinguish mechanical from thermal input is a critical component of peripheral somatosensory function. Polymodal C fibers respond to both stimuli. However, mechanosensitive, modality-specific fast-conducting tactile and nociceptor afferents theoretically carry information only about mechanical forces independent of the thermal environment. We hypothesize that the thermal environment can nonetheless modulate mechanical force sensibility in fibers that do not respond directly to change in temperature. To study this, fast-conducting mechanosensitive peripheral sensory fibers in male Sprague-Dawley rats were accessed at the soma in the dorsal root ganglia from T11 or L4/L5. Neuronal identification was performed using receptive field characteristics and passive and active electrical properties. Neurons responded to mechanical stimuli but failed to generate action potentials in response to changes in temperature alone, except for the tactile mechanical and cold sensitive neurons. Heat and cold ramps were utilized to determine temperature-induced modulation of response to mechanical stimuli. Mechanically evoked electrical activity in non-nociceptive, low-threshold mechanoreceptors (tactile afferents) decreased in response to changes in temperature while mechanically induced activity was increased in nociceptive, fast-conducting, high-threshold mechanoreceptors in response to the same changes in temperature. These data suggest that mechanical activation does not occur in isolation but rather that temperature changes appear to alter mechanical afferent activity and input to the central nervous system in a dynamic fashion. Further studies to understand the psychophysiological implications of thermal modulation of fast-conducting mechanical input to the spinal cord will provide greater insight into the implications of these findings.

Nerve injury induces a new profile of tactile and mechanical nociceptor input from undamaged peripheral afferents.

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 L4 dorsal root ganglion neurons of anesthetized young adult rats, 1 wk after L5 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 L4 distribution on the nerve-injured side compared with sham controls. Electrophysiology revealed that high-threshold mechanoreceptive cells of A-fiber conduction velocity in L4 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 L4 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.

Relationship between electrophysiological signature and defined sensory modality of trigeminal ganglion neurons in vivo.

The trigeminal ganglia (TG) innervate a heterogeneous set of highly sensitive and exposed tissues. Weak, innocuous stimuli can evoke pain as a normal response in some areas such as the cornea. This observation implies, however, the capability of low-threshold mechanoreceptors, inducing pain in the normal condition. To clarify this matter, the present study correlates the electrical signature (both fiber conduction velocity and somatic electrical properties) with receptor field, mechanical threshold, and temperature responsiveness of sensory afferents innervating tissues with dissimilar sensitivity (skin vs. cornea) in the trigeminal domain. Intracellular recordings were obtained in vivo from 148 neurons of the left TG of 62 mice. In 111 of these neurons, the peripheral receptor field was successfully localized: 96 of them innervated the hairy skin, while the remaining 15 innervated the cornea. The electrical signature was defined and peripheral responses correlated with tissue target. No high threshold neurons were found in the cornea. Moreover, the electrical signature of corneal afferents resembles nociceptive neurons in the skin. TG skin afferents showed similar membrane electrical signature and sensory modality as skin afferents from dorsal root ganglion, although TG afferents exhibited a shorter duration of afterhyperpolarization then those previously described in dorsal root ganglion. These data suggest than new or different ways to classify and study TG sensory neurons may be required.

Skin incision-induced receptive field responses of mechanosensitive peripheral neurons are developmentally regulated in the rat.

Maturation of the nervous system results in changes in both central and peripheral processing. To better understand responses to injury in the young, developmental differences in the acute response to incision were investigated in both tactile and nociceptive myelinated peripheral mechanosensitive afferent neurons in vivo. Neuronal intrasomal recordings were performed in juvenile and infant rats in 34 L5 dorsal root ganglia, and each neuron was phenotypically defined. Neurons had a mechanosensitive receptive field in the glabrous skin on the plantar surface of the hind paw, which was characterized at baseline and for up to 45 min after incision. Fundamental maturational differences in the effect of incision were clear: in high-threshold nociceptive mechanoreceptors, the mechanical threshold decreased immediately and the receptive field size increased rapidly in juvenile rats but not in infant rats. Additionally, a divergence in changes in the instantaneous response frequency of tactile afferents occurred between the two ages. These differences may help explain maturational differences in responses to peripheral injury and suggest that differences in central nervous system responses may be partially mitigated by spatially confined and frequency-dependent differences resulting from tactile and nociceptive mechanosensitive input.

Developmental differences in peripheral glabrous skin mechanosensory nerve receptive field and intracellular electrophysiologic properties: phenotypic characterization in infant and juvenile rats.

Developmental differences in peripheral neuron characteristics and functionality exist. Direct measurement of active and passive electrophysiologic and receptive field characteristics of single mechanosensitive neurons in glabrous skin was performed and phenotypic characterization of fiber subtypes was applied to analyze developmental differences in peripheral mechanosensitive afferents. After Institutional approval, male Sprague-Dawley infant (P7: postnatal day 7) and juvenile (P28) rats were anesthetized and single cell intracellular electrophysiology was performed in the dorsal root ganglion (DRG) soma of mechanosensitive cells with receptive field (RF) in the glabrous skin of the hindpaw. Passive and active electrical properties of the cells and RF size and characteristics determined. Fiber subtype classification was performed and developmental differences in fiber subtype properties analyzed. RF size was smaller at P7 for both low and high threshold mechanoreceptor (LTMR and HTMR) with no differences between A- and C-HTMR (AHTMR and CHTMR). The RF size was also correlated to anatomic location on glabrous skin, toes having smaller RF. Conduction velocity (CV) was adequate at P28 for AHTMR and CHTMR classification, but not at P7. Only width of the action potential at half height (D50) was significantly different between HTMR at P7, while D50, CV and amplitude of the AP were significant for HTMR at P28. RF size is determined in part by the RF distribution of the peripheral neuron. Developmental differences in RF size occur with larger RF sizes occurring in younger animals. This is consistent with RF size differences determined by measuring RF in the spinal cord, except the peripheral RF is much smaller, more refined, and in some cases pinpoint. Developmental differences make CV alone unreliable for neuron classification. Utilizing integration of all measured parameters allows classification of neurons into subtypes even at the younger ages. This will prove important in understanding changes that occur in the peripheral sensory afferents in the face of ongoing development and injury early in life.

Differing neurophysiologic mechanosensory input from glabrous and hairy skin in juvenile rats.

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.

Myelinated skin sensory neurons project extensively throughout adult mouse substantia gelatinosa.

The substantia gelatinosa (SG) of the dorsal horn of the spinal cord is a recipient zone for unmyelinated sensory neurons in adults. Recent studies of the central anatomy of physiologically identified skin sensory neurons in neonatal mice have shown that this region also receives substantial inputs from a variety of myelinated afferents. The present experiments were performed to determine whether these neonatal inputs represent a transient phenotype that retracts from the SG. Studies were conducted in an in vivo spinal cord preparation from adult mice; thoracic levels were targeted to facilitate comparisons with previous in vitro findings. We show that the SG continues to receive substantial projections from myelinated skin sensory neurons throughout life. A large population of myelinated nociceptors conducting in the upper A delta and low A beta range maintained extensive projections throughout all areas of the SG well into adulthood; the latter gave rise to dorsally recurving "flame"-shaped arbors extending into the marginal layer that were identical to afferents described in neonates and after nerve injury in adult rats. Furthermore, exquisitely sensitive down hair follicle afferents projected throughout the inner half of the SG (i.e., lamina IIi) and sent dense clusters of terminals well into the outer SG (IIo), where they intermingled with those of unmyelinated nociceptors. Arguments are presented that the SG likely plays a predominant role in tactile processing under normal conditions, but that this role switches rapidly to nociceptive-only during environmental exigencies imposed by temperature extremes.