Structure, vascularization, and innervation of the mystacial pad of the rat as revealed by the lectin Griffonia simplicifolia

Neurocircuitry underlying the antidepressant effect of retrograde facial botulinum toxin in mice

BACKGROUNDS

Botulinum toxin type A (BoNT/A) is extensively applied in spasticity and dystonia as it cleaves synaptosome-associated protein 25 (SNAP25) in the presynaptic terminals, thereby inhibiting neurotransmission. An increasing number of randomized clinical trials have suggested that glabellar BoNT/A injection improves depressive symptoms in patients with major depressive disorder (MDD). However, the underlying neuronal circuitry of BoNT/A-regulated depression remains largely uncharacterized.

RESULTS

Here, we modeled MDD using mice subjected to chronic restraint stress (CRS). By pre-injecting BoNT/A into the unilateral whisker intrinsic musculature (WIM), and performing behavioral testing, we showed that pre-injection of BoNT/A attenuated despair- and anhedonia-like phenotypes in CRS mice. By applying immunostaining of BoNT/A-cleaved SNAP25 (cl.SNAP25₁₉₇), subcellular spatial localization of SNAP25 with markers of cholinergic neurons (ChAT) and post-synaptic membrane (PSD95), and injection of monosynaptic retrograde tracer CTB-488-mixed BoNT/A to label the primary nucleus of the WIM, we demonstrated that BoNT/A axonal retrograde transported to the soma of whisker-innervating facial motoneurons (wFMNs) and subsequent transcytosis to synaptic terminals of second-order neurons induced central effects. Furthermore, using transsynaptic retrograde and monosynaptic antegrade viral neural circuit tracing with c-Fos brain mapping and co-staining of neural markers, we observed that the CRS-induced expression of c-Fos and CaMKII double-positive neurons in the ventrolateral periaqueductal grey (vlPAG), which sent afferents to wFMNs, was down-regulated 3 weeks after BoNT/A facial pre-administration. Strikingly, the repeated and targeted silencing of the wFMNs-projecting CaMKII-positive neurons in vlPAG with a chemogenetic approach via stereotactic injection of recombinant adeno-associated virus into specific brain regions of CRS mice mimicked the antidepressant-like action of BoNT/A pre-treatment. Conversely, repeated chemogenetic activation of this potential subpopulation counteracted the BoNT/A-improved significant antidepressant behavior.

CONCLUSION

We reported for the first time that BoNT/A inhibited the wFMNs-projecting vlPAG excitatory neurons through axonal retrograde transport and cell-to-cell transcytosis from the injected location of the WIM to regulate depressive-like phenotypes of CRS mice. For the limited and the reversibility of side effects, BoNT/A has substantial advantages and potential application in MDD.

Diversity of vibrissal follicle anatomy in cetaceans

Most cetaceans are born with vibrissae but they can be lost or reduced in adulthood, especially in odontocetes. Despite this, some species of odontocetes have been found to have functioning vibrissal follicles (including the follicle itself and any remaining vibrissal hair shaft) that play a role in mechanoreception, proprioception and electroreception. This reveals a greater diversity of vibrissal function in odontocetes than in any other mammalian group. However, we know very little about vibrissal follicle form and function across the Cetacea. Here, we qualitatively describe the gross vibrissal follicle anatomy of fetuses of three species of cetaceans, including two odontocetes: Atlantic white-sided dolphin (Lagenorhynchus acutus), harbour porpoise (Phocoena phocoena), and one mysticete: minke whale (Balaenoptera acutorostrata), and compared our findings to previous anatomical descriptions. All three species had few, short vibrissae contained within a relatively simple, single-part follicle, lacking in muscles. However, we observed differences in vibrissal number, follicle size and shape, and innervation distribution between the species. While all three species had nerve fibers around the follicles, the vibrissal follicles of Balaenoptera acutorostrata were innervated by a deep vibrissal nerve, and the nerve fibers of the odontocetes studied were looser and more branched. For example, in Lagenorhynchus acutus, branches of nerve fibers travelled parallel to the follicle, and innervated more superficial areas, rather than just the base. Our anatomical descriptions lend support to the observation that vibrissal morphology is diverse in cetaceans, and is worth further investigation to fully explore links between form and function.

The follicle-sinus complex of the bottlenose dolphin (Tursiops truncatus). Functional anatomy and possible evolutional significance of its somato-sensory innervation

Vibrissae are tactile hairs found mainly on the rostrum of most mammals. The follicle, which is surrounded by a large venous sinus, is called "follicle-sinus complex" (FSC). This complex is highly innervated by somatosensitive fibers and reached by visceromotor fibers that innervate the surrounding vessels. The surrounding striated muscles receive somatomotor fibers from the facial nerve. The bottlenose dolphin (Tursiops truncatus), a frequently described member of the delphinid family, possesses this organ only in the postnatal period. However, information on the function of the vibrissal complex in this latter species is scarce. Recently, psychophysical experiments on the river-living Guiana dolphin (Sotalia guianensis) revealed that the FSC could work as an electroreceptor in murky waters. In the present study, we analyzed the morphology and innervation of the FSC of newborn (n = 8) and adult (n = 3) bottlenose dolphins. We used Masson's trichrome stain and antibodies against neurofilament 200 kDa (NF 200), protein gene product (PGP 9.5), substance P (SP), calcitonin gene-related peptide, and tyrosine hydroxylase (TH) to characterize the FSC of the two age classes. Masson's trichrome staining revealed a structure almost identical to that of terrestrial mammals except for the fact that the FSC was occupied only by a venous sinus and that the vibrissal shaft lied within the follicle. Immunostaining for PGP 9.5 and NF 200 showed somatosensory fibers finishing high along the follicle with Merkel nerve endings and free nerve endings. We also found SP-positive fibers mostly in the surrounding blood vessels and TH both in the vessels and in the mesenchymal sheath. The FSC of the bottlenose dolphin, therefore, possesses a rich somatomotor innervation and a set of peptidergic visceromotor fibers. This anatomical disposition suggests a mechanoreceptor function in the newborns, possibly finalized to search for the opening of the mother's nipples. In the adult, however, this structure could change into a proprioceptive function in which the vibrissal shaft could provide information on the degree of rotation of the head. In the absence of psychophysical experiments in this species, the hypothesis of electroreception cannot be rejected.

Enhancing neuronal chloride extrusion rescues α2/α3 GABAA-mediated analgesia in neuropathic pain

Spinal disinhibition has been hypothesized to underlie pain hypersensitivity in neuropathic pain. Apparently contradictory mechanisms have been reported, raising questions on the best target to produce analgesia. Here, we show that nerve injury is associated with a reduction in the number of inhibitory synapses in the spinal dorsal horn. Paradoxically, this is accompanied by a BDNF-TrkB-mediated upregulation of synaptic GABAARs and by an α1-to-α2GABAAR subunit switch, providing a mechanistic rationale for the analgesic action of the α2,3GABAAR benzodiazepine-site ligand L838,417 after nerve injury. Yet, we demonstrate that impaired Cl- extrusion underlies the failure of L838,417 to induce analgesia at high doses due to a resulting collapse in Cl- gradient, dramatically limiting the benzodiazepine therapeutic window. In turn, enhancing KCC2 activity not only potentiated L838,417-induced analgesia, it rescued its analgesic potential at high doses, revealing a novel strategy for analgesia in pathological pain, by combined targeting of the appropriate GABAAR-subtypes and restoring Cl- homeostasis.

The Euler spiral of rat whiskers

This paper reports on an analytical study of the intrinsic shapes of 523 whiskers from 15 rats. We show that the variety of whiskers on a rat's cheek, each of which has different lengths and shapes, can be described by a simple mathematical equation such that each whisker is represented as an interval on the Euler spiral. When all the representative curves of mystacial vibrissae for a single rat are assembled together, they span an interval extending from one coiled domain of the Euler spiral to the other. We additionally find that each whisker makes nearly the same angle of 47^∘ with the normal to the spherical virtual surface formed by the tips of whiskers, which constitutes the rat's tactile sensory shroud or "search space." The implications of the linear curvature model for gaining insight into relationships between growth, form, and function are discussed.

Sensorimotor integration in the whisker somatosensory brain stem trigeminal loop

The rodent's vibrissal system is a useful model system for studying sensorimotor integration in perception. This integration determines the way in which sensory information is acquired by sensory organs and the motor commands that control them. The initial instance of sensorimotor integration in the whisker somatosensory system is implemented in the brain stem loop and may be essential to the way rodents explore and sense their environment. To examine the nature of these sensorimotor interactions, we recorded from lightly anesthetized rats in vivo and brain stem slices in vitro and isolated specific parts of this loop. We found that motor feedback to the vibrissal pad serves as a dynamic gain controller that controls the response of first-order sensory neurons by increasing and decreasing sensitivity to lower and higher tactile stimulus magnitudes, respectively. This delicate mechanism is mediated through tactile stimulus magnitude-dependent motor feedback. Conversely, tactile inputs affect the motor whisking output through influences on the rhythmic whisking circuitry, thus changing whisking kinetics. Similarly, tactile influences also modify the whisking amplitude through synaptic and intrinsic neuronal interaction in the facial nucleus, resulting in facilitation or suppression of whisking amplitude. These results point to the vast range of mechanisms underlying sensorimotor integration in the brain stem loop.NEW & NOTEWORTHY Sensorimotor integration is a process in which sensory and motor information is combined to control the flow of sensory information, as well as to adjust the motor system output. We found in the rodent's whisker somatosensory system mutual influences between tactile inputs and motor output, in which motor neurons control the flow of sensory information depending on their magnitude. Conversely, sensory information can control the magnitude and kinetics of whisker movement.

Representation of tactile scenes in the rodent barrel cortex

After half a century of research, the sensory features coded by neurons of the rodent barrel cortex remain poorly understood. Still, views of the sensory representation of whisker information are increasingly shifting from a labeled line representation of single-whisker deflections to a selectivity for specific elements of the complex statistics of the multi-whisker deflection patterns that take place during spontaneous rodent behavior - so called natural tactile scenes. Here we review the current knowledge regarding the coding of patterns of whisker stimuli by barrel cortex neurons, from responses to single-whisker deflections to the representation of complex tactile scenes. A number of multi-whisker tunings have already been identified, including center-surround feature extraction, angular tuning during edge-like multi-whisker deflections, and even tuning to specific statistical properties of the tactile scene such as the level of correlation across whiskers. However, a more general model of the representation of multi-whisker information in the barrel cortex is still missing. This is in part because of the lack of a human intuition regarding the perception emerging from a whisker system, but also because in contrast to other primary sensory cortices such as the visual cortex, the spatial feature selectivity of barrel cortex neurons rests on highly nonlinear interactions that remained hidden to classical receptive field approaches.

Anatomy and immunochemical characterization of the non-arterial peptidergic diffuse dural innervation of the rat and Rhesus monkey: Implications for functional regulation and treatment in migraine

Objective

The interplay between neuronal innervation and other cell types underlies the physiological functions of the dura mater and contributes to pathophysiological conditions such as migraine. We characterized the extensive, but understudied, non-arterial diffuse dural innervation (DDI) of the rat and Rhesus monkey.

Methods

We used a comprehensive integrated multi-molecular immunofluorescence labeling strategy to extensively profile the rat DDI and to a lesser extent that of the Rhesus monkey.

Results

The DDI was distributed across a dense, pervasive capillary network and included free nerve endings of peptidergic CGRP-expressing C fibers that were closely intertwined with noradrenergic (NA) sympathetic fibers and thin-caliber nonpeptidergic “C/Aδ” fibers. These newly identified C/Aδ fibers were unmyelinated, like C fibers, but expressed NF200, usually indicative of Aδ fibers, and uniquely co-labeled for the CGRP co-receptor, RAMP1. Slightly-larger caliber NF200-positive fibers co-labeled for myelin basic protein (MBP) and terminated as unbranched corpuscular endings. The DDI peptidergic fibers co-labeled for the lectin IB4 and expressed presumably excitatory α1-adrenergic receptors, as well as inhibitory 5HT1D receptors and the delta opioid receptor (δOR), but rarely the mu opioid receptor (µOR). Labeling for P2X3, TRPV1, TRPA1, and parasympathetic markers was not observed in the DDI.

Interpretation

These results suggest potential functional interactions, wherein peptidergic DDI fibers may be activated by stress-related sympathetic activity, resulting in CGRP release that could be detected in the circulation. CGRP may also activate nonpeptidergic C/Aδ fibers that are likely mechanosensitive or polymodal, leading to activation of post-synaptic pain transmission circuits. The distribution of α1-adrenergic receptors, RAMP1, and the unique expression of the δOR on CGRP-expressing DDI fibers suggest strategies for functional modulation and application to therapy.

Follicle Microstructure and Innervation Vary between Pinniped Micro- and Macrovibrissae

Histological data from terrestrial, semiaquatic, and fully aquatic mammal vibrissa (whisker) studies indicate that follicle microstructure and innervation vary across the mystacial vibrissal array (i.e. medial microvibrissae to lateral macrovibrissae). However, comparative data are lacking, and current histological studies on pinniped vibrissae only focus on the largest ventrolateral vibrissae. Consequently, we investigated the microstructure, medial-to-lateral innervation, and morphometric trends in harp seal (Pagophilus groenlandicus) vibrissal follicle-sinus complexes (F-SCs). The F-SCs were sectioned either longitudinally or in cross-section and stained with a modified Masson's trichrome stain (microstructure) or Bodian's silver stain (innervation). All F-SCs exhibited a tripartite blood organization system. The dermal capsule thickness, the distribution of major branches of the deep vibrissal nerve, and the hair shaft design were more symmetrical in medial F-SCs, but these features became more asymmetrical as the F-SCs became more lateral. Overall, the mean axon count was 1,221 ± 422.3 axons/F-SC and mean axon counts by column ranged from 550 ± 97.4 axons/F-SC (medially, column 11) to 1,632 ± 173.2 axons/F-SC (laterally, column 2). These values indicate a total of 117,216 axons innervating the entire mystacial vibrissal array. The mean axon count of lateral F-SCs was 1,533 ± 192.9 axons/ F-SC, which is similar to values reported in the literature for other pinniped F-SCs. Our data suggest that conventional studies that only examine the largest ventrolateral vibrissae may overestimate the total innervation by ∼20%. However, our study also accounts for variation in quantification methods and shows that conventional analyses likely only overestimate innervation by ∼10%. The relationship between axon count and cross-sectional F-SC surface area was nonlinear, and axon densities were consistent across the snout. Our data indicate that harp seals exhibit microstructural and innervational differences between their microvibrissae (columns 8-11) and macrovibrissae (columns 1-7). We hypothesize that this feature is conserved among pinnipeds and may result in functional compartmentalization within their mystacial vibrissal arrays.

Decoupling kinematics and mechanics reveals coding properties of trigeminal ganglion neurons in the rat vibrissal system

Tactile information available to the rat vibrissal system begins as external forces that cause whisker deformations, which in turn excite mechanoreceptors in the follicle. Despite the fundamental mechanical origin of tactile information, primary sensory neurons in the trigeminal ganglion (Vg) have often been described as encoding the kinematics (geometry) of object contact. Here we aimed to determine the extent to which Vg neurons encode the kinematics vs. mechanics of contact. We used models of whisker bending to quantify mechanical signals (forces and moments) at the whisker base while simultaneously monitoring whisker kinematics and recording single Vg units in both anesthetized rats and awake, body restrained rats. We employed a novel manual stimulation technique to deflect whiskers in a way that decouples kinematics from mechanics, and used Generalized Linear Models (GLMs) to show that Vg neurons more directly encode mechanical signals when the whisker is deflected in this decoupled stimulus space.

DOI:http://dx.doi.org/10.7554/eLife.13969.001

Animals must gather sensory information from the world around them and act on that information. Specialized sensory cells convert physical information from the environment into electrical signals that the brain can interpret. In the case of hearing, this physical information consists of changes in air pressure, and for vision, it is patterns of light bouncing off of objects.

Rodents rely heavily on touch information from their whiskers to explore their world. When a whisker touches an object, it deforms and bends. The first neurons to respond to whisker touch – so called primary sensory neurons – represent contact between the whisker and the object in the form of electrical signals, but exactly how they do this is unclear.

One possibility is that primary sensory neurons encode the movement of the whisker itself. Whenever a whisker touches an object, the whisker is deflected in a particular direction by a particular amount and at a particular speed. These movement-related features are referred to as the “kinematic” properties of whisker-object contact. Alternatively, these whisker sensory neurons might be more concerned with the forces at the base of the whisker caused by object contact. These forces are the “mechanical” properties of whisker-object contact.

Bush, Schroeder et al. set out to determine whether the electrical response of these whisker sensory neurons mainly encode kinematic or mechanical information. However, these two types of information are often closely related to each other: put simply, small whisker movements tend to accompany small forces and vice versa. Bush, Schroeder et al. therefore devised a method to deliver touch stimuli to the whiskers in a way that separates kinematic from mechanical information. Mathematical models were then developed to compare how well the neurons represent each type of information. The models showed that whisker sensory neurons generally encode mechanical signals more directly than kinematic ones.

This information adds to our understanding of how animals learn about the world through their senses. However, the analysis of Bush, Schroeder et al. relies on the long-standing simplification that whisker motion is two-dimensional, whereas in reality whiskers move in three dimensions. Therefore, a future challenge is to examine how sensory neurons represent information about touch, such as the location or shape of an object, during three-dimensional whisker-object contact.

DOI:http://dx.doi.org/10.7554/eLife.13969.002

Vibrissa Self-Motion and Touch Are Reliably Encoded along the Same Somatosensory Pathway from Brainstem through Thalamus

Active sensing involves the fusion of internally generated motor events with external sensation. For rodents, active somatosensation includes scanning the immediate environment with the mystacial vibrissae. In doing so, the vibrissae may touch an object at any angle in the whisk cycle. The representation of touch and vibrissa self-motion may in principle be encoded along separate pathways, or share a single pathway, from the periphery to cortex. Past studies established that the spike rates in neurons along the lemniscal pathway from receptors to cortex, which includes the principal trigeminal and ventral-posterior-medial thalamic nuclei, are substantially modulated by touch. In contrast, spike rates along the paralemniscal pathway, which includes the rostral spinal trigeminal interpolaris, posteromedial thalamic, and ventral zona incerta nuclei, are only weakly modulated by touch. Here we find that neurons along the lemniscal pathway robustly encode rhythmic whisking on a cycle-by-cycle basis, while encoding along the paralemniscal pathway is relatively poor. Thus, the representations of both touch and self-motion share one pathway. In fact, some individual neurons carry both signals, so that upstream neurons with a supralinear gain function could, in principle, demodulate these signals to recover the known decoding of touch as a function of vibrissa position in the whisk cycle.

Feedback in the brainstem: an excitatory disynaptic pathway for control of whisking

Sensorimotor processing relies on hierarchical neuronal circuits to mediate sensory-driven behaviors. In the mouse vibrissa system, trigeminal brainstem circuits are thought to mediate the first stage of vibrissa scanning control via sensory feedback that provides reflexive protraction in response to stimulation. However, these circuits are not well defined. Here we describe a complete disynaptic sensory receptor-to-muscle circuit for positive feedback in vibrissa movement. We identified a novel region of trigeminal brainstem, spinal trigeminal nucleus pars muralis, which contains a class of vGluT2+ excitatory projection neurons involved in vibrissa motor control. Complementary single- and dual-labeling with traditional and virus tracers demonstrate that these neurons both receive primary inputs from vibrissa sensory afferent fibers and send monosynaptic connections to facial nucleus motoneurons that directly innervate vibrissa musculature. These anatomical results suggest a general role of disynaptic architecture in fast positive feedback for motor output that drives active sensation.

Gephyrin clusters are absent from small diameter primary afferent terminals despite the presence of GABA(A) receptors

Whereas both GABA(A) receptors (GABA(A)Rs) and glycine receptors (GlyRs) play a role in control of dorsal horn neuron excitability, their relative contribution to inhibition of small diameter primary afferent terminals remains controversial. To address this, we designed an approach for quantitative analyses of the distribution of GABA(A)R-subunits, GlyR α1-subunit and their anchoring protein, gephyrin, on terminals of rat spinal sensory afferents identified by Calcitonin-Gene-Related-Peptide (CGRP) for peptidergic terminals, and by Isolectin-B4 (IB4) for nonpeptidergic terminals. The approach was designed for light microscopy, which is compatible with the mild fixation conditions necessary for immunodetection of several of these antigens. An algorithm was designed to recognize structures with dimensions similar to those of the microscope resolution. To avoid detecting false colocalization, the latter was considered significant only if the degree of pixel overlap exceeded that expected from randomly overlapping pixels given a hypergeometric distribution. We found that both CGRP(+) and IB4(+) terminals were devoid of GlyR α1-subunit and gephyrin. The α1 GABA(A)R was also absent from these terminals. In contrast, the GABA(A)R α2/α3/α5 and β3 subunits were significantly expressed in both terminal types, as were other GABA(A)R-associated-proteins (α-Dystroglycan/Neuroligin-2/Collybistin-2). Ultrastructural immunocytochemistry confirmed the presence of GABA(A)R β3 subunits in small afferent terminals. Real-time quantitative PCR (qRT-PCR) confirmed the results of light microscopy immunochemical analysis. These results indicate that dorsal horn inhibitory synapses follow different rules of organization at presynaptic versus postsynaptic sites (nociceptive afferent terminals vs inhibitory synapses on dorsal horn neurons). The absence of gephyrin clusters from primary afferent terminals suggests a more diffuse mode of GABA(A)-mediated transmission at presynaptic than at postsynaptic sites.

IB4-binding sensory neurons in the adult rat express a novel 3' UTR-extended isoform of CaMK4 that is associated with its localization to axons

Calcium/calmodulin-dependent protein kinase 4 (gene and transcript: CaMK4; protein: CaMKIV) is the nuclear effector of the Ca(2+) /calmodulin kinase (CaMK) pathway where it coordinates transcriptional responses. However, CaMKIV is present in the cytoplasm and axons of subpopulations of neurons, including some sensory neurons of the dorsal root ganglia (DRG), suggesting an extranuclear role for this protein. We observed that CaMKIV was expressed strongly in the cytoplasm and axons of a subpopulation of small-diameter DRG neurons, most likely cutaneous nociceptors by virtue of their binding the isolectin IB4. In IB4+ spinal nerve axons, 20% of CaMKIV was colocalized with the endocytic marker Rab7 in axons that highly expressed CAM-kinase-kinase (CAMKK), an upstream activator of CaMKIV, suggesting a role for CaMKIV in signaling though signaling endosomes. Using fluorescent in situ hybridization (FISH) with riboprobes, we also observed that small-diameter neurons expressed high levels of a novel 3' untranslated region (UTR) variant of CaMK4 mRNA. Using rapid amplification of cDNA ends (RACE), reverse-transcription polymerase chain reaction (RT-PCR) with gene-specific primers, and cDNA sequencing analyses we determined that the novel transcript contains an additional 10 kb beyond the annotated gene terminus to a highly conserved alternate polyadenylation site. Quantitative PCR (qPCR) analyses of fluorescent-activated cell sorted (FACS) DRG neurons confirmed that this 3'-UTR-extended variant was preferentially expressed in IB4-binding neurons. Computational analyses of the 3'-UTR sequence predict that UTR-extension introduces consensus sites for RNA-binding proteins (RBPs) including the embryonic lethal abnormal vision (ELAV)/Hu family proteins. We consider the possible implications of axonal CaMKIV in the context of the unique properties of IB4-binding DRG neurons.

Neuronal basis for object location in the vibrissa scanning sensorimotor system

An essential issue in perception is how the location of an object is estimated from tactile signals in the context of self-generated changes in sensor configuration. Here, we review the pathways and dynamics of neuronal signals that encode touch in the rodent vibrissa sensorimotor system. Rodents rhythmically scan an array of long, facial hairs across a region of interest. Behavioral evidence shows that these animals maintain knowledge of the azimuthal position of their vibrissae. Electrophysiological measurements have identified a reafferent signal of the azimuth that is coded in normalized coordinates, broadcast throughout primary sensory cortex and provides strong modulation of signals of vibrissa contact. Efferent signals in motor cortex report the range of the scan. Collectively, these signals allow the rodent to form a percept of object location.

Anatomical pathways involved in generating and sensing rhythmic whisker movements

The rodent whisker system is widely used as a model system for investigating sensorimotor integration, neural mechanisms of complex cognitive tasks, neural development, and robotics. The whisker pathways to the barrel cortex have received considerable attention. However, many subcortical structures are paramount to the whisker system. They contribute to important processes, like filtering out salient features, integration with other senses, and adaptation of the whisker system to the general behavioral state of the animal. We present here an overview of the brain regions and their connections involved in the whisker system. We do not only describe the anatomy and functional roles of the cerebral cortex, but also those of subcortical structures like the striatum, superior colliculus, cerebellum, pontomedullary reticular formation, zona incerta, and anterior pretectal nucleus as well as those of level setting systems like the cholinergic, histaminergic, serotonergic, and noradrenergic pathways. We conclude by discussing how these brain regions may affect each other and how they together may control the precise timing of whisker movements and coordinate whisker perception.

Collagenous skeleton of the rat mystacial pad

Anatomical and functional integrity of the rat mystacial pad (MP) is dependent on the intrinsic organization of its extracellular matrix. By using collagen autofluorescence, in the rat MP, we revealed a collagenous skeleton that interconnects whisker follicles, corium, and deep collagen layers. We suggest that this skeleton supports MP tissues, mediates force transmission from muscles to whiskers, facilitates whisker retraction after protraction, and limits MP extensibility.

The morphology of the rat vibrissal follicle-sinus complex revealed by three-dimensional computer-aided reconstruction

The vibrissal follicle-sinus complex (FSC) is a sensory receptor of the mammalian integumentary system that is located around the mouth. The purpose of the present study was to identify the actual 3-dimensional structure of the rat vibrissal FSC. Rat skin tissue was serially sectioned at a thickness of 10 μm and then stained with Masson's trichrome. The serial sections were reconstructed 3-dimensionally using Reconstruct software. The rat vibrissal follicle is a spindle-shaped structure that is embedded within a blood sinus and enveloped within a thick collagenous capsule. The vibrissal FSC is innervated by the deep vibrissal and superficial vibrissal nerves. The deep vibrissal nerve, travelling in the basal-to-apical direction, penetrates the thick collagenous capsule of the vibrissal FSC. The sinus system can be divided into a superior portion, known as the ring sinus, and an inferior portion, known as the cavernous sinus. The ring sinus contains a C-shaped structure, the ringwulst, which is suspended from the mesenchymal sheath of the follicle. Collagenous trabeculae can be seen in the cavernous sinus but not in the ring sinus. The ring sinus encircles the follicle obliquely and asymmetrically. The ringwulst encircles the follicle incompletely, in a C-shaped fashion. This study has demonstrated the previously underappreciated 3-dimensional structure of the vibrissal FSC, which differs from previously reported descriptions, and provides data that will enhance the understanding of vibrissal function.

A phenotypically restricted set of primary afferent nerve fibers innervate the bone versus skin: therapeutic opportunity for treating skeletal pain

Although musculoskeletal pain is one of the most common causes of chronic pain and physical disability in both developing and developed countries, relatively little is known about the nerve fibers and mechanisms that drive skeletal pain. Small diameter sensory nerve fibers, most of which are C-fiber nociceptors, can be separated into two broad populations: the peptide-rich and peptide-poor nerve fibers. Peptide-rich nerve fibers express substance P (SP) and calcitonin gene-related peptide (CGRP). In contrast, the peptide-poor nerve fibers bind to isolectin B4 (IB(4)) and express the purinergic receptor P(2)X(3) and Mas-related G protein-coupled receptor member d (Mrgprd). In the present report, we used mice in which the Mrgprd(+) nerve fibers express genetically encoded axonal tracers to determine the peptide-rich and peptide-poor sensory nerve fibers that innervate the glabrous skin of the hindpaw as compared to the bone marrow, mineralized bone and periosteum of the femur. Whereas the skin is richly innervated by CGRP(+), SP(+), P(2)X(3)(+) and Mrgprd(+) sensory nerve fibers, the bone marrow, mineralized bone and periosteum receive a significant innervation by SP(+) and CGRP(+), but not Mrgprd(+) and P(2)X(3)(+) nerve fibers. This lack of redundancy in the populations of C-fibers that innervate the bone may present a unique therapeutic opportunity for targeting skeletal pain as the peptide-rich and peptide-poor sensory nerve fibers generally express a different repertoire of receptors and channels to detect noxious stimuli. Thus, therapies that target the specific types of C-nerve fibers that innervate the bone may be uniquely effective in attenuating skeletal pain as compared to skin pain.

Postnatal changes in the Rexed lamination and markers of nociceptive afferents in the superficial dorsal horn of the rat

In this study, we investigated postnatal changes in Rexed's laminae and distribution of nociceptive afferents in the dorsal horn of the rat lumbar spinal cord at postnatal days 0, 5, 10, 15, 20, and 60. Transverse sections of the L4-L5 segments were processed for triple labeling with isolectin B4 (IB4)-binding as a marker of nonpeptidergic C-fibers, calcitonin gene-related peptide (CGRP) immunoreactivity to label peptidergic nociceptive afferents, and a fluorescent Nissl stain to visualize cells and lamination at different stages of postnatal development. The Nissl staining revealed that the thickness of lamina I (LI) and outer lamina II remained mostly unchanged from birth until adulthood. CGRP afferents terminated mostly in LI and the outer two-thirds of lamina II, whereas the termination area of fibers binding IB4 was centered on the middle one-third of lamina II at all ages studied. In absolute values, the overall width of the bands of intense CGRP and IB4 labeling increased with age but decreased as a percentage of the overall thickness of the dorsal horn with maturation. The overlap of CGRP termination area with that of IB4 afferents increased with age. The consequences of these findings are twofold. First, the size of the different laminae does not grow evenly across the dorsal horn. Second, CGRP and IB4 labeling cannot be considered per se to be reliable markers of lamination during development. These findings have implications for comparing data obtained in immature and mature tissues with respect to localization of structures in the dorsal horn.

Diversity in the neural circuitry of cold sensing revealed by genetic axonal labeling of transient receptor potential melastatin 8 neurons

Sensory nerves detect an extensive array of somatosensory stimuli, including environmental temperatures. Despite activating only a small cohort of sensory neurons, cold temperatures generate a variety of distinct sensations that range from pleasantly cool to painfully aching, prickling, and burning. Psychophysical and functional data show that cold responses are mediated by both C- and A delta-fibers with separate peripheral receptive zones, each of which likely provides one or more of these distinct cold sensations. With this diversity in the neural basis for cold, it is remarkable that the majority of cold responses in vivo are dependent on the cold and menthol receptor transient receptor potential melastatin 8 (TRPM8). TRPM8-null mice are deficient in temperature discrimination, detection of noxious cold temperatures, injury-evoked hypersensitivity to cold, and nocifensive responses to cooling compounds. To determine how TRPM8 plays such a critical yet diverse role in cold signaling, we generated mice expressing a genetically encoded axonal tracer in TRPM8 neurons. Based on tracer expression, we show that TRPM8 neurons bear the neurochemical hallmarks of both C- and A delta-fibers, and presumptive nociceptors and non-nociceptors. More strikingly, TRPM8 axons diffusely innervate the skin and oral cavity, terminating in peripheral zones that contain nerve endings mediating distinct perceptions of innocuous cool, noxious cold, and first- and second-cold pain. These results further demonstrate that the peripheral neural circuitry of cold sensing is cellularly and anatomically complex, yet suggests that cold fibers, caused by the diverse neuronal context of TRPM8 expression, use a single molecular sensor to convey a wide range of cold sensations.

Capsaicin receptor expression in the rat temporomandibular joint

Experimentally, temporomandibular joint (TMJ) nerve units respond to capsaicin, which is used clinically to treat TMJ pain. However, the existence of capsaicin receptors in the TMJ has not previously been clearly demonstrated. Immunohistochemical analysis has revealed the presence of transient receptor potential vanilloid subtype 1 (TRPV1) expression in the nerves and synovial lining cells of the TMJ. TRPV1-immunoreactive nerves are distributed in the synovial membrane of the joint capsule and provide branches to the joint compartment. The disc periphery is supplied by TRPV1 nerves that are mostly associated with small arterioles, and occasional nerves penetrate to the synovial lining layer. Double immunofluorescence has shown that many TRPV1-immunoreactive nerves are labeled with neuropeptide calcitonin gene-related peptide, whereas few are labeled with IB4-lectin. The results provide evidence for the presence of TRPV1 in both nerves and synovial lining cells, which might thus be involved in the mechanism of nociception and inflammation in the TMJ.

Topographically distinct epidermal nociceptive circuits revealed by axonal tracers targeted to Mrgprd

The brain receives sensory input from diverse peripheral tissues, including the skin, the body's largest sensory organ. Using genetically encoded axonal tracers expressed from the Mrgprd locus, we identify a subpopulation of nonpeptidergic, nociceptive neurons that project exclusively to the skin, and to no other peripheral tissue examined. Surprisingly, Mrgprd(+) innervation is restricted to the epidermis and absent from specialized sensory structures. Furthermore, Mrgprd(+) fibers terminate in a specific layer of the epidermis, the stratum granulosum. This termination zone is distinct from that innervated by most CGRP(+) neurons, revealing that peptidergic and nonpeptidergic epidermal innervation is spatially segregated. The central projections deriving from these distinct epidermal innervation zones terminate in adjacent laminae in the dorsal spinal cord. Thus, afferent input from different layers of the epidermis is conveyed by topographically segregated sensory circuits, suggesting that at least some aspects of sensory information processing may be organized along labeled lines.

Somatosensory organization and behavior in naked mole-rats: II. Peripheral structures, innervation, and selective lack of neuropeptides associated with thermoregulation and pain

African naked mole-rats are subterranean rodents that have a robust orienting response to stimulation of unique vibrissa-like body hairs that are widely spaced over an otherwise hairless skin. To determine whether these large body hairs have a specialized organization similar to facial vibrissae, the structure and innervation of facial vibrissa follicles, body hair follicles, and intervening skin in naked mole-rats was compared with that in rats and a furred African mole-rat species (the common mole-rat). Immunofluorescence and lectin-binding analyses revealed that the body hair follicles in naked mole-rats were exceptionally large and well innervated, similar to guard hairs of furred species. However, these body vibrissae lacked the anatomic specializations and unique types of innervation affiliated with follicle sinus complexes of facial vibrissae. In contrast to the furred species, naked mole-rats had a paucity of Abeta-fiber Merkel endings at all peripheral locations. Naked mole-rats also were completely lacking in cutaneous C-fibers immunoreactive for substance P and calcitonin gene-related peptide. In contrast, the hairless skin of the naked mole-rats had an exceptional abundance of presumptive Adelta-fibers. The unusual features of the cutaneous innervation in naked mole-rats are presumably adaptations to their subterranean environment and that they are the only known poikilothermic mammal. The features of this mammalian model system provide unique opportunities to discriminate mechanisms related to tactile spatial orientation, vascular regulation, and nociception.

Assessment of cutaneous innervation by skin biopsies

Skin biopsies that are immunostained to identify nerve fibers provide a new tool for assessing the small caliber nociceptors that terminate in the epidermis, as well as other cutaneous nerve fibers. Skin biopsies can be performed in multiple sites and can be repeated over time, so that a spatiotemporal profile of epidermal innervation can be constructed. This approach may help assess the progression of fiber loss in disease and of regeneration and re-innervation with treatment.

Innervation of the digit on the forepaw of the raccoon

The innervation of the digits on the raccoon forepaw was examined by using immunochemistry for protein gene product 9.5, calcitonin-gene related peptide, substance P, neuropeptide-Y, tyrosine hydroxylase, and neurofilament protein. The larger-caliber axons in the ventral glabrous skin terminate as Pacinian corpuscles deep in the dermis, small corpuscles and Merkel endings around the base of dermal papillae, and Merkel endings on rete pegs in dermal papillae. Extensive fine-caliber innervation terminates in the epidermis and on the microvasculature. The innervation is more dense in the distal than in the proximal volar pads. Pacinian endings are also concentrated in the transverse crease separating the distal and proximal pads. In the dorsal hairy skin, hair follicles are well innervated with piloneural complexes. Merkel innervation is located under slight epidermal elevations and in some large Merkel rete pegs located at the apex of transverse skin folds just proximal to the claw. No cutaneous Ruffini corpuscles were found anywhere on the digit. The claw is affiliated with dense medial and lateral beds of Pacinian endings, bouquets of highly branched Ruffini-like endings at the transition from the distal phalanx and unmyelinated innervation in the skin around the perimeter. Encapsulated endings are located at the lateral edge of the articular surface of the distal phalanx. Extensive fine-caliber innervation is affiliated with sweat glands and with the vasculature and is especially dense at presumptive arteriovenous sphincters. Virtually all of the sweat gland and vascular innervation is peptidergic, whereas most of the unmyelinated epidermal innervation is nonpeptidergic.

Structure and innervation of the vibrissal follicle-sinus complex in the Australian water rat, Hydromys chrysogaster

Light and electron microscopic techniques were used to examine the structure and innervation of the mystacial vibrissal follicle-sinus complex (F-SC) in the Australian water rat. The F-SCs of this semiaquatic rodent show the same morphologic elements described in terrestrial rats but differ in size, structure, and innervation. Most striking is the size of the water rat's caudal F-SCs, measuring 6.3 mm in length and 2.4 mm in diameter. The sinus system is divisible into a ring sinus and a cavernous sinus and shows a distinct asymmetry. At the highest level of the cavernous sinus, the outer root sheath forms a ridge in the direction of the trabeculae, which bind the ridge to the capsule. A ringwulst is present only in small and medium-sized F-SCs. The mean number of myelinated axons counted in the deep vibrissal nerve (DVN) of most caudal F-SCs was 537, indicating an innervation density of the water rat's vibrissal system at least 2.5 times as high as that of terrestrial rats. The total number of nerve fibers of the small superficial nerves was less than 10% of that of the DVN. These fibers innervate almost exclusively the area of the inner conical body. Structural specializations of the water rat F-SC are discussed as an analogous development in mammals adapted to the aquatic environment, primarily in terms of thermoregulation, whereas its high degree of innervation is assessed to lend support to the hypothesis that the vibrissal system is of special significance in aquatic mammals.

Anterograde transport of horseradish-peroxidase conjugated isolectin B4 from Griffonia simplicifolia I in spinal primary sensory neurons of the rat

Anterograde transport of the isolectin B4 from Griffonia simplicifolia I (B4) conjugated to horseradish peroxidase (HRP) was investigated in rat somatic and visceral primary sensory neurons at different spinal levels. Injection of B4-HRP into the L5 dorsal root ganglion (DRG) resulted in labelling in the sural nerve, but not in the gastrocnemius nerves. Free nerve endings and lanceolate-like nerve endings were labelled in the lateral hindpaw skin. Labelled fibres were also observed in the greater splanchnic nerve following B4-HRP injection into the T10-11 DRGs. Electron microscopic examination of the labelled nerves showed that B4-HRP labelled exclusively unmyelinated axons. In the spinal cord, labelling was observed in the superficial dorsal horn, and additionally, although much more sparse, in the medial and lateral collateral projections following injections into the T10-11 DRGs. The results suggest that B4-HRP should be a suitable anterograde tracer of unmyelinated cutaneous and splanchnic but not muscle primary afferent fibres.

Overexpression of nerve growth factor in skin causes preferential increases among innervation to specific sensory targets

The impact of increased levels of skin-derived nerve growth factor (NGF) neurotrophin on sensory and sympathetic innervation to the mouse mystacial pad and postero-orbital vibrissae was determined. Consistent with an approximate doubling of neuron number in trigeminal and superior cervical ganglia, many components of the sensory and sympathetic innervation were substantially enhanced. Although the increased number of neurons raised the possibility that all types of innervation were increased, immunohistochemical analysis indicated that enhanced NGF production had a differential effect upon sensory innervation, primarily increasing unmyelinated innervation. This increased innervation occurred in specific locations known to be innervated by small, unmyelinated fibers, suggesting that NGF modulated sensory innervation density, but not targeting. In contrast, sympathetic innervation was not only increased but also was distributed to some aberrant locations. In the intervibrissal fur of the mystacial pad, both the number of sensory axons and branches appeared increased, whereas in vibrissal follicle sinus complexes, only branching increased. In some areas, sensory ending density was lower than expected based upon the size of the source nerve bundles suggesting that many axons and branches were surviving but failing to form functional endings. Furthermore, the immunochemical profile of innervation was altered in some sensory populations as demonstrated by the coexistence of RT-97 neurofilament labeling in calcitonin gene-related peptide (CGRP) positive axons, by the loss of substance P colocalization in some CGRP axons, and by an absence of neuropeptide Y labeling in tyrosine hydroxylase positive sympathetic axons. Collectively, these results indicate that the NGF mediated increase in neuron number may be selective for particular sets of innervation and that increases among some populations may result from phenotypic switching.

Comprehensive immunofluorescence and lectin binding analysis of intervibrissal fur innervation in the mystacial pad of the rat

The innervation of the intervibrissal fur in the mystacial pad of the rat and mouse was examined by immunofluorescence with a wide variety of antibodies for neuronal related structural proteins, enzymes, and peptides as well as for lectin binding histofluorescence with Griffonia simplicifolia (GSA). Anti-protein gene product 9.5 (PGP) immunofluorescence labeled all sets of axons and endings. The innervation in the upper dermis and epidermis was distributed through a four tiered dermal plexus. From deep to superficial, the second tier was the source of all apparent myelinated mechanoreceptors, the third tier of nearly all the peptidergic and GSA binding innervation, and the fourth tier of nonpeptidergic GSA negative innervation (peptide-/GSA-). Three types of mechanoreceptors-Merkel, transverse lanceolate, and longitudinal lanceolate endings-innervated guard hair follicles. All had similar labeling characteristics for 160 kDa and 200 kDa neurofilament subunits, peripherin, carbonic anhydrase, synaptophysin, and S100. Palisades of longitudinal lanceolate endings were part of piloneural complexes along circumferentially oriented sets of transverse lanceolate endings, peptidergic free nerve endings (FNEs), and peptide-/GSA- FNEs. The longitudinal lanceolate endings were the only mechanoreceptors in the mystacial pad that had detectable calcitonin gene-related peptide. The epidermis contained four types of unmyelinated endings: simple free nerve endings (FNEs), penicillate endings, cluster endings and bush endings. Only the simple FNEs were clearly peptidergic. Virtually all others were peptide-/ GSA-. Each bush ending was actually an intermingled cluster of endings formed by several unmyelinated axons and occasionally an Adelta axon. In contrast to the other unmyelinated innervation to the epidermis, bush endings labeled with an antibody against the Schwann cell protein S100. The necks and mouths of follicles, as well as superficial vasculature, were innervated by a mixture of unmyelinated peptidergic and/or GSA labeled sensory and sympathetic axons. Small presumptive sweat glands were innervated by three sets of peptidergic axons of which one was immunoreactive for somatostatin. Potential functions of the various sets of innervation are discussed.

The innervation of the mystacial pad of the rat as revealed by PGP 9.5 immunofluorescence

The innervation of the mystacial pad in the rat was investigated with the aid of antihuman protein gene product (PGP) 9.5 immunofluorescence. PGP 9.5 is ubiquitin carboxyl-terminal hydrolase, which is distributed throughout neuronal cytoplasm. This technique revealed all previously known innervation as well as a wide variety of small-caliber axons and some endings of large-caliber afferents that had not been observed before. Newly revealed innervation affiliated with vibrissal-follicle sinus complexes included 1) fine-caliber, radially oriented processes in the epidermal rete ridge collar; 2) a loose network of fine-caliber, circumferentially arrayed processes in the centrifugal part of the mesenchymal sheath at the level of the ring sinus; 3) a loose haphazard network of fine-caliber and medium-caliber processes in the mesenchymal sheath and among the trabeculae of the cavernous sinus; 4) a loose network of circumferentially arrayed processes within the mesenchymal sheath of the cavernous sinus and in close proximity to the basement membrane; 5) a dense network of reticular-like endings provided by large-caliber afferents to the mesenchymal sheath in the upper part of the cavernous sinus; and 6) fine-caliber innervation to the dermal papilla at the base of all vibrissal shafts. In the intervibrissal skin, a dense distribution of fine-caliber individual and clustered profiles was detected in the epidermis. In addition to previously known innervation, Merkel endings were consistently observed in the epidermis at the mouths of guard hairs, loose networks of fine-caliber axons were found around the necks of occasional guard hairs, and fine-caliber profiles were frequently affiliated with vellus hairs. Vascular profiles were heavily innervated throughout the dermis. Axons and motor end plates of the facial nerve innervation to papillary muscles also were labeled. Transection of the infraorbital nerve eliminated all but the facial nerve innervation. Unilateral removal of the superior cervical ganglion eliminated the innervation to the dermal papillae but caused no other noticeable reduction. PGP 9.5-like immunofluorescence was also moderately expressed in apparent Schwann cells, in Merkel cells only in the external root sheath of vibrissal follicles, and in apparent dendritic and/or Langerhans cells usually located in the epidermis and occasionally in the follicles. PGP 9.5-like immunofluorescence persisted in highly vacuolated profiles along the usual courses of medium to large-caliber axons 2 weeks after nerve transection. The possible functional role of the newly discovered innervation is considered along with that of previously identified afferents.