Calcitonin gene-related peptide (CGRP) immunoreactivity in the neuroendocrine Merkel cells and nerve fibres of pig and human skin

CGRP physiology, pharmacology, and therapeutic targets: migraine and beyond

Calcitonin gene-related peptide (CGRP) is a neuropeptide with diverse physiological functions. Its two isoforms (α and β) are widely expressed throughout the body in sensory neurons as well as in other cell types, such as motor neurons and neuroendocrine cells. CGRP acts via at least two G protein-coupled receptors that form unusual complexes with receptor activity-modifying proteins. These are the CGRP receptor and the AMY1 receptor; in rodents, additional receptors come into play. Although CGRP is known to produce many effects, the precise molecular identity of the receptor(s) that mediates CGRP effects is seldom clear. Despite the many enigmas still in CGRP biology, therapeutics that target the CGRP axis to treat or prevent migraine are a bench-to-bedside success story. This review provides a contextual background on the regulation and sites of CGRP expression and CGRP receptor pharmacology. The physiological actions of CGRP in the nervous system are discussed, along with updates on CGRP actions in the cardiovascular, pulmonary, gastrointestinal, immune, hematopoietic, and reproductive systems and metabolic effects of CGRP in muscle and adipose tissues. We cover how CGRP in these systems is associated with disease states, most notably migraine. In this context, we discuss how CGRP actions in both the peripheral and central nervous systems provide a basis for therapeutic targeting of CGRP in migraine. Finally, we highlight potentially fertile ground for the development of additional therapeutics and combinatorial strategies that could be designed to modulate CGRP signaling for migraine and other diseases.

Anatomical contacts between sensory neurons and epidermal cells: an unrecognized anatomical network for neuro-immuno-cutaneous crosstalk

Sensory neurons innervating the skin are conventionally thought to be the sole transducers of touch, temperature, pain and itch. However, recent studies have shown that keratinocytes - like Merkel cells - act as sensory transducers, whether for innocuous or noxious mechanical, thermal or chemical stimuli, and communicate with intraepidermal free nerve endings via chemical synaptic contacts. This paradigm shift leads to consideration of the whole epidermis as a sensory epithelium. Sensory neurons additionally function as an efferent system. Through the release of neuropeptides in intimate neuroepidermal contact areas, they contribute to epidermal homeostasis and to the pathogenesis of inflammatory skin diseases. To counteract the dogma regarding neurocutaneous interactions, seen exclusively from the perspective of soluble and spreading mediators, this review highlights the essential contribution of the unrecognized anatomical contacts between sensory neurons and epidermal cells (keratinocytes, melanocytes, Langerhans cells and Merkel cells), which take part in the reciprocal dialogue between the skin, nervous system and immune system.

Ectopic Atoh1 expression drives Merkel cell production in embryonic, postnatal and adult mouse epidermis

Merkel cells are mechanosensitive skin cells whose production requires the basic helix-loop-helix transcription factor Atoh1. We induced ectopic Atoh1 expression in the skin of transgenic mice to determine whether Atoh1 was sufficient to create additional Merkel cells. In embryos, ectopic Atoh1 expression drove ectopic expression of the Merkel cell marker keratin 8 (K8) throughout the epidermis. Epidermal Atoh1 induction in adolescent mice similarly drove widespread K8 expression in glabrous skin of the paws, but in the whisker pads and body skin ectopic K8+ cells were confined to hair follicles and absent from interfollicular regions. Ectopic K8+ cells acquired several characteristics of mature Merkel cells in a time frame similar to that seen during postnatal development of normal Merkel cells. Although ectopic K8+ cell numbers decreased over time, small numbers of these cells remained in deep regions of body skin hair follicles at 3 months post-induction. In adult mice, greater numbers of ectopic K8+ cells were created by Atoh1 induction during anagen versus telogen and following disruption of Notch signaling by conditional deletion of Rbpj in the epidermis. Our data demonstrate that Atoh1 expression is sufficient to produce new Merkel cells in the epidermis, that epidermal cell competency to respond to Atoh1 varies by skin location, developmental age and hair cycle stage, and that the Notch pathway plays a key role in limiting epidermal cell competency to respond to Atoh1 expression.

Merkel cells and neurons keep in touch

The Merkel cell-neurite complex is a unique vertebrate touch receptor comprising two distinct cell types in the skin. Its presence in touch-sensitive skin areas was recognized more than a century ago, but the functions of each cell type in sensory transduction have been unclear. Three recent studies demonstrate that Merkel cells are mechanosensitive cells that function in touch transduction via Piezo2. One study concludes that Merkel cells, rather than sensory neurons, are principal sites of mechanotransduction, whereas two other studies report that both Merkel cells and neurons encode mechanical inputs. Together, these studies settle a long-standing debate on whether or not Merkel cells are mechanosensory cells, and enable future investigations of how these skin cells communicate with neurons.

Neurotransmitters and synaptic components in the Merkel cell-neurite complex, a gentle-touch receptor

Merkel cells are an enigmatic group of rare cells found in the skin of vertebrates. Most make contacts with somatosensory afferents to form Merkel cell-neurite complexes, which are gentle-touch receptors that initiate slowly adapting type I responses. The function of Merkel cells within the complex remains debated despite decades of research. Numerous anatomical studies demonstrate that Merkel cells form synaptic-like contacts with sensory afferent terminals. Moreover, recent molecular analysis reveals that Merkel cells express dozens of presynaptic molecules that are essential for synaptic vesicle release in neurons. Merkel cells also produce a host of neuroactive substances that can act as fast excitatory neurotransmitters or neuromodulators. Here, we review the major neurotransmitters found in Merkel cells and discuss these findings in relation to the potential function of Merkel cells in touch reception.

Calcitonin gene-related peptide inhibits chemokine production by human dermal microvascular endothelial cells

This study examined whether the sensory neuropeptide calcitonin gene-related peptide (CGRP) inhibits release of chemokines by dermal microvascular endothelial cells. Dermal blood vessels are associated with nerves containing CGRP, suggesting that CGRP-containing nerves may regulate cutaneous inflammation through effects on vessels. We examined CGRP effects on stimulated chemokine production by a human dermal microvascular endothelial cell line (HMEC-1) and primary human dermal microvascular endothelial cells (pHDMECs). HMEC-1 cells and pHDMECs expressed mRNA for components of the CGRP and adrenomedullin receptors and CGRP inhibited LPS-induced production of the chemokines CXCL8, CCL2, and CXCL1 by both HMEC-1 cells and pHDMECs. The receptor activity-modifying protein (RAMP)1/calcitonin receptor-like receptor (CL)-specific antagonists CGRP₈-₃₇ and BIBN4096BS, blocked this effect of CGRP in a dose-dependent manner. CGRP prevented LPS-induced IκBα degradation and NF-κB binding to the promoters of CXCL1, CXCL8 and CCL2 in HMEC-1 cells and Bay 11-7085, an inhibitor of NF-κB activation, suppressed LPS-induced production of CXCL1, CXCL8 and CCL2. Thus, the NF-κB pathway appears to be involved in CGRP-mediated suppression of chemokine production. Accordingly, CGRP treatment of LPS-stimulated HMEC-1 cells inhibited their ability to chemoattract human neutrophils and mononuclear cells. Elucidation of this pathway may suggest new avenues for therapeutic manipulation of cutaneous inflammation.

Evidence of fast serotonin transmission in frog slowly adapting type 1 responses

The Merkel cell-neurite (MCN) complex generates slowly adapting type 1 (SA1) response when mechanically stimulated. Both serotonin (5-HT) and glutamate have been implicated in the generation of normal SA1 responses, but previous studies have been inconclusive as to what their roles are or how synaptic transmission occurs. In this study, excised dorsal skin patches from common water frogs (Rana ridibunda) were stimulated by von Frey hairs during perfusion in a tissue bath, and single-unit spike activity was recorded from SA1 fibres. Serotonin had no significant effect on the SA1 response at low (10 µM) concentration, significantly increased activity in a force-independent manner at 100 µM, but decreased activity with reduced responsiveness to force at 1 mM. Glutamate showed no effect on the responsiveness to force at 100 µM. MDL 72222 (100 µM), an ionotropic 5-HT3 receptor antagonist, completely abolished the responsiveness to force, suggesting that serotonin is released from Merkel cells as a result of mechanical stimulation, and activated 5-HT3 receptors on the neurite. The metabotropic 5-HT2 receptor antagonist, ketanserin, greatly reduced the SA1 fibre's responsiveness to force, as did the non-specific glutamate receptor antagonist, kynurenic acid. This supports a role for serotonin and glutamate as neuromodulators in the MCN complex, possibly by activation and/or inhibition of signalling cascades in the Merkel cell associated with vesicle release. Additionally, it was observed that SA1 responses contained a force-independent component, similar to a dynamic response observed during mechanical vibrations.

Merkel cells

Merkel cells are post-mitotic cells scattered throughout the epidermis of vertebrates. They are particularly interesting because of the close connections that they develop with sensory nerve endings and the number of peptides they can secrete. These features suggest that they may make an important contribution to skin homeostasis and cutaneous nerve development. However, these cells remain mysterious because they are difficult to study. They have not been successfully cultured and cannot be isolated, severely hampering molecular biology and functional analysis. Merkel cells probably originate in the neural crest of avians and mammalians, and their "spontaneous" appearance in the epidermis may be caused by a neuron-independent epidermal differentiation process. Their functions are still unclear: they take part in mechanoreception or at least interact with neurons, but little is known about their interactions with other epidermal cells. This review provides a new look at these least-known cells of the skin. The numerous peptides they synthesize and release may allow them to communicate with many cells other than neurons, and it is plausible that Merkel cells play a key role in skin physiology and physiopathology.

Current considerations about Merkel cells

Since the discovery of Merkel cells by Friedrich S. Merkel in 1875, knowledge of their structure has increased with the progression of new technologies such as electron and laser microscopy, and immunohistochemical techniques. For most vertebrates, Merkel cells are located in the basal layer of the epidermis and characterized by dense-core granules that contain a variety of neuropeptides, plasma membrane spines and cytoskeletal filaments consisting of cytokeratins and desmosomes. The presence of the two latter structures would suggest that Merkel cells originate from the epidermis rather than from the neural crest, even though such a hypothesis is not unanimously accepted. The function of the Merkel cell is also very controversial. For a long time, it has been accepted that Merkel cells with associated nerve terminals act as mechanoreceptors although the transduction mechanism has not yet been elucidated. Merkel cells that do not make contact with nerve terminals have an endocrine function. The present review aims to shed new and comparative light on this field with an attempt to investigate the stimuli that Merkel cells are able to perceive.

Intermedin: a skin peptide that is downregulated in atopic dermatitis

Intermedin (IMD), also called adrenomedullin-2, is a peptide that belongs to the calcitonin/calcitonin gene-related peptide/amylin peptide family. IMD exerts many effects on the cardiovascular system, gastrointestinal tract, and central nervous system. Here, we analyzed the expression of the IMD peptide in human skin of healthy controls, in biopsies from lesional and non-lesional areas of atopic dermatitis (AD) skin, in cultured human keratinocytes, and in the HaCaT keratinocyte cell line at the transcriptional (quantitative reverse transcription-PCR) and translational (immunohistochemistry) level. IMD messenger RNA (mRNA) and protein could be detected in keratinocytes and human skin. Keratinocytes, nerve fibers, periglandular cells, arterial/arteriolar smooth muscle cells, and pericytes of dermal microvessels were intensely IMD-immunoreactive. The IMD mRNA was, compared to healthy skin, significantly reduced in lesional and non-lesional areas of AD skin. This was accompanied by a reduction of IMD immunoreactivity in pericytes of the upper dermis indicating that skin from AD patients is generally affected, and downregulation of IMD in AD skin is not a secondary phenomenon caused by acute inflammation but is a general characteristic of AD skin. These data further point to a role of IMD expressed by pericytes in conferring higher susceptibility of the skin of AD patients to inflammatory stimuli.

Human Merkel cells – aspects of cell biology, distribution and functions

Human Merkel cells were first described by Friedrich S. Merkel in 1875 and named "Tastzellen" (touch cells) assuming a sensory touch function within the skin. Only ultrastructural research revealed their characteristics such as dense-core granules, plasma membrane spines and dendrites as well as a loosely arranged cytoskeleton. Biochemical analysis identified the expression of very specific cytokeratins (most notably CK 20) allowing the immunohistochemical detection of Merkel cells. In humans, they occur within the basal epidermis, being concentrated in eccrine glandular ridges of glabrous skin and in Haarscheiben of hairy skin, within belt-like clusters of hair follicles, and in certain mucosal tissues. Within the human skin, the dense-core granules contain heterogeneously distributed neuropeptides, some of which might work as neurotransmitters through which Merkel cells and their associated nerves exert their classical function as slowly adapting mechanoreceptors type I. This is the case in the Haarscheiben, small sensory organs containing keratinocytes with a special program of differentiation that includes the expression of CK 17 and Ber-EP4. Other peptides may act as growth factors and thus might participate in growth, differentiation and homeostasis of cutaneous structures. It is not yet clear whether the Merkel cell carcinomas, aggressive skin carcinomas, indeed arise from Merkel cells. We summarize and discuss data on the distribution, function and heterogeneity of human Merkel cells in normal and diseased skin.

Immunohistochemical reactions of receptors to met-enkephalin, VIP, substance P, and CGRP located on Merkel cells in the rat sinus hair follicle

The role of Merkel cells in type I cutaneous mechanoreceptors remains enigmatic though mechanical transduction or neuromodulation function has been proposed. It has been shown that mammalian Merkel cells express immunohistochemical reactions of met-enkephalin, VIP, substance P, and CGRP, though the reactivity differs between species. If any one of these peptides acts as a transmitter or modulator for Merkel nerve terminals, these structures must have a specific receptor for the substance. We therefore studied the immunohistochemical localization of the above-mentioned neuropeptides and their receptors in Merkel cell-nerve endings in rat whisker pads. Specimens were doubly stained with polyclonal antibodies to neuropeptides and their receptors combined with a monoclonal antibody to cytokeratin 20, which was used for the labeling of Merkel cells. Merkel cells in the rat sinus hair follicles showed positive immunoreactions for all peptides studied, whereas the immunoreactions of receptors to these peptides were localized on Merkel cell membranes but not on the axon terminals. These results suggest that neuropeptides released from Merkel cells act on Merkel cells themselves by an autocrine mechanism.

Merkel cells, a new localization of prepro-orexin and orexin receptors

Orexins (OXA and OXB) are peptides derived from a common precursor called prepro-orexin. They act through G-protein receptors named orexin 1 receptor (OX(1)R) and orexin 2 receptor (OX(2)R). Orexins were first demonstrated in neurons of the lateral hypothalamus and found to be related to the control of food intake. However, it has been shown that they are widely distributed in both the nervous system and peripheral tissues, including endocrine organs such as the pituitary and adrenal glands. Merkel cells are neuroendocrine cells situated in the epidermis, tactile hairs and oral mucosa, and act as mechanoreceptors. Up to the present, various neuropeptides have been detected in these cells. The aim of the present study was to detect the presence of prepro-orexin and orexin receptors (OX(1)R and OX(2)R) in porcine Merkel cells using immunohistochemistry. Prepro-orexin was expressed in the cytoplasm of Merkel cells in the skin of the pig snout. Immunoreactivity for prepro-orexin was more intense in the mature side of the cell, where the dense-cored granules are accumulated. Epidermal nerve terminals associated with Merkel cells and dermal nerve fibres showed no immunostaining. Both orexin receptors (OX(1)R and OX(2)R) were also demonstrated in the cytoplasm of Merkel cells of pig snout skin. The finding of orexins and their receptors in Merkel cells suggests that they have an autocrine function. Further studies are needed to ascertain the significance of this function.

Friedrich Sigmund Merkel and his "Merkel cell", morphology, development, and physiology: review and new results

Merkel nerve endings are mechanoreceptors in the mammalian skin. They consist of large, pale cells with lobulated nuclei forming synapse-like contacts with enlarged terminal endings of myelinated nerve fibers. They were first described by F.S. Merkel in 1875. They are found in the skin and in those parts of the mucosa derived from the ectoderm. In mammals (apart from man), the largest accumulation of Merkel nerve endings is found in whiskers. In all vertebrates, Merkel nerve endings are located in the basal layer of the epidermis, apart from birds, where they are located in the dermis. Cytoskeletal filaments consisting of cytokeratins and osmiophilic granules containing a variety of neuropeptides are found in Merkel cells. In anseriform birds, groups of cells resembling Merkel cells, with discoid nerve terminals between cells, form Grandry corpuscles. There has been controversy over the origin of Merkel cells. Results from chick/quail chimeras show that, in birds, Merkel cells are a subpopulation of cells derived from the neural crest, which thus excludes their development from the epidermis. Most recently, also in mammals, conclusive evidence for a neural crest origin of Merkel cells has been obtained. Merkel cells and nerve terminals form mechanoreceptors. Calcium ions enter Merkel cells in response to mechanical stimuli, a process which triggers the release of calcium from intracellular stores resulting in exocytosis of neurotransmitter or neuromodulator. Recent results suggest that there may be glutamatergic transmission between Merkel cell and nerve terminal, which appears to be essential for the characteristic slowly adapting response of these receptors during maintained mechanical stimuli. Thus, we are convinced that Merkel cells with associated nerve terminals function as mechanoreceptor cells. Cells in the skin with a similar appearance as Merkel cells, but without contact to nerve terminals, are probably part of a diffuse neuroendocrine system and do not function as mechanoreceptors. Probably these cells, rather than those acting as mechanoreceptors, are the origin of a highly malignant skin cancer called Merkel cell carcinoma.

Spatial relationship between Merkel cells and Langerhans cells in human hair follicles

The distributions of Merkel cells and Langerhans cells within human hair follicles have been reported. However, there has been no description of the relationship between Merkel cells and Langerhans cells, which were discovered by 19th century German pathologists. Merkel cells and Langerhans cells share some similar characteristics such as the localization of human hair follicles, a close association with peripheral nerves and the expression of several neuropeptides. Merkel cells were stained with CK20 or CAM5.2, while Langerhans cells were stained with CD1a or S-100 protein. We thus immunohistochemically confirmed the preferential localization of Merkel cells and Langerhans cells in normal human hair follicles. Using a double staining technique, two- and three-dimensional observations demonstrated that a small proportion of Merkel cells were closely contacted with Langerhans cells below the sebaceous gland level, presumably indicating the bulge area. Merkel cells and Langerhans cells connected directly or approached each dendrite within the basal layer of the outer root sheath. For the first time, we demonstrated a close anatomical relationship between Merkel cells and Langerhans cells within the bulge area of human hair follicles where follicular stem cells may be present. These morphological observations suggest a functional interaction between follicular Merkel cells and Langerhans cells. We herein hypothesize that Merkel cells communicate with Langerhans cells by characteristic dendrites in which some neuropeptides or cytokines may be stored.

Calcitonin gene-related peptide immunoreactivity and afferent receptive properties of dorsal root ganglion neurones in guinea-pigs

To establish the afferent receptive properties of lumbosacral dorsal root ganglion (DRG) neurones that express calcitonin gene-related peptide (CGRP), intracellular recordings were made with fluorescent dye-filled electrodes in deeply anaesthetised young guinea-pigs. After determination of neuronal functional properties, dye was injected into the soma. CGRP-like immunoreactivity (CGRP-LI) was examined on histological sections of dye-marked neurones. Fourteen of 34 C-fibre neurones showed CGRP-LI. These included 10/21 C-fibre nociceptive neurones. All C-polymodal nociceptors in glabrous (n = 4) but none in hairy skin (n = 4) were positive. Positive C-fibre high threshold mechanoreceptive (HTM) units had receptive fields in dermal or deeper tissue. Four (n = 6) unresponsive or unidentified C-fibre units were positive. Neither C-fibre cooling sensitive (n = 4) nor C-fibre low threshold mechanoreceptive (LTM) units (n = 3) had CGRP-LI. Six of 23 A-fibre nociceptive cells were positive including one Aalpha/beta unit. Three of these positive cells had epidermal and three had dermal/deep receptive fields. Three of 36 A-fibre LTM units exhibited CGRP-LI; all were Aalpha/beta-fibre G hair units. All glabrous skin and muscle spindle units and in hairy skin slowly adapting and field units, and some G-hair units lacked CGRP-LI. CGRP-LI stained fibres were found in tissues containing receptive fields of positive DRG neurones: glabrous skin, near hair follicles and in skeletal muscle. A few substance P-labelled neurones did not exhibit CGRP-LI and vice versa. Thus CGRP expression was detected in under half the nociceptive neurones, was not limited to nociceptive neurones and apart from receptive properties was also related to location/depth in the tissues of a DRG neurone's peripheral terminals.

Expressions of putative neurotransmitters and neuronal growth related genes in Merkel cell-neurite complexes of the rats

Reverse transcription-polymerase chain reaction using 32P-labeled dCTP with specific primers for putative neurotransmitters related and neuronal growth-related genes (GAP-43, NGF and BDNF) were used to search for evidence of such substances in the Merkel cell. Merkel cell samples were made from sinus hairs in the facial skin of rats. The relative amount of mRNA in a tissue sample concentrated in Merkel cells was compared semiquantitatively to that from nearby tissue without Merkel cells. mRNAs for VIP, tyrosine hydroxylase, substance P were found in higher concentration in Merkel cells than in control tissues. mRNA for genes encoding pro-enkephalin, GAP-43, CGRP, NGF and BDNF were detected in the Merkel cell samples at concentration statistically equivalent to those found in the control tissues. It was concluded that the relative concentration of mRNAs for VIP, tyrosine hydroxylase and substance P is consistent with the possibility that Merkel cell acts as a possible transduction element in mechanical excitation of sense organs in which Merkel cells are present.

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.

The relationship between cutaneous C fibre type and antidromic vasodilatation in the rabbit and the rat

1. Skin blood flow was monitored during antidromic stimulation of identified cutaneous C fibres in fine filaments dissected from the saphenous nerve of anaesthetized rabbits and rats. The techniques used to monitor skin blood flow were laser Doppler perfusion imaging and laser Doppler flowmetry. 2. In the rabbit filaments a total of thirty-three C fibres were tested for their ability to produce antidromic vasodilatation. The only C fibres found to have vasodilator actions were of the polymodal nociceptor afferent class, and fourteen (50%) of the twenty-eight polymodal nociceptor units tested were vasoactive. The afferent receptive fields of polymodal nociceptor afferents were mapped carefully using suprathreshold mechanical stimuli, and there was a good correlation between afferent receptive field area and area of vasodilatation. 3. In the rat, eleven of the fifty-four C fibres antidromically stimulated had vasodilator actions. All eleven vasoactive C fibres were nociceptive and comprised seven polymodal nociceptor units, two heat nociceptor units and two incompletely classified nociceptor units. The area of increased blood flow was always coincident with the afferent field of the stimulated unit. 4. In the rat the vasodilator units were not evenly distributed over the saphenous nerve receptive field. Nine of the eleven vasoactive C fibres had receptive fields located on the foot or the digits, and only two were on the ankle or lower leg. Overall, the population of nociceptive C fibres was evenly distributed over the saphenous nerve receptive field. 5. In both the rabbit and the rat, a subclass of polymodal nociceptor afferents form the majority of the vasoactive units and will make the main contribution to axon reflex flare and other neurogenic inflammatory responses involving vasodilatation. The vasoactive polymodal nociceptor units tend to have relatively low mechanical sensitivity, although they have typical heat thresholds. In the rat heat nociceptor units also have vasodilator actions. However, such heat nociceptor units form a minor functional class of afferent C fibre in the rat saphenous nerve, and are not found in the rabbit saphenous nerve. 6. The findings from this study in the rabbit and the rat are compared with the situation in pig skin. The close relationship between afferent receptive field area and spread of flare across species is noted, and the way these measures increase with body size is discussed.

Immunohistochemical detection of human skin nerve fibers

The autonomic nervous system is involved in different functions such as transduction of afferent sensory inputs, trophic actions, modulation of immunologic events and thermoregulation. In the present investigation, we studied the pattern of human autonomic skin innervation with special reference to its relation to blood vessels, hair follicles, sweat glands and sensory receptors. For the first time, two clinically important areas have been compared: the skin of the forearm and of the face. Using indirect immunohistochemistry, we analyzed the distribution of calretinin (CR), calcitonin gene-related peptide (CGRP), neuropeptide Y (NPY), substance P (SP), neurokinin A (NKA), vasoactive intestinal peptide (VIP), nitric oxide synthase (NOS), tyrosine hydroxylase (TH), histamine, serotonin, enkephalin, and, enzyme histochemically, NADPH-diaphorase (NADPH-d). In the epidermis, we found nerve fibers containing SP, NKA and CGRP. In the dermis, SP-, CR-, VIP-, CGRP- and NKA-positive nerve fibers were detected. Particularly the large nerve fibers contained CR. VIP-positive fibers occurred especially around hair follicles and sweat glands. CGRP-positive nerve fibers were located close to the epidermal basal membrane, in the wall of blood vessels, and to a lesser extent around hair follicles. Immunoreactivity for SP and NKA in the dermis was observed predominantly in the papillary layer near the epidermal basal membrane. All neuropeptides tested in this study were also detected in the nerve fibers of the subcutis. Most of them were CGRP- and VIP-positive. They occurred in association with sweat glands and large arteries. NPY-positive nerve fibers are predominant in the wall of arteries, arterioles and veins. Nerve fibers containing NKA and SP were less common and identified only in the walls of large arteries in deeper dermal layers. In double-staining experiments, the NADPH-d reaction and reactivity to tubulin revealed a partial co-localization in nerve fibers, blood vessel walls, around glands and ganglionic cells. VIP-positive fibers were more common in the face skin than in the forearm. However, in forearm we detected more NPY-, CGRP-, NKA- and SP-positive nerve fibers than in face skin. These findings are important for future studies on skin disorders, such as sensory neuropathies, inflammatory reactions or allergic responses of human skin.

Cellular and subcellular distribution of 7B2 in porcine Merkel cells

BACKGROUND

Merkel cells are neuroendocrine cells located in the skin and oral mucosa of various mammalian species. These cells express multiple peptides as well as serotonin. Although the precise function of Merkel cells is still unknown, different studies support its role as mechano-electric transducer. 7B2 granin (secretogranin V) is a polypeptide isolated from the pituitary gland and present in the dense-cored granules of neuronal and paraneuronal cells.

METHODS

The expression of the 7B2 in Merkel cells of pig snout skin was analysed by immunohistochemical techniques. The streptavidin-biotin peroxidase complex procedure was employed for light microscopy. A postembedding method using immunoglobulin-colloidal gold complexes was employed for the ultrastructural studies.

RESULTS

Immunoreactivity for 7B2 was observed in virtually all Merkel cells, both in epidermis and vibrissae. The immunostaining was shown in the basal side of cytoplasms where neuroendocrine granules were accumulated. Immunoelectron microscopy allowed us to demonstrate that 7B2 labelling was located on the electrondense granules. Nuclei and epidermal nerve terminals associated with merkel cells did not show immunoreactivity.

CONCLUSIONS

The polypeptide 7B2 is present in the dense-cored granules of Merkel cells. This result is consistent with the possible role for 7B2 in secretory granules' processing. To our knowledge this is the first evidence of 7B2 protein in Merkel cells.

An immunohistochemical screening of neurochemical markers in fungiform papillae and taste buds of the anterior rat tongue

The occurrence and distribution of several neurochemical markers were investigated. Numerous nerve fibres were shown, using antibodies to protein gene product (PGP) 9.5, neurone-specific enolase, calcitonin gene-related peptide (CGRP), substance P. neurokinin A or protein S-100. The presence of vasoactive intestinal polypeptide (VIP), peptide histidine isoleucine amide (PHI), neuropeptide tyrosine, dopamine-beta-hydroxylase (DBH), cholecystokinin/gastrin, glutamate and galanin was more scarce. Nerve fibres containing these above-mentioned markers were found at several locations, i.e. in the epithelium, connective tissue, and around blood vessels. In the taste buds, numerous PGP 9.5, neurone-specific enolase-, CGRP-, substance P-, neurokinin A- and protein S-100-containing structures were found, but few VIP and galanin ones. No immunoreactivity was found with antibodies against somatostatin, bombesin, enkephalin or dynorphin. These findings extend knowledge about the general as well as the neurochemical messenger-based innervation of rat fungiform papillae, forming a firm basis for future functional investigations of normal, experimental and also clinical materials.

Immunohistochemical, electrophysiological, and electron microscopical study of rat fungiform taste buds after regeneration of chorda tympani through the non-gustatory lingual nerve

The sensory innervation of fungiform papillae on the rat dorsal tongue is derived from branches of two cranial nerves: the lingual branch of the trigeminal nerve which provides somatosensory innervation and the chorda tympani (CT) branch of the facial nerve, which provides innervation to the taste buds. Removal of the CT results in degeneration of the taste buds. Removal of both nerves results in reduction in size of fungiform papillae and an altered pattern of keratinization in its epithelium. Regeneration of nerves to the epithelium restores the pre-operative condition. Thus, in addition to their sensory functions, both the CT and lingual seem to exert trophic effects on the phenotypic expression of epithelial cells in the fungiform papillae. We severed both the CT and lingual nerves in rats and sutured the proximal stump of the CT to the distal stump of the lingual to promote regeneration of the CT along the lingual nerve pathway. At the same time, we prevented the proximal stump of the lingual from regenerating into the tongue. Our purpose was to determine whether and how the innervation pattern of the regenerated taste bud might be different from normal under these experimental conditions. We found that reinnervation by the CT through the lingual nerve occurs, that this restores the anatomical and functional integrity of the fungiform taste buds and papillae, and that some papillae, but not all, were richly innervated with subgemmal, extragemmal, and perigemmal neuron-specific enolase, calcitonin gene-related peptide, substance P, and neurokinin A-positive fibers. Moreover, responses to taste stimuli were recorded electrophysiologically from the CT.

Neurochemical markers in human cutaneous Merkel cells. An immunohistochemical investigation

Merkel cells (MCs) are specialized sensory cells widely distributed in the epithelia of vertebrates. A variable immunohistochemical pattern of neuronal and neurotransmitter markers has been demonstrated in MCs of several species including man. In the present study, we investigated the expression of neurochemical markers in a selected population of human cutaneous MCs by immunofluorescence. The structural neural proteins protein gene product 9.5 and neuron-specific enolase were found to be the most reliable markers for MC identification. Moreover, neurofilament immunoreactivity was shown in a small subset of epidermal MCs. Among the neurotransmitter markers, evidence for expression of calcitonin gene-related peptide, vasoactive intestinal polypeptide, peptide histidine isoleucine amide, neuropeptide Y, neurokinin A, galanin, substance P, somatostatin and phenylethanolamine N-methyltransferase was found. These immunoreactivities were highly variable as far as number of positive cells and staining intensity were concerned. The results indicate that a complex and heterogeneous immunophenotype can be expressed even within a homogeneous population of human MCs.

Neurochemical markers of human fungiform papillae and taste buds

The presence of distribution of several neurochemical markers in human fungiform papillae and taste buds were investigated by the immunohistochemical technique. The gustatory cells of the taste buds are in synaptic contact with sensory nerve endings, and considering the taste buds strictly as specialized sensory organs, the amounts and distribution of some of the neurochemical markers were different to what we expected. For example, few structures showed immunoreactivity to the tachykinins substance P (SP), calcitonin gene-related peptide (CGRP), and neurokinin A (NKA) also for the peptides vasoactive intestinal polypeptide (VIP), neuropeptide tyrosine (NPY) and galanin, low amounts of immunoreactivity occurred. On the other hand, using antibodies to protein gene product 9.5 (PGP 9.5), protein S-100, and glutamate, numerous nerve fibres and/or immunoreactive cells were found in the fungiform papillae, in the epithelium, in the connective tissue and around blood vessels, as well as in or near taste buds. Incubation with the antibodies against somatostatin, enkephalin, bombesin, peptide histidine isoleucine amide (PHI), cholecystokinin (CCK)/gastrin and dopamine-beta-hydroxylase (DBH) was negative for the fungiform papillae. In conclusion, the present study has shown several immunoreactive structures using antibodies against certain neurochemical markers. Further investigations will hopefully correlate these morphological findings with functional taste perception data. Future studies of patients with taste disorders or other pathological changes correlated with taste and tongue will also be of utmost importance.

Neuropeptides in skin

Neuropeptides are a heterogeneous group of more than 50 molecules that play a role in various cutaneous functions and diseases; they act as neuromodulators, neurotransmitters, neurohormones, and hormones. In the skin, neuropeptides are synthesized locally (i.e., in keratinocytes and in endothelial cells) and are transported by nerve fibers or immune cells (i.e., lymphocytes, monocytes, and polymorphonuclear cells). Specific receptors and binding sites for neuropeptides have been described in different cell lines in the skin (keratinocytes, endothelial cells, immune cells, fibroblasts). Many different biologic actions of neuropeptides have been demonstrated. Depletion of cutaneous neuropeptides (i.e., with capsaicin cream) or therapeutic use of neuropeptide agonists and/or antagonists may aid in the treatment of skin diseases.

Distribution and chemical phenotypes of neuroendocrine cells in the human anal canal

The presence, morphology and distribution of anal neuroendocrine cells were investigated with a panel of antisera and antibodies for neural markers, biogenic amines, and neuropeptides by the sensitive streptavidin-biotin-peroxidase immunocytochemistry, and coexistence patterns of neurochemically characterized neuroendocrine cells were examined by double immunofluorescence cytochemistry. In the colorectal zone, endocrine-like cells were immunoreactive for chromogranin A (CGA), serotonin (5-HT), pancreastatin (PST), peptide tyrosine tyrosine (PYY), glucagon-like peptide-1 (GLP-1), and somatostatin (SOM). Coexistence patterns of endocrine-like cell phenotypes with CGA and GLP-1 were heterogeneous. In the anal transitional zone (ATZ), endocrine-like cells were immunoreactive for CGA, 5-HT and PST. Furthermore, six new phenotypes of endocrine-like cells were characterized by their immunoreactivity for PYY, GLP-1, protein gene product 9.5 (PGP), calcitonin gene-related peptide (CGRP), neurotensin (NT), and SOM. All endocrine-like cell types in the ATZ were immunoreactive for CGA. In the squamous zone and perianal skin, CGA-immunopositive Merkel cells were also immunoreactive for CGRP, PST, NT and PGP. Neuroendocrine cells in the anal canal exhibit epithelial zone-related diversities in their neurochemical phenotypes and coexistence patterns, which may indicate specific regulatory functions. In the epithelium of the ATZ, which is regarded as metaplastic, endocrine-like cells expressed phenotypes characteristic of the neuroendocrine cells of the colorectal zone and the squamous zones, indicating a possible metaplastic origin of these cells.

Distribution patterns of the paraneuronal endocrine cells in the skin, gills and the airways of fishes as determined by immunohistochemical and histological methods

The neuro-endocrine cells of fish skin and respiratory surfaces, and their bioactive secretion as far as is known, are reviewed, and compared with similar elements in tetrapods, particularly amphibians. In the skin of teleost fish, immunohistochemistry has shown that Merkel cells react for serotonin, neuron-specific enolase and enkephalins. The pharmacology is not established in dipnoans or lampreys. In some teleosts, neuromasts react for substance P and leu-enkephalins; substance P is also reported from some ampullary organs (electroreceptors). Taste buds of teleosts may react for enkephalin and substance P. Basal cells of taste buds react for serotonin and neuron-specific enolase. Some unicellular skin glands of teleosts express bioactive compounds, including serotonin and some peptides; this ectopic expression is paralleled in amphibian skin glands. The dipnoan Protopterus has innervated pulmonary neuro-endocrine cells in the pneumatic duct region with dense-cored vesicles. In Polypterus and Amia the lungs have serotonin-positive neuro-endocrine cells that are apparently not innervated. In fish gills, a closed type of neuro-endocrine cell reacts for serotonin, an open type for enkephalins and some calcium-binding proteins (calbindin, calmodulin and S-100 protein). The functions of neuro-endocrine cells in fishes await investigation, but it is assumed they are regulatory.

Production and characterization of a monoclonal antibody against human calcitonin gene-related peptide (CGRP) and its immunohistochemical application to salivary glands

A monoclonal antibody (mAb), 129CD8 was raised against a C-terminal fragment (aa28-37) of alpha-human calcitonin gene-related peptide (CGRP) coupled to bovine serum albumin. The specificity of the monoclonal antibody 129CD8 was corroborated by dot immunobinding experiments, enzyme-linked immunoassay and immunostaining of tissue sections. In vitro studies showed that the mAb 129CD8 readily recognized the fragment 28-37 of alpha-human CGRP and to a slightly lesser degree whole alpha-human CGRP and the fragments containing the C-terminal part of the molecule. The mAb 129CD8 also recognized the beta-human CGRP but not the alpha-rat CGRP. The mAb 129CD8 did not react with substance P, katacalcin, calcitonin, amylin or fragments of alpha-human CGRP lacking the C-terminal part of the molecule. Immunocytochemical staining was performed on human skin, guinea-pig thyroid and salivary glands and the trigeminal ganglion, and rat thyroid gland. Our findings demonstrate, in keeping with previous studies, that in human skin, nerve fibres containing CGRP immunoreactivity are found in both epidermis and dermis. In accordance with previous investigators, the Merkel cells were immunoreactive for CGRP. In the guinea-pig and rat thyroid gland CGRP immunoreactivity was localized in the C-cells. The distribution of CGRP immunoreactivity in the guinea-pig salivary glands is different from that previously reported for rat salivary glands. In the guinea-pig trigeminal ganglion, CGRP immunoreactivity was localized mainly in small-sized neurons and fibres traversing the ganglion.(ABSTRACT TRUNCATED AT 250 WORDS)

Immunohistochemical studies of neurochemical markers in normal human buccal mucosa

The content of various substances, such as regulatory peptides, hormones and structural proteins, was investigated in normal buccal mucosa using indirect immunofluorescence. Thin nerve fibres, which from a morphological point of view were most probably sensory, showed immunoreactivity for substance P (SP), calcitonin gene-related peptide (CGRP), neuropeptide K (NPK) and neurokinin A (NKA). Also galanin (GAL), gamma-melanocyte stimulating hormone (gamma-MSH) and somatostatin (SOM) stained thin fibres were found in the propria, which were, however, few in number and the gamma-MSH staining was weak. CGRP, vasoactive intestinal polypeptide (VIP), peptide histidine isoleucine amide (PHI) and neuropeptide Y (NPY) immunoreactive nerve fibres were observed in close connection to blood vessels. SOM positive cells with processes were found, mostly scattered, in the connective tissue. A population of cells within the epithelium also showed somatostatin immunoreactivity. Protein S-100 (S-100) stained distinct populations of cells at two separate locations. In the propria, cells with one or two slender processes were seen, being mostly single but sometimes forming groups. In the epithelium, dendritic cells with many processes with or without 'spines' were observed, mainly located to the basal layer of the lamina epithelialis. Single nerve fibres and nerve bundles were also stained. Neurofilament (NF) positive fibres, singly and in bundles, as well as endorgan-like structures were seen. Neuron-specific enolase (NSE) and protein gene product 9.5 (PGP 9.5) both stained the same structures, namely single fibres, nerve bundles, nerves surrounding vessels and innervating muscles and glands (if present in the section), as well as Merkel cells. Also with these two markers endorgan-like structures were seen. No clear innervation of the epithelium could be observed with the markers used. No methionine-enkephalin (ENK) or synaptophysin (SYN) immunoreactive material was found.

Regional specificities in the distribution, chemical phenotypes, and coexistence patterns of neuropeptide containing nerve fibres in the human anal canal

Despite the pivotal clinical significance of the human anal canal, little is known about its total and specific innervation. This study assessed the comparative distribution and histotopology of nerve fibres immunoreactive for neural markers and a variety of regulatory active neuropeptides in the human anal canal by light microscopic immunohistochemistry. Depending on the epithelial zone and region of the anal canal, the neural elements were differentially immunoreactive for the pan-neural marker protein gene product 9.5, the catecholamine marker tyrosine hydroxylase, the neuroendocrine marker chromogranin A, and various neuropeptides. Protein gene product 9.5-immunoreactive nerve fibres were ubiquitously abundant in the anal canal. In the anal transitional zone, ectopic epithelial types were supplied by the same pattern of peptidergic nerves as the respective type of epithelium in normotopic location. In the dermis of the squamous zone and in the perianal epidermis, unusual distribution patterns of nerve fibres, referred to as areas of high nerve fibre density, were encountered. Double immunohistochemistry revealed region-specific coexistence patterns of neuropeptidergic nerve fibres, and novel peptide coexistence patterns were detected in anal nerve fibres. Subsets of nerve fibres formed close spatial relationships with chromogranin A-positive neuroendocrine cells, most frequently in the anal transitional zone. Chromogranin-A positive cells were shown to be present in the epithelium of perianal eccrine sweat glands. The differential distribution, peptide phenotypes and coexistence patterns of different nerve fibre populations in the human anal canal may reflect topospecific regulatory functions of neurally released neuropeptides in health and disease.

Regulation of Langerhans cell function by nerves containing calcitonin gene-related peptide

Several observations suggest interactions between the immune and nervous systems. Psoriasis and atopic dermatitis may worsen with anxiety and have been associated with anomalous neuropeptide regulation. Neurotransmitters affect lymphocyte function and lymphoid organs are innervated. Calcitonin gene-related peptide (CGRP) is a neuropeptide and vasodilator that modulates some macrophage functions, including antigen presentation in vitro. CGRP is associated with Langerhans cells (LC) in oesophageal mucosa, particularly during inflammation, is present in epidermal nerves and is associated with Merkel cells. We examined the ability of CGRP to modulate LC antigen-presenting function and asked if CGRP-containing nerves impinge on LC. We report here that CGRP-containing nerve fibres are intimately associated with LC in human epidermis and CGRP is found at the surface of some LC. In three functional assays CGRP inhibited LC antigen presentation. These findings indicate that CGRP may have immunomodulatory effects in vivo and suggest a locus of interaction between the nervous system and immunological function.

Immunohistochemical localization of neuron-specific enolase and calcitonin gene-related peptide in rat taste papillae

Calcitonin gene-related peptide-like and neuron-specific enolase-like immunoreactivity (CGRP-IR and NSE-IR) were surveyed immunohistochemically in the fungi-form, foliate and circumvallate papillae in rats. A dense CGRP-IR network (subgemmal and extragemmal) in the taste papillae is linked to the presence of taste buds, even though CGRP-IR fibers are rarely present in the taste buds. Three typical fiber populations were detected with these two markers. (a) A population of coarse NSE-IR intragemmal fibers characterized by thick neural swellings, never expressing CGRP-immunoreactivity. (b) A population of thin varicose intragemmal NSE/CGRP-IR fibers. (c) A population of subgemmal and extragemmal NSE-/CGRP-IR fibers that partly penetrated the epithelium. The common distribution of CGRP-IR and NSE-IR fibers at the base of taste buds, their differential distribution and morphology within taste buds, added to their restricted nature (gustatory or somatosensory) suggest that a population of CGRP-IR fibers undergoes a target-induced inhibition of its CGRP phenotype while entering the taste buds. The combined use of NSE and CGRP allowed a better characterization of nerve fibers within and between all three types of taste papillae. NSE was also a very good marker for a subtype of taste bud cells in the foliate and in the circumvallate papillae, but no such cells could be observed in the fungiform papillae.

Light microscopic immunoenzyme and electron microscopic immunogold cytochemistry reveal tachykinin immunoreactivity in Merkel cells of pig skin

Light microscopic (LM) immunoenzyme and electron microscopic (EM) immunogold cytochemistry were used to demonstrate the presence and subcellular distribution of tachykinin (substance P)-like immunoreactivity in Merkel cells of pig skin. Merkel cells of sinus hair follicles were strongly immunoreactive for tachykinins. In contrast, tachykinin-like immunoreactivity was absent from or very weak in epidermal Merkel cells while subepidermal and some intraepidermal nerve fibres were clearly immunopositive for tachykinins. Postembedding immunogold cytochemistry on the EM level revealed that tachykinin-like immunoreactivity was confined to the secretory granules in Merkel cells. Tachykinin immunoreactivity was clearly absent from the axon of the Merkel cell-axon complex. The selective presence of tachykinin immunoreactivity in secretory granules strongly suggests that tachykinins are synthesized in cutaneous Merkel cells, at least of pig. This is a further indication for the concept that the Merkel cell is a member of the diffuse neuroendocrine system. However, the functional role of tachykinins like substance P and neurokinin A and of other peptides present in Merkel cells remains enigmatic.