The primary role of skin nerve endings is to sense and respond to external factors, as well as to provide the body with an organized means of protection from environmental threats [1]. Afferent fibers, unmyelinated C-fibers, myelin-type Aδ fibers and autonomic nerve fibers are present in the skin, and are characterized by a dense distribution throughout all of its layers. Neuropeptides are released from these fibers, and are also part of the cutaneous neuroendocrine system and stimulated by nociceptive stimuli ^[2][3][2,3]. The role of neuropeptides (neuromodulators, neurotransmitters and neurohormones) in the regulation of lympho- cells, mast cells and other cells of the immune system consists in the transduction of neurological impulses from afferent nerve fibers to signals that can be read by immunocompetent cells, and this carries the potential to exacerbate the inflammatory response ^[4][5][6][7][4,5,6,7]. The observation that various chronic inflammatory skin disorders, e.g., atopic dermatitis and psoriasis, are characterized by enhanced neurotrophin expression and peptidergic nerve fibers supports these pathophysiologic phenomena [8]. Various chronic inflammatory skin disorders, such as atopic dermatitis, prurigo nodularis, rosacea and psoriasis, exhibit increased expression of neurotrophins and the presence of nerve fibers that contain peptides. These observations provide evidence that these diseases share similar pathophysiological mechanisms. The neuropeptides released from nerve fibers can stimulate keratinocytes, which then trigger the release of proinflammatory cytokines such as IL-1α, IL-6 and IL-8 in the epidermis ^[2][9][10][2,9,10]. The epidermis undergoes close interaction with nerve endings, as well as the epidermis and nerves, thus producing factors for mutual communication. Secreted from the skin nerve endings, neuropeptides, such as SP (substance P) and CGRP, bind to receptors on the surface of mast cells and activate them, leading to degranulation and the release of many pro-inflammatory cytokines and vasoactive amines. SP, CGRP and VIP are powerful histamine releasers from mast cells, and function via an independent reaction with IgE bound to the surface of mast cells. Tachykinins directly cause dilatation and increase capillary permeability, which leads to edema and indirectly causes erythema by releasing histamine from mast cells. The mediators released during mast cell degranulation increase inflammation. Mrgprs (Mas-related G-coupled protein receptors), TRPA1 and PAR-2 (protease-activated receptor 2) ^[11][12][11,12] play a significant role in inducing itching and inflammation. When the MrgprX1 receptor is activated, it triggers the degranulation of mast cells. This, in turn, can result in the development of neurogenic inflammation through communication with cutaneous and sensory nerve cells [4]. As a result of this physiological process within the skin, mediators are released directly from the cutaneous nerves and initiate an inflammatory response, which leads to erythema, swelling and pain.

Cutaneous neurogenic inflammation is a common element of chronic inflammatory skin disorders such as psoriasis, atopic dermatitis (AD), sensitive skin [13], rosacea [14], and hypertrophic scars [15].

The neuro-immuno-cutaneous (NIC) and neuro-immuno-cutaneous-endocrine (NICE) systems are based on a complex and ongoing communication network involving neuropeptides, cytokines, neurotransmitters, small molecules and other less defined factors, such as psychological stress [16]. These elements collaborate to maintain skin homeostasis, allowing the skin to detect and interpret environmental changes through the cutaneous neuroendocrine system, which uses chemical, physical and biological signals to regulate both local and global homeostasis. Neurogenic factors play an important role in the pathogenesis of skin inflammation, and there is a close relationship between the peripheral and central nervous systems, as well as between the endocrine and immune systems. The presence of numerous nerve cell endings in the skin, its rich vascularity, and the fact that it is the largest and most exposed organ to the action of harmful factors emphasizes its unique and important role in the pathogenesis and regulation local inflammation [1].

The expression of receptors for neuropeptides (NPs), such as SP, has been found on endothelial cells, where after the activation of NK-1R receptors, endothelial cell proliferation and vascularization occur, increasing the expression of (VCAM)-1. ICAM-1 expression is increased both by the direct action of SP via NK-1R and by TNF-α, IL-1 and IFN-γ ^{[4][17][18][19]}[4,17,18,19].

SP, together with CGRP released from peripheral nerve endings under the influence of a nociceptive stimulus, induces the translocation of P-selectin to the membranes of endothelial cells and the expression of E-selectin, intensifying inflammation. In addition, SP enhances the migration and endothelial adhesion of leukocytes and monocytes, and affects the vascular and cellular components of inflammation ^{[6][20][21][22]}[6,20,21,22].

The presence of neurotrophins (NTs) in the skin is crucial at every stage of the inflammatory response. Neuropeptides produced by cells present in the skin belong to three groups: opioid and non-opioid neuropeptides, and neurotrophins. Receptors for neurotrophins are found on keratinocytes, hair follicles, inflammatory cells such as T lymphocytes, macrophages, leukocytes and MCs. The most important neurotrophins produced in the skin include NT-3, NT-4, NGF (nerve growth factor) and BDNF (brain-derived neurotrophic factor). NGF is synthesized and released by keratinocytes, Merkel cells, fibroblasts and mast cells. NGF is the most important neurotrophic factor in dermal sensory nerves [6]. Skin diseases, which are clinically characterized by intense itching and histologically characterized by an increased number of nerve fibers in the skin, are regulated by neurotrophins ^[16][23][24][16,23,24].

The calcitonin gene-related peptide (CGRP) is one of the most prominent neuropeptides and is localized throughout the peripheral and central nervous system [25].

In thin unmyelinated sensory fibers in the dermal papillae, and in epidermal free nerve endings present in the dermal papillae, CGRP coexists with SP and causes pruritus. CGRP is also found in the perivascular region and is responsible for vasodilation in the skin without causing pruritus [26]. Neuroimmune reactions of the skin are bidirectional. The cutaneous nervous system can also be activated by cytokines released by immune cells [27]. The immune system takes note of pathogenic events through a set of receptors that recognize pathogen-associated molecular patterns (PAMPs), e.g., LPS and CpG, and damage-associated molecular patterns (DAMPs); some examples include high-mobility group box 1 (HMGB1), S100 proteins, and heat-shock proteins (HSPs) ^[28][29][30][28,29,30]. After binding with PAMPs or DAMPs, the described pattern-recognition receptors (PRRs), including Toll-like receptors (TLRs) and IL-1R, elicit inflammatory and immune responses through signaling to nuclear factor κB (NF-κB), thus inducing the expression of proinflammatory cytokines, for example, IL-1, -6, -31, IFN-γ (interferon-γ) and TNF-α (tumor necrosis factor-α) [31]. The cytokines that are released play the roles of ligands and activators of sensory nerves, and downstream neuronal effects take place. One example is IL-6, which triggers the expression of NGF and NT-3, 4 and 5; on the other hand, IL-31 exerts pruritic effects ^[32][33][34][32,33,34].

Melanocytes and sensory nerve endings interact via CGRP, which affects melanocytes by upregulating melanogenesis and increases melanocyte dendriticity by inducing keratinocyte-derived melanotropic factors [27]. CGRP affects the melanogenesis process when the skin is exposed to CGRP and when melanocytes are stimulated. During the melanogenesis process, the addition of CGRP-stimulated keratinocyte conditioned medium (CGRP-KCM) has been shown to stimulate melanogenesis; therefore, it is likely that keratinocytes produce melanotropic factors when stimulated with CGRP [6].

Substance P (SP) is a protein that consists of 10 amino acids. Together with neurokinin A and neurokinin B, it belongs to the family of neuropeptide tachykinins, whose wide range of activity is possible due to their presence in the nervous, digestive and immune systems. When substance P is released, it binds to its NK-1R receptors on various target cells. The linkages between neuropeptides and immune cells play an important role in the modulation of the inflammatory neurogenic process ^[35][36][35,36].

Substance P is released from the terminals of afferent unmyelinated C-fibers and myelin-type delta A-fibers in response to nociceptive stimulation [37], and plays a bigger part in itching than in pain [22]. It binds to keratinocytes or MCs [38], or induces the release of interleukins (as well as other cytokines) [39]. RWesearchers can distinguish three main directional effects of SP activity: vasodilation, the activation of B lymphocytes and the increased proliferation of keratinocytes and fibroblasts ^{[22][34][38][40][41]}[22,34,38,40,41]. SP plays an important role in AD, where the degranulation of mast cell granules leads to the release of proteases and histamine. The secondary mediators are leukotrienes and prostaglandins, while the substances secreted after the activation of MCs are interleukins such as IL-1, IL-2, IL-4, TNF-α and INF-γ ^[42][43][42,43]. At the level of the spinal cord, SP plays a role in pain neurotransmission and the modulation of autonomic stimuli. In the peripheral nervous system, SP receptors have been demonstrated on primary sensory neurons, where SP is regulated by nerve growth factor (NGF) [39]. SP initiates the degranulation of MCs, resulting in the release of activators of the inflammatory process and the hypervascularization and infiltration of mononuclear cells ^[34][38][41][34,38,41].

3. Receptors in Neuro-Immune Interaction

3.1. Neurokinin Receptor (NK-R)

Cells that are resident and temporarily present in the skin express different types of receptor for neuropeptides. SP, NKA and NKB bind to the G protein-coupled receptors NK-1R, NK-2R and NK-3R, respectively ^[44][52]. When NK-1R is activated via SP, it causes multiple signaling cascades that involve the degranulation of mast cells and release of proinflammatory mediators; examples include histamine, and NGF expression and the production of leukotriene B4 (LTB4) in keratinocytes, leading to neurogenic inflammation and pruritus ^[45][46][47][53,54,55]. Several studies have researched the role played by SP and NK-1R in the mechanism of itching in various diseases such as atopic dermatitis, psoriasis and chronic idiopathic urticaria (CSU) ^[48][49][56,57]. It is suggested that NGF and its receptor TrkA play a major role in pruritus and allergic diseases. In an active process of inflammation, NGF expression is markedly upregulated in nerves related to the inflamed area, while increased levels of NGF are associated with skin dermatoses such as psoriasis ^[50][58]. The role of NGF consists in the maintenance, proliferation and growth of nerve cells. The process of cutaneous inflammation involves the NGF-dependent production of SP, CGRP and other neurotransmitters and neuropeptides or molecules linked with nociception. Furthermore, NGF has a direct stimulating effect on the degranulation of mast cells, enhancing the count of mast cells in peripheral tissues and favoring the growth of myeloid cells ^[51][59], thus promoting the survival of several immune cells in the cutaneous system, including eosinophils, monocytes, neutrophils, T cells and macrophages. NGF also induces the proliferation and differentiation of B cells and encourages the release of histamine from basophils. NGF can also stimulate IL-1 expression in PC12 cells and suppress the production of LTC4 in human eosinophils ^[44][52][53][52,60,61].

3.2. Tropomyosin Receptor Kinase A

The first information about TrK receptors appeared in 1986. A family of tyrosine kinase receptors, which include TrkA, TrkB and TrkC, was isolated. Trk receptors are stimulated by multiple neurotrophins, including NGF, BDNF, NT-3 and NT-4 ^[44][54][52,62]. Keratinocytes are the most important NGF exit point in the skin. In addition, NGF is produced by immune cells and neurons in a dynamic inflammatory process ^[55][56][63,64]. Increased concentrations of cytosolic Ca²⁺ induce the release of neuropeptides from the sensory nerves in the skin. There are five important GPCRs that play leading roles in neurogenic inflammation, including PAR-2 and PAR-4, and the Mas-related G-coupled protein receptors C11, A3 and X ^[57][58][65,66], in addition to the temporary receptor capacity of the vanilloid TRPV1 and the ankyrin TRPA1 [11].

3.3. Mas-Related G-Coupled Protein Receptors (Mrgprs)

In the Mrgprs family, rwesearchers distinguish nine subgroups from MrgprA to MrgprH and MrgprX ^[59][67], which are characterized by low specificity for ligands and potentially high specificity for itching substances. This extensive group of receptors is characterized by low ligand specificity, and could have the best affinity for itch-inducing substances. Activation of the MrgprA3, C11 and X1 receptors is responsible for the peripheral itching sensation and scratching behavior. The receptors MrgprA3 and MrgprC11 are present on mast cells and on nerve endings, as well as on non-neuronal cells ^[60][68]. Activation of the Mrgpr receptor on mast cells causes itching and triggers strong scratching behavior, damaging the skin barrier, and thus, its immune homeostasis. When activated, MrgprX1 triggers the degranulation of mast cells and causes the engagement of sensory nerves and cutaneous cells in the development of neurogenic inflammation ^[61][69]. MrgprA3 and C11 are involved in the production of certain neuropeptides by sensitizing the TRPA1 and TRPV1 channels located on sensory nerve endings ^[62][63][70,71]. Mrgprs and the TRPA1 and PAR-2 receptors play a major role in itching and skin inflammation [4].

3.4. Transient Receptor Potential (TRP)

The temperature-sensitive channels, which are part of the transient receptor potential (TRP) superfamily, play a major part in the biology of the skin. Inflammatory processes within the skin are triggered by the activation of TRPV1 and TRPA1 receptors and result in the development of neurogenic inflammation in conditions such as psoriasis and AD ^{[64][65][66][67]}[72,73,74,75]. The produced neuropeptides affect skin cells that increase the expression of similar neuropeptide receptors, including microvascular and MC cells, which results in vasodilation, degranulation and the release of plasma proteins and white blood cells ^[4][68][4,76]. The increase in the level of Ca²⁺ in the cytosol causes the exocytosis of neuropeptides and inhibits or stimulates the potency of several inflammatory genes that encode cytokines, neuropeptides and matrix metalloproteinases (MMPs), playing a leading role in dermatitis ^[69][70][77,78]. Cation channels containing the TRP receptor are involved in the exocytosis of neuropeptides responsible for the mechanism of neurogenic inflammation. The temperature-dependent nociceptive cation channel TRPV1 responds to high temperature (>43 °C) and its agonist capsaicin, which is found in chilies peppers ^[71][79]. The activation of TRPV1 and rapid Ca²⁺ influx release neuropeptides such as SP and CGRP and contribute to neurogenic inflammation. During cutaneous neuritis, the released neuropeptides and other mediators sensitize or activate TRPV1, leading to the maintenance of CNI ^[6][72][6,80]. TRPV1 is present in skin cells, such as keratinocytes, mast cells and dendritic cells, that act as pain sensors and chemical stimuli ^[73][81]. TRPA1 modulates the inflammatory response in keratinocytes by intensifying the potency of pro-inflammatory cytokines and prostaglandin E2 (PGE₂) ^[74][82], which are involved in cutaneous inflammation and pruritus. They also activate the growth of HSP, which is responsible for the increase in pro-inflammatory cytokines in allergic skin diseases ^[75][76][83,84]. In conclusion, the activation of TRPA1 causes the production of several inflammatory mediators by keratinocytes. TRP ion channels are involved in cutaneous thermosensation, osmoregulation and inflammation, as well as cellular growth. When pathological conditions such as inflammation or tissue injury are present, TRP is involved in signaling painful and pruritic stimuli to the CNS. Therefore, the identification of ion channels that detect heat or cold provides major insight into the molecular foundations of neurogenic inflammation, pain and pruritus. Furthermore, some TRPs (TRPV1 and TRPV4) appear to play a direct part in peripheral neurogenic inflammation ^[77][78][79][85,86,87]. TRPV1 and TRPA1 play a leading role in neurogenic dermatitis by means of the release of neuropeptides (including SP and CGRP) and pro-inflammatory cytokines ^[72][80][80,88].

3.5. Role of Protease-Activated Receptors (PARs)

TRPV1, TRPA1 and proteases PAR-2 and PAR 4 cause an increase in intracellular Ca²⁺ (iCa²⁺) concentration, the exocytosis of neuropeptides, and the expression of pro-inflammatory genes, leading to the development of CNI ^[70][81][78,89]. PARs are G protein-coupled receptors, of which there are four different subtypes: PAR1-4. They are so-called ‘‘alarm receptors’’ that do not possess classic ligands, but are activated by N-terminal proteolytic cleavage and by environmental proteases. The activation of PAR2 and PAR4 has been attributed to itching or pain in atopic dermatitis. PAR2 is expressed by keratinocytes, ECs, MCs and sensory nerves. PAR 2 stimulation leads to the release of itch-inducing factors (e.g., ET-1, IL-33, TSLP and SP) from keratinocytes, EC, nerves and other neuroinflammatory mediators (TSLP and kallikrein) ^{[73][82][83][84]}[81,90,91,92]. There are receptors for TSLP and PAR2 on nerve endings. Proteases activate the pathways of C-fiber excitation by binding to the PAR2 receptor. In AD, as a result of mast cell degranulation, tryptase is released and nerve endings are stimulated, which leads to histamine-independent pruritus [27]. Sensory neurons and keratinocytes harness the power of PAR2, which is stimulated by proteases triggered by degenerated MCs. In AD, the levels of tryptase and PAR2 are elevated in the patient’s skin, and the excessive secretion of PAR2 in keratinocytes is sufficient to cause AD-like changes ^[12][84][12,92]. AR-2 activation results in the stimulation of TRPV4 channels in dorsal root ganglion (DRG) neurons and of the NF-κB pathway in keratinocytes ^[85][86][93,94]. After the secretion of proteases by MCs, activated PAR2 stimulates endogenous inflammation, itching and pain, which are dependent on CGRP and substance P in AD patients. In an addition, exogenous factors such as allergens can activate PAR-2, thereby contributing to itching and pain ^[80][87][88,95]. TRPV1, TRPA1, and PAR-2 and PAR-4 proteases are also present in cells that reside in and infiltrate the skin during CNI, which enhances skin signaling and may exacerbate AD ^[88][96].