The DRG and
TG contain calcium-binding proteins (CaBPs) such as parvalbumin, calretinin, S100 proteins and neurocalcin [13], [14], [15], [16], [17] and [18]. These CaBPs are localized in medium-sized to large neurons with myelinated axons. Previous immunohistochemical studies have demonstrated that encapsulated and unencapsulated corpuscular endings selleck compound in the skin and oral mucosa contain CaBPs [19], [20] and [21]. Thus, CaBP-containing neurons are considered to include low-threshold mechanoreceptors. In addition, large DRG neurons which contain CaBPs send their peripheral axons to muscle spindles [13], [14], [15] and [17]. These findings suggest that CaBPs are also markers for muscular proprioceptors in the DRG. However, cell bodies of proprioceptors innervating masticatory muscles and periodontal ligament are located in the mesencephalic trigeminal tract nucleus (Mes5). Because primary neurons in the Mes5 contain parvalbumin [22], [23] and [24], the CaBP is recognized to be a marker for primary proprioceptors
in the trigeminal nervous system. The development and survival of vertebrate neurons depends on neurotrophic factors such as nerve growth factor (NGF), brain derived neurotrophic factor (BDNF), neurotrophin-3 (NT3) and neurotrophin-4 (NT4) [25], [26], [27], [28], [29], [30], [31], [32], [33] and [34]. These neurotrophins are capable of promoting the survival of specific neuronal populations through their interaction with the tyrosine kinase receptors trkA, trkB and trkC [26], [28], [30], [33] and [34]. NGF binds to trkA. BDNF and LY2109761 purchase NT-4 signal through trkB. NT-3 binds to trkC and, to a lesser degree, trkB. Previous studies have demonstrated that the deletion of genes for these trks results in loss or decrease of various types of sensory neurons in the spinal
nervous system [25], [26], [27], [28], [29], [31] and [32]. In addition, the deficiency of intracellular proteins affects the development of nociceptive and proprioceptive neurons in the DRG [35] and [36]. The present review will focus on the developmental dependency of primary sensory neurons on neurotrophins and other proteins second in the trigeminal nervous system. The number of TG neurons is severely decreased in trk-knockout mice as compared to wildtype and heterozygous mice (82%, 39%, and 48% reduction for trkA, trkB and trkC, respectively) [37]. In trkA- or trkC-deficient mice, the number of TG neurons with a variety of cell body sizes is decreased. However, the absence of trkB causes the reduction of medium-sized and large TG neurons. The loss of NGF and trkA results in the disappearance of CGRP-containing neurons in the DRG and their central axons in the superficial laminae of the spinal cord [26] and [29]. In addition, CGRP-containing free nerve endings are absent in the skin of trkA null mutant mice.
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