In this study, transgenic mice in which membrane-linked enhanced green fluorescent protein (mGFP) is expressed from the Thy1.2 promoter were used. In these mice, a subpopulation of small to medium sized DRG neurons double stained for IB4 but not for CGRP. Most of the peripheral terminals traversed the dermis and ramify within the epidermis and form superficial terminals. Within the spinal cord, these afferents terminated exclusively within the substantia gelatinosa (SG). A second fibre type in the skin also expressed mGFP, and formed club-shaped endings towards the bases of hairs. Injury to the sciatic nerve resulted in mGFP loss from the SG ipsilateral to the nerve injury, but also in the corresponding region contralaterally. Together, these findings reveal the specificity of connectivity of a defined subpopulation of DRG sensory neurons innervating the epidermis and this will facilitate analysis of their physiological functions.
"However, many current promoters used to drive fluorescent protein expression result in labeling of large neuronal populations, which share a certain transmitter system or receptor type. Transgenic mice expressing fluorescent proteins only in subsets of spinal cord neurons could be an increasingly valuable tool to investigate spinal circuits on a finer scale (Belle et al., 2007; Heinke et al., 2004; Torsney et al., 2006). "
[Show abstract][Hide abstract] ABSTRACT: Two-photon microscopy enables high-resolution in vivo imaging of cellular morphology and activity, in particular of population activity in complex neuronal circuits. While two-photon imaging has been extensively used in a variety of brain regions in different species, in vivo application to the vertebrate spinal cord has lagged behind and only recently became feasible by adapting and refining the experimental preparations. A major experimental challenge for spinal cord imaging is adequate control of tissue movement, which meanwhile can be achieved by various means. One set of studies monitored structural dynamics of neuronal and glial cellular components in living animals using transgenic mice with specific expression of fluorescent proteins. Other studies employed in vivo calcium imaging for functional measurements of sensory-evoked responses in individual neurons of the dorsal horn circuitry, which at present is the only part of rodent spinal cord grey matter accessible for in vivo imaging. In a parallel approach, several research groups have applied two-photon imaging to sensorimotor circuits in the isolated spinal cord (in vitro) to provide complementary information and valuable new perspectives on the function of specific interneuron types in locomotor-related networks. In this review we summarize recent results from these types of high-resolution two-photon imaging studies in the spinal cord and provide experimental perspectives for improving and extending this approach in future applications.
"The other group (nonpeptidergic afferents) lacks these peptides, expresses the ATP-gated ion channel P2X3, and (at least in rodents) binds the lectin IB4 (Snider and McMahon, 1998). The majority (~70%) of epidermal afferents are nonpeptidergic, while about 30% are peptidergic (O'Brien et al., 1989; Lu et al., 2001; Belle et al., 2007). In contrast to the epidermis, which is largely avascular, the dermis is highly vascularized and densely innervated by peptidergic axons (Fundin et al., 1997). "
[Show abstract][Hide abstract] ABSTRACT: Sensory neurons that innervate the skin provide critical information about physical contact between the organism and the environment, including information about potentially-damaging stimuli that give rise to the sensation of pain. These afferents also contribute to the maintenance of tissue homeostasis, inflammation and wound healing, while sensitization of sensory afferents after injury results in painful hypersensitivity and protective behavior. In contrast to the traditional view of primary afferent terminals as the sole site of sensory transduction, recent reports have lead to the intriguing idea that cells of the skin play an active role in the transduction of sensory stimuli. The search for molecules that transduce different types of sensory stimuli (mechanical, heat, chemical) at the axon terminal has yielded a wide range of potential effectors, many of which are expressed by keratinocytes as well as neurons. Emerging evidence underscores the importance of nucleotide signaling through P2X ionotropic and P2Y metabotropic receptors in pain processing, and implicates nucleotide signaling as a critical form of communication between cells of the skin, immune cells and sensory neurons. It is of great interest to determine whether pathological changes in these mechanisms contribute to chronic pain in human disease states such as complex regional pain syndrome (CRPS). This review discusses recent advances in our understanding of communication mechanisms between cells of the skin and sensory axons in the transduction of sensory input leading to pain.
Brain Research Reviews 04/2009; 60(1):24-35. DOI:10.1016/j.brainresrev.2008.12.013 · 5.93 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The skin is innervated by two populations of unmyelinated sensory fibers, the peptidergic and nonpeptidergic, which transmit nociceptive information to the central nervous system. The peptidergic population expresses neuropeptides such as substance P (SP) and calcitonin gene-related peptide (CGRP) and has both cutaneous and visceral targets. The nonpeptidergic population expresses the purinergic receptor P2X(3), binds the isolectin B4 (IB4), and innervates mainly the epidermis. To date, the peptidergic nociceptor population in cutaneous tissue of the rat has been well characterized, whereas the nonpeptidergic innervation pattern has lacked an adequate description. To this aim, we used light microscopic immunocytochemistry to investigate the pattern of P2X(3)-immunoreactive (-IR) fiber innervation of both hairy and glabrous skin from male Sprague-Dawley rats. Our results show extensive P2X(3)-IR fibers throughout the upper and lower dermis. Thick bundles of P2X(3)-IR fibers were found to run in parallel with the dermal-epidermal junction and projected multiple thin collateral axons that penetrated the epidermal layer, creating a dense network of innervation throughout the entire epidermis. The distribution of P2X(3)-IR fibers in the epidermis was far more extensive than the distribution of CGRP-IR fibers. P2X(3)-IR fibers also innervate hair follicles but were rarely found in close proximity to glands and blood vessels. The present results suggest a primary role for P2X(3)-IR fibers in the detection of noxious stimuli in cutaneous tissue and provide an anatomical basis for future studies examining a possible functionally distinct role of nonpeptidergic nociceptors in the transmission of nociceptive signals.
The Journal of Comparative Neurology 06/2009; 514(6):555-66. DOI:10.1002/cne.22048 · 3.23 Impact Factor
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