Glial Cell Line-Derived Neurotrophic Factor Expression in Skin Alters the Mechanical Sensitivity of Cutaneous Nociceptors

Abstract

Neurons classified as nociceptors are dependent on nerve growth factor (NGF) during embryonic development, but a large subpopulation lose this dependence during embryonic and postnatal times and become responsive to the transforming growth factor ␤ family member, glial cell line-derived growth factor (GDNF). To elucidate the functional properties of GDNF-dependent nociceptors and distinguish them from nociceptors that retain NGF dependence, the cellular and physiologic properties of sensory neurons of wild-type and transgenic mice that overexpress GDNF in the skin (GDNF-OE) were analyzed using a skin, nerve, dorsal root ganglion, and spinal cord preparation, immunolabeling, and reverse transcriptase-PCR assays. Although an increase in peripheral conduction velocity of C-fibers in GDNF-OE mice was measured, other electrophysiological properties, including resting membrane potential and somal action potentials, were unchanged. We also show that isolectin B4 (IB4)-positive neurons, many of which are responsive to GDNF, exhibited significantly lower thresholds to mechanical stimulation relative to wild-type neurons. However, no change was observed in heat thresholds for the same population of cells. The increase in mechanical sensitivity was found to correlate with significant increases in acid-sensing ion channels 2A and 2B and transient receptor potential channel A1, which are thought to contribute to detection of mechanical stimuli. These data indicate that enhanced expression of GDNF in the skin can change mechanical sensitivity of IB4-positive nociceptive afferents and that this may occur through enhanced expression of specific types of channel proteins.

Myelinated Skin Sensory Neurons Project Extensively throughout Adult Mouse Substantia Gelatinosa

“…In accord with seminal in vivo studies in guinea pig (Sugiura et al, 1986) and in vitro studies in mice (Woodbury et al, 2001(Woodbury et al, , 2004Albers et al, 2006), all labeled C-nociceptors terminated in the SG and, to a lesser extent, marginal zone (lamina I). Most (12/13) were C-polymodal nociceptors that responded to moderately firm stroking of the RF with a fine paintbrush (mechanical threshold, 0.63 Ϯ 0.29 mN; range, 0.16 -1.24 mN) as well as heat (threshold, 42.2 Ϯ 3.2°C; range, 38 -46°C) and cold (threshold, 16.3 Ϯ 4.5°C; range, 10 -21°C).…”

“…Thus, these afferents exhibited a broader CV range than typically reported, and in the absence of information on central anatomy (below), those with the most slowly conducting axons could easily be mistaken for unmyelinated (C) low-threshold mechanoreceptors. Unlike the latter (n ϭ 2; not labeled) , all cells classified as D-HFAs responded equally well to both fast-and slow-moving stimuli and were silent during cooling (data not shown) (for C-LTMR cooling responses, see Albers et al, 2006) when care was taken to prevent skin movement.…”

Artemin Overexpression in Skin Enhances Expression of TRPV1 and TRPA1 in Cutaneous Sensory Neurons and Leads to Behavioral Sensitivity to Heat and Cold

“…ASICs are protongated sodium channels that are proposed to underlie acid-evoked pain and at least some aspects of mechanical sensation (Price et al, 2001;Benson et al, 2002;Krishtal, 2003;Jones et al, 2005;Albers et al, 2006). The relative expression of ASIC1, ASIC2a, ASIC2b, and ASIC3 was determined using RT-PCR assays of pooled lumbar ganglia.…”

“…For cutaneous sensory neurons, GDNF supports development of small neurons with nociceptive properties. These neurons express GFR␣1, are peptide poor, bind the lectin isolectin B4 (IB4), and some appear to be mechanonociceptors (Silverman and Kruger, 1988;Molliver et al, 1997;Bennett et al, 1998;Albers et al, 2006). The role of neurturin-responsive, GFR␣2-positive neurons is less clear.…”

Glial Cell Line-Derived Neurotrophic Factor Family Members Sensitize NociceptorsIn Vitroand Produce Thermal HyperalgesiaIn Vivo

“…Although initial studies of sensory neuron growth factors focused on their importance for developmental survival, we now know that they also regulate functional properties of sensory neurons Ritter et al, 2001;Albers et al, 2006). In both rodents and humans, in vivo injection of NGF produces thermal hypersensitivity within 30 min and mechanical sensitivity within hours (Lewin et al, 1994;Dyck et al, 1997).…”