Several groups (e.g. Bennett et al. 1998; Molliver et al. 1995, 1997) have divided small DRG neurones, hypothesised to be nociceptive, into two subpopulations, those that express trkA, the high-affinity receptor for nerve growth factor (NGF), and those that bind the isolectin IB4 and also express receptor components for glial derived neurotrophic factor (GDNF). Some co-localisation was found between these two markers. We have examined in the physiologically identified neurones: (1) the hypothesis that both groups are nociceptive; (2) the relative intensity of staining of both trkA-like immunoreactivity (trkA-LI) and IB4 binding in same neurones; and (3) the differences in properties of nociceptive neurones that are positive and negative for trkA and IB4.
All experimental procedures were carried out under licence according to the UK Animals (Scientific Procedure) Act of 1986. Wistar rats were deeply anaesthetized with pentobarbital (65 mg kg-1, I.P.) and injected regularly with the neuromuscular blocker pancuronium (0.5 mg kg-1, I.A.) always given with an additional dose (20 mg kg-1, I.A.) of anaesthetic. End-tidal CO2 and blood pressure were monitored throughout. After identification of sensory and electrophysiological properties in L5 DRG neurones, fluorescent dye was injected into the neurones. The animals were killed with an overdose of anaesthetic. Immunocytochemistry to show trkA-LI and IB4 binding was carried out on 7 µm sections of these neurones.
Most C-fibre units were trkA positive (nociceptive 9/11; unresponsive 7/10), and showed intense IB4 binding (nociceptive 8/11; unresponsive 9/12). Surprisingly, a weak inverse linear correlation between trkA-LI and IB4 binding intensity was found in those C-fibre neurones (n = 9) that were positive for both. All Aδ-fibre nociceptive units (13/13) and most Aα/β-fibre nociceptive units (15/18) showed trkA-LI, while no A-fibre nociceptive units (n = 33) were IB4 positive. No Aα/β-fibre low-threshold mechanoreceptive units (LTMs) showed either trkA or IB4, although some Aδ-fibre LTMs (D hair) were both weakly trkA-LI (2/4) and IB4 (3/4) positive. IB4-positive C-fibre neurones tended to have longer duration action potentials (APs) and AP rise times than IB4-negative neurones. In Aα/β nociceptive units, trkA-LI intensity was inversely related to conduction velocity and neurones with deeper receptive field in skin tended to have more intense trkA-LI than neurones with superficial receptor field.
Intense trkA was in most, but not all, nociceptive and unresponsive units within all ranges of conduction velocity, while intense IB4 binding was in most C-fibre nociceptive and unresponsive units. These patterns support the view that most nociceptive units may be NGF dependent, but raise the possibility that some C-fibre units may be influenced by both NGF and GDNF.S.N.L. received a grant from The Wellcome Trust.
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Molliver, D.C., Radeke, M.J., Feinstein, S.C. & Snider, W.D. (1995). J. Comp. Neurol. 361, 404.
Molliver, D.C., Wright, D.E., Leitner, M.L., Parsadanian, A.S., Doster, K., Wen, D., Yan, Q. & Snider, W.D. (1997). Neuron 19, 849.