Introduction: Hyperexcitability of primary sensory neurons and its most extreme form, spontaneous activity, are key cellular-level drivers of neuropathic pain. Though extensively studied in animal models of neuropathic pain and established as a phenomenon occurring in human primary sensory neurons, this altered electrophysiology has not been rigorously studied for human primary sensory neurons nor has its relationship to clinical symptoms of neuropathic pain been established [1-10].
Methods: The study was approved by the M.D. Anderson IRB. Written informed consent for participation was obtained from each tissue donor.
Human dorsal root ganglia and medical histories were obtained from patients undergoing oncological spine surgery that necessitated sacrifice of spinal nerve roots as part of standard of care. Clinical data regarding presence of radicular/neuropathic pain was obtained through retrospective review of medical records or collected at time of study enrollment.
Neurons were dissociated from surrounding tissue, briefly maintained in cell-culture (24-72 hours), and examined with whole-cell patch clamp techniques.
Results: Electrophysiological recordings were obtained from a total of 110 neurons, dissociated from 23 dorsal root ganglia, donated by 13 patients.
Spontaneous activity was noted in 15% (12/79) of neurons from ganglia with pain in a corresponding dermatome vs 0% (0/31) of neurons from pain free ganglia (P < 0.05)
Compared to neurons without spontaneous activity, human sensory neurons with spontaneous activity had significantly altered intrinsic membrane properties; depolarized resting membrane potential, hyperexcitability, and altered action potential kinetics (all P < 0.05).
Conclusions: Utilizing whole-cell patch clamp of dissociated human primary sensory neurons from patients both with and without neuropathic pain this study presents two important new findings: 1) first demonstration of a statistically significant association between in vitro spontaneous activity of dissociated human primary sensory neurons and neuropathic pain 2) the first characterization of the altered intrinsic membrane properties associated with spontaneous activity in human primary sensory neurons.
Patient Care: Unfortunately, several promising molecularly targeted treatments for neuropathic pain derived from discoveries in animal models have failed to translate into meaningful clinical treatments following unsuccessful clinical trials [11-13]. Though a variety of factors have been proposed as possible causes for these failures, basic cellular-level and molecular differences between animals and humans is commonly implicated [14, 15]. Given the contribution of these fundamental differences to failures in translation of molecular discoveries, it may be possible to achieve more efficient translation by validation of newly discovered molecular targets in human tissues or cells prior to engaging in clinical trials.
This study’s key result, confirmation that in vitro spontaneous activity of human sensory neurons is associated with clinical neuropathic pain, suggests spontaneous activity in cultured human sensory neurons could be used as an in vitro surrogate for neuropathic pain. This premise, combined with the presented human cell culture and electrophysiological methods makes it feasible to easily screen novel therapeutics for “analgesic” activity on human neurons prior to engaging in complex, costly human clinical trials.
This kind of in vitro screening holds promise in improvement in patient care by improving the efficiency of translating benchtop discoveries from animal models into effective clinical treatments and the presented research provides the foundations to achieve such an end.
Learning Objectives: By the conclusion of this session, participants should be able to:
1) Understand the utility and basic methodology of harvest, culture, and experimentation with human dorsal root ganglion neurons.
2) Understand the relationship of primary sensory neuron spontaneous action potential activity with human symptoms of neuropathic pain.
3) Understand the association of altered membrane properties associated with spontaneous activity
References: 1. Baumann, T.K., et al., Responses of adult human dorsal root ganglion neurons in culture to capsaicin and low pH. Pain, 1996. 65(1): p. 31-8.
2. Baumann, T.K., P. Chaudhary, and M.E. Martenson, Background potassium channel block and TRPV1 activation contribute to proton depolarization of sensory neurons from humans with neuropathic pain. Eur J Neurosci, 2004. 19(5): p. 1343-51.
3. Baumann, T.K. and M.E. Martenson, Spontaneous action potential discharge in cultured dorsal root ganglion neurons from patients with neuropathic pain. Proceedings of the 9th World Congress on Pain, 2000. 16: p. 101-108.
4. Burchiel, K.J., et al., Spontaneous activity of primary afferent neurons in diabetic BB/Wistar rats. A possible mechanism of chronic diabetic neuropathic pain. Diabetes, 1985. 34(11): p. 1210-3.
5. Devor, M., Ectopic discharge in Abeta afferents as a source of neuropathic pain. Exp Brain Res, 2009. 196(1): p. 115-28.
6. Djouhri, L., et al., Spontaneous pain, both neuropathic and inflammatory, is related to frequency of spontaneous firing in intact C-fiber nociceptors. J Neurosci, 2006. 26(4): p. 1281-92.
7. Kirk, E.J., Impulses in dorsal spinal nerve rootlets in cats and rabbits arising from dorsal root ganglia isolated from the periphery. J Comp Neurol, 1974. 155(2): p. 165-75.
8. Ma, C. and R.H. LaMotte, Multiple sites for generation of ectopic spontaneous activity in neurons of the chronically compressed dorsal root ganglion. J Neurosci, 2007. 27(51): p. 14059-68.
9. Study, R.E. and M.G. Kral, Spontaneous action potential activity in isolated dorsal root ganglion neurons from rats with a painful neuropathy. Pain, 1996. 65(2-3): p. 235-42.
10. Davidson, S., et al., Human sensory neurons: Membrane properties and sensitization by inflammatory mediators. Pain, 2014. 155(9): p. 1861-70.
11. Pacher, P. and G. Kunos, Modulating the endocannabinoid system in human health and disease--successes and failures. Febs j, 2013. 280(9): p. 1918-43.
12. Herbert, M.K. and P. Holzer, [Why are substance P(NK1)-receptor antagonists ineffective in pain treatment?]. Anaesthesist, 2002. 51(4): p. 308-19.
13. Hill, R., NK1 (substance P) receptor antagonists--why are they not analgesic in humans? Trends Pharmacol Sci, 2000. 21(7): p. 244-6.
14. Gereau, R.W.t., et al., A pain research agenda for the 21st century. J Pain, 2014. 15(12): p. 1203-14.
15. Borsook, D., et al., Lost but making progress--Where will new analgesic drugs come from? Sci Transl Med, 2014. 6(249): p. 249sr3.