Background & Aims

Efforts to develop novel treatments for chronic pain have been hindered by the lack of successful translation of compounds from preclinical models. It is necessary to develop new approaches to drug development to better link the human and rodent assays. We sought to achieve this aim through the validation of functional pain biomarkers as indicators of clinical target engagement. The human spinal somatosensory-evoked potential (SEP) is thought to be generated by the post-synaptic activation of wide-dynamic range (WDR) neurons in the spinal dorsal horn (SDH) following low-intensity electrical stimulation to activate A?-fibre afferents (low threshold). Capsaicin-induced central sensitisation increases the human spinal SEP and this effect attenuated by pregabalin. Spinal SEPs could provide a measure of the activation of the nociceptive system and how it interacts with standard-of-care painkillers without the need to activate nociceptors (so it would be tolerable as an assay).

Methods

We investigated the effects of pregabalin (a ligand of voltage-dependent calcium channels ?2? subunits), lacosamide (a non-selective sodium channel modulator) and tapentadol (a mu-opioid receptor agonist and noradrenaline reuptake inhibitor) on the amplitude of spinal SEPs in a dose-dependent manner. Adult male Wistar rats (n=60, 250-375g) were anaesthetised with isoflurane. Following a spinal laminectomy over L3-4, a linear multi-electrode silicon probe (64 channel, Cambridge Neurotech) was inserted into the dorsal horn. Electrical stimuli (4Hz x 250s low-intensity electrical stimuli and 3x higher intensity stimulus ramps) were delivered to the sciatic nerve in each 10min block, with recordings consisting of a 30min baseline period and up to 90min post-dose. Drugs were administered in a blinded manner following a block-randomised design (vehicle, 3, 10 and 30mg/Kg i.p).

Results

Low-intensity SEPs within the spinal dorsal horn have a characteristic depth profile (peak amplitude in lamina IV/V). The principle negative peak (N1 potential) of the SEP is consistent with peripheral conduction via A?-fibres. Waveform and multi-unit activity analyses of the higher intensity stimuli revealed graded recruitment of primary afferent fibres classes (A/C-fibres), each exhibiting distinct dorsoventral distribution patterns within the dorsal horn. Injection of 30mg/Kg lacosamide significantly reduced the amplitude of the N1 potential (two-way RM ANOVA, time x drug, P= 0.0003; primary endpoint). Injection of 10mg/Kg tapentadol also significantly reduced the amplitude of the N1 potential (two-way RM ANOVA, time x drug, P=0.0002; primary endpoint). Pregabalin did not show any significant effect on the N1 potential. The tapentadol-induced reduction of the N1 potential was shown to be maintained following spinal perfusion of tapentadol and this effect was reversible by naloxone.

Conclusions

To determine the spinal generator of the SEP and how tapentadol acts on the spinal cord to produce an analgesic effect, analysis of isolated single unit activity (clustered in Kilosort2 and Phy2) revealed a population of neurons that were activated by A?, A? and C-fibres. These neurons were further functionally identified using naturalistic stimulation (brush, pinch, von Frey), supporting a WDR neuronal generator of the rodent SEP. The stimulus-evoked activity was reversibly inhibited by tapentadol.
Together, these findings support the application of spinal SEPs in drug development to provide proof of interaction with the nociceptive system in an adaptable and translatable rodent<->human assay.

References

Di Lionardo, A. et al. Modulation of the N13 component of the somatosensory evoked potentials in an experimental model of central sensitization in humans. Scientific Reports | 11, (2021).

Presenting Author

Kenneth Steel

Poster Authors

Kenneth Steel

BSc

University of Bristol

Lead Author

Tony Blockeel

Lead Author

James Dunham

University of Bristol, United Kingdom.

Lead Author

Andrea Truini

University Sapienza

Lead Author

Caterina Leone

MD PhD

Department of Human Neuroscience, Sapienza University of Rome

Lead Author

Rolf-Detlef Treede

Heidelberg University

Lead Author

Jeff Krajewski

Eli Lilly and Company

Lead Author

Keith Phillips

Lead Author

Topics

  • Mechanisms: Biological-Systems (Physiology/Anatomy)