Sodium current properties differ in neonate and adult mouse superficial dorsal horn neurons

K.E. Farrell, M.A. Walsh, B.A. Graham, A.M. Brichta and R.J. Callister, Biomedical Sciences & Pharmacy, University of Newcastle, Callaghan, NSW 2308, Australia.

Superficial dorsal horn (SDH: laminae I-II) neurons are important for spinal processing of nociceptive information and their excitability is determined, in part, by the properties of voltage-gated sodium channels (I_Na). Recently, we have shown excitability and action potential (AP) properties in SDH neurons are altered during development (Walsh et al., 2009).

Purpose: To compare I_Na currents in neonate (P0-5) and adult (≥ P21) SDH neurons.

Methods: Mice were anaesthetized (Ketamine 100 mg/kg i.p.) and decapitated. Transverse slices were prepared from L3-5 spinal segments and whole-cell recordings were made (at 32°C) from visualized SDH neurons using a CsF-based internal.

Results: A fast activating and inactivating inward current was evoked by depolarising neurons from −60 to −20 mV. The peak amplitude of this current increased during development (1.34 ± 0.35 nA vs 6.58 ± 0.68 nA; neonates (n = 10) vs adults (n = 12)), and was completely abolished by 1 μM TTX in both neonates (n = 2) and adults (n = 3), thus confirming the current was mediated by I_Na. Time to peak and half width was slower in neonates (0.82 ± 0.08 ms vs 0.46 ± 0.06 ms; 1.40 ± 0.25 ms vs 0.43 ± 0.05 ms). I_Na activation voltage and peak current voltage also differed in neonatal and adult neurons (−50 mV vs −60 mV; −15 mV vs −25 mV).

Conclusion: These data show I_Na expression and kinetics differ in neonate and adult SDH neurons and provide an underlying mechanism for the differences observed previously in AP properties of developing SDH neurons.

Walsh MA, Graham BA, Brichta AM, Callister RJ. (2009) Journal of Neurophysiology 101: 1800-12.