Elsevier

Neuroscience

Volume 108, Issue 3, 14 December 2001, Pages 479-491
Neuroscience

Differences in the size of the somatic action potential overshoot between nociceptive and non-nociceptive dorsal root ganglion neurones in the guinea-pig

https://doi.org/10.1016/S0306-4522(01)00423-7Get rights and content

Abstract

Intracellular action potentials evoked by dorsal root stimulation were intracellularly recorded from L6 and S1 dorsal root ganglion neurones in deeply anaesthetised guinea-pigs in vivo. Units were classed as C, Aδ or Aα/β units and as nociceptive, low-threshold mechanoreceptive or unresponsive. Units with membrane potentials of at least −40 mV and action potentials with an amplitude of >20 mV were included.

Nociceptive neurones had significantly larger somatic action potential overshoots than low-threshold mechanoreceptors in C, Aδ and Aα/β units. A higher proportion of low-threshold mechanoreceptors than of nociceptors had action potentials that failed to overshoot in all conduction velocity groups. 60% of muscle spindle afferents failed to overshoot. The size of the overshoot was correlated positively with log10 action potential duration, log10 action potential rise time, log10 afterhyperpolarisation duration, action potential amplitude and membrane potential and negatively (weakly) with log10 conduction velocity.

We conclude that nociceptive neurones are more likely to have somatic action potential overshoots than low-threshold mechanoreceptors in any conduction velocity group. This effect was not due to electrode properties or conduction failure at site(s) of failure of action potential regeneration. Differences in overshoot may affect the influence of neuronal firing on cellular processes. If an overshooting action potential is used as a selection criterion, a bias towards nociceptive neurones is likely to occur. An overshooting action potential coupled with a long afterhyperpolarisation or broad action potential may help in identifying sensory neurones as nociceptive.

Section snippets

Experimental animals

Young female Dunkin Hartley guinea-pigs (weight 180–300 g which relates to ages of about 10–21 days) were anaesthetised initially with sodium pentobarbitone (50 mg/kg, i.p.). Core temperature was maintained at 36±0.5°C. Animals were artificially ventilated and end-tidal CO2 was monitored. Cannulation of the left carotid artery permitted intra-arterial (i.a.) injection of drugs. In most experiments, arterial blood pressure was recorded from the cannulated right carotid. Animals were maintained

Effects of electrodes

To establish whether AP amplitude was limited by the slower rise time of higher resistance electrodes filled with LiCl, we compared the AP amplitudes for all groups of neurones recorded using LiCl-filled electrodes with those recorded using KCl-filled electrodes. In Fig. 1A it can be seen that in nociceptive (C, Aδ and Aα/β) neurones, the mean AP overshoots in units recorded with LiCl-filled electrodes was not significantly different from that in units recorded with KCl electrodes.

In Fig. 1B,

Discussion

The present study has shown that nociceptive primary afferent DRG neurones are much more likely to have overshooting somatic APs than LTM units. Indeed only about 40% of muscle spindle afferent units showed overshooting somatic APs compared with 100% of the Aα/β nociceptive units. The difference in the extent of the overshoot between nociceptive and non-nociceptive neurones was clearly seen also in Aδ- and C-fibre units. Unresponsive units had distributions of overshoots similar to those of

Acknowledgements

This work was supported by a grant from the Wellcome Trust, UK. Our thanks also go to R. Meech and T. MacDonald in this department and to A.A. Harper (University of Dundee) for helpful discussions about this paper.

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