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