Association of somatic action potential shape with sensory receptive properties in guinea‐pig dorsal root ganglion neurones

Open Access
Publisher Website
Google Scholar
Abstract
1 Intracellular voltage recordings were made from the somata of L6 and S1 dorsal root ganglion (DRG) neurones at 28.5–31 °C in young guinea-pigs (150–300 g) anaesthetized with sodium pentobarbitone. Action potentials (APs) evoked by dorsal root stimulation were used to classify conduction velocities (CVs) as C, Aδ or Aα/β. Units with overshooting APs and membrane potentials (Vm) more negative than −40 mV were analysed: 40 C-, 45 Aδ- and 94 Aα/β-fibre units. 2 Sensory receptive properties were characterized as: (a) low-threshold mechanoreceptive (LTM) units (5 C-, 10 Aδ- and 57 Aα/β-fibre units); (b) nociceptive units, responding to noxious mechanical stimuli, some also to noxious heat (40 C-, 27 Aδ- and 27 Aα/β-fibre units); (c) unresponsive units that failed to respond to a variety of tests; and (d) C-fibre cooling-sensitive units (n= 4). LTM units made up about 8 % of identified C-fibre units, 36 % of identified Aδ-fibre units and > 73 % of identified Aα/β-fibre units. Compared with LTM units, the nociceptive units had APs that were longer on average by 3 times (C-fibre units), 1.7 times (Aδ-fibre units) and 1.4 times (Aα/β-fibre units). They also had significantly longer rise times (RTs) and fall times (FTs) in all CV ranges. Between Aα/β-nociceptors and Aα/β-LTMs there was a proportionately greater difference in RT than in FT. The duration of the afterhyperpolarization measured to 80 % recovery (AHP80) was also significantly longer in nociceptive than LTM neurones in all CV ranges: by 3 times (C-fibre units), 6.3 times (Aδ-fibre units) and 3.6 times (Aα/β-fibre units). The mean values of these variables in unresponsive units were similar to those of nociceptive units in each CV range; in C- and Aδ-fibre groups their mean AHP duration was even longer than in nociceptive units. 3 A-fibre LTM neurones were divided into Aδ- (D hair units, n= 8), and Aα/β- (G hair/field units, n= 22; T (tylotrich) hair units, n= 6; rapidly adapting (RA) glabrous units, n= 6; slowly adapting (SA) hairy and glabrous units, n= 2; and muscle spindle (MS) units n= 17). MS and SA units had the shortest duration APs, FTs and AHP80s of all these groups. The mean RT in D hair units was significantly longer than in all Aα/β LTM units combined. T hair units had the longest mean FT and AHP of all the A-LTM groups. The mean AHP was about 10 times longer in T hair units than in all other A-LTM units combined (significant), and was similar to that of A-fibre nociceptive neurones. 4 These differences in somatic AP shape may aid in distinguishing between LTM and nociceptive or unresponsive C- and Aδ-fibre units but probably not between nociceptive and unresponsive units. The differences seen may reflect differences in expression or activation of different types of ion channel.