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

Abstract

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. 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 (AHP 80) 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. 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 AHP 80 s 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. 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.