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

1. Intracellular voltage recordings were made from the somata of L6 and S1 dorsal root ganglion (DRG) neurones at 28.5-31 degrees C in young guinea-pi gs (150-300 g) anaesthetized with sodium pentobarbitone. Action potentials (APs) evoked by dorsal root stimulation were used to classify conduction ve locities (CVs) as C, A delta or A alpha/beta. Units with overshooting APs a nd membrane potentials (V-m) more negative than -40 mV were analysed: 40 C- , 45 A delta- and 94 A alpha/beta-fibre units. 2. Sensory receptive properties were characterized as: (a) low-threshold me chanoreceptive (LTM) units (5 C-, 10 A delta- and 57 A alpha/beta-fibre uni ts); (b) nociceptive units, responding to noxious mechanical stimuli, some also to noxious heat (40 C-, 27 A delta- and 27 A alpha/beta-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 id entified C-fibre units, 36% of identified A delta-fibre units and >73% of i dentified A alpha/beta-fibre units. Compared with LTM units, the nociceptiv e units had APs that were longer on average by 3 times (C-fibre units), 1.7 times (A delta-fibre units) and 1.4 times (A alpha/beta-fibre units). They also had significantly longer rise times (RTs) and fall times (FTs) in all CV ranges. Between A alpha/beta-nociceptors and A alpha/beta-LTMs there wa s a proportionately greater difference in RT than in FT. The duration of th e afterhyperpolarization measured to 80% recovery (AHP(80)) was also signif icantly longer in nociceptive than LTM neurones in all CV ranges: by 3 time s (C-fibre units), 6.3 times (A delta-fibre units) and 3.6 times (A alpha/b eta-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 A de lta-fibre groups their mean AHP duration was even longer than in nociceptiv e units. 3. A-fibre LTM neurones were divided into A delta- (D hair units, n = 8), a nd A alpha/beta- (G hair/field units, n = 22; T (tylotrich) hair units, n = 6; rapidly adapting (RB) glabrous units, n = 6; slowly adapting (SA) hairy and glabrous units, n = 2; and muscle spindle (MX) units n = 17). MS and S A 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 A alpha/ beta LTM units combined. T hair units had the longest mean FT and AHP of al l 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 th at of A-fibre nociceptive neurones. 4. These differences in somatic AP shape may aid in distinguishing between LTM and nociceptive or unresponsive C- and A delta-fibre units but probably not between nociceptive and unresponsive units. The differences seen may r eflect differences in expression or activation of different types of ion ch annel.