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

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, Adelta or Aalpha/beta. Units with overshooting APs and membrane potentials (Vm) more negative than -40 mV were analysed: 40 C-, 45 Adelta- and 94 Aalpha/beta-fibre units. 2. Sensory receptive properties were characterized as: (a) low-threshold mechanoreceptive (LTM) units (5 C-, 10 Adelta- and 57 Aalpha/beta-fibre units); (b) nociceptive units, responding to noxious mechanical stimuli, some also to noxious heat (40 C-, 27 Adelta- and 27 Aalpha/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 identified C-fibre units, 36 % of identified Adelta-fibre units and > 73 % of identified Aalpha/beta-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 (Adelta-fibre units) and 1.4 times (Aalpha/beta-fibre units). They also had significantly longer rise times (RTs) and fall times (FTs) in all CV ranges. Between Aalpha/beta-nociceptors and Aalpha/beta-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 (Adelta-fibre units) and 3.6 times (Aalpha/beta-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 Adelta-fibre groups their mean AHP duration was even longer than in nociceptive units. 3. A-fibre LTM neurones were divided into Adelta- (D hair units, n = 8), and Aalpha/beta- (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 Aalpha/beta 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 Adelta-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.

Figure 1. AP waveform to illustrate the variables measured

An intracellularly recorded somatic AP of an Aα/β-fibre neurone evoked by electrical stimulation of the dorsal root showing the electrophysiological parameters measured. These were membrane potential (V_m) (1), AP duration at base (APdB) (2), AP rise time (RT) (3), AP fall time (FT) (4), and AHP duration to 80 % recovery (AHP₈₀) (5).

Figure 4. AP variables of C- and A-fibre nociceptive and other types of unit

Scatterplots to show distributions of the variables with the median (horizontal line) superimposed in each case. For the C-fibre units, the data plotted on the left includes only cells with V_m more negative than −40 mV, and that on the right includes cells with V_m more negative than −30 mV. COOL (C-cells only), units that responded to cooling but had either no response or a weak response to mechanical stimulation; NOC, nociceptive units; UN, unresponsive units. Asterisks above the graph indicate the significant differences which in all cases were between that column and the LTM group, obtained using the Kruskall-Wallis test with Dunn's post hoc test, carried out between the four groups of C-cells, or between the three groups of Aδ- and Aα/β-cells. *P < 0.05, **P < 0.01, ***P < 0.001. The absence of asterisks indicates the absence of any significant difference.

Figure 5. AHP₈₀ and CV of C- and A-fibre nociceptive and other types of unit

Scatter plots to show distributions of the variables with the median (horizontal line) superimposed in each case. Details as for Fig. 4. Asterisks above the graph indicate the significant differences which in all cases were between that column and the LTM group: *P < 0.05, **P < 0.01, ***P < 0.001. The absence of asterisks indicates the absence of any significant difference.

Figure 7. AHP₈₀ and CV of A-LTM units with different sensory properties

Scatter plots of distributions of variables, with the median (horizontal bar) superimposed in each case, grouped according to the sensory properties of the neurones. The D hair units had Aδ-fibre CVs, and all other groups had Aα/β-fibre CVs. Asterisks above the graph (in A) indicate a significant difference between the data in that column and the muscle spindle group, except (in A and B) where indicated by a line linking groups that are significantly different: *P < 0.05, **P < 0.01, ***P < 0.001. The absence of asterisks indicates the absence of any significant difference, but note that since there were only two SA units, this group was excluded from statistical tests.

Figure 2. Examples of APs recorded from neurones with different sensory receptive properties

Examples of somatic APs evoked by dorsal root stimulation and recorded intracellularly from neurones with different receptor properties: C-fibres (A-C), Aδ-fibres (D-F) and Aα/β-fibres (G-J). The CV for each unit is shown. Note that the APs of C- and Aδ-nociceptive (HTM, A and D) and unresponsive (C and F) units are broader than C-LTM and D hair units (B and E), respectively, with a difference in RT between the C-HTM and C-LTM units (A and B). Note also the long AHPs in Aδ-nociceptive (D) and unresponsive (F) neurones compared with the very short AHP in the D hair unit (Aδ-LTM, E). Note the very fast AP and AHP in the Aα/β-LTM MS (muscle spindle) unit (H), the fast AP and AHP in the G hair unit (I), the relatively broad AP and long AHP in the Aα/β-HTM (nociceptive) unit (G) and the very long lasting AHP in the T hair unit (J).

Figure 3. Variability in somatic AP shape of nociceptive and LTM neurones

Somatic APs evoked intracellularly by dorsal root stimulation in C- and Aδ-nociceptive and LTM groups are superimposed with their peaks aligned to illustrate the range of AP configurations in each group. In the nociceptive groups there were too many units to superimpose, so examples of units spanning the entire range of AP shapes were selected (19 C-fibre units in A and 17 Aδ-fibre units in C). APs from all C-LTM units in the −40 mV group (n = 5) and all Aδ D hair units (n = 8) are shown in B and D, respectively. The scale bar in A applies also to B-D and represents 5 ms for C-fibre units and 2 ms for Aδ-fibre units.

Figure 6. AP variables of A-LTM units with different sensory properties

Scatter plots of distributions of variables, with the median (horizontal bar) superimposed in each case, grouped according to the sensory properties of the neurones. The D hair units had Aδ-fibre CVs, and all other groups had Aα/β-fibre CVs. G hair and field units were combined (see text). Asterisks above the graph indicate significant differences which in all cases were between the data in that column and the muscle spindle group. *P < 0.05, **P < 0.01, ***P < 0.001. The absence of asterisks indicates the absence of any significant difference but note that since there were only two SA units, this group was excluded from statistical tests.

Figure 8. Relationship of AHP duration to AP duration in subgroups of C- and A-fibre units

Plots of log₁₀AHP₈₀ against log₁₀AP duration (APdB) at base for the following: all nociceptive neurones (A), all LTM neurones (B), and for Aα/β-fibre LTM units (C) and all neurones both nociceptive and other types (D). The −40 mV group of C-cells was used throughout. In A, B and D, data from C-, Aδ- and Aα/β-fibre units are plotted with T hair data plotted separately in B. Symbols shown in B apply also to A and D, those in C relate to that graph only. Linear regression analysis was carried out on each group of cells plotted with separate symbols. Where a significant correlation was found, the linear regression line is plotted and the r² value is given, with the significance level indicated by asterisks: *P < 0.05, **P < 0.01, ***P < 0.001, ****P < 0.0001. There was no correlation for the nociceptive groups together or separately (A), but excluding T hair units there was correlation for all LTM units together (B), for Aα/β-LTM units alone (B) and for G hair/field units and MS units (C). There was no correlation for all C-cells but there was correlation for both Aδ- and Aα/β-cells (D).