SUMMARY1. Combining intracellular recording and dye-injection techniques permitted direct correlation of neuronal soma size with peripheral nerve conduction velocity in individual neurones of the L4 dorsal root ganglion (d.r.g.) of the anaesthetized 5-8-week-old rat.2. The conduction velocities fell into two main groups; those > 14 m/s (Ax and ft fibres) and those < 8 m/s (Ad and C fibres).3. Fibres with conduction velocities in the Ad range (2-2-8 m/s) in the sciatic nerve between the sciatic notch and the neuronal soma in the d.r.g. often conducted more slowly, that is in the C-fibre range (< 1-4 m/s), in the periphery from the tibial nerve to the sciatic notch.4. For the fast-conducting myelinated afferents, there was a loose positive correlation between cell size and the conduction velocity of the peripheral axon, whereas a clearer positive correlation existed between neuronal cell size and axonal conduction velocity both for Ad-and for C-fibre afferents.5. The relationship of the cell cross-sectional area (measured at the nucleolar level), to the cell volume for each neuronal soma was similar for the different conduction velocity groups.6. The somata of the fast-conducting myelinated Ax and A,8 fibres had a similar mean and range of cross-sectional areas to those of the large light cell population.7. The somata with Ad and C fibres were of a more uniform size and were restricted to the smaller cells within the ganglia. The mean and range of cross-sectional areas of the C cells was similar to those of the small dark cell population. Ad somata had a larger mean and range of cell sizes than those of the small dark cell population.8. The relationships of peripheral axon type to the morphological cell types are discussed.

SUMMARY1. The electrical characteristics of individual rat dorsal root ganglion neurones were studied and related to the peripheral axon conduction velocity and morphological cell type. Neurones were divided into four groups based on the conduction velocity of their peripheral axons (AX, 30-55 m/s; Aft, 14-30 m/s; Ad, 2-2-8 m/s and C < 1-4 m/s).2. Electrophysiological parameters examined included membrane potential, action potential amplitude and duration, after-potential height and duration, input resistance and the occurrence of time-dependent rectification.3. The mean duration ofthe somatic action potentials was found to be characteristic for each of the conduction velocity groupings. However, there was considerable overlap between groups. The fast-conducting (Ax) and slowly conducting (As) myelinated fibres had short-duration action potentials, within the ranges 0-49-135 and 0-5-1-7 ms at the base respectively. The Af and C cells had somatic action potentials with durations in the ranges of 0-6-2-9 and 0-6-7-4 ms respectively. The longer action potential durations could be related to the presence of an inflexion on the repolarizing phase seen in a third of Aft neurones (called Aft, neurones) and in all C neurones.4. The action potential overshoot was larger in C neurones and Aft1 neurones than in the other neurone groups.5. The mean duration of the after-hyperpolarization was several times greater in C neurones than in A neurones. AA neurones displayed the shortest and greatest amplitude after-hyperpolarizations. Large, long-lasting after-hyperpolarizations were not limited to neurones displaying an inflexion.6. The electrophysiological properties of the soma membrane of AA neurones closely resembled those of AA neurones, while in several respects those of C neurones resembled the Aft1 neuronal properties.7. The input resistance was found to be much greater in C than in A cells, although there was no significant difference between specific membrane resistance values calculated for the different groups. A number of A cells exhibited time-dependent rectification.

The hyperpolarization-activated current (Ih) has been implicated in nociception/pain, but its expression levels in nociceptors remained unknown. We recorded Ih magnitude and properties by voltage clamp from dorsal root ganglion (DRG) neurons in vivo, after classifying them as nociceptive or low-threshold-mechanoreceptors (LTMs) and as having C-, Aδ- or Aα/β-conduction velocities (CVs). For both nociceptors and LTMs, Ih amplitude and Ih density (at −100 mV) were significantly positively correlated with CV. Median Ih magnitudes and Ih density in neuronal subgroups were respectively: muscle spindle afferents (MSAs): −4.6 nA, −33 pA pF−1; cutaneous Aα/β LTMs: −2.2 nA, −20 pA pF−1; Aβ-nociceptors: −2.6 nA, −21 pA pF−1; both Aδ-LTMs and nociceptors: −1.3 nA, ~−14 pA pF−1; C-LTMs: −0.4 nA, −7.6 pA pF−1; and C-nociceptors: −0.26 nA, −5 pA pF−1. Ih activation slow time constants (slow τ values) were strongly correlated with fast τ values; both were shortest in MSAs. Most neurons had τ values consistent with HCN1-related Ih; others had τ values closer to HCN1+HCN2 channels, or HCN2 in the presence of cAMP. In contrast, median half-activation voltages (V0.5) of −80 to −86 mV for neuronal subgroups suggest contributions of HCN2 to Ih. τ values were unrelated to CV but were inversely correlated with Ih and Ih density for all non-MSA LTMs, and for Aδ-nociceptors. From activation curves ~2–7% of Ih would be activated at normal membrane potentials. The high Ih may be important for excitability of A-nociceptors (responsible for sharp/pricking-type pain) and Aα/β-LTMs (tactile sensations and proprioception). Underlying HCN expression in these subgroups therefore needs to be determined. Altered Ih expression and/or properties (e.g. in chronic/pathological pain states) may influence both nociceptor and LTM excitability.

We report here the expression and properties of the intermediate-conductance Ca²⁺-activated K⁺ (IK_Ca) channel in the GL-15 human glioblastoma cell line. Macroscopic IK_Ca currents on GL-15 cells displayed a mean amplitude of 7.2±0.8 pA/pF at 0 mV, at day 1 after plating. The current was inhibited by clotrimazole (CTL, IC₅₀=257 nM), TRAM-34 (IC₅₀=55 nM), and charybdotoxin (CTX, IC₅₀=10.3 nM). RT-PCR analysis demonstrated the expression of mRNA encoding the IK_Ca channel in GL-15 cells. Unitary currents recorded using the inside-out configuration had a conductance of 25 pS, a K_D for Ca²⁺ of 188 nM at -100 mV, and no voltage dependence. We tested whether the IKCa channel expression in GL-15 cells could be the result of an increased ERK activity. Inhibition of the ERK pathway with the MEK antagonist PD98059 (25 µM, for 5 days) virtually suppressed the IK_Ca current in GL-15 cells. PD98059 treatment also increased the length of cellular processes and up-regulated the astrocytic differentiative marker GFAP. A significant reduction of the IKCa current amplitude was also observed with time in culture, with mean currents of 7.17±0.75 pA/pF at 1-2 days, and 3.11±1.35 pA/pF at 5-6 days after plating. This time-dependent downregulation of the IK_Ca current was not accompanied by changes in the ERK activity, as assessed by immunoblot analysis. Semiquantitative RT-PCR analysis demonstrated a ~35% reduction of the IK_Ca channel mRNA resulting from ERK inhibition and a ~50% reduction with time in culture.

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