Delayed Depolarization and Slow Sodium Currents in Cutaneous Afferents

Intraaxonal recordings were obtained in vitro from the sural nerve (SN), the muscle branch of the anterior tibial nerve (ATN), or the deefferented ATN (dATN) in 5- to 7-wk-old rats. Whole-nerve sucrose gap recordings were obtained from the SN and the ATN. This allowed study of cutaneous (SN), mixed motor and muscle afferent (ATN), and isolated muscle afferent (dATN) axons.

Application of the potassium channel blocking agent 4-aminopyridine (4-AP) to ATN or dATN resulted in a slight prolongation of the action potential. In contrast, a distinct delayed depolarization followed the axonal action potential in cutaneous afferents (SN) exposed to 4-AP. The delayed depolarization could be induced by a single whole-nerve stimulus or by injection of constant-current depolarizing pulses into individual axons. The delayed depolarization often gave rise to bursts of action potentials and was followed by a prominent afterhyperpolarization (AHP).

In paired-pulse experiments on single SN axons, the recovery time (half-amplitude of the action potential) was 3.06 ± 1.82 (SE) ms (n = 12). After exposure to 4-AP the recovery time of the delayed depolarization was considerably longer (half-recovery time: 99.0 ± 28.3 ms; n = 15) than that of the action potential (18.8 ± 9.1 ms; n = 16).

Application of tetraethylammonium (TEA) to cutaneous or muscle afferents alone had little effect on single action potential waveform. However, TEA reduced the amplitude of the AHP elicited by a single stimulus in cutaneous afferent axons after exposure to 4-AP and resulted in repetitive spike discharge.

The delayed depolarization and spike burst activity induced by 4-AP in SN was present in Ca ^{-free solutions containing 1 mM ethylene glycol-bis(}β-aminoethyl ether)-N,N,N′,N′-tetraacetic acid and was not blocked by Cd ^{(1.0 mM).}

We obtained whole-cell patch-clamp recordings to study Na ^{currents from either randomly selected dorsal root ganglion neurons or cutaneous afferent neurons identified by retrograde labeling with Fluoro-Gold. The majority of the randomly selected neurons had a singular kinetically fast Na current. In contrast, no identified cutaneous afferent neurons had a singular fast Na current. Rather, they had a combination of kinetically separable fast and slow currents or a singular relatively slow Na current.}

These results demonstrate a difference in the sensitivity of myelinated cutaneous and muscle afferent axons to blockade of a 4-AP-sensitive K ^{channel. Cutaneous afferent axons give rise to a prominent depolarizing potential after the action potential, which is not present in the muscle afferent or motor axons. We propose that cutaneous afferent axons have kinetically slow Na channels not present in muscle afferent and efferent fibers, whose activation underlies the delayed depolarization and multiple spike discharge. The results indicate a difference in the Na channel organization of myelinated cutaneous versus muscle afferent axons and their cell bodies.}