Pregnenolone sulfate potentiates tetrodotoxin-resistant Na+ channels to increase the excitability of dural afferent neurons in rats

Background: Although peripheral administration of pregnenolone sulfate (PS) has been reported to produce pronociceptive effects, the mechanisms by which PS modulates the excitability of nociceptive neurons are poorly understood. Here, we report on the excitatory role of PS in peripheral nociceptive neurons, focusing on its effects on tetrodotoxin-resistant (TTX-R) Na⁺ channels.

Methods: TTX-R Na⁺ current (I_Na) mediated by Na_V1.8 was recorded from acutely isolated small-sized dural afferent neurons of rats, identified with the retrograde Fluorescent Dye DiI, using a whole-cell patch-clamp technique.

Results: Transcripts for Enzymes and transporters involved in PS biosynthesis were detected in the ophthalmic branch of the trigeminal ganglia. In voltage-clamp mode, PS preferentially potentiated the TTX-R persistent I_Na, a small non-inactivating current during sustained depolarization. PS shifted the voltage-inactivation relationship toward a depolarizing range. PS also delayed the onset of inactivation and accelerated the recovery from inactivation of TTX-R Na⁺ channels. Additionally, PS decreased the extent of use-dependent inhibition of TTX-R Na⁺ channels. In current-clamp mode, PS hyperpolarized dural afferent neurons by increasing the leak K⁺ conductance. Nevertheless, PS decreased the rheobase current-the minimum current required to generate action potentials-and increased the number of action potentials elicited by depolarizing current stimuli.

Conclusion: We have shown that the excitatory neurosteroid PS preferentially potentiates TTX-R persistent I_Na and reduces the inactivation of TTX-R Na⁺ channels, resulting in increased excitability of dural afferent neurons. The potential role of endogenous PS in migraine pathology warrants further investigation.