Chloroquine (CQ) and histamine are pruritogens commonly used to study itch in the mouse. A novel skin-nerve preparation was used to evaluate chloroquine (CQ)- and histamine-induced activation of afferent nerves in the dorsal thoracic skin of the mouse. All CQ sensitive nerves were C-fibres, and were also sensitive to histamine. The response to CQ, but not histamine, was largely absent in mrgpr-cluster Δ mice, supporting the hypothesis that CQ evokes itch largely via stimulation of MrgprA3 receptors. The CQ-induced action potential discharge was largely absent in phospholipase Cβ3 knockout animals. The CQ and histamine responses were not influenced by removal of TRPA1, TRPV1, TRPC3 or TRPC6, nor by the TRP channel blocker Ruthenium Red. The bouts of scratching in response to CQ were not different between wild-type and TRPA1-deficient mice. A selective inhibitor of the calcium-activated chloride channel TMEM16A, N-((4-methoxy)-2-naphthyl)-5-nitroanthranilic acid (MONNA), inhibited CQ-induced action potential discharge at itch nerve terminals and bouts of scratching by about 50%. Although TRPA1 and TRPV1 channels may be involved in the scratching responses to intradermal pruritogens, this is unlikely to be due to an effect at the nerve terminals, where chloride channels may play a more important role.

The action potential (AP) initiation in the nerve terminals and its subsequent AP conduction along the axons do not necessarily depend on the same subtypes of voltage-gated sodium channels (Na 1s). We evaluated the role of TTX-sensitive and TTX-resistant Na 1s in vagal afferent nociceptor nerves derived from jugular and nodose ganglia innervating the respiratory system. Single cell RT-PCR was performed on vagal afferent neurons retrogradely labelled from the guinea pig trachea. Almost all of the jugular neurons expressed the TTX-sensitive channel Na 1.7 along with TTX-resistant Na 1.8 and Na 1.9. Tracheal nodose neurons also expressed Na 1.7 but, less frequently, Na 1.8 and Na 1.9. Na 1.6 were expressed in ∼40% of the jugular and 25% of nodose tracheal neurons. Other Na 1 α subunits were only rarely expressed. Single fibre recordings were made from the vagal nodose and jugular nerve fibres innervating the trachea or lung in the isolated perfused vagally-innervated preparations that allowed for selective drug delivery to the nerve terminal compartment (AP initiation) or to the desheathed vagus nerve (AP conduction). AP initiation in jugular C-fibres was unaffected by TTX, although it was inhibited by Na 1.8 blocker (PF-01247324) and abolished by combination of TTX and PF-01247324. However, AP conduction in the majority of jugular C-fibres was abolished by TTX. By contrast, both AP initiation and conduction in nodose nociceptors was abolished by TTX or selective Na 1.7 blockers. Distinction between the effect of a drug with respect to inhibiting AP in the nerve terminals within the airways vs. at conduction sites along the vagus nerve is relevant to therapeutic strategies involving inhaled Na 1 blocking drugs.

Little is known about the neuronal voltage-gated sodium channels (NaVs) that control neurotransmission in the parasympathetic nervous system. We evaluated the expression of the a subunits of each of the nine NaVs in human, guinea pig, and mouse airway parasympathetic ganglia. We combined this information with a pharmacological analysis of selective NaV blockers on parasympathetic contractions of isolated airway smooth muscle. As would be expected from previous studies, tetrodotoxin potently blocked the parasympathetic responses in the airways of each species. Gene expression analysis showed that that NaV 1.7 was virtually the only tetrodotoxin-sensitive NaV1 gene expressed in guinea pig and human airway parasympathetic ganglia, where mouse ganglia expressed NaV1.1, 1.3, and 1.7. Using selective pharmacological blockers supported the gene expression results, showing that blocking NaV1.7 alone can abolish the responses in guinea pig and human bronchi, but not in mouse airways. To block the responses in mouse airways requires that NaV1.7 along with NaV1.1 and/or NaV1.3 is blocked. These results may suggest novel indications for NaV1.7-blocking drugs, in which there is an overactive parasympathetic drive, such as in asthma. The data also raise the potential concern of antiparasympathetic side effects for systemic NaV1.7 blockers.