NaV-igating the MAP from PACAP to excitement. Focus on "Activation of MEK/ERK signaling contributes to the PACAP-induced increase in guinea pig cardiac neuron excitability"

Nav1.7 via Promotion of ERK in the Trigeminal Ganglion Plays an Important Role in the Induction of Pulpitis Inflammatory Pain

The trigeminal ganglion (TG) refers to sensory neurons bodies that innervate the spinal cord and peripheral axons that innervate teeth. The tetrodotoxin-sensitive sodium (NA) channels (Nav1.7) play important roles in the pathophysiology of pain. In this study, we investigated the TG expression of Nav1.7 and extracellular signal-regulated kinase (ERK) in a rat model of pulpitis to explore the correlation between these channels and inflammatory pain. Pulpitis was confirmed by hematoxylin-eosin staining. In this study, we demonstrated that the reflex of rats to mechanical stimulation increases after pulp exposure and that the exposed rat molar pulp can upregulate the expression of Nav1.7 and ERK in the rat TG. Three days after rat pulp exposure, the expression levels of the two ion channels in the TG increased. TG target injection of PF04856264, a Nav1.7 inhibitor, dose-dependently increased the mechanical pain threshold and was able to inhibit ERK expression. TG target injection of PD98059, an ERK inhibitor, dose-dependently increased the mechanical pain threshold. These factors simultaneously resulted in the highest production. In this study, with the established link to inflammatory pain, we found that Nav1.7 and ERK both play important roles in the induction of inflammatory pain caused by pulpitis. We also found a correlation between the expression levels of Nav1.7 and ERK and the degree of inflammatory pain. Furthermore, ERK signaling pathways were promoted by the Nav1.7 in TG after pulpitis.

Activation of MEK/ERK signaling contributes to the PACAP-induced increase in guinea pig cardiac neuron excitability

Pituitary adenylate cyclase (PAC)-activating polypeptide (PACAP) peptides (Adcyap1) signaling at the selective PAC1 receptor (Adcyap1r1) participate in multiple homeostatic and stress-related responses, yet the cellular mechanisms underlying PACAP actions remain to be completely elucidated. PACAP/PAC₁ receptor signaling increases excitability of neurons within the guinea pig cardiac ganglia, and as these neurons are readily accessible, this neuronal system is particularly amenable to study of PACAP modulation of ionic conductances. The present study investigated how PACAP activation of MEK/ERK signaling contributed to the peptide-induced increase in cardiac neuron excitability. Treatment with the MEK inhibitor PD 98059 blocked PACAP-stimulated phosphorylated ERK and, in parallel, suppressed the increase in cardiac neuron excitability. However, PD 98059 did not blunt the ability of PACAP to enhance two inward ionic currents, one flowing through hyperpolarization-activated nonselective cationic channels (I_h) and another flowing through low-voltage-activated calcium channels (I_T), which support the peptide-induced increase in excitability. Thus a PACAP- and MEK/ERK-sensitive, voltage-dependent conductance(s), in addition to I_h and I_T, modulates neuronal excitability. Despite prior work implicating PACAP downregulation of the K_V4.2 potassium channel in modulation of excitability in other cells, treatment with the K_V4.2 current blocker 4-aminopyridine did not replicate the PACAP-induced increase in excitability in cardiac neurons. However, cardiac neurons express the ERK target, the Na_V1.7 sodium channel, and treatment with the selective Na_V1.7 channel inhibitor PF-04856264 decreased the PACAP modulation of excitability. From these results, PACAP/PAC1 activation of MEK/ERK signaling may phosphorylate the Na_V1.7 channel, enhancing sodium currents near the threshold, an action contributing to repetitive firing of the cardiac neurons exposed to PACAP.