Afferent neurons of the kidney with impaired firing pattern in inflammation - role of sodium currents?

Peripheral neurons with renal afferents exhibit a predominantly tonic firing pattern of higher frequency that is reduced to low frequencies (phasic firing pattern) in renal inflammation. We wanted to test the hypothesis that the reduction in firing activity during inflammation is due to high-activity tonic neurons switching from higher to low frequencies depending on altered sodium currents. We identified and cultivated afferent sensory neurons with renal projections from the dorsal root ganglia (Th11-L2). Cultivated neurons were incubated with the chemokine CXCL1 (1,5 nmol/ml) for 12 h. We characterized neurons as "tonic," i.e., sustained action potential (AP) firing, or "phasic," i.e., < 5 APs upon stimulation in the current clamp. Their membrane currents were investigated in a voltage clamp. Data analyzed: renal vs. non-renal and tonic vs. phasic neurons. Renal afferent neurons exposed to CXCL1 showed a decrease in tonic firing pattern (CXCL1: 35,6% vs. control: 57%, P < 0.05). Na+ and K+ currents were not different between control renal and non-renal DRG neurons. Phasic neurons exhibited higher Na+ and K+ currents than tonic resulting in shorter APs (3.7 ± 0.3 vs. 6.1 ± 0.6 ms, P < 0.01). In neurons incubated with CXCL1, Na+ and K+ peak current density increased in phasic (Na+: - 969 ± 47 vs. - 758 ± 47 nA/pF, P < 0.01; K+: 707 ± 22 vs. 558 ± 31 nA/pF, P < 0.01), but were unchanged in tonic neurons. Phasic neurons exposed to CXCL1 showed a broader range of Na+ currents ([- 365- - 1429 nA] vs. [- 412- - 4273 nA]; P < 0.05) similar to tonic neurons. After CXCL1 exposure, significant changes in phasic neurons were observed in sodium activation/inactivation as well as a wider distribution of Na+ currents characteristic of tonic neurons. These findings indicate a subgroup of tonic neurons besides mere tonic or phasic neurons exists able to exhibit a phasic activity pattern under pathological conditions.

Impaired excitability of renal afferent innervation after exposure to the inflammatory chemokine CXCL1.

Recently, we showed that renal afferent neurons exhibit a unique firing pattern, i.e., predominantly sustained firing, upon stimulation. Pathological conditions such as renal inflammation likely alter excitability of renal afferent neurons. Here, we tested whether the proinflammatory chemokine CXCL1 alters the firing pattern of renal afferent neurons. Rat dorsal root ganglion neurons (Th11-L2), retrogradely labeled with dicarbocyanine dye, were incubated with CXCL1 (20 h) or vehicle before patch-clamp recording. The firing pattern of neurons was characterized as tonic, i.e., sustained action potential (AP) firing, or phasic, i.e., <5 APs following current injection. Of the labeled renal afferents treated with vehicle, 58.9% exhibited a tonic firing pattern vs. 7.8%, in unlabeled, nonrenal neurons (P < 0.05). However, after exposure to CXCL1, significantly more phasic neurons were found among labeled renal neurons; hence the occurrence of tonic neurons with sustained firing upon electrical stimulation decreased (35.6 vs. 58.9%, P < 0.05). The firing frequency among tonic neurons was not statistically different between control and CXCL1-treated neurons. However, the lower firing frequency of phasic neurons was even further decreased with CXCL1 exposure [control: 1 AP/600 ms (1-2) vs. CXCL1: 1 AP/600 ms (1-1); P < 0.05; median (25th-75th percentile)]. Hence, CXCL1 shifted the firing pattern of renal afferents from a predominantly tonic to a more phasic firing pattern, suggesting that CXCL1 reduced the sensitivity of renal afferent units upon stimulation.

Sensory renal innervation: a kidney-specific firing activity due to a unique expression pattern of voltage-gated sodium channels?

Sensory neurons with afferent axons from the kidney are extraordinary in their response to electrical stimulation. More than 50% exhibit a tonic firing pattern, i.e., sustained action potential firing throughout depolarizing, pointing to an increased excitability, whereas nonrenal neurons show mainly a phasic response, i.e., less than five action potentials. Here we investigated whether these peculiar firing characteristics of renal afferent neurons are due to differences in the expression of voltage-gated sodium channels (Navs). Dorsal root ganglion (DRG) neurons from rats (Th11-L2) were recorded by the current-clamp technique and distinguished as "tonic" or "phasic." In voltage-clamp recordings, Navs were characterized by their tetrodotoxoxin (TTX) sensitivity, and their molecular identity was revealed by RT-PCR. The firing pattern of 66 DRG neurons (41 renal and 25 nonrenal) was investigated. Renal neurons exhibited more often a tonic firing pattern (56.1 vs. 12%). Tonic neurons showed a more positive threshold (-21.75 ± 1.43 vs.-29.33 ± 1.63 mV; P < 0.05), a higher overshoot (56.74 [53.6-60.96] vs. 46.79 mV [38.63-54.75]; P < 0.05) and longer action potential duration (4.61 [4.15-5.85] vs. 3.35 ms [2.12-5.67]; P < 0.05). These findings point to an increased presence of the TTX-resistant Navs 1.8 and 1.9. Furthermore, tonic neurons exhibited a relatively higher portion of TTX-resistant sodium currents. Interestingly, mRNA expression of TTX-resistant sodium channels was significantly increased in renal, predominantly tonic, DRG neurons. Hence, under physiological conditions, renal sensory neurons exhibit predominantly a firing pattern associated with higher excitability. Our findings support that this is due to an increased expression and activation of TTX-resistant Navs.