Abstract-Other than efferent sympathetic innervation, the kidney has peptidergic afferent fibers expressing TRPV1 receptors and releasing substance P. We tested the hypothesis that stimulation of afferent renal nerve activity with the TRPV1 agonist capsaicin inhibits efferent renal sympathetic nerve activity tonically by a neurokinin 1 receptordependant mechanism. Anesthetized Sprague-Dawley rats were instrumented as follows: (1) arterial and venous catheters for recording of blood pressure and heart rate and drug administration; (2) left-sided renal arterial catheter for selective intrarenal administration of the TRPV1 agonist capsaicin (3.3, 6.6, 10, 33*10 Ϫ7 M; 10 L; after 15, 30, 45, and 60 minutes, respectively) to stimulate afferent renal nerve activity; (3) right-sided bipolar electrode for continuous renal sympathetic nerve recording; and (4) specialized renal pelvic and renal artery catheters to separate pelvic from intrarenal afferent activity. Before and after intrarenal capsaicin application, increasing intravenous doses of the neurokinin 1 receptor blocker RP67580 were given. Intrarenal capsaicin decreased integrated renal sympathetic activity from 65.4Ϯ13.0 mV*s (baseline) to 12.8Ϯ3.2 mV*s (minimum; PϽ0.01). This sustained renal sympathetic inhibition reached its minimum within 70 minutes and was not directly linked to the transient electric afferent response to be expected with intrarenal capsaicin. Suppressed renal sympathetic activity transiently but completely recovered after intravenous administration of the neurokinin 1 blocker (maximum: 120.3Ϯ19.4 mV*s; PϽ0.01). Intrarenal afferent activity could be unequivocally separated from pelvic afferent activity. For the first time we provide direct evidence that afferent intrarenal nerves provide a tonically acting sympathoinhibitory system, which seems to be rather mediated by neurokinin release acting via neurokinin 1 receptor pathways rather than by electric afferent effects on central sympathetic outflow. Key Words: renal nerve Ⅲ afferent Ⅲ efferent Ⅲ TRPV1 Ⅲ NK 1 -receptor Ⅲ tonic inhibition T he kidney has a very complex sympathetic efferent and peptidergic afferent innervation 1 that recently became of increased interest as renal nerve ablation was introduced into the treatment of severely hypertensive patients. 2 However, especially the role of the afferent renal innervation in hypertension is still far from being fully understood. 3 We know that afferent renal nerve traffic is able to suppress the contralateral renal nerve activity by a sympathodepressory renorenal reflex that is altered in hypertension. 4 So far afferent nerve fibers involved in this reflex were said to be mainly projecting from the renal pelvis to the first neuron in the dorsal root ganglion, 5 although afferent nerve fibers are also found intrarenally in close vicinity to efferent sympathetic nerve fibers. 6 Furthermore, it is very likely that afferent nerve fibers are able to secrete transmitters, specifically substance P (SP) and calcitonin gene-related peptide (CGRP), ...
Freisinger W, Schatz J, Ditting T, Lampert A, Heinlein S, Lale N, Schmieder R, Veelken R. Sensory renal innervation: a kidney-specific firing activity due to a unique expression pattern of voltage-gated sodium channels? Am J Physiol Renal Physiol 304: F491-F497, 2013. First published January 2, 2013; doi:10.1152/ajprenal.00011.2012.-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 (Na vs). Dorsal root ganglion (DRG) neurons from rats were recorded by the current-clamp technique and distinguished as "tonic" or "phasic. .67]; P Ͻ 0.05). These findings point to an increased presence of the TTX-resistant Na vs 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 Na vs.
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 patchclamp recording. The firing pattern of neurons was characterized as tonic, i.e., sustained action potential (AP) firing, or phasic, i.e., Ͻ5 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 Ͻ 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 Ͻ 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 Ͻ 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. chemokine; CXCL1; renal afferent nerve; voltage-gated sodium channel; tonic; phasic; firing pattern OBSERVATIONS IN PATIENTS with renal failure and/or hypertension who were nephrectomized (7, 17) strongly suggest that renal sensory afferent innervation increases sympathetic-mediated vasoconstriction. Moreover, sympathetic nerve activity in patients after renal transplantation was only normalized with concomitant bilateral nephrectomy (17). Additional work in experimental models indicated (21, 23) that efferent neurogenic influences on cardiac pathology in renal insufficiency are mediated by renal afferent nerve activity (1).In experimental animals, afferent innervation of the kidney was reported to contribute to the sustained blood pressure increases when renal structural damage was present (3, 46). In contrast, renal afferent nerves were described to act protectively against salt-sensitive hypertension and the structural renal damage of high blood pressure (24,44). A more recent study using a more selective method of renal afferent denervation suggests that this might not be the case, but it could be shown that selective renal afferent denervation was able to blunt the development of deoxycorticosterone acetate-salt hypertension (12).Therefore, the benefit of afferent renal nerve ablation for the treatment of refractory hypertension remains controversial (31). In any case, the modulatory influence of afferent renal nerves on sympathetic tone is not well understood....
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