Activation Of Afferent C-fibers Research Articles

Increasing acclimation period improves the reproducibility of short-heating local thermal hyperemia

ObjectivesThe axon reflex (AR) can be induced by the activation of afferent C-fibers during local skin heating. The previously used long-heating local thermal hyperemia (LTH) protocols tested AR flare by normalizing to endothelial-mediated maximal vasodilatation to adjust capillary heterogeneity when the recording sites were randomly selected. The AR flare induced by short local heating can be reproducible without using the data from subsequent longer heating for normalization when the recording sites were fixed with holders in the same session. The aim of this study was to investigate the effect of acclimation period on the intersession reproducibility of short-heating LTH when the recording sites were relocated after a longer interval of 1–3days. MethodsAfter 30 or 60min acclimation, LTH with 5min heating was assessed on bilateral human forearms using single-point laser Doppler flowmetry. The test was repeated at the same recording sites again at the same time 1–3days later. Baseline and heating blood flux were analyzed and the data were expressed as different forms. Reproducibility of two tests was assessed using coefficient of variation (CV) and intra-class correlation coefficient (ICC) statistics. ResultsThe intersession reproducibility of peak cutaneous vascular conductance (CVC) (CV=18.38%, ICC=0.82), peak CVC change (CV=20.38%, ICC=0.83) and 4min area-under-the-curve (AUC) (CV=18.66%, ICC=0.75) of the right forearm and time to peak (CV=16.84%, ICC=0.52) of the left forearm were acceptable after 30min acclimation. When the acclimation period was increased to 60min, all of these data except 4min AUC of both sides reached an acceptable level. ConclusionsThe AR flare induced by short local heating is reproducible when the recording sites are relocated by a predefined rule. The reproducibility of LTH on right forearm is different from that on left forearm, and increasing acclimation period improves the reproducibility.

Axon reflex-related hyperemia induced by short local heating is reproducible

ObjectivesThe axon reflex (AR) flare is induced by antidromic activation of afferent C-fibers during nociceptive stimulation. This response has been suggested to be modulated by sympathetic activity and basal level of nitric oxide. In previously used protocols of local thermal hyperemia (LTH), AR flare has been used in combination with maximal vasodilatation to study the integrated endothelial function. The aim of this study was to investigate the intra-session reproducibility of short heating-induced AR flare, the specific neural-mediated portion of LTH, and to compare the reproducibility between different forms of data expression. MethodsShort-heating LTH was assessed using single-point laser Doppler flowmetry (LDF) on bilateral volar surface of the forearm in 10 men and 10 women. The blood flux measurement included a non-heating process for 5min, followed by a quick heating process from 33°C to 42°C for 5min. The test was repeated 45min later at the same recording sites with fixed holders. Baseline and heating blood flux were recorded and expressed as different forms of data. Reproducibility was assessed using coefficient of variation (CV) and intra-class correlation coefficient (ICC) statistics. ResultsThe reproducibility of peak cutaneous vascular conductance (CVC) (CV=16.02–17.31%, ICC=0.77–0.78), peak CVC change (CV=14.30–18.12%, ICC=0.80–0.86), and the 4min area-under-the-curve (CV=18.37–18.70%, ICC=0.60–0.78) was acceptable. The time to peak flux of each recording site ranged from 90 to 209s and all the peak fluxes have been achieved before 4min of heating. ConclusionsSingle-point LDF is a reproducible technique of assessing AR flare on volar surface of the forearm when the heating period is reduced to 5min and the recording sites are fixed. Using this new protocol, short-heating LTH has a potential to be used to evaluate the effects of acute physical or chemical interventions between two short-heating LTH tests to further explore the pathophysiological meaning of heating-induced AR flare.

Botulinum A Toxin Intravesical Injections in the Treatment of Painful Bladder Syndrome: A Pilot Study

ObjectiveWe evaluated the efficacy and tolerability of botulinum A toxin (BTX-A) intravesical injections in patients affected by painful bladder syndrome with increased urinary frequency, refractory to conventional treatment modalities. MethodsTwelve women and two men were prospectively included in the study. Under short general anaesthesia patients were given injections of 200U of commercially available BTX-A diluted in 20ml 0.9% NaCl. Injections were performed submucosally in the trigone and bladder floor under cystoscopic control. Voiding chart, the Visual Analog Scale (VAS) for pain, and urodynamics were performed before treatment and 1 and 3 mo afterward. ResultsOverall, 12 patients (85.7%) reported subjective improvement at 1 and 3 mo follow-up. The mean VAS score was significantly reduced at 1 and 3 mo after treatment (p<0.05 for both); at the same time points daytime and nighttime urinary frequency significantly decreased (p<0.01 and p<0.05, respectively), and bladder cystometric capacity significantly increased (p<0.01). Two patients reported incomplete bladder emptying. We did not detect any systemic side effects during or after treatment. ConclusionsThe results of this pilot study indicate that BTX-A intravesical injections are effective in the short-term management of painful bladder syndrome. By modulating afferent C-fiber activity within the bladder walls, BTX-A significantly improves urodynamic parameters and reduces bladder pain and urinary frequency.

The Effect and Mechanism of Intravesical Electrical Stimulation in Spinal Cord Injured Rats

Purpose: The objective of this study was to evaluate whether the effect on afferent c-fiber activity is the underlying mechanism of intravesical electrical stimulation (IVES) in spinal cord injured rats. Materials and Methods: Thirty five female Sprague-Dawley rats weighting 200-300g each were divided into the normal and spinalized groups. For the spinalized rats, we observed the c-fos expression, and we compared this in the non-IVES group with that in the IVES group. Cystometrograms were performed for all the groups via a suprapubic catheter. Results: After performing IVES in the normal and spinalized rats, the abnormal increases of the intercontraction interval (ICI) and the voiding pressure (VP) were reduced close to the normal range. In the spinalized rats, the number of c-fos positive cells in the dorsal commissure (DCM) decreased in the group that had IVES performed when compared with the non-IVES group. Conclusions: The IVES reduced the c-fos gene expression in the L6-S1 spinal cord segment and also the bladder hyperreflexia in the spinalized rats through the inhibition of afferent c-fiber activity, in addition to affecting the Aδ mechanoreceptors. (Korean J Urol 2006;47:303-309) ꠏꠏꠏꠏꠏꠏꠏꠏꠏꠏꠏꠏꠏꠏꠏꠏꠏꠏꠏꠏ

Mechanosensitive cardiac C-fiber response to changes in left ventricular filling, coronary perfusion pressure, hemorrhage, and volume expansion in rats.

Left ventricular (LV) end-diastolic pressure (LVEDP) increase due to volume expansion (VExp) enhances mechanosensitive vagal cardiac afferent C-fiber activity (CNFA), thus decreasing renal sympathetic nerve activity (RSNA). Hypotensive hemorrhage (hHem) attenuates RSNA despite decreased LVEDP. We hypothesized that CNFA increases with any change in LVEDP. Coronary perfusion pressure (CPP), supposedly affected in both conditions, might also be a stimulus of CNFA. VExp and hHem were performed in anesthetized male Sprague-Dawley rats while blood pressure, heart rate, and RSNA were measured. Cervical vagotomy abolished RSNA response in both reflex responses. Single-unit CNFA was recorded while LVEDP was changed. Rapid changes (+/- 4, +/-6, +/-8 mmHg) were obtained by graded occlusion of the caval vein and descending aorta. Prolonged changes were obtained by VExp and hHem. Furthermore, CNFA was recorded in a modified Langendorff heart while CPP was changed (70, 100, 40 mmHg). Rapid LVEDP changes increased CNFA [caval vein occlusion: +16 +/- 3 Hz (approximately +602%); aortic occlusion: +15 +/- 3 Hz (approximately +553%); 70 units; P < 0.05]. VExp and hHem (n = 6) increased CNFA [VExp: +10 +/- 4 Hz (approximately +1,033%); hHem: +10 +/- 2 Hz (approximately +1,225%); P < 0.05]. An increase in CPP increased CNFA [+2 +/- 1 Hz (approximately +225%); P < 0.05], whereas a decrease in CPP decreased CNFA [-0.8 +/- 0.4 Hz (approximately -50%); P < 0.05]. All C fibers recorded originated from the LV. CNFA increased with any LVEDP change but changed equidirectionally with CPP. Thus neither LVEDP nor CPP fully accounts directly for afferent C-fiber and reflex sympathetic responses. The intrinsic afferent stimuli and receptive fields accounting for reflex sympathoinhibition still remain cryptic.

NEUROMODULATION REDUCES C-FOS GENE EXPRESSION IN SPINALIZED RATS: A DOUBLE-BLIND RANDOMIZED STUDY

Neuromodulation of the sacral nerve roots is effective to treat various voiding dysfunctions, but the underlying mechanism of neuromodulation is not known. The objective of this study is to evaluate whether inhibition of afferent c-fiber activity is the underlying mechanism of sacral nerve root neuromodulation. Twenty-nine female Sprague-Dawley rats weighing 220 to 250 gm. were divided into 4 groups: normal control (normal rats without any procedure; n = 5), sham with saline (spinalized rats at T9 with saline bladder instillation; n = 7), sham with acetic acid (spinalized rats at T9 with acetic acid bladder instillation; n = 8) and stimulation group (spinalized rats at T9 with acetic acid bladder instillation plus electrical stimulation; n = 9). A cystometrogram was performed 10 days after spinal cord transection to confirm the development of bladder hyperreflexia. Bilateral electrode wires were implanted into S1 dorsal foramina and electrical stimulation was performed 8 hours a day for three weeks. The rats were perfused with 4% paraformaldehyde and an immunocytochemical method was used to stain fos-protein that was encoded by c-fos gene. A double-blind method was used in counting fos-protein positive neurons. Bladder hyperreflexia developed in all spinalized rats 10 days after spinal cord transection. Peak bladder pressure was found significantly reduced after neuromodulation (30.4 +/- 4.2 cm. water) compared with the same rats before neuromodulation (82.4 +/- 10.2 cm. water; p = 0. 007). The number of fos-protein positive neurons in the L6 spinal cord segment in the neuromodulation group (93.2 +/- 13.3 cells/section) decreased significantly when compared with the sham with acetic acid group (160.6 +/- 25.0 cells/section; p = 0.02). There was no significant difference in c-fos expression between the sham with saline group (90.5 +/- 15.6 cells/section) and the neuromodulation group (p = 0.92). Sacral dorsal root neuromodulation reduces c-fos gene expression and bladder hyperreflexia in spinalized rats, through inhibition of afferent c-fiber activity.

Role of substance P in the modulation of C-fiber-evoked responses of spinal dorsal horn neurons

Substance P (SP) as well as excitatory amino acids (EAAs) appear to be released in response to stimulation of primary afferent C-fibers. Activity at N-methyl- d-aspartate (NMDA) receptors is essential for wind-up (the progressive potentiation of C-fiber-evoked responses of single neurons in response to an electrical stimulation), however, the role of SP in wind-up is unclear. To address this, the effects of iontophoretically applied CP-99,994 (a NK-1 receptor antagonist), SP and SP(1–7) (an N-terminal breakdown product of SP), were compared on responses of spinal dorsal horn wide dynamic range (WDR) neurons of the rat. Post-stimulus time histograms (PSTH) were summed over 12 responses to low frequency (0.5 Hz) electrical stimulation of the cutaneous receptive field. Changes in responses of dorsal horn neurons were evaluated by monitoring C-fiber input, wind-up, and the total number of spikes evoked by C-fiber activity in response to the 12 stimuli. The NK-1 receptor antagonist CP-99,994 significantly inhibited the total number of C-spikes and caused a significant reduction in wind-up without changing the C-fiber input, indicating the involvement of NK-1 receptors in wind-up. Application of SP led to an overall increase in the total number of C-fiber evoked responses of dorsal horn neurons and ('-fiber input, however, wind-up, as defined, was significantly decreased following SP. In contrast, substance P(1–7) evoked a long-lasting increase in the total number of C-fiber-related spikes which was initially sustained by a long-lasting increase in the input followed by a longer lasting increase in wind-up, an effect opposite that of CP-99,994. As NMDA activity has been previously shown to be inhibited and then potentiated by SP N-terminal activity over a similar time interval, the present data are consistent with the mediation of wind-up by NMDA and its modulation by SP N-terminal activity. Release of SP in response to noxious stimulation may, therefore, increase primary afferent C-fiber activity (input) whereas an accumulation of SP N-terminal metabolites appears to potentiate wind-up, perhaps via positive modulation of EAA activity.

Dermatome mapping in the rat hindlimb by electrical stimulation of the spinal nerves

In rats treated with Evans blue (i.v.), electrical stimulation of the lumbar spinal nerves produced an extravasation zone in the skin. Stimulation of L 1 produced extravasation in the lower abdomen; that of L 2, in the cranial region of the hindlimb; that of L 3, in the ventral region of the hindlimb and the medial paw; that of L 4, in the lateral region of the hindlimb and the middle paw; that of L 5, in the caudal region of the hindlimb and the lateral paw; and that of L 6, in the caudal region of the hindlimb and the scrotum. Since the substance P antagonist FK224 inhibited the extravasation caused by stimulation of the sciatic nerve, the extravasation zones appeared to be related to antidromic activation of afferent C-fibers. Thus, a dermatome of afferent C-fibers was revealed in rats.

Role of leukocyte depletion in noncholinergic bronchoconstriction of guinea pigs

To test the role of leukocytes in the activation of afferent C-fibers in the lung, 33 guinea pigs, 18 control and 15 experimental or leukocyte depleted, were used. The leukocyte depletion was accomplished with an intraperitoneal injection of cyclophosphamide (100 mg/kg) 96 h prior to the study. On the day of the study, bronchial constriction was produced either by exsanguination ( n = 17) or by capsaicin injection (16 μg/kg, i.v.) ( n = 16) in anesthetized-paralyzed animals. Venous blood samples were collected for leukocyte counts. At 1–30 min following the above treatment, the maximal expiratory flow maneuver was performed and a decrease in the maximal expiratory flow at 50% baseline total lung capacity (żmax 50) was used as an index of bronchoconstriction. The leukocyte count decreased significantly following the pretreatment with cyclophosphamide [an average of 6217 ± 612 (control) vs. 2242 ± 334/mm 3 (experimental)]. Exsanguination caused a gradual decrease in V̇max 50 with time, indicating a temporal increase in bronchoconstriction. Capsaicin injection, on the other hand, caused an immediate (1 min) marked bronchoconstriction, which attenuated gradually with time. At a specific time point, leukocyte depletion did not produce any significant change in V̇max 50 compared to the control group for both types of bronchoconstrictions. Based on these results, we conclude that leukocytes may play an insignificant role in the bronchoconstriction caused by the activation of afferent C-fibers in guinea pig lungs under our experimental conditions.

Laser Doppler measurements of skin vasodilation elicited by percutaneous electrical stimulation of nociceptors in humans.

This study was designed to assess the quantitative features of the skin protecting mechanism of axon reflexes by monitoring stimulus related changes in skin blood flow using laser Doppler flowmetry. Skin vasodilatation induced by trains of brief transcutaneous electrical pulses (0.5 ms, 150 V) was strictly related in magnitude to the number of pulses (trains of 1-32). This vascular response was reduced by 95% when the skin had been pretreated by painting it for two days with a 1% alcoholic solution of capsaicin. The sensitivity of vascular responses to activation of afferent C-fibers indicates a role of this mechanism in the protection of the skin against noxious events.