Capsaicin-sensitive chemoceptive B-afferents: A neural system with dual sensory-efferent function

Low pH potentiates both capsaicin binding and channel gating of VR1 receptors

Capsaicin ion channels are highly expressed in peripheral nervous terminals and involved in pain and thermal sensations. One characteristic of the cloned VR1 receptor is its multimodal responses to various types of noxious stimuli. The channel is independently activated by capsaicin and related vanilloids at submicromolar range, by heat above 40 degrees C, and by protons at pH below 6.5. Furthermore, simultaneous applications of two or more stimuli lead to cross sensitization of the receptor, with an apparent increase in the sensitivity to any individual stimulus when applied alone. We studied here the mechanism underlying such cross-sensitization; in particular, between capsaicin and pH, two prototypical stimuli for the channel. By analyzing single-channel currents recorded from excised-patches expressing single recombinant VR1 receptors, we examined the effect of pH on burst properties of capsaicin activation at low concentrations and the effect on gating kinetics at high concentrations. Our results indicate that pH has dual effects on both capsaicin binding and channel gating. Lowering pH enhances the apparent binding affinity of capsaicin, promotes the occurrences of long openings and short closures, and stabilizes at least one of the open conformations of the channel. Our data also demonstrate that capsaicin binding and protonation of the receptor interact allosterically, where the effect of one can be offset by the effect of the other. These results provide important basis to further understand the nature of the activation pathways of the channel evoked by different stimuli as well as the general mechanism underling the cross-sensitization of pain.

Inhibitory effect of capsaicin on cholinergic transmission in ovine airways: evidence for non-cholinergic contractions

Electrical stimulation of ovine trachealis smooth muscle and bronchial ring segments induced neurogenic and monophasic atropine-sensitive contractions. Pretreatment of the tissues with capsaicin (100 microM) significantly reduced these contractions indicating a possible contribution of a peptidergic neurotransmitter to the contractions. The effect of capsaicin on electrically induced contractions was significantly inhibited by capsazepin indicating an action on vanilloid receptors. In both preparations, electrically induced contractions were not modified by tachykinin NK(1)- and NK(2)-receptor antagonists singly and in combination. It was therefore concluded that a component of the atropine-sensitive electrically induced contractions of ovine airways smooth muscles involved the release of a peptide neurotransmitter which is probably not a tachykinin. However, an action of capsaicin on prejunctional vanilloid receptors located on cholinergic nerves cannot be ruled out.

Modulation of synovial blood flow by the calcitonin gene-related peptide (CGRP) receptor antagonist, CGRP(8-37)

1. The effect of the calcitonin gene-related peptide (CGRP) receptor antagonist, CGRP(8-37) on blood flow in the knee joint of the anaesthetized rat was investigated. 2. Synovial blood flow in both exposed and intact, skin-covered knees was measured by laser Doppler perfusion imaging. 3. Topical application of CGRP(8-37) caused a dose-dependent fall in synovial blood flow in the exposed knee joint of the rat. At low (1.5 nmol) doses of CGRP(8-37) there was no significant effect on synovial blood flow. In rats treated with 7.5 nmol CGRP(8-37) there was a fall in synovial blood flow (maximum effect at 10 min: -28.8 +/- 4.6%; n=7), which returned to resting levels within 30 min. The highest dose (15 nmol) of antagonist used in this study caused a marked (maximum at 10 min: -35.6 +/- 9.3%; n=8), and prolonged (up to 30 min) fall in blood flow. 4. Ten days after surgical denervation, CGRP(9-37) (15 nmol, topical) had no significant effect on blood flow in the rat exposed knee joint (change in flux at 10 min: -5.1+/-3.6%; n=4). This suggests that CGRP(8-37) acts selectively to antagonize the actions of a neurally derived product, probably CGRP, on the rat synovial vasculature. 5. In skin-covered knee joints, intra-articular injection of CGRP(8-37) (15 nmol; bolus) elicited a significant fall in synovial blood flow (maximum effect at 10 min: -15.5 +/- 5.8%; n=6). 6. CGRP (0.01, 0.1 or 1.0 nmol; topical) caused a dose-dependent increase in exposed knee joint blood flow, which was attenuated by co-administration of 1.5 nmol CGRP(8-37). For example, 1 nmol CGRP elicited a peak increase in flux at 10 min of 94.7 +/- 31.8% (n=8) and 28.8 +/- 8.9% (n=7) in the absence and presence of CGRP(8-37), respectively. The vasodilator responses induced by acetylcholine (ACh) (10 nmol, topical; n=4-5) or sodium nitroprusside (SNP) (10 nmol, topical; n=4-5) were unaltered in the presence of CGRP(8-37) (1.5 nmol, topical). 7. Thus, the CGRP receptor antagonist CGRP(8-37) elicits vasoconstriction in the rat synovium. This suggests that the endogenous, basal release of CGRP may play a physiological role in the regulation of blood flow in the rat knee joint.

The vanilloid (capsaicin) receptor: receptor types and species differences

1. Capsaicin was postulated to exert its pharmacological actions by interacting at a specific recognition site (receptor) expressed predominantly by primary afferent neurons. 2. The actual existence of this long-sought "capsaicin-receptor" has recently been demonstrated by the specific binding of [3H]resiniferatoxin (RTX), an ultrapotent capsaicin analog with a unique spectra of actions. 3. Since homovanillic acid is the key structural motif shared by capsaicin and RTX, their recognition site appears to be best termed the vanilloid receptor. 4. Central (sensory ganglia and spinal cord) vanilloid receptors of the rat bind RTX with high affinity in a cooperative fashion; moreover, they recognize capsaicin with higher affinity than the competp6ive antagonist, capsazepine. Peripheral (urinary bladder, urethra, airways, colon) vanilloid receptors, by contrast, bind RTX with lower affinity in a noncooperative manner. An opposite affinity for capsazepine relative to capsaicin appears to distinguish vanilloid receptors in the urinary bladder from those present in the airways or colon. These findings imply heterogeneity in the properties of vanilloid receptors. 5. The affinity of [3H]RTX binding in vitro is influenced by reducing agents, suggesting an in vivo modulatory role for endogenous reducing agents in vanilloid receptor functions. 6. The size of central vanilloid receptors (270 kDa) as measured by radiation inactivation and the cooperative binding both suggest a receptor cluster with cooperating subunits. 7. RTX binds to vanilloid receptors with orders of magnitude higher affinity than capsaicin; its ability to induce cooperative binding is also more pronounced. These differences in receptor binding along with the pharmacokinetical differences in tissue equilibration and in plasma binding may form a rational basis to explain the peculiar spectrum of actions of RTX. 8. Guinea pig spinal cord and airway membranes bind RTX with lower affinity than rat tissues. The receptor density is, however, higher in the guinea pig in keeping with the marked sensitivity of this species to vanilloid actions. 9. The apparently low level of specific [3H]RTX binding sites in the hamster and rabbit is in accord with the resistance of these species to vanilloid actions. 10. In post-mortem human spinal cord specific [3H]RTX binding sites can be detected; their binding parameters are similar to those determined in guinea pig spinal cord. 11. The vanilloid receptor appears to display both intraspecies heterogeneity and marked interspecies differences. 12. As yet, it is known whether the vanilloid receptor is operated by endogenous ligands. It is not known either which receptor superfamily (if any) it belongs to. The [3H]RTX binding assay has, however, the potential of answering these questions.(ABSTRACT TRUNCATED AT 400 WORDS)

Release of calcitonin gene-related peptide-like immunoreactivity (CGRP-LI) from organs of the genitourinary tract in rats

Calcitonin gene-related peptide-like immunoreactivity (CGRP-LI) was detected in the rat urinary bladder, ureter and vas deferens and was depleted by systemic capsaicin desensitization. Exposure to capsaicin in vitro produced a prompt increase in CGRP-LI outflow in superfusates of these tissues, while a second application of the drug was ineffective indicating desensitization. These findings provide further evidence for a transmitter role of CGRP from peripheral endings of sensory nerves and the involvement of CGRP-LI in the specific motor response to capsaicin in the rat genitourinary tract.