Variable α2-adrenoceptor-mediated inhibition of bronchoconstriction and peptide release upon activation of pulmonary afferents

Renal sympathetic nerve activity modulates afferent renal nerve activity by PGE2-dependent activation of α1- and α2-adrenoceptors on renal sensory nerve fibers

Increasing efferent renal sympathetic nerve activity (ERSNA) increases afferent renal nerve activity (ARNA). To test whether the ERSNA-induced increases in ARNA involved norepinephrine activating α-adrenoceptors on the renal sensory nerves, we examined the effects of renal pelvic administration of the α1- and α2-adrenoceptor antagonists prazosin and rauwolscine on the ARNA responses to reflex increases in ERSNA (placing the rat's tail in 49°C water) and renal pelvic perfusion with norepinephrine in anesthetized rats. Hot tail increased ERSNA and ARNA, 6,930 ± 900 and 4,870 ± 670%·s (area under the curve ARNA vs. time). Renal pelvic perfusion with norepinephrine increased ARNA 1,870 ± 210%·s. Immunohistochemical studies showed that the sympathetic and sensory nerves were closely related in the pelvic wall. Renal pelvic perfusion with prazosin blocked and rauwolscine enhanced the ARNA responses to reflex increases in ERSNA and norepinephrine. Studies in a denervated renal pelvic wall preparation showed that norepinephrine increased substance P release, from 8 ± 1 to 16 ± 1 pg/min, and PGE2 release, from 77 ± 11 to 161 ± 23 pg/min, suggesting a role for PGE2 in the norepinephrine-induced activation of renal sensory nerves. Prazosin and indomethacin reduced and rauwolscine enhanced the norepinephrine-induced increases in substance P and PGE2. PGE2 enhanced the norepinephrine-induced activation of renal sensory nerves by stimulation of EP4 receptors. Interaction between ERSNA and ARNA is modulated by norepinephrine, which increases and decreases the activation of the renal sensory nerves by stimulating α1- and α2-adrenoceptors, respectively, on the renal pelvic sensory nerve fibers. Norepinephrine-induced activation of the sensory nerves is dependent on renal pelvic synthesis/release of PGE2.

Effect of sympathetic and parasympathetic mediators on the release of calcitonin gene-related peptide and prostaglandin E from rat dura mater, in vitro

Although not without controversy, an influence of the autonomic nervous system in headache is a matter for current debate. A possible contact site of autonomic and sensory nerves is the dura mater, where they form a dense network accompanying blood vessels. We investigated interactions between autonomic and nociceptive fibres by measuring release of calcitonin gene-related peptide (CGRP) and prostaglandin E2 (PGE2) from the dura mater, in vitro. The parasympathomimetic agent carbachol did not change basal release of CGRP or PGE2, whereas it diminished release induced by a mixture of inflammatory mediators. Norepinephrine did not change induced release of CGRP or PGE2, nor basal release of CGRP. However, basal release of PGE2 was enhanced by norepinephrine, and this enhancement was reduced by serotonin through 5-HT(1D) receptors. We conclude that sympathetic transmitters may control nociceptor sensitivity via increased basal PGE2 levels, a possible mechanism to facilitate headache generation. Parasympathetic transmitters may reduce enhanced nociceptor activity.

Modulation of CGRP and PGE2 release from isolated rat skin by alpha-adrenoceptors and kappa-opioid-receptors

Norepinephrine (NE) reduces the release of neuropeptides from central terminals of primary afferent neurones by presynaptic inhibition. We investigated whether NE also affects stimulus-induced intracutaneous calcitonin gene-related peptide (CGRP) and secondary prostaglandin E2 (PGE2) release. For comparison, kappa-opioid effects were examined. Antidromic electrical nerve stimulation resulted in significant increases in the release of CGRP and PGE2. The PGE2 release was prevented by selective activation of alpha2-adrenoceptors whereas the CGRP release was not changed. In contrast, selective kappa-opioid receptor activation diminished electrically evoked release of both CGRP and PGE2. We conclude that NE affected stimulated PGE2 release via alpha2-adrenoceptors on cells other than cutaneous afferents while kappa-opioid receptors are expressed in peripheral terminals of cutaneous afferents and their activation reduced CGRP release and secondary PGE2 formation.

Prejunctional control of pH 6-induced bronchoconstriction by NK1, NK2, mu-opioid, alpha2-adrenoceptor and glucocorticoid receptors in guinea-pig isolated perfused lung

This study investigated the release of calcitonin-gene related peptide-like (CGRP) immunoreactivity and bronchoconstriction induced by pH 6 buffer in guinea-pig isolated perfused lung. Both pH 6-induced CGRP-like immunoreactivity and bronchoconstriction were completely abolished after systemic pretreatment with capsaicin. Pretreatment with the NK2 receptor antagonist SR 48968 (5 x 10(-7)M) completely inhibited bronchoconstriction and significantly reduced the immunoreactivity induced by the pH 6 buffer. The NK1 antagonist SR 140333 (5 x 10(-7)M) and, to a lesser extent the NK1 antagonist CP 96345, morphine (5 x 10(-6)M), the alpha2-adrenoceptor agonist UK 14304 (10(-7)M) and betamethasone (10(-6)M) significantly reduced both pH 6-induced bronchial response and CGRP-like immunoreactivity overflow. The effects of morphine and UK14304 were partially reversed by naloxone (5 x 10(-5)M) and idazoxan (5 x 10(-50M). Therefore, NK1, NK2, mu-opioid, alpha2-adrenoceptor and glucocorticoid receptors seemed to have a prejunctional action on pH 6 buffer-induced CGRP-like immunoreactivity and bronchoconstriction.

Effects of three alpha 2-adrenoceptor agonists, rilmenidine, UK 14304 and clonidine on bradykinin- and substance P-induced airway microvascular leakage in guinea-pigs

The effects of three alpha 2-adrenoceptor agonists, clonidine, rilmenidine and UK 14304 on the increase of microvascular permeability induced by bradykinin or substance P in guinea-pigs airways have been studied in vivo. Extravasation of intravenously (i.v.) injected Evans blue dye was used as index of permeability. The effects of the three alpha 2-adrenoceptor agonists on the contraction induced by bradykinin (0.3 micrograms.kg-1, i.v.) and substance P (0.3 micrograms.kg-1, i.v.) were also studied on the isolated guinea-pig trachea. The increase of plasma exudation induced by bradykinin (0.3 micrograms.kg-1, i.v.) was inhibited partially by rilmenidine and UK 14304 (20 micrograms and 100 micrograms, intratracheally) respectively. These two substances had no action on the effects of substance P. The effects of rilmenidine and UK 14304 were abolished by alpha 2-blockers (idazoxan 1mg.kg-1 i.v. and RX 821001 100 micrograms.kg-1, i.v.), but they were not altered by the alpha 1-blocker prazosin (30 micrograms.kg-1, i.v.). Under similar conditions, clonidine or the alpha 1-adrenoceptor agonist methoxamine were without significant effects. In vitro, rilmenidine and UK 14304 inhibited partially the contractile effects of bradykinin but not those of substance P. To conclude, both rilmenidine and UK 14304 inhibit the bradykinin-induced increase of vascular permeability in the airways, and they probably do so on peptidergic nerve endings at the prejunctional level since these substances are without effect on substance P. The absence of activity of clonidine in our study might be due to a difference in spectrum of action on the several types of alpha 2-adrenergic or imidazole receptors.

Nerve-pulp interactions

Pulpal haemodynamics are naturally intermeshed with inflammatory responses. Cellular and humoral factors may be the vehicles that aid in physiological regulation, but when these systems are overly activated, they may lead to pathological changes. Sensory nerves may initiate inflammatory reactions when activated, and interestingly, recent findings show that vasoconstrictor nerves in the pulp can inhibit the release of neurally stored vasoactive and inflammatory mediators. Thus, there are options for endogenous control of inflammation. Perhaps a variation in the effectiveness of such control can explain why symptoms of hypersensitivity and pain are so unpredictable and individual. What naturally occurring agents are involved in early tissue changes and how do they act? Some agents exert their effects both on vessels and nerves. Thus, there is an intriguing mutual interplay between nerves and tissue reactions. A prolonged, painful stimulation may generate increased blood flow and inflammation, and vice versa, inflammation may lead to pain. This complexity of mechanisms generates many questions that need answers.

Inhibitory influence of sympathetic nerves on afferent nerve-induced extravasation in the rat incisor pulp upon direct electrical stimulation of the tooth

Previous studies have shown that sympathetic nerve stimulation reduces afferent nerve-induced vasodilation by mechanisms unrelated to vasoconstriction in the rat incisor pulp. The present investigation concerned whether similar modulatory mechanisms might also influence neurogenic plasma extravasation in dental pulp. Rat mandibular incisors were electrically stimulated and blood flow reactions in the pulp were recorded by laser Doppler flowmetry. Plasma extravasation in the incisor pulp, gingiva and lip were indirectly assessed by the Evans-blue method. Stimulation of teeth with 50 microA (5 min) did not cause increased dye accumulation in the stimulated pulps whereas stimulation with 100 microA significantly increased the dye content in ipsilateral pulps by 32% as compared to controls; 100 microA stimulation was without effect in unilaterally denervated animals. Tooth stimulation with 50 microA (5 min), in the presence of either the alpha-adrenergic blocker phenoxybenzamine (3 mg/kg), or the alpha 1-adrenergic antagonist prazosin (50 micrograms/kg), as well as in chronically sympathectomized animals, significantly increased the Evans-blue content in the stimulated pulps by 47, 83 and 86%, respectively. Application of short trains of impulses (same number of impulses as for the continuous stimulation but producing minimal vasoconstriction) resulted in some dye accumulation, which was enhanced in the ipsilateral pulps in the presence of prazosin (100 micrograms/kg) or after acute resection of the superior cervical sympathetic ganglion by 70 and 64%, respectively. The Evans-blue content in the lip and gingiva was uninfluenced by the tooth stimulation. The results indicate that activation of sympathetic nerves inhibits the afferent nerve-induced plasma extravasation in rat incisor pulp and this effect is mediated by alpha-adrenoceptors not associated with vasoconstriction.

Different effects of the K+ channel blockers 4-aminopyridine and charybdotoxin on sensory nerves in guinea-pig lung

In the isolated guinea-pig bronchus, the potassium channel blocking agent 4-aminopyridine (10(-4) M) caused a contraction which was abolished by capsaicin tachyphylaxis, suggesting involvement of sensory neuropeptides. Charybdotoxin (10(-8), 5 x 10(-8) M), which is a potent blocker of the high-conductance Ca(2+)-activated K+ channel in smooth muscle, caused slowly developing and long lasting bronchoconstriction, which was resistant to capsaicin tachyphylaxis. Neither 4-aminopyridine (10(-3), 10(-4) M) nor charybdotoxin (10(-8), 5 x 10(-8) M) had any significant effect on the bronchoconstriction induced by electrical field stimulation. Furthermore, charybdotoxin had no significant influence on the inhibitory effect of the alpha 2-adrenoceptor agonist SKF 35886 (5 x 10(-7) M) on the bronchoconstriction induced by electrical field stimulation. In the isolated perfused guinea-pig lung, 4-aminopyridine (3 x 10(-5) -10(-3) M) caused bronchoconstriction and enhanced both basal and (at 3 x 10(-5) M) vagal nerve stimulation-evoked calcitonin gene-related peptide outflow from pulmonary sensory nerves. In conclusion, 4-aminopyridine stimulated capsaicin-sensitive sensory neurons and enhanced the sensory activation induced by vagal nerve stimulation in guinea-pig lung. Charybdotoxin, on the other hand, caused bronchial contraction independently of capsaicin-sensitive nerves.

Activation of sympathetic nerves exerts an inhibitory influence on afferent nerve-induced vasodilation unrelated to vasoconstriction in rat dental pulp

In order to elucidate a possible influence of the sympathetic nervous system on afferent nerve function, rat mandibular incisors were electrically stimulated and blood flow changes monitored in the incisor pulp of untreated and sympathectomized animals by a laser Doppler flowmeter. Monopolar electrical stimulation of the tooth (200 microA, 5 ms, 40 Hz, 1 s) in normal animals resulted in a transient reduction in pulpal blood flow (PBF) (16% reduction, n = 10) followed by a small but long-lasting increase (11% increase). After administration of phenoxybenzamine or phentolamine (3 mg kg-1, i.v.) the initial dip in PBF was reduced by 59% (P < 0.001) while the subsequent increase was enhanced by 185% (P < 0.001). Similarly, infusion of prazosin (50 micrograms kg-1, i.v.) and idazoxan (0.5 mg kg-1, i.v.) significantly enhanced the increase in PBF by 118 and by 79%, respectively. In chronically sympathectomized animals the increase in PBF was 250% larger than that seen in untreated animals (P < 0.001). This increase in PBF was not further enhanced after alpha-adrenergic blockade. Acute resection of the superior cervical sympathetic ganglion, also resulted in some enhancement (by 56%) of the stimulation-induced increase in PBF (P < 0.01, n = 6). The increase in PBF was unaffected by infusion of timolol (150 micrograms kg-1) and atropine (1 mg kg-1) but was totally abolished by intravenous pre-treatment with capsaicin (1-3 mg kg-1). The present results suggest that activation of sympathetic nerves exerts inhibitory effects on the afferent nerve-induced vasodilation in the rat incisor pulp unrelated to sympathetic vasoconstriction.

Different ion channel mechanisms between low concentrations of capsaicin and high concentrations of capsaicin and nicotine regarding peptide release from pulmonary afferents

Vagal nerve stimulation (1 Hz for 1 min), capsaicin (10(-8) M and 10(-6) M), resiniferatoxin (3 x 10(-10) M) and nicotine (10(-4) M) evoked a non-cholinergic bronchoconstriction in the isolated perfused guinea-pig lung preparation. Simultaneously there was an increase in the perfusate levels of calcitonin gene-related peptide-like immunoreactivity, suggesting release from sensory nerves. Both the bronchoconstriction and peptide release evoked by a low concentration of capsaicin (10(-8) M) and that evoked by nerve stimulation were depressed by tetrodotoxin, suggesting involvement of Na+ channel dependent depolarization. Since the effects of capsaicin (10(-8) M) and vagal nerve stimulation were inhibited by omega-conotoxin but not influenced by nifedipine, the Ca(2+)-channel dependent is probably of N-type. Furthermore, the capsaicin analogue resiniferatoxin also evoked omega-conotoxin sensitive peptide release and bronchoconstriction. At the higher capsaicin concentration (10(-6) M), the functional response was only slightly inhibited by omega-conotoxin or tetrodotoxin indicating that capsaicin at this concentration evoked peptide release and functional effects through other mechanisms, probably involving Ca2+ fluxes in the non-selective cation channel associated with the proposed capsaicin receptor. The nicotine (10(-4) M) evoked peptide release and bronchoconstriction were only marginally influenced by omega-conotoxin or tetrodotoxin. It is concluded that the ion-channel mechanisms underlying the peptide releasing properties of antidromic nerve stimulation and low concentrations of capsaicin are similar and depend on action potential propagation, whereas capsaicin in high, toxic concentration and nicotine mainly act via receptor operated channels.

Release of calcitonin gene-related peptide from sensory neurons

CGRP is released from capsaicin-sensitive sensory neurons in a Ca(2+)-dependent manner in a variety of peripheral organs as well as from central terminals. The mechanisms for CGRP release by low concentrations of capsaicin, electrical antidromic nerve stimulation, and bradykinin have several similar characteristics regarding sensitivity to TTX, CTX, and alpha 2-adrenoceptor activation. High capsaicin concentration and nicotine evoke CGRP release via other mechanisms. The effects of capsaicin, resiniferatoxin, and SO2 are blocked by RR, which probably inhibits ion fluxes associated with capsaicin receptor activation. CGRP released upon irritation of peripheral branches of primary afferents may evoke a variety of cardiovascular actions and influence motility in the gastrointestinal and urogenital tracts.