Airway responses to capsaicin in guinea pigs: role of NK-1 and NK-2 neurokinin receptors

The role of NK-1 and NK-2 receptors on the pulmonary response to capsaicin in guinea pigs was evaluated using intravenous infusion of selective nonpeptide antagonists of NK 1 (CP 96345, 300 nmol/kg, and SR 140333, 300 nmol/kg) and NK-2 (SR 48968, 100 nmol/kg) neurokinin receptors. Maximal values of pulmonary dynamic elastance (Edyn) and pulmonary resistance (RL) after capsaicin infusion were significantly lower in the presence of SR 48968 (p < .005). Morphometric analysis of lungs obtained by quick-freezing showed significant attenuation of airway contraction and peribronchiolar edema formation in the presence of NK-2 antagonist (p < .001). When compared to guinea pigs that received only capsaicin, animals that received SR 140333 or CP 96345 showed lower values of Edyn, RL, airway contraction, and peribronchiolar edema, but only the difference in Edyn values was significant. The combination of NK-1 and NK-2 antagonists was not more effective than NK-2 antagonist alone in attenuating capsaicin effects. The results suggest that airway effects of capsaicin are mainly mediated by activation of NK-2 receptors although NK-1 receptors may also play a role.     

Are inner or outer hair cells the source of summating potentials recorded from the round window?

Previous studies indicated that antidromic stimulation of capsaicin-sensitive vagal afferent fibers activated, via peripheral release of tachykinins, nonadrenergic, noncholinergic parasympathetic ganglion neurons that mediate relaxations of guinea pig trachealis. On the basis of the effects of selective agonists and inhibition with a nonselective receptor antagonist (SR 48968), we speculated that tachykinin-mediated activation of neurokinin3 (NK3) receptors might be involved. Using the recently developed NK3-selective receptor antagonist SR 142801, we further assessed the role of NK3 receptors in these relaxant responses. Relaxations of the guinea pig trachea elicited by antidromic stimulation of capsaicin-sensitive vagal afferent nerves were markedly inhibited by 0.3 microM SR 142801 and were abolished by a combination of SR 142801 and either of the NK1-selective receptor antagonists SR 140333 and CP 99994 (0.3 microM each). The NK3 receptor antagonist had similar effects on the relaxant responses elicited by capsaicin and substance P, but it had no effect on relaxations of the trachealis elicited by electrical field stimulation of the postganglionic nerves that innervate the trachealis or by stimulation of the preganglionic parasympathetic vagal nerves that innervate the trachea. These results and the observation that the ganglion neurons that mediate these responses are densely innervated by substance P-containing nerve fibers lead us conclude that stimulation of capsaicin-sensitive visceral afferent fibers activates, upon peripheral release of tachykinins, nonadrenergic, noncholinergic inhibitory neurons innervating guinea pig trachealis via activation of both NK3 and NK1 receptors.     

Regulation of distal esophageal mucosal blood flow: the roles of nitric oxide and substance P

BACKGROUND: An increase in esophageal mucosal blood flow (MBF) may be an important protective mechanism against mucosal injury from noxious agents that are ingested or refluxed. This study investigated the changes in MBF and the regulation thereof after intraluminal application of noxious chemical stimuli. The role, if any, of substance P (SP) and nitric oxide (NO), two potent vasodilatory substances, and the vascular distribution of SP in the distal esophagus were evaluated. METHODS: Esophageal MBF was measured in anesthetized dogs with a laser Doppler flow probe attached to manometry and pH probes. MBF was measured before and after topical application of HCl (2 ml; 1N) or capsaicin (2 ml; 0.5%) in the distal esophagus. The effects on MBF of intraarterial SP and bradykinin were also determined. Pharmacologic antagonists and denervation procedures were used to delineate the mechanisms that regulate MBF. RESULTS: Sequential luminal applications of hydrochloric acid (HCl) or a single application of capsaicin increased MBF (p < 0.01). Topical intraluminal lidocaine blocked the response to capsaicin (p > 0.2) but not to HCl (p < 0.05). Abrupt increases in MBF occurred with intraarterial SP or bradykinin (p < 0.01). Neither atropine nor truncal vagotomy blocked the increase in MBF from these peptides or noxious stimuli. The NO synthesis antagonist NG-nitro-L-arginine methyl ester (L-NAME) blocked the response to bradykinin and attenuated the response to HCl (p < 0.05). NG-nitro-L-arginine methyl ester did not affect the response to SP or capsaicin. A substance P antagonist blocked the effects of both capsaicin (p > 0.6) and SP (p > 0.1) but not that of HCl (p < 0.01) or bradykinin (p > 0.01). CONCLUSIONS: Intraluminal applications of HCl or capsaicin appear to stimulate increases in esophageal MBF by different mechanisms. HCl produces an adaptive response that appears dependent on the paracrine effect of NO. Capsaicin-sensitive neurons mediate vasodilation through SP neurotransmission, independent of extrinsic vagal or cholinergic innervation.     

Stimulation of airway sensory nerves by cyclosporin A and FK506 in guinea-pig isolated bronchus

We have investigated the contractile property of cyclosporin A and FK506 in guinea-pig isolated bronchus. Cyclosporin A (10 microM) failed to significantly attenuate the excitatory non-adrenergic non-cholinergic (eNANC) and cholinergic contractile response (per cent methacholine Emax) induced by electrical field stimulation (EFS). In contrast, eNANC responses were significantly attenuated by both the neurokinin (NK)-1 and (NK)-2 receptor antagonists, N-acetyl-L-tryptophan 3,5-bis (trifluoromethyl)-benzyl and SR48968, respectively. Cyclosporin A and FK506 caused a concentration-dependent contraction in guinea-pig isolated bronchus, which was significantly attenuated by NK-1 and NK-2 receptor antagonists. The capsaicin receptor antagonist, capsazepine (10 microM) significantly reduced the contractile response to cyclosporin A and capsaicin, but not to FK506. The N-type calcium channel blocker, omega-Conotoxin (omegaCTX: 10 nM), significantly reduced the contractile response to FK506 and the eNANC response following EFS. In contrast, omega-CTX failed to significantly reduce the contractile potency to capsaicin or cyclosporin A. In bronchial preparations desensitized by repeated application of capsaicin (1 microM), the contractile responses to both cyclosporin A (100 microM) and FK506 (100 microM), were significantly reduced. In contrast, the contractile responses to substance P and neurokinin A (10 microM) were not altered. Furthermore, repeated application of cyclosporin A (100 microM) significantly inhibited the contractile response to capsaicin (1 microM). The findings from this study would indicate that cyclosporin A and FK506 mediate contraction of guinea-pig isolated bronchus secondary to the release of neuropeptides from airway sensory nerves. However, the release of sensory neuropeptides appears to be mediated via different mechanisms for cyclosporin A and FK506, the former by stimulation of the vanilloid receptor and the latter via opening of N-type calcium channels.     

Vagal, sympathetic and somatic sensory inputs to upper cervical (C1-C3) spinothalamic tract neurons in monkeys

1. Myocardial ischemia activates vagal and sympathetic cardiac afferent fibers. The purpose of this study was to determine a neuro physiological basis for cardiac pain referred to C1-C3 somatic dermatomes. We hypothesized that afferent fibers traveling in vagal or sympathetic nerves transmit nociceptive information to C1-C3 spinothalamic tract (STT) neurons. 2. Electrical stimulation of the left stellate ganglion to excite cardiopulmonary sympathetic afferent fibers increased extracellular activity of 44 of 77 C1-C3 STT neurons examined in 33 anesthetized male monkeys (Macaca fascicularis); responses increased as stimulus strength increased. Additionally, this stimulus inhibited 5 cells, increased/decreased activity of 2 cells, and did not affect 26 cells. 3. Electrical stimulation of the left (ipsilateral) thoracic vagus nerve excited 41 of 78 C1-C3 STT neurons, inhibited 4 neurons, increased/decreased activity of 2 neurons, and did not affect 31 neurons. Responses increased with increasing stimulus strength Contralateral vagal stimulation excited 7 of 39 cells tested, inhibited 4 cells and did not affect 28 cells. 4. Effects of stimulating one or more vagal branches were examined on 22 C1-C3 STT neurons excited by input from left thoracic vagus nerve. Stimulation of the cardiac branch excited 11 of 16 cells tested; stimulation of the recurrent laryngeal nerve excited 11 of 18 cells; stimulation of vagal fibers just rostral to the diaphragm excited 8 of 19 cells. 5. Excitatory somatic receptive fields ranged from small ipsilateral fields to large, sometimes bilateral or noncontinuous fields. Many fields included the ipsilateral neck and/or inferior jaw. Thirty-nine of 74 neurons examined were wide dynamic range (WDR), 21 were high threshold (HT), 6 were low threshold (LT), and 8 did not respond to brushing or noxious pinching of somatic tissues. Most (38 of 39) WDR cells responded to stimulation of the stellate ganglion or vagal fibers, as did 18 of 21 HT cells, 3 of 6 LT cells, and 2 of 8 cells unresponsive to brush or pinch stimuli. 6. Results of this study supported the concept that vagal and/ or sympathetic afferent activation of C1-C3 STT neurons might provide a neural mechanism for referred pain that originates in the heart or other visceral organs but is perceived in the neck and jaw region. Additionally, C1-C3 STT neurons processed sensory information from widespread regions of the body.     

Visceral pain: responses of the reticular formation neurons to gallbladder distension

Visceral projection (gallbladder distension) to the gigantocellular nucleus of the reticular formation of the cat was tested in neurons classified as pain (P), nonpain-pain (NP-P) and nonpain (NP) units, according to their responses to noxious and/or innocuous natural stimuli from the somatic areas. 96% of P neurons (23 out of 24) responded to gallbladder distension. Quantitative criteria showed comparable effectiveness of the somatic and visceral inputs. NP-P neurons reacted to the gallbladder stimulation in 71% of cases (22 out of 31); NP neurons were activated less effectively. Stimulation of either the central tegmental field or "nonspecific" thalamic nuclei evoked direct responses in 38% of P and 26% of NP-P units, which, in most of the P neurons were followed by excitatory and inhibitory phases. The duration of the latter was approximately one second and it greatly affected the responses of the units to somatic as well as to visceral inputs. A large proportion of P neurons responding to a visceral input documents the important role of the reticular formation in the mechanisms of visceral pain. Findings concerning comparable modifying influences upon reactions of P units both in the case of visceral and painful somatic afferentation indicated that similar control mechanisms could be involved.     

Chemosensitivity of nociceptive, mechanosensitive afferent nerve fibres in the guinea-pig ureter

The mechanosensitivity and chemosensitivity of afferent fibres were investigated in an in vitro preparation of the guinea-pig ureter. Electrophysiological recordings were obtained from 5 U-1 (low mechanical threshold, contraction-sensitive) and 74 U-2 units (high threshold). U-2 units had significant higher levels of spontaneous activity, lower conduction velocities, higher mechanical thresholds (U-1: 7 mmHg; U-2: 39 mmHg), less pronounced phasic responses and longer latencies in the response to distensions than the U-1 units. For chemical stimulation, guinea-pig urine (> 800 mosmol/L), bradykinin and capsaicin were applied intraluminally. The responses of U-1 units mainly corresponded to the contractions induced by the chemical stimulation. The vast majority of the U-2 units were excited by urine, bradykinin (threshold: 0.1-1 microM) and capsaicin (threshold: 0.03-0.3 microM). The responses to urine could be mimicked by high concentrations of potassium ions (> 200 mM), but not by an equiosmolar solution of NaCl, urea and mannitol. Chemical stimulation could also result in a transient sensitization of the U-2 units to mechanical stimuli. In the anaesthetized guinea-pig, pseudo-affective responses could be evoked by ureteric distension (threshold: 30-60 mmHg) and serosal application of capsaicin. Intraluminal application of urine in vivo did not evoke any reactions, suggesting that the responses of the U-2 units to urine might be due to an impaired barrier function of the urothelium in vitro. The data are in agreement with the hypothesis that U-2 units are visceral polymodal nociceptors. Since the U-1 units were also able to encode at least noxious mechanical stimuli, their involvement in visceral nociception cannot be excluded.     

Inhibition of sympathetic outflow by the angiotensin II receptor antagonist, eprosartan, but not by losartan, valsartan or irbesartan: relationship to differences in prejunctional angiotensin II receptor blockade

It is well established that angiotensin II can enhance sympathetic nervous system function by activating prejunctional angiotensin II type I (AT1) receptors located on sympathetic nerve terminals. Stimulation of these receptors enhances stimulus-evoked norepinephrine release, leading to increased activation of vascular alpha 1-adrenoceptors and consequently to enhanced vasoconstriction. In the present study, the effects of several chemically distinct nonpeptide angiotensin II receptor antagonists were evaluated on pressor responses evoked by activation of sympathetic outflow through spinal cord stimulation in the pithed rat. Stimulation of thoracolumbar sympathetic outflow in pithed rats produced frequency-dependent pressor responses. Infusion of sub-pressor doses of angiotensin II (40 ng/kg/min) shifted leftward the frequency-response curves for increases in blood pressure, indicating augmented sympathetic outflow. Furthermore, pressor responses resulting in spinal cord stimulation were inhibited by the peptide angiotensin II receptor antagonist, Sar1, Ile8 [angiotensin II] (10 micrograms/kg/min). These results confirm the existence of prejunctional angiotensin II receptors at the vascular neuroeffector junction that facilitate release of norepinephrine. The nonpeptide angiotensin II receptor antagonist, eprosartan (0.3 mg/kg i.v.), inhibited the pressor response induced by spinal cord stimulation in a manner similar to that observed with the peptide antagonist, Sar1, Ile8[angiotensin II]. In contrast, equivalent doses (0.3 mg/kg i.v.) of other nonpeptide angiotensin II receptor antagonists, such as losartan, valsartan, and irbesartan, had no effect on spinal cord stimulation of sympathetic outflow in the pithed rat. Although the mechanism by which eprosartan, but not the other nonpeptide angiotensin II receptor antagonists, inhibits sympathetic outflow in the pithed rat is unknown, one possibility is that eprosartan is a more effective antagonist of prejunctional angiotensin II receptors that augment neurotransmitter release. Because eprosartan is more effective in inhibiting sympathetic nervous system activity compared to other chemically distinct nonpeptide angiotensin II receptor antagonists, eprosartan may be more effective in lowering systolic blood pressure and in treating isolated systolic hypertension.     

Strychnine-dependent allodynia in the urethane-anesthetized rat is segmentally distributed and prevented by intrathecal glycine and betaine

The blockade of spinal glycine receptors with intrathecal strychnine produces a reversible allodynia-like state in the rat. Thus, hair deflection, in the presence of intrathecal strychnine, induces cardiovascular and motor withdrawal responses comparable with those evoked by noxious thermal, mechanical, or chemical stimulation in the absence of strychnine. In the present study, we mapped the cutaneous sites of abnormal sensitivity to hair deflection throughout the strychnine time course to investigate the segmental distribution of strychnine-induced allodynia. The ability of intrathecal glycine and the glycine derivative betaine to reverse strychnine-induced allodynia was also determined using dose-response analysis. Following intrathecal strychnine (40 micrograms), stroking the legs, flanks, lower back, and tail with a cotton-tipped applicator evoked a pronounced increase in mean arterial pressure, tachycardia, and an abrupt motor withdrawal response in urethane-anesthetized rats. These abnormal responses were only evoked by hair deflection at discrete sites, corresponding to the cutaneous dermatomes innervated by spinal segments near the site of strychnine injection. In rats with intrathecal catheters lying laterally in the subarachnoid space, allodynic sites were observed unilaterally on the ipsilateral side of intrathecal strychnine injection. Recovery from strychnine was complete by 30 min in all affected dermatomes. The cardiovascular and motor withdrawal responses to hair deflection were dose dependently inhibited by intrathecal glycine and intrathecal betaine. The ED50 (95% confidence interval) for intrathecal glycine was 609 (429-865) micrograms for the heart rate response, 694 (548-878) micrograms for the pressor response, and 549 (458-658) micrograms for the motor withdrawal response. The corresponding values for intrathecal betaine were 981 (509-1889), 1045 (740-1476), and 1083 (843-1391) micrograms, respectively. There was no difference in the effect of betaine on sensory-evoked cardiovascular and motor responses. Cortical electroencephalographic activity was not affected by intrathecal glycine or betaine, consistent with a spinal locus of action in reversing strychnine-induced allodynia. These results support the hypothesis that removal of spinal glycinergic modulation from low threshold afferent input with intrathecal strychnine results in segmentally localized, tactile-evoked allodynia.     

In vitro and in vivo pharmacology of S 16474, a novel dual tachykinin NK1 and NK2 receptor antagonist

Since tachykinins released from lung sensory nerve endings are thought to play a role in inflammatory diseases of airways via NK1 and NK2 receptors, dual tachykinin NK1 and NK2 receptor antagonists may have a great therapeutic potential. In vitro, the cyclopeptide S 16474 (cyclo-[Abo-Asp(D-Trp(Suc0Na)-Phe-N-(Me)Bzl)]) bound to both human tachykinin NK1 and NK2 receptors expressed in two lines of transfected Chinese hamster ovary cells (IC50 values 85 nM and 129 nM, respectively), while showing a poor affinity for the rat tachykinin NK1 receptor. S 16474 inhibited the contractions induced by substance P in isolated rabbit vena cava (pA2 7.0) and by neurokinin A in rabbit pulmonary artery (pA2 5.6). In vivo in anaesthetized guinea-pigs, S 16474 was found to dose dependently inhibit the bronchoconstrictions induced by intravenously administered substance P, neurokinin A and capsaicin. Plasma extravasation evoked in bronchi by endogenously released tachykinins under vagus nerve stimulation was abolished by S 16474 (10 mu mol/kg i.v.). These results demonstrate clearly that S 16474 is a tachykinin receptor antagonist exhibiting, in vitro and in vivo, a dual inhibitory effect on NK1 and NK2 receptors.     

Tachykinins may mediate capsaicin-induced, but not vagally induced motility in porcine antrum

Tachykinins are thought to be involved in extrinsic control of motility in the gastrointestinal tract. Using the isolated perfused porcine antrum with intact vagal innervation, we studied the effects of substance P, neurokinin A and capsaicin infusion, and electrical stimulation of the vagus nerves on antral motility without or with infusion of non-peptide antagonists for NK-1 receptors (CP96345) and NK-2 receptors (SR48968). Substance P and neurokinin A stimulated antral motility in a dose-dependent manner. The effect could be inhibited by atropine or a combination of the NK-1 and NK-2 receptor antagonists. Electrical stimulation of the vagus nerves and infusion of capsaicin (10(-5) M) stimulated antral motility. Vagally induced motility was not influenced by infusion of CP96345 and SR48968, whereas the effect of capsaicin was blocked. We conclude that tachykinins may be involved in regulation of antral motility through sensory nerves in the porcine antrum, but they do not seem to be involved in vagal regulation of antral motility.     

Cardiopulmonary sympathetic input excites primate cuneothalamic neurons: comparison with spinothalamic tract neurons

Stimulation of cardiopulmonary sympathetic afferent fibers excites thoracic and cervical spinothalamic tract (STT) cells that respond primarily to noxious somatic stimuli. Neurons in dorsal column nuclei respond primarily to innocuous somatic inputs, but noxious stimulation of pelvic viscera activates gracile neurons. The purpose of this study was to compare effects of thoracic visceral input on cuneothalamic and STT neurons. Stellate ganglia of 17 anesthetized monkeys (Macaca fascicularis) were stimulated electrically to activate cardiopulmonary sympathetic afferent fibers. Somatic receptive fields were manipulated with brush, tap, and pinch stimuli. Extracellular discharge rate was recorded for neurons antidromically activated from ventroposterolateral (VPL) thalamus. Stimulation of the ipsilateral stellate ganglion increased activity of 17 of 38 cuneothalamic neurons and of 1 gracilothalamic neuron with an upper body somatic field. Spinal cord transections showed that cardiopulmonary input to cuneothalamic neurons traveled in ipsilateral dorsal column and probably in dorsolateral funiculus. One of eight gracilothalamic neurons with lower body fields was inhibited by cardiopulmonary input, and none were excited. Stimulation of the ipsilateral stellate ganglion increased activity in 10 of 10 T3-T4 STT neurons. Evoked discharge rates, latencies to activation and durations of peristimulus histogram peaks were significantly less for cuneothalamic neurons compared with STT neurons. Furthermore, additional long latency peaks of activity developed in histograms for 6 of 10 STT neurons but never for cuneothalamic neurons. Contralateral cardiopulmonary sympathetic input did not excite cuneothalamic neurons but increased activity of 7 of 10 T3-T4 STT neurons. Most cuneothalamic neurons (24 of 31 cells tested) responded primarily to innocuous somatic stimuli, whereas STT neurons responded primarily or solely to noxious pinch of somatic fields. Neurons that responded to cardiopulmonary input most often had somatic fields located on proximal arm and chest. Results of this study showed that cardiopulmonary input was transmitted in dorsal pathways to cuneate nucleus and then to VPL thalamus and confirmed that STT neurons transmit nociceptive cardiopulmonary input to VPL thalamus. Differences in neuronal responses to noxious stimulation of cardiopulmonary sympathetic afferent fibers suggest that dorsal and ventrolateral pathways to VPL thalamus play different roles in the transmission and integration of nociceptive cardiac information.     

Human thalamic nucleus mediating taste and multiple other sensations related to ingestive behavior

Until now, taste was the only primary sensory modality for which the human central nervous system pathways were unknown. We report sensations evoked by stimulation at microampere current levels in the region of the human thalamic nucleus (ventralis caudalis parvocellularis internis) corresponding to the monkey taste relay nucleus. Stimulation in this region during awake neurosurgical procedures evoked special visceral/somatic (taste/pungent smell), general visceral (fullness of a hollow viscus), as well as painful and nonpainful general somatic sensations. General somatic or visceral sensation was evoked by stimulation at 80% of sites where special visceral/somatic sensation was evoked. These results suggest that primate taste relay mediates multiple sensations in addition to taste.     

Antinociceptive properties of propofol: involvement of spinal cord gamma-aminobutyric acid(A) receptors

In this study, we investigated the interaction of propofol (a compound used widely as an intravenous anesthetic) with gamma-aminobutyric acid(A) (GABA(A)) and delta opioid receptors at the level of the spinal cord. Nociceptive thresholds were measured in rats through the use of electrical current testing (ECT) and tail-flick latency. Full recovery from sedation occurred 36.3 +/- 1.7 min (mean +/- S.E.M.; n = 20) after 40 mg/kg propofol i.p. Forty minutes after administration, there was residual antinociception when assessed by ECT but not when assessed by noxious heat. The ECT antinociceptive effects of propofol at tail but not neck sites were suppressed by intrathecal injection of the GABA(A) antagonists bicuculline and SR-95531 and the delta opioid antagonist naltrindole. These results suggest that there is an interaction between propofol and antagonists at receptors in the caudal segments of the spinal cord responsible for tail innervation. Antagonist dose-response curves were compared with those for suppression of intrathecal midazolam-induced antinociception. All intrathecal antagonists reversed the antinociceptive effect of propofol with the same dose-response curves as those previously obtained for suppression of the effect of intrathecal midazolam. We conclude that propofol, when given intraperitoneally, produces antinociception in rats through an interaction with spinal GABA(A) receptors. This combination leads to activation of a spinal cord system involving a delta opioid receptor; the same mechanisms involved with midazolam-induced spinal antinociception.     

Spinal mechanisms underlying persistent pain and referred hyperalgesia in rats with an experimental ureteric stone

Spinal neurons processing information from the ureter have been characterized in rats 1-4 days after the implantation of an experimental ureteric stone and compared with those of normal rats. The effects of a conditioning noxious stimulation of the ureter in the presence of the hyperalgesia evoked by the calculosis also were examined. Extracellular recordings were performed at the T12-L1 segments of the spinal cord. In rats with calculosis, more neurons expressed a ureter input (53 vs. 42% in normal rats); such cells being more likely to show background activity, at a higher rate than normals (6.6 +/- 1.2 vs. 3.2 +/- 0.9 spikes/s; mean +/- SE) and increasing with the continuing presence of the stone. The threshold pressure for a ureteric response was higher than in normal rats (79 +/- 5 vs. 54 +/- 4 mmHg) but the neurons failed to encode increasing intensities of stimulation. Thirty-five percent of the neurons with exclusively innocuous somatic receptive fields had a ureter input in rats with calculosis, whereas none were seen in normal rats. A noxious ureteric distention applied to neurons with ureter input evoked a complex mixture of increases and decreases in somatic receptive field size and/or somatic input properties markedly different from the generalized increases in excitability seen when such a stimulus was applied to normal animals. We conclude that the presence of a ureteric stone evokes excitability changes of spinal neurons (enhanced background activity, greater number of ureter-driven cells, decreased threshold of convergent somatic receptive fields), which likely account for the referred hyperalgesia seen in rats with calculosis. However, further noxious visceral input occurring in the presence of persistent hyperalgesia produces selective changes that cannot be explained by a generalized excitability increase and suggest that the mechanisms underlying maintenance of hyperalgesia include alteration of both central inhibitory and excitatory systems.     

Interaction of intrathecally infused morphine and lidocaine in rats (part II): effects on the development of tolerance to morphine

BACKGROUND: There has been little information regarding the effects of local anesthetics on tolerance to opioids, although chronic use of combination of opioids and local anesthetics is popular for pain control. This study was designed to examine the effects of lidocaine on morphine tolerance to somatic and visceral antinociception. METHODS: Rats received a continuous intrathecal infusion of morphine (0.3-10 microg x kg(-1) x h(-1)), lidocaine (30-1000 microg x kg(-1). h(-1)), a combination of those, or saline. After 6- day infusion, intrathecal morphine challenge test (5 microg/10 microl) was performed, and time-response curve was constructed to assess the magnitude of tolerance. The tail flick (TF) test and colorectal distension (CD) test were used to measure somatic and visceral antinociceptive effects, respectively. RESULTS: Antinociceptive effects in the TF and CD tests caused by morphine challenge were reduced (P < 0.01) in the morphine infused groups. The magnitude of the tolerance was inversely associated with the amount of morphine infused. Lidocaine infusion induced no different change in the morphine challenge test from that seen in the saline infusion group. Development of tolerance was greater in morphine 3 microg x kg(-1) h(-1) than in morphine 0.75 microg x kg(-1) x h(-1) + lidocaine 150 microg x kg(-1) x h(-1) despite their similar antinociceptive effects during intrathecal infusion. The infusion of a low dose of morphine (0.3 microg kg(-1) x h(-1)) did not reduce the antinociceptive effects in the challenge test. CONCLUSION: Lidocaine in combination with morphine does not reduce tolerance to morphine nor develop cross-tolerance. The intrathecal infusion of morphine induced tolerance to somatic and visceral antinociception in a dose-dependent fashion.     

Intrafamilial clustering and 4-year follow-up of asymptomatic human T-cell leukaemia virus type I (HTLV-I) infection in Benin (West Africa)

Most primary sensory neurones depend on neurotrophins for survival. Mutant mice in which TrkA, the high-affinity receptor for nerve growth factor (NGF), has been inactivated lack nociceptive neurones in sensory ganglia and do not respond to noxious stimuli. The cornea of the eye is innervated by trigeminal neurones that are activated by noxious mechanical, thermal and chemical stimuli. In the human cornea, these stimuli evoke only sensations of pain. We have analysed the innervation pattern and the response to noxious stimulation of the cornea of trkA (-/-) mutant mice. Corneal nerves were stained with the gold chloride impregnation method. Corneal sensitivity to noxious stimuli was assessed by counting blinking movements evoked by von Frey hairs, topical application of saline at different temperatures and application of acetic acid and capsaicin at different concentrations. In the cornea of trkA (-/-) mutant animals, we observed a drastic reduction in the number of nerve trunks and branches in the corneal stroma. Furthermore, quantitative analysis of the number of thin nerve terminals revealed a marked decrease in the corneal epithelium of trkA (-/-) mice when compared to those present in wild type and trkA (+/-) animals. The blinking response of trkA (-/-) mice to mechanical, thermal and chemical noxious stimuli was also significantly reduced. These results indicate that the population of corneal sensory neurones is markedly depleted in trkA (-/-) mutant mice. However, a small portion of corneal sensory neurones survive in these mice suggesting that they may be NGF independent. On the basis of our results, we propose that these surviving cells are polymodal nociceptive neurones, sensitive to mechanical stimulation, noxious heat and acid.     

Generation of chromophore Tyr-containing mutants of the ribosomal protein L7/L12 from Escherichia coli by site-directed mutagenesis and characterization

1. The effects of tachykinins and capsaicin were studied by means of intracellular membrane potential and isometric tension recordings in the isolated trachea of the guinea-pig. 2. The basal membrane potential averaged -51 mV, and most preparations demonstrated spontaneous slow waves. Tetraethylammonium (TEA), a potassium channel blocker (8 x 10(-3) M), depolarized the membrane potential to -44 mV and induced a rhythmic activity. 3. In control solution, substance P (10(-8)-10(-6) M), [Nle10]-neurokinin A(4-10) (10(-8)-10(-6) M) and capsaicin (10(-7)-10(-6) M) induced concentration-dependent depolarizations which were statistically significant at the highest concentration tested (depolarization by 10(-6) M: 8, 11 and 16 mV for the NK1 agonist, the NK2 agonist and capsaicin, respectively). 4. In the presence of TEA (8 x 10(-3) M), the three substances induced depolarizations which were statistically significant at the highest concentration tested for substance P (10(-6) M) and at 10(-7) and 10(-6) M for both [Nle10]-neurokinin A(4-10) and capsaicin (depolarization by 10(-6) M: 11, 17 and 10 mV for substance P, [Nle10]neurokinin A(4-10) and capsaicin, respectively). 5. In the presence or absence of tetraethylammonium, [MePhe7]-neurokinin B (10(-8)-10(-6) M) did not induce any significant changes in membrane potential. 6. The depolarizing effects of substance P (10(-6) M) and [Nle10]-neurokinin A(4-10) (10(-6) M) were blocked only by the specific antagonists for NK1 and NK2 receptors, SR 140333 (10(-7) M) and SR 48968 (10(-7) M), respectively. The effects of capsaicin (10(-6) M) were partially inhibited by each antagonist and fully blocked by their combination. 7. Substance P (10(-9) to 10(-4) M), [Nle10]-neurokinin A(4-10) (10(-10) to 10(-5) M), [MePhe7]-neurokinin B and capsaicin (10(-7) to 10(-5) M) evoked concentration-dependent contractions. 8. The contractions to substance P were significantly inhibited by SR 140333 (10(-8) to 10(-6) M) but unaffected by SR 48968 (10(-8) to 10(-6) M). Furthermore, the response to [Nle10]-neurokinin A(4-10) was significantly inhibited by SR 48968 and unaffected by SR 140333 at the same concentrations. Although SR 48968 (10(-7) M) alone did not influence the effects of substance P, it potentiated the inhibitory effect of SR 140333 (10(-7) M). A similar synergetic effect of these two compounds was observed in the inhibition of the contractile response to [Nle10]-neurokinin A(4-10). 9. Neither SR 140333 (10(-7) M) nor SR 48968 (10(-7) M) alone influenced the contractions to [MePhe7]-neurokinin B and capsaicin. However, the combination of the two antagonists abolished the contractions to either peptide. 10. These results demonstrate that the stimulation of both NK1 and NK2 tachykinin-receptors induced contraction and depolarization of the guinea-pig tracheal smooth muscle and that both receptors were stimulated during the endogenous release of tachykinins by capsaicin. There was no evidence for a major role of NK3 receptors in the contractile and electrical activity of the guinea-pig isolated trachea.     

Effect of early cyclosporine levels on kidney allograft rejection

Experiments were performed on strips of mouse stomach and urinary bladder to characterize the receptors involved in the contractile responses of these tissues to neurokinins (substance P (SP), neurokinin A (NKA), neurokinin B (NKB), and neuropeptide gamma (NP gamma). The neurokinin receptors were characterized by using assays with selective agonists as well as peptide and nonpeptide antagonists and by applying the two Schild criteria for receptor classification, namely, the order of potency of agonists and the apparent affinity of competitive antagonists. The mouse stomach contains primarily NK1 and NK2 functional sites and possibly some NK3 receptors, whereas the urinary bladder possesses only the NK2 receptor. The rank order of potency of agonists in the stomach is Ac[Arg6,Sar9,Met(O2)11]SP-(6-11) > NKA > SP > [beta-Ala8]NKA-(4-10) > NKB > [MePhe7]NKB. Among the selective agonists, Ac[Arg6,Sar9,Met(O2)11]SP-(6-11) is more active than SP and [Sar9,Met(O2)11]SP on the NK1 receptor, whereas the order of potency on the NK2 receptor is NKA > NP gamma > or = [beta-Ala8]NKA-(4-10) > [Nle10]NKA-(4-10). The order of potency of agonists in the bladder is NP gamma > NKA > [beta-Ala8]NKA-(4-10). The myotropic responses mediated by NK1 selective agonists are blocked by SR 140333 (pA2 8.57) and those mediated by the NK2 selective agonists are inhibited by SR 48968 (pA2 9.05). RP 67580 (pA2 8.41) is more active than CP 99994 (pA2 6.06) on the mouse NK1 receptor. The NK1 receptor of the mouse shows, therefore, a pharmacological profile similar to that of the NK1 receptor of the rat. Similarly, MEN 10627 (pA2 9.20) is more active than R 396 (pA2 6.21), suggesting that the mouse NK2 receptor is similar to that of the rabbit. The mouse NK2 receptor of the urinary bladder shows similarity with that of the stomach, but is less sensitive to [beta-Ala8]NKA-(4-10).     

Capsaicin in the 4th ventricle abolishes retching and transmission of emetic vagal afferents to solitary nucleus neurons

Systemic tachykinin NK1 receptor antagonists and resiniferatoxin are known to abolish vomiting mediated by vagal afferents. Emetic vagal afferents have been shown to make synaptic contact with neurons in the medial solitary nucleus. These results suggest that substance P participates in the synapse as a mediator. To examine this possibility, the effects of 4th-ventricular application of capsaicin (0.033-33 mM, 20-30 microl) and resiniferatoxin (1.6-160 microM, 20-30 microl) on the activity of neurons in the medial solitary nucleus and fictive retching induced by vagal stimulation were observed in paralyzed decerebrate dogs. Capsaicin (33 mM) and resiniferatoxin (160 microM) initially increased the neuronal firing and occasionally produced retching, then abolished both neuronal and retching responses. However, stimulation of the medial solitary nucleus continued to provoke retching. Field potential changes in the medial solitary nucleus evoked by pulse-train vagal stimulation decreased in amplitude, but did not disappear. Latencies of neuronal firing and evoked potentials were about 300 ms. These results suggest that emetic vagal afferents are capsaicin-sensitive C fibers which may have substance P as an excitatory transmitter or modulator.