Endothelin-1 activates ETA receptors to cause reflex scratching in BALB/c mice

BACKGROUND AND PURPOSE

Endothelin-1 (ET-1) is present in murine and human skin and causes itch (pruritus) when injected in humans. This behavioural study examined the scratch reflex evoked by ET-1 in mice.

EXPERIMENTAL APPROACH

An automated detector was used to determine whether ET-1 causes reflex scratching, the behavioural correlate of itching, in BALB/c mice. Selective agonists and antagonists were used to probe the ET receptor(s) involved.

KEY RESULTS

ET-1 evoked dose-related reflex scratching lasting up to 20 min following intradermal injection (0.1-100 ng; 0.04-40 pmol). The ED(50) for ET-1 induced scratching was 2.1 ng and desensitization occurred with cumulative dosing. High doses of the ET(B) receptor agonist IRL1620 (10 microg; 5.5 nmol), also caused scratching (ED(50) 1.3 microg, 0.7 nmol). The ET(A) receptor antagonist BQ123 significantly reduced scratching evoked by ET-1 and IRL 1620, suggesting that both agonists caused scratching via an ET(A) receptor-dependent mechanism. The ET(B) receptor antagonist BQ788 significantly reduced scratching evoked by IRL1620 but had no effect on scratching evoked by ET-1. This indicated that activation of ET(B) receptors by high doses of ET(B) agonist, but not ET-1, can trigger scratching.

CONCLUSION AND IMPLICATIONS

ET-1 is a potent endogenous activator of reflex scratching (itch). Mechanisms for ET-induced scratching are considered, including direct action of ET-1 on pruriceptive nerve endings and indirect actions via release of endogenous mediators such as histamine from mast cells. ET-1 and ET(A) receptors, possibly also ET(B) receptors, are potential targets for developing specific anti-pruritic drugs to treat pruritic skin disorders such as atopic dermatitis.

Bilateral vagotomy or atropine pre-treatment reduces experimental diesel-soot induced lung inflammation

To investigate the role of the vagus nerve in acute inflammatory and cardiorespiratory responses to diesel particulate (DP) in the rat airway, we measured changes in respiration, blood pressure and neutrophils in lungs of urethane anesthetized Wistar rats 6-h post-instillation of DP (500 microg) and studied the effect of mid-cervical vagotomy or atropine (1 mg kg(-1)) pre-treatment. In conscious rats, we investigated DP, with and without atropine pre-treatment. DP increased neutrophil level in BAL (bronchoalveolar lavage) fluid from intact anesthetized rats to 2.5+/-0.7x10(6) cells (n=8), compared with saline instillation (0.3+/-0.1x10(6), n=7; P<0.05). Vagotomy reduced DP neutrophilia to 0.8+/-0.2x10(6) cells (n=8; P<0.05 vs. intact); atropine reduced DP-induced neutrophilia to 0.3+/-0.2x10(6) (n=4; P<0.05). In conscious rats, DP neutrophilia of 8.5+/-1.8x10(6), n=4, was reduced by pre-treatment with atropine to 2.2+/-1.2x10(6) cells, n=3. Hyperventilation occurred 6 h after DP in anesthetized rats with intact vagi, but not in bilaterally vagotomized or atropine pre-treated animals and was abolished by vagotomy (P<0.05, paired test). There were no significant differences in the other variables (mean blood pressure, heart rate and heart rate variability) measured before and 360 min after DP. In conclusion, DP activates a pro-inflammatory vago-vagal reflex which is reduced by atropine. Muscarinic ACh receptors in the rat lung are involved in DP-induced neutrophilia, and hence muscarinic antagonists may reduce airway and/or cardiovascular inflammation evoked by inhaled atmospheric DP in susceptible individuals.

A repetitive movement detector used for automatic monitoring and quantification of scratching in mice

We have designed an economical non-invasive movement detector for small animal studies and used it for monitoring and quantifying itch in mice. The system is based on a sensitive force transducer positioned below a recording platform holding a lightweight polystyrene recording box in which an animal is placed. A programmed micro-controller is used to discriminate between non-specific movement, grooming behaviour, and scratching movements made by the animal's hind limb. Following sub-dermal injection of histamine receptor agonists into the neck of a mouse, dose-related scratching occurred which was detected and quantified. There was 91% correlation between bouts of scratching as counted manually from playback of the video recording and recorded by the detector. The detector was also able rapidly to count the individual scratch movements of the hind limb that comprise a bout, with 95% accuracy in comparison with manual counting during slow motion playback of video tape, something that is impossible for an unaided observer to achieve because individual scratch movements are too fast to discriminate by eye. Separate detectors were used for the efficient non-invasive study of four animals simultaneously, and this number could easily be increased by adding more platforms. The system could also be modified to record the animal's position within the box, which would be of value in studies involving exploratory behaviour. In summary, the non-invasive multichannel repetitive movement detector will be very useful for accurate measurement of scratching during pruritus studies in small animals, with considerable savings in staff time and effort. It should therefore be a valuable tool for helping to investigate pruritus and in the evaluation of anti-pruritic drugs.

Involvement of histamine H4 and H1 receptors in scratching induced by histamine receptor agonists in Balb C mice

The role of histamine H(1), H(2), H(3) and H(4) receptors in acute itch induced by histamine was investigated in female BalbC mice. Scratching was induced by intradermal injections of pruritogen into the back of the neck and "itch" assessed by quantifying the scratching evoked. Histamine (0.03-80 micromol), histamine-trifluoromethyl-toluidine (HTMT, H(1) agonist, 0.002-2 micromol), clobenpropit (H(4) agonist, H(3) antagonist, 0.002-0.6 micromol) and to a lesser extent imetit (H(3)/H(4) agonist, 0.03-3 micromol) all induced dose-dependent scratching. Dimaprit (H(2) agonist, 0.04-40 micromol) did not cause scratching. Mepyramine (H(1) antagonist, 20 mg kg(-1), i.p.) reduced scratching evoked by histamine and HTMT, but not that caused by H(3) or H(4) agonists. Thioperamide (H(3)/H(4) antagonist, 20 mg kg(-1), i.p.) reduced scratching induced by histamine, H(3) and H(4) agonists, but not that caused by HTMT. The non-sedating H(1) antagonist, terfenadine, also significantly reduced the scratching induced by the H(1) agonist, HTMT. Cimetidine (H(2) antagonist, 20 mg kg(-1), i.p.) did not affect histamine-induced scratching. These results indicate that activation of histamine H(4) receptors causes itch in mice, in addition to the previously recognised role for H(1) receptors in evoking itch. Histamine H(4) receptor antagonists therefore merit investigation as antipruritic agents.

Anandamide induces cardiovascular and respiratory reflexes via vasosensory nerves in the anaesthetized rat

1. We tested the hypothesis that sensory nerves innervating blood vessels play a role in the local and systemic regulation of the cardiovascular and respiratory (CVR) systems. We measured CVR reflexes evoked by administration of anandamide (86 - 863 nmoles) and capsaicin (0.3 - 10 nmoles) into the hindlimb vasculature of anaesthetized rats. 2. Anandamide and capsaicin each caused a rapid dose-dependent reflex fall in blood pressure and an increase in ventilation when injected intra-arterially into the hindlimb. 3. Action of both agonists at the vanilloid receptor (VR1) on perivascular sensory nerves was investigated using capsazepine (1 mg kg(-1) i.a.) a competitive VR1 antagonist, ruthenium red (1 mg kg(-1) i.a.), a non-competitive antagonist at VR1, or a desensitizing dose of capsaicin (200 nmoles i.a.). The cannabinoid receptor antagonist SR141716 (1 mg kg(-1) i.a.) was used to determine agonist activity at the CB(1) receptor. 4. Capsazepine, ruthenium red, or acute VR1 desensitization by capsaicin-pretreatment, markedly attenuated the reflex CVR responses evoked by anandamide and capsaicin (P< 0.05; paired Student's t-test). Blockade of CB(1) had no significant effect on the responses to anandamide. 5. Local sectioning of the femoral and sciatic nerves attenuated CVR responses to anandamide and capsaicin (P< 0.05). Vagotomy or carotid sinus sectioning had no significant effect on anandamide- or capsaicin-induced responses. 6. These data demonstrate that both the endogenous cannabinoid, anandamide, and the vanilloid, capsaicin, evoke CVR reflexes when injected intra-arterially into the rat hindlimb. These responses appear to be mediated reflexly via VR1 located on sensory nerve endings within the hindlimb vasculature.

A comparison of effects measured with isotonic and isometric recording: I. Concentration-effect curves for agonists

Concentration-effect curves were obtained with carbachol tested on isolated preparations of guinea-pig ileum taken from adjacent sites in the same animal, one recorded isotonically, the other isometrically: similar experiments were made with histamine as agonist and with carbachol on rat uterus (in oestrus). The position and steepness of the curves was expressed as the values of [EC(50)] and the exponent, P: with carbachol or histamine on guinea-pig ileum the curves were significantly steeper with isotonic recording (P<0.02, sign test) and displaced towards lower concentrations (P<0.005) but there were significant correlations (P<0.05) between values obtained with tissues from the same animal. The curves for carbachol on the rat uterus were very steep: with isotonic recording the exponent (often eight or more) was consistently higher than with isometric (P<0.001): there was no significant displacement but there was a significant correlation (P<0.05) between values of [EC(50)] obtained with tissues from the same animal. Although the results obtained by the two methods are different, they are correlated. These effects are to be expected because with isotonic recording there can be no change in length until the tension exceeds the load and the tissue bulk sets an upper limit to shortening: the range within which an effect can be measured (the "operational window") is smaller. The observed effects on [EC(50)] and P have been reproduced with theoretical data.

A comparison of effects measured with isotonic and isometric recording: II. Concentration-effect curves for physiological antagonists

If one drug, B, antagonizes another, A, by producing the opposite physiological effect, the antagonist concentration-effect curves should be affected by the recording system, which limits the range of agonist responses. With pieces of isolated guinea-pig ileum taken from adjacent parts of the same animal, one recorded isotonically, the other isometrically with the same load, the isotonic IC(50) values for (-)isoprenaline opposing carbachol or histamine were lower than the isometric values (P<0.01) but there was a significant correlation between them (P<0.01): the isotonic curves were steeper (P<0.01) and there were wider shifts in IC(50) before increasing the agonist reduced the maximum relaxation. In similar experiments with pieces of rat uterus in oestrus from the same animal, the concentration-effect curves for carbachol opposed by increasing concentrations of (-)isoprenaline or (-)adrenaline had slightly lower EC(50) values with isometric recording but there was a significant correlation (P<0.01) with isotonic values. The antagonist effect (ratio of the EC(50) relative to that for the control) was higher with isotonic recording (P<0.01 for (-)isoprenaline, P<0.025 for (-)adrenaline) and all (27) curves were steeper than the corresponding isometric curve (P<0.001). The influence of the method of recording on the results is expected from the narrower operational window and smaller upper limit to relaxation with isotonic recording. A way of obtaining measurements of IC(50) against a standard agonist effect is suggested in an Appendix.

Adjuvant-induced joint inflammation causes very rapid transcription of beta-preprotachykinin and alpha-CGRP genes in innervating sensory ganglia

Neuropeptides synthesized in dorsal root ganglia (DRG) have been implicated in neurogenic inflammation and nociception in experimental and clinical inflammatory arthritis. We examined the very early changes in response to adjuvant injection in a rat model of unilateral tibio-tarsal joint inflammation and subsequent monoarthritis. Within 30 min of adjuvant injection ipsilateral swelling and hyperalgesia were apparent, and marked increases in beta-preprotachykinin-A (beta-PPT-A) and alpha-calcitonin gene-related peptide (CGRP)-encoding mRNAs were observed in small-diameter L5 DRG neurones innervating the affected joint. This response was augmented by recruitment of additional small-diameter DRG neurones expressing beta-PPT-A and CGRP transcripts. The increased mRNA was paralleled by initial increases in L5 DRG content of the protein products, substance P and calcitonin gene-related peptide. Within 15 min of adjuvant injection there were increases in electrical activity in sensory nerves innervating a joint. Blockade of this activity prevented the rapid induction in beta-PPT-A and CGRP mRNA expression in DRG neurones. Increased expression of heteronuclear (intron E) beta-PPT-A RNA suggests that increases in beta-PPT-A mRNA levels were, at least in part, due to transcription. Pre-treatment with the protein synthesis inhibitor cycloheximide had no effect upon the early rise in neuropeptide mRNAS: This and the rapid time course of these changes suggest that increased sensory neural discharge and activation of a latent modulator of transcription are involved.

Anandamide activates peripheral nociceptors in normal and arthritic rat knee joints

The effects of the endogenous cannabinoid anandamide were studied on peripheral, polymodal nociceptors recorded from normal and chronically inflamed (Freund's adjuvant) knee joint afferents in rats anaesthetized with pentobarbitone. Anandamide (860 nmol) caused a rapid, short lasting excitation of a sub-population of capsaicin-sensitive nociceptive afferents in normal knee joints (7.2+/-2.3 impulses s(-1); n=15 units from five animals). In arthritic joints there were 9.7+/-3.0 impulses s(-1) (n=11 from six animals), which was not significantly different from normal joints. The excitation was dose dependent (8.6 - 2900 nmol) and mediated by activation of the vanilloid receptor (VR(1)) as it was abolished by the VR1 antagonist capsazepine (1 mg kg(-1)). Our results show that anandamide, at high doses, can activate nociceptive afferents innervating the rat knee joints, in contrast with its widely described analgesic actions.

Nitric oxide modulates articular sensory discharge and responsiveness to bradykinin in normal and arthritic rats in vivo

Nitric oxide is implicated in peripheral nociceptive processing. This study determined the effects of the nitric oxide synthase inhibitor, L-NAME, on neural discharge from articular C-fibre afferents innervating normal and arthritic ankle joints in anaesthetized rats. Intra-arterial injection of L-NAME (10-20 mg kg(-1)) increased neural discharge in normal and arthritic ankle joints, whereas D-NAME (30 mg kg(-1)) had no effect. The excitation induced by L-NAME (20 mg kg(-1)) was reduced by co-injecting the nitric oxide precursor, L-arginine (50 mg kg(-1)). L-NAME (20 mg kg(-1)) also enhanced responsiveness to bradykinin (10 microg) but only in arthritic rats, whereas L-arginine (50 mg kg(-1)) reduced the excitation by bradykinin (30 microg) in both groups. These results provide evidence that nitric oxide modulates articular C-fibre activity and reduces responsiveness to bradykinin.

Hoe 140 and pseudo-irreversible antagonism in the rat vas deferens in vitro

The effects of bradykinin and the bradykinin B(2) receptor antagonists D-Arg-[Hyp(3),Thi(5,8),D-Phe(7)]-bradykinin (NPC 349) and D-Arg-[Hyp(3),Thi(5),D-Tic(7),Oic(8)]-bradykinin (Hoe 140) were examined in the electrically-stimulated rat vas deferens. Cumulative additions of bradykinin (1-3000 nM) produced two distinct responses: an enhancement in the magnitude of the basal electrically-induced twitch response (neurogenic response) and an increase in the baseline tension (musculotropic response). NPC 349 (10-100 microM) produced concentration-dependent surmountable rightward shifts of both the bradykinin neurogenic and musculotropic response curves. In contrast, while Hoe 140 (10-100 nM) caused an apparently surmountable antagonism of the bradykinin neurogenic response, it caused an apparent insurmountable antagonism of the bradykinin musculotropic response. Interestingly, co-incubation of Hoe 140 (30 nM) with NPC 349 (30 and 100 microM) resulted in a concentration-related upwards displacement of the Hoe 140-suppressed bradykinin musculotropic response curve. Thus, Hoe 140 can be described as a pseudo-irreversible antagonist against the bradykinin musculotropic response. No time-dependent changes were observed in the maximum bradykinin musculotropic response attainable when NPC 349 (100 microM) additions were made for the final 2 or 18 min of the Hoe 140 incubation (20 min). These findings indicate that slow reversibility of Hoe 140 from the bradykinin B(2) receptor is unlikely to be the mechanism responsible for the pseudo-irreversible antagonism of the bradykinin-induced musculotropic response. Instead, we propose an alternative explanation involving a third, unstable and inactive form of the bradykinin B(2) receptor.

Impaired cholinergic dilator response of resistance arteries isolated from patients with Raynaud's disease

AIMS

We examined the effect of cooling on the response to the endothelium-dependent and -independent dilators, acetylcholine (ACh) and sodium nitroprusside (SNP), respectively, in human microvessels in vitro, and compared the responses between Raynaud's disease (RD) patients and controls, in order to assess the pathogenic role of the endothelium in RD.

METHODS

Subcutaneous resistance arteries were dissected from gluteal fat biopsies taken from patients with RD (n=18) and from age-and sex-matched control subjects (n=17). Vessels were cannulated in a small vessel arteriograph, in which a pressure of 50 mmHg was maintained across the vessel wall. Cumulative concentration-response curves for ACh (10-10-10-4 m ) and SNP (10-10-10-3 m ) were generated in vessels at either 37 degrees C or 24 degrees C, with endothelium intact for ACh and removed for SNP (n=6 per group).

RESULTS

Neither dilator showed significant differences in sensitivity when comparing responses between vessels from RD patients and controls, at either temperature, but the maximal relaxation to ACh was depressed in vessels from RD patients compared with controls at 37 degrees C (Emax=45+/-13 in RD vs 89+/-4 in controls; P=0.004).

CONCLUSIONS

These results support the hypothesis that impaired endothelium-dependent vasodilatation is involved in the pathophysiology of RD.

Adenosine A1 receptor-mediated excitation of nociceptive afferents innervating the normal and arthritic rat knee joint

We tested the hypothesis that adenosine excites nociceptive primary afferents innervating the knee joint. Neuronal recordings were made from fine nerve filaments innervating the knee joint in rats anaesthetized with pentobarbitone. Drugs were injected close-arterially (i.a.) or into the articular space (i.art.). We studied normal and chronically inflamed arthritic joints, the latter 14-21 days after a single intra-articular injection of Freund's Complete Adjuvant, performed under halothane anaesthesia. Adenosine injected i.a. caused delayed (approximately 10 s) excitation of the majority of polymodal C-fibre afferents, and had similar effects when injected directly into the joint. Adenosine triphosphate (ATP) had biphasic effects on discharge, a fast (<1 s) excitation was followed by a delayed increase similar to that seen with adenosine. The adenosine A1 receptor agonists N6-cyclopentyladenosine (CPA) and N-[(1S,trans)-2-hydroxypentyl] adenosine (GR79236) also excited the C-fibre afferents. The A1 antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) antagonized the responses evoked by adenosine, CPA, and the delayed increase seen after ATP, indicating that excitation of the nociceptive afferents was mediated via adenosine A1 receptors. Adenosine and ATP evoked delayed excitatory effects of similar magnitude, regardless of whether or not the knee joint was chronically inflamed. The increased basal discharge observed in arthritic joints was unaffected by DPCPX, which implies that the increase in spontaneous activity associated with arthritis is unlikely to involve tonically released adenosine. The results support the hypothesis that adenosine excites primary afferent nociceptive nerve terminals in the rat knee joint, an effect mediated by adenosine A1 receptors. ATP, adenosine, and A1 receptors may play a role in generating the peripheral nociceptive (pain) signal.

P2X receptor-mediated excitation of nociceptive afferents in the normal and arthritic rat knee joint

1. We tested the hypothesis that functional P2X receptors are present on peripheral terminals of primary afferent articular nociceptors in the rat knee joint. Neural activity was recorded extracellularly from the medial articular nerve innervating the knee joint in rats anaesthetized with pentobarbitone. 2. The selective P2X receptor agonist, alphabeta methylene ATP (alphabetameATP), and the endogenous ligand, ATP, caused a rapid short-lasting excitation of a sub-population of C and Adelta nociceptive afferent nerves innervating normal knee joints when injected intra-arterially or intra-articularly, and this effect was antagonized by the non-selective P2 receptor antagonist PPADS. 3. Induction of a chronic (14-21 days) unilateral inflammatory arthritis of the knee joint using locally injected Freund's adjuvant neither increased or decreased responsiveness of joint nociceptors to alphabetameATP or ATP. 4. Our results support the hypothesis that alphabetameATP-sensitive P2X receptors are expressed on peripheral nociceptive afferents in the rat knee joint suggesting that they may be involved in the initiation of nociception and pain.

Functional studies in small arteries do not support a primary role for endothelin in the pathogenesis of Raynaud's disease

Endothelin-1 (ET-1) has been implicated in the pathogenesis of Raynaud's disease (RD). This study examined the effect of cooling on the response to ET-1 in human microvessels. Subcutaneous small arteries were dissected from gluteal fat biopsies taken from patients with RD (n = 20) and from age- and sex-matched control subjects (n = 17) and were cannulated in a small vessel arteriograph. Cumulative concentration-response curves to ET-1 (10(-12) to 3 x 10(-7) M) were obtained in vessels at 37 degrees C and 24 degrees C, with the endothelium either intact or removed (n = 6 per group). There were no significant differences in responses to ET-1 between RD patients and controls in either intact or denuded vessels, at either 37 degrees C or at 24 degrees C. There was, however, a significant endothelium-dependent interaction between the groups when the effect of temperature on the response to ET-1 was examined (p = 0.01; two-way ANOVA). Whereas cooling tended to reduce the sensitivity in RD, the opposite effect was observed in controls. Measurements of plasma ET-1 did not reveal any significant difference between patients with RD and healthy controls. These results suggest that ET-1 does not play a primary pathophysiologic role in RD. ET-1 might be responsible for mediating the prolonged vasospasm in RD, but secondary to another factor(s), such as impaired endothelium-dependent vasodilation.

Activation of P2X receptors for adenosine triphosphate evokes cardiorespiratory reflexes in anaesthetized rats

1. We tested the hypothesis that activation of P2X receptors associated with vagal afferent nerves can evoke a Bezold-Jarisch (B-J) depressor reflex in anaesthetized rats. 2. Injection of alphabeta-methylene ATP (alphabeta-MeATP; 0.6-600 nmol i.v.) evoked a dose-dependent B-J reflex comprising bradycardia, hypotension and apnoea in rats anaesthetized with pentobarbitone. Apnoea was commonly preceded by hyperventilation. Bilateral vagotomy significantly reduced the bradycardia and most of the apnoeic response without affecting hyperventilation, and unmasked a vasopressor response. Hypotension and apnoea were subject to desensitization, and ATP was about 100 times less potent than alphabeta-MeATP in evoking the B-J reflex. 3. ED50 values for responses to alphabeta-MeATP were: bradycardia 14.6 +/- 3.8 nmol; apnoea 47.1 +/- 8.5 nmol; hyperventilation 23.3 +/- 6.0 nmol, n = 14. The ED50 for apnoea was significantly greater than that for bradycardia or hyperventilation (P < 0.05). Atropine (2.8 micromol (kg body wt)-1 i.v.) antagonized the reflex bradycardia and hypotension. 4. The P2 antagonists suramin (14 micromol (kg body wt)-1 i.v.) and PPADS (17 micromol (kg body wt)-1 i.v.) antagonized the bradycardic and apnoeic components of the reflex response to alphabeta-MeATP, without reducing the vasopressor or hyperventilatory responses to the agonist. 5. Recordings from vagal afferents showed that pulmonary inflation receptors were activated by alphabeta-MeATP in 62 % of units recorded (ED50 22 +/- 5 nmol) and this was blocked by PPADS (17 micromol (kg body wt)-1 i.v.); unidentified vagal afferents were also activated. 6. alphabeta-MeATP activated carotid chemoreceptor afferents (ED50 23 +/- 9 nmol), an action that was unaffected by PPADS or suramin. 7. The results support the hypothesis that P2X receptor subtypes for ATP are associated with specific sensory nerves that form part of the homeostatic mechanism for cardiovascular and respiratory regulation and these receptors therefore have physiological, pathological and therapeutic significance.

Antagonist inhibition curves and the measurement of dissociation constants

1. Experiments carried out on guinea-pig isolated ileum with carbachol as agonist and diphenyl-acetoxyethyl-dimethyl-ethyl-ammonium (DADMEA) bromide as antagonist gave results which fit the theoretical relation between fractional inhibition (Q) of the effects of an agonist ([A]) and the concentration of a competitive antagonist ([B]): this also involves the Hill coefficient (logistic slope factor, P) for the agonist concentration-response curve and the degree of agonist stimulation, [A]/[A]50, where [A]50 produces a half-maximum response. 2. Values of IC50 and an exponent, P', can be obtained by fitting Q to [B] using a logistic approximation to the relation. Both P' and IC50 should be greater with higher agonist stimulation but the increase in P' may be masked by errors in extreme values of Q. Estimates of IC50, however, invariably increased with higher agonist stimulation but with a steep concentration-response curve (P > 1) and low agonist stimulation ([A]/[A]50 < 1, IC50 can be less than KD. 3. KD was calculated from the results in three ways; (i) by a least-squares fit of Q to [B] using the values of P and [A]/[A]50 calculated from the control concentration-response curve; (ii) from the value of IC50 for each line and the values of P and [A]/[A]50 and (iii) by using the agonist concentration-response curve to calculate the dose-ratio and estimate of KD for each in the presence of the antagonist. The methods gave similar results (nM: 11 experiments), 12.4 +/- 1.1 (i), 11.7 +/- 0.9 (ii), 14.8 +/- 1.6 (iii) but there are advantages in using methods (i) or (ii) rather than (iii). 4. The method by which KD is calculated is less important than the experimental design: the plan used in this work, with alternative small and large responses from the tissue, is very suitable for estimating KD with low concentrations of antagonists and small dose-ratios. Although it is not a sensitive test for competitive behaviour because only a small range of concentrations of antagonist is tested, the estimate of affinity should be free from complications involved in the use of higher concentrations of antagonist (and agonist) and the nature of the antagonism can always be checked by doing further experiments in the presence of a known competitive antagonist.

Selective blockade of M2 and M3 muscarinic receptors by hexahydrobenzyl-fourdapine and a comparison with zamifenacin

1. 4-Diphenylacetoxy-N-cyclohexylmethyl-piperidine HCl (hexahydro-benz-4DAP) is more active as an antagonist of carbachol at receptors in guinea-pig isolated ileum, log K (pA2) = 6.64 +/- 0.14 (s.e. 7 results), than at receptors in guinea-pig isolated atria, log K = 5.43 +/- 0.14 (7). In the presence of neostigmine bromide (0.2 microM) the value for atria was 5.62 +/- 0.19 (4), so the lower activity on atria cannot be attributed to hydrolysis of the compound by cholinesterases present in this tissue. 2. The limit of solubility of the free base in Krebs solution (pH 7.6) is about 50 microM for both hexahydrobenz-4DAP and benzyl-fourdapine (benz-4DAP). 3. In experiments on guinea-pig isolated ileum with hexahydro-benz-4DAP given together with 4-DAMP methobromide, the combined dose-ratio was consistent with competition: similar results were obtained with benz-4DAP. 4. In rats anaesthetized with pentobarbitone, hexahydro-benz-4DAP antagonized the effects of bethanechol on blood-pressure in doses that had little effect on heart rate or airflow. There was a limit to the effect which could be obtained, however, and the slopes of the Schild plots were less than one. The effects on rat blood-pressure had a half-life of at least 30 min. 5. In similar experiments with zamifenacin the slopes of the Schild plots were close to 1 and the compound was 10 to 20 times as active on blood-pressure at it was on heart-rate. 6. The limited solubility of the base probably accounts for the flat Schild plots obtained with hexahydro-benz-4DAP, which had about 10 fold selectivity for effects on blood-pressure and was more active than expected from tests on isolated ileum.

Induction of transcription factor AP2 mRNA expression in rat primary afferent neurons during acute inflammation

We have examined immediate early gene mRNA expression, using in situ hybridisation in innervating dorsal root ganglion (DRG) neurons following peripheral adjuvant injection. Neuronal expression of mRNAs encoding NGFI-A (nerve growth factor-induced), NGFI-B, c-jun, jun D and jun B was undetectable in untreated controls and was unchanged following adjuvant injection. AP-2 mRNA was expressed in the majority of DRG neurons in untreated controls and was significantly increased (217 +/- 43% control) 1 h after adjuvant injection. AP2 mRNA levels returned to control values by 2 h post-injection. AP-2 may form part of the early transcriptional response that induces neuropeptide gene expression in DRG after adjuvant injection.

Neuropeptide gene expression and capsaicin-sensitive primary afferents: maintenance and spread of adjuvant arthritis in the rat

1. Many experimental and clinical arthritides are characterized by their bilateral nature. There is strong evidence to suggest that this bilateral spread may be mediated by a neuronal mechanism. We have previously shown early and sustained induction of mRNAs encoding preprotachykinin (PPT) and calcitonin gene-related peptide (CGRP) in dorsal root ganglion (DRG) neurons innervating an inflamed, arthritic joint. We have now investigated the involvement of capsaicin-sensitive primary afferents and the expression of neuropeptide mRNAs in the maintenance and bilateral spread of mild adjuvant-induced arthritis in the rat. 2. Capsaicin was applied perineurally to either the left (Cap-L) or right (Cap-R) sciatic nerve of halothane-anaesthetized male Han Wistar rats. Two weeks after capsaicin lesioning, arthritis was induced by injection of Freund's complete adjuvant (FCA) around the left ankle at a dose that caused inflammation of the left ankle joint, and a delayed (14 days) contralateral (right) ankle arthritis. Arthritis was monitored for 15 days after injection, when animals were killed and the lumbar DRG dissected. PPT, CGRP, somatostatin (SS), and vasoactive intestinal polypeptide (VIP) mRNA expression was determined in L5 DRG using in situ hybridization. 3. Spread of inflammation/arthritis to the right limb was associated with bilateral rises in PPT and CGRP mRNA expression in L5 DRG. SS mRNA expression in right DRG was unaffected by spread of inflammation. FCA-L+Cap-L reduced left joint swelling and prevented spread of arthritis to the right joint when assessed by joint swelling. This inhibition of spread of arthritis was associated with significant reductions in all left L5 DRG neuropeptide mRNAs compared with controls, and the rise in right L5 DRG PPT mRNA expression seen in FCA-L-alone animals was blocked. FCA-L+Cap-R also reduced left joint swelling and prevented the spread of inflammation to the right ankle. This lesion prevented the rise in PPT and CGRP mRNA expression seen in right DRG with FCA-L alone. 4. These findings suggest a role for capsaicin-sensitive primary afferents and the primary afferent neuropeptides encoded by PPT and CGRP mRNA in the maintenance and spread of arthritis.

Endothelins and rat carotid body: autoradiographic and functional pharmacological studies

The effects of ET-1 and ET-3 on ventilation and carotid chemosensory discharge have been studied in rats anaesthetised with pentotarbitone. Autoradiographic studies were also performed in vitro to investigate the binding of [125I]ET-1 to rat carotid body, nodose ganglion and brain stem. ET-1 caused a dose-related hyperventilation that was abolished by cutting both carotid sinus nerves. Recordings of chemosensory discharge from the carotid sinus nerve confirmed that ET-1 caused chemoexcitation. ET-3 had only slight effects. The hyperventilation evoked by ET-1 was antagonised by the ETA receptor antagonist FR139317, but responses to hypoxia (10% oxygen) and to cyanide were unaffected. [125I]ET-1 bound to the carotid body, the nodose ganglion and to the brain stem, particularly in the region of the nucleus tractus solitarii. ET-1 binding in the carotid body was displaceable by FR139317, which is consistent with the functional evidence for ETA receptors in the carotid body. The effects of ET-1 on ventilation, coupled with the presence of ET binding sites in areas involved in respiratory and cardiovascular regulation, is consistent with a physiological role for ET in the control of respiration, but our evidence suggests that ET is not crucial for chemotransduction in acute hypoxia.

Endogenous glucocorticoids and the induction and spread of monoarthritis in the rat

Using mono- and bilateral tarsal arthritic models in the rat, we have previously shown increases in the expression of mRNAs encoding substance P and calcitonin gene-related peptide (CGRP) in primary sensory neurons innervating inflamed joints. Dorsal root ganglion (DRG) neuropeptide content in rats is altered by glucocorticoids, and since glucocorticoids regulate the expression of preprotachykinin (PPT) gene, the substance P precursor in other tissues, these effects may be mediated at the level of transcription. Indeed adrenalectomy potentiates disease in polyarthritis although the relationship to joint disease itself is unclear. Secretion of corticosterone in both mono- and bilaterally inflamed rats showed a loss of the normal diurnal nadir with no elevation of evening values. However, there were no changes in glucocorticoid target organs (adrenal gland, thymus and spleen) suggesting the stress was intermittent. Adrenalectomy in mono- and bilaterally inflamed rats did not significantly alter either the severity of inflammation or its spread. Bilaterally inflamed animals did, however, show reduced weight gain. Adrenalectomy had no effect on the induction of PPT and CGRP mRNA expression in innervating DRG neurons in monoarthritis (14 days after adjuvant injection), the unilateral increase in both PPT and CGRP mRNA expression in ADX animals being similar to SHAM arthritic rats. (PPT: ADX 140 +/- 13 left; 99 +/- 6 right % control; SHAM 160 +/- 22 left, 100 +/- 5 right % control. CGRP: ADX 177 +/- 6 left, 97 +/- 3 right % control; SHAM 147 +/- 21 left, 100 +/- 5 right % control).(ABSTRACT TRUNCATED AT 250 WORDS)

Local anaesthesia prevents acute inflammatory changes in neuropeptide messenger RNA expression in rat dorsal root ganglia neurons

Sensory neuropeptides, synthesised in dorsal root ganglia (DRG), are implicated in neurogenic inflammation and nociception in arthritis. Adjuvant monoarthritis increases primary afferent activity and alters expression of neuropeptide genes in DRG. We investigated the role of neural discharge in the early changes in neuropeptide gene expression. Adjuvant injection increased preprotachykinin (PPT) and calcitonin gene-related peptide (CGRP) messenger RNA (mRNA) after 8 h, whereas somatostatin mRNA expression remained unchanged, in innervating L5 DRG neurons. The changes in PPT mRNA expression were prevented by concurrent local anesthesia of the sciatic nerve. Our results suggest that electrical activity mediates, in part, the changes in DRG gene expression in response to acute inflammation.

A discrete adjuvant-induced monoarthritis in the rat: effects of adjuvant dose

Much evidence suggests an important role for the nervous system in the pathogenesis of peripheral inflammatory conditions such as arthritis. The classical adjuvant-induced arthritis model in the rat is a severe condition in which polyarthritis is accompanied by widespread systemic disease, complicating the interpretation of data. We have developed an adjuvant-induced arthritis of the tibio-tarsal joint in the rat, using a low dose of Freund's adjuvant administered locally. Initial inflammation is followed, after 14 days, by chronic monoarthritis which is maintained without generalised effects or loss of use of the limb until at least 30 days postinjection. A higher dose of adjuvant produces contralateral inflammation and arthritis, but only after 14 days, and without the complicating effects seen in classical adjuvant disease. Indirect measures of arthritis (joint circumference, inflammation score and nociceptive threshold) correlate closely with the histological state of the joint, supporting the use of these indices. This model can be easily exploited by alteration of adjuvant dose, to determine the roles of the nervous system in the pathogenesis, maintenance and symmetrical spread of inflammatory arthritis.

The effects of capsaicin, bradykinin, PGE2 and cicaprost on the discharge of articular sensory receptors in vitro

The responses of articular sensory receptors to capsaicin, bradykinin, PGE2, and the selective IP-receptor agonist cicaprost were studied in a rat isolated hindlimb in vitro preparation. Long-term maintenance of normal sensory receptor function was achieved in vitro under conditions of combined superfusion and slow perfusion. Response characteristics to mechanical or chemical stimuli on articular sensory receptors identified in this study did not differ to those reported in vivo. This preparation lacks complex effects mediated via spinal or central reflex mechanisms and allows greater control over the physiological environment of the receptors being studied. These results support the conclusion that the effects of capsaicin, bradykinin and the prostanoids are mediated by distinct pharmacological receptors associated with articular sensory nerve endings. The potent potentiating effects of cicaprost on bradykinin-induced excitation suggests that these actions are mediated via IP-receptors.

Prostanoid-induced potentiation of the excitatory and sensitizing effects of bradykinin on articular mechanonociceptors in the rat ankle joint

Responses of articular mechanonociceptors to intra-arterial injections of either bradykinin alone or in combination with prostaglandin E2, prostaglandin I2 or the selective I-type prostaglandin receptor agonist cicaprost were investigated electrophysiologically in anaesthetized rats. Bradykinin excited 76% of the mechanonociceptors studied and increased their responsiveness to repeated mechanical stimuli in 70% of units. Tachyphylaxis of these responses was evident in all cases. Injections of minimally effective doses of prostaglandin I2 or cicaprost excited the mechanonociceptors and increased their responsiveness to mechanical stimuli. Injections of prostaglandin E2 evoked only small increases in spontaneous discharge. Potentiation of bradykinin-evoked excitation was seen for combined injections of bradykinin with minimally effective or subthreshold doses of cicaprost in 86% of units, prostaglandin I2 in 40% of units and prostaglandin E2 in 56% of units. Combined injections of bradykinin and minimally effective or subthreshold doses of prostanoid agonist caused an increase in the responsiveness of mechanonociceptors to mechanical stimuli greater than that caused by either drug alone in 80% of units for cicaprost, 80% for prostaglandin I2 and 100% for prostaglandin E2. The relative potencies of prostaglandin I2, cicaprost and prostaglandin E2, suggest that prostanoid-induced excitation and sensitization of articular mechanonociceptors is mediated primarily by receptors for the naturally occurring prostanoid prostaglandin I2 (I-type P-receptors). Prostaglandin E2 may be important in potentiation of the sensitizing effects of bradykinin on mechanonociceptor responsiveness.

Excitation and inhibition of rat medial vestibular nucleus neurones by 5-hydroxytryptamine

The effects of 5-hydroxytryptamine (5-HT) and related compounds on the discharge rate of tonically active medial vestibular nucleus (MVN) neurones were studied in an in vitro slice preparation of the dorsal brainstem of the rat. The majority (87 of 107, 82%) of MVN neurones were excited by 5-HT. Nine cells (8%) showed a biphasic response to 5-HT, which consisted of a brief inhibition followed by excitation. Eleven cells (10%) were inhibited by 5-HT. The excitatory effects of 5-HT were mimicked by alpha-methyl-5-HT and antagonised by ketanserin and ritanserin, indicating the involvement of the 5-HT2 subtype of 5-HT receptor. In biphasic cells, blockade of 5-HT2 receptors by ketanserin reduced the excitatory component of the response and revealed an enhanced initial inhibition. The inhibitory effects in biphasic cells, and in cells that showed a pure inhibition in response to 5-HT, were blocked by pindobind-5-HT and mimicked by 8-hydroxy-2-(di-n-propylamino)-tetralin indicating the involvement of 5-HT1A receptors. The significance of these findings in relation to the effects of 5-HT on vestibular reflex function is discussed.

Increased expression of preprotachykinin, calcitonin gene-related peptide, but not vasoactive intestinal peptide messenger RNA in dorsal root ganglia during the development of adjuvant monoarthritis in the rat

Neuropeptides in dorsal root ganglia (DRG) have been implicated in the pathogenesis of pain and neurogenic inflammation in experimental and clinical arthritis. Recently we demonstrated increased levels of substance P (SP) and calcitonin gene-related peptide (CGRP) confined to innervating DRG in adjuvant-mediated monoarthritis. We have now investigated whether changes in peptide content are reflected in altered neuropeptide gene expression and the time course involved. Using in situ hybridization we found marked increases in expression of beta-preprotachykinin (PPT; 81 +/- 24% rise) and alpha-CGRP (44 +/- 6% rise) mRNAs in innervating (ipsilateral L5) DRG neurones only. These increases occurred at the onset of acute inflammation (8 h) and persisted until chronic arthritis developed after 14 days. There were no changes in the proportion of DRG neurones expressing PPT or CGRP mRNAs. Messenger RNA encoding vasoactive intestinal polypeptide (VIP) was not induced. These data suggest that increased synthesis of PPT and CGRP peptides in DRG may play a role in the pathogenesis both of adjuvant-mediated acute inflammation and chronic arthritis.

The contribution of charge to affinity at functional (M3) muscarinic receptors in guinea-pig ileum assessed from the effects of the carbon analogue of 4-DAMP methiodide

1. 4-Diphenylacetoxy-1:1-dimethyl cyclohexane (carbo-4-DAMP) is the carbon analogue of 4-diphenylacetoxy-N-methylpiperidine (4-DAMP) methiodide. The compounds differ only in that the quaternary nitrogen atom in 4-DAMP methiodide is replaced by a quaternary carbon atom, which is uncharged. 2. Carbo-4-DAMP appears to act competitively at functional (M3) muscarinic receptors in guinea-pig ileum. Estimates of log affinity constant are 6.0 at 30 degrees C and 5.9 at 37 degrees C, i.e. the compound has 0.1% of the affinity of 4-DAMP methobromide. 3. The absence of charge makes little difference to the conformation as determined by X-ray crystallography. The bond lengths and angles are very similar, though the bonds in the cyclohexane ring of carbo-4-DAMP are consistently slightly longer than those in the piperidinium ring of 4-DAMP methiodide, and the presence of the charge slightly reduces the space between molecules. 4. The difference between the affinities of 4-DAMP methobromide and carbo-4-DAMP indicates that the contribution of coulombic forces to the binding between 4-DAMP methiodide and muscarinic (M3) receptors is at least 17 kJ mol-1 (4.1 kcal mol-1) at 37 degrees C. How much this is an underestimate depends upon how much hydrophobic binding is greater with the uncharged compound.

Increase in substance P and CGRP, but not somatostatin content of innervating dorsal root ganglia in adjuvant monoarthritis in the rat

Neuropeptides, synthesized in dorsal root ganglia (DRG), are implicated in nociception and neurogenic inflammation. Alterations in DRG neuropeptide levels have been described in polyarthritic rats, but these models are associated with widespread systemic disease. Using mild adjuvant-mediated monoarthritis of the left carpal joint we found significant increases in substance P (+69%) and calcitonin gene-related peptide (CGRP; +204+), but not somatostatin in ipsilateral C6/7 DRG. Peptide levels in contralateral DRG and other ipsilateral DRG were unaltered. Substance P and CGRP in DRG may be of importance in the pathogenesis and maintenance of adjuvant arthritis.

Tonic activity of rat medial vestibular nucleus neurones in vitro and its inhibition by GABA

The spontaneous discharge of 48 medial vestibular nucleus (MVN) neurones was recorded extracellularly in horizontal slices of the rat brainstem in vitro. The mean tonic rate of discharge was 17.1 +/- 8.2 imp/s, similar to that observed by others in transverse (coronal) slices of the rat and guinea pig MVN. The tonic rate of discharge of individual MVN cells either increased or decreased after synaptic blockade in low Ca2+ media, suggesting that ongoing synaptic activity has an important influence on the spontaneous activity of MVN cells in vitro. However the persistence of tonic activity after synaptic blockade indicates that an intrinsic, pacemaker-like mechanism is involved in the generation of the tonic activity. GABA, muscimol, baclofen and 3-APA inhibited the tonic activity of all MVN cells tested. Bicuculline antagonised, and picrotoxin blocked, the inhibitory responses to muscimol, but the effects of GABA were only partially blocked in 50 microM picrotoxin. The effects of baclofen and 3-APA persisted in low Ca2+ media, and were antagonised by saclofen and phaclofen. Picrotoxin-resistant responses to GABA persisted in low Ca2+ media, and were also antagonised by saclofen. These results suggest that the inhibitory control of MVN neurones by GABA involves both the GABAA and GABAB subtypes of GABA receptor. GABAB receptors appear to be distributed both pre- and post-synaptically in the rat MVN. The possible significance of the intrinsic, tonic activity of MVN cells in normal vestibular function and in vestibular compensation, and the effects of GABA, are discussed.

Effects of paracetamol and aspirin on neural activity of joint mechanonociceptors in adjuvant arthritis

1. The effects of paracetamol and lysine acetylsalicylate (L-AS) on high-threshold mechanonociceptors have been investigated by recording neural activity from the inflamed ankle joint in anaesthetized rats with mild adjuvant-induced monoarthritis. 2. Paracetamol (50 mg kg-1, i.v.) and L-AS (100 mg kg-1, i.v., equivalent to 50 mg kg-1 aspirin) both caused a maximal reduction of about 40% in mechanically-evoked discharge and of 30% in ongoing (spontaneous) activity by about 15 min after the injection: a second dose of either drug did not have any significant additional effect on discharge. 3. The prostanoid IP receptor agonist, cicaprost (0.1-0.5 micrograms), increased both mechanically-evoked and ongoing discharge to pre-paracetamol levels when injected close-arterially 30-50 min after paracetamol, whereas prostaglandin E2 (PGE2) was relatively ineffective at restoring activity. 4. The results suggest that prostacyclin (PGI2) contributes to the sensitization of high-threshold joint mechanonociceptors in adjuvant-induced monoarthritis, and that paracetamol and L-AS both act to reduce discharge by inhibiting the synthesis of prostacyclin in the joint capsule. 5. Paracetamol has a direct peripheral action affecting joint capsule mechanonociceptors in rat adjuvant-induced arthritis which is very similar to that of the soluble aspirin preparation, L-AS. These findings, together with the existing literature concerning the anti-arthritic effects of paracetamol, are relevant to the treatment of chronic inflammatory disorders such as rheumatoid arthritis.

The role of PGE2 in the sensitization of mechanoreceptors in normal and inflamed ankle joints of the rat

The role of PGE2 in the sensitization of high-threshold tarsal joint mechanoreceptors (putative nociceptors) has been investigated in 11 arthritic and 16 normal rats. Injections of a low dose of Freund's complete adjuvant at multiple sites into the tissues surrounding the ankle joint induced a chronic unilateral monoarthritis in the injected limb. Measurements of both spontaneous activity and responses of tarsal joint mechanoreceptors to repeated graded mechanical stimuli were made. All of the mechanoreceptors examined had afferent fibres with conduction velocities in the C- or A-delta range. Using this new model of joint inflammation we have shown that lysine acetylsalicylate reduces the mechanical sensitivity of these joint mechanoreceptors and reduces the spontaneous activity in afferent nerve fibres. Prostaglandin E2 is unable to restore either the spontaneous activity in the afferent axon or the mechanical sensitivity of tarsal joint mechanoreceptors after lysine acetylsalicylate in the arthritic rat. Similarly, PGE2 does not sensitize or excite tarsal joint mechanoreceptors in the normal rat. In the normal rat, however, PGE2 potentiates the excitatory action of bradykinin and enhances the sensitizing effect of bradykinin on the responses of joint mechanoreceptors to mechanical stimulation when both substances are injected simultaneously. These results indicate that PGE2 is not important in the sensitization of these joint mechanoreceptors in this model of chronic joint inflammation but that in other circumstances PGE2 may be able to contribute to a sensitization of joint mechanoreceptors by enhancing the action of bradykinin.

PGI2-induced activation and sensitization of articular mechanonociceptors

The effects of PGE2 PGI2 and the stable PGI2 analogue cicaprost on the afferent discharge of ankle joint mechanonociceptors were studied in the anaesthetized rat. Close-arterial injection of PGI2 (0.01-0.1 micrograms) or cicaprost (0.05-5 micrograms) caused both sensitization to mechanical stimulation and excitation of the majority of mechanonociceptors, whereas PGE2 (0.03-3 micrograms) had only weak effects on a small number of nociceptive units. These results suggest the existence of specific PGI2 sensitive receptors (IP receptors) on rat sensory afferent nerves, and support the hypothesis that in the rat endogenous PGI2 plays an important role in the lowering of nociceptive thresholds in inflamed joints.

Localization of [125I]endothelin binding sites in the region of the carotid bifurcation and brainstem of the cat: possible baro- and chemoreceptor involvement

We have demonstrated by autoradiography, displaceable binding for [125I]endothelin-1 ([125I]ET-1), [125I]endothelin-2 ([125I]ET-2), and [125I]endothelin-3 ([125I]ET-3) in the cat carotid bifurcation as well as in the nucleus of the tractus solitarius, where baroreceptor and chemoreceptor afferents from the carotid body and sinus terminate. There was also significant binding in the nodose and superior cervical ganglia. Barosensory and chemosensory discharge was recorded from filaments of the carotid sinus nerve in cats anesthetized with pentobarbitone. Intra-carotid injection of ET-1 or ET-3 (4-402 pmoles) caused transient dose-related depression of baroreceptor discharge without any immediate effects on systemic blood pressure (BP) or heart rate; there was a delayed biphasic effect on BP. ET-1 had little effect on chemosensory discharge during the first 15 s post-injection, but there was a delayed (45-90 s) dose-related increase in discharge. The effects of all three ETs were qualitatively similar, and ET enhanced chemoexcitation evoked by either acetylcholine or sodium cyanide. Our results show that (a) ET binding sites are located in the baroreceptor and chemoreceptor afferent pathways and (b) ETs can influence afferent activity of baroreceptors and chemoreceptors. Further studies are needed to determine the significance of these findings, particularly with regard to reflex control of the cardiovascular system.

The effects of 5-HT on articular sensory receptors in normal and arthritic rats

1. The effects of intra arterial (i.a.) injections of 5-hydroxytryptamine (5-HT, 1-100 micrograms) on the discharge of (a) identified articular high threshold mechanoreceptors and (b) unidentified chemosensitive receptors in the ankle joint have been studied electrophysiologically in anaesthetized normal and arthritic rats. Recordings were made from a fine branch of the medial plantar nerve. 2. 5-HT increased the mechanical responsiveness of high threshold nociceptive mechanoreceptors with C and A delta fibre afferents in both normal and adjuvant-arthritic rats. Receptors in arthritic joints were more sensitive to 5-HT than were those from normal joints. 3. 5-HT produced a complex response from both types of articular receptors following i.a. injection. Two separate components were identified: (a) a fast transient burst of activity was obtained within 10 s of this injection in 66% of units from normal animals and 45% from arthritics, followed by (b) a delayed slow longer-lasting excitation seen in 62% of the units examined from normals and 77% of units from arthritic rats. 4. Increased mechanoreceptor responsiveness produced by 5-HT was reduced or abolished by the 5-HT3 receptor antagonists studied (MDL 72222, ICS 205-930, or GR 38032F, in single doses of 100 micrograms kg-1, i.a.). 5. Fast excitation showed marked tachyphylaxis and was antagonized by MDL 72222, ICS 205-930 or GR 38032F. It was unaffected by ketanserin (100 micrograms kg-1, i.a.). Delayed excitation was reduced or abolished by ketanserin but was unaffected by the 5-HT3-receptor antagonists. 6. Administration of MDL 72222, ICS 205-930 or GR 38032F caused short lasting (< 5 min) reductions in background activity from both types of unit recorded in arthritic rats, as well as in normal rats in which activity had increased following administration of 5-HT. Ketanserin caused similar reductions in background activity in chemosensitive units, but had no effect on mechanoreceptors. 7. At least two types of receptor are involved in the actions of 5-HT on articular sensory receptors with fine afferent fibres. Increased mechano-responsiveness involves a 5-HT3-receptor as does fast excitation. Delayed excitation probably involves a 5-HT2-receptor. Endogenous 5-HT appears not to play a crucial role in sensitization of high threshold mechanoreceptors in this model of chronic inflammation and arthritis, although its local release may potentiate the actions of other inflammatory mediators on sensory receptors in the ankle joint.

Arterial chemoreceptor involvement in salicylate-induced hyperventilation in rats

1. The extent to which peripheral arterial chemoreceptors are involved in the respiratory stimulant action of salicylates has been investigated in rats. 2. Injection of sodium salicylate (200 mg kg-1, single dose i.v.) caused a rapid transient hyperventilation that was not obtained when the carotid chemoreceptors were denervated by section of the carotid sinus nerves. A delayed (10 min) increase in respiration occurred regardless of whether or not the carotid nerves were sectioned. 3. Intravenous infusions of sodium salicylate (0.5 or 4 mg kg-1 min-1) caused hyperventilation in barbiturate-anaesthetized rats. The threshold dose for respiratory stimulation was significantly lower when the carotid sinus nerves were intact than when they were bilaterally sectioned, and the same pattern was observed following intravenous injections of sodium salicylate (cumulative doses) in anaesthetized and conscious rats. 4. Bilateral sectioning of the vagosympathetic nerve trunks did not significantly affect hyperventilation evoked by salicylate, suggesting that this response does not involve actions of salicylate on sensory receptors innervated by these nerves. 5. Administration of salicylate close-arterial to a carotid body, by local perfusion or cross-perfusion of a carotid sinus, led to an increase in respiration when the ipsilateral carotid nerve was intact, but not when it was sectioned. 6. Neuropharmacological studies on anaesthetized rats showed that chemosensory discharge, recorded from a sectioned carotid nerve, increased in response to salicylate injections with a similar dose-response pattern to the hyperventilation. Salicylate had no effect on baroreceptor discharge. 7. We conclude from our experiments that arterial chemoreceptors do contribute to salicylate-induced hyperventilation, and are almost exclusively responsible for the initial phase of the response in rats. Later increases in breathing are independent of reflexes from arterial chemoreceptors and result from actions at other sites, including the CNS. The therapeutic implications of our results are discussed.

Ganglioglomerular nerves influence responsiveness of cat carotid body chemoreceptors to almitrine

A bolus injection of almitrine bismesylate (0.5 mg.kg-1 i.v.) in anaesthetised artificially ventilated cats caused a significantly greater increase in carotid chemosensory discharge in animals with sectioned ipsilateral ganglioglomerular sympathetic nerves in comparison with a group in which these nerves were intact. Plasma levels of almitrine were similar in both groups. Responses to hypoxia and hypercapnia post-almitrine were also bigger if the ganglioglomerular nerves were cut. Domperidone (10-50 micrograms.kg-1 i.a), a dopamine D2 receptor antagonist, greatly increaed the responsiveness of chemoreceptors to almitrine in ganglioglomerular nerve-intact preparations. Almitrine-induced chemosensory activity was unaffected by illuminating the carotid bifurcation with light from a fibre optic lamp, regardless of whether or not the ganglioglomerular nerves were cut. It is concluded that almitrine may directly or indirectly activate an efferent pathway in the ganglioglomerular nerves to cause depression of chemoreceptor activity, possibly by releasing dopamine to act at D2 dopamine receptors in the carotid body.

A study of 5-HT-receptors associated with afferent nerves located in normal and inflamed rat ankle joints

Neural recordings were made from sensory fibres in a nerve supplying the ankle joint in normal rats and in rats with a novel monoarticular arthritis. The responses of mechanically and chemically sensitive units to intra-arterial injections of 5-HT were measured. In most cases the mechanosensitivity of sensory receptors in the ankle joint was not altered by 5-HT. However, 5-HT produced an increase in afferent activity in units which were identified as C-fibres on the basis of action potential amplitude and duration. The receptive fields of these chemosensitive units were not located. The responses of these units to 5-HT were dose dependent and were abolished by the 5-HT2-antogonist, ketanserin, but not by the 5-HT3-receptor antagonist, MDL 72222. The responses of chemosensitive units to injections of 5-HT were similar in normal and arthritic rats although the response was slightly prolonged in arthritic animals.

Characterization of opioid receptors in the cat carotid body involved in chemosensory depression in vivo

The effects of selective opioid receptor agonists and antagonists on neural discharge recorded from carotid body arterial chemoreceptors in vivo were studied in anaesthetized cats. Mean ID50 values were determined for each agonist and used to assess chemodepressant potency on intracarotid (i.c.) injection in animals artificially ventilated with air. [Met]enkephalin, [Leu]enkephalin, [D-Ala2, D-Leu5]enkephalin and [D-Pen2, D-Pen5]enkephalin were more potent chemodepressants than [D-Ala2, Me-Phe4, Gly-ol5]enkephalin, dynorphin (1-8) or ethylketocyclazocine; morphiceptin (mu-agonist) was inactive. The rank order of potency was compatible with the involvement of delta-opioid receptors in opioid-induced depression of chemosensory discharge. ICI 154129, a delta-opioid receptor antagonist, was used in fairly high doses and caused reversible dose-related antagonism of chemodepression induced by [Met]enkephalin. It also antagonized depression caused by single doses of [Leu]enkephalin, [D-Ala2, D-Leu5]enkephalin, [D-Ala2, Me-Phe4, Gly-ol5]enkephalin or dynorphin (1-8). ICI 174864, a more potent and selective delta-opioid receptor antagonist, also antagonized chemodepression induced by [Met]enkephalin or by the selective delta-receptor agonist [D-Pen2, D-Pen5]enkephalin. Comparison of background or 'spontaneous' chemosensory discharge during the 30 min periods immediately before and after injecting ICI 174864 (0.1-0.2 mg kg-1 i.c.) showed a significant increase in discharge in one experiment, but in four others discharge was either unaffected or decreased after the antagonist, which argues against a toxic depression of chemosensors by endogenous opioids under resting conditions in our preparation. Sensitivity of the carotid chemoreceptors to hypoxia (ventilating with 10% O2) was increased significantly after ICI 174864, which could be taken as evidence that endogenous opioids depress chemosensitivity during hypoxia. In contrast, responsiveness to hypercapnia was reduced after the antagonist, implying that endogenous opioids may potentiate chemoreceptor sensitivity during hypercapnia. The results obtained using 'selective' agonists and antagonists provide evidence that depression of chemosensory discharge caused by injected opioids involves a delta type of opioid receptor within the cat carotid body. Endogenous opioids may modulate arterial chemoreceptor sensitivity to physiological stimuli such as hypoxia and hypercapnia.

Pharmacological characterization of the receptor involved in chemoexcitation induced by adenosine

Experiments were performed on cats anaesthetized with pentobarbitone in which carotid body chemoreceptor activity was recorded from the peripheral end of a sectioned carotid nerve. Intracarotid (i.c.) injections of adenosine and its analogues, NECA (5'-N-ethylcarboxamidoadenosine), L-PIA(L-N6-phenylisopropyladenosine), and D-PIA(D-N6-phenylisopropyladenosine), caused dose-related increases in chemosensory discharge. The rank order of potency as chemoreceptor stimulants was: NECA greater than adenosine greater than L-PIA greater than D-PIA. Infusion of theophylline antagonized the chemoexcitatory effects of NECA, and infusion of 8-phenyltheophylline (8-PT), which is a more potent adenosine antagonist with less activity as a phosphodiesterase inhibitor, reduced the chemoexcitation induced by adenosine. Infusion of 8-PT (10 micrograms min-1 i.c.), a dose which substantially reduced the effect of injected adenosine, also reduced the sensitivity of carotid chemoreceptors to hypoxia (10% O2 for 4 min). It is concluded that the adenosine receptors in the cat carotid body which mediate chemosensory excitation are xanthine-sensitive and appear to be of the A2 sub-type. Adenosine, released within the carotid body by physiological stimuli, may be involved in chemoexcitation.

Changes in carotid body amine levels and effects of dopamine on respiration in rats treated neonatally with capsaicin

Dopamine levels in rat carotid bodies and the effects of intravenous dopamine injections on respiration in adult rats anaesthetized with pentobarbitone have been studied in animals which were treated with capsaicin neonatally. Levels of dopamine were five fold higher in the carotid bodies of capsaicin-treated rats as compared with vehicle-treated controls, but there was no significant difference between capsaicin-treated and vehicle-treated rats in their ID50 values for dopamine-induced respiratory depression. Domperidone, a dopamine D2-receptor antagonist, substantially reduced the respiratory depression caused by dopamine, both in capsaicin-treated and in control animals, suggesting that a D2-receptor was involved in the response. Cutting the carotid sinus nerves greatly reduced the ventilatory-depressant effect of dopamine, showing that sensory receptors, most probably arterial chemoreceptors, were responsible for most of the response. Substantially less reflex hyperventilation was evoked in capsaicin-treated rats by the peripheral chemoreceptor stimulants hypoxia and sodium cyanide, in comparison with the controls, and domperidone did not increase the responsiveness. About 80% of the reflex ventilatory change originated from carotid body chemoreceptors. The hypoventilation caused by breathing 100% O2 was not significantly different in capsaicin-treated rats when compared with controls. Domperidone substantially reduced this response in capsaicin-treated rats, but not in vehicle-treated animals. Dopamine-induced respiratory depression in capsaicin-treated rats was slightly enhanced, rather than reduced, by oxygen breathing; domperidone remained an effective antagonist of dopamine-induced ventilatory depression. Most of the reduction in respiration caused by dopamine in rats anaesthetized with pentobarbitone can be attributed to actions on a dopamine D2-receptor located in the carotid body. However, despite the increased levels of dopamine found in the carotid bodies, the reduced peripheral chemosensitivity observed in anaesthetized capsaicin-treated rats does not appear to result from a change in sensitivity to dopamine.

Increased sensitivity of rabbit carotid body chemoreceptors to dopamine after chronic treatment with domperidone

An increase in specific dopamine D2 receptor binding sites was observed in membranes prepared from the carotid bodies of rabbits treated for 8 weeks and then withdrawn for 4-9 days from the D2 antagonist domperidone (2-5 mg/kg per day). Recordings of chemoreceptor afferent discharge from the carotid body also revealed that this change in receptor density was accompanied by an increased sensitivity to the chemodepressant effects of exogenous dopamine. The chemoreceptor responsiveness of the carotid body to hypoxia is blunted in rabbits treated chronically with domperidone, but this can be restored to normal by an acute dose of the D2 antagonist. These experiments provide evidence that is compatible with a chemo-inhibitory role for endogenous dopamine in the rabbit's carotid body. Furthermore, these results suggest that the carotid body provides a useful model for the functional studies of dopamine D2 receptors.

Effects of the antagonists MDL 72222 and ketanserin on responses of cat carotid body chemoreceptors to 5-hydroxytryptamine

The effects of intracarotid (i.c.) injections of 5-hydroxytryptamine (5-HT; 1-50 micrograms) on carotid chemoreceptor activity recorded from the carotid sinus nerve have been studied in anaesthetized cats. Three separate components in the complex response of the chemoreceptors to injected 5-HT were identified. Firstly, a transient burst of activity was obtained during the injection period in 56% of the recordings. Secondly, in all the recordings a period of chemodepression commenced a few seconds after completing the injection and was usually dose-related. Thirdly, a delayed longer-lasting chemoexcitation occurred in many experiments, concomitant with a fall in systemic blood pressure. The neuronal 5-HT receptor antagonist MDL 72222 (10-100 micrograms kg-1, i.c.) virtually abolished the transient chemoexcitation evoked during 5-HT injections and also significantly increased the mean ID50 for 5-HT-induced chemodepression; in 37% of recordings 5-HT caused a dose-related chemoexcitation after the high dose of MDL 72222. Neither the delayed chemoexcitation nor the hypotension caused by 5-HT were much affected by the antagonist. MDL 72222 itself had a biphasic effect on chemosensory discharge, causing depression followed by a delayed excitation. The 5-HT2-receptor antagonist ketanserin (100 micrograms kg-1, i.c.) had no appreciable effect on the transient chemoexcitation evoked during 5-HT injections and caused a slight but significant increase in the mean ID50 for 5-HT-induced chemodepression. The delayed chemoexcitation and accompanying hypotension associated with 5-HT were both substantially reduced or abolished by the antagonist. Ketanserin itself caused a short-lasting period of chemoexcitation. All the effects of injected 5-HT on chemosensory discharge could be abolished by the combination of MDL 72222 and ketanserin (100 micrograms kg-1, i.c.). Neither MDL 72222 nor ketanserin had any significant effect upon the response of the carotid chemoreceptors to hypoxia. The rate at which discharge increased, and also the steady-state discharge before and during hypoxia, were unaffected by the antagonists, alone or in combination. At least two types of 5-HT receptor appeared to be involved in the response of carotid body chemoreceptors to 5-HT. Transient excitation and chemodepression were mediated via MDL 72222-sensitive (peripheral neuronal) receptors whereas the delayed chemoexcitation and associated hypotension involved a ketanserin-sensitive, presumably 5-HT2-, receptor. It appears unlikely that 5-HT plays a crucial role in chemoreception.