Daniel S. McQueen

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

The role of histamine H1, H2, H3 and H4 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 μmol), histamine‐trifluoromethyl‐toluidine (HTMT, H1 agonist, 0.002–2 μmol), clobenpropit (H4 agonist, H3 antagonist, 0.002–0.6 μmol) and to a lesser extent imetit (H3/H4 agonist, 0.03–3 μmol) all induced dose‐dependent scratching. Dimaprit (H2 agonist, 0.04–40 μmol) did not cause scratching. Mepyramine (H1 antagonist, 20 mg kg−1, i.p.) reduced scratching evoked by histamine and HTMT, but not that caused by H3 or H4 agonists. Thioperamide (H3/H4 antagonist, 20 mg kg−1, i.p.) reduced scratching induced by histamine, H3 and H4 agonists, but not that caused by HTMT. The non‐sedating H1 antagonist, terfenadine, also significantly reduced the scratching induced by the H1 agonist, HTMT. Cimetidine (H2 antagonist, 20 mg kg−1, i.p.) did not affect histamine‐induced scratching. These results indicate that activation of histamine H4 receptors causes itch in mice, in addition to the previously recognised role for H1 receptors in evoking itch. Histamine H4 receptor antagonists therefore merit investigation as antipruritic agents.

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.

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, αβ methylene ATP (αβmeATP), and the endogenous ligand, ATP, caused a rapid short‐lasting excitation of a sub‐population of C and Aδ 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 Freunds adjuvant neither increased or decreased responsiveness of joint nociceptors to αβmeATP or ATP. 4 Our results support the hypothesis that αβmeATP‐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.

Effect of adenosine on carotid chemoreceptor activity in the cat.

1 The effects of intracarotid (i.e.) injections or infusions of adenosine on chemoreceptor activity recorded from the peripheral end of a sectioned carotid sinus nerve have been studied in cats anaesthetized with pentobarbitone 2 Adenosine injections (0.1–100 μg) caused a rapid and marked increase of spontaneous chemoreceptor discharge, the intensity, duration and onset of which was dose‐dependent. Infusion of adenosine, 50 μg/min, also evoked an increase in discharge which persisted for the duration of the infusion 3 Both theophylline (1 mg i.e.) and aminophylline (1 mg i.e.) caused short‐lasting decreases in spontaneous discharge but did not prevent the excitatory effect of adenosine. Theophylline increased the excitatory action of adenosine 4 Naloxone (400 μg i.e.) antagonized the depressant effect of morphine on chemoreceptor discharge but not the excitatory action of adenosine 5 It is concluded that exogenous adenosine can excite the cat carotid chemoreceptors, an effect which is not prevented by theophylline in the doses studied. The physiological significance of the findings is discussed.

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.

Changes in the expression of NaV1.7, NaV1.8 and NaV1.9 in a distinct population of dorsal root ganglia innervating the rat knee joint in a model of chronic inflammatory joint pain

Voltage‐gated sodium channels play an essential role in regulating the excitability of nociceptive primary afferent neurones. In particular the tetrodotoxin‐sensitive (TTX‐S) NaV1.7 and the tetrodotoxin‐resistant (TTX‐R) NaV1.8 and NaV1.9 channels have been suggested to play a role in inflammatory pain. Previous work has revealed acute administration of inflammatory mediators, such as Freunds complete adjuvant (FCA) or carrageenan caused an upregulation in the levels of NaV1.7 and NaV1.8 protein in DRG (dorsal root ganglia) tissue up to 4 days post‐insult. In the present study, the expression of NaV1.7, NaV1.8 and NaV1.9 was examined over a 28 day timecourse during a rat model of FCA‐induced chronic inflammatory joint pain. Using the retrograde tracer Fast Blue (FB) and specific NaV1.7, NaV1.8 and NaV1.9 sodium channel antibodies, immunohistochemical staining techniques were used to study sodium channel expression in a distinct population of L3–L5 knee joint afferent DRGs. In the ganglia, counts were made of positively labelled cells in the FB population. The results demonstrate that, following FCA injection, NaV1.9 expression is upregulated at days 14, 21 and 28 post‐FCA, with NaV1.7 and NaV1.8 showing increased channel expression at days 14 and 28. These observations are accompanied by a unilateral joint hypersensitivity in the FCA‐injected knee indicated by a behavioural shift in weight distribution measured using an incapacitance tester. The increased presence of these channels suggests that NaV1.7, NaV1.8 and NaV1.9 play a role, at least in part, in the maintenance of chronic inflammatory pain several weeks after the initial insult.

Pharmacological characterization of the receptor involved in chemoexcitation induced by adenosine.

1 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. 2 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 > adenosine > L‐PIA > D‐PIA. 3 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. 4 Infusion of 8‐PT (10 μg 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). 5 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.

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

SummaryThe role of PGE2 in the sensitization of highthreshold tarsal joint mechanoreceptors (putative nociceptors) has been investigated in 11 arthritic and 16 normal rats. Injections of a low dose of Freunds 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-δ 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.

On the specificity and type of receptor involved in carotid body chemoreceptor activation by adenosine in the cat

1 Experiments were performed on cats anaesthetized with pentobarbitone in which carotid chemoreceptor activity was recorded from the peripheral end of a sectioned carotid sinus nerve. 2 Intracarotid injections of adenosine 5′‐triphosphate (ATP) (1–100 μg i.e.) caused a dose‐related increase in chemosensory discharge which was delayed in onset. 3 The adenosine uptake inhibitor dipyridamole potentiated the chemoexcitatory effects of injected adenosine and ATP. 4 The stable ATP analogue α‐β‐methylene ATP (10–100 μg i.c.) depressed chemoreceptor discharge, which suggests the presence of a P2‐purinoceptor in the carotid body, and provides evidence that the chemoexcitatory effect of ATP results from its hydrolysis to adenosine 5′‐phosphate (AMP)/adenosine. 5 Adenine, inosine, guanosine, cytidine and uridine had no appreciable effect on chemoreceptor discharge. 6 The adenosine R‐site agonists 2′‐chloroadenosine and N6‐methyladenosine had chemoexcitatory effects which were similar to those of adenosine, whereas the P‐site agonist 2′‐deoxyadenosine had no appreciable effect on discharge. 7 We conclude that the adenosine receptor in the cat carotid body has some of the characteristics of an R‐site receptor according to the classification of Londos & Wolff (1977).