Ketamine-xylazine anaesthesia and orofacial administration of substance P: A lethal combination in rats

BACKGROUND AND AIMS

Ketamine+xylazine mixture is a widely used anaesthetic in animal experiments. In rats anaesthetized with this mixture, we have shown that injection of carrageenan, a standard proinflammatory stimulus, into the cheek (intra-oral injection) induced oedema. A likely mediator of this oedema is substance P (SP), a major transmitter of sensory nerves in orofacial tissue. We have assessed the effects of intra-oral injection of SP in rats.

EXPERIMENTAL APPROACH

SP (50-1μg per rat) was injected intra-orally in male adult Holtzman or Wistar rats, anaesthetized with ketamine+xylazine. For comparison, histamine (50μg) and 5-HT (5μg) were similarly injected. Antagonists of SP (SR140333, 2mg/kg), of histamine (pyrilamine, 2mg/kg) or of 5-HT (pizotifen, 2mg/kg) were subcutaneously (s.c.) injected, 30min before the corresponding agonist. Oedema in the cheek was assessed by measuring tissue thickness with calipers.

RESULTS

Intra-oral injection of SP (1-50μg per rat) in Holtzman or Wistar rats anaesthetized with ketamine+xylazine induced, dose-dependently, death within 15min, accompanied by signs of excessive salivation. Rats pretreated with SR140333 were protected against SP-induced lethality and the excessive salivation. However, intra-oral injection of either histamine or 5-HT did not induce death, only a characteristic cheek oedema. These doses of SP injected into the hindpaws of conscious Holtzman and Wistar rats only induced oedema with no deaths. In rats anaesthetized with inhaled isoflurane, intra-oral SP (50μg) induced only cheek oedema, with no deaths or excessive salivation. This oedema was prevented by pre-treating rats with SR140333, pyrilamine and pizotifen.

CONCLUSION

It is likely that the deaths were due to excessive salivation induced by the particular combination of ketamine and SP. Our results are presented as a warning to other experimenters who might use these two otherwise non-toxic conditions and the consequent unexpected and needless loss of experimental animals.

Acrolein relaxes mouse isolated tracheal smooth muscle via a TRPA1-dependent mechanism

Airway sensory C-fibres express TRPA1 channels which have recently been identified as a key chemosensory receptor for acrolein, a toxic and highly prevalent component of smoke. TRPA1 likely plays an intermediary role in eliciting a range of effects induced by acrolein including cough and neurogenic inflammation. Currently, it is not known whether acrolein-induced activation of TRPA1 produces other airway effects including relaxation of mouse airway smooth muscle. The aims of this study were to examine the effects of acrolein on airway smooth muscle tone in mouse isolated trachea, and to characterise the cellular and molecular mechanisms underpinning the effects of acrolein. Isometric tension recording studies were conducted on mouse isolated tracheal segments to characterise acrolein-induced relaxation responses. Release of the relaxant PGE₂ was measured by EIA to examine its role in the response. Use of selective antagonists/inhibitors permitted pharmacological characterisation of the molecular and cellular mechanisms underlying this relaxation response. Acrolein induced dose-dependent relaxation responses in mouse isolated tracheal segments. Importantly, these relaxation responses were significantly inhibited by the TRPA1 antagonists AP-18 and HC-030031, an NK₁ receptor antagonist RP-67580, and the EP₂ receptor antagonist PF-04418948, whilst completely abolished by the non-selective COX inhibitor indomethacin. Acrolein also caused rapid PGE₂ release which was suppressed by HC-030031. In summary, acrolein induced a novel bronchodilator response in mouse airways. Pharmacologic studies indicate that acrolein-induced relaxation likely involves interplay between TRPA1-expressing airway sensory C-fibres, NK₁ receptor-expressing epithelial cells, and EP₂-receptor expressing airway smooth muscle cells.