This study was carried out to determine the effect of intravenous injection of anandamide on pulmonary C-fibre afferents and the cardiorespiratory reflexes. In anaesthetized, spontaneously breathing rats, intravenous bolus injection of anandamide near the right atrium immediately elicited the pulmonary chemoreflex responses, characterized by apnoea, bradycardia and hypotension. After perineural treatment of both cervical vagi with capsaicin to block the conduction of C-fibres, anandamide no longer evoked these reflex responses. In open-chest, and artificially ventilated rats, anandamide injection evoked an abrupt and intense discharge in vagal pulmonary C-fibres in a dose-dependent manner. After injection of the high dose, the fibre discharge generally started within 1 s, reached a peak in -2 s, and returned to baseline within 7 s. The stimulation of C-fibres by anandamide was completely and reversibly blocked by pretreatment with capsazepine, a competitive antagonist of the vanilloid type 1 receptor. Anandamide (0.4 mg kg-1) stimulated ∼93 % of pulmonary C-fibres that were activated by capsaicin at a much lower dose (0.6 μg kg-1); the response to anandamide showed similar intensity, but had slightly longer latency and duration than that to capsaicin. In conclusion, intravenous bolus injection of anandamide evokes a consistent and distinct stimulatory effect on pulmonary C-fibre terminals, and this effect appears to be mediated through an activation of the vanilloid type I receptor.
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