ROLE OF CAPSAICIN-SENSITIVE SENSORY NERVES IN MEDIATION OF THE CARDIOVASCULAR EFFECTS OF THE ESSENTIAL OIL OF CROTON ZEHNTNERI LEAVES IN ANAESTHETIZED RATS

SUMMARY 1 The essential oil of Croton zehntneri Pax et Hoffm. (EOCZ) contains anethole (42%) and estragole (46%), two isomers that share some chemical structural similarities with capsaicin. The present study investigated the cardiovascular effects of EOCZ and the role of capsaicin‐sensitive sensory nerve fibres in the mediation of these effects in anaesthetized rats. 2 Intravenous bolus injection of EOCZ (1–20 mg/kg) elicited dose‐dependent hypotension and bradycardia that were immediate and transient. Similar responses were also observed with anethole and estragole (both at 10 mg/kg). After cervical bivagotomy or perineural treatment of both cervical vagus nerves with capsaicin (250 mg/mL) to selectively block the conduction of sensory C‐fibres, both cardiovascular responses to EOCZ (10 mg/kg) were abolished. 3 Like capsaicin, an epigastric retrograde intra‐arterial injection of EOCZ (10 mg/kg, i.a.) into the femoral artery elicited a monophasic hypotensive response. This reflex response was blocked by either neonatal pretreatment with capsaicin (50 mg/kg, s.c.) or intrathecal injection of the substance P receptor antagonist RP 67580 (7.8 nmol, at the spinal level L5–L6), suggesting that it is mediated exclusively by substance P‐containing primary afferent fibres. 4 The cardiovascular responses to EOCZ (10 mg/kg, i.v.) were also significantly reduced by the selective vallinoid TPRV1 receptor antagonist capsazepine (1 mg/kg, i.v.). 5 It is concluded that i.v. administration of EOCZ in anaesthetized rats elicits a capsaicin‐like bradycardic and depressor reflex, which appears to be mediated by the activation of vallinoid TPRV1 receptors located on vagal sensory nerves. Like capsaicin, i.a. injection of EOCZ induces a spinally mediated sensory reflex.