Cardiovascular responses to intrathecal administration of endomorphins in anesthetized rats

Endomorphins (EMs), the endogenous, potent and selective μ-opioid receptor agonists, have been shown to decrease systemic arterial pressure (SAP) in rats after intravenous (i.v.) administration. In the present study, cardiovascular responses to intrathecal (i.t.) injection of EMs were investigated in urethane-anesthetized rats. It is noteworthy that EMs elicited decreases in SAP and heart rate (HR) in a dose-dependent manner; 10-300 nmol/kg were injected intrathecally. Furthermore, these vasodepressor and bradycardic effects were significantly antagonized by naloxone (0.5 mg/kg, i.t.). Interestingly, i.t. (5 mg/kg) or i.v. (50 mg/kg) administrations of N^ω-nitro-l-arginine methylester (l-NAME) attenuated the vasodepressor and bradycardic effects. Moreover, pretreatment of the rats with muscarinic receptor antagonist atropine (2 mg/kg, i.v.) and α-adrenoceptor antagonist phentolamine (1 mg/kg, i.v.) significantly reduced the vasodepressor effects of EMs. Nevertheless, pretreatment with β-adrenoceptor antagonist propranolol (2 mg/kg, i.v.) could only block the bradycardia effects induced by EMs, but had no significant effects on the hypotension. In summary, all the results suggested that i.t. administration of EMs decreased SAP and HR which were possibly mediated by the activation of opioid receptors in the rat spinal cord. In addition, nitric oxide (NO) release in both the spinal cord and in peripheral tissues might regulate the cardiovascular activities of EMs, and the muscarinic receptor and adrenoceptor played an important role in the regulation of the cardiovascular responses to i.t. administration of EMs.