Pharmacological characterization of an esophago-cardiovascular reflex in the rat

Yao, Dongyuan (1996) Pharmacological characterization of an esophago-cardiovascular reflex in the rat. Masters thesis, Memorial University of Newfoundland.

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Abstract

The distension of hollow viscera is known to evoke a range of autonomic visceromotor and behavioral responses subsumed under the term "pseudoaffective" reflex. Although some physiological properties of "pseudoaffective" reflex responses to esophageal distension have been previously described in the rat, only limited pharmacological data are currently available. The present investigation focused on an esophageal distension-evoked pressor/cardioaccelerator response in the urethane-anaesthetized rat. Our primary objectives were; i) to determine whether relevant sensory input from the esophagus to the CNS is conveyed by vagal or spinal afferents; and ii) to study the effects of antinociceptive agents on this reflex, and their sites of actions. Experiments were carried out on male, Sprague-Dawley rats (300-450 g), previously prepared for intrathecal (i.t.) injection at predetermined spinal segments, and anaesthetized with urethane (0.8 g + 20 mg/kg/h i.v.). Esophageal distension evoked a reproducible increase in arterial blood pressure and heart rate. Both components increased linearly with the log of inflation pressure (25-150 mmHg). Lower, being more effective than upper esophageal distension (100 mmHg for 20 sec), was insensitive to i.t. thoracic morphine (4-16 μg). However, i.v. morphine (1.0-4.0 mg/kg) produced a dose-dependent inhibition of the evoked responses. This esophago-cardiovascular reflex was: attenuated by unilateral, and abolished by bilateral cervical vagotomy; and inhibited by the i.t. thoracic, but not by i.t. lumbar or i.v. injection of dexmedetomidine (DX, 0.05-0.5 μg). The evoked response was inhibited by neonatal capsaicin, and by the topical administration of DX or morphine to the solitary complex (NTS). The pressor response persisted after i.v. pancuronium, scopolamine, and methscoplamine. The data indicate that: 1) the sensory information of this esophago-cardiovascular reflex is conveyed to the CNS via vagal afferents; 2) the efferent limb of this reflex is comprised of sympathetic cardioaccelerator- and vasoconstrictor-preganglionic pathways in the IML of the thoracic spinal cord; and 3) the internuncial connections are made up of bulbo-spinal pathways probably originating in the RVLM which in turn, receive input from the NTS. -- Keywords: esophageal distension; morphine; dexmedetomidine; intrathecal; solitary complex; spinal; vagal; pseudoaffective reflex; esophageal pain; rat

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