Modulation of the vagally mediated cardiorespiratory reflexes by medullary GABAB receptors in rats

Activation of GABA$ rm sb{B}$ receptors inhibits spontaneous and evoked neuronal activity in different regions of the central nervous system, including the nucleus tractus solitarius (NTS), the projection site of pulmonary vagal afferent fibres. These studies were designed to test the hypothesis that GABA$ rm sb{B}$ receptors in the NTS modulate the vagally mediated cardiorespiratory reflexes.Experiments were conducted on 53 spontaneously breathing, urethane-anesthetized (1.2-1.6 g/kg) and atropinized (0.2 mg/kg) rats. Tracheal pressure, tidal volume, mean arterial blood pressure, and the integrated diaphragmatic electromyogram (DiEMG) were recorded. The medulla was prepared for bilateral NTS injection of 110 nl of saline (vehicle), and 60 pmol baclofen, and 2.8 nmol CGP35348, a GABA$ rm sb{B}$ receptor agonist and antagonist, respectively. Injection sites were confirmed histologically.Results support the hypothesis, but suggest that the cardiorespiratory pattern and the vagally mediated reflexes are not tonically controlled by GABA$ rm sb{B}$ receptors under the present conditions. The different effects of baclofen on inspiratory time and expiratory time suggest a difference in the distribution of GABA$ rm sb{B}$ receptors in neuronal networks controlling each of these respiratory phases. (Abstract shortened by UMI.