The influence of baclofen on reflex circulatory reactions evoked by stimulation of the vagus nerve in the rabbit.

The purpose of our investigations was to investigate the effect of baclofen upon the reflex fall of arterial blood pressure and heart rate evoked by stimulation of bilateral vagus nerves. The experiments were carried out on 30 rabbits of both sexes and mixed breed weighing 3000-4500 g under urethane anaesthesia (1.5 g/kg). Mean arterial pressure and heart rate were measured in the ear middle artery by the electromanometer and registered by PC computer. Parameters of stimulation were: frequency 5c/s., duration-20 s., width of single rectangular pulse-1 msec., intensity of stimulation--multiplicity of threshold excitation (T). The intervals between stimulations were not shorter than 5 min. Baclofen was administrated intravenously (1.0 mg/kg). In the control group the facilitation of circulatory reaction was observed only at the smallest intensity of stimulation (1.0T-1.5T). When greater intensity of stimulations was applied the sum of unilateral effects was greater than the effect of simultaneous bilateral vagus nerve stimulation. This occlusion effect increased up to the stimulus value of 4-5T. After baclofen administration the facilitation of circulatory responses disappeared and only occlusion was observed. Baclofen increased the size of the area of active overlapping of the afferent innervation of the vagus nerve. Administration of baclofen produced an increase in mean arterial blood pressure (15 +/- 6 mm Hg) and heart rate (16 +/- 5 beats per minute) and decrease in the vagal reflex fall of mean arterial pressure and heart rate. Stimulating of the GABAB receptors markedly attenuates the baroreceptor reflex resulting in increase in mean arterial blood pressure and heart rate. These results support the hypothesis that GABA acts tonically on GABAB receptors to attenuate the baroreceptor reflex, thereby contributing to the regulation of circulatory parameters.