Involvement of I(1)-imidazoline receptors in baroreceptor reflex in the caudal ventrolateral medulla of rats

There is ample evidence to show the existence of center I(1)-imidazoline receptors that are involved in the regulation of cardiovascular activities. The purpose of this study was to examine the possible role of I(1)-imidazoline receptors and alpha(2)-adrenoceptors within the caudal ventrolateral medulla (CVLM) in mediating the baroreceptor reflex in anesthetized rats. Unilateral microinjection of idazoxan (2 nmol in 50 nl), a mixed antagonist of I(1)-imidazoline receptors and alpha(2)-adrenoceptors, into the CVLM significantly (P0.01) decreased blood pressure (BP), heart rate (HR), and the firing rate of presympathetic neurons in the rostral ventrolateral medulla (RVLM) by 21+/-6 mmHg, 25+/-5 beats per min and 3.5+/-0.9 spikes/s, respectively. Moreover, unilateral injection of idazoxan into the CVLM significantly (P0.01) reduced the inhibitory responses of the ipsilateral RVLM presympathetic neurons evoked by stimulation of aortic nerve and elevation of BP, and partially inhibited the neuronal cardiac cycle-related rhythm. Depressor responses evoked by aortic nerve stimulation were significantly (P0.01) attenuated 10 and 20 min after bilateral microinjection of idazoxan (2 nmol in 50 nl for each side) into the CVLM (-20+/-4 and -30+/-4 vs. -40+/-1 mmHg). However, injection of yohimbine (500 pmol in 50 nl), a selective alpha(2)-adrenoceptor antagonist, into the CVLM did not affect the resting cardiovascular activities and baroreceptor reflex. It is concluded that the CVLM I(1)-imidazoline receptors are involved in maintenance of tonic cardiovascular activities and transmission of the baroreceptor reflex.