Stimulation of α2-adrenergic receptors in nucleus tractus solitarius is required for the baroreceptor reflex

Historic aspects in the identification of the I1 receptor and the pharmacology of imidazolines

The central nervous system is involved in the control of arterial blood pressure. Stimulation of central alpha 2-adrenoceptors in the nucleus tractus solitarii (NTS) decreases sympathetic outflow, resulting in a fall in arterial blood pressure. One of the first antihypertensive substances with actions on the alpha 2-adrenoceptors of the NTS was alpha-methylnoradrenaline. Later on the imidazoline clonidine was developed for which numerous effects, mediated by alpha 2-adrenoceptors, in the CNS could be demonstrated. Since the centrally acting alpha 2-adrenoceptor agonists possess severe side effects, the development of more specific and selective centrally acting imidazolines resulted in the derivatives moxonidine and rilmenidine. The effects of the "second-generation imidazolines" could not be fully understood as alpha 2-adrenoceptor agonists. In the meantime, the rostral ventrolateral medulla (RVLM) has been identified as the site of action of the imidazolines and an I1-imidazoline binding site was characterized in this region. For the antihypertensive action of the imidazolines, agonism at the I1-imidazoline subtype seems to be responsible. In addition, an acid- and heat-stable endogenous substance, called clonidine displacing substance (CDS), was reported to bind at the putative I receptor. In 1992 a receptor protein for I receptors (70 kD) could be separated that is different from that of alpha 2-adrenoceptors. However, up to now we are still lacking the amino-acid sequence of the I receptor and its second messenger system.

Tonic stimulation of GABAB receptors in the nucleus tractus solitarius modulates the baroreceptor reflex

Previous studies have indicated that tonic stimulation of GABAB receptors in the nucleus tractus solitarius (NTS) contributes to the regulation of arterial blood pressure (AP). The present studies examined the hypotheses that (1) tonic stimulation of GABAB receptors in the NTS provides a tonic attenuation of the baroreceptor reflex and (2) enhanced stimulation of these GABAB receptors markedly attenuates the baroreceptor reflex resulting in an increase in AP. In chloralose-anesthetized rats electrical stimulation of the aortic depressor nerve elicited frequency-dependent decreases in AP and heart rate (HR). These responses were markedly attenuated, but not eliminated, by injection of the GABAB receptor agonist baclofen into the ipsilateral NTS. In contrast, the GABAA receptor agonist muscimol completely inhibited aortic depressor nerve-evoked responses. Blockade of GABAB receptors in the NTS by local injection of CGP-35348 elicited a dose-dependent decrease in AP, and a dose-dependent blockade of the pressor response elicited by injection of baclofen into the NTS. These results support the hypothesis that GABA acts tonically on GABAB receptors in the NTS to attenuate the baroreceptor reflex, thereby contributing to the regulation of AP.