Central alpha-adrenoceptors involved in cardiovascular regulation

It appears to be established that the central alpha-adrenoceptors involved in the control of cardiovascular function belong to the alpha 2-subtype. They are stimulated by catecholamines, clonidine, and other imidazoline derivatives but also by compounds of rather heterogenous structure. Activation of these central alpha 2-adrenoceptors results in an increase of cardiac vagal tone as well as in decreases of sympathetic nervous activity, blood pressure, and heart rate. alpha 2-Adrenoceptors in the nucleus of the solitary tract appear to be the main site of action of clonidine, followed, in decreasing order of likelihood, by those in the intermediolateral cell column of the spinal cord and the anterior hypothalamus. The location of the alpha 2-adrenoceptors relative to synapses is controversial. Two possibilities have so far been discussed: a location at the terminals of adrenergic neurons (presynaptic) or on the cell bodies or dendrites of not yet identified neurons innervated by adrenergic neurons (postsynaptic). In this paper, attention is drawn to another possibility suggesting a location at the terminals of nonadrenergic neurons.