Central cardiovascular effects of alpha adrenergic drugs: differences between catecholamines and imidazolines

To check whether the central hypotensive effect of alpha adrenergic agonists is linked with the stimulation of alpha-2 receptors, such drugs were administered directly to the nucleus reticularis lateralis, which is an important site for the hypotensive action of clonidine. These experiments were carried out by microinjections (0.5 microliter on each side) in normotensive cats anesthetized with pentobarbital. alpha-Methylnorepinephrine, a selective alpha-2 agonist (0.1-10 micrograms/kg) had no hypotensive effect in this region, whereas potent alpha-1 agonists such as cirazoline (0.01-1 micrograms/kg) and ST 587 (1-10 micrograms/kg), like clonidine, produced dose-dependent hypotensive effects. Our results suggest that alpha-2 selective catecholamines are not active in the nucleus reticularis lateralis region, whereas imidazolines induce a hypotensive effect whatever their affinity for one subtype of alpha adrenoceptors. Therefore, there may be some form of structure-activity relationship which would indicate the existence, in this particular region of the medulla oblongata, of sites preferring the imidazoline structure.