Central alpha-2 adrenoceptors regulate central and peripheral functions

Central alpha-2 adrenoceptors regulate a variety of functions including blood pressure, gastrointestinal activity, hormonal secretion, sleep-waking cycle, analgesia, anxiety and some aspects of the withdrawal reaction to opioids. Evidence has been provided that clonidine decreases blood pressure, inhibits salivary secretion and reduces gastrointestinal secretion and motility. Several alpha-2 adrenoceptor agonists reduce wakefulness and inhibit paradoxical sleep, whereas novel imidazoline derivatives, such as detomidine, display sedative and analgesic properties which have been related to the activation of central alpha-2 adrenoceptors. Recent data emphasize the importance of somatodendritic alpha-2 autoreceptors compared to presynaptic alpha-2 autoreceptors as physiological modulators of noradrenergic activity in the central nervous system. In addition, increasing experimental evidence has been provided on the relevant role played by central alpha-2 heteroreceptors (presynaptic and somatodendritic) in the regulation of several functions. Some effects mediated by the activation of alpha-2 adrenoceptors are very similar to those following stimulation of mu opioid receptors. This parallelism can be explained, at least in part, by the presence of these receptors on the same neurons on which they exert an inhibitory action mediated by the same cellular mechanisms. This review resumés the most prominent data about the role played by central alpha-2 adrenoceptors in the regulation of overall functions.