Dopamine: Biphasic dose responses

The present article indicates that dopamine and/or its agonists induce biph asic dose-response relationships for numerous endpoints. These include loco motion, pain sensitivity, blood pressure, prolactin secretion, oxytocin rel ease, heart rate, memory, and neuronal adenylate cyclase activity. Biphasic responses were reported predominantly with male Sprague-Dawley rats, but a lso with mice, dogs, monkeys, and humans. Regardless of the model or endpoi nt the maximum changes from the control were always modest being within the 10 to 80% range. The range of stimulatory responses was quite variable, ex tending from slightly greater than a factor of 10 for the endpoints such as memory, pain-vocalization, and diastolic blood pressure to the lob range f or prolactin release and the 10(8) range for oxytocin release. Mechanistic studies suggested that the stimulatory and inhibitory effects of dopamine a re mediated by different receptors or receptor subtypes having opposite act ions and different ligand affinities.