Development of receptors for dopamine and noradrenaline in rat brain.

In order to examine the receptor basis for the development of spontaneous locomotion and for the effects of clonidine in the infant rat, we measured the densities of alpha 1-adrenoceptors, alpha 2-adrenoceptors and D2-dopamine receptors in various brain regions of the developing infant Wistar rat. The mesolimbic D2-dopamine receptors paralleled the rise in spontaneous locomotion of the infant rat. No alpha 1-adrenoceptors were detectable at birth in any of the four regions examined (mesolimbic, hippocampus, frontal cortex, and hypothalamus), thus providing no basis for the locomotor-stimulating action of clonidine in the first week of life. Mesolimbic alpha 2-adrenoceptors matured between 21 and 28 days (at which time clonidine yields its usual sedating action).