The D3 and D4 subtypes are much less abundant than the D2 subtype and have a different distribution. D3 receptors are located predominantly in limbic regions, with low concentrations in the striatum, whereas D4 receptors are found in the frontal cortex, amygdala, mid-brain and medulla. The effects mediated by these receptors are not known, although an autoreceptor (presynaptic) role has been suggested.