Exposure to addictive drugs can cause long-lasting structural changes in neurons. Opioids decrease the size and the caliber of dendrites and soma of dopamine neurons of the ventral tegmental area.93 Repeated use of cocaine or amphetamine increases the number of dendritic branch points and spines on both medium spiny neurons in the nucleus accumbens and pyramidal neurons in the medial prefrontal cortex.94 Neurotrophic factors seem to be responsible for these changes. Modifications in the density of dendritic spines and neurotrophic factors have also been implicated in long-term potentiation and long-term depression by AMPA-sensitive glutamate receptors.