Amphetamines, like cocaine, increase the concentrations of dopamine in the synaptic gap. This heightens the response of the post-synaptic neuron. Also like cocaine, the altered dopamine function contributes to the addictiveness of amphetamines. (Kuczenski, R and D. Segal, D. Effects of methylphenidate on extracellular dopamine, serotonin, and norepinephrine; Comparison with amphetamine. J Neurochem 68 (5): 2032–7. 1997.)
Refer to Substances and the Brain for additional information.