The interaction between the neuromodulator adenosine and adenosine receptors on the surface of neurons modifies the neurons’ responses to neurotransmitters. The activated adenosine receptors alter the levels of small signaling molecules (ie, second messengers) in the cells. Depending on the receptors and cells involved, these changes can make it easier or more difficult for neurotransmitters to excite the cell. Adenosine’s activity is regulated by proteins called nucleoside transporters, which carry adenosine into and out of the cell. Alcohol interferes with the function of the adenosine system. For example, both acute and chronic alcohol exposure affect the function of the adenosine-carrying nucleoside transporters, thereby indirectly altering the second-messenger levels in the cells. Through this mechanism, adenosine may mediate some of alcohol’s effects, such as intoxication, motor incoordination, and sedation.