( However ( , ( we ( ( do ( know ( that ( caffeine ( affects ( some ( of ( ( the receptors ) ( in ( ( ( the brain ) , ) ( which ( ( in turn ) ( affect ( ( the metabolism ) ( of ( ( ( ( ( dopamine , ) serotonin ) , ) and ) norepinephrine ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) . ) ) ) )
(ROOT (S (ADVP (RB However)) (, ,) (NP (PRP we)) (VP (VBP do) (VP (VB know) (SBAR (IN that) (S (NP (NN caffeine)) (VP (VBZ affects) (NP (NP (DT some)) (PP (IN of) (NP (NP (DT the) (NNS receptors)) (PP (IN in) (NP (NP (DT the) (NN brain)) (, ,) (SBAR (WHNP (WDT which)) (S (PP (IN in) (NP (NN turn))) (VP (VBP affect) (NP (NP (DT the) (NN metabolism)) (PP (IN of) (NP (NP (NN dopamine)) (, ,) (NP (NN serotonin)) (, ,) (CC and) (NP (NN norepinephrine)))))))))))))))))) (. .)))
However, we do know that caffeine affects some of the receptors in the brain, which in turn affect the metabolism of dopamine, serotonin, and norepinephrine.
(ROOT (S (NP (NNP Addiction)) (VP (VBZ affects) (NP (NP (DT the) (JJ dopamine) (NNS receptors)) (PP (IN within) (NP (DT the) (NN brain))))) (. .)))