( But ( sometimes ( ( the cause ) ( ( is ( blockage ( of ( ( the ( dopamine receptors ) ) ( in ( the brain ) ) ) ) ) ) . ) ) ) )
(ROOT (S (CC But) (ADVP (RB sometimes)) (NP (DT the) (NN cause)) (VP (VBZ is) (NP (NP (NN blockage)) (PP (IN of) (NP (NP (DT the) (JJ dopamine) (NNS receptors)) (PP (IN in) (NP (DT the) (NN brain))))))) (. .)))
But sometimes the cause is blockage of the dopamine receptors in the brain.
(ROOT (S (NP (NNP Addiction)) (VP (VBZ affects) (NP (NP (DT the) (JJ dopamine) (NNS receptors)) (PP (IN within) (NP (DT the) (NN brain))))) (. .)))