Complex sentence: Serotonin competes with the catacholamines (specifically, with dopamine) for uptake by the brain; therefore, an imbalance of serotonin and dopamine can interfere with neurological functioning.
Split into simpler sentences: Serotonin competes with the catacholamines (specifically, with dopamine) for uptake by the brain. An imbalance of serotonin and dopamine can interfere with neurological functioning.