Dopamine, serotonin, acetylcholine and GABA (gamma-aminobutyric acid) are various types of neurotransmitters which are present in the synaptic knob of the neurons. Cocaine interferes with the transport of the neurotransmitter dopamine. Cocaine attaches to the dopamine transporter, the molecular channel that takes up free-floating dopamine from the synapse back into the sending neuron. As long as cocaine occupies the transporter, dopamine cannot re-enter the neuron. The concentration of dopamine hence builds up synapse, stimulating receiving-neuron receptors more and producing much greater dopamine impact on the receiving neurons than what occurs naturally. This causes a stimulating action on CNS, producing a sense of euphoria and increased energy. An excessive dosage of cocaine causes hallucinations.