NEURONS (nerve cells) communicate chemically by releasing and responding to a wide range of chemical substances, referred to in the aggregate as NEUROTRANSMITTERS. The process of neurotransmission refers to this form of chemical communication between cells of the central and peripheral nervous system at the anatomically specialized point of transmission, the SYNAPSE (synaptic junctions). Thus, it is convenient to conceive of "the" neurotransmitter for a specific instance of synaptic connections between neurons in one brain location (the source neurons) and their synaptic partner cells (the target neurons) in another neuronal location. For example, the phrase "dopaminergic neurons of the nigro-accumbens circuit" refers to the DOPAMINE-transmitting synaptic connections between the brain neurons of the substantia nigra and their targets in the NUCLEUS ACCUMBENS. Current concepts of neuro-transmission, however, require a broader view; they would consider as neurotransmitters all the chemical substances that a given neuron employs to signal the...