Formulated by Henry Dale in the 1940's, the original hypothesis was that each neuron releases only one type of neurotransmitter.  This idea has been shown to be false, as many neurons simultaneously release neuropeptides as well as more "classic" neurotransmitters (such as acetylcholine).  A more accurate representation of this principle is:  neurons release the same neurotransmitters at all of their synapses (i.e., a neuron does not release GABA from one set of terminals, and glutamate from another).