Considering neurotransmitters such as glutamate, GABA, dopamine etc..
then it is the receptor that defines the 'message' or action of the NT
(e.g. GABA mediates depolarisation in addition to its classical
inhibitory role, certain classes of metabotropic glutamate receptors
reduce glutamate release and depress synapses. Transmitters
themselves do nothing but activate a cellular pathway and therefore it
is the way these are organised in each cell that determines what the
message passed between cells will 'mean'.