Imaging neurotransmitters, not enzymes, receptors, or transporters

Dopamine, Serotonin, GABA, Glutamate, Histamine, Norepinephrine, Serine, Octopamine, … To generate spatial information about key neuromodulators and neurotransmitters, neurobiologists typically perform stainings against enzymes (e.g. TH for dopamine, GAD65-67 for GABA), receptors (e.g. Glutamate) or transporters (e.g. Norepinephrine, Glutamate). Why?