Cytochemistry of neurotransmitters: from statical analysis to the understanding of dynamic processes

In the past thirty years, cytochemical methods have allowed neuroscientists to identify and localize neuroactive molecules (neurotransmitters and neuropeptides), their receptors and their synthetic enzymes, and have advanced the understanding of many neuronal functions. Classic methods (histochemical and immunohistochemical techniques) have been used extensively to draw neurochemical maps in adult and developing nervous systems. As a consequence, many neuroactive molecules have become specific biochemical markers for neuronal systems. Double labelling techniques have greatly contributed to the discovery of the coexistence of two or more chemical compounds in the same cell. The in situ hybridization technique has recently become a productive addition to the tools available to the neuroscientist, especially when combined with immunocytochemistry to correlate mRNAs and protein expression. Even today, innovative roles for neurocytochemistry continue to be found. The newest approaches based on RT-PCR (reverse transcriptase-polymerase chain reaction) promise levels of sensitivity never reached before in in situ studies, and can provide simultaneous expression/functional data at the single-cell level.