Detection of Neuropeptides by Immunocytochemistry

Neuropeptides constitute the largest and most diverse class of signaling substances known in metazoans. Over the last 20 yr it has become apparent that neuropeptides have important roles as neurohormones, neuromodulators, cytokines, morphogenetic factors, and possibly in some cases, as true neurotransmitters. Each neuropeptide may even be multifunctional and exist in several isoforms in a given animal species. In the search for functions of neuropeptides, it has been critical to be able to localize sites of synthesis and release. Immunocytochemistry (ICC) has been instrumental in the accurate mapping of the cellular and subcellular distribution of neuropeptides in tissue. Other immunological assays, such as radioimmunoassay (RIA) and immuno-enzymatic assay (ELISA) provide powerful complements for quantification of neuropeptides. Several important discoveries related to neuropeptides have relied on ICC, for example: Different neuropeptides have very specific distributions in small populations of neurons (1 –3 ), neuropeptides are commonly colocalized with low-mol-wt neurotransmitters or other neuropeptides (4 ), the chemical diversity of neurons is far greater than previously suspected (2 ,3 ), and neuropeptide synthesis and release can be episodic (5 ).