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A major conceptual shift in neuroscience has been wrought by the realization that brain function is modulated by numerous chemicals in addition to classical neurotransmitters. Many of these informational substances are neuropeptides, originally studied in other contexts as hormones, "gut peptides," or growth factors. Their number presently exceeds 50 and most, if not all, alter behavior and mood states, although only endogenous analogs of psychoactive drugs like morphine, Valium, and phencyclidine have been well appreciated in this context. We now realize that their signal specificity resides in receptors (distinct classes of recognition molecules), rather than the close juxtaposition occurring at classical synapses. Rather precise brain distribution patterns for many neuropeptide receptors have been determined. A number of brain loci, many within emotion-mediating brain areas, are enriched with many types of neuropeptide receptors suggesting a convergence of information at these "nodes." Additionally, neuropeptide receptors occur on mobile cells of the immune system; monocytes can chemotax to numerous neuropeptides via processes shown by structure-activity analysis to be mediated by distinct receptors indistinguishable from those found in brain. Neuropeptides and their receptors thus join the brain, glands, and immune system in a network of communication between brain and body, probably representing the biochemical substrate of emotion.
Pert et al. (Thu,) studied this question.