ABSTRACT
One of the most significant trends in neuroscience over the past 15 years has been the steady increase in the number of novel chemical messengers discovered in neurons. The number of neural substances involved in synaptic transmission, modulation, or endocrine functions is thought to exceed 100, an estimate that many investigators regard as conservative. This remarkable increase in putative chemical messengers can be attributed to the virtual explosion of newly discovered neuropeptides acting as “nonconventional” signals. The designation “nonconventional” contrasts these agents with the “conventional” transmitters such as acetylcholine, γ-aminobutyric acid (GABA), or glutamate, which elicit responses in the millisecond time frame, usually through activation of ion channels. As will be pointed out in this chapter, peptides differ from conventional transmitters by their ability to diffuse considerable distances from sites of release and by their interaction with diverse second messenger systems. In other words, whereas the conventional transmitters may be regarded primarily as synaptic signals with well-defined and predictable functions, peptides function in a variety of contexts as transmitters, modulators, or hormones.
