**1. Introduction**

## **1.1. Neuropeptides**

Neuropeptides are intracellular peptides, composed of short chains of amino acids and found in brain tissue. They are often localized in axon terminals at synapses and are released as intercellular messengers that transmit information in the central nervous system, gastrointestinal tract etc. Many are also hormones released by nonneuronal cells. Neuropeptides can be divided and grouped according their site of synthesis and secretion or their structural or functional characteristics. Currently recognized neuropeptides include all hypothalamic releasing hormones, pituitary hormones, gastro-intestinal and brain peptides, some circulating hormones, opioide peptides, neurohypophyseal hormones etc (Siegel, 2006). Some neuropeptides are secreted by the nerve terminals with conventional neurotransmitters. But which are the differences between the classical neurotransmitters and the neuropeptides? The precursors of neuropeptides have at least 90 amino acids residues - larger than the precursors of the neurotransmitters. The synthesis of neuropeptides is carried in the neuronal soma and then is transported to the axonal ends. The secretion of neuropeptides requires lower concentration of intracellular Ca2+ in comparison to transmitters. After secretion the neuropeptides or their precursors are reused in the synapse. The concentration of the neuropeptides in the tissue is very low and they interact with the receptors at lower concentrations than neurotransmitters. Neuropeptides appearance and secretion are very plastic (Siegel, 2006). For example in pathological conditions, the number of endocrine cells that secrete neuropeptides can not only increase but also appear unusual locations as a result of additional stimulation (Gulubova et al., 2012).

© 2012 Tolekova et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Tolekova et al., licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
