![]() In addition to their neurotransmitter functions, some neuropeptides (eg, oxytocin and vasopressin) are released directly into the blood by neurons and act as hormones ( Chapter 10) others (eg, luteinizing hormone) act as hormones secreted by endocrine glands yet others (eg, cholecystokinin) act within peripheral organs such as the digestive system. ![]() Neuropeptides are found within the central nervous system and in the periphery, including both the sympathetic and parasympathetic nervous systems ( Chapter 9). Other signaling peptides in the nervous system, such as growth factors and cytokines ( Chapter 8), are generally distinguished from the neuropeptides even though they may have overlapping functions. The term neuropeptide is reserved for small proteins or polypeptides that act as neurotransmitters in the nervous system. Peptides are small proteins or polypeptides and thus composed of amino acids covalently linked by peptide bonds 7–1. However, much remains to be discovered about neuropeptide function because selective agonists and antagonists are still lacking for many neuropeptide receptors. More than 100 neuropeptide transmitters are known they play diverse roles in the nervous system, including regulation of sleep and arousal, emotion, reward, feeding and energy balance, pain and analgesia, and learning and memory. Moreover, many of the actions mediated by G proteins and second messengers alter the response properties of neurons resulting in “modulation” rather than simple excitation or inhibition. Stimulation of G protein–coupled receptors produces slower responses than stimulation of ligand-gated channels ( Chapter 4). They generally bind to G protein–linked receptors there are rare exceptions in which peptides, such as insulin, have receptors that are enzymes (eg, protein tyrosine kinases) and act in a neurotransmitter-like fashion. Neuropeptides are short proteins or polypeptides that serve as neurotransmitters. Neuropeptides and their receptors modulate many diverse functions of the central nervous system, including sleep, arousal, reward, feeding, pain, cognition, stress responses, and emotions. Neuropeptide transmitters act almost exclusively via activation of G protein–coupled receptors. Neuropeptides may diffuse for long distances within the extracellular space before binding to their specific receptors. Unlike small-molecule neurotransmitters, which are packaged in small synaptic vesicles, neuropeptides are generally packaged in large dense core vesicles both types of vesicles may be found in the same neuron. The “pre” refers to an N-terminal signal sequence that directs newly synthesized peptides into the regulated secretory pathway.Īs a result of alternative RNA splicing and differential cleavage of propeptides in different tissues, a single gene can give rise to diverse signaling peptides with distinct functions. Neuropeptides are synthesized as large precursor prepropeptides that undergo extensive posttranslational processing, which includes cleavages into smaller peptides and enzymatic modification. The synthesis of neuropeptides, like that of all proteins, requires the transcription of DNA and translation of the resulting messenger RNA (mRNA) into protein. Like the monoamines and acetylcholine, neuropeptide transmitters serve primarily modulatory roles in the nervous system. Other signaling peptides such as growth factors and cytokines are considered to be distinct even though they may have some overlapping functions. Neuropeptides are small proteins or polypeptides that serve as neurotransmitters in the nervous system generally acting via G protein–coupled receptors.
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