Cross reference: Neuropeptide Receptor Coelenterata Neuropeptides
Amphioxus Neuropeptides Lamprey Neuropeptides Substance P
Neurotransmitters in General Neuromodulators in General Ligands
"Neurons use many different chemical signals to communicate information, including neurotransmitters, peptides, cannabinoids, and even some gases, like nitric oxide."
"Neuropeptides are small protein-like molecules used by neurons to communicate with each other. They are neuronal signaling molecules, influence the activity of the brain in specific ways and are thus involved in particular brain functions...Neuropeptides are expressed and released by neurons, and mediate or modulate neuronal communication by acting on cell surface receptors."
"Peptide signals play a role in information processing that is different from that of conventional neurotransmitters, and many appear to be particularly associated with specific behaviours. For example, oxytocin and vasopressin have striking and specific effects on social behaviours, including maternal behaviour and pair bonding."
"Generally, peptides act at metabotropic or G-protein-coupled receptors expressed by selective populations of neurons. In essence they act as specific signals between one population of neurons and another.
Neurotransmitters generally affect the excitability of other neurons, by depolarising them or by hyperpolarising them. Peptides have much more diverse effects; amongst other things, they can affect gene expression, local blood flow, synaptogenesis, and glial cell morphology. Peptides tend to have prolonged actions, and some have striking effects on behaviour.
Neurons very often make both a conventional neurotransmitter (such as
glutamate, GABA or dopamine)
and one or more neuropeptides. Peptides are generally packaged in large
dense-core vesicles, and the co-existing neurotransmitters in small
synaptic vesicles. The large dense-core vesicles are often found in all
parts of a neuron, including the soma, dendrites, axonal swellings and nerve endings,
whereas the small synaptic vesicles are mainly found in clusters at
presynaptic locations. Release of the large vesicles and the small
vesicles is regulated differentially."
The Wiki link also gives an extensive list of links for many more neuropeptides.
NOTE: Many of the above 'neuropeptides' are referred to in the older literature as 'neurotransmitters'.
"The neuroendocrine peptides cholecystokinin (CCK) and gastrin, originally identified in mammals, are characterized by a common amidated C-terminal tetrapeptide sequence, Trp-Met-Asp-Phe.NH2, which also constitutes the minimal structure necessary for biological activity of both. Hence, it has been proposed that CCK and gastrin have evolved from a common ancestor. Although the occurrence of CCK/gastrin-related peptides has been suggested in representatives of several invertebrate phyla, the evidence, mostly based on immunoreactivity, has not been substantiated by peptide identification. Instead, CCK/gastrin-specific antibodies might be cross-reacting with Asp-Phe-amides, like the lymnaDFamides, isolated from the freshwater snail Lymnaea stagnalis.
Cionin, isolated from Ciona intestinalis, a representative of the protochordates that occupy a key position at the transition to vertebrates, so far represents the oldest genuine member of the CCK/gastrin family, dating the emergence of these peptides back to at least 500 million years ago. The CCK/gastrin family appears to be represented in the whole chordate phylum, and in addition to mammals, CCK and gastrin have recently been identified in a number of nonmammalian species representing the major vertebrate classes, including fishes, amphibians, reptiles, and birds.
This now makes it possible to consider the CCK/gastrin phylogeny based on structural information. A duplication of the ancestral gene appears to have already occurred before or during the appearance of cartilaginous fish, giving rise to two peptides most likely homologous to mammalian CCK and gastrin. Indicative of a function of gastrin, the acid secretory system appears to have developed concomitantly in sharks. The segregation of CCK and gastrin early in vertebrate evolution resembles the situation in other peptide families, in accordance with a suggested widespread pattern of multiplication within vertebrate peptide and protein families around 400 million years ago.
At the amphibian level, two separate peptide systems, resembling mammalian CCK and gastrin, have been characterized by identification of the mature bioactive peptides, cDNAs, gene structures, primary and secondary sites of gene expression, and their physiological actions. The overall gene structure, including exon/intron organization, is similar in all mammalian and nonmammalian CCK/gastrin genes. CCK is well conserved in all vertebrate species investigated, while the mammalian gastrins at first sight appear as a distinct group with little similarity to the nonmammalian gastrins outside the invariant C-terminal tetrapeptide and the C-terminal flanking peptide of the prohormone.
However, evidence indicates that the transition from nonmammalian to mammalian gastrin may not be as dramatic as first anticipated. In conclusion, the CCK/gastrin family appears to be represented in most, if not all, chordates, to which group it may also be limited. The two major classes, CCK and gastrin, probably arose as distinct peptide systems early in vertebrate history. While CCK is well conserved in all vertebrates, a major structural change of gastrin accompanied the transition to mammals."