Neuropeptides, small protein-like molecules, are used by neurons to communicate with each other. Many populations of neurons have distinctive biochemical phenotypes.
For example, in one subpopulation of about 3000 neurons in the arcuate nucleus of the hypothalamus, three anorectic peptides are co-expressed: α-melanocyte-stimulating hormone (α-MSH), galanin-like peptide, and cocaine-and-amphetamine-regulated transcript (CART). In another subpopulation two orexigenic peptides are co-expressed, neuropeptide Y and agouti-related peptide (AGRP).
These are not the only peptides in the arcuate nucleus; β-endorphin, dynorphin, enkephalin, galanin, ghrelin, growth-hormone releasing hormone, neurotensin, neuromedin U, and somatostatin are also expressed in subpopulations of arcuate neurons. These peptides are all released centrally and act on other neurons at specific receptors. The neuropeptide Y neurons also make the classical inhibitory neurotransmitter GABA.
Invertebrates also have many neuropeptides. CCAP has several functions including regulating heart rate, allatostatin and proctolin regulate food intake and growth, bursicon controls tanning of the cuticle and corazonin has a role in cuticle pigmentation and moulting.
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. The following is a list of neuroactive peptides coexisting with other neurotransmitters.
(also known as noradrenaline). In neurons of the A2 cell group in the nucleus of the solitary tract), norepinephrine co-exists with:
Galanin Enkephalin Neuropeptide Y
Somatostatin (in the hippocampus) Cholecystokinin Neuropeptide Y (in the arcuate nucleus)
VIP Substance P
Cholecystokinin Neurotensin Glucagon-like peptide-1 (in the nucleus accumbens)
Neuropeptide Y Neurotensin
Substance P TRH Enkephalin
Some neurons make several different peptides. For instance, Vasopressin co-exists with dynorphin and galanin in magnocellular neurons of the supraoptic nucleus and paraventricular nucleus, and with CRF (in parvocellular neurons of the paraventricular nucleus)
Oxytocin in the supraoptic nucleus co-exists with enkephalin, dynorphin, cocaine-and amphetamine regulated transcript (CART) and cholecystokinin.