Final answer:
Neuropeptides are neurotransmitters formed by amino acid chains, synthesized in the neuronal cell body, and released into synapses where they can activate receptors. After release, neuropeptides do not reenter the neuron but may be degraded or diffuse away. They can also act as hormones and serve as potent receptor agonists.
Step-by-step explanation:
A neuropeptide is a type of neurotransmitter that consists of chains of amino acids connected by peptide bonds. Unlike the neurotransmitters that are typically small molecules, neuropeptides can range in size, and some are quite short like met-enkephalin, which is five amino acids long, while others like beta-endorphin extend to 31 amino acids. Neuropeptides are synthesized in the cell body of a neuron and packed into vesicles. When a neuron is stimulated, these neuropeptides are secreted into the synaptic cleft, where they can activate various types of receptors on the post-synaptic cell or even serve as hormones elsewhere in the body, similar to VIP (vasoactive intestinal peptide) or substance P.
Neuropeptides bind to receptors on the surface of target cells since they cannot pass through the plasma membrane due to their water-soluble nature. Once released, these peptides are typically not recaptured by the neuron but may be degraded by enzymes or diffuse away from the synaptic site. Peptides derived from natural sources like substance P serve as receptor agonists and are potent in small quantities. They play significant roles within the central nervous system, influencing behavior and physiological responses.