Final answer:
Neuropeptide transmitters are synthesized as large proteins and cleaved into active forms, transported by axonal mechanisms, and mainly degraded by enzymes after release. Neurotransmitters are made from smaller precursors, stored in vesicles, and removed from the synaptic cleft via diffusion, degradation, or reuptake. Drugs can alter these processes, affecting neurotransmission.
Step-by-step explanation:
The synthesis, transport, and removal of neuropeptide transmitters differ from small-molecule neurotransmitters in several ways. Neuropeptides are synthesized as large precursor proteins in the cell body and are later cleaved into active neuropeptides, whereas neurotransmitters like acetylcholine are synthesized from smaller precursors and packaged into vesicles in the presynaptic element. The transport of neuropeptides to the synaptic cleft involves axonal transport mechanisms, while neurotransmitters are already present in the presynaptic terminal and are released into the synaptic cleft upon an action potential.
The removal of neurotransmitters from the synaptic cleft involves diffusion away from the synaptic cleft, degradation by enzymes, or reuptake by transport proteins in the presynaptic membrane. Neuropeptides, however, are less likely to be taken up again by the presynaptic neuron and are mainly degraded by enzymes. This is important because drugs that influence these processes, such as those inhibiting acetylcholinesterase in Alzheimer's patients, can significantly affect neurotransmission.
Additionally, neurotransmitters can also act as hormones when released into the bloodstream, as seen with norepinephrine and epinephrine, which are part of the adrenergic neurotransmitter system and can also act as hormones when released by the adrenal gland. The postsynaptic element that typically receives signals from a presynaptic neuron is the dendrites, where receptor proteins are located to bind the incoming neurotransmitters.