Final answer:
The feature most uniquely associated with metabotropic receptors is the generation of a second messenger, which leads to various cellular changes following the binding of a neurotransmitter.
Step-by-step explanation:
The most uniquely associated feature with a metabotropic receptor is 2) Second messenger. Unlike ionotropic receptors, which are ligand-gated ion channels that open immediately upon neurotransmitter binding, metabotropic receptors are part of a complex system that causes metabolic changes in the cell.
When a neurotransmitter binds to a metabotropic receptor, it leads to the activation of a G protein, which then moves and activates an effector protein. This, in turn, generates a second messenger, such as cyclic AMP (cAMP), which can cause various changes within the neuron, including the opening or closing of ion channels, alterations in metabolism, and changes in gene transcription. The other options presented - neurotransmitter, ligand-gated receptor, voltage-gated receptor, and synaptic vesicle - are all components involved in synaptic transmission but are not uniquely associated with a metabotropic receptor's function.