Question: Which best describes a neuromodulator? Need Assignment Help? Multiple choice question. A protein that binds to vesicles within presynaptic cells, that induces vesicles to fuse with the membrane when Ca2+ concentration increases. A compound released from a presynaptic cell that directly opens ligand-gated channels within the synapse on the postsynaptic cell.. A compound that indirectly alters the effectiveness of a synapse by altering release of, or responsiveness to, a neurotransmitter.