Which of the following is NOT one of the three classes of mechanisms by which neurotransmitters function?
a) compounds that have an indirect effect on membrane potential by means of second messengers
b) compounds that prevent inactivation of neurotransmitter at the synaptic cleft
c) lipid-soluble gases that can diffuse into the cell and bind to enzymes inside the cell
d) compounds that have a direct effect on membrane potential by affecting chemically regulated channels