Final answer:
G-protein coupled receptors are involved in indirect synapses, and ligand-gated channels are involved in direct synapses.
Step-by-step explanation:
G-protein coupled receptors (GPCRs) are involved in indirect synapses, where the neurotransmitter binds to the receptor, activating a G-protein, and leading to a cascade of intracellular events before the ultimate effect is achieved.
Ligand-gated channels, on the other hand, are involved in direct synapses. In this case, the binding of the neurotransmitter directly to the receptor on the ion channel causes a conformational change that opens or closes the channel, allowing ions to flow across the membrane, resulting in a rapid postsynaptic response.