Do antagonist-receptor complexes get absorbed into the cell?
When a neurotransmitter, like serotonin, binds to it's specific receptor, the ligand-receptor complex is not phagocytosed.  Picture the human cell membrane and think of the serotonin receptor like a long piece of rope that has been wound up, but is embedded inside the cell membrane forever.  After the serotonin binds, the structure of the receptor changes, causing lots of steps to start (like a domino effect) where molecules are released from the inner aspect of the receptor, thus carrying the message into the nucleus.