What is the mechanism of action of morphine at μ-opioid (mu) receptors?

  1. The receptor acts as a ligand-gated ion channel and allows the efflux of potassium from the cells, resulting in membrane hyperpolarisation and decreased neuronal excitability
  2. The receptor acts as a G-protein-coupled-receptor, with the G-protein activating adenylyl cyclase, resulting in increased intracellular cAMP and increased neuron excitability
  3. The receptor acts as a G-protein-coupled-receptor, with the G-protein resulting in the production of IP3 and DAG, which subsequently activate potassium efflux channels, resulting in membrane hyperpolarisation and decreased neuronal excitability
  4. The receptor acts as a G-protein coupled receptor, with the G-protein interacting directly with a potassium efflux channel, resulting in membrane hyperpolarisation and decreased neuronal excitability