Which sequence best describes events after opioid receptor activation?

Which sequence best describes events after opioid receptor activation?

• A. Activation of adenylyl cyclase followed by increase in cAMP
• B. Decrease in adenylyl cyclase leading to decreased cAMP, then decreased Ca influx and increased K+ influx
• C. Increase in Ca influx
• D. Activation of NMDA receptors

Answer: (B)

When opioid receptors are activated, they couple to Gi/o proteins which reduce the neuron's excitability. This signaling pathway inhibits adenylyl cyclase, leading to lower cAMP and decreased PKA activity. With less cAMP, calcium channels are less likely to open in response to depolarization, so calcium influx drops. At the same time, Gi/o activation increases potassium conductance through GIRK channels, causing more potassium to leave the cell and hyperpolarize the membrane, which further reduces neurotransmitter release. So the sequence is decreased adenylyl cyclase activity, lowered cAMP, reduced calcium influx, and increased potassium conductance (outward flow), yielding reduced neuronal activity. The other options describe opposite effects (increased AC and cAMP, more calcium influx, or NMDA receptor activation) that do not reflect how opioids diminish neuronal signaling.

When opioid receptors are activated, they couple to Gi/o proteins which reduce the neuron's excitability. This signaling pathway inhibits adenylyl cyclase, leading to lower cAMP and decreased PKA activity. With less cAMP, calcium channels are less likely to open in response to depolarization, so calcium influx drops. At the same time, Gi/o activation increases potassium conductance through GIRK channels, causing more potassium to leave the cell and hyperpolarize the membrane, which further reduces neurotransmitter release. So the sequence is decreased adenylyl cyclase activity, lowered cAMP, reduced calcium influx, and increased potassium conductance (outward flow), yielding reduced neuronal activity. The other options describe opposite effects (increased AC and cAMP, more calcium influx, or NMDA receptor activation) that do not reflect how opioids diminish neuronal signaling.