Opioids cause presynaptic ____ channels to close and postsynaptic ____ channels to open.

• A. Calcium; Potassium
• B. Sodium; Chloride
• C. Potassium; Calcium
• D. Magnesium; Sodium

Answer: (A)

Opioids dampen neuronal signaling by acting on G-protein–coupled receptors that modulate ion channels. At the presynaptic terminal, they inhibit voltage-gated calcium channels, reducing calcium influx when an action potential arrives. Less calcium means less neurotransmitter release, so excitatory signals in pain pathways are diminished. On the postsynaptic side, opioid-receptor activation opens potassium channels, allowing K+ to leave the cell, which hyperpolarizes the postsynaptic neuron and makes it harder to fire. This combination—decreased neurotransmitter release presynaptically and hyperpolarization postsynaptically—produces analgesia. Other ions like sodium or chloride aren’t the primary channels modulated by opioids in this context, so the correct pairing is presynaptic calcium channels closing and postsynaptic potassium channels opening.