Activation of mu opioid receptors inhibits transient high- and low-threshold Ca2+ currents, but spares a sustained current.

Opioids and opiates decrease the duration of action potentials and the amount of neurotransmitter released from sensory neurons. The mu-type opioid receptor, the binding site for morphine, is thought to act exclusively on K+ channels. Here, we show that activation of the mu receptor inhibits Ca2+ channels in rat sensory neurons; the effect is blocked by a mu antagonist and is not mimicked by kappa or delta receptor agonists. Both low-threshold (T-type) and high-threshold Ca2+ currents are partially suppressed. omega-Conotoxin-sensitive and omega-conotoxin-insensitive, high-threshold Ca2+ currents are inhibited. The kinetic effect on high-threshold current is like that caused by diminished rest potential: the transient component is selectively lost, whereas the sustained component is spared.