Our work assesses the effects of ? opioid receptor activation on high-threshold Ca2+/Ba2+ currents in freshly dispersed pyramidal neurons of the medial prefrontal cortex in rats. Application of the specific ? receptor agonist (D-Ala2, N-Me-Phe4, Gly5-ol)-enkephalin (DAMGO) at 1 muM decreased Ca2+ current amplitudes from 0.72 to 0.49 nA. The effect was abolished by naloxone and ?-Conotoxin GVIA. Inhibition was not abolished by strong depolarisation of the cell membrane. In addition, a macroscopic Ba2+ current recorded in cell-attached configuration was inhibited when DAMGO was applied outside the patch pipette. An adenylyl cyclase inhibitor (SQ 22536) and a protein kinase A inhibitor (H-89) decreased Ca2+ current amplitude. Moreover, the inhibitory effect of mu opioid receptors on Ca2+ currents required the activation of protein kinase A. We conclude that activation of mu opioid receptors in medial prefrontal cortex pyramidal neurons inhibits N type Ca2+ channel currents, and that protein kinase A is involved in this transduction pathway.