enkephalin--ala(2)-mephe(4)-gly(5)- and 9-(tetrahydro-2-furyl)-adenine

Our work assesses the effects of mu 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 mu receptor agonist (D-Ala2+, N-Me-Phe4+, Gly5+-ol)-enkephalin (DAMGO) at 1 microM decreased Ca2+ current amplitudes from 0.72 to 0.49 nA. The effect was abolished by naloxone and omega-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.

Topics: Adenine; Analgesics, Opioid; Animals; Animals, Newborn; Calcium Channel Blockers; Calcium Channels; Dose-Response Relationship, Radiation; Drug Interactions; Electric Stimulation; Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; Enzyme Inhibitors; Isoquinolines; Membrane Potentials; Naloxone; Narcotic Antagonists; omega-Conotoxin GVIA; Patch-Clamp Techniques; Prefrontal Cortex; Pyramidal Cells; Rats; Rats, Wistar; Receptors, Opioid, mu; Sulfonamides

A hallmark of native pacemaker channels is their regulation by neurotransmitters and hormones acting through the second messenger cAMP. In this study, we investigated the modulation of two cloned pacemaker channels, HCN1 and HCN2, by activation of coexpressed inhibitory G protein (Gi)-coupled (p-opioid) or stimulatory G protein (Gs)-coupled [serotonin 5-HT4(a)] receptors in Xenopus oocytes. Both receptors enhanced HCN2, but not HCN1 currents. Receptor activation increased HCN2 current amplitude, increased the activation rate sixfold and decreased the deactivation rate two-fold. In addition, the fully-activated current for HCN2 increased due to a receptor-induced increase of the maximal conductance. These effects were inhibited by 9-(tetrahydro-2'-furyl)adenine (SQ22536), were independent of protein kinases A and C and could be explained by a cAMP-induced shift of the voltage dependence of activation by 15 mV to more positive potentials. The pathway through which these effects occurred involved Gbetagamma-activation of adenylyl cyclase and, in the case of the p-opioid receptor, required co-expression of Galphas. The effect of the 5-HT4(a)-receptor, in part caused by its constitutive activity, occurred directly through Galphas-activation. This suggests that 5-HT4(a) receptors may contribute to functional heterogeneity of pacemaker currents (Ih) in those neurons in which 5-HT4(a)R and HCN2 coexist.

Topics: Adenine; Animals; Cyclic AMP; Cyclic AMP-Dependent Protein Kinases; Cyclic Nucleotide-Gated Cation Channels; Electric Conductivity; Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; Female; Gene Expression; GTP-Binding Protein alpha Subunits, Gi-Go; GTP-Binding Protein alpha Subunits, Gs; Humans; Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels; Ion Channels; Membrane Potentials; Muscle Proteins; Nerve Tissue Proteins; Oocytes; Potassium Channels; Protein Kinase C; Receptors, Opioid, mu; Receptors, Serotonin; Transfection; Xenopus