Order of application determines the interaction between phorbol esters and GTP-gamma-S in dorsal raphe neurons: evidence that the effect of 5-HT is modified upstream of the G protein Ca channel interaction

PMA counteracts G protein actions on CaV2.2 channels in rat sympathetic neurons

Protein kinase C (PKC)-induced phosphorylation and G protein-mediated inhibition of Ca(V)2.2 N-type Ca2+ channels counteract exerting opposing modulatory responses at the channel level. At present, the most striking question remaining is whether prominent enhancement of the Ca2+ current (I(Ca)) observed under PKC activation arises from relief of G-protein tonic inhibition. Here, by using patch-clamp methods in superior cervical ganglion (SCG) neurons of rat, we show the following: First, that PKC activation by phorbol-12-myristate-13-acetate (PMA) not only counteracts mutually with noradrenaline (NA) and GTPgammaS-induced I(Ca) inhibition, but also reverses current inhibition by Gbetagamma subunits over-expression. Second, that PMA increases I(Ca) beyond the enhancement expected by sole removal of the G protein-mediated tonic inhibition. Accordingly, PMA increases conductance through N-type Ca2+ channels, unlike the G protein inhibitor GDPbetaS. Together, our results support that PMA-induced phosphorylation produces changes in I(Ca) that cannot be accounted for by prevention of G protein inhibition. They may have important implications in reinterpretation of existing data with PMA. Furthermore, counteracting modulation of ion channels and reversibility within a short time frame are better support for a dynamic system with short-term adaptive responses.

Effect of pertussis toxin and N-ethylmaleimide on voltage-dependent and -independent calcium current modulation in serotonergic neurons

Introduction of GTP-gamma-S into a neuronal cell spontaneously results in G-protein activation. A possible contribution to this mechanism is that some receptors have a constitutive activity that stimulates GDP/GTP exchange resulting in increased GTPase activity of G-protein alpha subunits, leading to a facilitation of GTP-gamma-S binding. It follows that partial or complete uncoupling of receptors and G-proteins could inhibit Ca(2+) current modulation by GTP-gamma-S. This possibility was tested in acutely isolated rat dorsal raphe neurons by uncoupling the receptor and G-protein using N-ethylmaleimide and pertussis toxin. Since these compounds have been suggested to differentially block voltage-dependent inhibition, relative to voltage-independent, we investigated whether the apparent voltage-independent component of Ca(2+) channel modulation by 5-hydroxytryptamine (5-HT) shares the same mechanism as the voltage-dependent component. N-ethylmaleimide inhibited the response to 5-HT by about 50% but had no effect on the response to GTP-gamma-S. In dorsal raphe neurons 28.9% of the total response to 5-HT was voltage-independent. N-ethylmaleimide had identical effects on the voltage-dependent and -independent components as measured by tail current inhibition. The response to 5-HT was completely sensitive to pertussis toxin, and completely uncoupling the receptors and G-proteins did not affect the maximal response to GTP-gamma-S. Our results suggest that the apparent voltage-independent component of Ca(2+) channel modulation by 5-HT in dorsal raphe neurons might share the same mechanism as does the voltage-dependent component. In addition, these experiments provided evidence that partial or even complete uncoupling of receptors and G-proteins did not affect Ca(2+) current modulation by direct activators of G-proteins.

A critical protein kinase C phosphorylation site on the 5-HT(1A) receptor controlling coupling to N-type calcium channels

The importance of specific protein kinase C (PKC) sites for modulation of the inhibitory coupling of 5-HT(1A) receptors to N-type Ca(2+) channels was examined using patch-clamp techniques in F11 rat dorsal root ganglion x mouse neuroblastoma hybrid cells. The PKC activator phorbol 12-myristate 13-acetate (PMA, 10 nM) reduced by 28.6 +/- 6.8 % 5-HT-mediated, but not GTP-gamma-S-induced, inhibition of Ca(2+) current, whereas a higher concentration of PMA (500 nM) inhibited both the actions of 5-HT and GTP-gamma-S. 5-HT(1A) receptor expression plasmids with or without mutation of a single PKC site in the second intracellular loop (i2, T149A) or of three PKC sites located in the third intracellular loop (i3, T229A-S253G-T343A) were stably transfected into F11 cells. The T149A 5 HT(1A) receptor inhibited forskolin-stimulated cyclic AMP levels but was largely uncoupled from Ca(2+) channel modulation. In one (i2) clone a response rate to 5-HT of 31.6 % was obtained. The T149A mutant displayed markedly reduced sensitivity to PMA (10 nM) compared to wild-type 5-HT(1A) receptors, with only a 13.4 +/- 3 % reduction in 5-HT-induced channel inhibition; when exposed to 500 nM PMA, reductions in the action of 5-HT were comparable to those of the wild-type receptor. By contrast, the i3 mutant displayed comparable sensitivity to the wild-type 5-HT(1A) receptor to either concentration of PMA. PMA at 10 nM exhibited a similar uncoupling effect on the response of the endogenous opiate receptor to the agonist D-alanine-5-leucine-enkephalin (DADLE) in wild-type and T149A mutant-expressing clones. The T149 site of the 5-HT(1A) receptor is crucial for receptor uncoupling by sub-maximal PKC activation while at maximal PKC activation, downstream sites uncouple G proteins from the N-type Ca(2+) channel.

Antagonizing effect of protein kinase C activation on the mu-opioid agonist-induced inhibition of high voltage-activated calcium current in rat periaqueductal gray neuron

Opioids have been thought to induce analgesia by activating the descending pain control system, especially at the level of periaqueductal gray, and regulate the neurotransmitter release through the inhibition of calcium channel. In the present study, the modulatory effects of protein kinase C and protein kinase A on the mu-opioid agonist-induced inhibition of the high-voltage activated calcium current were examined in the acutely dissociated rat periaqueductal gray neurons with the nystatin-perforated patch-clamp technique. Among 505 neurons tested, the barium current passing through the high-voltage activated calcium channels of 172 neurons (34%) were inhibited by 32+/-3% with the application of an mu-opioid agonist, [D-Ala(2),N-MePhe(4),Gly(5)-ol]-enkephalin (DAMGO, 1 microM). The barium currents itself and the DAMGO-induced inhibitory effects were not affected by the application of either an adenylate cyclase activator (forskolin, 1 microM) or a protein kinase inhibitor (staurosporin, 10 nM) for 2 min. The DAMGO inhibition was completely and irreversibly antagonized by the application of a protein kinase C activator, phorbol-12-myristate-13-acetate (PMA, 1 microM) for 2 min without any alteration of the barium current itself. However, the antagonizing effect of PMA was completely abolished by the application of 10 nM staurosporin for 2 min. After then, PMA did not show the antagonizing effect any more. Inversely, when staurosporin was applied before PMA, the antagonizing effect of PMA was also not shown. These results demonstrate that the mu-opioid agonist-induced inhibition of the periaqueductal gray neuronal high-voltage activated calcium current can be antagonized by protein kinase C activation. This finding may provide us a significant clue to understand the action mechanism of opioid-induced analgesia in the periaqueductal gray.