Roles of protein kinase A and C in the opioid potentiation of the GABAA response in rat periaqueductal gray neuron.

The periaqueductal gray (PAG) is the main target site of the opioid-induced analgesia. The present study was designed to examine the roles of protein kinase A (PKA) and C (PKC) in the opioid-induced modulation of the currents activated by an inhibitory neurotransmitter, gamma-aminobutyric acid (GABA). The PAG neurons were acutely isolated and voltage-clamped under the nystatin-perforated patch-clamp mode. The GABA-activated current was sensitively blocked by a GABA(A) receptor antagonist, bicuculline, and selectively carried by chloride ions. The GABA(A) receptor-activated Cl(-) current was potentiated by a mu-opioid receptor agonist, [D-Ala(2),N-MePhe(4),Gly(5)-ol]-enkephalin acetate (DAMGO). The GABA response was also potentiated by phorbol-12-myristate-13-acetate (PMA). Pretreatment with PMA occluded the DAMGO potentiation. However, both chelerythrine and 2-[1-(3-dimethylaminopropyl)indol-3-yl]-3-(indol-3-yl) maleimide (GF109203X) also potentiated the GABA response. Pretreatment with chelerythrine or GF109203X also occluded the DAMGO potentiation. Meanwhile, the GABA response was potentiated by N-(2-[p-bromocinnamylamino]-ethyl)-5-isoquinolinesulfonamide (H-89), while not altered by forskolin. Pretreatment with H-89 occluded the potentiation effect of DAMGO on the GABA response. In addition, the DAMGO effect was completely blocked by pretreatment with forskolin. From the result, it can be suggested that activation of mu-opioid receptor potentiates the GABA(A) response through the mediation of PKA inhibition, and that PKC is not directly involved in the action mechanism of DAMGO.

Role of protein kinase C in opioid modulation of glycine-gated Cl(-) current in rat periaqueductal gray neuron.

The Role of protein kinase C in the modulatory effect of a mu-opioid receptor agonist, [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]enkephalin (DAMGO), on the glycine-gated Cl(-) current was examined in acutely dissociated rat periaqueductal gray neurons. Using the nystatin-perforated patch-clamp technique, the neurons were voltage-clamped at -60 mV. The glycine-gated Cl(-) current (I(Gly)) was sensitive to strychnine. On pretreatment with 1 microM DAMGO, the 30-microM glycine response increased with time and showed a maximum amplitude of 209+/-37% of control. After a protein kinase C activator, phorbol-12-myristate-13-acetate (PMA, 0.1 microM) as pretreatment, I(Gly) increased to 138+/-6% of control. The DAMGO potentiation of I(Gly) was not altered by coapplication with PMA. Although protein kinase C inhibitors, chelerythrine (3 microM) and 2-[1-(3-dimethylaminopropyl)indol-3-yl]-3-(indol-3-yl) maleimide (GF109203X, 1 microM), did not alter I(Gly), the DAMGO-induced potentiation of I(Gly) was reduced to 161+/-21% or 164+/-31% of the control after coapplication with chelerythrine or GF109203X, respectively. These results indicate that the potentiation of I(Gly) by a mu-opioid receptor agonist is partly mediated by activation of protein kinase C.