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

Electrophysiological Actions of N/OFQ

Whilst the nociceptin/orphanin FQ (N/OFQ) receptor (NOP) has similar intracellular coupling mechanisms to opioid receptors, it has distinct modulatory effects on physiological functions such as pain. These actions range from agonistic to antagonistic interactions with classical opioids within the spinal cord and brain, respectively. Understanding the electrophysiological actions of N/OFQ has been crucial in ascertaining the mechanisms by which these agonistic and antagonistic interactions occur. These similarities and differences between N/OFQ and opioids are due to the relative location of NOP versus opioid receptors on specific neuronal elements within these CNS regions. These mechanisms result in varied cellular actions including postsynaptic modulation of ion channels and presynaptic regulation of neurotransmitter release.

Molecular Changes in Opioid Addiction: The Role of Adenylyl Cyclase and cAMP/PKA System

For centuries, opiate analgesics have had a considerable presence in the treatment of moderate to severe pain. While effective in providing analgesia, opiates are notorious in exerting many undesirable adverse reactions. The receptor targets and the intracellular effectors of opioids have largely been identified. Furthermore, much of the mechanisms underlying the development of tolerance, dependence, and withdrawal have been delineated. Thus, there is a focus on developing novel compounds or strategies in mitigating or avoiding the development of tolerance, dependence, and withdrawal. This review focuses on the adenylyl cyclase and cyclic adenosine 3,5-monophosphate (cAMP)/protein kinase A (AC/cAMP/PKA) system as the central player in mediating the acute and chronic effects of opioids. This chapter also reviews the neuronal adaptive changes in the locus coeruleus, amygdala, periaqueductal gray, and ventral tegmental area induced by acute and chronic actions of opioid because these neuronal adaptive changes in these regions may underlie the behavioral changes observed in opiate users and abusers.

Transcription factor REST negatively influences the protein kinase C-dependent up-regulation of human mu-opioid receptor gene transcription

Mu-opioid receptor expression increases during neurogenesis, regulates the survival of maturing neurons and is implicated in ischemia-induced neuronal death. The repressor element 1 silencing transcription factor (REST), a regulator of a subset of genes in differentiating and post-mitotic neurons, is involved in its transcriptional repression. Extracellular signaling molecules and mechanisms that control the human mu-opioid receptor (hMOR) gene transcription are not clearly understood. We examined the role of protein kinase C (PKC) on hMOR transcription in a model of neuronal cells and in the context of the potential influence of REST. In native SH-SY5Y neuroblastoma cells, PKC activation with phorbol 12-myristate 13-acetate (PMA, 16 nM, 24h) down-regulated hMOR transcription and concomitantly elevated the REST binding activity to repressor element 1 of the hMOR promoter. In contrast, PMA activated hMOR gene transcription when REST expression was knocked down by an antisense strategy or by retinoic acid-induced cell differentiation. PMA acts through a PKC-dependent pathway requiring downstream MAP kinases and the transcription factor AP-1. In a series of hMOR-luciferase promoter/reporter constructs transfected into SH-SY5Y cells and PC12 cells, PMA up-regulated hMOR transcription in PC12 cells lacking REST, and in SH-SY5Y cells either transfected with constructs deficient in the REST DNA binding element or when REST was down-regulated in retinoic acid-differentiated cells. These findings help explain how hMOR transcription is regulated and may clarify its contribution to epigenetic modifications and reprogramming of differentiated neuronal cells exposed to PKC-activating agents.

Anxiolytic-like effects of sinapic acid in mice

Sinapic acid is a phenylpropanoid compound and is found in various herbal materials and high-bran cereals. With the exception of its antioxidant activities, the pharmacological properties of sinapic acid have been rarely reported. The purpose of this study was to characterize the putative anxiolytic-like properties of sinapic acid using an elevated plus-maze (EPM) and hole-board test. Control mice were orally treated with an equal volume of vehicle (10% Tween 80 solution), and positive control mice were treated with diazepam (1 mg/kg, i.p.). Sinapic acid (4 mg/kg, p.o.) significantly increased the percentages of time spent in the open arms of the EPM test (P<0.05). In the hole-board test, sinapic acid also significantly increased the number of head-dips at 4 mg/kg (P<0.05). In addition, the anxiolytic-like properties of sinapic acid examined in the EPM test were blocked by flumazenil or bicuculline, which are GABA(A) antagonists. Moreover, sinapic acid markedly potentiated GABA current in single cortical neurons in a dose-dependant manner, and reactive I(GABA) increased to 1.8 times at 1 muM of sinapic acid. These results suggested that sinapic acid is a prominent anxiolytic agent, and that its anxiolytic-like effects are mediated via GABA(A) receptors and potentiating Cl(-) currents.

The ameliorating effect of oroxylin A on scopolamine-induced memory impairment in mice

Oroxylin A is a flavonoid and was originally isolated from the root of Scutellaria baicalensis Georgi., one of the most important medicinal herbs in traditional Chinese medicine. The aim of this study was to investigate the ameliorating effects of oroxylin A on memory impairment using the passive avoidance test, the Y-maze test, and the Morris water maze test in mice. Drug-induced amnesia was induced by administering scopolamine (1 mg/kg, i.p.) or diazepam (1 mg/kg, i.p.). Oroxylin A (5 mg/kg) significantly reversed cognitive impairments in mice by passive avoidance and the Y-maze testing (P<.05). Oroxylin A also improved escape latencies in training trials and increased swimming times and distances within the target zone of the Morris water maze (P<.05). Moreover, the ameliorating effects of oroxylin A were antagonized by both muscimol and diazepam (0.25 mg/kg, i.p., respectively), which are GABA(A) receptor agonists. Furthermore, oroxylin A (100 microM) was found to inhibit GABA-induced inward Cl(-) current in a single cortical neuron. These results suggest that oroxylin A may be useful for the treatment of cognitive impairments induced by cholinergic dysfunction via the GABAergic nervous system.

Opioid mu receptor activation inhibits sodium currents in prefrontal cortical neurons via a protein kinase A- and C-dependent mechanism

Opioid transmission in the medial prefrontal cortex is involved in mood regulation and is altered by drug dependency. However, the mechanism by which ionic channels in cortical neurons are controlled by mu opioid receptors has not been elucidated. In this study, the effect of mu opioid receptor activation on voltage-dependent Na(+) currents was assessed in medial prefrontal cortical neurons. In 66 out of 98 nonpyramidal neurons, the application of 1 microM of DAMGO ([D-Ala(2), N-Me-Phe(4), Gly(5)-OL]-enkephalin), a specific mu receptor agonist, caused a decrease in the Na(+) current amplitude to approximately 79% of that observed in controls (half blocking concentration = 0.094 microM). Moreover, DAMGO decreased the maximum current activation rate, prolonged its time-dependent inactivation, and shifted the half inactivation voltage from -63.4 mV to -71.5 mV. DAMGO prolonged the time constant of recovery from inactivation from 5.4 ms to 7.4 ms. The DAMGO-evoked inhibition of Na(+) current was attenuated when GDP-beta-S (0.4 mM, Guanosine 5-[beta-thio]diphosphate trilithium salt) was included in the intracellular solution. Inhibitors of kinase A and C greatly attenuated the DAMGO-induced inhibition, while adenylyl cyclase and kinase C activators mirrored the DAMGO inhibitory effect. Na(+) currents in pyramidal neurons were insensitive to DAMGO. We conclude that the activation of mu opioid receptors inhibits the voltage-dependent Na(+) currents expressed in nonpyramidal neurons of the medial prefrontal cortex, and that kinases A and C are involved in this inhibitory pathway.

Opioid inhibition of GABAergic neurotransmission in mechanically isolated rat periaqueductal gray neurons

The descending pain control system is activated by opioid peptides mainly at the midbrain periaqueductal gray (PAG). Although activation of presynaptic opioid receptors has been reported to inhibit gamma-aminobutyric acid (GABA) release, the exact electrophysiological mechanisms are controversial. To elucidate the mechanisms involved in the opioid modulation of presynaptic GABA release, we isolated single PAG neurons with functionally intact synaptic terminals by a mechanical dissociation in the absence of enzyme. With the conventional whole-cell recording mode under the voltage-clamp conditions, the spontaneous miniature inhibitory postsynaptic currents (mIPSCs) were recorded. Bicuculline completely and reversibly blocked mIPSCs. A specific mu-opioid agonist, [d-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin (DAMGO), reversibly reduced the frequency of mIPSCs without any alteration of amplitude. The inhibitory effect of DAMGO was blocked by N-ethylmaleimide. Blockade of presynaptic Ca(2+) influx by cadmium or depletion of extracellular Ca(2+) did not alter the DAMGO inhibition. In addition, K(+) channels blockers, Ba(2+) or 4-aminopyridine, did not affect the DAMGO effect. The present study indicates that activation of presynaptic mu-opioid receptors coupled to G-proteins inhibits GABA release through unknown intracellular mechanisms downstream of Ca(2+) influx.

Activation of protein kinase C antagonizes the opioid inhibition of calcium current in rat spinal dorsal horn neurons

Spinal dorsal horn (SDH) is one of important regions in both nociceptive transmission and antinociception. Opioid peptides produce analgesia via regulation of neurotransmitter release through modulation of voltage-dependent Ca(2+) channel (VDCC) in neuronal tissues. The modulatory effect of micro-opioid receptor (MOR) activation on VDCC was investigated in acutely isolated rat SDH neurons under the conventional whole-cell patch-clamp recording mode. The Ba(2+) current passing through VDCC was reversibly inhibited by a MOR agonist, [D-Ala(2),N-MePhe(4),Gly(5)-ol]-enkephalin (DAMGO, 1 microM). Among 108 SDH neurons tested, VDCC of 39 neurons (36%) were inhibited by MOR activation, while other 69 neurons (64%) were not affected. The L-, N-, P/Q-, and R-type VDCC components shared 58.4+/-18.9%, 29.3+/-12.1%, 8.7+/-7.2%, and 3.4+/-4.8% of the total VDCC, respectively. Among VDCC subtypes inhibited by MOR activation, L- and N-types were 61.4+/-12.8% and 30.7+/-14.4%, respectively, while both P/Q- and R-types were 7.9+/-11.8%. A depolarizing pre-pulse increased the amplitude of VDCC and suppressed most of the inhibitory effect of MOR activation. Application of 1 microM phorbol-12-myristate-13-acetate completely antagonized the inhibitory effect of MOR activation without any alteration of basal VDCC amplitude. In contrast, the response of MOR activation was not altered by application of 4-alpha-phorbol (1 microM), 2-[3-Dimethylaminopropyl]indol-3-yl]-3-(indol-3-yl) maleimide (GF109203X, 1 microM), forskolin (1 microM), N-(2-[p-Bromocinnamylamino]ethyl)-5-isoquinolinesulfonamide hydrochloride (H-89, 1 microM). These results indicate that activation of MOR coupled to G-proteins inhibits VDCC, and that this G-protein-mediated inhibition is antagonized by PKC-dependent phosphorylation.

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.