Biomedical subjects

Steven W Johnson

Publications and source records attributed to Steven W Johnson.

At least 19 recordsLinked to original sources

PubMed2006

Subthalamic stimulation evokes complex EPSCs in the rat substantia nigra pars reticulata in vitro.

The subthalamic nucleus (STN) plays an important role in movement control by exerting its excitatory influence on the substantia nigra pars reticulata (SNR), a major output structure of the basal ganglia. Moreover, excessive burst firing of SNR neurons seen in Parkinson's disease has been attributed to excessive transmission in the subthalamonigral pathway. Using the 'blind' whole-cell patch clamp recording technique in rat brain slices, we found that focal electrical stimulation of the STN evoked complex, long-duration excitatory postsynaptic currents (EPSCs) in SNR neurons. Complex EPSCs lasted 200-500 ms and consisted of an initial monosynaptic EPSC followed by a series of late EPSCs superimposed on a slow inward shift in holding current. Focal stimulation of regions outside the STN failed to evoke complex EPSCs. The late component of complex EPSCs was markedly reduced by ionotropic glutamate receptor antagonists (2-amino-5-phosphonopentanoic acid and 6-cyano-7-nitro-quinoxalone) and by a GABAA receptor agonist (isoguvacine) when these agents were applied directly to the STN using a fast-flow microapplicator. Moreover, the complex EPSC was greatly enhanced by bath application of the GABAA receptor antagonists picrotoxin or bicuculline. These data suggest that recurrent glutamate synapses in the STN generate polysynaptic, complex EPSCs that are under tonic inhibition by GABA. Because complex EPSCs are expected to generate bursts of action potentials in SNR neurons, we suggest that complex EPSCs may contribute to the pathological burst firing that is associated with the symptoms of Parkinson's disease.

2-Amino-5-phosphonovalerate↗

PubMed2006

Dopamine-mediated actions of ephedrine in the rat substantia nigra.

Although ephedrine is a centrally active stimulant, its effect on midbrain dopamine neurons is not known. To study the effect of ephedrine on dopamine-containing cells, current-clamp microelectrode recordings were made from substantia nigra pars compacta (SNC) neurons in horizontal brain slice preparations. Ephedrine (100-1000 microM) slowed spontaneous firing and produced a modest concentration-dependent hyperpolarization of membrane potential (EC50 279 microM), with a concomitant net decrease in membrane resistance. These effects were blocked by the D2-like dopamine antagonist sulpiride (1 microM). Electrically evoked inhibitory synaptic potentials mediated by GABAB receptors were reduced 28% by ephedrine. However, ephedrine did not reduce fast synaptic potentials mediated by GABAA or ionotropic glutamate receptors. Inhibition of the GABAB response appeared to be mediated by a postsynaptic mechanism because ephedrine also reduced baclofen-induced hyperpolarization by 28%. Both ephedrine-induced hyperpolarization and inhibition of baclofen-induced hyperpolarization were abolished when slices were superfused with the tyrosine hydroxylase inhibitor alpha-methyl-para-tyrosine (AMPT). Despite perfusion with AMPT, the ability of ephedrine to cause hyperpolarization was restored after perfusing the slice with dopamine (30 microM). Taken together, these results suggest that ephedrine causes hyperpolarization and suppresses GABAB receptor-mediated effects by releasing endogenous dopamine. However, the high concentrations required to observe these effects in vitro suggest that biologically relevant central effects of ephedrine are more likely to be mediated either by non-dopamine systems, such as those involving noradrenaline, or by dopamine systems outside the SNC.

Action Potentials↗

PubMed2005

Macrophage migration inhibitory factor is a determinant of hypoxia-induced apoptosis in colon cancer cell lines.

Hypoxia contributes to cytotoxic chemotherapy and radiation resistance and may play a role in the efficacy of antiangiogenesis cancer therapy. We have generated a series of cell lines derived from the colon adenocarcinoma models HT29 and HCT116 by exposing cells in vitro to repeated sublethal periods of profound hypoxia. These cell lines have altered sensitivity to hypoxia-induced apoptosis: those derived from HT29 are resistant, whereas those from HCT116 are more susceptible. We used cDNA selected subtractive hybridization to identify novel genes mediating sensitivity to hypoxia-induced apoptosis and isolated macrophage migration inhibitory factor (MIF) from the hypoxia-conditioned cell lines. MIF expression correlates with susceptibility of the cell lines to apoptosis. In hypoxia-resistant cells, the induction of apoptosis by hypoxia can be restored by the addition of exogenous recombinant MIF protein, suggesting that resistance may result in part from down-regulation of MIF production possibly through an autocrine loop. Inhibition of MIF using small interfering RNA in the susceptible lines conferred resistance to hypoxia-induced cell death. The relative expression of MIF in the hypoxia-conditioned cells implanted s.c. in severe combined immunodeficient mice in vivo was similar to that observed in vitro. In an analysis of 12 unrelated colon tumor cell lines, MIF expression and response to hypoxia varied widely. Cell lines in which MIF was inducible by hypoxia were more sensitive to oxaliplatin. In human colon tumor specimens analyzed by immunohistochemistry, MIF expression was similarly variable. There was no detectable expression of MIF in normal colon mucosa or adenoma but positive staining in all carcinomas tested. Taken together, these data indicate that MIF may be a determinant of hypoxia-induced apoptosis in vitro and that its variable expression in human colon cancers may indicate a functional role in vivo. We suggest that MIF expression in colorectal cancer may be a marker of susceptibility to therapies that may depend on induction of hypoxia, possibly including antiangiogenic therapy.

Apoptosis↗

PubMed2005

Dopamine depletion alters responses to glutamate and GABA in the rat subthalamic nucleus.

We used whole-cell recordings to compare currents evoked by glutamate and GABA receptor agonists in subthalamic nucleus neurons located ipsilateral and contralateral to unilateral 6-hydroxydopamine (6-OHDA) injections into the substantia nigra zona compacta. The ratio of currents evoked by AMPA (0.6 microM) and NMDA (20 microM) was significantly greater in neurons recorded ipsilateral to 6-OHDA lesions compared with the ratio of currents recorded in control (contralateral) neurons. Both the GABA(A) agonist isoguvacine (20 microM) and the GABA(B) agonist baclofen (10 microM) evoked significantly greater outward currents in subthalamic nucleus neurons ipsilateral to the lesion compared to contralateral neurons. We conclude that chronic dopamine depletion up-regulates expression of GABA receptors and shifts the functional expression of ionotropic glutamate receptor subtype from NMDA to AMPA receptors in subthalamic nucleus.

Animals↗

PubMed2005

NMDA enhances a depolarization-activated inward current in subthalamic neurons.

Previous studies have shown that N-methyl-D-aspartate (NMDA) receptor stimulation evokes Ca2+- and Na+-dependent burst firing in subthalamic nucleus (STN) neurons. Using whole-cell patch pipettes to record currents under voltage-clamp, we identified a time-dependent depolarization-activated inward current (DIC) that may underlie NMDA-induced burst firing in STN neurons in rat brain slices. Continuous superfusion with NMDA (20 microM) elicited a marked TTX-insensitive inward current when the membrane was depolarized to the level of -70 or -50 mV, from a holding potential of -100 mV. This current had a long duration, and its peak amplitude occurred at a test potential of -60 mV. DIC could not be evoked using the non-NMDA receptor agonist D,L-alpha-amino-3-hydroxy-5-methylisoxalone-4-propionic acid (AMPA). DIC was blocked by either intracellular BAPTA or by removal of extracellular Ca2+, but selective blockers of T-type (mibefradil), L-type (nifedipine) and N-type (omega-conotoxin GVIA) Ca2+ channels did not. Perfusing slices with a low extracellular concentration of sodium abolished the NMDA-induced DIC, implying that both Ca2+ and Na+ are necessary for the expression of DIC. Transient receptor potential (TRP) channel blockers flufenamic acid and SKF96365 severely reduced DIC amplitude, whereas NMDA-gated currents were either increased or were unchanged. These results suggest that the activation of NMDA receptors enhances a Ca2+-activated non-selective cation current that may be mediated by a member of the TRP channel family in STN neurons.

Animals↗

PubMed2004

Multiple mechanisms underlie burst firing in rat midbrain dopamine neurons in vitro.

Both apamin and NMDA evoke burst firing in dopamine neurons recorded intracellularly in slices of rat midbrain. However, apamin-induced bursting required injection of depolarizing currents, and was mimicked by Bay K8644 and blocked by nifedipine. In contrast, NMDA-induced bursting required hyperpolarizing currents and was not blocked by nifedipine. Our results show that burst firing can be evoked in dopamine neurons via two different mechanisms.

Action Potentials↗

PubMed2004

Calcium-dependent subthreshold oscillations determine bursting activity induced by N-methyl-D-aspartate in rat subthalamic neurons in vitro.

We used whole-cell patch recordings in current clamp to investigate the ionic dependence of burst firing induced by N-methyl-d-aspartate (NMDA) in neurons of the subthalamic nucleus (STN) in slices of rat brain. NMDA (20 microm) converted single-spike firing to burst firing in 87% of STN neurons tested. NMDA-induced bursting was blocked by AP5 (50 microm), and was not mimicked by the non-NMDA receptor agonist AMPA (0.6 microm). Tetrodotoxin (1 microm) converted bursts to oscillations of membrane potential, which were most robust when oscillations ranged between -50 and -70 mV. The NMDA bursts were blocked by an elevated extracellular concentration of Mg(2+), but superfusate containing no added Mg(2+) either reduced or increased burst firing, depending upon the amount of intracellular current injection. Block of K(+) conductances by apamin and tetraethylammonium prolonged burst duration, but iberiotoxin had no effect. NMDA-induced burst firing and membrane oscillations were completely blocked by superfusate containing no added Ca(2+), and they were significantly reduced when patch pipettes contained BAPTA. Selective antagonists for T-type (mibefradil, 10 microm), L-type (nifedipine, 3 microm), and N-type (omega-conotoxin GVIA, 1 micro m) Ca(2+) channels had no effect on NMDA burst firing. Superfusate containing a low concentration of Na(+) (20 mm) completely abolished NMDA-induced burst firing. Flufenamic acid (10 microm), which blocks current mediated by Ca(2+)-activated nonselective cation channels (I(CAN)), reversibly abolished NMDA-depended bursting. These results are consistent with the hypothesis that NMDA-induced burst firing in STN neurons requires activation of either an I(CAN) or a Na(+)-Ca(2+) exchanger.

Animals↗

PubMed2003

Synaptic plasticity in rat subthalamic nucleus induced by high-frequency stimulation.

The technique of deep brain stimulation (DBS) has become a preferred surgical choice for the treatment of advanced Parkinson's disease. The subthalamic nucleus (STN) is presently the most promising target for such DBS. In this study, whole-cell patch-clamp recordings were made from 46 STN neurons in rat brain slices to examine the effect of high-frequency stimulation (HFS) of the STN on glutamatergic synaptic transmission in STN neurons. HFS, consisting of trains of stimuli at a frequency of 100 Hz for 1 min, produced three types of synaptic plasticity in 17 STN neurons. First, HFS of the STN induced short-term potentiation (STP) of evoked postsynaptic current (EPSC) amplitude in four neurons. STP was associated with a reduction in the EPSC paired-pulse ratio, suggesting a presynaptic site of action. Second, HFS of the STN generated long-term potentiation (LTP) of EPSC amplitude in eight neurons. Although the EPSC paired-pulse ratio was reduced transiently in the first 2 min following HFS, ratios measured 6-20 min after HFS were unchanged from control. This suggests that LTP is maintained by a postsynaptic mechanism. Third, HFS produced long-term depression (LTD) of EPSC amplitude in five STN neurons. LTD was associated with a significant increase in EPSC paired-pulse ratios, indicating a presynaptic site of action. These results suggest that HFS can produce long-term changes in the efficacy of synaptic transmission in the STN. HFS-induced synaptic plasticity might be one mechanism underlying the effectiveness of DBS in the STN as a treatment of advanced Parkinson's disease.

6-Cyano-7-nitroquinoxaline-2,3-dione↗

PubMed2003

Modulation of calcium-activated potassium small conductance (SK) current in rat dopamine neurons of the ventral tegmental area.

Block of calcium-sensitive potassium SK current (gKCa([SK])) by apamin or bicuculline methiodide potentiates burst firing in dopamine neurons in the presence of N-methyl-D-aspartate (NMDA). The purpose of this study was to test the hypothesis that calcium entry through NMDA-gated channels can potentiate gKCa([SK]) in dopamine neurons in the ventral tegmental area. We used microelectrodes to record an outward tail current that was evoked by membrane depolarization under single-electrode voltage-clamp. Using bicuculline methiodide (50 microM) as a reversible inhibitor of gKCa([SK]), we found that NMDA (15 microM) reduced the peak amplitude of the outward tail current by 39%. Contrary to expectations, our results suggest that stimulation of NMDA receptors reduces the calcium-activated potassium gKCa([SK]), an effect that could facilitate NMDA-dependent burst firing.

Animals↗

PubMed2003

Group II metabotropic glutamate receptor modulation of excitatory transmission in rat subthalamic nucleus.

Patch pipettes were used to record currents in whole-cell configuration to study the effects of group II metabotropic glutamate receptor (mGluR) stimulation on synaptic transmission in slices of rat subthalamic nucleus. Evoked glutamatergic excitatory postsynaptic currents (EPSCs) were reversibly reduced by the selective group II mGluR agonist (2'S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG IV) in a concentration-dependent manner, with an IC50 of 0.19 +/- 0.05 microM. DCG IV (1 microM) had no effect on inhibitory postsynaptic currents mediated by GABA. DCG IV-induced inhibition of EPSCs was reversed by the selective group II mGluR antagonist LY 341495 (100 nM) and mimicked by another selective group II agonist (2S,1'S,2'S)-2-(carboxycyclopropyl)glycine (L-CCG-I). Inhibition of EPSC amplitude by DCG IV and L-CCG-I was associated with an increase in the paired-pulse ratio of EPSCs. The protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (2 microM) reduced the inhibitory effect of DCG IV on EPSCs. However, the response to DCG IV was not affected by the protein kinase A (PKA) activator forskolin (20 microM), by the adenylyl cyclase inhibitor MDL 12230A (20 microM), or by the phosphodiesterase inhibitor Ro 20-1724 (50 microM). DCG IV-induced inhibition of EPSCs was reduced by the non-selective protein kinase inhibitors H-7 (100 microM), H-8 (50 microM) and HA-1004 (100 microM). These results suggest that group II mGluR stimulation acts presynaptically to inhibit glutamate release by a PKC-dependent mechanism in the subthalamic nucleus.

1-(5-Isoquinolinesulfonyl)-2-Methylpiperazine↗

PubMed2003

A modeling study suggests complementary roles for GABAA and NMDA receptors and the SK channel in regulating the firing pattern in midbrain dopamine neurons.

Midbrain dopaminergic (DA) neurons in vivo exhibit two major firing patterns: single-spike firing and burst firing. The firing pattern expressed is dependent on both the intrinsic properties of the neurons and their excitatory and inhibitory synaptic inputs. Experimental data suggest that the activation of N-methyl-D-aspartate (NMDA) and GABAA receptors is a crucial contributor to the initiation and suppression of burst firing, respectively, and that blocking Ca(2+)-activated potassium SK channels can facilitate burst firing. A multi-compartmental model of a DA neuron with a branching structure was developed and calibrated based on in vitro experimental data to explore the effects of different levels of activation of NMDA and GABAA receptors as well as the modulation of the SK current on the firing activity. The simulated tonic activation of GABAA receptors was calibrated by taking into account the difference in the electrotonic properties in vivo versus in vitro. Although NMDA-evoked currents are required for burst generation in the model, currents evoked by GABAA-receptor activation can also regulate the firing pattern. For example, the model predicts that increasing the level of NMDA receptor activation can produce excessive depolarization that prevents burst firing, but a concurrent increase in the activation of GABAA receptors can restore burst firing. Another prediction of the model is that blocking the SK channel current in vivo will facilitate bursting, but not as robustly as blocking the GABAA receptors.

Animals↗

PubMed2003

Gene expression patterns in ovarian carcinomas.

We used DNA microarrays to characterize the global gene expression patterns in surface epithelial cancers of the ovary. We identified groups of genes that distinguished the clear cell subtype from other ovarian carcinomas, grade I and II from grade III serous papillary carcinomas, and ovarian from breast carcinomas. Six clear cell carcinomas were distinguished from 36 other ovarian carcinomas (predominantly serous papillary) based on their gene expression patterns. The differences may yield insights into the worse prognosis and therapeutic resistance associated with clear cell carcinomas. A comparison of the gene expression patterns in the ovarian cancers to published data of gene expression in breast cancers revealed a large number of differentially expressed genes. We identified a group of 62 genes that correctly classified all 125 breast and ovarian cancer specimens. Among the best discriminators more highly expressed in the ovarian carcinomas were PAX8 (paired box gene 8), mesothelin, and ephrin-B1 (EFNB1). Although estrogen receptor was expressed in both the ovarian and breast cancers, genes that are coregulated with the estrogen receptor in breast cancers, including GATA-3, LIV-1, and X-box binding protein 1, did not show a similar pattern of coexpression in the ovarian cancers.

Adenocarcinoma↗

PubMed2003

Gene expression profiling of malignant mesothelioma.

PURPOSE: Malignant mesothelioma is a uniformly fatal cancer of the pleural and peritoneal spaces. Several challenging clinical problems include poor understanding of the pathophysiology, inaccurate diagnosis from tissue samples, and unsuccessful treatment strategies. The purpose of this study was to use microarray analysis to identify specific gene expression changes in mesothelioma compared with normal mesothelium. EXPERIMENTAL DESIGN: We performed gene expression analysis on mesothelioma tissue specimens from 16 patients and compared these to 4 control pleural tissue samples using cDNA microarray filters with 4132 clones. Multiple normalization and analysis approaches were used. Quantitative reverse transcription-PCR and immunohistochemistry were used to validate results. RESULTS: Genes (166) were significantly up-regulated, and 26 were down-regulated. Validation of 18 genes using real-time PCR confirmed array predictions in every case. Analysis revealed activation of several key pathways including genes involved in glucose metabolism, mRNA translation, and cytoskeletal remodeling. Expression profiling identified processes likely responsible for 18-fluoro-2-deoxy-glucose uptake and tumor localization by positron emission tomography, and a role for hypoxia-inducible factor-1 was suggested. Potentially important up-regulated genes included gp96, lung resistance-related protein, galectin-3 binding protein, the M(r) 67,000 laminin receptor (on tumor vessels), and voltage-dependent anion channels. Prospective testing using reverse transcription-PCR confirmed up-regulation of these novel markers. CONCLUSIONS: Expression profiling revealed marked up-regulation of energy, protein translation, and cytoskeletal remodeling pathways in mesothelioma. Additional genes that could be important in our understanding of the pathogenesis of mesothelioma, aiding in diagnosis, or improving targets for therapy were also identified.

Aged↗

PubMed2003

Current status of oxaliplatin in colorectal cancer.

Novel platinum compounds have been sought with the goal of identifying molecules active in cisplatin-resistant cancers such as colorectal cancer. Some 30 years ago it was shown that varying the structure of the coordination complex could enhance the activity of platinum compounds against preclinical models. The diaminocyclohexane ligand, now embodied in the structure of oxaliplatin, was the most promising of a series of such analogues. Oxaliplatin has recently been approved in the United States for the treatment of refractory colorectal cancer. Studies in the initial treatment of advanced disease indicate that its activity when used in combination with 5-fluorouracil (5-FU) is at least as good as standard therapy for this stage of the disease. Novel combinations have been reported in phase II studies, and adjuvant trials have been completed in the United States and Europe. It is reasonable to conclude that there are now two accepted regimens for the treatment of colorectal cancer, irinotecan/5-FU and oxaliplatin/5-FU. Early studies using both regimens sequentially are of interest, and phase I trials of three-drug combinations have been presented. Current efforts are directed toward individualizing therapy through predictive analyses of the targets and metabolic pathways of all three agents in pharmacogenetic and pharmacogenomic studies.

Antineoplastic Agents↗

PubMed2003

Dopamine receptor supersensitivity in rat subthalamus after 6-hydroxydopamine lesions.

The subthalamic nucleus (STN) receives direct dopaminergic innervation from the substantia nigra pars compacta, but the importance of this input in the pathophysiology of parkinsonism remains to be determined. We used whole-cell patch-clamp recordings in brain slices to study presynaptic dopaminergic modulation of synaptic inputs to the STN in unilateral 6-hydroxydopamine (6-OHDA)-lesioned rats. Here, we report that dopamine was more potent for inhibiting GABA IPSCs and glutamate EPSCs in the STN ipsilateral to the lesion, and was less potent for suppressing IPSCs and EPSCs in the STN contralateral to the lesion, compared with the effects of dopamine in control STN. Dopamine reduced IPSCs with an IC50 value of 20.9 +/- 3.6 microM in control STN, whereas IC50 values were 0.83 +/- 0.15 and 55.1 +/- 11.1 microM in STN ipsilateral and contralateral to 6-OHDA lesions, respectively. Dopamine also inhibited EPSCs with an IC50 value of 12.8 +/- 2.8 microM in control STN, whereas IC50 values were 4.5 +/- 0.9 and 41.6 +/- 9.8 microM in STN ipsilateral and contralateral to 6-OHDA lesions, respectively. Results with paired stimuli to evoke EPSCs and IPSCs suggest that endogenous dopamine acts presynaptically to inhibit transmitter release in the STN. These results show that chronic dopamine denervation significantly alters the regulation of synaptic input to the STN. Our results also suggest that the STN may be an important target for levodopa therapy in Parkinson's disease.

6-Cyano-7-nitroquinoxaline-2,3-dione↗

PubMed2002

Group I metabotropic glutamate receptor-mediated enhancement of dopamine cell burst firing in rat ventral tegmental area in vitro.

Metabotropic glutamate receptor (mGluR) agonist t-ACPD produced concentration-dependent enhancement of NMDA/apamin-induced burst firing and membrane oscillations in dopamine cells, recorded intracellularly from ventral tegmental area in the rat midbrain slice. Such effects were blocked reversibly by mGluR antagonist MCPG, and mimicked by the selective agonist for group I (but not group II and III) mGluRs. Our results point out a selective involvement of group I mGluRs in facilitating burst firing of midbrain dopamine cells.

Action Potentials↗

PubMed2002

Excitatory effects of dopamine on subthalamic nucleus neurons: in vitro study of rats pretreated with 6-hydroxydopamine and levodopa.

Increased output from the subthalamic nucleus (STN) following chronic dopamine depletion has been linked to the rigidity and tremor seen in Parkinson's disease (PD). We used extracellular microelectrode recordings from rat brain slices to investigate effects of dopamine on STN neurons. In brain slices prepared from rats that received unilateral 6-hydroxydopamine (6-OHDA) treatment, the spontaneous firing rate of STN neurons was reduced by 63%, and the firing pattern was more irregular, compared to STN neurons from normal rats. However, treatment with levodopa (50 mg/kg, i.p., daily) for 4 weeks normalized the firing rate and pattern of STN neurons in the 6-OHDA-treated rats. Dopamine (3-300 microM), added to the superfusate, significantly increased the firing rates of STN neurons in a concentration-dependent fashion, and also produced a more regular firing pattern in 6-OHDA-lesioned tissue. This excitatory effect of dopamine was mimicked by a D2 receptor agonist (quinpirole), and was reduced by the D2 antagonists haloperidol, clozapine and sulpiride. Antagonists of the D1 receptor (SCH-23390) and ionotropic glutamatergic receptors (CNQX and AP5) could not block the effect of dopamine on firing rate. These results suggest that dopamine exerts a direct excitatory influence on STN neurons via the activation of D2-like receptors.

Animals↗

PubMed2002

Presynaptic modulation of synaptic transmission by opioid receptor in rat subthalamic nucleus in vitro.

Presynaptic modulation of synaptic transmission in rat subthalamic nucleus (STN) neurons was investigated using whole-cell patch-clamp recordings in brain slices. Evoked GABAergic inhibitory postsynaptic currents (IPSCs) were reversibly reduced by methionine enkephalin (ME) with an IC(50) value of 1.1 +/- 0.3 microM. The action of ME was mimicked by the mu-selective agonist [D-Ala(2), N-Me-Phe(4), Gly(5)-ol]-enkephalin (DAMGO), and was partially blocked by the mu-selective antagonists naloxonazine and D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH(2) (CTAP). Evoked GABA(A) IPSCs were also inhibited by the delta-selective agonist [D-Pen(2,5)]-enkephalin (DPDPE), but not by the kappa-selective agonist (+)-(5 alpha,7 alpha,8 beta)-N-methyl-N-[7-(1-pyrrolidinyl)-1-oxaspiro[4.5]dec-8-yl]-benzeneacetamide (U-69593) and the orphan receptor agonist orphanin FQ/nociceptin (OFQ). DPDPE-induced inhibition was completely blocked by the delta-selective antagonist N,N-diallyl-Tyr-Aib-Aib-Phe-Leu-OH (ICI 174,864). ME, DAMGO and DPDPE increased the paired-pulse ratio of IPSCs. Evoked excitatory postsynaptic currents (EPSCs) were reversibly reduced by ME with an IC(50) value of 0.35 +/- 0.14 microM. Inhibition by ME was associated with an increase in the paired-pulse ratio of EPSCs. The action of ME was mimicked by DAMGO, and blocked by naloxonazine. DPDPE had little effect on evoked EPSCs. Neither U-69593 nor OFQ had any effect. ME significantly decreased the frequency of spontaneous miniature EPSCs (mEPSCs) without change in their amplitude. The action of ME was mimicked by DAMGO. DPDPE had no effect. The presynaptic voltage-dependent potassium conductance blocker 4-aminopyridine (4-AP, 100 microM) abolished the inhibitory effects of ME on evoked IPSCs and EPSCs. In contrast, 4-AP only partially blocked the actions of baclofen. These results suggest that opioids inhibit inhibitory synaptic transmission in the STN through the activation of presynaptic mu- and delta- receptors. In contrast, inhibition of excitatory synaptic inputs to the STN occurs through the activation of only mu-receptors. Both inhibitions may be mediated by blockade of voltage-dependent potassium conductance.

Animals↗