PubMed Health⌕ Search

Biomedical subjects

S W Johnson

Publications and source records attributed to S W Johnson.

At least 19 recordsLinked to original sources
PubMed2006

Gene expression profiling of human ovarian tumours.

There is currently a lack of reliable diagnostic and prognostic markers for ovarian cancer. We established gene expression profiles for 120 human ovarian tumours to identify determinants of histologic subtype, grade and degree of malignancy. Unsupervised cluster analysis of the most variable set of expression data resulted in three major tumour groups. One consisted predominantly of benign tumours, one contained mostly malignant tumours, and one was comprised of a mixture of borderline and malignant tumours. Using two supervised approaches, we identified a set of genes that distinguished the benign, borderline and malignant phenotypes. These algorithms were unable to establish profiles for histologic subtype or grade. To validate these findings, the expression of 21 candidate genes selected from these analyses was measured by quantitative RT-PCR using an independent set of tumour samples. Hierarchical clustering of these data resulted in two major groups, one benign and one malignant, with the borderline tumours interspersed between the two groups. These results indicate that borderline ovarian tumours may be classified as either benign or malignant, and that this classifier could be useful for predicting the clinical course of borderline tumours. Immunohistochemical analysis also demonstrated increased expression of CD24 antigen in malignant versus benign tumour tissue. The data that we have generated will contribute to a growing body of expression data that more accurately define the biologic and clinical characteristics of ovarian cancers.

Adenocarcinoma, Clear Cell↗
PubMed2005

Identification of genes associated with platinum drug sensitivity and resistance in human ovarian cancer cells.

Platinum-based chemotherapeutic regimens are ultimately unsuccessful due to intrinsic or acquired drug resistance. Understanding the molecular basis for platinum drug sensitivity/resistance is necessary for the development of new drugs and therapeutic regimens. In an effort to identify such determinants, we evaluated the expression of approximately 4000 genes using cDNA microarray screening in a panel of 14 unrelated human ovarian cancer cell lines derived from patients who were either untreated or treated with platinum-based chemotherapy. These data were analysed relative to the sensitivities of the cells to four platinum drugs (cis-diamminedichloroplatinum (cisplatin), carboplatin, DACH-(oxalato)platinum (II) (oxaliplatin) and cis-diamminedichloro (2-methylpyridine) platinum (II) (AMD473)) as well as the proliferation rate of the cells. Correlation analysis of the microarray data with respect to drug sensitivity and resistance revealed a significant association of Stat1 expression with decreased sensitivity to cisplatin (r=0.65) and AMD473 (r=0.76). These results were confirmed by quantitative RT-PCR and Western blot analyses. To study the functional significance of these findings, the full-length Stat1 cDNA was transfected into drug-sensitive A2780 human ovarian cancer cells. The resulting clones that exhibited increased Stat1 expression were three- to five-fold resistant to cisplatin and AMD473 as compared to the parental cells. The effect of inhibiting Jak/Stat signalling on platinum drug sensitivity was investigated using the Janus kinase inhibitor, AG490. Pretreatment of platinum-resistant cells with AG490 resulted in significant increased sensitivity to AMD473, but not to cisplatin or oxaliplatin. Overall, the results indicate that cDNA microarray analysis may be used successfully to identify determinants of drug sensitivity/resistance and future functional studies of other candidate genes from this database may lead to an increased understanding of the drug resistance phenotype.

Cell Line, Tumor↗
PubMed2005

Dopamine receptors set the pattern of activity generated in subthalamic neurons.

Information processing in the brain requires adequate background neuronal activity. As Parkinson's disease progresses, patients typically become akinetic; the death of dopaminergic neurons leads to a dopamine-depleted state, which disrupts information processing related to movement in a brain area called the basal ganglia. Using agonists of dopamine receptors in the D1 and D2 families on rat brain slices, we show that dopamine receptors in these two families govern the firing pattern of neurons in the subthalamic nucleus, a crucial part of the basal ganglia. We propose a conceptual frame, based on specific properties of dopamine receptors, to account for the dominance of different background firing patterns in normal and dopamine-depleted states.

Animals↗
PubMed2003

Metabotropic glutamate and muscarinic cholinergic receptor-mediated preferential inhibition of N-methyl-D-aspartate component of transmissions in rat ventral tegmental area.

Presynaptic inhibition is one of the major control mechanisms in the CNS. Our laboratory recently reported that presynaptic GABA(B) and adenosine A(1) receptors mediate a preferential inhibition on N-methyl-D-aspartate receptor-mediated excitatory postsynaptic currents recorded in rat midbrain dopamine neurons. Here we extended these findings to metabotropic glutamate and muscarinic cholinergic receptors. Intracellular voltage clamp recordings were made from dopamine neurons in rat ventral tegmental area in slice preparations. (+/-)-1-Aminocyclopentane-trans-1,3-dicarboxylic acid (agonist for groups I and II metabotropic glutamate receptors) and L(+)-2-amino-4-phosphonobutyric acid (L-AP4; agonist for group III metabotropic glutamate receptors) were significantly more potent for inhibiting N-methyl-D-aspartate receptor-mediated excitatory postsynaptic currents, as compared with inhibition of excitatory postsynaptic currents mediated by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors. Such preferential inhibition of the N-methyl-D-aspartate component was also observed for muscarine (agonist for muscarinic cholinergic receptors). Inhibitory effects of (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid, L-AP4, and muscarine were blocked reversibly by their respective antagonists [(RS)-alpha-methyl-4-carboxyphenylglycine, (RS)-alpha-methyl-4-phosphonophenylglycine, and 1,1-dimethyl-4-diphenylacetoxypiperidinium iodide]. In addition, all three agonists increased the ratio of excitatory postsynaptic currents in paired-pulse studies and did not reduce currents induced by exogenous N-methyl-D-aspartate and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid. Interestingly, the glutamate release stimulator 4-aminopyridine (30 microM) and the glutamate uptake inhibitor L-anti-endo-3,4-methanopyrrolidine dicarboxylate (300 microM) preferentially increased the amplitude of N-methyl-D-aspartate excitatory postsynaptic currents.Thus, agonists for metabotropic glutamate and muscarinic cholinergic receptors act presynaptically to cause a preferential reduction in the N-methyl-D-aspartate component of excitatory synaptic transmissions. Together with the evidence for GABA(B) and adenosine A(1) receptor-mediated preferential inhibition of the N-methyl-D-aspartate component, the present results suggest that limiting glutamate spillover onto postsynaptic N-methyl-D-aspartate receptors may be a general rule for presynaptic modulation in midbrain dopamine neurons.

Animals↗
PubMed2003

Presynaptic inhibition of synaptic transmission by adenosine in rat subthalamic nucleus in vitro.

Whole-cell patch clamp recordings were made from the subthalamic nucleus in rat brain slice preparations to examine the effect of adenosine on inhibitory and excitatory synaptic transmission. Adenosine reversibly inhibited both GABA-mediated inhibitory and glutamate-mediated excitatory postsynaptic currents. Adenosine at 100 microM reduced the amplitude of inhibitory and excitatory postsynaptic currents by 42+/-5% and 34+/-6%, respectively. Reductions in the amplitude of both inhibitory and excitatory postsynaptic currents were accompanied by increases in paired-pulse ratios. In addition, adenosine decreased the frequency of spontaneous miniature excitatory postsynaptic currents but had no effect on their amplitude. These results are consistent with a presynaptic site of action. The adenosine A(1) receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine completely reversed the adenosine-induced attenuation of inhibitory and excitatory postsynaptic currents, but 8-cyclopentyl-1,3-dipropylxanthine alone had no effect on synaptic currents evoked at 0.1 Hz. However, 8-cyclopentyl-1,3-dipropylxanthine inhibited a time-dependent depression of excitatory postsynaptic currents that was normally observed in response to a 5 Hz train of stimuli, suggesting that endogenous adenosine could be released during higher frequencies of stimulation. These results suggest that adenosine inhibits synaptic release of GABA and glutamate by stimulation of presynaptic A(1) receptors in the subthalamic nucleus.

Adenosine↗
PubMed2003

Dual modulation of gabaergic transmission by metabotropic glutamate receptors in rat ventral tegmental area.

The effects of metabotropic glutamate receptor (mGluR) activation on non-dopamine (putative GABAergic) neurons and inhibitory synaptic transmission in the ventral tegmental area were examined using intracellular recordings from rat midbrain slices. Perfusion of (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (t-ACPD; agonist for group I and II mGluRs), but not L-amino-4-phosphonobutyric acid (L-AP4; agonist for group III mGluRs), produced membrane depolarization (current clamp) and inward current (voltage clamp) in non-dopamine neurons. The t-ACPD-induced depolarization was concentration-dependent (concentration producing 50% maximal depolarization [EC(50)]=6.1+/-2.5 microM), and was blocked by the antagonist (+/-)-alpha-methyl-4-carboxyphenylglycine, but not by tetrodotoxin and ionotropic glutamate-receptor antagonists. The t-ACPD-evoked responses were mimicked comparably by selective group I mGluR agonist (RS)-3,5-dihydroxyphenylglycine (DHPG). Furthermore, the DHPG-induced depolarization in non-dopamine neurons was greatly reduced by mGluR1-specific antagonist 7(hydroxyimino)cyclopropachromen-1a-carboxylate ethyl ester. When recorded in dopamine neurons, the frequency of spontaneous GABA(A) receptor-mediated inhibitory postsynaptic potentials was increased by t-ACPD but not L-AP4. However, the amplitude of evoked inhibitory postsynaptic currents in dopamine neurons was reduced by all three group mGluR agonists. These results reveal a dual modulation of mGLuR activation on inhibitory transmission in midbrain ventral tegmental area: enhancing putative GABAergic neuronal excitability and thus potentiating tonic inhibitory synaptic transmission while reducing evoked synaptic transmission at inhibitory terminals.

Anesthetics, Local↗
PubMed2001

Glycine receptor-mediated inhibition of dopamine and non-dopamine neurons of the rat ventral tegmental area in vitro.

Dopaminergic and non-dopaminergic neurons of the ventral tegmental area (VTA) were recorded intracellularly in slices of rat midbrain. Glycine (0.1-3 mM) caused a strychnine-sensitive and chloride-dependent reduction in membrane input resistance in both types of neuron. However, glycine also reduced the frequency of spontaneous bicuculline-sensitive inhibitory postsynaptic potentials (IPSPs) when recorded in dopaminergic cells. We conclude that glycine inhibits both types of VTA neuron by activating a strychnine-sensitive chloride conductance. Our data also raise the possibility that glycine could increase dopamine output from the VTA by a mechanism of disinhibition.

Action Potentials↗
PubMed2001

Potentiation of GABA(A) receptor agonists by GABA uptake inhibitors in the rat ventral midbrain.

Whole-cell patch recordings were made from dopamine-containing neurons in the ventral tegmental area (VTA) and substantia nigra zona compacta (SNC). Isoguvacine evoked an outward current (at -60 mV) in a concentration-dependent manner with an EC50 of 62+/-8 microM. The gamma-aminobutyric acid (GABA) uptake inhibitor 1-(2(((diphenylmethylene)imino)oxy)ethyl)-1,2,5,6-tetrahydro-3-pyridinecarboxylic acid hydrochloride (NO 711) (3 microM) shifted the isoguvacine concentration-response curve to the left, with a new EC50 of 22+/-4 microM. L-Arginine (3 mM) also shifted the isoguvacine concentration-response curve to the left, with a new EC50 of 29+/-5 microM. L-Arginine (3 mM) increased the currents evoked by GABA (100 microM) and muscimol (1 microM) by 208% and 261%, respectively. The GABA uptake inhibitor 4,5,6,7,-tetrahydroisoxazolo[4,5-c]-pyridin-3-ol hydrobromide (THPO) (300 microM) not only mimicked but also occluded the ability of L-arginine (3 mM) to potentiate currents evoked by isoguvacine. Equimolar replacement of Na+ with choline increased GABA-evoked currents, suggesting that a low Na+ concentration has an inhibitory effect on GABA transport. Low Na+ concentration (25 mM) inhibited isoguvacine currents but still occluded the potentiating effects of L-arginine. We conclude that GABA uptake inhibitors potentiate the actions of the GABA(A) receptor agonists, isoguvacine and muscimol, probably because they are effective substrates for GABA transporters in the ventral midbrain.

Animals↗
PubMed2001

Presynaptic GABA(B) receptors inhibit synaptic inputs to rat subthalamic neurons.

Effects of baclofen on synaptic transmission were studied in rat subthalamic neurons using whole-cell patch clamp recording from brain slices. Focal electrical stimulation of the brain slice evoked GABAergic inhibitory postsynaptic currents and glutamatergic excitatory postsynaptic currents. Baclofen reduced the amplitude of evoked inhibitory postsynaptic currents in a concentration-dependent manner with an IC(50) of 0.6+/-0.2 microM. Evoked excitatory postsynaptic currents were also reduced by baclofen concentration-dependently (IC(50) of 1.6+/-0.2 microM), but baclofen was more potent at reducing the GABA(A) receptor inhibitory postsynaptic currents. The GABA(B) receptor antagonist CGP 35348 blocked these inhibitory effects of baclofen on evoked inhibitory and excitatory postsynaptic currents. Baclofen increased the paired-pulse ratios of evoked inhibitory and excitatory postsynaptic currents. Furthermore, baclofen reduced the frequency of spontaneous miniature excitatory postsynaptic currents, but had no effect on their amplitude. These results provide evidence for presence of presynaptic GABA(B) receptors that modulate both GABA and glutamate release from afferent terminals in the subthalamus.

Animals↗
PubMed2001

GDNF acutely modulates excitability and A-type K(+) channels in midbrain dopaminergic neurons.

Glial cell line-derived neurotrophic factor (GDNF) prevents lesion-induced death of midbrain dopaminergic neurons, but its function in normal brain remains uncertain. Here we show that GDNF acutely and reversibly potentiated the excitability of cultured midbrain neurons by inhibiting transient A-type K(+) channels. The effects of GDNF were limited to large, tyrosine hydroxylase (TH)-positive dopaminergic neurons, and were mediated by mitogen associated protein (MAP) kinase. Application of GDNF also elicited a MAP kinase-dependent enhancement of the excitability in dopaminergic neurons in midbrain slice. These results demonstrate an acute regulation of GDNF on ion channels and its underlying signaling mechanism, and reveal an unexpected role of GDNF in normal midbrain dopaminergic neurons.

4-Aminopyridine↗
PubMed2000

Increased nucleotide excision repair in cisplatin-resistant ovarian cancer cells: role of ERCC1-XPF.

Increased platinum-DNA adduct removal has been shown by several DNA repair assays to be associated with cisplatin resistance in the A2780/C-series human ovarian cancer model system. In the present study, we provide further evidence that the resistance phenotype of these cell lines is due, in part, to enhanced nucleotide excision repair (NER). Cisplatin resistance was found to be associated with increased UV resistance. Northern blot analysis revealed that increased expression of ERCC1 was also associated with cisplatin resistance in this panel. Several other NER genes were found to be constitutively overexpressed in the most resistant cell line, C200, as compared with the parental A2780 cells. A plasmid substrate containing a site-specific cisplatin adduct was used to measure the nucleotide excision activity of cell extracts prepared from cisplatin-sensitive and -resistant cells. Using this in vitro assay, extracts prepared from C200 cells exhibited approximately 3-fold more activity than extracts prepared from A2780 cells, similar to the difference in UV sensitivity. Complementation of A2780 extracts with ERCC1-XPF protein resulted in approximately 2-fold increased activity, but had little effect on excision in C200 extracts. Overall, these results support a role for the ERCC1-XPF endonuclease as a determinant of increased NER in this cisplatin resistance model.

Antineoplastic Agents↗
PubMed2000

The phosphatidylinositol 3-kinase/AKT signal transduction pathway plays a critical role in the expression of p21WAF1/CIP1/SDI1 induced by cisplatin and paclitaxel.

The cyclin-dependent kinase inhibitor p21WAF1/CIP1/SD11 (p21) plays a crucial role in DNA repair, cell differentiation, and apoptosis through regulation of the cell cycle. A2780 human ovarian carcinoma cells, which are sensitive to cisplatin and paclitaxel, express wild-type p53 and exhibit a p53-mediated increase in p21 in response to the chemotherapeutic agents. Here, we demonstrate that phosphatidylinositol 3-kinase (PI3K) and its downstream targets serine/threonine kinases AKT1 and AKT2 (AKT), are required for the full induction of p21 in A2780 cells treated with cisplatin or paclitaxel. Inactivation of the PI3K/AKT signal transduction pathway either by its specific inhibitor LY294002 or by expression of dominant negative AKT inhibited p21 expression but had no inhibitory effect on the expression of the proapoptotic protein BAX by cisplatin and paclitaxel treatment. In addition, overexpression of wild-type or constitutively active AKT in A2780 cells sustained the regulation of p21 induction or increased the level of p21 expression, respectively. Experiments with additional ovarian carcinoma cell lines revealed that PI3K is involved in the expression of p21 induced by cisplatin or paclitaxel in OVCAR-10 cells, which have wild-type p53, but not in OVCAR-5 cells, which lack functional p53. These data indicate that the PI3K/AKT signal transduction pathway mediates p21 expression and suggest that this pathway contributes to cell cycle regulation promoted by p53 in response to drug-induced stress. However, inactivation of PI3K/AKT signaling did not result in significant alteration of the drug sensitivity of A2780 cells, suggesting that the cell death induced by cisplatin or paclitaxel proceeds independently of cell protective effects of PI3K and AKT.

Antineoplastic Agents↗
PubMed2000

Presynaptic dopamine D2 and muscarine M3 receptors inhibit excitatory and inhibitory transmission to rat subthalamic neurones in vitro.

Whole-cell patch-clamp recordings were made from subthalamic nucleus (STN) neurones in brain slices from rats. Stimulation with bipolar electrodes evoked synaptic currents mediated by glutamate (EPSCs) and GABAA (IPSCs) receptors. Dopamine reversibly reduced the amplitude of GABAA IPSCs by up to 48 % with an IC50 value of 3.4 +/- 0.8 microM. The dopamine D2 receptor agonist quinpirole, but not the D1 receptor agonist SKF 82958, also inhibited GABAA IPSCs. This effect was completely reversed by the D2 receptor antagonist sulpiride but not by SCH 23390, a D1 antagonist. Muscarine reversibly reduced the amplitude of GABAA IPSCs by up to 70 % with an IC50 value of 0.6 +/- 0.1 microM. Inhibition of IPSCs by muscarine was completely blocked by scopolamine (10 microM), a muscarinic receptor antagonist. The M3 muscarinic receptor antagonist 4-DAMP effectively reversed muscarine-induced inhibition of IPSCs with an IC50 of 0.11 +/- 0.03 microM. Although the M1 receptor antagonist pirenzepine also reversed the inhibition of IPSCs by muscarine, this effect was only observed at relatively high concentrations (IC50 = 21.7 +/- 9.4 microM). Dopamine and muscarine both increased the paired-pulse ratio of GABAA IPSCs. Neither agent produced sustained changes in postsynaptic holding current. Glutamate EPSCs were also inhibited reversibly by dopamine (by up to 29%; IC50 = 16 +/- 3 microM) and muscarine (by up to 41%; IC50 = 1.0 +/- 0.4 microM). However, both agents were more potent and efficacious for reducing GABA IPSCs compared with glutamate EPSCs. These results suggest that the most significant effect of dopamine and muscarine in the STN is to reduce inhibitory synaptic input by acting at presynaptic dopamine D2 and muscarinic M3 receptors, respectively.

Animals↗
PubMed2000

Clinical pharmacokinetics and administration of established platinum drugs.

We review the pharmacology and clinical administration of the commonly used platinum-based anticancer drugs cisplatin and carboplatin, and the more recently approved diamminocyclohexane-based oxaliplatin. The development of analogues of cisplatin has been focused upon identifying compounds with less toxicity and with a different spectrum of activity. Carboplatin exemplifies the former, while the initial data with oxaliplatin support its activity in cisplatin-resistant tumours. The clinical pharmacokinetics of the drugs are reviewed. Incorporation of these data into the design of clinical regimens has permitted individualised therapy with carboplatin, and has enhanced safety. Additional investigation of the pharmacodynamics of all of these agents is expected to result in their selective application. The clinical effects of these analogues are discussed.

Animals↗
PubMed1999

Identification of a chromosome 3p14.3-21.1 gene, APPL, encoding an adaptor molecule that interacts with the oncoprotein-serine/threonine kinase AKT2.

AKT2 is a serine/threonine kinase implicated in human ovarian and pancreatic cancers. AKT2 is activated by a variety of growth factors and insulin via phosphatidylinositol 3-kinase (PI3K). However, its normal cellular role is not well understood. To gain insight into the function of AKT2, we performed yeast two-hybrid system to screen for interacting proteins. Using this technique, we identified a novel interactor, designated APPL, which contains a pleckstrin homology (PH) domain, a phosphotyrosine binding (PTB) domain and a leucine zipper, classes of motifs defined in signaling molecules as functional interaction domains with specific targets. The PH domain of APPL shows similarity to those found in GTPase-activating proteins such as oligophrenin-1 and Graf, whereas its PTB domain exhibits homology with CED-6, an adaptor protein that promotes engulfment of apoptotic cells, and IB1, a transactivator of the GLUT2 gene. APPL is highly expressed in skeletal muscle, heart, ovary and pancreas, tissues in which AKT2 mRNA is abundant. APPL interacts with the inactive form of AKT2; moreover, APPL binds to the PI3K catalytic subunit, p110alpha. These data suggest that APPL is an adaptor that may tether inactive AKT2 to p110alpha in the cytoplasm and thereby may expedite recruitment of AKT2 and p110alpha to the cell membrane upon mitogenic stimulation. Furthermore, the APPL gene was mapped to human chromosome 3p14.3-p21.1, where deletions and other rearrangements have often been reported in a variety of tumor types. The identification of APPL may facilitate further analysis of the physiological and oncogenic activities of AKT2.

Amino Acid Sequence↗
PubMed1999

Decreased cisplatin damage-dependent DNA synthesis in cellular extracts of mismatch repair deficient cells.

The proficiency of both nucleotide excision repair (NER) and DNA mismatch repair (MMR) influences cellular sensitivity to cisplatin (cis-diamminedichloroplatinum). To gain further insight into how MMR may influence platinum drug sensitivity, the effect of loss of MMR on repair synthesis was measured in vitro by a commonly used method that relies on whole-cell extracts to drive [alpha-32P]dATP incorporation into cisplatin-damaged plasmid DNA. Extracts evaluated include those from cells with or without functional hMLH1 (HCT116+ch2 versus HCT116+ch3, respectively) and hMSH2 (HEC59 versus HEC59+ch2, respectively). Loss of MMR in the HCT116 system was associated with a 2.8-fold reduction in cisplatin damage-specific DNA synthesis, whereas it was associated with a 3.0-fold reduction in the HEC59 system, suggesting that a decrease in the ability to repair cisplatin-damaged DNA accompanies loss of MMR. An in vitro DNA excision assay that utilized a substrate containing a site-specific cisplatin adduct was performed. Using this highly NER-specific assay, no significant difference was apparent between the extracts derived from NER-proficient versus -deficient cells. These and other data lead us to suggest that the increase in apparent repair synthesis in platinum-damaged plasmids by extracts from MMR-proficient versus -deficient cellular extracts may reflect a distinct and possibly adverse DNA synthetic process rather than productive NER.

Base Pair Mismatch↗
PubMed1999

Presynaptic inhibition preferentially reduces in NMDA receptor-mediated component of transmission in rat midbrain dopamine neurons.

We used patch pipettes to record whole-cell currents from single dopamine neurons in slices of rat midbrain. Pharmacological methods were used to isolate EPSCs evoked by focal electrical stimulation. Baclofen was significantly more potent for inhibiting NMDA receptor-mediated EPSCs (IC50=0.24 microM) compared with inhibition of EPSCs mediated by AMPA receptors (IC50=1.72 microM). The increased potency of baclofen for inhibiting the NMDA component persisted in superfusate that contained zero Mg2+ and when postsynaptic K+ conductances were reduced by Cs+ and QX-314. Effects of baclofen on EPSCs were blocked by the GABA(B) receptor antagonist CGP-35348. Adenosine was 20 fold more potent for reducing the NMDA component of transmission (IC50=31 microM) compared with inhibition of AMPA receptor-mediated EPSCs (IC50=654 microM). Effects of adenosine on EPSCs were blocked by the A1 receptor antagonist DPCPX. Both baclofen and adenosine significantly increased the ratio of EPSCs in paired-pulse studies, suggesting presynaptic sites of action. Although adenosine (1 mM) did not reduce currents evoked by exogenous NMDA (10 microM), baclofen (1 microM) reduced NMDA currents by 29%. Neither baclofen nor adenosine altered currents evoked by exogenous AMPA (1 microM). We conclude that adenosine acts at presynaptic A1 receptors to cause a preferential reduction in the NMDA component of synaptic transmission. In contrast, baclofen preferentially reduces NMDA EPSCs by acting at both pre- and postsynaptic GABA(B) receptors. By regulating NMDA receptor function, A1 and GABA(B) receptors may play important roles in regulating the excitability of dopamine neurons.

Adenosine↗