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P Calabresi

Publications and source records attributed to P Calabresi.

At least 181 records · Page 10Linked to original sources
PubMed1991

Involvement of GABA systems in feedback regulation of glutamate-and GABA-mediated synaptic potentials in rat neostriatum.

1. Neostriatal neurones were recorded intracellularly from a rat corticostriatal slice preparation. Depolarizing postsynaptic potentials (DPSPs) were evoked by either cortical or intrastriatal stimulation. 2. Kynurenic acid (600 microM), an antagonist of excitatory amino acids, reduced the cortically-evoked DPSPs by 88% while the intrastriatally evoked potentials were reduced by 48%. Bicuculline (100 microM) produced only a slight inhibition of the cortically evoked DPSPs (12%), but clearly depressed intrastriatal potentials (52%). 3. The effects of (-)-baclofen, a gamma-aminobutyric acid (GABA)B receptor agonist, were studied on the cortically evoked DPSPs. In all the tested neurones (-)-baclofen, added to the superfusion medium, caused a concentration-dependent decrease of these potentials (half-maximal effect (EC50) = 800 nM). This effect was not affected by bicuculline. (-)-Baclofen did not change the membrane potential, the input resistance, current-evoked firing frequency, or postsynaptic responses to exogenously applied glutamate. 4. The effects of (-)-baclofen on the DPSPs were compared to those produced by application of GABA and muscimol. GABA and muscimol decreased the DPSPs and caused a membrane depolarization coupled with a decrease of the membrane resistance. Bicuculline (100 microM) blocked the GABA-induced changes of the membrane potential and of the resistance, but not the decrease of the synaptic potentials. All the effects produced by muscimol were blocked by bicuculline. 5. Following intrastriatal stimulation a residual kynurenate-insensitive potential persisted; this potential was blocked by bicuculline (100 microM). (-)-Baclofen produced a dose-dependent decrease of this potential (EC50 = 800 nM). The postsynaptic responses to exogenously applied GABA were unchanged by (-)-baclofen. 6. The amplitude of kynurenate and bicuculline-sensitive DPSPs were stable at a frequency of 0.1 Hz. At frequencies between 0.3 and 3 Hz both these potentials were attenuated with the second stimulus and after about five stimuli a steady state was reached. Membrane potential and input resistance were not affected by these frequencies of stimulation. 7. Application of the GABA uptake inhibitor nipecotic acid (100-300 microM) clearly reduced the amplitude of both kynurenate-and bicuculline-sensitive DPSPs evoked at low frequencies of stimulation (0.01-0.3 Hz), but had lower effects at higher stimulation rates (1-3 Hz). Application of nipecotic acid increased the duration of membrane responses to exogenously applied GABA.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1990

Benzylacyclouridine reverses azidothymidine-induced marrow suppression without impairment of anti-human immunodeficiency virus activity.

Increased extracellular concentrations of uridine (Urd) have been reported to reduce, in vitro, azidothymidine (AZT)-induced inhibition of human granulocyte-macrophage progenitor cells without impairment of its antihuman immunodeficiency virus (HIV) activity. Because of the clinical toxicities associated with chronic Urd administration, the ability of benzylacyclouridine (BAU) to effect, in vivo, AZT-induced anemia and leukopenia was assessed. This agent inhibits Urd catabolism and, in vivo, increases the plasma concentration of Urd in a dose-dependent manner, without Urd-related toxicity. In mice rendered anemic and leukopenic by the administration of AZT for 28 days in drinking water (1.5 mg/mL), the continued administration of AZT plus daily BAU (300 mg/kg, orally) partially reversed AZT-induced anemia and leukopenia (P less than .05), increased peripheral reticulocytes (to 4.9%, P less than .01), increased cellularity in the marrow, and improved megaloblastosis. When coadministered with AZT from the onset of drug administration, BAU reduced AZT-induced marrow toxicity. In vitro, at a concentration of 100 mumol/L, BAU possesses minimal anti-HIV activity and has no effect on the ability of AZT to reverse the HIV-induced cytopathic effect in MT4 cells. The clinical and biochemical implications of these findings are discussed.

Anemia↗
PubMed1990

Different effect of benzylacyclouridine on the toxic and therapeutic effects of azidothymidine in mice.

It has been reported that in vitro uridine (Urd) can reverse azidothymidine (AZT) cytotoxicity without decreasing anti-human immunodeficiency virus (HIV) activity. Our studies in mice have shown that daily oral doses of benzylacyclouridine (BAU), an inhibitor of Urd breakdown, also reduces AZT hematologic toxicity, presumably by elevating the plasma concentration of Urd. We now extend these murine studies and report the effect of various doses of exogenous Urd, various doses of BAU, or the combination of BAU and Urd, administered daily, on AZT-induced toxicity. In mice receiving concomitant AZT, daily doses of Urd of 1,000 to 2,000 mg/kg increase peripheral reticulocytes and slightly reduce AZT-induced hematologic toxicity. However, the range of effective doses is narrow, and higher doses of Urd (greater than 3,000 mg/kg/d) significantly enhance hematologic toxicity. At its most effective dose, (2,000 mg/kg/d), Urd produces 28% mortality. In contrast, BAU doses up to 300 mg/kg/d reduced AZT-related hematologic toxicity in a dose-dependent manner without mortality. Higher daily doses of BAU and the combination of BAU with low doses of Urd were not more effective. Studies conducted in mice infected with the Rauscher murine leukemia virus (RLV) indicate that BAU does not impair the antiretroviral effect of AZT when administered at doses that reduce AZT-induced anemia and leukopenia. These findings may be significant for the treatment of patients with acquired immunodeficiency syndrome (AIDS) and AIDS-related complex.

Acquired Immunodeficiency Syndrome↗
PubMed1990

Oral zidovudine, continuous-infusion fluorouracil, and oral leucovorin calcium: a phase I study.

A phase I clinical, pharmacologic, and biochemical evaluation of escalating oral zidovudine (AZT) given over 2 days with a fixed dose of continuous-infusion fluorouracil (800 mg/m2 per day X 3 days) and oral leucovorin calcium was performed. Eighteen patients were treated with doses of AZT ranging from 1.0 to 9.0 g/m2 per day. Nausea and vomiting were dose limiting, with a maximally tolerated dose of 7.5 g/m2 per day. Rash and mucositis occurred but were not dose limiting. A dose-related increase in peak plasma levels of AZT was observed, and the alpha half-life of AZT in plasma (75 min) was unaffected by these high doses. At doses above 4.0 g/m2 per day, trough levels significantly increased, perhaps reflecting prolonged absorption from the gut. No responses were observed; however, a significant increase in DNA single-strand breaks was observed in peripheral blood cells after a threshold dose of 4.0 g/m2 per day, confirming a biological effect of AZT in this regimen. Further trials with an intravenous formulation capable of maintaining plasma levels and circumventing dose-limiting toxicity are warranted.

Administration, Oral↗
PubMed1990

Endogenous GABA mediates presynaptic inhibition of spontaneous and evoked excitatory synaptic potentials in the rat neostriatum.

The effect of the blockade of the gamma-aminobutyric acid (GABA) uptake system on the amplitude of glutamatergic synaptic potentials was studied by using a corticostriatal slice preparation. Nipecotic acid (0.1-1 mM), a GABA uptake blocker, produced a dose-dependent decrease of the amplitude of kynurenate-sensitive excitatory synaptic potentials recorded in the neostriatum following cortical stimulation. Nipecotic acid did not affect the postsynaptic responses to exogenously applied glutamate. The presynaptic effect of endogenous GABA was bicuculline-resistant and was mimicked by baclofen (0.3-3 microM). This effect was not blocked by phaclofen (0.5-1 mM). These findings show that phaclofen-insensitive GABAB receptors, activated by endogenous GABA, mediate presynaptic inhibition of cortical glutamatergic inputs in the neostriatum.

Animals↗
PubMed1990

5-Fluorouracil enhances azidothymidine cytotoxicity: in vitro, in vivo, and biochemical studies.

Preliminary in vitro studies suggest that the combination of 5-fluorouracil (FUra) and 3'-azido-3'-deoxythymidine (AZT) is more cytotoxic than either agent alone. Therefore, a biochemical and therapeutic evaluation of this combination was initiated. Quantitation of the cytotoxicity of FUra plus AZT against the growth of HCT-8 cells in vitro revealed that 1 microM FUra (approximately 10% inhibitory concentration) increased the cytotoxicity of AZT and decreased its 50% inhibitory concentration by 60%. Similarly, incubating HCT-8 cells in 5 microM AZT (approximately 10% inhibitory concentration) decreased the 50% inhibitory concentration of FUra by over 50%. Biochemical analysis indicated that AZT did not affect FUra-induced inhibition of thymidylate synthase or [3H]-FUra incorporation into nucleic acids. In contrast, incubation in 5 microM FUra increased the incorporation of [3H]-AZT (5 microM) into the nucleic acid fraction of these cells by 52% (P less than 0.05). Therapeutic evaluation of this combination in athymic (nude) mice bearing HCT-8 xenographs revealed that, while weekly FUra (85 mg/kg) or AZT (600 mg/kg) exerts minimal antineoplastic activity (after 4 courses, treatment/control = 0.81 and 0.70, respectively), their combination, at the same doses, inhibited tumor growth by nearly 70% (P less than 0.01 versus FUra alone). FUra-related host toxicity was not increased by the addition of AZT. Higher doses of FUra alone were not more effective than FUra plus AZT. In vivo, AZT did not affect the incorporation of [3H]-FUra into the nucleic acid fraction of various murine tissues, including HCT-8 xenografts. FUra, however, increased [3H]-AZT incorporation into nucleic acids in a tissue-specific manner. In the presence of FUra, the incorporation of [3H]-AZT in spleen, liver, and gut increased 40, 21, and 4%, respectively, while in HCT-8 xenografts [3H]-AZT incorporation increased more than 2-fold. Analysis of the activities of selected enzymes involved in pyrimidine metabolism suggests that this tissue-specific effect may be related to the pyrimidine salvage capacity of these tissues. These findings are described in light of their potential impact on human colon cancer chemotherapy.

Animals↗
PubMed1990

Effects of 1 alpha, 25-dihydroxyvitamin D3 on the human chronic myelogenous leukemia cell line RWLeu-4.

The effects of 1 alpha, 25-dihydroxyvitamin D3 (VD3) on proliferation, differentiation, and macromolecular synthesis in the new Philadelphia chromosome-positive chronic myelogenous leukemia cell line, RWLeu-4, were investigated. Binding of [3H]VD3 was saturable, with approximately 2000-3000 sites/cell, and half-maximal binding occurring at 0.21-0.33 nM. Treatment of RWLeu-4 cells with VD3 induced 24R-hydroxylase activity, a marker of vitamin D3 responsiveness in many tissues. Exposure of RWLeu-4 cells to VD3 also inhibited proliferation and DNA synthesis with a 50% effective dose of 3.5-10 nM within 72 h; in addition, protein and RNA synthesis were inhibited by VD3 treatment. Exposure of RWLeu-4 cells to 5 nM VD3 for 72 h caused 50% of the cells to differentiate into macrophage/monocyte type cells as judged by nitroblue tetrazolium staining and adherence to plastic. Progressive expression of cell surface maturation-specific antigens of the monocyte/macrophage lineage was induced by treatment of RWLeu-4 cells with VD3 for 24 to 72 h at doses that inhibited cellular proliferation. c-myc RNA, which is constitutively expressed in RWLeu-4 cells, increased after 0.5 h of treatment with 50 nM VD3 and then rapidly decreased to barely detectable levels after 4 h of treatment. Finally, the in vitro tyrosine kinase activity associated with the p210bcr-abl oncogene product was decreased approximately 50% by VD3 treatment. Because of the presence of a functional receptor-effector system for VD3 and multiple biological responses to the hormone, these cells provide a unique model system with which to probe the specific effects of VD3 on cell growth and differentiation in chronic myelogenous leukemia.

Calcitriol↗
PubMed1990

Effects of glycine on neurons in the rat substantia nigra zona compacta: in vitro electrophysiological study.

The effects of bath-applied glycine to substantia nigra zona compacta neurons of rat were investigated by intracellular recording techniques in vitro. Superfusion of glycine (1 mM) in the medium hyperpolarized 53% of the neurons recorded with KCl electrodes, whereas 32% of the cells were depolarized. The remaining 15% of neurons was hyperpolarized and then depolarized by the amino acid. In spite of these membrane changes, the action potential firing was depressed. Both hyperpolarization and depolarization were correlated to an outward and an inward current, respectively, when recording in single-electrode voltage-clamp mode. In response to bath application of glycine, the neurons showed a concentration-dependent conductance increase. Micromolar concentrations of glycine (100-300 microM) in the superfusion medium produced a membrane hyperpolarization (outward current) in most of the tested cells, whereas millimolar concentration of amino acid could cause depolarization (inward current) in the same neurons. When the recording electrodes contained K acetate, only hyperpolarizations (outward current) were produced. The potential and current changes and the increase in membrane conductance produced by glycine showed little desensitization when neurons were recorded with K acetate electrodes. The mean reversal potential for the membrane hyperpolarization was -80 mV with intracellular electrodes containing KCl and -84 mV with electrodes containing K acetate. The mean null potential for the depolarizing effect was -46 mV. The reversal potential for the glycinergic responses was shifted to less negative values upon lowering the extracellular concentration of chloride or increasing the extracellular concentration of potassium. Strychnine (1-10 microM) reversibly antagonized both the conductance increase and the membrane changes produced by glycine. Bath application of bicuculline (100 microM) and picrotoxin (200 microM) did not affect glycine responses, while depressing the actions of GABA and muscimol. It is concluded the glycine exerts an inhibition on substantia nigra zona compacta neurons by acting on strychnine-sensitive receptors. The membrane effects produced by glycine result from the activation of a chloride current. In addition, the simultaneous involvement of potassium ions may contribute to the overall effects of glycine.

Action Potentials↗
PubMed1990

Heterogeneity of potential for hematogenous metastasis in a human pancreatic carcinoma.

Human pancreatic carcinoma, a disease with grave prognosis, frequently metastasizes to the liver, with detrimental consequences for the host. Good models of experimental metastasis for this disease are lacking. We describe a model of hepatic metastasis from the fast-growing variant (FG) of the human pancreatic carcinoma COLO 357. We also show that the slow-growing variant (SG) of COLO 357 lacks the potential for forming hepatic and pulmonary metastases following injection into the spleen of the nude mouse. This expression of heterogeneity of potential for hematogenous metastases can be exploited by pursuing studies aiming at identifying differences between the cells with and without metastatic potential.

Animals↗
PubMed1990

A phase I-II trial of continuous-infusion cisplatin, continuous-infusion 5-fluorouracil, and VP-16 in colorectal carcinoma.

Twenty-nine evaluable patients with colorectal adenocarcinoma were treated in a phase I-II trial of combination chemotherapy with a 72-h continuous infusion of cisplatin (CDDP) and 5-fluorouracil (5-FU) with an infusion of VP-16 given at 24 and 48 h after the start of therapy. There were five (17 +/- 14%) partial responses lasting 2-6 months (median, 3). Three of these responses occurred among the 10 previously untreated patients. The toxicity of this regimen was pronounced. Four of eight patients with severe neutropenia required hospitalization for infections, two of which were life-threatening; one of six patients with severe thrombocytopenia had a life-threatening hemorrhagic complication; and four patients experienced severe, dose-limiting fatigue. These complications occurred principally with CDDP and VP-16 at doses above 27.5 mg/m2/day and 110 mg/m2/dose, respectively. Mucositis occurred in six patients and limited the dose of 5-FU to 1,300 mg/m2/day. Although the response rate appeared to be high in previously untreated patients, the minimal palliative benefit of treatment and the brief duration of the responses do not compensate for the toxicity observed.

Adenocarcinoma↗
PubMed1990

Actions of TNF and IFN-gamma on angiogenesis in vitro.

We have developed a model system for studying angiogenesis in which microvascular fragments and myofibroblasts (Mf) isolated from lipid tissues are grown in co-culture. We have found that Mf induce capillary formation by producing an endothelial cell growth factor and by secreting an extracellular matrix that causes endothelial cells to form a cordlike structure. This system appeared to be well suited for examining the effects of vasoactive substances such as the TNF and INF-gamma on capillary growth. TNF-alpha,beta, and IFN-gamma not only significantly inhibited capillary growth induced by Mf, but also blocked capillary development induced by fibroblast growth factors (FGF), well-known potent angiogenic factors. Recently, we have found that platelet-derived growth factor (PDGF) enhances in vitro capillary formation, probably at least in part by acting on Mf. Just as with FGF, capillary growth in the presence of PDGF was almost completely blocked by IFN-gamma. We examined the mode by which IFN-gamma inhibits angiogenesis and found that IFN-gamma inhibits both the proliferation of endothelial cells and collagen(s) synthesis by Mf. These actions of TNF or IFN-gamma could limit vascular formation in solid tumors.

Animals↗
PubMed1990

Responses of rat substantia nigra compacta neurones to L-DOPA.

1. The electrophysiological actions of L-DOPA were studied on substantia nigra dopamine-containing neurones by use of intracellular recordings, current and voltage clamp, in vitro. 2. L-DOPA (3-300 microM), applied by superfusion, decreased the spontaneous firing activity of the neurones, hyperpolarized the membrane potential and produced an outward current. 3. These effects of L-DOPA were graded and were antagonized by sulpiride, a D2-receptor antagonist. 4. The L-DOPA activated membrane hyperpolarization persisted in slices exposed to tetrodotoxin while it was markedly reduced or abolished in solutions with no calcium plus cobalt (2 mM). 5. In slices treated with carbidopa, an inhibitor of the aromatic amino acid decarboxylase enzyme, the actions of L-DOPA were greatly reduced. 6. We propose that L-DOPA inhibits the firing of substantia nigra zona compacta neurones by being converted to dopamine which is then released by these cells.

Animals↗
PubMed1990

Synaptic and intrinsic control of membrane excitability of neostriatal neurons. I. An in vivo analysis.

1. The relationship between membrane properties of neostriatal neurons and spontaneous and evoked synaptic potentials was studied with the use of intracellular recordings from anesthetized rats. Most of these neurons showed regular or irregular spontaneous depolarizing potentials that only in a few cases triggered action potentials at resting level. 2. The stimulation of the ipsilateral substantia nigra or of the sensorimotor cortex produced a relatively fast depolarizing post-synaptic potential (EPSP). In some cells this potential was followed by an inhibitory period that appeared as an hyperpolarization when the cell was depolarized from the resting level (inhibitory postsynaptic potential, IPSP). A late and long-lasting depolarization (LD) followed the EPSP or the EPSP-IPSP sequence. 3. Repetitive discharge with little adaptation was observed during direct depolarization. Most of the neurons tested for current-voltage (I-V) relationship showed nonlinearity of the input resistance in the hyperpolarizing direction. Spontaneous and evoked EPSPs were decreased in their amplitude and duration when the membrane potential was held at levels more hyperpolarized than -85 mV because of the strong rectification at these levels of hyperpolarization. 4. Local microiontophoretic application of bicuculline (BIC) or systemic administration of BIC and pentylenetetrazole (PTZ) produced a reduction of the IPSPs. The reduction of the inhibitory transmission caused a strong increase of the LD. The current-evoked firing pattern was not greatly altered. 5. The intracellular application of cesium increased the amplitude and the duration of the spontaneous depolarizations that triggered bursts of action potentials under this condition. Spikes were broadened and the rectification in the hyperpolarization direction was reduced. 6. Iontophoretically applied cadmium strongly depressed the amplitude of the spontaneous and evoked postsynaptic potentials. During cadmium application, nigral stimulation produced constant latency, all-or-none spikes in the absence of any synaptic potential. 7. Repetitive stimulation of the ipsilateral substantia nigra by electrical shocks (5 Hz, 25 s) produced a progressive and reversible decrease of the spontaneous depolarizing potentials (SDPs) and a decrease of the firing rate. In the same cells, when the train of stimulation was delivered in the ipsilateral cortex, a membrane depolarization coupled with an increase of the firing rate was observed. 8. We conclude that although synaptic circuits mediate a phasic inhibition in neostriatum, the low level of spontaneous firing of most neostriatal neurons is mainly because of the effects that membrane properties exert on the spontaneous and the evoked synaptic depolarizations in the striatum.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗
PubMed1990

Synaptic and intrinsic control of membrane excitability of neostriatal neurons. II. An in vitro analysis.

1. The effects of intrinsic membrane properties on the spontaneous and synaptically evoked activity of neostriatal neurons were studied in an in vitro slice preparation with the use of intracellular recordings. The recorded neurons did not show spontaneous action potentials at rest; depolarizing current pulses triggered a tonic firing pattern. 2. Subthreshold spontaneous depolarizing potentials (SDPs) were observed in 52% of the recorded neurons. The amplitude of these potentials at rest ranged between 2 and 15 mV, and their duration between 4 and 100 ms. The frequency and the amplitude of the SDPs were functions of the membrane potential: membrane depolarization by constant positive current increased the frequency of the SDPs and reduced their amplitude; hyperpolarization of the membrane decreased their frequency and increased their amplitude. Often, at membrane potentials more negative than -90 mV, SDPs were completely suppressed. 3. SDPs were blocked by low calcium-cobalt containing solutions. In the presence of tetrodotoxin (TTX, 1-3 microM), SDPs were completely abolished in 50% of the tested neurons; in the remaining neurons, small (1-4 mV) TTX-resistant SDPs were observed. In most of the neurons, bicuculline (BIC, 10-100 microM) and low concentrations of tetanus toxin (5-10 micrograms/ml) did not clearly affect the SDPs. Higher concentrations of tetanus toxin (100 micrograms/ml) blocked the SDPs as well as the synaptic potentials evoked by intrastriatal stimulation. 4. At resting membrane potential, intrastriatal stimulation produced a fast depolarizing postsynaptic potential (EPSP) that was reduced by BIC (10-100 microM). The relationship between EPSP amplitude and membrane potential was studied either by utilizing K(+)-chloride electrodes or by the use of cesium-chloride electrodes. In both these cases, the reversal potential for the EPSPs was between 0 and -14 mV. In cesium-loaded neurons, the decrease of the EPSP, usually observed at negative membrane potentials (below -85 mV), was clearly reduced. Internal cesium prolonged the duration of the SDPs and the EPSPs evoked by intrastriatal stimulation. 5. The relationship between spontaneous and evoked synaptic activity and membrane potential was studied in the presence of different external potassium blockers. 4-Aminopyridine (4AP, 0.1-1 mM) increased the EPSP amplitude and the frequency of the SDPs, but did not decrease membrane rectification and the shunt of the EPSPs present at negative membrane potentials. On the contrary, rectification of the membrane and the shunt of the EPSPs below -85 mV were clearly reduced by tetraethylammonium (TEA, 10-20 mM).(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗
PubMed1990

Kainic acid on neostriatal neurons intracellularly recorded in vitro: electrophysiological evidence for differential neuronal sensitivity.

The electrophysiological effects produced by different concentrations of kainic acid (KA) were studied by utilizing intracellular recordings from neostriatal slices. In most of the recorded cells (81%), concentrations of KA ranging between 10 and 300 nM produced reversible and dose-dependent membrane depolarizations. Higher concentrations of this agonist caused larger depolarizations and changes of the membrane properties of the recorded neurons not reversible during the time of recording. In a smaller percentage (19%) of the recorded cells, 10-100 nM KA did not produce significant membrane depolarizations; in these neurons, the depolarizations produced by higher concentrations of KA were small and reversible. The 2 populations of neurons showed similar electrophysiological properties and did not reveal differential sensitivity to quisqualic acid (QUIS; 10-30 microM) or to NMDA (10-30 microM). Tetrodotoxin (TTX; 1 microM) did not reduce the depolarizations produced by KA and by NMDA. Low-calcium, cobalt-containing solutions abolished the effects produced by NMDA, but not the KA-induced depolarizations. Kynurenic acid (500 microM) significantly antagonized the depolarizations produced by KA and reduced the changes of the membrane properties caused by high doses of this agonist. In several neurons, KA induced bicuculline-sensitive synaptic depolarizing potentials. Our findings suggest the presence of 2 subpopulations of neostriatal neurons showing differential postsynaptic sensitivity to KA. The differential sensitivity of neostriatal neurons to KA might influence the responses of these cells to glutamatergic cortical inputs and the degenerative changes observed in neostriatal neurons in some pathological conditions.

Action Potentials↗
PubMed1990

Muscarine depolarizes rat substantia nigra zona compacta and ventral tegmental neurons in vitro through M1-like receptors.

Intracellular recordings were made from presumed dopamine-containing neurons in slices of rat mesencephalon. Muscarine (3-100 microM) increased the rate of spontaneous action potentials; it also caused a membrane depolarization and, in voltage-clamp, an inward current. Concentration-effect curves to muscarine were shifted rightwards by pirenzepine (0.03-1 microM) with an estimated KD of 14 nM. The inward current caused by muscarine was voltage-dependent. Between about -50 and (-)-65 mV it was associated with a decrease in membrane conductance, but between -70 and -110 mV it was unaccompanied by any change in membrane conductance. Muscarine was without effect on the action potential afterhyperpolarization, or on a slowly developing inward current evoked by step hyperpolarizations of up to 20 mV from -45 mV. Muscarinic depolarizations or inward currents were reduced reversibly or abolished by a low calcium (0.25 mM)/high magnesium (10 mM) solution. It is concluded that muscarinic excitation of dopaminergic neurons is mediated by M1-like receptors.

Animals↗