Characterization of thyrotropin-induced increase in iodothyronine monodeiodinating activity in mice.

To further characterize the effect of TSH administration on thyroid iodothyronine monodeiodinating activity, we have evaluated the in vitro conversion of T4 to T3 (outer ring deiodination) and T3 to 3,3'-diiodothyronine (T2; inner ring deiodination) by mouse thyroid, liver, and kidney homogenates, comparing tissues from TSH-treated mice (0.1-200 mU bovine TSH, ip, for 1-3 days) with tissues from saline-treated controls. The in vitro conversion activity was studied in the presence of 1-20 mM dithiothreitol; most of the studies were carried out at 4 mM. Studies were carried out at optimal pH 6.5 for outer ring and 7.8 for inner ring deiodination. The iodothyronine monodeiodinase in mouse thyroid is similar to the ones in liver and kidney. It is heat labile (inactivated at 56 C for 5 min), inhibited by propylthiouracil (0.2 mM) and ipodate (0.2 mM), and unaffected by methimazole (up to 20 mM), ascorbate (up to 0.1 M) or KI (up to 20 mM). The mean +/- SE baseline rates of T4 to T3 and T3 to T2 conversion were 100 +/- 6.3 and 56.5 +/- 2.9 pmol/mg thyroid protein X 30 min at 37 C, respectively. A significant increase in each conversion activity was found after TSH treatment (0.2 U, ip, daily for 3 days); T4 to T3 conversion rose to 282 +/- 15.4, and T3 to T2 increased to 153 +/- 7.4 pmol/mg thyroid protein (P less than 0.001). A 12.8% increase in thyroid weight was found in the TSH-treated group (P less than 0.03 compared with saline control group). Similar but less marked increased in monodeiodinating activities were seen in the liver. A minimal but significant increase in inner ring monodeiodination with no significant increase in T4 to T3 converting activity was found in kidney, which, in the mouse, has markedly less outer ring deiodinase than liver or thyroid. The iodothyronine monodeiodinating activities did not increase until 12 h in thyroid and 48 h in liver after the first dose of TSH. Significant increases in T4 to T3 and T3 to T2 conversion were seen with doses of TSH as low as 0.1 mU (ip, daily for 3 days), and there was a linear dose-response thereafter. The decay of the increased iodothyronine monodeiodinating activities after a single dose of TSH (0.2 U) appeared to be linear, with a decay t 1/2 of 1.3 days for T4 to T3 conversion and about 1.0 day for T3 to T2 conversion.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of SCH 58261, an adenosine A(2A) receptor antagonist, on quinpirole-induced turning in 6-hydroxydopamine-lesioned rats. Lack of tolerance after chronic caffeine intake.

In unilaterally 6-hydroxydopamine (6-OHDA)-lesioned rats, a rodent model of Parkinson's disease (PD), the adenosine A(2A) receptor antagonist SCH 58261 significantly increased (+180%, p <.01) the number of rotations induced by a low dose of quinpirole (a dopamine D(2) receptor agonist), while it did not significantly modify the effects of a comparably low dose of SKF 38393 (a dopamine D(1) receptor agonist). The dose-dependent potentiating effects of SCH 58261 on quinpirole-induced turning were similar in caffeine-treated rats (1 g/l in drinking water over 14 days) and control animals (tap water). The selective potentiating effects of SCH 58261 on D(2)-dependent turning confirm the existence of a potent and specific A(2A)/D(2) receptor-receptor interaction. The persistence of the potentiating effects of SCH 58261 after chronic caffeine intake suggests that no tolerance should develop towards the antiparkinsonian effects of adenosine A(2A) receptor antagonists with chronic treatment.

The selective mGlu(5) receptor agonist CHPG inhibits quinpirole-induced turning in 6-hydroxydopamine-lesioned rats and modulates the binding characteristics of dopamine D(2) receptors in the rat striatum: interactions with adenosine A(2a) receptors.

In 6-hydroxydopamine-lesioned rats, the selective mGlu(5) receptor agonist (RS)-2-Cholro-5-Hydroxyphenylglycine (CHPG, 1-6 microg/10 microl intracerebroventricularly) significantly inhibited contralateral turning induced by quinpirole and, to a lesser extent, that induced by SKF 38393. The inhibitory effects of CHPG on quinpirole-induced turning were significantly potentiated by an adenosine A(2A) receptor agonist (CGS 21680, 0.2 mg/kg IP) and attenuated by an A(2A) receptor antagonist (SCH 58261, 1 mg/kg IP). In rat striatal membranes, CHPG (100-1,000 nM) significantly reduced the affinity of the high-affinity state of D(2) receptors for the agonist, an effect potentiated by CGS 21680 (30 nM). These results show the occurrence of functional interactions among mGlu(5), adenosine A(2A), and dopamine D(2) receptors in the regulation of striatal functioning, and suggest that mGlu(5) receptors may be regarded as alternative/integrative targets for the development of therapeutic strategies in the treatment of Parkinson's disease.

Adenosine A2A and group I metabotropic glutamate receptors synergistically modulate the binding characteristics of dopamine D2 receptors in the rat striatum.

There is experimental evidence for the existence of interactions between metabotropic glutamate (mGlu), adenosine and dopamine receptors in the striatum. In membrane preparations from rat striatum the group I and II mGlu receptor agonist 1-aminocyclopentane-1S-3R-dicarboxylic acid (1S-3R-ACPD) was found to modulate the binding characteristics of D2 receptors in a similar manner as the A2A receptor agonist 2-[p-(2-carboxyethyl)phenthylamino]-5'-N-ethylcarboxamidoadenosine (CGS 21680), with a significant decrease in the affinity of the high-affinity state of D2 receptors for dopamine. The effect of 1S-3R-ACPD was mimicked by (+/-)-trans-ACPD (t-ACPD; a racemic mixture of 1S-3R-ACPD and its inactive isomer 1R-3S-ACPD) and by the selective group I mGlu receptor agonist 3,5-dihydroxyphenylglycine (DHPG) and it was counteracted by the selective group I mGlu receptor antagonist 1-aminoindan-1,5-dicarboxilic acid (AIDA), but not by the the group II and III mGlu receptor antagonist (RS)-alpha-methyl-4-tetrazolylphenylglycine (MTPG) or the adenosine receptor antagonist 8-phenyltheophylline. Furthermore, a strong synergistic effect was observed when the striatal membranes were exposed to both CGS 21680 and 1S-3R-ACPD. In agreement with the biochemical results, in unilaterally 6-OH-dopamine lesioned rats 1S-3R-ACPD counteracted the turning behaviour induced by the D2 receptor agonist quinpirole, but not by the D1 receptor agonist SKF 38393, and it synergistically potentiated the antagonistic effect of CGS 21680 on quinpirole-induced turning behaviour.

The stimulation of cholecystokinin receptors in the rostral nucleus accumbens significantly antagonizes the EEG and behavioural effects induced by phencyclidine in rats.

The influence of cholecystokinin (CCK), bilaterally injected into the rostral nucleus accumbens, on the EEG and behavioural effects induced by phencyclidine (PCP) has been studied in rats. CCK (10 ng) significantly inhibited PCP-induced EEG effects (increase of spectral power with respect to pre-drug tracing; increase of relative power distribution in the slowest frequency bands), and behavioural effects (circling and ataxia). The inhibitory effects of CCK were completely antagonized by 1 ng PD 135-158, a selective CCKB receptor antagonist, but not by lorglumide (1 microgram), a selective CCKA receptor antagonist. Since the effects induced by PCP in rodents have been proposed to be an experimental correlate of the psychotic symptoms it induces in humans, these results indicate that CCK may act as a neuroleptic. They also suggest that CCKB receptors located in the rostral nucleus accumbens may be involved in the neuroleptic-like activity of CCK.

The mGlu5 receptor agonist CHPG stimulates striatal glutamate release: possible involvement of A2A receptors.

The intrastriatal perfusion of the selective metabotropic glutamate (mGlu)5 receptor agonist (RS)-2-chloro-5-hydroxy-phenylglycine (CHPG, 1000 microM) significantly increased (approximately + 100%, p < 0.05) glutamate extracellular levels with respect to basal values. The potentiating effect of CHPG was prevented by the selective mGlu5 receptor antagonist 2-methyl-6(phenyl-ethynyl)-pyridine (MPEP, 250 microM)) and by the adenosine A2A receptor antagonist SCH 58261 (2 mg/kg, i.p.). The results show that mGlu5 receptors are involved in the regulation of striatal glutamate release and suggest an involvement of adenosine A2A receptors in mGlu5 receptor-mediated effects.

Age-related decline in the functional response of striatal group I mGlu receptors.

In order to verify whether striatal group I metabotropic glutamate (mGlu) receptors undergo functional alteration in ageing, the effects induced by the selective agonist 3,5-dihydroxyphenylglycine (DHPG) in the striatum of young (3 months) and aged (24-25 months old) rats were compared. The ability of DHPG to stimulate phosphoinositide (PI) hydrolysis (striatal slices), to influence striatal dopamine release (in vivo microdialysis) and to potentiate the effects of NMDA on extracellular field potential amplitude (extracellular recordings on striatal slices) was reduced in the striatum of old vs young rats. These results show an age-dependent reduction in the functional response of striatal group I mGlu receptors, which may be one of the factors underlying the reduced ability aged striatum to integrate information.

The intrastratial injection of an adenosine A(2) receptor antagonist prevents frontal cortex EEG abnormalities in a rat model of Huntington's disease.

The influence of 3,7-dimethyl-1-propargylxanthine (DMPX) an adenosine A(2) receptor antagonist, was studied in the quinolinic acid (QA) model of Huntington's disease. Male Wistar rats received bilateral intrastriatal injections of QA (300 nmol) alone or plus DMPX (0.02, 0.2 and 2 microg). At the dose of 0.2 microg, DMPX completely prevented QA-induced EEG abnormalities at the level of frontal cortex. The results support the hypothesis of a neuroprotective role of adenosine A(2) receptor antagonists.

Age-related alteration of the adenosine/dopamine balance in the rat striatum.

An antagonistic interaction between adenosine A2A- and dopamine D2-receptors has been described. Radioligand binding experiments showed a predominant reduction in the number of D2 vs. A2A-receptors in the striatum of aged compared to young rats. The A2A-receptor-mediated antagonistic modulation of D2-receptor binding remained unchanged in aged animals. In striatal homogenates a significant increase in adenosine and no change in dopamine content was found in aged vs. young rats. These results reveal the existence of an age-dependent imbalance of adenosine vs. dopamine in favor of adenosine, which involves both presynaptic and postsynaptic mechanisms.

Adenosine A1 and A2 receptor agonists significantly prevent the electroencephalographic effects induced by MK-801 in rats.

Both N6-cyclopentyladenosine (CPA, adenosine A1 receptor agonist) and 2-[4-(2-carboxyethyl)phenethylamino]-5'-N-ethylcarboxamido-adenosi ne (CGS 21680, adenosine A2 receptor agonist) inhibited the electroencephalographic (EEG) effects induced by the noncompetitive NMDA receptor antagonist (+)-5-methyl-10,11-dihydro-5H-dibenzo-(a,d)cyclohepten-5,10-imine maleate (MK-801) in rats. While the inhibitory effects of CPA were evident at doses (0.1 and 0.5 mg/kg i.p.) devoid of intrinsic behavioral effects, CGS 21680 was effective only when administered at depressant doses (2 mg/kg i.p.). Since the effects induced by NMDA receptor antagonists may be regarded as a model of psychosis, these results suggest a possible role of adenosine receptor agonists as antipsychotics.

Selective agonists of metabotropic glutamate receptors elicit significant EEG effects when infused in the nucleus accumbens of rats.

The effect of intra-accumbens infusion of selective group I ((S)-3,5-dihydroxyphenylglycine, DHPG), group II ((2S,3S,4S)-CCG/(2S,1'S,2'S)-2-(carboxycyclopropyl)glycine, L-CCG-I) and group III ((L-(+)-2-amino-4-phosphonobutyric acid, L-AP4) metabotropic glutamate (mGlu) receptor agonists was studied in male Wistar rats. A computerised electroencephalographic (EEG) power spectral analysis was performed. While DHPG (400 nmoles) induced EEG and behavioural limbic seizures, L-CCG-I (400 nmoles) and L-AP4 (800 nmoles) induced a 'depressant' EEG with an increase in relative power in the slow-frequency bands and a decrease in relative power in the high-frequency bands) and behavioural effects. These results show for the first time that the stimulation of groups I, II and III mGlu receptors located in the nucleus accumbens significantly influences the EEG tracing in rats.

Adenosine A2A receptor stimulation enhances striatal extracellular glutamate levels in rats.

The influence of CGS 21680, an adenosine A2A receptor agonist, on striatal glutamate extracellular levels was tested in a microdialysis study in rats. CGS 21680 (10 mu M), infused intrastriatally through the microdialysis probe, greatly enhanced glutamate extracellular levels. These results show that striatal adenosine A2A receptors are involved in the regulation of striatal glutamate extracellular levels. They also suggest that adenosine A2A receptor antagonists may possess neuroprotective effects in models of striatal excitotoxicity.

SCH 58261 (an adenosine A(2A) receptor antagonist) reduces, only at low doses, K(+)-evoked glutamate release in the striatum.

The aim of the present work was to determine whether systemic administration of the adenosine A(2A) receptor antagonist, SCH 58261 (7-(2-phenylethyl)-5-amino-2-(2-furyl)-pyrazolo-[4,3-e]-1,2,4,triazolo[1,5-c]pyrimidine), could modulate striatal glutamate outflow in the rat. Microdialysis experiments were performed in male Wistar rats implanted with microdialysis probes in the striatum. Pretreatment (15 min before) with SCH 58261 (0.01 and 0.1, but not 1 mg/kg intraperitoneally) significantly prevented K(+)-stimulated glutamate release. These results suggest that SCH 58261 could possess neuroprotective effects in the low dose range, while, at higher doses, the occurrence of additional mechanisms may limit the neuroprotective potential of this drug.

CGS 21680 antagonizes motor hyperactivity in a rat model of Huntington's disease.

The influence of CGS 21680 (2-[p-(2-carboxyethyl)phenethylamino]-5'-N-ethylcarboxamido-adenos ine), an adenosine A2 receptor agonist, was tested in an animal model of Huntington's disease. Male Wistar rats received bilateral intrastriatal injections of quinolinic acid and then, 1 and 2 weeks later, they were treated with intrastriatal CGS 21680 (3 micrograms/2 microliters) or saline. While quinolinic acid-lesioned rats not treated with CGS 21680 showed the typical motor hyperresponsiveness to d-amphetamine (1 mg/kg i.p.), the intrastriatal injection of CGS 21680 completely prevented this effect.

Stimulation of adenosine A1 receptors prevents the EEG arousal due to dopamine D1 receptor activation in rabbits.

The influence of adenosine A1 (N6-cyclopentyladenosine, CPA) and A2 (2-[4-(2-carboxylethyl)phenethylamino]-5'-N-ethylcarboxamido -adenosine hydrochloride, CGS 21680) receptor agonists on SKF 38393-induced electroencephalographic (EEG) arousal was studied in rabbits. While CPA (0.1 mg/kg i.v.) significantly prevented the EEG effects of SKF 38393, CGS 21680 (0.2 mg/kg i.v.) did not affect them. These results demonstrate that adenosine A1 receptors can modulate dopamine D1 receptor-induced EEG arousal and show, for the first time, that adenosine-dopamine interactions are involved in brain functions other than motor activity.

Adenosine A1 and A2A receptor antagonists stimulate motor activity: evidence for an increased effectiveness in aged rats.

The motor effects of selective adenosine A1 and A2A receptor antagonists were tested in young (2 months) and aged (24 months) Wistar rats. In young rats, both the selective A2A receptor antagonist 5-amino-7-(2-phenylethyl)-2-2(2-furyl)-pyrazolo[4,3-e]-1,2,4-triazo++ + lo[1,5-c]pyrimidine (SCH 58261, minimal effective dose 2 mg/kg intraperitoneally (i.p.)) and the selective A1 receptor antagonist 8-cyclopentyl-1,3-dimethylxanthine (CPT, minimal effective dose 1.2 mg/kg i.p.) stimulated motor activity. In old rats, both compounds induced significant motor activation starting from doses lower than those required in young animals. Specifically, the minimal effective doses of SCH 58261 and CPT in aged rats were 1 and 0.6 mg/kg i.p, respectively. The results indicate that both adenosine A1 and A2A receptors play a functional role in the control of motor activity, and, therefore, the blockade of both receptor subtypes is involved in the motor stimulating properties of methylxanthines. Also the evidence indicates, for the first time, that in aged animals the motor inhibitory adenosinergic tone seems to be increased with respect to young animals.

Adenosine A1 receptor blockade selectively potentiates the motor effects induced by dopamine D1 receptor stimulation in rodents.

An antagonistic interaction between adenosine A1 and dopamine D1 receptors has previously been found in the basal ganglia. However, direct evidence for a selective adenosine A1 antagonist-induced potentiation of dopamine D1-mediated motor activation is lacking. The systemic administration of the adenosine A1 antagonist 8-cyclopentyl-1,3-dimethylxanthine significantly potentiated the motor activating properties of the systemically administered dopamine D1 agonist SKF 38393 in both reserpinized mice and unilaterally 6-hydroxy-dopamine-lesioned rats. However, 8-cyclopentyl-1, 3-dimethylxanthine did not modify the motor effects of the dopamine D2 agonist quinpirole. The present work shows that an antagonistic interaction between adenosine A1 and dopamine D1 receptors may be involved in the motor activating effects of adenosine antagonists, like caffeine.

Evidence for the occurrence of depressant EEG effects after stimulation of dopamine D3 receptors: a computerized study in rabbits.

The putative dopamine D3 receptor agonist, (+/-) 7-OH-di-n-propylaminotetralin (+/- 7-OH-DPAT), induced depressant effects on rabbit EEG at the dose of 1 mg/kg i.v. Bromocriptine, a preferential dopamine D2 receptor agonist, induced EEG activation at the dose of 0.5 mg/kg i.v. Although the lack of very selective ligands makes it difficult to discriminate between D2- and D3- dependent effects, these findings suggest that -unlike D2 receptors-dopamine D3 receptors may mediate depressant effects on the electrocorticogram.

Baseline electroencephalographic tracings in rabbits differ significantly according to 'acute' or 'chronic' preparation. A computerized study.

Baseline electroencephalographic (EEG) tracings recorded from 'acute' and 'chronic' rabbits were compared by computerized spectral analysis. The results showed that baseline EEG activity of rabbits differ significantly and markedly according to acute or chronic preparation. It is suggested that this finding should be taken into account when studying the EEG effects of centrally acting drugs.