Brain adenosine receptors in Maudsley reactive and non-reactive rats

Desensitization and internalization of adenosine A1 receptors in rat brain by in vivo treatment with R-PIA: involvement of coated vesicles

Chronic treatment of rats with R-PIA 'in vivo' desensitized adenosine A1 receptor-mediated inhibition of adenylyl cyclase in brain plasma membranes and increased basal and forskolin-stimulated adenylyl cyclase. The adenosine A1 receptor agonist CHA (cyclohexyl adenosine) inhibited forskolin-stimulated adenylyl cyclase in synaptic plasma membranes from control rats but failed to do so in membranes isolated from rats treated with R-PIA. This loss of response was accompanied with a significant decrease in both, total number of adenosine A1 receptors and steady-state level of alpha-Gi in synaptic plasma membranes. An increase in the steady-state level of alpha-Gs in synaptic plasma membranes was also observed by R-PIA treatment. Concurrently, a significant increase of adenosine A1 receptors was observed in microsomes and coated vesicles. These results demonstrate adenosine A1 receptor desensitization in rat brain by 'in vivo' treatment with R-PIA and suggest a role for coated vesicles in the internalization of G-protein coupled receptors.

On the significance of brain extracellular uric acid detected with in-vivo monitoring techniques: a review

The concentration of uric acid [UA] in the extracellular fluid (ECF) estimated with in-vivo voltammetry and microdialysis data is compared for probes of different diameters from the day of implantation (acute) to several days (chronic) or even months after surgery. For small probes (diameter < 160 microns) the acute [UA] of ca. 5 microM decreased significantly to ca. 1 microM under chronic conditions. For larger probes (e.g., 320-microns diameter) the acute [UA] was also ca. 5 microM, but this value significantly increased to ca. 50 microM under chronic conditions. Associated with this difference in [UA], there were parallel differences in the extent of gliosis around the probes. These findings are discussed in terms of possible sources of extracellular UA and their implications for in-vivo monitoring techniques in behaving animals.

Potential use of DPCPX as probe for in vivo localization of brain A1 adenosine receptors

The suitability of (3H)DPCPX (8-cyclopentyl-1,3-dipropylxanthine), a xanthine derivative, as an vivo probe for labelling adenosine A1 receptors was studied in rats. [3H]DPCPX (nM) penetrated largely into the brain (0.8% of the injected dose per gram of brain tissue 5 min after injection). Brain concentrations stayed at a plateau level from 5 to 15 min after the injection. The distribution in the different brain regions was heterogeneous with the highest amount of [3H]DPCPX in cerebellum and hippocampus and the lowest concentrations in hypothalamus and brain stem. Displacement (45-70% of total radioactivity) was obtained by the injection of 250 nM of cold DPCPX or cyclopentylxanthine, an analog of DPCPX. The ex vivo autoradiographic distribution of [3H]DPCPX was similar to the in vitro autoradiographic distribution of tritiated A1 adenosine receptor ligand as [3H]CHA. These results suggest the potential use of DPCPX for further in vivo investigation of A1 adenosine receptors with techniques such as positron emission tomography.

Bovine brain coated vesicles contain adenosine A1 receptors. Presence of adenylate cyclase coupled to the receptor

Clathrin-coated vesicles purified from bovine brain express adenosine A1 receptor binding activity. N6-Cyclohexyl[3H]adenosine [( 3H]CHA), an agonist for the A1 receptor, binds specifically to coated vesicles. High and low agonist affinity states of the receptor for the radioligand [3H]CHA with KD values of 0.18 and 4.4 nM, respectively, were detected. The high purity of coated vesicles was established by assays for biochemical markers and by electron microscopy. Binding competition experiments using agonists (N6CHA, N-cyclopentyladenosine, 5'-(N-ethylcarboxamido)adenosine, and N6-[(R)- and N6-[(S)-phenylisopropyl]adenosine) and antagonists (theophylline, 3-isobutyl-1-methylxanthine, and caffeine) confirmed the typical adenosine A1 nature of the binding site. This binding site presents stereospecificity for N6-phenylisopropyladenosine, showing 33 times more affinity for N6-[(R)- than for N6-[(S)-phenylisopropyl]adenosine. The specific binding of [3H]CHA in coated vesicles is regulated by guanine nucleotides. [3H]CHA specific binding was decreased by 70% in the presence of the hydrolysis-resistant GTP analogue guanyl-5-yl-imidodiphosphate. Bovine brain coated vesicles present adenylate cyclase activity. This activity was modulated by forskolin and CHA. The results of this study support the evidence that adenosine A1 receptors present in coated vesicles are coupled to adenylate cyclase activity through a Gi protein.

Benzodiazepine anti-conflict effects in Maudsley reactive (MR/Har) and non-reactive (MNRA/Har) rats

The Maudsley Reactive (MR/Har) and Non-Reactive (MNRA/Har) rat strains, bred originally by Broadhurst for differences in Open Field Defecation, also differ in their control (i.e., non-drug) behavior in the Conditioned Suppression of Drinking (CSD) conflict procedure, a second "model" behavior for the study of anxiety and/or emotionality in rats. The present studies compared the effects of diazepam and alprazolam on CSD behavior in these two strains of rats. In daily 10-min sessions, water-deprived rats were trained to drink from a tube that was occasionally electrified (0.2-0.5 mA), electrification being signaled by a tone. Both diazepam and alprazolam increased punished responding in a dose-related manner. The per cent increase in punished responding (for diazepam only) was comparable in the two strains; however, both statistical and empirical approaches indicated that the magnitude of the anti-conflict effect of benzodiazepines in MNRA/Har versus MR/Har rats was not related to differences in baseline (i.e., non-drug) punished responding. Based on the absolute change in shocks received, rats of the MNRA/Har strain exhibited a significantly greater anti-conflict effect following diazepam or alprazolam treatment than did rats of the MR/Har strain. These findings further the hypothesis that the behavioral differences exhibited by Maudsley MR/Har and MNRA/Har rat strains may constitute a genetically-based "animal model" for the study of emotionality and/or anxiety.

MR/Har and MNRA/Har Maudsley rat strains: differential response to chlordiazepoxide in a conflict task

The Maudsley Reactive (MR/Har) and Non-Reactive (MNRA/Har) rat strains, selectively bred for differences in open field defecation, have also been shown to differ in their baseline behavior in the Conditioned Suppression of Drinking (CSD) procedure, a second "model" behavior for the study of anxiety and/or emotionality in rats. The present studies were designed to compare the responsiveness of these two strains to the typical antianxiety agent chlordiazepoxide in the CSD paradigm. In daily 10-minute sessions, water-deprived rats were trained to drink from a tube that was occasionally electrified (0.5 mA), electrification being signaled by a tone. Consistent with previous reports, after several weeks of CSD testing, MNRA/Har rats accepted significantly more shocks than did MR/Har rats during control (nondrug) sessions. In both strains, the number of shocks accepted was inversely related to the intensity of the shock used (0.25-1.0 mA), with MNRA/Har rats accepting significantly more shocks than MR/Har rats at all intensities examined. The effects of various doses (1.25-28.4 mg/kg, IP) of chlordiazepoxide were determined in subjects of the MNRA/Har strain at the original training intensity (0.5 mA), while a lower intensity (0.25 mA) was utilized in MR/Har rats. Although punished responding in control (i.e., nondrug) CSD sessions did not differ under these conditions, MNRA/Har rats were found to be more responsive to the anticonflict effects of chlordiazepoxide than rats of the MR/Har strain. This strain difference in anticonflict efficacy of chlordiazepoxide was quite dramatic, with MNRA/Har rats accepting twice as many shocks as MR/Har rats following maximally effective doses of chlordiazepoxide. Low doses of chlordiazepoxide increased water intake slightly, while higher doses decreased water intake. Surprisingly, the chlordiazepoxide-induced depression of water intake was greater in rats of the MR/Har strain. Thus, these Maudsley Reactive and Non-Reactive rat strains, bred originally for their differences in open field behavior, also differ markedly in their responsiveness to chlordiazepoxide in the CSD paradigm. These findings further support the hypothesis that the MR/Har and MNRAHar rat strains may represent a genetically-based "animal model" for the study of emotionality and/or anxiety.