Role of adenosine A1 receptors in the modulation of dopamine D1 and adenosine A2a receptor signaling in the neostriatum

Adenosine is known to modulate the function of neostriatal neurons. Adenosine acting on A(2A) receptors increases the phosphorylation of dopamine- and cAMP-regulated phosphoprotein of M(r) 32 kDa (DARPP-32) at Thr34 (the cAMP-dependent protein kinase [PKA] site) in striatopallidal neurons, and opposes dopamine D2 receptor signaling. In contrast, the role of adenosine A(1) receptors in the regulation of dopamine/DARPP-32 signaling is not clearly understood. Here, we investigated the effect of adenosine A(1) receptors on D(1), D(2) and A(2A) receptor signaling using mouse neostriatal slices. An A(1) receptor agonist, 2-chloro-N(6)-cyclopentyladenosine (100 nM), caused a transient increase, followed by a transient decrease, in DARPP-32 Thr34 phosphorylation. Our data support the following model for the actions of the A(1) receptor agonist. The A(1) receptor-induced early increase in Thr34 phosphorylation was mediated by presynaptic inhibition of dopamine release, and the subsequent removal of tonic inhibition by D(2) receptors of A(2A) receptor/G(olf)/cAMP/PKA signaling. The A(1) receptor-induced late decrease in Thr34 phosphorylation was mediated by a postsynaptic G(i) mechanism, resulting in inhibition of D(1) and A(2A) receptor-coupled G(olf)/cAMP/PKA signaling in direct and indirect pathway neurons, respectively. In conclusion, A(1) receptors play a major modulatory role in dopamine and adenosine receptor signaling.

Influence of manganese on the release of neurotransmitters in rat striatum

On the basis of the evidence that manganese may be released along with glutamate into the extracellular space in the hippocampus and amygdala, the release of manganese and its influence in the striatum was examined by using the in vivo microdialysis method in the present study. The release of 54Mn previously taken up by the striatum into the extracellular space was enhanced during stimulation with 100 mM KCl. This enhancement of 54Mn release into the striatal extracellular space was inhibited by addition of 1 micro M tetrodotoxin. When the rat striatum was perfused with artificial CSF containing 200 nM MnCl(2), the levels of GABA in the perfusate were remarkably decreased, while the levels of glutamate, aspartate, and glycine in the perfusate were not appreciably decreased. These results suggest that manganese released into the synaptic cleft in a calcium- and impulse-dependent manner may influence GABA release in the striatum.

Manganese influences the levels of neurotransmitters in synapses in rat brain

54Mn previously taken up by the amygdala is released along with known neurotransmitters into the extracellular space during stimulation with 100 mM KCl. The possibility of manganese release from neuron terminals in a calcium- and impulse-dependent manner was examined by using the in vivo microdialysis method in the present study. The increase of (54)Mn release into the amygdalar extracellular space during stimulation with high K(+) was inhibited by addition of 1 microM tetrodotoxin. This increase of (54)Mn release into the extracellular space by stimulation with high K(+) was also observed in the hippocampus, but not in the substantia nigra. The increment of glutamate in the extracellular space during stimulation with high K(+) was highly correlated with that of (54)Mn, suggesting that manganese is concurrently released with glutamate from neuron terminals. The level of (54)Mn in the extracellular space in the hippocampus was increased with that of glutamate, but not with those of GABA and glycine, during stimulation with 100 mM KCl in the presence of 30 microM kainate. This increase was more marked than during stimulation with 30 microM kainate alone. It is likely that manganese is released from glutamatergic neuron terminals. When the rat hippocampus was perfused with artificial cerebrospinal fluid containing 20 or 200 nM MnCl(2), the levels of glutamate, aspartate and GABA in the perfusate were dose-dependently decreased during perfusion with manganese. The present findings demonstrate that manganese released into the synaptic cleft may influence synaptic neurotransmission.

Manganese concentration in mouse brain after intravenous injection

Abnormal deposit of manganese (Mn) in the basal ganglia is often observed in patients with chronic liver failure and patients receiving long-term parenteral nutrition. Based on the data that (54)Mn is transported into the brain efficiently via a transferrin-independent uptake system, Mn concentration in mouse brain was determined after intravenous (iv) injection (2 mg Mn/kg/day x 5 times) of either MnCl(2) or pH 8.6 buffer-treated MnCl(2), which has a higher affinity for transferrin than untreated MnCl(2). Brain Mn concentration was significantly increased in either case. Brain Mn concentration of MnCl(2) group was significantly higher than that of pH 8.6 buffer-treated MnCl(2) groups. Mn concentration in the caudate putamen of MnCl(2) group was also significantly higher than that of pH 8.6 buffer-treated MnCl(2) group. Ninety seconds after a single injection of MnCl(2) (2 mg Mn/kg), brain Mn concentration was remarkably increased with blood Mn level, but not 1 hr after injection. On the other hand, hepatic Mn concentration was remarkably high 1 hr after injection and the brain Mn concentration was increased again at 24 hr, followed by decrease of the hepatic Mn concentration. Relative Mn concentrations in the brain 90 sec after injection were different from those 24 hr after injection, suggesting that the mechanism of the increase of brain Mn concentration via blood Mn level is different from that via the redistribution from the liver. Mn ion and/or Mn bound to low molecular weight compounds may be involved in abnormal Mn uptake in the brain.

Binding properties of various metals to blood components and serum proteins: a multitracer study

The binding affinity of various trace elements to blood components and the pH-dependence of the binding affinity of the elements to serum proteins were examined using the radioactive multitracer technique. The binding affinity of 13 elements (Be, V, Mn, Zn, Se, Rb, Sr, Ce, Eu, Gd, Tm, Yb, and Lu) was simultaneously determined by gamma-ray spectrometry. The blood drawn from rats was separated into plasma, corpuscles, and erythrocyte ghosts. It was found that Be, Sr, Mn, and Zn bind highly to the plasma proteins. V and Se were highly bound to the corpuscles, and Se to the erythrocyte ghosts as well. Similar binding percentages of rare earth elements (Ce, Eu, Gd, Tm, Yb, and Lu) were found for each of the blood components, with the highest percentages being observed for plasma proteins. Albumin, beta-globulin, gamma-globulin, apotransferrin, and holotransferrin were examined in the study on the affinity of individual serum proteins. The pH dependence of the affinity of metal ions to the serum proteins in the pH range of 6.4-8.5 was examined using ultrafiltration and gamma-ray spectrometry. Each element showed a characteristic binding affinity to each serum protein, depending on pH. The results are discussed in terms of the chelating ability of metal ions and the nature of the serum proteins.