Enhanced release of adenosine under cell-damaging conditions in the developing and adult mouse hippocampus

Adenosine, ketogenic diet and epilepsy: the emerging therapeutic relationship between metabolism and brain activity

For many years the neuromodulator adenosine has been recognized as an endogenous anticonvulsant molecule and termed a “retaliatory metabolite.” As the core molecule of ATP, adenosine forms a unique link between cell energy and neuronal excitability. In parallel, a ketogenic (high-fat, low-carbohydrate) diet is a metabolic therapy that influences neuronal activity significantly, and ketogenic diets have been used successfully to treat medically-refractory epilepsy, particularly in children, for decades. To date the key neural mechanisms underlying the success of dietary therapy are unclear, hindering development of analogous pharmacological solutions. Similarly, adenosine receptor–based therapies for epilepsy and myriad other disorders remain elusive. In this review we explore the physiological regulation of adenosine as an anticonvulsant strategy and suggest a critical role for adenosine in the success of ketogenic diet therapy for epilepsy. While the current focus is on the regulation of adenosine, ketogenic metabolism and epilepsy, the therapeutic implications extend to acute and chronic neurological disorders as diverse as brain injury, inflammatory and neuropathic pain, autism and hyperdopaminergic disorders. Emerging evidence for broad clinical relevance of the metabolic regulation of adenosine will be discussed.

Presynaptic adenosine A1 receptors modulate excitatory synaptic transmission in the posterior piriform cortex in rats

The effect of adenosine on the fEPSP was examined in the lateral olfactory tract (Ia input) and associative tract (Ib input) of the rat piriform cortex. The fEPSP evoked in the Ia input showed paired-pulse facilitation, while that in the Ib input showed paired-pulse depression, suggesting a lower resting release probability in the Ia input. This was supported by results showing that MK801 blocked the NMDA receptor-induced fEPSP more rapidly in the Ib input than the Ia input. Adenosine caused dose-dependent inhibition of the fEPSP in both inputs, the sensitivity being higher in the Ib input. This effect was mimicked by the A(1) receptor agonist, CHA, and antagonized by co-application of the A(1) receptor antagonist, DPCPX, showing that adenosine was acting at A(1) receptors. Application of DPCPX alone caused an increase in the fEPSP, the increase being larger in the Ia input. DPCPX also caused paired-pulse depression in both inputs, and the paired-pulse ratio measured in its presence was very similar in both inputs. These results suggest there is a lower endogenous concentration of adenosine in the Ib sublayer than the Ia sublayer, which might account for the native difference in the resting release probability of the two inputs. The adenosine-induced inhibition of the fEPSP in both inputs was associated with a significant reduction in the rate at which MK801 blocked NMDA receptor-mediated fEPSP activity, suggesting a presynaptic location of the A(1) receptors. Blocking of N-, P/Q-type calcium channels occluded the inhibition by adenosine, indicating that they are downstream effectors of presynaptic A(1) receptor activation.

Adenosine A2A receptors in diffuse dermal fibrosis: pathogenic role in human dermal fibroblasts and in a murine model of scleroderma

OBJECTIVE

Adenosine regulates inflammation and tissue repair, and adenosine A2A receptors promote wound healing by stimulating collagen matrix production. We therefore examined whether adenosine A2A receptors contribute to the pathogenesis of dermal fibrosis.

METHODS

Collagen production by primary human dermal fibroblasts was analyzed by real-time polymerase chain reaction, 14C-proline incorporation, and Sircol assay. Intracellular signaling for dermal collagen production was investigated using inhibitors of MEK-1 and by demonstration of ERK phosphorylation. In vivo effects were studied in a bleomycin-induced dermal fibrosis model using adenosine A2A receptor-deficient wild-type littermate mice, C57BL/6 mice, and mice treated with adenosine A2A receptor antagonist. Morphometric features and levels of hydroxyproline were determined as measures of dermal fibrosis.

RESULTS

Adenosine A2A receptor occupancy promoted collagen production by primary human dermal fibroblasts, which was blocked by adenosine A2A, but not A1 or A2B, receptor antagonism. Adenosine A2A receptor ligation stimulated ERK phosphorylation, and A2A receptor-mediated collagen production by dermal fibroblasts was blocked by MEK-1 inhibitors. Adenosine A2A receptor-deficient and A2A receptor antagonist-treated mice were protected from developing bleomycin-induced dermal fibrosis.

CONCLUSION

These results demonstrate that adenosine A2A receptors play an active role in the pathogenesis of dermal fibrosis and suggest a novel therapeutic target in the treatment and prevention of dermal fibrosis in diseases such as scleroderma.

Uteroplacental insufficiency affects epigenetic determinants of chromatin structure in brains of neonatal and juvenile IUGR rats

Intrauterine growth retardation (IUGR) increases the risk of neuroendocrine reprogramming. In the rat, IUGR leads to persistent changes in cerebral mRNA levels. This suggests lasting alterations in IUGR cerebral transcriptional regulation, which may result from changes in chromatin structure. Candidate nutritional triggers for these changes include altered cerebral zinc and one-carbon metabolite levels. We hypothesized that IUGR affects cerebral chromatin structure in neonatal and postnatal rat brains. Rats were rendered IUGR by bilateral uterine artery ligation; controls (Con) underwent sham surgery. At day of life 0 (d0), we measured cerebral DNA methylation, histone acetylation, expression of chromatin-affecting enzymes, and cerebral levels of one-carbon metabolites and zinc. At day of life 21 (d21), we measured cerebral DNA methylation and histone acetylation, as well as the caloric content of Con and IUGR rat breast milk. At d0, IUGR significantly decreased genome-wide and CpG island methylation, as well as increased histone 3 lysine 9 (H3/K9) and histone 3 lysine 14 (H3/K14) acetylation in the hippocampus and periventricular white matter, respectively. IUGR also decreased expression of the chromatin-affecting enzymes DNA methyltransferase 1 (DNMT1), methyl-CpG binding protein 2 (MeCP2), and histone deacetylase (HDAC)1 in association with increased cerebral levels of zinc. In d21 female IUGR rats, cerebral CpG DNA methylation remained lower, whereas H3/K9 and H3/K14 hyperacetylation persisted in hippocampus and white matter, respectively. In d21 male rats, IUGR decreased acetylation of H3/K9 and H3/K14 in these respective regions compared with controls. Despite these differences, caloric, fat, and protein content were similar in breast milk from Con and IUGR dams. We conclude that IUGR results in postnatal changes in cerebral chromatin structure and that these changes are sex specific.

GABA Release Modified by Adenosine Receptors in Mouse Hippocampal Slices under Normal and Ischemic Conditions

The excitatory glutamatergic neurons in the hippocampus are modulated by inhibitory GABA-releasing interneurons. The neuromodulator adenosine is known to inhibit the presynaptic release of neurotransmitters and to hyperpolarize postsynaptic neurons in the hippocampus, which would imply that it is an endogenous protective agent against cerebral ischemia and excitotoxic neuronal damage. Interactions of the GABAergic and adenosinergic systems in regulating neuronal excitability in the hippocampus is of crucial importance, particularly under cell-damaging conditions. We now characterized the effects of adenosine receptor agonists and antagonists on the release of preloaded [3H]GABA from hippocampal slices prepared from adult (3-month-old) mice, using a superfusion system. The effects were tested both under normal conditions and in ischemia induced by omitting glucose and oxygen from the superfusion medium. Basal and K+ -evoked GABA release in the hippocampus were depressed by adenosinergic compounds. Under normal conditions activation of both adenosine A1 and A2A receptors by the agonists R(-)N6-(2-phenylisopropyl)adenosine and CGS 21680 inhibited the K+ -evoked release, which effects were blocked by their specific antagonists, 8-cyclopentyl-1,3-dipropyl-xanthine and 3,7-dimethyl-1-propargylxanthine, respectively. Under ischemic conditions the release of both GABA and adenosine is markedly enhanced. The above receptor agonists then depressed both the basal and K+ -evoked GABA release, only the action of A2A receptors being however receptor-mediated. The demonstrated depression of GABA release by adenosine in the hippocampus could be deleterious to neurons and contribute to excitotoxicity.