Expression of adenosine A1 receptors in cultured neurons from fetal rat brain

Purinergic signaling in embryonic and stem cell development

Nucleotides are of crucial importance as carriers of energy in all organisms. However, the concept that in addition to their intracellular roles, nucleotides act as extracellular ligands specifically on receptors of the plasma membrane took longer to be accepted. Purinergic signaling exerted by purines and pyrimidines, principally ATP and adenosine, occurs throughout embryologic development in a wide variety of organisms, including amphibians, birds, and mammals. Cellular signaling, mediated by ATP, is present in development at very early stages, e.g., gastrulation of Xenopus and germ layer definition of chick embryo cells. Purinergic receptor expression and functions have been studied in the development of many organs, including the heart, eye, skeletal muscle and the nervous system. In vitro studies with stem cells revealed that purinergic receptors are involved in the processes of proliferation, differentiation, and phenotype determination of differentiated cells. Thus, nucleotides are able to induce various intracellular signaling pathways via crosstalk with other bioactive molecules acting on growth factor and neurotransmitter receptors. Since normal development is disturbed by dysfunction of purinergic signaling in animal models, further studies are needed to elucidate the functions of purinoceptor subtypes in developmental processes.

Sex differences in the neurotoxic effects of adenosine A1 receptor antagonism during ethanol withdrawal: reversal with an A1 receptor agonist or an NMDA receptor antagonist

BACKGROUND

Neuronal adaptations that occur during chronic ethanol (EtOH) exposure have been observed to sensitize the brain to excitotoxic insult during withdrawal. The adenosine receptor system warrants further examination in this regard, as recent evidence has implicated adenosine receptor involvement in the behavioral effects of both EtOH exposure and withdrawal.

METHODS

The current studies examined effects of adenosine A(1) receptor manipulation on neuronal injury in EtOH-naive and EtOH-withdrawn male and female rat hippocampal slice cultures. EtOH-naive and EtOH pretreated (43.1 to 26.9 mM from days 5 to 15 DIV) cultures were exposed to the A(1) receptor agonist 2-Chloro-N(6)-cyclopentyladenosine (CCPA; 10 nM), the A(1) receptor antagonist 8-Cyclopentyl-1,3-dipropylxanthine (DPCPX;10 nM), or the N-methyl-D-aspartate (NMDA) receptor antagonist D,L,-2-amino-5-phosphovalerate (APV; 20 microM) at 15 days in vitro (DIV). Cytotoxicity was measured in the primary neuronal layers of the dentate gyrus, CA3 and CA1 hippocampal regions by quantification of propidium iodide (PI) fluorescence after 24 hours. Immunohistochemical analysis of A(1) receptor abundance was conducted in EtOH-naive and EtOH pretreated slice cultures at 15 DIV.

RESULTS

Twenty-four hour exposure to DPCPX in EtOH-naive slice cultures did not produced neurotoxicity in any region of slice cultures. Though withdrawal from 10 day EtOH exposure produced no toxicity in either male or female slice cultures, exposure to DPCPX during 24 hours of EtOH withdrawal produced a marked increase in PI uptake in all hippocampal culture subregions in female cultures (to approximately 160% of control values). A significant effect for sex was observed in the CA1 region such that toxicity in females cultures exposed to the A(1) antagonist during withdrawal was greater than that observed in male cultures. These effects of DPCPX in EtOH withdrawn female and male slices were prevented by co-exposure to either the A(1) agonist CCPA or the NMDA receptor antagonist APV for 24 hours. No differences in the abundance of A(1) receptors were observed in male and female EtOH-naive or EtOH pretreated cultures.

CONCLUSIONS

The current findings suggest that the female hippocampus possesses an innate sensitivity to effects of EtOH exposure and withdrawal on neuronal excitability that is independent of hormonal influences. Further, this sex difference is not related to effects of EtOH exposure on A(1) receptor abundance, but likely reflects increased NMDA receptor-mediated signaling downstream of A(1) inhibition in females.

Adenosine A(1) receptor in cultured neurons from rat cerebral cortex: colocalization with adenosine deaminase

Adenosine A(1) receptors (A(1)Rs) have been characterized in primary cultures of neurons from cerebral cortex. The specific adenosine A(1) antagonist 8-cyclopentyl-1,3-[(3)H]dipropylxanthine bound to both membranes and intact cells. When saturation experiments were performed in membranes, a K(D) value of 0.76 nM and a B(max) of 57 fmol/mg of protein were obtained. Competition assays revealed a pharmacological profile characteristic of A(1)Rs. The presence of this receptor was further confirmed by RT-PCR analysis. The expression of the receptor showed no significant changes during the period of culture studied, up to 12 days in vitro. A(1)R agonist inhibited forskolin-stimulated adenylyl cyclase, showing the functional coupling of these receptors with the effector. alphaG(i1, 2) protein level, detected by immunoblot, presented an increase during the period of culture. This increase correlated with an increase in the mRNA level of alphaG(i1) but not alphaG(i2). By immunochemical assays, it is shown that these receptors are expressed in both the neuronal cell body and the proximal dendrites. Colocalization of A(1)Rs with microtubule-associated protein 2 and cell surface adenosine deaminase was shown by confocal microscopy. The high degree of colocalization observed between A(1)Rs and ectoadenosine deaminase in neurons could suggest an important role of the enzyme in adenosine-mediated neuromodulation.

Non-selective effects of adenosine A1 receptor ligands on energy metabolism and macromolecular biosynthesis in cultured central neurons

To investigate the effects of adenosine A1 receptor activation on energy metabolism and RNA and protein biosynthesis in central neurons, cultured neurons from the rat forebrain were exposed for 1 hr to 72 hr to various concentrations (10 nM-100 microM) of the selective A1 receptor agonist 2-chloro-N6-cyclopentyladenosine (CCPA) or the A1 receptor antagonist 8-cyclopentyltheophylline (CPT). At all concentrations tested, the adenosinergic compounds did not affect cell viability within 72 hr of treatment, except for CPT, which reduced viability by 19.7% when used at the concentration of 100 microM. Energy metabolism was analysed by studying the specific uptake of 2-D-[3H]deoxyglucose ([3H]2DG). Rates of RNA and protein biosynthesis were assessed by the measurement of [3H]uridine and [3H]leucine incorporation, respectively. Neuronal [3H]2DG uptake was increased by 16% (P < 0.01) after 8 hr in the presence of 100 microM CCPA, whereas 100 microM CPT for 24 hr also increased [3H]2DG uptake (8%, P < 0.01). At these concentrations, both ligands inhibited [3H]uridine incorporation after a 3-hr treatment by 92% and 30%, respectively. CCPA never altered [3H]leucine incorporation when compared to controls, and CPT significantly inhibited protein synthesis only at 10-100 microM. Additional experiments to analyse the influence of A1 ligands on the transport of [3H]2DG, [3H]leucine and [3H]uridine suggested that CCPA and CPT, which interact functionally with adenosine receptors by regulating cyclic AMP production in this model, are able to alter energy metabolism and RNA synthesis in central neurons in a nonspecific manner by interacting with glucose and uridine transporters.