Calcium signals in the nucleus accumbens: Activation of astrocytes by ATP and succinate

Abstract

BackgroundAccumulating evidence suggests that glial signalling is activated by different brain functions. However, knowledge regarding molecular mechanisms of activation or their relation to neuronal activity is limited. The purpose of the present study is to identify the characteristics of ATP-evoked glial signalling in the brain reward area, the nucleus accumbens (NAc), and thereby to explore the action of citric acid cycle intermediate succinate (SUC).ResultsWe described the burst-like propagation of Ca2+ transients evoked by ATP in acute NAc slices from rat brain. Co-localization of the ATP-evoked Ca2+ signalling with immunoreactivities of the astroglia-specific gap junction forming channel protein connexin43 (Cx43) and the glial fibrillary acidic protein (GFAP) indicated that the responsive cells were a subpopulation of Cx43 and GFAP immunoreactive astrocytes. The ATP-evoked Ca2+ transients were present under the blockade of neuronal activity, but were inhibited by Ca2+ store depletion and antagonism of the G protein coupled purinergic P2Y1 receptor subtype-specific antagonist MRS2179. Similarly, Ca2+ transients evoked by the P2Y1 receptor subtype-specific agonist 2-(Methylthio)adenosine 5'-diphosphate were also blocked by MRS2179. These characteristics implied that intercellular Ca2+ signalling originated from the release of Ca2+ from internal stores, triggered by the activation of P2Y1 receptors. Inhibition by the gap junction blockers carbenoxolone and flufenamic acid and by an antibody raised against the gating-associated segment of Cx43 suggested that intercellular Ca2+ signalling proceeded through gap junctions. We demonstrated for the first time that extracellular SUC also evoked Ca2+ transients (EC50 = 50-60 μM) in about 15% of the ATP-responsive NAc astrocytes. By contrast to glial cells, electrophysiologically identified NAc neurons surrounded by ATP-responsive astrocytes were not activated simultaneously.ConclusionsWe concluded, therefore, that ATP- and SUC-sensitive Ca2+ transients appear to represent a signalling layer independent of NAc neurons. This previously unrecognised glial action of SUC, a major cellular energy metabolite, may play a role in linking metabolism to Ca2+ signalling in astrocytic networks under physiological and pathological conditions such as exercise and metabolic diseases.

Extensive astrocyte synchronization advances neuronal coupling in slow wave activity in vivo

“…Since inhibition of neuronal Cx36 may also result in reduced synchronization 42 , we opted to specifically block the astrocytic Cx43 isoform with a specific antibody against its gating peptide segment 43 , which has previously been demonstrated to inhibit Cx43 function 16, 39 . The Cx43 antibody (0.75 mg/ml, 10 μl) was applied to the cortical surface 24 hours before the experiment to ensure antibody penetration into layer 2.…”

Activation of astroglial calcium signaling by endogenous metabolites succinate and gamma-hydroxybutyrate in the nucleus accumbens

“…Activation of the presumed brain type SUC receptor (as an analogy to SUCNR1 we reference this putative receptor as SUCBR1) by agonists SUC or GHB evokes repetitive Ca 2+ transients in astrocytes independently of neuronal signaling (Molnár et al, 2009, 2011). A sub-population of SUCBR1-triggered repetitive Ca 2+ transients can also be activated by ATP making Ca 2+ signaling more robust.…”

“…We also observed for the first time the activation of repetitive astroglial Ca 2+ transients in response to the major intermediate of the citric acids cycle succinic acid (SUC). The apparent EC 50 (50–60 μM) of SUC effect is within the range of physiological plasma SUC concentration (Molnár et al, 2011), since the concentration of SUC in plasma increases from 5 up to 125 μM with exercise, metabolic acidosis or hyperglycemic metabolic states (Krebs, 1950; Nordmann and Nordmann, 1961; Hochachka and Dressendorfer, 1976; Kushnir et al, 2001; Forni et al, 2005; Sadagopan et al, 2007). These data suggest that SUC-responsive Ca 2+ transients may also have a regulatory role for cellular energy supply.…”

Polyamidoamine dendrimer impairs mitochondrial oxidation in brain tissue

“…Furthermore, we also reported on astroglial signaling independent of neuronal activity in acute brain slices isolated from the rat nucleus accumbens [10]. In general, neurons are more susceptible to oxidative injury than astrocytes, due to their limited antioxidant capacity [11].…”

“…Ψ MITO is also coupled to intracellular Ca 2+ regulation [11,13,14,16]. Changes in intracellular Ca 2+ level indicate activation of neuronal [17,18] or astroglial cells [10,19,20]. …”