α1-noradrenergic cAMP signaling in astrocytes of the murine olfactory bulb

cAMP is a ubiquitous second messenger and plays a central role in the regulation of gene expression and synaptic transmission. While calcium imaging is a well-established technique with a variety of different chemical and genetically encoded sensors, cAMP sensors have been lacking until recently and hence cAMP signaling is not well studied in astrocytes so far. Flamindo2 from Odaka et al. (PLOS One 9:6, 2014) is a genetically encoded cAMP sensor, which fluorescence intensity decreases in response to increased cAMP levels. In this study we investigated noradrenergic cAMP signaling in astrocytes of the main olfactory bulb by confocal cAMP imaging using Flamindo2 in acute brain slices.The olfactory bulb is innervated by noradrenergic projections from the locus coeruleus as main source of norepinephrine (NE). Those noradrenergic terminals are found in the glomerular layer and modulate neuronal plasticity and thus olfactory processing. Noradrenalin induces cAMP signals in the astrocytes of the olfactory bulb via α1-, α2- and β⁠-adrenergic receptors. Although the α1-receptor is known to be coupled to Gq, application α1-receptor agonist phenylephrine (PE) leads to dose-dependent cAMP-signals in the astrocytes, suggesting activation of adenylyl cyclases (AC). Experiments conducted in Ca2+ -free environment showed that PE-induced cAMP signals were Ca2+-dependent. In addition, blockage of Ca2+-sensitive AC subtypes suppressed PE-evoked cAMP signals, indicating that PE-evoked increases in Ca2+ led to activation of Ca2+-sensitive AC subtypes and subsequent production of cAMP.