Over the past decade astroglia has come to be recognised as an active player in information processing in the central nervous system. It has been implicated in a variety of vital functions such the regulation of breathing as we have previously reported (Gourine et al., 2010). In order to elucidate the varied and important roles of astrocytes in different functional contexts, optogenetic approaches are proving invaluable for selective astrocyte stimulation. Whilst not electrically excitable, astrocytes display excitability based on [Ca2+]i increases following metabotropic receptor activation and Ca2+ mobilisation from stores and/or Ca2+ entry from the extracellular space. Our initial experiments employing light sensitive ion channels made use of the Ca2+ permeability of the channelrhodopsin variant ChR2(H134R) to excite astrocytes and initiate glial [Ca2+]i waves which self-propagate through release of ATP. Experiments in nil external Ca2+ and using thapsigargin (1µM) showed that ChR2(H143R)-induced Ca2+ excitation of astrocytes involves Ca2+ release from intracellular stores. Currently we are exploring other opsins such as CatCh as tools for control of astrocytic [Ca2+]i (Kleinlogel et al., 2011). An approach more closely simulating astrocytic cell physiology employs light-activated G-protein-coupled receptors. We adapted the rhodopsin-adrenoceptor (AR) chimaeras Opto-α1-AR and Opto-β2-AR (Airan et al., 2009) for selective expression in astrocytes using lenti- and adenoviral vectors and a transcriptionally enhanced, shortened glial fibrillary acidic protein promoter (Liu et al., 2008). We are specifically interested in the communication between astrocytes and central noradrenergic neurones. To study this interaction, confocal [Ca2+]i measurements and whole cell patch clamp recordings in organotypic rat brainstem slices were carried out and revealed that optogenetic excitation of astrocytes leads to powerful excitation of locus coeruleus (LC) or C1 neurones (a ventral group of pre-sympathetic catecholaminergic neurones). This response was delayed (~60-100 sec) and could be prevented by the glycogen metabolism blocker 1,4-dideoxy-1,4-imino-D-arabinitol (DAB; 500μM), indicating that it may be triggered by lactate released by activated astrocytes. We then investigated the effects of optogenetic stimulation of astrocytes on noradrenaline (NA) release measured by fast scan cyclic voltammetry. Opto-X-AR expressed in astrocytes in organotypic brain slices containing LC were activated with blue light flashes (15 – 25 Hz). Stimulation of astrocytes evoked robust NA release from neurones which was blocked by the P2Y purinoceptor antagonist MRS2179 (10 μM) and by DAB, in line with the hypothesis that astrocytes in the LC signal via ATP and lactate secretion to evoke neuronal NA release. To further confirm this, ATP (400 μM) or L-lactate (0.4 mM) were bath applied and mimicked the effect of astrocytic light stimulation on NA release. The effect of ATP was inhibited by co-application of MRS2179 or DAB. This suggests that activation of purinergic receptors on astrocytes may trigger lactate production and release, and that extracellular lactate may be the principle messenger leading to neuronal NA release. Consistent with this idea, activation of opto-AR in cultured dissociated astrocytes led to intracellular acidification as revealed by confocal measurement of the ratiometric pH indicator SNARF-5, and this acidification could be prevented by DAB. The signalling pathway between astrocytes and noradrenergic neurones may be important in various brain areas as in vivo optogenetic stimulation of astrocytes in ventral brainstem areas containing noradrenergic and adrenergic neurones activates sympathetic outflow. Molecular mechanisms of this signalling pathway are currently under investigation.