SUMMARY Astrocytes perform crucial supportive functions, including neurotransmitter clearance, ion buffering and metabolite delivery. They can also influence blood flow and neuronal activity by releasing gliotransmitters in response to intracellular Ca2+ transients. However, little is known about how astrocytes are engaged during different behaviors in vivo. Here we demonstrate that norepinephrine primes astrocytes to detect changes in cortical network activity. We show in mice that locomotion triggers simultaneous activation of astrocyte networks in multiple brain regions. This global stimulation of astrocytes was inhibited by alpha-adrenoceptor antagonists and abolished by depletion of norepinephrine from the brain. Although astrocytes in visual cortex of awake mice were rarely engaged when neurons were activated by light stimulation alone, pairing norepinephrine release with light stimulation markedly enhanced astrocyte Ca2+ signaling. Our findings indicate that norepinephrine shifts the gain of astrocyte networks according to behavioral state, enabling astrocytes to respond to local changes in neuronal activity.

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Norepinephrine (NE) causes an increase in the frequency of inhibitory postsynaptic potentials in CA1 pyramidal neurons in vitro. The possibility that this increase in tonic inhibition is caused by an excitatory effect on inhibitory interneurons was investigated through whole-cell recordings from pyramidal cells and both whole-cell and cell-attached patch recordings from visualized interneurons in acute slices of rat hippocampus. Adrenergic agonists caused a large increase in the frequency and amplitude of spontaneous IPSCs recorded from pyramidal cells in the presence of ionotropic glutamate receptor blockers, but they had no effect on either the frequency or the amplitude of action potential-independent miniature IPSCs recorded in tetrodotoxin. This effect was mediated primarily by an alpha adrenoceptor, although a slight beta adrenoceptor-dependent increase in IPSCs was also observed. NE caused interneurons located in all strata to depolarize and begin firing action potentials. Many of these cells had axons that ramified throughout the stratum pyramidale, suggesting that they are responsible for the IPSCs observed in pyramidal neurons. This depolarization was also mediated by an alpha adrenoceptor and was blocked by a selective alpha 1- but not a selective alpha 2-adrenoceptor antagonist. However, a slight beta adrenoceptor-dependent depolarization was detected in those interneurons that displayed time-dependent inward rectification. In the presence of a beta antagonist, NE induced an inward current that reversed near the predicted K+ equilibrium potential and was not affected by changes in intracellular Cl- concentration. In the presence of an alpha 1 antagonist, NE induced an inwardly rectifying current at potentials negative to approximately -70 mV that did not reverse (between -130 and -60 mV), characteristics similar to the hyperpolarization-activated current (lh). However, the depolarizing action of NE is attributable primarily to the alpha 1 adrenoceptor-mediated decrease in K+ conductance and not the beta adrenoceptor-dependent increase in lh. These results provide evidence that NE increases action potential-dependent IPSCs in pyramidal neurons by depolarizing surrounding inhibitory interneurons. This potent excitatory action of NE on multiple classes of hippocampal interneurons may contribute to the NE-induced decrease in the spontaneous activity of pyramidal neurons and the antiepileptic effects of NE observed in vivo.

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alpha(1)-Adrenergic receptors (ARs) are not well defined in the central nervous system. The particular cell types and areas that express these receptors are uncertain because of the lack of high avidity antibodies and selective ligands. We have developed transgenic mice that either systemically overexpress the human alpha(1A)-AR subtype fused with the enhanced green fluorescent protein (EGFP) or express the EGFP protein alone under the control of the mouse alpha(1A)-AR promoter. We confirm our transgenic model against the alpha(1A)-AR knockout mouse, which expresses the LacZ gene in place of the coding region for the alpha(1A)-AR. By using these models, we have now determined cellular localization of the alpha(1A)-AR in the brain, at the protein level. The alpha(1A)-AR or the EGFP protein is expressed prominently in neuronal cells in the cerebral cortex, hippocampus, hypothalamus, midbrain, pontine olivary nuclei, trigeminal nuclei, cerebellum, and spinal cord. The types of neurons were diverse, and the alpha(1A)-AR colocalized with markers for glutamic acid decarboxylase (GAD), gamma-aminobutyric acid (GABA), and N-methyl-D-aspartate (NMDA) receptors. Recordings from alpha(1A)-AR EGFP-expressing cells in the stratum oriens of the hippocampal CA1 region confirmed that these cells were interneurons. We could not detect expression of the alpha(1A)-AR in mature astrocytes, oligodendrocytes, or cerebral blood vessels, but we could detect the alpha(1A)-AR in oligodendrocyte progenitors. We conclude that the alpha(1A)-AR is abundant in the brain, expressed in various types of neurons, and may regulate the function of oligodendrocyte progenitors, interneurons, GABA, and NMDA receptor containing neurons.