The conservation of sleep across all animal species suggests that sleep serves a vital function. We here report that sleep has a critical function in ensuring metabolic homeostasis. Using real-time assessments of tetramethylammonium diffusion and two-photon imaging in live mice, we show that natural sleep or anesthesia are associated with a 60% increase in the interstitial space, resulting in a striking increase in convective exchange of cerebrospinal fluid with interstitial fluid. In turn, convective fluxes of interstitial fluid increased the rate of β-amyloid clearance during sleep. Thus, the restorative function of sleep may be a consequence of the enhanced removal of potentially neurotoxic waste products that accumulate in the awake central nervous system.

Defining the microanatomic differences between the human brain and that of other mammals is key to understanding its unique computational power. Although much effort has been devoted to comparative studies of neurons, astrocytes have received far less attention. We report here that protoplasmic astrocytes in human neocortex are 2.6-fold larger in diameter and extend 10-fold more GFAP (glial fibrillary acidic protein)-positive primary processes than their rodent counterparts. In cortical slices prepared from acutely resected surgical tissue, protoplasmic astrocytes propagate Ca 2ϩ waves with a speed of 36 m/s, approximately fourfold faster than rodent. Human astrocytes also transiently increase cystosolic Ca 2ϩ in response to glutamatergic and purinergic receptor agonists. The human neocortex also harbors several anatomically defined subclasses of astrocytes not represented in rodents. These include a population of astrocytes that reside in layers 5-6 and extend long fibers characterized by regularly spaced varicosities. Another specialized type of astrocyte, the interlaminar astrocyte, abundantly populates the superficial cortical layers and extends long processes without varicosities to cortical layers 3 and 4. Human fibrous astrocytes resemble their rodent counterpart but are larger in diameter. Thus, human cortical astrocytes are both larger, and structurally both more complex and more diverse, than those of rodents. On this basis, we posit that this astrocytic complexity has permitted the increased functional competence of the adult human brain.

Objective In the brain, protein waste removal is partly performed by paravascular pathways that facilitate convective exchange of water and soluble contents between cerebrospinal and interstitial fluids. Several lines of evidence suggest that bulk flow drainage via the glymphatic system is driven by cerebrovascular pulsation, and is dependent on astroglial water channels that line paravascular cerebrospinal fluid (CSF) pathways. The Objective of this study was to evaluate whether the efficiency of CSF-ISF exchange and interstitial solute clearance is impaired in the aging brain. Methods CSF-ISF exchange was evaluated by in vivo and ex vivo fluorescence microscopy while interstitial solute clearance was evaluated by radio-tracer clearance assays in young (2–3 month), middle age (10–12 month) and old (18–20 month) wild type mice. The relationship between age-related changes in the expression of the astrocytic water channel aquaporin-4 (AQP4) and changes in glymphatic pathway function were evaluated by immunofluorescence. Results Advancing age was associated with a dramatic decline in the efficiency of exchange between the subarachnoid CSF and the brain parenchyma. Relative to the young, clearance of intraparechamally injected amyloid β was impaired by 40% in the old mice. A 27% reduction in the vessel wall pulsatility of intracortical arterioles and widespread loss of perivascular AQP4 polarization along the penetrating arteries accompanied the decline in CSF-ISF exchange. Interpretation We propose that impaired glymphatic clearance contributes to cognitive decline among the elderly and may represent a novel therapeutic target for the treatment of neurodegenerative diseases associated with accumulation of mis-folded protein aggregates.

Hypersynchronous neuronal firing is a hallmark of epilepsy, but the mechanisms underlying simultaneous activation of multiple neurons remains unknown. Epileptic discharges are in part initiated by a local depolarization shift that drives groups of neurons into synchronous bursting. In an attempt to define the cellular basis for hypersynchronous bursting activity, we studied the occurrence of paroxysmal depolarization shifts after suppressing synaptic activity by TTX and voltage-gated Ca 2+ channel blockers. Here we report that paroxysmal depolarization shifts can be initiated by release of glutamate from extrasynaptic sources or by photolysis of caged Ca 2+ in astrocytes. Two-photon imaging of live exposed cortex revealed that several anti-epileptics, including valproate, gabapentin and phenytoin, reduced the ability of astrocytes to transmit Ca 2+ signaling. Our results reveal an unanticipated key role for astrocytes in seizure activity. As such, these findings identify astrocytes as a proximal target for the treatment of epileptic disorders.Epilepsy is a neurological disorder in which normal brain function is disrupted as a consequence of intensive burst activity from groups of neurons1. Epilepsies result from long-lasting plastic changes in the brain affecting the expression of receptors and channels, and involve sprouting and reorganization of synapses, as well as reactive gliosis2 ,3 . Several lines of evidence suggest a key role of glutamate in the pathogenesis of epilepsy. Local or systemic administration of glutamate agonists triggers excessive neuronal firing, whereas glutamate receptor (GluR) antagonists have anticonvulsant properties 4 . The observation that astrocytes release glutamate via a regulated Ca 2+ dependent mechanism 5-8 prompted us to hypothesize that glutamate released by astrocytes plays a causal role in synchronous firing of large populations of neurons.Paroxysmal depolarization shifts (PDSs) are abnormal prolonged depolarizations with repetitive spiking and are reflected as interictal discharges in the electroencephalogram 2, 3. We report here that glutamate released by astrocytes can trigger PDSs in several models of experimental seizure. A unifying feature of seizure activity was its consistent association Corresponding author: Guo-Feng Tian (Guo-Feng_Tian@URMC.Rochester.edu). * These authors contributed equally to this work. NIH Public Access RESULTS PDSs can be triggered by an action potential-independent mechanismTo examine the cellular mechanism underlying PDSs, we patched CA1 pyramidal neurons in rat hippocampal slices exposed to 4-aminopyridine (4-AP). 4-AP is a K + channel blocker that induces intense electrical discharges in slices9 and seizure activity in experimental animals 10 . All slices exposed to 4-AP (61 slices from 23 rats) exhibited epileptiform bursting activity expressed as transient episodes of neuronal depolarizations eliciting trains of action potentials (Fig. 1a). Bath application of TTX promptly eliminated neuronal firing (Fig. 1b). Unexpectedly, the...

Although astrocytes are the most abundant cell type in the brain, evidence for their activation during physiological sensory activity is lacking. Here we show that whisker stimulation evokes increases in astrocytic cytosolic calcium (Ca(2+)) within the barrel cortex of adult mice. Increases in astrocytic Ca(2+) were a function of the frequency of stimulation, occurred within several seconds and were inhibited by metabotropic glutamate receptor antagonists. To distinguish between synaptic input and output, local synaptic activity in cortical layer 2 was silenced by iontophoresis of AMPA and NMDA receptor antagonists. The antagonists did not reduce astrocytic Ca(2+) responses despite a marked reduction in excitatory postsynaptic currents in response to whisker stimulation. These findings indicate that astrocytes respond to synaptic input, by means of spillover or ectopic release of glutamate, and that increases in astrocytic Ca(2+) occur independently of postsynaptic excitatory activity.