Incidentally neurons do express glycogen synthase, which however is kept inactive by proteasomal-dependent mechanisms; when articially activated it causes neuronal apoptosis (1823). Physiological activity affects glycogen levels in astrocytes; it is controlled by norepinephrine, serotonin, and VIP. Glycogen increases in sleep and anesthesia and decreases in awake and active brain, while glycogen mobilization helps to sustain neuronal activity (127, 246, 1047). Glycogenolysis is also critically important for maintenance of LTP (1706) and for memory consolidation (573). Are these physiological effects associated only with the glycogen function as an energy reserve? Total amount of glycogen in the brain is very low, being in a range of 0.51.5 g, which is 100 times less than in the liver (245). This amount of glycogen cannot sustain the brain activity for any reasonable time, and it might e

In the hippocampus the number of enwrapped synapses varies between 60 and 90% (1802, 1888). In the cerebellum, complex appendages emanating from the processes of Bergmann glial cells (FIGURE 30) cover almost 90% of synapses formed by climbing bers and ~65% of synapses formed by parallel bers on the Purkinje neuron (609, 1903). Degree of coverage also varies between different types of synapses. In the hippocampus, astrocytes cover ~50% of small macular synapses, while ~90% of large mushroom spines and perforated synapses are enwrapped with astrocytic processes (1888). In organotypic slices, 97% of complex synapses and 78% of simple synapses have astroglial coverage (1024). At the synapses formed by mossy bers on granule neurons, coverage of individual synapses is ~15% (1128, 1711); however, astrocytes also cover whole glomeruli thus isolating them (1451).

The perisynaptic astroglial processes covering synapses are exceedingly thin, their proles being on average (cid:4)200 nm (and often (cid:4)100 nm) in diameter, although they have an exceedingly large surface (1451). The perisynaptic astroglial membrane represents the major part of the cell surface area, accounting for ~80% of the total astroglial plasmalemma, while contributing only a minor fraction (~4 10%) to the cellular volume; as a result, astroglial perisynaptic processes have an extremely high surface to volume ratio (~25 (cid:11)m(cid:3)1; Ref. 608). Perisynaptic astroglial membrane is densely packed with receptors, ion channels, and multiple transporters; ion uxes are particularly important for generation of local Ca2(cid:2) and Na(cid:2) signals, which couple homeostatic astroglial cascades with neuronal activity (1813). The perisynaptic processes are plastic structures; for example, they rapidly ensheath dendrites and synapses in neurons newly generated in denta

The metabolic role of astroglia and distribution of energy consumption between neurons and glia are far form being resolved. Futhermore, the metabolic plasticity of the brain may dynamically affect neuronal-glial energetic balance and lactate turnover. F. Regulation of Synaptic Connectivity and Synaptic Transmission 1. Astroglial synaptic coverage