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.

Parkinson's disease is the most common serious movement disorder in the world, affecting about 1% of adults older than 60 years. The disease is attributed to selective loss of neurons in the substantia nigra, and its cause is enigmatic in most individuals. Symptoms of Parkinson's disease respond in varying degrees to drugs, and surgery offers hope for patients no longer adequately controlled in this manner. The high prevalence of the disease, and important advances in its management, mean that generalists need to have a working knowledge of this disorder. This Seminar covers the basics, from terminology to aspects of diagnosis, treatment, and pathogenesis.

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Little is known about the expression and possible functions of unopposed gap junction hemichannels in the brain. Emerging evidence suggests that gap junction hemichannels can act as stand-alone functional channels in astrocytes. With immunocytochemistry, dye uptake, and HPLC measurements, we show that astrocytes in vitro express functional hemichannels that can mediate robust efflux of glutamate and aspartate. Functional hemichannels were confirmed by passage of extracellular lucifer yellow (LY) into astrocytes in nominal divalent cation-free solution (DCFS) and the ability to block this passage with gap junction blocking agents. Glutamate/aspartate release (or LY loading) in DCFS was blocked by multivalent cations (Ca2+, Ba2+, Sr2+, Mg2+, and La3+) and by gap junction blocking agents (carbenoxolone, octanol, heptanol, flufenamic acid, and 18alpha-glycyrrhetinic acid) with affinities close to those reported for blockade of gap junction intercellular communication. Glutamate efflux via hemichannels was also accompanied by greatly reduced glutamate uptake. Glutamate release in DCFS, however, was not significantly mediated by reversal of the glutamate transporter: release did not saturate and was not blocked by glutamate transporter blockers. Control experiments in DCFS precluded glutamate release by volume-sensitive anion channels, P2X7 purinergic receptor pores, or general purinergic receptor activation. Blocking intracellular Ca2+ mobilization by BAPTA-AM or thapsigargin did not inhibit glutamate release in DCFS. Divalent cation removal also induced glutamate release from intact CNS white matter (acutely isolated optic nerve) that was blocked by carbenoxolone, suggesting the existence of functional hemichannels in situ. Our results indicated that astrocyte hemichannels could influence CNS levels of extracellular glutamate with implications for normal and pathological brain function.

White matter of the mammalian CNS suffers irreversible injury when subjected to anoxia/ischemia. However, the mechanisms of anoxic injury in central myelinated tracts are not well understood. Although white matter injury depends on the presence of extracellular Ca2+, the mode of entry of Ca2+ into cells has not been fully characterized. We studied the mechanisms of anoxic injury using the in vitro rat optic nerve, a representative central white matter tract. Functional integrity of the nerves was monitored electrophysiologically by quantitatively measuring the area under the compound action potential, which recovered to 33.5 +/- 9.3% of control after a standard 60 min anoxic insult. Reducing Na+ influx through voltage-gated Na+ channels during anoxia by applying Na+ channel blockers (TTX, saxitoxin) substantially improved recovery; TTX was protective even at concentrations that had little effect on the control compound action potential. Conversely, increasing Na+ channel permeability during anoxia with veratridine resulted in greater injury. Manipulating the transmembrane Na+ gradient at various times before or during anoxia greatly affected the degree of resulting injury; applying zero-Na+ solution (choline or Li+ substituted) before anoxia significantly improved recovery; paradoxically, the same solution applied after the start of anoxia resulted in more injury than control. Thus, ionic conditions that favored reversal of the normal transmembrane Na+ gradient during anoxia promoted injury, suggesting that Ca2+ loading might occur via reverse operation of the Na+)-Ca2+ exchanger. Na(+)-Ca2+ exchanger blockers (bepridil, benzamil, dichlorobenzamil) significantly protected the optic nerve from anoxic injury. Together, these results suggest the following sequence of events leading to anoxic injury in the rat optic nerve: anoxia causes rapid depletion of ATP and membrane depolarization leading to Na+ influx through incompletely inactivated Na+ channels. The resulting rise in the intracellular [Na+], coupled with membrane depolarization, causes damaging levels of Ca2+ to be admitted into the intracellular compartment through reverse operation of the Na(+)-Ca2+ exchanger. These observations emphasize that differences in the pathophysiology of gray and white matter anoxic injury are likely to necessitate multiple strategies for optimal CNS protection.

The brain contains glycogen but at low concentration compared with liver and muscle. In the adult brain, glycogen is found predominantly in astrocytes. Astrocyte glycogen content is modulated by a number of factors including some neurotransmitters and ambient glucose concentration. Compelling evidence indicates that astrocyte glycogen breaks down during hypoglycemia to lactate that is transferred to adjacent neurons or axons where it is used aerobically as fuel. In the case of CNS white matter, this source of energy can extend axon function for 20 min or longer. Likewise, during periods of intense neural activity when energy demand exceeds glucose supply, astrocyte glycogen is degraded to lactate, a portion of which is transferred to axons for fuel. Astrocyte glycogen, therefore, offers some protection against hypoglycemic neural injury and ensures that neurons and axons can maintain their function during very intense periods of activation. These emerging principles about the roles of astrocyte glycogen contradict the long held belief that this metabolic pool has little or no functional significance.

CNS glycogen, contained predominantly in astrocytes, can be converted to a monocarboxylate and transported to axons as an energy source during aglycaemia. We analysed glycogen regulation and the role of glycogen in supporting neural activity in adult mouse optic nerve, a favourable white matter preparation. Axon function was quantified by measuring the compound action potential (CAP) area. During aglycaemia, axon function persisted for 20 min, then declined in conjunction with glycogen content. Lactate fully supported CAPs in the absence of glucose, but was unable to sustain glycogen content; thus, axon failure occurred rapidly when lactate was withdrawn. Glycogen content in the steady state was directly proportional to bath glucose concentration. Increasing [K + ] o to 10 mM caused a rapid decrease in glycogen content. Latency to onset of CAP failure during aglycaemia was directly proportional to glycogen content and varied from about 2 to 30 min. Intense neural activity reduced glycogen content in the presence of 10 mM bath glucose and CAP area gradually declined. CAP area declined more rapidly during high frequency stimulation if monocarboxylate transport was inhibited. This suggested that astrocytic glycogen was broken down to a monocarboxylate(s) that was used by rapidly discharging axons. Likewise, depleting glycogen by brief periods of high frequency axon stimulation accelerated onset of CAP decline during aglycaemia. In summary, these experiments indicated that glycogen content was under dynamic control and that glycogen was used to support the energy needs of CNS axons during both physiological as well as pathological processes.

We tested the hypothesis that astrocytic glycogen sustains axon function during and enhances axon survival after 60 min of glucose deprivation. Axon function in the rat optic nerve (RON), a CNS white matter tract, was monitored by measuring the area of the stimulus-evoked compound action potential (CAP). Switching to glucose-free artificial CSF (aCSF) had no effect on the CAP area for ϳ30 min, after which the CAP rapidly failed. Exposure to glucose-free aCSF for 60 min caused irreversible injury, which was measured as incomplete recovery of the CAP. Glycogen content of the RON fell to a low stable level 30 min after glucose withdrawal, compatible with rapid use in the absence of glucose. An increase of glycogen content induced by high-glucose pretreatment increased the latency to CAP failure and improved CAP recovery. Conversely, a decrease of glycogen content induced by norepinephrine pretreatment decreased the latency to CAP failure and reduced CAP recovery. To determine whether lactate represented the fuel derived from glycogen and shuttled to axons, we used the lactate transport blockers quercetin, ␣-cyano-4-hydroxycinnamic acid (4-CIN), and p-chloromercuribenzene sulfonic acid ( pCMBS). All transport blockers, when applied during glucose withdrawal, decreased latency to CAP failure and decreased CAP recovery. The inhibitors 4-CIN and pCMBS, but not quercetin, blocked lactate uptake by axons. These results indicated that, in the absence of glucose, astrocytic glycogen was broken down to lactate, which was transferred to axons for fuel. Key words: astrocytes; ␣-cyano-4-hydroxycinnamate; glucose; hypoglycemia; lactate; p-chloromercuribenzene sulfonic acid; quercetin; rat optic nerveThe function of brain glycogen is not well understood. Glycogen turns over rapidly in the brain, however, and turnover is enhanced when adjacent neural activity is increased (Orkand et al., 1973;Pentreath and Kai-Kai, 1982;Swanson et al., 1992). It is appealing to imagine that glycogen might serve to provide fuel to the brain when glucose is in short supply. Indeed, astrocytic glycogen in vitro is degraded rapidly when glucose is withdrawn (Dringen et al., 1993), and glycogen falls rapidly in vivo during ischemia, with a time course that is closely related to the depletion of ATP and the accumulation of lactate (Swanson et al., 1989a). These observations are consistent with the action of glycogen as a fuel source during glucose shortage, but they do not prove this hypothesis. Glycogen content varies by a factor of two or more between brain regions [it is highest in the brainstem and cerebellum and lowest in the striatum and white matter (Swanson et al., 1989a)]. Energy metabolism also varies significantly between different brain regions (Sokoloff et al., 1977). Therefore, glycogen could be more protective against glucose depletion in some areas than in others.Given all of the above, it is natural to wonder whether glycogen can enhance the survival and function of brain tissue in the absence of glucose. Surprisingly, only a single st...