Astrocytic clasmatodendrosis in hippocampal organ culture

Mechanisms by which astrocytes are irreversibly injured from ischemic brain injury remain incompletely defined. More than 90 years ago Alzheimer showed that astrocytes lose their distal processes (i.e., undergo "clasmatodendrosis") when irreversibly injured by a reduction in blood flow, a process shown by Friede and van Houten (1961) to be due to energy failure and acidosis. Such alterations in astrocytic morphology can relate directly to changes in cell function. However, astrocytic clasmatodendrosis has largely been lost to the modern literature, perhaps because of a inability to study it under controlled conditions. In the present study, novel four-dimensional (4D)and digital deblurring imaging of glial fibrillary acidic protein (GFAP) immunostaining changes in hippocampal organ cultures (HOTCs) were used to establish an in vitro model of astrocytic clasmatodendrosis. Also, astrocytes in primary culture were transfected with green fluorescent protein (GFP) to show the occurrence of clasmatodendrosis via a parallel and separate means. In HOTCs, a significant reduction in astrocytic process length occurred 15 min (and remained for 60 min) after exposure to acidic Ringer's and mitochondrial inhibition in the pyramidal cell body layer. Time-lapsed images of primary cultures showed thinning of cell processes within 15 min of exposure to acidic Ringer's and mitochondrial inhibition. Distal processes subsequently broke away but retained their fluorescence for minutes before disintegrating along with their parent cell bodies. This report shows the spatiotemporal occurrence of clasmatodendrosis in astrocytes of HOTCs closely parallels that seen in vivo. Thus, HOTCs, where microenvironmental conditions can be controlled and single, identified cells can be followed in space and time, can be applied to study the interrelations between energy metabolism and pH that result in clasmatodendrosis.

Prostaglandin E receptor subtypes in cultured rat microglia and their role in reducing lipopolysaccharide-induced interleukin-1beta production

Prostaglandins (PGs) are potent modulators of brain function under normal and pathological conditions. The diverse effects of PGs are due to the various actions of specific receptor subtypes for these prostanoids. Recent work has shown that PGE2, while generally considered a proinflammatory molecule, reduces microglial activation and thus has an antiinflammatory effect on these cells. To gain further insight to the mechanisms by which PGE2 influences the activation of microglia, we investigated PGE receptor subtype, i.e., EP1, EP2, EP3, and EP4, expression and function in cultured rat microglia. RT-PCR showed the presence of the EP1 and EP2 but not EP3 and EP4 receptor subtypes. Sequencing confirmed their identity with previously published receptor subtypes. PGE2 and the EP1 agonist 17-phenyl trinor PGE2 but not the EP3 agonist sulprostone elicited reversible intracellular [Ca2+] increases in microglia as measured by fura-2. PGE2 and the EP2/EP4-specific agonists 11-deoxy-PGE1 and 19-hydroxy-PGE2 but not the EP4-selective agonist 1-hydroxy-PGE1 induced dose-dependent production of cyclic AMP (cAMP). Interleukin (IL)-1beta production, a marker of activated microglia, was also measured following lipopolysaccharide exposure in the presence or absence of the receptor subtype agonists. PGE2 and the EP2 agonists reduced IL-1beta production. IL-1beta production was unchanged by EP1, EP3, and EP4 agonists. The adenylyl cyclase activator forskolin and the cAMP analogue dibutyryl cAMP also reduced IL-1beta production. Thus, the inhibitory effects of PGE2 on microglia are mediated by the EP2 receptor subtype, and the signaling mechanism of this effect is likely via cAMP. These results show that the effects of PGE2 on microglia are receptor subtype-specific. Furthermore, they suggest that specific and selective manipulation of the effects of PGs on microglia and, as a result, brain function may be possible.

Prostaglandin E2 and 4-aminopyridine prevent the lipopolysaccharide-induced outwardly rectifying potassium current and interleukin-1beta production in cultured rat microglia

Brain inflammation includes microglial activation and enhanced production of diffusible chemical mediators, including prostaglandin E2. Prostaglandin E2 is generally considered a proinflammatory molecule, but it also promotes neuronal survival and down-regulates some aspects of microglial activation. It remains unknown, however, if and how prostaglandin E2 prevents microglial activation. In primary culture, microglial activation is predicted by a characteristic pattern of whole-cell potassium currents and interleukin-1beta production. We investigated if prostaglandin E2 could alter these currents and, if so, whether these currents are necessary for microglial activation. Microglia were isolated from mixed cell cultures prepared from neonatal rat brains and exposed to 0-10 microM prostaglandin E2 and lipopolysaccharide for 24 h. Currents were elicited by using standard patch-clamp technique, and interleukin-1beta production was measured by ELISA. Peak outward current densities in microglia treated with lipopolysaccharide plus prostaglandin E2 (10 nM) were reduced significantly from those of cells treated with lipopolysaccharide alone. Prostaglandin E2 and 4-aminopyridine (a blocker of outward potassium currents) also significantly reduced interleukin-1beta production. Thus, although prostaglandin E2 is classified generally as a proinflammatory chemical, it has complex roles in brain inflammation that include preventing microglial activation, perhaps by reducing the outward potassium current.

Calcium waves precede electrophysiological changes of spreading depression in hippocampal organ cultures

Although intercellular Ca2+ waves resemble spreading depression (SD) and occur in hippocampal organ cultures (HOTCs), SD has not been reported in these cultures. Accordingly, electrophysiological and Ca2+ imaging techniques were used to examine potential interrelations between Ca2+ waves and electrophysiological changes of SD. Our results show, for the first time, that HOTCs can support SD. Furthermore, two distinct Ca2+ waves were found to precede SD. The first traveled >100 micron/sec along the pyramidal cell dendritic layer. The second subsequently traveled mostly perpendicular to the pyramidal cell layer from CA3 (or CA1) but also in all directions from its area of initiation. This second, slower wave spread with the interstitial DC change of SD at millimeters per minute but always ahead of it by 6-16 sec. Heptanol, which uncouples gap junctions, blocked both of these Ca2+ waves and SD. Thus, two types of Ca2+ waves occur with the initiation and propagation of SD. The first might reflect interneuronal changes linked by gap junctions, whereas the second might stem from interastrocyte changes linked via similar connections. Because individual cells can be followed in space and time for protracted periods in HOTCs, this preparation may be ideal for studies designed to explore not only the mechanisms of SD but also the long-term consequences of SD, such as ischemic tolerance.

Cytoskeletal actin gates a Cl- channel in neocortical astrocytes

Increases in astroglial Cl- conductance accompany changes in cell morphology and disassembly of cytoskeletal actin, but Cl- channels underlying these conductance increases have not been described. We characterize an outwardly rectifying Cl- channel in rodent neocortical cultured astrocytes and describe how cell shape and cytoskeletal actin modulate channel gating. In inside-out patch-clamp recordings from cultured astrocytes, outwardly rectifying Cl- channels either were spontaneously active or inducible in quiescent patches by depolarizing voltage steps. Average single-channel conductance was 36 pS between -60 and -80 mV and was 75 pS between 60 and 80 mV in symmetrical (150 mM NaCl) solutions. The permeability ratio (PNa/PCl) was 0.14 at lower ionic strength but increased at higher salt concentrations. Both ATP and 4, 4-diisothiocyanostilbene-2,2'-disulfonic acid produced a flicker block, whereas Zn2+ produced complete inhibition of channel activity. The frequency of observing both spontaneous and inducible Cl- channel activity was markedly higher in stellate than in flat, polygonally shaped astrocytes. In addition, cytoskeletal actin modulated channel open-state probability (PO) and conductance at negative membrane potentials, controlling the degree of outward rectification. Direct application of phalloidin, which stabilizes actin, preserved low PO and promoted lower conductance levels at negative potentials. Lower PO also was induced by direct application of polymerized actin. The actions of phalloidin and actin were reversed by coapplication of gelsolin and cytochalasin D, respectively. These results provide the first report of an outwardly rectifying Cl- channel in neocortical astrocytes and demonstrate how changes in cell shape and cytoskeletal actin may control Cl- conductance in these cells.

Neuronal nitric oxide synthase expression is induced in neocortical astrocytes after spreading depression

Spreading depression (SD) confers either increased susceptibility to ischemic injury or a delayed protection. Because nitric oxide modulates ischemic injury, we investigated if altered expression of nitric oxide synthase (NOS) by SD could account for the effect of SD on ischemia. Furthermore, the identity of cells expressing NOS after SD is important, since SD results in heterogeneous, cell type-specific changes in intracellular environment, which can control NOS activity. Immunohistochemical, computer-based image analyses and Western blotting show that the number of neuronal NOS (nNOS)-positive cells in the somatosensory cortex was significantly increased at 6 hours and 3 days after SD (P < 0.05 and 0.01, respectively), whereas inducible NOS expression remained unchanged. Double-labeling of nNOS and glial fibrillary acidic protein identified these nNOS-positive cells as astrocytes. The effect of altered NO production on induced nNOS expression was examined by treating rats with sodium nitroprusside or NA-nitro-L-arginine methyl ester (LNAM) during SD. Increased nNOS expression was prevented by sodium nitroprusside and phenylephrine or phenylephrine alone, but not LNAM. Because SD increased astrocytic nNOS expression at time points correlating with both ischemic hypersensitivity and ischemic tolerance, the ability of SD to modulate ischemic injury must be complex, perhaps involving NOS but other factors as well.

Astroglial acid-base dynamics in hyperglycemic and normoglycemic global ischemia

Biochemical, histological, and physiological evidence suggest strongly that astrocytes may either defend or damage brain tissue, depending on the brain carbohydrate content preceding global ischemia (28,43). This paper will first review the concept of acidosis in ischemia and the possible role of severe, compartmentalized astrocytic acidosis in pan necrosis. Results are then presented demonstrating that astrocytes are also capable of maintaining an alkaline intracellular pH (pHi) during normoglycemic global ischemia. Mechanisms underlying depolarization-dependent astroglial alkalosis are then reviewed. Recent experiments indicate that bicarbonate (HCO3-) transport is a major mechanism by which astroglia not only alkalinize their interior but also acidify the interstitium. Maintenance of alkalosis during normoglycemic ischemia supports the hypothesis that astroglial HCO3- transport might ultimately protect neurons from excitotoxicity in ischemia without infarction (17). Inhibition of astroglial HCO3- transport may be a critical and requisite event, ultimately leading to compartmentalized astroglial acidosis and irreversible injury to all cell types.

Reactive astrocytosis from excitotoxic injury in hippocampal organ culture parallels that seen in vivo

Reactive astrocytes influence not only the severity of brain injury, but also the capacity of brain to reshape itself with learning. Mechanisms responsible for astrogliosis remain unknown but might be best studied in vitro, where improved access and visualization permit application of modern molecular and cellular techniques. We have begun to explore whether gliosis might be studied in hippocampal organotypic cultures (HOTCs), where potential cell-to-cell interactions are preserved and the advantages of an in vitro preparation are still realized. Following HOTC exposure to N-methyl-D-aspartate (NMDA), dose-dependent changes occurred in the optical density (OD) values for the astrocytic immunohistochemical [immunostaining (IS)] markers glial fibrillary acidic protein (GFAP) and vimentin. Exposure of HOTCs to NMDA (10 microM) caused selective death in the CA1 hippocampal region and the dentate gyrus. It also significantly increased GFAP IS and vimentin IS OD values in these regions. Increased GFAP IS and vimentin IS OD values were also seen in CA3, a hippocampal region that displayed no cell death. Light microscopic examination revealed hypertrophied GFAP and vimentin IS cells, characteristic of reactive astrocytes. Cellular proliferation, as assessed by proliferating cell nuclear antigen IS, was also significantly increased in all three of these hippocampal regions. In contrast, exposure of HOTCs to a noninjurious level of NMDA (1 microM) caused only minor changes in GFAP IS and vimentin IS OD values but a significantly reduced cellular proliferation in all HOTC regions. These results show that reactive astrocytosis from excitotoxic injury of HOTC parallels changes seen in vivo after global ischemia. Furthermore, since resting astroglia within HOTCs are also similar to their counterparts in vivo, HOTCs may be used to examine mechanisms by which these cells are transformed into reactive species within tissue that resembles intact brain.

Long-term elevation of cyclooxygenase-2, but not lipoxygenase, in regions synaptically distant from spreading depression

Eicosanoids, produced from arachidonic acid by cyclooxygenases (COXs) and lipoxygenases (LIPOXs), are involved in numerous brain processes. To explore if brief and noninjurious stimuli chronically alter expression of these enzymes, we examined the induction of COX-2 and LIPOX expression following unilateral neocortical spreading depression (SD). Expression was examined over time and in regions not experiencing SD (hippocampus) but synaptically connected to the site of stimulation (cortex). One hundred six male Wistar rats had SD induced via microinjection of 0.5 M KCl (0.5 M NaCl for sham) into left parietal cortex every 9 minutes for 1 or 3 hours. One hour before SD some animals received dexamethasone (Dex), mepacrine (Mep), indomethacin (Indo), nordihydroguaiaretic acid (Ndga), phenylephrine (Pe), sodium nitroprusside (Snp) with Pe, or N omega-nitro-L-arginine methyl ester (Lnam). Animals survived for 0, 3, or 6 hours, or 1, 2, 3, 7, 14, 21, or 28 days. Brains were processed immunohistochemically for COX-2 and LIPOX, and the optical density (OD) of the left and right cortex, dentate gyrus (DG), CA3, and CA1 immunoreactivity (IR) were measured. Induction was expressed as the log of left divided by right side OD for each region. COX-2 IR in the left cortex was elevated rapidly and was sustained for 21 days following SD. COX-2 IR was also elevated in the ipsilateral hippocampus not experiencing SD, with the rank order of induction as follows: DG > CA3 > CA1. Dex, Snp, and/or Pe significantly reduced the induction of COX-2. No changes in LIPOX IR were observed. These results show that long-term changes in COX-2 expression are induced by SD and these changes decrease with synaptic distance. Benign stimuli increase COX-2 expression and thus may influence brain function for extended periods and at distant locations.

Eicosanoids and nitric oxide influence induction of reactive gliosis from spreading depression in microglia but not astrocytes

Microglia and astrocytes are transformed into reactive glia (RG) by brain disease and normal function. Eicosanoids and nitric oxide (NO), two intercellular mediators, may influence gliosis. We investigated how drugs that alter production of these paracrine signals effect induction of glial reactivity from spreading depression. Unilateral (left) neocortical spreading depression was induced in 95 halothane anesthetized rats by intracortical injections of 0.5 M KCl, with or without drug treatment (five animals/group). Immunohistochemical staining (IS) intensity using the OX-42 and anti-glial fibrillary acidic protein (GFAP) antibodies determined reactivity in microglia and astrocytes, respectively. After 3 days, brains were processed for OX-42 and GFAP-IS and mean optical densities (OD) of IS were measured. Average OD's (for OX-42) and the log ratio (left/right) of OD's (OX-42 and GFAP) were compared to normal animals. Spreading depression induced significant log ratios for both OX-42- and GFAP-IS (P's < 0.01). However, dexamethasone (a glucocorticoid), nordihydroguaiaretic acid (a lipoxygenase inhibitor), and nitroprusside (a NO donor) prevented significant left sided and log ratio OD values for microglia (P's > 0.05). L-Name, a NO synthase inhibitor, caused significant increases in left and right OD's for microglia (P's < 0.05). Mepacrine, a phospholipase A2 inhibitor, Indomethacin, a cyclooxygenase inhibitor, and phenylephrine, an adrenergic agonist, did not prevent induction of significant OX-42 log ratios (P's < 0.01, 0.05, 0.01), and resulted in increases in left side OD's (P's < 0.01, 0.05, 0.05). Significant GFAP log ratios occurred after spreading depression in all drug groups, P's < 0.01. Thus, induction of reactivity in microglia is more sensitive to eicosanoids and NO than in astrocytes.

Whole-cell chloride currents in rat astrocytes accompany changes in cell morphology

Astrocytes can change shape dramatically in response to increased physiological and pathological demands, yet the functional consequences of morphological change are unknown. We report the expression of Cl- currents after manipulations that alter astrocyte morphology. Whole-cell Cl- currents were elicited after (1) rounding up cells by brief exposure to trypsin; (2) converting cells from a flat polygonal to a process-bearing (stellate) morphology by exposure to serum-free Ringer's solution; and (3) swelling cells by exposure to hypo-osmotic solution. Zero-current potentials approximated the Nernst for Cl-, and rectification usually followed that predicted by the constant-field equation. We observed heterogeneity in the activation and inactivation kinetics, as well as in the relative degree of outward versus inward rectification. Cl- conductances were inhibited by 4, 4-diisothiocyanostilbene-2,2'-disulfonic acid (200 microM) and by Zn2+ (1 mM). Whole-cell Cl- currents were not expressed in cells without structural change. We investigated whether changes in cytoskeletal actin accompanying changes in astrocytic morphology play a role in the induction of shape-dependent Cl- currents. Cytochalasins, which disrupt actin polymers by enhancing actin-ATP hydrolysis, elicited whole-cell Cl- conductances in flat, polygonal astrocytes. In stellate cells, elevated intracellular Ca2+ (2 microM), which can depolymerize actin, enhanced Cl- currents, and high intracellular ATP (5 mM), required for repolymerization, reduced Cl- currents. Modulation of Cl- current by Ca2+ and ATP was blocked by concurrent whole-cell dialysis with phalloidin and DNase, respectively. Phalloidin stabilizes actin polymers and DNase inhibits actin polymerization. Dialysis with phalloidin also prevented hypo-osmotically activated Cl- currents. These results demonstrate how the expression of astrocyte Cl- currents can be dependent on cell morphology, the structure of actin, Ca2+ homeostasis, and metabolism.

Characterization of inducible cyclooxygenase in rat brain

Considerable debate exists regarding the cellular source of prostaglandins in the mammalian central nervous system (CNS). At least two forms of prostaglandin endoperoxide synthase, or cyclooxygenase (COX), the principal enzyme in the biosynthesis of these mediators, are known to exist. Both forms have been identified in the CNS, but only the distribution of COX 1 has been mapped in detail. In this study, we used Western blot analysis and immunohistochemistry to describe the biochemical characterization and anatomical distribution of the second, mitogen-inducible form of this enzyme, COX 2 in the rat brain. COX 2-like immunoreactive (COX 2-ir) staining occurred in dendrites and cell bodies of neurons, structures that are typically postsynaptic. It was noted in distinct portions of specific cortical laminae and subcortical nuclei. The distribution in the CNS was quite different from COX 1. COX 2-ir neurons were primarily observed in the cortex and allocortical structures, such as the hippocampal formation and amygdala. Within the amygdala, neurons were primarily observed in the caudal and posterior part of the deep and cortical nuclei. In the diencephalon, COX 2-ir cells were also observed in the paraventricular nucleus of the hypothalamus and in the nuclei of the anteroventral region surrounding the third ventricle, including the vascular organ of the lamina terminalis. COX 2-ir neurons were also observed in the subparafascicular nucleus, the medial zona incerta, and pretectal area. In the brainstem, COX 2-ir neurons were observed in the dorsal raphe nucleus, the nucleus of the brachium of the inferior colliculus, and in the region of the subcoeruleus. The distribution of COX 2-ir neurons in the CNS suggests that COX 2 may be involved in processing and integration of visceral and special sensory input and in elaboration of the autonomic, endocrine, and behavioral responses.

Determination of extracellular bicarbonate and carbon dioxide concentrations in brain slices using carbonate and pH-selective microelectrodes

The extracellular pH of the brain is subject to shifts during neural activity. To understand these pH changes, it is necessary to measure [H+], [HCO3-], [CO3(2-)] and [CO2]. In principle, this can be accomplished using CO3(2-) and pH-sensitive microelectrodes; however, interference from HCO3- and Cl-, and physiological changes in [HCO3-], complicate measurements with CO3(2-) electrodes. Calibration requires knowledge of slope response, interference constants and corrections for [HCO3-] shifts. We show that when [HCO3-] is altered at constant [CO2] in the absence of Cl-, the HCO3- interference cancels and the Nikolsky equation reduces to the Nernst equation for CO3(2-). Measurement of CO3(2-) slope response by this method yielded a value of 28.5 +/- 0.72 mV per decade change in [CO3(2-)]. In Cl(-)-containing solutions, interference coefficient for HCO3- and Cl- were determined by altering [HCO3-] at constant [CO2], changing [CO2] at constant [HCO3-], then solving the simultaneous Nikolsky equations for each transition. The mean interference constants corresponded to selectivity ratios of 245:1 and 1150:1 for CO3(2-) over HCO3- and Cl- respectively. To correct for possible changes in [HCO3-], the equilibrium relation between CO3(2-) and HCO3- was substituted into the Nikolsky equation to yield an equation in [CO3(2-)] and [H+]. By simultaneously measuring shifts in [H+] with a pH microelectrode, this equation is readily solved for [CO3(2-)]. These methods were tested by measuring [HCO3-] and [CO2] in experimental solutions, and in the extracellular fluid of rat hippocampal slices.

Neocortical spreading depression provokes the expression of c-fos protein-like immunoreactivity within trigeminal nucleus caudalis via trigeminovascular mechanisms

The effects of neocortical spreading depression (SD) on the expression of immunoreactive c-fos protein were examined within the superficial laminae of trigeminal nucleus caudalis (TNC), a brainstem region processing nociceptive information. KCl was microinjected into the left parietal cortex at 9 min intervals over 1 hr, and SD was detected by a shift in interstitial DC potential within adjacent frontal cortex. The stained cells in lower brainstem and upper cervical spinal cord were counted on both sides after tissues were sectioned (50 microns) and processed for c-fos protein-like immunoreactivity (LI) using a rabbit polyclonal antiserum. C-fos protein-LI was visualized in the ventrolateral TNC, chiefly in laminae I and Ilo and predominantly within spinal segment C1-2 (e.g., -1.5 to -4.5 mm from obex) ipsilaterally. SD significantly increased cell staining within ipsilateral TNC. The ratio of cells in laminae I and Ilo on the left: right sides was 1.32 +/- 0.13 after 1 M KCl, as compared to 1.06 +/- 0.05 in control animals receiving 1 M NaCl instead of KCl microinjections (p < 0.01). The ratio was reduced to an insignificant difference after chronic surgical transection of meningeal afferents and recurrent SD (1.09 +/- 0.11). Pretreatment with intravenous sumatriptan, a 5-HT1-like receptor agonist that selectively blocks meningeal C-fibers and attenuates c-fos protein-LI within TNC after noxious meningeal stimulation, also reduced the ratio to an insignificant difference (1.10 +/- 0.09). Sumatriptan or chronic surgical transection of meningeal afferents, however, did not reduce the ability of KCl microinjections to induce SD. On the other hand, combined hyperoxia and hypercapnia not only reduced the number of evoked SDs from 6.3 +/- 1.0 to 2.5 +/- 1.2 after 0.15 M KCl microinjection, but also significantly (p < 0.01) reduced associated c-fos protein-LI in TNC. These data indicate that multiple neocortical SDs activate cells within TNC. The increase in c-fos protein-LI, observed predominantly ipsilaterally, was probably mediated by SD-induced stimulation of ipsilaterally projecting unmyelinated C-fibers innervating the meninges. If true, this is the first report demonstrating that neurophysiological events within cerebral cortex can activate brainstem regions involved in the processing of nociceptive information via trigeminovascular mechanisms.

Intracellular pH rises and astrocytes swell after portacaval anastomosis in rats

The basis for astrocytic swelling after the early period after portacaval anastomosis (PCA) is poorly defined. In other eukaryotic cells intracellular pH (pHi) and volume are determined, in part, by the same general mechanisms, yet how astrocytic pHi varies with enlargement of these cells after PCA is unknown. Therefore, direct measurements of pHi in astrocytes were made and compared with pericapillary astrocytic area as determined from electron micrographs in rats 5-8 days after PCA. Astrocytic area (n = 14 measurements for each group) was found to be significantly (P less than 0.0009) greater in PCA animals (n = 3) than in sham-operated control animals. (n = 3). Double-barrel pH-sensitive microelectrodes were used to measure pHi in neocortical cells defined by electrophysiological criteria to be astrocytic. Astrocytes (n = 25) from PCA animals (n = 5) had a resting membrane potential of 72 +/- 5 mV (mean +/- SD) and an pHi of 7.11 +/- 0.11 while comparable cells (n = 12) from sham-operated controls (n = 2) had a membrane potential of 81 +/- 6 mV and an pHi of 7.00 +/- 0.10. Astrocytes from PCA animals were significantly more depolarized (P less than 0.001) and alkaline (P less than 0.009), at a time when they were also significantly larger than those from sham-operated controls. Astrocytes are known to become more alkaline when they are activated by brief depolarizing stimuli. However, this is the first demonstration that such an interrelationship can also exist for steady-state conditions of these cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Focal elevations in neocortical interstitial K+ produced by stimulation of the fastigial nucleus in rat

We studied whether K+, a potent cerebrovasodilator released by active neurons, participates in the increase in cortical cerebral blood flow (CBF) elicited by stimulation of the cerebellar fastigial nucleus (FN). Rats were anesthetized by continuous administration of halothane (1-3%), paralyzed and artificially ventilated. FN was stimulated electrically (8 s trains, 50 Hz, 5-10 V) through microelectrodes positioned stereotaxically. K+o (mM) was measured in sensory cortex by K(+)-sensitive micropipettes. In some experiments neocortical CBF was monitored continuously by laser-doppler flowmetry. Stimulation of the FN produced significant increases in K+o that averaged 0.91 +/- 0.16 mM (range 0.5-2.9 mM; n = 19) and were confined to sites corresponding to the intermediate cortical laminae (P less than 0.05, ANOVA). To determine whether such K+o elevations were able to produce increases in CBF comparable to those elicited by FN stimulation, cortical K+o was increased by superfusing the sensory cortex with 20-30 mM K+ in Ringer. K+o elevations of 2.8 +/- 0.6 mM increased CBF by 17 +/- 2% (n = 5), an increase considerably smaller than that elicited by FN stimulation in cerebral cortex. We conclude that K+ is unlikely to mediate the cortical cerebrovasodilation. Furthermore, the restricted spatial distribution of the K+o increase indicates that the cortical neural activity evoked by FN stimulation is highly focal. Thus the findings support the hypothesis that, in cortex, the vasodilation is mediated by activation of a restricted group of neural elements, perhaps neurons in laminae III-IV.

Carbonic acid buffer species measured in real time with an intracellular microelectrode array

Carbonic acid buffer anions, HCO3- and CO3(2-), play an instrumental role in a host of vital processes in animal cells and tissues. Yet study of carbonic acid buffer species is hampered because no means are available to simultaneously monitor them at a cellular level in a rapid and dynamic fashion. An ion-selective cocktail, previously reported to measure changes in bicarbonate activity (alpha HCO3-), was instead shown to be principally selective for alpha CO3(2-). Ion-selective micropipettes (ISMs) based on this exchanger and consisting of a 3:1:6 (volume) mixture of tri-n-octylpropylammonium chloride, 1-octanol, and trifluoroacetyl-p-butylbenzene showed no significant interference from bicarbonate, chloride, phosphate, ascorbate, lactate, glutamate, acetate, or hydroxyl ions at concentrations expected in vivo. Intracellular and triple-barrel ISMs, consisting of a CO3(2-) sensitive, pH-sensitive, and reference barrel, were fabricated. Skeletal muscle cells (n = 17) were penetrated in vivo and showed values of 74 +/- 7 mV for membrane potential, 6.94 +/- 0.09 pHi, and 11 +/- 5 microM intracellular alpha CO3(2-), from which intracellular alpha HCO3- of 25 +/- 10 mM and CO2 tension of 120 +/- 55 Torr were calculated. All ion measurements reached a new steady state in 9 +/- 2 s after cell penetration. Thus measurements of intracellular alpha CO3(2-) and pH and associated levels of alpha HCO3(2-) and CO2 tension can be determined in biological tissues and cells with a spatial and temporal resolution previously unattainable.

Spreading depression increases immunohistochemical staining of glial fibrillary acidic protein

Reactive astrocytosis is a process by which astrocytes respond to brain injury by showing an increase in glial fibrillary acidic protein (GFAP) staining that is associated with hypertrophy and/or hyperplasia of these cells. Because spreading depression (SD) is a perturbation uncomplicated by neuronal necrosis and is seen in both in vivo and in vitro neural structures, we sought to determine whether SD was a sufficient stimulus to induce enhanced GFAP staining. SD was elicited in anesthetized rats by application of KCI to parietal cortex for 3 hr; equimolar NaCI was applied to contralateral cortex. SD was confirmed by monitoring DC potentials in frontal neocortices. Animals were allowed to recover for 48 hr, and their brains were processed for semiquantitative and computer-based analyses of GFAP staining intensity. Experimental GFAP staining was referenced to contralateral control levels. Neocortical SD (13-37 SDs) was associated with a significant (p less than 10(-4)), 43% increase in GFAP staining intensity, which remained statistically greater than normal for more than 2 weeks. If SD was inhibited by combined hyperoxia and hypercarbia, only a nonsignificant (p greater than 0.20), 7% increase in GFAP staining was seen. Thus, SD may be a useful physiologic process with which to begin to explore the cellular mechanisms that induce the transformation of normal astrocytes into reactive species.

Ionic concomitants of astroglial transformation to reactive species

Brain injury, including ischemia, changes normal astrocytes into reactive species that hypertrophy and begin to proliferate. Understanding the mechanisms that underlie these changes could lead to new abilities to promote regeneration and retard neural degeneration after ischemia. Because ionic changes occur after nonneural cells are exposed to mitogens, we have begun to examine the ionic changes that may trigger reactive gliosis. We showed that two changes thought to be important for mitogenesis, elevation of interstitial potassium or intracellular pH, are correlated with reactive gliosis as indicated by increased immunohistochemical staining for glial fibrillary acidic protein. This relation was seen after activation of cerebral cortex by recurrent spreading depression but not by physiologic stimulation. Deoxyribonucleic acid synthesis occurs in fibroblasts only when intracellular potassium exceeds 90 mM, a level seen in astrocytes only during spreading depression. Thus, our results support the contention that a threshold level of potassium (and pH) must be exceeded in eukaryotic cells before proliferation or anabolism will proceed.

Astrocytic acidosis in hyperglycemic and complete ischemia

Nearly complete brain ischemia under normoglycemic conditions results in death of only selectively vulnerable neurons. With prior elevation of brain glucose, such injury is enhanced to include pancellular necrosis (i.e., infarction), perhaps because an associated, severe lactic acidosis preferentially injures astrocytes. However, no direct physiologic measurements exist to support this hypothesis. Therefore, we used microelectrodes to measure intracellular pH and passive electrical properties of cortical astrocytes as a first approach to characterizing the physiologic behavior of these cells during hyperglycemic and complete ischemia, conditions that produce infarction in reperfused brain. Anesthesized rats (n = 26) were made extremely hyperglycemic (blood glucose, 51.4 +/- 2.8 mM) so as to create potentially the most extreme acidic conditions possible; then ischemia was induced by cardiac arrest. Two loci more acidic than the interstitial space (6.17-6.20 pH) were found. The more acidic locus [4.30 +/- 0.19 (n = 5); range: 3.82-4.89] was occasionally seen at the onset of anoxic depolarization, 3-7 min after cardiac arrest. The less acidic locus [5.30 +/- 0.07 (n = 53); range 4.46-5.93)] was seen 5-46 min after cardiac arrest. A small negative change in DC potential [8 +/- 1 mV (n = 5); range -3 to -12 mV and 7 +/- 2 mV (n = 53); range +3 to -31 mV, respectively] was always seen upon impalement of acidic loci, suggesting cellular penetration. In a separate group of five animals, electrical characteristics of these cells were specifically measured (n = 17): membrane potential was -12 +/- 0.2 mV (range -3 to -24 mV), input resistance was 114 +/- 16 M omega (range 25-250 M omega), and time constant was 4.4 +/- 0.4 ms (range 3.0-7.9 ms). Injection of horseradish peroxidase into cells from either animal group uniformly stained degenerating astrocytes. These findings establish previously unrecognized properties of ischemic astrocytes that may be prerequisites for infarction from nearly complete ischemia: the capacity to develop profound cellular acidosis and a concomitant reduction in cell membrane ion permeability.

The effects of extracellular acidosis on neurons and glia in vitro

Cerebral lactic acid, a product of ischemic anaerobic glycolysis, may directly contribute to ischemic brain damage in vivo. In this study we evaluated the effects of extracellular acid exposure on 7-day-old cultures of embryonic rat forebrain. Mixed neuronal and glial cultures were exposed to either lactic or hydrochloric acid to compare the toxicities of relatively permeable and impermeable acids. Neurons were relatively resistant to extra-cellular HCl acidosis, often surviving 10-min exposures to pH 3.8. In the same cultures, immunochemically defined astrocytes survived 10-min HCl exposures to a maximum acidity of pH 4.2. Similarly, axonal bundles defasciculated in HCl-titrated media below pH 4.4, although their constituent fibers often survived pH 3.8. Cell death occurred at higher pH in cultures subjected to lactic acidosis than in those exposed to HCl. Over half of forebrain neurons and glia subjected for 10 min to lactic acidification failed to survive exposure to pH 4.9. Longer 1-h lactic acid incubations resulted in cell death below pH 5.2. The potent cytotoxicity of lactic acid may be a direct result of the relatively rapid transfer of its neutral protonated form across cell membranes. This process would rapidly deplete intracellular buffer stores, resulting in unchecked cytosolic acidification. Neuronal and glial death from extracellular acidosis may therefore be a function of both the degree and the rapidity of intracellular acidification.

Intracellular pH transients of mammalian astrocytes

Intracellular pH (pHi) is an important physiologic variable that both reflects and influences cell function. Glial cells are known to alter their functional state in response to a variety of stimuli and accordingly may be expected to display corresponding shifts in pHi. We used fine-tipped, double-barreled, pH-sensitive microelectrodes to continuously monitor pHi in glial cells in vivo from rat frontal cortex. Cells were identified as glia by a high membrane potential and lack of injury discharge or synaptic potentials. Continuous, stable recordings of pHi from astrocytes were obtained for up to 80 min but typically lasted for approximately 10 min. Resting pHi was 7.04 +/- 0.02 with a membrane potential of 73 +/- 0.9 mV (mean +/- SEM; n = 51). With cortical stimulation, glia depolarized and became more alkaline by 0.05-0.40 pH (n = 50). During spreading depression (SD), glia shifted more alkaline by 0.11-0.78 pH (n = 26). After stimulation or SD, glia repolarized and pHi became more acidic than at resting levels. Superfusion of the cortical surface with 0.5-2 mM Ba2+ caused glia to hyperpolarize during stimulation and completely abolished the intracellular alkaline response. The predominant pH response of the interstitial space during stimulation or SD was a slow acidification. With superfusion of Ba2+ an early stimulus-evoked interstitial alkaline shift was revealed. The mechanism of the intracellular alkaline shift is likely to involve active extrusion of acid. However, internal consumption of protons cannot be excluded. The sensitivity of the response to Ba2+ suggests that it is triggered by membrane depolarization. These results suggest that glial pHi is normally modulated by the level of local neuronal activity.

Acid-induced injury in elasmobranch brain

Sodium lactate injection into rat brain produces coagulation necrosis consistent with infarction when pH0 is held at less than or equal to 5.30 for 20 min. Such injury may result from excessive astroglial acidification. If true, then brain damage from acidosis in elasmobranchs might evolve differently since glial reaction there to another necrotizing injury, exposure to extreme cold, is dissimilar from that seen in mammals. Accordingly, pH0 was monitored and sodium lactate (pH 4.00-7.00) injected into skate (Raja erinacea) cerebella. Necrosis was seen only when pH0 was less than or equal to 4.86 for 20 min; and pH0 rise after injections was unaffected by those which destroyed brain, and not slowed as in rat. Thus elasmobranchs are less susceptible to irreversible brain injury from acidosis, a capacity which may result from their lower body temperature compared to mammals.

Compartmentation of acid-base balance in brain during complete ischemia

During near complete hyperglycemic brain ischemia, brain lactate levels rise in excess of 16-18 mmol/K and are associated with severe brain infarction. Analyses of pHo, Pt(CO2), and total brain lactate under these circumstances suggest that H+, HCO3, and lactate become unequally distributed between cells and the interstitial space and, perhaps, even between different types of brain cells. In addition, to whatever physiological advantages it may generate, such compartmentalization may be a factor leading to cell death in brain ischemia.

Intracellular pH of astrocytes increases rapidly with cortical stimulation

Modulation of intracellular pH is widely implicated in the control of cell growth and metabolism, yet little is known about intracellular pH and brain function. To determine how stimulation of brain may affect the intracellular pH of mammalian glial cells, rat cortical astrocytes were studied for the first time in vivo using pH-sensitive electrodes of submicron caliber. Stimulation of the cortical surface caused a cytoplasmic alkaline shift of tenths of a pH within seconds. Cessation of induced electrical activity was followed by pH recovery and a small acid rebound. Recordings obtained during cortical-spreading depression revealed similar but generally larger intracellular pH shifts. Production of metabolic acids is known to occur when the brain is stimulated and has led to the long-held presumption that brain cells accordingly become more acidic. The observation that glia initially become more alkaline during electrical activity is thus paradoxical. The correlation of glial alkalinization with evoked electrical activity suggests that modulation of intracellular pH of glia may have important functional implications.

Light and electron microscopic evaluation of hydrogen ion-induced brain necrosis

Excessive accumulation of hydrogen ions in the brain may play a pivotal role in initiating the necrosis seen in infarction and following hyperglycemic augmentation of ischemic brain damage. To examine possible mechanisms involved in hydrogen ion-induced necrosis, sequential structural changes in rat brain were examined following intracortical injection of sodium lactate solution (pH 4.5), as compared with injections at pH 7.3. Following pH 7.3 injection, neuronal swelling developed between 1 and 6 h, but only a needle track wound surrounded by a thin rim of necrotic neurons and vacuolated neuropil was present 24 h after injection. In contrast, pH 4.5 injection produced neuronal necrosis as soon as 1 h after injection, followed by necrosis of astrocytes and intravascular thrombi at 3 and 6 h. Alterations common to both groups included vascular permeability to horseradish peroxidase, dilation of extracellular spaces, astrocyte swelling, capillary compression, and vascular stasis. These data suggest that neurons, astrocytes, and endothelia can be directly damaged by increased acid in the interstitial space. Lethal injury initially appeared to affect neurons, while subsequent astrocyte necrosis and vascular occlusion may damage tissue by secondary ischemia.

Hydrogen ions kill brain at concentrations reached in ischemia

Elevation of brain glucose before the onset of nearly complete ischemia leads to increased lactic acid within brain. When excessive, such acidosis may be a necessary factor for converting selective neuronal loss to brain infarction from nearly complete ischemia. To examine the potential neurotoxicity of excessive lactic acid concentrations, we microinjected (0.5 microliter/min) 150 mM sodium lactate solutions (adjusted to 6.50-4.00 pH) for 20 min into parietal cortex of anesthetized rats. Interstitial pH (pH0) was monitored with hydrogen ion-selective microelectrodes. Animals were allowed to recover for 24 h before injection zones were examined with the light microscope. Injectants produced brain necrosis in a histological pattern resembling ischemic infarction only when pH0 was less than or equal to 5.30. Nonlethal injections showed only needle tract injuries. Abrupt deterioration of brain acid-base homeostatic mechanisms correlated with necrosis since pH0 returned to baseline more slowly after lethal tissue injections than after nonlethal ones. The slowed return of pH0 to baseline after the severely acidic injections may reflect altered function of plasma membrane antiport systems for pH regulation and loss of brain hydrogen ion buffers.

Acid-induced changes of brain protein buffering

Excessive cellular acidosis is thought to enhance destruction of brain from ischemia. Protein denaturation may contribute to such injury although the behavior of brain proteins to acidosis is poorly defined. As a first approach to detect acid-induced changes in brain proteins and to characterize buffer content, homogenates were acidified for 20 min (as low as pH 3.1), returned to baseline pH (6.9), and then titrated. Titration curves show a significant (P less than 0.0001) and permanent increase in buffer content compared to controls when pH of acid exposure was 4.5-3.7 or less. Since acidity of pH 4.5 is rarely, if ever, achieved in vivo, protein denaturation from acidity alone is unlikely to account for necrosis of brain from ischemia.

Bicarbonate and ammonia changes in brain during spreading depression

An alkaline, followed by an acid-going transient, characterizes acid-base changes in the interstitial space during spreading depression in a variety of brain structures. In rat, such changes are associated with a significant rise in brain lactate content. How brain proton buffers behave during spreading depression is unknown. Techniques to significantly improve the response time of gas permeable membrane semimicroelectrodes for carbon dioxide and ammonia are reported. Measurements with such electrodes, when coupled to measurements of hydrogen ion concentration (from microelectrodes), permit rapid changes to be determined in bicarbonate concentration or ammonia and ammonium ion concentration, respectively. Bicarbonate concentration fell from 30 +/- 1 (n = 16) to 14 +/- 1 mM (n = 16) during spreading depression. On the other hand, ammonia concentration rose from 2.3 +/- 0.1 to 4.4 +/- 0.3 microM (n = 17) while ammonium ion concentration rose from 116 +/- 11 (n = 17) to 382 +/- 30 microM (n = 17) during spreading depression. Bicarbonate changes probably reflect titration of brain bicarbonate stores by accumulated lactic acid. Similar physicochemical changes do not explain the rise in ammonia and ammonium ion concentrations. Instead, elevation of the latter can only result from an increase in ammonia content of the interstitial space.

In vivo measurement of regional brain and tumor pH using [14C]dimethyloxazolidinedione and quantitative autoradiography. II: Characterization of the extracellular fluid compartment using pH-sensitive microelectrodes and [14C]sucrose

We measured the extracellular (interstitial) pH (pHe) of RG-2 rat gliomas using H+-sensitive microelectrodes and estimated the volume of tumor extracellular space based on the tissue-plasma ratio of [14C]sucrose. The average RG-2 pHe was 7.63 +/- 0.15 (mean +/- SD, n = 6), whereas the average pHe of contralateral brain tissue was 7.34 +/- 0.10 (n = 3) and arterial pH was 7.36 +/- 0.02. RG-2 extracellular space water volume was estimated to be 0.3 ml water/g tissue. In separate experiments in normal, nontumored rats, intracellular pH (pHi) was calculated for nine gray and white matter regions based on measurements of tissue and plasma [14C]dimethyloxazolidinedione concentration. pHi values ranged from 6.80 to 6.94, and no consistent gray-white differences were observed. Our data suggest that tumor pHi is not more acidic than that of normal brain tissue and that the observed alkalinity of primary brain tumors is due to the presence of a large alkaline extracellular space.

Carbonic acid buffer changes during complete brain ischemia

Simultaneous measurements of tissue PCO2 (PtCO2), interstitial H+ concentration ([H+]o), and tissue lactate content were used to examine changes in interstitial HCO3- concentration ([HCO3-]o) during complete ischemia. In normoglycemic rats (blood glucose of 6-8 mM; neocortical ischemic-induced lactate content 8-12 mmol/kg) [H+]o increased from 7.22 +/- 0.02 to 6.79 +/- 0.02 pH (n = 3). By contrast, in hyperglycemic rats (blood glucose 18-75 mM; ischemic-induced lactate content 19-31 mmol/kg) [H+]o rose by a significantly larger amount to 6.19 +/- 0.02 pH (n = 7). Given that HCO3- is the predominant interstitial H+ buffer, changes in peak PtCO2 show why peak [H+]o were bimodally distributed compared with lactate content. Between 8 and 12 mmol/kg lactate, when peak PtCO2 rose from 99 to 186 Torr but [H+]o was constant at 6.79 pH, calculated [HCO3-]o increased from 11.9 to 21.9 mM. Then after transitional changes, peak PtCO2 and [H+]o remained constant at 389 +/- 9 Torr (n = 7) and 6.19 pH despite the fact that tissue lactate ranged from 19 to 31 mmol/kg lactate, respectively; [HCO3-]o must have remained constant at 12.3 +/- 0.7 mM (n = 7). Since ischemic brain continued to produce another 12 more mmol/kg of lactic acid above 19 mmol/kg lactate without further changes in PtCO2 or [H+]o, H+ and HCO3- must have been heterogeneously compartmented. The continued lactic acid production occurred in a compartment that occupied 36% of neocortical space. This compartment is likely to represent glial cells.

Hydrogen ion buffering during complete brain ischemia

As a first step to quantify [H+] changes in brain during ischemia we used H+-selective microelectrodes and enzyme fluorometric techniques to describe the relationship between interstitial [H+] ([H+]o) and peak tissue lactate after cardiac arrest. We found a step function relationship between [H+]o and tissue lactate rather than the linear titration expected in a homogeneous protein solution. Within a blood glucose range from 3-7 mM, brain lactate rose from 8-13 mmol/kg along with a rise in [H+]o of 99 +/- 6 nM(0.44 +/- 0.02 pH). At higher blood glucose levels (17-80 mM), brain lactate accumulated to levels of 16-31 mmol/kg; concurrently [H+]o rose by 608 +/- 16 nM (1.07 +/- 0.02 pH). The unchanging level of [H+]o between 8-13 and 16-31 mmol/kg lactate implies that [H+]o is at a steady-state, but not equilibrium with respect to [H+] in other brain compartments. We propose that ion-transport characteristics of astroglia account for the observed relationship of [H+]o to tissue lactate during complete ischemia and suggest that brain infarction develops after plasma membranes in brain cells can no longer transport ions to regulate [H+].

Alkaline and acid transients in cerebellar microenvironment

1. Extracellular pH (pHo) was measured in the cerebellar cortex of the rat using a recently developed liquid membrane ion-selective micropipette (ISM). pHo was determined during stimulus-evoked neuronal activity, elevated extracellular potassium concentration, [K+]o, spreading depression (SD), and complete ischemia. In many experiments [K+]o was simultaneously determined. 2. A train of local surface stimuli (LOC) produced an initial alkaline shift in pHo from a base line of 7.20-7.30 to 7.25-7.35. This was followed by a long-lasting acid phase that reached a plateau of 7.05-7.15 after 64 s of stimulation. pHo decrease was related to stimulus frequency, intensity, and duration. 3. Superfusion with Ringer solution containing manganese ions rapidly abolished parallel fiber-induced Purkinje cell synaptic depolarization together with the alkaline shifts while enhancing the acid shifts. 4. Superfusion of the cerebellar cortex with Ringer solution containing increasingly elevated [K+] progressively lowered pHo to a plateau of 6.95-7.05. The acidification occurred in the presence of ouabain but was reversed on return to the normal [K+]o or with the addition of the glycolytic blocker, fluoride. Stimulus-evoked alkaline shifts were enhanced by K+-Ringer superfusion. These experiments suggested that the acid shift was due to the metabolic production of an anion, possibly lactate. 5. Elevation of [K+]o above 8-12 mM often produced oscillation in pHo and [K+]o with a period of about 40 s. Sometimes these oscillations ended in a spontaneous SD or SD could be evoked by stimulation. Under these conditions of raised [K+]o, the SD consisted of a very pronounced alkaline transient followed by a small, long-lasting acid shift. When SD was induced by conditioning the cerebellum with proprionate or lowered NaCl, the alkaline phase was reduced and the acid enhanced. 6. Complete ischemia began with a progressive decrease of pHo and rise in [K+]o. When [K+]o reached 12 mM, a second more rapid rise in [K+]o to 40 mM or more occurred. This was correlated with 0.1-0.2 pHo transient increase similar to that seen during SD. pHo eventually reached a plateau of 6.60-6.80, close to neutrality. 7. Superfusion with Ringer solution containing acetazolamide immediately altered pHo homeostasis by increasing base-line pHo by about 0.10 and enhanced the induced pHo changes. These results suggest that carbonic anhydrase (CA) is important for acute buffering of the brain extracellular microenvironment. 8. The above results were interpreted in terms of changes in extracellular strong ion concentration differences ( [SID]o), extracellular concentration of total weak acid ( [Atot]o) and partial pressure of CO2 (Pco2) in the brain microenvironment. The results indicate that neuronal activity produces changes in many of the constituents of the microenvironment.

Sodium liquid ion exchanger microelectrode used to measure large extracellular sodium transients

A new liquid membrane microelectrode has been developed that is easily fabricated and can measure fast sodium transients in the presence of potassium interference. It responds to a sudden change in sodium activity within 1 second. The electrode has been used to provide the first direct evidence of large sodium transients in the extracellular space of the brain of the catfish.