The time course of evoked depolarization of cortical glial cells

Changes in the volume of the intercellular space of the cerebral cortex in conditions of peripheral stimulation in rats

Changes in the volume of the intercellular space of the rat cerebral cortex in response to peripheral repetitive stimulation were studied. The volume of the intercellular space and its changes were assessed by a modification of the four-electrode impedance method. The results suggest that evoked electrical activity in the cerebral cortex was accompanied by 3-5% decreases in the volume of the intercellular space.

Regulation and modulation of pH in the brain

The regulation of pH is a vital homeostatic function shared by all tissues. Mechanisms that govern H+ in the intracellular and extracellular fluid are especially important in the brain, because electrical activity can elicit rapid pH changes in both compartments. These acid-base transients may in turn influence neural activity by affecting a variety of ion channels. The mechanisms responsible for the regulation of intracellular pH in brain are similar to those of other tissues and are comprised principally of forms of Na+/H+ exchange, Na+-driven Cl-/HCO3- exchange, Na+-HCO3- cotransport, and passive Cl-/HCO3- exchange. Differences in the expression or efficacy of these mechanisms have been noted among the functionally and morphologically diverse neurons and glial cells that have been studied. Molecular identification of transporter isoforms has revealed heterogeneity among brain regions and cell types. Neural activity gives rise to an assortment of extracellular and intracellular pH shifts that originate from a variety of mechanisms. Intracellular pH shifts in neurons and glia have been linked to Ca2+ transport, activation of acid extrusion systems, and the accumulation of metabolic products. Extracellular pH shifts can occur within milliseconds of neural activity, arise from an assortment of mechanisms, and are governed by the activity of extracellular carbonic anhydrase. The functional significance of these compartmental, activity-dependent pH shifts is discussed.

Effects of extracellular potassium on glycogen stores of astrocytes in vitro

Astrocyte-enriched and meningeal cell cultures of the rat cerebral cortex were prepared, and their glycogen content was measured after 10-90 min under control (2.5 mM) concentrations of potassium after prefeeding with 20 mM glucose. No net change in glycogen level was noted in either culture over this period. Cell cultures were then exposed to increased concentrations of potassium (5, 10, and 15 mM), and their glycogen content was measured after 10-90 min. Both types of cell culture showed complex and variable changes in glycogen content. In general, increased potassium concentrations caused astrocyte glycogen stores to be reduced at physiological increases of potassium levels (from 2.5 to 5 mM and above), although a period of resynthesis was evident at all potassium concentrations. Meningeal cell glycogen levels were highly variable and only affected by high (10 and 15 mM) levels of potassium. These results are discussed with respect to the theory that changes in the external potassium concentration caused by neuronal activity might act as a signal controlling astrocyte glycogen stores.

Contribution of glia and neurons to the surface-negative potentials of the cerebral cortex during its electrical stimulation

In 20 cats anaesthetized with pentobarbital the suprasylvian gyrus was stimulated by single stimuli or by trains of 50 s stimuli and the potentials from the cortical surface and the intracellular potentials from glial and nerve cells were recorded. Glial cells were identified according to conventional electrophysiological criteria: the absence of action potentials and postsynaptic potentials; slow depolarization in response to electrical stimulation. The slow negativity of direct response to a single stimulus is similar in shape and time course to the depolarization of the cortical glial cells and is unlike the hyperpolarization of the cortical neurons. Quantitative analysis showed that the basic part of the slow negativity is the glial component, whereas the neuronal component--inhibitory postsynaptic potential--plays a much lesser role. The negative shift of the potential on the cortical surface evoked by its high-frequency stimulation is similar in shape and time course to the depolarization shift of the membrane potential of the cortical glial cells (the mean value and standard error of time to peak for glial depolarization were 567.6 +/- 26.8 ms and 427 +/- 24 ms for negative shift of potential). (The results are based on recordings from 37 cells.) The negative shift decays much quicker; it is not similar in shape and time course to the hyperpolarization shift of the neuronal membrane potentials (the mean value and standard error of time to peak for inhibitory postsynaptic potential was 44.9 +/- 4.5 ms). According to the quantitative analysis, the negative shift of the potential reflects mainly the depolarization of the cortical glial cells. The contribution of the hyperpolarization of neurons to the surface-negative shift can be distinctly observed during the first 0.2-0.3 s of stimulation. It is supposed that accumulation of K+ ions in intercellular clefts results in depolarization of glial syncytium, which is reflected on the cortical surface as a slow negativity and a negative shift of the potential.

Prolonged negative surface potentials of the cat sensomotor cortex and responses of neurons and glial cells

Evoked potentials to stimulation of the ventrolateral and intralaminar thalamic nuclei, the surface of the sensomotor cortex, and the pyramidal pathways, derived from the same point, and also corresponding postsynaptic responses of pyramidal neurons were studied in acute experiments on cats anesthetized with ether or superficially with pentobarbital (25-30 mg/kg, intraperitoneally), and immobilized with muscle relaxants. Surface application of strychnine inhibits the slow negative potential arising in response to direct and primary responses, and the corresponding slow potentials of the IPSP. The action of iontophoretic application of strychnine on IPSP of pyramidal neurons and responses of cortical glial cells also were studied. Both methods of application of strychnine block mainly the early component of the IPSP, during which the input resistance is significantly lower than that during the late component, evidence of their different genesis. The results of the investigation show that slow negative potentials are a reflection of hyperpolarization of pyramidal neurons, and that the separate components of the responses have a common genesis.

Sodium, potassium, and metabolic studies of glial, liver, and kidney nuclei under anoxic-ischemic conditions

Glial, liver, and kidney nuclei were studied under anoxic-ischemic conditions. It was found that glial nuclei were significantly depleted of sodium and potassium as early as 15 seconds after the insult, whereas liver and kidney nuclei were not. There was a concomitant significant drop in high-energy substrates (ATP, P-creatine, glucose) in both glial and brain homogenate, whereas these substrates remained relatively constant in kidney and began to drop only at the four-minute time point in liver. These unique observations in glial nuclei may have important implications in the glial swelling that is seen in ischemic states.

Blockade of cortical spreading depression in electrically and chemically stimulated areas of cerebral cortex in rats

Penetration of cortical spreading depression (SD) into epileptic foci established in the cerebral cortex by penicillin or Metrazol and into electrically stimulated cortical regions was studied in anaesthetized rats. SD suppressed the activity of penicillin foci with low rates of interictal discharge (0.3 Hz) but did not invade more active foci (1 Hz) or foci triggered by electrical stimulation (1-3 Hz). Metrazol foci did not block SD propagation unless stimulated at 6-10 Hz. Repetitive direct cortical responses elicited by 0.05-0.1 msec pulses blocked SD propagation when applied at 6-10 Hz for 5-20 min. The SD blockade covered an area 3-5 mm in diameter around the bipolar stimulating electrodes. The block outlasted the stimulation for several minutes but was fully reversible. New stimulation reinstated the SD blockade after a shorter latency and at lower stimulus intensities and rates. Interaction of the blocked cortical area and SD resulted in anomalous SD propagation, characterized by reentry or circle waves, returning through or around the stimulated region to the recovered cortex. The dynamics of the onset and offset of blocking suggests that SD propagation is prevented by enhanced K+ reabsorption which rapidly removes the K+ ions penetrating the stimulated area from the SD wave front. The interactive phenomena, particularly SD circulation around an epileptic focus, may account for periodic changes of ictal and interictal activity found in some types of focal epilepsy.

Human auditory sustained potentials. I. The nature of the response

In response to a sustained toneburst a negative baseline shift can be recorded from the human fronto-central scalp regions with an onset latency of approximately 150 msec. This auditory sustained potential is distinct both in its scalp distribution and in its stimulus relationships from the transient response occurring at the onset or offset of the toneburst. It differs from the contingent negative variation in that it can occur in the absence of attention or during sleep. Attention to the auditory stimulus can increase the amplitude of the sustained potential, possibly through the addition of an extra negative potential related to auditory expectancy or uncertainty.

Glial-neural interaction demonstrated by the injection of Na+ and Li+ into cortical glia

Injection of Na+ or Li+ into cortical glia evokes glial depolarization, discharge of adjacent neurons, and vascular pulsation. The effects can be explained by the extrusion of K+ from glia after cation injection, glial swelling, and the slow removal of the cation from glia. The data suggest that the reduced rate of reuptake of K+ into Na+-loaded glia results in epileptiform firing of neurons, and support the hypothesis that glia function to buffer the environment of neurons.

Ultrastructural comparisons of neurons of supraoptic and circularis nuclei in normal and dehydrated rats

A quantitative ultrastructural investigation was undertaken to compare the nucleus circularis (NC) and supraoptic nucleus (SON) of the rat both under normal and water-deprived conditions. NC was found to have dramatically more of its cells and membrane surface involved in direct soma-somatic contact than the SON. Water deprivation, even for one day, brought about a significant increase in both percentage of cells and membrane surface in contact in both nuclei, apparently by the retraction of fine glial processes from between the somata. The normal NC was made up of only one ultrastructurally identifiable cell type. The normal NC had no cells showing expanded endoplasmic reticulum, although these were seen following 5 days (but not 1 day) of water deprivation. The normal SON did have 4.4% of its cells showing expanded endoplasmic reticulum. This percentage significantly increased following water deprivation. The vesicle population per area of cytoplasm was very similar between the two normal nuclei. One day of water deprivation brought about a significant increase in less than 800 A vesicles in NC but not the SON. Five days of water deprivation resulted in a significant decrease in the lysosomal population per unit area in both nuclei. Vesicle changes have been discussed in relation to the volume changes in the cells.

Intercellular relationships in the external glial limiting membrane of the neocortex of the cat and rat

The external glial limiting membrane of the cerebral cortex appears to be a complete astrocytic mantle covering the pial surface of the molecular layer. It consists of flattened cell bodies arranged singly or in small groups spaced about 100 mu apart and multitudes of interdigitating processes arrayed in layers. The glial mantle is thicker in the sulci than on the gyri. It is covered externally by a basal lamina which is associated with collagenous fibrils and cells of the pia mater. The extracellular space in aldehyde-perfused material appears as a regular, electron-lucent interval 150 A wide between adjacent cell membranes. Gap junctions are frequently encountered in the external glial limiting membrane; desmosomes are present between astrocytic processes but are seen much less often.

Undershoots following stimulus-induced rises of extracellular potassium concentration in cerebral cortex of cat

Extracellular potassium activity (ak) was recorded with potassium-sensitive electrodes in the sensorimotor cortex of cats. Resting activity was 2.8--3.4 mEquiv/l. Electric stimulation of the cortical surface and the nucleus ventroposterolateralis of the thalamus brought about an increase in aK followed by an undershoot and return to normal value. The lowest observed value of aK was 2.1 mEquiv./l. Size and duration (range 0.5--4 min) of the undershoots of aK increased with increasing peak amplitudes of the preceding rise in aK. Following the rise in aK, a period of reduced neuronal activity was observed which usually shorter lasting than the decrease in extracellular aK. An undershoot of aK and a concomitant reduction of neuronal discharge frequency can also occur in immediate response to antidromic stimulation of the pyramidal tract. To compare the K+ redistribution at normal and reduced levels of aK electrophoretic K+ signals were produced with constant current pulses from a proximate KCl-filled capillary. Both amplitudes and half times of decay of these K+ signals were found to decrease during the phase of poststimulatory undershoot in aK (19 and 23% respectively). It is suggested that an activated reuptake of potassium contributes to the decrease in extracellular aK in addition to inhibitory processes.

Some quantitative observations upon the responses of neuroglial cells which follow axotomy of adjacent neurones

1. The dry mass and nucleic acid content of freshly isolated neuroglial cells and of their nucleoli were measured by interference microscopy and ultraviolet absorption microspectrography. The incorporation of tritiated nucleosides and of an amino acid was followed autoradiographically.2. After hypoglossal axotomy in the adult rat hypertrophy of astrocytes of the hypoglossal nucleus occurred in a biphasic manner. The first phase lasted from days 1-10 and was accompanied by a small degree of astrocytic hyperplasia, and the second from days 20-80. Hypertrophy of oligodendrocytes accompanied the second phase of the astrocytic response.3. When severed axons failed to reinnervate denervated muscle, the second phase of the astrocytic response was markedly reduced and the hypertrophy of oligodendrocytes did not occur.4. If the severed axons re-innervated denervated muscle after a controlled delay, the second phase of the astrocytic response and the oligodendroglial hypertrophy was also delayed.5. Injection of botulinum toxin into the tongue caused changes in astrocytes and oligondendrocytes closely resembling those found after axotomy.6. Transient astrocytic hypertrophy occurred in the uninjured right hypoglossal nucleus, and had a different time course to the changes occurring on the injured side.7. The results are discussed in relation to changes in the metabolism and to alterations in the dendritic fields of injured neurones, previously measured in these circumstances.

The contribution by glial cells to surface recordings from the optic nerve of an amphibian

1. The contribution by glial cells to surface recordings has been examined in the optic nerve of the amphibian Necturus maculosus. The method of current injection was employed selectively to alter the membrane potential of glial cells without affecting that of the axons. The resulting changes in potential were recorded simultaneously from the surface of the nerve using the sucrose gap method and intracellularly from a glial cell near the gap.2. The sucrose gap method recorded 40% of the changes in glial membrane potential. This percentage was not affected when the current electrode was inserted into different glial cells while maintaining the recording conditions constant.3. Following axonal degeneration, produced by removing the eye 2-3 months earlier, the percentage contribution by glia increased to 84%.4. By measuring sucrose gap responses to changes in K(o) it was possible to estimate that the sucrose gap method recorded 31-60% of changes in axonal membrane potential. It was also determined that the axons, unlike glial cells, are relatively insensitive to reductions in K(o). Surface responses to decreases in external potassium thus reflect the magnitude of the glial contribution.5. It is concluded that changes in glial membrane potential contribute about as much to surface recordings from the optic nerve of Necturus as do equivalent changes in axonal membrane potential. The contributions by the glial cells and axons are related to the relative volumes of tissue they respectively occupy. The significance of these findings to the analysis of surface recordings from the mammalian brain is discussed. Since mammalian glial cells, like those in Amphibia and the leech, become depolarized during neuronal activity and on the basis of electron microscopic evidence appear to be electrically coupled, it is likely that they contribute to the electroencephalogram.

Neuroglia: biophysical properties and physiologic function

The membrane time constant of neocortical glial cells is abolut 385 microseconds, less than one-twentieth the known value for the Betz cell. Glial membrane specific resistance is low (approximately 200 to 500 ohm centimeters squared. Neuroglial cells are ideally suited to buffer the immediate extraneuronal space at areas of synaptic contact against the increases in external potassium ion concentration that accompany postsynaptic and spike activity and to minimize the spread of potassium ions to other pre- and postsynaptic regions.