Effects of general anesthetics on intercellular communications mediated by gap junctions between astrocytes in primary culture

Role of gap junction in the expression of morphine-induced antinociception

The present study was undertaken to investigate whether gap junctional communication could be involved in morphine-induced antinociceptive response using blockers of the gap junctional channel, carbenoxolone and Gap27. Intrathecal pretreatment with either carbenoxolone or Gap27 caused a dose-dependent attenuation of morphine-induced antinociception. Furthermore, the dose-response line for morphine-induced antinociception was shifted to the right by 2.53-fold following intrathecal treatment with carbenoxolone. These findings suggest that gap-junctional-dependent communication in the mouse spinal cord may play, at least in part, a role in the expression of morphine-induced antinociception.

Memory in astrocytes: a hypothesis

Background

Recent work has indicated an increasingly complex role for astrocytes in the central nervous system. Astrocytes are now known to exchange information with neurons at synaptic junctions and to alter the information processing capabilities of the neurons. As an extension of this trend a hypothesis was proposed that astrocytes function to store information. To explore this idea the ion channels in biological membranes were compared to models known as cellular automata. These comparisons were made to test the hypothesis that ion channels in the membranes of astrocytes form a dynamic information storage device.

Results

Two dimensional cellular automata were found to behave similarly to ion channels in a membrane when they function at the boundary between order and chaos. The length of time information is stored in this class of cellular automata is exponentially related to the number of units. Therefore the length of time biological ion channels store information was plotted versus the estimated number of ion channels in the tissue. This analysis indicates that there is an exponential relationship between memory and the number of ion channels. Extrapolation of this relationship to the estimated number of ion channels in the astrocytes of a human brain indicates that memory can be stored in this system for an entire life span. Interestingly, this information is not affixed to any physical structure, but is stored as an organization of the activity of the ion channels. Further analysis of two dimensional cellular automata also demonstrates that these systems have both associative and temporal memory capabilities.

Conclusion

It is concluded that astrocytes may serve as a dynamic information sink for neurons. The memory in the astrocytes is stored by organizing the activity of ion channels and is not associated with a physical location such as a synapse. In order for this form of memory to be of significant duration it is necessary that the ion channels in the astrocyte syncytium be electrically in contact with each other. This function may be served by astrocyte gap junctions and suggests that agents that selectively block these gap junctions should disrupt memory.

Cortical spreading depression (CSD)-induced tolerance to transient focal cerebral ischemia in halothane anesthetized rats is affected by anesthetic level but not ATP-sensitive potassium channels

We investigated the participation of ATP-sensitive potassium (K(ATP)) channels, adenosine A1 receptors, and the effects of different levels of halothane anesthesia in the development of CSD-induced ischemic tolerance. To elicit CSD, 0.5 M KCl was applied for 2 h to the right hemisphere of halothane anesthetized male Wistar rats. The inhalation concentration of halothane during CSD was maintained at 0.5% (n = 8), 1.0% (n = 8), or 2.0% (n = 8). For control animals, saline was applied instead of KCl (n = 8). To inhibit K(ATP) channels or adenosine A1 receptors, glibenclamide (0.1 mg/kg icv; n = 8), 5-hydroxydeconaoate (5-HD; 100 mg/kg ip; n = 12), or 8-Cyclopentyl-1, 3-dipropylxanthine (DPCPX) (1.0 mg/kg ip; n = 8) was applied before preconditioning during 1.0% halothane anesthesia. Temporary occlusion (120 min) of the right middle cerebral artery was induced 4 days after preconditioning and the infarct volume was measured. Preconditioning elicited under 1.0% halothane reduced cortical infarct volume from 277 +/- 15 mm3 in the control group to 159 +/- 14 mm3 in the CSD group (mean +/- SEM, P < 0.05). In contrast, CSD induced during inhalation of 0.5% or 2.0% halothane did not confer ischemic tolerance. The reduction in infarct area with CSD during inhalation of 1% halothane was not changed in animals treated with glibenclamide or 5-HD or DPCPX. These results uncover a crucial role of halothane level but not of K(ATP) channels or adenosine A1 receptors in the preconditioning effects of CSD.

Cholinergic action on cortical glial cells in vivo

This study aims at understanding complex interactions between cortical neurons, glia and blood supply developing during the transition from slow-wave sleep to wakefulness. In spite of essential advances from in vitro and culture preparations, the basic mechanisms of glial interactions with their cellular and ionic environment had remained uninvestigated in vivo. Here we approach this issue by performing simultaneous intracellular recordings of cortical neurons and glia, together with measurements of cerebral blood flow (CBF), extracellular K+ concentrations and local field potentials in both anesthetized (ketamine-xylazine) and naturally behaving cats. Under anesthesia, cortical activation was elicited with electric stimulation of cholinergic nuclei (pedunculopontine tegmental in the brainstem and/or nucleus basalis in the basal forebrain). Iontophoretic application of acetylcholine on the recorded cells was also used. In the vast majority of cases (> 80%) glial cells were hyperpolarized during electric stimulation or spontaneous activation. This result was also obtained in all cases where iontophoresis was used or when glutamatergic kainate/quisqualate receptors were blocked with 6-cyano-7-nitroquinoxaline-2,3-dione. The glial hyperpolarization was associated with steady neuronal depolarization, increased CBF, lower extracellular K+ concentration, increased membrane resistance, decreased membrane capacitance and persistent positive DC field potentials. In some cases of cortical activation (< 20%), glial cells displayed sustained depolarizing potentials, in parallel with neuronal depolarization, decreased CBF and more negative DC field potentials. The above-mentioned effects of cholinergic activation were blocked by the muscarinic antagonist scopolamine. We propose that the glial response to cholinergic activation results from the balance between the direct hyperpolarizing action of acetylcholine and the depolarizing modulation of glutamate from the neighboring neurons, in addition to the modulation of the interglial communication pathway and/or the ionic traffic across blood vessels.

Controlling pathological pain by adenovirally driven spinal production of the anti-inflammatory cytokine, interleukin-10

Gene therapy for the control of pain has, to date, targeted neurons. However, recent evidence supports that spinal cord glia are critical to the creation and maintenance of pain facilitation through the release of proinflammatory cytokines. Because of the ability of interleukin-10 (IL-10) to suppress proinflammatory cytokines, we tested whether an adenoviral vector encoding human IL-10 (AD-h-IL10) would block and reverse pain facilitation. Three pain models were examined, all of which are mediated by spinal pro-inflammatory cytokines. Acute intrathecal administration of rat IL-10 protein itself briefly reversed chronic constriction injury-induced mechanical allodynia and thermal hyperalgesia. The transient reversal caused by IL-10 protein paralleled the half-life of human IL-10 protein in the intrathecal space (t(1/2) approximately 2 h). IL-10 gene therapy both prevented and reversed thermal hyperalgesia and mechanical allodynia, without affecting basal responses to thermal or mechanical stimuli. Extra-territorial, as well as territorial, pain changes were reversed by this treatment. Intrathecal AD-h-IL10 injected over lumbosacral spinal cord led to elevated lumbosacral cerebrospinal fluid (CSF) levels of human IL-10, with far less human IL-10 observed in cervical CSF. In keeping with IL-10's known anti-inflammatory actions, AD-h-IL10 lowered CSF levels of IL-1, relative to control AD. These studies support that this gene therapy approach provides an alternative to neuronally focused drug and gene therapies for clinical pain control.

Controlling neuropathic pain by adeno-associated virus driven production of the anti-inflammatory cytokine, interleukin-10

Despite many decades of drug development, effective therapies for neuropathic pain remain elusive. The recent recognition of spinal cord glia and glial pro-inflammatory cytokines as important contributors to neuropathic pain suggests an alternative therapeutic strategy; that is, targeting glial activation or its downstream consequences. While several glial-selective drugs have been successful in controlling neuropathic pain in animal models, none are optimal for human use. Thus the aim of the present studies was to explore a novel approach for controlling neuropathic pain. Here, an adeno-associated viral (serotype II; AAV2) vector was created that encodes the anti-inflammatory cytokine, interleukin-10 (IL-10). This anti-inflammatory cytokine is known to suppress the production of pro-inflammatory cytokines. Upon intrathecal administration, this novel AAV2-IL-10 vector was successful in transiently preventing and reversing neuropathic pain. Intrathecal administration of an AAV2 vector encoding beta-galactosidase revealed that AAV2 preferentially infects meningeal cells surrounding the CSF space. Taken together, these data provide initial support that intrathecal gene therapy to drive the production of IL-10 may prove to be an efficacious treatment for neuropathic pain.

Depolarisation phenomena in traumatic and ischaemic brain injury

1. Cortical spreading depression is a non-physiological global depolarisation of neurones and astrocytes that can be initiated with varying degrees of difficulty in the normally perfused cerebral cortex in the experimental laboratory. Induction is typically with electrical stimulation, needling of the cerebral cortex, or superfusion of isotonic or more concentrated potassium chloride solution. The phenomenon propagates across the cerebral cortex at a rate of 2-5 mm per minute, and is accompanied by marked but transient increases in cerebral blood flow, in local tissue oxygen tension, and most probably in metabolic rate. 2. Peri-infarct depolarisation is also a depolarisation event affecting neurones and glia, with an electrophysiological basis similar or identical to CSD, but occurring spontaneously in the ischaemic penumbra or boundary zone in focal cerebral cortical ischaemia. Most such events arise from the edge of the ischaemic core, and propagate throughout the penumbra, at a rate similar to that of cortical spreading depression. 3. Cortical spreading depression in the normally perfused cortex does not result in histological damage whereas peri-infarct depolarisations augment neuronal damage in the penumbra, and are believed by many authors to constitute an important, or the principal, mechanism by which electrophysiological penumbra progressively deteriorates, ultimately undergoing terminal depolarisation and thus recruitment into an expanded core lesion. 4. There is some experimental evidence to suggest that under some circumstances induction of episodes of cortical spreading depression can confer protection against subsequent ischaemic insults. 5. Although cortical spreading depression and peri-infarct depolarisations have been extensively studied in the experimental in vivo models, there is now clear evidence that depolarisations also occur and propagate in the human brain in areas surrounding a focus of traumatic contusion. 6. Whether such events in the injured human brain represent cortical spreading depression or peri-infarct depolarisation is unclear. However, invasive and probably non-invasive monitoring methods are available which may serve to distinguish which event has occurred. 7. Much further work will be needed to examine the relationship of depolarisation events in the injured brain with outcome from cerebral ischaemia or head injury, to examine the factors which influence the frequency of depolarisation events, and to determine which depolarisation events in the human brain augment the injury and should be prevented.

General anesthetics attenuate gap junction coupling in P19 cell line

Gap junction communication is widespread throughout the mammalian nervous system among neurons as well as glia. We addressed the hypothesis that general anesthetics attenuate gap junction mediated coupling in P19 cell line that can differentiate into neuronal-like cells and astrocytes and oligodendrocytes. We characterized the extent of dye coupling over time in the P19 cell line using colocalization of chlormethylbenzamido-1,1 dioctadecyl-3,3,3',3'-tetramethylindocarbocyamine (CM-DiI) and calcein-AM in donor and recipient cells in cocultures. After seeding, the gap junction permeant dye calcein spreads from donor to recipient cells. CM-DiI and calcein fluorescence identified donor and recipient cells, respectively. The extent of intercellular connections was evaluated using cell counting and flow cytometry up to 2 hr after treatment. Clinically relevant concentrations of the intravenous anesthetics propofol (15 microM) and thiopental (10 microM) attenuated gap junction permeability in P19 cell cultures. In contrast, halothane, a volatile anesthetic in a concentration (0.64 mM) relevant to its free aqueous EC50 had no effect on gap junction coupling; however, very high halothane concentrations (2.8 mM) blocked dye transfer by approximately 90%. The results indicate that halothane concentrations pertinent to clinical anesthesia were unable to attenuate gap junction communication in a cell line that can express neuronal and glial gap junction proteins; however, clinically relevant concentrations of propofol and thiopental depressed gap junction coupling.

A role for proinflammatory cytokines and fractalkine in analgesia, tolerance, and subsequent pain facilitation induced by chronic intrathecal morphine

The present experiments examined the role of spinal proinflammatory cytokines [interleukin-1beta (IL-1)] and chemokines (fractalkine) in acute analgesia and in the development of analgesic tolerance, thermal hyperalgesia, and tactile allodynia in response to chronic intrathecal morphine. Chronic (5 d), but not acute (1 d), intrathecal morphine was associated with a rapid increase in proinflammatory cytokine protein and/or mRNA in dorsal spinal cord and lumbosacral CSF. To determine whether IL-1 release modulates the effects of morphine, intrathecal morphine was coadministered with intrathecal IL-1 receptor antagonist (IL-1ra). This regimen potentiated acute morphine analgesia and inhibited the development of hyperalgesia, allodynia, and analgesic tolerance. Similarly, intrathecal IL-1ra administered after the establishment of morphine tolerance reversed hyperalgesia and prevented the additional development of tolerance and allodynia. Fractalkine also appears to modulate the effects of intrathecal morphine because coadministration of morphine with intrathecal neutralizing antibody against the fractalkine receptor (CX3CR1) potentiated acute morphine analgesia and attenuated the development of tolerance, hyperalgesia, and allodynia. Fractalkine may be exerting these effects via IL-1 because fractalkine (CX3CL1) induced the release of IL-1 from acutely isolated dorsal spinal cord in vitro. Finally, gene therapy with an adenoviral vector encoding for the release of the anti-inflammatory cytokine IL-10 also potentiated acute morphine analgesia and attenuated the development of tolerance, hyperalgesia, and allodynia. Taken together, these results suggest that IL-1 and fractalkine are endogenous regulators of morphine analgesia and are involved in the increases in pain sensitivity that occur after chronic opiates.

HIV-1 gp120 stimulates proinflammatory cytokine-mediated pain facilitation via activation of nitric oxide synthase-I (nNOS)

It has become clear that spinal cord glia (microglia and astrocytes) importantly contribute to the creation of exaggerated pain responses. One model used to study this is peri-spinal (intrathecal, i.t.) administration of gp120, an envelope protein of HIV-1 known to activate glia. Previous studies demonstrated that i.t. gp120 produces pain facilitation via the release of glial proinflammatory cytokines. The present series of studies tested whether spinal nitric oxide (NO) contributes to i.t. gp120-induced mechanical allodynia and, if so, what effect NO has on spinal proinflammatory cytokines. gp120 stimulation of acutely isolated lumbar dorsal spinal cords released NO as well as proinflammatory cytokines (tumor necrosis factor-alpha, interleukin-1beta (IL1), interleukin-6 (IL6)), thus identifying NO as a candidate mediator of gp120-induced behavioral effects. Behaviorally, identical effects were observed when gp120-induced mechanical allodynia was challenged by i.t. pre-treatment with either a broad-spectrum nitric oxide synthase (NOS) inhibitor (L-NAME) or 7-NINA, a selective inhibitor of NOS type-I (nNOS). Both abolished gp120-induced mechanical allodynia. While the literature pre-dominantly documents that proinflammatory cytokines stimulate the production of NO rather than the reverse, here we show that gp120-induced NO increases proinflammatory cytokine mRNA levels (RT-PCR) and both protein expression and protein release (serial ELISA). Furthermore, gp120 increases mRNA for IL1 converting enzyme and matrix metalloproteinase-9, enzymes responsible for activation and release of proinflammatory cytokines.

Ethanol suppression of ventral tegmental area GABA neuron electrical transmission involves N-methyl-D-aspartate receptors

Ventral tegmental area (VTA) GABA neurons are critical substrates modulating the mesocorticolimbic dopamine system implicated in natural and drug reward. The aim of this study was to evaluate the effects of ethanol on glutamatergic and GABAergic modulation of VTA GABA neuron electrical synaptic transmission. We evaluated the effects of systemic ethanol (0.05-2.0 g/kg i.p.), the N-methyl-D-aspartate (NMDA) receptor antagonist dizocilpine (MK-801; 0.05-0.2 mg/kg i.v.), the connexin-36 gap junction blocker quinidine (5-20 mg/kg i.v.), the fast-acting barbiturate methohexital (Brevital; 5-10 mg/kg i.v.), and the benzodiazepine chlordiazepoxide (Librium; 5-10 mg/kg i.v.), as well as in situ VTA administration of NMDA and the GABA(A) receptor agonist muscimol, on VTA GABA neuron spontaneous activity and internal capsule stimulus-induced poststimulus spike discharges (ICPSDs). Systemic ethanol, quinidine, and dizocilpine reduced, whereas local NMDA enhanced, and the systemic and local GABA(A) receptor modulators did not significantly alter VTA GABA neuron ICPSDs. Ethanol potentiated dizocilpine inhibition of VTA GABA neuron ICPSDs, but not quinidine inhibition. In situ microelectrophoretic application of dopamine markedly enhanced VTA GABA neuron firing rate (131%), spike duration (124%), and spike coupling, which were blocked by systemic quinidine. These findings indicate that VTA GABA neurons are coupled electrically via gap junctions and that the inhibitory effect of ethanol on electrical transmission is primarily via inhibition of NMDA receptor-mediated excitation, not via enhancement of GABA receptor-mediated inhibition. Thus, the rewarding properties of ethanol may result from inhibitory effects on excitatory glutamatergic neurotransmission between electrically coupled networks of midbrain GABA neurons.

Attenuation of Gap-Junction-Mediated Signaling Facilitated Anesthetic Effect of Sevoflurane in the Central Nervous System of Rats

Accumulating evidence suggests that reduction of intrinsic excitability or synaptic excitation and/or an enhancement of synaptic inhibition underlie the general anesthetic condition. Besides chemical synapse, neurons could communicate with each other by electrical coupling via gap-junctions. We hypothesized that inhibition of cell-to-cell signaling through gap-junction in the central nervous system (CNS) is involved in the anesthetic mechanism of volatile anesthetics. The minimum alveolar concentration (MAC) of sevoflurane was measured after the intracerebroventricular (ICV) or intrathecal (IT) administration of carbenoxolone (CBX), a gap-junction inhibitor, in vivo. The spontaneous oscillation in membrane currents of locus coeruleus neurons that results from electrical coupling between neurons was also recorded from young rat pontine slices by the patch clamp method, and the effect of sevoflurane on this oscillation was examined in vitro. The ICV administration of CBX (125 and 250 micro g/rat) significantly reduced the MAC of sevoflurane dose-dependently, whereas IT injection failed to inhibit the MAC. Sevoflurane at clinically relevant concentrations (0.1-0.5 mM) suppressed the spontaneous oscillation in membrane current concentration-dependently. These suppressions were significant at 0.5 mM with both amplitude and frequency. We suggest that suppression of gap-junction-mediated signaling in the CNS is involved in the anesthetic-induced immobilization by sevoflurane.

IMPLICATIONS

The intracerebroventricular administration of the gap-junction inhibitor, carbenoxolone, reduced the MAC of sevoflurane, and sevoflurane suppressed the signaling through gap-junctions in the central nervous system. The inhibition of gap-junctions may be one of the mechanisms and the site of action of sevoflurane.

Desflurane affords greater protection than halothane against focal cerebral ischaemia in the rat

BACKGROUND

We studied the potential neuroprotective effects of halothane and desflurane, compared with the awake state, on infarct size following 2 h of intraluminal middle cerebral artery occlusion (MCAo) and 22 h of reperfusion.

METHODS

Male Sprague-Dawley rats were anaesthetized with desflurane or halothane, intubated, and mechanically ventilated. Mean arterial pressure (MAP), blood gases, and pH were controlled. Body temperature was maintained at 37.5-38 degrees C. Animals were assigned to one of four groups according to the anaesthetic type (halothane or desflurane) and the duration of anaesthesia: "short-duration", during the preparation only; "long-duration", during both preparation and ischaemia. Twenty-four hours after MCAo, infarcts were visualized by staining with 2,3,5-triphenyltetrazolium chloride. Two additional groups of rats were subjected to the same protocol as that of long-duration halothane and long-duration desflurane with additional pericranial temperature measurements made.

RESULTS

Physiological parameters were comparable between the groups but MAP was higher (P<0.0001) in the short-duration groups. In the short-duration groups, cerebral infarct volumes were not significantly different between anaesthetics (short-duration halothane: 288 (61) mm(3), mean (SD); short-duration desflurane: 269 (71) mm(3), P>0.56). Compared with the awake state (short-duration groups), halothane and desflurane significantly reduced infarct volumes (long-duration halothane: 199 (54) mm(3), P<0.0047 vs short-duration halothane; long-duration desflurane: 121 (55) mm(3), P<0.0001 vs short-duration desflurane). The mean infarct volume in the long-duration desflurane group was significantly lower than that in the long-duration halothane group (P<0.0053). Pericranial temperatures were similar in the desflurane and halothane long-duration groups (P>0.17).

CONCLUSIONS

In rats, desflurane-induced neuroprotection against focal cerebral ischaemia was greater than that conferred by halothane.

Spinal glia and proinflammatory cytokines mediate mirror-image neuropathic pain in rats

Mirror-image allodynia is a mysterious phenomenon that occurs in association with many clinical pain syndromes. Allodynia refers to pain in response to light touch/pressure stimuli, which normally are perceived as innocuous. Mirror-image allodynia arises from the healthy body region contralateral to the actual site of trauma/inflammation. Virtually nothing is known about the mechanisms underlying such pain. A recently developed animal model of inflammatory neuropathy reliably produces mirror-image allodynia, thus allowing this pain phenomenon to be analyzed. In this sciatic inflammatory neuropathy (SIN) model, decreased response threshold to tactile stimuli (mechanical allodynia) develops in rats after microinjection of immune activators around one healthy sciatic nerve at mid-thigh level. Low level immune activation produces unilateral allodynia ipsilateral to the site of sciatic inflammation; more intense immune activation produces bilateral (ipsilateral + mirror image) allodynia. The present studies demonstrate that both ipsilateral and mirror-image SIN-induced allodynias are (1) reversed by intrathecal (peri-spinal) delivery of fluorocitrate, a glial metabolic inhibitor; (2) prevented and reversed by intrathecal CNI-1493, an inhibitor of p38 mitogen-activated kinases implicated in proinflammatory cytokine production and signaling; and (3) prevented or reversed by intrathecal proinflammatory cytokine antagonists specific for interleukin-1, tumor necrosis factor, or interleukin-6. Reversal of ipsilateral and mirror-image allodynias was rapid and complete even when SIN was maintained constantly for 2 weeks before proinflammatory cytokine antagonist administration. These results provide the first evidence that ipsilateral and mirror-image inflammatory neuropathy pain are created both acutely and chronically through glial and proinflammatory cytokine actions.

Spreading and synchronous depressions of cortical activity in acutely injured human brain

BACKGROUND AND PURPOSE

Cortical spreading depression (CSD) has been much studied experimentally but never demonstrated unequivocally in human neocortex by direct electrophysiological recording. A similar phenomenon, peri-infarct depolarization, occurs in experimental models of stroke and causes the infarct to enlarge. Our current understanding of the mechanisms of deterioration in the days after major traumatic or ischemic brain injury in humans has not yielded any effective, novel drug treatment. This study sought clear evidence for the occurrence and propagation of CSD in the injured human brain.

METHODS

In 14 patients undergoing neurosurgery after head injury or intracranial hemorrhage, we placed electrocorticographic (ECoG) electrodes near foci of damaged cortical tissue.

RESULTS

Transient episodes of depressed ECoG activity that propagated across the cortex at rates in the range of 0.6 to 5.0 mm/min were observed in 5 patients; this rate of propagation is characteristic of CSD. We also observed, in 8 of the 14 patients, transient depressions of ECoG amplitude that appeared essentially simultaneous in all recording channels, without clear evidence of spread.

CONCLUSIONS

These results indicate that CSD or similar events occur in the injured human brain and are more frequent than previously suggested. On the basis of these observations, we suggest that the related phenomenon, peri-infarct depolarization, is indeed likely to occur in boundary zones in the ischemic human cerebral cortex.

Cortical spreading depression in the feline brain following sustained and transient stimuli studied using diffusion-weighted imaging

Cortical spreading depression (CSD) was induced by transient (10 min) applications of KCl in agar upon the cortical surface of alpha-chloralose anaesthetised cats. Its features were compared with CSD resulting from sustained applications of crystalline KCl through a mapping of the apparent diffusion coefficient (ADC) using diffusion-weighted echo planar imaging (DWI) over a poststimulus period of 60-100 min. Individual CSD events were computationally detected with the aid of Savitzky-Golay smoothing applied to critically sampled data derived from regions of interest (ROIs) made up of 2 x 2 pixel matrices. The latter were consistently placed at three selected sites on the suprasylvian gyrus (SG) and six sites on the marginal gyrus (MG). The CSD events thus detected were then quantitatively characterised for each ROI using the original time series. Both stimuli consistently elicited similar spreading patterns of initial, primary CSD events that propagated over the SG and marginal MG and were restricted to the hemispheres on which the stimuli were applied. There followed secondary events over smaller extents of cortical surface. Sustained stimuli elicited primary and secondary CSD events with similar amplitudes of ADC deflection that were distributed around a single mean. The ADC deflections were also conserved in peak amplitude throughout the course of their propagation. The initial primary event showed a poststimulus latency of 1.1 +/- 0.1 min. Successive secondary events followed at longer, but uniform, time intervals of around 10 min. Primary and secondary CSDs showed significantly different velocities of conduction (3.32 +/- 0.43 mm min(-1) vs. 2.11 +/- 0.21 mm min(-1), respectively; n = 5) across the cerebral hemisphere. In contrast, transient stimuli produced significantly fewer numbers of CSD events (3.8 +/- 0.5 events per animal, n = 5) than did sustained stimuli (7.4 +/- 0.5 events per animal, mean +/- S.E.M., n = 5, P = 0.002). The peak ADC deflection of their primary CSD events declined by approximately 30 % as they propagated from their initiation site to the interhemispheric boundary. The primary CSD event following a transient stimulus showed a latency of 1.4 +/- 0.1 min. It was followed by successive and smaller secondary ADC deflections that were separated by progressively longer time intervals. Conduction velocities of secondary events were similar to those of primary events. Conduction velocities of both primary and secondary events were slower than their counterparts following a sustained stimulus. ADC changes associated with CSD thus persist at times well after stimulus withdrawal and vary markedly with the nature of the initiating stimulus even in brain regions remote from the stimulus site.

The Astrocentric Hypothesis: proposed role of astrocytes in consciousness and memory formation

Consciousness is self-awareness. This process is closely associated with attention and working memory, a special form of short-term memory, which is vital when solving explicit task. Edelman has equated consciousness as the "remembered present" to highlight the importance of this form of memory (G.M. Edelman, Bright Air, Brilliant Fire, Basic Books, New York, 1992). The majority of other memories are recollections of past events that are encoded, stored, and brought back into consciousness if appropriate for solving new problems. Encoding prior experiences into memories is based on the salience of each event (A.R. Damasio, Descartes' Error, G.P. Putnam's Sons, New York, 1994; G.M. Edelman, Bright Air, Brilliant Fire, Basic Books, New York, 1992). It is proposed that protoplasmic astrocytes bind attended sensory information into consciousness and store encoded memories. This conclusion is supported by research conducted by gliobiologist over the past 15 years.

Investigation of feline brain anatomy for the detection of cortical spreading depression with magnetic resonance imaging

Cortical spreading depression (CSD) and peri-infarct depolarisation (PID) are related phenomena that have been associated with the human clinical syndromes of migraine (CSD), head injury and stroke (PID). Nevertheless the existence of CSD in man remains controversial, despite the detection of this phenomenon in the brains of most, if not all, other animal species investigated. This failure to unambiguously detect CSD clinically may be at least partly due to the anatomically complex, gyrencephalic structure of the human brain. This study was designed to establish conditions for the study of CSD in the brain of a gyrencephalic species using the noninvasive technique of magnetic resonance imaging (MRI). The 3-dimensional (3D) gyrencephalic anatomy of the cat brain was examined to determine the imaging conditions necessary to detect CSD events. Orthogonal transverse, sagittal and horizontal T1-weighted image slices showed that the marginal and suprasylvian gyri were the most appropriate cortical structures to study CSD. This was in view of (1) their simple geometry: (2) their lengthy extent of grey matter orientated rostrocaudally in the cortex: (3) their separation by a sulcus across which CSD spread could be studied and (4) the discontinuity in the grey matter in these regions between the right and left hemispheres dorsal to the corpus callosum. The structure suggested by the T1-weighted images was corroborated by systematic diffusion tensor imaging to map the fractional anisotropy and diffusion trace. Thus a single horizontal image plane could visualise the neighbouring suprasylvian and marginal gyri of both cerebral hemispheres, whereas its complex shape and position ruled out the ectosylvian gyrus for CSD studies. With the horizontal imaging plane, CSD events were reproducibly detected by animating successive diffusion-weighted MR images following local KCl stimulation of the cortical surface. In single image frames, CSD detection and characterisation required image subtraction or statistical mapping methods that, nevertheless, yielded concordant results. In repeat experiments, CSD events were qualitatively similar in appearance whether elicited by sustained or transient KCl applications. Our experimental approach thus successfully describes cat brain anatomy in vivo, and elucidates the necessary conditions for the application of MRI methods to detect CSD propagation.

The effects of anesthetics on cortical spreading depression elicitation and c-fos expression in rats

The effects of anesthetics on the generation of cortical spreading depression (CSD) were investigated. Volatile anesthetics halothane, isoflurane, sevoflurane (0.5, 1.0, and 2.0 MAC), and the intravenous anesthetic pentobarbital were studied. Cortical spreading depression was induced by 3M-KCl applied to a surface of brain cortex for 30 minutes. Direct current (DC) potential was recorded, and the number, amplitude, and duration of CSDs were observed. With increasing concentrations of each volatile anesthetic, there was a dose-related reduction in CSD frequency but not in CSD amplitude. At 2.0 MAC of sevoflurane the suppression of CSD was less than with the other volatile anesthetics. In addition, the influence of anesthetics on expression of c-fos mRNA was investigated. Additional animals anesthetized by isoflurane or sevoflurane were studied. Five CSDs were elicited by electric stimulation (0.5 mV, 1 second) in each animal. In situ hybridization with 35S-labeled oligonucleotides was used to evaluate the level of c-fos mRNA. The expression of c-fos was observed in the hemisphere in which CSD was elicited, but there was no difference in expression of c-fos among the groups. We conclude that volatile anesthetics can induce suppression of CSD elicitation in a dose dependent manner, but that at high concentrations sevoflurane is significantly less effective than other volatile agents. Pentobarbital has the least effect on KCl-induced CSD. These data suggest that the choice of anesthetics can impact the results of studies examining membrane depolarization and the ionic changes initiated by CSD.

A quantitative analysis of cortical spreading depression events in the feline brain characterized with diffusion-weighted MRI

Cortical spreading depression (CSD) in the gyrencephalic cat brain was detected with diffusion-weighted echoplanar (DWEP) magnetic resonance imaging (4-8/min for 1-2 hours) using a horizontal imaging plane through the suprasylvian (SG) and marginal gyri. A t-statistic mapping technique allowed a quantitative characterization of the passage of events through single-image pixels (0.15 mm(2)), thus providing a resolution unavailable to previous studies in which time-dependent changes instead were derived from averaging data over relatively large ROIs. Using the enhanced analysis, CSD events initiated by KCl could be quantified for the first time as primary or secondary according to their spatial and temporal features. Primary events covered 26.2 +/- 9.9 mm(2)of cortical surface (mean +/- SD, n = 7 experiments) and propagated rapidly (3.5 +/- 0.65 mm * min(-1)) with a hemispherical geometry. In contrast, the subsequent secondary events were multiple, spatially restricted (covering 7.6 +/- 4.6 mm(2), P < 0.005), slower in propagation (2.6 +/- 0.41 mm * min(-1), P < 0.012), and often confined to the originating gyrus (26 out of 59 events). However, both event types were associated with significantly reduced apparent diffusion coefficients (ADCs; from 800 to approximately 660 x 10(-6) mm(2)* s(-1), P < 0.05) that were similar for both primary (21 +/- 5.1%) and secondary waves (18 +/- 7. 7%) and that had similar durations (full width at half-maximal height: 86 +/- 17 vs. 79 +/- 20 seconds, respectively). These findings associate CSD for the first time with two categories of ADC disturbance that are similar in amplitude and duration but that differ in spatial extent, velocity, and extensiveness of spread.

Factors influencing the frequency of fluorescence transients as markers of peri-infarct depolarizations in focal cerebral ischemia

BACKGROUND AND PURPOSE

Peri-infarct depolarizations (PIDs) that occur in ischemic boundary zones of the cerebral cortex of experimental animals have been shown to promote rather than simply to indicate the evolution of the lesion and are especially prominent in the rat. To study the influence of one factor, species, on PID incidence, we compared the frequency of PIDs in a primate species, the squirrel monkey, with that in the cat after middle cerebral artery occlusion. Plasma glucose was reviewed as a possible cause of interexperiment variability in the cat experiments.

METHODS

In open-skull experiments under chloralose anesthesia, changes in cortical fluorescence believed to indicate NADH/NAD(+) redox state, as markers of PIDs, were recorded by serial imaging of the cortical surface in vivo for 4 hours after middle cerebral artery occlusion.

RESULTS

Fluorescence transients occurred in squirrel monkeys at a frequency (mean+/-SD) of 0.7+/-0.8 hours(-1) (n=5), which was not significantly less than in that observed in cats (1.3+/-1.6 hours(-1), n=8). Data from the cat experiments indicated a relationship between number of transients (dependent) and plasma glucose, with a striking increase in PID frequency in association with values of mean postocclusion plasma glucose <4.1 mmol/L (Mann-Whitney U=15.0, P=0.034); this observation agrees well with other published findings.

CONCLUSIONS

Transient changes in fluorescence strongly suggestive of peri-infarct depolarizations, either transient or terminal, occur and propagate in the ischemic cerebral cortex of a nonhuman primate. The results also suggest that the relationship of frequency of peri-infarct depolarizations with plasma glucose requires further examination, to confirm the finding and to determine a safe lower limit for a target range for control of plasma glucose if insulin is used in the management of patients with cerebral ischemia.

Cortical spreading depression in the gyrencephalic feline brain studied by magnetic resonance imaging

1.Time-lapse diffusion-weighted magnetic resonance imaging (DWI) was used to detect and characterize complex waves of cortical spreading depression (CSD) evoked with KCL placed upon the suprasylvian gyrus of anaesthetized cats. 2. The time-lapse representations successfully demonstrated primary CSD waves that propagated with elliptical wavefronts selectively over the ipsilateral cerebral hemispheres with a velocity of 3.8 +/- 0.70 mm min(-1) (mean +/- S.E.M. of 5 experiments). 3. In contrast, the succeeding secondary waves often remained within the originating gyrus, were slower (velocity 2.0 +/- 0.18 mm min(-1), more fragmented and varied in number. 4. Computed traces of the apparent diffusion coefficients (ADCs) showed negative deflections followed by monotonic decays (amplitudes: primary wave, -19.9 +/- 2.8%; subsequent waves, -13.6 +/- 1.9% duration at half-maximal decay, 150-200 s) when determined from regions of interest (ROIs) through which both primary and succeeding CSD waves propagated. 5. The passage of both the primary and the succeeding waves often correlated with transient DC potential deflections recorded from the suprasylvian gyrus. 6. The detailed waveforms of the ADC and the T2*-weighted (blood oxygenation level-dependent: BOLD) traces showed a clear reciprocal correlation. These imaging features that reflect disturbances in cellular water balance agree closely with BOLD measurements that followed the propagation velocities of the first and subsequent CSD events. They also provide a close physiological correlate for clinical observations of cortical blood flow disturbances associated with human migraine.

An intrinsic neuronal-like network in the rat pineal gland

Recent studies have shown that in rat pineal glands kept in vitro action potential-producing cell clusters are demonstrable. To test whether the clusters interact, multiple-unit recordings were carried out simultaneously from different clusters, with or without electrical stimulation. Clusters with rhythmic burst activity exhibit highly synchronized firing and electrical stimulation of one cluster elicits an immediate response in another one, apparently involving synapses but not gap junctions. It is hypothesized that the interacting clusters form a network. As the firing is affected by norepinephrine, acetylcholine and Ca2+, the network may monitor the interstitial concentrations of these substances and spread this information in the gland, to modulate melatonin secretion.

Halothane as a neuroprotectant during constant stimulation of the perforant path

PURPOSE

To determine the neuroprotective effects of halothane during constant stimulation of the perforant path.

METHODS

Male Sprague-Dawley rats had electrodes implanted into the perforant path and dentate granule cell layer under halothane anaesthesia (1-2% in oxygen). They were then divided into four groups. In group 1 (n = 9), the perforant path was stimulated at 20 Hz for 2 h under halothane anaesthesia (1-2%). In group 2 (n = 3), the animals were unstimulated but maintained under halothane anaesthesia (1-2%) for 2 h with the electrodes in place. Both groups 1 and 2 had the electrodes removed and were then allowed to recover fully from the anaesthetic. In groups 3 and 4, the electrodes were held in place with dental acrylic. Both of these groups were allowed to recover fully from anaesthesia. In group 3 (n = 3), 24-48 h after recovery from anaesthesia, the perforant path was stimulated at 20 Hz for 2 h. Group 4 (n = 3) received no stimulation. After 14-17 days, the rats were killed, and morphometry and cell counts were performed on the hippocampi from rats in groups 1 and 2.

RESULTS

Cell densities were not significantly different between control (group 2), unstimulated rats, and animals stimulated under halothane anaesthesia (group 1). Stimulation in the unanaesthetised rats resulted in severe neuronal loss in hilus, CA1, and CA3.

CONCLUSIONS

Halothane protects hippocampal neurons against damage induced by constant stimulation of the perforant path.

Gap junctional communication and pharmacological heterogeneity in astrocytes cultured from the rat striatum

Indo-1 and fluo-3 imaging techniques were used to investigate the role of gap junctions in the changes in cytosolic calcium concentrations ([Ca2+]i) induced by several receptor agonists. Subpopulations of confluent cultured astrocytes from the rat striatum were superfused with submaximal concentrations of endothelin-1 (Et1) and the alpha 1-adrenergic and muscarinic receptor agonists, methoxamine and carbachol, respectively. 2. Combined binding and autoradiographic studies indicated that all striatal astrocytes possess binding sites for Et1. In contrast, alpha 1-adrenergic and muscarinic binding sites were found to be heterogeneously distributed. In agreement with these findings, Et1 induced fast calcium responses in all cells while only subsets of striatal astrocytes responded to the application of methoxamine or carbachol. 3. Halothane, heptanol and octanol, which are commonly used as gap junction inhibitors, drastically reduced the amplitude of Et1-induced calcium responses. In contrast, 18-alpha-glycyrrhetinic acid (alpha GA) used at a concentration known to block gap junction permeability in astrocytes had no significant effect on the amplitude of these calcium responses. 4. As demonstrated by quantitative and topological analysis, Et1 application similarly increased [Ca2+]i levels in all astrocytes in both the absence and presence of alpha GA. 5. In control conditions, subpopulations of cells responding to methoxamine or carbachol exhibited two main types of calcium responses which differed in their shape and kinetic characteristics. In the presence of alpha GA the number of cells responding to these receptor agonists was significantly reduced. Indeed, responses characterized by their long latency, slow rise time and weak amplitude disappeared in the presence of alpha GA while responses with short latency and fast rise time were preserved. 6. These results indicate that permeable gap junction channels tend to attenuate the pharmacological and functional heterogeneity of populations of astrocytes, while their inhibition restricts calcium responses in astrocytes expressing high densities of transmitter receptors coupled to phospholipase C.

Immunorecognition, ultrastructure and phosphorylation status of astrocytic gap junctions and connexin43 in rat brain after cerebral focal ischaemia

Gap junctions between astrocytes support a functional syncytium that is thought to play an important role in neural homeostasis. In order to investigate regulation of this syncytium and of connexin43 (Cx43), a principal astrocytic gap junction protein, we determined the sequelae of gap junction and Cx43 disposition in a rat cerebral focal ischaemia model with various ischaemia/reperfusion times using sequence-specific anti-Cx43 antibodies (designated 13-8300, 18A, 16A and 71-0700) that exhibit differential recognition of Cx43, perhaps reflecting functional aspects of gap junctions. Antibody 13-8300 specifically detects only an unphosphorylated form of Cx43 in both Western blots and tissue sections. In hypothalamus after brief (15 min) ischaemic injury, Cx43 at intact gap junctions undergoes dephosphorylation, accompanied by reduced epitope recognition by antibodies 16A and 71-0700. Tissue examined 24 h after reperfusion showed that these effects were reversible. Astrocytic gap junction internalization occurring 1 h after ischaemia was accompanied by decreased immunodetection with 13-8300. At this time, gap junctions were absent in the ischaemic core, coinciding with a loss of Cx43 recognition with 18A and 13-8300, but elevated labelling of internalized Cx43 with 16A and 71-0700. Unphosphorylated Cx43 persisted at intact gap junctions confined to a thin corridor at the ischaemic penumbra which contained presumptive apoptotic cell profiles. Similar results were obtained in ischaemic striatum and cerebral cortex, though with a delayed time course that depended on the severity of the ischaemic insult. These results demonstrate that astrocytic Cx43 epitope masking, dephosphorylation and cellular redistribution occur after ischaemic brain injury, proceed as a temporally and spatially ordered sequence of events and culminate in differential patterns of Cx43 modification and sequestration at the lesion centre and periphery. These observations suggest an attempt by astrocytes in the vicinity of injury to remodel the junctional syncytium according to altered tissue homeostatic requirements.

The concept of chemical neurotransmission--variations on the theme

The present concept of chemical neurotransmission occurring purely through synaptic transmission has dominated neurobiological thinking for about the last 40 years. According to this conventional view neurotransmitters are substances that are synthesized within the neurones, liberated into the synaptic cleft after stimulation of the nerve, and that finally elicit a biologically plausible response in the postsynaptic target cell or the nerve terminal itself. This concept undoubtedly comprises the main body of interneuronal chemical signalling. However, a large amount of evidence, obtained during the last two decades, suggests that there are a number of parallel mechanisms, which may essentially participate in neuronal signalling, or at least modulate it. Thus, the recent progress of research has provided the following compelling evidence: 1) a large variety of substances, some of them synthesized in non-neuronal cells, actually participate actively in neuronal signalling; 2) functional connections in brain are not determined by the synaptic connections only; 3) glial cells have an active and fundamental role in signal transmission; and 4) the signalling properties and mechanisms of each neurone are constantly under functional and structural regulation. The aim of this review is to present shortly some of the central concepts and/or mechanisms that have risen during the last two decades. Also the functional and/or clinical relevance of these mechanisms is addressed briefly.

Isoflurane and pentobarbital reduce the frequency of transient ischemic depolarizations during focal ischemia in rats

Repetitive transient ischemic depolarizations (IDs) during focal cerebral ischemia are thought to contribute to ischemic damage. Isoflurane and pentobarbital reduce injury (versus the nonanesthetized state) after focal cerebral ischemia. The mechanism by which these drugs reduce injury is not known. This protective effect might be mediated by a reduction in the number of IDs. We measured the frequency of IDs during focal cerebral ischemia in animals anesthetized with isoflurane or pentobarbital and compared it with that in N2O/fentanyl anesthetized animals and in animals in which the N-methyl-D-aspartate receptor antagonist MK801 (dizocilpine) was given. Focal cerebral ischemia was induced by the occlusion of the middle cerebral artery for a period of 2 h. Cortical infarct volumes were determined after 3 h of reperfusion by image analysis of 2,3,5-triphenyl tetrazolium-stained coronal brain sections. The infarct volume was significantly greater in the N2O/fentanyl group than in the other three groups. Infarct volumes in the isoflurane, pentobarbital, and MK801 groups were similar. The frequency of IDs was significantly greater in the N2O/fentanyl group than in the other three groups, and was the least in the MK801 group. There was a direct correlation between the number of IDs and the volume of tissue injury. The data indicate that the protective effect of isoflurane and pentobarbital might, in part, be determined by their ability to reduce IDs during focal ischemia. However, the observation that the infarct volume was similar in the MK801, isoflurane, and pentobarbital groups, despite a greater frequency of IDs in the latter two groups, suggests that mechanisms other than a simple reduction in the number of IDs probably also play a role in anesthetic-mediated cerebral protection.

IMPLICATIONS

Transient ischemic depolarizations during focal ischemia contribute to brain injury. Both isoflurane and pentobarbital reduced the frequency of these depolarizations. Isoflurane- and pentobarbital-mediated reduction in the frequency of depolarizations might, in part, mediate the previously documented neuroprotective effect of these drugs.

Altered gap junctional communication, intercellular signaling, and growth in cultured astrocytes deficient in connexin43

Astrocytes are characterized by extensive intercellular communication mediated primarily by gap junction channels composed of connexin43. To examine this junctional protein in astrocytic functions, astrocytes were cultured from embryonic mice with a null mutation in the connexin43 gene (Reaume et al.: Science 267:1831-1834, 1995). Using anti-Cx43 antibodies, immunoblotting and immunostaining indicated that homozygous null astrocytes were devoid of Cx43. They are also deficient in intercellular dye transfer. Astrocytes cultured from heterozygous embryos express significantly lower Cx43 compared to wild type, and their dye coupling is reduced. Markers of glial differentiation, such as glial fibrillary acidic protein and S100, appeared similar in all genotypes. Measurement of intercellular calcium concentration following mechanical stimulation of confluent astrocytes revealed that the number of cells affected by a rise in intracellular calcium was reduced in homozygous cultures compared to wild type. In fact, the calcium response in homozygous astrocytes was similar to that observed in wild-type astrocytes in the presence of a gap junction blocker. The growth rate of astrocytes lacking Cx43 was reduced compared to wild-type astrocytes. These results suggest that gap junctional intercellular communication mediated by Cx43 is not critical for astrocyte differentiation but is likely involved in the regulation of intercellular calcium signaling and cell growth.

Effects of volatile anaesthetics on the membrane potential and ion channels of cultured neocortical astrocytes

Volatile anaesthetics cause changes in the membrane resting potential of central neurons. This effect probably arises from actions on neuronal ion channels, but may also involve alterations in the ion composition of the extracellular space. Since glial cells play a key role in regulating the extracellular ion composition in the brains of mammals, we analyzed the effects of halothane, isoflurane and enflurane on the membrane conductances and ion channels of cultured cortical astrocytes. Astrocytes were dissociated from the neocortex of 0-2-day old rats and grown in culture for 3-4 weeks. Anaesthetic-induced changes in the membrane potential were recorded in the whole cell current-clamp configuration of the patch-clamp technique. We further studied the effects of halothane and enflurane on single ion channels in excised membrane patches. At concentrations corresponding to 1-2 MAC (1 MAC induces general anaesthesia in 50% of the patients and rats), membrane potentials recorded in the presence of enflurane, isoflurane and halothane did not differ significantly from the control values. At higher concentrations, effects of enflurane and halothane, but not of isoflurane, were statistically significant. Single-channel recordings revealed that halothane and enflurane activated a high conductance anion channel, which possibly mediated the effects observed during whole cell recordings. In less than 10% of the membrane patches, volatile anaesthetics either increased or decreased the mean open time of K+-selective ion channels without altering single-channel conductances. In summary, it seems unlikely that the actions of volatile anaesthetics described here are involved in the state of general anaesthesia. Statistically significant effects occurred at concentrations ten times higher than those required to cause half-maximal depression of action potential firing of neocortical neurons in cultured brain slices. However, it cannot be excluded that the changes observed in the membrane conductance of cortical astrocytes disturb the physiological function of these cells, thereby influencing the membrane resting potential of neurons.

Reduction of infarct volume by halothane: effect on cerebral blood flow or perifocal spreading depression-like depolarizations

Halothane is a strong inhibitor of potassium evoked spreading depression (SD) in cats. In the current study, we investigate halothane effects on induction of perifocal SD-like depolarizations, CBF, and infarct evolution in focal ischemia. Calomel and platinum electrodes measured cortical direct current potential and CBF in ectosylvian, suprasylvian, and marginal gyri. Left middle cerebral artery occlusion (MCAO) induced permanent focal ischemia for 16 hours in artificially ventilated cats (30% oxygen, 70% nitrous oxide) under halothane (0.75%, n = 8) or alpha-chloralose anesthesia (60 mg/kg intravenously, n = 7). Under alpha-chloralose, MCAO induced severe ischemia in ectosylvian and suprasylvian gyri(mean CBF < 10 mL/100 g/min), and direct current potentials turned immediately into terminal depolarization. In marginal gyri, CBF reduction was mild (more than 20 mL/100 g/min), and in six of seven animals, frequent SD-like depolarizations turned into terminal depolarization at a later stage of the experiments. Under halothane, MCAO induced severe ischemia (less than 10 mL/100 g/min) and immediate terminal depolarization only in ectosylvian gyrus. In suprasylvian gyrus, residual CBF remained significantly higher (more than 10 mL/100 g/min) than under alpha-chloralose, whereas in marginal gyri, CBF did not differ between groups. Compared with chloralose, the number of transient depolarizations was significantly reduced in marginal gyrus, and in suprasylvian gyrus transient but significantly longer depolarizations than in marginal gyrus were recorded. Except for one animal, transient depolarizations did not turn into terminal depolarization under halothane, and infarct volume reduction was particularly seen in suprasylvian gyrus. We conclude that halothane, the most commonly used anesthetic in studies of experimental brain ischemia, has protective properties, which may depend on both cerebrovascular and electrophysiologic influences.

Lack of biocytin transfer at gap junctions in the chicken vestibular nuclei

In vivo experiments were designed to test for functional gap junctions at 'mixed' synapses that were morphologically characterized between the large-diameter, primary vestibular fibers and second-order vestibular neurons in the chicken, Gallus gallus. In previous intracellular recordings and dye injections into these neurons from brain slice preparations of chick embryos (E15/16) and also newborn hatchlings (HI-2), no evidence was obtained for functional gap junctions. Therefore, biocytin, a low molecular weight tracer that permeates gap junction channels, was extracellularly applied to either the ampullary nerves or to the vestibular ganglion of 3-6 day old hatchlings and adult chickens (9 months). This procedure resulted in the uptake of the dye and heavy staining of both the thick and thin fibers composing the vestibular nerve and in loading of vestibular efferent neurons. However, no dye transfer was observed between the large-diameter, primary vestibular fibers and second-order vestibular neurons. This observation, which was performed using a relatively non-invasive approach on intact animals, suggests that the gap junctions at these mixed synapses are probably not functional under the conditions of these experiments.

Effects of volatile anesthetics on hippocampal slice metabolism, response to anoxia with and without glucose

Anesthetics attenuate ischemic damage and so are often not used when preparing hippocampal slices for studies of ischemic or anoxic damage. In this study, we tested whether halothane, ether or isoflurane, when used briefly during slice preparation, have persistent effects on slice ATP metabolism, protein synthesis or morphology. We also tested the effects of anoxia with and without glucose on these slices. Five minutes of anoxia without glucose (anoxia-aglycemia) caused a dramatic depletion of ATP to less than 22% of control levels, a persistent inhibition of neuronal protein synthesis to less than 10% of control rates and severe morphological damage. Slices prepared using volatile anesthetics showed the same degree of damage due to anoxia-aglycemia, when compared with untreated tissue. In contrast, 5 min anoxia caused a 40% decrease in ATP levels in untreated tissue, but did not damage protein synthesis or morphology. While isoflurane-treated tissue responded identically to anoxia as untreated tissue, both halothane and ether prevented the anoxic ATP fall. These findings suggest that while halothane and ether may have persistent effects on slice responses to anoxia, isoflurane is a good candidate anesthetic for slice preparation procedures.

Plasma albumin induces calcium waves in rat cortical astrocytes

Changes in intracellular calcium were monitored in cultured cortical astrocytes stimulated with albumin. Albumin elicited intracellular calcium mobilisation from intracellular stores, inducing repetitive intracellular calcium oscillations. The oscillations were not blocked by ryanodine, a blocker of the Ca-induced Ca release mechanism, and the release occurred from the same store as is accessed by glutamate and bradykinin, both of which release calcium by an IP3-dependent mechanism. Calcium signals induced by albumin appear therefore to occur via a pure IP3-dependent mechanism. When albumin was applied to confluent monolayers of astrocytes, the oscillations in individual cells were initially unsynchronised, but after several minutes of application, the Ca2 oscillations were observed to synchronise and spread through the astrocyte network as a wave. These intercellular calcium waves were inhibited by the gap junction blocker halothane. Using the fluorescence recovery after photobleaching (FRAP) technique, we demonstrate that the development of propagated waves with prolonged exposure to albumin does not result from an increase in cell coupling. The development of calcium waves on exposure to albumin may be important in the formation of glial scars in the CNS after breakdown of the blood-brain barrier.

Heptanol but not fluoroacetate prevents the propagation of spreading depression in rat hippocampal slices

We investigated whether heptanol and other long-chain alcohols that are known to block gap junctions interfere with the generation or the propagation of spreading depression (SD). Waves of SD were triggered by micro-injection of concentrated KCl solution in stratum (s.) radiatum of CA1 of rat hippocampal tissue slices. DC-coupled recordings of extracellular potential (V0) were made at the injection and at a second site approximately 1 mm distant in st. radiatum and sometimes also in st. pyramidale. Extracellular excitatory postsynaptic potentials (fEPSPs) were evoked by stimulation of the Schaffer collateral bundle; in some experiments, antidromic population spikes were evoked by stimulation of the alveus. Bath application of 3 mM heptanol or 5 mM hexanol completely and reversibly prevented the propagation of the SD-related potential shift (delta V0) without abolishing the delta V0 at the injection site. Octanol (1 mM) had a similar but less reliably reversible effect. fEPSPs were depressed by approximately 30% by heptanol and octanol, 65% by hexanol. Antidromic population spikes were depressed by 30%. In isolated, patchclamped CA1 pyramidal neurons, heptanol partially and reversibly depressed voltage-dependent Na currents possibly explaining the slight depression of antidromic spikes and, by acting on presynaptic action potentials, also the depression of fEPSPs. Fluoroacetate (FAc), a putative selective blocker of glial metabolism, first induced multiple spike firing in response to single afferent volleys and then severely suppressed synaptic transmission (confirming earlier reports) without depressing the antidromic population spike. FAc did not inhibit SD propagation. The effect of alkyl alcohols is compatible with the idea that the opening of normally closed neuronal gap junctions is required for SD propagation. Alternative possible explanations include interference with the lipid phase of neuron membranes. The absence of SD inhibition by FAc confirms that synaptic transmission is not necessary for the propagation of SD, and it suggests that normally functioning glial cells are not essential for SD generation or propagation.

Mechanisms of etomidate potentiation of GABAA receptor-gated currents in cultured postnatal hippocampal neurons

The effect of etomidate, an imidazole general anesthetic, on GABAA receptor function was studied in cultured hippocampal neurons. At a clinically relevant concentration of 4.1 microM, etomidate shifts the GABA dose response to the left (ED50 shift from 10.2 to 5.2 microM), with no change in the maximum current evoked by saturating concentrations of GABA. At a higher concentration of 82 microM, etomidate directly induces current in the absence of GABA. Etomidate selectively increases the amplitude and prolongs the duration of miniature inhibitory postsynaptic currents without significant effects on miniature inhibitory postsynaptic currents. The combined effects of etomidate on miniature inhibitory postsynaptic current amplitude and duration enhance the total charge transfer by 280% during a spontaneous synaptic event. Analysis of single channels opened by GABA indicates that 8.2 microM etomidate increases the probability of channels being open 13-fold and increases the effective channel open time two-fold. Given the present understanding of central inhibitory synapses, the effect of etomidate on channel kinetics is most likely to be the predominant mechanism which influences the synaptic function. In addition, etomidate, through its modulation of both channel kinetics and open probability, is likely to have a large impact on extrasynaptic GABAA receptor function.

Propofol influences the electroretinogram to a lesser degree than thiopentone

BACKGROUND

The Electroretinogram (ERG) is used clinically to assess the function of retina. Anaesthetic agents are known to affect ERG, and as anaesthesia is often needed in children and uncooperative patients, knowledge about its effects is of clinical importance. Barbiturates selectively depress ERG components, and we compared thiopentone with propofol to assess if the latter preserved retinal function better.

METHODS

Ten pigs, average weight 17 kg (SD +/- 2 kg) were anaesthetized randomly with propofol 10 mg kg-1 or thiopentone 30 mg kg-1. Anaesthesia was maintained by 65% nitrous oxide in oxygen and continuous infusion of the induction agent, i.e. 10 mg kg-1 h-1 of propofol, or 10 mg kg-1 h-1 for the first hour, then 5 mg kg-1 h-1 of thiopentone, with doses being based on pilot studies. After an interval of one week the programme was repeated using the other agent. After 40 minutes dark-adaptation, responses to single flashes of graded intensities from a xenon flashlamp were recorded at five-minute intervals. The a- and b-wave amplitudes and implicit times (time to peak), and a-wave slopes were determined.

RESULTS

The b-wave implicit time was significantly shorter during propofol anaesthesia than when using thiopentone. The effect was most pronounced at the lowest intensities (P < 0.01). No statistically significant differences were found in the amplitudes of the b-waves. The a-wave appeared at lower stimulus intensity (P < 0.05) and the a-wave slopes were significantly steeper (P < 0.01) during propofol anaesthesia.

CONCLUSION

Propofol accordingly appeared to preserve the photoreceptor response better than thiopentone, and may therefore be considered to be more suitable for ERG recordings than thiopentone.

Halothane, but not alpha-chloralose, blocks potassium-evoked cortical spreading depression in cats

The effects of two anesthetics, halothane and alpha-chloralose, on induction of spreading depression and on extracellular glutamate elevation after intracortical potassium administration were investigated in artificially ventilated (30% oxygen/70% nitrous oxide) cats. High potassium concentrations were achieved using either direct KCl injections (7 microliters, 150 mM via a micropipette) or microdialysis by supplementing 100, 300 or 500 mM KCl, respectively, for 10 min to the perfusion solution (Ringer's). Changes of the cortical DC potential were recorded adjacent (1-2 mm: electrode DC1) and distant (6-7 mm: electrode DC2) to the injection site. Either under halothane (0.75% in the respiratory gas mixture) or under alpha-chloralose (60 mg/kg i.v.) anesthesia, prolonged negative shifts of the DC potential reflecting the elevated potassium levels after KCl injection were measured near the injection site (electrode DC1). In contrast, spreading depressions (transient short DC deflections) were almost exclusively observed under alpha-chloralose. Spreading depressions recorded with electrode DC1 were superimposed on the prolonged negative DC shifts and they propagated frequently to the more distant site (DC2). Upon KCl administration, dose dependent elevations of extracellular glutamate were measured. These elevations were not significantly altered by the type of anesthesia. Our results suggest that in cats, spreading depression induction is affected by anesthesia, i.e., spreading depression induction is inhibited by halothane as compared to alpha-chloralose. Furthermore, factors other than glutamate or high potassium seem to contribute to spreading depression induction.

Astrocytes exhibit regional specificity in gap-junction coupling

Astrocytes are coupled to each other via gap-junctions both in vivo and in vitro. Gap-junction coupling is essential to a number of astrocyte functions including the spatial buffering of extracellular K+ and the propagation of Ca2+ waves. Using fluorescence recovery after photo-bleach, we quantitatively assayed and compared the coupling of astrocytes cultured from six different central nervous system (CNS) regions in the rat: spinal cord, cortex, hypothalamus, hippocampus, optic nerve, and cerebellum. The degree of fluorescence recovery (% recovery) and time constant of recovery (tau) served as quantitative indicators of coupling strength. Gap-junction coupling differed markedly between CNS regions. Coupling was weakest in astrocytes derived from spinal cord (43% recovery, tau approximately 400 s) and strongest in astrocytes from optic nerve (91% recovery, tau approximately 226 s) and cerebellum (95% recovery, tau approximately 100 s). As indicated by the degree of recovery, coupling strength among CNS regions could be ranked as follows: spinal cord < cortex < hypothalamus < hippocampus = optic nerve = cerebellum. Gap-junction coupling also differed between CNS regions with respect to its sensitivity to inhibition by the uncoupling agent octanol. Kd values for 50% inhibition by octanol ranged from 188 microM in spinal cord astrocytes to 654 microM in hippocampal astrocytes. Sensitivity of gap-junctions to octanol could be ranked as follows: spinal cord = cortex = hypothalamus > cerebellum > optic nerve > hippocampus. The observed differences in coupling indicate differences in the number of gap-junction connections in astrocytes cultured from the six CNS regions. These differences may reflect the adaptation of astrocytes to varying functional requirements in different CNS regions.

Connexin37 forms high conductance gap junction channels with subconductance state activity and selective dye and ionic permeabilities

Gap junctions are thought to mediate the direct intercellular coupling of adjacent cells by the open-closed gating of an aqueous pore permeable to ions and molecules of up to 1 kDa or 10-14 A in diameter. We symmetrically altered the ionic composition or asymmetrically added 6-carboxyfluorescein (6-CF, M(r) = 376), a fluorescent tracer, to pairs of connexin37-transfected mouse neuro2A cells to examine the ionic and dye permeability of human connexin37 channels. We demonstrate that the 300-pS channel formed by connexin37 has an effective relative anion/cation permeability ratio of 0.43, directly converts to at least one intermediate (63 pS) subconductance state, and that 6-CF dye transfer is accompanied by a 24% decrease in unitary channel conductance. These observations favor a new interpretation of the gap junction pore consistent with direct ion-channel interactions or electrostatic charge effects common to more conventional multistate ion channels. These results have distinct implications about the different forms of intercellular signaling (cationic, ionic, and/or biochemical) that can occur depending on the expression and conformation of the connexin channel proteins.

Direct signaling from astrocytes to neurons in cultures of mammalian brain cells

Although astrocytes have been considered to be supportive, rather than transmissive, in the adult nervous system, recent studies have challenged this assumption by demonstrating that astrocytes possess functional neurotransmitter receptors. Astrocytes are now shown to directly modulate the free cytosolic calcium, and hence transmission characteristics, of neighboring neurons. When a focal electric field potential was applied to single astrocytes in mixed cultures of rat forebrain astrocytes and neurons, a prompt elevation of calcium occurred in the target cell. This in turn triggered a wave of calcium increase, which propagated from astrocyte to astrocyte. Neurons resting on these astrocytes responded with large increases in their concentration of cytosolic calcium. The gap junction blocker octanol attenuated the neuronal response, which suggests that the astrocytic-neuronal signaling is mediated through intercellular connections rather than synaptically. This neuronal response to local astrocytic stimulation may mediate local intercellular communication within the brain.

Halothane and isoflurane enhance basal and carbachol-stimulated inositol(1,4,5)triphosphate formation in SH-SY5Y human neuroblastoma cells

The cellular mechanisms underlying the clinical effects of volatile anaesthetics remain unknown, although the plasma membrane and its associated proteins are likely targets. One such protein is the enzyme phospholipase C (PLC), which catalyses the formation of the second messenger inositol(1,4,5)triphosphate [Ins(1,4,5)P3]. Using SH-SY5Y human neuroblastoma cells we have demonstrated that halothane (0.50, 0.75 and 1.00%) enhances basal Ins(1,4,5)P3 mass formation approximately 1.8-fold. Halothane also caused a dose-dependent enhancement of carbachol-stimulated biphasic Ins(1,4,5)P3 formation at both the peak (half-maximal stimulation, EC50 = 0.76%) and plateau (EC50 = 0.74%) phases. At 1%, halothane did not alter the affinity for carbachol at either the peak (IC50: air = 9.4 +/- 1.5, halothane = 12.7 +/- 1.0 microM) or plateau (EC50: air = 11.7 +/- 1.2, halothane = 11.6 +/- 1.0 microM) phase, but did increase the maximum Ins(1,4,5)P3 response at both phases (air vs halothane: peak, 79.9 +/- 0.5 vs 124.8 +/- 2.5; plateau, 33.2 +/- 0.5 vs 47.9 +/- 0.6 pmol/mg protein). Isoflurane (2%) also enhanced basal and carbachol-stimulated Ins(1,4,5)P3 formation 2-fold and 1.5-fold, respectively. In summary, clinically relevant doses of the volatile anaesthetics halothane and isoflurane enhance basal and carbachol-stimulated Ins(1,4,5)P3 formation. Thus, activation of PLC, and subsequent potential Ins(1,4,5)P3-mediated rises in intracellular calcium, could play a part in the cellular mechanisms of volatile agent-induced anaesthesia.