Pathophysiology of pH and Ca2+ in bloodstream and brain

Severe hypocalcemia and seizures after normalization of pCO2 in a patient with severe bronchopulmonary dysplasia and permissive hypercapnia

Objectives

To describe the association between the rapid normalization of pCO2 after intubation in a patient with severe bronchopulmonary dysplasia managed with permissive hypercapnia, with the risk of developing hypocalcemia and seizures, and to make health care providers aware of this risk in similar cases.

Case presentation

An extreme premature infant, born at 25 weeks of gestational age (GA), developed a severe bronchopulmonary dysplasia (BPD) and, after several extubation failures could be managed with non-invasive ventilation and permissive hypercapnia, with capillary pCO2 of up to 80 mmHg and pH >7.20. At 46 postmenstrual age (PMA) he was intubated because of severe hypercapnia and compensating metabolic alkalosis. About 20 h after intubation, after normalization of pH and pCO2, he developed hypocalcemia and seizures, that remitted after iCa normalization. A comparison between arterial and capillary blood gases showed a significantly greater correlation between pH and iCa in arterial than in capillary samples.

Conclusions

Our findings emphasize the importance of avoiding the abrupt reduction of pCO2 and the close monitoring of acute metabolic changes after its correction in chronic patients with permissive hypercapnia, as well as the potential superiority of arterial samples over capillaries to improve the precision of this control.

A role for ATP-sensitive potassium channels in the anticonvulsant effects of triamterene in mice

There are reports indicating that diuretics including chlorothiazide, furosemide, ethacrynic acid, amiloride and bumetanide can have anticonvulsant properties. Intracellular acidification appears to be a mechanism for the anticonvulsant action of some diuretics. This study was conducted to investigate whether or not triamterene, a K(+)-sparing diuretic, can generate protection against seizures induced by intravenous or intraperitoneal pentylenetetrazole (PTZ) models. And to see if, triamterene can withstand maximal electroshock seizure (MES) in mice. We also investigated to see if there is any connection between triamterene's anti-seizure effect and ATP-sensitive K(+) (KATP) channels. Five days triamterene oral administration (10, 20 and 40 mg/kg), significantly increased clonic seizure threshold which was induced by intravenous pentylenetetrazole. Triamterene (10, 20 and 40 mg/kg) treatment also increased the latency of clonic seizure and decreased its frequency in intraperitoneal PTZ model. Administration of triamterene (20 mg/kg) also decreased the incidence of tonic seizure in MES-induced seizure. Co-administration of a KATP sensitive channel blocker, glibenclamide, in the 6th day, 60 min before intravenous PTZ blocked triamterene's anticonvulsant effect. A KATP sensitive channel opener, diazoxide, enhanced triamterene's anti-seizure effect in both intravenous PTZ or MES seizure models. At the end, triamterene exerts anticonvulsant effect in 3 seizure models of mice including intravenous PTZ, intraperitoneal PTZ and MES. The anti-seizure effect of triamterene probably is induced through KATP channels.

Hyperventilation-induced nystagmus in vestibular neuritis: pattern and clinical implication

BACKGROUND

It was the aim of this study to investigate the pattern of evolution of hyperventilation-induced nystagmus (HIN) in vestibular neuritis (VN) and to determine whether HIN influences the dizziness outcome at the last follow-up visit.

METHODS

Fifty-three consecutive patients with VN underwent a quantitative vestibular function test including hyperventilation and the Korean version of the Dizziness Handicap Inventory during the acute period and the follow-up visit.

RESULTS

The incidence of HIN was higher in the acute (62%, 33/53) than in the chronic (17%, 9/53) stages of VN. Approximately 70% (6/9) of patients who continued to have persistent HIN at the last follow-up reported dizziness compared to only 27% (12/44) of patients who had no HIN. Patients who complained of persistent dizziness were significantly more likely to have persistent HIN and high Korean Dizziness Handicap Inventory scores at the last follow-up compared with patients who did not suffer from dizziness. In terms of the degree of recovery of dizziness, patients with HIN initially beating toward the contralesional side exhibited significantly more improvement than patients with HIN initially beating toward the ipsilesional side.

CONCLUSIONS

The presence of either HIN beating toward the ipsilesional side at the acute stage of VN or persistent HIN at the follow-up visit is associated with persistent dizziness.

Rational design of surface/interface chemistry for quantitative in vivo monitoring of brain chemistry

To understand the molecular basis of brain functions, researchers would like to be able to quantitatively monitor the levels of neurochemicals in the extracellular fluid in vivo. However, the chemical and physiological complexity of the central nervous system (CNS) presents challenges for the development of these analytical methods. This Account describes the rational design and careful construction of electrodes and nanoparticles with specific surface/interface chemistry for quantitative in vivo monitoring of brain chemistry. We used the redox nature of neurochemicals at the electrode/electrolyte interface to establish a basis for monitoring specific neurochemicals. Carbon nanotubes provide an electrode/electrolyte interface for the selective oxidation of ascorbate, and we have developed both in vivo voltammetry and an online electrochemical detecting system for continuously monitoring this molecule in the CNS. Although Ca(2+) and Mg(2+) are involved in a number of neurochemical signaling processes, they are still difficult to detect in the CNS. These divalent cations can enhance electrocatalytic oxidation of NADH at an electrode modified with toluidine blue O. We used this property to develop online electrochemical detection systems for simultaneous measurements of Ca(2+) and Mg(2+) and for continuous selective monitoring of Mg(2+) in the CNS. We have also harnessed biological schemes for neurosensing in the brain to design other monitoring systems. By taking advantage of the distinct reaction properties of dopamine (DA), we have developed a nonoxidative mechanism for DA sensing and a system that can potentially be used for continuously sensing of DA release. Using "artificial peroxidase" (Prussian blue) to replace a natural peroxidase (horseradish peroxidase, HRP), our online system can simultaneously detect basal levels of glucose and lactate. By substituting oxidases with dehydrogenases, we have used enzyme-based biosensing schemes to develop a physiologically relevant system for detecting glucose and lactate in rat brain. Because of their unique optical properties and modifiable surfaces, gold nanoparticles (Au-NPs) have provided a platform of colorimetric assay for in vivo cerebral glucose quantification. We designed and modified the surfaces of Au-NPs and then used a sequence of reactions to produce hydroxyl radicals from glucose.

Increased serum levels of the brain damage marker S100B after apnea in trained breath-hold divers: a study including respiratory and cardiovascular observations

The concentration of the protein S100B in serum is used as a brain damage marker in various conditions. We wanted to investigate whether a voluntary, prolonged apnea in trained breath-hold divers resulted in an increase of S100B in serum. Nine trained breath-hold divers performed a protocol mimicking the procedures they use during breath-hold training and competition, including extensive preapneic hyperventilation and glossopharyngeal insufflation, in order to perform a maximum-duration apnea, i.e., “static apnea” (average: 335 s, range: 281–403 s). Arterial blood samples were collected and cardiovascular variables recorded. Arterial partial pressures of O2 and CO2 (PaO2 and PaCO2) were 128 Torr and 20 Torr, respectively, at the start of apnea. The degree of asphyxia at the end of apnea was considerable, with PaO2 and PaCO2 reaching 28 Torr and 45 Torr, respectively. The concentration of S100B in serum transiently increased from 0.066 μg/l at the start of apnea to 0.083 μg/l after the apnea ( P < 0.05). The increase in S100B is attributed to the asphyxia or to other physiological responses to apnea, for example, increased blood pressure, and probably indicates a temporary opening of the blood-brain barrier. It is not possible to conclude that the observed increase in S100B levels in serum after a maximal-duration apnea reflects a serious injury to the brain, although the results raise concerns considering negative long-term effects. At the least, the results indicate that prolonged, voluntary apnea affects the integrity of the central nervous system and do not preclude cumulative effects.

Neurobiology of repeated transcranial magnetic stimulation in the treatment of anxiety: a critical review

Transcranial magnetic stimulation (TMS) has been applied to a growing number of psychiatric disorders as a neurophysiological probe, a primary brain-mapping tool, and a candidate treatment. Although most investigations have focused on the treatment of major depression, increasing attention has been paid to anxiety disorders. The aim of this study is to summarize published findings about the application of TMS as a putative treatment for anxiety disorders. TMS neurophysiological and mapping findings, both clinical and preclinical, have been included when relevant. We searched Medline, PsycInfo, and the Cochrane Library from 1980 to January 2009 for the terms 'generalized anxiety disorder', 'social anxiety disorder', 'social phobia', 'panic', 'anxiety', or 'posttraumatic stress disorder' in combination with 'TMS', 'cortex excitability', 'rTMS', 'motor threshold', 'motor evoked potential', 'cortical silent period', 'intracortical inhibition', 'neuroimaging', or 'intracortical facilitation'. Most of the therapeutic experiences with repetitive TMS available in the literature are in the form of case reports, not controlled or blinded studies. Stimulation of the right dorsolateral prefrontal cortex, especially at high frequencies, has been reported to reduce anxiety symptoms in posttraumatic stress disorder and panic disorder; nevertheless, results are mixed. A specific role for the right dorsolateral prefrontal cortex in the posttraumatic stress disorder symptom core can be hypothesized. TMS remains an investigational intervention that has not yet gained approval for the clinical treatment of any anxiety disorder. Clinical sham-controlled trials are scarce. Many of these trials have supported the idea that TMS has a significant effect, but in some studies, the effect is small and short lived. The neurobiological correlates suggest possible efficacy for the treatment of social anxiety that still has to be investigated.

Brain temperature and pH measured by (1)H chemical shift imaging of a thulium agent

Temperature and pH are two of the most important physiological parameters and are believed to be tightly regulated because they are intricately related to energy metabolism in living organisms. Temperature and/or pH data in mammalian brain are scarce, however, mainly because of lack of precise and non-invasive methods. At 11.7 T, we demonstrate that a thulium-based macrocyclic complex infused through the bloodstream can be used to obtain temperature and pH maps of rat brain in vivo by (1)H chemical shift imaging (CSI) of the sensor itself in conjunction with a multi-parametric model that depends on several proton resonances of the sensor. Accuracies of temperature and pH determination with the thulium sensor - which has a predominantly extracellular presence - depend on stable signals during the course of the CSI experiment as well as redundancy for temperature and pH sensitivities contained within the observed signals. The thulium-based method compared well with other methods for temperature ((1)H MRS of N-acetylaspartate and water; copper-constantan thermocouple wire) and pH ((31)P MRS of inorganic phosphate and phosphocreatine) assessment, as established by in vitro and in vivo studies. In vitro studies in phantoms with two compartments of different pH value observed under different ambient temperature conditions generated precise temperature and pH distribution maps. In vivo studies in alpha-chloralose-anesthetized and renal-ligated rats revealed temperature (33-34 degrees C) and pH (7.3-7.4) distributions in the cerebral cortex that are in agreement with observations by other methods. These results show that the thulium sensor can be used to measure temperature and pH distributions in rat brain in vivo simultaneously and accurately using Biosensor Imaging of Redundant Deviation in Shifts (BIRDS).

Central neurogenic hyperventilation treated with intravenous fentanyl followed by transdermal application

Central neurogenic hyperventilation (CNH) is a rare clinical condition that is sometimes difficult to treat. We report a 51-year-old female patient who was successfully treated with intravenous fentanyl followed by transdermal fentanyl. She had a transient epileptic episode with a temporary loss of consciousness. Immediately before her admission to the intensive care unit (ICU), her Pa(CO2) and pH were 6.7 mmHg and 7.64, respectively. Rebreathing from a paper bag and the intravenous administration of diazepam failed to improve the decreased Pa(CO2). Therefore, we administered intravenous fentanyl, at the rate of 50 microg x h(-1). Two days after her admission to the ICU, the Pa(CO2) had increased gradually to 22.9 mmHg, and the pH to 7.50. Although infiltration of recurrent lymphoma to the brain became apparent, she remained active, without epilepsy or loss of consciousness, in a general ward for 1 month with transdermal fentanyl, treatment until she again became drowsy; she died on hospital day 58. Transdermal fentanyl seems to be a good palliative measure to treat CNH in patients who have advanced neoplasms.

Differential contribution of storage pools to the extracellular amount of accumbal dopamine in high and low responders to novelty: effects of reserpine

The present study examined the effects of reserpine on the extracellular concentration of accumbal dopamine in high responders (HR) and low responders (LR) to novelty rats. Reserpine reduced the baseline concentration of extracellular accumbal dopamine more in HR than in LR, indicating that the dopamine release is more dependent on reserpine-sensitive storage vesicles in non-challenged HR than in non-challenged LR. In addition, reserpine reduced the novelty-induced increase of the extracellular concentration of accumbal dopamine in LR, but not in HR, indicating that the dopamine release in response to novelty depends on reserpine-sensitive storage vesicles only in LR, not in HR. Our data clearly demonstrate that HR and LR differ in the characteristics of those monoaminergic storage vesicles that mediate accumbal dopamine release.

Excitability of human motor and visual cortex before, during, and after hyperventilation

In humans, hyperventilation (HV) has various effects on systemic physiology and, in particular, on neuronal excitability and synaptic transmission. However, it is far from clear how the effects of HV are mediated at the cortical level. In this study we investigated the effects of HV-induced hypocapnia on primary motor (M1) and visual cortex (V1) excitability. We used 1) motor threshold (MT) and phosphene threshold (PT) and 2) stimulus-response (S-R) curves (i.e., recruitment curves) as measures of excitability. In the motor cortex, we additionally investigated 3) the intrinsic inhibitory and facilitatory neuronal circuits using a short-interval paired-pulse paradigm. Measurements were performed before, during, and after 10 min of HV (resulting in a minimum end-tidal Pco(2) of 15 Torr). HV significantly increased motor-evoked potential (MEP) amplitudes, particularly at lower transcranial magnetic stimulation (TMS) intensities. Paired-pulse stimulation indicated that HV decreases intracortical inhibition (ICI) without changing intracortical facilitation. The results suggestthat low Pco(2) levels modulate, in particular, the intrinsic neuronal circuits of ICI, which are largely mediated by neurons containing gamma-aminobutyric acid. Modulation of MT probably resulted from alterations of Na(+) channel conductances. A significant decrease of PT, together with higher intensity of phosphenes at low stimulus intensities, furthermore suggested that HV acts on the excitability of M1 and V1 in a comparable fashion. This finding implies that HV also affects other brain structures besides the corticospinal motor system. The further exploration of these physiological mechanisms may contribute to the understanding of the various HV-related clinical phenomenona.

Hypocapnia reduces the T wave of the electrocardiogram in normal human subjects

During voluntary hyperventilation in unanesthetized humans, hypocapnia causes coronary vasoconstriction and decreased oxygen (O2) supply and availability to the heart. This can induce local epicardial coronary artery spasm in susceptible patients. Its diagnostic potential for detection of early heart disease is unclear. This is because such hypocapnia produces an inconsistent and irreproducible effect on electrocardiogram (ECG) in healthy subjects. To resolve this inconsistency, we have applied two new experimental techniques in normal, healthy subjects to measure the effects of hypocapnia on their ECG: mechanical hyperventilation and averaging of multiple ECG cycles. In 15 normal subjects, we show that hypocapnia (20 ± 1 mmHg) significantly reduced mean T wave amplitude by 0.1 ± 0.0 mV. Hypocapnia also increased mean heart rate by 4 beats/min without significantly altering blood pressure, ionized calcium or potassium levels, or the R wave or other features of the ECG. We therefore provide the first unequivocal demonstration that hypocapnia does consistently reduce T wave amplitude in normal, healthy subjects.

Effects of adenosine receptor antagonists on pial arteriolar dilation during carbon dioxide inhalation

The role of adenosine in the cerebrovascular response to carbon dioxide inhalation was evaluated in two sets of experiments. The pial circulation was recorded by a Laser-Doppler flow probe placed over a closed cranial window in methoxyflurane anesthetized rats. Topical application of the nonselective adenosine receptor antagonist caffeine (1 mM), the selective A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX,1 microM), or the selective A2A receptor antagonist 4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo[2,3-a]triazin-5-yl amino]ethyl) phenol (ZM 241385, 1 microM) all failed to affect mean arterial blood pressure, basal cerebral blood flow, or the carbon dioxide-evoked hyperemia. Systemically administered caffeine (20 mg/kg) also had no significant effects. However, following the systemic administration of the nonselective nitric oxide synthase inhibitor, NG-nitro-L-arginine methyl ester (L-NAME, 20 mg/kg), the topical application of both caffeine and ZM 241385 (but not DPCPX) significantly reduced the carbon dioxide-evoked hyperemia. L-NAME (20 mg/kg) administered intravenously, evoked a significant increase in mean arterial blood pressure, a slow progressive decline in cerebral blood flow and, during brief (60-90 s) periods of 7.5% carbon dioxide inhalation, a significant decrease in arterial blood pressure. L-NAME failed to reduce the carbon dioxide-evoked increase in cerebral blood flow as measured by the area under the curve (AUC), although it did reduce the peak flow response. Topically applied L-NAME (1 mM) failed to alter mean arterial blood pressure, basal cerebral blood flow, or the carbon dioxide-evoked increases in cerebral blood flow. In a second series of experiments, we evaluated the ability of 10% carbon dioxide inhalation for 8 min to elicit a release of adenosine from the cerebral cortex. Adenosine levels in the cortical superfusates rose significantly during periods of carbon dioxide inhalation. The data suggest that following the removal of the confounding effects of nitric oxide, which are unlikely to be mediated locally, a significant contribution by adenosine A2A receptor activation to the carbon dioxide-evoked cortical hyperemia was evident.

Ion regulation in the brain: implications for pathophysiology

Ions in the brain are regulated independently from plasma levels by active transport across choroid plexus epithelium and cerebral capillary endothelium, assisted by astrocytes. In "resting" brain tissue, extracellular potassium ([K+]o) is lower and [H]o is higher (i.e., pHo is lower) than elsewhere in the body. This difference probably helps to maintain the stability of cerebral function because both high [K]o and low [H+]o enhance neuron excitability. Decrease in osmolarity enhances synaptic transmission and neuronal excitability whereas increased osmolarity has the opposite effect. Iso-osmotic low Na+ concentration also enhances voltage-dependent Ca2+ currents and synaptic transmission. Hypertonicity is the main cause of diabetic coma. In normally functioning brain tissue, the fluctuations in ion levels are limited, but intense neuronal excitation causes [K+]o to rise and [Na+]o, [Ca2+]o to fall. When excessive excitation, defective inhibition, energy failure, mechanical trauma, or blood-brain barrier defects drive ion levels beyond normal limits, positive feedback can develop as abnormal ion distributions influence neuron function, which in turn aggravates ion maldistribution. Computer simulation confirmed that elevation of [K+]o can lead to such a vicious circle and ignite seizures, spreading depression (SD), or hypoxic SD-like depolarization (anoxic depolarization).

Two mechanisms that raise free intracellular calcium in rat hippocampal neurons during hypoosmotic and low NaCl treatment

Previous studies have shown that exposing hippocampal slices to low osmolarity (pi(o)) or to low extracellular NaCl concentration ([NaCl](o)) enhances synaptic transmission and also causes interstitial calcium ([Ca(2+)](o)) to decrease. Reduction of [Ca(2+)](o) suggests cellular uptake and could explain the potentiation of synaptic transmission. We measured intracellular calcium activity ([Ca(2+)](i)) using fluorescent indicator dyes. In CA1 hippocampal pyramidal neurons in tissue slices, lowering pi(o) by approximately 70 mOsm caused "resting" [Ca(2+)](i) as well as synaptically or directly stimulated transient increases of calcium activity (Delta[Ca(2+)](i)) to transiently decrease and then to increase. In dissociated cells, lowering pi(o) by approximately 70 mOsm caused [Ca(2+)](i) to almost double on average from 83 to 155 nM. The increase of [Ca(2+)](i) was not significantly correlated with hypotonic cell swelling. Isoosmotic (mannitol- or sucrose-substituted) lowering of [NaCl](o), which did not cause cell swelling, also raised [Ca(2+)](i). Substituting NaCl with choline-Cl or Na-methyl-sulfate did not affect [Ca(2+)](i). In neurons bathed in calcium-free medium, lowering pi(o) caused a milder increase of [Ca(2+)](i), which was correlated with cell swelling, but in the absence of external Ca(2+), isotonic lowering of [NaCl](o) triggered only a brief, transient response. We conclude that decrease of extracellular ionic strength (i.e., in both low pi(o) and low [NaCl](o)) causes a net influx of Ca(2+) from the extracellular medium whereas cell swelling, or the increase in membrane tension, is a signal for the release of Ca(2+) from intracellular stores.

Increased excitability of the human corticospinal system with hyperventilation

OBJECTIVES

Hyperventilation is effective in inducing generalized spike-wave discharges in patients with absence seizures and improves visual function and normalizes visual function in patients with multiple sclerosis. Hyperventilation increases the excitability of cutaneous and motor axons. In experimental animals, hyperventilation increases excitability of hippocampal neurons. There is however no direct evidence of a hyperventilation-induced increase in neuronal excitability within the central nervous system in humans. In this study we determined the effects of hyperventilation on the human corticospinal system.

METHODS

We studied the effects of hyperventilation on (1) motor evoked potentials (MEPs) induced by transcranial magnetic pulse stimulation of the motor cortex and (2) F-wave responses. Six subjects were studied.

RESULTS

Hyperventilation resulting in an end-tidal pCO2 of 15 mm Hg or less enhanced the amplitude of the MEP and resulted in a shortened onset latency. F-wave amplitudes were enhanced without any change in onset latency.

CONCLUSIONS

These findings indicate that hyperventilation increases the excitability of the human corticospinal system. A hyperventilation-induced increase in excitability within the central nervous system may account for clinical phenomena such as facilitation of spike-wave discharges.

Vestibular disease unmasked by hyperventilation

Hyperventilation-induced dizziness is often thought to be psychogenic, but its effects in the presence of known vestibular disease have not been adequately examined. In this study hyperventilation was tested in two models of vestibular disease. These were, first, patients with profound unilateral vestibular deficit (prior translabyrinthine acoustic neuroma resection [postsurgery group]) and, second, patients with variable unilateral vestibular deficit (unoperated unilateral acoustic neuroma [presurgery group]). Patients were hyperventilated for 90 seconds. Using infrared videonystagmography, 100% of the 32 postsurgery patients and 82% of the 28 presurgery patients developed nystagmus with hyperventilation. Hyperventilation was more sensitive than head shake for eliciting nystagmus in these models. The false-positive rate for nystagmus in 29 normal volunteers was 3.5% for hyperventilation and 10% for head shake. Our results show that hyperventilation can unmask underlying vestibular disease.

Extracellular acidosis and high levels of carbon dioxide suppress synaptic transmission and prevent the induction of long-term potentiation in the CA1 region of rat hippocampal slices

Long-term potentiation (LTP) is a long-lasting increase in synaptic strength induced by high frequency stimulation. LTP may participate in learning and memory formation. In many synaptic systems, LTP is dependent on intact function of N-methyl-D-aspartate (NMDA) receptors. NMDA receptors may be inhibited in different conditions involving also extracellular acidosis. A decrease in the extracellular pH accompanies many pathological states such as ischemia, hypoxia, and the CNS injury. The study was designed to determine whether comparable extracellular acid-base imbalances are able to interfere with the LTP induction. Hippocampal slices from adult rats were stimulated with high frequency stimulation (1 x 100 Hz/1 s) at Schaffer collateral-commissural synaptic system in the environment with different pH (6.7-7.8) and the field responses were recorded in CA1. Acidosis was achieved by supplying excessive CO2 or by HCO3-decrease in standard bicarbonate-containing buffer or by a direct acidification of the buffer containing Na-HEPES. Invariably, all forms of acidification suppressed the efficacy of normal, low frequency synaptic transmission and prevented the induction of LTP in a reversible manner; i.e., after reperfusion of the slices at pH 7.3 and restimulation, there was a return of synaptic transmission back to baseline, and a significant amount of LTP occurred. In contrast, alkalization to pH 7.8, although enhancing synaptic transmission efficacy, did not further increase the LTP magnitude compared to control environment with pH 7.3. The results suggest that extracellular acidosis associated with several pathological conditions in the CNS may significantly diminish the LTP induction, and thus negatively affect all physiological processes that utilize LTP.

Regional variation in the effects of nicotine on catecholamine overflow in rat brain

The effects of acute, repeated intermittent and continuous administration of nicotine on the overflow of noradrenaline in the ventral hippocampus and dopamine in the nucleus accumbens and striatum have been studied. Daily injections of nicotine (0.4 mg/kg(-1) for 5 days) enhanced noradrenaline and dopamine overflow in the ventral hippocampus and nucleus accumbens respectively (P < 0.01 and P < 0.05) but not dopamine in the striatum in response to a nicotine challenge. The responses in the ventral hippocampus and nucleus accumbens were attenuated (P < 0.01) by the constant infusion of nicotine at a dose of 1 mg kg(-1) per day; the dopamine response in the striatum required a higher dose (4 mg kg(-1) per day) before desensitisation was observed. The data suggest that the dopamine projections to the striatum are less sensitive to both stimulation and desensitisation by nicotine than the catecholamine projections to the ventral hippocampus and nucleus accumbens.

Lowering of extracellular pH suppresses low-Mg(2+)-induces seizures in combined entorhinal cortex-hippocampal slices

Lowering [Mg2+]o induces epileptiform bursting in hippocampus and entorhinal cortex (EC), presumably by activation of N-methyl-D-aspartate (NMDA) receptors. Since increasing [H+]o has been shown to reduce NMDA receptor activation, we hypothesized that this could contribute to anticonvulsant actions of acidic pH. To test this, we studied the effects of raising extracellular PCO2 (20.6%, pH = 6.7) or lowering extracellular pH (6.7 or 6.2) on low-Mg(2+)-induced epileptiform discharges. Lowering the pH to 6.7 by either means increased the interval between seizure-like events (SLEs), decreased the maximal amplitude of SLEs, and if the site of seizure generation was at a distance from the recording site, acidification slowed the rate of seizure propagation. In contrast, the duration of SLEs was unaffected by acidic pH or high PCO2. Raising PCO2 or lowering pH to 6.7 also blocked early (8-10 min) but not late (> 20 min) phases of status-like discharges. All effects of the extracellular pH changes were fully reversible. Further lowering of extracellular pH to 6.2 completely and reversibly blocked both SLEs and status-like discharges. Our data show that the effects of high PCO2 and low pH on seizures in the EC in vitro may be dose-dependent and consistent with induction by proton blockade of NMDA receptors. Thus, blockade of NMDA currents by protons may be an important component of the anticonvulsant action of extracellular acidosis. The results also suggest that acidosis may be a desirable property for new antiepileptic treatments.

Release of endogenous GABA in the posterior hypothalamus of the conscious rat; effects of drugs and experimentally induced blood pressure changes

Push-pull superfusion was used to investigate the release of endogenous GABA in the posterior hypothalamus of the conscious, freely moving rat at basal conditions and in response to centrally applied drugs or to peripherally induced blood pressure changes. After an initial, exponential decline, the release rate of GABA remained fairly constant for many hours. Fluctuations in the release rate of GABA point to the existence of an ultradian rhythm with an approximate frequency of 1 cycle/65 min. Hypothalamic superfusion with a potassium-rich (50 or 90 mmol/l) artificial cerebrospinal fluid led to a concentration-dependent increase in the GABA release. The release of GABA was also enhanced by veratridine (1 or 10 mumol/l) in a concentration-dependent way. Hypothalamic superfusion with the neutrotoxin tetrodotoxin (1 mumol/l) led to a long-lasting decrease in the GABA release. The rise in blood pressure (45 mmHg) elicited by an intravenous infusion of noradrenaline was associated with an increased release rate of GABA in the hypothalamus. Hypotension produced by nitroprusside (25 mmHg) led to a counteracting decrease in hypothalamic GABA outflow. The findings suggest that approximately 45% of the basal outflow of GABA found in the superfusate are released from GABA-ergic neurons of the posterior hypothalamus. The release rate of GABA fluctuates according to an ultradian rhythm. The modified release of GABA in response to experimentally induced blood pressure changes suggests that, in the posterior hypothalamus of the conscious rat, GABAergic neurons are involved in cardiovascular control and possess a hypotensive function.

Influence of tetrodotoxin and calcium on changes in extracellular dopamine levels evoked by systemic nicotine

The influence of tetrodotoxin (TTX) and calcium on the increase of extracellular dopamine (DA) levels in the nucleus accumbens (NAcc), evoked by the systemic administration of nicotine, cocaine and d-amphetamine, have been studied in conscious, freely moving rats using in vivo microdialysis. TTX (10(-6) M), administered via the dialysis probe, completely abolished (P < 0.01) the elevations in extracellular DA, DOPAC and HVA seen following nicotine (0.4 mg/kg SC). The removal of calcium with the inclusion of diaminoethanetetraacetic acid (EDTA 10(-4) M) in the Ringer solution was also associated with inhibition (P < 0.01) of the nicotine-induced changes in these parameters. The systemic administration of cocaine (15 mg/kg IP) and d-amphetamine (0.5 mg/kg SC) caused elevations in extracellular DA (P < 0.01) accompanied by significant decreases (P < 0.01) in HVA levels. DOPAC levels were also significantly (P < 0.01) lowered by d-amphetamine treatment. The presence of TTX and removal of calcium with addition of EDTA completely abolished the changes in NAcc DA and HVA induced by cocaine. TTX had no influence on the d-amphetamine evoked responses in NAcc DA. However, the metabolites, which were markedly reduced by the TTX, were not further decreased by the systemic administration of d-amphetamine. NAcc DA was significantly (P < 0.01) raised following d-amphetamine in the absence of calcium and presence of EDTA. However, this was significantly (P < 0.01) attenuated in comparison to that seen in the presence of calcium. The results support the conclusion that, at the dose tested, nicotine evokes increases in extracellular NAcc DA levels by calcium and impulse-dependent mechanisms.(ABSTRACT TRUNCATED AT 250 WORDS)

Increased pH and inorganic phosphate in temporal seizure foci demonstrated by [31P]MRS

To investigate alterations of brain metabolism associated with temporal lobe epilepsy, [31P]MRS studies were performed on the anterotemporal lobes of patients with medically refractory complex partial seizures. Interictally, the pH was significantly more alkaline in the temporal lobe ipsilateral to the seizure focus (7.25 vs. 7.08, p less than 0.05), and the inorganic phosphorous concentration was greater on the side of the epileptogenic focus (1.9 vs. 1.1 mM, p less than 0.05). These changes in pH and inorganic phosphate may represent metabolic alterations secondary to seizures. Alternatively, because alkalosis enhances neural excitability and may enhance seizure activity, the increased pH of the seizure focus may provide insight into the pathophysiologic mechanism of epileptic seizures.

Influence of extracellular and intracellular pH on GABA-gated chloride conductance in crayfish muscle fibres

The effect of intracellular and extracellular pH on GABA-gated Cl- conductance was studied using H(+)-selective microelectrodes and a three-microelectrode voltage clamp in crayfish leg opener muscle fibres in bicarbonate-free solutions. Experimental variation of intracellular pH in the range 6.4-8.0 did not affect the GABA-gated conductance. In contrast to this, the GABA-gated conductance was sensitive to changes in external pH. Raising the external pH from 7.4 to 8.4 decreased the GABA-gated peak conductance observed immediately following application of GABA by 30%, and a change from 7.4 to 6.4 produced an increase of 26%. The effect of extracellular pH on the GABA-gated peak conductance was approximately linear in the pH range 6.4-8.9. A slight decrease in the slope of the pH-conductance relationship was evident in the pH range 5.4-6.4. The desensitization of the GABA-gated conductance was also affected by external pH. At pH 6.9 the conductance produced by 1 mM GABA showed a desensitization of about 15%, and at pH 8.9 this value was 34%. Raising the external pH in the presence of GABA decreased the GABA-gated peak conductance and increased the fractional desensitization, while lowering the external pH produced opposite effects, and was capable of repriming the conductance from a desensitized state to the non-desensitized state. The above results show that the GABA-gated conductance is sensitive to changes in external pH in the physiological range, and suggest that pH-dependent changes in the postsynaptic efficacy of GABA-mediated inhibition may contribute to H+ modulation of neuronal excitability.

Prolonged postischemic hyperventilation reduces acute neuronal damage after 15 min of cardiac arrest in the dog

Hyperventilation is commonly used as a constituent of antiedematous therapy after global cerebral ischemia. The effect of hyperventilation on brain functions, however, is complex, and a number of mechanisms involved remains unclear. In this study, we attempted to determine whether postischemic hyperventilation influences acute neuronal changes developing during recirculation. Two groups of dogs underwent 15 min of cardiac arrest and cardiopulmonary resuscitation with an 8 h survival. After resuscitation, in group A the internal environment was maintained in the physiological ranges. In group B the animals were artificially hyperventilated maintaining a high level of respiratory alkalosis during recirculation. Histopathological examination of the vulnerable structures was performed using the Nauta degenerating method and the argyrophilic neurons were counted. Statistically significant amelioration in group B suggests that postischemic hyperventilation may act as a neuroprotective factor.

Importance of the calcium content infused during microdialysis for the effects induced by D2 agonists on the release of dopamine in the striatum of the rat

Brain microdialysis was used to investigate whether different calcium concentrations (1.2 and 3.4 mmol/l) of the perfusion fluid influenced the effects of D2 agonists on the release of dopamine in the striatum. We used the D2 agonists (-)N-0437 and (+)PHNO. After both local and systemic administration of (-)N-0437 and (+)PHNO, differences were apparent between their effects at 1.2 mmol/l calcium and 3.4 mmol/l calcium Ringer's solution. Although the drugs induced a similar maximal decrease in the release of dopamine with both calcium concentrations, the potency of the effect was significantly greater at 1.2 mmol/l when compared to 3.4 mmol/l calcium Ringer's solution. Thus, when measuring pharmacological effects of dopaminergic agents, it seems essential to use a Ringer's solution containing the physiological calcium concentration in brain microdialysis.

3,4-Diaminopyridine-evoked noradrenaline release in rat hippocampus: role of Na+ entry on Ca2+ pools and of protein kinase C

Slices of rat hippocampus, preincubated with [3H]noradrenaline [(3H]NA), were superfused continuously and stimulated by addition of 3,4-diaminopyridine (3,4-DAP; 100 microM) for 10 min to the superfusion medium. An overflow of 3H evoked by 3,4-DAP (representing [3H]NA release) was measurable not only in the presence but also in the absence of extracellular Ca2+. Both the protein kinase C (PKC) activator 4 beta-phorbol 12,13-dibutyrate (4 beta-PDB) and the PKC inhibitor polymyxin B, affected mainly the evoked release in the absence of extracellular Ca2+ in a facilitatory or inhibitory manner, respectively. Moreover, in the absence of extracellular Ca2+, both the 3,4-DAP-evoked [3H]NA release and the facilitatory effect of 4 beta-PDB were abolished in the presence of tetrodotoxin or in the absence of Na+ in the superfusion medium. Ruthenium red, a blocker of mitochondrial Ca2+ reuptake, potently increased 3,4-DAP-evoked [3H]NA release in Ca(2+)-free EGTA-containing medium. The facilitatory effects of ruthenium red and 4 beta-PDB were additive. From these and earlier observations we conclude (1) that the mechanism of 3,4-DAP-evoked [3H]NA release involves both Ca2+ influx into the nerve terminals and mobilization of intraneuronal Ca2+ pools. Most probably Ca2+ release from cytoplasmic Ca2+ stores (e.g. endoplasmic reticular pools or mitochondria) is induced by Na+ ions entering the nerve endings during 3,4-DAP-evoked repetitive action potentials. (2) The facilitatory effect of phorbol ester on 3,4-DAP-evoked NA release appears to be mediated not by changes in Ca2+ influx, but by enhancement of intraneuronal events distal to Na+ ion entry and increased intracellular Ca2+ availability.

Dopaminergic-cholinergic interactions in the striatum: the critical significance of calcium concentrations in brain microdialysis

Brain microdialysis experiments were performed to assess the effects of calcium (1.2 mmol/l and 3.4 mmol/l) in the perfusion solution on a variety of pharmacological treatments known to affect the release of dopamine (DA) and/or acetylcholine (ACh). Intrastriatal infusion of the muscarinic receptor agonist oxotremorine (100 microM), the selective dopamine D-2 receptor agonist (-)-N-0437 (1 microM), and the indirect DA agonists (+)amphetamine (10 microM) and nomifensine (1 microM) via the dialysis probe did not affect the overflow of ACh when the perfusion fluid contained 3.4 mmol/l calcium. In contrast, these compounds produced pronounced decreases in the overflow of ACh at 1.2 mmol/l calcium. Intrastriatal infusion of the muscarinic receptor antagonist atropine (1 microM) increased the output of ACh both at 1.2 mmol/l and 3.4 mmol/l calcium. The selective DA D-2 receptor antagonist (-)-sulpiride (1 microM) did not affect the overflow of ACh at either calcium concentration. Infusion of oxotremorine and atropine had no effect on the overflow of DA at either 1.2 mmol/l at 3.4 mmol/l calcium. (-)-N-0437 decreased and (-)-sulpiride increased DA overflow, both effects being independent of the calcium concentration in the perfusion fluid. Nomifensine and (+)amphetamine caused relatively (but not absolutely) larger increases in the overflow of DA at 1.2 mmol/l calcium. These findings emphasize the critical importance of the calcium concentration of the perfusion fluid in determining the nature of pharmacological responses in microdialysis experiments, and demonstrate that locally applied dopaminergic drugs can modulate striatal cholinergic function.

Spinal dorsal horn neurons in elevated extracellular calcium: cell properties and spontaneous discharges

Recordings were made from neurons in the dorsal horn (DH), and from dorsal and ventral roots (DRs and VRs) of isolated spinal cords of infant mice. Raising calcium concentration ([Ca2+]) in the organ bath from 1.2 to 2.4 mmol/l resulted in a slight hyperpolarization, elevation of threshold current (rheobase), and augmentation of excitatory postsynaptic potentials (EPSPs). In many cells EPSPs acquired a much prolonged late phase. Orthodromic stimulation evoked in some DH neurons an action potential that had the same threshold as, and coincided in time with, the 'dorsal horn response' (DHR) recorded from DR. In spinal cords bathed in elevated [Ca2+], DR recordings showed irregularly recurring spontaneous waves, and DH neurons generated spontaneous EPSPs, often with spikes. Some neurons fired irregularly timed spontaneous action potentials that did not appear triggered by EPSPs. In less than 50% of the neurons the spontaneous EPSPs coincided in time with the spontaneous DR waves. The action potentials that appeared without EPSP were fired independently from DR activity. These observations confirm that elevation of interstitial free calcium concentration results in strong enhancement of excitatory transmission, especially of an EPSP of much extended duration. Virtually all neurons showed increased spontaneous activity in high [Ca2+], but only a minority appeared recruited into the synchronized discharges that are detectable as spontaneous waves in DR and VR recordings.

Spontaneous activity induced in isolated mouse spinal cord by high extracellular calcium and by low extracellular magnesium

Spontaneous discharges were observed in recordings from dorsal and ventral roots (DRs and VRs) of hemisected mouse spinal cords in vitro when bath [Ca2+] was raised from the control level of 1.2 to 1.8 or 2.4 mmol/l, and when bath [Mg2+] was lowered resembling drug-induced 'interictal' discharges described earlier. Maximum discharge frequency was reached at 2.4 or 3.6 mmol/l [Ca2+] while at higher concentrations mean frequency diminished but mean amplitude still increased somewhat. The frequency distribution of wave amplitudes suggests recruitment of neurons by groups or assemblies. The pacemaker is located in the dorsal spinal quadrant. The GABAA receptor antagonists picrotoxin and bicuculline in low concentration blocked the spontaneous activity, indicating an obligatory GABAergic link in the pacemaker circuit. The NMDA antagonists D.L-APV and D-APV in high concentration variably depressed but did not abolish spontaneous activity. The propensity of spinal neuron assemblies for self-paced discharge may contribute to the pathologic irritability of injured spinal cords.

Changes in extracellular calcium and magnesium and synaptic transmission in isolated mouse spinal cord

Hemisected mouse spinal cords were maintained in vitro and the concentrations of calcium [CA2+] and of magnesium [Mg2+] in the bath were varied. In control solution containing 1.2 mM of both [Ca2+] and [Mg2+] stimulation of a dorsal root (DR) evoked in an adjacent DR an initial fiber volley representing 'input'; a postsynaptic compound spike recorded by volume conduction from dorsal gray matter, the dorsal horn response (DHR); and a slow dorsal root potential (DRP). In the ventral root of the stimulated segment a monosynaptic reflex (VRR1) was evoked. The fiber volley was enhanced by lowering [Ca2+] or [Mg2+] and depressed when either ion concentration was raised. The DRP, DHR and VRR1 were enhanced in low [Mg2+] and in moderately elevated [Ca2+]. At 1.8 mM [Ca2+] and above, the 'classical' dorsal root reflex (DRR) and a GABA-dependent delayed VR reflex (VRR2) appeared. Transmission of reflexes was maximal between 2.4 and 3.6 mM [Ca2+], while DRP was maximal at about 4.8 mM. In elevated [Mg2+] and in low [Ca2+] all synaptically transmitted responses (DRR, DRP and VRR) were depressed. The influence of both [Ca2+] and [Mg2+] was stronger on DRP, DRR and VRR2 than on DHR and VRR1. The effects of simultaneous changes of [Ca2+] and [Mg2+] partially cancelled each other. We conclude that low to moderately elevated [Ca2+] mainly influences the release of transmitter from presynaptic terminals; at very high [Ca2+] the depression of neuronal excitability dominates. The effects of Ca2+ and Mg2+ on GABAergic transmission are especially marked.

Brain fluid calcium concentration and response to acute hypercalcaemia during development in the rat

1. In vivo measurements of plasma, cerebrospinal fluid (CSF) and brain interstitial fluid (ISF) ionic Ca2+ concentrations ([Ca2+]) have been made in anaesthetized rats aged between 19 days gestation and adult using calcium-selective microelectrodes. Total calcium concentration ([ Ca]) has also been determined in plasma and CSF samples by atomic absorption spectrometry. 2. Under control conditions, plasma, CSF and ISF [Ca2+] showed a small, but significant, decrease with age. Plasma and CSF, but not ISF, showed a transient hypocalcaemia at birth. After birth there were no significant differences between plasma, CSF and ISF [Ca2+], except in adult rats where CSF [Ca2+] was significantly lower than plasma [Ca2+]. The age-related changes in CSF and ISF [Ca2+] were small and it is uncertain as to whether they may have any functional significance. 3. Under control conditions, plasma and CSF [Ca] also declined with age. The fall in plasma [Ca] paralleled the changes in plasma [Ca2+]. The decrease in CSF [Ca] was steeper than that in [Ca2+] and indicated a higher proportion of protein-bound or complexed calcium in the CSF of young when compared to old rats. 4. Acute plasma hypercalcaemia was induced by intramuscular injections of calcium gluconate and consequent changes in CSF or ISF [Ca2+] were monitored in vivo. There was very weak regulation of CSF and ISF [Ca2+] at 21 days gestation, which may reflect placental control over fetal plasma calcium. Soon after birth there was good regulation in both ISF and CSF [Ca2+]. CSF [Ca] was also measured during hypercalcaemia in samples from post-natal rats and there was similar regulation to that in CSF [Ca2+]. 5. It is concluded that under control conditions, during rat development, CSF and ISF [Ca2+] closely follow changes in plasma [Ca2+], but that soon after birth homeostatic mechanisms develop to prevent large fluctuations in brain fluid calcium.

Concentration of carbon dioxide, interstitial pH and synaptic transmission in hippocampal formation of the rat

1. Interstitial pH (pHo) was measured with ion-selective microelectrodes in the fascia dentata of rats anaesthetized with urethane, while CO2 levels were controlled by varying pulmonary ventilation and CO2 content of inspired air. In the CA1 sector of hippocampal tissue slices in vitro pHo was similarly measured and altered by varying CO2 in the gas phase, or by adding HCl or NaOH to the artificial cerebrospinal fluid (ACSF) of the bath, or by changing the concentration of HCO3-. 2. Orthodromically evoked compound action potentials ('population spikes') were depressed in hypercapnia and increased in hypocapnia. In the fascia dentata of intact brains the population spike of the granule cells varied on average by more than 40% of control amplitude for each 0.1 change of pHo. In the CA1 zone of tissue slices in vitro, the change of population spike amplitude was approximately 30% per pH change of 0.1 caused by altered CO2 or HCO3- concentration, but only about 15% per pH change of 0.1 when HCl or NaOH were administered. 3. In anaesthetized rats the focal synaptic potential (FEPSP) evoked by a given stimulus intensity was weakly influenced by varying [CO2]; in tissue slices weak effects on FEPSP were inconsistent. In hippocampus both in situ and in vitro the population spike triggered by a given magnitude of FEPSP increased in hypocapnia and decreased in hypercapnia. This suggests that the main effect of CO2 is on the electric excitability of postsynaptic cells, with minor or no effect on transmitter release and on the interaction of the transmitter with its receptors. 4. Hypercapnia of anaesthetized rats was usually associated with a slight increase of [K+]o in the fascia dentata. Tissue [Ca2+]o changed little and not consistently. Neither of these two ions, nor concomitant changes of blood pressure or tissue partial pressure of oxygen, (Pt, O2), could account for the effects of pH on neuronal excitability. 5. The results show that increasing the extracellular concentration of H+ ions has a moderately depressant effect on the firing threshold of hippocampal neurones. The more powerful effects of elevated [CO2] and of lowered [HCO3-] may probably be explained by a direct effect on the neuronal membrane. The brain, by regulating breathing, controls its own excitability.

Epileptiform activity induced by alkalosis in rat neocortical slices: block by antagonists of N-methyl-D-aspartate

The effects of changing extracellular pH on epileptiform activity induced by the removal of magnesium ions from the perfusing medium were studied. The proportion of bicarbonate in the artificial cerebrospinal fluid and of CO2 in the gas mixture were altered to mimic metabolic and respiratory acid-base disturbances. Changes in pH of 0.2 unit from control produced marked effects. Epileptiform activity was enhanced by alkalosis and diminished by acidosis. In normal magnesium-containing medium metabolic alkalosis (pH greater than 7.8) induced spontaneous epileptiform activity that was blocked by selective N-methyl-D-aspartate antagonists. The relevance of these findings to acid/base changes in clinical epilepsy is discussed.