Coordinated Acetylcholine Release in Prefrontal Cortex and Hippocampus Is Associated with Arousal and Reward on Distinct Timescales

Cholinergic neurotransmission throughout the neocortex and hippocampus regulates arousal, learning, and attention. However, owing to the poorly characterized timing and location of acetylcholine release, its detailed behavioral functions remain unclear. Using electrochemical biosensors chronically implanted in mice, we made continuous measurements of the spatiotemporal dynamics of acetylcholine release across multiple behavioral states. We found that tonic levels of acetylcholine release were coordinated between the prefrontal cortex and hippocampus and maximal during training on a rewarded working memory task. Tonic release also increased during REM sleep but was contingent on subsequent wakefulness. In contrast, coordinated phasic acetylcholine release occurred only during the memory task and was strongly localized to reward delivery areas without being contingent on trial outcome. These results show that coordinated acetylcholine release between the prefrontal cortex and hippocampus is associated with reward and arousal on distinct timescales, providing dual mechanisms to support learned behavior acquisition during cognitive task performance.

Dissociable effects of antipsychotics on ketamine-induced changes in regional oxygenation and inter-regional coherence of low frequency oxygen fluctuations in the rat

Typical and atypical antipsychotics have been shown to alleviate N-methyl-D-aspartate (NMDA) receptor antagonist-induced BOLD signals in healthy humans and animals to differing degrees; factors that might relate to their different molecular mechanisms and clinical profiles. Recent studies have also extended these investigations to the analysis of resting state functional connectivity measures of BOLD signals in different brain regions. Using constant potential amperometry, we examined the effects of the NMDA receptor antagonist S-(+)-ketamine on tissue oxygen levels in medial prefrontal cortex (mPFC) and medial ventral striatum (mVS), and temporal coherence of low-frequency oxygen fluctuations between these regions in freely moving rats. Furthermore, we assessed the extent to which the atypical antipsychotic clozapine and the typical antipsychotic haloperidol could modulate the effects of S-(+)-ketamine on these measures. Acute S-(+)-ketamine (5-25 mg/kg) produced dose-dependent increases in both tissue O2 levels and coherence. Although effects of clozapine and haloperidol alone were relatively minor, their effects on ketamine-induced signals were markedly more distinct. Clozapine dose-dependently attenuated the absolute S-(+)-ketamine (25 mg/kg) O2 signal in both regions, and also attenuated ketamine-induced increases in regional coherence. Haloperidol had no effect on the absolute ketamine O2 signal yet potentiated increases in regional coherence. The dissociable effects of haloperidol and clozapine on ketamine-induced hyperoxygenation and mPFC-mVS coherence elucidate potentially important mechanistic differences between these classes of pharmacology. This study demonstrates for the first time that in vivo amperometry can measure both regional brain tissue O2 levels and inter-regional coherence, advancing BOLD-like measurements of functional connectivity into awake, unconstrained animals.

Changes in reward-related signals in the rat nucleus accumbens measured by in vivo oxygen amperometry are consistent with fMRI BOLD responses in man

Real-time in vivo oxygen amperometry, a technique that allows measurement of regional brain tissue oxygen (O(2)) has been previously shown to bear relationship to the BOLD signal measured with functional magnetic resonance imaging (fMRI) protocols. In the present study, O(2) amperometry was applied to the study of reward processing in the rat nucleus accumbens to validate the technique with a behavioural process known to cause robust signals in human neuroimaging studies. After acquisition of a cued-lever pressing task a robust increase in O(2) tissue levels was observed in the nucleus accumbens specifically following a correct lever press to the rewarded cue. This O(2) signal was modulated by cue reversal but not lever reversal, by differences in reward magnitudes and by the motivational state of the animal consistent with previous reports of the role of the nucleus accumbens in both the anticipation and representation of reward value. Moreover, this modulation by reward value was related more to the expected incentive value rather than the hedonic value of reward, also consistent with previous reports of accumbens coding of "wanting" of reward. Altogether, these results show striking similarities to those obtained in human fMRI studies suggesting the use of oxygen amperometry as a valid surrogate for fMRI in animals performing cognitive tasks, and a powerful approach to bridge between different techniques of measurement of brain function.

Characterisation of carbon paste electrodes for real-time amperometric monitoring of brain tissue oxygen

Tissue O₂ can be monitored using a variety of electrochemical techniques and electrodes. In vitro and in vivo characterisation studies for O₂ reduction at carbon paste electrodes (CPEs) using constant potential amperometry (CPA) are presented. Cyclic voltammetry indicated that an applied potential of -650 mV is required for O₂ reduction at CPEs. High sensitivity (-1.49 ± 0.01 nA/μM), low detection limit (ca. 0.1 μM) and good linear response characteristics (R² > 0.99) were observed in calibration experiments performed at this potential. There was also no effect of pH, temperature, and ion changes, and no dependence upon flow/fluid convection (stirring). Several compounds (e.g. dopamine and its metabolites) present in brain extracellular fluid were tested at physiological concentrations and shown not to interfere with the CPA O₂ signal. In vivo experiments confirmed a sub-second response time observed in vitro and demonstrated long-term stability extending over twelve weeks, with minimal O₂ consumption (ca. 1 nmol/h). These results indicate that CPEs operating amperometrically at a constant potential of -650 mV (vs. SCE) can be used reliably to continuously monitor brain extracellular tissue O₂.

Diverse and often opposite behavioural effects of NMDA receptor antagonists in rats: implications for "NMDA antagonist modelling" of schizophrenia

RATIONALE

Little attention has been paid to the relative equivalence of behavioural effects of NMDA receptor antagonists in rodents, with different compounds often used interchangeably to "model" aspects of schizophrenia in preclinical studies.

OBJECTIVES

To further resolve such conjecture, the present study systematically compared eight different NMDA receptor antagonists: MK-801, PCP, ketamine, memantine, SDZ 220,581, Ro 25-6981, CP 101-606 and NVP-AAM077, in a series of variable interval (VI) schedules of reinforcement. Aspects of motivation as indexed in these tasks may well be impaired in schizophrenia and undoubtedly impact on the capacity to perform more complex, explicit tasks of cognition.

METHODS AND RESULTS

An initial locomotor activity assessment demonstrated that all antagonists tested, except the NR2A-subunit preferring antagonist NVP-AAM077, induced hyperactivity, albeit of greatly differing magnitudes, qualities and temporal profiles. Three distinct patterns of antagonist effect were evident from the VI assays used: a uniform decrease in responding produced by (S)-(+)-ketamine, memantine and NVP-AAM077, a uniform increase in responding caused by the NR2B-subunit preferring antagonists Ro 25-6981 and CP 101-606, and variable bidirectional effects of PCP, SDZ 220,581 and MK-801.

CONCLUSION

Despite nominally common mechanisms of action and often presumed biological equivalence, the NMDA antagonists tested produced very diverse effects on the expression of instrumental action. Other aspects of responding were left intact, including switching and matching behaviours, and the ability to respond to conditional stimuli. The implications of such findings with regard to animal modelling of schizophrenic psychotic symptoms are manifold.

Group II metabotropic glutamate receptor antagonists LY341495 and LY366457 increase locomotor activity in mice

The group II metabotropic glutamate receptor (mGluR) antagonists LY341495 and LY366457 were profiled for their effects on locomotor activity in mice. Both compounds significantly increased locomotor activity. Observational studies showed that rearing was also selectively increased. LY366457-induced hyperactivity was significantly attenuated by the selective D1 dopamine receptor antagonist SCH23390 and also by the D2 dopamine receptor antagonist haloperidol but only at doses that significantly suppressed spontaneous locomotion. The selective 5-HT(2A) antagonist MDL100907 had no effect on LY366457-induced hyperactivity, while the less selective 5-HT(2A-C) antagonist ritanserin had only a modest effect. In all cases, the doses of antagonists that reduced the locomotor response to LY366457 were greater than those previously shown to reduce the locomotor response to the psychostimulants amphetamine and cocaine and MK-801. Pretreatment with reserpine also significantly attenuated the response to LY366457, possibly implicating a monoaminergic substrate in the mediation of this effect. The phenomenonology and pharmacology of the locomotor activation induced by the mGluR antagonists differs markedly from that induced by locomotor stimulants such as amphetamine, cocaine or MK-801. These results suggest that group II mGluRs may be involved in the tonic suppression of locomotor and exploratory activity, and this suppression can be disinhibited in the presence of a group II mGluR antagonist.

Role of 5-HT(1A) and 5-HT(1B) receptors in the mediation of behavior in the forced swim test in mice

The purpose of this study was to further examine the effect of activation of 5-HT(1A) and 5-HT(1B) receptors in the forced swim test in mice and to determine if activation of these receptors played a role in the mediation of the effects of the tricyclic antidepressant imipramine. The 5-HT(1A) agonist 8-OH-DPAT decreased immobility in the forced swim test in mice as previously described. Both the selective 5-HT(1B) agonist anpirtoline (1.25-5 mg/kg) and mixed 5-HT(1A/B) agonist RU24969 (0.6-2.5 mg/kg) significantly increased time spent swimming in the FST.

Selective imidazoline I2 ligands do not show antidepressant-like activity in the forced swim test in mice

Clonidine is an adrenergic agonist with high affinity for alpha2 adrenoceptors that also has affinity for imidazoline receptors. Clonidine has previously been shown to reduce immobility in the forced swim test (FST) in mice. In the present study, this effect was blocked by idazoxan (0.06 mg/kg s.c.) and by yohimbine (1.0 mg/kg s.c.) suggesting that clonidine's effects in this test are mediated via its action at alpha2 sites. Imidazoline I2 site ligands have been shown to inhibit monoamine oxidase and thus may also have antidepressant activity. Three compounds with selective affinity for I2 receptors (BU224, BU239, BDF 8082) were also tested in the FST. These compounds showed no activity either alone or in combination with a subthreshold dose of imipramine in the FST. These results suggest that I2 receptor ligands do not show antidepressant-like activity in the FST in mice. Furthermore the activity of the mixed alpha2/I1 agonist clonidine is most likely to be due to its action at alpha2 sites.

Control and Modeling of Fluid Leakoff During Hydraulic Fracturing

Summary

This work examines the variability in fluid leakoff rates measured under static and dynamic conditions. Laboratory-generated-data are compared to field-measureddata, and the conditions under which static and dynamic data should be used for fracture design are examined. Control of fluid leakoff in both the low-permeability matrix and highly permeable natural fractures is examined on formation cores under a variety of conditions. The control mechanism offered by various fluid-loss additives isevaluated by examining the fluid-matrix and fluid-filter-cake interactions.

Introduction

Fluid leakoff during hydraulic fracturing can exceed 70%of the injected volume if not controlled properly. A consequence of high leakoff can be the severe curtailment of production because of formation matrix damage, adverse formation fluid interactions, or altered fracturegeometry. An overwhelming amount of fluid can berequired to achieve a desired fracture geometry in a massive hydraulic fracturing treatment; thus, fluid efficiency cangovern the economic success of the treatment. A knowledge of the leakoff characteristics of aparticular formation is essential both to select proper fluid particular formation is essential both to select proper fluid loss control measures for the treatment and to predictfracture geometry. Advances in pressure analysis have made possible theestimation of formation fluid-leakoff rates from pressuredecline following injection. The method depends ona knowledge of gross fracture height; therefore, the method is best applied in formations with a large net permeable height. Using changes in fracture gradient to permeable height. Using changes in fracture gradient to estimate leakoff rates during pumping has been proposedrecently. Leakoff rates obtained from field measurements areimportant not only because they provide realistic numbersfor the prediction of fracture geometry and job design, but also because they provide a yardstick for laboratory measurements of leakoff and the development of fluid-loss control methods and additives. When such field data are not available, laboratory wall-building coefficient (C.) and spurt data are generated by applying the fluid in question to formation core samples. The conditions under which these tests are run candictate the resulting leakoff coefficient. Apart from effortsto simulate actual pumping conditions, wide variations canresult from simple static testing procedures. In the workdescribed herein, several factors affecting the outcomeof fluid-loss tests have been identified. Once those factors are controlled satisfactorily, various dynamic methodsof testing are compared. An attempt is made to correlate field-measured leak offdata with laboratory results. Both field and laboratory datashow the overriding influence of leakoff to natural fractures and/or high-permeability streaks. Once the highleakoff to these areas is curbed, leakoff to thelow-permeability matrix is influenced strongly by the shearrate within the fracture, particularly in high-rate jobs inthe early part of the pad where fracture widths arerelatively small. This can lead to changing leakoff ratesthroughout the treatment. To account for variations in C, during a treatment, Crawford 4 suggests multiplyinglaboratory values by a factor of. Characteristics of a particular formation can dictate the fluid-loss additive or combination of additives required to achieve necessary leakoff control. Particulate additivesare essential in controlling leakoff to high-permeability streaks or formations, while liquid hydrocarbon additives function well in low-permeability matrix control. The interaction of each of these additives with the formation and filter cake affects their performance. This work looks indetail at the interaction of hydrocarbon with complexed fluids and low-permeability formations.

Discussion

The leakoff rate during a hydraulic fracturing treatmenthas a marked effect on the final geometry of the fracture. With higher-efficiency fluids, less fluid is required toachieve a desired fracture length. For example, when thefracture is contained within zone, decreasing C., from0.001 to 0.0001 ft/min 1/2 [0.03 to 0.003 cm/min 1/2]increases the efficiency from 30 to 90%, and makes itpossible to use one-third as much fluid to achieve a desired possible to use one-third as much fluid to achieve a desired fracture length (Fig. 1). Using the same example, increasing fracturing fluidefficiency also increases closure time (Fig. 2). Ninety-percent fluid efficiencies alone become impractical, sinceit is desirable for the fracture to close on the proppantbefore gel degradation to minimize proppant settling. Ina large number of fracture treatments, where the fluidstability and/or break rate is tailored to 12 to 24 hours, fluid efficiencies of no greater than 70% are advisable.

JPT

P. 1071

Chemical Model for the Rheological Behavior of Crosslinked Fluid Systems

Summary

The use of crosslinking agents to improve viscosity in polysaccharide polymer fluids is a widespread practice in hydraulic fracturing. The viscosity obtained from the use of a particular crosslinking agent depends entirely on the parameters of the in-situ chemical reaction to be performed at the wellsite. The major parameters encountered, such as concentration of polymer and crosslinking agent, pH, temperature, and shear regimen, will dictate the apparent viscosity of the product generated by the reaction. A mechanistic model for this crosslinking reaction is presented along with a description of the general effects of concentration, pH, temperature, and shear levels. Macroscopic observation of an ideal "complexed" gel is discussed using the most significant reaction parameters. Data show that the rheological properties of a crosslinked fracturing fluid are time-dependent and vary widely, depending on the reaction parameters to be encountered at the wellsite during a fracture treatment.

Introduction

Since their introduction as stimulation fluids to the industry in 1968, the use of crosslinked fracturing fluids has grown steadily. Today, they account for approximately 35% of the total volume of aqueous gels used in stimulation treatments. These fluids provide several advantages over non-crosslinked gels:greater viscosity per pound of polymer,friction reduction,wider fractures,better sand transport,more viscosity in high-temperature applications, andversatility and adaptability to a wide variety of treatment conditions.

Before a comparison between various crosslinked fluids can be made, it should be recognized that the rheological data are highly dependent on the experimental conditions under which they were obtained. One of our primary objectives is to emphasize the importance of some of the experimental conditions. There are many water-soluble polymers that can be crosslinked with a variety of crosslinking agents to form fracturing fluids. However, only a rather limited number of polysaccharide gelling agents have found extensive commercial application in fracturing fluids. Table 1 shows the many chemical elements that have been used successfully to crosslink polysaccharides materials. Each element has its own unique pH. oxidation state, and concentration range for optimal crosslink formation. Although many metals require specific salt and/or chelated derivatives as the delivery form, the resulting crosslinked gels exhibit many common properties. This paper is restricted to the natural polysaccharides (cellulose and guar gum) and their nonionic derivatives (Fig. 1). We use examples of crosslinking agents from Table 1 to illustrate the effect of shear, pH. temperature, and type of coordination on the general properties exhibited by crosslinked fluids.

Experimental Procedure

Viscosity measurements were made on a Model 50 or Model 39 Fann viscometer using a variety of bob and sleeve combinations as described in Ref. 10. The crosslinking reactions were performed by first prehydrating a 0.48 to 0.72 wt% solution of the base polymer (40 to 60 lbm/1,000 gal) in a blender for 30 minutes in the presence of an adequate buffer concentration to control pH. The ph-control agents used as buffers include fumaric acid, hydrochloric acid, acetic acid, formic acid, sodium bicarbonate, sodium carbonate, and sodium hydroxide.

JPT

P. 315^