Mathematical resolution of mixed in vivo voltammetry signals. Models, equipment, assessment by simultaneous microdialysis sampling

MK-801-induced behavioral and dopaminergic responses in the shell part of the nucleus accumbens in adult male rats are disrupted after neonatal blockade of the ventral subiculum

The present study was conducted in the context of animal modeling of schizophrenia. It investigated in adult male rats, after transient neonatal blockade of the ventral subiculum (VSub), the impact of a very specific non-competitive antagonist of NMDA receptors (MK-801) on locomotor activity and dopaminergic (DAergic) responses in the dorsomedial shell part of the nucleus accumbens (Nacc), a striatal subregion described as the common target region for antipsychotics. The functional neonatal inactivation of the VSub was achieved by local microinjection of tetrodotoxin (TTX) at postnatal day 8 (PND8). Control pups were microinjected with the solvent phosphate buffered saline (PBS). Locomotor responses and DAergic variations in the dorsomedial shell part of the Nacc were measured simultaneously using in vivo voltammetry in awake, freely moving male animals after sc administration of MK-801. The following results were obtained: 1) a dose-dependent increase in locomotor activity in PBS and TTX animals, greater in TTX rats/PBS rats; and 2) divergent DAergic responses for PBS and TTX animals. A decrease in DA levels with a return to around basal values was observed in PBS animals. An increase in DA levels was obtained in TTX animals. The present data suggest that neonatal blockade of the VSub results in disruption in NMDA glutamatergic transmission, causing a disturbance in DA release in the dorsomedial shell in adults male rats. In the context of animal modeling of schizophrenia using the same approach it would be interesting to investigate possible changes in postsynaptic NMDA receptors-related proteins in the dorsomedial shell region in the Nacc.

Postnatal functional inactivation of the ventral subiculum enhances dopaminergic responses in the core part of the nucleus accumbens following ketamine injection in adult rats

For almost two decades schizophrenia has been considered to be a functional disconnection disorder. This functional disconnectivity between several brain regions could have a neurodevelopmental origin. Various approaches suggest the ventral subiculum (SUB) is a particular target region for neurodevelopemental disturbances in schizophrenia. It is also commonly acknowledged that there is a striatal dopaminergic (DA) dysregulation in schizophrenia which may depend on a subiculo-striatal disconnection involving glutamatergic NMDA receptors. The present study was designed to investigate, in adult rats, the effects of the non-competitive NMDA receptor antagonist ketamine on DA responses in the ventral striatum, or, more specifically, the core part of the nucleus accumbens (Nacc), following postnatal functional inactivation of the SUB. Functional inactivation of the left SUB was carried out by local tetrodotoxin (TTX) microinjection at postnatal day 8 (PND8), i.e. at a critical point in the neurodevelopmental period. DA variations were recorded using in vivo voltammetry in freely moving adult rats (11 weeks). Locomotor activity was recorded simultaneously with the extracellular levels of DA in the core part of the Nacc. Data obtained during the present study showed that after administration of ketamine, the two indexes were higher in TTX animals than PBS animals, the suggestion being that animals microinjected with TTX in the left SUB at PND8 present greater reactivity to ketamine than animals microinjected with PBS. These findings could provide new information regarding the involvement of NMDA glutamatergic receptors in the core part of the Nacc in the pathophysiology of schizophrenia.

Direct in Vivo Electrochemical Detection of Resting Dopamine Using Poly(3,4-ethylenedioxythiophene)/Carbon Nanotube Functionalized Microelectrodes

Dopamine (DA) is a monoamine neurotransmitter responsible for the maintenance of a variety of vital life functions. In vivo DA signaling occurs over multiple time scales, from subsecond phasic release due to dopamine neuron firing to tonic release responsible for long-term DA concentration changes over minutes to hours. Due to the complex, multifaceted nature of DA signaling, analytical sensing technology must be capable of recording DA from multiple locations and over multiple time scales. Decades of research has focused on improving in vivo detection capabilities for subsecond phasic DA, but the accurate detection of absolute resting DA levels in real time has proven challenging. We have developed a poly(3,4-ethylenedioxythiophene) (PEDOT)-based nanocomposite coating that exhibits excellent DA sensing capabilities for resting DA. PEDOT/functionalized carbon nanotube (PEDOT/CNT)-coated carbon fiber microelectrodes (CFEs) are capable of directly measuring resting DA using square wave voltammetry (SWV) with high sensitivity and selectivity. Incorporation of a PEDOT/CNT coating significantly increases the sensitivity for the detection of resting DA by a factor of 422. SWV measurements performed at PEDOT/CNT-functionalized CFEs implanted in the rat dorsal striatum reveal the absolute basal DA concentration to be 82 ± 6 nM. Systemic administration of the dopamine transporter inhibitor nomifensine increases resting DA to a maximum 207 ± 16 nM at 28 ± 2 min following injection. PEDOT/CNT was also functionalized onto individual gold electrode sites along silicon microelectrode arrays (MEAs) to produce a multisite DA sensing electrode. MEA implantation allows for the quantification of basal DA from different brain regions with excellent spatial resolution. SWV detection paired with PEDOT/CNT functionalization is highly adaptable and shows great promise for tonic DA detection with high spatial and temporal resolution.

Dopaminergic responses in the core part of the nucleus accumbens to subcutaneous MK801 administration are increased following postnatal transient blockade of the prefrontal cortex

Schizophrenia is a complex and devastating neuropsychiatric disease thought to result from impaired connectivity between several integrative regions, stemming from developmental failures. In particular, the left prefrontal cortex of schizophrenia patients seems to be targeted by such early developmental disturbances. Data obtained over the last three decades support the hypothesis of a dopaminergic dysfunction in schizophrenia. Striatal dopaminergic dysregulation in schizophrenia may result from a dysconnection between the prefrontal cortex and the striatum (dorsal and ventral) involving glutamatergic N-methyl-d-aspartate (NMDA) receptors. In the context of animal modeling of the pathophysiology of schizophrenia, the present study was designed to investigate the effects of MK 801 (dizocilpine) on locomotor activity and dopaminergic responses in the left core part of the nucleus accumbens (ventral striatum) in adult rats following neonatal tetrodotoxin inactivation of the left prefrontal cortex (infralimbic/prelimbic region) at postnatal day 8. Dopaminergic variations were recorded in the nucleus accumbens by means of in vivo voltammetry in freely moving adult animals. Following MK 801 administration, and in comparison to control (PBS) animals, animals microinjected with tetrodotoxin display locomotor hyperactivity and increased extracellular dopamine levels in the core part of the nucleus accumbens. These findings suggest neonatal functional inactivation of the prefrontal cortex may lead to a dysregulation of dopamine release in the core part of the nucleus accumbens involving NMDA receptors. The results obtained may provide new insight into the involvement of NMDA receptors in the pathophysiology of schizophrenia and suggest that future studies should look carefully at the core of the nucleus accumbens.

Neonatal Prefrontal Inactivation Results in Reversed Dopaminergic Responses in the Shell Subregion of the Nucleus Accumbens to NMDA Antagonists

Striatal dopaminergic dysregulation in schizophrenia could result from a prefronto-striatal dysconnectivity, of neurodevelopmental origin, involving N-methyl-d-aspartate (NMDA) receptors. The dorsomedian shell part of the nucleus accumbens is a striatal subregion of particular interest inasmuch as it has been described as the common target region for antipsychotics. Moreover, NMDA receptors located on the dopaminergic endings have been reported in the shell. The present study examines in adult rats the effects of early functional inactivation of the left prefrontal cortex on behavioral and dopaminergic responses in the dorsomedian shell part of the nucleus accumbens following administration of two noncompetitive NMDA receptor antagonists, ketamine, and dizocilpine (MK-801). The results showed that postnatal blockade of the prefrontal cortex led to increased locomotor activity as well as increased extracellular dopamine levels in the dorsomedian shell following administration of both noncompetitive NMDA receptor antagonists, and, more markedly, after treatment with the more specific one, MK-801, whereas decreased dopaminergic levels were observed in respective controls. These data suggest a link between NMDA receptor dysfunctioning and dopamine dysregulation at the level of the dorsomedian shell part of the nucleus accumbens. They may help to understand the pathophysiology of schizophrenia in a neurodevelopmental perspective.

Ketamine increases striatal dopamine release and hyperlocomotion in adult rats after postnatal functional blockade of the prefrontal cortex

Schizophrenia is a complex psychiatric disorder that may result from defective connectivity, of neurodevelopmental origin, between several integrative brain regions. Different anomalies consistent with brain development failures have been observed in patients' left prefrontal cortex (PFC). A striatal dopaminergic functional disturbance is also commonly acknowledged in schizophrenia and could be related to a dysfunctioning of dopamine-glutamate interactions. Non-competitive NMDA antagonists, such as ketamine, can induce psychotic symptoms in healthy individuals and worsen these symptoms in patients with schizophrenia. Our study set out to investigate the consequences of neonatal functional blockade of the PFC for dopaminergic and behavioral reactivity to ketamine in adult rats. Following tetrodotoxin (TTX) inactivation of the left PFC at postnatal day 8, dopaminergic responses induced by ketamine (5 mg/kg, 10 mg/kg, 20 mg/kg sc) were monitored using in vivo voltammetry in the left part of the dorsal striatum in freely moving adult rats. Dopaminergic responses and locomotor activity were followed in parallel. Compared to PBS animals, in rats microinjected with TTX, ketamine challenge induced a greater release of dopamine in the dorsal striatum for the highest dose (20 mg/kg sc) and the intermediate dose (10mg/kg sc). A higher increase in locomotor activity in TTX animals was observed only for the highest dose of ketamine (20 mg/kg sc). These data suggest transient inactivation of the PFC during early development results in greater behavioral and striatal dopaminergic reactivity to ketamine in adulthood. Our study provides an anatomo-functional framework that may contribute toward a better understanding of the involvement of NMDA glutamatergic receptors in schizophrenia.

Discrete Fourier Transform as applicable technique in electrochemical detection of hydrazine using multi-walled carbon nanotube/polyacrylonitrile ceramic fiber as working electrode

Effect of "Discrete Fourier Transform" (DFT) is studied for electrochemical detection of some electroactive species using multi-walled carbon nanotube/polyacrylonitrile ceramic fiber as ultra micro electrode. Based on DFT theory, two independent phases i.e. the imaginary and real phases are evaluated during the oxidation/reduction of the quasi-reversible or irreversible electroactive species, revealing the independent components of imaginary (IImaginary) and real (IReal) currents. The results show that, in different electrochemical modes such as cyclic voltammetry (CV), the contribution of DFT to the electrochemical signals significantly improves the detection limit of the electrochemical technique. More sensitive signals are obtained at high scan rates according to the combination of electrochemical techniques with the DFT theory. The reliability of DFT algorithm was evaluated for rapid determination of trace amount of hydrazine (N2H4) at a scan rate up to 800 V s(-1). In this study, the amounts of phase and amplitude were estimated to 1.69 and 31.57, respectively. The detection limit of hydrazine was 4.13×10(-9) M. The application of this technique was also evaluated for determination of hydrazine in different industrial wastewater samples.

Early prefrontal functional blockade in rats results in schizophrenia-related anomalies in behavior and dopamine

Growing evidence suggests schizophrenia may arise from abnormalities in early brain development. The prefrontal cortex (PFC) stands out as one of the main regions affected in schizophrenia. Latent inhibition, an interesting cognitive marker for schizophrenia, has been found in some studies to be reduced in acute patients. It is generally widely accepted that there is a dopaminergic dysfunctioning in schizophrenia. Moreover, several authors have reported that the psychostimulant, D-amphetamine (D-AMP), exacerbates symptoms in patients with schizophrenia. We explored in rats the effects in adulthood of neonatal transient inactivation of the PFC on behavioral and neurochemical anomalies associated with schizophrenia. Following tetrodotoxin (TTX) inactivation of the left PFC at postnatal day 8, latent inhibition-related dopaminergic responses and dopaminergic reactivity to D-AMP were monitored using in vivo voltammetry in the left core part of the nucleus accumbens in adult freely moving rats. Dopaminergic responses and behavioral responses were followed in parallel. Prefrontal neonatal inactivation resulted in disrupted behavioral responses of latent inhibition and latent inhibition-related dopaminergic responses in the core subregion. After D-AMP challenge, the highest dose (1.5 mg/kg i.p.) induced a greater dopamine increase in the core in rats microinjected with TTX, and a parallel increase in locomotor activity, suggesting that following prefrontal neonatal TTX inactivation animals display a greater behavioral and dopaminergic reactivity to D-AMP. Transitory inactivation of the PFC early in the postnatal developmental period leads to behavioral and neurochemical changes in adulthood that are meaningful for schizophrenia modeling. The data obtained may help our understanding of the pathophysiology of this disabling disorder.

Latent inhibition-related dopaminergic responses in the nucleus accumbens are disrupted following neonatal transient inactivation of the ventral subiculum

Schizophrenia would result from a defective connectivity between several integrative regions as a consequence of neurodevelopmental failure. Various anomalies reminiscent of early brain development disturbances have been observed in patients' left ventral subiculum of the hippocampus (SUB). Numerous data support the hypothesis of a functional dopaminergic dysregulation in schizophrenia. The common target structure for the action of antipsychotics appears to be a subregion of the ventral striatum, the dorsomedial shell part of the nucleus accumbens. Latent inhibition, a cognitive marker of interest for schizophrenia, has been found to be disrupted in acute patients. The present study set out to investigate the consequences of a neonatal functional inactivation of the left SUB by tetrodotoxin (TTX) in 8-day-old rats for the latent inhibition-related dopaminergic responses, as monitored by in vivo voltammetry in freely moving adult animals (11 weeks) in the left core and dorsomedial shell parts of the nucleus accumbens in an olfactory aversion procedure. Results obtained during the retention session of a three-stage latent inhibition protocol showed that the postnatal unilateral functional blockade of the SUB was followed in pre-exposed TTX-conditioned adult rats by a disruption of the behavioral expression of latent inhibition and induced a total and a partial reversal of the latent inhibition-related dopaminergic responses in the dorsomedial shell and core parts of the nucleus accumbens, respectively. The present data suggest that neonatal inactivation of the SUB has more marked consequences for the dopaminergic responses recorded in the dorsomedial shell part, than in the core part of the nucleus accumbens. These findings may provide new insight into the pathophysiology of schizophrenia.

Dopamine signaling in the nucleus accumbens of animals self-administering drugs of abuse

Abuse of psychoactive substances can lead to drug addiction. In animals, addiction is best modeled by drug self-administration paradigms. It has been proposed that the crucial common denominator for the development of drug addiction is the ability of drugs of abuse to increase extracellular concentrations of dopamine in the nucleus accumbens (NAcc). Studies using in vivo microdialysis and chronoamperometry in the behaving animal have demonstrated that drugs of abuse increase tonic dopamine concentrations in the NAcc. However, it is known that dopamine neurons respond to reward-related stimuli on a subsecond timescale. Thus, it is necessary to collect neurochemical information with this level of temporal resolution, as achieved with in vivo fast-scan cyclic voltammetry (FSCV), to fully understand the role of phasic dopamine release in normal behavior and drug addiction. We review studies that investigated the effects of drugs of abuse on NAcc dopamine levels in freely moving animals using in vivo microdialysis, chronoamperometry, and FSCV. After a brief introduction of dopamine signal transduction and anatomy and a section on current theories on the role of dopamine in natural goal-directed behavior, a discussion of techniques for the in vivo assessment of extracellular dopamine in behaving animals is presented. Then, we review studies using these techniques to investigate changes in phasic and tonic dopamine signaling in the NAcc during (1) response-dependent and -independent administration of abused drugs, (2) the presentation of drug-conditioned stimuli and operant behavior in self-administration paradigms, (3) drug withdrawal, and (4) cue-induced reinstatement of drug seeking. These results are then integrated with current ideas on the role of dopamine in addiction with an emphasis on a model illustrating phasic and tonic NAcc dopamine signaling during different stages of drug addiction. This model predicts that phasic dopamine release in response to drug-related stimuli will be enhanced over stimuli associated with natural reinforcers, which may result in aberrant goal-directed behaviors contributing to drug addiction.

Differential influence of the ventral subiculum on dopaminergic responses observed in core and dorsomedial shell subregions of the nucleus accumbens in latent inhibition

It has previously been reported that dopamine (DA) responses observed in the core and dorsomedial shell parts of the nucleus accumbens (Nacc) in latent inhibition (LI) are dependent on the left entorhinal cortex (ENT). The present study was designed to investigate the influence of the left ventral subiculum (SUB) closely linked to the ENT on the DA responses obtained in the Nacc during LI, using an aversive conditioned olfactory paradigm and in vivo voltammetry in freely moving rats. In the first (pre-exposure) session, functional blockade of the left SUB was achieved by local microinjection of tetrodotoxin (TTX). In the second session, rats were aversively conditioned to banana odor, the conditional stimulus (CS). In the retention (test) session the results were as follows: (1) pre-exposed (PE) conditioned animals microinjected with TTX, displayed aversion toward the CS; (2) in the core part of the Nacc, for PE-TTX-conditioned rats as for non-pre-exposed (NPE) conditioned animals, DA levels remained close to the baseline whereas DA variations in both groups were significantly different from the DA increases observed in PE-conditioned rats microinjected with the solvent (phosphate-buffered saline (PBS)); (3) in the shell part of the Nacc, for PE-TTX-conditioned rats, DA variations were close to or above the baseline. They were situated between the rapid DA increases observed in NPE-conditioned animals and the transient DA decreases obtained in PE-PBS-conditioned animals. These findings suggest that, in parallel to the left ENT, the left SUB controls DA LI-related responses in the Nacc. The present data may also offer new insight into the pathophysiology of schizophrenia.

Neonatal functional blockade of the entorhinal cortex results in disruption of accumbal dopaminergic responses observed in latent inhibition paradigm in adult rats

Latent inhibition (LI) has been found to be disrupted in non-treated patients with schizophrenia. Dopaminergic (DAergic) dysfunctioning is generally acknowledged to occur in schizophrenia. Various abnormalities in the entorhinal cortex (ENT) have been described in patients with schizophrenia. Numerous data also suggest that schizophrenia has a neurodevelopmental origin. The present study was designed to test the hypothesis that reversible inactivation of the ENT during neonatal development results in disrupted DA responses characteristic of LI in adult rats. Tetrodotoxin (TTX) was microinjected locally in the left ENT at postnatal day 8 (PND8). DA variations were recorded in the dorsomedial shell and core parts of the nucleus accumbens (Nacc) using in vivo voltammetry in freely-moving grown-up rats in a LI paradigm. In the first session the animals were pre-exposed (PE) to the conditional stimulus (banana odour) alone. In the second they were aversively conditioned to banana odour. In the third (test) session the following results were obtained in PE animals subjected to temporary inactivation of the ENT at PND8: (1) aversive behaviour was observed in TTX-PE conditioned animals; (2) DA variations in the dorsomedial shell and core parts of the Nacc were similar in TTX-PE and non-pre-exposed conditioned rats. These findings strongly suggest that neonatal disconnection of the ENT disrupts LI in adult animals. They may further our understanding of the pathophysiology of schizophrenia.

Striatal dopaminergic responses observed in latent inhibition are dependent on the hippocampal ventral subicular region

We showed recently that behavioural and striatal dopaminergic (DA) responses obtained in latent inhibition are crucially dependent on the parahippocampal region, the entorhinal cortex. In the present study, we investigated the influence exerted by the hippocampal ventral subicular region (SUB) on the DA responses in the anterior part of the dorsal striatum using in vivo voltammetry in freely moving rats and the same latent inhibition paradigm. To that end, the left SUB was temporarily blocked with tetrodotoxin (TTX) during pre-exposure to a new olfactory stimulus (banana odour). During the second session the animals were aversively conditioned to banana odour. With respect to the results obtained during the test session (third presentation of banana odour), similar changes in behaviour and DA levels were obtained in control and conditioned rats microinjected with the solvent, phosphate-buffered saline (PBS), in the SUB, consistently with a latent inhibition phenomenon. In contrast, after reversible inactivation of the SUB during the pre-exposure session, TTX-pre-exposed conditioned animals displayed aversive behaviour in the test session, and anterior striatal DA variations in these animals differed significantly from those obtained in pre-exposed rats injected locally with PBS. Striatal DA variations obtained in conditioned animals microinjected with TTX were also significantly different from those observed in conditioned non-pre-exposed animals. The present data suggest that, in parallel to the entorhinal cortex, the SUB regulates the latent inhibition-related behavioural and DA responses in the anterior part of the dorsal striatum. These data may provide new insight into the pathophysiology of schizophrenic psychoses.

Simultaneous real-time amperometric measurement of catecholamines and serotonin at carbon fibre 'dident' microelectrodes

Amperometry allows real-time measurement of in vivo electrochemical signals, albeit with no capacity to identify the constituents of the signal. In the present study, differential amperometry was used to monitor catecholamine and serotonin (5-HT) simultaneously at the same location. 'Dident' carbon fibre microelectrodes (microelectrodes with two working electrodes in a single assembly) were constructed and coated with Nafion to prevent poisoning on contact with brain tissue. One electrode (channel A) was held at +200 mV versus Ag/AgCl to monitor catecholamines selectively. This potential is too low to oxidise 5-HT. The second electrode (channel B), recording faradaic current at +500 mV versus Ag/AgCl, measured the sum of catecholamine and 5-HT oxidation. The 5-HT current component was the difference of channel B minus channel A. With appropriate balancing of the two channels, it is possible to record catecholamines and 5-HT simultaneously at the same dident microelectrode. Examples of measurements in striatum, cortex and locus coeruleus are shown.

Influence of the entorhinal cortex on accumbal and striatal dopaminergic responses in a latent inhibition paradigm

The use of latent inhibition paradigms is one means of investigating the involvement of mesencephalic dopaminergic (DA) neurons in cognitive processes. We have shown recently that DA neurons reaching the core and the dorsomedial shell parts of the nucleus accumbens and the anterior part of the striatum are differentially involved in latent inhibition. In other respects, theoretical, behavioral and anatomo-functional data suggest that the entorhinal cortex (ENT) may control latent inhibition expression. In this study, using in vivo voltammetry in freely moving rats, we investigated the influence of the ENT on the DA responses obtained in the core and dorsomedial shell parts of the nucleus accumbens and the anterior part of the striatum. For this purpose a reversible inactivation of the left ENT was achieved by the local microinjection of tetrodotoxin, 3 h before pre-exposure to the conditional stimulus (banana odour). During the second session, animals were aversively conditioned to banana odour. Results obtained during the third session (test session), in animals submitted to the reversible blockade of the ENT before the first session were as follows: (1) pre-exposed conditioned animals displayed behavioral aversive responses; (2) where core DA responses were concerned, responses were situated between those observed in pre-exposed and non-pre-exposed conditioned animals; (3) by contrast, where the dorsomedial shell part of the nucleus accumbens and the anterior striatum were concerned, DA variations were not statistically different in pre-exposed and non-pre-exposed conditioned rats. These data suggest that the left ENT exerts a crucial influence over the latent-inhibition-related DA responses in the left dorsomedial shell part of the nucleus accumbens and the left anterior part of the striatum, whereas one or more other brain regions control DA variations in the left core part of the nucleus accumbens. These data may help us to understand the pathophysiology of schizophrenic psychoses.

Differential involvement of dopamine in the anterior and posterior parts of the dorsal striatum in latent inhibition

The involvement of mesostriatal dopaminergic neurons in cognitive operations is not well understood, and needs to be further clarified. The use of latent inhibition paradigms is a means of investigating cognitive processes. In this study, we investigated the involvement in latent inhibition of dopaminergic inputs in the anterior part and posterior part of the dorsal striatum. The latent inhibition phenomenon was observed in a conditioned olfactory aversion paradigm. Changes in extracellular dopamine levels induced by the conditioned olfactory stimulus (banana odor) were monitored in the two parts of the dorsal striatum in the left hemisphere after pre-exposure to the olfactory stimulus using in vivo voltammetry in freely moving rats. During the conditioning session animals received either an i.p. injection of NaCl (0.9%) (control groups) or an i.p. injection of LiCl (0.15 M) (conditioned groups). Dopamine variations and place preference or aversion toward the stimulus were analyzed simultaneously in pre-exposed and non-pre-exposed animals. Data collected during the retention (test) session were as follows. Where the anterior part of the striatum was concerned, similar enhancements in dopamine levels (+100%) were obtained in pre-exposed and non-pre-exposed control animals, as well as in the pre-exposed experimental animals. In contrast, dopamine levels in the non-pre-exposed experimental group (conditioned animals) remained fairly consistently close to the baseline after the presentation of the olfactory stimulus. Where the posterior part of the striatum was concerned, increases in extracellular dopamine levels were similar (+50%) for the different groups. The present results suggested that dopaminergic neurons reaching the anterior part of the dorsal striatum are implicated in the latent inhibition phenomenon and affective perception, whereas dopaminergic terminals in the posterior part of the dorsal striatum appeared to be involved neither in latent inhibition nor in affective perception of the stimulus, seeming only to be affected by the intrinsic properties of the stimulus. Cognitive as well as affective deficits have been reported in patients with schizophrenia. Thus the present data may be considered in the context of the pathophysiology of schizophrenic psychoses.

Gastric mucosal blood flow changes in Helicobacter pylori infection and NSAID-induced gastric injury

BACKGROUND

The impact of H. pylori infection on gastric mucosal blood flow and NSAID-induced gastric damage is unclear.

AIM

To study the effects of H. pylori infection on gastric mucosal blood flow, both at basal conditions and after NSAID exposure, and its relation with mucosal damage and nitric oxide production.

METHODS

Gastric mucosal blood flow, nitric oxide production and gastric damage were assessed in time after H. pylori SS1 or E. coli inoculation in mice. Experiments were conducted in basal conditions or after oral exposure to indomethacin (20 mg/kg).

RESULTS

H. pylori infected mice exhibited a significant increase in gastric blood flow and gastric nitric oxide production 1 week after infection, but those parameters returned to basal levels by 4 weeks. NSAID challenge elicited a similar reduction in gastric blood flow [25-35%] in H. pylori-infected and control animals. However, only 1 week H. pylori-infected mice, which exhibited a significant baseline hyperemia, were able to maintain gastric blood flow values within the normal range after NSAID exposure. NSAID-induced gastric damage was increased in H. pylori-infected mice by 4 weeks, but not 1 week after infection.

CONCLUSIONS

Underlying H. pylori infection aggravates acute NSAID-induced gastric damage. However, at early phases, gastric hyperemia associated with increased nitric oxide production may exert some protective role.

Modeling fast dopamine neurotransmission in the nucleus accumbens during behavior

Recent advances in electrophysiology and voltammetry permit monitoring of dopamine (DA) neuronal activity in real time in the brain of awake animals. Studies using these approaches demonstrate that behaviorally relevant events elicit characteristic patterns of electrical activity in midbrain DA neurons as well as large, transient changes in extracellular DA in the nucleus accumbens (NAc). In addition to providing insight into the role of the DA system in the processing of motor, motivational, and sensory information, the new findings also shed light on fast DA neurotransmission in a behavioral context. This report, (1). summarizes the information obtained by electrophysiological and real-time voltammetric approaches and (2). describes a general model of phasic DA signaling in the NAc that links the observed changes in DA electrical activity and extracellular dynamics. The analysis demonstrates that the behaviorally evoked DA transients are governed by similar mechanisms as those produced by short trains of electrical stimulation. Thus, action potential-dependent release and presynaptic uptake are primary determinants of functional DA levels in the brain during behavior. Interestingly, the model predicts that the same burst of electrical activity generated at DA cell bodies produces markedly different DA dynamics in forebrain projection fields. The distinct changes result from heterogeneous release and uptake rates and may underlie region-specific effects of DA. Auto- and heteroreceptors, as well as other sites of presynaptic control, could further modulate the DA transients.

In vivo spectroscopy: a novel approach for simultaneously estimating nitric oxide and hemodynamic parameters in the rat brain

Nitric oxide (NO) is a versatile molecule involved in a wide range of biological processes. Under physiological conditions, NO reacts with oxyhemoglobin (OxyHb) to form methemoglobin (MetHb) at a very high rate. Microdialysis studies have used hemoglobin solutions as a trapping method to quantify NO in vivo. The methodology described here uses the microcapillary network with endogenous OxyHb instead of microdialysis probe with exogenous OxyHb for monitoring MetHb as an indirect index of NO levels by in vivo spectroscopy using optical fibers. This new method has been validated in rat cerebral cortex by the infusion of NO or well-known drug-induced changes in NO concentration (NMDA agonists and a NO-synthase inhibitor) and by comparing results with simultaneous voltammetric recordings. Results indicate that this spectroscopy technique is able to record large increases in MetHb levels and to detect reductions of its basal levels. In addition, data show that similar changes and kinetics can be observed with both techniques. Thus, intravascular MetHb can be used as an indirect index of NO levels. It is proposed that in vivo spectroscopy may be a useful tool to gain insight into the roles of NO in hemodynamic parameters and in other physiological processes such as the regulation of the mitochondrial respiratory chain.

Dissociation in the involvement of dopaminergic neurons innervating the core and shell subregions of the nucleus accumbens in latent inhibition and affective perception

Mesencephalic dopaminergic neurons have been found to be involved in affective processes. Their implication in cognitive processes appears less well understood. The use of latent inhibition paradigms is a means of studying these kinds of processes. In this study, we investigated the involvement of dopaminergic projections in the core, the dorsomedial shell and the ventromedial shell of the nucleus accumbens, in latent inhibition in olfactory aversive learning. Variations in extracellular dopamine levels induced by an aversively conditioned olfactory stimulus were monitored in the three parts of the nucleus accumbens in the left hemisphere, after pre-exposure to the olfactory stimulus using in vivo voltammetry in freely moving rats. The parallel between dopamine changes and place preference or aversion toward the stimulus were analyzed in pre-exposed and non-pre-exposed animals. Results showed that dopaminergic neurons innervating the nucleus accumbens are differentially involved in the latent inhibition phenomenon. Dopaminergic neurons innervating the core and the dorsomedial shell subregions of the nucleus accumbens appeared to be involved in latent inhibition processes, unlike those reaching the ventromedial shell. Nonetheless dopamine in the ventromedial shell was found to be involved in affective perception of the stimulus.The present data suggest that dopaminergic neurons innervating the three nucleus accumbens subregions are functionally related to networks involved in parallel processing of the cognitive and affective values of environmental information, and that interaction between these systems, at some levels, may lead to a given behavioral output. These data may provide new insights into the pathophysiology of schizophrenic psychoses.

Specificity of amygdalostriatal interactions in the involvement of mesencephalic dopaminergic neurons in affective perception

We have recently shown that dopaminergic responses to an attractive or an aversive stimulus were respectively increased and decreased in the core part of the nucleus accumbens and the ventromedial dorsal striatum. By contrast, increases in dopaminergic responses were obtained in the shell part of the nucleus accumbens with stimuli of both affective values. In addition, the involvement of the basolateral amygdala in affective processes has been reported by several authors. Anatomo-functional relationships between the basolateral amygdala and striatal structures have also been described. Thus, in the present work we studied the regulation by the basolateral amygdala of affective dopaminergic responses in the two parts of the nucleus accumbens (core and shell) and the ventromedial dorsal striatum. More precisely, variations in extracellular levels of dopamine induced by an attractive or an aversive olfactory stimulus were studied using in vivo voltammetry in freely moving rats. Changes in dopamine levels in the three left striatal regions were measured after functional blockade of the ipsilateral basolateral amygdala with tetrodotoxin. Changes in place attraction or aversion toward the stimulus were studied in parallel to dopamine variations. The results obtained suggest a specific regulation of affective dopaminergic responses in the two parts of the nucleus accumbens by the basolateral amygdala and a lack of influence of the basolateral amygdala on the ventromedial dorsal striatum. The results suggest that attraction or aversion toward a stimulus are correlated with dopamine variations in the core of the nucleus accumbens and that the basolateral amygdala controls affective behavioural responses. These data may provide new insights into the pathophysiology of schizophrenic psychoses.

Transient increases in catecholaminergic activity in medial prefrontal cortex and nucleus accumbens shell during novelty

Voltammetric recordings with electrochemically modified carbon-fiber electrodes were obtained from specific regions of the forebrain in rats given free-choice access to a novel environment. Entry into novelty increased the catechol signal in the medial prefrontal cortex and shell of the nucleus accumbens by more than 100%, but had no consistent effect in either the neostriatum or accumbal core. In both the medial prefrontal cortex and accumbal shell, moreover, the novelty-induced increase in catecholaminergic activity was detectable only during the initial entry into the novel compartment and did not reappear when animals returned to the familiar environment. These results support increasing evidence for a functional distinction between the accumbal core and shell, with the latter having been linked to brain reward mechanisms. The results also indicate that novelty activates, albeit very transiently, some of the same neurochemical systems believed to play a critical role in the reinforcing effects of certain drugs of abuse.

In vivo monitoring of brain neurotransmitter release for the assessment of neuroendocrine interactions

1. The neurotransmitter mechanisms regulating neuroendocrine processes have been traditionally inferred from the effects of drugs purportedly acting through specific transmitter systems. The direct appraisal of changes in endogenous neuromediators had to rely initially on analyses of brain samples obtained post-morten. 2. Currently, a more physiological assessment is available through the monitoring ot the extracellular levels of neurotransmitters and their metabolites in discrete brain areas of living animals. Two methodologies, namely in vivo voltammetry and microdialysis, are being increasingly used for this purpose. This article summarizes their principles, relative merits, and limitations and presents some relevant applications. 3. Thus, microdialysis data show a differential response in the amphetamine-induced dopamine release in the nucleus accumbens in adult male and female rats castrated prepuberally. Given their high time-resolution, in vivo electrochemistry techniques seem especially suited for studying the fast, non-genomic effects of steroid hormones. This is illustrated by the voltammetric detection of a rapid release of dopamine in the corpus striatum induced by progesterone in males. 4. These methodologies should be regarded as complementary tools for the assessment of the neurochemical correlates of neuroendocrine interactions.

Voltammetric and microdialysis monitoring of brain monoamine neurotransmitter release during sociosexual interactions

The monoamine neurotransmitters have long been ascribed important modulatory actions on male sexual behavior by a wealth of pharmacological studies. Methodological developments have now made possible the assessment of the extracellular levels of amine transmitters and their metabolites in discrete brain areas of sexually behaving animals using in vivo voltammetry and microdialysis. Studies in our and other laboratories consistently show increased dopamine release in forebrain structures known to be involved in mating activity, including the nucleus accumbens and the medial preoptic area, during both the appetitive (i.e., non-contact exposure to sexual stimuli) and consummatory phases of this behavior. Serotonin utilization seems to be mainly related to consummatory events. These findings are consistent with the pharmacological evidence as well as previous ex vivo work. The state of sexual inactivity that follows unrestricted mating associates with increased dopamine turnover in the preoptic area. According to the available information, it could reflect some blockade of dopaminergic receptors, possibly involving prolactin. No disturbance of ongoing sexual behavior was observed during the neurochemical monitoring sessions with either methodology. These studies show voltammetry and microdialysis as powerful complementary tools for the assessment of sociosexual interactions.

Brain microdialysis and its application for the study of animal behaviour

Microdialysis is a sampling method that is used to determine the extracellular concentration of neurotransmitters in the brain. The method can be applied to conscious and unrestrained animals and is very suitable for the study of the chemistry of endogenous behaviour. This article reviews the contribution that microdialysis made to our understanding of the chemistry of behaviour. Methodological and practical considerations such as the implantation time and the use of guide cannulas are reviewed. The question whether neurotransmitters and related metabolites in dialysates reflect true synaptic release is critically discussed. There is much evidence that dopamine, noradrenaline, acetylcholine and serotonin in dialysates are related to neurotransmission, but there is serious doubt whether this is the case with amino acid transmitters such as GABA, glutamate and aspartate. Until now far over 100 papers appeared that used microdialysis in behavioural studies. Behavioural activation, the sleep-awake cycle and diurnal rhythms were subject of several of these studies. Various workers have described neurochemical changes in the brain that are related to feeding. Other studies were concerned with sexual behaviour and the sexual cycle in females. Parturition, maternal behaviour and offspring recognition have been studied in a series of microdialysis studies carried out in sheep. An overview is given of the microdialysis studies that were carried out to understand the biochemistry of stress. In this respect dopamine and noradrenaline have received much attention. A great number of microdialysis studies dealt with the role of dopamine in self-stimulation, reward and aversive emotions. It is concluded that microdialysis is at presently the most versatile and practical method to study the chemistry of behaviour and it is to be expected that it will soon be a routine methodology in behavioural research. Finally, perspectives and possible future developments of the methods are discussed.

Asymmetrical involvement of mesolimbic dopaminergic neurons in affective perception

Pharmacological studies suggest that increases and decreases in dopamine levels in the nucleus accumbens contribute to positive and negative affective states, respectively. In vivo neurochemical investigations have led to contradictory conclusions, since increases and decreases in dopamine release have been observed in aversive situations. Clinical and experimental observations argue for a hemispheric asymmetry in the processing of appetitive and aversive stimuli. Mesolimbic dopaminergic neurons are part of integrative networks which appear specifically organized in the right and left hemispheres. Dopaminergic neurons may thus be involved in affective processes but in a different manner in the two hemispheres. We tested this hypothesis in the nucleus accumbens of male rats using in vivo voltammetry and a conditioned aversion paradigm to an olfactory stimulus. We found that dopaminergic responses were similar in the two hemispheres following the initial encounter with the stimulus. After conditioning, however, dopaminergic responses to a naturally attractive olfactory stimulus were more elevated in the right nucleus accumbens and responses to an aversive stimulus more marked in the left nucleus. In addition, dopaminergic responses displayed an intraaccumbal regionalization, in particular opposite variations were obtained in the core and shell subterritories in response to the aversive situation. These results may provide new insights in the understanding of the relative contribution of the two hemispheres in affective perception in normal and psychopathological conditions.

Neurobiological correlates of masculine sexual behavior

The experimental analysis of the neuroendocrine interactions regulating sexual behavior has traditionally relied on studying the effects of CNS lesions and pharmacological treatments with hormones or drugs purportedly acting through specific neurotransmitter systems. New methodological developments have allowed the assessment of several indices of neural function in experimental animals, particularly the rat, as they relate to behavioral changes. In the field of sexual behavior, ex vivo analyses have been used to measure markers of energy metabolism, such as 2-deoxyglucose uptake and Na,K-ATPase activity, the tissue content of neurotransmitters and metabolites, the levels of steroid receptors and neurosteroids, and immediate-early gene expression products in different areas of the CNS. In vivo studies have monitored brain electrical activity and temperature, as well as the extracellular levels of neurotransmitters and metabolites by cerebrospinal fluid sampling, push-pull perfusion and, especially, electrochemical recordings and microdialysis, in the course of mating and exposure to various relevant stimuli. The findings with the different methodologies are generally consistent and agree with those of previous surgical and pharmacological manipulations. They provide data on temporal relationships between neurobiological and behavioral events and suggest new interpretations for different aspects of the male copulatory pattern.

Artificial neural network and classical least-squares methods for neurotransmitter mixture analysis

Identification of individual components in biological mixtures can be a difficult problem regardless of the analytical method employed. In this work, Raman spectroscopy was chosen as a prototype analytical method due to its inherent versatility and applicability to aqueous media, making it useful for the study of biological samples. Artificial neural networks (ANNs) and the classical least-squares (CLS) method were used to identify and quantify the Raman spectra of the small-molecule neurotransmitters and mixtures of such molecules. The transfer functions used by a network, as well as the architecture of a network, played an important role in the ability of the network to identify the Raman spectra of individual neurotransmitters and the Raman spectra of neurotransmitter mixtures. Specifically, networks using sigmoid and hyperbolic tangent transfer functions generalized better from the mixtures in the training data set to those in the testing data sets than networks using sine functions. Networks with connections that permit the local processing of inputs generally performed better than other networks on all the testing data sets. and better than the CLS method of curve fitting, on novel spectra of some neurotransmitters. The CLS method was found to perform well on noisy, shifted, and difference spectra.

Changes in monoamine turnover in forebrain areas associated with masculine sexual behavior: a microdialysis study

This report compares the changes in the main dopamine (DA) and serotonin (5-HT) metabolites, respectively dihydroxyphenylacetic acid (DOPAC) and 5-hydroxy indoleacetic acid (5-HIAA) in three relatively close brain regions, namely the nucleus accumbens (ACB), the medial preoptic area, and the medial basal hypothalamus (MBH), as well as DA in the ACB, of copulating male rats. All these neurochemicals remained fairly stable when the animals were exposed to non sexual social stimuli (castrated females), and they increased during mating with receptive females. There were regional differences in these copulation-related changes, however, with those in the MBH being shorter-lived. There were also differences in the time-course of the changes in DOPAC and 5-HIAA the latter being slower. It is suggested that they reflect the involvement of the DA and 5-HT innervation of diencephalic structures in, respectively the appetitive and consummatory/satiation mechanisms of sexual behavior. The physiological relevance of these neurochemical changes is supported by the lack of differences between the standard measures for sexual behavior recorded before surgery and during the dialysis session.

Formation and clearance of interstitial metabolites of dopamine and serotonin in the rat striatum: an in vivo microdialysis study

In vivo microdialysis was employed in order to characterize the steady-state kinetics of the turnover of specific dopamine and serotonin metabolites in the rat striatum 48 h after surgery. Inhibitors of monoamine oxidase (MAO; pargyline) and catechol-O-methyltransferase (COMT; Ro 40-7592) were administered, either separately or in conjunction, at doses sufficient to block these enzymes in the CNS. In some experiments, the acid metabolite carrier was blocked with probenecid. Temporal changes were then observed in the efflux of interstitial dopamine, 3-methoxytyramine (3-MT), 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), and 5-hydroxyindoleacetic acid (5-HIAA). The fractional rate constants for the accumulation or disappearance of the metabolites could be determined after pharmacological blockade of catabolic enzymes or the acid metabolite carrier. Interstitial 5-HIAA was found to be cleared with a half-life of approximately 2 h. After blockade of either MAO or COMT, HVA disappeared with a half-life of 17 min. Experiments employing probenecid suggested that some of the interstitial HVA was cleared by the acid metabolite carrier, the remainder being cleared by a probenecid-insensitive process, possibly conjugation. After MAO inhibition, DOPAC disappeared with an apparent half-life of 11.3 min. The rate of 3-MT accumulation after pargyline indicated that the majority of interstitial HVA (> 95%) is formed from DOPAC rather than 3-MT. The formation of 3-MT from interstitial dopamine, calculated from the accumulation rate of 3-MT after pargyline, appeared to follow first-order kinetics (k = 0.1 min-1).