In vivo selective monitoring of basal levels of cerebral dopamine using voltammetry with Nafion modified (NA-CRO) carbon fibre micro-electrodes

A novel carbon tipped single micro-optrode for combined optogenetics and electrophysiology

Optical microelectrodes (optrodes) are used in neuroscience to transmit light into the brain of a genetically modified animal to evoke and record electrical activity from light-sensitive neurons. Our novel micro-optrode solution integrates a light-transmitting 125 micrometer optical fiber and a 9 micrometer carbon monofilament to form an electrical lead element, which is contained in a borosilicate glass sheathing coaxial arrangement ending with a micrometer-sized carbon tip. This novel unit design is stiff and slender enough to be used for targeting deep brain areas, and may cause less tissue damage compared with previous models. The center-positioned carbon fiber is less prone to light-induced artifacts than side-lit metal microelectrodes previously presented. The carbon tip is capable of not only recording electrical signals of neuronal origin but can also provide valuable surface area for electron transfer, which is essential in electrochemical (voltammetry, amperometry) or microbiosensor applications. We present details of design and manufacture as well as operational examples of the newly developed single micro-optrode, which includes assessments of 1) carbon tip length-impedance relationship, 2) light transmission capabilities, 3) photoelectric artifacts in carbon fibers, 4) responses to dopamine using fast-scan cyclic voltammetry in vivo, and 5) optogenetic stimulation and spike or local field potential recording from the rat brain transfected with channelrhodopsin-2. With this work, we demonstrate that our novel carbon tipped single micro-optrode may open up new avenues for use in optogenetic stimulation when needing to be combined with extracellular recording, electrochemical, or microbiosensor measurements performed on a millisecond basis.

Impulse-dependent extracellular resting dopamine concentration in rat striatum in vivo

The ambient resting dopamine (DA) concentration in brain regulates cognition and motivation. Despite its importance, resting DA level in vivo remains elusive. Here, by high-frequency stimulation of the medial forebrain bundle and immediately following the stimulus-induced DA overflow, we recorded a DA "undershoot" which is a temporal reduction of DA concentration to a level below the baseline. Based on the DA undershoot, we predicted a resting DA concentration of ∼73nM in rat striatum in vivo. Simulation studies suggested that removing basal DA by DAT during the post-stimulation inhibition of tonic DA release caused the DA undershoot, and the resting concentration of DA modulated the kinetics of the evoked DA transient. The DA undershoot was eliminated by either blocking D2 receptors with haloperidol or blocking the DA transporter (DAT) with cocaine. Therefore, the impulse-dependent resting DA concentration is in the tens of nanomolar range and is modulated by the presynaptic D2 receptors and the DAT in vivo.

Selective D3 Receptor Antagonist SB-277011-A Potentiates the Effect of Cocaine on Extracellular Dopamine in the Nucleus Accumbens: a Dual Core-Shell Voltammetry Study in Anesthetized Rats

Dopamine (DA) D3 receptors have been associated with drug intake and abuse and selectively distribute in the brain circuits responding to drug administration. Here we examined the effects of an acute systemic administration of cocaine (15 mg/kg) alone or preceded by treatment with the selective D3 receptor antagonist SB-277011-A (10 mg/kg) on DA levels concurrently in the rat nucleus accumbens shell and core sub-regions (NAcshell and NAccore, respectively). It is shown that cocaine increases extracellular DA in both compartments and that blocking D3 receptors with SB-277011-A, although the latter is devoid of dopaminergic effects per se, potentiates these effects. No differences in the amplitude of the response were observed between NAcshell and NAccore compartments, though the dopaminergic response in the NAcshell was transient whereas that in the NAccore rose slowly to reach a plateau. These results demonstrate the feasibility to use multiprobe voltammetry to measure discrete monoaminergic responses in discrete areas of the brain and confirm the effect of D3 receptors antagonist at modifying the neurochemical effects of cocaine.

Medial prefrontal cortical alpha1 adrenoreceptor modulation of the nucleus accumbens dopamine response to stress in Long-Evans rats

RATIONALE

The medial prefrontal cortex (PFC) receives stress-sensitive dopamine (DA) and noradrenergic (NE) projections from the ventral tegmental area and locus coeruleus, respectively, and evidence from various sources point to a complex functional interaction between these two systems. Stress will also stimulate DA transmission in the nucleus accumbens (NAcc), and our previous work has shown that this response is under the indirect inhibitory control of a DA-sensitive mechanism in PFC.

OBJECTIVE

We examined the possibility that the NAcc DA stress response is also modulated by prefrontal cortical NE.

MATERIALS AND METHODS

We used voltammetry to study in freely behaving rats the effects of local applications of alpha(1) (benoxathian 0.1, 1, 10 nmol), alpha(2) (SKF86466), and beta(1/2) (alprenolol) receptor selective antagonists into the PFC on the NAcc DA response to tail-pinch stress.

RESULTS

The NAcc DA stress response was dose-dependently inhibited by local PFC blockade of alpha(1) receptors. Additional tests revealed, however, that the DA stress response in NAcc is unaffected after local alpha(1) receptor activation with cirazoline. Furthermore, at equivalent doses, neither alpha(2) nor beta(1/2) receptor blockade significantly affected the NAcc DA stress response.

CONCLUSIONS

These data indicate that stress-induced activation of subcortical DA transmission is modulated by the NE input to PFC acting at alpha(1) receptors. They suggest that, under normal circumstances, this system exerts a facilitatory or enabling influence on the NAcc DA stress response.

Influence of naturally occurring variations in maternal care on prepulse inhibition of acoustic startle and the medial prefrontal cortical dopamine response to stress in adult rats

In rats, naturally occurring variations in maternal care contribute to the development of individual differences in the behavioral and neuroendocrine responses to stress during adulthood. The dopamine (DA) projection to the medial prefrontal cortex (mPFC) plays an important role in mediating stress responsivity and is thought to be involved also in regulating sensorimotor gating. In the present study, we compared prepulse inhibition (PPI) of acoustic startle as well as the left and right mPFC DA stress responses in the adult offspring of high- and low-licking/grooming (LG) dams. Our data indicate that the offspring of low-LG animals are impaired on measures of PPI compared with high-LG animals. We also observed in low-LG animals a significant blunting of the mPFC DA stress responses that was lateralized to the right hemisphere, whereas in high-LG animals, the left and right mPFC DA stress responses were equally attenuated. Although mPFC levels of DA transporter did not differ between the two groups of animals, mPFC levels of catechol-O-methyl transferase immunoreactivity of low-LG animals were significantly lower than those of high-LG animals. These data provide evidence that variations in maternal care can lead to lasting changes in mPFC DA responsivity to stress and suggest the possibility that such changes in mesocorticolimbic DA function can also lead to deficits in sensorimotor gating.

Effects of tissue trauma on the characteristics of microdialysis zero-net-flux method sampling neurotransmitters

Microdialysis has been used for studying neurochemistry in brain regions that respond to afferent inputs or administered drugs. As the knowledge derived from and concerning microdialysis grows, so do the concerns over its invasiveness and, hence, the credibility of resulting data. Recent experimental and theoretical studies impugned the validity of the microdialysis zero-net-flux (ZNF) method in measuring brain extracellular neurotransmitters, suggesting that the tissue trauma resulting from probe implantation seriously compromises its worth. This paper developed a theoretical model to study the influences of two categories of tissue trauma on microdialysis ZNF operation: (1) morphological alterations in tissue extracellular structure and (2) physiological impairment of neurotransmitter release and uptake processes. Model results show that alterations of tissue extracellular structure negligibly affect the accuracy of the ZNF method in determining the basal level of extracellular neurotransmitter but do affect the fundamental characteristics of microdialysis: the extraction efficiency and relative recovery. An inhibited or damaged neurotransmitter uptake process always decreases the efficiency of microdialysis extraction, but rise of the relative recovery of neurotransmitters with the same uptake inhibition/damage occurs only when there is far more damage to the neurotransmitter release than to the uptake process in the tissue. A criterion for this rising trend of microdialysis relative recovery is discussed in terms of trauma parameters and neurotransmitter uptake inhibition.

Evidence on extracellular dopamine level in rat striatum: implications for the validity of quantitative microdialysis

Microdialysis zero-net-flux (ZNF) method is commonly used to monitor drug-induced changes in neurotransmitter baseline and release/uptake processes. Recent studies in this field suggest that microdialysis ZNF method seriously underestimates the resting concentration of extracellular dopamine in the rat neostriatum because probe implantation preferentially damages nearby dopamine release sites and that dopamine uptake inhibition increases the relative recovery of dopamine by microdialysis. This study assessed the validity of these claims by examining current data on extracellular dopamine levels at rest and after drug application obtained by voltammetry, a technique thought to induce less tissue disruption than microdialysis. To obtain the extracellular baseline value for dopamine from the evoked overflow data, we modified the existing dopamine kinetic model to suit both the resting and stimulated circumstances. It was found that dopamine uptake inhibition did in fact decrease the microdialysis relative recovery of dopamine, implying that the average basal extracellular dopamine level is within the range of 7-20 nm in rat striatum. This study concludes that the microdialysis ZNF method indeed underestimates the extracellular dopamine concentration, although not by as much as had been thought. Chronic microdialysis damages both neurotransmitter release and uptake, but it does so in a somewhat relative and proportional way for both processes. Thus the validity of the microdialysis ZNF method is not seriously undermined.

A microphysiometer for simultaneous measurement of changes in extracellular glucose, lactate, oxygen, and acidification rate

A microphysiometer capable of measuring changes in extracellular glucose, lactate, oxygen, and acidification rate has been developed by incorporating modified electrodes into a standard Cytosensor Microphysiometer plunger. Glucose and lactate are measured indirectly at platinum electrodes by amperometric oxidation of hydrogen peroxide, which is produced from catalysis of glucose and lactate at films containing their respective entrapped oxidase. Oxygen is measured amperometrically at a platinum electrode coated with a Nafion film, while the acidification rate is measured potentiometrically by a Cytosensor Microphysiometer. Analytical information is obtained during the Cytosensor stop-flow cycles, where the electrodes measure changes in the extracellular medium corresponding to the consumption or production of the analyte by the cells. Modification of the Cytosensor plunger for multianalyte determination is described, and the operation of the technique is illustrated by the simultaneous measurement of all four analytes during the addition of fluoride and DNP to Chinese hamster ovary cells and fluoride and antimycin A to mouse fibroblast cells. Cell metabolic recovery and dynamics after exposure to agents can also be observed in specific cases.

Influence of early postnatal rearing conditions on mesocorticolimbic dopamine and behavioural responses to psychostimulants and stressors in adult rats

While many experiment with drugs, relatively few individuals develop a true addiction. We hypothesized that, in rats, such individual differences in the actions of addictive drugs might be determined by postnatal rearing conditions. To test this idea, we investigated whether stimulant- and stress-induced activation of nucleus accumbens dopamine transmission and dopamine-dependent behaviours might differ among adults rats that had been either repeatedly subjected to prolonged maternal separation or a brief handling procedure or left undisturbed (non-handled) during the first 14 days of life. We found that, in comparison with their handled counterparts, maternally separated and non-handled animals are hyperactive when placed in a novel setting, display a dose-dependent higher sensitivity to cocaine-induced locomotor activity and respond to a mild stressor (tail-pinch) with significantly greater increases in nucleus accumbens dopamine levels. In addition, maternally separated animals were found to sensitize to the locomotor stimulant action of amphetamine when repeatedly stressed under conditions that failed to sensitize handled and non-handled animals. Finally, quantitative receptor autoradiography revealed a lower density of nucleus accumbens-core and striatal dopamine transporter sites in maternally separated animals. Interestingly, we also found greatly reduced D(3) dopamine receptor binding and mRNA levels in the nucleus accumbens-shell of handled animals. Together, these findings provide compelling evidence that disruptions in early postnatal rearing conditions can lead to profound and lasting changes in the responsiveness of mesocorticolimbic dopamine neurons to stress and psychostimulants, and suggest a neurobiological basis for individual differences in vulnerability to compulsive drug taking.

Microelectrochemical sensors for in vivo brain analysis: an investigation of procedures for modifying Pt electrodes using Nafion

Various Nafion coating procedures were examined in order to design a simple and reproducible coating method to maximise permselective characteristics, and thus eliminate signals from electroactive interferents, in sensors designed for direct in vivo measurements in the brain. Interferents investigated included ascorbic acid (AA), the principal endogenous electroactive interferent present in the brain, and uric acid. Application of the Nafion (5% commercial solution) using a thermally annealing procedure involving 5 pre-coats, and 2 subsequent dip-bake layers resulted in elimination of interferent signals. It also produced complete blocking of the signal for the neurotransmitter dopamine. The optimum time and temperature for annealing was found to be 5 min at 210 degrees C. An examination of shelf life over two weeks indicated negligible AA interference over this period. Preliminary investigations with respect to the potential use of these Nafion-modified Pt electrodes in the design of implantable, first generation, peroxide detecting biosensors indicated that the modified electrode had no effect on O2 permeability but did produce a significant decrease in H2O2 sensitivity. While this may preclude their use in biosensor development they may be more suitable for detection of gaseous neurochemicals such as nitric oxide.

Basolateral amygdala modulation of the nucleus accumbens dopamine response to stress: role of the medial prefrontal cortex

The basolateral amygdala (BLA) is involved in modulating affective responses to stress and, along with the nucleus accumbens (NAc) and medial prefrontal cortex (mPFC), receives a stress-responsive dopamine (DA) projection from the ventral tegmental area. The present study was undertaken to characterize the role of BLA DA D1 and D2/D3 receptor subtypes in modulating the NAc and mPFC DA responses to stress. Voltammetry was used to monitor, in freely behaving rats, stress-induced DA release in NAc or mPFC after injection of D1 (SCH 23390) or D2/D3 (raclopride) receptor antagonist into BLA. Intra-BLA SCH 23390 injection potentiated stress-induced NAc DA release but attenuated the mPFC DA stress response; raclopride had no effect on either the NAc or mPFC DA responses to stress. Based on these results, we also examined the possibility that BLA can indirectly modulate the NAc DA stress response via its projection to mPFC. To do so we studied the effects of intra-mPFC co-administration of D1 (SKF 38393) and D2/D3 (quinpirole) receptor agonists on the potentiated NAc DA stress response resulting from intra-BLA SCH 23390 injection. Alone, mPFC D1 and D2/D3 receptor co-activation had no effect on stress-induced NAc DA release, but did prevent the potentiated NAc DA stress response produced by BLA D1 receptor blockade. These findings indicate that BLA DA modulates the NAc and mPFC DA stress responses via activation of the D1 receptor subtype. They also suggest that BLA DA modulates stress-induced NAc DA release indirectly by modulating the mPFC DA response to stress.

Enhanced nucleus accumbens dopamine and plasma corticosterone stress responses in adult rats with neonatal excitotoxic lesions to the medial prefrontal cortex

The medial prefrontal cortex modulates the nucleus accumbens dopamine response to stress and has been implicated in feedback regulation of hypothalamic-pituitary-adrenal axis activation by stress. Here we report on the effects of bilateral neonatal (postnatal day 7) ibotenate-induced lesions to the medial prefrontal cortex on nucleus accumbens dopamine and neuroendocrine function in adult rats. Voltammetry was used to monitor the dopamine response to each of five, once-daily exposures to tail-pinch stress whereas alterations in neuroendocrine function were determined from the plasma corticosterone response to a single 20-min episode of restraint stress. Potential lesion-induced deficits in sensory-motor gating were assessed by measuring prepulse inhibition of the acoustic startle response before and after repeated stress. Our data show that each daily stress episode elicited larger and longer-lasting dopamine increases in prefrontal cortex-lesioned animals than in sham-lesioned controls. Furthermore, greater stress-induced elevations in plasma corticosterone were seen in lesioned animals than in their sham-lesioned counterparts. However, while repeated stress potentiated startle responses in animals of both groups, there was no effect of lesion on the amplitude or on prepulse inhibition of the startle response.Together, these findings indicate that neonatal prefrontal cortex damage can lead to changes in mesolimbic dopamine and neuroendocrine function during adulthood. They also add to a growing body of experimental and clinical evidence implicating abnormal prefrontal cortex neuronal development in the pathophysiology of schizophrenia and other disorders linked to central dopamine dysfunction.

Effects of alkylating agents on dopamine D(3) receptors in rat brain: selective protection by dopamine

Dopamine D(3) receptors are structurally highly homologous to other D(2)-like dopamine receptors, but differ from them pharmacologically. D(3) receptors are notably resistant to alkylation by 1-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ), which readily alkylates D(2) receptors. We compared EEDQ with N-(p-isothiocyanatophenethyl)spiperone (NIPS), a selective D(2)-like receptor alkylating agent, for effects on D(3) and D(2) receptors in rat brain using autoradiographic analysis. Neither agent occluded D(3) receptors in vivo at doses that produced substantial blockade of D(2) receptors, even after catecholamine-depleting pretreatments. In vitro, however, D(3) receptors were readily alkylated by both NIPS (IC(50)=40 nM) and EEDQ (IC(50)=12 microM). These effects on D(3) sites were blocked by nM concentrations of dopamine, whereas microM concentrations were required to protect D(2) receptors from the alkylating agents. The findings are consistent with the view that alkylation of D(3) receptors in vivo is prevented by its high affinity for even minor concentrations of endogenous dopamine.

High-speed voltammetry: dual measurement of dopamine and serotonin

A high-speed voltammetric system was designed and tested for dual measurement of dopamine (DA) and serotonin (5-HT) at 250-ms intervals. First, an anodic-cathodic square-wave pulse was delivered to activate the electrode (carbon fiber, 7 microm in diameter), then DA and 5-HT oxidation currents (current intensity) were measured when potentials were stepped from 100 to 250 mV and 300 to 450 mV, respectively. To isolate DA and 5-HT current intensities, the current observed at 100 mV was subtracted from that at 250 mV, and the current observed at 300 mV was subtracted from that at 450 mV, respectively. Measurements were performed every 250 ms. In vitro, DA and 5-HT current intensities increased with increasing concentrations of DA and 5-HT, respectively. The DA current intensity was not affected by the addition of the DA metabolite 3,4-dihydroxyphenylacetic acid (10(-6) M) or ascorbic acid (10(-5) M), but the 5-HT current intensity was affected by the addition of the 5-HT metabolite 5-hydroxyindoleacetic acid (10(-6) M) or uric acid (10(-5) M). Electrodes were used for several months without any change in sensitivity. In vivo, following intraperitoneal injection of L-DOPA to rats, an increase in striatal DA release was observed but there was no increase in release of 5-HT. Following intraperitoneal injection of 5-hydroxytryptamine there was an increase in 5-HT release but not DA release. This high-speed system was capable of obtaining stable, long-term dual measurements of DA and 5-HT in vitro and in vivo.

Effects of medial prefrontal cortical injections of GABA receptor agonists and antagonists on the local and nucleus accumbens dopamine responses to stress

Stress stimulates dopamine (DA) release in nucleus accumbens (NAcc) but will do so more strongly in medial prefrontal cortex (PFC). Evidence indicates, however, that the cortical DA response to stress acts to dampen the concurrent increase in NAcc DA release. In the present study, we used voltammetry to investigate the role of PFC GABA in regulating the NAcc DA response to stress. The results of Experiment 1 show that the NAcc stress response is inhibited following bilateral cortical microinjections of baclofen (GABAB receptor agonist). While phaclofen (GABAB receptor antagonist) blocked the effect of baclofen, it had no significant effect of its own. Intra-PFC injections of muscimol (GABAA receptor agonist) and bicuculline (GABAA receptor antagonist) had no effect on the DA stress response in NAcc. In Experiment 2, we explored the possibility that GABA influences the NAcc DA stress response indirectly by modulating stress-induced DA release in PFC. None of the drugs tested had an effect on the PFC stress response at a dose (1 nmol) that produced reliable effects on the NAcc stress response. At an order of magnitude higher dose, however, locally applied phaclofen and muscimol enhanced and attenuated, respectively, the DA stress response in PFC. These results were validated in Experiment 3 by showing that intra-PFC injections of GBR-12395 (DA uptake blocker) and quinpirole (D2/D3 receptor agonist) dose-dependently enhanced and inhibited, respectively, the local DA stress response. Together, these findings indicate that increased GABA transmission in PFC exerts an inhibitory influence on the NAcc DA response to stress, and that this action is mediated primarily but not exclusively by GABAB receptors which may be located both on cortical output neurons and on DA terminals.

Relationships between stress-induced increases in medial prefrontal cortical dopamine and plasma corticosterone levels in rats: role of cerebral laterality

In the present study, in vivo voltammetry was used to monitor changes in dopamine levels in the left and right medial prefrontal cortex of rats exposed to mild physical and psychological stress. These were 2 min of tail-pinch and 15 min exposure to cat odour, respectively. Fourteen male Long Evans rats with bilateral carbon fibre recording electrodes were tested on four consecutive days, and records obtained in each medial prefrontal cortex for each stressor. A week later, animals underwent a 20 min restraint stress, with plasma samples taken at 0, 20 and 80 min to determine stress-induced corticosterone responses. It was found that dopamine responses to tail-pinch were significantly longer-lasting in the left hemisphere than in the right, while this asymmetry was not present for the dopamine response to cat odour. Stress-induced dopamine increases elicited by the two stressors were significantly correlated only in the right medial prefrontal cortex. Restraint stress-induced increases in plasma corticosterone were positively correlated with dopaminergic responses to tail-pinch, but were only related to dopamine cat odour responses when individual asymmetries favoured the right medial prefrontal cortex. The data suggest that asymmetric mesocortical dopamine activation depends on the type of stress, and that regulation of dopamine responses to both types of stress is most tightly coupled in the right hemisphere. While neuroendocrine and dopaminergic stress responses are positively linked, this relationship is only asymmetrical for the psychological stressor, suggesting a specialized role for right cortical mechanisms in the integration of emotional and physiological responses to stressful situations. A preliminary report of this work was presented at the Society for Neuroscience meeting in Washington DC, November, 1996.

NMDA receptors in nucleus accumbens modulate stress-induced dopamine release in nucleus accumbens and ventral tegmental area

Converging evidence suggests that dopamine (DA) transmission in nucleus accumbens (NAcc) is modulated locally by an excitatory amino acid (EAA)-containing input possibly originating in medial prefrontal cortex (PFC). In the present study, we examined the effects of intra-NAcc administration of EAA receptor antagonists on stress-induced increases of NAcc DA levels and of dendritically released DA in the ventral tegmental area (VTA). Local injection of the NMDA receptor antagonist-AP-5 (0.05, 0.5, and 5.0 nmoles)-dose-dependently potentiated increases in NAcc DA levels elicited by 15 min of restraint stress. In contrast, local application of equivalent doses of the kainate/AMPA receptor antagonist-DNQX-failed to alter the NAcc DA stress response reliably. In a separate experiment, we found that intra-NAcc injection of AP-5 also potentiated stress-induced increases in VTA DA levels. These results indicate that EAAs acting at NMDA receptors in NAcc can modulate stress-induced DA release in this region. Our data indicate, however, that this action exerts an inhibitory influence on the NAcc DA stress response, suggesting that the relevant population of NMDA receptors are not located on NAcc DA terminals. The fact that intra-NAcc AP-5 injections also potentiated the DA stress response in VTA suggests instead an action mediated by NMDA receptors located on NAcc neurons that feedback, directly or indirectly, to cell bodies of the mesocorticolimbic DA system.

Influence of perinatal factors on the nucleus accumbens dopamine response to repeated stress during adulthood: an electrochemical study in the rat

Evidence from animal studies suggests that a period of anoxia to the fetus, a consequence common to many birth complications, results in long-term alterations in ventral mesencephalic dopamine function. Long-term functional changes in these dopamine neurons, in particular those that innervate the nucleus accumbens, also occur when animals are repeatedly stressed. In the present study, we examined the possibility that a period of anoxia during a Cesarean section birth can later alter the development of stress-induced sensitization of dopamine transmission in the nucleus accumbens. Dams were decapitated on the last day of gestation and the entire uterus was removed by Cesarean section. Pups were then delivered either immediately (Cesarean section group) or were immersed in a 37 degrees C saline bath for 3.5 or 13.5 min (Cesarean section+anoxia groups) before delivery of the pups. A fourth group of pups that were born vaginally served as controls (Vaginal group). Three to four months postnatally, animals from each group were implanted with monoamine-selective carbon-fiber electrodes into the nucleus accumbens. Voltammetry was used to monitor the dopamine response to each of five consecutive, once daily, 15-min exposures to tail-pinch stress. The results show that the first exposure to stress elicited dopamine signal increases of comparable amplitudes and durations in all animals. However, when compared to the initial stress response, the fourth and fifth exposures to tail-pinch elicited significantly longer-lasting dopamine responses in animals born by Cesarean section, either with or without added anoxia. In contrast, there was no significant day-to-day enhancement of the stress response in control, vaginally born animals. The findings reported here provide experimental support for the idea that birth complications may contribute to the pathophysiology of psychiatric disorders, in particular those that involve central dopamine dysfunction, such as schizophrenia. Specifically, our results suggest that subtle alterations in birth procedure may be sufficient to increase the sensitivity of mesolimbic dopamine neurons to the effects of repeated stress in the adult animal.

Medial prefrontal cortical D1 receptor modulation of the meso-accumbens dopamine response to stress: an electrochemical study in freely-behaving rats

Voltammetry was used to study the role of prefrontal cortex (PFC) dopamine (DA) in modulating the nucleus accumbens (NAcc) DA response to stress. Signal increases elicited in NAcc by 15 min of restraint were monitored in freely-behaving rats following intra-PFC microinjections of D1 and D2 receptor-selective drugs. The exact site of injection was first determined by assessing the electrochemical response to stress at two dorsal-ventral levels of PFC. Consistent with previous reports, a pronounced stress response was observed ventrally at sites within the infralimbic PFC but not dorsally within the superficial layers of PFC. When microinjected into the infralimbic PFC, the D1 receptor antagonist SCH 23390 significantly enhanced the NAcc stress response. While the D1 receptor agonist SKF 38393 tended to decrease the NAcc stress response, it failed to do so reliably. Neither sulpiride (D2 receptor antagonist) nor quinpirole (D2 receptor agonist) had a significant effect. Finally, systemic administration of the selective DA uptake inhibitor GBR 12909 dose-dependently potentiated stress-induced signal increases in NAcc and in PFC, indicating that the electrochemical responses to stress in both regions were due primarily to increases in extracellular DA levels. Together, these data add to other evidence indicating that the PFC exerts an inhibitory influence on subcortical DA transmission. Specifically, the present results suggest that the NAcc DA response to stress is dampened by the concurrent activation of meso-PFC DA neurons and that this action is mediated, at least in part, by D1 receptors in PFC.

Electrochemical evidence of increased dopamine transmission in prefrontal cortex and nucleus accumbens elicited by ventral tegmental mu-opioid receptor activation in freely behaving rats

Chronoamperometry was used in combination with monoamine-selective electrodes to monitor, in nucleus accumbens (NAcc) and prefrontal cortex (PFC) of freely behaving rats, changes in dopamine (DA)-like electrochemical signals elicited by unilateral ventral tegmental microinjections of the selective mu-opioid receptor agonist D-Ala, N-Me-Phe-Gly-Ol-Enkephalin (DAMGO; 0.01, 0.1, and 1.0 nmol). The results show that DAMGO dose-dependently increased electrochemical signals both in Nacc and PFC within a few minutes of injection. While DAMGO elicited signal increases of comparable amplitudes in both regions, the increases recorded in PFC were significantly longer lasting than those in NAcc; at the highest dose tested (1.0 nmol), DAMGO produced signal increases that lasted (mean +/- sem) 129 +/- 7.3 min in PFC and 96 +/- 12.5 min in NAcc. Pretreatment with the opioid receptor antagonist, naloxone (2 mg/kg, sc), significantly attenuated the peak amplitude and reduced the duration of DAMGO-induced (0.1 nmol) signal increases both in PFC and NAcc. In contrast, pretreatment with apomorphine (50 micrograms/kg, sc), a D1/D2 DA receptor agonist, significantly reduced the duration and the rate of rise of the signal increases in both regions but had little effect on the peak increases in signal. Unilateral ventral tegmental DAMGO administration (0.01, 0.1, and 1.0 nmol) also caused dose-dependent increases in contraversive circling the duration of which approximated that of the signal increases recorded in NAcc. However, differences in the time courses of DAMGO-induced contraversive circling and signal increases in NAcc suggest that the behavioral stimulant effect of ventral tegmental mu-opioid receptor activation may not be mediated exclusively by meso-NAcc DA neurons. The results of this study suggest that enkephalins modulate the activity of meso-PFC DA neurons and that behaviorally relevant activation of mu-opioid receptors in the ventral tegmental area increases DA transmission in PFC to a same, if not to a greater extent as in NAcc. These findings are discussed in relation to evidence indicating that the response of meso-NAcc DA neurons to a variety of stimuli, including drugs of abuse, is indirectly regulated by a DA-sensitive neurons in PFC.

Simultaneous, selective detection of catecholaminergic and indolaminergic signals using cyclic voltammetry with treated micro-sensor

Selective and simultaneous voltammetric analysis of catechols and indoles in vivo and in vitro has until now been feasible only by means of 'slow' scanning methods (scan speed in tens of seconds) such as differential pulse (DPV) and differential normal pulse voltammetry in conjunction with electrically and/or chemically treated carbon-fiber micro-electrodes (mCFE). Faster electrochemical techniques, such as chronoamperometry and cyclic voltammetry (CV), allow more rapid (seconds or fractions of a second) and frequent measurements of these chemicals. However, these methods show poor sensitivity and selectivity in the presence of different electroactive compounds with similar oxidation potentials. In order to analyze whether the lack of sensitivity and selectivity of the fast voltammetric methods results from the rapidity of the measurement or from the use of untreated sensors, the methods of CV (scan speed: 1000 mV/s) and DPV (scan speed: 10 mV/s) have been applied with either untreated or electrically treated mCFE to analyze the in vitro oxidation potential and current values of DA and 5-HT. When associated with untreated mCFE, neither method was able to separate and selectively detect the two compounds dissolved together in an inert vehicle; the voltammogram recorded resulted in a single broad oxidation signal. In contrast, when these techniques were performed with electrically treated mCFE, oxidation signals for DA (peak A) and 5-HT (peak B) were monitored simultaneously at approximately + 65 mV and + 240 mV, with DPV respectively, and at + 120 mV and + 300 mV with CV, respectively. Additionally, CV with treated mCFE on anesthetized rats, simultaneously monitored two striatal signals at approximately + 100 mV and + 300 mV. The oxidation values (Em) and current levels (nA) of these peaks remained stable during control recordings. The current levels were selectively increased by peripheral injection of fluphenazine (DA antagonist) or of 5-hydroxytryptophan (precursor of serotonin). The chemical nature of these two peaks may therefore be considered catecholaminergic and indolaminergic, respectively. Hence, this report provides the first evidence for the feasibility of concomitant in vitro analysis of DA and 5-HT using a rapid scanning method such as CV. In addition, the values of current level (nA) obtained with CV-mCFE for DA and 5-HT are comparable to those monitored with DPV-mCFE, supporting the view that treatment of the sensor is a key point for increasing the selectivity and the sensitivity of these voltammetric techniques. The feasibility of using CV with electrically treated mCFE for fast in vivo analysis of catechol and indole activities is also demonstrated.

Short-range differential pulse voltammetry for fast, selective analysis of basal levels of cerebral compounds in vivo

Differential pulse voltammetry (DPV) with pretreated biosensors (carbon fibre microelectrodes (mCFE), 10-30 microns diameter) allows selective in vivo measurement of basal endogenous levels of dopamine (DA), serotonin (5-HT), their metabolites (dihydroxyphenylacetic acid, DOPAC; 5-hydroxyindoleacetic acid, 5-HIAA), and neuropeptides. We have now modified DPV in order to reduce the time of analysis from tens of seconds to 1-2 s without losing selectivity. We call this newly reported method short-range differential pulse voltammetry (SRDPV). Simply, while in DPV the complete oxidation peak is recorded, SRDPV measures only the top of each oxidation peak. For example, to monitor peak 2 which corresponds to the in vivo oxidation of extracellular DOPAC and occurs at approximately +85 +/- 10 mV, the initial (Ei) and final (Ef) potentials applied with DPV were -100 mV and +200 mV, respectively, while they were +75 mV (Ei) and +95 mV (Ef) with SRDPV. At the typical scan range of 10 mV.s-1, the effective time of measurement was 30 s for DPV and 2 s for SRDPV. A similar procedure was performed to analyze peak 3 (5-HIAA, occurring at +230 +/- 11 mV) with Ei + 50 mV and Ef + 350 mV for DPV, or +220 mV and +240 mV for SRDPV. DPV and SRDPV were compared in vitro by quantitating DOPAC and 5-HIAA in solutions of increasing concentrations (chosen on the basis of the suggested in vivo content of these two compounds). Data indicated that similar sensitivity and selectivity were obtained with both methods at all concentrations, supporting the applicability of SRDPV for in vitro studies. In vivo experiments were performed in anesthetized adult male rats prepared for voltammetry by inserting the electrically pretreated biosensor (mCFE) into the striatum. DPV measurements were performed automatically every 3-5 min and were alternated every 10-20 min with a sequence of 5-10 SRDPV scans performed every 10-30 s. Subsequent pharmacological or electrical manipulations of the two biogenic amine systems studied were monitored by alternate use of DPV and SRDPV. The data presented support the capability of SRDPV with pretreated biosensors to measure in vivo electroactive compounds with selectivity and sensitivity comparable to that of DPV, but with improved time resolution.