Oral administration of a specific kynurenic acid synthesis (KAT II) inhibitor attenuates evoked glutamate release in rat prefrontal cortex.

Cognitive deficits represent core symptoms in schizophrenia (SZ) and predict patient outcome; however, they remain poorly treated by current antipsychotic drugs. Elevated levels of the endogenous alpha7 nicotinic receptor negative allosteric modulator and NMDA receptor antagonist, kynurenic acid (KYNA), are commonly seen in post-mortem tissue and cerebrospinal fluid of patients with SZ. When acutely or chronically elevated in rodents, KYNA produces cognitive deficits similar to those seen in the disease, making down-regulation of KYNA, via inhibition of kynurenine aminotransferase II (KAT II), a potential treatment strategy. We determined, in adult Wistar rats, if the orally available KAT II inhibitor BFF816 a) prevents KYNA elevations in prefrontal cortex (PFC) after a systemic kynurenine injection and b) reverses the kynurenine-induced attenuation of evoked prefrontal glutamate release caused by stimulation of the nucleus accumbens shell (NAcSh). Systemic injection of kynurenine (25 or 100 mg/kg, i.p.) increased KYNA levels in PFC (532% and 1104% of baseline, respectively). NMDA infusions (0.15 µg/0.5 µL) into NAcSh raised prefrontal glutamate levels more than 30-fold above baseline. The two doses of kynurenine reduced evoked glutamate release in PFC (by 43% and 94%, respectively, compared to NMDA alone). Co-administration of BFF816 (30 or 100 mg/kg, p.o.) with kynurenine (25 mg/kg, i.p.) attenuated the neosynthesis of KYNA and dose-dependently restored NMDA-stimulated glutamate release in the PFC (16% and 69%, respectively). The ability to prevent KYNA neosynthesis and to normalize evoked glutamate release in PFC justifies further development of KAT II inhibitors for the treatment of cognitive deficits in SZ.

Positive allosteric modulators of the α7 nicotinic acetylcholine receptor potentiate glutamate release in the prefrontal cortex of freely-moving rats.

Positive allosteric modulators (PAMs) of α7 nicotinic acetylcholine receptors (α7nAChRs) exhibit pro-cognitive effects in animal models of schizophrenia and are targets for the discovery of cognition-enhancing drugs. However, little is known about their in vivo mechanism of action because such studies have been performed in vitro. Here we test the hypothesis that PAMs' potentiation of glutamate release in prefrontal cortex depends upon the level of endogenous cholinergic activity. NMDA stimulation of the nucleus accumbens shell (0.05-0.30 µg in 0.5 µL) increased extracellular choline (0.87 ± 0.15 - 1.73 ± 0.31 µM) and glutamate (0.15 µg, 3.79 ± 0.87 µM) in medial prefrontal cortex, and the glutamate release was prevented by local infusions of MLA (6.75 µg, 0.19 ± 0.06 µM). The lower dose (1 mg/kg) of AVL3288 (type I) potentiated the glutamate release to a greater degree after the high dose of NMDA (0.30 µg; 84.7% increase vs AVL vehicle) versus the low dose of NMDA (0.05 µg; 24.2% increase), whereas glutamate release was inhibited when the high dose of NMDA was combined with the high dose of AVL3288 (64.2% decrease). In contrast, PNU120596 (type II) only potentiated glutamate release when the high dose (9 mg/kg) was combined with the low dose of NMDA (0.05 µg; 211% increase from PNU vehicle). Collectively, the results suggest a potential in vivo mechanism for the pro-cognitive effects of PAMs and provide the proof-of-concept for the continued focus on allosteric modulation of cortical α7nAChRs for cognition-enhancing drug development.

Transient inactivation of the ventral hippocampus in neonatal rats impairs the mesolimbic regulation of prefrontal glutamate release in adulthood.

Cognitive deficits in schizophrenia (SZ) reflect maturational disruptions within a neural system that includes the ventral hippocampus (VH), nucleus accumbens (NAc), basal forebrain, and prefrontal cortex (PFC). A better understanding of these changes may reveal drug targets for more efficacious cognition enhancers. We have utilized an animal model in which the above distributed system is altered, during a sensitive period of development, by transiently inactivating the VH and its efferent projections. We determined the ability of NAc shell activation to evoke prefrontal glutamate release in adult male Wistar rats that had received saline (Sal) or tetrodotoxin (TTX) as neonates (PD7) or as adolescents (PD32). The nucleus accumbens shell (NAcSh) was activated by NMDA infusions (0.05-0.30 µg/0.5 µL). Basal and evoked glutamate levels were measured amperometrically using a glutamate-sensitive microelectrode. There were no differences in basal glutamate levels among the groups tested (overall 1.41 ± 0.26 uM). However, the dose-related stimulation of prefrontal glutamate levels seen in control rats treated with saline on PD7 (4.31 ± 0.22 µM after 0.15 µg) was markedly attenuated in rats treated with TTX on PD7 (0.45 ± 0.12 µM after 0.15 µg). This effect was age-dependent as infusions of TTX on PD32 did not alter the NMDA-induced increases in glutamate release (4.10 ± 0.37 µM after 0.15 µg). Collectively, these findings reveal that transient inactivation of VH transmission, during a sensitive period of development, leads to a functional mesolimbic-cortical disconnection that produces neurochemical and ultimately cognitive impairments resembling those seen in SZ.

Localized infusions of the partial alpha 7 nicotinic receptor agonist SSR180711 evoke rapid and transient increases in prefrontal glutamate release.

The ability of local infusions of the alpha 7 nicotinic acetycholine receptor (α7 nAChR) partial agonist SSR180711 to evoke glutamate release in prefrontal cortex was determined in awake rats using a microelectrode array. Infusions of SSR180711 produced dose-dependent increases in glutamate levels. The lower dose (1.0µg in 0.4µL) evoked a rapid rise (∼1.0s) in glutamate (1.41±0.30µM above baseline). The higher dose (5.0µg) produced a similarly rapid, yet larger increase (3.51±0.36µM above baseline). After each dose, the glutamate signal was cleared to basal levels within 7-18s. SSR180711-evoked glutamate was mediated by the α7 nAChR as co-infusion of the selective α7 nAChR antagonist α-bungarotoxin (10.0µM)+SSR1808711 (5.0µg) reduced the effect of 5.0µg alone by 87% (2.62 vs. 0.35µM). Finally, the clearance of the SSR180711 (5.0µg)-evoked glutamate was bidirectionally affected by drugs that inhibited (threo-beta-benzyl-oxy-aspartate (TßOA), 100.0µM) or facilitated (ceftriaxalone, 200mg/kg, i.p.) excitatory amino acid transporters. TßOA slowed both the clearance (s) and rate of clearance (µM/s) by 10-fold, particularly at the mid-late stages of the return to baseline. Ceftriaxone reduced the magnitude of the SSR180711-evoked increase by 65%. These results demonstrate that pharmacological stimulation of α7 nAChRs within the prefrontal cortex is sufficient to evoke rapid yet transient increases in glutamate levels. Such increases may underlie the cognition-enhancing effects of the drug in animals; further justifying studies on the use of α7 nAChR-positive modulators in treating cognition-impairing disorders in humans.

Early developmental elevations of brain kynurenic acid impair cognitive flexibility in adults: reversal with galantamine.

Levels of kynurenic acid (KYNA), an endogenous α7 nicotinic acetylcholine receptor (α7nAChR) antagonist, are elevated in the brain of patients with schizophrenia (SZ) and might contribute to the pathophysiology and cognitive deficits seen in the disorder. As developmental vulnerabilities contribute to the etiology of SZ, we determined, in rats, the effects of perinatal increases in KYNA on brain chemistry and cognitive flexibility. KYNA's bioprecursor l-kynurenine (100mg/day) was fed to dams from gestational day 15 to postnatal day 21 (PD21). Offspring were then given regular chow until adulthood. Control rats received unadulterated mash. Brain tissue levels of KYNA were measured at PD2 and PD21, and extracellular levels of KYNA and glutamate were determined by microdialysis in the prefrontal cortex in adulthood (PD56-80). In other adult rats, the effects of perinatal l-kynurenine administration on cognitive flexibility were assessed using an attentional set-shifting task. l-Kynurenine treatment raised forebrain KYNA levels ∼3-fold at PD2 and ∼2.5-fold at PD21. At PD56-80, extracellular prefrontal KYNA levels were moderately but significantly elevated (+12%), whereas extracellular glutamate levels were not different from controls. Set-shifting was selectively impaired by perinatal exposure to l-kynurenine, as treated rats acquired the discrimination and intra-dimensional shift at the same rate as controls, yet exhibited marked deficits in the initial reversal and extra-dimensional shift. Acute administration of the α7nAChR-positive modulator galantamine (3.0mg/kg, i.p.) restored performance to control levels. These results validate early developmental exposure to l-kynurenine as a novel, naturalistic animal model for studying cognitive deficits in SZ.

High throughput analysis of grape genetic diversity as a tool for germplasm collection management.

Using 20 SSR markers well scattered across the 19 grape chromosomes, we analyzed 4,370 accessions of the INRA grape repository at Vassal, mostly cultivars of Vitis vinifera subsp. sativa (3,727), but also accessions of V. vinifera subsp. sylvestris (80), interspecific hybrids (364), and rootstocks (199). The analysis revealed 2,836 SSR single profiles: 2,323 sativa cultivars, 72 wild individuals (sylvestris), 306 interspecific hybrids, and 135 rootstocks, corresponding to 2,739 different cultivars in all. A total of 524 alleles were detected, with a mean of 26.20 alleles per locus. For the 2,323 cultivars of V. vinifera, 338 alleles were detected with a mean of 16.9 alleles per locus. The mean genetic diversity (GDI) was 0.797 and the level of heterozygosity was 0.76, with broad variation from 0.20 to 1. Interspecific hybrids and rootstocks were more heterozygous and more diverse (GDI = 0.839 and 0.865, respectively) than V. vinifera cultivars (GDI = 0.769), Vitis vinifera subsp. sylvestris being the least divergent with GDI = 0.708. Principal coordinates analysis distinguished the four groups. Slight clonal polymorphism was detected. The limit between clonal variation and cultivar polymorphism was set at four allelic differences out of 40. SSR markers were useful as a complementary tool to traditional ampelography for cultivar identification. Finally, a set of nine SSR markers was defined that was sufficient to distinguish 99.8% of the analyzed accessions. This set is suitable for routine characterization and will be valuable for germplasm management.

Cortical kynurenic acid bi-directionally modulates prefrontal glutamate levels as assessed by microdialysis and rapid electrochemistry.

Using two in vivo methods, microdialysis and rapid in situ electrochemistry, this study examined the modulation of extracellular glutamate levels by endogenously produced kynurenic acid (KYNA) in the prefrontal cortex (PFC) of awake rats. Measured by microdialysis, i.p. administration of KYNA's bioprecursor L-kynurenine dose-dependently elevated extracellular KYNA and reduced extracellular glutamate (nadir after 50 mg/kg kynurenine: 60% decrease from baseline values). This dose-dependent decrease in glutamate levels was also seen using a glutamate-sensitive microelectrode array (MEA) (31% decrease following 50 mg/kg kynurenine). The kynurenine-induced reduction in glutamate was blocked (microdialysis) or attenuated (MEA) by co-administration of galantamine (3 mg/kg i.p.), a drug that competes with KYNA at an allosteric potentiating site of the alpha 7 nicotinic acetylcholine receptor. In separate experiments, extracellular glutamate levels were measured by MEA following the local perfusion (45 min) of the PFC with kynurenine (2.5 microM) or the selective KYNA biosynthesis inhibitor S-ethylsulfonylbenzoylalanine (S-ESBA; 5 mM). In agreement with previous microdialysis studies, local kynurenine application produced a reversible reduction in glutamate (nadir: -29%), whereas perfusion with S-ESBA increased glutamate levels reversibly (maximum: +38%). Collectively, these results demonstrate that fluctuations in the biosynthesis of KYNA in the PFC bi-directionally modulate extracellular glutamate levels, and that qualitatively very similar data are obtained by microdialysis and MEA. Since KYNA levels are elevated in the PFC of individuals with schizophrenia, and since prefrontal glutamatergic and nicotinic transmission mediate cognitive flexibility, normalization of KYNA levels in the PFC may constitute an effective treatment strategy for alleviating cognitive deficits in schizophrenia.

Astrocyte-derived kynurenic acid modulates basal and evoked cortical acetylcholine release.

We tested the hypothesis that fluctuations in the levels of kynurenic acid (KYNA), an endogenous antagonist of the alpha7 nicotinic acetylcholine (ACh) receptor, modulate extracellular ACh levels in the medial prefrontal cortex in rats. Decreases in cortical KYNA levels were achieved by local perfusion of S-ESBA, a selective inhibitor of the astrocytic enzyme kynurenine aminotransferase II (KAT II), which catalyses the formation of KYNA from its precursor L-kynurenine. At 5 mm, S-ESBA caused a 30% reduction in extracellular KYNA levels, which was accompanied by a two-threefold increase in basal cortical ACh levels. Co-perfusion of KYNA in the endogenous range (100 nm), which by itself tended to reduce basal ACh levels, blocked the ability of S-ESBA to raise extracellular ACh levels. KYNA perfusion (100 nm) also prevented the evoked ACh release caused by d-amphetamine (2.0 mg/kg). This effect was duplicated by the systemic administration of kynurenine (50 mg/kg), which resulted in a significant increase in cortical KYNA formation. Jointly, these data indicate that astrocytes, by producing and releasing KYNA, have the ability to modulate cortical cholinergic neurotransmission under both basal and stimulated conditions. As cortical KYNA levels are elevated in individuals with schizophrenia, and in light of the established role of cortical ACh in executive functions, our findings suggest that drugs capable of attenuating the production of KYNA may be of benefit in the treatment of cognitive deficits in schizophrenia.

Prefrontal cortical modulation of acetylcholine release in posterior parietal cortex.

Attentional processing is a crucial early stage in cognition and is subject to "top-down" regulation by prefrontal cortex (PFC). Top-down regulation involves modification of input processing in cortical and subcortical areas, including the posterior parietal cortex (PPC). Cortical cholinergic inputs, originating from the basal forebrain cholinergic system, have been demonstrated to mediate important aspects of attentional processing. The present study investigated the ability of cholinergic and glutamatergic transmission within PFC to regulate acetylcholine (ACh) release in PPC. The first set of experiments demonstrated increases in ACh efflux in PPC following AMPA administration into the PFC. These increases were antagonized by co-administration of the AMPA receptor antagonist DNQX into the PFC. The second set of experiments demonstrated that administration of carbachol, but not nicotine, into the PFC also increased ACh efflux in PPC. The effects of carbachol were attenuated by co-administration (into PFC) of a muscarinic antagonist (atropine) and partially attenuated by the nicotine antagonist mecamylamine and DNQX. Perfusion of carbachol, nicotine, or AMPA into the PPC did not affect PFC ACh efflux, suggesting that these cortical interactions are not bi-directional. These studies demonstrate the capacity of the PFC to regulate ACh release in the PPC via glutamatergic and cholinergic prefrontal mechanisms. Prefrontal regulation of ACh release elsewhere in the cortex is hypothesized to contribute to the cognitive optimization of input processing.

Neuronal activity in the nucleus accumbens is necessary for performance-related increases in cortical acetylcholine release.

In vivo microdialysis was used to determine the necessity of neuronal activity in the nucleus accumbens (NAC) for task-induced increases in cortical acetylcholine (ACh) efflux. Rats were trained in a behavioral task in which they were required to perform a defined number of licks of a citric acid solution in order to gain access to a palatable, cheese-flavored food. Upon reaching a consistent level of performance, rats were implanted with microdialysis cannula in the medial prefrontal cortex (mPFC) and either the ipsilateral shell of the NAC or in the dorsal striatum (STR; control site). Dialysis samples from the mPFC were analyzed for ACh concentrations and samples from the NAC were analyzed for dopamine (DA) concentrations. Performance in the task was associated with increases in both ACh efflux in the cortex (150-200%) and DA efflux in the NAC (50-75%). These increases were blocked by administration of tetrodotoxin (TTX; 1.0 microM) via reverse dialysis into the NAC. Administration of TTX into the dorsal STR control site was ineffective in blocking performance-associated increases in cortical ACh. The D2 antagonist sulpiride (10 or 100 microM) administered into the NAC via reverse dialysis was ineffective in blocking increases in cortical ACh efflux. The present data reveal that neuronal activity in the NAC is necessary for behaviorally induced increases in cortical ACh efflux and that this activation does not require increases in D2 receptor activity.

Differential cortical acetylcholine release in rats performing a sustained attention task versus behavioral control tasks that do not explicitly tax attention.

The present study used microdialysis techniques to compare acetylcholine release in the frontoparietal cortex of rats performing in a task requiring sustained attention with that of rats performing in two control procedures. The two control procedures were a fixed-interval 9-s schedule of reinforcement assessing primarily the effects of operant responding and comparable reward rates, and an operant procedure designed to test the effects of lever extension to prompt responding. These two control procedures involved comparable sensory-motor and motivational variables to those of the sustained attention task, but did not explicitly tax attentional processes. Performance of the sustained attention task was associated with a significant increase in cortical acetylcholine efflux, reaching a maximum of nearly 140%. Performance of the two control procedures was associated with significantly smaller (approximately 50%) increases in cortical acetylcholine release. This robust dissociation between attentional and control performance-associated increases in cortical acetylcholine release resulted, in part, from the elimination of the pre-task transfer of the animals into the operant chambers and the associated increases in acetylcholine release observed in previous studies. The present results support the hypothesis that demands on attentional performance, as opposed to the frequency of lever pressing, reward delivery and other task-related variables, selectively activate the basal forebrain corticopetal cholinergic system.

The effects of manipulations of attentional demand on cortical acetylcholine release.

In vivo microdialysis was used to measure acetylcholine (ACh) efflux in the frontoparietal cortex while rats performed in one of two operant tasks. One task was designed and validated to generate measures of sustained attention, while the other task was designed to minimize explicit demands on sustained attentional resources (low-demand task). Transferring animals from the baseline environment into the operant chambers robustly increased cortical ACh efflux regardless of subsequent task demands. Performance in the sustained attention task further increased frontoparietal ACh efflux, and these increases were not observed when animals were simply exposed to the operant chamber without task performance. Manipulations of the task parameters within a session, to either increase or decrease explicit demands on sustained attention, were not associated with fluctuations in ACh efflux. Unexpectedly, performance in the low-demand task was also associated with significant increases in ACh efflux that were similar to those observed during the sustained attention task. However, widespread depletions of cortical cholinergic inputs produced by intra-basalis infusions of 192 IgG-saporin failed to impair performance in the low-demand task, suggesting that cholinergic transmission is not necessary for performance in this task. The present results indicate that although a wider range of instrumental processes than previously hypothesized are associated with increases in cortical ACh release, the dependence of performance on the integrity of cortical cholinergic inputs may be limited to tasks with explicit attentional demands.

Psychotogenic properties of benzodiazepine receptor inverse agonists.

The neurochemical, behavioral, and cognitive effects of the benzodiazepine receptor partial inverse agonist beta-carboline FG 7142 (FG), a drug traditionally described as exhibiting 'anxiogenic' effects, are proposed to model core components of present theories of the neuronal mechanisms of schizophrenia. FG activates the mesolimbic dopaminergic system and, via increases in dopaminergic activity in the nucleus accumbens, disinhibits corticopetal cholinergic projections. The latter effect of FG is hypothesized to mediate the hyperattentional impairments that contribute to the development of psychotic cognition. Furthermore, the FG-induced abnormal overprocessing of conditioned stimuli and contexts provides an explanation of the 'anxiogenic' effects of FG. The FG-induced increases in the activity of cortical cholinergic inputs and the FG-induced cognitive impairments in rats and monkeys were demonstrated to be attenuated by the administration of typical and atypical antipsychotic drugs. Compared to the classic psychotogenic drugs amphetamine and phencyclidine, the effects of FG serve as an alternative psychotogenic manipulation in research focusing on the cortical and cognitive aspects of current theories of schizophrenia.

The cognitive neuroscience of sustained attention: where top-down meets bottom-up.

The psychological construct 'sustained attention' describes a fundamental component of attention characterized by the subject's readiness to detect rarely and unpredictably occurring signals over prolonged periods of time. Human imaging studies have demonstrated that activation of frontal and parietal cortical areas, mostly in the right hemisphere, are associated with sustained attention performance. Animal neuroscientific research has focused on cortical afferent systems, particularly on the cholinergic inputs originating in the basal forebrain, as crucial components of the neuronal network mediating sustained attentional performance. Sustained attention performance-associated activation of the basal forebrain corticopetal cholinergic system is conceptualized as a component of the 'top-down' processes initiated by activation of the 'anterior attention system' and designed to mediate knowledge-driven detection and selection of target stimuli. Activated cortical cholinergic inputs facilitate these processes, particularly under taxing attentional conditions, by enhancing cortical sensory and sensory-associational information processing, including the filtering of noise and distractors. Collectively, the findings from human and animal studies provide the basis for a relatively precise description of the neuronal circuits mediating sustained attention, and the dissociation between these circuits and those mediating the 'arousal' components of attention.

Dissociations between the effects of intra-accumbens administration of amphetamine and exposure to a novel environment on accumbens dopamine and cortical acetylcholine release.

Previous research has demonstrated an interaction between the effects of amphetamine and exposure to a novel environment on the activity of neurons in the nucleus accumbens. Given a model in which these accumbens efferents gate the excitability of basal forebrain cholinergic corticopetal neurons, the administration of intra-accumbens amphetamine was hypothesized to potentiate the increase in cortical acetylcholine produced by introduction to a novel environment. Dual probe microdialysis revealed no synergistic interactions between exposure to a novel environment and amphetamine on nucleus accumbens dopamine or cortical acetylcholine efflux. This finding indicates that exposure to a novel environment failed to recruit the telencephalic activation of the nucleus accumbens presumably necessary to reveal modulatory effects of accumbens dopaminergic transmission on cortical acetylcholine release.

Amphetamine-stimulated cortical acetylcholine release: role of the basal forebrain.

Systemic administration of amphetamine results in increases in the release of acetylcholine in the cortex. Basal forebrain mediation of this effect was examined in three experiments using microdialysis in freely-moving rats. Experiment 1 examined whether dopamine receptor activity within the basal forebrain was necessary for amphetamine-induced increase in cortical acetylcholine by examining whether intra-basalis perfusion of dopamine antagonists attenuates this increase. Systemic administration of 2.0 mg/kg amphetamine increased dopamine efflux within the basal forebrain nearly 700% above basal levels. However, the increase in cortical acetylcholine efflux following amphetamine administration was unaffected by intra-basalis perfusions of high concentrations of D1- (100 microM SCH 23390) or D2-like (100 microM sulpiride) dopamine receptor antagonists. Experiments 2 and 3 determined whether glutamatergic or GABAergic local modulation of the excitability of the basal forebrain cholinergic neurons influences the ability of systemic amphetamine to increase cortical acetylcholine efflux. In Experiment 2, perfusion of kynurenate (1.0 mM), a non-selective glutamate receptor antagonist, into the basal forebrain attenuated the increase in cortical acetylcholine produced by amphetamine. Experiment 3 revealed that positive modulation of GABAergic transmission by bilateral intra-basalis infusion of the benzodiazepine receptor agonist chlordiazepoxide (40 microg/hemisphere) also attenuated the amphetamine-stimulated increase in cortical acetylcholine efflux. These data suggest that amphetamine increases cortical acetylcholine release via a complex neuronal network rather than simply increasing basal forebrain D1 or D2 receptor activity.

Basal forebrain glutamatergic modulation of cortical acetylcholine release.

The mediation of cortical ACh release by basal forebrain glutamate receptors was studied in awake rats fitted with microdialysis probes in medial prefrontal cortex and ipsilateral basal forebrain. Repeated presentation of a stimulus consisting of exposure to darkness with the opportunity to consume a sweetened cereal resulted in a transient increase in cortical ACh efflux. This stimulated release was dependent on basal forebrain glutamate receptor activity as intrabasalis perfusion with the ionotropic glutamate receptor antagonist kynurenate (1.0 mM) markedly attenuated darkness/cereal-induced ACh release. Activation of AMPA/kainate receptors by intrabasalis perfusion of kainate (100 microM) was sufficient to increase cortical ACh efflux even under basal (nonstimulated) conditions. This effect of kainate was blocked by coperfusion with the antagonist DNQX (0.1-5.0 mM). Stimulation of NMDA receptors with intrabasalis perfusion of NMDA (50 or 200 microM) did not increase basal cortical ACh efflux. However, perfusion of NMDA in rats following exposure to the darkness/cereal stimulus resulted in a potentiation of both the magnitude and duration of stimulated cortical ACh efflux. Moreover, intrabasalis perfusion of the higher dose of NMDA resulted in a rapid increase in cortical ACh efflux even before presentation of the darkness/cereal stimulus, suggesting an anticipatory change in the excitability of basal forebrain cholinergic neurons. These data demonstrate that basal forebrain glutamate receptors contribute to the stimulation of cortical ACh efflux in response to behavioral stimuli. The specific roles of basal forebrain glutamate receptor subtypes in mediating cortical ACh release differ and depend on the level of activity of basal forebrain cholinergic neurons.

Effects of intra-accumbens infusions of amphetamine or cis-flupenthixol on sustained attention performance in rats.

Attentional abilities, as reflected by performance in sustained, selective, or divided attention tasks, have been extensively demonstrated to depend on the integrity of cortical cholinergic inputs and their basal forebrain afferent circuits. As nucleus accumbens (NAC) efferent projections have been hypothesized to modulate the excitability of basal forebrain corticopetal projections, the effects of drug-induced modulation of NAC dopaminergic transmission on sustained attention performance in rats were assessed. Unexpectedly, bilateral infusions of amphetamine (AMPH; 0, 3.0, 10.0, 30.0 microg/microl/hemisphere) into the shell region of the NAC did not affect sustained attention performance. In contrast, microinjections of the non-selective dopamine (DA) receptor antagonist cis-flupenthixol (FLU; 0, 3.0, 10.0, 25.0 microg/microl/hemisphere) into the NAC of a separate group of animals impaired the detection of both signals and non-signals, as reflected by decreases in the relative numbers of hits and correct rejections. As the effects of neither AMPH nor FLU followed the predictions made on the basis of current hypotheses about the modulation of basal forebrain neuronal activity by NAC DA, the present results challenge such hypotheses and indicate that the conditions which implicate NAC DA in attentional performance remain unsettled.

Repeated pretreatment with amphetamine sensitizes increases in cortical acetylcholine release.

RATIONALE: Previous studies on the attentional effects of repeated psychostimulant administration in rats suggested the possibility that these effects are mediated via increases in the efficacy of psychostimulants to stimulate cortical acetylcholine (ACh) release. Furthermore, neurochemical data have raised the possibility that increases in nucleus accumbens (NAC) dopamine (DA) release trans-synaptically increase the excitability of basal forebrain corticopetal cholinergic projections, thereby supporting speculations about relationships between the effects of repeated psychostimulant administration on NAC DA and cortical ACh release. OBJECTIVES: To determine whether repeated exposure to amphetamine would potentiate the stimulating effects of the drug on cortical ACh and NAC DA efflux. METHODS: Rats were implanted with microdialysis guide cannula in the medial prefrontal cortex and the shell region of the ipsilateral NAC. Amphetamine (2.0 mg/kg i.p.) or saline (0.9%) was administered every other day for 10 days, for a total of five injections. ACh and DA efflux and locomotor activity were measured on the day of the first and last injections of this pretreatment regimen. All animals were retested following a challenge dose of amphetamine (2.0 mg/kg i.p.) given 10 and 19 days after the last pretreatment injection. RESULTS: The initial injections of amphetamine stimulated ACh and DA efflux and locomotor behavior in both groups. The pretreatment with amphetamine potentiated the ability of the drug to stimulate cortical ACh efflux on day 19 of the withdrawal period. The pretreatment with amphetamine also increased the effects of the challenge dose on motoric activity on day 10. Pretreatment with amphetamine did not result in a significant augmentation of the amphetamine-induced increase in DA efflux in the NAC. CONCLUSIONS: Pretreatment with amphetamine sensitizes the ability of amphetamine to stimulate cortical ACh efflux. These results support the hypothesis that sensitized release of cortical ACh mediated the previously observed hyperattentional impairments in amphetamine pretreated rats. Sensitized cortical ACh release following repeated exposure to psychostimulants may mediate the overprocessing of addictive drug-related stimuli, thus contributing to repeated compulsive addictive drug use.

Increases in cortical acetylcholine release during sustained attention performance in rats.

Acetylcholine (ACh) efflux in the frontoparietal cortex was studied with in vivo microdialysis while rats performed in an operant task designed to assess sustained attention. Transferring animals from the baseline environment into the operant chambers elicited a robust increase in cortical ACh efflux that persisted throughout the 18-min pre-task period. Subsequent performance in the 36-min sustained attention task was associated with further significant increases in frontoparietal ACh efflux, while the termination of the task resulted in a delayed decline in ACh levels. Upon the 12-min presentation of a visual distracter (flashing houselight, 0.5 Hz) during task performance, animals initially developed a significant response bias to the left lever in the first 6-min distracter block, reflecting a reduction of attentional effort. Under continued conditions of increased attentional demand, performance recovered during the second 6-min distracter block. This return to attentional processing was accompanied by an increase in cortical ACh efflux, suggesting that the augmentation of attentional demand produced by the distracter elicited further increases in ACh release. The enhancement of cortical ACh efflux observed prior to task performance implies the presence of complex relationships between cortical ACh release and anticipatory and/or contextual factors related to operant performance and attentional processing. This finding, along with the further increases in cortical ACh efflux associated with task performance, extends hypotheses regarding the crucial role of cortical cholinergic transmission for attentional functions. Furthermore, the effects of the distracter stimulus provide evidence for a direct relationship between attentional effort and cortical ACh release.