Acetylcholine release from the frontal cortex during exploratory activity

Multicore fiber optic imaging reveals that astrocyte calcium activity in the mouse cerebral cortex is modulated by internal motivational state

Astrocytes are a direct target of neuromodulators and can influence neuronal activity on broad spatial and temporal scales in response to a rise in cytosolic calcium. However, our knowledge about how astrocytes are recruited during different animal behaviors remains limited. To measure astrocyte activity calcium in vivo during normative behaviors, we utilize a high-resolution, long working distance multicore fiber optic imaging system that allows visualization of individual astrocyte calcium transients in the cerebral cortex of freely moving mice. We define the spatiotemporal dynamics of astrocyte calcium changes during diverse behaviors, ranging from sleep-wake cycles to the exploration of novel objects, showing that their activity is more variable and less synchronous than apparent in head-immobilized imaging conditions. In accordance with their molecular diversity, individual astrocytes often exhibit distinct thresholds and activity patterns during explorative behaviors, allowing temporal encoding across the astrocyte network. Astrocyte calcium events were induced by noradrenergic and cholinergic systems and modulated by internal state. The distinct activity patterns exhibited by astrocytes provides a means to vary their neuromodulatory influence in different behavioral contexts and internal states.

Multicore fiber optic imaging reveals that astrocyte calcium activity in the cerebral cortex is modulated by internal motivational state

Astrocytes are a direct target of neuromodulators and can influence neuronal activity on broad spatial and temporal scales through their close proximity to synapses. However, our knowledge about how astrocytes are functionally recruited during different animal behaviors and their diverse effects on the CNS remains limited. To enable measurement of astrocyte activity patterns in vivo during normative behaviors, we developed a high-resolution, long working distance, multi-core fiber optic imaging platform that allows visualization of cortical astrocyte calcium transients through a cranial window in freely moving mice. Using this platform, we defined the spatiotemporal dynamics of astrocytes during diverse behaviors, ranging from circadian fluctuations to novelty exploration, showing that astrocyte activity patterns are more variable and less synchronous than apparent in head-immobilized imaging conditions. Although the activity of astrocytes in visual cortex was highly synchronized during quiescence to arousal transitions, individual astrocytes often exhibited distinct thresholds and activity patterns during explorative behaviors, in accordance with their molecular diversity, allowing temporal sequencing across the astrocyte network. Imaging astrocyte activity during self-initiated behaviors revealed that noradrenergic and cholinergic systems act synergistically to recruit astrocytes during state transitions associated with arousal and attention, which was profoundly modulated by internal state. The distinct activity patterns exhibited by astrocytes in the cerebral cortex may provide a means to vary their neuromodulatory influence in response to different behaviors and internal states.

Can the frequency of defensive posture adoption influence the expression of active behavior of adult rats?

During the youth, rats spend a long time in play fighting (PF), and asymmetries are observed in terms of the frequency of adoption of the supine posture. This study investigated the effects of asymmetry in adopting supine posture during PF on the time spent in locomotor activity-related contextual behaviors during youth and adulthood. Male Wistar rats were divided into three groups according to the number of times of their adoption of a defensive supine (S) posture: low (LS, n = 10), intermediary (IS, n = 10) and high (HS, n = 10). Rats aged 21-62 days were placed in voluntary physical active cages (VPAC) with running wheels. The time spent in social play (SP), play fighting (PF), voluntary physical activity (VPA), spontaneous physical activity (SPA) and resting (Rest) were recorded daily. During adulthood (63-92 days of age), rats were placed individually in the VPAC, with daily quantification of VPA. The average time spent in PF was not different among the groups, but was age-dependent. LS and IS rats spent more time in VPA than the HS rats, between days 49-62 (intragroup analyses) and 72-78 (individual analyses) days of age. However, LS and IS animals spent less time in SPA than HS rats from 42-48 days of age. In conclusion, the expressions of VPA and SPA are influenced by the asymmetries in adoption of the supine posture of members within a social group, remaining even after the separation of the social groups.

Cholinergic Modulation Promotes Attentional Modulation in Primary Visual Cortex- A Modeling Study

Attention greatly influences sensory neural processing by enhancing firing rates of neurons that represent the attended stimuli and by modulating their tuning properties. The cholinergic system is believed to partly mediate the attention contingent improvement of cortical processing by influencing neuronal excitability, synaptic transmission and neural network characteristics. Here, we used a biophysically based model to investigate the mechanisms by which cholinergic system influences sensory information processing in the primary visual cortex (V1) layer 4C. The physiological properties and architectures of our model were inspired by experimental data and include feed-forward input from dorsal lateral geniculate nucleus that sets up orientation preference in V1 neural responses. When including a cholinergic drive, we found significant sharpening in orientation selectivity, desynchronization of LFP gamma power and spike-field coherence, decreased response variability and correlation reduction mostly by influencing intracortical interactions and by increasing inhibitory drive. Our results indicated that these effects emerged due to changes specific to the behavior of the inhibitory neurons. The behavior of our model closely resembles the effects of attention on neural activities in monkey V1. Our model suggests precise mechanisms through which cholinergic modulation may mediate the effects of attention in the visual cortex.

Ecotoxicity Assessment of Fe3O4 Magnetic Nanoparticle Exposure in Adult Zebrafish at an Environmental Pertinent Concentration by Behavioral and Biochemical Testing

Magnetic Nanoparticles (MNPs) are widely being investigated as novel promising multifunctional agents, specifically in the fields of development for theranostics, electronics, waste water treatment, cosmetics, and energy storage devices. Unique, superior, and indispensable properties of magnetization, heat transfer, and melting temperature make MNPs emerge in the field of therapeutics in future healthcare industries. However, MNPs ecotoxicity as well as behavioral toxicity is still unexplored. Ecotoxicity analysis may assist investigate MNPs uptake mechanism and its influence on bioavailability under a given set of environmental factors, which can be followed to investigate the biomagnification of MNPs in the environment and health risk possessed by them in an ecological food chain. In this study, we attempted to determine the behavioral changes in zebrafishes at low (1 ppm) or high (10 ppm) concentration levels of Fe₃O₄ MNPs. The synthesized Fe₃O₄ MNPs sized at 15 nm were characterized by the transmission electron microscope (TEM), the superconducting quantum interference device (SQUID) magnetometer, and the multiple behavior tests for novel tank, mirror biting, conspecific social interaction, shoaling, circadian rhythm, and short-term memory of zebrafish under MNPs chronic exposure were demonstrated. Low concentration MNP exposure did not trigger alteration for majority behavioral and biochemical tests in adult zebrafish. However, tight shoal groups were observed at a high concentration of MNPs exposure along with a modest reduction in fish exploratory behavior and a significant reduction in conspecific social interaction behavior. By using enzyme-linked immunosorbent assays (ELISA), we found a high dose of MNPs exposure significantly elevated cortisol, acetylcholine, and catalase levels while reducing serotonin, acetylcholine esterase, and dopamine levels in the brain. Our data demonstrates chronic MNPs exposure at an environmentally-relevant dose is relatively safe by supporting evidence from an array of behavioral and biochemical tests. This combinational approach using behavioral and biochemical tests would be helpful for understanding the MNPs association with anticipated colloids and particles effecting bioavailability and uptake into cells and organisms.

Vascular Cognitive Impairment Linked to Brain Endothelium Inflammation in Early Stages of Heart Failure in Mice

BACKGROUND

Although advanced heart failure (HF) is a clinically documented risk factor for vascular cognitive impairment, the occurrence and pathomechanisms of vascular cognitive impairment in early stages of HF are equivocal. Here, we characterize vascular cognitive impairment in the early stages of HF development and assess whether cerebral hypoperfusion or prothrombotic conditions are involved.

METHODS AND RESULTS

Tgαq*44 mice with slowly developing isolated HF triggered by cardiomyocyte-specific overexpression of G-αq*44 protein were studied before the end-stage HF, at the ages of 3, 6, and 10 months: before left ventricle dysfunction; at the stage of early left ventricle diastolic dysfunction (with preserved ejection fraction); and left ventricle diastolic/systolic dysfunction, respectively. In 6- to 10-month-old but not in 3-month-old Tgαq*44 mice, behavioral and cognitive impairment was identified with compromised blood-brain barrier permeability, most significantly in brain cortex, that was associated with myelin sheet loss and changes in astrocytes and microglia. Brain endothelial cells displayed increased E-selectin immunoreactivity, which was accompanied by increased amyloid-β1-42 accumulation in piriform cortex and increased cortical oxidative stress (8-OHdG immunoreactivity). Resting cerebral blood flow measured by magnetic resonance imaging in vivo was preserved, but ex vivo NO-dependent cortical arteriole flow regulation was impaired. Platelet hyperreactivity was present in 3- to 10-month-old Tgαq*44 mice, but it was not associated with increased platelet-dependent thrombogenicity.

CONCLUSIONS

We report for the first time that vascular cognitive impairment is already present in the early stage of HF development, even before left ventricle systolic dysfunction. The underlying pathomechanism, independent of brain hypoperfusion, involves preceding platelet hyperreactivity and brain endothelium inflammatory activation.

Open-field exposure facilitates consummatory extinction

During extinction, the organism learns that a conditioned stimulus or a conditioned response is no longer associated with an unconditioned stimulus, and as a consequence, a decrement in the response is presented. The exposure to novel situations (e.g. exploration of a novel open field) has been used widely to modulate (i.e. either enhance or deteriorate) learning and memory. The aim of the present study was to test whether open-field exposure could modulate consummatory extinction. The results indicated that open-field exposure accelerated the extinction response (i.e. experimental animals provided novelty exposure had lower consummatory behavior than control animals) when applied before - but not after - the first extinction trial, or when applied before the second extinction trial. The results suggest that environmental treatments such as novelty exposure provide a valuable, nonpharmacological alternative to potentially modulate extinction processes.

Chlorella sorokiniana Extract Improves Short-Term Memory in Rats

Increasing evidence shows that eukaryotic microalgae and, in particular, the green microalga Chlorella, can be used as natural sources to obtain a whole variety of compounds, such as omega (ω)-3 and ω-6 polyunsatured fatty acids (PUFAs). Although either beneficial or toxic effects of Chlorella sorokiniana have been mainly attributed to its specific ω-3 and ω-6 PUFAs content, the underlying molecular pathways remain to be elucidated yet. Here, we investigate the effects of an acute oral administration of a lipid extract of Chlorella sorokiniana, containing mainly ω-3 and ω-6 PUFAs, on cognitive, emotional and social behaviour in rats, analysing possible underlying neurochemical alterations. Our results showed improved short-term memory in Chlorella sorokiniana-treated rats compared to controls, without any differences in exploratory performance, locomotor activity, anxiety profile and depressive-like behaviour. On the other hand, while the social behaviour of Chlorella sorokiniana-treated animals was significantly decreased, no effects on aggressivity were observed. Neurochemical investigations showed region-specific effects, consisting in an elevation of noradrenaline (NA) and serotonin (5-HT) content in hippocampus, but not in the prefrontal cortex and striatum. In conclusion, our results point towards a beneficial effect of Chlorella sorokiniana extract on short-term memory, but also highlight the need of caution in the use of this natural supplement due to its possible masked toxic effects.

Cholinesterase inhibitors, donepezil and rivastigmine, attenuate spatial memory and cognitive flexibility impairment induced by acute ethanol in the Barnes maze task in rats

Central cholinergic dysfunction contributes to acute spatial memory deficits produced by ethanol administration. Donepezil and rivastigmine elevate acetylcholine levels in the synaptic cleft through the inhibition of cholinesterases—enzymes involved in acetylcholine degradation. The aim of our study was to reveal whether donepezil (acetylcholinesterase inhibitor) and rivastigmine (also butyrylcholinesterase inhibitor) attenuate spatial memory impairment as induced by acute ethanol administration in the Barnes maze task (primary latency and number of errors in finding the escape box) in rats. Additionally, we compared the influence of these drugs on ethanol-disturbed memory. In the first experiment, the dose of ethanol (1.75 g/kg, i.p.) was selected that impaired spatial memory, but did not induce motor impairment. Next, we studied the influence of donepezil (1 and 3 mg/kg, i.p.), as well as rivastigmine (0.5 and 1 mg/kg, i.p.), given either before the probe trial or the reversal learning on ethanol-induced memory impairment. Our study demonstrated that these drugs, when given before the probe trial, were equally effective in attenuating ethanol-induced impairment in both test situations, whereas rivastigmine, at both doses (0.5 and 1 mg/kg, i.p.), and donepezil only at a higher dose (3 mg/kg, i.p.) given prior the reversal learning, attenuated the ethanol-induced impairment in cognitive flexibility. Thus, rivastigmine appears to exert more beneficial effect than donepezil in reversing ethanol-induced cognitive impairments—probably due to its wider spectrum of activity. In conclusion, the ethanol-induced spatial memory impairment may be attenuated by pharmacological manipulation of central cholinergic neurotransmission.

Investigations on light induced stress model and on the role of Phyllanthus amarus in attenuation of stress related depression with focus on 5HT2Am- RNA expression

Background

There is an increasing evidence that psychological stress and depression trigger changes in various biochemical parameters in animals and human subjects. Chronic stress in rats, and psychosocial stress in humans, is implicated in the pathophysiology of mood and anxiety disorders.

Purpose

The current study was been designed to investigate the behavioral, anatomical status of rat brain and expression of serotonin receptor (5HT_2A) mRNA related to pathophysiology of stress after high light (HL) illumination and to evaluate the effect of Phyllanthus amarus (PA) over such stress induced changes in mid brain and prefrontal cortex region of rat brain.

Methods

Bright light illumination was used to induce stress in wistar rats. Established methods were used to analyse biochemical and histopathological tests.

Results

PA administration to HL stressed animals for 7 days prevented stress-induced oxidative damage, as evidenced by significant enhancement of key endogenous antioxidant defense components. HL stress also caused reduced activities of membrane bound enzymes in synaptic membrane along with reduced levels of lipid profile and glycoprotein in the midbrain region. Stress induced changes in the locomotor activity, time spent for exploring the center of the arena, frequency of rearing and grooming, and frequency of facing the corner of the arena, were altered on PA administration.

Conclusion

The findings suggest that the therapeutic efficacy of PA may be mediated, atleast partially, via reversal of oxidative damage. Further the study demonstrates the promising intervention by PA on the mRNA expression of 5HT 2A in the brain. These preliminary results pave the way for further validation of PA against stress.

Impaired hippocampal acetylcholine release parallels spatial memory deficits in Tg2576 mice subjected to basal forebrain cholinergic degeneration

The Alzheimer's disease (AD) mouse model Tg2576 overexpresses an AD associated mutant variant of human APP and accumulates amyloid beta (Aβ) in an age-dependent manner. Using the selective cholinergic immunotoxin mu p75-saporin (SAP), we induced a partial basal forebrain cholinergic degeneration (BFCD) in 3 months old male Tg2576 mice to co-express cholinergic degeneration with Aβ overexpression as these characteristics constitutes key hallmarks of AD. At 9 months, SAP lesioned Tg2576 mice were cognitively impaired in two spatial paradigms addressing working memory and mid to long-term memory. Conversely, there was no deterioration of cognitive functioning in sham lesioned Tg2576 mice or wild type littermates (wt) receiving the immunotoxin. At 10 months of age, release of acetylcholine (ACh) was addressed by microdialysis in conscious mice. Scopolamine-induced increases in hippocampal ACh efflux was significantly reduced in SAP lesioned Tg2576 mice compared to sham lesioned Tg2576 mice. Intriguingly, there was no significant difference in ACh efflux between wt treatment groups. Following SAP treatment, choline acetyltransferase activity was reduced in the hippocampus and frontal cortex and the reduction was comparable between groups. Our results suggest that partial BFCD acts collectively with increased levels of Aβ to induce cognitive decline and to compromise cholinergic release. Tg2576 mice with BFCD may constitute a new and suitable AD mouse model to study the interrelations between cholinergic deficits and amsyloid deposition.

Translating the N-methyl-D-aspartate receptor antagonist model of schizophrenia to treatments for cognitive impairment in schizophrenia

The N-methyl-D-aspartate receptor (NMDAR) antagonists, phencyclidine (PCP), dizocilpine (MK-801), or ketamine, given subchronically (sc) to rodents and primates, produce prolonged deficits in cognitive function, including novel object recognition (NOR), an analog of human declarative memory, one of the cognitive domains impaired in schizophrenia. Atypical antipsychotic drugs (AAPDs) have been reported to improve declarative memory in some patients with schizophrenia, as well as to ameliorate and prevent the NOR deficit in rodents following scNMDAR antagonist treatment. While the efficacy of AAPDs to improve cognitive impairment in schizophrenia (CIS) is limited, at best, and controversial, single doses of all currently available AAPDs so far tested transiently restore NOR in rodents following scNMDAR antagonist treatment. Typical antipsychotic drugs (APDs), e.g. haloperidol and perphenazine, are ineffective in this rodent model, and may be less effective as treatments of some domains of CIS. Serotonergic mechanisms, including, but not limited to serotonin (5-HT)2A and 5-HT7 antagonism, 5-HT(1A), and GABA(A) agonism, contribute to the efficacy of the AAPDs in the scNMDAR antagonist rodent models, which are relevant to the loss of GABA interneuron/hyperglutamate hypothesis of the etiology of CIS. The ability of sub-effective doses of the atypical APDs to ameliorate NOR in the scNMDAR-treated rodents can be restored by the addition of a sub-effective dose of the 5-HT(1A) partial agonist, tandospirone, or the 5-HT7 antagonist, SB269970. The mGluR2/3 agonist, LY379268, which itself is unable to restore NOR in the scNMDAR-treated rodents, can also restore NOR when given with lurasidone, an AAPD. Enhancing cortical and hippocampal dopamine and acetylcholine efflux, or both, may contribute to the restoration of NOR by the atypical APDs. Importantly, co-administration of lurasidone, tandospirone, or SB269970, with PCP, to rodents, at doses 5-10 fold greater than those acutely effective to restore NOR following scNMDAR treatment, prevents the effect of scPCP to produce an enduring deficit in NOR. This difference in dosage may be relevant to utilizing AAPDs to prevent the onset of CIS in individuals at high risk for developing schizophrenia. The scNMDAR paradigm may be useful for identifying possible means to treat and prevent CIS.

Behavioral deficits induced by lead exposure are accompanied by serotonergic and cholinergic alterations in the prefrontal cortex

The effects of long-term lead (Pb) exposure producing a blood Pb concentration of lower than 20 μg/dL, i.e. below that associated with overt neurological deficits in occupationally exposed individuals, was studied in adult rats. In order to assess gender differences, we performed parallel behavioral experiments in male and female rats. Exposure to Pb acetate (50 ppm in drinking water) for 6 months induced motor and cognitive alterations, however these effects were gender- and task-dependent. Chronic lead exposure impaired spatial learning assessed in the Morris water maze test (MWM) in both genders, whereas it only induced hyperactivity in the open field and impaired motor coordination in the rotarod test, only in male rats. Hyperactivity in male rats was accompanied by an increase in extracellular level of acetylcholine in the prefrontal cortex. Extracellular dopamine concentration in the prefrontal cortex was unaffected by lead exposure whereas serotonin concentration in the same brain area was significantly decreased in both male and female rats exposed to lead. These results unveil new molecular mechanisms underlying neuropsychiatric alterations induced by chronic lead exposure.

NADPH oxidase elevations in pyramidal neurons drive psychosocial stress-induced neuropathology

Oxidative stress is thought to be involved in the development of behavioral and histopathological alterations in animal models of psychosis. Here we investigate the causal contribution of reactive oxygen species generation by the phagocyte NADPH oxidase NOX2 to neuropathological alterations in a rat model of chronic psychosocial stress. In rats exposed to social isolation, the earliest neuropathological alterations were signs of oxidative stress and appearance of NOX2. Alterations in behavior, increase in glutamate levels and loss of parvalbumin were detectable after 4 weeks of social isolation. The expression of the NOX2 subunit p47(phox) was markedly increased in pyramidal neurons of isolated rats, but below detection threshold in GABAergic neurons, astrocytes and microglia. Rats with a loss of function mutation in the NOX2 subunit p47(phox) were protected from behavioral and neuropathological alterations induced by social isolation. To test reversibility, we applied the antioxidant/NOX inhibitor apocynin after initiation of social isolation for a time period of 3 weeks. Apocynin reversed behavioral alterations fully when applied after 4 weeks of social isolation, but only partially after 7 weeks. Our results demonstrate that social isolation induces rapid elevations of the NOX2 complex in the brain. Expression of the enzyme complex was strongest in pyramidal neurons and a loss of function mutation prevented neuropathology induced by social isolation. Finally, at least at early stages, pharmacological targeting of NOX2 activity might reverse behavioral alterations.

Modularity of mind and the role of incentive motivation in representing novelty

Animal and human brains contain a myriad of mental representations that have to be successfully tracked within fractions of a second in a large number of situations. This retrieval process is hard to explain without postulating the massive modularity of cognition. Assuming that the mind is massively modular, it is then necessary to understand how cognitive modules can efficiently represent dynamic environments-in which some modules may have to deal with change-induced novelty and uncertainty. Novelty of a stimulus is a problem for a module when unknown, significant stimuli do not satisfy the module's processing criteria-or domain specificity-and cannot therefore be included in its database. It is suggested that the brain mechanisms of incentive motivation, recruited when faced with novelty and uncertainty, induce transient variations in the domain specificity of cognitive modules in order to allow them to process information they were not prepared to learn. It is hypothesised that the behavioural transitions leading from exploratory activity to habit formation are correlated with (and possibly caused by) the organism's ability to counter novelty-induced uncertainty.

Recognition of novel objects and their location in rats with selective cholinergic lesion of the medial septum

The importance of cholinergic neurons projecting from the medial septum (MS) of the basal forebrain to the hippocampus in memory function has been controversial. The aim of this study was to determine whether loss of cholinergic neurons in the MS disrupts object and/or object location recognition in male Sprague-Dawley rats. Animals received intraseptal injections of either vehicle, or the selective cholinergic immunotoxin 192 IgG-saporin (SAP). 14 days later, rats were tested for novel object recognition (NOR). Twenty-four hours later, these same rats were tested for object location recognition (OLR) (recognition of a familiar object moved to a novel location). Intraseptal injections of SAP produced an 86% decrease in choline acetyltransferase (ChAT) activity in the hippocampus, and a 31% decrease in ChAT activity in the frontal cortex. SAP lesion had no significant effect on NOR, but produced a significant impairment in OLR in these same rats. The results support a role for septo-hippocampal cholinergic projections in memory for the location of objects, but not for novel object recognition.

Cortical cholinergic abnormalities contribute to the amnesic state induced by pyrithiamine-induced thiamine deficiency in the rat

Although the key neuropathology associated with diencephalic amnesia is lesions to the thalamus and/or mammillary bodies, functional deactivation of the hippocampus and associated cortical regions also appear to contribute to the memory dysfunction. For example, there is loss of forebrain cholinergic neurons and alterations in stimulated acetylcholine (ACh) levels in the hippocampus and cortex in animal models of diencephalic amnesia associated with thiamine deficiency. In the present study, the pyrithiamine-induced thiamine deficiency rat model was used to assess the functional relationships between thalamic pathology, behavioral impairment, ACh efflux and cholinergic innervation of the hippocampus and cortex. In pyrithiamine-induced thiamine deficiency-treated rats, ACh efflux during behavioral testing was blunted to differing degrees in the hippocampus, medial frontal cortex and retrosplenial cortex. In addition, significant reductions in cholinergic fiber densities were observed in each of these regions. However, only hippocampal cholinergic fiber density correlated significantly with ACh efflux in the same region, suggesting that the reduction in cortical ACh efflux in cases of diencephalic amnesia cannot be fully explained by a loss of cholinergic fiber innervation. This notion supports the emerging theory that the functional consequences of the distal effects of lesions go beyond simple deafferentation. Specifically, some frontal cortical regions exhibit hypersensitivity to deafferentation that is only detected during behavioral and/or physiological demand.

Regulation of cortical acetylcholine release: insights from in vivo microdialysis studies

Acetylcholine release links the activity of presynaptic neurons with their postsynaptic targets and thus represents the intercellular correlate of cholinergic neurotransmission. Here, we review the regulation and functional significance of acetylcholine release in the mammalian cerebral cortex, with a particular emphasis on information derived from in vivo microdialysis studies over the past three decades. This information is integrated with anatomical and behavioral data to derive conclusions regarding the role of cortical cholinergic transmission in normal behavioral and how its dysregulation may contribute to cognitive correlates of several neuropsychiatric conditions. Some unresolved issues regarding the regulation and significance of cortical acetylcholine release and the promise of new methodology for advancing our knowledge in this area are also briefly discussed.

Basal forebrain lactate release and promotion of cortical arousal during prolonged waking is attenuated in aging

The wake-promoting basal forebrain (BF) is critically involved in sustaining cortical arousal. In the present study, we investigated how aging affects the capacity of the BF to cope with continuous activation during prolonged waking. Increased neuronal activity induces lactate release in the activated brain area, and BF stimulation increases cortical arousal. We used in vivo microdialysis to measure lactate levels in the BF, and electroencephalography (EEG) to measure cortical arousal, during 3 h sleep deprivation (SD) in three age groups of rats. Lactate increased during SD in young but not in aged (middle-aged and old) rats. The increase in high-frequency (HF) EEG theta power (7-9 Hz), a marker of cortical arousal and active waking, was attenuated in the aged. Furthermore, a positive correlation between BF lactate release and HF EEG theta increase was found in young but not in aged rats. We hypothesized that these age-related attenuations result from reduced capacity of the BF to respond to increased neuronal activation. This was tested by stimulating the BF with glutamate receptor agonist NMDA. Whereas BF stimulation increased waking in young and old rats, lactate increase and the HF EEG theta increase were attenuated in the old. Also, the homeostatic increase in sleep intensity after SD was attenuated in aged rats. Our results suggest that an age-related attenuation in BF function reduces cortical arousal during prolonged waking. As the quality of waking is important in regulating the subsequent sleep, reduced cortical arousal during SD may contribute to the age-related reduction in sleep intensity.

Involvement of NOX2 in the development of behavioral and pathologic alterations in isolated rats

BACKGROUND

Social stress leads to oxidative stress in the central nervous system, contributing to the development of mental disorders. Loss of parvalbumin in interneurons is an important feature of these diseases. We studied the role of the superoxide-producing nicotinamide adenosine dinucleotide phosphate (NADPH) oxidase 2 (NOX2) in rats exposed to social isolation.

METHODS

Male rats were kept for 7 weeks in group or in social isolation (n = 6-10 per group). Behavioral tests, immunohistochemistry, and analysis of NOX2 expression were performed at the end of social isolation. Apocynin was given in the drinking water (5 mg/kg/day).

RESULTS

NOX2 was below detection level in the brains of control animals, whereas it was highly expressed in isolated rats, particularly in nucleus accumbens and prefrontal cortex. Indirect markers of oxidative stress (oxidized nucleic acid 8-hydroxy-2'-deoxyguanosine, redox-sensitive transcription factor c-fos, and hypoxia-inducible factor-1alpha) were increased after social isolation in brain areas with high NOX2 expression. An increase in immunoreactive microglia suggested that oxidative stress could be in part due to NOX2 activation in microglia. In response to social isolation, rats showed increased locomotor activity, decreased discrimination, signs of oxidative stress in neurons, and loss of parvalbumin-immunoreactivity. Treatment of isolated rats with the antioxidant/NOX inhibitor apocynin prevented the behavioral and histopathological alterations induced by social isolation.

CONCLUSIONS

Our data suggest that NOX2-derived oxidative stress is involved in loss of parvalbumin immunoreactivity and development of behavioral alterations after social isolation. These results provide a molecular mechanism for the coupling between social stress and brain oxidative stress, as well as potential new therapeutic avenues.

Heavy metal uranium affects the brain cholinergic system in rat following sub-chronic and chronic exposure

Uranium is a heavy metal naturally present in the environment that may be chronically ingested by the population. Previous studies have shown that uranium is present in the brain and alters behaviour, notably locomotor activity, sensorimotor ability, sleep/wake cycle and the memory process, but also metabolism of neurotransmitters. The cholinergic system mediates many cognitive systems, including those disturbed after chronic exposure to uranium i.e., spatial memory, sleep/wake cycle and locomotor activity. The objective of this study was to assess whether these disorders follow uranium-induced alteration of the cholinergic system. In comparison with 40 control rats, 40 rats drank 40 mg/L uranyl nitrate for 1.5 or 9 months. Cortex and hippocampus were removed and gene expression and protein level were analysed to determine potential changes in cholinergic receptors and acetylcholine levels. The expression of genes showed various alterations in the two brain areas after short- and long-term exposure. Nevertheless, protein levels of the choline acetyltransferase enzyme (ChAT), the vesicular transporter of acetylcholine (VAChT) and the nicotinic receptor beta2 sub-unit (nAChRbeta2) were unmodified in all cases of the experiment and muscarinic receptor type 1 (m1AChR) protein level was disturbed only after 9 months of exposure in the cortex (-30%). Acetylcholine levels were unchanged in the hippocampus after 1.5 and 9 months, but were decreased in the cortex after 1.5 months only (-22%). Acetylcholinesterase (AChE) activity was also unchanged in the hippocampus but decreased in the cortex after 1.5 and 9 months (-16% and -18%, respectively). Taken together, these data indicate that the cholinergic system is a target of uranium exposure in a structure-dependent and time-dependent manner. These cholinergic alterations could participate in behavioural impairments.

Memory impairment induced by an interfering task is reverted by pre-frontal cortex lesions: a possible role for an inhibitory process in memory suppression in mice

Interference theory refers to the idea that forgetting occurs because the recall of certain items interferes with the recall of other items. Recently, it has been proposed that interference is due to an inhibitory control mechanism, triggered by competing memories, that ultimately causes forgetting [Anderson MC (2003) Rethinking interference theory: Executive control and the mechanisms of forgetting. J Mem Lang 49:415-4453]. In the present research we study the interference process by submitting CD1 mice to two different hippocampal-dependent tasks: a place object recognition task (PORT) and a step-through inhibitory avoidance task (IA). Our results show a mutual interference between PORT and IA. To elucidate the possible neural mechanism underlying the interference process, we submit hippocampus- and prefrontal cortex-lesioned mice to PORT immediately before IA training. Results from these experiments show that prefrontal cortex lesions completely revert the impairing effect exerted by PORT administration on IA memory, while hippocampus lesions, that as expected impair memory for both PORT and IA, increase this effect. Altogether our results suggest that interference-induced forgetting is driven by an inhibitory control mechanism through activation of hippocampus-prefrontal cortex circuitry. The hippocampus seems to be crucial for storing information related to both behavioral tasks. Competition between memories triggers the inhibitory control mechanism, by activating prefrontal cortex, and induces memory suppression.

Telencephalon: Neocortex

The neocortex is an ultracomplex, six-layered structure that develops from the dorsal palliai sector of the telencephalic hemispheres (Figs. 2.24, 2.25, 11.1). All mammals, including monotremes and marsupials, possess a neocortex, but in reptiles, i.e. the ancestors of mammals, only a three-layered neocortical primordium is present [509, 511]. The term neocortex refers to its late phylogenetic appearance, in comparison to the “palaeocortical” olfactory cortex and the “archicortical” hippocampal cortex, both of which are present in all amniotes [509].

Subsequent exposure to the choline uptake enhancer MKC-231 antagonizes phencyclidine-induced behavioral deficits and reduction in septal cholinergic neurons in rats

This study examined the effects of subsequent, subchronic, treatment with choline uptake enhancer MKC-231 on the behavioral and cellular deficits induced by repeated PCP exposure in rats. Prior subchronic PCP exposure resulted in increased locomotion following an acute PCP or cocaine challenge, but resulted in decreased locomotor activity in response to a carbachol-challenge. MKC-231 significantly antagonized the alterations in the locomotor responses to cocaine and carbachol, but not to PCP. In the novel object recognition test, repeated PCP exposure caused cognitive deficits in rats, and the PCP-induced cognitive deficits were antagonized by MKC-231. In contrast, no effects of PCP exposure were shown in the repeated passive avoidance test. Furthermore, repeated PCP exposure decreased a number of choline acetyltransferase (ChAT)-positive cells in the medial septum and increased dynorphin A expression in the ventral striatum. Moreover, MKC-231 significantly antagonized the changes in septal ChAT-positive cells, but not the changes in ventrostriatal dynorphin A expression. These results suggest that MKC-231 could be a therapeutic drug for the treatment of schizophrenia.

Neurobiological mechanisms for the regulation of mammalian sleep-wake behavior: reinterpretation of historical evidence and inclusion of contemporary cellular and molecular evidence

At its most basic level, the function of mammalian sleep can be described as a restorative process of the brain and body; recently, however, progressive research has revealed a host of vital functions to which sleep is essential. Although many excellent reviews on sleep behavior have been published, none have incorporated contemporary studies examining the molecular mechanisms that govern the various stages of sleep. Utilizing a holistic approach, this review is focused on the basic mechanisms involved in the transition from wakefulness, initiation of sleep and the subsequent generation of slow-wave sleep and rapid eye movement (REM) sleep. Additionally, using recent molecular studies and experimental evidence that provides a direct link to sleep as a behavior, we have developed a new model, the cellular-molecular-network model, explaining the mechanisms responsible for regulating REM sleep. By analyzing the fundamental neurobiological mechanisms responsible for the generation and maintenance of sleep-wake behavior in mammals, we intend to provide a broader understanding of our present knowledge in the field of sleep research.

The pharmacology, neuroanatomy and neurogenetics of one-trial object recognition in rodents

Rats and mice are attracted by novel objects. They readily approach novel objects and explore them with their vibrissae, nose and forepaws. It is assumed that such a single explorative episode leaves a lasting and complex memory trace, which includes information about the features of the object explored, as well as where and even when the object was encountered. Indeed, it has been shown that rodents are able to discriminate a novel from a familiar object (one-trial object recognition), can detect a mismatch between the past and present location of a familiar object (one-trial object-place recognition), and can discriminate different objects in terms of their relative recency (temporal order memory), i.e., which one of two objects has been encountered earlier. Since the novelty-preference paradigm is very versatile and has some advantages compared to several other memory tasks, such as the water maze, it has become a powerful tool in current neurophamacological, neuroanatomical and neurogenetical memory research using both rats and mice. This review is intended to provide a comprehensive summary on key findings delineating the brain structures, neurotransmitters, molecular mechanisms and genes involved in encoding, consolidation, storage and retrieval of different forms of one-trial object memory in rats and mice.

Cholinergic cells in the nucleus basalis of mice express the N-methyl-d-aspartate-receptor subunit NR2C and its replacement by the NR2B subunit enhances frontal and amygdaloid acetylcholine levels

It is known that glutamatergic and cholinergic systems interact functionally at the level of the cholinergic basal forebrain. The N-methyl-d-aspartate receptor (NMDA-R) is a multiprotein complex composed of NR1, NR2 and/or NR3 subunits. The subunit composition of NMDA-R of cholinergic cells in the nucleus basalis has not yet been investigated. Here, by means of choline acetyl transferase and NR2B or NR2C double staining, we demonstrate that mice express both the NR2C and NR2B subunits in nucleus basalis cholinergic cells. We generated NR2C-2B mutant mice in which an insertion of NR2B cDNA into the gene locus of the NR2C gene replaced NR2C by NR2B expression throughout the brain. This NR2C-2B mutant was used to examine whether a subunit exchange in cholinergic neurons would affect acetylcholine (ACh) content in several brain structures. We found increased ACh levels in the frontal cortex and amygdala in the brains of NR2C-2B mutant mice. Brain ACh has been implicated in neuroplasticity, novelty-induced arousal and encoding of novel stimuli. We therefore assessed behavioral habituation to novel environments and objects as well as object recognition in NR2C-2B subunit exchange mice. The behavioral analysis did not indicate any gross behavioral alteration in the mutant mice compared with the wildtype mice. Our results show that the NR2C by NR2B subunit exchange in mice affects ACh content in two target areas of the nucleus basalis.

Environmental enrichment reduces the function of D1 dopamine receptors in the prefrontal cortex of the rat

Environmental enrichment produces changes in spontaneous and psychostimulant-induced motor activity. Dopamine in the prefrontal cortex (PFC), through the activation of D1 receptors, has been suggested to play a role in modulating motor activity. The present study investigated the effects of environmental enrichment on spontaneous motor activity, prefrontal acetylcholine release following local D1 receptor stimulation and D1 receptor expression in the PFC. Male wistar rats (3 months of age) were housed in enriched or isolated conditions during 90 days. Animals were then implanted with guide cannulae to perform microdialysis experiments in the PFC. Spontaneous motor activity and acetylcholine extracellular concentrations were monitored simultaneously. Also spontaneous motor activity was measured in an open field. On completion of the experiments, the density of D1 receptors in the PFC was studied by immunocytochemistry. Rats housed in an enriched environment showed significantly lower spontaneous motor activity in the open field compared to isolated animals. Perfusion of the D1 agonist SKF38393 (50 microM; 40 min) in the PFC produced long lasting increases of spontaneous motor activity and of local dialysate concentrations of acetylcholine in both groups of rats. However, increases of both motor activity and acetylcholine concentrations were significantly lower in enriched compared to isolated animals. Moreover, the density of D1 receptors in the PFC was significantly reduced in animals housed in an enriched environment. These results are the first evidence suggesting that environmental enrichment during adult life changes the function of D1 dopamine receptors in the PFC.

Microdialysis as a tool in local pharmacodynamics

In many cases the clinical outcome of therapy needs to be determined by the drug concentration in the tissue compartment in which the pharmacological effect occurs rather than in the plasma. Microdialysis is an in vivo technique that allows direct measurement of unbound tissue concentrations and permits monitoring of the biochemical and physiological effects of drugs throughout the body. Microdialysis was first used in pharmacodynamic research to study neurotransmission, and this remains its most common application in the field. In this review, we give an overview of the principles, techniques, and applications of microdialysis in pharmacodynamic studies of local physiological events, including measurement of endogenous substances such as acetylcholine, catecholamines, serotonin, amino acids, peptides, glucose, lactate, glycerol, and hormones. Microdialysis coupled with systemic drug administration also permits the more intensive examination of the pharmacotherapeutic effect of drugs on extracellular levels of endogenous substances in peripheral compartments and blood. Selected examples of the physiological effects and mechanisms of action of drugs are also discussed, as are the advantages and limitations of this method. It is concluded that microdialysis is a reliable technique for the measurement of local events, which makes it an attractive tool for local pharmacodynamic research.

Cholinergic involvement in the cortical and hippocampal Fos expression induced in the rat by placement in a novel environment

Placing rats into a series of novel environments induced vigorous c-fos expression in the infralimbic, anterior cingulate and retrosplenial cortices, and in the hippocampus. Pretreatment with the antimuscarinic drugs scopolamine and atropine was able to greatly suppress novelty-induced Fos expression at these sites. Placement into the novel environments also induced Fos expression in the habenula and the paraventricular thalamic nucleus, but the response at these sites did not appear to be sensitive to cholinergic blockade. These findings suggest that cholinergic mechanisms play an important role in ability of behavioral events to influence cortical and hippocampal immediate-early gene expression and are consistent with the possibility that some of the effects of anticholinergic drugs on placticity and learning may be mediated through alterations in the expression of these genes.

Effects of the metabotropic glutamate receptor agonist, ACPD, on the extracellular concentrations of GABA and acetylcholine in the prefrontal cortex of the rat during the normal process of aging

The aim of the present study was to investigate the effects of activation of metabotropic glutamate receptors (mGluR) on the extracellular concentrations of GABA and acetylcholine in the prefrontal cortex of freely moving rats of different groups of age. Perfusion, through the microdialysis probe, of the agonist of mGluR, (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD; 100, 500 and 1000 microM), in the prefrontal cortex of young rats produced a dose-related increase of the dialysate concentrations of GABA. The effects of perfusion of ACPD on the concentrations of GABA were attenuated in middle-aged rats. In the prefrontal cortex of aged rats, perfusion of ACPD produced no changes in dialysate concentrations of GABA at any of the doses used. Conversely, perfusion of ACPD (100, 500 and 1000 microM) in the prefrontal cortex of young, middle-aged and aged rats did not modify the dialysate concentrations of acetylcholine. Basal concentrations of acetylcholine in the prefrontal cortex of middle-aged and aged rats were significantly lower than those in young rats. In contrast, basal dialysate concentrations of GABA were not significantly different in young, middle-aged and aged rats. These results suggest that the interaction GABA-glutamate in the prefrontal cortex, mediated by mGluRs, changes with age.

Inhibition of acetylcholine-induced activation of extracellular regulated protein kinase prevents the encoding of an inhibitory avoidance response in the rat

It has been demonstrated that the forebrain cholinergic system and the extracellular regulated kinase signal transduction pathway are involved in the mechanisms of learning, encoding, and storage of information. We investigated the involvement of the cholinergic and glutamatergic systems projecting to the medial prefrontal cortex and ventral hippocampus and of the extracellular regulated kinase signal transduction pathway in the acquisition and recall of the step-down inhibitory avoidance response in the rat, a relatively simple behavioral test acquired in a one-trial session. To this aim we studied by microdialysis the release of acetylcholine and glutamate, and by immunohistochemistry the activation of extracellular regulated kinase during acquisition, encoding and recall of the behavior. Cholinergic, but not glutamatergic, neurons projecting to the medial prefrontal cortex and ventral hippocampus were activated during acquisition of the task, as shown by increase in cortical and hippocampal acetylcholine release. Released acetylcholine in turn activated extracellular regulated kinase in neurons located in the target structures, since the muscarinic receptor antagonist scopolamine blocked extracellular regulated kinase activation. Both increased acetylcholine release and extracellular regulated kinase activation were necessary for memory formation, as administration of scopolamine and of extracellular regulated kinase inhibitors was followed by blockade of extracellular regulated kinase activation and amnesia. Our data indicate that a critical function of the learning-associated increase in acetylcholine release is to promote the activation of the extracellular regulated kinase signal transduction pathway and help understanding the role of these systems in the encoding of an inhibitory avoidance memory.

Effects of isoflurane on prefrontal acetylcholine release and hypothalamic Fos response in young adult and aged rats

This experiment investigated the influence of age on prefrontal acetylcholine (ACh) release and Fos response in the hypothalamic paraventricular nucleus and the nucleus tractus solitarius (NTS) of rats following isoflurane anesthesia. It is known that isoflurane decreases acetylcholine release in most brain regions. In the present study, we found that the level of prefrontal acetylcholine was significantly lower in 28-month-old rats (14% of baseline) than in 3-month-old rats (38% of baseline) during 2 h of isoflurane anesthesia (P < 0.05). The old rat group showed significantly greater Fos induction in the paraventricular nucleus compared to the young adult rat group (P < 0.05), indicating that the old rats were subjected to stress. No difference in Fos response was noted in the nucleus tractus solitarius. The old rats displayed a significant increase in feeding behavior during the 3-h recovery period (P < 0.05), but there was no difference in overall acetylcholine levels. Taken together, these findings suggest that isoflurane anesthesia influences old rats more profoundly than young adult rats with regard to reductions in acetylcholine release and stress responses. This may have implications for understanding the development of postoperative delirium in aged patients.

Excitotoxic lesions of the pedunculopontine tegmental nucleus in rats impair performance on a test of sustained attention

Recent research has suggested that the pontomesencephalic tegmentum might be an important part of a network underlying sustained attention. The largest structure of the pontomesencephalic tegmentum is the pedunculopontine tegmental nucleus, which has ascending connections to thalamus and with corticostriatal systems. In this experiment we examined the performance of rats following bilateral excitotoxic lesions of the pedunculopontine tegmental nucleus on a test of sustained attention previously used to examine frontal cortical function. After an initial period of darkness, the rats had to attend continuously to a dim stimulus light that would, at unpredictable intervals, become transiently brighter. During this period of increased stimulus brightness the rats could press a lever to obtain a food reward. Rats were trained to a criterion level of performance before lesions were made. After surgery, sham lesioned rats (n=7) resumed accurate responding, with an average successful detection rate of approximately 70%. Pedunculopontine lesioned rats (n=7), however, only achieved a successful detection rate of approximately 40%. When the duration of the bright target stimulus was increased from 1.5 to 4 s, the performance of the pedunculopontine lesioned rats significantly improved. The observation that an increase in brightness duration caused a marked improvement in lesioned rats' performance suggests strongly that the impairment was in attention rather than motor ability or simple sensory processing. These data are taken to be consistent with the hypothesis that the pedunculopontine tegmental nucleus is an important part of a network maintaining attention.

Neurokinin-1 receptor antagonism by SR140333: enhanced in vivo ACh in the hippocampus and promnestic post-trial effects

Substance P (SP) has memory-promoting, reinforcing and anxiolytic-like effects when applied systemically or centrally. Such effects may be mediated by the neurokinin-1 (NK-1) receptor, since SP preferentially binds to this receptor. We measured the effects of a selective non-peptide NK-1 receptor antagonist, SR140333 (1, 3 and 9 mg/kg i.p.) on ACh levels in frontal cortex, amygdala and hippocampus by microdialysis and HPLC. Levels of ACh in the hippocampus increased dose-dependently immediately after treatment. The same doses of SR140333 given post-trial had minor facilitative effects on inhibitory avoidance learning and open-field habituation, but did not have reinforcing effects in a conditioned place preference (CPP) task. The selective action of NK-1 receptor antagonism on hippocampal ACh may be related to its positive influence on learning.

Changes in acetylcholine extracellular levels during cognitive processes

Measuring the changes in neurotransmitter extracellular levels in discrete brain areas is considered a tool for identifying the neuronal systems involved in specific behavioral responses or cognitive processes. Acetylcholine (ACh) is the first neurotransmitter whose diffusion from the central nervous system was investigated and whose extracellular levels variations were correlated to changes in neuronal activity. This was done initially by means of the cup technique and then by the microdialysis technique. The latter, notwithstanding some technical limitations, makes it possible to detect variations in extracellular levels of ACh in unrestrained, behaving animals. This review summarizes and discusses the results obtained investigating the changes in ACh release during performance of operant tasks, exposition to novel stimuli, locomotor activity, and the performance of spatial memory tasks, working memory, and place preference memory tasks. Activation of the forebrain cholinergic system has been demonstrated in many tasks and conditions in which the environment requires the animal to analyze novel stimuli that may represent a threat or offer a reward. The sustained cholinergic activation, demonstrated by high levels of extracellular ACh observed during the behavioral paradigms, indicates that many behaviors occur within or require the facilitation provided by the cholinergic system to the operation of pertinent neuronal pathways.

Biphasic effects of ethanol on acetylcholine release in the rat prefrontal cortex

Low doses of ethanol (0.5 g/kg i.p.) increased, while higher doses (1 g/kg i.p.) reduced acetylcholine (ACh) release in the rat prefrontal cortex (PFC). Ethanol (50-300 mM) applied in the nucleus basalis through a second dialysis probe caused concentration-dependent biphasic changes in prefrontocortical ACh release. Ethanol apparently acts on cholinergic fibers to modulate ACh transmission in the PFC. These results could be of relevance for the bidirectional modulation of working memory by ethanol.

Selective antagonism at dopamine D3 receptors enhances monoaminergic and cholinergic neurotransmission in the rat anterior cingulate cortex

Recent neuroanatomical and functional investigations focusing on dopamine (DA) D(3) receptors have suggested a potential role of this receptor in psychiatric diseases such as schizophrenia and drug dependence. In line with the key role of the prefrontal cortex in psychiatric disorders, the present study aimed at assessing the effects of the acute systemic administration of the selective DA D(3) receptor antagonist SB-277011-A on the in vivo extracellular levels of monoamines (DA, norepinephrine (NE), and serotonin (5-HT)) and acetylcholine (ACh) in the anterior cingulate subregion of the medial prefrontal cortex. The in vivo neurochemical profile of SB-277011-A (10 mg/kg, i.p.) in the anterior cingulate cortex was compared with both typical and atypical antipsychotics including clozapine (10 mg/kg, s.c.), olanzapine (10 mg/kg, s.c.), sulpiride (10 mg/kg, s.c.), and haloperidol (0.5 mg/kg, s.c.). The acute administration of SB-277011-A, clozapine, and olanzapine produced a significant increase in extracellular levels of DA, NE, and ACh without affecting levels of 5-HT. Sulpiride also significantly increased extracellular DA, but with a delayed onset over SB-277011-A, clozapine, and olanzapine. In contrast, haloperidol failed to alter any of the three monoamines and ACh in the anterior cingulate cortex. These findings add to a growing body of evidence suggesting a differentiation between typical and atypical antipsychotic drugs (APDs) in the anterior cingulate cortex and a role of DA D(3) receptors in desired antipsychotic drug profile. Similar to their effects on DA and NE, SB-277011-A, clozapine, and olanzapine increased extracellular levels of ACh, whereas haloperidol and sulpiride did not alter ACh. The results obtained in the present study provide evidence of the important role of DA D(3) receptors in the effect of pharmacotherapeutic agents that are used for the treatment of psychiatric disorders such as schizophrenia and drug dependence.

Investigation on acetylcholine, aspartate, glutamate and GABA extracellular levels from ventral hippocampus during repeated exploratory activity in the rat

The extracellular levels of aspartate, glutamate, gamma-aminobutyric acid (GABA), and acetylcholine (ACh) were investigated by microdialysis, coupled with HPLC, in the ventral hippocampus of rats during two 30-min exploration periods. Motor activity was monitored. During exploration I, an increase in motor activity associated with a 315% increase in aspartate, 181% in glutamate, and 264% in ACh levels, occurred during the first 10 min. The increase in GABA level reached a maximum of 257% during the second 10 min. The neurotransmitter levels returned to basal values within 40 min. During exploration II, 1 h later, a smaller increase in neurotransmitter levels and motor activity was observed. In both explorations, the increase in neurotransmitter levels was completely abolished by 1 and 3 microM TTX. A statistically significant relationship was found between neurotransmitter extracellular levels and motor activity, for aspartate and glutamate in exploration I, and for ACh in exploration I and II. In conclusion, exploratory activity is associated with or depends on the activation of neuronal systems in the ventral hippocampus releasing aspartate, glutamate, GABA, and ACh. The activation is dampened by habituation.