Selective responsiveness of medial prefrontal cortex neurons to the meaningful stimulus with a low probability of occurrence in rats

Medial prefrontal cortex lesions disrupt prepotent action selection signals in dorsomedial striatum

The ability to inhibit or adapt unwanted actions or movements is a critical feature of almost all forms of behavior. Many have attributed this ability to frontal brain areas such as the anterior cingulate cortex (ACC) and the medial prefrontal cortex (mPFC), but the exact contribution of each brain region is often debated because their functions are not examined in animals performing the same task. Recently, we have shown that ACC signals a need for cognitive control and is crucial for the adaptation of action selection signals in dorsomedial striatum (DMS) in rats performing a stop-change task. Here, we show that unlike ACC, the prelimbic region of mPFC does not disrupt the inhibition or adaption of an action plan at either the level of behavior or downstream firing in DMS. Instead, lesions to mPFC correlate with changes in DMS signals involved in action initiation and disrupt performance on GO trials while improving performance on STOP trials.

Subdividing Stress Groups into Eustress and Distress Groups Using Laterality Index Calculated from Brain Hemodynamic Response

A stress group should be subdivided into eustress (low-stress) and distress (high-stress) groups to better evaluate personal cognitive abilities and mental/physical health. However, it is challenging because of the inconsistent pattern in brain activation. We aimed to ascertain the necessity of subdividing the stress groups. The stress group was screened by salivary alpha-amylase (sAA) and then, the brain’s hemodynamic reactions were measured by functional near-infrared spectroscopy (fNIRS) based on the near-infrared biosensor. We compared the two stress subgroups categorized by sAA using a newly designed emotional stimulus-response paradigm with an international affective picture system (IAPS) to enhance hemodynamic signals induced by the target effect. We calculated the laterality index for stress (LIS) from the measured signals to identify the dominantly activated cortex in both the subgroups. Both the stress groups exhibited brain activity in the right frontal cortex. Specifically, the eustress group exhibited the largest brain activity, whereas the distress group exhibited recessive brain activity, regardless of positive or negative stimuli. LIS values were larger in the order of the eustress, control, and distress groups; this indicates that the stress group can be divided into eustress and distress groups. We built a foundation for subdividing stress groups into eustress and distress groups using fNIRS.

NMDA receptor-dependent plasticity in the nucleus accumbens connects reward-predictive cues to approach responses

Learning associations between environmental cues and rewards is a fundamental adaptive function. Via such learning, reward-predictive cues come to activate approach to locations where reward is available. The nucleus accumbens (NAc) is essential for cued approach behavior in trained subjects, and cue-evoked excitations in NAc neurons are critical for the expression of this behavior. Excitatory synapses within the NAc undergo synaptic plasticity that presumably contributes to cued approach acquisition, but a direct link between synaptic plasticity within the NAc and the development of cue-evoked neural activity during learning has not been established. Here we show that, with repeated cue-reward pairings, cue-evoked excitations in the NAc emerge and grow in the trials prior to the detectable expression of cued approach behavior. We demonstrate that the growth of these signals requires NMDA receptor-dependent plasticity within the NAc, revealing a neural mechanism by which the NAc participates in learning of conditioned reward-seeking behaviors.

Conditioned stimuli elicit phasic changes in nucleus accumbens (NAc) firing that invigorate approach responses to predicted rewards. Here the authors show that NAc neurons acquire cue-evoked responses during learning as a result of excitatory plasticity within the NAc.

Central role for the insular cortex in mediating conditioned responses to anticipatory cues

Reward-related circuits are fundamental for initiating feeding on the basis of food-predicting cues, whereas gustatory circuits are believed to be involved in the evaluation of food during consumption. However, accumulating evidence challenges such a rigid separation. The insular cortex (IC), an area largely studied in rodents for its role in taste processing, is involved in representing anticipatory cues. Although IC responses to anticipatory cues are well established, the role of IC cue-related activity in mediating feeding behaviors is poorly understood. Here, we examined the involvement of the IC in the expression of cue-triggered food approach in mice trained with a Pavlovian conditioning paradigm. We observed a significant change in neuronal firing during presentation of the cue. Pharmacological silencing of the IC inhibited food port approach. Such a behavior could be recapitulated by temporally selective inactivation during the cue. These findings represent the first evidence, to our knowledge, that cue-evoked neuronal activity in the mouse IC modulates behavioral output, and demonstrate a causal link between cue responses and feeding behaviors.

Differential contributions of infralimbic prefrontal cortex and nucleus accumbens during reward-based learning and extinction

Using environmental cues for the prediction of future events is essential for survival. Such cue-outcome associations are thought to depend on mesolimbic circuitry involving the nucleus accumbens (NAc) and prefrontal cortex (PFC). Several studies have identified roles for both NAc and PFC in the expression of stable goal-directed behaviors, but much remains unknown about their roles during learning of such behaviors. To further address this question, we used in vivo oxygen amperometry, a proxy for blood oxygen level-dependent (BOLD) signal measurement in human functional magnetic resonance imaging, in rats performing a cued lever-pressing task requiring discrimination between a rewarded and nonrewarded cue. Simultaneous oxygen recordings were obtained from infralimbic PFC (IFC) and NAc throughout both acquisition and extinction of this task. Activation of NAc was specifically observed following rewarded cue onset during the entire acquisition phase and also during the first days of extinction. In contrast, IFC activated only during the earliest periods of acquisition and extinction, more specifically to the nonrewarded cue. Thus, in vivo oxygen amperometry permits a novel, stable form of longitudinal analysis of brain activity in behaving animals, allowing dissociation of the roles of different brain regions over time during learning of reward-driven instrumental action. The present results offer a unique temporal perspective on how NAc may promote actions directed toward anticipated positive outcome throughout learning, while IFC might suppress actions that no longer result in reward, but only during critical periods of learning.

Inactivation of the basolateral amygdala during opiate reward learning disinhibits prelimbic cortical neurons and modulates associative memory extinction

RATIONALE

Neurons within the basolateral amygdala (BLA) and prelimbic cortex (PLC) are involved in associative learning during morphine reward memory recall and extinction. However, the nature by which the BLA regulates PLC neuronal encoding of associative opiate reward learning is not presently understood.

OBJECTIVE

The purpose of this study was to examine the functional effects of reversible inactivation of the BLA on behavioral and neuronal activity patterns in the PLC during either the acquisition or extinction phases of opiate reward memory processing.

METHODS

Using a combination of in vivo neuronal population recordings in the rat PLC and pharmacological inactivation of the BLA during a place conditioning procedure, we examined the functional impact of BLA inactivation during the acquisition, recall, and extinction of opiate reward memory.

RESULTS

Inactivation of the BLA caused an increase in the spontaneous firing and bursting activity of PLC neurons. Inactivation of the BLA during the acquisition phase of opiate reward conditioning caused a subsequent acceleration in the extinction of the previously learned opiate reward memory and behavioral aversions to morphine-paired environments. While BLA inactivation during extinction training led to a delay in extinction memory recall.

CONCLUSIONS

Our findings demonstrate a functional link between the BLA and neuronal populations in the PLC specifically during the acquisition and extinction phases of opiate reward memory and suggest that BLA input to the PLC modulates the processing of opiate-related extinction memory.

Auditory cortex stimulation to suppress tinnitus: mechanisms and strategies

Brain stimulation is an important method used to modulate neural activity and suppress tinnitus. Several auditory and non-auditory brain regions have been targeted for stimulation. This paper reviews recent progress on auditory cortex (AC) stimulation to suppress tinnitus and its underlying neural mechanisms and stimulation strategies. At the same time, the author provides his opinions and hypotheses on both animal and human models. The author also proposes a medial geniculate body (MGB)-thalamic reticular nucleus (TRN)-Gating mechanism to reflect tinnitus-related neural information coming from upstream and downstream projection structures. The upstream structures include the lower auditory brainstem and midbrain structures. The downstream structures include the AC and certain limbic centers. Both upstream and downstream information is involved in a dynamic gating mechanism in the MGB together with the TRN. When abnormal gating occurs at the thalamic level, the spilled-out information interacts with the AC to generate tinnitus. The tinnitus signals at the MGB-TRN-Gating may be modulated by different forms of stimulations including brain stimulation. Each stimulation acts as a gain modulator to control the level of tinnitus signals at the MGB-TRN-Gate. This hypothesis may explain why different types of stimulation can induce tinnitus suppression. Depending on the tinnitus etiology, MGB-TRN-Gating may be different in levels and dynamics, which cause variability in tinnitus suppression induced by different gain controllers. This may explain why the induced suppression of tinnitus by one type of stimulation varies across individual patients.

Selective effects of perinatal ethanol exposure in medial prefrontal cortex and nucleus accumbens

Ethanol exposure during development is the leading known cause of mental retardation and can result in characteristic physiological and cognitive deficits, often termed Fetal Alcohol Spectrum Disorders (FASD). Previous behavioral findings using rat models of FASD have suggested that there are changes in the nucleus accumbens (NAC) and medial prefrontal cortex (mPFC) following ethanol exposure during development. This study used a rat model of FASD to evaluate dendritic morphology in both the NAC and mPFC and cell number in the NAC. Dendritic morphology in mPFC and NAC was assessed using a modified Golgi stain and analyzed via three dimensional reconstructions with Neurolucida (MBF Bioscience). Cell counts in the NAC (shell and core) were determined using an unbiased stereology procedure (Stereo Investigator (MBF Bioscience)). Perinatal ethanol exposure did not affect neuronal or glial cell population numbers in the NAC. Ethanol exposure produced a sexually dimorphic effect on dendritic branching at one point along the NAC dendrites but was without effect on all other measures of dendritic morphology in the NAC. In contrast, spine density was reduced and distribution was significantly altered in layer II/III neurons of the mPFC following ethanol exposure. Ethanol exposure during development was also associated with an increase in soma size in the mPFC. These findings suggest that previously observed sexually dimorphic changes in activation of the NAC in a rat model of FASD may be due to altered input from the mPFC.

Prefrontal and monoaminergic contributions to stop-signal task performance in rats

Defining the neural and neurochemical substrates of response inhibition is of crucial importance for the study and treatment of pathologies characterized by impulsivity such as attention-deficit/hyperactivity disorder and addiction. The stop-signal task (SST) is one of the most popular paradigms used to study the speed and efficacy of inhibitory processes in humans and other animals. Here we investigated the effect of temporarily inactivating different prefrontal subregions in the rat by means of muscimol microinfusions on SST performance. We found that dorsomedial prefrontal cortical areas are important for inhibiting an already initiated response. We also investigated the possible neural substrates of the selective noradrenaline reuptake inhibitor atomoxetine via its local microinfusion into different subregions of the rat prefrontal cortex. Our results show that both orbitofrontal and dorsal prelimbic cortices mediate the beneficial effects of atomoxetine on SST performance. To assess the neurochemical specificity of these effects, we infused the α2-adrenergic agonist guanfacine and the D(1)/D(2) antagonist α-flupenthixol in dorsal prelimbic cortex to interfere with noradrenergic and dopaminergic neurotransmission, respectively. Guanfacine, which modulates noradrenergic neurotransmission, selectively impaired stopping, whereas blocking dopaminergic receptors by α-flupenthixol infusion prolonged go reaction time only, confirming the important role of noradrenergic neurotransmission in response inhibition. These results show that, similar to humans, distinct networks play important roles during SST performance in the rat and that they are differentially modulated by noradrenergic and dopaminergic neurotransmission. This study advances our understanding of the neuroanatomical and neurochemical determinants of impulsivity, which are relevant for a range of psychiatric disorders.

Acquisition of glucose-conditioned flavor preference requires the activation of dopamine D1-like receptors within the medial prefrontal cortex in rats

In this study, we investigated the role of dopamine transmission within the medial prefrontal cortex (mPFC) in flavor preference learning induced by post-oral glucose. In Experiment 1, rats were trained with a flavor (CS+) paired with intragastric (IG) infusions of 8% glucose and a different flavor (CS-) paired with IG water infusions. The CS+ preference was evaluated in two-bottle tests following bilateral injection of the dopamine D1-like receptor antagonist, SCH23390, into the mPFC at total doses of 0, 12 and 24nmol. SCH23390 produced dose-dependent reductions in CS+ intake but did not block the CS+ preference. In Experiment 2, new rats were injected daily in the mPFC with either saline or SCH23390 (12nmol), prior to training sessions with CS+/IG glucose and CS-/IG water. In the two-bottle choice tests, SCH rats, unlike the Control rats, failed to prefer the CS+ (50% vs. 74%). Collectively, the results show that D1-like receptor activation in the medial prefrontal cortex plays a crucial role in the acquisition of flavor preference learning induced by the post-oral reinforcing properties of glucose.

Activity-dependent depression of medial prefrontal cortex inputs to accumbens neurons by the basolateral amygdala

The encoding of reward-predictive stimuli by neurons in the nucleus accumbens (NAcc) depends on integrated synaptic activity from the basolateral amygdala (BLA) and medial prefrontal cortex (mPFC) afferent inputs. In a previous study, we found that single electrical stimulation pulses applied to the BLA facilitate mPFC-evoked spiking in NAcc neurons in a timing-dependent manner, presumably by a fast glutamatergic mechanism. In the present study, the ability of repetitive BLA activation to modulate synaptic inputs to NAcc neurons through dopamine- or N-methyl-D-aspartate (NMDA)-dependent mechanisms is characterized. NAcc neurons receiving excitatory input from both mPFC and BLA were recorded in urethane-anesthetized rats. Train stimulation of the BLA depressed mPFC-evoked spiking in these neurons. This was not attributable to mechanisms involving NMDA or dopamine D1, D2, D3 or D5 receptors, since blockade of these receptors did not affect the BLA-mediated depression. BLA-mediated depression was only evident when the BLA stimulation evoked spikes in the recorded neuron; thus, depolarization of the recorded neuron may be critical for this effect. The ability of the BLA to suppress mPFC-to-NAcc signaling may be a mechanism by which normal or pathologically heightened emotional states disrupt goal-directed behavior in favor of emotionally-driven responses.

Basolateral amygdala neurons facilitate reward-seeking behavior by exciting nucleus accumbens neurons

Both the nucleus accumbens (NAc) and basolateral amygdala (BLA) contribute to learned behavioral choice. Neurons in both structures that encode reward-predictive cues may underlie the decision to respond to such cues, but the neural circuits by which the BLA influences reward-seeking behavior have not been established. Here, we test the hypothesis that the BLA drives NAc neuronal responses to reward-predictive cues. First, using a disconnection experiment, we show that the BLA and dopamine projections to the NAc interact to promote the reward-seeking behavioral response. Next, we demonstrate that BLA neuronal responses to cues precede those of NAc neurons and that cue-evoked excitation of NAc neurons depends on BLA input. These results indicate that BLA input is required for dopamine to enhance the cue-evoked firing of NAc neurons and that this enhanced firing promotes reward-seeking behavior.

Contributions of the amygdala and medial prefrontal cortex to incentive cue responding

Reward-seeking behavior is controlled by neuronal circuits that include the basolateral nucleus of amygdala (BLA), medial prefrontal cortex (mPFC), nucleus accumbens (NAc) and ventral tegmental area. Using a discriminative stimulus (DS) task in which an intermittently presented cue (DS) directs rats to make an operant response for sucrose, we previously demonstrated that dopamine receptor antagonism in the NAc reduced reinforced cue responding, whereas general inactivation of the NAc increased behavioral responding in the absence of the cue. Because they send major glutamatergic projections to the NAc, the BLA and mPFC may also contribute to reward-seeking behaviors modulated by the NAc. In this study we compare the effects of BLA and mPFC inactivation on rats' performance of a DS task. BLA inactivation by combined GABA(A) and GABA(B) agonists impaired cue responding with minimal effects on operant behavior in the absence of cues. Dorsal medial prefrontal cortex (dmPFC) inactivation also inhibited cue-evoked reward-seeking. In contrast, ventral medial prefrontal cortex (vmPFC) inactivation disinhibited responding to unrewarded cues with less influence on reinforced cue responding. These findings demonstrate that the BLA and dmPFC facilitate cue-evoked reward-seeking, whereas, in the same task the vmPFC exerts inhibitory control over unrewarded behaviors.

Dorsomedial prefrontal cortex contribution to behavioral and nucleus accumbens neuronal responses to incentive cues

Cue-elicited phasic changes in firing of nucleus accumbens (NAc) neurons can facilitate reward-seeking behavior. Here, we test the hypothesis that the medial prefrontal cortex (mPFC), which sends a dense glutamatergic projection to the NAc core, contributes to NAc neuronal firing responses to reward-predictive cues. Rats trained to perform an operant response to a cue for sucrose were implanted with recording electrodes in the core of the NAc and microinjection cannulas in the dorsal mPFC (dmPFC). The cue-evoked firing of NAc neurons was reduced by bilateral injection of GABA(A) and GABA(B) agonists into the dmPFC concomitant with loss of behavioral responding to the cue. In addition, unilateral dmPFC inactivation reduced ipsilateral cue excitations and contralateral cue inhibitions. These findings indicate that cue-evoked excitations and inhibitions of NAc core neurons depend on dmPFC projections to the NAc and that these phasic changes contribute to the behavioral response to reward-predictive cues.

Understanding decision-making deficits in neurological conditions: insights from models of natural action selection

Models of natural action selection implicate fronto-striatal circuits in both motor and cognitive 'actions'. Dysfunction of these circuits leads to decision-making deficits in various populations. We review how computational models provide insights into the mechanistic basis for these deficits in Parkinson's patients and those with ventromedial frontal damage. We then consider implications of the models for understanding behaviour and cognition in attention-deficit/hyperactivity disorder (ADHD). Incorporation of cortical noradrenaline function into the model improves action selection in noisy environments and accounts for response variability in ADHD. We close with more general clinical implications.

Non-sensory cortical and subcortical connections of the primary auditory cortex in Mongolian gerbils: bottom-up and top-down processing of neuronal information via field AI

In the present study, we will provide further anatomical evidence that the primary auditory cortex (field AI) is not only involved in sensory processing of its own modality, but also in complex bottom-up and top-down processing of multimodal information. We have recently shown that AI in the Mongolian gerbil (Meriones unguiculatus) has substantial connections with non-auditory sensory and multisensory brain structures [Budinger, E., Heil, P., Hess, A., Scheich, H., 2006. Multisensory processing via early cortical stages: Connections of the primary auditory cortical field with other sensory systems. Neuroscience 143, 1065-1083]. Here we will report about the direct connections of AI with non-sensory cortical areas and subcortical structures. We approached this issue by means of the axonal transport of the sensitive bidirectional neuronal tracers fluorescein-labelled (FD) and tetramethylrhodamine-labelled dextran (TMRD), which were simultaneously injected into different frequency regions of the gerbil's AI. Of the total number of retrogradely labelled cell bodies found in non-sensory brain areas, which identify cells of origin of direct projections to AI, approximately 24% were in cortical areas and 76% in subcortical structures. Of the cell bodies in the cortical areas, about 4.4% were located in the orbital, 11.1% in the infralimbic medial prefrontal (areas DPC, IL), 18.2% in the cingulate (3.2% in CG1, 2.9% in CG2, 12.1% in CG3), 9.5% in the frontal association (area Fr2), 12.0% in the insular (areas AI, DI), 10.8% in the retrosplenial, and 34.0% in the perirhinal cortex. The cortical regions with retrogradely labelled cells, as well as the entorhinal cortex, also contained anterogradely labelled axons and their terminations, which means that they are also target areas of direct projections from AI. The laminar pattern of corticocortical connections indicates that AI receives primarily cortical feedback-type inputs and projects in a feedforward manner to its target areas. The high number of double-labelled somata, the non-topographic distribution of single FD- and TMRD-labelled somata, and the overlapping spatial distribution of FD- and TMRD-labelled axonal elements suggest rather non-tonotopic connections between AI and the multimodal cortices. Of the labelled cell bodies in the subcortical structures, about 38.8% were located in the ipsilateral basal forebrain (10.6% in the lateral amygdala LA, 11.5% in the globus pallidus GP, 3.7% in the ventral pallidum VPa, 13.0% in the nucleus basalis NB), 13.1% in the ipsi- and contralateral diencephalon (6.4% in the posterior paraventricular thalamic nuclei, 6.7% in the hypothalamic area), and 48.1% in the midbrain (20.0% in the ipsilateral substantia nigra, 9.8% in the ipsi- and contralateral ventral tegmental area, 5.0% in the ipsi- and contralateral locus coeruleus, 13.3% the ipsi- and contralateral dorsal raphe nuclei). Thus, the majority of subcortical inputs to AI was related to different neurotransmitter systems. Anterograde labelling was only found in some ipsilateral basal forebrain structures, namely, the LA, basolateral amygdala, GP, VPa, and NB. As for the cortex, the proportion and spatial distribution of single FD-, TMRD-, and double-labelled neuronal elements suggests rather non-tonotopic connections between AI and the neuromodulatory subcortical structures.

Differential effects of the D- and L- isomers of amphetamine on pharmacological MRI BOLD contrast in the rat

RATIONALE

The D - and L-amphetamine sulphate isomers are used in the formulation of Adderall XR(R), which is effective in the treatment of attention-deficit hyperactivity disorder (ADHD). The effects of these isomers on brain activity has not been examined using neuroimaging.

OBJECTIVES

This study determines the pharmacological magnetic resonance imaging blood-oxygenation-level-dependent (BOLD) response in rat brain regions after administration of each isomer.

MATERIALS AND METHODS

Rats were individually placed into a 2.35 T Bruker magnet for 60 min to achieve basal recording of variation in signal intensity. Either saline (n = 9), D-amphetamine sulphate (2 mg/kg, i.p.; n = 9) or L: -amphetamine sulphate (4 mg/kg, i.p.; n = 9) were administered, and recording continued for a further 90 min. Data were analysed for BOLD effects using statistical parametric maps. Blood pressure, blood gases and respiratory rate were monitored during scanning.

RESULTS

The isomers show overlapping effects on the BOLD responses in areas including nucleus accumbens, medial entorhinal cortex, colliculi, field CA1 of hippocampus and thalamic nuclei. The L-isomer produced greater global changes in the positive BOLD response than the D-isomer, including the somatosensory and motor cortices and frontal brain regions such as the orbitofrontal cortices, prelimbic and infralimbic cortex which were not observed with the D-isomer.

CONCLUSIONS

The amphetamine isomers produce different BOLD responses in brain areas related to cognition, pleasure, pain processing and motor control probably because of variations on brain amine systems such as dopamine and noradrenaline. The isomers may, therefore, have distinct actions on brain regions affected in ADHD patients.

Guanfacine produces differential effects in frontal cortex compared with striatum: assessed by phMRI BOLD contrast

RATIONALE

Guanfacine (an alpha-(2A) adrenoreceptor agonist) is a drug of benefit in the treatment of attention deficit hyperactivity disorder (ADHD) (Taylor FB, Russo J, J Clin Psychopharmacol 21:223-228, 2001). Assessment of this drug using neuroimaging will provide information about the brain regions involved in its effects.

OBJECTIVES

The pharmacological magnetic resonance imaging blood oxygenation level dependent (BOLD) response was determined in rat brain regions following administration of guanfacine.

METHODS

Male rats were individually placed into a 2.35 T Bruker magnet for 60 min to achieve basal recording of changes in signal intensity. Either saline (n = 9) or guanfacine (0.3 mg/kg, i.p.; n = 9) was then administered and recording was continued for a further 90 min. Data were analysed for BOLD effects using statistical parametric maps. Respiration rate, blood pressure and blood gases were monitored and remained constant throughout scanning.

RESULTS

The main changes observed were negative BOLD effects in the caudate putamen and nucleus accumbens with positive BOLD effects in frontal association, prelimbic and motor cortex areas.

CONCLUSIONS

These data suggest that guanfacine can decrease neuronal activity in the caudate while increasing frontal cortex activity. This ability to change neuronal activity in specific areas of rat brain that are known to be impaired in ADHD (Solanto MV, Behav Brain Res 130:65-71, 2002) may contribute to guanfacine's beneficial effects.

Tripartite relationship among P300, clinical features and brain structure in neuroleptic-naive schizophrenia

Auditory P300 abnormalities in schizophrenia patients have been repeatedly reported by many studies. However, reported relationships among P300 abnormalities, clinical features and other biological variables, such as abnormalities in structural brain imaging, are notably discrepant. This is partially due to the inclusion of patients who have had long-term administration of neuroleptics and those from whom this treatment has been withdrawn. The present study measures event-related potentials in 13 neuroleptic-naive schizophrenia patients using an auditory oddball paradigm to clarify the relationships among P300 amplitude, clinical features and brain structure. All patients underwent computed tomography to estimate the area of the right and left frontal cortical sulci and Sylvian fissures. Clinical symptoms were assessed using the Positive And Negative Syndrome Scale. The high correlation coefficients were obtained between P300 amplitude and the anxiety/depression factor score (r = -0.77), the positive factor score (r = -0.58) and between P300 amplitude and the area ratios of the fronto-temporal region (r = -0.66). These findings show that fronto-temporal region and P300 amplitude are closely related to the earliest stage of illness even in neuroleptic-naive patients.

Tetrodotoxin-dependent glutamate release in the rat nucleus accumbens during concurrent presentation of appetitive and conditioned aversive stimuli

In vivo microdialysis combined with a high-performance liquid chromatography was used to monitor extracellular glutamate (GLU) levels in the nucleus accumbens (N.Acc) of Sprague-Dawley rats during their behavioral responses to the concurrent presentation of appetitive and conditioned aversive stimuli. The presentation of a highly palatable diet followed by a tone previously paired with footshock to rats trained to take a pellet of the diet under these experimental conditions resulted in a marked and short lasting increase in extracellular glutamate, whereas the tone alone had no effect. A similar increase of the glutamate release was observed during the presentation of a piece of rubber instead of the diet. In both cases, the increase in extracellular glutamate was completely prevented by intra-accumbal infusions through the dialysis probe of 1 microM tetrodotoxin (a voltage-dependent Na(+) channel blocker), whereas (S)-4-carboxyphenylglycine (a cystine/glutamate exchange blocker, 5 microM) had no effect. The data obtained suggest that behavioral responses to unpredicted change in motivational value of expected reward appear to be associated with an increase of the extracellular glutamate level in the nucleus accumbens, and impulse-dependent synaptic release, rather than non-vesicular glutamate release via cystine/glutamate exchange, is responsible for this phenomenon.

Electrical stimulation of medial prefrontal cortex reduces conditioned fear in a temporally specific manner

The authors recently showed that extinction of auditory fear conditioning leads to potentiation of tone-evoked activity of neurons in the infralimbic (IL) subregion of the medial prefrontal cortex, suggesting that IL inhibits fear after extinction (M. R. Milad, & G. J. Quirk, 2002). In support of this finding, pairing conditioned tones with brief (300-ms) electrical stimulation of IL reduces conditioned freezing. The present study showed that IL stimulation inhibits freezing if given 0.1 s after tone onset (the latency of tone-evoked responses) but has no effect if given either 1 s before or 1 s after tone onset. This suggests that IL gates the response of downstream structures such as the amygdala to fear stimuli.

Anticipatory cues differentially provoke in vivo peptidergic and monoaminergic release at the medial prefrontal cortex

Like primary reinforcers, the anticipation of reward ought to affect neurochemical release in brain regions, such as the medial prefrontal cortex (mPFC), which are associated with appraisal processes. To assess the neurochemical changes associated with anticipation, rats were exposed to the pairing of auditory (60-dB white noise), visual, and olfactory cues with the daily presentation of a palatable snack (Cue Relevant group). Rats of a second group were similarly trained, but for a 2-week period, the snack was no longer provided following cue presentation (Extinction group). In the third condition, the presentation of the snack and cues was uncorrelated (Cue Irrelevant group). Analyses of dialysates collected in vivo from the mPFC revealed that release of corticotropin-releasing hormone (CRH), gastrin-releasing peptide (GRP), and the 5-HT catabolite, 5-hydroxyindole acetic acid (5-HIAA), had increased bilaterally in response to the anticipatory cues, whereas DA release increased only within the right mPFC. In the case of CRH and GRP, these increases were also apparent in the extinction condition, despite the fact that behavioral arousal to the anticipatory cues (increased exploration, rearing, grooming, and vigilance) was only evident in the Cue Relevant condition. In contrast, the elevated DA and 5-HIAA were apparent exclusively in the Cue Relevant condition. Thus, CRH and GRP systems may serve to allocate salience and/or incentive reward value to biologically significant stimuli or reflect the emotional response to the anticipatory stimulus. The activity of DA and 5-HT neurons, in contrast, is more closely aligned with the cognitive appraisal of predictor stimuli.

Noradrenergic action in prefrontal cortex in the late stage of memory consolidation

These experiments investigated the role of the noradrenergic system in the late stage of memory consolidation and in particular its action at beta receptors in the prelimbic region (PL) of the prefrontal cortex in the hours after training. Rats were trained in a rapidly acquired, appetitively motivated foraging task based on olfactory discrimination. They were injected with a beta adrenergic receptor antagonist into the PL 5 min or 2 h after training and tested 48 h later. Rats injected at 2 h showed amnesia, whereas those injected at 5 min had good retention, equivalent to saline-injected controls. Monitoring extracellular noradrenaline efflux in PL by in vivo microdialysis during the first hours after training revealed a significant increase shortly after training, with a rapid return to baseline, and then another increase around the 2-h posttraining time window. Pseudo-trained rats showed a smaller early efflux and did not show the second wave of efflux at 2 h. These results confirm earlier pharmacological and immunohistochemical studies suggesting a delayed role of noradrenaline in a late phase of long-term memory consolidation and the engagement of the PL during these consolidation processes.

Medial agranular cortex activity related to event-related potential generation in the rat

Neuronal firings and local field potentials were recorded in the medial agranular cortex (Fr2) together with surface event-related potentials (ERPs) during an auditory oddball task in the rat. 10/32 Fr2 neurons showed a sustained increase in firing frequency in response to target tone (100-615 ms from the tone onset) with an activation peak corresponding to surface P100 component. At the intra-cortical recording site, local P100 component was observed with greater peak amplitude than that on the surface. No conspicuous neuronal response was evoked by non-target tone. The results suggest that the rat Fr2 is involved in stimulus discrimination and generation of ERPs during the oddball task.