Firing characteristics of deep layer neurons in prefrontal cortex in rats performing spatial working memory tasks

Involvement of ventral pallidum in prefrontal cortex-dependent aspects of spatial working memory

Ventral pallidum (VP) is an important source of limbic input to medial thalamus. Three studies examined the role of VP in spatial memory tasks impaired by medial thalamic lesions. In the 1st study, rats with VP lesions were impaired performing delayed matching trained with retractable levers (DMRL), a measure sensitive to prefrontal (but not hippocampal) damage. The 2nd study demonstrated dose-dependent DMRL impairment following microinjection of gamma-aminobutyric acidA, glutamate, or mu-opioid agonists in VP. In the 3rd study, VP lesions had no effect on varying choice radial-maze delayed nonmatching, a measure sensitive to hippocampal (but not prefrontal) lesions. These results suggest a common role in spatial memory for VP and other components of prefrontal-ventral striatopallidothalamic circuits distinct from hippocampal function.

Role of active movement in place-specific firing of hippocampal neurons

The extent of external and internal factors contributing to location-specific firing of hippocampal place cells is currently unclear. We investigated the role of active movement in location-specific firing by comparing spatial firing patterns of hippocampal neurons, while rats either ran freely or rode a motorized cart on the same circular track. Most neurons changed their spatial firing patterns across the two navigation conditions ("remapping"), and they were stably maintained across repeated active or passive navigation sessions. These results show that active movement is a critical factor in determining place-specific firing of hippocampal neurons. This could explain why passive displacement is not an effective way of acquiring spatial knowledge for subsequent active navigation in an unfamiliar environment.

An integrate-and-fire model of prefrontal cortex neuronal activity during performance of goal-directed decision making

The orbital frontal cortex appears to be involved in learning the rules of goal-directed behavior necessary to perform the correct actions based on perception to accomplish different tasks. The activity of orbitofrontal neurons changes dependent upon the specific task or goal involved, but the functional role of this activity in performance of specific tasks has not been fully determined. Here we present a model of prefrontal cortex function using networks of integrate-and-fire neurons arranged in minicolumns. This network model forms associations between representations of sensory input and motor actions, and uses these associations to guide goal-directed behavior. The selection of goal-directed actions involves convergence of the spread of activity from the goal representation with the spread of activity from the current state. This spiking network model provides a biological implementation of the action selection process used in reinforcement learning theory. The spiking activity shows properties similar to recordings of orbitofrontal neurons during task performance.

Bursting of prefrontal cortex neurons in awake rats is regulated by metabotropic glutamate 5 (mGlu5) receptors: rate-dependent influence and interaction with NMDA receptors

Metabotropic glutamate 5 (mGlu5) receptors have been recently implicated in prefrontal cortex (PFC)-dependent executive functions because inhibition of mGlu5 receptors impairs working memory and worsens cognitive-impairing effects of NMDA receptor antagonists. To better understand the mechanisms by which mGlu5 receptors influence PFC function, we examined the effects of selective mGlu5 receptor antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP), given alone or in combination with the NMDA receptor antagonist MK801, on ensemble single unit activity in the medial PFC (mPFC) of behaving rats. MPEP decreased the spontaneous burst activity of the majority of mPFC neurons. This inhibition was selective for the most active cells because greater decreases were observed in neurons with higher baseline firing rates. MPEP augmented the effects of MK801 on burst activity, variability of spike firing and random spike activity. These findings demonstrate that in awake animals mGlu5 receptors regulate the function of PFC neurons by two related mechanisms: (i) rate-dependent excitatory influence on spontaneous burst activity; and (ii) potentiation of NMDA receptor mediated effects on firing rate and burst activity. These mechanisms support the idea that modulation of mGlu5 receptors may provide a pharmacological strategy for fine-tuning the temporal pattern of firing of PFC neurons.

Dopamine excites nucleus accumbens neurons through the differential modulation of glutamate and GABA release

Afferent activity into the nucleus accumbens (NAc) occurs in bursts of action potentials. However, it is unclear how synapses in this nucleus respond to such bursts, or how these responses are altered by dopamine (DA). I examined the effects of DA on excitatory and inhibitory responses to trains of stimuli in rat NAc slices. Both EPSCs and IPSCs showed use-dependent depression during trains. Although DA inhibited both glutamate and GABA release in the NAc, it differentially inhibited release during trains. The inhibition of IPSCs persisted throughout the train of stimuli, whereas the inhibition of EPSCs progressively diminished. This differential modulation may be explained by a calcium-dependent change in the recovery from depression at the GABA synapses, where DA acts by decreasing Ca2+ entry. Thus, at later stages of sustained stimulation, DA preferentially inhibits GABA release, producing a net excitatory effect during bursts suggesting a mechanism for enhancing the contrast between competing inputs into the NAc, as well as for affecting long-term plasticity in this structure.

Coding for spatial goals in the prelimbic/infralimbic area of the rat frontal cortex

Finding one's way in space requires a distributed neural network to support accurate spatial navigation. In the rat, this network likely includes the hippocampus and its place cells. Although such cells allow the organism to locate itself in the environment, an additional mechanism is required to specify the animal's goal. Here, we show that firing activity of neurons in medial prefrontal cortex (mPFC) reflects the motivational salience of places. We recorded mPFC neurons from rats performing a place navigation task, and found that a substantial proportion of cells in the prelimbic/infralimbic area had place fields. A much smaller proportion of cells with such properties was found in the dorsal anterior cingulate area. Furthermore, the distribution of place fields in prelimbic/infralimbic cells was not homogeneous: goal locations were overrepresented. Because such locations were spatially dissociated from rewards, we suggest that mPFC neurons might be responsible for encoding the rat's goals, a process necessary for path planning.

Activation of metabotropic glutamate 2/3 receptors reverses the effects of NMDA receptor hypofunction on prefrontal cortex unit activity in awake rats

Systemic exposure to N-methyl-d-aspartate (NMDA) receptor antagonists can lead to psychosis and prefrontal cortex (PFC)-dependent behavioral impairments. Agonists of metabotropic glutamate 2/3 (mGlu2/3) receptors ameliorate the adverse behavioral effects of NMDA antagonists in humans and laboratory animals, and are being considered as a novel treatment for some symptoms of schizophrenia. Despite the compelling behavioral data, the cellular mechanisms by which potentiation of mGlu2/3 receptor function attenuates the effects of NMDA receptor hypofunction remain unclear. In freely moving rats, we recorded the response of medial PFC (prelimbic) single units to treatment with the NMDA antagonist MK801 and assessed the dose-dependent effects of pre- or posttreatment with the mGlu2/3 receptor agonist LY354740 on this response. NMDA receptor antagonist-induced behavioral stereotypy was measured during recording because it may relate to the psychotomimetic properties of this treatment and is dependent on the functional integrity of the PFC. In most PFC neurons, systemic administration of MK801 increased the spontaneous firing rate, decreased the variability of spike trains, and disrupted patterns of spontaneous bursts. Given alone, LY354740 (1, 3, and 10 mg/kg) decreased spontaneous activity of PFC neurons at the highest dose. Pre- or posttreatment with LY354740 blocked MK801-induced changes on firing rate, burst activity, and variability of spike activity. These physiological changes coincided with a reduction in MK801-induced behavioral stereotypy by LY354740. These data indicate that activation of mGlu2/3 receptors reduces the disruptive effects of NMDA receptor hypofunction on the spontaneous spike activity and bursting of PFC neurons. This mechanism may provide a physiological basis for reversal of NMDA antagonist-induced behaviors by mGlu2/3 agonists.

Parallel processing across neural systems: Implications for a multiple memory system hypothesis

A common conceptualization of the organization of memory systems in brain is that different types of memory are mediated by distinct neural systems. Strong support for this view comes from studies that show double (or triple) dissociations between spatial, response, and emotional memories following selective lesions of hippocampus, striatum, and the amygdala. Here, we examine the extent to which hippocampal and striatal neural activity patterns support the multiple memory systems view. A comparison is made between hippocampal and striatal neural correlates with behavior during asymptotic performance of spatial and response maze tasks. Location- (or place), movement, and reward-specific firing patterns were found in both structures regardless of the task demands. Many, but not all, place fields of hippocampal and striatal neurons were similarly affected by changes in the visual and reward context regardless of the cognitive demands. Also, many, but not all, hippocampal and striatal movement-sensitive neurons showed significant changes in their behavioral correlates after a change in visual context, irrespective of cognitive strategy. Similar partial reorganization was observed following manipulations of the reward condition for cells recorded from both structures, again regardless of task. Assuming that representations that persist across context changes reflect learned information, we make the following conclusions. First, the consistent pattern of partial reorganization supports a view that the analysis of spatial, response, and reinforcement information is accomplished via an error-driven, or match-mismatch, algorithm across neural systems. Second, task-relevant processing occurs continuously within hippocampus and striatum regardless of the cognitive demands of the task. Third, given the high degree of parallel processing across allegedly different memory systems, we propose that different neural systems may effectively compete for control of a behavioral expression system. The strength of the influence of any one neural system on behavioral output is likely modulated by factors such as motivation, experience, or hormone status.

Medial prefrontal cortex is involved in spatial temporal order memory but not spatial recognition memory in tests relying on spontaneous exploration in rats

The present study describes two novel tasks relying on spontaneous patterns of exploration in a radial-arm maze that can be used to assess spatial recognition memory and spatial temporal order memory (i.e. memory for the order in which places have been visited) in the rat. In the recognition memory task, rats were permitted to freely explore two arms in the maze on a first trial and one 'familiar' arm and one novelly located arm on a second trial 105 min later. In the temporal order memory task, rats were permitted to explore two arms in the maze on a first trial, two novel arms on a second trial 60 min later, and one 'older familiar' arm and one 'more recent familiar' arm on a third trial 45 min later. Using these tasks, we found that rats direct greater exploration at a novel than a familiar arm location, thus showing long-term spatial recognition memory, and at an older familiar arm than a more recent familiar arm, thus showing long-term spatial temporal order memory. Lidocaine inactivation of the mPFC prior to the final trial in each task disrupted performance on the temporal order but not the recognition memory task, thereby demonstrating a role for the mPFC in the retrieval and/or use of temporal order information but not in spatial memory per se. These findings highlight the specific involvement of the rat mPFC in temporal order memory and have important implications for a broader understanding of mPFC function.

Prefrontal cortex function, quasi-physiological stimuli, and synaptic plasticity

The prelimbic area of rat medial frontal cortex may be functionally analogous to human/primate dorsolateral prefrontal cortex. This area may be involved in selective attention to the external stimuli and the coupling of the attention to a repertory of actions. It was suggested that this function may rely on a form of long-term memory [Biol. Rev. 77 (2002) 563]. Indeed, during learning of this type of behavior, a portion of prelimbic neurons persistently change their firing characteristics [Prog. Brain Res. 126 (2000) 287]. It is therefore important to study long-term potentiation (LTP) and depression (LTD) in rat prelimbic neurons. In this article, the author first briefly reviews recent findings on the prefrontal cortex function and discusses that the prefrontal cortex may be involved in long-term memory. Second, the author will show some new results which indicate that quasi-physiological patterns of stimuli mimicking prelimbic neuronal activity during behavior can induce LTP in prelimbic pyramidal neuron synapses. These results suggest that prelimbic neuronal activity during behavior may lastingly modify prelimbic synaptic efficacy.

NMDA receptor hypofunction produces concomitant firing rate potentiation and burst activity reduction in the prefrontal cortex

Cognitive deficits associated with frontal lobe dysfunction are a determinant of long-term disability in schizophrenia and are not effectively treated with available medications. Clinical studies show that many aspects of these deficits are transiently induced in healthy individuals treated with N-methyl-D-aspartate (NMDA) antagonists. These findings and recent genetic linkage studies strongly implicate NMDA receptor deficiency in schizophrenia and suggest that reversing this deficiency is pertinent to treating the cognitive symptoms of schizophrenia. Despite the wealth of behavioral data on the effects of NMDA antagonist treatment in humans and laboratory animals, there is a fundamental lack of understanding about the mechanisms by which a general state of NMDA deficiency influences the function of cortical neurons. Using ensemble recording in freely moving rats, we found that NMDA antagonist treatment, at doses that impaired working memory, potentiated the firing rate of most prefrontal cortex neurons. This potentiation, which correlated with expression of behavioral stereotypy, resulted from an increased number of irregularly discharged single spikes. Concurrent with the increase in spike activity, there was a significant reduction in organized bursting activity. These results identify two distinct mechanisms by which NMDA receptor deficiency may disrupt frontal lobe function: an increase in disorganized spike activity, which may enhance cortical noise and transmission of disinformation; and a decrease in burst activity, which reduces transmission efficacy of cortical neurons. These findings provide a physiological basis for the NMDA receptor deficiency model of schizophrenia and may clarify the nature of cortical dysfunction in this disease.

Regulation of persistent activity by background inhibition in an in vitro model of a cortical microcircuit

We combined in vitro intracellular recording from prefrontal cortical neurons with simulated synaptic activity of a layer 5 prefrontal microcircuit using a dynamic clamp. During simulated in vivo background conditions, the cell responded to a brief depolarization with a sequence of spikes that outlasted the depolarization, mimicking the activity of a cell recorded during the delay period of a working memory task in the behaving monkey. The onset of sustained activity depended on the number of action potentials elicited by the cue-like depolarization. Too few spikes failed to provide enough NMDA drive to elicit sustained reverberations; too many spikes activated a slow intrinsic hyperpolarization current that prevented spiking; an intermediate number of spikes produced sustained activity. When high dopamine levels were simulated by depolarizing the cell and by increasing the amount of NMDA current, the cell exhibited spontaneous 'up-states' that terminated by the activation of a slow intrinsic hyperpolarizing current. The firing rate during the delay period could be effectively modulated by the standard deviation of the inhibitory background synaptic noise without significant changes in the background firing rate before cue onset. These results suggest that the balance between fast feedback inhibition and slower AMPA and NMDA feedback excitation is critical in initiating persistent activity and that the maintenance of persistent activity may be regulated by the amount of correlated background inhibition.

Dynamics of population code for working memory in the prefrontal cortex

Some neurons (delay cells) in the prefrontal cortex elevate their activities throughout the time period during which the animal is required to remember past events and prepare future behavior, suggesting that working memory is mediated by continuous neural activity. It is unknown, however, how working memory is represented within a population of prefrontal cortical neurons. We recorded from neuronal ensembles in the prefrontal cortex as rats learned a new delayed alternation task. Ensemble activities changed in parallel with behavioral learning so that they increasingly allowed correct decoding of previous and future goal choices. In well-trained rats, considerable decoding was possible based on only a few neurons and after removing continuously active delay cells. These results show that neural activity in the prefrontal cortex changes dynamically during new task learning so that working memory is robustly represented and that working memory can be mediated by sequential activation of different neural populations.

The NMDA-to-AMPA ratio at synapses onto layer 2/3 pyramidal neurons is conserved across prefrontal and visual cortices

To better understand regulation of N-methyl-d-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor complements across the cortex, and to investigate NMDA receptor (NMDAR)-based models of persistent activity, we compared NMDA/AMPA ratios in prefrontal (PFC) and visual cortex (VC) in rat. Whole cell voltage-clamp responses were recorded in brain slices from layer 2/3 pyramidal cells of the medial PFC and VC of rats aged p16-p21. Mixed miniature excitatory postsynaptic currents (mEPSCs) having AMPA receptor (AMPAR)- and NMDAR-mediated components were isolated in nominally 0 Mg2+ ACSF. Averaged mEPSCs were well-fit by double exponentials. No significant differences in the NMDA/AMPA ratio (PFC: 27 +/- 1%; VC: 28 +/- 3%), peak mEPSC amplitude (PFC: 19.1 +/- 1 pA; VC: 17.5 +/- 0.7 pA), NMDAR decay kinetics (PFC: 69 +/- 8 ms; VC: 67 +/- 6 ms), or degree of correlation between NMDAR- and AMPAR-mediated mEPSC components were found between the areas (PFC: n = 27; VC: n = 28). Recordings from older rats (p26-29) also showed no differences. EPSCs were evoked extracellularly in 2 mM Mg2+ at depolarized potentials; although the average NMDA/AMPA ratio was larger than that observed for mEPSCs, the ratio was similar in the two regions. In nominally 0 Mg2+ and in the presence of CNQX, spontaneous activation of NMDAR increased recording noise and produced a small tonic depolarization which was similar in both areas. We conclude that this basic property of excitatory transmission is conserved across PFC and VC synapses and is therefore unlikely to contribute to differences in firing patterns observed in vivo in the two regions.

Potentiation of prelimbic field potentials during and seconds after trains of excitations in the rat hippocampo-prefrontal pathway

Field potentials were recorded in the prelimbic cortex of anaesthetized rats after excitations of the hippocampo-prefrontal pathway. Stimuli were delivered to the hippocampal CA1 region and short-term changes of field potential amplitudes were observed in two situations. (1) Amplitudes were monitored during trains of stimulations given at frequencies between 1 and 20 Hz. Within trains, potentiation was followed by depression. Both types of changes were frequency dependent. (2) The time course of recovery from within-train plasticity was obtained from field potentials evoked at varying intervals after trains. This revealed a post-train potentiation having a maximum after 2-4 s and lasting for approximately 10 s. The maximal post-train potentiation was nearly independent of the frequency of the preceding train.

Infralimbic D2 receptor influences on anxiety-like behavior and active memory/attention in CD-1 mice

Ventromedial prefrontal cortical (vmPFC) dopamine (DA) influences attentional aspects of cognition and anxiety-like behavioral responding in rodents. The present study investigated the role of D2 receptors on spontaneous alternation in the Y-maze and anxiety-like behavior in a two-trial elevated plus-maze (EPM) procedure in CD-1 mice following vmPFC infusions of the D2 antagonist, sulpiride, and the D2 agonist, quinpirole. Pretrial 1 quinpirole infusions did not influence any anxiety measure (with the exception that the lowest dose increased protected stretch attends), but reduced protected exploration activity (closed-arm entry/time ratios and wall rearing). In Trial 2 24 h later (no injection), quinpirole exerted an anxiolytic behavioral profile relative to Trial 2 control mice (enhanced open-arm entry/time ratios, unprotected head dips), with no effects on protected exploration or risk assessment activity. Pretrial 1 sulpiride infusions enhanced unprotected exploration (open-arm entry/time ratios, unprotected stretch attend, and head dips), but did not influence protected exploration or risk assessment in the EPM. In Trial 2, 24 h later (no injection), sulpiride extended this anxiolytic profile to reduced protected exploration and risk assessment activity (closed-time ratio, protected stretch attend, and head dips). In the Y-maze, whereas quinpirole disrupted alternation performance (5- and 10-nmol dose) concomitant with marked repetitive same-arm returns (SAR) at the highest dose, sulpiride disrupted alternation performance concomitant with marked repetitive SAR behavior at the lowest dose only. These data indicate that although infralimbic (IL) quinpirole and sulpiride infusions similarly disrupted alternation performance in the Y-maze and reduced Trial 2 anxiety-like responding in the EPM, these drugs differentially produced these effects.

Effects of medial prefrontal cortex cytotoxic lesions in mice

Mice (C57BL/6J strain, females) with cytotoxic lesions of the medial wall of the prefrontal cortex were given a battery of tests to assess emotional, species-typical, cognitive, motor and other behaviours. Lesioned mice showed a profile of reduced anxiety, both on a plus-maze, and a similar, novel test, the successive alleys. There was no evidence, however, for attenuation of anxiety in tests of hyponeophagia, and lesioned mice, like controls, preferred the black to the white area of an enclosed alley. Their locomotor activity tended to be higher than that of the controls, particularly when the test surroundings were novel or relatively so. Species-typical behaviours were similar to those of control mice, except lesioned mice displaced ('burrowed') less food pellets from a tube in their home cage. They were not impaired at learning a spatial Y-maze reference memory task, which is profoundly affected by cytotoxic hippocampal lesions in the same strain, or at learning a multi-trial passive avoidance test. Their strength and co-ordination in motor performance tests was also normal. The results show that cytotoxic medial prefrontal cortex lesions in mice produce a clear but restricted anxiolytic action. The marked reduction in burrowing, in the absence of any detectable impairment of motor ability, demonstrates the sensitivity of this behavioural index.

The role of rat medial frontal cortex in effort-based decision making

We conducted a series of experiments to elucidate the role of rat medial frontal cortex (MFC) (including prelimbic, infralimbic, and cingulate cortices) in effort-based decision making. Rats were trained on a cost-benefit T-maze task in which they could either choose to climb a barrier to obtain a high reward in one arm (HR arm) or could obtain a small reward in the other with no barrier present (LR arm). Before surgery, all animals were selecting the HR arm on almost every trial. However, after excitotoxic lesions to MFC, the rats shifted to selecting the LR arm on almost every trial. This was not caused by a spatial memory or motor deficit because the same rats returned to selecting the HR arm when the amount of effort needed to be expended to obtain reward in either arm was equalized by putting an identical barrier in the LR arm. Additional experiments demonstrated that the change in effort-based decisions observed in the rats was not caused by a complete insensitivity to reward and effort because they returned to choosing the HR arm if either the cost was reduced (by making the barrier smaller) or the benefit was increased (increasing the food ratio differential). Instead, the MFC lesion shifted the animals' decision criterion, making them more likely to choose the LR arm than the sham-lesioned animals. These results imply that medial frontal cortex is important for allowing the animal to put in more work to obtain greater rewards.

Correlated discharges among putative pyramidal neurons and interneurons in the primate prefrontal cortex

Neurophysiological recordings have revealed that the discharges of nearby cortical cells are positively correlated in time scales that range from millisecond synchronization of action potentials to much slower firing rate co-variations, evident in rates averaged over hundreds of milliseconds. The presence of correlated firing can offer insights into the patterns of connectivity between neurons; however, few models of population coding have taken account of the neuronal diversity present in cerebral cortex, notably a distinction between inhibitory and excitatory cells. We addressed this question in the monkey dorsolateral prefrontal cortex by recording neuronal activity from multiple micro-electrodes, typically spaced 0.2-0.3 mm apart. Putative excitatory and inhibitory neurons were distinguished based on their action potential waveform and baseline discharge rate. We tested each pair of simultaneously recorded neurons for presence of significant cross-correlation peaks and measured the correlation of their averaged firing rates in successive trials. When observed, cross-correlation peaks were centered at time 0, indicating synchronous firing consistent with two neurons receiving common input. Discharges in pairs of putative inhibitory interneurons were found to be significantly more strongly correlated than in pairs of putative excitatory cells. The degree of correlated firing was also higher for neurons with similar spatial receptive fields and neurons active in the same epochs of the behavioral task. These factors were important in predicting the strength of both short time scale (<5 ms) correlations and of trial-to-trial discharge rate covariations. Correlated firing was only marginally accounted for by motor and behavioral variations between trials. Our findings suggest that nearby inhibitory neurons are more tightly synchronized than excitatory ones and account for much of the correlated discharges commonly observed in undifferentiated cortical networks. In contrast, the discharge of pyramidal neurons, the sole projection cells of the cerebral cortex, appears largely independent, suggesting that correlated firing may be a property confined within local circuits and only to a lesser degree propagated to distant cortical areas and modules.

Memory trace in prefrontal cortex: theory for the cognitive switch

The dorsolateral prefrontal cortex in human and non-human primates functions as the highest-order executor for the perception-action cycle. According to this view, when perceptual stimuli from the environment are novel or complex, the dorsolateral prefrontal cortex serves to set consciously a goal-directed scheme which broadly determines an action repertory to meet the particular demand from the environment. In this respect, the dorsolateral prefrontal cortex is a short-term activation device with the properties of a cognitive switch', because it couples a particular set of perceptual stimuli to a particular set of actions. Here, I suggest that, in order for the organism to react systematically to the environment, neural traces for the switch function must be stored in the brain. Thus, the highest-order, perception-action interface function of the dorsolateral prefrontal cortex per se depends on permanently stored neural traces in the dorsolateral prefrontal cortex and related structures. Such a memory system may be located functionally between two of the well-documented memory systems in the brain: the declarative memory system and the procedural memory system. Finally, based on available neurophysiological data, the possible mechanisms underlying the formation of cognitive switch traces are proposed.

Deep layer prefrontal cortex unit discharge in a cue-controlled open-field environment in the freely-moving rat

The activity of single units in prefrontal cortex (prelimbic and anterior cingulate subregions) was recorded as rats performed a 'pellet-chasing' task in a cue-controlled, open-field environment in which the position of a single salient cue card was manipulated. Spike train analyses revealed three different types of unit. The first type was characterized by rhythmic bursts of spiking with inter-burst intervals of approximately 200 ms (66% of units), the second by bursts with inter-burst intervals of approximately 80 ms (33% of units), and the third by non-rhythmic firing characteristics (33% of units). None of the units had spatially-selective firing characteristics, nor were their discharge patterns affected by manipulation of the cue card. Instead, the firing of the units had multiple behavioural correlates that occurred as the rat explored the environment. These results are in line with previous studies that suggest that prefrontal cortex unit discharge is not related to spatial processing but to behaviours necessary for exploration.

Plasticity in the projection from the anterior thalamic nuclei to the anterior cingulate cortex in the rat in vivo: paired-pulse facilitation, long-term potentiation and short-term depression

Several neurophysiological and computational theories of the rodent navigational system suggest that the differing cortices of the frontal lobe and thalamus share information and therefore undergo changes in synaptic strength. We examine here for the first time three forms of synaptic plasticity in the projection from the anterior thalamic nuclei to the anterior cingulate cortex: we demonstrate that this projection is capable of expressing paired-pulse facilitation, long-term potentiation, and short-term depression. Furthermore, input/output curves show that field excitatory post-synaptic potential amplitude increased at all stimulus intensities following high-frequency stimulation. These findings add important information to our understanding of synaptic plasticity in this important pathway, which has been widely hypothesized to play important roles in memory and spatial representation in the rodent.

Position sensitivity in phasically discharging nucleus accumbens neurons of rats alternating between tasks requiring complementary types of spatial cues

To determine how hippocampal location-selective discharges might influence downstream structures for navigation, nucleus accumbens neurons were recorded in rats alternating between two tasks guided respectively by lit cues in the maze or by extramaze room cues. Of 144 phasically active neurons, 80 showed significant behavioral correlates including displacements, immobility prior to, or after reward delivery, as well as turning, similar to previous reports. Nine neurons were position-selective, 22 were sensitive to task and platform changes and 40 others were both. Although the accumbens neurons showed the same behavioral correlate in two or four functionally equivalent locations, these responses were stronger at some of these places, evidence for position sensitivity. To test whether position responses were selective for room versus platform cues, the experimental platform was rotated while the rat performed each of the two tasks. This revealed responses to changes in position relative to both platform and room cues, despite the fact that previous studies had shown that place responses of hippocampal neurons recorded in the same task are anchored to room cues only. After these manipulations and shifts between the two tasks, the responses varied among simultaneously recorded neurons, and even in single neurons in alternating visits to reward sites. Again this contrasts with the uniformity of place responses of hippocampal neurons recorded in this same task. Thus accumbens position responses may derive from hippocampal inputs, while responses to context changes are more likely to derive from other signals or intrinsic processing. Considering the accumbens as a limbic-motor interface, we conclude that position-modulated behavioral responses in the accumbens may be intermediate between the allocentric reference frame of position-selective discharges in the hippocampus and the egocentric coding required to organize movement control. The conflicting responses among simultaneously recorded neurons could reflect competition processes serving as substrates for action selection and learning.

The hippocampal formation--orbitomedial prefrontal cortex circuit in the attentional control of active memory

The long held view that the hippocampal formation is not only essential, but also solely responsible for declarative memory in humans (and by analogy non-human primates) has come into question. Based on extensive reciprocal connection patterns between the hippocampal formation and the orbitoventromedial prefrontal cortex in primates and rats, a central role for the hippocampal formation in the attentional control of behavior is emerging. In this paper, evidence is reviewed showing that the hippocampal-orbitomedial prefrontal cortex circuit may be involved in attentional monitoring of the internal sensorium. This attentional monitoring system, in a sense, is the working memory of viscero-emotional processing. The hippocampal formation can thus be viewed as a discrepancy detector with respect to the relative activational status of cognitive/emotional set in the orbitomedial prefrontal cortex. Discrepancies between the current representation of the internal milieu and the "just-prior" representation held "on-line" in orbitomedial prefrontal cortex associative working memory, are signaled from the hippocampus to the prefrontal cortex prospective attentional systems to activate, process, and reconcile internal (past) with external (present) environments, and finally to effectively alter active working emotional "sets" to exert cognitive-emotional control of behavior.

Neurons in rat medial prefrontal cortex show anticipatory rate changes to predictable differential rewards in a spatial memory task

The present study electrophysiologically examined the contribution of prelimbic and infralimbic neurons in the medial prefrontal cortex (mPFC) to integration of reward and spatial information while rats performed multiple memory trials on a differentially rewarded eight arm radial maze. Alternate arms consistently held one of two different reward amounts. Similar to previous examinations of the rat mPFC, few cells showed discrete place fields or altered firing during a delay period. The most common behavioral correlate was a change in neuronal firing rate prior to reward acquisition at arm ends. A small number of reward-related cells differentiated between high and low reward arms. The presence of neurons that anticipate expected reward consequences based on information about the spatial environment is consistent with the hypothesis that the mPFC is part of a neural system which merges spatial information with its motivational significance.

Stimulation of prefrontal cortex at physiologically relevant frequencies inhibits dopamine release in the nucleus accumbens

The prefrontal cortex (PFC) is thought to provide an excitatory influence on the output of mesoaccumbens dopamine neurons. The evidence for this influence primarily arises from findings in the rat that chemical or high-intensity and high-frequency (60-200 Hz) electrical stimulations of PFC increase burst activity of midbrain dopamine neurons, and augment terminal release of dopamine in the nucleus accumbens. However, PFC neurons in animals that are engaged in PFC-dependent cognitive tasks increase their firing frequency from a baseline of 1-3 Hz to 7-10 Hz, suggesting that the commonly used high-frequency stimulation parameters of the PFC may not be relevant to the behavioral states that are associated with PFC activation. We investigated the influence of PFC activation at lower physiologically relevant frequencies on the release of dopamine in the nucleus accumbens. Using rapid (5-min) microdialysis measures of extracellular dopamine in the nucleus accumbens, we found that although PFC stimulation at 60 Hz produces the expected increases in accumbal dopamine release, the same amplitude of PFC stimulation at 10 Hz significantly decreased these levels. These results indicate that activation of PFC, at frequencies that are associated with increased cognitive demand on this region, inhibits the mesoaccumbens dopamine system.

Amygdala regulation of nucleus accumbens dopamine output is governed by the prefrontal cortex

A dynamic interaction between the prefrontal cortex (PFC), amygdala, and nucleus accumbens (NAc) may be fundamental to regulation of goal-directed behavior by affective and cognitive processes. This study demonstrates that a mechanism for this triadic relationship is an inhibitory control by prefrontal cortex on accumbal dopamine release during amygdala activation. In freely moving rats, microstimulation of basolateral amygdala at intensities that produced mild behavioral activation produced an expected rapid increase in glutamate efflux in the prefrontal cortex and the nucleus accumbens shell region of the ventral striatum. However, during the stimulation, dopamine release increased only in the prefrontal cortex, not in the nucleus accumbens. An increase in accumbal dopamine release was observed during the stimulation if glutamate activation in the prefrontal cortex was inhibited at either presynaptic or postsynaptic levels. Some behaviors expressed during the stimulation were intensified in animals in which prefrontal cortex glutamate activation was blocked. In addition, these animals continued to express stimulus-induced behaviors after the termination of stimulation, whereas normal poststimulus behaviors such as ambulation and grooming were not displayed as frequently. Considering that dopamine neurotransmission in the nucleus accumbens is thought to play an integral role in goal-directed motor behavior, these findings suggest that the prefrontal cortex influences the behavioral impact of amygdala activation via a concomitant active suppression of accumbal dopamine release. Absence of this cortical influence appears to result in an aberrant pattern of behavioral expression in response to amygdala activation, including behavioral perseveration after stimulus termination.

Position and behavioral modulation of synchronization of hippocampal and accumbens neuronal discharges in freely moving rats

To understand how hippocampal signals are processed by downstream neurons, we analyzed the relative timing between neuronal discharges in simultaneous recordings in the hippocampus and nucleus accumbens of rats performing in a plus maze. In all, 154 pairs of cells (composed of 65 hippocampal and 56 accumbens neurons) were examined during the 1 s period prior to reward delivery. Cross-correlation analyses over a +/- 300-ms window with 10-ms bins revealed that 108 pairs had at least one significant histogram bin (P < 0.01). The most frequently occurring peaks of hippocampal firing prior to accumbens discharges appeared at latencies from -30-0 ms, corresponding to published values of the latency of the hippocampal pathway to the nucleus accumbens. Other peaks appeared most often at latencies multiples of about 110 ms prior to and after this, corresponding to theta rhythmicity. Since firing synchronization can result from several types of connectivity patterns (such as common inputs), a group of 18 hippocampus-accumbens pairs was selected as those most likely to have monosynaptic connections. The criterion was the presence of at least one highly significant peak (P < 0.001) at latencies corresponding to field potentials evoked in the accumbens by hippocampal stimulation. A significant peak occurred on all four maze arms for only one of these cell pairs, indicating positional modulation for the others. In addition, behavior dependence of the synchrony between these nucleus accumbens and hippocampus neurons was examined by studying data in relation to three different synchronization points: reward box arrival, box departure, and arrival at the center of the maze. This indicates that the functional connectivity between hippocampal and accumbens neurons was stronger when the rat was near reward areas. Ten of the hippocampal neurons in these 18 cell pairs showed 9-Hz (theta) rhythmic activity in autocorrelation analyses. Of these 10 cells, cross-correlograms from eight hippocampal-accumbens pairs also showed theta rhythmicity. Overall, these results indicate that the synchrony between hippocampus and nucleus accumbens neurons is modulated by spatial position and behavior, and theta rhythm may play an important role for this synchronization.

Plasticity at hippocampal to prefrontal cortex synapses: dual roles in working memory and consolidation

The involvement of the hippocampus and the prefrontal cortex in cognitive processes and particularly in learning and memory has been known for a long time. However, the specific role of the projection which connects these two structures has remained elusive. The existence of a direct monosynaptic pathway from the ventral CA1 region of the hippocampus and subiculum to specific areas of the prefrontal cortex provides a useful model for conceptualizing the functional operations of hippocampal-prefrontal cortex communication in learning and memory. It is known now that hippocampal to prefrontal cortex synapses are modifiable synapses and can express different forms of plasticity, including long-term potentiation, long-term depression, and depotentiation. Here we review these findings and focus on recent studies that start to relate synaptic plasticity in the hippocampo-prefrontal cortex pathway to two specific aspects of learning and memory, i.e., the consolidation of information and working memory. The available evidence suggests that functional interactions between the hippocampus and prefrontal cortex in cognition and memory are more complex than previously anticipated, with the possibility for bidirectional regulation of synaptic strength as a function of the specific demands of tasks.

Haloperidol and clozapine increase neural activity in the rat prefrontal cortex

Haloperidol and clozapine have been widely used to alleviate schizophrenic symptoms, but their physiological effects in the prefrontal cortex (PFC) are not known. Effects of haloperidol and clozapine on single unit activity were investigated in the medial PFC of anesthetized rats. Injection (intraperitoneal) of haloperidol (1 mg/kg) or clozapine (20 mg/kg) significantly elevated discharge rates of PFC neurons. Considering that hypofrontality is one characteristic of schizophrenic symptoms, these results raise the possibility that enhancement of PFC neural activity contributes to therapeutic effects of haloperidol and clozapine.

Temporally structured replay of awake hippocampal ensemble activity during rapid eye movement sleep

Human dreaming occurs during rapid eye movement (REM) sleep. To investigate the structure of neural activity during REM sleep, we simultaneously recorded the activity of multiple neurons in the rat hippocampus during both sleep and awake behavior. We show that temporally sequenced ensemble firing rate patterns reflecting tens of seconds to minutes of behavioral experience are reproduced during REM episodes at an equivalent timescale. Furthermore, within such REM episodes behavior-dependent modulation of the subcortically driven theta rhythm is also reproduced. These results demonstrate that long temporal sequences of patterned multineuronal activity suggestive of episodic memory traces are reactivated during REM sleep. Such reactivation may be important for memory processing and provides a basis for the electrophysiological examination of the content of dream states.

Hippocampal neurons encode information about different types of memory episodes occurring in the same location

Firing patterns of hippocampal complex-spike neurons were examined for the capacity to encode information important to the memory demands of a task even when the overt behavior and location of the animal are held constant. Neuronal activity was recorded as rats continuously alternated left and right turns from the central stem of a modified T maze. Two-thirds of the cells fired differentially as the rat traversed the common stem on left-turn and right-turn trials, even when potentially confounding variations in running speed, heading, and position on the stem were taken into account. Other cells fired differentially on the two trial types in combination with behavioral and spatial factors or appeared to fire similarly on both trial types. This pattern of results suggests that hippocampal representations encode some of the information necessary for representing specific memory episodes.

Relationship among discharges of neighboring neurons in the rat prefrontal cortex during spatial working memory tasks

The relationship among discharges of neurons that were recorded simultaneously with tetrodes in the rat medial prefrontal cortex was analyzed. Spatial working memory tasks were divided into several distinct stages based on the behavioral correlates of individual neurons, and interneuronal correlation of signal (mean discharge rate at each stage) and noise (trial-to-trial deviation from the signal) was calculated. Behavioral correlates of neighboring neurons were quite heterogeneous and, accordingly, average signal correlation was relatively low ( approximately 0.16). Noise correlation was even lower ( approximately 0.06), but neuronal noise was more correlated among the neurons with similar signals. Spikes underlying the signal and noise correlation among the prefrontal cortical neurons were loosely synchronized over a few hundred milliseconds. These results suggest that neighboring prefrontal cortical neurons process largely independent information and have weakly correlated noise and that precisely synchronized spikes play a relatively minor role in producing the correlated signal and noise among these neurons.

Sustained visual attention performance-associated prefrontal neuronal activity: evidence for cholinergic modulation

Cortical cholinergic inputs are hypothesized to mediate attentional functions. The present experiment was designed to determine the single unit activity of neurons within the medial prefrontal cortex (mPFC) of rats performing a sustained visual attention task. Demands on attentional performance were varied by the presentation of a visual distractor. The contribution of cholinergic afferents of the mPFC to performance-associated unit activity within this area was determined by recording neuronal activity before and after unilateral cholinergic deafferentation using intracortical infusion of the immunotoxin 192 IgG-saporin. Presentation of the visual distractor resulted in a decrease in the detection of brief, unpredictable visual signals. As predicted, the unilateral loss of cholinergic inputs within the recording area of the mPFC did not affect sustained attentional performance. Cholinergic deafferentation, however, resulted in a decrease in the overall firing rate of medial prefrontal neurons and a substantial reduction in the proportion of neurons whose firing patterns correlated with specific aspects of behavioral performance. Furthermore, cholinergic deafferentation attenuated the frequency and amplitude of increased mPFC neuronal firing rates that were associated with the presentation of the visual distractor. The main findings from this experiment suggest that cholinergic inputs to the mPFC strongly influence spontaneous and behaviorally correlated single unit activity and mediate increases in neuronal activity associated with enhanced demands for attentional processing, all of which may be fundamental aspects in the maintenance of attentional performance.

Concurrent modulation of anxiety and memory

We have previously shown that the ventromedial prefrontal cortex (vmPFC) is involved in spontaneous working memory and anxiety-related behaviour in CD-1 mice. Specifically, pretrial microinjection of the kappa(1) agonist, U-69,593, in the infralimbic (IL) area of the vmPFC produced a robust anxiolytic behavioural profile in the elevated plus-maze and enhanced spontaneous working memory in the Y-maze. In the present study we sought to determine whether these effects were specific to IL kappa receptors. We hypothesized that microinjection of the kappa antagonist, norBNI, in the IL cortex would influence anxiety and spontaneous memory in an opposite direction to the effects produced by the kappa(1) agonist. In week 1, transfer-latency reference memory and anxiety were tested in the elevated plus-maze in two separate trials with an intertrial interval of 24 h. In week 2, spontaneous working memory was tested in the Y-maze followed immediately by defensive/withdrawal anxiety in the open field for one half of the animals in each group, and the other half was tested in reverse order. Pretreatment with one injection of vehicle, 1, 5 or 10 nmol/0.5 microl norBNI in the IL cortex dose-dependently reduced transfer-latencies and produced an anxiogenic behavioural profile in the first elevated plus-maze trial. Following a 24 h delay, transfer-latency reference memory was not influenced, but a robust anxiogenic behavioural profile was observed in the second no-injection anxiety trial in the elevated plus-maze relative to control animals. In week 2, the same groups of mice were again pretreated with one injection of the same doses of norBNI in the IL cortex and tested in the open field and Y-maze. NorBNI pretreatment was anxiogenic in the defensive/withdrawal anxiety test and disrupted spontaneous working memory regardless of testing order. The present results show the influence of kappa receptor modulation on anxiety induction and spontaneous working memory. These results also support the hypothesis that immediate memory processing may modulate the induction of anxiety-related behaviours.

The effects of local application of D2 selective dopaminergic drugs into the medial prefrontal cortex of rats in a delayed spatial choice task

The study examined the effects of modulation of dopamine D2 receptors-mediated neurotransmission in the rat's prefrontal cortex (PFC) on storage and executive components of working memory. Rats were trained on delayed (delay interval, 3 s) and non-delayed choice in a U-maze. The prominence of proactive interference was evaluated by sorting errors in a current trial on the basis of animal reactions in a preceding trial. The erroneous runs to the same arm of the maze as in the previous trial were identified as the repetitions (RE) and the erroneous runs to the other arm in comparison with the previous trial were classified as alternations (AE). The bilateral microinfusion of D2 agonists PPHT (0.004 microg, 0.04 microg, 0.4 microg/1 microl) into medial wall of the PFC produced a dose-dependent increase in the error rate of the delayed-response task and did not influence non-delayed choice. In delay condition PPHT enhanced the perseverative tendencies (the rate of RE was significantly higher than the rate of AE), in non-delayed choice the erroneous performance was mainly represented by AE. In contrast, the infusion of D2-receptor antagonist sulpiride (0.03 microg, 0.3 microg, 3 microg/1 microl) increased the accuracy of delayed choice and changed the mode of intertrial dependence-rats made significantly more AE than RE. The results are discussed in terms of the involvement of D2 receptor dependent transmission of the PFC in different cognitive processes related to the delayed performance in U-maze (within-trial short-term storage of information versus dynamic control of between-trials working memory processing).

Functional role of rat prelimbic-infralimbic cortices in spatial memory: evidence for their involvement in attention and behavioural flexibility

The involvement of the medial prefrontal cortex (mPFC), and more particularly the prelimbic and infralimbic cortices (PL-IL area), in spatial memory remains controversial. The present study investigates the effects of neurotoxic lesions restricted to the PL-IL area of the mPFC in rats trained in two different spatial tasks. In experiment 1, PL-IL lesioned rats showed normal acquisition of a delayed non-matching to position task. They were also able to plan their responses for a prospective strategy but were transiently disrupted when the initial delay was extended. In experiment 2, rats were trained to locate one baited box among 13 identical boxes distributed on a circular arena. Lesioned rats performed normally when trained from a single start position but were severely disrupted when four start positions were used. A probe trial showed this deficit was not due to failure to learn the goal location. The addition of a proximal cue signalling the goal box helped lesioned rats to directly open the goal box, but did not compensate for greater distances that they travelled to reach it. Results from both experiments indicate that the PL-IL area is directly involved neither in allocentric spatial representations nor prospective memory and is not specifically involved in working memory. This area seems more likely to be involved in both attentional processes and behavioural flexibility that may be important for processing information for working memory as well as for spatial memory.

Disturbance of rat lever-press learning by hippocampo-prefrontal disconnection

To determine whether the medial prefrontal cortex (PFC), ventral hippocampus and hippocampo-PFC pathway are involved in operant lever-press learning, we conducted lidocaine injections to these brain sites. Rats were injected immediately after lever-press acquisition in the first training, and the second 5-min test the next day. Results showed the response rate of either PFC- or ventral hippocampus-inactivated rats to be lower than that of control rats in the test the next day. Rats having lidocaine injected into the unilateral ventral hippocampus combined with contralateral medial PFC also showed lower response rate in their tests. These results suggest that hippocampo-PFC disconnection disturbs operant learning.

Dopamine-mediated stabilization of delay-period activity in a network model of prefrontal cortex

The prefrontal cortex (PFC) is critically involved in working memory, which underlies memory-guided, goal-directed behavior. During working-memory tasks, PFC neurons exhibit sustained elevated activity, which may reflect the active holding of goal-related information or the preparation of forthcoming actions. Dopamine via the D1 receptor strongly modulates both this sustained (delay-period) activity and behavioral performance in working-memory tasks. However, the function of dopamine during delay-period activity and the underlying neural mechanisms are only poorly understood. Recently we proposed that dopamine might stabilize active neural representations in PFC circuits during tasks involving working memory and render them robust against interfering stimuli and noise. To further test this idea and to examine the dopamine-modulated ionic currents that could give rise to increased stability of neural representations, we developed a network model of the PFC consisting of multicompartment neurons equipped with Hodgkin-Huxley-like channel kinetics that could reproduce in vitro whole cell and in vivo recordings from PFC neurons. Dopaminergic effects on intrinsic ionic and synaptic conductances were implemented in the model based on in vitro data. Simulated dopamine strongly enhanced high, delay-type activity but not low, spontaneous activity in the model network. Furthermore the strength of an afferent stimulation needed to disrupt delay-type activity increased with the magnitude of the dopamine-induced shifts in network parameters, making the currently active representation much more stable. Stability could be increased by dopamine-induced enhancements of the persistent Na(+) and N-methyl-D-aspartate (NMDA) conductances. Stability also was enhanced by a reduction in AMPA conductances. The increase in GABA(A) conductances that occurs after stimulation of dopaminergic D1 receptors was necessary in this context to prevent uncontrolled, spontaneous switches into high-activity states (i.e., spontaneous activation of task-irrelevant representations). In conclusion, the dopamine-induced changes in the biophysical properties of intrinsic ionic and synaptic conductances conjointly acted to highly increase stability of activated representations in PFC networks and at the same time retain control over network behavior and thus preserve its ability to adequately respond to task-related stimuli. Predictions of the model can be tested in vivo by locally applying specific D1 receptor, NMDA, or GABA(A) antagonists while recording from PFC neurons in delayed reaction-type tasks with interfering stimuli.

A neural systems analysis of adaptive navigation

In the field of the neurobiology of learning, significant emphasis has been placed on understanding neural plasticity within a single structure (or synapse type) as it relates to a particular type of learning mediated by a particular brain area. To appreciate fully the breadth of the plasticity responsible for complex learning phenomena, it is imperative that we also examine the neural mechanisms of the behavioral instantiation of learned information, how motivational systems interact, and how past memories affect the learning process. To address this issue, we describe a model of complex learning (rodent adaptive navigation) that could be used to study dynamically interactive neural systems. Adaptive navigation depends on the efficient integration of external and internal sensory information with motivational systems to arrive at the most effective cognitive and/or behavioral strategies. We present evidence consistent with the view that during navigation: 1) the limbic thalamus and limbic cortex is primarily responsible for the integration of current and expected sensory information, 2) the hippocampal-septal-hypothalamic system provides a mechanism whereby motivational perspectives bias sensory processing, and 3) the amygdala-prefrontal-striatal circuit allows animals to evaluate the expected reinforcement consequences of context-dependent behavioral responses. Although much remains to be determined regarding the nature of the interactions among neural systems, new insights have emerged regarding the mechanisms that underlie flexible and adaptive behavioral responses.

Analysis of the connectional organization of neural systems associated with the hippocampus in rats

The hippocampus of the rat enjoys a central significance for researchers interested in the neural mechanisms of memory and spatial information processing. Many of the theoretical models advanced to explain function in this system, however, do not reflect the wealth of information on the connectivity of these structures, and employ greatly simplified treatments of its complex connectivity. We were interested in whether a more analytical approach, which begins with analysis of the connectivity of the system, might provide insights complementary to those derived by synthetic models. Accordingly, we collated detailed neuroanatomical information about the connectivity of the hippocampal system in the rat, and analysed the resulting data. Analyses of connectivity based on a variety of different analytical techniques have recently been used to elucidate the global organization of other systems in the macaque and cat, and have given rise to successful predictions. We applied non-metric multidimensional scaling and non-parametric cluster analysis to our summary matrix of connection data. The analyses produced organizational schemes that were consistent with known physiological properties and provided the basis for making tentative predictions of the further structures that may contain 'place' and 'head-direction' cells, which structures we identify. The consistency between the analyses of connectivity and the distribution of physiological properties across the system suggests that functional relationships are constrained by the organization of the connectivity of the system, and so that structure and function are linked at the systems level.

Induction of stable long-term depression in vivo in the hippocampal-prefrontal cortex pathway

We studied excitatory field potentials in the medial prefrontal cortex (mPFC, prelimbic area) to electrostimulation of the ventral hippocampus (CA1/subicular region) in the anaesthetized rat. Nine hundred stimulus trains (5 pulses at 250 Hz) applied at 1 Hz to the ventral hippocampus significantly and persistently depressed the amplitude and maximal slope ( approximately 55% for each index) of the prelimbic field potentials, but did not change the latency of the maximal slope or peak negativity. Twelve stimulus trains (50 pulses at 250 Hz) applied subsequently at 0.1 Hz restored the depression back to control level, and this reversible depression was maintained for at least 13 h. Cumulative depressive effects on the prelimbic field potential amplitude and maximal slope were observed upon addition of stimulus trains in the hippocampus. An important implication of the results is that the direct pathway from the hippocampus to the mPFC in the rat retains long-term depression (LTD) as a neuroplastic form in vivo. This form could cooperate with long-term potentiation (LTP) and such a bi-directional synaptic plasticity in the prefrontal cortex contributes to how cortical neural networks store information.