The role of rat dorsomedial prefrontal cortex in spatial working memory

Neurophysiology of rule switching in the corticostriatal circuit

The ability to adjust behavioral responses to cues in a changing environment is crucial for survival. Activity in the medial Prefrontal Cortex (mPFC) is thought to both represent rules to guide behavior as well as detect and resolve conflicts between rules in changing contingencies. While lesion and pharmacological studies have supported a crucial role for mPFC in this type of set-shifting, an understanding of how mPFC represents current rules or detects and resolves conflict between different rules is still unclear. Meanwhile, medial dorsal striatum (mDS) receives major projections from mPFC and neural activity of mDS is closely linked to action selection, making the mDS a potential major player for enacting rule-guided action policies. However, exactly what is signaled by mPFC and how this impacts neural signals in mDS is not well known. In this review, we will summarize what is known about neural signals of rules and set shifting in both prefrontal cortex and dorsal striatum, as well as provide questions and directions for future experiments.

Cholinergic and ghrelinergic receptors and KCNQ channels in the medial PFC regulate the expression of palatability

The medial prefrontal cortex (mPFC) is a key brain region for the control of consummatory behavior. Neuronal activity in this area is modulated when rats initiate consummatory licking and reversible inactivations eliminate reward contrast effects and reduce a measure of palatability, the duration of licking bouts. Together, these data suggest the hypothesis that rhythmic neuronal activity in the mPFC is crucial for the control of consummatory behavior. The muscarinic cholinergic system is known to regulate membrane excitability and control low-frequency rhythmic activity in the mPFC. Muscarinic receptors (mAChRs) act through KCNQ (Kv7) potassium channels, which have recently been linked to the orexigenic peptide ghrelin. To understand if drugs that act on KCNQ channels within the mPFC have effects on consummatory behavior, we made infusions of several muscarinic drugs (scopolamine, oxotremorine, physostigmine), the KCNQ channel blocker XE-991, and ghrelin into the mPFC and evaluated their effects on consummatory behavior. A consistent finding across all drugs was an effect on the duration of licking bouts when animals consume solutions with a relatively high concentration of sucrose. The muscarinic antagonist scopolamine reduced bout durations, both systemically and intra-cortically. By contrast, the muscarinic agonist oxotremorine, the cholinesterase inhibitor physostigmine, the KCNQ channel blocker XE-991, and ghrelin all increased the durations of licking bouts when infused into the mPFC. Our findings suggest that cholinergic and ghrelinergic signaling in the mPFC, acting through KCNQ channels, regulates the expression of palatability.

Neural correlates of rules and conflict in medial prefrontal cortex during decision and feedback epochs

The ability to properly adjust behavioral responses to cues in a changing environment is crucial for survival. Activity in the medial Prefrontal Cortex (mPFC) is thought to both represent rules to guide behavior as well as detect and resolve conflicts between rules in changing contingencies. However, while lesion and pharmacological studies have supported a crucial role for mPFC in this type of set-shifting, an understanding of how mPFC represents current rules or detects and resolves conflict between different rules is unclear. Here, we directly address the role of rat mPFC in shifting rule based behavioral strategies using a novel behavioral task designed to tease apart neural signatures of rules, conflict and direction. We demonstrate that activity of single neurons in rat mPFC represent distinct rules. Further, we show increased firing on high conflict trials in a separate population of mPFC neurons. Reduced firing in both populations of neurons was associated with poor performance. Moreover, activity in both populations increased and decreased firing during the outcome epoch when reward was and was not delivered on correct and incorrect trials, respectively. In addition, outcome firing was modulated by the current rule and the degree of conflict associated with the previous decision. These results promote a greater understanding of the role that mPFC plays in switching between rules, signaling both rule and conflict to promote improved behavioral performance.

CBP-Dependent memory consolidation in the prefrontal cortex supports object-location learning

Recognition of an object's location in space is supported by hippocampus-dependent recollection. Converging evidence strongly suggests that the interplay between the prefrontal cortex and hippocampus is critical for spatial memory. Lesion, pharmacological, and genetic studies have been successful in dissecting the role of plasticity in the hippocampal circuit in a variety of neural processes relevant to spatial memory, including memory for the location of objects. However, prefrontal mechanisms underlying spatial memory are less well understood. Here, we show that an acute hypofunction of the cyclic-AMP regulatory element binding protein (CREB) Binding Protein (CBP) histone acetyltransferase (HAT) in the medial prefrontal cortex (mPFC) results in delay-dependent disruption of object-location memory. These data suggest that mechanisms involving CBP HAT-mediated lysine acetylation of nuclear proteins support selectively long-term encoding in the mPFC circuits. Evidence from the object-location task suggests that long-term memory encoding within the mPFC complements hippocampus-dependent spatial memory mechanisms and may be critical for broader network integration of information necessary for an assessment of subtle spatial differences to guide appropriate behavioral response during retrieval of spatial memories.

The medial prefrontal cortex is crucial for the maintenance of persistent licking and the expression of incentive contrast

We examined the role of the medial prefrontal cortex (mPFC) in reward processing and the control of consummatory behavior. Rats were trained in an operant licking procedure in which they received alternating access to solutions with relatively high and low levels of sucrose (20 and 4%, w/v). Each level of sucrose was available for fixed intervals of 30 s over 30 min test sessions. Over several days of training, rats came to lick persistently when the high level of sucrose was available and suppressed licking when the low level of sucrose was available. Pharmacological inactivations of the mPFC, specifically the rostral part of the prelimbic area, greatly reduced intake of the higher value fluid and only slightly increased intake of the lower value fluid. In addition, the inactivations altered within-session patterns and microstructural measures of licking. Rats licked equally for the high and low levels of sucrose at the beginning of the test sessions and “relearned” to reduce intake of the low value fluid over the test sessions. Durations of licking bouts (clusters of licks with inter-lick intervals <0.5 s) were reduced for the high value fluid and there were many more brief licking bouts (<1 s) when the low value fluid was available. These effects were verified using an alternative approach (optogenetic silencing using archaerhodopsin) and were distinct from inactivation of the ventral striatum, which simply increased overall intake. Our findings suggest that the mPFC is crucial for the maintenance of persistent licking and the expression of learned feeding strategies.

Functional connectivity changes during a working memory task in rat via NMF analysis

Working memory (WM) is necessary in higher cognition. The brain as a complex network is formed by interconnections among neurons. Connectivity results in neural dynamics to support cognition. The first aim is to investigate connectivity dynamics in medial prefrontal cortex (mPFC) networks during WM. As brain neural activity is sparse, the second aim is to find the intrinsic connectivity property in a feature space. Using multi-channel electrode recording techniques, spikes were simultaneously obtained from mPFC of rats that performed a Y-maze WM task. Continuous time series converted from spikes were embedded in a low-dimensional space by non-negative matrix factorization (NMF). mPFC network in original space was constructed by measuring connections among neurons. And the same network in NMF space was constructed by computing connectivity values between the extracted NMF components. Causal density (Cd) and global efficiency (E) were estimated to present the network property. The results showed that Cd and E significantly peaked in the interval right before the maze choice point in correct trials. However, the increase did not emerge in error trials. Additionally, Cd and E in two spaces displayed similar trends in correct trials. The difference was that the measures in NMF space were significantly greater than those in original space. Our findings indicated that the anticipatory changes in mPFC networks may have an effect on future WM behavioral choices. Moreover, the NMF analysis achieves a better characterization for a brain network.

Mistakes were made: neural mechanisms for the adaptive control of action initiation by the medial prefrontal cortex

Studies in rats, monkeys and humans have established that the medial prefrontal cortex is crucial for the ability to exert adaptive control over behavior. Here, we review studies on the role of the rat medial prefrontal cortex in adaptive control, with a focus on simple reaction time tasks that can be easily used across species and have clinical relevance. The performance of these tasks is associated with neural activity in the medial prefrontal cortex that reflects stimulus detection, action timing, and outcome monitoring. We describe rhythmic neural activity that occurs when animals initiate a temporally extended action. Such rhythmic activity is coterminous with major changes in population spike activity. Testing animals over a series of sessions with varying pre-stimulus intervals showed that the signals adapt to the current temporal demands of the task. Disruptions of rhythmic neural activity occur on error trials (premature responding) and lead to a persistent encoding of the error and a subsequent change in behavioral performance (i.e. post-error slowing). Analysis of simultaneously recorded spike activity suggests that the presence of strong theta rhythms is coterminous with altered network dynamics, and might serve as a mechanism for adaptive control. Computational modeling suggests that these signals may enable learning from errors. Together, our findings contribute to an emerging literature and provide a new perspective on the neuronal mechanisms for the adaptive control of action.

Behavioral flexibility in rats and mice: contributions of distinct frontocortical regions

Research examining the contribution of genetics to behavior is increasingly focused on higher order behavioral and cognitive processes including the ability to modify behaviors when environmental demands change. The frontal cortices of mammals, including rodents, subserve a diverse set of behavioral and cognitive functions including motor planning, social behavior, evaluation of expected outcomes and working memory, which may be particularly sensitive to genetic factors and interactions with experience (e.g. stress). Behavioral flexibility is a core attribute of these functions. This review orients readers to the current landscape of the literature on the frontocortical bases of behavioral flexibility in rodent laboratory experiments. Studies are divided into three broad categories: reversal learning, inhibitory learning and set-shifting. Functional dissociations within the broader scope of behavioral flexibility are reviewed, followed by discussion of the associations between specific components of frontal cortex and specific aspects of relevant behavioral processes. Finally, the authors identify open questions that need to be addressed to better establish the constituents of frontal cortex underlying behavioral flexibility.

Incoordination between spikes and LFPs in Aβ1-42-mediated memory deficits in rats

Alzheimer's disease (AD) is a neurodegenerative disease that gradually induces cognitive deficits. Impairments of working memory have been typically observed in AD. It is well known that spikes and local field potentials (LFPs) as well as the coordination between them encode information in normal brain function. However, the abnormal coordination between spikes and LFPs in the cognitive deficits of AD has remained largely unexplored. As amyloid-β peptide (Aβ) is a causative factor for the cognitive impairments of AD, developing a mechanistic understanding of the contribution of Aβ to cognitive impairments may yield important insights into the pathophysiology of AD. In the present study, we simultaneously recorded spikes and LFPs from multiple electrodes implanted in the prefrontal cortex of rats (control and intra-hippocampal Aβ injection group) that performed a Y-maze working memory task. The information changes in spikes and LFPs during the task were assessed by calculation of entropy. Then the coordination between spikes and LFPs was estimated by the correlation of LFP entropy and spike entropy. Compared with the control group, the Aβ group showed significantly weaker coordination between spikes and LFPs. Our results indicate that the incoordination between spikes and LFPs may provide a potential mechanism for the cognitive deficits in working memory of AD.

Transient inactivation of the medial prefrontal cortex impairs performance on a working memory-dependent conditional discrimination task

The rodent medial prefrontal cortex (mPFC) has been implicated in working memory function; lesions and inactivation of this region have been shown to result in impairments in spatial working memory (WM) tasks. Our laboratory has developed a tactile-visual conditional discrimination (CD) task, which uses floor insert cues to signal the correct goal-arm choice in a T maze. This task can be manipulated by altering the floor insert cues to be present throughout the trial (CDSTANDARD) or to be present only at the beginning of the trial (CDWM), thus making the task either WM-independent or WM-dependent, respectively. This ability to manipulate the working memory demand of the task while holding all other task features constant allows us to rule out the possibility that confounding performance variables contribute to the observed impairment. A previous study from our lab showed that mPFC inactivation did not impair performance on CDSTANDARD, confirming that mPFC inactivation does not induce sensorimotor or motivational deficits that could impact task performance. To examine whether mPFC inactivation impairs CDWM, the current study transiently inactivated the mPFC with bilateral microinfusions of muscimol immediately prior to testing on the CDWM task. As predicted, CDWM task performance was significantly impaired during the muscimol-infusion session compared with the control saline-infusion sessions. Together with our previous demonstration that the mPFC in not required for CDSTANDARD, these results not only confirm that the mPFC is crucial for working memory, but also set the stage for using the task-comparison approach to investigate corticolimbic interactions during working memory.

Complex neural codes in rat prelimbic cortex are stable across days on a spatial decision task

The rodent prelimbic cortex has been shown to play an important role in cognitive processing, and has been implicated in encoding many different parameters relevant to solving decision-making tasks. However, it is not known how the prelimbic cortex represents all these disparate variables, and if they are simultaneously represented when the task requires it. In order to investigate this question, we trained rats to run the Multiple-T Left Right Alternate (MT-LRA) task and recorded multi-unit ensembles from their prelimbic regions. Significant populations of cells in the prelimbic cortex represented the strategy controlling reward receipt on a given lap, whether the animal chose to go right or left on a given lap, and whether the animal made a correct decision or an error on a given lap. These populations overlapped in the cells recorded, with several cells demonstrating differential firing to all three variables. The spatial and strategic firing patterns of individual prelimbic cells were highly conserved across several days of running this task, indicating that each cell encoded the same information across days.

Neural structures underlying set-shifting: roles of medial prefrontal cortex and anterior cingulate cortex

Impaired attentional set-shifting and inflexible decision-making are problems frequently observed during normal aging and in several psychiatric disorders. To understand the neuropathophysiology of underlying inflexible behavior, animal models of attentional set-shifting have been developed to mimic tasks such as the Wisconsin Card Sorting Task (WCST), which tap into a number of cognitive functions including stimulus-response encoding, working memory, attention, error detection, and conflict resolution. Here, we review many of these tasks in several different species and speculate on how prefrontal cortex and anterior cingulate cortex might contribute to normal performance during set-shifting.

Reward-related activity in the medial prefrontal cortex is driven by consumption

An emerging literature suggests that the medial prefrontal cortex (mPFC) is crucial for the ability to track behavioral outcomes over time and has a critical role in successful foraging. Here, we examine this issue by analyzing changes in neuronal spike activity and local field potentials in the rat mPFC in relation to the consumption of rewarding stimuli. Using multi-electrode recording methods, we simultaneously recorded from ensembles of neurons and field potentials in the mPFC during the performance of an operant-delayed alternation task and a variable-interval licking procedure. In both tasks, we found that consummatory behavior (licking) activates many mPFC neurons and is associated with theta-band phase locking by mPFC field potentials. Many neurons that were modulated by the delivery of reward were also modulated when rats emitted bouts of licks during the period of consumption. The majority of these licking-modulated neurons were found in the rostral part of the prelimbic cortex, a region that is heavily interconnected with the gustatory insular cortex and projects to subcortical feeding-related centers. Based on the tight coupling between spike activity, theta-band phase locking, and licking behavior, we suggest that reward-related activity in the mPFC is driven by consummatory behavior.

The role of medial prefrontal cortex in memory and decision making

Some have claimed that the medial prefrontal cortex (mPFC) mediates decision making. Others suggest mPFC is selectively involved in the retrieval of remote long-term memory. Yet others suggests mPFC supports memory and consolidation on time scales ranging from seconds to days. How can all these roles be reconciled? We propose that the function of the mPFC is to learn associations between context, locations, events, and corresponding adaptive responses, particularly emotional responses. Thus, the ubiquitous involvement of mPFC in both memory and decision making may be due to the fact that almost all such tasks entail the ability to recall the best action or emotional response to specific events in a particular place and time. An interaction between multiple memory systems may explain the changing importance of mPFC to different types of memories over time. In particular, mPFC likely relies on the hippocampus to support rapid learning and memory consolidation.

Contrasting effects on path integration after hippocampal damage in humans and rats

The hippocampus and other medial temporal lobe structures have been linked to both memory and spatial cognition, but it has been unclear how these ideas are connected. We carried out parallel studies of path integration in patients with medial temporal lobe lesions and rats with hippocampal lesions. Subjects entered a circular arena without vision, searched for a target, and then attempted to return to the start location. Patients performed accurately, and as well as controls, so long as the outward path was relatively direct and the target was found within 20 s. In sharp contrast, rats with hippocampal lesions were impaired, even when the outward path was shorter than 1 m, involved no turns, and the target was found within 3 s. We suggest that patients succeeded because performance could be supported by working memory and that patients and rats differ after hippocampal lesions in their ability to construct a coherent working memory of spatial environments.

Memory in aged mice is rescued by enhanced expression of the GluN2B subunit of the NMDA receptor

The GluN2B subunit of the N-methyl-d-aspartate (NMDA) receptor shows age-related declines in expression across the frontal cortex and hippocampus. This decline is strongly correlated to age-related memory declines. This study was designed to determine if increasing GluN2B subunit expression in the frontal lobe or hippocampus would improve memory in aged mice. Mice were injected bilaterally with either the GluN2B vector, containing cDNA specific for the GluN2B subunit and enhanced green fluorescent protein (eGFP); a control vector or vehicle. Spatial memory, cognitive flexibility, and associative memory were assessed using the Morris water maze. Aged mice, with increased GluN2B subunit expression, exhibited improved long-term spatial memory, comparable to young mice. However, memory was rescued on different days in the Morris water maze; early for hippocampal GluN2B subunit enrichment and later for the frontal lobe. A higher concentration of the GluN2B antagonist, Ro 25-6981, was required to impair long-term spatial memory in aged mice with enhanced GluN2B expression, as compared to aged controls, suggesting there was an increase in the number of GluN2B-containing NMDA receptors. In addition, hippocampal slices from aged mice with increased GluN2B subunit expression exhibited enhanced NMDA receptor-mediated excitatory post-synaptic potentials (EPSP). Treatment with Ro 25-6981 showed that a greater proportion of the NMDA receptor-mediated EPSP was due to the GluN2B subunit in these animals, as compared to aged controls. These results suggest that increasing the production of the GluN2B subunit in aged animals enhances memory and synaptic transmission. Therapies that enhance GluN2B subunit expression within the aged brain may be useful for ameliorating age-related memory declines.

Inactivating anterior insular cortex reduces risk taking

We often have to make risky decisions between alternatives with outcomes that can be better or worse than the outcomes of safer alternatives. Although previous studies have implicated various brain regions in risky decision making, it remains unknown which regions are crucial for balancing whether to take a risk or play it safe. Here, we focused on the anterior insular cortex (AIC), the causal involvement of which in risky decision making is still unclear, although human imaging studies have reported AIC activation in various gambling tasks. We investigated the effects of temporarily inactivating the AIC on rats' risk preference in two types of gambling tasks, one in which risk arose in reward amount and one in which it arose in reward delay. As a control within the same subjects, we inactivated the adjacent orbitofrontal cortex (OFC), which is well known to affect risk preference. In both gambling tasks, AIC inactivation decreased risk preference whereas OFC inactivation increased it. In risk-free control situations, AIC and OFC inactivations did not affect decision making. These results suggest that the AIC is causally involved in risky decision making and promotes risk taking. The AIC and OFC may be crucial for the opposing motives of whether to take a risk or avoid it.

Noradrenergic control of error perseveration in medial prefrontal cortex

The medial prefrontal cortex (mPFC) plays a key role in behavioral variability, action monitoring, and inhibitory control. The functional role of mPFC may change over the lifespan due to a number of aging-related issues, including dendritic regression, increased cAMP signaling, and reductions in the efficacy of neuromodulators to influence mPFC processing. A key neurotransmitter in mPFC is norepinephrine. Previous studies have reported aging-related changes in the sensitivity of mPFC-dependent tasks to noradrenergic agonist drugs, such as guanfacine. Here, we assessed the effects of yohimbine, an alpha-2 noradrenergic antagonist, in cohorts of younger and older rats in a classic test of spatial working memory (using a T-maze). Older rats (23–29 mo.) were impaired by a lower dose of yohimbine compared to younger animals (5–10 mo.). To determine if the drug acts on alpha-2 noradrenergic receptors in mPFC and if its effects are specific to memory-guided performance, we made infusions of yohimbine into mPFC of a cohort of young rats (6 mo.) using an operant delayed response task. The task involved testing rats in blocks of trials with memory- and stimulus-guided performance. Yohimbine selectively impaired memory-guided performance and was associated with error perseveration. Infusions of muscimol (a GABA-A agonist) at the same sites also selectively impaired memory-guided performance, but did not lead to error perseveration. Based on these results, we propose several potential interpretations for the role for the noradrenergic system in the performance of delayed response tasks, including the encoding of previous response locations, task rules (i.e., using a win-stay strategy instead of a win-shift strategy), and performance monitoring (e.g., prospective encoding of outcomes).

The role of medial prefrontal cortex in memory and decision making

Some have claimed that the medial prefrontal cortex (mPFC) mediates decision making. Others suggest mPFC is selectively involved in the retrieval of remote long-term memory. Yet others suggests mPFC supports memory and consolidation on time scales ranging from seconds to days. How can all these roles be reconciled? We propose that the function of the mPFC is to learn associations between context, locations, events, and corresponding adaptive responses, particularly emotional responses. Thus, the ubiquitous involvement of mPFC in both memory and decision making may be due to the fact that almost all such tasks entail the ability to recall the best action or emotional response to specific events in a particular place and time. An interaction between multiple memory systems may explain the changing importance of mPFC to different types of memories over time. In particular, mPFC likely relies on the hippocampus to support rapid learning and memory consolidation.

Anticipatory activity in rat medial prefrontal cortex during a working memory task

OBJECTIVE

Working memory is a key cognitive function in which the prefrontal cortex plays a crucial role. This study aimed to show the firing patterns of a neuronal population in the prefrontal cortex of the rat in a working memory task and to explore how a neuronal ensemble encodes a working memory event.

METHODS

Sprague-Dawley rats were trained in a Y-maze until they reached an 80% correct rate in a working memory task. Then a 16-channel microelectrode array was implanted in the prefrontal cortex. After recovery, neuronal population activity was recorded during the task, using the Cerebus data-acquisition system. Spatio-temporal trains of action potentials were obtained from the original neuronal population signals.

RESULTS

During the Y-maze working memory task, some neurons showed significantly increased firing rates and evident neuronal ensemble activity. Moreover, the anticipatory activity was associated with the delayed alternate choice of the upcoming movement. In correct trials, the averaged pre-event firing rate (10.86 ± 1.82 spikes/bin) was higher than the post-event rate (8.17 ± 1.15 spikes/bin) (P<0.05). However, in incorrect trials, the rates did not differ.

CONCLUSION

The results indicate that the anticipatory activity of a neuronal ensemble in the prefrontal cortex may play a role in encoding working memory events.

Working with memory: evidence for a role for the medial prefrontal cortex in performance monitoring during spatial delayed alternation

Neuronal spike activity was recorded in the medial prefrontal cortex (mPFC) as rats performed an operant spatial delayed alternation task. The sensitivities of neurons to choice, outcome, and temporal information-related aspects of the task were examined. About one-third of neurons were sensitive to the location of delayed responding while animals were at one of two spatially distinct response ports. However, many fewer neurons (<10%) maintained choice information over the delay, each exhibiting persistent differences in firing rates for only a portion of the delay. Another third of cells encoded information about behavioral outcomes, and some of these neurons (>20% of all cells) fired at distinct rates in advance of correct and incorrect responses (i.e., prospective encoding of outcome). Other cells were sensitive to reward-related feedback stimuli (>20%), the outcome of the preceding trial (retrospective encoding, 5-10%), and/or the time since a trial was last performed (10-20%). An anatomical analysis of the recording sites found that cells that were sensitive to choice, temporal, and outcome information were commingled within the middle layers of the mPFC. Together, our results suggest that spatial processing is only part of what drives mPFC neurons to become active during spatial working memory tasks. We propose that the primary role of mPFC in these tasks is to monitor behavioral performance by encoding information about recent trial outcomes to guide expectations and responses on the current trial. By encoding these variables, the mPFC is able to exert control over action and ensure that tasks are performed effectively and efficiently.

Characterizing cognitive aging of working memory and executive function in animal models

Executive functions supported by prefrontal cortical (PFC) systems provide essential control and planning mechanisms to guide goal-directed behavior. As such, age-related alterations in executive functions can mediate profound and widespread deficits on a diverse array of neurocognitive processes. Many of the critical neuroanatomical and functional characteristics of prefrontal cortex are preserved in rodents, allowing for meaningful cross species comparisons relevant to the study of cognitive aging. In particular, as rodents lend themselves to genetic, cellular and biochemical approaches, rodent models of executive function stand to significantly contribute to our understanding of the critical neurobiological mechanisms that mediate decline of executive processes across the lifespan. Moreover, rodent analogs of executive functions that decline in human aging represent an essential component of a targeted, rational approach for developing and testing effective treatment and prevention therapies for age-related cognitive decline. This paper reviews behavioral approaches used to study executive function in rodents, with a focus on those assays that share a foundation in the psychological and neuroanatomical constructs important for human aging. A particular emphasis is placed on behavioral approaches used to assess working memory and cognitive flexibility, which are sensitive to decline with age across species and for which strong rodent models currently exist. In addition, other approaches in rodent behavior that have potential for providing analogs to functions that reliably decline to human aging (e.g., information processing speed) are discussed.

Lost in transition: aging-related changes in executive control by the medial prefrontal cortex

Neural correlates of aging in the medial prefrontal cortex (mPFC) were studied using an operant delayed response task. The task used blocks of trials with memory-guided (delayed alternation) and visually-guided (stimulus-response) responding. Older rats (24 months) performed at a slow pace compared with younger rats (6 months). They wasted time engaged in nonessential behaviors (e.g., licking on spouts beyond the period of reward delivery) and were slow to respond at the end of the delay period. Aged mPFC neurons showed normal spatial processing. They differed from neurons in younger rats by having reduced modulations by imperative stimuli indicating reward availability and reduced activity associated with response latencies for reward collection. Older rats showed reduced sensitivity to imperative stimuli at three levels of neural activity: reduced fractions of neurons with changes in firing rate around the stimulus, reduced correlation over neurons at the time of the stimulus as measured with analysis of population activity, and reduced amplitudes of event-related fluctuations in intracortical field potentials at the time of the imperative stimulus. Our findings suggest that aging alters the encoding of time-sensitive information and impairs the ability of prefrontal networks to keep subjects "on task."

Prefrontal cortical mechanisms underlying delayed alternation in mice

The prefrontal cortex (PFC) has been implicated in the maintenance of task-relevant information during goal-directed behavior. Using a combination of lesions, local inactivation, and optogenetics, we investigated the functional role of the medial prefrontal cortex (mPFC) in mice with a novel operant delayed alternation task. Task difficulty was manipulated by changing the duration of the delay between two sequential actions. In experiment 1, we showed that excitotoxic lesions of the mPFC impaired acquisition of delayed alternation with long delays (16 s), whereas lesions of the dorsal hippocampus and ventral striatum, areas connected with the PFC, did not produce any deficits. Lesions of dorsal hippocampus, however, significantly impaired reversal learning when the rule was changed from alternation to repetition. In experiment 2, we showed that local infusions of muscimol (an agonist of the GABA(A) receptor) into mPFC impaired performance even when the animal was well trained, suggesting that the mPFC is critical not only for acquisition but also for successful performance. In experiment 3, to examine the mechanisms underlying the role of GABAergic inhibition, we used Cre-inducible Channelrhodopsin-2 to activate parvalbumin (PV)-expressing GABAergic interneurons in the mPFC of PV-Cre transgenic mice as they performed the task. Using whole cell patch-clamp recording, we demonstrated that activation of PV-expressing interneurons in vitro with blue light in brain slices reliably produced spiking and inhibited nearby pyramidal projection neurons. With similar stimulation parameters, in vivo stimulation significantly impaired delayed alternation performance. Together these results demonstrate a critical role for the mPFC in the acquisition and performance of the delayed alternation task.

Impact of prenatal ischemia on behavior, cognitive abilities and neuroanatomy in adult rats with white matter damage

Early brain damage, such as white matter damage (WMD), resulting from perinatal hypoxia-ischemia in preterm and low birth weight infants represents a high risk factor for mortality and chronic disabilities, including sensory, motor, behavioral and cognitive disorders. In previous studies, we developed a model of WMD based on prenatal ischemia (PI), induced by unilateral ligation of uterine artery at E17 in pregnant rats. We have shown that PI reproduced some of the main deficits observed in preterm infants, such as white and gray matter damage, myelination deficits, locomotor, sensorimotor, and short-term memory impairments, as well as related musculoskeletal and neuroanatomical histopathologies [1-3]. Here, we determined the deleterious impact of PI on several behavioral and cognitive abilities in adult rats, as well as on the neuroanatomical substratum in various related brain areas. Adult PI rats exhibited spontaneous exploratory and motor hyperactivity, deficits in information encoding, and deficits in short- and long-term object memory tasks, but no impairments in spatial learning or working memory in watermaze tasks. These results were in accordance with white matter injury and damage in the medial and lateral entorhinal cortices, as detected by axonal degeneration, astrogliosis and neuronal density. Although there was astrogliosis and axonal degeneration in the fornix, hippocampus and cingulate cortex, neuronal density in the hippocampus and cingulate cortex was not affected by PI. Levels of spontaneous hyperactivity, deficits in object memory tasks, neuronal density in the medial and lateral entorhinal cortices, and astrogliosis in the fornix correlated with birth weight in PI rats. Thus, this rodent model of WMD based on PI appears to recapitulate the main neurobehavioral and neuroanatomical human deficits often observed in preterm children with a perinatal history of ischemia.

Space representation in the prefrontal cortex

The representation of space and its function in the prefrontal cortex have been examined using a variety of behavioral tasks. Among them, since the delayed-response task requires the temporary maintenance of spatial information, this task has been used to examine the mechanisms of spatial representation. In addition, the concept of working memory to explain prefrontal functions has helped us to understand the nature and functions of space representation in the prefrontal cortex. The detailed analysis of delay-period activity observed in spatial working memory tasks has provided important information for understanding space representation in the prefrontal cortex. Directional delay-period activity has been shown to be a neural correlate of the mechanism for temporarily maintaining information and represent spatial information for the visual cue and the saccade. In addition, many task-related prefrontal neurons exhibit spatially selective activities. These neurons are also important components of spatial information processing. In fact, information flow from sensory-related neurons to motor-related neurons has been demonstrated, along with a change in spatial representation as the trial progresses. The dynamic functional interactions among neurons exhibiting different task-related activities and representing different aspects of information could play an essential role in information processing. In addition, information provided from other cortical or subcortical areas might also be necessary for the representation of space in the prefrontal cortex. To better understand the representation of space and its function in the prefrontal cortex, we need to understand the nature of functional interactions between the prefrontal cortex and other cortical and subcortical areas.

Persistent activity in a cortical-to-subcortical circuit: bridging the temporal gap in trace eyelid conditioning

We have addressed the source and nature of the persistent neural activity that bridges the stimulus-free gap between the conditioned stimulus (CS) and unconditioned stimulus (US) during trace eyelid conditioning. Previous work has demonstrated that this persistent activity is necessary for trace eyelid conditioning: CS-elicited activity in mossy fiber inputs to the cerebellum does not extend into the stimulus-free trace interval, which precludes the cerebellar learning that mediates conditioned response expression. In behaving rabbits we used in vivo recordings from a region of medial prefrontal cortex (mPFC) that is necessary for trace eyelid conditioning to test the hypothesis that neurons there generate activity that persists beyond CS offset. These recordings revealed two patterns of activity during the trace interval that would enable cerebellar learning. Activity in some cells began during the tone CS and persisted to overlap with the US, whereas in other cells, activity began during the stimulus-free trace interval. Injection of anterograde tracers into this same region of mPFC revealed dense labeling in the pontine nuclei, where recordings also revealed tone-evoked persistent activity during trace conditioning. These data suggest a corticopontine pathway that provides an input to the cerebellum during trace conditioning trials that bridges the temporal gap between the CS and US to engage cerebellar learning. As such, trace eyelid conditioning represents a well-characterized and experimentally tractable system that can facilitate mechanistic analyses of cortical persistent activity and how it is used by downstream brain structures to influence behavior.

Neuroanatomical, sensorimotor and cognitive deficits in adult rats with white matter injury following prenatal ischemia

Perinatal brain injury including white matter damage (WMD) is highly related to sensory, motor or cognitive impairments in humans born prematurely. Our aim was to examine the neuroanatomical, functional and behavioral changes in adult rats that experienced prenatal ischemia (PI), thereby inducing WMD. PI was induced by unilateral uterine artery ligation at E17 in pregnant rats. We assessed performances in gait, cognitive abilities and topographical organization of maps, and neuronal and glial density in primary motor and somatosensory cortices, the hippocampus and prefrontal cortex, as well as axonal degeneration and astrogliosis in white matter tracts. We found WMD in corpus callosum and brainstem, and associated with the hippocampus and somatosensory cortex, but not the motor cortex after PI. PI rats exhibited mild locomotor impairments associated with minor signs of spasticity. Motor map organization and neuronal density were normal in PI rats, contrasting with major somatosensory map disorganization, reduced neuronal density, and a marked reduction of inhibitory interneurons. PI rats exhibited spontaneous hyperactivity in open-field test and short-term memory deficits associated with abnormal neuronal density in related brain areas. Thus, this model reproduces in adult PI rats the main deficits observed in infants with a perinatal history of hypoxia-ischemia and WMD.

An analysis of rat prefrontal cortex in mediating executive function

While it is acknowledged that species specific differences are an implicit condition of comparative studies, rodent models of prefrontal function serve a significant role in the acquisition of converging evidence on prefrontal function across levels of analysis and research techniques. The purpose of the present review is to examine whether the prefrontal cortex (PFC) in rats supports a variety of processes associated with executive function including working memory, temporal processing, planning (prospective coding), flexibility, rule learning, and decision making. Therefore, in this review we examined changes associated with working memory processes for spatial locations, visual objects, odors, tastes, and response domains or attributes, temporal processes including temporal order, sequence learning, prospective coding, behavioral flexibility associated with reversal learning and set shifting, paired associate learning, and decision making based on effort, time discounting, and uncertainty following damage to the PFC in rats. In addition, potential parallel processes of executive function in monkeys and humans based on several theories of subregional differentiation within the PFC will be presented. Specifically, theories based on domain or attribute specificity (Goldman-Rakic, 1996), level of processing (Petrides, 1996), rule learning based on complexity (Wise, Murray, & Gerfen, 1996), executive functions based on connectivity with other brain regions associated with top-down control (Miller & Cohen, 2001), are presented and applied to PFC function in rats with the aim of understanding subregional specificity in the rat PFC. The data suggest that there is subregional specificity within the PFC of rats, monkey and humans and there are parallel cognitive functions of the different subregions of the PFC in rats, monkeys and humans.

Gene regulation in the rat prefrontal cortex after learning with or without cholinergic insult

The prefrontal cortex is essential for a wide variety of higher functions, including attention and memory. Cholinergic neurons are thought to be of prime importance in the modulation of these processes. Degeneration of forebrain cholinergic neurons has been linked to several neurological disorders. The present study was designed to identify genes and networks in rat prefrontal cortex that are associated with learning and cholinergic-loss-memory deficit. Affymetrix microarray technology was used to screen gene expression changes in rats submitted or not to 192 IgG-saporin immunolesion of cholinergic basal forebrain and trained in spatial/object novelty tasks. Results showed learning processes were associated with significant expression of genes, which were organized in several clusters of highly correlated genes and would be involved in biological processes such as intracellular signaling process, transcription regulation, and filament organization and axon guidance. Memory loss following cortical cholinergic deafferentation was associated with significant expression of genes belonging to only one clearly delineated cluster and would be involved in biological processes related to cytoskeleton organization and proliferation, and glial and vascular remodeling, i.e., in processes associated with brain repair after injury.