Topographic organization of the striatal and thalamic connections of rat medial agranular cortex

Sex-dependent noradrenergic modulation of premotor cortex during decision-making

Rodent premotor cortex (M2) integrates information from sensory and cognitive networks for action planning during goal-directed decision-making. M2 function is regulated by cortical inputs and ascending neuromodulators, including norepinephrine (NE) released from the locus coeruleus (LC). LC-NE has been shown to modulate the signal-to-noise ratio of neural representations in target cortical regions, increasing the salience of relevant stimuli. Using rats performing a two-alternative forced choice task after administration of a β-noradrenergic antagonist (propranolol), we show that β-noradrenergic signaling is necessary for effective action plan signals in anterior M2. Loss of β-noradrenergic signaling results in failure to suppress irrelevant action plans in anterior M2 disrupting decoding of cue-related information, delaying decision times, and increasing trial omissions, particularly in females. Furthermore, we identify a potential mechanism for the sex bias in behavioral and neural changes after propranolol administration via differential expression of β2 noradrenergic receptor RNA across sexes in anterior M2, particularly on local inhibitory neurons. Overall, we show a critical role for β-noradrenergic signaling in anterior M2 during decision-making by suppressing irrelevant information to enable efficient action planning and decision-making.

Chronic H3R activation reduces L-Dopa-induced dyskinesia, normalizes cortical GABA and glutamate levels, and increases striatal dopamine D1R mRNA expression in 6-hydroxydopamine-lesioned male rats

RATIONALE

Dyskinesias induced by L-3,4-dihydroxyphenylalanine, L-Dopa (LIDs), are the major complication in the pharmacological treatment of Parkinson's disease. LIDs induce overactivity of the glutamatergic cortico-striatal projections, and drugs that reduce glutamatergic overactivity exert antidyskinetic actions. Chronic administration of immepip, agonist at histamine H₃ receptors (H₃R), reduces LIDs and diminishes GABA and glutamate content in striatal dialysates (Avila-Luna et al., Psychopharmacology 236: 1937-1948, 2019).

OBJECTIVES AND METHODS

In rats unilaterally lesioned with 6-hydroxydopamine in the substantia nigra pars compacta (SNc), we examined whether the chronic administration of immepip and their withdrawal modify LIDs, the effect of L-Dopa on glutamate and GABA content, and mRNA levels of dopamine D₁ receptors (D₁Rs) and H₃Rs in the cerebral cortex and striatum.

RESULTS

The administration of L-Dopa for 21 days induced LIDs. This effect was accompanied by increased GABA and glutamate levels in the cerebral cortex ipsi and contralateral to the lesioned SNc, and immepip administration prevented (GABA) or reduced (glutamate) these actions. In the striatum, GABA content increased in the ipsilateral nucleus, an effect prevented by immepip. L-Dopa administration had no significant effects on striatal glutamate levels. In lesioned and L-Dopa-treated animals, D₁R mRNA decreased in the ipsilateral striatum, an effect prevented by immepip administration.

CONCLUSIONS

Our results indicate that chronic H₃R activation reduces LIDs and the overactivity of glutamatergic cortico-striatal projections, providing further evidence for an interaction between D₁Rs and H₃Rs in the cortex and striatum under normal and pathological conditions.

Striatal spatial heterogeneity, clustering, and white matter association of GFAP+ astrocytes in a mouse model of Huntington's disease

Introduction

Huntington's disease (HD) is a neurodegenerative disease that primarily affects the striatum, a brain region that controls movement and some forms of cognition. Neuronal dysfunction and loss in HD is accompanied by increased astrocyte density and astrocyte pathology. Astrocytes are a heterogeneous population classified into multiple subtypes depending on the expression of different gene markers. Studying whether mutant Huntingtin (HTT) alters specific subtypes of astrocytes is necessary to understand their relative contribution to HD.

Methods

Here, we studied whether astrocytes expressing two different markers; glial fibrillary acidic protein (GFAP), associated with astrocyte activation, and S100 calcium-binding protein B (S100B), a marker of matured astrocytes and inflammation, were differentially altered in HD.

Results

First, we found three distinct populations in the striatum of WT and symptomatic zQ175 mice: GFAP⁺, S100B⁺, and dual GFAP⁺S100B⁺. The number of GFAP⁺ and S100B⁺ astrocytes throughout the striatum was increased in HD mice compared to WT, coinciding with an increase in HTT aggregation. Overlap between GFAP and S100B staining was expected, but dual GFAP⁺S100B⁺ astrocytes only accounted for less than 10% of all tested astrocytes and the number of GFAP⁺S100B⁺ astrocytes did not differ between WT and HD, suggesting that GFAP⁺ astrocytes and S100B⁺ astrocytes are distinct types of astrocytes. Interestingly, a spatial characterization of these astrocyte subtypes in HD mice showed that while S100B⁺ were homogeneously distributed throughout the striatum, GFAP⁺ preferentially accumulated in "patches" in the dorsomedial (dm) striatum, a region associated with goal-directed behaviors. In addition, GFAP⁺ astrocytes in the dm striatum of zQ175 mice showed increased clustering and association with white matter fascicles and were preferentially located in areas with low HTT aggregate load.

Discussion

In summary, we showed that GFAP⁺ and S100B⁺ astrocyte subtypes are distinctly affected in HD and exist in distinct spatial arrangements that may offer new insights to the function of these specific astrocytes subtypes and their potential implications in HD pathology.

Structural and functional organization of the midline and intralaminar nuclei of the thalamus

The midline and intralaminar nuclei of the thalamus form a major part of the "limbic thalamus;" that is, thalamic structures anatomically and functionally linked with the limbic forebrain. The midline nuclei consist of the paraventricular (PV) and paratenial nuclei, dorsally and the rhomboid and nucleus reuniens (RE), ventrally. The rostral intralaminar nuclei (ILt) consist of the central medial (CM), paracentral (PC) and central lateral (CL) nuclei. We presently concentrate on RE, PV, CM and CL nuclei of the thalamus. The nucleus reuniens receives a diverse array of input from limbic-related sites, and predominantly projects to the hippocampus and to "limbic" cortices. The RE participates in various cognitive functions including spatial working memory, executive functions (attention, behavioral flexibility) and affect/fear behavior. The PV receives significant limbic-related afferents, particularly the hypothalamus, and mainly distributes to "affective" structures of the forebrain including the bed nucleus of stria terminalis, nucleus accumbens and the amygdala. Accordingly, PV serves a critical role in "motivated behaviors" such as arousal, feeding/consummatory behavior and drug addiction. The rostral ILt receives both limbic and sensorimotor-related input and distributes widely over limbic and motor regions of the frontal cortex-and throughout the dorsal striatum. The intralaminar thalamus is critical for maintaining consciousness and directly participates in various sensorimotor functions (visuospatial or reaction time tasks) and cognitive tasks involving striatal-cortical interactions. As discussed herein, while each of the midline and intralaminar nuclei are anatomically and functionally distinct, they collectively serve a vital role in several affective, cognitive and executive behaviors - as major components of a brainstem-diencephalic-thalamocortical circuitry.

Concomitant Processing of Choice and Outcome in Frontal Corticostriatal Ensembles Correlates with Performance of Rats

The frontal cortex-basal ganglia network plays a pivotal role in adaptive goal-directed behaviors. Medial frontal cortex (MFC) encodes information about choices and outcomes into sequential activation of neural population, or neural trajectory. While MFC projects to the dorsal striatum (DS), whether DS also displays temporally coordinated activity remains unknown. We studied this question by simultaneously recording neural ensembles in the MFC and DS of rodents performing an outcome-based alternative choice task. We found that the two regions exhibited highly parallel evolution of neural trajectories, transforming choice information into outcome-related information. When the two trajectories were highly correlated, spike synchrony was task-dependently modulated in some MFC-DS neuron pairs. Our results suggest that neural trajectories concomitantly process decision-relevant information in MFC and DS with increased spike synchrony between these regions.

Role of the Dopaminergic System in the Striatum and Its Association With Functional Recovery or Rehabilitation After Brain Injury

Disabilities are estimated to occur in approximately 2% of survivors of traumatic brain injury (TBI) worldwide, and disability may persist even decades after brain injury. Facilitation or modulation of functional recovery is an important goal of rehabilitation in all patients who survive severe TBI. However, this recovery tends to vary among patients because it is affected by the biological and physical characteristics of the patients; the types, doses, and application regimens of the drugs used; and clinical indications. In clinical practice, diverse dopaminergic drugs with various dosing and application procedures are used for TBI. Previous studies have shown that dopamine (DA) neurotransmission is disrupted following moderate to severe TBI and have reported beneficial effects of drugs that affect the dopaminergic system. However, the mechanisms of action of dopaminergic drugs have not been completely clarified, partly because dopaminergic receptor activation can lead to restoration of the pathway of the corticobasal ganglia after injury in brain structures with high densities of these receptors. This review aims to provide an overview of the functionality of the dopaminergic system in the striatum and its roles in functional recovery or rehabilitation after TBI.

Retrosplenial Cortical Connectivity with Frontal Basal Ganglia Networks

Previous studies of the retrosplenial cortex (RSC) have focused on its role in navigation and memory, consistent with its well-established medial temporal connections, but recent evidence also suggests a role for this region in reward and decision making. Because function is determined largely by anatomical connections, and to better understand the anatomy of RSC, we used tract-tracing methods to examine the anatomical connectivity between the rat RSC and frontostriatal networks (canonical reward and decision-making circuits). We find that, among frontal cortical regions, RSC bidirectionally connects most strongly with the ACC, but also with an area of the central-medial orbito-frontal cortex. RSC projects to the dorsomedial striatum, and its terminal fields are virtually encompassed by the frontal-striatal projection zone, suggestive of functional convergence through the basal ganglia. This overlap is driven by ACC, prelimbic cortex, and orbito-frontal cortex, all of which contribute to goal-directed decision making, suggesting that the RSC is involved in similar processes.

Neural Circuits for Reward

Innate behaviors often viewed as genetically predetermined behaviors. However, in the environment animals often are subjected to external stimuli conflicting with those. Thus, animals subsequently need to change those behaviors to survive and reproduce. In the brain, the reward pathway is well-known for its role to adjust behaviors according to external stimuli, or rewards. However, only recently the relationship between reward pathway and innate behavior begins to be explored. In this review, we summarize the recent data on this subject from rodent studies which suggest an important role of this crosstalk between circuits involved in reward pathway and innate behaviors. We also discuss some of the neurotransmitters and neuromodulators underlying this crosstalk and the related mechanisms.

Motor cortex can directly drive the globus pallidus neurons in a projection neuron type-dependent manner in the rat

The basal ganglia are critical for the control of motor behaviors and for reinforcement learning. Here, we demonstrate in rats that primary and secondary motor areas (M1 and M2) make functional synaptic connections in the globus pallidus (GP), not usually thought of as an input site of the basal ganglia. Morphological observation revealed that the density of axonal boutons from motor cortices in the GP was 47% and 78% of that in the subthalamic nucleus (STN) from M1 and M2, respectively. Cortical excitation of GP neurons was comparable to that of STN neurons in slice preparations. FoxP2-expressing arkypallidal neurons were preferentially innervated by the motor cortex. The connection probability of cortico-pallidal innervation was higher for M2 than M1. These results suggest that cortico-pallidal innervation is an additional excitatory input to the basal ganglia, and that it can affect behaviors via the cortex-basal ganglia-thalamus motor loop.

Enhanced Population Coding for Rewarded Choices in the Medial Frontal Cortex of the Mouse

Instrumental behavior is characterized by the selection of actions based on the degree to which they lead to a desired outcome. However, we lack a detailed understanding of how rewarded actions are reinforced and preferentially implemented. In rodents, the medial frontal cortex is hypothesized to play an important role in this process, based in part on its capacity to encode chosen actions and their outcomes. We therefore asked how neural representations of choice and outcome might interact to facilitate instrumental behavior. To investigate this question, we imaged neural ensemble activity in layer 2/3 of the secondary motor region (M2) while mice engaged in a two-choice auditory discrimination task with probabilistic outcomes. Correct choices could result in one of three reward amounts (single, double or omitted reward), which allowed us to measure neural and behavioral effects of reward magnitude, as well as its categorical presence or absence. Single-unit and population decoding analyses revealed a consistent influence of outcome on choice signals in M2. Specifically, rewarded choices were more robustly encoded relative to unrewarded choices, with little dependence on the exact magnitude of reinforcement. Our results provide insight into the integration of past choices and outcomes in the rodent brain during instrumental behavior.

Functional and anatomical relationships between the medial precentral cortex, dorsal striatum, and head direction cell circuitry. II. Neuroanatomical studies

An animal's directional heading within its environment is encoded by the activity of head direction (HD) cells. In rodents, these neurons are found primarily within the limbic system in the interconnected structures that form the limbic HD circuit. In our accompanying report in this issue, we describe two HD cell populations located outside of this circuit in the medial precentral cortex (PrCM) and dorsal striatum (DS). These extralimbic areas receive their HD signals from the limbic system but do not provide critical input or feedback to limbic HD cells (Mehlman ML, Winter SS, Valerio S, Taube JS. J Neurophysiol 121: 350-370, 2019.). In this report, we complement our previous lesion and recording experiments with a series of neuroanatomical tracing studies in rats designed to examine patterns of connectivity between the PrCM, DS, limbic HD circuit, and related spatial processing circuitry. Retrograde tracing revealed that the DS receives direct input from numerous structures known to contain HD cells and/or other spatially tuned cell types. Importantly, these projections preferentially target and converge within the most medial portion of the DS, the same area in which we previously recorded HD cells. The PrCM receives direct input from a subset of these spatial processing structures. Anterograde tracing identified indirect pathways that could permit the PrCM and DS to convey self-motion information to the limbic HD circuit. These tracing studies reveal the anatomical basis for the functional relationships observed in our lesion and recording experiments. Collectively, these findings expand our understanding of how spatial processing circuitry functionally and anatomically extends beyond the limbic system into the PrCM and DS. NEW & NOTEWORTHY Head direction (HD) cells are located primarily within the limbic system, but small populations of extralimbic HD cells are found in the medial precentral cortex (PrCM) and dorsal striatum (DS). The neuroanatomical tracing experiments reported here explored the pathways capable of transmitting the HD signal to these extralimbic areas. We found that projections arising from numerous spatial processing structures converge within portions of the PrCM and DS that contain HD cells.

Pyramidal Cell Subtypes and Their Synaptic Connections in Layer 5 of Rat Frontal Cortex

The frontal cortical areas make a coordinated response that generates appropriate behavior commands, using individual local circuits with corticostriatal and corticocortical connections in longer time scales than sensory areas. In secondary motor cortex (M2), situated between the prefrontal and primary motor areas, major subtypes of layer 5 corticostriatal cells are crossed-corticostriatal (CCS) cells innervating both sides of striatum, and corticopontine (CPn) cells projecting to the ipsilateral striatum and pontine nuclei. CCS cells innervate CPn cells unidirectionally: the former are therefore hierarchically higher than the latter among L5 corticostriatal cells. CCS cells project directly to both frontal and nonfrontal areas. On the other hand, CPn cells innervate the thalamus and layer 1a of frontal areas, where thalamic fibers relaying basal ganglia outputs are distributed. Thus, CCS cells can make activities of frontal areas in concert with those of nonfrontal area using corticocortical loops, whereas CPn cells are more involved in closed corticostriatal loops than CCS cells. Since reciprocal connections between CPn cells with facilitatory synapses may be related to persistent activity, CPn cells play a key role of longer time constant processes in corticostriatal as well as in corticocortical loops between the frontal areas.

Mediodorsal Thalamic Neurons Mirror the Activity of Medial Prefrontal Neurons Responding to Movement and Reinforcement during a Dynamic DNMTP Task

The mediodorsal nucleus (MD) interacts with medial prefrontal cortex (mPFC) to support learning and adaptive decision-making. MD receives driver (layer 5) and modulatory (layer 6) projections from PFC and is the main source of driver thalamic projections to middle cortical layers of PFC. Little is known about the activity of MD neurons and their influence on PFC during decision-making. We recorded MD neurons in rats performing a dynamic delayed nonmatching to position (dDNMTP) task and compared results to a previous study of mPFC with the same task (Onos et al., 2016). Criterion event-related responses were observed for 22% (254/1179) of neurons recorded in MD, 237 (93%) of which exhibited activity consistent with mPFC response types. More MD than mPFC neurons exhibited responses related to movement (45% vs. 29%) and reinforcement (51% vs. 27%). MD had few responses related to lever presses, and none related to preparation or memory delay, which constituted 43% of event-related activity in mPFC. Comparison of averaged normalized population activity and population response times confirmed the broad similarity of common response types in MD and mPFC and revealed differences in the onset and offset of some response types. Our results show that MD represents information about actions and outcomes essential for decision-making during dDNMTP, consistent with evidence from lesion studies that MD supports reward-based learning and action-selection. These findings support the hypothesis that MD reinforces task-relevant neural activity in PFC that gives rise to adaptive behavior.

Sensory Processing in the Dorsolateral Striatum: The Contribution of Thalamostriatal Pathways

The dorsal striatum has two functionally-defined subdivisions: a dorsomedial striatum (DMS) region involved in mediating goal-directed behaviors that require conscious effort, and a dorsolateral striatum (DLS) region involved in the execution of habitual behaviors in a familiar sensory context. Consistent with its presumed role in forming stimulus-response (S-R) associations, neurons in DLS receive massive inputs from sensorimotor cortex and are responsive to both active and passive sensory stimulation. While several studies have established that corticostriatal inputs contribute to the stimulus-induced responses observed in the DLS, there is growing awareness that the thalamus has a significant role in conveying sensory-related information to DLS and other parts of the striatum. The thalamostriatal projections to DLS originate mainly from the caudal intralaminar region, which contains the parafascicular (Pf) nucleus, and from higher-order thalamic nuclei such as the medial part of the posterior (POm) nucleus. Based on recent findings, we hypothesize that the thalamostriatal projections from these two regions exert opposing influences on the expression of behavioral habits. This article reviews the subcortical circuits that regulate the transmission of sensory information through these thalamostriatal projection systems, and describes the evidence that indicates these circuits could be manipulated to ameliorate the symptoms of Parkinson's disease (PD) and related neurological disorders.

Secondary Motor Cortex: Where 'Sensory' Meets 'Motor' in the Rodent Frontal Cortex

In rodents, the medial aspect of the secondary motor cortex (M2) is known by other names, including medial agranular cortex (AGm), medial precentral cortex (PrCm), and frontal orienting field (FOF). As a subdivision of the medial prefrontal cortex (mPFC), M2 can be defined by a distinct set of afferent and efferent connections, microstimulation responses, and lesion outcomes. However, the behavioral role of M2 remains mysterious. Here, we focus on evidence from rodent studies, highlighting recent findings of early and context-dependent choice-related activity in M2 during voluntary behavior. Based on the current understanding, we suggest that a major function for M2 is to flexibly map antecedent signals such as sensory cues to motor actions, thereby enabling adaptive choice behavior.

Unilateral lesions of the dorsocentral striatum (DCS) disrupt spatial and temporal characteristics of food protection behavior

Spatial and temporal information processing provide a foundation for higher cognitive functions. The survival of animals depends on integrating spatial and temporal information to organize behavior. In general, previous research has focused on only one source of information processing; however, there is evidence to support a convergence in the processing of egocentric-spatial and temporal information within a cortico-striatal system of structures. The current study evaluated the contributions of the dorsocentral striatum (DCS) to egocentric-spatial and temporal (within the seconds-to-minutes range) processing of information using a food protection task. Long-Evans rats received unilateral NMDA lesions of the DCS followed by testing in a food protection task. Performance in this task is mediated by the motivation of the animal to consume a food item, their perception of the time required to consume a food item, their sensory ability to process egocentric cues, and their motor ability to evade an incoming conspecific. Unilateral DCS lesions were shown to impact both spatial and temporal characteristics of food protection. These results suggest that the DCS may be a critical structure for the integration of egocentric-spatial and temporal information within the interval timing range.

Theta Rhythm of the Hippocampus: Subcortical Control and Functional Significance

The theta rhythm is the largest extracellular synchronous signal that can be recorded from the mammalian brain and has been strongly implicated in mnemonic processes of the hippocampus. We describe (a) ascending brain stem–forebrain systems involved in controlling theta and nontheta (desynchronization) states of the hippocampal electroencephalogram; (b) theta rhythmically discharging cells in several structures of Papez's circuit and their possible functional significance, specifically with respect to head direction cells in this same circuit; and (c) the role of nucleus reuniens of the thalamus as a major interface between the medial prefrontal cortex and hippocampus and as a prominent source of afferent limbic information to the hippocampus. We suggest that the hippocampus receives two main types of input: theta rhythm from ascending brain stem– diencephaloseptal systems and information bearing mainly from thalamocortical/cortical systems. The temporal convergence of activity of these two systems results in the encoding of information in the hippocampus, primarily reaching it from the entorhinal cortex and nucleus reuniens.

Inhibition of mammalian target of rapamycin activation in the rostral anterior cingulate cortex attenuates pain-related aversion in rats

Pain is a complex experience that comprises both sensory and affective dimensions. Mammalian target of rapamycin (mTOR) plays an important role in the modulation of neuronal plasticity associated with the pathogenesis of pain sensation. However, the role of mTOR in pain affect is unclear. Using a formalin-induced conditioned place avoidance (F-CPA) test, the current study investigated the effects of the mTOR specific inhibitor rapamycin on noxious stimulation induced aversion in the rostral anterior cingulate cortex (rACC). Intraplantar injection of 5% formalin was associated with significant activation of mTOR, as well as p70 ribosomal S6 protein (p70S6K), its downstream effector, in the rACC. The inhibition of mTOR activation with rapamycin disrupted pain-related aversion; however, this inhibition did not affect formalin-induced spontaneous nociceptive behaviors in rats. These findings demonstrated for the first time that mTOR and its downstream pathway in the rACC contribute to the induction of pain-related negative emotion.

Limbic circuitry of the midline thalamus

The thalamus was subdivided into three major groups: sensorimotor nuclei (or principal/relay nuclei), limbic nuclei and nuclei bridging these two domains. Limbic nuclei of thalamus (or 'limbic thalamus') consist of the anterior nuclei, midline nuclei, medial division of the mediodorsal nucleus (MDm) and central medial nucleus (CM) of the intralaminar complex. The midline nuclei include the paraventricular (PV) and paratenial (PT) nuclei, dorsally, and the reuniens (RE) and rhomboid (RH) nuclei, ventrally. The 'limbic' thalamic nuclei predominantly connect with limbic-related structures and serve a direct role in limbic-associated functions. Regarding the midline nuclei, RE/RH mainly target limbic cortical structures, particularly the hippocampus and the medial prefrontal cortex. Accordingly, RE/RH participate in functions involving interactions of the HF and mPFC. By contrast, PV/PT mainly project to limbic subcortical structures, particularly the amygdala and nucleus accumbens, and hence are critically involved in affective behaviors such as stress/anxiety, feeding behavior, and drug seeking activities. The anatomical/functional characteristics of MDm and CM are very similar to those of the midline nuclei and hence the collection of nuclei extending dorsoventrally along the midline/paramidline of the thalamus constitute the core of the 'limbic thalamus'.

A mesoscale connectome of the mouse brain

Comprehensive knowledge of the brain's wiring diagram is fundamental for understanding how the nervous system processes information at both local and global scales. However, with the singular exception of the C. elegans microscale connectome, there are no complete connectivity data sets in other species. Here we report a brain-wide, cellular-level, mesoscale connectome for the mouse. The Allen Mouse Brain Connectivity Atlas uses enhanced green fluorescent protein (EGFP)-expressing adeno-associated viral vectors to trace axonal projections from defined regions and cell types, and high-throughput serial two-photon tomography to image the EGFP-labelled axons throughout the brain. This systematic and standardized approach allows spatial registration of individual experiments into a common three dimensional (3D) reference space, resulting in a whole-brain connectivity matrix. A computational model yields insights into connectional strength distribution, symmetry and other network properties. Virtual tractography illustrates 3D topography among interconnected regions. Cortico-thalamic pathway analysis demonstrates segregation and integration of parallel pathways. The Allen Mouse Brain Connectivity Atlas is a freely available, foundational resource for structural and functional investigations into the neural circuits that support behavioural and cognitive processes in health and disease.

AAV vector-mediated secretion of chondroitinase provides a sensitive tracer for axonal arborisations

As part of a project to express chondroitinase ABC (ChABC) in neurons of the central nervous system, we have inserted a modified ChABC gene into an adeno-associated viral (AAV) vector and injected it into the vibrissal motor cortex in adult rats to determine the extent and distribution of expression of the enzyme. A similar vector for expression of green fluorescent protein (GFP) was injected into the same location. For each vector, two versions with minor differences were used, giving similar results. After 4 weeks, the brains were stained to show GFP and products of chondroitinase digestion. Chondroitinase was widely expressed, and the AAV-ChABC and AAV-GFP vectors gave similar expression patterns in many respects, consistent with the known projections from the directly transduced neurons in vibrissal motor cortex and adjacent cingulate cortex. In addition, diffusion of vector to deeper neuronal populations led to labelling of remote projection fields which was much more extensive with AAV-ChABC than with AAV-GFP. The most notable of these populations are inferred to be neurons of cortical layer 6, projecting widely in the thalamus, and neurons of the anterior pole of the hippocampus, projecting through most of the hippocampus. We conclude that, whereas GFP does not label the thinnest axonal branches of some neuronal types, chondroitinase is efficiently secreted from these arborisations and enables their extent to be sensitively visualised. After 12 weeks, chondroitinase expression was undiminished.

Searching for the holy grail: temporally informative firing patterns in the rat

This chapter reviews our work from the past decade investigating cortical and striatal firing patterns in rats while they time intervals in the multi-seconds range. We have found that both cortical and striatal firing rates contain information that the rat can use to identify how much time has elapsed both from trial onset and from the onset of an active response state. I describe findings showing that the striatal neurons that are modulated by time are also modulated by overt behaviors, suggesting that time modulates the strength of motor coding in the striatum, rather than being represented as an abstract quantity in isolation. I also describe work showing that there are a variety of temporally informative activity patterns in pre-motor cortex, and argue that the heterogeneity of these patterns can enhance an organism's temporal estimate. Finally, I describe recent behavioral work from my lab in which the simultaneous cueing of multiple durations leads to a scalar temporal expectation at an intermediate time, providing strong support for a monotonic representation of time.

Hippocampus, time, and memory--a retrospective analysis

In 1984, there was considerable evidence that the hippocampus was important for spatial learning and some evidence that it was also involved in duration discrimination. The article "Hippocampus, Time, and Memory" (Meck, Church, & Olton, 1984), however, was the first to isolate the effects of hippocampal damage on specific stages of temporal processing. In this review, to celebrate the 30th anniversary of Behavioral Neuroscience, we look back on factors that contributed to the long-lasting influence of this article. The major results were that a fimbria-fornix lesion (a) interferes with the ability to retain information in temporal working memory, and (b) distorts the content of temporal reference memory, but (c) did not decrease sensitivity to signal duration. This was the first lesion experiment in which the results were interpreted by a well-developed theory of behavior (scalar timing theory). It has led to extensive research on the role of the hippocampus in temporal processing by many investigators. The most important ones are the development of computational models with plausible neural mechanisms (such as the striatal beat-frequency model of interval timing), the use of multiple behavioral measures of timing, and empirical research on the neural mechanisms of timing and temporal memory using ensemble recording of neurons in prefrontal-striatal-hippocampal circuits.

Medial premotor cortex shows a reduction in inhibitory markers and mediates recovery in a mouse model of focal stroke

BACKGROUND AND PURPOSE

Motor recovery after ischemic stroke in primary motor cortex is thought to occur in part through training-enhanced reorganization in undamaged premotor areas, enabled by reductions in cortical inhibition. Here we used a mouse model of focal cortical stroke and a double-lesion approach to test the idea that a medial premotor area (medial agranular cortex [AGm]) reorganizes to mediate recovery of prehension, and that this reorganization is associated with a reduction in inhibitory interneuron markers.

METHODS

C57Bl/6 mice were trained to perform a skilled prehension task to an asymptotic level of performance after which they underwent photocoagulation-induced stroke in the caudal forelimb area. The mice were then retrained and inhibitory interneuron immunofluorescence was assessed in prechosen, anatomically defined neocortical areas. Mice then underwent a second photocoagulation-induced stroke in AGm.

RESULTS

Focal caudal forelimb area stroke led to a decrement in skilled prehension. Training-associated recovery of prehension was associated with a reduction in parvalbumin, calretinin, and calbindin expression in AGm. Subsequent infarction of AGm led to reinstatement of the original deficit.

CONCLUSIONS

We conclude that with training, AGm can reorganize after a focal motor stroke and serve as a new control area for prehension. Reduced inhibition may represent a marker for reorganization or it is necessary for reorganization to occur. Our mouse model, with all of the attendant genetic benefits, may allow us to determine at the cellular and molecular levels how behavioral training and endogenous plasticity interact to mediate recovery.

Activation of Anterior Cingulate Cortex Extracellular Signal-Regulated Kinase-1 and -2 (ERK1/2) Regulates Acetic Acid-Induced, Pain-Related Anxiety in Adult Female Mice

In visceral pain, anxiety and pain occur simultaneously, but the etiogenesis of this effect is not yet well-described. The anterior cingulate cortex (ACC) is known to be associated with the affective response to noxious stimuli. The aim of the current study is to define the role of ACC extracellular signal-regulated (ERK)-1 and-2 (ERK1/2) activity in the development of pain-related anxiety/depression and the nocifensive response in acetic acid (AA)-elicited visceral pain. The model of visceral pain was created by intraperitoneal (ip) injection of AA to female Kunming mice. We found that AA injection resulted in a dynamic, bilateral ERK1/2 activation pattern in the ACC. Inhibition of ERK1/2 activation 2 hr after AA injection by subcutaneous (sc) injection of the mitogen-activating extracellular kinase (MEK) inhibitor, SL327, had no effect on the nocifensive responses, but did attenuate anxiety-like behavior, as determined by elevated plus-maze and open-field testing results. These data suggest that AA-induced visceral pain activates expression of ACC ERK1/2, which regulates visceral pain-related anxiety, but not the nocifensive response.

Activation of anterior cingulate cortex produces inhibitory effects on noxious mechanical and electrical stimuli-evoked responses in rat spinal WDR neurons

In present study, in vivo electrophysiological techniques were applied to examine the effects of anterior cingulate cortex (ACC) activation on mechanical and electrical stimuli-evoked responses in rat spinal cord wide-dynamic-range (WDR) neurons. We found that bilateral ACC electrical stimulation (100Hz, 20V, 20s) had different effects on neuronal responses to brush, pressure and pinch stimuli (10s). The brush-evoked neuronal responses at baseline, post 1min and post 5min were 60.8±15.0, 59.2±15.4 and 60.0±19.3 spikes/10s, respectively (n=10, P>0.05 vs. baseline). The pressure-evoked neuronal responses at baseline, post 1min and post 5min were 77.8±11.9, 38.0±7.8 and 45.8±7.6 spikes/10s, respectively (n=10, P<0.05 vs. baseline). The pinch-evoked neuronal responses at baseline, post 1min and post 5min were 137.6±16.7, 62.6±17.5 and 68.8±15.0 spikes/10s, respectively (n=10, P<0.05 vs. baseline). Furthermore, ACC stimulation generated distinct effects on the different components of wind-up response. The total numbers of late response (LR) and after-discharge (AD), but not early response (ER), significantly decreased. Collectively, the present study demonstrated that short-term ACC activation could generate long-term inhibitory effects on the responses of WDR neurons to noxious mechanical (pressure and pinch) and electrical stimuli. The results indicated that ACC activation could negatively regulate noxious information ascending from spinal cord with long-term effect, providing potential neuronal substrate for the modulation of ACC activation on nociception.

A heterogeneous population code for elapsed time in rat medial agranular cortex

The neural mechanisms underlying the temporal control of behavior are largely unknown. Here we recorded from medial agranular cortex neurons in rats while they freely behaved in a temporal production task, the peak-interval procedure. Due to variability in estimating the time of food availability, robust responding typically bracketed the expected duration, starting some time before and ending some time after the signaled delay. These response periods provided analytic "steady state" windows during which subjects actively indicated their temporal expectation of food availability. Remarkably, during these response periods, a variety of firing patterns were seen that could be broadly described as ramps, peaks, and dips, with different slopes, directions, and times at which maxima or minima occur. Regularized linear discriminant analysis indicated that these patterns provided sufficiently reliable information to discriminate the elapsed duration of responding within these response periods. Modeling this across neuron variability showed that the utilization of ramps, dips, and peaks, with different slopes and minimal/maximal rates at different times, led to a substantial improvement in temporal prediction errors, suggesting that heterogeneity in the neural representation of elapsed time may facilitate temporally controlled behavior.

Reversible Inactivation of Rat Premotor Cortex Impairs Temporal Preparation, but not Inhibitory Control, During Simple Reaction-Time Performance

Previous studies by our lab and others have established a role for medial areas of the prefrontal cortex (mPFC) in the top-down control of action during simple reaction-time (RT) tasks. However, the neural circuits that allow mPFC to influence activity in the motor system have remained unclear. In the present study, we used a combination of tract-tracing and reversible inactivation methods to examine the role of a motor-related area in the rat frontal cortex, called the rostral forelimb area (RFA), in the top-down control of action. Neural tracing studies involved used electrical microstimulation to identify RFA and injections of biotinylated dextran amines (BDA) to map out connections of RFA with other parts of the frontal cortex. Connections were found between RFA and mPFC, the agranular insular cortex, and the primary motor cortex. Reversible inactivations using muscimol infusions into RFA increased response times and eliminated delay-dependent speeding, but did not increase premature responding. These results are markedly different from what is obtained when muscimol is infused into mPFC, which leads to excessive premature responding and a reduction of RTs to stimuli at short delays (Narayanan et al., 2006). We also tested animals during the RT task after inactivating the agranular insular cortex, which contains neurons that projects to and receives from RFA and mPFC, and found no effects on RT performance. Together, these studies suggest that RFA is a premotor region in the rat frontal cortex that competes with mPFC to control action selection. We suggest that RFA controls the threshold that is used to initiate responding and generates prepotent excitation over responding that is crucial for temporal preparation.

Thalamic afferent and efferent connectivity to cerebral cortical areas with direct projections to identified subgroups of trigeminal premotoneurons in the rat

The roles of supramedullary brain mechanisms involved in the control of jaw movements are not fully understood. To address this issue, a series of retrograde (Fluorogold, FG) and anterograde (biotinylated dextran amine, BDA) tract-tracing studies were done in rats. At first, we identified projection patterns from defined sensorimotor cortical areas to subgroups of trigeminal premotoneurons that are located in defined brainstem areas. Focal injections of FG into these brainstem areas revealed that the rostralmost part of lateral agranular cortex (rmost-Agl), the rostralmost part of medial agranular cortex (rmost-Agm), and the rostralmost part of primary somatosensory cortex (rmost-S1) preferentially project to brainstem areas containing jaw-closing premotoneurons, jaw-opening premotoneurons and a mixture of both types of premotoneurons, respectively. The thalamic reciprocal connectivities to rmost-Agl, rmost-Agm, and rmost-S1 were then investigated following cortical injections of FG or BDA. We found many retrogradely FG-labeled neurons and large numbers of axons and terminals labeled anterogradely with BDA in the dorsal thalamus mainly on the side ipsilateral to the injection sites. The rmost-Agl had strong connections with the ventral lateral nucleus (VL), ventromedial nucleus (VM), parafascicular nucleus, and posterior nucleus (Po); the rmost-Agm with the ventral anterior nucleus, VL, VM, central lateral nucleus, paracentral nucleus, central medial nucleus, mediodorsal nucleus and Po; and the rmost-S1 with the ventral posteromedial nucleus and Po. The present results suggest that the descending multiple pathways from the cerebral cortex to jaw-closing and jaw-opening premotoneurons have unique functional roles in jaw movement motor control.

Evidence of action sequence chunking in goal-directed instrumental conditioning and its dependence on the dorsomedial prefrontal cortex

The current study investigated the contribution of the dorsomedial prefrontal cortex (dmPFC) to instrumental action selection. We found that cell body lesions of the dmPFC, centered on the medial agranular area, spared rats' ability to choose between actions based on either the value or the discriminative stimulus properties of an outcome. We next examined the effects of these lesions on action sequence learning using a concurrent bidirectional heterogeneous chain task in which the identity of the reward delivered was determined by the order in which the two lever press actions were performed. Although both lesioned rats and sham controls learned to perform the task, we found that they relied on different behavioral strategies to do so. In subsequent tests, rats in the sham group were able to withhold their performance of a sequence when either its associated outcome was devalued or the contingency between that sequence and its outcome was degraded by delivering the outcome noncontingently. Interestingly, lesioned rats failed to reorganize their performance at the action sequence level and, rather, were found to withhold their performance of the terminal response in the sequence that had earned the devalued outcome relative to the more distal response, suggesting that they represented the elements of the sequence as distinct behavioral units. These findings demonstrate that rats can use sequence-level representations, or action chunks, to organize their behavior in a goal-directed manner and indicate that the dmPFC plays a critical role in this process.

Cortical connections of the rat lateral posterior thalamic nucleus

Spatial processing related to directed attention is thought to be mediated by a specific cortical-basal ganglia-thalamic-cortical network in the rat. Key components of this network are associative cortical areas medial agranular cortex (AGm) and posterior parietal cortex (PPC), dorsocentral striatum (DCS), and lateral posterior (LP) thalamic nucleus, all of which are interconnected. Previously, we found that thalamostriatal projections reaching DCS arise from separate populations of neurons of the mediorostral part of LP (LPMR). The far medial LPMR (fmLPMR) terminates in central DCS, a projection area of AGm, whereas central LPMR terminates in dorsal DCS, a projection area of PPC. This represents segregated regional convergence in DCS from different sources of thalamic and cortical inputs. In the present study, thalamocortical and corticothalamic projections arising from and terminating in LPMR and neighboring thalamic nuclei were studied by anterograde and retrograde tracing techniques in order to further understand the anatomical basis of this neural circuitry. A significant finding was that within LPMR, separate neuronal populations provide thalamic inputs to AGm or PPC and that these cortical areas project to separate regions in LPMR, from which they receive thalamic inputs. Other cortical areas adjacent to AGm or PPC also demonstrated reciprocal connections with LP or surrounding nuclei in a topographic manner. Our findings suggest that the cortical-basal ganglia-thalamic network mediating directed attention in the rat is formed by multiple loops, each having reciprocal connections that are organized in a precise and segregated topographical manner.

Basal ganglia contributions to adaptive navigation

The striatum has long been considered to be selectively important for nondeclarative, procedural types of memory. This stands in contrast with spatial context processing that is typically attributed to hippocampus. Neurophysiological evidence from studies of the neural mechanisms of adaptive navigation reveals that distinct neural systems such as the striatum and hippocampus continuously process task relevant information regardless of the current cognitive strategy. For example, both striatal and hippocampal neural representations reflect spatial location, directional heading, reward, and egocentric movement features of a test situation in an experience-dependent way, and independent of task demands. Thus, continual parallel processing across memory systems may be the norm rather than the exception. It is suggested that neuromodulators, such as dopamine, may serve to differentially regulate learning-induced neural plasticity mechanisms within these memory systems such that the most successful form of neural processing exerts the strongest control over response selection functions. In this way, dopamine may serve to optimize behavioral choices in the face of changing environmental demands during navigation.

Region and sex differences in constituent dopamine neurons and immunoreactivity for intracellular estrogen and androgen receptors in mesocortical projections in rats

Many cortical and prefrontal functions show sex differences in their development, adult capacity, and dysfunction in disorders like schizophrenia. Correlations between circulating gonadal hormones and certain prefrontal functions have also been identified in humans and experimental animal models. Although multiple mechanisms may be involved, such hormone sensitivities/sex differences could be related to gonadal steroid actions on another regulator of cortical/prefrontal cortical function, the mesocortical dopamine system. Thus, although it is well known that perturbations in prefrontal dopamine signaling induce behavioral deficits, it is also known that several endpoints of these afferents are sensitive to gonadal steroids and/or are sexually dimorphic. This study explored possible substrates for this in two ways: by comparing the distributions of immunoreactivity for intracellular estrogen (alpha and beta) and androgen receptors among retrogradely labeled dopaminergic and nondopaminergic mesocortical neurons projecting to prefrontal, premotor, and primary motor cortices, areas in which male rat dopamine axons are differentially hormone-sensitive; and by comparing anatomical data in males and females. These analyses revealed region-, cell-, and sex-specific specializations in receptor localization that paralleled established patterns of mesocortical hormone sensitivity, including the androgen sensitivity of dopamine axons and dopamine-dependent functions in prefrontal cortex. It was also found that the proportions of dopamine neurons making up mesocortical projections were approximately 30% in males, whereas in females, significantly more constituent cells were dopaminergic. Together, these features may be part of the neurobiology giving mesocortical afferents their hormone sensitivities and/or sex differences in physiology, function, and dysfunction in disease.

Quantification of synaptic density in corticostriatal projections from rat medial agranular cortex

Medial agranular cortex (AGm) has a prominent bilateral projection to the dorsocentral striatum (DCS). We wished to develop a normal baseline by which to assess neuronal plasticity in this corticostriatal system in rats with neglect resulting from a unilateral lesion in AGm, followed by treatment with agents that promote sprouting and functional recovery in other systems. Injections of biotinylated dextran amine were made into AGm in normal rats, and unbiased sampling was used to quantify the density of axons and axonal varicosities present in DCS (the latter represent presynaptic profiles). Labeling density in contralateral DCS is approximately half of that seen in ipsilateral DCS (this ratio is 0.50 for axons, 0.55 for varicosities). The ratio of varicosities is stable over a greater than seven-fold range of absolute densities. There is no consistent relationship between the absolute density of axons and axon varicosities; however, the ratio measures are strongly correlated. We conclude that changes in the contralateral/ipsilateral ratio of axon density after experimental treatments do reflect changes in synaptic density, but axon varicosities are likely to be the most sensitive anatomical parameter by which to assess plasticity at the light microscopic level.

Striatal projections from the rat lateral posterior thalamic nucleus

The dorsocentral striatum (DCS) has been implicated as an associative striatal area receiving inputs from several cortical areas including medial agranular cortex (AGm), posterior parietal cortex (PPC), and visual association cortex to form a cortical-subcortical circuit involved in directed attention and neglect. The lateral posterior thalamic nucleus (LP) may also play a role in directed attention and neglect because LP has robust reciprocal connections with these cortical areas and projects to DCS. We used anterograde axonal tracing to map thalamostriatal projections from LP and surrounding thalamic nuclei, with a focus on projections to DCS. The thalamic nuclei investigated included LP, laterodorsal thalamic nucleus (LD), central lateral nucleus (CL), and posterior thalamic nucleus (Po). We found that the mediorostral part of LP (LPMR) projects strongly to DCS as well as to the dorsal peripheral region of the striatum. Further, there is topography within LPMR and DCS such that the far medial LPMR projects to the central region of DCS (projection area of AGm) and the central LPMR projects to the dorsal region of DCS (projection area of PPC and Oc2M). In contrast, the laterorostral part of LP (LPLR) and other thalamic nuclei surrounding LP project to dorsolateral to dorsomedial peripheral regions of the striatum but do not project to DCS. These findings indicate that DCS is a region of convergence for thalamostriatal and corticostriatal projections from regions that are themselves interconnected, serving as the key element of the corticostriatal-thalamic network mediating spatial processing and directed attention.

Hippocampal and neocortical interactions during context discrimination: electrophysiological evidence from the rat

There is substantial evidence that hippocampus plays an important role in the processing of contextual information. Its specific role, however, remains unclear. One possibility is that single hippocampal neurons represent context information so that local circuits can construct representations of the current context, and the context that is expected based on past experience. Population codes derived from input by multiple local circuits may then engage match-mismatch algorithms that compare current and expected context information to determine the extent to which an expected context has changed. The results of such match-mismatch comparisons can be used to discriminate contexts. When context changes are detected, efferent messages may be passed on to connected neocortical areas so that informed "decisions" regarding future behavioral and cognitive strategies can be made. Here, a brief review describes evidence that a primary consequence of hippocampal processing is the discrimination of meaningful contexts. Then, the functional significance of neocortical circuits that likely receive hippocampal output messages are described in terms of their contribution to the control of ongoing behavioral and cognitive strategy, especially during active navigation. It is clear from this systems view that studies of spatial navigation continue to provide researchers with an excellent model of hippocampal-neocortical interactions during learning.

Anatomical analysis of afferent projections to the medial prefrontal cortex in the rat

The medial prefrontal cortex (mPFC) has been associated with diverse functions including attentional processes, visceromotor activity, decision making, goal directed behavior, and working memory. Using retrograde tracing techniques, we examined, compared, and contrasted afferent projections to the four divisions of the mPFC in the rat: the medial (frontal) agranular (AGm), anterior cingulate (AC), prelimbic (PL), and infralimbic (IL) cortices. Each division of the mPFC receives a unique set of afferent projections. There is a shift dorsoventrally along the mPFC from predominantly sensorimotor input to the dorsal mPFC (AGm and dorsal AC) to primarily 'limbic' input to the ventral mPFC (PL and IL). The AGm and dorsal AC receive afferent projections from widespread areas of the cortex (and associated thalamic nuclei) representing all sensory modalities. This information is presumably integrated at, and utilized by, the dorsal mPFC in goal directed actions. In contrast with the dorsal mPFC, the ventral mPFC receives significantly less cortical input overall and afferents from limbic as opposed to sensorimotor regions of cortex. The main sources of afferent projections to PL/IL are from the orbitomedial prefrontal, agranular insular, perirhinal and entorhinal cortices, the hippocampus, the claustrum, the medial basal forebrain, the basal nuclei of amygdala, the midline thalamus and monoaminergic nuclei of the brainstem. With a few exceptions, there are few projections from the hypothalamus to the dorsal or ventral mPFC. Accordingly, subcortical limbic information mainly reaches the mPFC via the midline thalamus and basal nuclei of amygdala. As discussed herein, based on patterns of afferent (as well as efferent) projections, PL is positioned to serve a direct role in cognitive functions homologous to dorsolateral PFC of primates, whereas IL appears to represent a visceromotor center homologous to the orbitomedial PFC of primates.

Association between cerebral glutamate and human behaviour: the sensation seeking personality trait

INTRODUCTION

Brain imaging studies have linked the anterior cingulate cortex (ACC) to motivation, drive, and personality traits like novelty and sensation seeking. Animal studies have shown glutamatergic neurotransmission to be important in ACC function as well as motivated behaviour. However, the role of glutamate in related personality traits like sensation seeking has not been investigated in humans.

METHODS

The associations between sensation seeking personality scores and absolute glutamate concentrations in the ACC and the hippocampal region measured by 3-Tesla proton magnetic resonance spectroscopy (1H-MRS) were investigated.

RESULTS

ACC glutamate concentration was negatively correlated with the sensation seeking sum score and the experience seeking subscore. A weak negative correlation was also observed between the hippocampal glutamate and the sensation seeking sum score. The reexamination of the glutamate concentration after 4 weeks revealed a similar relationship with sensation seeking.

DISCUSSION

Although preliminary, the results are in line with the key role of the ACC for motivation and executive control and with the impact of glutamate on novelty related behaviour observed in animal experiments. The role of the hippocampus for novelty processing is discussed. Glutamate measurement with 1H-MRS may facilitate the understanding of biological underpinnings of personality traits and psychiatric diseases associated with dysfunctions in motivation and drive.

Interactions among the medial prefrontal cortex, hippocampus and midline thalamus in emotional and cognitive processing in the rat

The medial prefrontal cortex (mPFC) participates in several higher order functions including selective attention, visceromotor control, decision making and goal-directed behaviors. We discuss the role of the infralimbic cortex (IL) in visceromotor control and the prelimbic cortex (PL) in cognition and their interactions in goal-directed behaviors in the rat. The PL strongly interconnects with a relatively small group of structures that, like PL, subserve cognition, and together have been designated the 'PL circuit.' These structures primarily include the hippocampus, insular cortex, nucleus accumbens, basolateral nucleus of the amygdala, the mediodorsal and reuniens nuclei of the thalamus and the ventral tegmental area of the midbrain. Lesions of each of these structures, like those of PL, produce deficits in delayed response tasks and memory. The PL (and ventral anterior cingulate cortex) (AC) of rats is ideally positioned to integrate current and past information, including its affective qualities, and act on it through its projections to the ventral striatum/ventral pallidum. We further discuss the role of nucleus reuniens of thalamus as a major interface between the mPFC and the hippocampus, and as a prominent source of afferent limbic information to the mPFC and hippocampus. We suggest that the IL of rats is functionally homologous to the orbitomedial cortex of primates and the prelimbic (and ventral AC) cortex to the lateral/dorsolateral cortex of primates, and that the IL/PL complex of rats exerts significant control over emotional and cognitive aspects of goal-directed behavior.

Increased cholecystokinin release in the rat anterior cingulate cortex during carrageenan-induced arthritis

Several human and animal studies indicate that the anterior cingulate cortex (ACC) plays an important role in the affective component of pain. The neuropeptide cholecystokinin (CCK) is especially abundant in the ACC. CCK has been suggested to be involved in the mediation of anxiety and in the modulation of opioid effects in the spinal cord and medulla oblongata. However, its possible role in pain transmission or modulation in the brain is far less clear. In this study, a model of subchronic inflammatory pain in rats, carrageenan-induced monoarthritis, was used to study the effect of pain on the release of CCK-like immunoreactivity (CCK-LI) in the ACC. Pain-related behaviour quantified by weight bearing and stance scoring, as well as inflammation measured by ankle oedema, was increased for at least 24 h after carrageenan injection with a maximum at 5 h. Using microdialysis in freely moving rats, extracellular concentrations of CCK-LI was measured in the ACC during a time period when the animals showed significant pain behaviour. In animals with carrageenan-induced arthritis, both basal and potassium-evoked release of CCK-LI were significantly increased compared to controls. HPLC analysis of dialysates from the ACC during potassium stimulation showed that the main part of the immunoreactive material was sulphated CCK-8. Because CCK has been implicated in anxiety, we suggest that an altered CCK-ergic activity in the ACC may be of importance for the affective component of pain, but an involvement in the modulation of nociception is also possible.

Auditory inhibitory gating in medial prefrontal cortex: Single unit and local field potential analysis

Medial prefrontal cortex is a crucial region involved in inhibitory processes. Damage to the medial prefrontal cortex can lead to loss of normal inhibitory control over motor, sensory, emotional and cognitive functions. The goal of the present study was to examine the basic properties of inhibitory gating in this brain region in rats. Inhibitory gating has recently been proposed as a neurophysiological assay for sensory filters in higher brain regions that potentially enable or disable information throughput. This perspective has important clinical relevance due to the findings that gating is dramatically impaired in individuals with emotional and cognitive impairments (i.e. schizophrenia). We used the standard inhibitory gating two-tone paradigm with a 500 ms interval between tones and a 10 s interval between tone pairs. We recorded both single unit and local field potentials from chronic microwire arrays implanted in the medial prefrontal cortex. We investigated short-term (within session) and long-term (between session) variability of auditory gating and additionally examined how altering the interval between the tones influenced the potency of the inhibition. The local field potentials displayed greater variability with a reduction in the amplitudes of the tone responses over both the short and long-term time windows. The decrease across sessions was most intense for the second tone response (test tone) leading to a more robust gating (lower T/C ratio). Surprisingly, single unit responses of different varieties retained similar levels of auditory responsiveness and inhibition in both the short and long-term analysis. Neural inhibition decreased monotonically related to the increase in intertone interval. This change in gating was most consistent in the local field potentials. Subsets of single unit responses did not show the lack of inhibition even for the longer intertone intervals tested (4 s interval). These findings support the idea that the medial prefrontal cortex is an important site where early inhibitory functions reside and potentially mediate psychological processes.

Organization of anterior cingulate and frontal cortical projections to the anterior and laterodorsal thalamic nuclei in the rat

The anterior and laterodorsal thalamic nuclei provide massive projections to the anterior cingulate and frontal cortices in the rat. However, the organization of reciprocal corticothalamic projections has not yet been studied comprehensively. In the present study, we clarified the organization of anterior cingulate and frontal cortical projections to the anterior and laterodorsal thalamic nuclei, using retrograde and anterograde axonal transport methods. The anteromedial nucleus (AM) receives mainly ipsilateral projections from the prelimbic and medial orbital cortices and bilateral projections from the anterior cingulate and secondary motor cortices. The projections from the anterior cingulate cortex are organized such that the rostrocaudal axis of the AM corresponds to the rostrocaudal axis of the cortex, whereas those from the secondary motor cortex are organized such that the rostrocaudal axis of the AM corresponds to the caudorostral axis of the cortex. The ventromedial part of the anteroventral nucleus receives ipsilateral projections from the anterior cingulate cortex and bilateral projections from the secondary motor cortex, in a topographic manner similar to the projections to the AM. The ventromedial part of the laterodorsal nucleus (LD) receives ipsilateral projections from the anterior cingulate and secondary motor cortices. The projections are roughly organized such that more dorsal and ventral regions within the ventromedial LD receive projections preferentially from the anterior cingulate cortex. The difference in anterior cingulate and frontal cortical projections to the anterior and laterodorsal nuclei may suggest that each thalamic nucleus plays a different functional role in spatial memory processing.