The effects of discrete cingulum bundle lesions in the rat on the acquisition and performance of two tests of spatial working memory

The Granular Retrosplenial Cortex Is Necessary in Male Rats for Object-Location Associative Learning and Memory, But Not Spatial Working Memory or Visual Discrimination and Reversal, in the Touchscreen Operant Chamber

The retrosplenial cortex (RSC) is a hub of diverse afferent and efferent projections thought to be involved in associative learning. RSC shows early pathology in mild cognitive impairment and Alzheimer's disease (AD), which impairs associative learning. To understand and develop therapies for diseases such as AD, animal models are essential. Given the importance of human RSC in object-location associative learning and the success of object-location associative paradigms in human studies and in the clinic, it would be of considerable value to establish a translational model of object-location learning for the rodent. For this reason, we sought to test the role of RSC in object-location learning in male rats using the object-location paired-associates learning (PAL) touchscreen task. First, increased cFos immunoreactivity was observed in granular RSC following PAL training when compared with extended pretraining controls. Following this, RSC lesions following PAL acquisition were used to explore the necessity of the RSC in object-location associative learning and memory and two tasks involving only one modality: trial-unique nonmatching-to-location for spatial working memory and pairwise visual discrimination/reversal. RSC lesions impaired both memory for learned paired-associates and learning of new object-location associations but did not affect performance in either the spatial or visual single-modality tasks. These findings provide evidence that RSC is necessary for object-location learning and less so for learning and memory involving the individual modalities therein.

White matter abnormalities associated with ADHD outcomes in adulthood

It remains unclear if previously reported structural abnormalities in children with ADHD are present in adulthood regardless of clinical outcome. In this study, we examined the extent to which focal-rather than diffuse-abnormalities in fiber collinearity of 18 major white matter tracts could distinguish 126 adults with rigorously diagnosed childhood ADHD (ADHD; mean age [SD] = 34.3 [3.6] years; F/M = 12/114) from 58 adults without ADHD histories (non-ADHD; mean age [SD] = 33.9 [4.1] years; F/M = 5/53) and if any of these abnormalities were greater for those with persisting ADHD symptomatology. To this end, a tract profile approach was used. After accounting for age, sex, handedness, and comorbidities, a MANCOVA revealed a main effect of group (ADHD < non-ADHD; F_[18,155] = 2.1; p = 0.007) on fractional anisotropy (FA, a measure of fiber collinearity and/or integrity), in focal portions of white matter tracts involved in visuospatial processing and memory (i.e., anterior portion of the left inferior longitudinal fasciculus, and middle portion of the left and right cingulum angular bundle). Only abnormalities in the anterior portion of the left inferior longitudinal fasciculus distinguished probands with persisting versus desisting ADHD symptomatology, suggesting that abnormalities in the cingulum angular bundle might reflect "scarring" effects of childhood ADHD. To our knowledge, this is the first study using a tract profile approach to identify focal or widespread structural abnormalities in adults with ADHD rigorously diagnosed in childhood.

Organisation of cingulum bundle fibres connecting the anterior thalamic nuclei with the rodent anterior cingulate and retrosplenial cortices

Despite considerable interest in the properties of the cingulum bundle, descriptions of the composition of this major pathway in the rodent brain have not kept pace with advances in tract tracing. Using complementary approaches in rats and mice, this study examined the dense, reciprocal connections the anterior thalamic nuclei have with the cingulate and retrosplenial cortices, connections thought to be major contributors to the rodent cingulum bundle. The rat data came from a mixture of fluorescent and viral tracers, some injected directly into the bundle. The mouse data were collated from the Allen Mouse Brain Atlas. The projections from the three major anterior thalamic nuclei occupied much of the external medullary stratum of the cingulum bundle, where they were concentrated in its more medial portions. These anterior thalamic projections formed a rostral-reaching basket of efferents prior to joining the cingulum bundle, with anteromedial efferents taking the most rostral routes, often reaching the genu of the corpus callosum, while anterodorsal efferents took the least rostral route. In contrast, the return cortico-anterior thalamic projections frequently crossed directly through the bundle or briefly joined the internal stratum of the cingulum bundle, often entering the internal capsule before reaching the anterior thalamus. These analyses confirm that anterior thalamic connections comprise an important component of the rodent cingulum bundle, while also demonstrating the very different routes used by thalamo-cortical and cortico-thalamic projections. This information reveals how the composition of the cingulum bundle alters along its length.

The cingulum bundle: Anatomy, function, and dysfunction

The cingulum bundle is a prominent white matter tract that interconnects frontal, parietal, and medial temporal sites, while also linking subcortical nuclei to the cingulate gyrus. Despite its apparent continuity, the cingulum's composition continually changes as fibres join and leave the bundle. To help understand its complex structure, this review begins with detailed, comparative descriptions of the multiple connections comprising the cingulum bundle. Next, the impact of cingulum bundle damage in rats, monkeys, and humans is analysed. Despite causing extensive anatomical disconnections, cingulum bundle lesions typically produce only mild deficits, highlighting the importance of parallel pathways and the distributed nature of its various functions. Meanwhile, non-invasive imaging implicates the cingulum bundle in executive control, emotion, pain (dorsal cingulum), and episodic memory (parahippocampal cingulum), while clinical studies reveal cingulum abnormalities in numerous conditions, including schizophrenia, depression, post-traumatic stress disorder, obsessive compulsive disorder, autism spectrum disorder, Mild Cognitive Impairment, and Alzheimer's disease. Understanding the seemingly diverse contributions of the cingulum will require better ways of isolating pathways within this highly complex tract.

Hippocampal - diencephalic - cingulate networks for memory and emotion: An anatomical guide

This review brings together current knowledge from tract tracing studies to update and reconsider those limbic connections initially highlighted by Papez (1937) for their presumed role in emotion. These connections link hippocampal and parahippocampal regions with the mammillary bodies, the anterior thalamic nuclei, and the cingulate gyrus, all structures now strongly implicated in memory functions. An additional goal of this review is to describe the routes taken by the various connections within this network. The original descriptions of these limbic connections saw their interconnecting pathways forming a serial circuit that began and finished in the hippocampal formation. It is now clear that, with the exception of the mammillary bodies, these various sites are multiply interconnected with each other, including many reciprocal connections. In addition, these same connections are topographically organised, creating further subsystems. This complex pattern of connectivity helps to explain the difficulty of interpreting the functional outcome of damage to any individual site within the network. For these same reasons, Papez' initial concept of a loop beginning and ending in the hippocampal formation needs to be seen as a much more complex system of hippocampal-diencephalic-cingulate connections. The functions of these multiple interactions might be better viewed as principally providing efferent information from the posterior medial temporal lobe. Both a subcortical diencephalic route (via the fornix) and a cortical cingulate route (via retrosplenial cortex) can be distinguished. These routes provide indirect pathways for hippocampal interactions with prefrontal cortex, with the preponderance of both sets of connections arising from the more posterior hippocampal regions. These multi-stage connections complement the direct hippocampal projections to prefrontal cortex, which principally arise from the anterior hippocampus, thereby creating longitudinal functional differences along the anterior-posterior plane of the hippocampus.

Selective neuronal degeneration in the retrosplenial cortex impairs the recall of contextual fear memory

The retrosplenial cortex (RSC) is one of the largest cortical areas in rodents, and is subdivided in two main regions, A29 and A30, according to their cytoarchitectural organization and connectivities. However, very little is known about the functional activity of each RSC subdivision during the execution of complex cognitive tasks. Here, we used a well-established fear learning protocol that induced long-lasting contextual fear memory and showed that during evocation of the fear memory, the expression of early growth response gene 1 was up-regulated in A30, and in other brain areas implicated in fear and spatial memory, however, was down-regulated in A29, including layers IV and V. To search for the participation of A29 on fear memory, we triggered selective degeneration of neurons within cortical layers IV and V of A29 by using a non-invasive protocol that takes advantage of the vulnerability that these neurons have MK801-toxicity and the modulation of this neurodegeneration by testosterone. Application of 5 mg/kg MK801 in intact males induced negligible neuronal degeneration of A29 neurons and had no impact on fear memory retrieval. However, in orchiectomized rats, 5 mg/kg MK801 induced overt degeneration of layers IV-V neurons of A29, significantly impairing fear memory recall. Degeneration of A29 neurons did not affect exploratory or anxiety-related behavior nor altered unconditioned freezing. Importantly, protecting A29 neurons from MK801-toxicity by testosterone preserved fear memory recall in orchiectomized rats. Thus, neurons within cortical layers IV-V of A29 are critically required for efficient retrieval of contextual fear memory.

The role of the anterior cingulate cortex in choices based on reward value and reward contingency

Although several studies have emphasized the role of the anterior cingulate cortex (ACC) in associating actions with reward value, its role in guiding choices on the basis of changes in reward value has not been assessed. Accordingly, we compared rhesus monkeys with ACC lesions and controls on object- and action-based reinforcer devaluation tasks. Monkeys were required to associate an object or an action with one of two reward outcomes, and we assessed the monkey's shift in choices of objects or actions after changes in the value of 1 outcome. No group differences emerged on either task. For comparison, we tested the same monkeys on their ability to make choices guided by reward contingency in object- and action-based reversal learning tasks. Monkeys with ACC lesions were impaired in using rewarded trials to sustain the selection of the correct object during object reversal learning. They were also impaired in using errors to guide choices in action reversal learning. These data indicate that the role of the ACC is not restricted to linking specific actions with reward outcomes, as previously reported. Instead, the data suggest a more general role for the ACC in using information about reward and nonreward to sustain effective choice behavior.

Impaired head direction cell representation in the anterodorsal thalamus after lesions of the retrosplenial cortex

The retrosplenial cortex (RSP), a brain region frequently linked to processes of spatial navigation, contains neurons that discharge as a function of a rat's head direction (HD). HD cells have been identified throughout the limbic system including the anterodorsal thalamus (ADN) and postsubiculum (PoS), both of which are reciprocally connected to the RSP. The functional relationship between HD cells in the RSP and those found in other limbic regions is presently unknown, but given the intimate connectivity between the RSP and regions such as the ADN and PoS, and the reported loss of spatial orientation in rodents and humans with RSP damage, it is likely that the RSP plays an important role in processing the limbic HD signal. To test this hypothesis, we produced neurotoxic or electrolytic lesions of the RSP and recorded HD cells in the ADN of female Long-Evans rats. HD cells remained present in the ADN after RSP lesions, but the stability of their preferred firing directions was significantly reduced even in the presence of a salient visual landmark. Subsequent tests revealed that lesions of the RSP moderately impaired landmark control over the cells' preferred firing directions, but spared the cells directional stability when animals were required to update their orientation using self-movement cues. Together, these results suggest that the RSP plays a prominent role in processing landmark information for accurate HD cell orientation and may explain the poor directional sense in humans that follows damage to the RSP.

NMDA receptor involvement in spatial delayed alternation in developing rats

Two experiments examined the effect of the noncompetitive NMDA receptor antagonist, dizocilpine maleate (MK-801), on spatial working memory during development. Rats were trained on spatial delayed alternation (SDA) in a T-maze after ip administration of 0.06 mg/kg MK-801, 0.1 mg/kg MK-801, or saline on postnatal days (P) P23 and P33 (Experiment 1), or following bilateral intrahippocampal administration of 2.5 or 5.0 microg per side MK-801 or saline on P26 (Experiment 2). In Experiment 1, MK-801 dose-dependently impaired SDA learning at both ages. Because the same doses of systemic MK-801 have no effect on T-maze position discrimination learning, impairment of SDA by MK-801 likely reflects disruption of spatial working memory. Both doses of MK-801 abolished acquisition of SDA performance in Experiment 2. Disruption of hippocampal plasticity may account for the effects produced by systemic MK-801 administration. These results confirm and extend earlier lesion studies by implicating plasticity of hippocampal neurons in the ontogeny of spatial delayed alternation.

Functional relevant loss of long association fibre tracts integrity in early Alzheimer's disease

The aim of our study was to quantify the structural integrity of the long association fibre tracts in early Alzheimer's disease (AD) and to correlate the findings with the cognitive performance of the patients. We conducted region-of-interest-based analyses of color-coded diffusion-tensor imaging in 12 patients with early AD (age 69.8+/-8.0 years; MMSE 25.3+/-1.8) and 16 age- and education-matched healthy controls. Early AD patients showed significantly decreased fractional anisotropy (FA) of the cingulate bundles and the inferior fronto-occipital fascicles bilaterally, whereas FA values of the superior longitudinal fascicles (second division) did not differ significantly between patients and controls. Neuropsychological performance of patients in the verbal episodic memory test domain correlated significantly with disturbances of left cingulate fibre tract integrity. Reduced left cingulate bundle integrity was most strongly correlated with impaired performance in a verbal recognition task (Spearman's rho=0.81, P=0.001). Moreover, Boston naming test performance also correlated with the left cingulate bundle integrity (Spearman's rho=0.71, P=0.009). These findings suggest substantial disturbances of the structural connectivity within long association fibre tracts, especially the cingulate bundles and the inferior fronto-occipital fascicles, in early AD and highlight the important role of the cingulate bundles in verbal recognition.

Color-coded diffusion-tensor-imaging of posterior cingulate fiber tracts in mild cognitive impairment

Different processes like microvascular dysfunction, free radical toxicity, beta-amyloid deposits, and Wallerian degeneration can cause functionally relevant disturbances of cerebral neuronal networks by myelin degeneration. Color-coded diffusion-tensor-imaging (ccDTI) allows the structural identification and quantification of myelinated fiber tracts. Particularly, posterior cingulate fiber tracts, which are regarded as important neuronal substrates of the network representing memory processing can be localized only imprecisely by conventional magnetic resonance imaging techniques. The posterior cingulate bundles were assessed by ccDTI in 17 patients with amnestic mild cognitive impairment (MCI), 25 patients with Alzheimer's dementia (DAT), and 21 age-matched controls. Additionally, DTI values were correlated with memory performance in the delayed verbal recall test. Fractional anisotropy and mean diffusivity differed significantly between MCI and controls, as well as between DAT and controls. Performance in the delayed verbal recall test of the entire study group correlated significantly with posterior cingulate bundle anisotropy and diffusivity. Using ccDTI seems, hence, a favorable strategy to detect and quantify the structural integrity of posterior cingulate white matter in MCI. Alterations of DTI parameters substantiate the involvement of white matter pathology in the development of MCI. Moreover, ccDTI could serve as in vivo method to investigate age and disease-related myelin alterations as potential morphological substrates of cognitive dysfunction.

A reaffirmation of the retrosplenial contribution to rodent navigation: reviewing the influences of lesion, strain, and task

Retrosplenial cortex (RS) is situated both anatomically and functionally between neocortical and limbic structures involved in spatial navigation. Initial anatomical, electrophysiological and behavioural evidence in both humans and rodents strongly suggested a role for RS in spatial navigation as well. Later studies using more selective cytotoxic lesions in rodents, however, cast doubt on earlier RS studies by failing to find spatial deficits following RS lesions. Contrasting reports from behavioural results on spatial tasks following RS damage have continued to be reported during the past decade. That RS does indeed contribute spatial behaviour even in rodents has been recently reaffirmed. The ambiguity surrounding RS is shown to result from differences in the choice of spatial tasks and rat strains between studies that find RS deficits and those that do not. The reconciliation of behavioural results following RS lesions strengthens the view that RS forms a part of the neural circuitry that underlies spatial navigation.

Testing the importance of the retrosplenial navigation system: lesion size but not strain matters: a reply to Harker and Whishaw

In their review on the retrosplenial cortex Harker and Whishaw [Neurosci Biobehav Rev, 2004] claim that there is continued disagreement over the importance of this region for navigation. They argue that discrepancies in the published effects of retrosplenial lesions reflect two principal variables, choice of rat strain and choice of spatial task. In this reply, evidence is provided showing that Harker and Whishaw [Neurosci Biobehav Rev, 2004] have created a misleading impression and, in fact, there is a clear consensus that the rat retrosplenial cortex is necessary for navigation. Likewise, there is no dispute that the effects of retrosplenial lesions will differ for different tests of spatial learning. While Harker and Whishaw [Neurosci Biobehav Rev, 2004] also conclude that choice of rat strain has a critical impact on whether a lesion-induced deficit is found, a comprehensive review of the published data shows no systematic strain difference. There is, however, growing evidence that when interpreting the effects of retrosplenial lesions, account should be given of the lesion method and its interaction with lesion size.

A two-dimensional neuropsychology of defense: fear/anxiety and defensive distance

We present in this paper a picture of the neural systems controlling defense that updates and simplifies Gray's "Neuropsychology of Anxiety". It is based on two behavioural dimensions: 'defensive distance' as defined by the Blanchards and 'defensive direction'. Defensive direction is a categorical dimension with avoidance of threat corresponding to fear and approach to threat corresponding to anxiety. These two psychological dimensions are mapped to underlying neural dimensions. Defensive distance is mapped to neural level, with the shortest defensive distances involving the lowest neural level (periaqueductal grey) and the largest defensive distances the highest neural level (prefrontal cortex). Defensive direction is mapped to separate parallel streams that run across these levels. A significant departure from prior models is the proposal that both fear and anxiety are represented at all levels. The theory is presented in a simplified form that does not incorporate the interactions that must occur between non-adjacent levels of the system. It also requires expansion to include the dimension of escapability of threat. Our current development and these proposed future extensions do not change the core concepts originally proposed by Gray and, we argue, demonstrate their enduring value.

Finding your way in the dark: the retrosplenial cortex contributes to spatial memory and navigation without visual cues

Path integration is presumed to rely on self-motion cues to identify locations in space and is subject to cumulative error. The authors tested the hypothesis that rats use memory to reduce such errors and that the retrosplenial cortex contributes to this process. Rats were trained for 1 week to hoard food in an arena after beginning a trial from a fixed starting location; probe trials were then conducted in which they began a trial from a novel place in light or darkness. After control injections, rats searched around the training location, showing normal spatial memory. Inactivation of the retrosplenial cortex disrupted this search preference. To assess accuracy during navigation, rats were then trained to perform multiple trials daily, with a fixed or a different starting location in light or darkness. Retrosplenial cortex inactivation impaired accuracy in darkness. The retrosplenial cortex may provide mnemonic information, which decreases errors when navigating in the dark.

Identifying cortical inputs to the rat hippocampus that subserve allocentric spatial processes: a simple problem with a complex answer

A consideration of the cortical projections to the hippocampus provides a number of candidate regions that might provide distal sensory information needed for allocentric processing. Prominent among the input regions are the entorhinal cortex, the perirhinal cortex, the postrhinal cortex, and the retrosplenial cortex. A review of these sites reveals the surprising fact that in spite of their anatomical connections, removal of the perirhinal and postrhinal cortices has little or no effect on spatial tasks and hence does not functionally disconnect the hippocampus. Extensive retrosplenial lesions have only mild effects, and even lesions of the entorhinal cortex only partially mimic the effects of hippocampal lesions upon tests of spatial memory. In contrast, studies using c-fos imaging support the involvement of the entorhinal, postrhinal, and retrosplenial cortices, but not the perirhinal cortex. It is argued that there exist multiple aspects of spatial memory, and this is reflected in the multiple routes by which cortical information can reach the hippocampus. One consequence is that lesions in a single site often have surprisingly mild effects on standard spatial tests.

Parallel recovery of MK-801-induced spatial learning impairment and neuronal injury in male mice

The relationship between spatial learning impairment and reversible neuronal injury in the posterior cingulate/retrosplenial (PC/RS) cortex induced by MK-801 in male mice was studied using a four-corner holeboard task. Mice were dosed with 1 mg/kg MK-801 and tested on acquisition of a new "baited" hole at 5 or 12 h posttreatment. Acquisition in drugged mice was impaired at 5 h, but not at 12 h posttreatment. Their retention performances were unaffected 24 h after either the 5 or 12 h posttreatment acquisition sessions. MK-801 (1 mg/kg) was found to induce locomotor hyperactivity and some sensorimotor impairment at 5 h posttreatment. which could have contributed to the acquisition deficit. However, nonassociative effects of the drug were not prominent because this same dose did not impair holeboard performance at 5 h posttreatment when the task was well learned. Histologic experiments showed that many injured neurons (containing cytoplasmic vacuoles) were present in the PC/RS cortex at 5 h posttreatment but the reaction was essentially reversed at 12 h posttreatment. The results suggest that the acquisition impairment and neuronal injury induced by MK-801 evolve and recover in parallel according to a similar time schedule.

Comparing the effects of selective cingulate cortex lesions and cingulum bundle lesions on water maze performance by rats

The ability of rats to learn the location of a hidden platform in a swim maze was compared in animals with excitotoxic lesions of the anterior or posterior (retrosplenial) cingulate cortex or radiofrequency lesions of the cingulum bundle or fimbria-fornix. Performance of this allocentric spatial task was unaffected by the posterior cingulate cortex lesions, while anterior cingulate cortex damage produced only a mild acquisition deficit. Transection of the fornix and lesions of the cingulum bundle produced similar patterns of impairment on initial acquisition, but the cingulum bundle lesions had less effect on reversal of the task. The results from the water maze, and from a subsequent T-maze alternation task, indicate that cingulum bundle lesions can produce a spatial deficit that is similar, but milder, to that observed after fornix transection. The results of the excitotoxic lesions suggest that previous studies examining conventional cingulate lesions may have been influenced by damage to adjacent fibre tracts, such as the cingulum bundle.

Recognition memory in rats--II. Neuroanatomical substrates

A discussion of the neuroanatomical systems thought to be of importance for the mediation of recognition memory in the rat warrants consideration of different, but not necessarily exclusive concepts. An important concept is the hypothesis that a dichotomy in the neural systems mediating spatial and non-spatial (item) memory exists in the rat. We have adopted a model of recognition memory suggesting that information about previously encountered items is stored in a dynamic pattern of neural activity and not in a localized representation. These patterns are features of distributed neuronal networks and different networks may process different forms of recognition memory. Two parallel-distributed neuronal networks are proposed in the rat. Network 1 is essential for the processing of non-spatial/item recognition memory processes and incorporates the cortical association areas such as TE1, TE2 and TE3, the rhinal cortices, the mediodorsal thalamic nucleus and prefrontal cortical areas. Network 2 comprises the hippocampus, mamillary bodies, anterior thalamic nuclei and medial prefrontal areas, especially the prelimbic cortex, and is suggested to be pivotal for the processing of spatial recognition memory.

Effects of radiofrequency versus neurotoxic cingulate lesions on spatial reversal learning in mice

Mice with radiofrequency (RF) lesions of the posterior (PC) or anterior (AC) cingulate cortex were trained on spatial discrimination reversal learning in a T-maze. The results were compared with those obtained in an earlier study after ibotenic acid (IBO) cingulate lesions. PC-RF lesions facilitated the initial discrimination and first reversal, whereas they retarded subsequent reversals; in contrast, PC-IBO lesions yielded a deficit on the initial discrimination and first reversal, but had no effect on subsequent reversals. AC-IBO, but not AC-RF lesions, precluded the formation of a learning set across reversals. These data suggest that cingulum transection, which accompanies RF but not IBO lesions, can mask or even antagonize the specific effects of cingulate damage. Consequently, inferences made from the effects of conventional lesions to assess and distinguish the functions of the two cingulate areas appear subject to caution.