Effects of mammillary body lesions on spatial reference and working memory tasks

Electrophysiological Properties of the Medial Mammillary Bodies across the Sleep-Wake Cycle

The medial mammillary bodies (MBs) play an important role in the formation of spatial memories; their dense inputs from hippocampal and brainstem regions makes them well placed to integrate movement-related and spatial information, which is then extended to the anterior thalamic nuclei and beyond to the cortex. While the anatomical connectivity of the medial MBs has been well studied, much less is known about their physiological properties, particularly in freely moving animals. We therefore carried out a comprehensive characterization of medial MB electrophysiology across arousal states by concurrently recording from the medial MB and the CA1 field of the hippocampus in male rats. In agreement with previous studies, we found medial MB neurons to have firing rates modulated by running speed and angular head velocity, as well as theta-entrained firing. We extended the characterization of MB neuron electrophysiology in three key ways: (1) we identified a subset of neurons (25%) that exhibit dominant bursting activity; (2) we showed that ∼30% of theta-entrained neurons exhibit robust theta cycle skipping, a firing characteristic that implicates them in a network for prospective coding of position; and (3) a considerable proportion of medial MB units showed sharp-wave ripple (SWR) responsive firing (∼37%). The functional heterogeneity of MB electrophysiology reinforces their role as an integrative node for mnemonic processing and identifies potential roles for the MBs in memory consolidation through propagation of SWR-responsive activity to the anterior thalamus and prospective coding in the form of theta cycle skipping.

Neurochemistry of the mammillary body

The mammillary body (MB) is a component of the extended hippocampal system and many studies have shown that its functions are vital for mnemonic processes. Together with other subcortical structures, such as the anterior thalamic nuclei and tegmental nuclei of Gudden, the MB plays a crucial role in the processing of spatial and working memory, as well as navigation in rats. The aim of this paper is to review the distribution of various substances in the MB of the rat, with a description of their possible physiological roles. The following groups of substances are reviewed: (1) classical neurotransmitters (glutamate and other excitatory transmitters, gamma-aminobutyric acid, acetylcholine, serotonin, and dopamine), (2) neuropeptides (enkephalins, substance P, cocaine- and amphetamine-regulated transcript, neurotensin, neuropeptide Y, somatostatin, orexins, and galanin), and (3) other substances (calcium-binding proteins and calcium sensor proteins). This detailed description of the chemical parcellation may facilitate a better understanding of the MB functions and its complex relations with other structures of the extended hippocampal system.

Reduced brain mammillary body volumes and memory deficits in adolescents who have undergone the Fontan procedure

BACKGROUND

Adolescents with single ventricle heart disease (SVHD) who have undergone the Fontan procedure show cognitive/memory deficits. Mammillary bodies are key brain sites that regulate memory; however, their integrity in SVHD is unclear. We evaluated mammillary body (MB) volumes and their associations with cognitive/memory scores in SVHD and controls.

METHODS

Brain MRI data were collected from 63 adolescents (25 SVHD; 38 controls) using a 3.0-Tesla MRI scanner. Cognition and memory were assessed using Montreal Cognitive Assessment (MoCA) and Wide Range Assessment of Memory and Learning 2. MB volumes were calculated and compared between groups (ANCOVA, covariates: age, sex, and total brain volume [TBV]). Partial correlations and linear regression were performed to examine associations between volumes and cognitive scores (covariates: age, sex, and TBV).

RESULTS

SVHD group showed significantly lower MoCA and WRAML2 scores over controls. MB volumes were significantly reduced in SVHD over controls. After controlling for age, sex, and TBV, MB volumes correlated with MoCA and delayed memory recall scores in SVHD and controls.

CONCLUSION

Adolescents with SVHD show reduced MB volumes associated with cognitive/memory deficits. Potential mechanisms of volume losses may include developmental and/or hypoxic/ischemic-induced processes. Providers should screen for cognitive deficits and explore possible interventions to improve memory.

Dietary Galacto-Oligosaccharides and Resistant Starch Protect Against Altered CB1 and 5-HT1A and 2A Receptor Densities in Rat Brain: Implications for Preventing Cognitive and Appetite Dysfunction During a High-Fat Diet

SCOPE

A high-fat, but low-fiber, diet is associated with obesity and cognitive dysfunction, while dietary fiber supplementation can improve cognition.

METHODS AND RESULTS

This study examines whether dietary fibers, galacto-oligosaccharides (GOS) and resistant starch (RS), could prevent high-fat (HF)-diet-induced alterations in neurotransmitter receptor densities in brain regions associated with cognition and appetite. Rats are fed a HF diet, HF diet with GOS, HF diet with RS, or a low-fat (LF, control) diet for 4 weeks. Cannabinoid CB1 (CB1R) and 5HT_1A (5HT_1A R) and 5-HT_2A (5HT_2A R) receptor binding densities are examined. In the hippocampus and hypothalamus, a HF diet significantly increases CB1R binding, while HF + GOS and HF + RS diets prevented this increase. HF diet also increases hippocampal and hypothalamic 5-HT_1A R binding, while HF + GOS and HF + RS prevented the alterations. Increased 5-HT_2A binding is prevented by HF + GOS and HF + RS in the medial mammillary nucleus.

CONCLUSIONS

These results demonstrate that increased CB1R, 5-HT_1A R and 5-HT_2A R induced by a HF diet can be prevented by GOS and RS supplementation in brain regions involved in cognition and appetite. Therefore, increased fiber intake may have beneficial effects on improving learning and memory, as well as reducing excessive appetite, during the chronic consumption of a HF (standard Western) diet.

Quantitative Anatomic Analysis of the Transcallosal-Transchoroidal Approach and the Transcallosal-Subchoroidal Approach to the Floor of the Third Ventricle: An Anatomic Study

OBJECTIVE

To compare transcallosal-transchoroidal and transcallosal-subchoroidal approaches to the ipsilateral and contralateral edges of the floor of the third ventricle using quantitative analyses.

METHODS

Five formalin-fixed cadaveric human heads (10 sides) were examined under the operating microscope. Quantitative measurements (area of surgical freedom and angle of attack) were obtained using 3-T magnetic resonance imaging and a StealthStation image guidance system. The limits of the surgical approaches were shown by touching a probe to 6 designated points on the floor of the third ventricle.

RESULTS

The transchoroidal approach provided greater surgical freedom than the subchoroidal approach to access ipsilateral and contralateral middle landmarks at the edges of the floor of the third ventricle in both longitudinal and horizontal planes (P ≤ 0.03). No significant difference between the 2 approaches was found in accessing the anterior and posterior landmarks of the third ventricle in each plane. The surgical freedom to the contralateral anterior, middle, and posterior landmarks was greater than to the ipsilateral landmarks in both the transchoroidal and subchoroidal approaches.

CONCLUSIONS

The transcallosal-transchoroidal approach, compared with the transcallosal-subchoroidal approach, may provide better exposure and require less retraction for removal of ipsilateral or contralateral lesions located in the midbrain or hypothalamus and situated near the floor of the third ventricle. The contralateral transcallosal approach with either the transchoroidal or subchoroidal approach may provide good surgical freedom for removal of lesions located near the floor of the third ventricle, such as lesions in the midbrain.

Lesions within the head direction system reduce retrosplenial c-fos expression but do not impair performance on a radial-arm maze task

The lateral mammillary nuclei are a central structure within the head direction system yet there is still relatively little known about how these nuclei contribute to spatial performance. In the present study, rats with selective neurotoxic lesions of the lateral mammillary nuclei were tested on a working memory task in a radial-arm maze. This task requires animals to distinguish between eight radially-oriented arms and remember which arms they have entered within a session. Even though it might have been predicted that this task would heavily tax the head direction system, the lesion rats performed equivalently to their surgical controls on this task; no deficit emerged even when the task was made more difficult by rotating the maze mid-way through testing in order to reduce reliance on intramaze cues. Rats were subsequently tested in the dark to increase the use of internally generated direction cues but the lesion rats remained unimpaired. In contrast, the lateral mammillary nuclei lesions were found to decrease retrosplenial c-Fos levels. These results would suggest that the head direction system is not required for the acquisition of the standard radial-arm maze task. It would also suggest that small decreases in retrosplenial c-Fos are not sufficient to produce behavioural impairments.

The effect of pharmacological inactivation of the mammillary body and anterior thalamic nuclei on hippocampal theta rhythm in urethane-anesthetized rats

The mammillary body (MB) and the anterior thalamic nuclei (ATN) are closely related structures, which take part in learning and memory processes. However, the exact role of these structures has remained unclear. In both structures neurons firing according to hippocampal theta rhythm have been found, mainly in the medial mammillary nucleus (MM) and anteroventral thalamic nucleus (AV). These neurons are driven by descending projections from the hippocampal formation and are thought to convey theta rhythm back to the hippocampus (HP). We argue that the MB-ATN axis not only relays theta signal, but may also modulate it. To examine it, we performed a pharmacological inactivation of the MM and AV by local infusion of procaine, and measured changes in theta activity in selected structures of the extended hippocampal system in urethane-anesthetized rats. The inactivation of the MM resulted in decrease in EEG power in the HP and AV, the most evidently in the lower theta frequency bands, i.e. 3-5Hz in the HP (down to 9.2% in 3- to 4-Hz band and 37.6% in 4- to 5-Hz band, in comparison to the power in the control conditions) and 3-4Hz in the AV (down to 24.9%). After the AV inactivation, hippocampal EEG power decreased in theta frequency bands of 3-8Hz (down to 61.6% in 6- to 7-Hz band and 69.4% in 7- to 8-Hz band). Our results suggest that the role of the MB-ATN axis in regulating theta rhythm signaling may be much more important than has been speculated so far.

Mammillary Bodies in Alzheimer's Disease: A Golgi and Electron Microscope Study

Alzheimer's disease (AD) is a progressive neurodegenerative disorder, characterized by irreversible memory decline, concerning no rarely spatial memory and orientation, alterations of the mood and personality, gradual loss of motor skills, and substantial loss of capacities obtained by previous long education. We attempted to describe the morphological findings of the mammillary bodies in early cases of AD. Samples were processed for electron microscopy and silver impregnation techniques. The nuclei of the mammillary bodies demonstrated a substantial decrease in the neuronal population and marked abbreviation of dendritic arbors. Decrease in spine density and morphological abnormalities of dendritic spines was also seen. Synaptic alterations were prominent. Alzheimer's pathology, such as deposits of amyloid-β peptide and neurofibrillary degeneration, was minimal. Electron microscopy revealed mitochondrial alterations and fragmentation of Golgi apparatus, associated frequently with synaptic pathology.

Supramammillary serotonin reduction alters place learning and concomitant hippocampal, septal, and supramammillar theta activity in a Morris water maze

Hippocampal theta activity is related to spatial information processing, and high-frequency theta activity, in particular, has been linked to efficient spatial memory performance. Theta activity is regulated by the synchronizing ascending system (SAS), which includes mesencephalic and diencephalic relays. The supramamillary nucleus (SUMn) is located between the reticularis pontis oralis and the medial septum (MS), in close relation with the posterior hypothalamic nucleus (PHn), all of which are part of this ascending system. It has been proposed that the SUMn plays a role in the modulation of hippocampal theta-frequency; this could occur through direct connections between the SUMn and the hippocampus or through the influence of the SUMn on the MS. Serotonergic raphe neurons prominently innervate the hippocampus and several components of the SAS, including the SUMn. Serotonin desynchronizes hippocampal theta activity, and it has been proposed that serotonin may regulate learning through the modulation of hippocampal synchrony. In agreement with this hypothesis, serotonin depletion in the SUMn/PHn results in deficient spatial learning and alterations in CA1 theta activity-related learning in a Morris water maze. Because it has been reported that SUMn inactivation with lidocaine impairs the consolidation of reference memory, we asked whether changes in hippocampal theta activity related to learning would occur through serotonin depletion in the SUMn, together with deficiencies in memory. We infused 5,7-DHT bilaterally into the SUMn in rats and evaluated place learning in the standard Morris water maze task. Hippocampal (CA1 and dentate gyrus), septal and SUMn EEG were recorded during training of the test. The EEG power in each region and the coherence between the different regions were evaluated. Serotonin depletion in the SUMn induced deficient spatial learning and altered the expression of hippocampal high-frequency theta activity. These results provide evidence in support of a role for serotonin as a modulator of hippocampal learning, acting through changes in the synchronicity evoked in several relays of the SAS.

How do mammillary body inputs contribute to anterior thalamic function?

It has long been assumed that the main function of the mammillary bodies is to provide a relay for indirect hippocampal inputs to the anterior thalamic nuclei. Such models afford the mammillary bodies no independent role in memory and overlook the importance of their other, non-hippocampal, inputs. This review focuses on recent advances that herald a new understanding of the importance of the mammillary bodies, and their inputs from the limbic midbrain, for anterior thalamic function. It has become apparent that the mammillary bodies' contribution to memory is not dependent on afferents from the subicular complex. Rather, the ventral tegmental nucleus of Gudden is a vital source of inputs that support memory processes within the medial mammillary bodies. In parallel, the lateral mammillary bodies, via their connections with the dorsal tegmental nucleus of Gudden, are critical for generating head-direction signals. These two parallel, but distinct, information streams converge on the anterior thalamic nuclei and support different aspects of spatial memory.

Behavioral assays with mouse models of Alzheimer's disease: practical considerations and guidelines

In Alzheimer's disease (AD) basic research and drug discovery, mouse models are essential resources for uncovering biological mechanisms, validating molecular targets and screening potential compounds. Both transgenic and non-genetically modified mouse models enable access to different types of AD-like pathology in vivo. Although there is a wealth of genetic and biochemical studies on proposed AD pathogenic pathways, as a disease that centrally features cognitive failure, the ultimate readout for any interventions should be measures of learning and memory. This is particularly important given the lack of knowledge on disease etiology - assessment by cognitive assays offers the advantage of targeting relevant memory systems without requiring assumptions about pathogenesis. A multitude of behavioral assays are available for assessing cognitive functioning in mouse models, including ones specific for hippocampal-dependent learning and memory. Here we review the basics of available transgenic and non-transgenic AD mouse models and detail three well-established behavioral tasks commonly used for testing hippocampal-dependent cognition in mice - contextual fear conditioning, radial arm water maze and Morris water maze. In particular, we discuss the practical considerations, requirements and caveats of these behavioral testing paradigms.

Affective brain areas and sleep-disordered breathing

The neural damage accompanying the hypoxia, reduced perfusion, and other consequences of sleep-disordered breathing, found in obstructive sleep apnea, heart failure, and congenital central hypoventilation syndrome (CCHS), appears in areas that serve multiple functions, including emotional drives to breathe, and involve systems that serve affective, cardiovascular, and breathing roles. The damage, assessed with structural magnetic resonance imaging (MRI) procedures, shows tissue loss or water content and diffusion changes indicative of injury, and impaired axonal integrity between structures; damage is preferentially unilateral. Functional MRI responses in affected areas also are time- or amplitude-distorted to ventilatory or autonomic challenges. Among the structures injured are the insular, cingulate, and ventral medial prefrontal cortices, as well as cerebellar deep nuclei and cortex, anterior hypothalamus, caudal raphé, ventrolateral medulla, portions of the basal ganglia and, in CCHS, the locus coeruleus. Caudal raphé and locus coeruleus injury have the potential to modify serotonergic and adrenergic modulation of upper airway and arousal characteristics, as well as affective drive to breathe. Since both axons and gray matter show injury, the consequences to function, especially to autonomic, cognitive, and mood regulation, are major. Several of the affected rostral sites mediate aspects of dyspnea, especially in CCHS, while others participate in initiation of inspiration after central breathing pauses, and the medullary injury can impair baroreflex and breathing control. The ancillary injury associated with sleep-disordered breathing to central structures can elicit multiple other distortions in cardiovascular, cognitive, and emotional functions in addition to effects on breathing regulation.

Sleep-disordered breathing: effects on brain structure and function

Sleep-disordered breathing is accompanied by neural injury that affects a wide range of physiological systems which include processes for sensing chemoreception and airflow, driving respiratory musculature, timing circuitry for coordination of breathing patterning, and integration of blood pressure mechanisms with respiration. The damage also occurs in regions mediating emotion and mood, as well as areas regulating memory and cognitive functioning, and appears in structures that serve significant glycemic control processes. The injured structures include brain areas involved in hormone release and action of major neurotransmitters, including those playing a role in depression. The injury is reflected in a range of structural magnetic resonance procedures, and also appears as functional distortions of evoked activity in brain areas mediating vital autonomic and breathing functions. The damage is preferentially unilateral, and includes axonal projections; the asymmetry of the injury poses unique concerns for sympathetic discharge and potential consequences for arrhythmia. Sleep-disordered breathing should be viewed as a condition that includes central nervous system injury and impaired function; the processes underlying injury remain unclear.

Gut-brain chemokine changes in portal hypertensive rats

BACKGROUND

Hepatic encephalopathy is a syndrome whose physiopathology is poorly understood; therefore, current diagnostic tests are imperfect and modern therapy is nonspecific. Particularly, it has been suggested that inflammation plays an important role in the pathogenesis of portal hypertensive encephalopathy in the rat.

AIM

We have studied an experimental model of portal hypertension based on a triple partial portal vein ligation in the rat to verify this hypothesis.

METHODS

One month after portal hypertension we assayed in the splanchnic area (liver, small bowel and mesenteric lymph nodes) and in the central nervous system (hippocampus and cerebellum) fractalkine (CX3CL1) and stromal cell-derived factor alpha (SDF1-α) as well as their respective receptors (CX3CR1 and CXCR4) because of their key role in inflammatory processes.

RESULTS

The significant increase of fractalkine in mesenteric lymph nodes (P<0.05) and its receptor (CX3CR1) in the small bowel (P<0.05) and hippocampus (P<0.01), associated with the increased expression of SDF1-α in the hippocampus (P<0.01) and the cerebellum (P<0.01) suggest that prehepatic portal hypertension in the rat induces important alterations in the expression of chemokines in the gut-brain axis.

CONCLUSION

The present study revealed that portal hypertension is associated with splanchnic-brain inflammatory alterations mediated by chemokines.

Portal hypertension in 18-month-old rats: memory deficits and brain metabolic activity

Portal hypertension is a major complication of cirrhosis that frequently leads to a neuropsychiatric disorder that affects cognition. We compared the performance of 18-month-old prehepatic portal hypertensive rats (PH) and 18-month-old normal rats (CO) in spatial short-term and reference memory tasks in the Morris water maze and in active avoidance task. The PH group showed worse spatial short-term memory than the CO group. Also, the PH group tended to perform worse than the CO group in the reference memory task, but it presented a correct acquisition of the active avoidance task. We assessed the brain metabolic activity of the animals by means of cytochrome c-oxidase (COx) histochemistry. We found that the PH group developed prefrontal dysfunction characterized by increased COx activity in this region compared to the CO group. Similar results were found in the medial mammillary nucleus and dentate gyrus, whereas the CA1 area, bed nucleus of the stria terminalis, and supramammillary nucleus showed lower COx activity in the PH group as compared to the CO group. We conclude that the 18-month-old portal hypertensive rats present spatial memory impairment without alteration of implicit learning. This deficit could be related to the alteration of the metabolic activity of the brain regions involved in the processing of spatial memories.

Preserved learning about allocentric cues but impaired flexible memory expression in rats with hippocampal lesions

Several studies have shown that slight modifications in the standard reference spatial memory procedure normally used for allocentric learning in the Morris water maze and the radial maze, can overcome the classic deficit in allocentric navigation typically observed in rats with hippocampal damage. In these special paradigms, however, there is only intramaze manipulation of a salient stimulus. The present study was designed to investigate whether extramaze manipulations produce a similar outcome. With this aim a four-arm plus-shaped maze and a reference spatial memory paradigm were used, in which the goal arm was marked in two ways: by a prominent extramaze cue (intermittent light), which maintained a constant relation with the goal, and by the extramaze constellation of stimuli around the maze. Experiment 1 showed that, unlike the standard version of the task, using this special training procedure hippocampally-damaged rats could learn a place response as quickly as control animals; importantly, one day after reaching criterion, lesioned and control subjects performed the task perfectly during a transfer test in which the salient extramaze stimulus used during the acquisition was removed. However, although acquisition deficit was overcomed in these lesioned animals, a profound deficit in retention was detected 15 days later. Experiment 2 suggests that although under our special paradigm hippocampal rats can learn a place response, spatial memory only can be expressed when the requisites of behavioral flexibility are minimal. These findings suggest that, under certain circumstances, extrahippocampal structures are sufficient for building a coherent allocentric representation of space; however, flexible memory expression is dependent, fundamentally, on hippocampal functioning.

Re-evaluating the role of the mammillary bodies in memory

Although the mammillary bodies were among the first brain regions to be implicated in amnesia, the functional importance of this structure for memory has been questioned over the intervening years. Recent patient studies have, however, re-established the mammillary bodies, and their projections to the anterior thalamus via the mammillothalamic tract, as being crucial for recollective memory. Complementary animal research has also made substantial advances in recent years by determining the electrophysiological, neurochemical, anatomical and functional properties of the mammillary bodies. Mammillary body and mammillothalamic tract lesions in rats impair performance on a number of spatial memory tasks and these deficits are consistent with impoverished spatial encoding. The mammillary bodies have traditionally been considered a hippocampal relay which is consistent with the equivalent deficits seen following lesions of the mammillary bodies or their major efferents, the mammillothalamic tract. However, recent findings suggest that the mammillary bodies may have a role in memory that is independent of their hippocampal formation afferents; instead, the ventral tegmental nucleus of Gudden could be providing critical mammillary body inputs needed to support mnemonic processes. Finally, it is now apparent that the medial and lateral mammillary nuclei should be considered separately and initial research indicates that the medial mammillary nucleus is predominantly responsible for the spatial memory deficits following mammillary body lesions in rats.

Sexually dimorphic c-Fos expression following spatial working memory in young and adult rats

The sex differences in the functional contribution of brain substrates were explored following acquisition of a spatial working memory task using quantification of c-Fos protein. Rats of both sexes were trained during adolescence and adulthood in Morris water maze using a hidden escape platform with different daily location. Two control groups for each sex and age were added to explore the c-Fos activation not specific to the memory process. These were a free-swimming group (yoked control) and a handled control (CO) group. Behaviorally, no age differences were found in number of days required by males to acquire the task, but females showed a delay in acquisition during adolescence (P30) that improved in adulthood (P90). Both sexes showed a learning-related increase in Fos immunoreactivity in the anterodorsal and anteroventral thalamus and medial and lateral mammillary nuclei during adolescence. Higher levels of learning-related Fos immunoreactivity were found in the infralimbic cortex, CA3 and CA1 only in females. During adulthood the common activated region was the prelimbic cortex with the addition of the infralimbic cortex in the male group and the lateral mammillary nucleus in the female group. These results indicated sex and age differences in brain functioning following working memory task. However, they could not be necessarily linked with differences in performance since similar results were found between males and females during adulthood. The activation of common and interrelated structures suggests that these structures are involved in spatial processing but it also highlights the relevance of developmental changes for understanding the memory process.

Basal and learning task-related brain oxidative metabolism in cirrhotic rats

Hepatic encephalopathy is a neurological complication observed in patients with liver disease. Subjects with hepatic encephalopathy can develop memory alterations. In order to investigate brain oxidative metabolism in an animal model of chronic cirrhosis and its modification after spatial working memory task, we determined the neural metabolic activity of several brain limbic system regions by cytochrome oxidase (COx) histochemistry and assessed the spatial working memory in the Morris water maze of rats with cirrhosis by administration of thioacetamide. This COx histochemistry was done in cirrhotic and control rats under basal conditions and after the spatial working memory task. The histochemical results showed differences in basal COx activity between control and cirrhotic rats in hippocampal and thalamic regions. In cirrhotic rats basal COx activity was increased in the CA1 and CA3 areas of the hippocampus and reduced in the anterodorsal and anteroventral thalamic nuclei. We found impaired spatial working memory in animals with cirrhosis. These animals showed absence of metabolic activation of the CA3 hippocampal subfield and the lateral mammillary nucleus and disturbance of COx activity in the medial mammillary nucleus and the anteroventral thalamus. These findings suggest that cirrhotic rats show spatial working memory deficits that could be related to the alteration of metabolic activity of neural regions thought to be involved in the processing of spatial memories.

Mammillary bodies and fornix fibers are injured in heart failure

Cognitive abnormalities, including memory deficits, are common in heart failure (HF). Brain structures, including the hippocampus, fornix, and thalamus participate in memory processing, and most show structural injury and functional deficits in HF. The mammillary bodies and fornix play essential roles in spatial and working memory processing, interact with other structures, and may also be injured in HF. We assessed mammillary body volumes and cross-sectional fornix areas in 17 HF and 50 control subjects using high-resolution T1-weighted magnetic resonance images. Mammillary body volumes and fornix cross-sectional areas were significantly reduced bilaterally in HF, and these differences remained after controlling age, gender, and intracranial volume. Mammillary body and fornix injury may contribute to the compromised spatial and working memory deficits in HF. Pathological processes eliciting the damage may include injury accompanying hypoxic/ischemic processes in pathologic HF perfusion and breathing, and thiamine deficiency accompanying diuretic use and nutritional mal-absorption in the condition.

Reduced mammillary body volume in patients with obstructive sleep apnea

Obstructive sleep apnea (OSA) patients show compromised emotional and cognitive functions, including anterograde memory deficits. While some memory inadequacies in OSA may result from earlier-described structural deficits in the hippocampus, mammillary body injury also could contribute, since these structures receive projections from the hippocampus via the fornix, project heavily to the anterior thalamus, and have been implicated in other conditions with memory deficiencies, such as Korsakoff's syndrome. However, volume loss in mammillary bodies has not been reported in OSA, likely a consequence of logistic difficulties in size assessment. We evaluated mammillary body volumes in 43 OSA (mean age+/-S.D., 46.9+/-9.2 years; mean apnea-hypopnea-index+/-S.D., 31.2+/-19.9 events/h) and 66 control subjects (age, 47.3+/-8.9 years). Two high-resolution T1-weighted image volumes were collected on a 3.0 T magnetic resonance scanner, averaged to improve signal-to-noise, and reoriented (without warping) into a common space. Brain sections containing both mammillary bodies were oversampled, and the bodies were manually traced and volumes calculated. OSA patients showed significantly reduced left, right, and combined mammillary body volumes compared with control subjects, after partitioning for age, gender, and head size (multivariate linear model, p<0.05). Left-side mammillary bodies showed greater volume reduction than the right side. Diminished mammillary body volume in OSA patients may be associated with memory and spatial orientation deficits found in the syndrome. The mechanisms contributing to the volume loss are unclear, but may relate to hypoxic/ischemic processes, possibly assisted by nutritional deficiencies in the syndrome.

Gating of Sema3E/PlexinD1 signaling by neuropilin-1 switches axonal repulsion to attraction during brain development

The establishment of functional neural circuits requires the guidance of axons in response to the actions of secreted and cell-surface molecules such as the semaphorins. Semaphorin 3E and its receptor PlexinD1 are expressed in the brain, but their functions are unknown. Here, we show that Sema3E/PlexinD1 signaling plays an important role in initial development of descending axon tracts in the forebrain. Early errors in axonal projections are reflected in behavioral deficits in Sema3E null mutant mice. Two distinct signaling mechanisms can be distinguished downstream of Sema3E. On corticofugal and striatonigral neurons expressing PlexinD1 but not Neuropilin-1, Sema3E acts as a repellent. In contrast, on subiculo-mammillary neurons coexpressing PlexinD1 and Neuropilin-1, Sema3E acts as an attractant. The extracellular domain of Neuropilin-1 is sufficient to convert repulsive signaling by PlexinD1 to attraction. Our data therefore reveal a "gating" function of neuropilins in semaphorin-plexin signaling during the assembly of forebrain neuronal circuits.

Spatial memory alterations in three models of hepatic encephalopathy

A behavioural evaluation was carried out on three chronic models of hepatic encephalopathy: two models of type B HE, portacaval shunt (PCS) and portal hypertension (PH) and one of type C HE with cirrhosis and portal hypertension from thioacetamide intoxication (TAA). The tasks selected cover a wide range of behaviours related to: locomotion (rotarod-accelerod test), anxiety (open field and elevated plus maze) and memory (Morris water maze). The results indicate that neither locomotor activity nor anxiety was affected in our models, in comparison with their respective controls. However, this is not the case for the mnesic tasks. Hence, the PCS and TAA groups displayed a severe alteration in spatial reference memory and cannot correctly perform the Morris maze task, while this alteration is less severe in the PH group. On the contrary, the PH group revealed a deficit in spatial working memory, like the TAA group, but this does not occur in subjects with PCS. These results reveal a double dissociation in spatial reference memory and spatial working memory between the PCS and PH groups, which would be of great interest to study about cerebral causes and substrates of the alterations accompanying HE.

Induction of c-Fos expression in the mammillary bodies, anterior thalamus and dorsal hippocampus after fear conditioning

The aim of the present study was to provide further evidence on the role of particular subdivisions of the mammillary bodies, anterior thalamus and dorsal hippocampus to contextual and auditory fear conditioning. We used c-Fos expression as a marker of neuronal activation to compare rats that received tone-footshock pairings in a distinctive context (conditioned group) to rats being exposed to both the context and the auditory CS without receiving footshocks (unconditioned group), and naïve rats that were only handled. Fos immunoreactivity was significantly increased only in the anterodorsal thalamic nucleus and the lateral mammillary nucleus of the conditioned group. However, the dorsal hippocampus showed the highest density of c-Fos positive nuclei in the naïve group as compared to the other groups. Together, our data support previous studies indicating a particular involvement of the mammillary bodies and anterior thalamus in fear conditioning.

Astroglial distribution and sexual differences in neural metabolism in mammillary bodies

The sexual differences in cerebral nuclei are produced by the organizational and the activational function of gonadal hormones. The different performances by male and female rats in memory tasks requiring use of the mammillary bodies (MBs), could be due to structural and functional sexual dimorphic differences. Our work quantifies the number of glial fibrillary acidic protein immunoreactive (GFAP-IR) astrocytes, and neuronal metabolic activity measured by the cytochrome oxidase (CO) histochemistry in the MBs in rats of both sexes. We find that there is no difference in astroglial number in the medial mammillary nucleus (MMN) and in the lateral mammillary nucleus (LMN) of males, females in estrus and diestrus adult rats. However, we do find statistically significant differences between the sexes in the neuronal oxidative metabolism influenced by the estrous cycle. We, therefore, conclude that there are functional and not structural sex differences in the MBs.

Distribution of substance P reveals a novel subdivision in the hippocampus of parasitic South American cowbirds

Parasitic cowbirds monitor potential hosts' nests and return to lay when appropriate, a task that is likely to involve spatial recall. Seasonal and sexual behavioral variations in the cowbirds correlate with anatomical changes in the hippocampal formation. During the breeding season, parasites have larger hippocampal formations than nonparasites. In parasitic species in which females alone perform nest bookkeeping, females have larger hippocampal formations than males. We investigated the distribution of the neuropeptide substance P (SP) in three sympatric cowbirds: two obligate parasites (shiny cowbird and screaming cowbird) and one nonparasite (bay-winged cowbird). Distribution of SP was similar to that in other songbirds, except for a previously undescribed field of dense SP-rich terminals within the hippocampus that we call the hippocampal SP terminal field (SPh). We found robust species differences in the volume of this new area, measured relative to the remainder of the telencephalon. SPh was largest in the generalist parasite (shiny cowbird) and smallest in the nonparasitic species (bay-winged cowbird). In the specialist parasite (screaming cowbird), SPh was smaller than in the generalist parasite but larger than in the nonparasitic species. SPh overlaps with two subdivisions described in the pigeon that have been related to the mammalian dentate gyrus and subiculum. The area containing SPh receives a major input from the lateral mammillary nucleus, which is probably the avian equivalent of the mammalian supramammillary nucleus (SUM), the main source of extrinsic SP input to mammalian hippocampus. SPh may be the termination of a pathway homologous to the SP-rich projection from SUM to the hippocampus in mammals.

Effects of systemic lupus erythematosus on spatial cognition and cerebral regional metabolic reactivity in BxSB lupus-prone mice

Brain-reactive auto-antibodies appear as key elements in the progressive CNS disturbances associated with systemic lupus erythematosus. The BxSB lupus prone mice are a model of this pathology, in which a gene located on the Y chromosome provokes a sex specific morbidity in males. This study was aimed to establish and characterize the relationships between behavioral disorders, neurological deficiencies and the aged-related immunological perturbations in this murine model. For this purpose, spatial and motor abilities were evaluated in male and female mice at six and 26 weeks of age. The results showed that the older males were greatly altered in their spatial abilities while the young ones and the females, whatever their age, were not. None of the animals had motor skill and motor learning disabilities. These spatial alterations were associated with modifications of basal neuronal activity measured by the cytochrome oxidase histochemical method in several areas directly or indirectly involved in spatial behavior, such as the hippocampus, the amygdala, the parietal and perirhinal cortex. Immunological study allowed us to correlate the behavioral abnormalities to the appearance of antibodies reactivities against cellular and nuclear components.

Dissociable memory effects after medial thalamus lesions in the rat

Variable neuropathology in cases of diencephalic amnesia has led to uncertainty in identifying key thalamic nuclei and their potential role in learning and memory. Based on the principal neural connections of the medial thalamus, the current study tested the hypothesis that different aggregates of thalamic nuclei contribute to separate memory systems. Lesions of the anterior thalamic aggregate (AT), which comprises the anterodorsal, anteromedial and anteroventral nuclei produced substantial deficits in both working and reference spatial memory in a radial arm maze task in rats, supporting the view that the AT is an integral part of a hippocampal memory system. Lesions to the lateral thalamic aggregate (LT), which comprises the intralaminar nuclei (centrolateral, paracentral and rostral central medial nuclei) and lateral mediodorsal thalamic nuclei (lateral and paralamellar nuclei) produced a mild working memory impairment only, while lesions to the posteromedial thalamic aggregate (MT), which comprises the central and medial mediodorsal thalamic nuclei and the intermediodorsal nucleus had no effect on radial arm maze performance. In contrast, only MT lesions impaired learning associated with memory for reward value, consistent with the idea that the MT contributes to an amygdala memory system. Compared with chance discrimination, the control and AT groups, but not MT or LT groups, showed evidence for temporal order memory for two recently presented objects; all groups showed intact object recognition for novel vs. familiar objects. These new dissociations show that different medial thalamic aggregates participate in multiple memory systems and reinforce the idea that memory deficits in diencephalic amnesics may vary as a function of the relative involvement of different thalamic regions.

Genetic ablation of the mammillary bodies in the Foxb1 mutant mouse leads to selective deficit of spatial working memory

Mammillary bodies and the mammillothalamic tract are parts of a classic neural circuitry that has been implicated in severe memory disturbances accompanying Korsakoff's syndrome. However, the specific role of mammillary bodies in memory functions remains controversial, often being considered as just an extension of the hippocampal memory system. To study this issue we used mutant mice with a targeted mutation in the transcription factor gene Foxb1. These mice suffer perinatal degeneration of the medial and most of the lateral mammillary nuclei, as well as of the mammillothalamic bundle. Foxb1 mutant mice showed no deficits in such hippocampal-dependent tasks as contextual fear conditioning and social transmission of food preference. They were also not impaired in the spatial reference memory test in the radial arm maze. However, Foxb1 mutants showed deficits in the task for spatial navigation within the Barnes maze. Furthermore, they showed impairments in spatial working memory tasks such as the spontaneous alternation and the working memory test in the radial arm maze. Thus, our behavioural analysis of Foxb1 mutants suggests that the medial mammillary nuclei and mammillothalamic tract play a role in a specific subset of spatial tasks, which require combined use of both spatial and working memory functions. Therefore, the mammillary bodies and the mammillothalamic tract may form an important route through which the working memory circuitry receives spatial information from the hippocampus.

Transient spatial deficit associated with bilateral lesions of the lateral mammillary nuclei

The mammillary bodies have long been implicated in spatial memory, and lesions of this structure in rats can impair some spatial memory tasks. The mammillary bodies, however, comprise two main nuclei that have different electrophysiological and anatomical properties. It is therefore possible that they have different functions. The present study determined whether selective lesions of one of these components, the lateral mammillary nucleus, are sufficient to induce spatial memory deficits. While selective lateral mammillary nuclei lesions induced deficits on a working memory task in the water maze, this impairment was milder and not as persistent as that seen with complete mammillary body lesions. Furthermore, lateral mammillary nuclei lesions did not impair T-maze alternation, which is sensitive to complete mammillary body lesions. From these results it appears that lesions confined to the lateral mammillary nuclei are sufficient to produce mild impairments when rapid, new spatial learning is at a premium. At the same time, the remaining mammillary nuclei also contribute to spatial learning, though this may be in a qualitatively different manner.

Effect of reversible inactivation of the supramammillary nucleus on spatial learning and memory in rats

Memory includes processes such as acquisition, consolidation and retrieval. Reference memory (RM) and working memory (WM) are two kinds of memory that can be assessed in rodents using spatial tasks, especially using the Morris water maze. The Morris water maze is particularly sensitive to hippocampal lesions. The supramammillary nucleus (SuM) has strong links with the hippocampus and septum. The role of the SuM on spatial learning is controversial. In the present study, involvement of SuM in the different steps of spatial RM and WM was investigated in the Morris water maze using reversible inactivation of SuM with lidocaine. Lidocaine (0.5 microl, 4%) was injected into the SuM through a guide cannula implanted above the SuM. The rats were trained on RM and WM versions of the Morris water maze. SuM was inactivated before training or immediately after training or before the probe trial of retrieval tests. Reversible inactivation of the SuM impaired consolidation of RM, and of consolidation and retrieval of WM. Therefore, it seems that activity of SuM neurons plays a role in spatial RM and WM learning and memory in the rat.

Involvement of the mammillary bodies in spatial working memory revealed by cytochrome oxidase activity

In view of the inconclusive findings relating the nuclei of the mammillary bodies (MB) with spatial memory, we evaluated the oxidative metabolic activity of the medial and lateral nuclei of the mammillary bodies (MB) after training young rats (30 days) of both sexes in the Morris water maze. Different groups were trained in spatial working (WM) or reference memory (RM) tasks, respectively. The corresponding naïve groups swam for the same amount of time as the trained groups but without the escape platform. Control groups were added that had not been manipulated in any way. No sex-related differences were detected in the working memory task although males exhibited better reference memory than females. Cytochrome oxidase (CO) activity, an endogenous metabolic marker for neuronal activity, was measured in all the groups. CO activity increased significantly in both MB nuclei of male and female rats only in the spatial working memory group. In addition, high CO activity in the lateral nucleus of the MB was linearly correlated with lower escape latencies in both sexes after training in the working memory task. No CO activity changes were found in the basolateral amygdala (BL) in any of the experimental groups. This nucleus was used as a control brain region because of its participation in emotional behavior. The results suggest a specific role of the MB nuclei in spatial working memory in both sexes.

Lesions of the mammillothalamic tract impair the acquisition of spatial but not nonspatial contextual conditional discriminations

This study examined the influence of selective mammillothalamic tract lesions in rats on the acquisition of two kinds of contextual conditional discrimination: one involving two contexts (A and B) that differed in their visuo-spatial properties and another involving two contexts (C and D) that differed in temperature. In contexts A (and C) presentations of a tone were paired with food whereas presentations of a clicker were not; and in contexts B (and D) presentations of the clicker were paired with food whereas those of the tone were not. Mammillothalamic tract lesions disrupted initial acquisition of the conditional discrimination involving visual contexts (A and B), but not the formally equivalent discrimination involving thermal contexts (C and D). These results provide support for the suggestion that mammillothalamic tract lesions disrupt visuo-spatial encoding.

Evidence of a spatial encoding deficit in rats with lesions of the mammillary bodies or mammillothalamic tract

The present study sought to identify the role of the mammillary bodies and their projections to the anterior thalamic nuclei for spatial memory. Rats with either selective, neurotoxic mammillary body lesions or discrete mammillothalamic tract lesions were tested on various spatial working memory tasks. Tests using the T-maze, radial-arm maze, and water maze were manipulated to compare three possible theories of mammillary body function by increasing proactive interference, increasing retention interval, and taxing the rapid processing of novel spatial stimuli. On T-maze alternation and radial-arm maze tasks, both lesion groups were initially impaired but seemed to recover. Transfer tests revealed, however, a more permanent change in performance, suggesting a failure to use distal (allocentric) cues. Consistent with this, both groups were also impaired at matching-to-place in the water maze and showed little improvement with practice. Nevertheless, once the lesion groups had been trained on a task, they were not affected differentially either by an increase of proactive interference or by retention intervals of up to 30 min. Although both mammillary body and mammillothalamic tract lesions resulted in similar impairments, the mammillothalamic tract group was the more affected when acquiring new spatial information. Together, these results suggest that mammillary body damage causes an encoding deficit when learning new spatial tasks, resulting in a suboptimal mode of performance, which may reflect a loss of directional heading information.

Long-lasting effects of neonatal dexamethasone treatment on spatial learning and hippocampal synaptic plasticity: involvement of the NMDA receptor complex

The effects of neonatal dexamethasone (DEX) treatment on spatial learning and hippocampal synaptic plasticity were investigated in adult rats. Spatial learning in reference and working memory versions of the Morris maze was impaired in DEX-treated rats. In hippocampal slices of DEX rats, long-term depression was facilitated and potentiation was impaired. Paired-pulse facilitation was normal, suggesting a postsynaptic defect as cause of the learning and plasticity deficits. Western blot analysis of hippocampal postsynaptic densities (PSD) revealed a reduction in NR2B subunit protein, whereas the abundance of the other major N-methyl-D-aspartate (NMDA) receptor subunits (NR1, NR2A), AMPA receptor subunits (GluR2/3), scaffolding proteins, and Ca2+/calmodulin-dependent protein kinase II (alphaCaMKII) were unaltered. This selective reduction in NR2B likely resulted from altered receptor assembly rather than subunit expression, because the abundance of NR2B in the homogenate and crude synaptosomal fractions was unaltered. In addition, the activity of alphaCaMKII, an NMDA receptor complex associated protein kinase, was increased in PSD of DEX rats. The results indicate that neonatal treatment with DEX causes alterations in composition and function of the hippocampal NMDA receptor complex that persist into adulthood. These alterations likely explain the deficits in hippocampal synaptic plasticity and spatial learning induced by neonatal DEX treatment.

Distribution of melanin-concentrating hormone neurons projecting to the medial mammillary nucleus

The melanin-concentrating hormone and neuropeptide glutamic acid-isoleucine are expressed in neurons located mainly in the hypothalamus that project widely throughout the CNS. One of the melanin-concentrating hormone main targets is the medial mammillary nucleus, but the exact origin of these fibers is unknown. We observed melanin-concentrating hormone and neuropeptide glutamic acid-isoleucine immunoreactive fibers coursing throughout the mammillary complex, showing higher density in the pars lateralis of the medial mammillary nucleus, while the lateral mammillary nucleus showed sparse melanin-concentrating hormone innervation. The origins of these afferents were determined by using implant of the retrograde tracer True Blue in the medial mammillary nucleus. Double-labeled neurons were observed in the lateral hypothalamic area, rostromedial zona incerta and dorsal tuberomammillary nucleus. A considerable population of retrogradely labeled melanin-concentrating hormone perikaryal profiles was also immunoreactive to neuropeptide glutamic acid-isoleucine (74+/-15% to 85+/-15%). The afferents from the lateral hypothalamic area, rostromedial zona incerta and dorsal tuberomammillary nucleus to the medial mammillary nucleus were confirmed using implant of the anterograde tracer Phaseolus vulgaris leucoagglutinin. In addition, using double-labeled immunohistochemistry, we found no co-localization between neurons expressing melanin-concentrating hormone and adenosine deaminase (histaminergic marker) in the dorsal tuberomammillary nucleus. We hypothesize that these melanin-concentrating hormone projections participate in spatial memory process mediated by the medial mammillary nucleus. These pathways would enable the animal to look for food during the initial moments of appetite stimulation.

c-Fos expression in supramammillary and medial mammillary nuclei following spatial reference and working memory tasks

To investigate brain substrates of spatial memory, neuronal expression of c-Fos protein was studied. Two groups of rats were trained in two spatial memory tasks in the Morris water maze, where the rats have to apply a reference memory rule or a working memory rule. In addition to the experimental groups, two control groups were used to study c-fos activation not specific to the memory processes studied. After immunohistochemical procedures, the number of c-Fos positive neuronal nuclei was quantified in the mammillary body (MB) region (medial mammillary nucleus [MMn] and supramammillary nucleus [SuM]). The results have shown that some MMn neurons expressed c-Fos nuclear immunoreactivity related to spatial working memory but not to spatial reference memory. The increased number of c-Fos immunoreactive neuronal nuclei in the SuM was related to spatial training but not to either working or reference memory demands of the tasks.