Mode of [14C] 2-deoxy-D-glucose uptake into retrosplenial cortex and other memory-related structures of the monkey during a delayed response

Voluntary Behavior and Training Conditions Modulate in vivo Extracellular Glucose and Lactate in the Mouse Primary Motor Cortex

Learning or performing new behaviors requires significant neuronal signaling and is metabolically demanding. The metabolic cost of performing a behavior is mitigated by exposure and practice which result in diminished signaling and metabolic requirements. We examined the impact of novel and habituated wheel running, as well as effortful behaviors on the modulation of extracellular glucose and lactate using biosensors inserted in the primary motor cortex of mice. We found that motor behaviors produce increases in extracellular lactate and decreases in extracellular glucose in the primary motor cortex. These effects were modulated by experience, novelty and intensity of the behavior. The increase in extracellular lactate appears to be strongly associated with novelty of a behavior as well as the difficulty of performing a behavior. Our observations are consistent with the view that a main function of aerobic glycolysis is not to fuel the current neuronal activity but to sustain new bio-infrastructure as learning changes neural networks, chiefly through the shuttling of glucose derived carbons into the pentose phosphate pathway for the biosynthesis of nucleotides.

Intraretrosplenial Cortical Grafts of Fetal Cholinergic Neurons and the Restoration of Spatial Memory Function

The retrosplenial cortex (RSC) receives cholinergic afferent fibers from the medial septal nucleus and diagonal band of Broca (DBB) by way of the cingulate bundle and the fornix. Bilateral lesions of both the cingulate and fornix pathways result in a complete depletion of cholinergic input to the RSC. In the present study we have examined the effects of transplanting cholinergic neurons from fetal rat pups to the RSC of adult rats following lesions of the cingulate bundle and fornix. The animals with lesions exhibited severe spatial memory impairments with a complete loss of extrinsic cholinergic afferents to the RSC. Animals with intraretrosplenial cortical transplants exhibited significant improvements in learning and memory performance as revealed by decreased escape latencies in spatial reference memory tests, increased numbers of platform crossings in spatial navigation tests, and a higher percentage of correct choices in a spatial working memory task. These improvements appeared to be cholinergically mediated because atropine administration significantly disrupted spatial navigation performance. The survival of the transplanted cholinergic neurons and their innervation of the RSC were characterized using a monoclonal antibody to choline acetyltransferase (ChAT). The staining of graft-derived ChAT-positive fibers also revealed a pattern of innervation that mimicked that of the cholinergic input in normal animals. These results indicate that intraretrosplenial cortical transplants of cholinergic neurons can rectify spatial memory deficits produced by the loss of intrinsic cholinergic afferents from the medial septal nucleus. Copyright © 1997 Elsevier Science Inc.

Subspecialization in the human posterior medial cortex

The posterior medial cortex (PMC) is particularly poorly understood. Its neural activity changes have been related to highly disparate mental processes. We therefore investigated PMC properties with a data-driven exploratory approach. First, we subdivided the PMC by whole-brain coactivation profiles. Second, functional connectivity of the ensuing PMC regions was compared by task-constrained meta-analytic coactivation mapping (MACM) and task-unconstrained resting-state correlations (RSFC). Third, PMC regions were functionally described by forward/reverse functional inference. A precuneal cluster was mostly connected to the intraparietal sulcus, frontal eye fields, and right temporo-parietal junction; associated with attention and motor tasks. A ventral posterior cingulate cortex (PCC) cluster was mostly connected to the ventromedial prefrontal cortex and middle left inferior parietal cortex (IPC); associated with facial appraisal and language tasks. A dorsal PCC cluster was mostly connected to the dorsomedial prefrontal cortex, anterior/posterior IPC, posterior midcingulate cortex, and left dorsolateral prefrontal cortex; associated with delay discounting. A cluster in the retrosplenial cortex was mostly connected to the anterior thalamus and hippocampus. Furthermore, all PMC clusters were congruently coupled with the default mode network according to task-unconstrained but not task-constrained connectivity. We thus identified distinct regions in the PMC and characterized their neural networks and functional implications.

3-Monoiodothyronamine: the rationale for its action as an endogenous adrenergic-blocking neuromodulator

The investigations reported here were designed to gain insights into the role of 3-monoiodothyronamine (T1AM) in the brain, where the amine was originally identified and characterized. Extensive deiodinase studies indicated that T1AM was derived from the T4 metabolite, reverse triiodothyronine (revT3), while functional studies provided well-confirmed evidence that T1AM has strong adrenergic-blocking effects. Because a state of adrenergic overactivity prevails when triiodothyronine (T3) concentrations become excessive, the possibility that T3's metabolic partner, revT3, might give rise to an antagonist of those T3 actions was thought to be reasonable. All T1AM studies thus far have required use of pharmacological doses. Therefore we considered that choosing a physiological site of action was a priority and focused on the locus coeruleus (LC), the major noradrenergic control center in the brain. Site-directed injections of T1AM into the LC elicited a significant, dose-dependent neuronal firing rate change in a subset of adrenergic neurons with an EC(50)=2.7 microM, a dose well within the physiological range. Further evidence for its physiological actions came from autoradiographic images obtained following intravenous carrier-free (125)I-labeled T1AM injection. These showed that the amine bound with high affinity to the LC and to other selected brain nuclei, each of which is both an LC target and a known T3 binding site. This new evidence points to a physiological role for T1AM as an endogenous adrenergic-blocking neuromodulator in the central noradrenergic system.

Thinking about repairing thinking

The work of Sinden et al. suggests that it may be possible to produce improvement in the “highest” areas of brain function by transplanting brain tissue. What appears to be the limiting factor is not the complexity of the mental process under consideration but the discreteness of the lesion which causes the impairment and the appropriateness and accuracy of placement of the grafted tissue.

Therapeutic uses for neural grafts: Progress slowed but not abandoned

In spite of Stein and Glasier's justifiable conclusion that initial optimism concerning the immediate clinical applicability of neural transplantation was premature, there exists much experimental evidence to support the potential for incorporating this procedure into a therapeutic arsenal in the future. To realize this potential will require continued evolution of our knowledge at multiple levels of the clinical and basic neurosciences.

The structure, operation, and functionality of intracerebral grafts

The concept of structure, operation, and functionality, as they may be understood by clinicians or researchers using neural transplantation techniques, are briefly defined. Following Stein & Glasier, we emphasize that the question of whether an intracerebral graft is really functional should be addressed not only in terms of what such a graft does in a given brain structure, but also in terms of what it does at the level of the organism.

The NGF superfamily of neurotrophins: Potential treatment for Alzheimer's and Parkinson's disease

Stein & Glasier suggest embryonic neural tissue grafts as a potential treatment strategy for Alzheimer's and Parkinson's disease. As an alternative, we suggest that the family of nerve growth factor-related neurotrophins and their trk (tyrosine kinase) receptors underlie cholinergic basal forebrain (CBF) and dopaminergic substantia nigra neuron degeneration in these diseases, respectively. Therefore, treatment approaches for these disorders could utilize neurotrophins.

Some practical and theoretical issues concerning fetal brain tissue grafts as therapy for brain dysfunctions

Grafts of embryonic neural tissue into the brains of adult patients are currently being used to treat Parkinson's disease and are under serious consideration as therapy for a variety of other degenerative and traumatic disorders. This target article evaluates the use of transplants to promote recovery from brain injury and highlights the kinds of questions and problems that must be addressed before this form of therapy is routinely applied. It has been argued that neural transplantation can promote functional recovery through the replacement of damaged nerve cells, the reestablishment of specific nerve pathways lost as a result of injury, the release of specific neurotransmitters, or the production of factors that promote neuronal growth. The latter two mechanisms, which need not rely on anatomical connections to the host brain, are open to examination for nonsurgical, less intrusive therapeutic use. Certain subjective judgments used to select patients who will receive grafts and in assessment of the outcome of graft therapy make it difficult to evaluate the procedure. In addition, little long-term assessment of transplant efficacy and effect has been done in nonhuman primates. Carefully controlled human studies, with multiple testing paradigms, are also needed to establish the efficacy of transplant therapy.

Models of neurological defects and defects in neurological models

The transition from research to patient following advances in transplantation research is likely to be disappointing unless it includes a better understanding of critically relevant characteristics of the neurological disorder and improvements in the animal models, particularly the behavioral features. The appropriateness of the model has less to do with the species than with how the species is used.

(n-3) polyunsaturated fatty acid deficiency reduces the expression of both isoforms of the brain glucose transporter GLUT1 in rats

The altered neuron activity of rats deficient in (n-3) PUFAs may be due in part to a decrease in brain glucose utilization and glucose transport. We measured the glucose transporter protein GLUT1 isoforms at the blood-brain barrier (55-kDa) and in astrocytes (45-kDa) by Western immunoblotting and their mRNA by real time RT-PCR analysis in the cerebral cortex of adult male rats fed diets lacking (n-3) fatty acids (1st generation). The neuron glucose transporter GLUT3 was also assayed. The fatty acids in the phosphatidylcholine (PC), ethanolamine phosphoglycerolipid (EPG), and phosphatidylserine (PS) fractions of isolated microvessels and homogenates of the cerebral cortex were determined. The levels of (n-6) PUFAs [mainly arachidonic acid, 20:4(n-6)] in the phospholipid fractions of microvessels were higher and the levels of (n-3) PUFAs [mainly docosahexaenoic acid, 22:6(n-3)] were lower than in cerebral cortex homogenates. The microvessels and cortex of rats fed the (n-3) PUFA-deficient diet had 50% of the control 22:6(n-3) contents; 22:6(n-3) was replaced by 22:5(n-6). The 55-kDa GLUT1 immunoreactivity in (n-3) PUFA-deficient microvessels was decreased (down 25%, P < 0.01), as was the 45 kDa-GLUT1 in the homogenate (down 30%, P < 0.01). But the amount of immunoreactivity of GLUT3 did not change. The amount of GLUT1 mRNA was not affected by the (n-3) PUFA-deficient diet. These results suggest that the decreased glucose utilization in the cerebral cortex of (n-3) PUFA-deficient rats is due to reduced amounts of the 2 isoforms of GLUT1, indicating post-transcriptional regulation of GLUT1 synthesis.

Glucose transporter plasticity during memory processing

Various types of learning, including operant conditioning, induce an increase in cellular activation concomitant with an increase in local cerebral glucose utilization (LCGU). This increase is mediated by increased cerebral blood flow or changes in brain capillary density and diameter. Because glucose transporters are ultimately responsible for glucose uptake, we examined their plastic expression in response to cellular activation. In vitro and in vivo studies have demonstrated that cerebral glucose transporter 1 (GLUT1) expression consistently parallels changes in LCGU. The present study is the first to investigate the effect of memory processing on glucose transporters expression. Changes in GLUT expression produced by training in an operant conditioning task were measured in the brain of CD1 mice. Using semi-quantitative immunohistochemistry, Western blot and real time RT-PCR the cerebral GLUT1 and GLUT3 expression was quantified immediately, 220 min and 24 h following training. Relative to sham-trained and naive controls, operant conditioning training induced an immediate increase in GLUT1 immunoreactivity level in the hippocampus CA1 pyramidal cells as well as in the sensorimotor cortex. At longer post-learning delays, GLUT1 immunoreactivity decreased in the sensorimotor cortex and putamen. Parallel to the changes in protein levels, hippocampus GLUT1 mRNA level also increased immediately following learning. No effect of learning was found on hippocampal GLUT3 protein or mRNA expression. Measures of changes in glucose transporters expression present a link between cellular activation and glucose metabolism. The learning-induced localized increases in GLUT1 protein as well as mRNA levels observed in the present study confirm the previous findings that GLUT1 expression is plastic and respond to changes in cellular metabolic demands.

Posterior cingulate, precuneal and retrosplenial cortices: cytology and components of the neural network correlates of consciousness

Neuronal aggregates involved in conscious awareness are not evenly distributed throughout the CNS but comprise key components referred to as the neural network correlates of consciousness (NNCC). A critical node in this network is the posterior cingulate, precuneal, and retrosplenial cortices. The cytological and neurochemical composition of this region is reviewed in relation to the Brodmann map. This region has the highest level of cortical glucose metabolism and cytochrome c oxidase activity. Monkey studies suggest that the anterior thalamic projection likely drives retrosplenial and posterior cingulate cortex metabolism and that the midbrain projection to the anteroventral thalamic nucleus is a key coupling site between the brainstem system for arousal and cortical systems for cognitive processing and awareness. The pivotal role of the posterior cingulate, precuneal, and retrosplenial cortices in consciousness is demonstrated with posterior cingulate epilepsy cases, midcingulate lesions that de-afferent this region and are associated with unilateral sensory neglect, observations from stroke and vegetative state patients, alterations in blood flow during sleep, and the actions of general anesthetics. Since this region is critically involved in self reflection, it is not surprising that it is similarly a site for the NNCC. Interestingly, information processing during complex cognitive tasks and during aversive sensations such as pain induces efforts to terminate self reflection and result in decreased processing in posterior cingulate and precuneal cortices.

Retrosplenial cortex lesions of area Rgb (but not of area Rga) impair spatial learning and memory in the rat

The retrosplenial cortex, which is situated in a critical position in the flow of information between the hippocampal formation and the neocortex, contributes to spatial memory, but no studies have examined the distinct contribution of each area of the retrosplenial cortex to this behavior. This study tests the hypothesis that the two areas of the retrosplenial granular cortex play distinct roles in spatial learning and memory. Adult, male Sprague-Dawley rats with small, bilateral lesions (ibotenic acid) of the retrosplenial granular cortex were tested for 2 weeks in a repeated acquisition water maze task. Compared to controls, rats with complete lesions of the retrosplenial granular b cortex (Rgb) were slightly, but significantly impaired, whereas rats with lesions of the retrosplenial granular a cortex (Rga) displayed no impairment. Further, the Rgb-lesioned (but not the Rga-lesioned) group was impaired in the probe trials at the end of the first week of training. All animals were tested in the same paradigm for a second week to determine if the learning and memory impairment in the Rgb-lesioned rats simply reflected "delayed learning." All animals improved their maze performance during the second week of testing, but the Rgb-lesioned group still had no preference for the correct quadrant in the probe trial. Together, these data indicate that Rgb plays a small, independent role in spatial learning and memory. Further, although selective lesions of Rga or Rgb do not cause a large deficit in learning, concomitant destruction of both areas causes a much greater impairment in learning than would be predicted from their independent contributions. The data highlight the unique and complex contribution of each area of the retrosplenial cortex to behavior.

Connections of the retrosplenial granular b cortex in the rat

Although the retrosplenial granular b cortex (Rgb) is situated in a critical position between the hippocampal formation and the neocortex, surprisingly few studies have examined its connections carefully. The present experiments use both anterograde and retrograde tracing techniques to characterize the connections of Rgb. The main cortical projections from Rgb are to the caudal part of the anterior cingulate cortex, area 18b, retrosplenial granular a cortex (Rga), and postsubiculum, and less dense terminal fields are present in the prelimbic and caudal occipital cortices. The major subcortical projections are to the anterior thalamic nuclei and the rostral pontine nuclei, and very small terminal fields are present in the caudal dorsomedial part of the striatum, the reuniens and reticular nuclei of the thalamus, and the mammillary bodies. Contralaterally, Rgb primarily projects to itself, i.e., homotypically, and more sparsely projects to Rga and postsubiculum. In general, the axons from Rgb terminate ipsilaterally in cortical layers I and III-V and contralaterally in layer V, with a smaller number of terminals in layers I and VI. Thalamic projections from Rgb target the anteroventral and laterodorsal nuclei of the thalamus, with only a few axons terminating in the anterodorsal nucleus, the reticular nucleus, and the nucleus reuniens of the thalamus. Rgb is innervated by the anterior cingulate cortex, precentral agranular cortex, cortical area 18b, dorsal subiculum, and postsubiculum. Subcortical projections to Rgb originate mainly in the claustrum, the horizontal limb of the diagonal band of Broca, and the anterior thalamic nuclei. These data demonstrate that, in the rat, Rgb is a major nodal point for the integration and subsequent distribution of information to and from the hippocampal formation, the midline limbic and visual cortices, and the thalamus. Thus, similarly to the entorhinal cortex, Rgb in the rat is a prominent gateway for information exchange between the hippocampal formation and other limbic areas of the brain.

Cytology of human caudomedial cingulate, retrosplenial, and caudal parahippocampal cortices

Brodmann showed areas 26, 29, 30, 23, and 31 on the human posterior cingulate gyrus without marking sulcal areas. Histologic studies of retrosplenial areas 29 and 30 identify them on the ventral bank of the cingulate gyrus (CGv), whereas standardized atlases show area 30 on the surface of the caudomedial region. This study evaluates all areas on the CGv and caudomedial region with rigorous cytologic criteria in coronal and oblique sections Nissl stained or immunoreacted for neuron-specific nuclear binding protein and nonphosphorylated neurofilament proteins (NFP-ir). Ectosplenial area 26 has a granular layer with few large pyramidal neurons below. Lateral area 29 (29l) has a dense granular layer II-IV and undifferentiated layers V and VI. Medial area 29 (29m) has a layer III of medium and NFP-ir pyramids and a layer IV with some large, NFP-ir pyramidal neurons that distinguish it from areas 29l, 30, and 27. Although area 29m is primarily on the CGv, a terminal branch can extend onto the caudomedial lobule. Area 30 is dysgranular with a variable thickness layer IV that is interrupted by large NFP-ir neurons in layers IIIc and Va. Although area 30 does not appear on the surface of the caudomedial lobule, a terminal branch can form less that 1% of this gyrus. Area 23a is isocortex with a clear layer IV and large, NFP-ir neurons in layers IIIc and Va. Area 23b is similar to area 23a but with a thicker layer IV, more large neurons in layer Va, and a higher density of NFP-ir neurons in layer III. The caudomedial gyral surface is composed of areas 23a and 23b and a caudal extension of area 31. Although posterior area 27 and the parasubiculum are similar to rostral levels, posterior area 36' differs from rostral area 36. Subregional flat maps show that retrosplenial cortex is on the CGv, most of the surface of caudomedial cortex is areas 23a, 23b, and 31, and the retrosplenial/parahippocampal border is at the ventral edge of the splenium. Thus, Brodmann's map understates the rostral extent of retrosplenial cortex, overstates its caudoventral extent, and abridges the caudomedial extent of area 23.

Fos expression in the rostral thalamic nuclei and associated cortical regions in response to different spatial memory tests

Using the quantification of the Fos protein as an indicator of neuronal activation, we studied the involvement of the rostral thalamic nuclei and associated structures in different spatial memory tasks in two experiments. In both experiments, tasks were matched for sensorimotor factors but differed in their spatial and mnemonic demands. In Experiment 1, matched groups of rats either ran in a standard eight-arm radial maze or ran up and down just one arm of the maze while the number of runs and rewards were matched across pairs of rats. In Experiment 2, both groups were trained on the eight-arm radial maze but in different rooms. On the test day, one group was moved so that both groups now performed the same radial-maze task in the same room but for one group the extramaze cues were novel. There were significant increases in Fos in all three of the anterior thalamic nuclei (anterodorsal, anteroventral and anteromedial) as well as the adjacent nucleus reuniens and rostral reticular thalamic nucleus, in both the eight-arm versus one-arm condition (Experiment 1) and the novel room versus familiar room condition (Experiment 2). There were no significant differences in the mediodorsal thalamic nucleus in either experiment. The more spatially demanding task in each experiment also resulted in increased Fos expression in the subicular complex (postsubiculum, presubiculum and parasubiculum), as well as in the prelimbic cortex. Performing the standard radial-arm maze task also produced significant Fos increases in both rostral and caudal levels of the retrosplenial cortex when compared to rats running up and down a single arm in the same maze (Experiment 1); performing the task in a novel room did not, however, result in any further Fos increases in this region (Experiment 2). The specificity of the changes in levels of Fos was shown by a lack of any consistent difference in levels in six control sites.The present results reveal a group of anatomically related structures that work together in the intact rat brain during tasks that tax allocentric spatial working memory.

Localization of amino acids, neuropeptides and cholinergic markers in neurons of the septum-diagonal band complex projecting to the retrosplenial granular cortex of the rat

Retrograde labelling was combined with immunohistochemistry to localize neurons containing choline acetyltransferase, gamma-aminobutyric acid (GABA), glutamate, leu-enkephalin, neurotensin, and substance P-like immunoreactivity in the projection pathways from the septum-diagonal band complex to the retrosplenial granular cortex in the rat. Injections of horseradish peroxidase conjugated to subunit B of cholera toxin (CT-HRP) into the retrosplenial granular cortex resulted in retrogradely labelled neurons in the ipsilateral nuclei of the diagonal band of Broca, especially in the horizonatal nucleus of the diagonal band, and small numbers of CT-HRP-labelled neurons were also found in the medial septal nucleus. In the horizontal and vertical nuclei of the diagonal band of Broca, 90-95% of CT-HRP-labelled neurons (35-45 per section) were immunoreactive for choline acetyltransferase and small numbers of retrogradely labelled neurons (2 to 4-5 per section) were also immunoreactive for GABA, glutamate, neurotensin, leu-enkephalin, or substance P. In the medial septal nucleus approximately 75-80% of the retrogradely labelled neurons (8-10 per section) were immunoreactive for choline acetyltransferase and up to 25% of the CT-HRP labelled neurons (1-3 per section) in the medial septal nucleus also displayed GABA-, glutamate-, neurotensin-, leu-enkephalin-, or substance P-immunoreactivity. These results suggest that the complexity of the neurotransmitter(s)/neuromodulator(s) of septum-diagonal band complex projections to the retrosplenial granular cortex should be taken into account when considering the mechanisms of cortical activation.

Multivariate analysis of laminar patterns of neurodegeneration in posterior cingulate cortex in Alzheimer's disease

Posterior cingulate cortex is the site of earliest reductions in glucose metabolism and qualitatively different laminar patterns of neurodegeneration in Alzheimer's disease (AD). This study used multivariate analyses of area 23 in 72 cases of definite AD to assess relationships between laminar patterns of neurodegeneration, neurofibrillary tangle (NFT) and senile plaque (SP) densities, age of disease onset and duration, and apolipoprotein E (ApoE) genotype. No age-related changes in neurons occurred over four decades in 17 controls and regression analysis of all AD cases showed no relationships between neuron, SP, and tau-immunoreactive NFT densities. Principal components analysis of neurons in layers III-Va and eigenvector projections showed five subgroups. The subgroups were independent because each had a full range of disease durations and qualitatively different laminar patterns in degeneration suggested disease subtypes (ST). Cases with most severe neuron losses (STSevere) had an early onset, most SP, and highest proportion of ApoE epsilon4 homozygotes. Changes in the distribution of NFT were similar over disease course in two subtypes and NFT did not account for most neurodegeneration. In STII-V with moderate neuron loss in most layers, cases with no NFT had a disease duration of 3.5 +/- 0.9 years (mean +/- SEM), those with most in layers IIIc or Va had a duration of 7.3 +/- 1 years, and those with most in layers II-IIIab had a duration of 12.1 +/- 1 years. In STSevere, cases with highest NFT densities in layers II-IIIab also were late stage. Finally, epsilon4 homozygotes were most frequent in STSevere, but four statistical tests showed that this risk is not directly involved in neurodegeneration. In conclusion, multivariate pattern recognition shows that AD is composed of independent neuropathological subtypes and NFT in area 23 do not account for most neuron losses.

Cerebral cortex pathology in aging and Alzheimer's disease: a quantitative survey of large hospital-based geriatric and psychiatric cohorts

In order to explore the relationships between the involvement of specific neuronal populations and cognitive deterioration, and to compare the hierarchical patterns of cortical involvement in normal brain aging and Alzheimer's disease, over 1200 brains from elderly subjects without cognitive deficits, as well as from patients with age-associated memory impairment and Alzheimer's disease, were examined. Our results suggest that the neuropathological changes associated with normal brain aging and Alzheimer's disease affect select cortical circuits at different points in time. Extensive hippocampal alterations are correlated with age-associated memory impairment, whereas substantial neurofibrillary tangle formation in neocortical association areas of the temporal lobe is a prerequisite for the development of Alzheimer's disease. Despite several lines of evidence involving amyloid deposit in the pathogenesis of Alzheimer's disease and Down's syndrome, our observations indicate that there is no correlation between senile plaque densities and degree of dementia in both disorders. In contrast to younger elderly cases, in the ninth and tenth decades of life, there is a differential cortical involvement in that parietal and cingulate areas are early affected in the course of Alzheimer's disease, and neocortical senile plaques densities are strongly correlated with the severity of dementia. Moreover, Alzheimer's disease symptomatology is characterized in these very old patients by high neurofibrillary tangle densities in the anterior CA1 field, but not in the entorhinal cortex and inferior temporal cortex. These observations are discussed in the light of the hypothesis of global corticocortical disconnection and with respect to the notion of selective neuronal vulnerability in Alzheimer's disease.

Retrosplenial/presubicular continuum in primates: a developmental approach in fetal macaques using neurotensin and parvalbumin as markers

In spite of numerous hodological and neuropsychological studies emphasizing the multimodal connections and integrative functions of the retrosplenial cortex in primates, the precise fate of its caudoventral extent and the composition of the merging area with the hippocampal formation remain a matter of debate. We reported previously how the anlage of the retrosplenial cortex merges with the immature presubicular zone in the fetal rhesus monkey at the end of the first trimester of gestation. In the present study, this caudal area was further defined on a chemoarchitectonic basis, particularly during the late prenatal and perinatal stages, which correspond to the development of the cingulate sulcus and temporal gyri, and the differentiation of the retrosplenial/subicular complex. Neurotensin (NT), a pyramidal cell marker in the limbic cortex, and parvalbumin (PV), a marker of a subset of inhibitory local circuit neurons in the hippocampal formation, were used as immunocytochemical markers. According to distinct chemoarchitectural patterns, (1) areas 29 l and 29 m of the retrosplenial cortex formed a triangle-shaped ventral expansion which merged with a similar but dorsal expansion of the pre/parasubicular fields. A temporal extension of area 29 m down to area TH could not be detected. The pre/parasubiculum contributed with area 29 m to the lateral bank of the calcarine sulcus as far as the most caudal extent of the hippocampal formation. (2) The lamina principalis interna of the presubiculum was well individualized and did not appear as a simple horizontal shift of adjoining fields. (3) NT and PV displayed a distinct temporal profile of development. NT was already expressed in the pyramidal cells of the prospective retrosplenial cortex and ventral hippocampal formation at E47 (term 165 days). Major pathways of the hippocampal formation and retrosplenial cortex (fimbria, fornix, angular and cingulum bundles) were progressively labeled indicating early developing projections. A large set of NT-positive afferents reached the retrosplenial cortex between E114 and E120. Their laminar distribution was compatible with a thalamic or a subicular origin. (4) The development of PV expression was delayed until the last quarter of gestation, supporting its proposal as a signal of functional onset. The developmental fate and the particular connections of the presubiculum suggest that its functional importance should be further investigated during infancy and adulthood.

Cerebral glucose metabolism and memory in aged rhesus macaques

Positron emission tomography and the glucose metabolic tracer [18F]fluorodeoxyglucose were used to evaluate the relationship between regional cerebral metabolic rates for glucose (rCMRglc), age, and performance on a delayed response (DR) test of memory in the aged monkey. Eleven aged animals, 21-26-years old, were included in the analysis. Regional CMRglc, normalized to values for the entire brain, were determined for the dorsal prefrontal cortex, orbitofrontal cortex, hippocampus, and temporal cortex. The aged animals exhibited significant DR deficits relative to a cohort of normal young monkeys. Variability in DR performance among the aged subjects was significantly correlated with relative hippocampal rCMRglc, and chronological age was a reliable predictor of orbitofrontal rCMRglc ratios. This pattern of results suggests that DR impairments in the aged monkey may partly reflect age-related dysfunction distributed among multiple limbic system structures that participate in normal learning and memory. Overall, the findings support the use of positron emission tomography in efforts to define the relationship between cognitive performance, age, and brain physiology in nonhuman primates.

Metabolic reduction in the posterior cingulate cortex in very early Alzheimer's disease

This study investigated cerebral glucose metabolism in very early Alzheimer's disease, before a clinical diagnosis of probable Alzheimer's disease is possible, using [18F]fluorodeoxyglucose positron emission tomography. First, 66 patients with probable Alzheimer's disease with a spectrum of dementia severity (Mini-Mental State Examination score, 0-23) were recruited and studied. Cortical metabolic activity was analyzed topographically using three-dimensional stereotactic surface projections. Regression analysis was performed for each brain pixel to predict metabolic patterns of very early disease. Predictions were tested prospectively in a group of 8 patients who complained only of memory impairment without general cognitive decline (Mini-Mental State Examination score, 25 +/- 1) at the time of scanning but whose condition later progressed to probable Alzheimer's disease. Both results were compared to cerebral metabolic activity in 22 age-similar normal control subjects. Prediction and analysis of actual patients consistently indicated marked metabolic reduction (21-22%) in the posterior cingulate cortex and cinguloparietal transitional area in patients with very early Alzheimer's disease. Mean metabolic reduction in the posterior cingulate cortex was significantly greater than that in the lateral neocortices or parahippocampal cortex. The result suggests a functional importance for the posterior cingulate cortex in impairment of learning and memory, which is a feature of very early Alzheimer's disease.

Intra-retrosplenial cortical grafts of cholinergic neurons: functional incorporation and restoration of high affinity choline uptake

Fetal septal neurons transplanted into the deafferented retrosplenial cortex (RSC) of rats have been shown to reinnervate the host brain and ameliorate spatial memory deficits. In the present study we examined the effects of implanting cholinergic neurons on high affinity choline uptake (HACU) in the denervated RSC and the correlational relationship between this cholinergic parameter and the level of behavioral recovery. Three groups of animals were used: 1) normal control rats (NC), 2) rats with lesions of the fornix and cingulate pathways (FX), and 3) lesioned rats with fetal septal grafts in the RSC (RSCsep-TPL). We found that intra-RSC septal grafts produced significant increases in HACU, and that recovery of HACU was significantly correlated with the improvements in the performance of spatial reference memory, spatial navigation, and spatial working memory tasks. We have also investigated the ability of the host brain to modulate the activity of the implanted neurons. In particular we evaluated the effect of the animals' performance in a 6-arm radial maze task on high affinity choline uptake (HACU). Animals in each of the NC, FX, and RSCsep-TPL groups were randomly assigned one of the following subgroups: 1) rats that performed the maze task before the determination of HACU (BEH), or 2) rats that did not perform the maze task before the determination of HACU (NON-BEH). Significant increases were observed in the NC and RSCsep-TPL groups, but not in the FX animals, indicating that fetal septal grafts in the RSC can become functionally incorporated with the host neural circuitry, and that the activity of the implanted cholinergic neurons can be modulated by the host brain.

Visuospatial working memory in patients with schizophrenia

Recent investigations have documented abnormalities in working memory related processes in schizophrenics on tasks assessing the central executive component of this cognitive model. This preliminary study investigated the function of another component of the working memory system, the visuospatial scratch pad in schizophrenia. The "scratch pad's" passive visual store--responsible for the temporary retention of visual material--was assessed via a computerized spatial delayed response task, whereas its active spatial rehearsal subsystem--specialized for retaining the temporal properties--was explored through visual block span. To assess elemental visual spatial abilities we used the Judgment of Line Orientation test. Thirty-two schizophrenics and 27 controls were tested. Although we discovered the basic perceptual abilities of patients to be intact, we determined that whenever memory was necessitated on spatial tasks, patients demonstrated marked deficits. This pattern of cognitive dysfunction is consistent with impairments in a neural network involving prefrontal and/or posterior brain regions in schizophrenia.

Radioactive 2-DG incorporation patterns in the mesial frontal cortex of task-performing monkeys

The pattern of radioactive 2-deoxy-D-glucose (2-DG) uptake in the rostral mesial cortex was investigated in seven hemispheres of four task-performing monkeys (a delayed-response task performed with a forelimb). A two-dimensional 2-DG map was constructed from frontal sections. Blob-like 2-DG incorporation sites (2-DG active sites) were observed in single frontal sections, e.g., in the anterior cingulate gyrus (CiG) and supplementary and primary motor cortices in the mesial surface, and around the superior precentral sulcus in the premotor area. Blob-like 2-DG incorporation sites were also observed in the medial part of the dorsal frontal cortex near the midline. However, most of these blob-like 2-DG active sites were revealed in fact not to be blobs. They formed rostrocaudally continuous streaks when they were constructed in a two-dimensional map. Streaks fused with one another in some areas, and gave off branches in other areas. These 2-DG uptake patterns were similar between the paired left and right hemispheres of three brains. It is highly probable that these 2-DG active streaks (or blobs) reflected neuronal activity related to somatomotor and/or eye movements, because the 2-DG-labeled areas included motor, premotor, supplementary motor, and possibly part of the supplementary eye fields. It is also probable that this 2-DG incorporation was related to cognitive or memory functions, because neuronal activity related to performance of a delayed-response was reported in the rostral mesial cortex and in the CiG.

The spinal cord as an alternative model for nerve tissue graft

The spinal cord provides an alternative model for nerve tissue grafting experiments. Anatomo-functional correlations are easier to make here than in any other region of the CNS because of a direct implication of spinal cord neurons in sensorimotor activities. Lesions can be easily performed to isolate spinal cord neurons from descending inputs. The anatomy of descending monoaminergic systems is well defined and these systems offer a favourable paradigm for lesion-graft experiments.

Multiple obstacles to gene therapy in the brain

Neuwelt et al. have proposed gene-transfer experiments utilizing an animal model that offers many important advantages for investigating the feasibility of gene therapy in the human brain. A variety of tissues concerning the viral vector and mode of delivery of the corrective genes need to be resolved, however, before such therapy is scientifically supportable.

Principles of brain tissue engineering

It is often presumed that effects of neural tissue transplants are due to release of neurotransmitter. In many cases, however, effects attributed to transplants may be related to phenomena such as trophic effects mediated by glial cells or even tissue reactions to injury. Any conclusion regarding causation of graft effects must be based on the control groups or other comparisons used. In human clinical studies, for example, comparing the same subject before and after transplantation allows for many interpretations of the causes of clinical changes.

Lessons on transplant survival from a successful model system

Studies on the snailMelampusreveal that connectivity is crucial to the survival of transplanted ganglia. Transplanted CNS ganglia can innervate targets or induce supernumerary structures. Neuron survival is optimized by the neural incorporation that occurs when a transplanted ganglion is substituted for an excised ganglion. Better provision for the trophic requirements of neurons will improve the success of mammalian fetal transplants.

Repairing the brain: Trophic factor or transplant?

Three experiments on neural grafting with adult rat hosts are described. Working memory impairments were produced by lesioning the hippocampus or severing its connections with the septum by ablating the fimbria-fornix. The results suggest that the survival and growth of a neural graft, whether an autograft or a xenograft, is not a necessary condition for functional recovery on a task tapping working memory.

Will brain tissue grafts become an important therapy to restore visual function in cerebrally blind patients?

Grafting embryonic brain tissue into the brain of patients with visual field loss due to cerebral lesions may become a method to restore visual function. This method is not without risk, however, and will only be considered in cases of complete blindness after bilateral occipital lesions, when other, risk-free neuropsychological methods fail.

Difficulties inherent in the restoration of dynamically reactive brain systems

The responses displayed by an injured or diseased nervous system are complex. Some of the responses may effect a functional reorganization of the affected neural circuitry. Strategies aimed at the restoration of function, whether or not these involve transplantation, need to recognize the innate reactive capacity of the nervous system to damage. More successful strategies will probably incorporate, rather than ignore, the adaptive responses of the compromised neural systems.

Elegant studies of transplant-derived repair of cognitive performance

Cholinergic-rich grafts have been shown to be effective in restoring maze-learning deficits in rats with lesions of the forebrain cholinergic projection system. However, the relevance of those studies to developing novel therapies for Alzheimer's disease is questioned.

Neural transplants are grey matters

The lesion and transplantation data cited by Sinden et al., when considered in tandem, seem to harbor an internal inconsistency, raising questions of false localization of function. The extrapolation of such data to cognitive impairment and potential treatment strategies in Alzheimer's disease is problematic. Patients with focal basal forebrain lesions (e.g., anterior communicating artery aneurysm rupture) might be a more appropriate target population.

Immunobiology of neural transplants and functional incorporation of grafted dopamine neurons

In contrast to the views put forth by Stein & Glasier, we support the use of inbred strains of rodents in studies of the immunobiology of neural transplants. Inbred strains demonstrate homology of the major histocompatibility complex (MHC). Virtually all experimental work in transplantation immunology is performed using inbred strains, yet very few published studies of immune rejection in intracerebral grafts have used inbred animals.

Local and global gene therapy in the central nervous system

For focal neurodegenerative diseases or brain tumors, localized delivery of protein or genetic vectors may be sufficient to alleviate symptoms, halt disease progression, or even cure the disease. One may circumvent the limitation imposed by the blood-brain barrier by transplantation of genetically altered cell grafts or focal inoculation of virus or protein. However, permanent gene replacement therapy for diseases affecting the entire brain will require global delivery of genetic vectors. The neurotoxicity of currently available viral vectors and the transient nature of transgene expression invivomust be overcome before their use in human gene therapy becomes clinically applicable.

Neural grafting in human disease versus animal models: Cautionary notes

Over the past two decades, research on neural transplantation in animal models of neurodegeneration has provided provocative in sights into the therapeutic use of grafted tissue for various neurological diseases. Although great strides have been made and functional benefits gained in these animal models, much information is still needed with regard to transplantation in human patients. Several factors are unique to human disease, for example, age of the recipient, duration of disease, and drug interaction with grafted cells; these need to be explored before grafting can be considered a safe and effective therapeutic tool.

Building a rational foundation for neural transplantation

The neural transplantation research described by Sinden and colleagues provides part of the rationale for the clinical application of neural transplantation. The authors are asked to clarify their view of the role of the cholinergic system in cognition, to address extrahippocampal damage caused by transient forebrain ischemia, and to consider the effects of delayed neural degeneration in their structure-function analysis.

Intraretrosplenial grafts of cholinergic neurons and spatial memory function

The transplantation of cholinergic neurons into the hippocampal formation has been well characterized. We describe our studies on the effects of cholinergic transplants in the retrosplenial cortex. These transplants were capable of ameliorating spatial navigation deficits in rats with septohippocampal lesions. In addition, we provide evidence for the modulation of transplanted neurons by the host brain.

Gene therapy and neural grafting: Keeping the message switched on

A major problem in developing an effective gene therapy for the nervous system lies in understanding the principles that maintain or turn off the expression of genes following their transfer into the CNS.

Therapeutic neural transplantation: Boon or boondoggle?

Despite reports of recovery of function after neural transplantation, the biological interactions between transplanted neurons and the host brain that are necessary to mediate recovery are unclear at present. One source of confusion is in the variety of models and protocols used in these studies. It is suggested that multisite experimentation using standard protocols, models, and recovery criteria would be helpful in moving neural transplantation from the laboratory to the clinic.

The ethics of fetal tissue grafting should be considered along with the science

In addition to the scientific and medical issues surrounding the use of fetal tissue transplants, the ethical implications should be considered. Two major ethical issues are relevant. The first of these is whether this experimental procedure can be justified on the basis of potential benefit to the patient. The second is whether the use of tissue obtained from intentionally aborted fetuses can be justified in the context of historical and existing guidelines for the protection of human subjects. The separation of ethical decisions from medical practice and scientific research is necessary to prevent the exploitation of innocent human life.

Gene therapy for neurodegenerative disorders and malignant brain tumors

Gene therapy approaches have great promise in the treatment of neurodegenerative disorders and malignant brain tumors. Neuwelt et al. review available viral-mediated gene therapy methods and their blood-brain-barrier (BBB) disruption delivery technique, briefly mentioning nonviral mediated gene therapy methods. This commentary discussed the BBB disruption delivery technique, viral and nonviral mediated gene therapy approaches to Parkinson's disease, and the potential use of antisense oligo to suppress malignant brain tumors.

Behavioral effects of neural grafts: Action still in search of a mechanism

This commentary reviews data supporting circuitry reconstruction, replacement neurotransmitters, and trophic action as mechanisms whereby transplants promote recovery of function. Issue is taken with the thesis of Sinden et al. that adequate data exist to indicate that reconstruction of hippocampal circuitry damaged by hypoxia with CA1 transplants is a confirmed mechanism whereby these transplants produce recovery. Sinden et al.'s and Stein & Glasier's proposal that there is definitive evidence showing that all transplants produce trophic effects is also questioned.

Neural transplantation, cognitive aging and speech

Research on neural transplantation has great potential societal importance in part because of the expanding proportion of the population that is elderly. Transplantation studies can benefit from the guidance of research on cognitive aging, especially in connection with the assessment of behavioral outcomes. Speech for example, might be explored using avian models.

Pathway rewiring with neural transplantation

A lesion to the brain is not necessary for a successful neural transplantation. Embryonic Purkinje cells placed on the surface of an uninjured adult cerebellum can develop and migrate into the host molecular layer. Both the Purkinje cells that migrated into the host cerebellum and those that remained in the graft were innervated by collateral sprouting of adult intact climbing fibers.