The limbic system in Mammalian brain evolution.

Previous accounts of mammalian brain allometry have relied largely on data from primates, insectivores and bats. Here we examine scaling of brain structures in carnivores, ungulates, xenarthrans and sirenians, taxa chosen to maximize potential olfactory and limbic system variability. The data were compared to known scaling of the same structures in bats, insectivores and primates. Fundamental patterns in brain scaling were similar across all taxa. Marine mammals with reduced olfactory bulbs also had reduced limbic systems overall, particularly in those structures receiving direct olfactory input. In all species, a limbic factor with olfactory and non-olfactory components was observed. Primates, insectivores, ungulate and marine mammals collectively demonstrate an inverse relationship between isocortex and limbic volumes, but terrestrial carnivores have high relative volumes of both, and bats low relative volumes of both. We discuss developmental processes that may provide the mechanistic bases for understanding these findings.

Somatosensory areas of manatee cerebral cortex: histochemical characterization and functional implications.

A histochemical and cytoarchitectural analysis was completed for the neocortex of the Florida manatee in order to localize primary sensory areas and particularly primary somatosensory cortex (SI). Based on the location of cytochrome oxidase-dense staining in flattened cortex preparations, preliminary functional divisions were assigned for SI with the face represented laterally followed by the flipper, body and tail representations proceeding medially. The neonate exhibited four distinct patches in the frontoparietal cortex (presumptive SI), whereas juvenile and adult specimens demonstrated a distinct pattern in which cytochrome oxidase-dense staining appeared to be blended into one large patch extending dorsomedially. This differential staining between younger versus older more developed animals was also seen on coronal sections stained for cytochrome oxidase, myelin, or Nissl bodies. These were systematically analyzed in order to accurately localize the laminar and cytoarchitectural extent of cytochrome oxidase staining. Overall, SI appears to span seven cytoarchitectural areas to which we have assigned presumptive functional representations based on the relative locations of cytochrome oxidase-dense staining.

Overlap and interdigitation of cortical and thalamic afferents to dorsocentral striatum in the rat.

Dorsocentral striatum (DCS) is an associative region necessary for directed attention in rats. DCS is defined as the main region in which axons from ipsilateral medial agranular cortex (AGm) terminate within the striatum. In this double-labeling study, we placed a green axonal tracer in area AGm and a red one in an additional brain region. We examined the spatial relationship between terminals from area AGm and other portions of the cortical-basal ganglia-thalamic-cortical network involved in directed attention and its dysfunction, hemispatial neglect, in the rat. These include lateral agranular cortex (AGl), posterior parietal cortex (PPC), ventrolateral orbital cortex (VLO), and secondary visual cortex (Oc2M). One important finding is the presence of a dense focus of labeled axons within DCS after injections in cortical area PPC or Oc2M. In these foci, axons from PPC or Oc2M extensively overlap and interdigitate with axons from cortical area AGm. Additionally, retrograde labeling of striatal neurons, along with double anterograde labeling, suggests that axons from cortical area AGm and AGl cross and possibly make contact with the dendritic processes of single medium spiny neurons. Axons from thalamic nucleus LP were observed to form a dense band dorsal to DCS which is similar to that seen following PPC injections, and a significant number of LP axons were also observed within DCS. Projections from thalamic nucleus VL are present in the dense dorsolateral AGm band that abuts the external capsule, are densest in the dorsolateral striatum, and were not observed in DCS. These results extend previous findings that DCS receives input from diverse cortical areas and thalamic nuclei which are themselves interconnected.

A rodent model for investigating the neurobiology of contralateral neglect.

BACKGROUND: Contralateral neglect is a common and disabling sequela of right hemisphere strokes. Neglect involves attentional and cognitive deficits, including distortions of contralateral spatial and personal awareness. There are no established successful therapies for neglect, and treatment is often complicated by anosognosia. The disturbances associated with neglect are debilitating to patients and their families, and presence of neglect is a strong predictor of poor prognosis for recovery. OBJECTIVE: The present report reviews findings from 20 years of research using a rat model of neglect. In the rat, 2 cortical areas that are linked by corticocortical connections have been identified as having a major role in neglect, and these correspond to frontal and parietal fields in primates. These 2 cortical areas also have convergent projections to the dorsocentral striatum, which has been implicated as a crucial subcortical component of the cortical-striatal-thalamic circuitry involved in directed attention and neglect. We discuss the role of the dorsocentral striatum in neglect and recovery and present evidence that induced axonal sprouting may promote functional recovery following cortical lesions that produce neglect. CONCLUSIONS: The rodent model of neglect captures some of the essential behavioral and anatomic features of neglect in humans. This model has helped reveal the pathophysiology of neglect, has suggested a crucial role of the striatum in recovery from neglect, and is being used to investigate potential therapeutic approaches.

Infusion of apomorphine into the dorsocentral striatum produces acute drug-induced recovery from neglect produced by unilateral medial agranular cortex lesions in rats.

Previous studies have shown that systemic administration of apomorphine is effective in producing acute drug-induced recovery from neglect induced by unilateral medial agranular cortex (AGm) lesions. More recent studies have demonstrated that recovery from neglect may be due to plastic changes occurring in the dorsal central striatum (DCS). Further, lesions of the DCS produce neglect that does not respond to systemic administration of apomorphine, suggesting that this area may be crucial for the therapeutic effects of apomorphine. In the present study, the behavioral effects of apomorphine infused into the DCS of animals with AGm lesion-induced neglect were examined to determine whether the DCS is a site of drug action. An infusion of 0.375 micro g of apomorphine into the DCS, but not a lateral striatal control area, was effective in producing acute recovery from neglect. The results of this study support the crucial role of the DCS in recovery from neglect induced by unilateral AGm lesions and suggest that the DCS may be an important site of action for the therapeutic effects of apomorphine. Because dopamine agonist therapy has been shown to be effective in humans with neglect, the results of the current study may represent an important step in the development of future pharmacotherapies.

The associative striatum: cortical and thalamic projections to the dorsocentral striatum in rats.

Corticostriatal projections to the dorsocentral striatum (DCS) were investigated using retrograde fluorescent axonal tracing. The DCS is of interest because of its role in directed attention and recovery from multimodal hemispatial neglect following cortical lesions of medial agranular cortex (AGm), an association area that is its major source of cortical input. A key finding was that the multimodal posterior parietal cortex (PPC) also contributes substantial input to DCS. This is significant because PPC and AGm are linked by corticocortical connections and are both critical components of the circuitry involved in spatial processing and directed attention. Other cortical areas providing input to DCS include visual association areas, lateral agranular cortex and orbital cortex. These areas also have reciprocal connections with AGm and PPC. Less consistent labeling was seen in somatic sensorimotor areas FL, HL and Par 1. Thalamic afferents to DCS are prominent from the intralaminar, ventrolateral, mediodorsal, ventromedial, laterodorsal (LD) and lateral posterior (LP) nuclei. Collectively, these nuclei constitute the sources of thalamic input to cortical areas AGm and PPC. Nuclei LD and LP are only labeled with injections in dorsal DCS, the site of major input from PPC, and PPC receives its thalamic input from LD and LP. We conclude that DCS receives inputs from cortical and thalamic areas that are themselves linked by corticocortical and thalamocortical connections. These findings support the hypothesis that DCS is a key component of an associative network of cortical, striatal and thalamic regions involved in multimodal processing and directed attention.

Effects of light deprivation on recovery from neglect and extinction induced by unilateral lesions of the medial agranular cortex and dorsocentral striatum.

A number of previous studies have indicated that an environmental manipulation, 48 h of light deprivation (LD), produces virtually complete and permanent behavioral recovery of function from neglect induced by medial agranular cortex (AGm) lesions. LD-induced behavioral recovery from neglect is correlated with physiological changes in the dorsolateral striatum, an area that contains the projection zone of AGm efferents in the dorsocentral striatum (DCS). In this study, the behavioral effects of 48 h of LD on subjects with either unilateral DCS, AGm, or combined AGm/DCS lesions were investigated to examine whether the integrity of the DCS is crucial for behavioral recovery from neglect and whether LD will have a therapeutic effect on extinction deficits. Subjects were tested for extinction to bilateral simultaneous stimulation of the forepaws, and visual, auditory and tactile neglect. Forty-eight hours of LD failed to produce behavioral recovery from neglect in rats with DCS lesions, or a therapeutic affect on extinction deficits in any of the groups. The results of this study further support the crucial role of the DCS in recovery from neglect induced by AGm lesions and suggests that the DCS may be the crucial site for the mechanisms leading to LD-induced recovery. Further, the ineffectiveness of LD on extinction suggests that components of the neglect syndrome are dissociable and may require different therapeutic interventions.

Unilateral destruction of the dorsocentral striatum in rats produces neglect but not extinction to bilateral simultaneous stimulation.

A number of previous studies have indicated that lesions of the medial agranular cortex (AGm) in rats induce multimodal neglect and extinction to bilateral simultaneous stimulation (extinction), the two major symptoms of the neglect syndrome in humans. A recent study demonstrated that lesions of dorsocentral striatum (DCS), the site of AGm projections to the striatum, produce multimodal neglect qualitatively similar to that found with AGm lesions. In the present study, the behavioral effects of unilateral DCS lesions were examined in more detail for the major manifestations of neglect: hemineglect, extinction, and allesthesia/allokinesia. Subjects were tested for extinction to bilateral simultaneous stimulation of the forepaws three times a week for 3 weeks. Neglect testing occurred twice weekly and the subjects were tested for the presence of neglect by rating the magnitude of orientation to visual, tactile, and auditory stimulation. The results indicated that DCS operates, while demonstrating severe neglect, failed to demonstrate extinction or allesthesia/allokinesia. These findings suggest that the neural mechanisms that underlie neglect and extinction are dissociable in this system. A better understanding of the neural mechanisms that underlie extinction is particularly important because humans that have recovered from neglect often continue to demonstrate the debilitating symptoms of extinction.

Tactile hairs on the postcranial body in Florida manatees: a Mammalian lateral line?

Previous reports have suggested that the sparsely distributed hairs found on the entire postcranial body of sirenians are all sinus type tactile hairs. This would represent a unique arrangement because no other mammal has been reported to possess tactile hairs except on restricted regions of the body, primarily the face. In order to investigate this issue further, hair counts were made systematically in three Florida manatees (Trichechus manatus latirostris), and hair follicle microanatomy was studied in 110 specimens gathered from 9 animals. We found that the postcranial body possesses approximately 1500 hairs per side, and hair density decreases from dorsal to ventral. External hair length ranged from 2-9 mm, and most hairs were separated from their nearest neighbor by 20-40 mm, resulting in an independent domain of movement for each hair. All hairs exhibited the anatomical characteristics of follicle-sinus complexes typical of tactile hairs, including a dense connective tissue capsule containing an elongated circumferential blood sinus and innervation by 20-50 axons which ascend the mesenchymal sheath. We conclude that this represents a unique distributed underwater tactile system capable of conveying detailed and significant external information concerning approaching animals, water currents and possibly the presence of large stationary features of the environment. Such a system would be analogous to the lateral line in fish, and would be particularly useful in the turbid habitat frequented by Florida manatees.

Microanatomy of facial vibrissae in the Florida manatee: the basis for specialized sensory function and oripulation.

Sirenians, including Florida manatees, possess an array of hairs and bristles on the face. These are distributed in a pattern involving nine distinct regions of the face, unlike that of any other mammalian order. Some of these bristles and hairs are known to be used in tactile exploration and in grasping behaviors. In the present study we characterized the microanatomical structure of the hair and bristle follicles from the nine regions of the face. All follicles had the attributes of vibrissae, including a dense connective tissue capsule, prominent blood sinus complex, and substantial innervation. Each of the nine regions of the face exhibited a distinct combination of these morphological attributes, congruent with the previous designation of these regions based on location and external morphological criteria. The present data suggest that perioral bristles in manatees might have a tactile sensory role much like that of vibrissae in other mammals, in addition to their documented role in grasping of plants during feeding. Such a combination of motor and sensory usages would be unique to sirenians. Finally, we speculate that the facial hairs and bristles may play a role in hydrodynamic reception.

Cortical layer VII and persistent subplate cells in mammalian brains.

Layer VII is the deepest cortical layer in rats, and consists of a thin layer of persistent subplate cells overlain by a cell-sparse, myelin-rich stratum through which many corticocortical axons travel. Layer VII neurons participate in local and long-distance corticocortical connections. The present study was undertaken to determine whether layer VII is a typical feature in rodent brains, and to determine which other mammalian taxa exhibit a layer VII. The adult brains of 144 species from 22 orders were examined. Of these, 43 species in 6 orders exhibit a layer VII. These include the sciurognath Rodentia, Insectivora, Paucituberculata, Paramelemorphia, some Xenarthra, and some Chiroptera. In all taxa interstitial cells were observed scattered throughout the white matter. The observed distribution of layer VII in this sample of mammalian taxa suggests that layer VII is a typical feature in some orders, but is not present in most orders. The heterogeneous distribution of layer VII in the Rodentia and Chiroptera suggests that species-level developmental dynamics are involved. It is hypothesized that the timing of subplate apoptosis in relation to the establishment of corticocortical connections is the major factor that determines whether layer VII is present in the adult stage.

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

The rostral and caudal portions of rat medial agranular cortex (AGm) play different functional roles. To refine the anatomical framework for understanding these differences, axonal tracers were used to map the topography of the connections of AGm with the striatum and thalamus. The striatal projections follow mediolateral and rostrocaudal gradients that correspond to the locations of the neurons of origin within AGm. Projections from rostral AGm are widespread and dense rostrally, then coalesce into a circumscribed dorsocentral region at the level of the pre-commissural septal nuclei. Projections from mid and caudal AGm are less widespread and less dense, and are focused more caudally. Striatal projections from the adjacent anterior cingulate and lateral agranular areas overlap those of AGm but are concentrated more medially and laterally, respectively. Thalamic connections of AGm are organized so that more caudal portions of AGm have connections with progressively more lateral and caudal regions of the thalamus, and the full extent of AGm is connected with the ventrolateral (VL) nucleus. Rostral AGm is interconnected with the lateral portion of the mediodorsal nucleus (MD1), VL, and the central lateral (CL), paracentral (PC), central medial, rhomboid and ventromedial nuclei. Caudal AGm has robust connections with VL, the posterior, lateral posterior and lateral dorsal nuclei, but little or none with MD1, CL/PC and VM. These differences in the subcortical connections of rostral and caudal AGm parallel their known differences in corticocortical connections, and represent another basis for experimental explorations of the functional roles of these cortical territories.

Interstitial cells of the white matter in the inferior parietal cortex in schizophrenia: An unbiased cell-counting study.

Previous studies have found an increased density of the interstitial cells of the white matter (ICWMs) in the frontal and temporal cortex in schizophrenia. Some data suggested this abnormality was restricted to a subgroup of patients, whose clinical features were consistent with the presence of the deficit syndrome. Clinical studies suggest deficit features are due to an abnormality in a cortical-subcortical circuit that includes dorsolateral prefrontal and inferior parietal cortex. We compared the density of ICWMs labeled for MAP2 immunoreactivity in Brodmann area 39 (inferior parietal cortex) from nine schizophrenia subjects (three deficit and six nondeficit) and nine matched controls using an unbiased cell-counting technique. The density of ICWMs was significantly greater in the deficit syndrome subjects compared to the nondeficit schizophrenia group (respective means +/- SEM, 0.22 +/- 0.04, and 0. 13 +/- 0.02; P < 0.05). The density of ICWMs in the deficit group was also significantly greater (P < 0.05) than that of the control group (0.09 +/- 0.02), but the nondeficit and control groups were not significantly different. These findings 1) confirm that an abnormal placement of neurons in the white matter is found in schizophrenia, 2) provide evidence for a microscopic anatomical abnormality in the inferior parietal cortex, and 3) suggest the ICWM abnormality may be confined to deficit patients.

Forebrain connections of medial agranular cortex in the prairie vole, Microtus ochrogaster.

Fluorescent axonal tracers were used to investigate the connections of medial agranular cortex (frontal area 2, Fr2) in male prairie voles. The rostral and caudal portions of Fr2 (rFr2 and cFr2) have distinct but partially overlapping patterns of connections. Thalamic labeling after cFr2 injections was present in anteromedial nucleus (AM), ventrolateral nucleus (VL), lateral segment, mediodorsal nucleus (MDl), centrolateral nucleus (CL), ventromedial nucleus (VM), posterior nucleus (Po) and lateral posterior nucleus (LP). A band of labeled cells involving CL, central medial nucleus (CM) and rhomboid nucleus (Rh) formed a halo around the periphery of submedial (gelatinosus) nucleus (Sm). Within cFr2 there is a rostrocaudal gradient whereby projections from VL and MDl become progressively sparser caudally, whereas those from LP and Po become denser. Rostral Fr2 receives afferents from a similar group of thalamic nuclei, but has denser innervation from VL and MDl, lacks afferents from LP, and receives less input from nuclei around the periphery of Sm. Caudal Fr2 has extensive cortical connections including orbital cortex, rostral Fr2, Fr1, caudal parietal area 1 (Par1), parietal area 2 (Par2), and posterior parietal, retrosplenial and visual areas. Rostral Fr2 has similar connections with areas Fr1, Par1 and Par2; orbital connections focused in ventrolateral orbital cortex (VLO); connections with caudal Fr2; greatly reduced connections with posterior parietal cortex and the visual areas; and no connections with retrosplenial cortex. The axons linking rFr2 and cFr2 with each other and with other cortical areas travel predominately in the deep gray matter of layers VI and VII rather than in the white matter. Projections to the dorsal striatum from rFr2 are widespread in the head of the caudate, become progressively restricted to a dorsocentral focus more caudally, and disappear by the level of the anterior commissure. The projections from cFr2 are largely restricted to a focal dorsocentral region of the striatum and to the dorsolateral margin of the caudatoputamen. In comparison to area Fr2, the laterally adjacent area Fr1 has thalamic and cortical connections which are markedly restricted. Area Fr1 receives thalamic input from nuclei VL, anteroventral nucleus (AV), CL and Po, but none from mediodorsal nucleus (MD) or LP, and its input from VM is reduced. Cortical afferents to Fr1 originate from areas Fr2, caudal Par1 and Par2. Medial agranular cortex of prairie voles has a pattern of connections largely similar to that seen in rats, suggesting that area Fr2 in prairie voles is part of a cortical network that may mediate complex behaviors involving spatial orientation.

Change in size of replaced amalgam restorations: a methodological study.

The purpose of this study was to evaluate the pre- and postoperative size of amalgam restorations that were scheduled for replacement. A video imaging system in combination with specially designed devices to align the stone casts prepared from impressions of the teeth gave reproducible results for both an in vitro and in vivo series. The in vivo series showed that removal of the occlusal part of amalgam restorations could be done without significantly increasing the size of the cavity in the tooth, provided the restorations did not have marginal discrepancies. If marginal discrepancies or recurrent caries were present, the replacement restorations showed a statistically significant increase in size compared with the original restorations.

Disconnection of medial agranular and posterior parietal cortex produces multimodal neglect in rats.

Two cortical areas in rats have been found to be important in directed attention and spatial processing: the medial agranular cortex (AGm), the rodent analog of the frontal eye fields; and the posterior parietal cortex (PPC), the rodent analog of area 7 in primates. As in primates, unilateral destruction of either of these cortical association areas produces severe contralesional neglect of visual, auditory, and tactile stimulation. AGm and PPC are reciprocally interconnected by longitudinally oriented axons traveling in layer VI of the cortex. Their trajectory provides a unique opportunity to examine the effects of disconnection of these two areas. The key question is whether these two regions function independently or as components of a cortical network for directed attention. Unilateral disconnection of the PPC and AGm was achieved via transverse knife-cuts extending through layer VI of cortex, and the disconnection verified by tract-tracing methods. The knife-cuts produced severe multimodal neglect and allesthesia/allokinesia. The deficits produced by the knife-cuts were virtually identical to those produced by unilateral destruction of these regions. The control operates, which received knife-cuts that spared the interconnections between the AGm and PPC, were unimpaired. The results indicate that AGm and PPC in rats function as parts of a cortical system for directed attention.

Layer VII and the gray matter trajectories of corticocortical axons in rats.

The trajectory of long distance intrahemispheric corticocortical axons has been investigated using the anterograde fluorescent axonal tracer fluororuby. Most axons of this kind were found to travel through the gray matter of layers VI and VII rather than in the white matter. The cell-sparse zone immediately superficial to layer VII contains a dense aggregate of longitudinally directed axons. Corticocortical axons traveling in the mediolateral plane also utilize the deep gray matter predominately. Layer VII neurons are persistent remnants of the subplate in rats. Based on our retrograde labeling results, they are involved in long distance as well as local corticocortical connections. Layer VII neurons are often labeled in a more continuous pattern after cortical injections of retrograde tracers than neurons of layers II, III and V, which are labeled in a patchy manner.

Neuronal connections of orbital cortex in rats: topography of cortical and thalamic afferents.

The cortical and thalamic afferent connections of rat orbital cortex were investigated using fluorescent retrograde axonal tracers. Each of the four orbital areas has a distinct pattern of connections. Corticocortical connections involving the ventral and ventrolateral orbital areas are more extensive than those of the medial and lateral orbital areas. The medial orbital area has cortical connections with the cingulate, medial agranular (Fr2) and posterior parietal (PPC) cortices. The ventral orbital area has connections with the cingulate area, area Fr2, secondary somatic sensory area Par2, PPC, and visual areas Oc2M and Oc2L. The ventrolateral orbital area (VLO) receives cortical input from insular cortex, area Fr2, somatic sensory areas Par1 and Par2, PPC and Oc2L. The lateral orbital area has cortical connections limited to the agranular and granular insular areas, and Par2. Thalamic afferents to the four orbital fields are also topographically organized, and are focused in the submedial and mediodorsal nuclei. The ventrolateral orbital area receives input from the entirety of the submedial nucleus, whereas the other orbital areas receive input from its periphery only. Each orbital area is connected with a particular segment of the mediodorsal nucleus. The medial orbital area receives its principal thalamic afferents from the parataenial nucleus, the dorsocentral portion of the mediodorsal nucleus, and the ventromedial portion of the submedial nucleus. The ventral orbital area receives input from the lateral segment of the mediodorsal nucleus, the rostromedial portion of the submedial nucleus, and the central lateral nucleus. Thalamic afferents to the ventrolateral orbital area arise from the entirety of the submedial nucleus and from the lateral segment of the mediodorsal nucleus. The lateral orbital area receives thalamic afferents from the central segment of the mediodorsal nucleus, the ventral portion of the submedial nucleus, and the ventromedial nucleus. The paraventricular, ventromedial, rhomboid and reuniens nuclei also provide additional input to the four orbital areas. The connections of the ventrolateral orbital area are interpreted in the context of its role in directed attention and allocentric spatial localization. The present findings provide anatomical support for the view that areas Fr2, PPC and VLO comprise a cortical network mediating such functions.

Manatee cerebral cortex: cytoarchitecture of the caudal region in Trichechus manatus latirostris.

In several brains of the Florida manatee, Trichechus manatus latirostris, the architecture of caudal regions of cerebral cortex was examined in order to complete a map of cortical areas in the brain of this unique herbivore. Through observation of sections stained for Nissl substance, myelinated axons, acetylcholinesterase and cytochrome oxidase, we have identified 11 new cortical areas based on qualitative cytoarchitectural appearance and measurements of laminar thicknesses, for a total of 24 such cortical areas in manatee cerebral cortex. Some areas exhibit poorly differentiated laminae while in others there are 6 clearly demarcated layers, often with sublaminar organization. Some previously identified areas were found to extend into the region caudal to the vertically oriented lateral fissure. As in other mammalian brains, cortical areas in manatees are organized in concentric rings of allocortex, mesocortex, and isocortex. Putative functional roles have been assigned to most of the identified areas based on location, architecture, behavioral and anatomical considerations, and extrapolation from other taxa in which functional mapping has been done.

Thalamocortical projections activated by phrenic nerve afferents in the cat.

This study identified thalamocortical projections activated by respiratory afferents. Cortical evoked potentials were recorded in the right primary somatosensory cortex of the cat following electrical stimulation of the left C5 root of the phrenic nerve. The majority of primary sites were located in the vicinity of the postcruciate dimple, in area 3a near the 3a/3b border, corresponding to the trunk region of the cortical body map. Retrograde fluorescent tracers injected at the sites of primary activation produced labeled cells in the oralis nucleus of the ventroposterior complex [4]. Control injections made in adjacent cortical areas not activated by phrenic stimulation resulted in labeling in the ventroposterior complex which did not overlap that seen with injections of primary activation sites. We conclude that respiratory muscle afferents in the phrenic nerve elicit activity in the trunk region of primary somatosensory cortex via specific thalamocortical projections originating in the oralis portion of the thalamic ventroposterior complex.