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.

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.

Topographic organization in the corticocortical connections of medial agranular cortex in rats.

Medial agranular cortex (AGm) is a narrow, longitudinally oriented region known to have extensive corticortical connections. The rostral and caudal portions of AGm exhibit functional differences that may involve these connections. Therefore we have examined the rostrocaudal organization of the afferent cortical connections of AGm by using fluorescent tracers, to determine whether there are significant differences between rostral and caudal AGm. Mediolateral patterns have also been examined in order to compare the pattern of corticocortical connections of AGm to those of the laterally adjacent lateral agranular cortex (AGl) and medially adjacent anterior cingulate area (AC). In the rostrocaudal domain, there are notable patterns in the connections of AGm with somatic sensorimotor, visual, and retrosplenial cortex. Rostral AGm receives extensive afferents from the caudal part of somatic sensorimotor area Par I, whereas caudal AGm receives input largely from the hindlimb cortex (area HL). Middle portions of AGm show an intermediate condition, indicating a continuously changing pattern rather than the presence of sharp border zones. The whole of the second somatic sensorimotor area Par II projects to rostral AGm, whereas caudal AGm receives input only from the caudal portion of Par II. Visual cortex projections to AGm originate in areas Oc1, Oc2L and Oc2M. Connections of rostral AGm with visual cortex are noticeably less dense than those of mid and caudal AGm, and are focused in area Oc2L. The granular visual area Oc1 projects almost exclusively to mid and caudal AGm. Retrosplenial cortex has more extensive connections with caudal AGm than with rostral AGm, and the agranular and granular retrosplenial subregions are both involved. Other cortical connections of AGm show little or no apparent rostrocaudal topography. These include afferents from orbital, perirhinal, and entorhinal cortex, all of which are bilateral in origin. In the mediolateral dimension, AGm has more extensive corticocortical connections than either AGl or AC. Of these three neighboring areas, only AGm has connections with the somatic sensorimotor, visual, retrosplenial and orbital cortices. In keeping with its role as primary motor cortex, AGl is predominantly connected with area Par I of somatic sensorimotor cortex, specifically rostral Par I. AGl receives no input from visual or retrosplenial cortex. Anterior cingulate cortex has connections with visual area Oc2 and with retrosplenial cortex, but none with somatic sensorimotor cortex. Orbital cortex projections are sparse to AGl and do not appear to involve AC.(ABSTRACT TRUNCATED AT 400 WORDS)

The three-dimensional morphology of afferent terminal fields in the rat dentate gyrus: periodic variation.

The laminated afferent terminal fields in the rat dentate gyrus molecular layer exhibit differential histochemical staining properties. Dependent variables (e.g., field geometrics, stain intensity) based on this organization can allow evaluation of the effect of independent variables upon the integrity of each afferent, but anatomically justifiable procedures for matching measurement regions across animals are necessary. We describe such a procedure and a serendipitous observation on the normal organization of the terminal fields. From a 10 X 10 mediolateral X anteroposterior array of measurement points per animal, it was determined that each afferent field, and the total molecular layer, exhibits periodic variation in width relative to the granule cell layer. Thus, to reduce statistical variability, either a high dependent variable sampling rate, or sampling within a region of naturally low variability, is suggested. Evidence for such a region is presented, and possible consequences of this novel topology of afferentation are discussed.

Efferent connections of dorsal and ventral agranular insular cortex in the hamster, Mesocricetus auratus.

The anterior portion of rodent agranular insular cortex consists of a ventral periallocortical region (AIv) and a dorsal proisocortical region (AId). Each of these two cortical areas has distinct efferent connections, but in certain brain areas their projection fields are partially or wholly overlapping. Bilateral projections to layers I, III and VI of medial frontal cortex originate in the dorsal agranular insular cortex and terminate in the prelimbic, anterior cingulate and medial precentral areas; those originating in ventral agranular insular cortex terminate in the medial orbital, infralimbic and prelimbic areas. The dorsal and ventral regions of the agranular insular cortex project topographically to the ipsilateral cortex bordering the rhinal fissure, which includes the posterior primary olfactory, posterior agranular insular, perirhinal and lateral entorhinal areas. Fibers to these lateral cortical areas were found to travel in a cell-free zone, between cortical layer VI and the claustrum, which corresponds to the extreme capsule. The dorsal and ventral regions send commissural projections to layer I, lamina dissecans and outer layer V, and layer VI of the contralateral homotopical cortex, via the corpus callosum. Projections from the ventral and dorsal regions of the agranular insular cortex to the caudatoputamen are topographically arranged and terminate in finger-like patches. The ventral, but not the dorsal region, projects to the ventral striatum and ventral pallidum. The thalamic projections of the ventral and dorsal regions are largely overlapping, with projections from both to the ipsilateral reticular nucleus and bilaterally to the rhomboid, mediodorsal, gelatinosus and ventromedial nuclei. The heaviest projection is that to the full anteroposterior extent of the medial segment of the mediodorsal nucleus. Brainstem areas receiving projections from the ventral and dorsal regions include the lateral hypothalamus, substantia nigra pars compacta, ventral tegmental area and dorsal raphe nucleus. In addition, the ventral region projects to the periaqueductal gray and the dorsal region projects to the parabrachial and ventral pontine nuclei. These efferent connections largely reciprocate the afferent connections of the ventral and dorsal agranular insular cortex, and provide further support for the concept that these regions are portions of an outer ring of limbic cortex which plays a critical role in the expression of motivated, species-typical behaviors.

Afferent connections of dorsal and ventral agranular insular cortex in the hamster Mesocricetus auratus.

The agranular insular cortex is transitional in location and structure between the ventrally adjacent olfactory allocortex primutivus and dorsally adjacent sensory-motor isocortex. Its ventral anterior division receives major afferent projections from olfactory areas of the limbic system (posterior primary olfactory cortex, posterolateral cortical amygdaloid nucleus and lateral entorhinal cortex) while its dorsal anterior division does so from non-olfactory limbic areas (lateral and basolateral amygdaloid nuclei). The medial segment of the mediodorsal thalamic nucleus projects to both the ventral and dorsal divisions of the agranular insular cortex, to the former from its anterior portion and to the latter from its posterior portion. Other thalamic inputs to the two divisions arise from the gelatinosus, central medial, rhomboid and parafascicular nuclei. The dorsal division, but not the ventral division, receives input from neurons in the lateral hypothalamus and posterior hypothalamus. The medial frontal cortex projects topographically and bilaterally upon both ventral and dorsal anterior insular cortex, to the former from the ventrally located medial orbital and infralimbic areas, to the latter from the dorsally-located anterior cingulate and medial precentral areas, and to both from the intermediately located prelimbic area. Similarly, the ipsilateral posterior agranular insular cortex and perirhinal cortex project in a topographic manner upon the two divisions of the agranular insular cortex. Commissural input to both divisions originates from pyramidal neurons in the respective contralateral homotopical cortical area. In each case, pyramidal neurons in layer V contribute 90% of this projection and 10% arises from layer III pyramidals. In the brainstem, the dorsal raphe nucleus projects to the ventral and dorsal divisions of the agranular insular cortex and the parabrachial nucleus projects to the dorsal division. Based on their cytoarchitecture, pattern of afferent connections and known functional properties, we consider the ventral and dorsal divisions of the agranular insular cortex to be, respectively, periallocortical and proisocortical portions of the limbic cortex.

Acetylcholinesterase stain intensity variation in the rat dentate gyrus: a quantitative description based on digital image analysis.

Three-dimensional patterns of variation in the intensity of acetylcholinesterase histochemical staining and the width of stain-defined subregions were quantified for the dentate gyrus of the adult male Long-Evans rat. Matched tissue sections sampled through the central hippocampal formation of five rats were measured with a digital image analysis computer system. The width and stain intensity were determined for defined portions of the dentate gyrus related to gross acetylcholinesterase staining patterns and the known distribution of dentate afferents. Normalized values reflecting stain intensity at defined positions within this standardized sampling array were examined to investigate regional differences in acetylcholinesterase distribution along the primary dendritic axis of dentate granule neurons. The data illustrate quantitative differences in the partitioning of acetylcholinesterase as a function of intrahippocampal position. The variation is more pronounced in the septal-temporal axis than the granule cell layer crest-tip axis. Furthermore, the septal-temporal variations in acetylcholinesterase intensity demonstrate some independence according to proximal-distal location within the molecular layer. The results suggest that acetylcholinesterase distribution within the dentate gyrus may reflect local physiological characteristics of those afferent systems related to this enzyme, including but not necessarily limited to those that are specifically cholinergic.

Cerebral cortical evoked potentials elicited by cat intercostal muscle mechanoreceptors.

Intercostal muscle afferents discharge in response to changes in intercostal muscle mechanics and have spinal and brain stem projections. It was hypothesized that intercostal muscle mechanoreceptors also project to the sensorimotor cortex. In cats, the proximal muscle branch of an intercostal nerve was used for electrical stimulation. The mechanical stimulation was stretch of an isolated intercostal space. The sensorimotor cortex was mapped with a surface ball electrode. Primary cortical evoked potentials (CEP) were found in area 3a of the sensorimotor cortex with mechanical and electrical stimulation. The CEP was elicited with the smallest stretch amplitude used, 50 microns. The CEP response showed little increase beyond 300-microns stretch. The CEP elicited by 50-microns stretch suggests an initial cortical activation by intercostal muscle spindles. The minimal increase in CEP amplitude with stretch > 300 microns suggests that the CEP response is primarily due to muscle spindle recruitment. The increase in amplitude beyond this stretch may be due to recruitment of tendon organs. These results demonstrate a short-latency projection of intercostal muscle mechanoreceptors to the sensorimotor region of the cerebral cortex. This cortical activation may be involved in respiratory sensations and/or transcortical reflex responses to changes in respiratory muscle mechanics.

Combined retrograde and anterograde tracing of neuronal connections: Fluoro-Gold and autoradiography.

We have combined the retrograde Fluoro-Gold (FG) and anterograde autoradiographic (AR) procedures to yield a sensitive high resolution technique by which afferent and efferent connections can be visualized from a single intracerebral injection site. Combined FG/AR sections show excellent results, with no apparent loss of signal compared to performing either procedure alone. Since the FG label is intense, the two labels may be viewed simultaneously by superimposing low level darkfield illumination from below the specimen with fluorescence illumination from above. This combined procedure is useful in the analysis of reciprocal connections involving small spatial domains, such as patchy corticocortical connections. Due to the high signal to noise ratio of both labels, this material is ideally suited for quantitative assessment using automated image analysis.

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.

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.

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.

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.

Bilateral destruction of the ventrolateral orbital cortex produces allocentric but not egocentric spatial deficits in rats.

Previous studies have implicated the ventrolateral orbital cortex (VLO) in spatial attention and orientation. Unilateral destruction of the VLO has been found to produce severe multimodal neglect to unilateral stimulation which is qualitatively quite similar to that found following unilateral destruction of either the medial agranular or posterior parietal cortices. A series of anatomical studies have shown that the VLO is reciprocally interconnected with both the medial agranular cortex and the posterior parietal cortex, which are involved in egocentric and allocentric spatial processing respectively. However, the role of the VLO in either egocentric or allocentric spatial processing has never been directly examined. The present study directly examined the role of the VLO in spatial learning by examining the effects of bilateral VLO destruction on performance in both egocentric (adjacent-arm maze task) and allocentric (cheeseboard task) spatial tasks. Subjects in either the cheese board task or the adjacent arm maze were given presurgical maze training and then were assigned to one of three surgical groups: a bilateral VLO group, a lesion control group which received bilateral destruction of the laterally adjacent lateral orbital cortex which has a quite different pattern of connectivity than the VLO, or a sham operated control group. The results indicated that the VLO operates were significantly impaired in the cheeseboard task (allocentric task) relative to controls, but displayed no deficits in the adjacent-arm maze (egocentric task), a pattern of results similar to those found for the posterior parietal cortex. The results of the present study strongly support the contention that the VLO is a component of the cortical circuitry for spatial processing in rodents.

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.

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.

Projection of phrenic nerve afferents to the cat sensorimotor cortex.

The projection of phrenic nerve afferents to the sensorimotor cortex was studied in cats. The results of these experiments demonstrate that stimulation of phrenic nerve afferents elicits cortical evoked potentials (CEPs) in the sensorimotor cortex of cats. Cortical foci for CEPs classified as primary were found in areas 3b, 3a and 4 gamma. These foci were located medial to forelimb and lateral to hindlimb afferent representations in the sensorimotor cortex.

Layer VII of rodent cerebral cortex.

The cerebral isocortex is usually considered to be a 6-layered structure. Our anatomical findings suggest that layer VII be recognized as a distinct entity in rodent isocortex. This conclusion is based on cytoarchitectural, fiberarchitectural, connectional and developmental data.

Afferent connections of medial precentral cortex in the rat.

The principal sources of input to rat medial precentral cortex (PCm), as revealed by iontophoretically applied horseradish peroxidase, are: the ventral lateral, mediodorsal, central lateral and ventromedial thalamic nuclei; contralateral PCm; ipsilateral cortical areas related to vision, somatic sensation and audition; retrosplenial and ventrolateral orbital cortex. These results are compared to findings in monkeys and discussed in relation to the phenomenon of polymodal neglect of hemispace.

Thalamocortical connections of rat posterior parietal cortex.

The neuronal connections of rat posterior parietal cortex (PPC) have been examined using retrograde fluorescent axonal tracers. We have found that PPC receives thalamic input predominantly from the lateral posterior and lateral dorsal nuclei, and not from the ventrobasal nucleus, which projects to the rostrally adjacent hindlimb cortex, or from the dorsal lateral geniculate nucleus, which projects to the caudally adjacent visual association area. PPC has reciprocal corticocortical connections with medial agranular cortex and orbital cortex; together, these three cortical areas may function as a network for directed attention in rats.