Topography and architecture of visual and somatosensory areas of the agouti.

We analyzed the organization of the somatosensory and visual cortices of the agouti, a diurnal rodent with a relatively big brain, using a combination of multiunit microelectrode recordings and histological techniques including myelin and cytochrome oxidase staining. We found multiple representations of the sensory periphery in the parietal, temporal, and occipital lobes. While the agouti's primary (V1) and secondary visual areas seemed to lack any obvious modular arrangement, such as blobs or stripes, which are found in some primates and carnivores, the primary somatosensory area (S1) was internally subdivided in discrete regions, isomorphically associated with peripheral structures. Our results confirm and extend previous reports on this species, and provide additional data to understand how variations in lifestyle can influence brain organization in rodents.

Morphometric variability of nicotinamide adenine dinucleotide phosphate diaphorase neurons in the primary sensory areas of the rat.

Even though there is great regional variation in the distribution of inhibitory neurons in the mammalian isocortex, relatively little is known about their morphological differences across areal borders. To obtain a better understanding of particularities of inhibitory circuits in cortical areas that correspond to different sensory modalities, we investigated the morphometric differences of a subset of inhibitory neurons reactive to the enzyme nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d) within the primary auditory (A1), somatosensory (S1), and visual (V1) areas of the rat. One hundred and twenty NADPH-d-reactive neurons from cortical layer IV (40 cells in each cortical area) were reconstructed using the Neurolucida system. We collected morphometric data on cell body area, dendritic field area, number of dendrites per branching order, total dendritic length, dendritic complexity (Sholl analysis), and fractal dimension. To characterize different cell groups based on morphology, we performed a cluster analysis based on the previously mentioned parameters and searched for correlations among these variables. Morphometric analysis of NADPH-d neurons allowed us to distinguish three groups of cells, corresponding to the three analyzed areas. S1 neurons have a higher morphological complexity than those found in both A1 and V1. The difference among these groups, based on cluster analysis, was mainly related to the size and complexity of dendritic branching. A principal component analysis (PCA) applied to the data showed that area of dendritic field and fractal dimension are the parameters mostly responsible for dataset variance among the three areas. Our results suggest that the nitrergic cortical circuitry of primary sensory areas of the rat is differentially specialized, probably reflecting peculiarities of both habit and behavior of the species.

S1 to S2 hind- and forelimb projections in the agouti somatosensory cortex: axon fragments morphological analysis.

The integration of cutaneous, proprioceptive, and motor information in area S2 seems to be essential for manual object recognition and motor control. Part of the inputs to S2 comes from area S1. However no detailed investigations of the morphology of this projection are available. In the present study we describe and quantify the morphology of axon fragments of S1 to S2 ipsilateral projections in the agouti somatosensory cortex. Two groups of projecting axon arbors in S2 were individually reconstructed in three dimensions using Neurolucida, after a single electrophysiological guided BDA injection in either the forelimb (n=4) or the hindlimb (n=4). Electrophysiological mapping was performed 15 days after injections, allowing the localization of S2. Cluster analysis of 40 fragments after hindlimb and 40 after forelimb distinguished two clusters of terminals designated as type I and type II. On average, Type I fragments had greater surface areas and segment lengths than type II fragments, whereas type II fragments had higher number of terminal boutons, number of segments and branching points/mm than type I fragments. Type I corresponded to 58% of the axons projecting from the hindlimb representation in S1 whereas 63% of the sample originating from the forelimb representation in S1 corresponded to type II axons. The results suggest possible parallel processing by two stereotyped classes of axon terminals in the S1 to S2 projections that may represent at least part of the circuitry groundwork associated with distinct somatomotor skills of these limbs in agoutis.

Selenite and selenate effects on mercury (Hg(2+)) uptake and distribution in tilapia, Oreochromis niloticus L., assessed by chronic bioassay.

Aquatic organisms are considered excellent biomarkers of mercury (Hg) occurrence in the environment. Selenium (Se) acts in antagonism to this metal, stimulating its elimination, and reducing its toxicity. In this paper, tilapia (Oreochromis niloticus) were chronically acclimated in sub-lethal Hg(2+), Hg(2+) + Se(4+) and Hg(2+) + Se(6+) concentrations. Distribution and bioaccumulation of both elements were evaluated in fish tissues. The kidney was the main target of the Hg and Se uptake, and the presence of Hg induced the Se hepatic elimination. The Hg bioaccumulation in the gill, spleen and heart were higher in the presence of Se(6+) than in the presence of Se(4+).

Architectonic subdivisions of the amygdalar complex of a primitive marsupial (Didelphis aurita).

The architecture of the amygdaloid complex of a marsupial, the opossum Didelphis aurita, was analyzed using classical stains like Nissl staining and myelin (Gallyas) staining, and enzyme histochemistry for acetylcholinesterase and NADPH-diaphorase. Most of the subdivisions of the amygdaloid complex described in eutherian mammals were identified in the opossum brain. NADPH-diaphorase revealed reactivity in the neuropil of nearly all amygdaloid subdivisions with different intensities, allowing the identification of the medial and lateral subdivisions of the cortical posterior nucleus and the lateral subdivision of the lateral nucleus. The lateral, central, basolateral and basomedial nuclei exhibited acetylcholinesterase positivity, which provided a useful chemoarchitectural criterion for the identification of the anterior basolateral nucleus. Myelin stain allowed the identification of the medial subdivision of the lateral nucleus, and resulted in intense staining of the medial subdivisions of the central nucleus. The medial, posterior, and cortical nuclei, as well as the amygdalopiriform area did not exhibit positivity for myelin staining. On the basis of cyto- and chemoarchitectural criteria, the present study highlights that the opossum amygdaloid complex shares similarities with that of other species, thus supporting the idea that the organization of the amygdala is part of a basic plan conserved through mammalian evolution.

The organizational variability of the rodent somatosensory cortex.

Rodentia is the largest mammalian order, with more than 2,000 species displaying a great diversity of morphological characteristics and living in different ecological niches (terrestrial, semi-aquatic, arboreal and fossorial). Analysis of the organization of the somatosensory areas in six species of rodents allowed us to demonstrate that although these species share a similar neocortical blueprint with other eutherian mammals, important differences exist between homologous areas across different species, probably as a function of both lifestyle and peripheral sensory specializations typical of each species. We based this generalization on a phylogenetic comparison of the intrinsic organization of the primary somatosensory area (SI) across representatives of different rodent suborders. This analysis revealed considerable structural variability, including the differential expansion of cortical representation of specific body parts (cortical amplification) as well as the parcellation of areas into processing modules.

Callosal axon arbors in the limb representations of the somatosensory cortex (SI) in the agouti (Dasyprocta primnolopha).

The present report compares the morphology of callosal axon arbors projecting from and to the hind- or forelimb representations in the primary somatosensory cortex (SI) of the agouti (Dasyprocta primnolopha), a large, lisencephlic Brazilian rodent that uses forelimb coordination for feeding. Callosal axons were labeled after single pressure (n = 6) or iontophoretic injections (n = 2) of the neuronal tracer biotinylated dextran amine (BDA, 10 kD), either into the hind- (n = 4) or forelimb (n = 4) representations of SI, as identified by electrophysiological recording. Sixty-nine labeled axon fragments located across all layers of contralateral SI representations of the hindlimb (n = 35) and forelimb (n = 34) were analyzed. Quantitative morphometric features such as densities of branching points and boutons, segments length, branching angles, and terminal field areas were measured. Cluster analysis of these values revealed the existence of two types of axon terminals: Type I (46.4%), less branched and more widespread, and Type II (53.6%), more branched and compact. Both axon types were asymmetrically distributed; Type I axonal fragments being more frequent in hindlimb (71.9%) vs. forelimb (28.13%) representation, while most of Type II axonal arbors were found in the forelimb representation (67.56%). We concluded that the sets of callosal axon connecting fore- and hindlimb regions in SI are morphometrically distinct from each other. As callosal projections in somatosensory and motor cortices seem to be essential for bimanual interaction, we suggest that the morphological specialization of callosal axons in SI of the agouti may be correlated with this particular function.

The brain decade in debate: VI. Sensory and motor maps: dynamics and plasticity

This article is an edited transcription of a virtual symposium promoted by the Brazilian Society of Neuroscience and Behavior (SBNeC). Although the dynamics of sensory and motor representations have been one of the most studied features of the central nervous system, the actual mechanisms of brain plasticity that underlie the dynamic nature of sensory and motor maps are not entirely unraveled. Our discussion began with the notion that the processing of sensory information depends on many different cortical areas. Some of them are arranged topographically and others have non-topographic (analytical) properties. Besides a sensory component, every cortical area has an efferent output that can be mapped and can influence motor behavior. Although new behaviors might be related to modifications of the sensory or motor representations in a given cortical area, they can also be the result of the acquired ability to make new associations between specific sensory cues and certain movements, a type of learning known as conditioning motor learning. Many types of learning are directly related to the emotional or cognitive context in which a new behavior is acquired. This has been demonstrated by paradigms in which the receptive field properties of cortical neurons are modified when an animal is engaged in a given discrimination task or when a triggering feature is paired with an aversive stimulus. The role of the cholinergic input from the nucleus basalis to the neocortex was also highlighted as one important component of the circuits responsible for the context-dependent changes that can be induced in cortical maps

The brain decade in debate: VI. Sensory and motor maps: dynamics and plasticity.

This article is an edited transcription of a virtual symposium promoted by the Brazilian Society of Neuroscience and Behavior (SBNeC). Although the dynamics of sensory and motor representations have been one of the most studied features of the central nervous system, the actual mechanisms of brain plasticity that underlie the dynamic nature of sensory and motor maps are not entirely unraveled. Our discussion began with the notion that the processing of sensory information depends on many different cortical areas. Some of them are arranged topographically and others have non-topographic (analytical) properties. Besides a sensory component, every cortical area has an efferent output that can be mapped and can influence motor behavior. Although new behaviors might be related to modifications of the sensory or motor representations in a given cortical area, they can also be the result of the acquired ability to make new associations between specific sensory cues and certain movements, a type of learning known as conditioning motor learning. Many types of learning are directly related to the emotional or cognitive context in which a new behavior is acquired. This has been demonstrated by paradigms in which the receptive field properties of cortical neurons are modified when an animal is engaged in a given discrimination task or when a triggering feature is paired with an aversive stimulus. The role of the cholinergic input from the nucleus basalis to the neocortex was also highlighted as one important component of the circuits responsible for the context-dependent changes that can be induced in cortical maps.

The barrel field of the adult mouse SmI cortex as revealed by NADPH-diaphorase histochemistry.

The main goal of the present work was to investigate the pattern of NADPH-diaphorase activity in the somatosensory cortex of the adult mouse. Our results show that this enzyme, which is responsible for the production of the neuronal messenger nitric oxide, is abundant within the neuropil of SmI cortex, revealing the complete pattern of barrel fields. A previous study, however, had reported that NADPH-diaphorase reactivity within the barrels was transient, disappearing after the second postnatal week. We hypothesize that the massive occurrence of NADPH-diaphorase in the barrel fields of the adult mouse brain is related to the potential for plastic changes in the somatosensory cortex that is maintained throughout maturity.

Distribution of NADPH-diaphorase cells in visual and somatosensory cortex in four mammalian species.

The distribution of the well-labeled nicotinamide adenine dinucleotide phosphate diaphorase (NADPHd) Type I neurons was evaluated in the isocortex of four mammalian species: the Didelphis opossum, the Monodelphis opossum, the rat and the marmoset. In Didelphis opossum, laminar distribution was examined in tangential and non-tangential sections. The density increases from superficial to deep layers of the gray matter. In rats' tangential sections, infragranular and supragranular layers have higher density than layer IV. Cell density measurements in the visual and the somatosensory cortices were compared in tangential sections from flattened hemispheres of the four species. Somatosensory areas were identified histochemically in rat (barrel fields) and marmoset (S1 and S2/PV). In the opossums, areas S1 and S2/PV were identified by multiunit recording. Except in the rat, primary visual cortex (V1) was labeled histochemically by NADPHd and/or cytochrome oxidase. In the four species, cell density in somatosensory cortex was significantly higher than in visual cortex. Taken together these results demonstrate that NADPHd Type I neurons are not homogeneously distributed in the isocortex of these mammals. In conclusion, the tangential distribution of Type I neurons in the sensory areas examined, but not its laminar distribution, was similar in the four species. Given that rats, marmosets and opossums are distantly related species, and that the latter are considered to have more 'generalized' brains, it is conceivable that this pattern of tangential distribution of Type I neurons is a general feature of mammalian isocortex.

NADPH-diaphorase activity in area 17 on the squirrel monkey visual cortex: neuropil pattern, cell morphology and laminar distribution

We studied the distribution of NADPH-diaphorase activity in the visual cortex of normal adult New World monkeys (Saimiri sciureus) using the malic enzyme "indirect" method. NADPH-diaphorase neuropil activity had a heterogeneous distribution. In coronal sections, it had a clear laminar pattern that was coincident with Nissl-stained layers. In tangential sections, we observed blobs in supragranular layers of V1 and stripes throughout the entire V2. We quantified and compared the tangential distribution of NADPH-diaphorase and cytochrome oxidade blobs in adjacent sections of the supragramular layers of V1. Although their spatial distributions were rather similar, the two enzymes did not always overlap. The histochemical reaction also revealed two different types of stained cells: a slightly stained subpopulation and a subgroup of deeply stained neurons resembling a Golgi impregnation. These neurons were sparsely spined non-pyramidal cells. Their dendritic arbors were very well stained but their axons were not always evident. In the gray matter, heavily stained neurons showed different dendritic arbor morphologies. However, most of the strongly reactive cells lay in the subjacent white matter, where they presented a more homogenous morphology. Our results demonstrate that the pattern of NADPH-diaphorase activity is similar to that previously described in Old World monkeys.

NADPH-diaphorase activity in area 17 of the squirrel monkey visual cortex: neuropil pattern, cell morphology and laminar distribution.

We studied the distribution of NADPH-diaphorase activity in the visual cortex of normal adult New World monkeys (Saimiri sciureus) using the malic enzyme "indirect" method. NADPH-diaphorase neuropil activity had a heterogeneous distribution. In coronal sections, it had a clear laminar pattern that was coincident with Nissl-stained layers. In tangential sections, we observed blobs in supragranular layers of V1 and stripes throughout the entire V2. We quantified and compared the tangential distribution of NADPH-diaphorase and cytochrome oxidase blobs in adjacent sections of the supragranular layers of V1. Although their spatial distributions were rather similar, the two enzymes did not always overlap. The histochemical reaction also revealed two different types of stained cells: a slightly stained subpopulation and a subgroup of deeply stained neurons resembling a Golgi impregnation. These neurons were sparsely spined non-pyramidal cells. Their dendritic arbors were very well stained but their axons were not always evident. In the gray matter, heavily stained neurons showed different dendritic arbor morphologies. However, most of the strongly reactive cells lay in the subjacent white matter, where they presented a more homogenous morphology. Our results demonstrate that the pattern of NADPH-diaphorase activity is similar to that previously described in Old World monkeys.

NADPH diaphorase histochemistry as a marker for barrels in rat somatosensory cortex.

The primary somatosensory area (S1) of rodents presents multicellular units called barrels which can be identified by different techniques (e.g., Nissl staining, cytochrome oxidase or succinate dehydrogenase histochemistry). We applied NADPH diaphorase histochemistry to tangential sections of rat neocortex to determine if the reactive neuropil also shows the same remarkable array observed with other techniques. We demonstrated NADPH diaphorase-positive barrels in all hemispheres tested. The barrels are recognized as patches where the neuropil is most reactive. Each barrel is separated from the other by a less labeled neuropil. Many NADPH diaphorase-positive neurons are seen along the section, but very few cells are found in the barrel field. In this region, most of the labeled neurons are localized in the less reactive region between the barrels, although a few NADPH diaphorase-positive cells can also be found inside the barrels.

NADPH diaphorase histochemistry as a marker for barrels in rat somatosensory cortex

The primary somatosensory area (S1) of rodents presents multicellular units called barrels which can be identified by different techniques (e.g., Nissl staining, cytochrome oxidase or succinate dehydrogenase histochemistry). We applied NADPH diaphorase histochemistry to tangential sections of rat neocortex to determine if the reactive neuropil also shows the same remarkable array observed with other techniques. We demonstrated NADPH diaphorase-positive barrels in all hemispheres tested. The barrels are recognized as patches where the neuropil is most reactive. Each barrel is separated from the other by a less labeled neuropil. Many NADPH diaphorase-positive neurons are seen along the section, but very few cells are found in the barrel fild. In this region, most of the labeled neurons are localized in the less reactive region between the barrels, although a few NADPH diaphorase-positive cells can also be found insede the barrels

Distribution of NADPH-diaphorase-positive neurons in the opossum neocortex.

The distribution of NADPH-diaphorase reactive cells were evaluated both in horizontal sections of a flattened cortex and in transversal sections of the opossum (Didelphis marsupialis) neocortex. The tangential distribution of labeled cells behind the orbitalis fissure was denser in the rostral vs caudal regions and in the lateral vs medial regions. Transversal sections revealed that most of the positive neurons are in the grey matter, although 1/4 of this population is located in the underlying white matter. This pattern of neuronal distribution is similar to that previously described in rodents, but quite different from that observed in higher mammals such as the cat and primates.

Distribution of NADPH-diaphorase-positive neurons in the opossum neocortex

The distribution of NADPH-diaphorase reactive cells was evalutated both in horizontal sections of a flattended cortex and in transversal sections of the opossum (Didelphis marsupialis) neocortex. The tangential distribution of labeled cells behind the orbitalis fissure was denser in the rostral vs caudal regions and in the lateral vs medial regions. Transversal sections revealed that most of the positive neurons are in the grey matter, although 1/4 of this population is located in the underlying white matter. This pattern of neuronal distribution is similar to that previously described in rodents, but quite different from that observed in higher mammals such as the cat and primates

Evidence for recrossing of the pathway from the retina to the ipsilateral nucleus of the optic tract.

Single-unit recordings of the nucleus of the optic tract (NOT) under visual stimulation were performed in 5 opossums. Most of the units were directionally selective. Receptive fields for the contralateral eye lie mainly in the contralateral field while those for the ipsilateral eye were mainly in the ipsilateral field. As the nasal retina does not project ipsilaterally, recrossing must occur in the pathway from the retina to the ipsilateral NOT. Possible sites for this recrossing are discussed.

Evidence for recrossing of the pathway from the retina to the ipsilateral nucleus of the optic tract

Single-unit recordings of the nucleous of the optic tract (NOT) under visual stimulation were performed in 5 opossums. Most of the units were directionally selective. Receptive fields for the contralateral eye lie mainly in the contralateral field while those for the ipsilateral eye were mainly in the ipsilateral field. As the nasal does not project ipsilaterally, recrossing must occur in the pathway from the retina to the ipsilateral NOT. Possible sites for this recrossing are discussed

Visual response properties of pretectal units in the nucleus of the optic tract of the opossum.

Single-units were recorded from the nucleus of the optic tract. Most of the units showed excitation in response to random check patterns presented on a tangent screen to the contralateral eye, moving in a temporal to nasal direction and/or inhibition in the opposite direction. The excitatory response to the temporal to nasal movement, observed in most units, was unchanged throughout the range of speeds tested, except for a decrease at the slowest (0.6 deg/s) and fastest (150 deg/s) speeds. On the other hand, the inhibitory responses evoked by a nasal to temporal movement, had a peak between 3 and 16 deg/s which decreased towards both extremes. An average of 45% of the units were influenced by the stimulation of the ipsilateral eye. In one third of them the response was very weak. In the remainder, the mean frequency of spikes in one direction of the horizontal movement was more than twice that in the opposite stimulus direction. In the great majority of these units, stimulation of each eye yielded the same overall pattern of directionality, that is, movement of the stimulus towards the recording side led to excitation and/or movement in the reverse direction led to inhibition. Inhibition was stronger than excitation in most ipsilaterally responding units. Excitatory responses elicited by the ipsilateral eye were always weaker than those by the contralateral but in a few cases the ipsilateral inhibitory component was more prominent than the contralateral one.