Distinct Chronic Post-Viral Lung Diseases upon Infection with Influenza or Parainfluenza Viruses Differentially Impact Superinfection Outcome

Chronic obstructive pulmonary disease (COPD) and asthma remain prevalent human lung diseases. Variability in epithelial and inflammatory components that results in pathologic heterogeneity complicates the development of treatments for these diseases. Early childhood infection with parainfluenza virus or respiratory syncytial virus is strongly associated with the development of asthma and COPD later in life, and exacerbations of these diseases correlate with the presence of viral RNA in the lung. Well-characterized animal models of postviral chronic lung diseases are necessary to study the underlying mechanisms of viral-related COPD and asthma and to develop appropriate therapies. In this study, we cross-analyzed chronic lung disease caused by infection with Sendai virus (SeV) or influenza A virus in mice. Differences were observed in lesion composition and inflammatory profiles between SeV- and influenza A virus-induced long-term lung disease. In addition, a primary SeV infection led to worsened pathologic findings on secondary heterologous viral challenge, whereas the reversed infection scheme protected against disease in response to a secondary viral challenge >1 month after the primary infection. These data demonstrate the differential effect of primary viral infections in the susceptibility to disease exacerbation in response to a different secondary viral infection and highlight the usefulness of these viral models as tools to understand the underlying mechanisms that mediate distinct chronic postviral lung diseases.

Imitation Strategies in Callosotomized Patients

Imitation is a human ability rooted in early life. It allows people to interact with each other by observing and reproducing simple and complex movements alike. Imitation can occur in at least two forms: the rst, de ned as anatomical, seems to be based primarily on the mental construct of the "body schema" because the imitating movement corresponds precisely to the imitated movement in bodily terms, but not in terms of spatial compatibility. For example, a right arm movement of a model is imitated with a right arm movement by a facing imitator in a spatially incompatible fashion. The other form, de ned as specular or mirror-mode, involves a spatially compatible matching between imitated and imitating movements, as when an imitator moves her right arm upon viewing a corresponding left arm movement of a facing model (Chiavarino et al., 2007). In a previous study, healthy subjects showed a slight (61%) preference for the specular mode when freely imitating meaningful and meaningless gestures, whereas they strongly preferred the anatomical mode (93%) when given an intentionally ambiguous instruction such as "use the same (or the opposite) limb as the model" (Pierpaoli et al., 2014). In the present investigation it has been shown that callosotomized patients tended to favour the mirror-mode in both the free (66%) and the instructed condition (61% responses in driven sessions) regardless instructions given by the experimenter. Moreover, present data suggest that the extent of the callosotomy may in uence the patient's performance.

The effect of HIV-1 Vif polymorphisms on A3G anti-viral activity in an in vivo mouse model

Humans encode seven APOBEC3 proteins (A-H), with A3G, 3F and 3H as the major factors restricting HIV-1 replication. HIV-1, however, encodes Vif, which counteracts A3 proteins by chaperoning them to the proteasome where they are degraded. Vif polymorphisms found in HIV-1s isolated from infected patients have varying anti-A3G potency when assayed in vitro, but there are few studies demonstrating this in in vivo models. Here, we created Friend murine leukemia viruses encoding vif alleles that were previously shown to differentially neutralize A3G in cell culture or that were originally found in primary HIV-1 isolates. Infection of transgenic mice expressing different levels of human A3G showed that these naturally occurring Vif variants differed in their ability to counteract A3G during in vivo infection, although the effects on viral replication were not identical to those seen in cultured cells. We also found that the polymorphic Vifs that attenuated viral replication reverted to wild type only in A3G transgenic mice. Finally, we found that the level of A3G-mediated deamination was inversely correlated with the level of viral replication. This animal model should be useful for studying the functional significance of naturally occurring vif polymorphisms, as well as viral evolution in the presence of A3G.

Further evidence for the topography and connectivity of the corpus callosum: an FMRI study of patients with partial callosal resection

BACKGROUND AND PURPOSE

This functional MRI study was designed to describe activated fiber topography and trajectories in the corpus callosum (CC) of six patients carrying different degree of partial callosal resection.

METHODS

Patients receiving gustatory, tactile, and visual stimulation according to a block-design protocol were scanned in a 1.5 Tesla magnet. Diffusion tensor imaging (DTI) data were also acquired to visualize spared interhemispheric fibers.

RESULTS

Taste stimuli evoked bilateral activation of the primary gustatory area in all patients and foci in the anterior CC, when spared. Tactile stimuli to the hand evoked bilateral foci in the primary somatosensory area in patients with an intact posterior callosal body and only contralateral in the other patients. Callosal foci occurred in the CC body, if spared. In patients with an intact splenium central visual stimulation induced bilateral activation of the primary visual area as well as foci in the splenium itself.

CONCLUSION

Present data show that interhemispheric fibers linking sensory areas crossed through the CC at the sites where the different sensory stimuli evoked activation foci, and that topography of callosal foci evoked by sensory stimulation in spared CC portions is consistent with that previously observed in subjects with intact CC.

Anatomical or mirror mode imitation? A behavioral approach

Imitation can occur in at least two forms: one, which can be defined as anatomical, is based primarily on the mental construct of the body schema and allows recognition of correspondences between own body anatomy and that of other individuals. The other form, defined as specular or mirror mode, is most probably based on the allocation of some form of attention to the same region of the environmental space both by model and imitator, and to the objects it contains. This study investigated the behavioral strategy of imitation in normal subjects, to assess whether they carried out task instructions using an anatomical or a mirror perspective. Twenty seven adults were asked to imitate intransitive meaningful and meaningless gestures shown by a model in video clips. Instructions about how to perform them were provided before each trial. Trials were free (intended to produce spontaneous imitation) or driven (intended to produce anatomical imitation); further driven trials were administered to verify participants' knowledge of bodily laterality and were used as control. Performances were interpreted as anatomical or mirror imitation, according to the observation of anatomical or spatial reference frames between stimulus and imitator. The results revealed that in spontaneous imitation the mirror mode was more frequent (61% of responses), in line with previous studies. The novel finding was the prevalence (93% of responses) of anatomical imitation in tasks involving detailed driven instructions.

Contribution of the corpus callosum to bilateral representation of the trunk midline in the human brain: an fMRI study of callosotomized patients

Human brain studies have shown that the cutaneous receptors of trunk regions close to the midline are represented in the first somatosensory cortex (SI) of both hemispheres. The present study aims to establish whether in humans, as in non-human primates, the bilateral representation of the trunk midline in area SI depends on the corpus callosum. Data were obtained from eight callosotomized patients: three with complete callosal resection, one with a partial posterior resection including the splenium and the callosal trunk, and four with partial anterior resections sparing the splenium and in one case also the posterior part of the callosal trunk. The investigation was carried out with functional magnetic resonance imaging. Unilateral tactile stimulation was applied by rubbing ventral trunk regions close to the midline (about 20 x 10 cm in width) with a soft cotton pad (frequency 1 Hz). Cortical activation foci elicited by unilateral stimulation of cutaneous regions adjacent to the midline were detected in the contralateral post-central gyrus (PCG), in a region corresponding to the trunk ventral midline representation zone of area SI, as described in a previous study of intact subjects. In most patients, activation foci were also found in the ipsilateral PCG, again as in subjects with an intact corpus callosum. The data confirm that the skin regions adjacent to the trunk midline are represented bilaterally in SI, and indicate that ipsilateral activation is at least partially independent of the corpus callosum.

Amyotrophic lateral sclerosis and sports: a case-control study

An increased incidence of amyotrophic lateral sclerosis (ALS) amongst soccer players in Italy has recently been reported. A case-control study (300 cases and 300 matched controls) was conducted to explore the association between ALS and physical/sports activities, with specific reference to trauma-related risk. Neither the practice of competitive sports nor sports-related traumas were found to be associated with an increased risk of ALS. The practice of physical activities or sports is not per se a risk factor for ALS. Our results exclude sports-related microtraumas as etiopathogenic factors in the natural history of ALS.

Glutamic acid decarboxylase immunoreactivity in callosal projecting neurons of cat and rat somatic sensory areas

The distribution of GABAergic callosally projecting neurons was analysed in the somatic sensory areas of cat and rat cerebral cortex by combining retrograde tracing of nerve cell bodies and glutamic acid decarboxylase (GAD) immunocytochemistry. A retrograde tracer (colloidal gold- labelled wheat germ agglutinin conjugated to enzymatically inactive horseradish peroxidase) was injected in the first or second somatic sensory area. Brain sections were processed for the simultaneous visualisation of the retrograde tracer and GAD immunoreactivity. In all animals, double-labelled neurons were found in the hemisphere contralateral to the injection site (double-labelled callosal neurons). Their proportion was similar in both species (0.8% of all retrogradely-labelled neurons in cat, 0.7% in rat). These results: 1) confirm the existence of a small proportion of GABAergic callosally projecting neurons in rat somatic sensory cortices; 2) indicate the presence of a small but significant proportion of GAD-positive callosally projecting neurons in cat somatic sensory cortices; and 3) show that the proportion of GAD-positive callosal neurons is similar in the two species.

Posterior corpus callosum and interhemispheric transfer of somatosensory information: an fMRI and neuropsychological study of a partially callosotomized patient

Interhemispheric somatosensory transfer was studied by functional magnetic resonance imaging (fMRI) and neuropsychological tests in a patient who underwent resection of the corpus callosum (CC) for drug-resistant epilepsy in two stages. The first resection involved the anterior half of the body of CC and the second, its posterior half and the splenium. For the fMRI study, the hand was stimulated with a rough sponge. The neuropsychological tests included: Tactile Naming Test (TNT), Same-Different Recognition Test (SDRT), and Tactile Finger Localization Test (intra- and intermanual tasks, TFLT). The patient was studied 1 week before and then 6 months and 1 year after the second surgery. Before this operation, unilateral tactile stimulation of either hand activated contralaterally the first (SI) and second (SII) somatosensory areas and the posterior parietal (PP) cortex, and SII and PP cortex ipsilaterally. All three tests were performed without errors. In both postoperative sessions, somatosensory activation was observed in contralateral SI, SII, and PP cortex, but not in ipsilateral SII and PP cortex. Performance was 100% correct in the TNT for the right hand, but below chance for the left; in the other tests, it was below chance except for TFLT in the intramanual task. This case provides the direct demonstration that activation of SII and PP cortex to stimulation of the ipsilateral hand and normal interhemispheric transfer of tactile information require the integrity of the posterior body of the CC.

Taste laterality in the split brain

Two patients with corpus callosum resection, one complete and the other sparing the genu and the rostrum, were tested for discrimination of three basic taste stimuli (sour, bitter, salty) applied to the right or left sides of the tongue. Responses were made by pointing with either hand to written words or images of visual objects corresponding to the stimuli, a language-based discrimination. In both patients, response accuracy was significantly above chance for both hemitongues but there was a significant advantage for the left side. Reaction time was shorter for left stimuli than for right stimuli but the difference was not significant. Eight normal controls matched for age with the patients performed equally well with right and left hemitongue stimuli and so did a third callosotomy patient with sparing of the posterior callosum, including the splenium. Tactile and visual tests showed that the left hemisphere was responsible for language-based responses in the first two patients. The results confirm and extend previous findings in another callosotomy patient, indicating that: (i) taste information from either side of the tongue can reach the left hemisphere in the absence of the corpus callosum; (ii) the ipsilateral input from the tongue to the left hemisphere is more potent functionally than the contralateral input and (iii) in the normal brain, the corpus callosum, specifically its posterior part including the splenium, appears to equalize the effects of the ipsilateral and contralateral gustatory inputs on the left hemisphere. Taken together with evidence about lateralized taste deficits following unilateral cortical lesions, the results also suggest that the gustatory pathways from tongue to cortex are bilaterally-distributed with an ipsilateral predominance that may be subject to individual variations.

Role of the corpus callosum in the somatosensory activation of the ipsilateral cerebral cortex: an fMRI study of callosotomized patients

To verify whether the activation of the posterior parietal and parietal opercular cortices to tactile stimulation of the ipsilateral hand is mediated by the corpus callosum, a functional magnetic resonance imaging (fMRI, 1.0 tesla) study was performed in 12 control and 12 callosotomized subjects (three with total and nine with partial resection). Eleven patients were also submitted to the tactile naming test. In all subjects, unilateral tactile stimulation provoked a signal increase temporally correlated with the stimulus in three cortical regions of the contralateral hemisphere. One corresponded to the first somatosensory area, the second was in the posterior parietal cortex, and the third in the parietal opercular cortex. In controls, activation was also observed in the ipsilateral posterior parietal and parietal opercular cortices, in regions anatomically corresponding to those activated contralaterally. In callosotomized subjects, activation in the ipsilateral hemisphere was observed only in two patients with splenium and posterior body intact. These two patients and another four with the entire splenium and variable portions of the posterior body unsectioned named objects explored with the right and left hand without errors. This ability was impaired in the other patients. The present physiological and anatomical data indicate that in humans activation of the posterior parietal and parietal opercular cortices in the hemisphere ipsilateral to the stimulated hand is mediated by the corpus callosum, and that the commissural fibres involved probably cross the midline in the posterior third of its body.

Localization of the first and second somatosensory areas in the human cerebral cortex with functional MR imaging

BACKGROUND AND PURPOSE

Our objective was to map by means of a conventional mid-field (1.0 T) MR imaging system the somatosensory areas activated by unilateral tactile stimulation of the hand, with particular attention to the areas of the ipsilateral hemisphere.

METHODS

Single-shot echo-planar T2*-weighted imaging sequences were performed in 12 healthy volunteers to acquire 10 contiguous 7-mm-thick sections parallel to the coronal and axial planes during tactile stimulation of the hand. The stimulation paradigm consisted of brushing the subjects' palm and fingers with a rough sponge at a frequency of about 1 Hz.

RESULTS

Stimulation provoked a signal increase (about 2% to 5%) that temporally corresponded to the stimulus in several cortical regions of both hemispheres. Contralaterally, activation foci were in the anterior parietal cortex in an area presumably corresponding to the hand representation zone of the first somatosensory cortex, in the posterior parietal cortex, and in the parietal opercular cortex forming the upper bank of the sylvian sulcus and probably corresponding to the second somatosensory cortex. Activation foci were also observed in the frontal cortex. Ipsilaterally, activated areas were in regions of the posterior parietal and opercular cortices roughly symmetrical to those activated in the contralateral hemisphere. The same activation pattern was observed in all subjects.

CONCLUSION

The activated areas of the somatosensory cortex described in the present study corresponded to those reported in other studies with magnetoelectroencephalography, positron emission tomography, and higher-field functional MR imaging. An additional area of activation in the ipsilateral parietal operculum, unnoticed in other functional MR imaging studies, was also observed.

The cerebral ventricles, the animal spirits and the dawn of brain localization of function

This paper reviews the early history of brain localization of function. It analyses the doctrines professed in ancient times by philosophers and physicians, who believed that brain functions were carried out in the cerebral ventricles by the psychic pneuma, or animal spirit, a sort of special and light substance endowed with the power to perform sensory, motor and mental activities. This theory, conceived in the Classic Age and called "ventricular-pneumatic doctrine", evolved in the 4th-5th centuries A.D. into the "three-cell theory", according to which each cerebral ventricle was the seat of a specific function, and contained a unique type of spirit with the power to perform that function. The three-cell theory represents the earliest attempt to localize different mind functions in separate brain sites and was held true by Byzantine, Arabian and Western Latin scholars well beyond the Renaissance. The paper is subdivided into an Introduction and eight sections. The first two sections report a brief history of the philosophical and medical doctrines about the pneuma as mediator of all vital functions, the ventricular-pneumatic doctrine elaborated by Galen of Pergamon, and his theory of nerve physiology based on the assumption that the pneuma, set in motion by active brain movements and flowing in the hollow nerves, could transfer sensations from the sense organs to the anterior ventricles, and motor commands from the posterior ventricle to the muscles. The third and fourth sections trace the ways in which these doctrines were transmitted to the Byzantine physicians and then to the Arabs, until they reached the Latin West. Here, throughout the Middle Ages they not only formed the background of medical and natural philosophy, but also influenced Christian theologians. The fifth section is devoted to the ventricular localization of mind faculties, called internal senses by Arabian and Western Latin scholars. Most authors recognized three basic internal senses: imagination, cognition and memory, and generally localized imagination in the anterior ventricle, cognition in the middle and memory in the posterior one, while other scholars adopted complex lists including up to seven faculties, each carried out by a specific type of animal spirit and localized, or sub-localized, in different ventricular sites according to complex topographical patterns. This section reports more than sixty patterns of ventricular localization from various authors (summarized in a Table), the rationale of complex ventricular localization, and the naive interpretations of Medieval physicians and surgeons of the impairment of the internal senses caused by brain disease and trauma. The sixth section deals with the decline of the three-cell theory, which was first challenged in the early 16th century and then drastically revised by several Renaissance and post-Renaissance experimentalists, anatomists and philosophers, although some remnants of the Galenic pneumatic neurophysiology survived in medicine until the 18th century. The penultimate section analyses bibliographical data on the earliest localizationists and shows that, independently of chronological priority, Nemesius of Emesa was the source of the pattern of ventricular localization of function adopted by later Byzantines and by the Arabs, and then transmitted to Latin Western scholars. The last section discusses the legacy of the three-cell theory to later generations of neuroscientists.

Glutamate decarboxylase immunoreactivity in corticocortical projecting neurons of rat somatic sensory cortex

Combined retrograde tracing and immunocytochemical experiments were carried out on rats to ascertain whether corticocortical projecting neurons in the somatic sensory areas are immunoreactive to an antiserum against glutamate decarboxylase. Injections of a retrograde tracer (colloidal gold-labelled wheat germ agglutinin conjugated to enzymatically inactive horseradish peroxidase) in the first somatic sensory area labelled neurons in the injected area, in the second somatic sensory area, and in the parietoventral area of the ipsilateral hemisphere. The topographical and laminar distribution of these retrogradely-labelled corticocortical neurons in the first and second somatic sensory areas and in the parietoventral area was in line with a previous description (Fabri M. and Burton H. (1991b) J. comp. Neurol. 311, 405-424). In sections processed for the simultaneous visualization of the retrograde tracer and glutamate decarboxylase immunoreactivity, a number of neurons were double-labelled. Double-labelled neurons were most numerous in the first somatic sensory cortex, where they accounted for 5% of all retrogradely-labelled neurons. Outside this region, double-labelled cells were observed in the second somatic sensory cortex and in the parietoventral cortex, where they amounted respectively to 2.8% and 2.3% of all corticocortical neurons labelled in these two areas. Glutamate decarboxylase-immunopositive corticocortical neurons were mainly concentrated in the infragranular layers (73.8% of all double-labelled neurons in the first somatic sensory area, 81.7% in the second somatic sensory area, and 76.5% in the parietoventral area). The results indicate the presence of a small but significant contingent of GABAergic inhibitory neurons in the associative connections of the somatic sensory areas.

Laminar pattern of termination of the ipsilateral cortical projection from SII to SI in cats

The present light and electron microscopic experiments were carried out on the first somatic sensory area (SI) of cats to determine the laminar distribution of axon terminals from the ipsilateral second somatic sensory area (SII) and to identify the types of synapses between these terminals and the neuronal elements of SI. Phaseolus vulgaris-leucoagglutinin (PHA-L) was iontophoretically injected into multiple sites and at different cortical depths of the forepaw representation zone of SII. Fixed brain blocks containing the injected SII and ipsilateral SI were cut into slices and processed immunocytochemically to stain PHA-L-filled fibers and terminals. Light microscopic examination of SI revealed patches of anterograde labeling in the forepaw representation zone, concentrated mainly in supragranular layers. In these layers, thin immunolabeled fibers branched extensively and formed a dense plexus that was more prominent in layers II and I. Conversely, the infragranular layers contained fragments of vertically oriented thick fibers that rarely emitted axon collaterals. PHA-L-labeled axons had numerous swellings along their course, interpreted as boutons en passant, and stalked boutons. Of 19,661 labeled terminals (17,833 beads and 1,828 stalked boutons), 84.74% were observed in supragranular layers, with the highest concentration in layer II (33.15%) and lower in layers I (26.27%) and III (25.30%). The proportion of terminals was lower in layers IV (6.49%) and V (5.45%) and lowest in layer VI (3.32%). These counts also showed that boutons en passant were the majority (90.70%) and stalked boutons, the minority (9.30%). The ratio of these two types of presynaptic specializations was similar (9:1) in all six layers. Electron microscopic examination of the labeled regions of SI showed that both axon swellings and stalked boutons formed synapses of the asymmetric type with SI neuronal elements. The majority (85.37%) of a sample of 130 labeled terminals synapsed on SI neurons in layers I-III. The identified postsynaptic profiles were dendritic spines (61.11%) or medium-sized and small dendrites (38.89%). These results are discussed in relation to those of a companion study on the laminar pattern of the projection from SI to SII of cats (P. Barbaresi, A. Minelli, and T. Manzoni, 1994, J. Comp. Neurol. 343:582-596). Based on the anatomical organization of these reciprocal connections, there seems to be no clear hierarchicalal relationship between SI and SII in cats.

Topographical relations between ipsilateral cortical afferents and callosal neurons in the second somatic sensory area of cats

Experiments were carried out on the second somatic sensory area (SII) of cats to study 1) the laminar distribution of axon terminals from the ipsilateral first somatic sensory cortex (SI); and 2) the topographical relations between their terminal field and the callosal neurons projecting to the contralateral homotopic cortex. To label simultaneously in SII both ipsilateral cortical afferents and callosal cells, cats were given iontophoretic injections of Phaseolus vulgaris-leucoagglutinin (PHA-L) in the forepaw zone of ipsilateral SI, and pressure injections of horseradish peroxidase (HRP) in the same zone of contralateral SII. The possibility that ipsilateral cortical axon terminals synapse callosal neurons was investigated with the electron microscope by combining lesion-induced degeneration with retrograde HRP labelling. Fibers and terminations immunolabelled with PHA-L from ipsilateral SI were distributed in SII in a typical patchy pattern and were mostly concentrated in supragranular layers. Labelled fibers formed a very dense plexus in layer III and ramified densely also in layers I and II. Labelled axon terminals were both en passant and single-stalked boutons. Counts of 8,303 PHA-L-labelled terminals of either type showed that 82.40% were in supragranular layers. The highest concentration was in layer III (43.99%), followed by layers II (30.32%) and I (8.09%). The remaining terminals were distributed among layers IV (6.96%), V (4.93%), and VI (5.68%). The same region of SII containing anterogradely labelled axons and terminals also contained numerous neurons retrogradely labelled with HRP from contralateral SII. Callosal projection neurons were pyramidal, dwelt mainly in layer III, and were distributed tangentially in periodic patches. Patches of anterograde and retrograde labelling either interdigitated or overlapped both areally and laminarly. In the zones of overlap, numerous PHA-L-labelled axon terminals were seen in close apposition to HRP-labelled pyramidal cell dendrites. Combined HRP-electron microscopic degeneration experiments showed that in SII axon terminals from ipsilateral SI form asymmetric synapses with HRP-labelled dendrites and dendritic spines pertaining to callosal projection neurons. These results are discussed in relation to the layering and function of the SI to SII projection, and to the evidence that SII neurons projecting to the homotopic area of the contralateral hemisphere have direct access to the sensory information transmitted from ipsilateral SI.

Calcitonin gene-related peptide (CGRP) in the cat neocortex: evidence for a sparse but widespread network of immunoreactive fibers

The morphology, and laminar and topographic distribution of fibers containing calcitonin gene-related peptide (CGRP) immunoreactivity were studied by light and electron microscopic methods in the cerebral cortex of adult cats using a rabbit antiserum raised against the C-terminal region of the rat alpha-CGRP. At the light microscopic level, a sparse number of CGRP-positive fibers were observed in the frontal, parietal, and occipital cortices. They showed numerous irregularly spaced varicosities, were mostly oriented vertically, and in rare cases gave rise to boutons terminaux as they ascended toward the pial surface. At the border between layers I and II, they branched into horizontal fibers that could be followed for several hundred microns in layer I and gave rise to terminal clusters of boutons. In some sections, CGRP-positive fibers were seen in close association with blood vessels. At the electron microscopic level, CGRP immunoreactivity was found in axon terminals containing few mitochondria and clear synaptic vesicles. CGRP-positive axon terminals were very sparse, and mainly of small size. The majority formed conventional synapses, all of the asymmetric type. CGRP-positive fibers showed an uneven topographic distribution through the cortical mantle, with the frontal areas exhibiting the highest density and the occipital cortex the lowest. These results show that CGRP-containing axons are more widely distributed than previously thought since they were observed in all the cortical areas examined, and cast some doubts on the hypothesis that the functional role of this peptide is restricted to the processing of visceral sensory information.(ABSTRACT TRUNCATED AT 250 WORDS)

Substance P-containing pyramidal neurons in the cat somatic sensory cortex

Light and electron microscopic immunocytochemical methods were used to verify the possibility that neocortical pyramidal neurons in the first somatic sensory cortex of cats contain substance P. At the light microscopic level, substance P-positive neurons accounted for about 3% of all cortical neurons, and the vast majority were nonpyramidal cells. However, 10% of substance P-positive neurons had a large conical cell body, a prominent apical dendrite directed toward the pia, and basal dendrites, thus suggesting they are pyramidal neurons. These neurons were in layers III and V. At the electron microscopic level, the majority of immunoreactive axon terminals formed symmetric synapses, but some substance P-positive axon terminals made asymmetric synapses. Labelled dendritic spines were also present. Combined retrograde transport-immunocytochemical experiments were also carried out to study whether substance P-positive neurons are projection neurons. Colloidal gold-labelled wheat germ agglutinin conjugated to enzymatically inactive horseradish peroxidase was injected either in the first somatic sensory cortex or in the dorsal column nuclei. In the somatic sensory cortex contralateral to the injection sites, a few substance P-positive neurons in layers III and V also contained black granules, indicative of retrograde transport. This indicates that some substance P-positive neurons project to cortical and subcortical targets. We have therefore identified a subpopulation of substance P-positive neurons that have most of the features of pyramidal neurons, are the probable source of immunoreactive axon terminals forming asymmetric synapses on dendritic spines, and project to the contralateral somatic sensory cortex and dorsal column nuclei. These characteristics fulfill the criteria required for classifying a cortical neuron as pyramidal.

Thalamic connections of the second somatic sensory area in cats studied with anterograde and retrograde tract-tracing techniques

The thalamic connections of the second somatosensory area in the anterior ectosylvian gyrus of cats have been investigated using the retrograde tracer horseradish peroxidase and the anterograde tracer Phaseolus vulgaris leucoagglutinin. Horseradish peroxidase was injected iontophoretically in several somatotopic zones of the second somatosensory area map of six cats. Sites of horseradish peroxidase delivery were identified preliminarily by recording with microelectrodes the responses of neurons to skin stimulation. Phaseolus vulgaris leucoagglutinin was iontophoretically injected within the ventrobasal complex (one cat) or in the posterior complex (one cat). Horseradish peroxidase injections into cytoarchitectonic area SII retrogradely labeled neurons in the ipsilateral ventrobasal complex and in the posterior complex. Counts of labeled neurons from the ipsilateral thalamus showed that the overwhelming majority of horseradish peroxidase-labeled neurons were in the ventrobasal complex (96.3-96.9%) and few were in the posterior complex (3.1-3.7%). Neurons labeled in the ventrobasal complex were observed throughout the anteroposterior extent of the nucleus, while their mediolateral distribution varied with the site of horseradish peroxidase delivery in the body map of the second somatosensory area, which indicates that the projections from the ventrobasal complex to the second somatosensory area are somatotopically organized. In the cat in which the horseradish peroxidase injection involved both the second somatosensory area proper and the second somatosensory area medial, which lies in the lower bank of suprasylvian sulcus, labeled neurons were almost as numerous in the ventrobasal complex as in the posterior complex. Phaseolus vulgaris leucoagglutinin injected in the ventrobasal complex anterogradely labeled thalamocortical fibers in the ipsilateral anterior ectosylvian gyrus. In this case, patches of labeled fibers and terminals were distributed exclusively within the cytoarchitectonic borders of the second somatosensory area proper. Labeled terminals were numerous in layer IV and lower layer III, but terminal boutons and fibers with axonal swellings, probably forming synapses en passant, were frequently observed also in layers VI and I. Injection of Phaseolus vulgaris leucoagglutinin in the posterior complex labeled thalamocortical fibers in two distinct regions in the ipsilateral anterior ectosylvian gyrus, one lying laterally and the other medially, which correspond, respectively, to the fourth somatosensory area and the second somatosensory area medial. In both areas the densest plexus of labeled fibers and axon terminals was in layer IV and lower layer III, but numerous labeled fibers and terminals were also observed in layer I. In this case, only rare fragments of labeled fibers were present in second somatosensory area proper, but no labeled terminals could be observed.

Matching of receptive fields in the association projections from SI to SII of cats

Anatomical and electrophysiological experiments were performed on cats to investigate the pattern of divergence and convergence in the association projections from the first (SI) to the second (SII) somatic sensory cortex and to ascertain whether diverging and converging fibre components from SI have receptive fields (RFs) matching those of target neurons in SII. In the first group of six cats, a single deposit of horseradish peroxidase (HRP) was iontophoretically placed (2-4 microA for 20 minutes) into an electrophysiologically identified site of the SII map: the digit (3 cats), forepaw (2 cats), and arm (1 cat) zones. The forelimb representation in ipsilateral SI was subsequently explored with microelectrodes and RFs from small clusters of neurons systematically mapped. Planar maps of this area were reconstructed with the aid of a computer from serial sections, to correlate on the tangential plane the topographical distribution of retrogradely labelled association neurons with the physiological map of the forelimb. Since diverging projections were observed from a zone of SI to multiple zones of SII, double-labelling experiments were carried out in a second group of three cats, in which two retrograde fluorescent dyes (diamidino yellow and fast blue) were injected by pressure into two different sites of the SII map, to ascertain whether SI sends diverging projections by branching axons. HRP injections in SII retrogradely labelled a discrete number of association neurons in SI. Their distribution area was several tens of times wider than that covered by the injection site. This suggests that a remarkable amount of divergence and convergence exists in the association projections from SI to SII. Despite the substantial difference in the extent of the injected and labelled areas, RFs of afferent and target neurons corresponded closely. Injections covering a small region within a single digit zone of SII labelled neurons throughout the entire representation of the same digit in SI, while neurons labelled in somatotopically inappropriate zones were rare. RFs mapped from several sites of the labelled region in SI were individually smaller than the RF mapped from the injection site in SII, but the overall size of afferent RFs encompassed that of target neurons. Divergence and convergence in the SI projections to SII zones representing more proximal portions of the forelimb may be even greater since HRP injections in the forepaw and arm zones of SII labelled a number of neurons also in the digit zone of SI, providing the RFs mapped from the injection sites were sufficiently wide to include the digits.(ABSTRACT TRUNCATED AT 400 WORDS)

Glutamate-positive neurons and axon terminals in cat sensory cortex: a correlative light and electron microscopic study

Immunocytochemical methods were used to perform a correlative light and electron microscopic study of neurons and axon terminals immunoreactive to the antiglutamate (Glu) serum of Hepler et al. ('88) in the visual and somatic sensory areas of cats. At the light microscopic level, numerous Glu-positive neurons were found in all layers except layer I of both cortical areas. On the basis of the dendritic staining of Glu-positive cells, two major morphological categories were found: pyramidal cells, which were the most frequent type of immunostained neuron, and multipolar neurons, which were more numerous in layer IV of area 17 than in any other layer. A large number of Glu-positive neurons, however, did not display dendritic labelling and were considered unidentified neurons. Counts of labelled neurons were performed in the striate cortex; approximately 40% were Glu-positive. Numerous lightly stained punctate structures were observed in all cortical layers: the majority of these Glu-positive puncta were in the neuropil. After resectioning the plastic sections for electron microscopy it was observed that: 1) the majority of neurons unidentifiable at light microscopic level were indeed pyramidal neurons except in layer IV of area 17, where many stained cells were probably spiny stellate neurons. Some Glu-positive neurons, however, exhibited clear ultrastructural features of nonspiny nonpyramidal cells; 2) all synaptic contacts made by Glu-positive axon terminals were of the asymmetric type, but not all asymmetric synaptic contacts were labelled. The vast majority of postsynaptic targets of Glu-positive axons were unlabelled dendritic spines and shafts. The present results provide further evidence that Glu (or a closely related compound) is probably the neurotransmitter of numerous excitatory neurons in the neocortex.

Callosal connections of the somatic sensory areas II and IV in the cat

The homotopic and heterotopic callosal connections in the forelimb representations of the second (SII) and fourth (SIV) somatic sensory areas of cats were investigated by means of the axonal transport of horseradish peroxidase (HRP) in conjunction with microelectrode recording. The tracer was injected in the electrophysiologically identified hand and/or digit zone of SII (six cats) or SIV (four cats). The homotopic area in the contralateral hemisphere was explored with microelectrodes in five animals (three injected in SII and two in SIV) to map neuronal receptive fields. The aim was to correlate in the same experimental case the topography of labelled callosal neurons with the physiological map of the forelimb. Labelled cells and recording sites were plotted on planar maps reconstructed with the aid of a computer from serial coronal sections from the anterior ectosylvian gyrus. After SII injections, labelled callosal neurons were observed throughout the forelimb representation in the contralateral area, but in the tangential plane their distribution was uneven. Each somatotopic zone composing the forelimb map, that is, the arm, hand, and digit zones, contained several subzones in which callosal neurons were either dense or rare. Microelectrode explorations showed that receptive fields mapped from callosal and relatively acallosal subzones representing the same body part were similar in extent and location. After SIV injections, labelled callosal neurons were observed throughout the forelimb and proximal body representation of the contralateral area. Although slight regional variations in the density of labelled cells were apparent, no subzones bare of callosal labelling were observed in SIV. In both SII and SIV, callosal neurons were concentrated mainly in layer III, but a significant number was also evident in the infragranular layers. After HRP injections in the digit zone of SII or SIV, labelled cell bodies were also observed in heterotopic areas of the contralateral hemisphere. Most of these neurons were clustered in the medial bank of the coronal sulcus and in two other heterotopic cortical regions lying, respectively, in the anterior suprasylvian sulcus and in the lateral branch of the ansate sulcus. Some callosal cells interconnecting SII and SIV were also labelled. The results show that the distal forelimb zones in SII and SIV are callosally connected with the respective homotopic zones and with several somatosensory fields located heterotopically in the contralateral hemisphere.

Immunocytochemical evidence for glutamatergic cortico-cortical connections in monkeys

Retrograde transport of horseradish peroxidase and immunocytochemical visualization of glutamate (Glu) were combined to investigate the neurotransmitter used by cortico-cortical neurons in the first (SI) and second (SII) somatic sensory areas of macaque monkeys. The majority of association and callosal neurons in SI and SII were immunoreactive for an antiGlu serum: evidence was therefore obtained in support of a role for Glu, or a closely related compound, as the synaptic transmitter used by cortico-cortical fibers.

Glutamate-positive corticocortical neurons in the somatic sensory areas I and II of cats

Combined retrograde transport-immunocytochemical experiments were carried out on cats to study the morphology, laminar distribution, and percentages of corticocortical projecting neurons of somatosensory area I (SI) and II (SII) showing immunoreactivity to an antiserum raised against the amino acid glutamate (Glu). A previously characterized anti-Glu serum (Conti et al., 1987a, b; Hepler et al., 1987) was used in conjunction with HRP. This tracer was injected either in SI to label retrogradely neurons in ipsilateral SII (SII-SI association neurons) and contralateral SI (SI-SI callosal neurons) or in SII to label retrogradely neurons in ipsilateral SI (SI-SII association neurons) and contralateral SII (SII-SII callosal neurons). In sections from SI and SII processed for simultaneous visualization of Glu and HRP (Bowker et al., 1982), and containing the cells from which every one of the 4 corticocortical projections arise, 3 types of labeled neurons were observed: (1) single-labeled neurons showing the homogeneous brown immunoreaction product of Glu (Glu-positive neurons); (2) single-labeled neurons containing the granular black reaction product of retrogradely transported HRP (Glu-negative, association or callosal neurons); and (3) double-labeled neurons in which both the black HRP granules and the brown immunostaining were present (Glu-positive, association or callosal neurons). Double-labeled neurons were all pyramidal in shape and were distributed intermingled with Glu-negative corticocortical neurons in all layers of SI and SII known to give rise to association and callosal projections. Counts from 25-micron-thick sections showed that of 432 association and callosal neurons sampled from SI and SII, 214 (49.5%) were Glu-negative and 218 (50.5%) Glu-positive. In counts carried out on 5-micron-thick sections, the percentage of Glu-positive corticocortical neurons raised to about 70%. The 2 populations of single- and double-labeled corticocortical neurons showed no difference in their perikaryal cross-sectional areas. The present results show that a large fraction of association and callosal neurons of SI and SII are immunoreactive for Glu, and, therefore, these neurons probably use this excitatory amino acid, or a closely related compound, as neurotransmitter.

Demonstration of glutamate-positive axon terminals forming asymmetric synapses in cat neocortex

Electron microscopic examination of sections immunocytochemically processed with an anti-glutamate serum reveals that many asymmetric synapses in the cat neocortex contain elevated levels of immunodetectable glutamate. These labelled axon terminals are likely to use glutamate as neurotransmitter. Axon terminals forming symmetric contacts were never labelled. Since glutamate is known to exert potent excitatory effects on neocortical neurons, the present finding gives immunocytochemical evidence that asymmetric synapses are excitatory.

D-[3H]aspartate retrograde labelling of callosal and association neurones of somatosensory areas I and II of cats

Experiments were carried out on cats to ascertain whether corticocortical neurones of somatosensory areas I (SI) and II (SII) could be labelled by retrograde axonal transport of D-[3H]aspartate (D-[3H]Asp). This tritiated enantiomer of the amino acid aspartate is (1) taken up selectively by axon terminals of neurones releasing aspartate and/or glutamate as excitatory neurotransmitter, (2) retrogradely transported and accumulated in perikarya, (3) not metabolized, and (4) visualized by autoradiography. A solution of D-[3H]Asp was injected in eight cats in the trunk and forelimb zones of SI (two cats) or in the forelimb zone of SII (six cats). In order to compare the labelling patterns obtained with D-[3H]Asp with those resulting after injection of a nonselective neuronal tracer, horseradish peroxidase (HRP) was delivered mixed with the radioactive tracer in seven of the eight cats. Furthermore, six additional animals received HRP injections in SI (three cats; trunk and forelimb zones) or SII (three cats; forelimb zone). D-[3H]Asp retrograde labelling of perikarya was absent from the ipsilateral thalamus of all cats injected with the radioactive tracer but a dense terminal plexus of anterogradely labelled corticothalamic fibres from SI and SII was observed, overlapping the distribution area of thalamocortical neurones retrogradely labelled with HRP from the same areas. D-[3H]Asp-labelled neurones were present in ipsilateral SII (SII-SI association neurones) in cats injected in SI. In these animals a bundle of radioactive fibres was observed in the rostral portion of the corpus callosum entering the contralateral hemisphere. There, neurones retrogradely labelled with silver grains were present in SI (SI-SI callosal neurones). Association and callosal neurones labelled from SI showed a topographical distribution similar to that of neurones retrogradely labelled with HRP. The laminar patterns of corticocortical neurones labelled with D-[3H]Asp or with HRP were also similar, with one exception. In the inner half of layer II, SII-SI association neurones and SI-SI callosal neurones labelled with the radioactive marker were much less numerous than those labelled with HRP. In cats injected in SII, D-[3H]Asp retrogradely labelled cells were present in ipsilateral SI (SI-SII association neurones). Their topographical and laminar distribution overlapped that of neurones labelled with HRP but, as in cats injected in SI, association neurones labelled with silver grains were unusually rare in the inner layer III.(ABSTRACT TRUNCATED AT 400 WORDS)

Callosal projections from area SII to SI in monkeys: anatomical organization and comparison with association projections

The present research was aimed at ascertaining in the macaque monkey the reciprocity of the heterotopical callosal connections between SI and SII, with particular regard to the connectivity of the hand representation, and at comparing the topographical and laminar pattern of these callosal connections with those of association connections entertained by these areas. Horseradish peroxidase (HRP) was unilaterally injected into area SI in five monkeys. The sites of HRP delivery included the trunk and the hand zones preliminarily identified by recording multi-unit responses to peripheral stimulation by means of microelectrodes. Anterograde and retrograde labelling was studied in SII of both sides. The results showed the complete reciprocity of the heterotopical callosal connections between SI and SII. In the latter area both callosal axon terminals and neurones were found, which were labelled from either the trunk or the hand zone of contralateral SI. Labelling of callosal axon terminals occurred mainly in layer IV and in the lowermost part of layer III. Labelled callosal neurones were mainly in the lower half of layer III, whereas few occurred in infragranular layers. Topographically, the distribution of callosal terminals and cell bodies duplicated the distribution of association terminals and cell bodies labelled in SII on the side ipsilateral to HRP injection. The laminar pattern of termination of association fibres from SI was similar to that of callosal fibres. However, the distribution of association-projecting neurones in SII showed a striking difference from that of callosal-projecting neurones. Unlike the latter neurones, which were mainly located in supragranular layers, association cell bodies overwhelmingly dwelt in layers V and VI and were less numerous in layers II and III. This laminar pattern of association SII-SI cells corresponds to the "feed-backward" model and fits the laminar pattern of their axon terminations (Friedman: Brain Res. 273: 147-151, '83). The association and callosal inputs and outputs of area SII are discussed in relation to the function of the forward and backward type of reciprocal connections entertained with SI in the ipsilateral hemisphere and to the function of SII in the interhemispheric exchange of somatosensory information.

D-[3H]aspartate retrograde labelling of association neurones in area SI of the cat

Injection of D-[3H]aspartate into area SII of cats retrogradely labelled association cells in area SI. Numerous intensely labelled cells were found in layer II and in the upper layer III but labelling was scanty in other layers. In contrast, association neurones labelled by horseradish peroxidase injected in area SII mixed with the radioactive marker were also numerous in the other sublaminae of layer III and in infragranular layers of SI. Association neurones of the outer laminae of this area are likely to use aspartate and/or glutamate as neurotransmitter(s).

Bilateral receptive fields and callosal connectivity of the body midline representation in the first somatosensory area of primates

Experiments were performed in order to study the receptive field (RF) organization and the callosal connectivity of the trunk representation zone in areas 3b and 1 of the postcentral cortex of macaque monkeys. Multiunit microelectrode recordings showed that neurons responding to tactile stimulation of bilateral RFs across the midline of the body were contained in three topographically distinct zones of the trunk map. In one zone, at the junction between cytoarchitectonic areas 3b and 1, RFs straddled the dorsal midline of the trunk. In the other two zones, one located caudally in area 1 in front of the postcentral dimple, and the other rostrally in area 3b in the depth of the posterior bank of the central sulcus, RFs straddled the ventral midline of the trunk. The first one and the other two zones are referred to here as the dorsal and the ventral midline representation zones, respectively. Elsewhere in the trunk map, neurons responded only to stimulation of contralateral RFs. The callosal connectivity of the trunk map was studied by means of the transport of horseradish peroxidase (HRP). Multiple injections of HRP in electrophysiologically identified sites of the trunk representation in one hemisphere labeled both callosal fiber terminals and callosally projecting neurons in the contralateral homotopic cortex. Dense patches of callosal neurons intensely labeled with HRP were present in the cortical regions representing the body midlines and were distributed for the most part in layer III. Some neurons lightly labeled with HRP were scattered in other zones of the trunk map. Callosal terminations were densest within the midline zones and very sparse or absent in the lateral trunk zones. Correlation of physiological and anatomical data obtained either separately or from the same animal demonstrated that cortical regions containing bilateral-field neurons also contained the highest density of labeled callosal terminations and neurons. This correlation suggests a role for the corpus callosum in the perception of the body midline, either by generating the bilateral RFs of these neurons or by coordinating the activity of the regions containing neurons with thalamically generated bilateral RFs.

[Topography of the thalamo-cortical projections on trunk representation demonstrated by fluorescent neuro-tracers]

With the aim to study the detailed topography of the thalamo-cortical neurones projecting to the trunk representation zone of the first somatosensory area (SI), punctate injections of three different fluorescent tracers (Evans Blue, Nuclear Yellow and Fast Blue) were performed in the three physiologically defined subareas forming the trunk region of SI. These injections resulted in the labelling of three different cell aggregates, narrow in dorsoventral and mediolateral extent but elongated rostrocaudally, located in topographically distinct regions of the nucleus ventralis posterio-lateralis. The results suggest that the highly organized topography of the trunk representation of area SI is imposed by the thalamo-cortical input from VPL.

[Callosal connections of the somatosensory area in the primate: anatomical and electrophysiological studies]

In order to study the callosal connections of the hand sensory field of the second somatosensory area of the monkey, experiments were carried out by combining the method of retrograde neuronal tracing with microelectrode recording. In six monkeys, Macaca Irus, single or multiple (5-8) injections of horseradish peroxidase (HRP) were performed into the cortex of the parietal operculum of one side. Neurones retrogradely labelled with HRP (callosal neurones) were found in the post-central gyrus and in the parietal operculum of the contralateral hemisphere. Microelectrode recording from this hemisphere showed that the cortical zones of both the first and the second somatosensory area containing neurones excited by sensory stimulation of the contralateral hand also contained HRP-positive neurones.

Topography and receptive field organization of the body midline representation in the ventrobasal complex of the cat

The topography and receptive field (RF) organization of neurones in the trunk zone of the thalamic ventrobasal complex (VB) projecting to the homologous zone of the ipsilateral first somatosensory area (SI) were studied in the cat by performing experiments of retrograde neuronal tracing and microelectrode recording. Punctate cortical injections of small amounts of either horseradish peroxidase or fluorescent tracers (Evans Blue, Nuclear Yellow and Fast Blue) retrogradely labelled cell aggregates lying in the dorsal half of a VB region interposed between subnucleus VPL1 and VPLm. Aggregates of labelled cells were narrow in dorsoventral and mediolateral extent and elongated rostrocaudally. The distribution of VB cells projecting to the cortical subareas representing the dorsal midline, lateral trunk and ventral midline of the body in area SI, was established by injecting a different fluorescent marker into a physiologically defined site in each subarea. These injections resulted in labelling of three different cell aggregates located in topographically distinct regions of the VB trunk zone. Each aggregate of labelled cells only projected to one cortical subarea. Microelectrode analysis of cell populations of the VB trunk zone showed that neurones lying in regions projecting to dorsal and ventral midline zones of area SI had bilateral RFs, straddling the dorsal and the ventral midline of the body respectively. Neurones lying in the region projecting to the lateral trunk representation of area SI had contralateral RFs located on the lateral surface of the trunk. The results suggest that the detailed topography of the trunk map in the area SI and the bilaterality of the cortical representation of the body midlines, described in previous experiments, is imposed by the thalamocortical input from the VB.

Callosal mechanism for the interhemispheric transfer of hand somatosensory information in the monkey

The retrograde transport of horseradish peroxidase (HRP) was combined with extracellular microelectrode recording from single and multiple-neurones to study the anatomical and functional organization of the callosal connections of the hand sensory projection field in the parietal operculum of monkeys (Macaca Irus). In 3 animals anaesthetized with ketamine, a single injection of HRP (0.5 microliter) was delivered into the cortex forming the upper bank of the sylvian sulcus at a site where neuronal responses to somatic sensory stimulation of the hand were recorded. In the ipsilateral hemisphere, retrogradely HRP-labelled cells were found in the cortex of the post-central gyrus and in the thalamic nuclei ventralis posteroinferior and pulvinar oralis. In the contralateral hemisphere HRP-labelled neurones were present in the opercular cortex lying dorsal, and slightly caudal, to the posterior pole of the insula. Few scattered callosal neurones were also found in the post-central gyrus. In 3 other animals, multiple injections (5-8; 0.5 microliter each) of HRP were performed in the parietal operculum. In the ipsilateral hemisphere, retrogradely labelled cells were present in the post-central gyrus and in the following thalamic nuclei: ventralis posteroinferior, pulvinar oralis and medialis, ventralis posteromedialis and posterior complex. Few labelled cells were also present in the ventral part of the nucleus ventralis posterolateralis. In the contralateral hemisphere, numerous callosal cells were labelled with HRP. These cells were found, with regional variations in density, in wide regions of the buried and exposed cortex of the parietal operculum and in the post-central gyrus. These 3 monkeys were subjected to microelectrode mapping experiments (N2O and halothane anaesthesia) to explore the peripheral receptive fields of neurones in the parietal operculum and post-central gyrus contralateral to the injected side. HRP labelled callosal neurones were found in regions of the second and first somatosensory cortical areas which also contained units driven from the contralateral hand.

Axonal branching in the periaqueductal gray projections to the thalamus: a fluorescent retrograde double-labeling study in the cat

The double-labeling technique based on the retrograde axonal transport of fluorescent tracers (Evans blue, EB; Fast blue, FB; Nuclear yellow, NY) was used in the cat in order to investigate the occurrence of axonal branching in the periaqueductal gray (PAG) projections to some thalamic nuclei (n. ventralis postero-lateralis, VPL; n. ventralis postero-medialis, VPM; n. parafascicularis, Pf). In a first group of cats, FB and EB were injected, respectively, within the right and left VPM. In another two groups of cats, FB injections into Pf were combined with either EB or NY injections within VPL or VPM. Double-labeled neurons were found within the PAG only in the animals of the first group. The present results show that some PAG neurons project bilaterally to VPM by means of axons collaterals.

Periaqueductal grey projection to the ventrobasal complex in the cat: an horseradish peroxidase study

Horseradish peroxidase (HRP) was injected within the thalamic ventrobasal complex of 14 cats. The aim was to ascertain whether the periaqueductal grey matter (PAG) sends fibres to this complex. Retrogradely labelled cells were found within the PAG following HRP delivery either in the nucleus ventralis posterolateralis (VPL) or ventralis posteromedialis (VPM). PAG-VPL projection is only ipsilateral and arises mainly from lateral PAG, PAG-VPM projection is bilateral and originates from latero-ventral regions of the central grey. The hypothesis that PAG might control the activity of ventrobasal nociceptive neurones is proposed.

Callosal projections from the two body midlines

1. Horseradish peroxidase (HRP) was injected within the proximal limb and trunk representation zones of the first somatosensory area (SI) of 16 cats. The tangential and laminar distributions of retrogradely labelled neurones (callosal neurones) of the contralateral homotopic cortex were studied. This cortex was explored with microelectrodes on the day after HRP delivery to relate the distribution of callosal neurones to the electrophysiological map of the trunk. 2. Callosal neurones were found in the contralateral SI area mainly in layer III, but also many in layer VI, especially following large HRP injections, and very few in the other layers. Callosal neurones of layer III were mostly pyramidal, those of layer VI pyramidal and non-pyramidal. Many neurones were intensely stained by HRP, and cell details, such as fine dendritic branchings, spines, and axon collaterals, could be seen. 3. Callosal neurones are grouped within two regions located, respectively, in the rostral and caudal parts of the exteroceptive trunk map. The rostral region overlaps the representation of the dorsal midline (cytoarchitectonic field 3b) and the second one that of the ventral body midline (cytoarchitectonic field 2). The cortex intermediate between these two fields contains rare callosal cells and receives afferences from the lateral trunk surface. Few or no callosal cells were found within the proximal limb zones. Neurones recorded from the two midline zones have bilateral receptive fields straddling either the back or the ventral surface of the trunk. 4. It is concluded that the interhemispheric fusion between the two hemibody representations in areas SI is brought about by the mutual callosal links which the two midline zones entertain with their contralateral homologues.

The anatomical substrate of callosal messages from SI and SII in the cat

Horseradish peroxidase (HRP) was injected into the first (SI) or second (SII) somatosensory areas of 21 adult cats. The radial and tangential (normal and parallel to the pial surface, respectively) distribution and morphology of the callosal neurons were studied. HRP injections were combined with single unit recording in the contralateral cortex in order to determine which part of the somatosensory periphery is represented within the regions containing callosal neurons, the callosal (efferent) zones, in SI and SII. The callosal zone of SI extends over the trunk and part of the forepaw representation. In the forepaw and hindlimb representations callosal neurons projecting only to the contralateral SII are found, while in the trunk representation callosal neurons projecting to contralateral SI or SII are found. The callosal zone in SII extends widely throughout the forepaw representation in this area and projects to the contralateral SII but not to SI. In both SI and SII the callosal neurons are mainly located in layer III. A few of them are also found in layer VI. They are very rare in other layers. Callosal neurons in layer III are mostly pyramidal but exceptionally stellate; in layer VI they are pyramidal, triangular, and occasionally stellate. These data indicate that transformations of the cortical somatosensory maps are achieved in the message sent through the corpus callosum. These transformations are i) determined by the extent and location of the callosal zones and perhaps by the distribution of callosal neurons within them, ii) different in different areas, iii) different in a same area, according to the cortical targets to which they are conveyed. The existence of callosal connections originated from areas of distal forepaw representation supplies a possible anatomical substrate for those types of intermanual transfer of tactile learning which depend upon the integrity of the corpus callosum.

Anatomical and functional aspects of the associative projections from somatic area SI to SII

1. Electrophysiological and morphological (retrograde axonal transport of horseradish peroxidase, HRP) experiments have been carried out in the cat in order to study the associative projections from area SI to ipsilateral SII. 2. Microelectrode recordings were performed in the forepaw focus of SII both in normal (64 units) and in SI-undercut (51 units) cats. 29.6% of the neurons recorded in the unoperated and 29.4% of those collected in the operated cats were excited by electric stimulation of the ipsilateral SI (forepaw focus). In both preparations almost all such units were endowed with large (either contra- or bilateral) receptive fields (RF). Cell population recorded in the SI-undercut cats showed no significant impairment to peripheral stimuli and/or changes in the size of the RFs. 3. From the forepaw focus of SI, 150 units have been recorded and tested by stimulation of the homologous focus of the ipsilateral SII. Eight of them were fired antidromically and thus identified as association cells. Their RFs were very small and located only in the digits of the contralateral forepaw. 4. Both single or multiple HRP injections were performed in SII. Retrogradely labelled cells were found in the ipsilateral SI. The great majority of association cells are pyramids and dwell mainly in layer III. In spite of the large diffusion of the exogenous reaction product in the injected SII and of the presence of retrogradely labelled cells anywhere in the ipsilateral thalamic VB complex, the distribution of association cells is unequal throughout SI since they strongly predominate in the digit zone of the forepaw representation.

Callosal transfer of impulses originating from superficial and deep nerves of the cat forelimb

1. Experiments were performed in 18 chloralose-anaesthetized, curarized cats in order to study the callosal transfer of somatic information originated in exteroceptive and proprioceptive receptors. Several cutaneous and deep nerves of the forelimb were prepared and stimulated with graded intensities, so as to activate selectively afferent fibres pertaining to the different groups of Lloyd's classification. Simultaneous records were taken (and averaged on-line by means of a multichannel analyzer) from the distal end of a cut dorsal rootlet (C7-C8), from the cerebral cortex (SI, SII or area 3a, according to the experiment) and from the somesthetic callosal region (SCR). 2. The low-threshold afferent fibres (Group II) of cutaneous origin were found to have a wide projection to the SCR, with the maximal density in its middle portion. Some of the fastest corticocallosal impulses are relayed monosynaptically at cortical level. Plots of the amplitude of cortical and callosal responses as a function of stimulus strength showed that both central responses have the same threshold and exhibit a parallel, sharply-rising amplitude increase, thus suggesting that the cortico-callosal re-transmission system for afferent impulses of cutaneous origin is very powerful in nature. Impulses elicited in afferent fibres of higher threshold (Group III) do not enhance the cortical and callosal positive waves provoked by Group II afferent volleys. 3. Afferent fibres of deep origin were also found to send a wide projection to the SCR, although less substantial than that of cutaneous fibres. Stimulation of the deep radial nerve elicited mass responses in the whole SCR, provided the strength of stimuli was high enough to engage the Group II fibres. Only in the central portion of the SCR were small potentials recorded in response to pure Group I volleys of DRN. Experiments performed with selective stimulation of pure muscular branches of forelimb deep nerves as well as of articular and mixed (muscular and articular) branches gave evidence making it possible to ascertain the origin of deep afferent fibres projecting to the SCR. Stimulation of the forelimb muscular branches with strength provoking full activation of Group I afferent and additional engagement of those of Group II, did not provoke mass responses in the whole extent of the SCR. In order to obtain callosal potentials upon stimulation of pure muscular nerves, it was necessary to increase the stimulus strength at or above the threshold for Group III fibres. On the contrary, the same callosal foci unresponsive to Group I and II muscular afferent volleys exhibited clear-cut responses to stimulation of the lowest-threshold Group I and/or Group II afferents of articular and mixed nerves. From the results it might be inferred that only proprioceptive information originating from articular receptors and from extrafusal muscular afferents has access to the callosal interhemispheric transfer.