H. Van der Loos

Organization of the projections from barrel cortex to thalamus in mice studied with Phaseolus vulgaris-leucoagglutinin and HRP

SummaryIn order to elucidate the geometric organization of projections from the barrel cortex to the thalamus, iontophoretic injections of the anterograde tracer Phaseolus vulgaris-leucoagglutinin were made. The injections were confined to one barrel column (i.e. barrel in layer IV + cortical tissue above and below it). Axonal terminations could be demonstrated in three thalamic nuclei: reticularis (RT), ventrobasalis (VB) and posterior (PO). Anterograde terminal labelling was obtained in RT + VB; in PO only; or in RT + VB + PO. The terminals labelled in PO were much larger than those in RT and VB. The termination areas in RT, VB and PO were shaped like rods which have a rostro-caudal orientation. These cortico-thalamic projections are discretely and topographically organized. The clearest such arrangement was found in VB. Here, projections from the A row of barrels in BF terminate dorsally, whereas those from the C row end ventrally. Barrel A1 projects to the lateral part of VB, whereas A4, to more medial parts; other rows are arranged similarly. These results were compared with the distribution of thalamo-cortical projection neurons that were labelled after iontophoretic HRP injections in individual barrels. We concluded that the corticothalamic projections originating from one barrel column contact an arc of barreloids in VB.

Mouse somatosensory cortex: Alterations in the barrelfield following receptor injury at different early postnatal ages

Abstract The destruction of a row—row C—of follicles of the mystacial vibrissae in 106 mice during early postnatal life caused alterations of the barrels in the contralateral first somatosensory area of the cortex that corresponded to the lesioned sites. It is to be noted that barrel neurones are placed three synapses away from the periphery. Destruction of follicles on postnatal day 0 to 6 inclusive produced alterations of corresponding barrels in the cerebral hemispheres of the animals killed when 12 days old. This suggests that in the development of the somatosensory system of the mouse there is a critical period, during which peripheral insult leads to abnormal cytoarchitectonic cortical patterns. These patterns varied and were categorised, a given type being predominant for a given day of lesion. After lesions on postnatal day 0, a narrow, elongated barrel-like territory occupies the lesion-related domain. After lesions on postnatal day 1, this domain is reduced to an elongated island of cells in disarray, or is absent (barrelless territory). Lesions on postnatal day 2 caused a pattern that is transitional between the barrel less territory and the poorly defined barrels, which are predominant following lesions on postnatal day 3. Lesions on days 4, 5 or 6 produced (abnormally) sharply defined barrels. Whereas all previous modifications were confined to the hemisphere contralateral to the lesion, the last lesion-group displayed enlarged septa in both hemispheres. Wherever the lesion-related domain occupies an area smaller than the normal barrel row which it replaces, the adjacent rows ‘fill in’. Surface area measurements and statistical analysis allowed us to make the following point: (i) After lesions from day 0 to day 3 the surface area of the cortical lesion-related domain was reduced; it was only after lesions on days 2 and 3 that the increase in area of the neighbouring rows offset this reduction. (ii) Alterations due to lesions from birth to day 6 do not follow a gradual progression toward normal, as described by others. Indeed, the greatest reduction of the lesion-related domain was found following lesions on day 1 and not day 0. (iii) Following lesions on days 4, 5 or 6, an increase in area of the lesion-related septa corresponded to a reduction in area of the lesion-related barrels. Tentative interpretations of our findings include collateral sprouting of intact peripheral sensory nerve fibres, degeneration of neurones whose peripheral, sensory terminals have been destroyed, peripheral and central regeneration of lesioned Gasserian neurones, a modified balance in the competition of the terminals for postsynaptic space, atrophy of neurones that receive a decreasing number of terminals or impulses, loss of the remodelling capacity of a centre after its invasion by synapses, stabilisation of early exuberantly distributed callosal terminals, and an altered parcellation of the thalamocortical fibres from the onset of their ingrowth into the cortex.

Organization of feedback and feedforward projections of the barrel cortex: a PHA-L study in the mouse.

SummaryIn order to analyze the organization of the efferent projections of single barrel columns (BC, i.e. a barrel in layer IV of parietal cortex plus the cortical tissue above and below it), we made small iontophoretic injections of the anterograde tracer Phaseolus vulgaris leucoagglutinin in the barrel cortex of 20 adult mice. On the basis of reconstructions of the sites of terminal labelling, the brain regions receiving projections from the barrel cortex could be identified and classified in five groups. Each group is characterized by the topography of the distribution of efferents arising from a single BC. The projections to the trigeminal sensory complex are point to point: i.e. one BC projects only to the site of termination of the primary sensory neurons innervating the corresponding whisker follicle. In the ventrobasal thalamic nucleus BC projections are not restricted to the corresponding barreloid; instead they contact parts of barreloids belonging to one arc. In the reticular and posterior thalamic nuclei the projections from a row of BCs converge to a collective termination site, whereas in the superior colliculus the projections from an arc of BCs converge to a common termination site. There is a complete overlap of BC projections in restricted zones within SII, motor cortex, perirhinal cortex, contralateral barrelfield, caudoputamen and pons. The organization of the efferents from the barrel cortex demonstrates a contrast between feedback and feedforward projections from this important area of neocortex.

Plasticity in the barrel cortex of the adult mouse: effects of peripheral deprivation on GAD-immunoreactivity.

SummaryThe whisker-to-barrel pathway of the adult mouse was used in a study on the effects of peripheral sensory deprivation on GAD-immunoreactivity in the somatosensory cortex. At varying periods of time after removal of a set of vibrissal follicles, mice were processed for immunohistochemistry using an antibody against GAD. In sections tangential to the cortical surface we observed, in the barrels whose follicles were removed, decreased immunoreactivity as early as three days after surgery. The decrease was due to a lesser numerical density of stained puncta and to less intense staining of those remaining. GAD-positive somata were also less intensely stained, whereas their number did not seem to be changed. The changes, apparent at 3 days after the surgery, were restricted to the barrels corresponding to the removed follicles and were maximal at 2–4 weeks. At longer survival times (until 7 months) the immunoreactivity returned to normal, coincident with the regeneration of peripheral nerve fibres in the absence of their follicles. We conclude that GAD-immunoreactivity in the barrel cortex swiftly reacts to modifications of neuronal activity evoked in the periphery.

Altered Sensory Processing in the Somatosensory Cortex of the Mouse Mutant Barrelless

Mice homozygous for the barrelless (brl) mutation, mapped here to chromosome 11, lack barrel-shaped arrays of cell clusters termed “barrels” in the primary somatosensory cortex. Deoxyglucose uptake demonstrated that the topology of the cortical whisker representation is nevertheless preserved. Anterograde tracers revealed a lack of spatial segregation of thalamic afferents into individual barrel territories, and single-cell recordings demonstrated a lack of temporal discrimination of center from surround information. Thus, structural segregation of thalamic inputs is not essential to generate topological order in the somatosensory cortex, but it is required for discrete spatiotemporal relay of sensory information to the cortex.

Ultrastructure of giant and small thalamic terminals of cortical origin: a study of the projections from the barrel cortex in mice using Phaseolus vulgaris leuco-agglutinin (PHA-L).

SummaryBy means of tracing with the lectin Phaseolusvulgaris leucoagglutinin (PHA-L), we examined in the thalamus of the mouse, the axon terminals of fibers originating in the barrel cortex. Vibratome sections of the brain were subjected to PHA-L immunocytochemistry and processed for light and electron microscopy. We observed small (0.5–0.8 μm in diameter) varicosities of labeled fibers in the nucleus ventrobasalis (VB) and the nucleus posterior (PO) as well as labeled giant terminals (3–5 μm in diameter) in PO. The analysis involved examination of serial sections and computer-aided reconstruction of several terminals. The small varicosities in VB appear to be small axon terminals forming distinct asymmetric synapses with small dendritic profiles. Some labeled terminals are apposed to, but not synaptically related with, the cell bodies of neurons in VB that are retrogradely labeled with PHA-L. The small varicosities seen with the light microscope in PO are terminals forming asymmetric synapses with dendritic shafts. The giant terminals in PO appear as large, vesicle-filled profiles forming part of synaptic glomeruli, i.e. complexes of one corticothalamic terminal engulfing several excrescences of a single dendrite. A giant terminal forms several asymmetric synapses (about 8) with these excrescences, as well as numerous (up to 15) puncta adhaerentia. The glomeruli are enveloped in glial lamellae, and they are often found at the bifurcations of primary dendritic segments. We suggest that the small terminals in VB are in the service of feedback signalling from the barrel cortex to its principal thalamic relay nucleus; the functional importance of this projection may reside in increased spatio-temporal discrimination. We interpret the giant terminals in PO as elements serving feed-forward processing, allowing the barrel cortex to influence, via PO, parts of the motor pathway modulating the animals ongoing behavior.

Early lesions of mouse vibrissal follicles: Their influence on dendrite orientation in the cortical barrelfield

SummaryThere is a statistically significant order in the tangential orientation of stellate cell dendrites, both spiny and smooth, in layer IV of the barrelfield of the mouse parietal cortex. Neurones situated in a barrel side have most of their dendrites oriented towards the barrel hollow; those situated in the hollow preferentially orient their dendrites parallel to the long axis of the barrel. A quantitative measure of the orientation of individual dendrites in barrelfields of 60-day old mice was obtained using a semiautomatic computer-microscope and a minicomputer. In the same manner, the dendrite orientation of layer IV stellate cells was determined in barrelfields, whose (cytoarchitectonic) pattern had been experimentally altered through lesions of the middle row of the mystacial vibrissal follicles at birth. The dendrites of these cells are oriented in harmony with the novel parcellation of the cortex. Consequently, for cells in the altered areas of the barrelfields, the dendrite orientation is different from that of cells with identical positions in a normal field (see Fig. 8). We tentatively interpret these findings as an adaptation of dendrite orientation to an altered pattern of thalamic input to layer IV that, in turn, is a consequence of the peripheral manipulation.

Plasticity in the barrel cortex of the adult mouse: transient increase of GAD-immunoreactivity following sensory stimulation

SummarySensory experience during perinatal life and adulthood modifies physiological and anatomical characteristics of the central nervous system. So far, this phenomenon has been studied in situations of complete or partial sensory deprivation. We here report that increased sensory stimulation, during four days, of a number of whisker follicles on the face of the adult mouse results in an increased immunoreactivity of glutamic acid decarboxylase (the biosynthetic enzyme of the inhibitory neurotransmitter GABA) in the somatosensory cortex of the adult mouse. Effects were limited to a column of tissue corresponding to the representation of the stimulated follicles and lasted two days beyond stimulation. These findings suggest that sensory stimulation transiently modifies local cortical processing.

Monoaminergic afferents to cortex modulate structural plasticity in the barrelfield of the mouse

Electrolytic lesions of the follicles of a set of mystacial vibrissae, and their innervation, of the mouse placed during the early postnatal period result in a modification in appearance of the corresponding and of adjacent barrels in the somatosensory cortex of the adult animal. These changes can be evoked during the first 6 days of postnatal life--the so-called critical period. The pattern of these modifications varies with the age of the animal at which the lesion was placed. In order to evaluate the contribution of the monoaminergic cortical input to this type of plasticity, the noradrenergic and/or serotonergic afferents to the cerebral cortex of newborn mice were destroyed by systemic administration of various selective neurotoxic drugs (6-hydroxydopamine, 5,7-dihydroxytryptamine, N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine). The animals were then subjected, on postnatal day 3 (P3; P0 = day of birth), to a lesion of the follicles of the large, caudal mystacial vibrissae of row C. Control animals were injected with vehicle solution only but had the same follicles lesioned. Compared with animals with intact monoaminergic afferents, those treated with neurotoxins showed a different changed barrel pattern, i.e. one that corresponded to a pattern normally obtained after a lesion placed at an earlier stage of development, i.e. at P2 or P1. Thus, monoaminergic depletion of the cortex results in a retardation of the maturation of the parietal cortex as defined by its plastic response to peripheral nerve injury.(ABSTRACT TRUNCATED AT 250 WORDS)

Tangential orientation and spatial order in dendrites of cat auditory cortex: A computer microscope study of Golgi-impregnated material

SummaryIn the tangential plane (parallel to the pial surface) dendrites in the primary auditory cortex (A1) of cat were found to exhibit preferentially oriented growth. This was shown by means of a computer microscope study of Golgi-Cox stained neurons as seen in 100 μm and 300 μm thick tangential sections. Two techniques were used to represent the 3-dimensional structure of dendrites: the “dendritic stick” and the “dendritic trumpet”. The former dismembers a dendrite into its individual segments; the latter considers a dendrite as an entity and represents it by its centroid, its moments and the spatial dispersion of its branches. Both statistical and Fourier analyses of the data show that within the tangential plane there is a significant and consistent orientation of the dendritic sticks in a dorso-ventral direction which seems correlated with the cortical isofrequency contours observed in electrophysiological maps of the A1 region. The dendritic trumpet analyses also show a distinctly non-random vertical distribution of pyramidal cell basal dendrites but not of stellate cell dendrites.