Thalamocortical Projections Research Articles

Auditory thalamic nuclei projections to the temporal cortex in the rat

Thalamocortical projections from the auditory thalamic nuclei were examined systematically in the rat, including those from the dorsal division (MGD) of the medial geniculate body (MG), which were less clearly determined in previous studies. Injections of biocytin confined in each thalamic nucleus revealed characteristic features of projections in terms of cortical areas and layers of termination. In contrast to exclusively selective projections to cortical area Te1 from the ventral division (MGV) of the MG, diffuse and selective terminations were observed in the projections from the dorsal (MGD) and medial divisions (MGM) of the MG and the suprageniculate nucleus (SG). Diffuse termination was continuous in layer I or VI of the temporal cortex, while selective termination was in layers III and IV of discrete cortical areas. In addition to diffuse termination in the upper half of layer I of cortical areas Te1, Te2d and Te3v, the MGD and SG projections formed plexuses of axons selectively in lower layer III and layer IV of Te2d and Te3v. The SG projections targeted further the dorsal bank of the perirhinal cortex (PRh), while the MGD projections targeted in part the ventral fringe of Te1. The MGM projections terminated diffusely in layer VI of Te1 and Te3v, and selectively in lower layer III and layer IV of the rostral part of Te3v. Diffuse projections to layers I and VI from the SG and MGM extended in cortical regions over the dorsal fringe of Te1. Selective dense projections to middle cortical layers of Te2d and Te3v (especially its rostral part) indicate the existence of auditory areas, which could be involved in cross-modal interaction with visual and somatosensory system, respectively. Diffuse projections are supposed to bind information processings in these areas and the primary auditory area (Te1).

Dual representation of pain in the operculo-insular cortex in humans.

We report the response properties of the suprasylvian opercular and insular cortices to a painful stimulation delivered by a CO(2) laser recorded by depth intracerebral electrodes in epileptic patients. We defined two cortical areas of activation in the operculo-insular cortex in response to a painful laser stimulation: a suprasylvian opercular area, where we recorded responses peaking 140-170 ms after a painful stimulation (N140-P170), and a deeper insular area, where responses with a similar pattern peaked 180-230 ms after the stimulus (N180-P230). The average delay of 50 ms measured between the opercular and insular responses may reflect either sequential activation of the suprasylvian cortex then of the insula via corticocortical connections, or direct activation of the insula by inputs conveyed via thalamocortical projections through distinct fibres with different conduction times. We also recorded similar insular and opercular responses in the hemisphere ipsilateral to the stimulation, peaking 15 ms later than contralateral responses; this delay is compatible with transcallosal input transmission between these cortices. The mean stereotactic coordinates of the suprasylvian opercular N140-P170 and insular N180-P230 responses were found to be very similar to those of the maximal blood-flow responses to pain reported by previous PET and functional MRI studies in these cortical areas. We were able to distinguish the suprasylvian opercular and insular cortices in terms of response latencies evoked by a painful stimulus and in terms of stereotactic coordinates of the sources of these responses. The sequential timing of activation of the suprasylvian and insular cortices shown in this study thus complements in the time domain the spatial information provided by neuroimaging studies of the cortical processing of pain. It strongly suggests that these cortical areas are those responding with the shortest latency to peripheral pain inputs in the human brain.

The impact of thalamo-cortical projections on activity spread in cortex

To what extent does thalamo-cortical connectivity influence global patterns of macroscopical activity spread in cat cortex? We address this question by simulation of activity flow in a network model representing cortical areas and thalamic nuclei as nodes with all or none activity. The connectivity schemes employed are based on data from anatomical tracer studies. For two networks, with and without thalamic nuclei, we investigate how well physiological experiments from strychnine neuronography can be reproduced. Our results indicate that thalamic interaction does influence cortical activity spread. However, while the relation between physiological activity spread and anatomy of the cortico–cortical connectivity is significant, the relation between activity spread and the anatomy of cortico–thalamic connections is not.

Unilateral pallidotomy for Parkinson’s disease disrupts ocular fixation

Although some motor functions of the basal ganglia have been well studied, the oculomotor functions are not well established. We studied eye movements in patients with Parkinson’s disease (PD) undergoing pallidotomy to assess the role of the globus pallidus interna (GPi) in oculomotor control. Horizontal visually guided, gap and predictive saccades as well as ocular fixation were studied in patients with advanced PD before and 1 month after unilateral pallidotomy, and in healthy controls on two occasions 1 month apart. There was no difference in saccadic latency or accuracy, the number of saccadic anticipations or the ability to generate predictive saccades between the two assessments for either patients or controls. The number and amplitude of square wave jerks during ocular fixation however increased significantly in patients after pallidotomy. The results imply altered function of frontal or prefrontal cortical regions involved in ocular fixation resulting from a disruption to inhibitory pallidal influences on thalamocortical projections. The posteroventral GPi however appears not to be involved in externally controlled or predictive saccadic function.

Functional Subregions in Primary Auditory Cortex Defined by Thalamocortical Terminal Arbors: An Electrophysiological and Anterograde Labeling Study

Several functional maps have been described in primary auditory cortex, including those related to frequency, tuning, latency, binaurality, and intensity. Many of these maps are arranged in a discontinuous or patchy manner. Similarly, thalamocortical projections arising from the ventral division of the medial geniculate body to the primary auditory cortex are also patchy. We used anterograde labeling and electrophysiological methods to examine the relationship between thalamocortical patches and auditory cortical maps. Biotinylated dextran-amine was deposited into physiologically characterized sites in the ventral division of the medial geniculate body of New Zealand white rabbits. Approximately 7 d later, the animal was again anesthetized and the ipsilateral auditory cortex was mapped with tungsten microelectrodes. Multi-unit physiological data were obtained for the following characteristics: best frequency (BF), binaurality, response type, latency, sharpness of tuning, and threshold. Immunocytochemical methods were used to reveal the injection site in the ventral division of the medial geniculate body as well as the anterogradely labeled thalamocortical afferents in the auditory cortex. In 86% of the cases (12 of 14), entry into a thalamocortical patch was associated with a marked change in physiological responses. A consistent BF and binaural class were usually observed within a patch. The patches appear to innervate distinct functional regions coding frequency and binaurality. A model is presented showing how patchy thalamocortical projections participate in the formation of tonotopic and binaural maps in primary auditory cortex.

Analysis of the structural bases of information processing in the basal ganglia: the spatial organization of thalamocortical projections in the dog brain.

Analysis of the architectonics of the thalamic projection systems of the caudate nucleus, putamen, and nucleus accumbens using axonal transport of retrograde markers was performed to study the morphological substrate for information processing in the striatum of the dog brain. The striatal nuclei contained areas through which segregated conduction of motor (dorsal segments of the caudate nucleus and putamen) and limbic (medial segment of the nucleus accumbens) information could occur, along with areas (ventral segments of the caudate nucleus and putamen, lateral segment of the nucleus accumbens) in which convergent conduction of functionally diverse information processed in the thalamic nuclei could occur. These data make a significant contribution to our understanding of the anatomical aspects of both the functional heterogeneity of the basal ganglia and integrative information processing occurring within them, which underlies the mechanisms organizing adaptive behavior.

Arousal systems.

The brain contains autochthonous neural systems that evoke waking from sleep in response to sensory stimuli, prolong or enhance arousal in response to special stimuli, and also generate and maintain wakefulness regardless of sensory stimuli during the active part of the day. Through ascending projections to the cortex, these arousal systems stimulate cortical activation, characterized by high frequency gamma and low frequency rhythmic theta activity, and through descending projections to the spinal cord, they stimulate muscle tonus along with sensory-motor responsiveness and activity. They are comprised of neuronal aggregates within the brainstem reticular formation, thalamus, posterior hypothalamus and basal forebrain, and they utilize multiple different neurotransmitters. Within the brainstem, neurons of the reticular formation, which predominantly utilize glutamate as a neurotransmitter, stimulate cortical activation by exciting the widespread projecting neurons of the nonspecific thalamo-cortical projection system, which similarly utilize glutamate, and neurons of the ventral extra-thalamic relay systems located in the posterior hypothalamus and basal forebrain, many of which also utilize glutamate. In addition, these systems have descending projections by which they can enhance or modulate muscle tonus and activity. Articulating with these are cholinergic neurons of the ponto-mesencephalic tegmentum and basal forebrain that promote cortical activation during waking and also during rapid eye movement sleep (REMS), in association therein with muscle atonia. Dopaminergic ventral mesencephalic neurons stimulate a highly motivated and positively rewarding state during waking and may also do so during REMS. In contrast, noradrenergic locus coeruleus neurons promote an aroused waking state and prevent REMS as well as slow wave sleep (SWS). Serotonergic raphe neurons promote a seemingly quiet or satiated waking state, which though exclusive of REMS, can actually be conducive to SWS. Histaminergic neurons of the posterior hypothalamus act like noradrenergic neurons in enforcing waking and are joined by neurons in the region that contain orexin, a neuropeptide recently shown to maintain waking and in absentia to be responsible for narcolepsy, or the inability to maintain wakefulness. These multiple arousal systems are grossly redundant, since no one system is absolutely necessary for the occurrence of waking; yet they are differentiated, since each plays a special role in waking and sleep. During SWS, they are submitted to an inhibitory influence arising in part at least from particular GABAergic neurons co-distributed with many neurons of the arousal systems and also concentrated within the basal forebrain and adjacent preoptic region.

The early topography of thalamocortical projections is shifted in Ebf1 and Dlx1/2 mutant mice.

The prevailing model to explain the formation of topographic projections in the nervous system stipulates that this process is governed by information located within the projecting and targeted structures. In mammals, different thalamic nuclei establish highly ordered projections with specific neocortical domains and the mechanisms controlling the initial topography of these projections remain to be characterized. To address this issue, we examined Ebf1(-/-) embryos in which a subset of thalamic axons does not reach the neocortex. We show that the projections that do form between thalamic nuclei and neocortical domains have a shifted topography, in the absence of regionalization defects in the thalamus or neocortex. This shift is first detected inside the basal ganglia, a structure on the path of thalamic axons, and which develops abnormally in Ebf1(-/-) embryos. A similar shift in the topography of thalamocortical axons inside the basal ganglia and neocortex was observed in Dlx1/2(-/-) embryos, which also have an abnormal basal ganglia development. Furthermore, Dlx1 and Dlx2 are not expressed in the dorsal thalamus or in cortical projections neurons. Thus, our study shows that: (1) different thalamic nuclei do not establish projections independently of each other; (2) a shift in thalamocortical topography can occur in the absence of major regionalization defects in the dorsal thalamus and neocortex; and (3) the basal ganglia may contain decision points for thalamic axons' pathfinding and topographic organization. These observations suggest that the topography of thalamocortical projections is not strictly determined by cues located within the neocortex and may be regulated by the relative positioning of thalamic axons inside the basal ganglia.

Developmental expression of EphA4-tyrosine kinase receptor in the mouse brain and spinal cord

Eph receptor tyrosine kinases and their ephrin ligands are involved in some of the most important steps during the development of the central nervous system, including cell migration, axon guidance, topographic mapping and synapse formation. Moreover, in the adult, they have been implicated in plasticity and regulation of neural stem cell function. One member of the Eph family, EphA4, can bind to both classes of ephrins and may have multiple functions in nervous system development. In order to look for potential sites of EphA4 action during central nervous system development, we conducted a spatio-temporal analysis of EphA4 protein expression. We used immunohistochemistry in preference to in situ hybridization because of the high likelihood that EphA4 protein is expressed on axon tracts, long distances from EphA4 mRNA. In the telencephalon, EphA4 was expressed in the developing cortex from embryonic day 11 (E11) and, later, on major cortical tracts including the corpus callosum and cortico-spinal tract. Robust EphA4 expression was also found in the hippocampus and fornix, and cells and tracts in the striatum. In the diencephalon, the thalamus, the hypothalamus and thalamo-cortical projection were strongly positive. In the mesencephalon, a number of different nuclei were weakly positive, most prominently the red nucleus. In the rhombencephalon, many nuclei were strongly positive including the cerebellum and one of its afferent paths, the inferior cerebellar peduncle, as well as the olivary region. In the spinal cord, there was a dynamic pattern of expression through development, with persistent expression in the dorsal funiculus and ventral grey matter.

Normal Development of Embryonic Thalamocortical Connectivity in the Absence of Evoked Synaptic Activity

This study is concerned with the role of impulse activity and synaptic transmission in early thalamocortical development. Disruption of the gene encoding SNAP-25, a component of the soluble N-ethylmaleimide-sensitive factor attachment protein (SNAP) receptor complex required for regulated neuroexocytosis, eliminates evoked but not spontaneous neurotransmitter release (Washbourne et al., 2002). The Snap25 null mutant mouse provides an opportunity to test whether synaptic activity is required for prenatal neural development. We found that evoked release is not needed for at least the gross formation of the embryonic forebrain, because the major features of the diencephalon and telencephalon were normal in the null mutant mouse. However, half of the homozygous mutants showed undulation of the cortical plate, which in the most severely affected brains was accompanied by a marked reduction of calbindin-immunoreactive neurons. Carbocyanine dye tracing of the thalamocortical fiber pathway revealed normal growth kinetics and fasciculation patterns between embryonic days 17.5 and 19. As in normal mice, mutant thalamocortical axons reach the cortex, accumulate below the cortical plate, and then start to extend side-branches in the subplate and deep cortical plate. Multiple carbocyanine dye placements in the cortical convexity revealed normal overall topography of both early thalamocortical and corticofugal projections. Electrophysiological recordings from thalamocortical slices confirmed that thalamic axons were capable of conducting action potentials to the cortex. Thus, our data suggest that axonal growth and early topographic arrangement of these fiber pathways do not rely on activity-dependent mechanisms requiring evoked neurotransmitter release. Intercellular communication mediated by constitutive secretion of transmitters or growth factors, however, might play a part.

Prenatal development of neural excitation in rat thalamocortical projections studied by optical recording

To elucidate the formation of early thalamocortical synapses we recorded optical images with voltage-sensitive dyes from the cerebral cortex of prenatal rats by selective thalamic stimulation of thalamocortical slice preparations. At embryonic day (E) 17, thalamic stimulation elicited excitation that rapidly propagated through the internal capsule to the cortex. These responses lasted less than 15 ms, and were not affected by the application of glutamate receptor antagonists, suggesting that they might reflect presynaptic fiber responses. At E18, long-lasting (more than 300 ms) responses appeared in the internal capsule and in subplate. By E19, long-lasting responses increased in the cortical subplate. By E21, shortly before birth, the deep cortical layers were also activated in addition to the subplate. These long-lasting responses seen in the internal capsule and subplate were blocked by the antagonist perfusion, but the first spike-like responses still remained. The laminar location of the responses was confirmed in the same slices by Nissl staining and subplate cells were labeled by birthdating with bromodeoxyuridine at E13. Our results demonstrate that there is a few days delay between the arrival of thalamocortical axons at the subplate at E16 and the appearance of functional thalamocortical synaptic transmission at E19. Since thalamocortical connections are already functional within the subplate and in the deep cortical plate at embryonic ages, prenatal thalamocortical synaptic connections could influence cortical circuit formation before birth.

Few intrinsic connections cross the hand-face border of area 3b of New World monkeys.

After long-standing loss of afferents from the hand, the hand representation in area 3b of the somatosensory cortex of monkeys becomes responsive to touch on the face. Because the reactivation of deprived hand cortex by the face inputs could depend on axonal connections across the hand-face border, we determined the extent of such connections in New World marmosets, owl monkeys, and squirrel monkeys. Small injections of anatomic tracers were placed in the hand or the face representations after these representations were identified by microelectrode recordings. The positions of retrogradely labeled neurons were plotted in processed brain sections cut parallel to the brain surface, and their locations were related to anatomic isomorphs of the hand and face representations revealed in adjacent brain sections stained for myelin. In these sections, the hand-face border was clearly visualized as a myelin-poor septum. The intrinsic connections of area 3b labeled by injections in either the hand or face representations were almost completely confined to their respective representation, and very few neurons projected across the border. In addition, neurons in the somatosensory thalamus labeled by injections in either face or hand representations were confined to either VPM, representing the face, or the hand subnucleus of VPL. Thus the reactivation of hand cortex by face stimulation does not depend on a previously existing network of intrinsic cortical connections across the hand-face border, or mismatched thalamocortical projections.

Miswiring of Limbic Thalamocortical Projections in the Absence of Ephrin-A5

Axon guidance cues of the ephrin ligand family have been hypothesized to regulate the formation of thalamocortical connections, but in vivo evidence for such a role has not been examined directly. To test whether ephrin-mediated repulsive cues participate in sorting the projections originating from distinct thalamic nuclei, we analyzed the organization of somatosensory and anterior cingulate afferents postnatally in mice lacking ephrin-A5 gene expression. Projections from ventrobasal and laterodorsal nuclei to their respective sensory and limbic cortical areas developed normally. However, a portion of limbic thalamic neurons from the laterodorsal nucleus also formed additional projections to somatosensory cortical territories, thus maintaining inappropriate dual projections to multiple cortical regions. These results suggest that ephrin-A5 is not required for the formation of normal cortical projections from the appropriate thalamic nuclei, but rather acts as a guidance cue that restricts limbic thalamic axons from inappropriate neocortical regions.

Projection neurons and interneurons in the lateral geniculate nucleus undergo distinct forms of degeneration ranging from retrograde and transsynaptic apoptosis to transient atrophy after cortical ablation in rat

The cytological responses of thalamic interneurons to selective degeneration of thalamocortical projection neurons after cortical damage in the adult brain are poorly understood. We used a unilateral neocortical lesion model (occipital cortex ablation) in the adult rat to test the hypothesis that interneurons and projection neurons in the lateral geniculate nucleus undergo distinct forms of degeneration. In situ nuclear DNA fragmentation in neurons in the lateral geniculate occurs maximally at 7 days postlesion. Geniculocortical projection neurons that are identified by the retrograde tracer Fluorogold die primarily with a morphology of endstage apoptosis prominent at 7 days postlesion. In contrast, interneurons, identified by their particular nuclear ultrastructure and by glutamic acid decarboxylase immunoreactivity, undergo an atrophic vacuolar pathology starting early during the period of projection neuron death and peaking after the projection neuron death is complete. This degeneration of interneurons is transient, because these neurons exhibit structural recovery and their numbers are not changed significantly postlesion. A rare subset of interneurons (less than one in 100 interneurons and less than one in 100 apoptotic cells) undergoes apoptosis concurrently with the projection neurons. We conclude that different types of neurons within the same thalamic nucleus respond differently to focal cortical target deprivation. Unlike the apoptosis-prone projection neurons, most interneurons undergo transient transsynaptic atrophy and recovery rather than cell death. Nevertheless, a small subset of lateral geniculate interneurons undergoes transsynaptic apoptosis in response to projection neuron apoptosis. The pathological responses of thalamic neurons to cortical trauma vary depending on cell type.

Thalamic Relay Nuclei of the Basal Ganglia Form Both Reciprocal and Nonreciprocal Cortical Connections, Linking Multiple Frontal Cortical Areas

Thalamic relay nuclei transmit basal ganglia output to the frontal cortex, forming the last link in corticobasal ganglia circuitry. The thalamus regulates cortical activity through differential laminar connections, providing not only feedback, but also initiating "feedforward" loops, via nonreciprocal projections, that influence higher cortical areas. This study examines the organization of thalamic connections with cortex from basal ganglia relay nuclei, including ventral anterior (VA), ventral lateral (VL), and mediodorsal (MD) nuclei, in the Macaque monkey. Anterograde and bidirectional tracer injections ([3H]-amino acids, dextran conjugates of Fluorescein, Lucifer Yellow or FluoroRuby, or wheat germ agglutinin) into discrete VA/VL, MD, and frontal cortical sites demonstrate specific thalamocortical connections. VL projections target caudal motor areas (primary, supplementary, and caudal premotor areas), whereas VA projections target more rostral premotor areas (including cingulate and presupplementary motor areas) and MD projects to dorsolateral and orbital prefrontal cortices. Thalamocortical projections innervate cortical layers I and III, and to a lesser extent, layer V. In motor areas layer I projections are more extensive than those to layer III (and V). The complex laminar organization of projections from specific thalamic sites suggests differential regulation of cortical function. Injections of bidirectional tracers into thalamic and frontal cortical sites also show that in comparison to thalamocortical projections, corticothalamic projections to VA-VL and MD are more widespread. These findings demonstrate both reciprocal and nonreciprocal components to the thalamo-cortico-thalamic relay. Together, these experiments indicate a dual role for VA-VL and MD nuclei: (1) to relay basal ganglia output within specific cortical circuits and (2) to mediate information flow between cortical circuits.

Selective Action of Orexin (Hypocretin) on Nonspecific Thalamocortical Projection Neurons

As is evident from the pathological consequences of its absence in narcolepsy, orexin (hypocretin) appears to be critical for the maintenance of wakefulness. Via diffuse projections through the brain, orexin-containing neurons in the hypothalamus may act on a number of wake-promoting systems. Among these are the intralaminar and midline thalamic nuclei, which project in turn in a widespread manner to the cerebral cortex within the nonspecific thalamocortical projection system. Testing the effect of orexin in rat brain slices, in two nuclei of this system, centromedial (CM) nuclei and rhomboid nuclei, we found that it depolarized and excited all neurons tested through a direct postsynaptic action. An additional analysis of this effect in CM neurons indicates that it results from the decrease of a potassium conductance. By a detailed comparison of the effects of orexin A and B, we established that orexin B was more potent than orexin A, indicating the probable mediation by orexin type 2 receptors. In contrast to its effect on the nonspecific thalamocortical projection neurons, orexin had no effect on the specific sensory relay neurons of the somatic, ventral posterolateral, and visual dorsal lateral geniculate nuclei. Orexin differs in this regard from norepinephrine and acetylcholine, to which neurons in the specific and nonspecific systems are sensitive. Orexin may thus act in the thalamus to promote wakefulness by exciting neurons of the nonspecific thalamocortical projection system, which, through widespread projections to the cerebral cortex, stimulate and maintain cortical activation.

Basal-ganglia 'projections' to the prefrontal cortex of the primate.

We used retrograde transneuronal transport of the McIntyre-B strain of herpes simplex virus type 1 to examine the extent and organization of basal-ganglia-thalamocortical projections to five regions of prefrontal cortex in the cebus monkey (Cebus apella): medial and lateral area 9 (9m and 9l), dorsal and ventral area 46 (46d and 46v) and lateral area 12 (12l). All of these prefrontal areas were found to be targets of basal-ganglia output that originated in the internal segment of the globus pallidus (GPi) and/or the pars reticulata of the substantia nigra (SNpr). Approximately one-third of the total volume of these nuclei was directed toward prefrontal cortex, a volume comparable to that directed at the cortical motor areas. The origins of the outputs to different prefrontal areas were topographically organized. Different portions of SNpr (the rostral and caudal thirds) projected to areas 9m and 12l. Similarly, different output nuclei (GPi and SNpr) projected to adjacent portions of the same cytoarchitectonic field (46d and 46v). Furthermore, the outputs to prefrontal areas were segregated from those to motor areas of cortex. Thus, basal-ganglia outputs to prefrontal cortex are both extensive and topographically organized, forming a rich anatomical substrate for basal-ganglia influences on the cognitive operations of the frontal lobe.

Distinct Actions ofEmx1,Emx2, andPax6in Regulating the Specification of Areas in the Developing Neocortex

The mammalian neocortex is organized into subdivisions referred to as areas that are distinguished from one another by differences in architecture, axonal connections, and function. The transcription factors EMX1, EMX2, and PAX6 have been proposed to regulate arealization. Emx1 and Emx2 are expressed by progenitor cells in a low rostrolateral to high caudomedial gradient across the embryonic neocortex, and Pax6 is expressed in a high rostrolateral to low caudomedial gradient. Recent evidence has suggested that EMX2 and PAX6 have a role in the genetic regulation of arealization. Here we use a panel of seven genes (Cad6, Cad8, Id2, RZRbeta, p75, EphA7, and ephrin-A5) representative of a broad range of proteins as complementary markers of positional identity to obtain a more thorough assessment of the suggested roles for EMX2 and PAX6 in arealization, and in addition to assess the proposed but untested role for EMX1 in arealization. Orderly changes in the size and positioning of domains of marker expression in Emx2 and Pax6 mutants strongly imply that rostrolateral areas (motor and somatosensory) are expanded, whereas caudomedial areas (visual) are reduced in Emx2 mutants and that opposite effects occur in Pax6 mutants, consistent with their opposing gradients of expression. In contrast, patterns of marker expression, as well as the distribution of area-specific thalamocortical projections, appear normal in Emx1 mutants, indicating that they do not exhibit changes in arealization. This lack of a defined role for EMX1 in arealization is supported by our finding of similar shifts in patterns of marker expression in Emx1; Emx2 double mutants as in Emx2 mutants. Thus, our findings indicate that EMX2 and PAX6 regulate, in opposing manners, arealization of the neocortex and impart positional identity to cortical cells, whereas EMX1 appears not to have a role in this process.

Reduction of early thalamic input alters adult corticocortical connectivity

The functional specificity of mammalian isocortex requires that precise connections be established between cortical areas and their targets. While recent studies of cortical development have focused on intrinsic specification, the role of extrinsic factors has received considerably less attention. In the present study, we examined how early removal of thalamic input affects the development of visual corticocortical connections. Hamster pups received ablations of visual thalamic nuclei on the day of birth. At 30 days of age, an injection of horseradish peroxidase (HRP) was placed into the area of cortex deafferented by the early thalamic ablation to retrogradely label adult corticocortical connections. Ablated animals displayed a significant increase in the number of corticocortical connections compared to control animals. The increased connectivity in ablated animals was primarily due to a significant increase in the number of corticocortical projections arising from non-visual areas. These results demonstrate that an intact thalamocortical projection is necessary for the development of normal cortical connectivity.

Ephrins regulate the formation of terminal axonal arbors during the development of thalamocortical projections.

The development of connections between thalamic afferents and their cortical target cells occurs in a highly precise manner. Thalamic axons enter the cortex through deep cortical layers, then stop their growth in layer 4 and elaborate terminal arbors specifically within this layer. The mechanisms that underlie target layer recognition for thalamocortical projections are not known. We compared the growth pattern of thalamic explants cultured on membrane substrates purified from cortical layer 4, the main recipient layer for thalamic axons, and cortical layer 5, a non-target layer. Thalamic axons exhibited a reduced growth rate and an increased branching density on their appropriate target membranes compared with non-target substrate. When confronted with alternating stripes of both membrane substrates, thalamic axons grew preferentially on their target membrane stripes. Enzymatic treatment of cortical membranes revealed that growth, branching and guidance of thalamic axons are independently regulated by attractive and repulsive cues differentially expressed in distinct cortical layers. These results indicate that multiple membrane-associated molecules collectively contribute to the laminar targeting of thalamic afferents. Furthermore, we found that interfering with the function of Eph tyrosine kinase receptors and their ligands, ephrins, abolished the preferential branching of thalamic axons on their target membranes, and that recombinant ephrin-A5 ligand elicited a branch-promoting activity on thalamic axons. We conclude that interactions between Eph receptors and ephrins mediate branch formation of thalamic axons and thereby may play a role in the establishment of layer-specific thalamocortical connections.