Afferents In Layer Research Articles

Lateral geniculate neurons projecting to primary visual cortex show ocular dominance plasticity in adult mice.

Experience-dependent plasticity in the mature visual system is widely considered to be cortical. Using chronic two-photon Ca2+ imaging of thalamic afferents in layer 1 of binocular visual cortex, we provide evidence against this tenet: the respective dorsal lateral geniculate nucleus (dLGN) cells showed pronounced ocular dominance (OD) shifts after monocular deprivation in adult mice. Most (86%), but not all, of dLGN cell boutons were monocular during normal visual experience. Following deprivation, initially deprived-eye-dominated boutons reduced or lost their visual responsiveness to that eye and frequently became responsive to the non-deprived eye. This cannot be explained by eye-specific cortical changes propagating to dLGN via cortico-thalamic feedback because the shift in dLGN responses was largely resistant to cortical inactivation using the GABAA receptor agonist muscimol. Our data suggest that OD shifts observed in the binocular visual cortex of adult mice may at least partially reflect plasticity of eye-specific inputs onto dLGN neurons.

Cortical dopaminergic innervation among humans, chimpanzees, and macaque monkeys: A comparative study

In this study, we assessed the possibility that humans differ from other primate species in the supply of dopamine to the frontal cortex. To this end, quantitative comparative analyses were performed among humans, chimpanzees, and macaques using immunohistochemical methods to visualize tyrosine hydroxylase–immunoreactive axons within the cerebral cortex. Axon densities and neuron densities were quantified using computer-assisted stereology. Prefrontal areas 9 and 32 were chosen for evaluation due to their roles in higher-order executive functions and theory of mind, respectively. Primary motor cortex (area 4) was also evaluated because it is not directly associated with cognition. We did not find an overt quantitative increase in cortical dopaminergic innervation in humans relative to the other primates examined. However, several differences in cortical dopaminergic innervation were observed among species which may have functional implications. Specifically, humans exhibited a sublaminar pattern of innervation in layer I of areas 9 and 32 that differed from that of macaques and chimpanzees. Analysis of axon length density to neuron density among species revealed that humans and chimpanzees together deviated from macaques in having increased dopaminergic afferents in layers III and V/VI of areas 9 and 32, but there were no phylogenetic differences in area 4. Finally, morphological specializations of axon coils that may be indicative of cortical plasticity events were observed in humans and chimpanzees, but not macaques. Our findings suggest significant modifications of dopamine's role in cortical organization occurred in the evolution of the apes, with further changes in the descent of humans.

Altered zincergic innervation of the developing primary somatosensory cortex in monoamine oxidase-A knockout mice

Genetic inactivation of monoamine oxidase-A (MAO-A) significantly elevates levels of serotonin (5-HT) during early development and causes a disruption in the compartmented organization of thalamocortical axon terminals in layer 4 of the somatosensory cortex. In order to determine whether corticocortical innervation of the primary somatosensory cortex is also affected by this mutation, we examined the distribution of zinc-containing axon terminals (terminals known to originate from within the cortex) in the developing somatosensory cortex of MAO-A knockout mice, at postnatal days (PD) 3, 5, 6, 8, 10, 12, 15, 28, and 60. In layer 4 of wild-type mice, histochemical staining for zinc respected barrel-specific compartments at all ages beyond PD 5. By contrast, zinc staining in MAO-A knockout mice did not exhibit signs of barrel compartmentation at any age. Across cortical layers, substantial developmental changes in the distribution of zinc-containing terminals were observed in wild-type mice up until PD 12, at which time the mature lamina-specific pattern of zinc staining was achieved. Similar changes were observed in the somatosensory cortex of MAO-A knockout mice, except that its developmental time course was significantly compressed, with zincergic innervation achieving a mature appearance by PD 8. These results provide evidence that an excess of monoamines, most likely 5-HT, dramatically perturbs the columnar organization of intracortical zincergic afferents in layer 4 and significantly accelerates the appearance of a mature laminar pattern of zinc-containing corticocortical terminals.

TrkB-like immunoreactivity is present on geniculocortical afferents in layer IV of kitten primary visual cortex.

Exogenous administration of the neurotrophins brain-derived neurotrophic factor (BDNF) or neurotrophin-4/5 (NT-4/5), or blockade of their endogenous actions, have been reported to affect the anatomic organization and physiological responses of neurons in developing mammalian primary visual cortex. Experimental alteration of levels of these neurotrophic factors can also influence the morphology of the geniculocortical afferents that project from the lateral geniculate nucleus (LGN) to primary visual cortex. BDNF and NT-4/5 are ligands of the TrkB tyrosine kinase receptor. Although multiple populations of cortical neurons express TrkB, it is not known whether geniculocortical afferents express this receptor on their axon branches in visual cortex. We have anatomically labeled geniculocortical afferents of postnatal day 40 kittens with the anterograde neuronal tracer Phaseolus vulgaris leucoagglutinin (PHA-L) and performed double-label immunofluorescence with a panel of anti-TrkB antibodies. Confocal microscopy and object-based colocalization analysis were used to measure levels of TrkB-like immunoreactivity (IR) on geniculocortical afferents in layer IV of primary visual cortex. By using a conservative analysis involving a comparison of measured colocalization with the amount of colocalization expected based on random overlap of TrkB puncta and PHA-L--labeled afferents, 3 of 5 anti-TrkB antibodies tested showed significant colocalization with the geniculocortical axons. Results for the other two antibodies were indeterminate. The indices obtained for colocalization of TrkB and geniculocortical afferents were also compared with the equivalent index obtained for GAD65, a protein that has a similar overall expression pattern to that of TrkB but is not expressed on geniculocortical axons. This analysis indicated that TrkB was present on geniculocortical axons for all five TrkB antibodies tested. TrkB-like IR was also observed on neuronal somata in the LGN. These results indicate that TrkB receptors on geniculocortical afferents are potential mediators of the actions of BDNF and NT-4/5 in developing visual cortex.

Isthmotectal connections in plethodontid salamanders

In the plethodontid salamander species Plethodon jordani and Hydromantes italicus, the morphology and connectivity of isthmic cells were investigated by means of intracellular staining with biocytin. Dendritic arborization, axonal pathways, and size and morphology of telodendritic structures in the fiber layers of the optic tectum were determined. The latter were studied electron microscopically. The majority of isthmic neurons project to both tectal hemispheres, each cell forming telodendritic structures of different extent in the ipsilateral and contralateral hemisphere. These structures are column-like with diameters of about 60 microm. The ipsilateral terminals extend through layers 1-3 of the tectal white matter and intermingle with terminals of contralateral retinal afferents in layers 1-3, as well as with ipsilateral retinal afferents in layer 3. The corresponding contralateral telodendritic structures are confined to layer 1 and are not in direct contact with ipsilateral retinal afferents in layer 3. Both telodendritic structures are located in the rostral two-thirds of the optic tectum, which are binocularly innervated. The rostrocaudal and mediolateral sites of isthmic telodendra are in register with the direct retinal map in each tectal hemisphere. Few isthmic cells project to the caudal one-third of the optic tectum, which is monocularly driven. These cells form only one axonal terminal in the ipsilateral tectal hemisphere. Size and structure of these ipsilateral terminals are similar to the ipsilateral telodendra of bilaterally projecting isthmic neurons.

Timecourse of development of the wallaby trigeminal pathway: III. Thalamocortical and corticothalamic projections.

The development of trigeminal projections between the thalamus and cortex has been investigated in the marsupial mammal, the wallaby, by using a carbocyanine dye, horseradish peroxidase conjugated to wheat germ agglutinin (WGA-HRP), Neurobiotin, and biocytin as pathway tracers. The appearance of whisker-related patterns in the cortex in relation to their appearance in the brainstem and thalamus was examined, as was the presence or absence of a waiting period for thalamocortical afferents and the identity of the first cortical cells to project to the thalamus. Thalamic afferents first reached the cortex at postnatal day (P) 15 and were distributed up to the deep edge of the compact cell zone in the superficial cortical plate throughout development, in both dye and WGA-HRP labelled material, with no evidence of a waiting period. The initial corticothalamic projection, detected by retrograde transport of WGA-HRP from the thalamus, occurred at P60 from layer 5 cells. This was confirmed by labelling of corticothalamic axons after cortical injections of Neurobiotin and biocytin. Scattered, labelled cells seen before P60 after dye labelling from the thalamus presumably resulted from transcellular labelling via thalamic afferents. Clustering of afferents in layer 4 and cell bodies and their dendrites in layers 5 and 6 first occurred simultaneously at P76. There is a sequential onset of pattern formation, first in brainstem, then in thalamus, and finally in cortex, with a long delay between afferent arrival and pattern formation at each level. Independent regulation at each level, likely depending on target maturation, is suggested.

Distribution of GABAergic elements postsynaptic to ventroposteromedial thalamic projections in layer IV of rat barrel cortex.

The spatial synaptic pattern formed by boutons, originating in the ventroposteromedial thalamic nucleus, with GABAergic neurons in the rat barrel cortex was mapped. The aim was to shed light on the structural basis by which inhibitory circuits may be activated at the first stage of cortical information processing. The thalamic afferent projection was labelled by anterograde transport of Phaseolus vulgaris leucoagglutinin (PHA-L), whereas the GABAergic targets in layer IV of the rat barrel cortex was visualized by postembedding GABA immunogold-labelling or by pre-embedding parvalbumin immunocytochemistry. In the first set of experiments, we mapped barrels, contained in single ultrathin sections, by means of a computer-controlled electron microscope stage in their entire layer IV representation. From a total of 1199 asymmetric PHA-L-labelled synapses, only 98 were on GABAergic elements, mainly on dendritic shafts. This corresponded to 8.2% of all synapses counted. These synapses on GABAergic targets were essentially homogeneously distributed without a reliable relationship to barrel subdivisions, i.e., hollow versus wall; or layer IVa versus layer IVb. In the second part of the study, we demonstrated that parvalbumin-containing neurons represent the major GABAergic cell type targetted by thalamic afferents in layer IV of the barrel cortex, since all parvalbumin-positive cells investigated received multiple synaptic contacts (up to eight synapses per neuron) from the ventroposteromedial thalamic nucleus. These results imply that interneurons responsible for perisomatic inhibition (basket and chandelier cells known to contain parvalbumin) are likely to be strongly excited by thalamic afferents, despite the relatively low proportion of thalamic synapses on GABAergic elements compared to spines of principal cells, and participate in the early stages of cortical sensory information processing.

Effects of visual deprivation on the postnatal development of the geniculocortical projection in kittens

In kittens reared with either monocular, binocular or reverse suture, beginning before the physiological eyelid opening (around one week of age) and lasting until after one month, the cortical laminar distribution of geniculocortical afferents to area 17 was examined by using orthograde transport of wheat germ agglutinin conjugated with horseradish peroxidase, and compared with that in normal kittens. In normal kittens, at birth, the afferents were distributed most densely in layer I and, to a lesser extent, widely from the upper part of layer II to layers V or VI. After one month, the afferents were found mainly in and around layer IV and very sparsely in layer I. Neither binocular nor monocular suture affected this normal development. In contrast, when the present procedure of monocular suture had been followed by opening the sutured lid and suturing the other lid (reverse suture) for one week, the distribution was altered. The density of the afferents in layer I was increased while the labelled terminals in deeper layers were as segregated in and around layer IV as observed in normal kittens. Such increase in density of the afferents resulted only when the injected tracer covered the medial or intermediate part of the C complex of the lateral geniculate nucleus. To confirm these findings, geniculate neurons retrogradely labelled by horesradish peroxidase injections into layer I of area 17 were examined in normal and reverse-sutured kittens. In both kinds of kittens, the labelled neurons were dense in the C complex, and absent or sparse in the A laminae. But, the number was higher in reverse-sutured kittens. These results suggest an involvement of geniculocortical layer I projections in reorganization of neuronal circuits in the visual cortex.

Role of visual experience in promoting segregation of eye dominance patches in the visual cortex of the cat.

Transneuronal autoradiography was used to study the role of visual experience in the development of ocular dominance patches in the cat. In order to assess quantitatively the effects of visual deprivation, image analysis was used to measure the profiles of grain density in layer IV. Fourier power spectra of these profiles were computed to give a measure of the amplitudes and frequencies of the fluctuations in grain density that were present. Deprivation of normal patterned vision by binocular lid suture or by rearing in total darkness from shortly after birth abolished the dominant periodicity (of about 1.1 mm) in the distribution of left and right eye afferents in layer IV of area 17. A dominant periodicity of about 2.2 mm was, however, present in area 18 of both normal and dark-reared animals. Visual deprivation was not able to reverse segregation. One animal reared normally for 6 weeks was placed in the dark for a further 28 weeks and showed normal periodicities in the distribution of geniculate inputs to area 17. Another animal given 128 hours of experience and kept in the dark for the rest of the time until it was 12 weeks old also showed normal segregation. To determine the minimum amount of visual experience necessary for segregation to occur, four animals were given 8-, 24-, 48-, and 128-hour periods of visual experience and were studied at 12 weeks of age. Eight hours of experience had no detectable effect on segregation; periodicities of intermediate amplitude were present in animals that received 24 and 48 hours of experience, while 128 hours of experience resulted in periodicities of normal amplitude. Recovery from visual deprivation was studied by rearing kittens from birth in the dark for varying periods and then returning them to the normally lit colony room for periods of 6 to 22 weeks. Recovery from 6 weeks of dark rearing was found to be complete; much less recovery occurred following periods of 8 to 25 weeks of initial deprivation, and no recovery at all occurred after 30 weeks of deprivation. It is concluded that the spontaneous activity present in the geniculocortical afferents of dark-reared and lid-sutured cats is not adequate to drive normal periodic segregation in area 17, though it can do so in area 18. Between 48 and 128 hours of visual experience, given before 8 weeks of age, appears to be necessary and sufficient for normal periodic segregation of geniculate afferents in area 17 of the cat.

Increase in the density of geniculocortical afferents to layer I after reverse suture in kittens

In kittens reared with one eye lid sutured shut from birth until after one of age, geniculocortical afferents in layer I of area 17 are as sparse as obtained from normal kittens of the same age; however, when such a procedure was followed by opening the sutured lid and suturing the other lid, the afferents in layer I, examined one week later, were found much denser. These findings suggest that geniculocortical afferents in layer I may play inpart in proliferative reorganization of the visual cortex.

Afferent specific localization of muscarinic acetylcholine receptors in cingulate cortex

The laminar distribution of acetylcholine receptors in rat cingulate cortex and their localization to axons of neurons in the anterior thalamic nuclei (ATN) were evaluated with the muscarinic antagonist [3H]propylbenzilylcholine mustard (PrBCM) in vitro. Specific binding of PrBCM in granular area 29 was heterogeneous, with a 57% variation from the highest binding in layer Ia to the lowest in layer II-III. In contrast, binding in area 24 was homogeneous, with only a 14% variation. The heterogeneity of PrBCM binding almost exactly duplicated the distribution of termination of ATN afferents in layers I to IV of area 29c. Four experiments indicated that 50% of the excess binding in layers Ia and IV was due to axonal receptor sites. First, ATN lesions abolished 41% and 27% of total specific binding in layers Ia and IV, respectively. Second, an undercut procedure that totally deafferented layers I to Va showed changes similar to those following ATN lesions, suggesting that other afferents to these layers may not have muscarinic receptors associated with them. Third, the sequence of losses in receptor binding and acetylcholinesterase (AChE) activity was evaluated 2, 3, 5, 9, and 14 days following ATN lesions. Since AChE was present in ATN axons, as evidenced by early postlesion losses, the correlation of both losses as well as previous analyses of axon degeneration in this cortex confirmed that these receptors were in axons. Fourth, binding peaks in layers Ia and IV remained in area 29c following destruction of virtually all neurons with the neurotoxin ibotenic acid. This is the first evidence that the activity of a major neocortical thalamic afferent may be regulated by axonal acetylcholine receptors.

Ocular dominance in layer IV of the cat's visual cortex and the effects of monocular deprivation.

1. The relation between the physiological pattern of ocular dominance and the anatomical distribution of geniculocortical afferents serving each eye was studied in layer IV of the primary visual cortex of normal and monocularly deprived cats. 2. One eye was injected with radioactive label. After allowing sufficient time for transeuronal transport, micro-electrode recordings were made, and the geniculocoritcal afferents serving the injected eye were located autoradiographically. 3. In layer IV of normal cats, cell were clustered according to eye preference, and fewer cells were binocularly driven than in other layers. Points of transition between groups of cells dominated by one eye and those dominated by the other were marked with electrolytic lesions. A good correspondence was found between the location of cells dominated by the injected eye and the patches of radioactively labelled geniculocortical afferents. 4. Following prolonged early monocular deprivation, the patches of geniculocortical afferents in layer IV serving the deprived eye were smaller, and those serving the non-deprived eye larger, than normal. Again there was a coincidence between the patches of radioactively labelled afferents and the location of cells dominated by the injected eye. 5. The deprived eye was found to dominate a substantial fraction (22%) of cortical cells in the fourth layer. In other cortical layers, only 7% of the cells were dominated by the deprived eye. 6. These findings suggest that the thalamocortical projection is physically rearranged as a consequence of monocular deprivation, as has been demonstrated for layer IVc of the monkey's visual cortex (Hubel, Wiesel & Le Vay, 1977).