Edward L. White

A quantitative study of thalamocortical and other synapses involving the apical dendrites of corticothalamic projection cells in mouse SmI cortex

SummaryLesion-induced degeneration was combined with the retrograde transport of horseradish peroxidase (HRP) to examine the thalamocortical and other synaptical relationships of corticothalamic projection cells in mouse SmI cortex. Injections containing 40% HRP were placed in the ventrobasal thalamus and the next day, electrolytic lesions were made of the injection site. About four days later, the animals were perfused with aldehydes and SmI cortex ipsilateral to the injection and lesion sites was tissue chopped and reacted for HRP. The somata of HRP-filled corticothalamic cells in SmI cortex had diameters of about 10 μm and occurred in the upper half of layer VI and in the lower half of layer V; their apical dendrites usually terminated within or just below layer IV, but in some instances, extended nearly to the pial surface. Reconstructions of serial thin sections through the layer IV portions of the apical dendrites of seven well-labelled corticothalamic cells showed them to form 7 to 20% of their synapses with degenerating thalamocortical axon terminals. In contrast, the proportion of thalamocortical synapses formed by the layer IV dendrites of corticostriatal (Hersch & White, in preparation) and corticocortical (White & Hersch, 1981) projection cells ranges from 0.3 to 0.9% and from 1.5 to 6.8%, respectively. We interpret these findings to indicate that pyramidal cell dendrites are specified to form characteristic proportions of thalamocortical synapses.

Three-dimensional aspects and synaptic relationships of a Golgi-impregnated spiny stellate cell reconstructed from serial thin sections

SummaryA Golgi-impregnated spiny stellate cell was selected from layer IV of SmI cortex in a mouse whose ipsilateral ventrobasal complex had been lesioned. The neuron was gold-toned, thin sectioned and then reconstructed in three dimensions using wooden sheets of appropriate thickness. These procedures enabled the numbers and distribution of thalamocortical and other synapses onto the reconstructed neuron to be determined. Results show the cell body to be roughly spherical and to receive 49 symmetrical synapses and four synapses which are intermediate between the asymmetrical and symmetrical type. A single, clearly asymmetrical axosomatic synapse is made by a degenerating, thalamocortical axon terminal. Five primary dendrites and their branches were reconstructed and, interestingly, these processes are distinctly elliptical in cross-section. The reconstructed dendrites receive 68 symmetrical synapses onto their shafts and 373 synapses onto spines of which 359 are asymmetrical and 14 symmetrical. Forty-eight, or about 13%, of the asymmetrical axospinous synapses are made by degenerating, thalamocortical axon terminals. An intriguing finding is that in many regions of the dendritic tree, two or more spines involved in thalamocortical synapses are attached to the dendritic shaft at intervals of 5±0.5 μm.

The identification of thalamocortical axon terminals in barrels of mouse Sml cortex using immunohistochemistry of anterogradely transported lectin (Phaseolus vulgaris-leucoagglutinin)

The anterograde transport and immunohistochemical demonstration of the lectin, Phaseolus vulgaris-leucoagglutinin (PHA-L) has been used to label thalamocortical axon terminals in barrels of mouse SmI cortex. The reaction product is visible with both the light and electron microscopes so that the distribution of axons and the types of synapses they form can be determined.

A survey of morphogenesis during the early postnatal period in PMBSF barrels of mouse SmI cortex with emphasis on barrel D4

The long term goal of this work is to understand synaptogenesis in homologous regions of the cerebral cortex, i.e. a whisker barrel. Hemispheres of aldehyde perfused mice, at various ages from P6 to P65 (DOB = P0; three each), were osmicated and sectioned at 40 microns parallel to the pia. Barrels were identified, mapped and measured in sections through mid-level layer IV, and then embedded for electron microscopy. The main findings were: (1) Cell bodies and large diameter dendrites thin out in barrel hollows from P6 to P8. (2) Degeneration occurs primarily from P6 to P11, peaking on P8. (3) Single synapses from narrow, tubular axons predominate before P14; afterwards, multiple synapses from bag-like terminals increase in number. (4) The number of spines increases dramatically between P9 and P12. (5) Asymmetrical and symmetrical synapses occur at all ages studied; their junction lengths are not significantly different at any age. (6) Asymmetrical synapse density increases rapidly from P6 to P8, slowly from P9 to P.12, sharply between P13 and P14 along with patterned whisking, slowly to P20 and drops in adults. (7) Synapses onto spiny and non-spiny stellate cell bodies increase markedly from P10 to P20. (8) Changes in density of asymmetrical synapses in neuropil and of symmetrical synapses on spiny stellate cell bodies follow similar sequences but the sequence in neuropil is 72 h earlier. (9) When barrel size is taken into account, synaptogenesis is monotonic, increasing sharply in the second postnatal week followed by a slower increase into adulthood.

Thalamocortical synapses with corticothalamic projection neurons in mouse SmI cortex : electron microscopic demonstration of a monosynaptic feedback loop

Abstract Synapses between degenerating thalamocortical axon terminals and corticothalamic projection neurons labeled by the retrograde transport of horseradish peroxidase have been identified in mouse SmI cortex. This monosynaptic thalamocortical-corticothalamic feedback loop is suggested to be involved in reinforcing differences in the levels of excitation between parallel components of thalamocortical projections.

Termination of Thalamic Afferents in the Cerebral Cortex

For most of the past century, investigations of thalamocortical relations have employed various light microscopic approaches to elucidate and to refine topographic relationships between corresponding areas of the thalamus and cortex. However, because of the limited resolving power of the light microscope, information on the precise relationships of thalamic afferents with cortical nerve cells could not be obtained. The scope of investigations into thalamocortical relations was greatly broadened during the 1960s for, by then, electron microscopy had made it possible to visualize synapses between thalamic afferents and their target cells in cortex. However, the capability of seeing thalamocortical synapses does not of itself permit the identification of postsynaptic cell types, and it was not until late in the 1970s that approaches were elaborated which held out the promise of revealing this crucial aspect of thalamocortical relations (see review by White, 1979). The past few years have seen the fulfillment of that promise: in several regions of neocortex, cells which receive input directly from the thalamus have been conclusively identified (e.g., Peters et al., 1979; White, 1978). Other recent studies have focused on various quantitative aspects of thalamocortical synaptic relationships (e.g., Hersch and White, 1981b; White and Hersch, 1982). The purpose of the present review is to critically evaluate these and related findings, which in the relatively short period since the publication of a previous review on the subject (White, 1979), have significantly enriched our understanding of thalamocortical organization.

Quantification of synapses formed with apical dendrites of Golgi-impregnated pyramidal cells: variability in thalamocortical inputs, but consistency in the ratios of asymmetrical to symmetrical synapses.

Abstract Five pyramidal cells from the posteromedial barrel subfield of mouse SmI cortex were labeled by Golgi impregnation and then gold-toned and de-impregnated ( Fairen, Peters & Saldanha, 1977 ). Subsequently, 40 to 70 μm-long segments of their apical dendrites occurring in layer IV were graphically reconstructed from serial thin sections to determine the distribution of their synapses. Thalamocortical synapses onto these dendritic segments were identified by lesion-induced degeneration. The synaptic pattern of the pyramidal cell apical dendrites was consistent with previous reports in that most synapses occurred on spines and were asymmetrical and the smaller number of shaft synapses were primarily symmetrical. Some axospinous synapses were formed by degenerating thalamocortical axon terminals. The proportion of thalamocortical synapses onto reconstructed dendritic segments was different for different neurons. For example, thalamocortical axon terminals formed 15% of the synapses involving the spines of the reconstructed segment from a medium superficial layer V pyramidal cell and 10% of the synapses onto portions of the segment from a large layer VI pyramidal cell. In contrast, reconstructed dendritic segments of three other layer VI pyramidal cells formed no more than one thalamocortical synapse. An analysis of the distribution of synapses onto reconstructed dendritic segments revealed that the segments of 3 medium and large pyramidal cells had a ratio of about 12.5 asymmetrical synapses per symmetrical synapse, whereas the segments of 2 small pyramidal cells had ratios of only 6.5 asymmetrical synapses per symmetrical synapse. That these ratios fall into 2 distinct groups suggests that the relative number of asymmetrical and symmetrical synapses is stereotyped within populations of neurons.

Changes in mouse barrel synapses consequent to sensory deprivation from birth.

Neonatal sensory deprivation induced by whisker trimming affects significantly the functional organization of receptive fields in adult barrel cortex. In this study, the effects of deprivation on thalamocortical synapses and on asymmetrical and symmetrical synapses not of thalamic origin were examined. Thalamocortical synapses were labeled by lesion‐induced degeneration in adult (postnatal day 60) mice subjected to whisker trimming from birth, other synaptic types were unlabeled. Brains were processed for electron microscopy, and numerical densities of synapses were evaluated by using stereologic approaches for whisker trimmed vs. control animals. Results demonstrated no change in nonthalamic, asymmetrical synapses; however, a decrease of 52% in the numerical density of symmetrical synapses (46.3 vs. 88.5 million per mm3; Z = −2.121; P < 0.05) and a decrease of 43% in the numerical density of thalamocortical synapses (57.5 vs. 102.33 million per mm3; Z = −2.121; P < 0.05) were observed after deprivation. Thus, experience‐dependent plasticity of receptive fields in barrel cortex involves directly axons of both extrinsic and intracortical origin. The proportion of thalamocortical axospinous to axodendritic synapses was the same in control vs. deprived animals: in each instance, 80% of the synapses were axospinous (Z = 0.85; P = 0.2). These results suggest that neither excitatory neurons, whose thalamocortical synapses are primarily axospinous, nor inhibitory neurons, whose thalamocortical synapses are mainly axodendritic (White [1989] Cortical Circuits. Synaptic Organization of the Cerebral Cortex; Structure, Function, and Theory. 1989; Boston: Birkhauser), are affected preferentially by the deprivation‐associated decrease in thalamocortical synapses. J. Comp. Neurol. 457:75–86, 2003.

Synapses between identified neuronal elements. An electron microscopic demonstration of degenerating axon terminals synapsing with Golgi impregnated neurons

Abstract Golgi impregnated neurons can be gold-toned, so that in the electron microscope their profiles are marked by gold particles. It is shown that this technique can be used to determine which neurons in the cerebral cortex synapse with degenerating axon terminals. Thus, both the pre- and post-synaptic components of thalamocortical synapses can be identified.

Synaptic sequences in mouse SmI cortex involving pyramidal cells labeled by retrograde filling with horseradish peroxidase.

A method combining anterograde degeneration and the retrograde transport of horseradish peroxidase (HRP) has been used to study synapses involving pyramidal cells in mouse SmI cortex. Neurons labeled with HRP are so well filled that even their finer processes, such as dendritic spines and axon collaterals, are clearly visible with both the light and electron microscopes. Results indicate that pyramidal cells projecting from SmI to ipsilateral MsI cortex receive thalamocortical input and have local axon collaterals which form asymmetrical synapses with spines and with varicose, non-spiny dendrites.