Intrinsic circuitry: synapses involving the local axon collaterals of corticocortical projection neurons in the mouse primary somatosensory cortex

Pyramidal neurons in the mouse SmI cortex were labeled by the retrograde transport of horseradish peroxidase (HRP) injected into the ipsilateral MsI cortex. Terminals of the local axon collaterals of these neurons (CC terminals) were identified in SmI, and their distribution and synaptic connectivity were examined. To avoid confusion, terminals in SmI cortex labeled by the anterograde transport of HRP injected into MsI were eliminated by lesion-induced degeneration. Lesions of MsI were made 24 hours after the injection of HRP; postlesion survival time was 4 days. Most CC axon terminals occurred in layers III and V where they formed asymmetrical synapses. Of 139 CC synapses in layer III and 104 in layer V, approximately 13% were formed with dendritic shafts. Reconstruction of 19 of these dendrites from serial thin sections showed them to originate from both spiny and nonspiny neurons. Most synapses of CC terminals (about 87%) were onto dendritic spines. In contrast, White and Keller (1987) demonstrated that terminals belonging to the local axon collaterals of corticothalamic (CT) projection cells synapse mainly with dendritic shafts of nonspiny neurons: 92% onto shafts, the remainder onto spines. The distribution of asymmetical synapses onto spines and dendritic shafts was analyzed for neuropil in layers III, IV, and V. Depending on the layer, from 34 to 46% of the asymmetrical synapses in the neuropil were onto dendritic shafts. Results showing that CC and CT terminals form proportions of axodendritic vs. axospinous synapses that differ from each other, and from the neuropil, indicate that local axon collaterals are highly selective with regard to their postsynaptic elements.     

Organization of Pyramidal Cell Apical Dendrites and Composition of Dendritic Clusters in the Mouse: Emphasis on Primary Motor Cortex

It has been proposed that neurons in sensory cortices are organized into modules that centre on clusters of apical dendrites belonging to layer V pyramidal neurons. In the present study, sections reacted for microtubule-associated protein (MAPP) were examined in order to determine the three-dimensional inter-relationships of pyramidal cell dendrites in mouse primary motor cortex (MsI) cortex. Results indicate that pyramidal cell dendrites in MsI cortex can be interpreted to be arranged in a modular fashion, and that these modules are organized similarly to those in the sensory areas of the cortex. Also included in the present study are experiments designed to determine if the clusters of apical dendrites, around which the modules are centred, are composed of dendrites belonging to one or to more than one type of projection cell. Callosal neurons in MsI cortex were labelled by the retrograde transport of horseradish peroxidase deposited onto severed callosal fibres in the contralateral hemisphere. Examination of tangential thin sections through layer IV of MsI cortex shows clusters of apical dendrites in which every dendrite is labelled with horseradish peroxidase. Adjacent clusters are composed of unlabelled dendrites, suggesting that the apical dendrites of callosal neurons aggregate to form clusters that are composed exclusively of dendrites belonging to this type of projection cell. These findings suggest a hitherto unsuspected degree of specificity in the cellular composition of cortical modules.     

The Convergence of Axon Terminals from the Mediodorsal Thalamic Nucleus and Ventral Tegmental Area on Pyramidal Cells in Layer V of the Rat Prelimbic Cortex

We investigated the ultrastructural basis of the synaptic convergence of afferent fibres from the mediodorsal thalamic nucleus (MD) and the ventral tegmental area (VTA) on the prefrontal cortical neurons of the rat by examining the synaptic relationships between thalamocortical or tegmentocortical terminals labelled with anterograde markers [lesion-induced degeneration or transport of wheat germ agglutinin conjugated to horseradish peroxidase (WGA—HRP)] and randomly selected unlabelled apical dendrites of layer V pyramidal cells in the prelimbic cortex. WGA—HRP-labelled terminals from the VTA ranged in diameter from 0.7 to 2.8 μm and established synaptic contacts with large dendritic profiles, i.e. proximal segments of apical dendritic shafts and spines from layer V pyramidal cells. Symmetrical synapses, i.e. inhibitory synapses, were more often seen than asymmetrical ones. Degenerating terminals from the MD formed asymmetrical synapses on dendritic spines or occasionally on small dendritic shafts of apical dendrites from layer V pyramidal cells, which received tegmentocortical synapses, mostly within layer III. Thalamocortical synapses were more distally distributed over common apical dendrites than tegmentocortical synapses, although some of them overlapped. The numerical density of direct synaptic inputs from the MD and VTA was low. These results suggest that fibres from the VTA exert their inhibitory effects directly on pyramidal cells in layer V via synaptic junctions with apical dendrites of these pyramidal cells, and that the tegmentocortical fibres are in an ideal anatomical position to modulate the reverberatory circuits between the MD and the prelimbic cortex.     

The ultrastructural characteristics of the pyramidal callosal neurons of layer III in the primary auditory area (A1) of the cat cortex]

An electron microscope study of retrogradely labelled pyramidal neurons in layer III of the primary auditory cortex (AI) after HRP injections into the contralateral AI has been carried out in cats. From 4 to 10 synapses were usually revealed on somatic profiles of these callosal neurons. Synapses occupied 20.0% of the somatic surface of these neurons. All of the revealed synapses on the somata of callosal neurons had symmetric contacts and were formed by axon terminals with small elongated synaptic vesicles. Average length of these synaptic contacts was 1.6 microns. In layer III anterogradely labelled terminals of callosal fibres were also revealed. The majority of them contained large round synaptic vesicles and formed asymmetric contacts on spines. Three labelled axon terminals with small elongated vesicles were found to form symmetric axo-somatic synapses on callosal neurons of layer III.     

Interlaminar connections of rat visual cortex: An ultrastructural study

Thermal lesions were made in layers I, II, and upper part of layer III of rat visual cortex. The distribution of degenerating axons and axon terminals in layers IV, V, and VI was studied using electron microscopic techniques. Following supragranular thermal lesions, the majority of degenerating axon terminals were found in layer V, with extension into the adjacent part of layer VI. Neural profiles postsynaptic to degenerating axon terminals were found in these layers in the following distribution: 81.7% on spines of small to medium size dendrites; 18.2% on dendrite shafts; and <1% on neuronal perikarya. Few degenerating terminals were found on or near apical dendrites. Degenerating terminals were identified on shafts of stellate-type dendrites found in the upper part of layer V. Degenerating axons oriented parallel to the cortical surface were found most often in deep layer IV and upper layer V. Degenerating axons were also seen in axon bundles coursing vertically through layer IV. Approximately 10% of the terminals within a grid square have undergone degeneration; no clustering of degenerating terminals was found in vertical or transverse sections through layers V and VI. We suggest that most axon terminals arising from pyramidal neurons in layers II and upper III synapse with spines and shafts of dendrite branches originating from pyramidal neurons in layer V and perhaps VI.     

Synaptic termination of afferents from the ventrolateral nucleus of the thalamus in the cat motor cortex. A light and electron microscopy study

The synaptic termination in the cat motor cortex of afferents from the ventrolateral nucleus of the thalamus (VL) has been studied with experimental light and electron microscopic methods. The distribution of normal synapses on motor cortex pyramidal, stellate, and Betz cells was also examined. Synapses in the motor cortex can be classified into two general types. The first and most prominent type contains flat vesicles, lacks a compact postsynaptic density, and corresponds to Colonnier's ('68) symmetrical synapse. Stellate neurons receive synapses of both types on their cell bodies and proximal dendritic shafts, while pyramidal cells have only symmetrical synapses at these sites. The dendritic spines of both stellate and pyramidal cells are contacted by predominantly asymmetrical synapses. Betz cells, like smaller pyramidal neurons, receive only symmetrical synapses on their cell bodies. The proximal portions of the Betz cells apical dendrites, however, receive both asymmetrical and symmetrical synapses. Following VL lesions, degenerating synapses were mainly found in three cortical layers: the upper third of layer I (18%), layer III (66%), and layer VI (13%). Degenerating synapses were not seen in the lower two-thirds of layer I or in layer II, and were only rarely seen in layer V (3%). Ninety-one percent of the VL synapses were found on spines and 8% on stellate-type dendritic shafts. Stellate cell bodies rarely received VL synapses (1%) and none occurred on pyramidal or Betz cell bodies and their proximal dendrites. A VL synapse within layer III was found on two dendritic spines of a Betz cell apical dendrite. Thus, part of the VL input to layer III synapses on the processes of both motor cortex output neurons (Betz cells in layer V) and cortical interneurons (stellate cells in layer III).     

Synapses of extrinsic and intrinsic origin made by callosal projection neurons in mouse visual cortex

Neurons in areas 17/18a and 17/18b of mouse cerebral cortex were labeled by the retrograde transport of horseradish peroxidase (HRP) transported from severed callosal axons in the contralateral hemisphere. Terminals of the local axon collaterals of labeled neurons (intrinsic terminals) were identified in the border regions of area 17 with areas 18a and 18b, and their distribution and synaptic connectivity were determined. Also examined were the synaptic connections of extrinsic callosal axon terminals labeled by lesion-induced degeneration consequent to the severing of callosal fibers. A postlesion survival time of 3 days was chosen because by this time the extrinsic terminals were all degenerating, whereas the intrinsic terminals were labeled by horseradish peroxidase. Both intrinsic and extrinsic callosal axon terminals occurred in all layers of the cortex where, with rare exception, they formed asymmetrical synapses. Layers II and III contained the highest concentrations of intrinsic and extrinsic callosal axon terminals. Analyses of serial thin sections through layers II and III in both areas 17/18a and 17/18b yielded similar results: 97% of the intrinsic (1,412 total sample) and of the extrinsic (414 total sample) callosal axon terminals synapsed onto dendritic spines, likely those of pyramidal neurons; the remainder synapsed onto dendritic shafts of both spiny and nonspiny neurons. Thus, the synaptic output patterns of intrinsic vs. extrinsic callosal axon terminals are strikingly similar. Moreover, the high proportion of axospinous synapses formed by both types of terminal (97%) contrasts with the proportion of asymmetrical axospinous synapses that occurs in the surrounding neuropil where about 64% of the asymmetrical synapses are onto spines. This result is in accord with previous quantitative studies of the synaptic connectivities of callosal projection neurons in mouse somatosensory cortex, and lends additional weight to the hypothesis that axonal pathways are highly selective for the types of elements with which they synapse.     

Synapses made by axons of callosal projection neurons in mouse somatosensory cortex: emphasis on intrinsic connections

This is one of a series of papers aimed at identifying the synaptic output patterns of the local and distant projections of subgroups of pyramidal neurons. The subgroups are defined by the target site to which their main axon projects. Pyramidal neurons in areas 1 and 40 of mouse cerebral cortex were labeled by the retrograde transport of horseradish peroxidase (HRP) transported from severed callosal axons in the contralateral hemisphere. Terminals of the local axon collaterals of these neurons ("intrinsic" terminals) were identified in somatosensory areas 1 and 40, and their distribution and synaptic connectivity were examined. Also examined were the synaptic connections of "extrinsic" callosal axon terminals labeled by lesion induced degeneration consequent to the severing of callosal fibers. A post-lesion survival time of 3 days was chosen because by this time the extrinsic terminals were all degenerating, whereas the intrinsic terminals were labeled by HRP. Both intrinsic and extrinsic callosal axon terminals occurred in all layers of the cortex where they formed only asymmetrical synapses. Layers II and III contained the highest concentrations of both types of callosal axon terminal. Analyses of serial thin sections through layers II and III in both areas 1 and 40 yielded similar results: 97% of the extrinsic (277 total sample) and of the intrinsic (1215 total sample) callosal axon terminals synapsed onto dendritic spines, likely those of pyramidal neurons; the remainder synapsed onto dendritic shafts of both spiny and nonspiny neurons. Thus the synaptic output patterns of intrinsic vs. extrinsic callosal axon terminals are strikingly similar. Moreover, the high proportion of axospinous synapses formed by both types of terminal contrasts with the proportion of asymmetrical, axospinous synapses that occur in the surrounding neuropil where only about 80% of the asymmetrical synapses are onto spines. This result is in accord with previous quantitative studies of the synaptic connectivities of both extrinsic and intrinsic axonal pathways in the cortex (White and Keller, 1989: Cortical Circuits; Boston: Birkhauser): in all instances, axonal pathways are highly selective for the types of elements with which they synapse.     

A quantitative study of the thalamocortical and other synapses in layer IV of pyramidal cells projecting from mouse SmI cortex to the caudate-putamen nucleus

The thalamocortical and other synapses of the apical dendrites of corticostriatal projection neurons in mouse primary somatosensory cortex (SmI) were examined by combining anterograde degeneration with the retrograde transport of horseradish peroxidase (HRP). Electrolytic lesions were made in the ventrobasal thalamus, followed 3 days later by injections of 40% HRP into the ipsilateral caudate-putamen nucleus. The next day, the mice were perfused and the SmI cortex ipsilateral to the lesion and injection sites was chopped at 125-μm and reacted for HRP using a CoCl₂-DAB method. HRP-labeled corticostriatal cells in SmI cortex were medium-sized pyramidal cells, having somata located in the superficial portion of layer V and apical dendrites extending into layer I. Seven corticostriatal cells were serially thin sectioned and the layer IV portions of their apical dendrites were reconstructed. Each apical dendrite formed only one or two thalamocortical synapses (0.3 to 0.9% of their synapses in layer IV) indicating that corticostriatal neurons may be minimally responsive to direct synaptic input from the specific thalamic nuclei. Each apical dendrite formed about 12.6 asymmetrical synapses for every symmetrical synapse, suggesting that the relative numbers of excitatory and inhibitory synapses impinging on apical dendrites belonging to an individual class of neurons may be specified.     

Callosal terminals in the rat prefrontal cortex: Synaptic targets and association with GABA-immunoreactive structures

The callosal projections of the cerebral cortex play an important role in the functional integration of the two hemispheres, and the anatomy of these connections has been extensively studied in primary sensory and motor regions. In the present investigation, we examined the synaptic targets of callosal terminals in a limbic association area, the prefrontal cortex (PFC) in the rat. In addition, we examined the relationship of callosal afferents to GABA local circuit neurons within the PFC. Callosal terminals were labeled by either anterograde transport of Phaseolus vulgaris leucoagglutinin from superficial or deep layers or by anterograde degeneration following electrolytic lesion of the contralateral PFC. Callosal terminals in either the superficial or deep layers labeled by either method formed primarily asymmetric axo-spinous synapses (approximately 95%), while the remainder formed axo-dendritic synapses. Some of the dendrites postsynaptic to callosal terminals exhibited a morphology characteristic of local circuit neurons. This observation was confirmed in tissue immunolabeled for GABA, in which degenerating callosal terminals sometimes formed asymmetric synapses on GABA-labeled dendrites. In addition, GABA-labeled terminals and callosal afferents were sometimes observed to converge onto common postsynaptic dendritic shafts or spines within the PFC. These results indicate that callosal terminals in limbic association cortex, consistent with sensory and motor cortices, primarily target the spines of pyramidal neurons. In addition, the results suggest that callosal afferents to the PFC interact with GABA local circuit neurons at multiple levels. Specifically, a proportion of callosal terminals appear to provide excitatory drive to GABA cells, while GABA terminals may modulate the excitation from callosal inputs to the distal dendrites and spines of PFC pyramidal neurons. Synapse 29:193–205, 1998. © 1998 Wiley-Liss, Inc.     

A study of the organization of apical dendrites in the somatic sensory cortex of the rat

In the parietal cortex of the rat, sections cut tangentially show that profiles of medium and large apical dendrites are grouped into clusters. The number of apical dendrites in each cluster is variable and the usual separation between individual clusters is about 50 μ. Despite these variations the pattern does not appear to be random. Reconstructions from one micron serial sections show that neurons giving rise to the ascending dendrites forming clusters are located at different levels in layer V. The cell bodies of these neurons are arranged vertically below their dendrites and show a tendency to form groups. All of the neurons have apical dendrites that ascend through the cortex with a few secondary branches occurring close to the base. The principal secondary branching begins in layer III and spreads obliquely up through layer I. Furthermore, beginning in the inferior region of layer III apical dendrites are added to the clusters at their peripheries. These are from layer III pyramids. It is clear that the superior aspects of the cluster arrangements must intermingle with those of the neurons in adjacent clusters. The neuropil surrounding the dendrites forming clusters appears to contain a few smaller dendrites. Small unmyelinated axons are the most frequent component of the surrounding neuropil and these form terminals which synapse on the spines and trunks of the clustered dendrites. There is no obvious function that can be ascribed to the clusters other than they may form a component of the columnar organization in cortex described in part by physiological techniques.     

Ultrastructural characteristics of callosal neurons in deep layers of the primary auditory cortex (AI) in cat]

An electron microscope study of retrogradely labelled nonpyramidal neurons has been carried out in layers V-VI of the primary auditory cortex (AI) after HRP injections into the contralateral AI of cats. From 2 to 9 synapses were usually revealed on somatic profiles of these callosal neurons. Synapses occupied 15.8 +/- 1.7% (on the average) of the somatic surface of these neurons. All of the revealed synapses on the somata of these callosal neurons had symmetric contacts and were formed by axon terminals with small elongated synaptic vesicles. An average length of these synaptic contacts in sections was 1.6 +/- 0.1 mm. HRP-labelled axon terminals of callosal fibres in layers V-VI contained round synaptic vesicles and formed asymmetric synapses on spines and dendrites. Possible functional significance of axo-somatic synapses in formation of impulsation patterns of the callosal neurons is discussed.     

Electron microscopic evidence that axon terminals from the mediodorsal thalamic nucleus make direct synaptic contacts with callosal cells in the prelimbic cortex of the rat

A combined study of anterograde axonal degeneration and HRP retrograde labeling has shown that there exist monosynaptic connections between afferent fibers from the mediodorsal thalamic nucleus (MD) and callosal cells in the prelimbic cortex of the rat. Degenerating axon terminals from MD made asymmetrical synaptic contacts with dendritic spines from apical dendrites of layer III pyramidal cells that were retrogradely labeled with HRP after its injection into the prelimbic cortex contralateral to MD lesions.     

Distribution of corticocortical and thalamocortical synapses on identified motor cortical neurons in the cat: Golgi, electron microscopic and degeneration study

Details of the terminal connection of corticocortical and thalamocortical fibers on pyramidal and stellate neurons in the cat motor cortex were studied using the electron microscope in combination with the Golgi and axonal degeneration techniques. Corticocortical terminals were examined in 23 identified neurons of which 11 were pyramidal and 12 were stellate. Stellate neurons located in layer III received many degenerating terminals (average 8.4 +/- 2.2 per unit length of dendrite (ULD)) and the majority of these (95%) were found on the proximal dendrites or on the cell bodies. The pyramidal neurons received fewer degenerating terminals (average 2.1 +/- 0.27/ULD) and these were located on more distal dendritic shafts or on dendritic spines. The majority of these synapses were of the asymmetric type. Thalamocortical terminals were examined in 9 pyramidal and 9 stellate neurons. Pyramidal neurons received many terminals (average 6.0 +/- 1.23/ULD) and these were found on the basal as well as the apical dendrites and on dendrite spines. Stellate neurons received fewer terminals (average 4.2 +/- 0.64/ULD) and were located primarily on proximal dendritic shafts. The majority of these synapses were of the asymmetric type. The functional role of these synapses is discussed in relation to the physiological results reported in the preceding paper.     

Structure of the piriform cortex of the opossum. II. Fine structure of cell bodies and neuropil

The fine structure of cell bodies and neuropil in the piriform cortex of the opossum has been examined. A close similarity in ultrastructure of many features has been demonstrated between this pylogenetically old cortex in a primitive mammal and the neocortex of higher mammals. Cell bodies of pyramidal cells are very similar to those in the neocortex: The nucleus is pale with a smooth surface, the cytoplasm has a modest number of organelles, and the soma receives a small number of exclusively symmetrical synapses. Semilunar cells, which have apical but no basal den-drites, are very similar to pyramidal cells in ultrastructure of their cell bodies. Two populations of neurons with nonpyramidal ultrastructural features have been distinguished: (1) cells in layer III that closely resemble the well-known large multipolar cells in neocortex by virtue of a large number of symmetrical and asymmetrical somatic synapses and long cisterns of rough endoplasmic reticulum (ER); and (2) large cells in layer I with very few somatic synapses, a large number of mitochondria, and short cisterns of rough ER that may correspond to cells with somatic appendages described with the Golgi method. Large numbers of profiles are found in all layers that contain round vesicles and make asymmetrical synapses onto dendritic spines, and occasionally, dendritic shafts. Theseprofileshavedistinctly different morphological features in layer Ia, in which olfactory bulb afferents are concentrated, and in layers Ib, II, and III, which contain terminals of association and commis-sural fibers. A smaller number of profiles containing pleomorphic vesicles make symmetrical contacts onto initial segments, dendritic shafts, cell bodies, and occasionally, dendritic spines. Most dendritic spines in all layers are small to medium in size (0.3–1.2 μm) and presumably originate from pyramidal cells. In layer Ia, however, large, flattened spines are also present which appear to originate from semi-lunar cells. In layer III, and to a lesser extent other layers, large irregular spines are present that may be branched appendages on dendrites of complex appendage cells (Haberly, 1983).     

Thalamocortical synapses of pyramidal cells which project from SmI to MsI cortex in the mouse

Electrolytic lesions were made of the nucleus ventralis posterior pars lateralis thalami and of the nucleus posterior thalami in male CD/1 mice to label thalamocortical axon terminals in mouse SmI cortex. Pyramidal cells in layers II through V of SmI cortex were labeled by the retrograde transport of horseradis peroxidase injected into ipsilateral MsI. Numerous pyramidal cells, particularly in the more superficial layers of SmI cortex, were filled with reaction product so that even their dendritic spines and local axon collaterals were clearly visible. Six well-filled pyramidal cells with somata in layers III and IV were serial thin-sectioned, and portions of their dendrites in layer IV were examined with the electron microscope to determine the distribution of their thalamocortical and other synapses. It was found that, in general, different dendrites of a single pyramidal cell formed similar proportions of thalamocortical synapses and that the six pyramidal cells, as a group, also formed similar proportions of thalamocortical synapses with their dendrites in layer IV. In contrast, when the thalamocortical connectivity of the six cells was considered as a function of their depths within the cortex, a clear trend was seen for the proportion of thalamocortical synapses to increase with increasing proximity of the cell body to layer IV. A hypothesis based on the timing of developmental events is proposed to account for this observation.     

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.     

An electron microscopic study of the origins of the projections of the first auditory area of the cortex (AI) to the medial geniculate body in cats

An electron microscope study of retrogradely labelled neurons in layer VI of the primary auditory cortex (AI) after injection of the horseradish peroxidase to the medial geniculate body was carried out in cats. Three-eight synapses (4.6 +/- 0.6 at an average) were revealed on the somata profiles of these retrogradely labelled cortico-geniculate neurons. Synapses occupied 10.8 +/- 1.0% of the somatic profile of cortico-geniculate neurons. Almost all (98.7%) of these axosomatic synapses had symmetrical contacts and were formed by axonal terminals with small elongated synaptic vesicles. HRP retrogradely labelled axonal terminals of geniculo-cortical fibres were revealed in neuropil of layer VI. They contained large round synaptic vesicles and formed asymmetrical synapses, mainly on spines. The role of axo-somatic synapses in regulation of the activity of cortico-geniculate neurons was discussed.     

Morphological and numerical analysis of synaptic interactions between neurons in deep and superficial layers of the entorhinal cortex of the rat

Neurons providing connections between the deep and superficial layers of the entorhinal cortex (EC) constitute a pivotal link in the network underlying reverberation and gating of neuronal activity in the entorhinal-hippocampal system. To learn more of these deep-to-superficial neurons and their targets, we applied the tracer Neurobiotin pericellularly in layer V of the medial EC of 12 rats. Labeled axons in the superficial layers were studied with light and electron microscopy, and their synaptic organization recorded. Neurobiotin-labeled layer V neurons displayed "Golgi-like" staining. Two major cell types were distinguished among these neurons: (1) pyramidal neurons with apical spiny dendrites traversing all layers and ramifying in layer I, and (2) horizontal neurons with dendrites confined to the deep layers. Labeled axons ramified profusely in layer III, superficially in layer II and deep in layer I. Analysis of labeled axon terminals in layers I-II and III showed that most synapses (95%) were asymmetrical. Of these synapses, 56% occurred with spines (presumably belonging to principal neurons) and 44% with dendritic shafts (presumably interneurons). A small fraction of the synapses (5%) was of the symmetrical type. Such synapses were mainly seen on dendritic shafts. We found in two sections a symmetrical synapse on a spine. These findings suggest that the deep to superficial projection is mainly excitatory in nature, and that these fibers subserve both excitation and feed-forward inhibition. There is an additional, much weaker, inhibitory component in this projection, which may have a disinhibitory effect on the entorhinal network in the superficial layers.     

Synaptic relationships between axon terminals from the mediodorsal thalamic nucleus and layer III pyramidal cells in the prelimbic cortex of the rat

A combined study of anterograde axonal degeneration and Golgi electron microscopic technique was designed to examine the distribution and density of axon terminals from the mediodorsal thalamic nucleus (MD) over layer III pyramidal cells in the prelimbic cortex of the rat. The reconstructive analysis of serial ultrathin sections of gold-toned apical and basal dendrites of layer III pyramidal cells showed that degenerating thalamocortical axon terminals from MD formed asymmetrical synaptic contacts predominantly with dendritic spines of the identified basal dendrites as well as apical dendrites. There was little difference in the numerical density of thalamocortical synapses from MD per unit length of both apical and basal dendrites.