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).     

Ultrastructural analysis of synaptic relationships of intracellularly stained pyramidal cell axons in piriform cortex

Axons of pyramidal cells in piriform cortex stained by intracellular injection of horseradish peroxidase (HRP) have been analyzed by light and electron microscopy. Myelinated primary axons give rise to extensive, very fine caliber (0.2 micron) unmyelinated collaterals with stereotyped radiating branching patterns. Serial section electron microscopic analysis of the stained portions of the collateral systems (initial 1-2 mm) revealed that they give rise to synaptic contacts on dendritic spines and shafts. These synapses typically contain compact clusters of large, predominantly spherical synaptic vesicles subjacent to asymmetrical contacts with heavy postsynaptic densities. On the basis of comparisons with Golgi material and intracellularly stained dendrites, it was concluded that dendritic spines receiving synapses from the proximal portions of pyramidal cell axon collaterals originate primarily from pyramidal cell basal dendrites. Postsynaptic dendritic shafts contacted closely resemble dendrites of probable GABAergic neurons identified in antibody and [3H]-GABA uptake studies. Electron microscopic examination of pyramidal cell axon initial segments revealed a high density of symmetrical synaptic contacts on their surfaces. Synaptic vesicles in the presynaptic boutons were small and flattened. It is concluded that pyramidal cells synaptically interact over short distances with other pyramidal cells via basal dendrites and with deep nonpyramidal cells that probably include GABAergic cells mediating a feedback inhibition. This contrasts with long associational projections of pyramidal cells that terminate predominantly on apical dendrites of other pyramidal cells.     

Potentiation of late components in olfactory bulb and piriform cortex requires activation of cortical association fibers

Previous research has demonstrated that repeated high-frequency stimulation of the granule cell layer of the olfactory bulb (OB) produces an enduring potentiation of late components (PLC) in potentials evoked in the OB and piriform cortex (PC), while leaving the monosynaptic EPSP produced by OB mitral cells in PC pyramidal cells unaltered. Two experiments were conducted using male Long-Evans rats with chronically implanted electrodes to assess the relative contribution to this potentiation of the two main fiber systems that interconnect the OB and PC: the lateral olfactory tract (LOT), which contains mitral cell axons that synapse on PC pyramidal cells, and the PC association fiber system, which consists of the axons of PC pyramidal cells that synapse on several cell populations within the PC and on granule cells in the OB. The results indicate that stimulation of PC association fibers is both necessary and sufficient to duplicate the pattern of potentiation seen following OB stimulation in previous experiments. LOT stimulation had no consistent effect, and coactivation of the LOT and PC association fibers was no more effective than activation of PC association fibers alone. Possible mechanisms underlying this effect are discussed, including (1) long-term potentiation (LTP) at synapses made by the axons of PC pyramidal cells on neurons in the OB and PC; and (2) repetitive firing in PC pyramidal cells due to regenerative excitation in a population of deep cells in the PC and endopiriform nucleus.     

Ultrastructure of synaptic remodeling in piriform cortex of adult rats after neonatal olfactory bulb removal: an immunocytochemical study

The purpose of this investigation was to study possible remodeling in synaptic structures of the piriform cortex (PC) of adult rats following neonatal deafferentation by removal of the olfactory bulb (OB) at birth. Emphasis was placed on possible qualitative changes in the ultrastructure and immunocytochemical localization of cholecystokinin (CCK, a possible excitatory neurotransmitter or modulator) and glutamic acid decarboxylase (GAD, precursor enzyme to the inhibitory transmitter GABA) in axons, terminals, and synaptic complexes. Light microscopic results in normal adult material show that GAD-positive terminals form a dense band subjacent to the lateral olfactory tract (LOT), become less dense in deeper Ib, and are rare in layer II. Following deafferentation, GAD-positive terminals appear denser and more homogeneously distributed throughout layer I and are also more prevalent in layer II. Ultrastructural results of normals and controls indicate GAD-positive terminals normally contain pleomorphic or flattened vesicles and form symmetric contacts onto dendritic shafts and branches throughout layer I. In deafferented layer I not only do there appear to be greater numbers of symmetric GAD-positive contacts, but in contrast to normals, asymmetric contacts mainly onto spines are now present. Light microscopic results from deafferented material also show an apparent proliferation with spread or sprouting of CCK-positive fibers or axonlike structures mainly into layer Ia, whereas these fibers are normally observed only in the LOT and are generally few in number. Also in normals the few CCK-positive terminals in the area subjacent to the LOT contain flattened or pleomorphic vesicles and form symmetric contacts. Deafferentation results in CCK-positive terminals throughout layer I with a greater frequency of synaptic contacts which now also include a few asymmetric contacts onto spines. The findings clearly show modifications in synaptic patterns of immunocytochemical-labeled terminals that might be compatible with the process of atypical reinnervation of deafferented postsynaptic sites and possible ingrowth of new axons.     

Aging in the rat olfactory system: relative stability of piriform cortex contrasts with changes in olfactory bulb and olfactory epithelium

Previous studies have quantified growth and atrophy of the olfactory bulb and olfactory epithelium of the Sprague-Dawley rat from maturity to senescence. Major events occurring in these structures include changes in the volume of mitral cells and changes in the number of septal olfactory receptors. These effects are large, consist of a growth phase followed by atrophy, and are temporally related in that events in the olfactory epithelium precede those in the mitral cells. A hypothesis of aging based on transneuronal effects would predict that these changes would be similarly transmitted to the next synaptic station in the olfactory pathway. Therefore, cells and synapses of the piriform cortex were studied in rats 3, 12, 18, 24, 27, 30, and 33 months of age. Alternate Vibratome sections through brains perfused with mixed aldehydes were processed for light and electron microscopy. No significant age effects were found for the volumes of cortical laminae Ia and Ib. Both numerical and surface density of synaptic apposition zones in layer Ia, formed primarily by mitral cell axons, were stable with age. A modest (18%) but significant decline in the proportion of layer Ia occupied by dendrites and spines was mirrored by an increase in the proportion of glial processes; no change in the proportion of axons and terminals was observed. Neither nuclear volume, nor soma volume, nor numerical density of layer II neurons changed with age. Thus, contacts made in the piriform cortex by mitral cell axons remain relatively stable in senescence, despite the marked volumetric changes in the mitral cell somata, changes which were confirmed again in this study. Age-related dendritic regression in layer II neurons may be attributable to functional deafferentation subsequent to reduced receptor input to mitral cells.     

Selective suppression of intrinsic but not afferent fiber synaptic transmission by baclofen in the piriform (olfactory) cortex

The GABA_B agonist baclofen has been shown to suppress synaptic transmission in subregions of the hippocampus and in the piriform (olfactory) cortex. Here we report a laminar selectivity of suppression of synaptic potentials in the olfactory cortex. In brain slice preparations, baclofen suppresses extracellularly recorded field potentials at the intrinsic fiber synapses proximal to the superficial pyramidal cell bodies (layer Ib) while leaving the af ferent fiber synaptic potentials recorded at the distal dendrites (layer Ia) little affected. This dose-dependent selective suppression of intrinsic fiber synaptic transmission is also correlated with an increase of paired-pulse facilitation. These results suggest that afferent and intrinsic synaptic inputs may be differentially modulated by the activation of GABA_B receptors and that this selective suppression is at least partially mediated via a presynaptic mechanism.     

Ultrastuctural studies of the primate lateral geniculate nucleus: Morphology and spatial relationships of axon terminals arising from the retina,visual cortex (area 17), superior colliculus,parabigminal nucleus,and pretectum of Galago crassicaudatus

The electron microscopic autoradiographic tracing method has been used to examine the morphology and postsynaptic relationships of five projections (retina, cortical area 17, superior colliculus (tectal), parabigeminal nucleus, and pretectum) to the dorsal lateral geniculate nucleus of the greater bush baby galago crassicaudatus. Retinal terminals have been examined in the contralaterally innervated layer of each of the three matched pairs [parvi-(X-cell), magno- (Y--cell), and koniocellular (small, W-cell)] of geniculate layers. These terminals are large and contain pale mitochondria and round vesicles (RLPs). RLPs are presynaptic to juxtasomatic regions of parvi-and magnocellular neurons. In contrast, RLPs innervate more distal regions of konicellular neurons. Labeled cortical, tectal, and parabigeminal terminals are relatively small and contain round vesicles na dark mitochondria. Cortical terminals in each of the three representative layers are presynaptic to small diameter dendrites. No convergence of cortical and retinal terminals has been seen in any layer. Labeled tectal and parabigminal terminals are found primarily in the koniocellular layers, but the latter are also seen in all other layers. Tectal and parabigeminal terminals have been observed converging with retinal terminals on dendrites of some koniocellular neurons. Labeled pretectogeniculate terminals contain densely packed pleomorphic vesicles, dark mitochondria, and a dark cytoplasmic matric. These terminals, which are present in each of the representative layers, are presynaptic to conventional dendrites and profiles containing loosely despersed pleomorphic vesicles and a pale cytoplasmic matrix. © 1994 Wiley-Liss, Inc.     

Distribution and ultrastructure of neurons in opossum piriform cortex displaying immunoreactivity to GABA and GAD and high-affinity tritiated GABA uptake

GABAergic neurons have been identified in the piriform cortex of the opossum at light and electron microscopic levels by immunocytochemical localization of GABA and the GABA-synthesizing enzyme glutamic acid decarboxylase and by autoradiographic visualization of high-affinity 3H-GABA uptake. Four major neuron populations have been distinguished on the basis of soma size, shape, and segregation at specific depths and locations: large horizontal cells in layer Ia of the anterior piriform cortex, small globular cells with thin dendrites concentrated in layers Ib and II of the posterior piriform cortex, and multipolar and fusiform cells concentrated in the deep part of layer III in anterior and posterior parts of the piriform cortex and the subjacent endopiriform nucleus. All four populations were well visualized with both antisera, but the large layer Ia horizontal cells displayed only very light 3H-GABA uptake, thus suggesting a lack of local axon collaterals or lack of high-affinity GABA uptake sites. The large, ultrastructurally distinctive somata of layer Ia horizontal cells receive a very small number of symmetrical synapses; the thin, axonlike dendrites of small globular cells are exclusively postsynaptic and receive large numbers of both symmetrical and asymmetrical synapses, in contrast to somata which receive a small number of both types; and the deep multipolar and fusiform cells receive a highly variable number of symmetrical and asymmetrical synapses on somata and proximal dendrites. Labeled puncta of axon terminal dimensions were found in large numbers in the neuropil surrounding pyramidal cell somata in layer II and in the endopiriform nucleus. Moderately large numbers of labeled puncta were found in layer I at the depth of pyramidal cell apical dendrites with greater numbers in layer Ia at the depth of distal apical segments than in layer Ib. High-affinity GABA uptake was demonstrated in the termination zone of the projection from the anterior olfactory nucleus to the anterior piriform cortex. Cell bodies of origin of this projection displayed heavy retrograde labeling with 3H-GABA. Matching neuropil and cellular labeling was demonstrated with the GABA-BSA antiserum but not with the GAD antiserum, thus suggesting that GABA is normally present in these cells but is taken up from the neuropil rather than synthesized. No comparable high-affinity GABA uptake was demonstrated in the association fiber systems that originate in the piriform cortex.(ABSTRACT TRUNCATED AT 400 WORDS)     

Physiological characterization of layer III non-pyramidal neurons in piriform (olfactory) cortex of rat

We performed whole-cell recordings of layer III non-pyramidal neurons in the piriform cortex of Sprague–Dawley rats. For comparison purposes, recordings were made from deep pyramidal cells, which are also present in layer III. These two cell types could be distinguished both anatomically and physiologically. Anatomically, the layer III non-pyramidal neuron displayed smooth beady dendrites, while deep pyramidal cells showed thicker dendrites with spines. The dendrites of the layer III non-pyramidal neuron also tended to be restricted to layer III while deep pyramidal cells had long apical dendrites that spanned layers I and II. Although the resting membrane potentials of both cell types were very similar, significant differences were noted in other physiological measures. Layer III non-pyramidal neurons typically displayed higher input resistances, faster time constants, smaller spike amplitudes, shorter spike widths, and higher spike thresholds. In addition, layer III non-pyramidal neurons were able to spike at much higher rates when stimulated with the same level of threshold normalized current injection. The most dramatic differences in physiology were seen in the pattern of spiking in response to increasing levels of positive constant current pulses. Layer III non-pyramidal neurons showed qualitatively different responses at low and high levels of stimulation. At low levels, spikes occurred with long latency and the firing frequency increased throughout the duration of the current pulse. At high levels, non-pyramidal neurons started spiking with short latency, followed by a decrease in firing frequency, which in turn was followed by an increase in firing frequency. Deep pyramidal neurons differed dramatically from this pattern, displaying a qualitatively similar response at all levels of current injection. This response was characterized by short latency spikes and spike adaptation for the duration of the current pulse.     

Serotoninergic terminals: Ultrastructure and synaptic interaction with catecholamine-containing neurons in the medial nuclei of the solitary tracts

The ultrastructural morphology of serotoninergic terminals and their synaptic relation with catecholaminergic neurons were examined in the medial nuclei of the solitary tracts (m-NTS) using combined autoradiographic and immunocytochemical methods. Adult rats were pretreated with a monoamine oxidase inhibitor and subjected to a 2-hour intraventricular infusion of 50 nM tritiated 5-hydroxytryptamine (³H-5HT). At the termination of the infusion, the brains were fixed by aortic arch perfusion with a mixture of 4% paraformaldehyde and 0.5% glutaraldehyde. Coronal Vibratome sections through the NTS and more rostral raphe nuclei were immunocytochemically labeled with specific antiserum to serotonin or tyrosine hydroxylase and then processed for autoradiography. By light microscopy, concentrations of reduced silver grains indicating uptake of ³H-5HT usually paralleled the localization of peroxidase immunoreactivity for serotonin in neuronal perikarya of the rostral raphe nuclei and in varicosities in the brainstem. The ³H-5HT-containing varicosities were found throughout the medial and commissural portions of the NTS, where they were frequently associated with processes showing immunoreactivity for the catecholamine-synthesizing enzyme tyrosine hydroxylase. Ultrastructural examination of the m-NTS revealed that the silver grains for ³H-5HT were accumulated over axon terminals. The 5HT-labeled terminals contained a heterogeneous population of vesicles and formed both symmetric and asymmetric synapses with dendrites. The recipient dendrites were either unlabeled or showed immunoreactivity for tyrosine hydroxylase. These findings support a direct serotoninergic modulation of catecholaminergic neurons within the rat m-NTS.     

Catecholamine innervation of the basal forebrain. III. Olfactory bulb, anterior olfactory nuclei, olfactory tubercle and piriform cortex.

The catecholamine innervation of the olfactory bulb, anterior olfactory nuclei, olfactory tubercle and piriform cortex was studied in the rat using biochemical analysis and fluorescence histochemistry. Biochemical studies demonstrate a moderate norepinephrine (NE) content in all olfactory structures, a high dopamine (DA) content in the olfactory tubercle and a low DA content in the olfactory bulb, anterior olfactory nucleus and piriform cortex. Following locus coeruleus lesions NE content decreases 71% in the olfactory bulb, 82% in the anterior olfactory nucleus, 62% in olfactory tubercle and 77% in piriform cortex...     

The topographic organization of associational fibers of the olfactory system in the rat, including centrifugal fibers to the olfactory bulb

This study analyzed the topographic organization of the associational fibers within the olfactory cortex of the rat, by using the autoradiographic method. Small injections of 3H-leucine were placed in all of the subdivisions of the olfactory cortex, to label selectively the fibers arising in each area. Intracortical fibers were identified from all of the olfactory cortical areas except the olfactory tubercle and were classified into two major systems (the layer Ib system and the layer II-deep Ib system) on the basis of their laminar pattern of termination (see Luskin and Price, '83). The layer Ib fiber system arises in the anterior olfactory nucleus, piriform cortex, and lateral entorhinal area, and is broadly organized in relation to the lateral olfactory tract. Cortical areas deep to or near the lateral olfactory tract are preferentially interconnected with areas near the tract, while parts of the cortex lateral and caudal to the lateral olfactory tract are most heavily interconnected with areas lateral, caudal, and medial to the tract. Commissural projections from the anterior olfactory nucleus and the anterior piriform cortex match some (but not all) components of the ipsilateral layer Ib fiber system. The layer II-deep Ib fiber system arises in three small areas--the ventral tenia tecta, the dorsal peduncular cortex, and the periamygdaloid cortex. The fibers from the ventral tenia tecta terminate in layer II of the anterior olfactory nucleus and are topographically organized. The fibers from the dorsal peduncular cortex and the periamygdaloid cortex are more widely distributed, especially in the lateral and caudal parts of the cortex. Two other intracortical projections do not fit into either of these fiber systems. The nucleus of the lateral olfactory tract projects bilaterally to the islands of Calleja and the medial edge of the anterior piriform cortex. The anterior cortical nucleus projects to many parts of the olfactory cortex, but the fibers end in both superficial and deep parts of layer I (layer Ia and Ib). There are projections from several of the olfactory cortical areas to the cortical areas surrounding the olfactory cortex. Virtually all of the olfactory areas also project to the ventral and dorsal endopiriform nuclei deep to the piriform cortex and/or to the polymorph zone deep to the olfactory tubercle. In addition, projections have been demonstrated to the deep amygdaloid nuclei, especially from the more ventromedial and caudal parts of the olfactory cortex.     

Functional deprivation promotes amyloid plaque pathogenesis in Tg2576 mouse olfactory bulb and piriform cortex

Cerebral hypometabolism and amyloid accumulation are principal neuropathological manifestations of Alzheimer’s disease (AD). Whether and how brain/neuronal activity might modulate certain pathological processes of AD are interesting topics of recent clinical and basic research in the field, and may be of potential medical relevance in regard to both the disease etiology and intervention. Using the Tg2576 transgenic mouse model of AD, this study characterized a promotive effect of neuronal hypoactivity associated with functional deprivation on amyloid plaque pathogenesis in the olfactory pathway. Unilateral naris‐occlusion caused β‐secretase‐1 (BACE1) elevation in neuronal terminals in the deprived relative to the non‐deprived bulb and piriform cortex in young adult mice. In parallel with the overall age‐related plaque development in the forebrain, locally increased BACE1 immunoreactivity co‐occurred with amyloid deposition first in the piriform cortex then within the bulb, more prominent on the deprived relative to the non‐deprived side. Biochemical analyses confirmed elevated BACE1 protein levels, enzymatic activity and products in the deprived relative to non‐deprived bulbs. Plaque‐associated BACE1 immunoreactivity in the bulb and piriform cortex was localized preferentially to swollen/sprouting glutamatergic axonal terminals, with Aβ immunoreactivity occurring inside as well as around these terminals. Together, these findings suggest that functional deprivation or neuronal hypoactivity facilitates amyloid plaque formation in the forebrain in a transgenic model of AD, which operates synergistically with age effect. The data also implicate an intrinsic association of amyloid accumulation and plaque formation with progressive axonal pathology.     

The laminar distribution of intracortical fibers originating in the olfactory cortex of the rat

In this study, the autoradiographic method for tracing axonal connections was used to identify the laminar distribution of intracortical fibers originating in the olfactory cortical areas of the rat. Most of the projections can be divided into two major fiber systems with different laminar patterns of termination. The first of these, termed the layer Ib fiber system, arises in the anterior olfactory nucleus, the anterior and posterior piriform cortex, and the lateral entorhinal cortex, and terminates predominantly in layer Ib and, in many cases, layer III of the entire olfactory cortex. The second system, termed the layer II-deep Ib fiber system, originates in three relatively small olfactory cortical areas-the dorsal peduncular cortex, the ventral tenia tecta, and the periamygdaloid cortex and terminates in and around the cells of layer II in most parts of the olfactory cortex. There is significant overlap in the laminar distribution of the two systems, although the distinction between them is readily apparent. Within the layer Ib fiber system there are relatively slight but consistent differences in the lamination of fibers from different areas. The fibers from the anterior olfactory nucleus are concentrated in the deep part of layer Ib while those from the anterior piriform cortex are concentrated in the superficial part of this layer. The fibers from the posterior piriform cortex tend to be densest in the middle of layer Ib. These differences are maintained in all areas of termination of each set of fibers, both ipsilaterally and contra-laterally. In addition, intracortical fibers from the anterior cortical nucleus of the amygdala are distributed throughout layer I, including layer la and Ib. Fibers from the nucleus of the lateral olfactory tract terminate bilaterally around the cells of the islands of Callej a and the medial edge of the anterior piriform cortex.     

Ultrastructural localization of immunoreactivity in the developing piriform cortex

The purpose of this study was to determine the ultrastructural basis for the immunoreactivity patterns in synaptic structures during development in layers I and II of the piriform cortex (PC) of rats. Antisera to cholecystokinin (CCK) and glutamic acid decarboxylase (GAD) were used at several different postnatal days (PN) and in adults to describe the distribution, characteristics, and relative frequency of labeled profiles--especially axons and terminals--with emphasis on details of the synaptic contacts. GAD-positive terminals occur from PN 2 to adulthood but only form contacts in deeper sublayers (Ib and II) initially. Contacts increase in layer I after PN 6 and are reduced in layer II after PN 21 when the GAD-labeled terminals and synapses take on adult features with flattened vesicles and symmetric contacts. CCK-labeled terminals are present in deeper sublayers at PN 2 but are few and rarely form contacts. Both terminals and contacts increase between PN 2 and 9, taking on distinctive shapes and vesicle morphology by PN 13. At PN 21 and older, CCK terminals have mainly flattened vesicles and mostly form symmetric contacts onto dendrites and somata in deeper layers (Ib and II). Superficial sublayer Ia has very few CCK-labeled synapses and axons. Thus immunoreactivity occurs in terminals prior to synapse formation; labeling of the presynaptic specializations precedes subsequent maturation; synaptic vesicle morphology and membrane specializations are similar for the vast majority of both CCK and GAD terminals; inhibitory (GABA) synapses are established sooner than the possibly excitatory CCK synapses; a deep to superficial gradient of synaptogenesis is associated with GAD-positive terminals in the PC; and the labeling patterns may be related to critical developmental or synaptogenic periods.     

Ultrastructural analyses of afferent terminals in the subthalamic nucleus of the cat with a combined degeneration and horseradish peroxidase tracing method

The synaptic organization of the feline subthalamic nucleus (STN) was studied electron microscopically. Following horseradish peroxidase (HRP) injections into the globus pallidus (GP) and electrolytic lesions of the nucleus tegmenti pedunculopontinus pars compacta (TCP) in the same cat, both degenerating and HRP-labeled terminals were found in the STN with abundant retrogradely HRP-labeled neurons. Degenerating terminals of TPC origin were medium-sized and characterized by asymmetric synaptic contacts. They synapsed widely on the STN neuronal surface, including the somata, dendrites of varying dimensions, dendritic spines and vesicle-containing processes. They formed 25.1%, 65.1%, 4.7%, and 4.7%, respectively, of all TPC efferent terminals. Some of the postsynaptic components were labeled with HRP. Occasionally both degenerating terminals and HRP-labeled terminals were in synaptic contact with the same HRP-labeled neuron: therefore, afferents of TPC and GP converge on the same STN projection neuron. In order to discover the origin of these HRP-labeled terminals, a mixed solution containing HRP and kainic acid was injected into the GP. Numerous degenerating terminals were observed to synapse with HRP-labeled STN neurons, but no HRP-labeled terminal was observed. These degenerating terminals were similar in appearance to the above-mentioned HRP-labeled terminals. They were characterized by their relatively large size, predominantly symmetric synapses, and preferential distribution on the somata and large or medium-sized dendrites. They formed 39.6%, 20.1%, and 31.1%, respectively, of all GP efferent terminals. Therefore, it became clear that both the HRP-labeled terminals of the first experiment and the degenerating terminals of the second experiment originated from the GP. Following surgical ablations of the primary sensorimotor cortex (Cx), some axon terminals in the STN showed degeneration. These degenerating terminals were small and formed asymmetric synapses mainly with dendritic spines, small dendrites and vesicle-containing processes. They formed 48.0%, 28.0%, and 12.0%, respectively, of all Cx efferent terminals. These electron microscopic investigations reveal the convergence of TPC and GP afferents and that STN projection neurons relay the TPC and pallidal inputs directly to the GP.     

Architecture and intrinsic connections of the prefrontal cortex in the rhesus monkey

An investigation of the architectonic organization and intrinsic connections of the prefrontal cortex was conducted in rhesus monkeys. Cytoarchitectonic analysis indicates that in the prefrontal cortex there are two trends of gradual change in laminar characteristics that can be traced from limbic periallocortex towards isocortical areas. The stepwise change in laminar features is characterized by the emergence and gradual increase in the width of granular layer IV, by an increase in the size of pyramidal cells in layers III and V, and by a higher cell-packing density in the supragranular layers. Myeloarchitectonic analysis reveals that the limbic areas are poorly myelinated, adjacent areas have a diffuse myelin content confined to the deep layers, and in isocortices the myelinated fibers are distributed in organized horizontal bands (of Baillarger) and a vertical plexus. Using the above architectonic criteria, we observed that one of the architectonic trends takes a radial basoventral course from the periallocortex in the caudal orbitofrontal region to the adjacent proisocortex and then to area 13. The next stage of architectonic regions includes orbital areas 12, 11, and 14, which is followed by area 10, lateral area 12, and the rostral part of ventral area 46. The last group includes the caudal part of ventral area 46 and ventral area 8. The other trend takes a mediodorsal course from the periallocortex around the rostral portion of the corpus callosum to the adjacent proisocortical areas 24, 25, and 32 and then to the medially situated isocortical areas 9, 10, and 14. The next stage includes lateral areas 10 and 9 and the rostral part of dorsal area 46. The last group includes the caudal part of dorsal area 46 and dorsal area 8. The interconnections of subdivisions of the basoventral and mediodorsal cortices were studied with the aid of anterograde and retrograde tracers. Within each trend a given area projects in two directions: to adjoining regions belonging to succeeding architectonic stages on the one hand, and to nearby regions from the preceding architectonic stage on the other. In each direction there is more than one region involved in this projection system, paralleling the radial nature of architectonic change. Periallo- and proisocortices have widespread intrinsic connections, whereas isocortices situated at a distance from limbic areas, such as area 8, have restricted connections. Most interconnections are limited to areas within the same architectonic trend. However, there are links between cortices from the two trends, and these seem to occur between areas that are at a similar stage of architectonic differentiation. The results suggest that there are two architectonically, and perhaps functionally, distinct axes within the prefrontal cortex. The earliest stages within each axis, which have widespread connections, may have a global role in neural processing. On the other hand, the latest stages, which have restricted connections, may have a more specific role in processes associated with the frontal lobe.