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

Distribution of GABAergic Neurons and VGluT1 and VGAT Immunoreactive Boutons in the Ferret (Mustela putorius) Piriform Cortex and Endopiriform Nucleus. Comparison With Visual Areas 17, 18 and 19

We studied the cellular organization of the piriform network [comprising the piriform cortex (PC) and endopiriform nucleus (EP)] of the ferret (Mustela putorius)-a highly excitable region prone to seizures-and, more specifically, the distribution and morphology of different types of gamma-aminobutyric acid (GABA)ergic neurons, and the distribution and ratio of glutamatergic and GABAergic boutons, and we compared our findings to those in primary visual area 17, and secondary areas 18 and 19. We accomplished this by using cytochrome oxidase and immunohistochemistry for mature neuronal nuclei (NeuN), GABAergic neurons [glutamic acid decarboxylase-67 (GAD67), calretinin (CR) and parvalbumin (PV)], and for excitatory (vesicular glutamate transporter 1; VGluT1) and inhibitory (vesicular GABA transporter; VGAT) boutons. In the ferret, the cellular organization of the piriform network is similar to that described in other species such as cats, rats and opossums although some differences also exist. GABAergic immunolabeling showed similarities between cortical layers I-III of the PC and visual areas, such as the relative distribution of GABAergic neurons and the density and area of VGluT1- and VGAT-immunoreactive boutons. However, multiple differences between the piriform network and visual areas (layers I-VI) were found, such as the percentage of GABAergic neurons with respect to the total number of neurons and the ratio of VGluT1- and VGAT-immunoreactive boutons. These findings are relevant to better understand the high excitability of the piriform network.

Quantitative analysis of axon collaterals of single pyramidal cells of the anterior piriform cortex of the guinea pig

BACKGROUND

The role of the piriform cortex (PC) in olfactory information processing remains largely unknown. The anterior part of the piriform cortex (APC) has been the focus of cortical-level studies of olfactory coding, and associative processes have attracted considerable attention as an important part in odor discrimination and olfactory information processing. Associational connections of pyramidal cells in the guinea pig APC were studied by direct visualization of axons stained and quantitatively analyzed by intracellular biocytin injection in vivo.

RESULTS

The observations illustrated that axon collaterals of the individual cells were widely and spatially distributed within the PC, and sometimes also showed a long associational projection to the olfactory bulb (OB). The data showed that long associational axons were both rostrally and caudally directed throughout the PC, and the intrinsic associational fibers of pyramidal cells in the APC are omnidirectional connections in the PC. Within the PC, associational axons typically followed rather linear trajectories and irregular bouton distributions. Quantitative data of the axon collaterals of two pyramidal cells in the APC showed that the average length of axonal collaterals was 101 mm, out of which 79 mm (78% of total length) were distributed in the PC. The average number of boutons was 8926 and 7101, respectively, with 79% of the total number of boutons being distributed in the PC. The percentage of the total area of the APC and the posterior piriform cortex occupied by the average distribution region of the axon collaterals of two superficial pyramidal (SP) cells was about 18 and 5%, respectively.

CONCLUSION

Our results demonstrate that omnidirectional connection of pyramidal cells in the APC provides a substrate for recurrent processes. These findings indicate that the axon collaterals of SP cells in the PC could make synaptic contacts with all granule cells in the OB. This study provides the morphological evidence for understanding the mechanisms of information processing and associative memory in the APC.

Grid cells and cortical representation

One of the grand challenges in neuroscience is to comprehend neural computation in the association cortices, the parts of the cortex that have shown the largest expansion and differentiation during mammalian evolution and that are thought to contribute profoundly to the emergence of advanced cognition in humans. In this Review, we use grid cells in the medial entorhinal cortex as a gateway to understand network computation at a stage of cortical processing in which firing patterns are shaped not primarily by incoming sensory signals but to a large extent by the intrinsic properties of the local circuit.

Coding and transformations in the olfactory system

How is sensory information represented in the brain? A long-standing debate in neural coding is whether and how timing of spikes conveys information to downstream neurons. Although we know that neurons in the olfactory bulb (OB) exhibit rich temporal dynamics, the functional relevance of temporal coding remains hotly debated. Recent recording experiments in awake behaving animals have elucidated highly organized temporal structures of activity in the OB. In addition, the analysis of neural circuits in the piriform cortex (PC) demonstrated the importance of not only OB afferent inputs but also intrinsic PC neural circuits in shaping odor responses. Furthermore, new experiments involving stimulation of the OB with specific temporal patterns allowed for testing the relevance of temporal codes. Together, these studies suggest that the relative timing of neuronal activity in the OB conveys odor information and that neural circuits in the PC possess various mechanisms to decode temporal patterns of OB input.

Gain control of γ frequency activation by a novel feed forward disinhibitory loop: implications for normal and epileptic neural activity

The inhibition of excitatory (pyramidal) neurons directly dampens their activity resulting in a suppression of neural network output. The inhibition of inhibitory cells is more complex. Inhibitory drive is known to gate neural network synchrony, but there is also a widely held view that it may augment excitability by reducing inhibitory cell activity, a process termed disinhibition. Surprisingly, however, disinhibition has never been demonstrated to be an important mechanism that augments or drives the activity of excitatory neurons in a functioning neural circuit. Using voltage sensitive dye imaging (VSDI) we show that 20–80 Hz stimulus trains, β–γ activation, of the olfactory cortex pyramidal cells in layer II leads to a subsequent reduction in inhibitory interneuron activity that augments the efficacy of the initial stimulus. This disinhibition occurs with a lag of about 150–250 ms after the initial excitation of the layer 2 pyramidal cell layer. In addition, activation of the endopiriform nucleus also arises just before the disinhibitory phase with a lag of about 40–80 ms. Preventing the spread of action potentials from layer II stopped the excitation of the endopiriform nucleus, abolished the disinhibitory activity, and reduced the excitation of layer II cells. After the induction of experimental epilepsy the disinhibition was more intense with a concomitant increase in excitatory cell activity. Our observations provide the first evidence of feed forward disinhibition loop that augments excitatory neurotransmission, a mechanism that could play an important role in the development of epileptic seizures.

Olfactory consciousness and gamma oscillation couplings across the olfactory bulb, olfactory cortex, and orbitofrontal cortex

The orbitofrontal cortex receives multi-modality sensory inputs, including olfactory input, and is thought to be involved in conscious perception of the olfactory image of objects. Generation of olfactory consciousness may require neuronal circuit mechanisms for the “binding” of distributed neuronal activities, with each constituent neuron representing a specific component of an olfactory percept. The shortest neuronal pathway for odor signals to reach the orbitofrontal cortex is olfactory sensory neuron—olfactory bulb—olfactory cortex—orbitofrontal cortex, but other pathways exist, including transthalamic pathways. Here, we review studies on the structural organization and functional properties of the shortest pathway, and propose a model of neuronal circuit mechanisms underlying the temporal bindings of distributed neuronal activities in the olfactory cortex. We describe a hypothesis that suggests functional roles of gamma oscillations in the bindings. This hypothesis proposes that two types of projection neurons in the olfactory bulb, tufted cells and mitral cells, play distinct functional roles in bindings at neuronal circuits in the olfactory cortex: tufted cells provide specificity-projecting circuits which send odor information with early-onset fast gamma synchronization, while mitral cells give rise to dispersedly-projecting feed-forward binding circuits which transmit the response synchronization timing with later-onset slow gamma synchronization. This hypothesis also suggests a sequence of bindings in the olfactory cortex: a small-scale binding by the early-phase fast gamma synchrony of tufted cell inputs followed by a larger-scale binding due to the later-onset slow gamma synchrony of mitral cell inputs. We discuss that behavioral state, including wakefulness and sleep, regulates gamma oscillation couplings across the olfactory bulb, olfactory cortex, and orbitofrontal cortex.

Olfactory cortex generates synchronized top-down inputs to the olfactory bulb during slow-wave sleep

The olfactory cortex is functionally isolated from the external odor world during slow-wave sleep. However, the neuronal activity pattern in the olfactory cortex and its functional roles during slow-wave sleep are not well understood. Here, we demonstrate in freely behaving rats that the anterior piriform cortex, a major area of the olfactory cortex, repeatedly generates sharp waves that are accompanied by synchronized discharges of numerous cortical neurons. Olfactory cortex sharp waves occurred relatively independently of hippocampal sharp waves. Current source density analysis showed that sharp wave generation involved the participation of recurrent association fiber synapses to pyramidal cells in the olfactory cortex. During slow-wave sleep, the olfactory bulb showed sharp waves that were in synchrony with olfactory cortex sharp waves, indicating that olfactory cortex sharp waves drove synchronized top-down inputs to the olfactory bulb. Based on these results, we speculate that the olfactory cortex sharp waves may play a role in the reorganization of bulbar neuronal circuits during slow-wave sleep.

GABAergic inhibitory interneurons in the posterior piriform cortex of the GAD67-GFP mouse

Gamma-aminobutyric acid (GABA)-releasing inhibitory interneurons, a critical component of cortical circuitry, are involved in myriad known functional roles. However, information regarding the cytoarchitectural, physiological, and molecular properties of interneurons in posterior piriform cortex (PPC) is sparse. Taking advantage of the glutamic acid decarboxylase (GAD)67-enhanced green fluorescent protein (EGFP) mouse, we used in vitro whole-cell patch-clamp techniques to record from GABAergic interneurons across all 3 layers of PPC and, subsequently, to reconstruct their morphology. For the first time, 5 groups of interneurons are identified, whose firing types are defined based on those described within neocortex. Interestingly, each interneuron group with a distinct firing type also exhibits unique morphological properties, laminar distributions, and excitatory synaptic properties. The dendritic and axonal processes demonstrate subtype-specific orientations and a differential expression of spines and boutons, respectively. In addition, the active and passive electrophysiological properties of these cells show marked intergroup differences. Immunohistochemical techniques revealed a laminar-specific distribution of calcium-binding proteins and vasoactive intestinal peptide (VIP) expression. Surprisingly, excitatory synaptic properties in several groups lack target-specific differences seen in neocortical circuits, reflecting a circuit arranged with less complexity. These data aid in the identification of PPC interneurons and allow us to make well-supported postulations about their functional properties.

An early critical period for long-term plasticity and structural modification of sensory synapses in olfactory cortex

Critical periods for plasticity of thalamic sensory inputs play an important role in developing neocortical circuits. During an early postnatal time window, pyramidal cells of visual, auditory, and somatosensory cortex undergo structural refinement and possess an enhanced ability for activity-dependent synaptic plasticity. In olfactory cortex, however, pyramidal cells receive direct sensory input from the olfactory bulb, and it is unclear whether the development of olfactory sensory circuits is governed by a critical period. Here, we show that NMDA receptor-dependent long-term potentiation and dendritic spine maturation occur only during a brief postnatal time window at sensory synapses of olfactory cortex pyramidal cells. In contrast, associational synapses onto the same cells retain the capacity for plasticity into adulthood.

Acute changes in the neuronal expression of GABA and glutamate decarboxylase isoforms in the rat piriform cortex following status epilepticus

The piriform cortex (PC) is the largest region of the mammalian olfactory cortex with strong connections to other limbic structures, including the amygdala, hippocampus, and entorhinal cortex. In addition to its functional importance in the classification of olfactory stimuli, the PC has been implicated in the study of memory processing, spread of excitatory information, and the facilitation and propagation of seizures within the limbic system. Previous data from the kindling model of epilepsy indicated that alterations in GABAergic inhibition in the transition zone between the anterior and posterior PC, termed here central PC, are particularly involved in the processes underlying seizure propagation. In the present study we studied alterations in GABAergic neurons in different parts of the PC following seizures induced by kainate or pilocarpine in rats. GABA neurons were labeled either immunohistochemically for GABA or its synthesizing enzyme glutamate decarboxylase (GAD) or by in situ hybridization using antisense probes for GAD65 and GAD67 mRNAs. For comparison with the PC, labeled neurons were examined in the basolateral amygdala, substantia nigra pars reticulata, and the hippocampal formation. In the PC of controls, immunohistochemical labeling for GABA and GAD yielded consistently higher neuronal densities in most cell layers than labeling for GAD65 or GAD67 mRNAs, indicating a low basal activity of these neurons. Eight hours following kainate- or pilocarpine-induced seizures, severe neuronal damage was observed in the PC. Counting of GABA neurons in the PC demonstrated significant decreases in densities of neurons labeled for GABA or GAD proteins. However, a significantly increased density of neurons labeled for GAD65 and GAD67 mRNAs was determined in layer II of the central PC, indicating that a subpopulation of remaining neurons up-regulated the mRNAs for the GAD isoenzymes. One likely explanation for this finding is that remaining GABA neurons in layer II of the central PC maintain high levels of activity to control the increased excitability of the region. In line with previous studies, an up-regulation of GAD67 mRNA, but not GAD65 mRNA, was observed in dentate granule cells following seizures, whereas no indication of such up-regulation was determined for the other brain regions examined. The data substantiate the particular susceptibility of the central PC to seizure-induced plasticity and indicate that this brain region provides an interesting tool to study the regulation of GAD isoenzymes.

Quantitative analysis of axon collaterals of single superficial pyramidal cells in layer IIb of the piriform cortex of the guinea pig

To understand the functional organization of the piriform cortex (PC), the axon collaterals of three pyramidal cells in layer IIb of the anterior PC and one pyramidal cell in layer IIb of the posterior PC were labeled and quantitatively analyzed by intracellular biocytin injection in the guinea pig. Single pyramidal cells in the anterior and posterior PCs have widely distributed axon collaterals, which exhibit little tendency for patchy concentrations inside as well as outside the PC. The total lengths of the axon collaterals of the three fully analyzed pyramidal cells ranged from 68 to 156 mm, more than 50% of which were distributed in the PC. The total number of boutons of the three cells ranged from 6000 to 14,000, 5000-7000 of which were distributed in the PC. It was estimated that individual pyramidal cells in layer IIb form synaptic contacts with 2200 to 3000 other pyramidal cells in the PC, indicating that single pyramidal cells in layer IIb receive input from a large number of other pyramidal cells. This high connectivity of the network of pyramidal cells in the PC can be regarded as the neural network operating parallel distributed processing, which may play an important role in experience-induced enhancement in odorant discrimination in the PC.

Quantitative analysis of axon collaterals of single cells in layer III of the piriform cortex of the guinea pig

Recent physiological and morphological studies suggest that the piriform cortex (PC) functions like the association areas of the neocortex rather than the typical primary sensory area as was previously assumed. The axon connection patterns of single cells are important for understanding the functional organization of the PC. The axon collaterals of three single pyramidal cells and one spiny multipolar cell in layer III of the PC were labeled and quantitatively analyzed by intracellular injections of biocytin in guinea pigs. The individual pyramidal and spiny multipolar cells have highly distributed axon collaterals, which display little tendency for patchy concentrations, within the PC and multiple higher order behavior/reward/contextual-related areas, such as the prefrontal cortex, amygdaloid nuclei, and entorhinal cortex. For the pyramidal cells, the average length of axonal collaterals is 143 mm; the average number of boutons is 12,930. For the spiny multipolar cell, the length of the axonal collaterals is 88 mm; the number of boutons is 7,052. The pyramidal cells in the anterior subdivision of the PC (APC) have both rostrally and caudally directed intrinsic association fibers, whereas the pyramidal and spiny multipolar cells in the posterior subdivision (PPC) have predominantly caudally directed intrinsic association fibers in the PC. Our results reveal that the connection patterns of single cells in layer III resemble those of pyramidal cells in layer II, suggesting that the PC performs correlative functions analogous to those in the association area of other sensory systems. The rostrally-to-caudally directed connections in the APC provide a substrate for the recurrent process, whereas largely caudally directed connections in the PPC suggest the dominance of the feed-forward process.

A new subdivision of anterior piriform cortex and associated deep nucleus with novel features of interest for olfaction and epilepsy

The anterior part of the piriform cortex (the APC) has been the focus of cortical-level studies of olfactory coding and associative processes and has attracted considerable attention as a result of a unique capacity to initiate generalized tonic-clonic seizures. Based on analysis of cytoarchitecture, connections, and immunocytochemical markers, a new subdivision of the APC and an associated deep nucleus are distinguished in the rat. As a result of its ventrorostral location in the APC, the new subdivision is termed the APC(VR). The deep nucleus is termed the pre-endopiriform nucleus (pEn) based on location and certain parallels to the endopiriform nucleus. The APC(VR) has unique features of interest for normal function: immunostaining suggests that it receives input from tufted cells in the olfactory bulb in addition to mitral cells, and it provides a heavy, rather selective projection from the piriform cortex to the ventrolateral orbital cortex (VLO), a prefrontal area where chemosensory, visual, and spatial information converges. The APC(VR) also has di- and tri-synaptic projections to the VLO via the pEn and the submedial thalamic nucleus. The pEn is of particular interest from a pathological standpoint because it corresponds in location to the physiologically defined "deep piriform cortex" ("area tempestas") from which convulsants initiate temporal lobe seizures, and blockade reduces ischemic damage to the hippocampus. Immunostaining revealed novel features of the pEn and APC(VR) that could alter excitability, including a near-absence of gamma-aminobutyric acid (GABA)ergic "cartridge" endings on axon initial segments, few cholecystokinin (CCK)-positive basket cells, and very low gamma-aminobutyric acid transporter-1 (GAT1)-like immunoreactivity. Normal functions of the APC(VR)-pEn may require a shaping of neuronal activity by inhibitory processes in a fashion that renders this region susceptible to pathological behavior.

New features of connectivity in piriform cortex visualized by intracellular injection of pyramidal cells suggest that "primary" olfactory cortex functions like "association" cortex in other sensory systems

Associational connections of pyramidal cells in rat posterior piriform cortex were studied by direct visualization of axons stained by intracellular injection in vivo. The results revealed that individual cells have widespread axonal arbors that extend over nearly the full length of the cerebral hemisphere. Within piriform cortex these arbors are highly distributed with no regularly arranged patchy concentrations like those associated with the columnar organization in other primary sensory areas (i.e., where periodically arranged sets of cells have common response properties, inputs, and outputs). A lack of columnar organization was also indicated by a marked disparity in the intrinsic projection patterns of neighboring injected cells. Analysis of axonal branching patterns, bouton distributions, and dendritic arbors suggested that each pyramidal cell makes a small number of synaptic contacts on a large number (>1000) of other cells in piriform cortex at disparate locations. Axons from individual pyramidal cells also arborize extensively within many neighboring cortical areas, most of which send strong projections back to piriform cortex. These include areas involved in high-order functions in prefrontal, amygdaloid, entorhinal, and perirhinal cortex, to which there are few projections from other primary sensory areas. Our results suggest that piriform cortex performs correlative functions analogous to those in association areas of neocortex rather than those typical of primary sensory areas with which it has been traditionally classed. Findings from other studies suggest that the olfactory bulb subserves functions performed by primary areas in other sensory systems.

Characteristics of plateau activity during the latent period prior to epileptiform discharges in slices from rat piriform cortex

The deep piriform region has an unusually high seizure susceptibility. Voltage imaging previously located the sites of epileptiform discharge onset in slices of rat piriform cortex and revealed the spatiotemporal pattern of development of two types of electrical activity during the latent period prior to discharge onset. A ramplike depolarization (onset activity) appears at the site of discharge onset. Onset activity is preceded by a sustained low-amplitude depolarization (plateau activity) at another site, which shows little if any overlap with the site of onset. Because synaptic blockade at either of these two sites blocks discharges, it was proposed that both forms of latent period activity are necessary for the generation of epileptiform discharges and that the onset and plateau sites work together in the amplification of electrical activity. The capacity for amplification was examined here by studying subthreshold responses in slices of piriform cortex using two different in vitro models of epilepsy. Under some conditions electrically evoked responses showed a nonlinear dependence on stimulus current, suggesting amplification by strong polysynaptic excitatory responses. The sites of plateau and onset activity were mapped for different in vitro models of epilepsy and different sites of stimulation. These experiments showed that the site of plateau activity expanded into deep layers of neighboring neocortex in parallel with expansions of the onset site into neocortex. These results provide further evidence that interactions between the sites of onset and plateau activity play an important role in the initiation of epileptiform discharges. The site of plateau activity showed little variation with different stimulation sites in the piriform cortex, but when stimulation was applied in the endopiriform nucleus (in the sites of onset of plateau activity), plateau activity had a lower amplitude and became distributed over a much wider area. These results indicate that in the initiation of epileptiform discharges, the location of the circuit that generates plateau activity is not rigidly defined but can exhibit flexibility.

Projections from the lateral vestibular nucleus to the upper cervical spinal cord of the cat: A correlative light and electron microscopic study of axon terminals stained with PHA-L

Vestibulospinal axon collaterals in C1 and C2 were stained following injections of Phaseolus vulgaris leucoagglutinin (PHA-L) into the lateral vestibular nucleus (LVN). The distribution and geometry of collaterals within three regions of the ventral horn were determined at the light microscopic level. These processes were subsequently examined at the electron microscopic level to define the relationship between their ultrastructural characteristics and their geometry and location. All round or elliptical varicosities, whose diameters exceeded the diameter of the adjacent axon shaft by a factor of two, as measured at the light microscopic level, contained synaptic vesicles and contacted dendrites or somata. These varicosities accounted for 82% of labelled axon terminals found at the electron microscopic level. Thus, axon terminals stained with PHA-L can be identified reliably at the light microscopic level, but synaptic density will be slightly underestimated. One-hundred and thirty-eight axon terminals were classified as excitatory or inhibitory on the basis of well-established morphological criteria (e.g., vesicle shape). Placed in the context of previous physiological observations describing the excitatory or inhibitory actions of medial and lateral vestibulospinal tract (MVST and LVST) neurons, our results suggest that projections from the LVN to the ipsilateral ventral horn originate primarily from the LVST. These connections are excitatory. Ipsilateral connections via the MVST are inhibitory and are largely confined to a region near the border of laminae VII and VIII. Most axon terminals in the contralateral ventral horn were inhibitory. This result indicates that the LVN is the source of a specific subset of crossed MVST axons with inputs from the posterior semicircular canal.

Differences in the distribution of GABA- and GAD-immunoreactive neurons in the anterior and posterior piriform cortex of rats

There is accumulating evidence of anterior-posterior differences in the susceptibility of the piriform cortex to seizure induction and to functional alterations in response to seizures elicited from other limbic brain regions, but the reasons for such differences along the anterior-posterior axis of the piriform cortex are not clear. In the present study, GABAergic neurons have been identified in the piriform cortex of the rat at light microscopic level by immunocytochemical localization of GABA and the GABA-synthesizing enzyme glutamic acid decarboxylase. A monoclonal antibody to GABA and, for comparison, polyclonal antibodies to GABA and glutamic acid decarboxylase were used for this purpose. In both anterior and posterior piriform cortex, the highest number and density of GABA-immunoreactive cells was found in layer II. Lower density of GABAergic cells was found in layers I and III and the subjacent endopiriform cortex. When cells were quantified in 19 corresponding sections of the piriform cortex, covering most of anterior-posterior extension of this region, there appeared to be an increased density of GABAergic neurons in sections near to or within the transition zone between anterior and posterior piriform cortex. A more detailed analysis at 4 section levels in the anterior and posterior piriform cortex and the transition zone between the 2 parts substantiated a significantly higher density of GABAergic neurons in the transition zone, which was predominantly due to increased numbers of cells in layers II and III. We propose that the transition zone between anterior and posterior piriform cortex is a location where numerous GABAergic interneurons regulate the activity of neighbouring deep pyramidal cells which receive dense excitatory input from both the olfactory bulb and distant pyramidal cells in the more anterior and posterior parts of the piriform cortex at the same time, thus increasing the risk of paroxysmal activation within this restricted area. This proposal is in line with recent observations of increased susceptibility to epileptiform activation and to kindling-induced neurochemical alterations within the transition zone between anterior and posterior piriform cortex.

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.

Organization of the projections from the pericruciate cortex to the pontomedullary brainstem of the cat: a study using the anterograde tracer Phaseolus vulgaris-leucoagglutinin

The anterograde tracer Phaseolus vulgaris-leucoagglutinin (PHA-L) was used to study the distribution and density of the projections that originate from four identified subdivisions of the pericruciate cortex (namely, the forelimb and hind limb representations of area 4, area 6a beta, and area 6a gamma) and that terminate in the pontomedullary brainstem in the cat. Injections of PHA-L in all areas of the pericruciate cortex labelled numerous fibers and their terminal swellings in the brainstem. The major target regions of all four cortical areas were the pontine nuclei and the pontomedullary reticular formation (PMRF). Injections into both the forelimb and hind limb representations of area 4 and into area 6a beta resulted in a dense pattern of terminal labelling in restricted regions of the medial and lateral parts of the ipsilateral pontine nuclei. The labelling following the area 6a beta injection was spatially distinct from that seen following the area 4 injections. Injections into the forelimb representation of area 4 as well as into area 6a beta and 6a gamma resulted in the labelling of numerous terminal swellings bilaterally in the PMRF; in contrast, there were few labelled terminal swellings in the PMRF following injections into the hind limb representation of area 4. Terminal swellings on individual corticoreticular fibers were far less densely aggregated than those in the pontine nuclei. The dense pattern of innervation to restricted regions of the pontine nuclei supports previous suggestions that the corticopontine projections retain a high degree of topographical specificity that could be used in the control of discrete voluntary movements. In contrast, the more diffuse pattern of the projections to the PMRF may facilitate the selection and activation of the complex postural patterns that accompany voluntary movement.

Intrinsic association fiber system of the piriform cortex: a quantitative study based on a cholera toxin B subunit tracing in the rat

By using retrograde and anterograde transport of the B subunit of cholera toxin (CTb), we examined quantitatively the association fiber systems, i.e., the collaterals of pyramidal cell axons, that reciprocally connect both the rostral and the caudal parts of the piriform cortex (PC). Well-defined CTb injections were obtained in layers Ib or II-III of the rostral and the caudal parts of the PC. Using precision counting, we determined the proportion of cellular profiles in layers II and III that gave rise to association fibers and thus demonstrated a predominance of rostrocaudal fibers over the caudorostral ones. Our data also support a precise laminar organization of the PC in which the rostrocaudal fibers originated mainly from layer II and the caudorostral fibers primarily from layer III. Cholera toxin injections into layer Ib produced a peak of labeled profiles 2 mm from the site, indicating that a large proportion of the association fibers from layer II travel for at least 2 mm and then synapse in layer Ib. At either end of the PC, the association projections with respect to olfactory processing, propagation of the activity within the PC, and the possible role of intrinsic fibers in olfactory memory.

The role of the piriform cortex in kindling

In epilepsy research, there is growing interest in the role of the piriform cortex (PC) in the development and maintenance of limbic kindling and other types of limbic epileptogenesis leading to complex partial seizures, i.e. the most common type of seizures in human epilepsy. The PC ("primary olfactory cortex") is the largest area of the mammalian olfactory cortex and receives direct projections from the olfactory bulb via the lateral olfactory tract (LOT). Beside the obvious involvement in olfactory perception and discrimination, the PC, because of its unique intrinsic associative fiber system and its various connections to and from other limbic nuclei, has been implicated in the study of memory processing, spread of excitatory waves, and in the study of brain disorders such as epilepsy with particular emphasis on the kindling model of temporal lobe epilepsy with complex partial seizures. The interest in the kindling model is based primarily on the following observations. (1) The PC contains the most susceptible neural circuits of all forebrain regions for electrical (or chemical) induction of limbic seizures. (2) During electrical stimulation of other limbic brain regions, broad and large afterdischarges can be observed in the ipsilateral PC, indicating that the PC is activated early during the kindling process. (3) The interictal discharge, which many consider to be the hallmark of epilepsy, originates in the PC, independent of which structure serves as the kindled focus. (4) Autoradiographic studies of cerebral metabolism in rat amygdala kindling show that, during focal seizures, the area which exhibits the most consistent increase in glucose utilization is the ipsilateral paleocortex, particularly the PC. (5) During the commonly short initial afterdischarges induced by stimulation of the amygdala at the early stages of kindling, the PC is the first region that exhibits induction of immediate-early genes, such as c-fos. (6) The PC is the most sensitive brain structure to brain damage by continuous or frequent stimulation of the amygdala or hippocampus. (7) Amygdala kindling leads to a circumscribed loss of GABAergic neurons in the ipsilateral PC, which is likely to explain the increase in excitability of PC pyramidal neurons during kindling. (8) Kindling of the amygdala or hippocampus induces astrogliosis in the PC, indicating neuronal death in this brain region. Furthermore, activation of microglia is seen in the PC after amygdala kindling. (9) Complete bilateral lesions of the PC block the generalization of seizures upon kindling from the hippocampus or olfactory bulb. Incomplete or unilateral lesions are less effective in this regard, but large unilateral lesions of the PC and adjacent endopiriform nucleus markedly increase the threshold for induction of focal seizures from stimulation of the basolateral amygdala (BLA) prior to and after kindling, indicating that the PC critically contributes to regulation of excitability in the amygdala. (10) Potentiation of GABAergic neurotransmission in the PC markedly increases the threshold for induction of kindled seizures via stimulation of the BLA, again indicating a critical role of the PC in regulation of seizure susceptibility of the amygdala. Microinjections of NMDA antagonists or sodium channel blockers into the PC block seizure generalization during kindling development. (11) Neurophysiological studies on the amygdala-PC slice preparation from kindled rats showed that kindling of the amygdala induces long-lasting changes in synaptic efficacy in the ipsilateral PC, including spontaneous discharges and enhanced susceptibility to evoked burst responses. The epileptiform potentials in PC slice preparations from kindled rats seem to originate in neuron at the deep boundary of PC. Spontaneous firing and enhanced excitability of PC neurons in response to kindling from other sites is also seen in vivo, substantiating the fact that kindling induces long-lasting changes in the PC c

Associative synaptic potentials in the piriform cortex of the isolated guinea-pig brain in vitro

The involvement of local and remote associative fibres in the generation of piriform cortex synaptic potentials was investigated in the isolated guinea-pig brain maintained in vitro by arterial perfusion by implementing current source density analysis (CSD) on cortical field potential profiles. Previous hypotheses were verified using acute surgical isolation of piriform cortical areas to study different synaptic events separately. Stimulation of the lateral olfactory tract activated associative potentials throughout the piriform cortex. In the anterior piriform cortex, the current sinks responsible for the generation of associative potentials were located in the superficial portion of layer Ib and in layer III. In the posterior piriform cortex, two associative events were observed: an early sink located in the superficial part of layer Ib, followed by a sink in the deep part of the same layer. In the anterior piriform cortex, local associative synaptic potentials were separated from the component carried by long projective fibres by surgically isolating a small area of cortex monosynaptically activated by lateral olfactory tract stimulation. In this patch of lateral olfactory tract-connected anterior piriform cortex, local associative sinks were observed in the superficial Ib layer and in layer III. Monosynaptic activation of the isolated patch of anterior piriform cortex induced purely associative potentials throughout the piriform cortex. These potentials were mediated by the synaptic activation of apical dendrites in the superficial Ib layer and selectively abolished by severing the long associative fibres. The anterior piriform cortex layer III sink and the posterior piriform cortex deep Ib associative component were evoked by the activation of large population spikes in the monosynaptic anterior piriform cortex and the disynaptic posterior piriform cortex response respectively. These two sinks are presumably generated locally through a polysynaptic circuit, whose activation depends on the degree of cortical excitation. Olfactory signal processing in the guinea-pig piriform cortex during states of normal excitability is supported by the interactions between associative inputs impinging on the synapses located separately on the dendrites of pyramidal neurons. An increase in the synchronization of piriform cortex neuron discharge activates usually silent local circuit synapses.

Synaptic relationships involving local axon collaterals of pyramidal neurons in the cat motor cortex

The intracortical synaptic relationships of pyramidal neurons in the cat motor cortex were studied by intracellular recording and labeling techniques. Neurons that responded with monosynaptic excitatory postsynaptic potentials (EPSPs) to microstimulation in the somatosensory cortex were identified by intracellular recordings. Long-term potentiation (LTP) was evoked in all of these neurons (n = 15), following tetanic stimulation (50 Hz, 5 s) of their afferents from the somatosensory cortex. Three of these cells (cells A-C) were identified as pyramidal neurons, following intracellular injections of Neurobiotin. The intracortical axon collaterals of these labeled cells arborized extensively, forming terminal clusters both in close proximity to the parent soma and along their long, horizontal branches. Terminal clusters in both the proximal and in the distal termination zones of each of the cells were studied by electron microscopy. In their proximal arborization zones, the axon collaterals of the labeled pyramidal neurons synapsed preferentially with dendritic spines belonging to other pyramidal cells. In contrast, in their distal terminal clusters, the axon collaterals of each of the cells formed synapses in different proportions with different postsynaptic targets. The distal axon collaterals of cell A formed 86% of their synapses with pyramidal neurons; those of cell B formed 64% of their synapses with pyramidal cells, the remaining synapses with the dendritic shafts and somata of nonpyramidal neurons, and those of cell C provided most of their output (68%) to nonpyramidal, presumably inhibitory neurons. These findings suggest a high selectivity of intrinsic axon collaterals to form specific patterns of synapses. The patterns of synaptic interactions formed by these intrinsic axon collaterals may be a substrate for shaping and modulating representation maps in the motor cortex.

Axon hillocks and initial segments in spinal trigeminal nucleus with emphasis on synapses including axo-axo-axonic contacts

As a part of a continuing study of the feline spinal trigeminal nucleus, the fine structure and synaptic arrangements on the axon hillock and axon initial segment of neurons in this region are described here. Transmission electron microscopy has been used to characterize qualitatively the axon hillock and initial segment and associated synapses in pars interpolaris. Axon hillocks and initial segments are easily identified in continuity with somata or as isolated profiles in the neuropil, and they receive synaptic contacts: these we regard as axo-axonic. The presynaptic terminals contain either mainly round or mainly flattened synaptic vesicles and have Type I (asymmetric) or Type II (symmetric) thickenings respectively at their contacts with the axon hillock or initial segment. I report here also the unusual arrangement of three separate axons in a serial synaptic complex. Some of the round vesicle Type I contacts onto the axon hillock-initial segment region also receive Type II contacts from one or more flattened vesicle terminals, thus forming an axo-axo-axonic complex. These flattened vesicle terminals lack the usual features of a presynaptic dendrite. It has been shown that in this nucleus some round vesicle terminals, especially those postsynaptic to flattened vesicle terminals, are primary afferents from the periphery. Therefore the round vesicle terminal presynaptic to the axon hillock-initial segment region, some of which are included in the axo-axo-axonic complex may also be a primary afferent directly contacting the spike generator area of the relay neuron and under presynaptic control of a flattened vesicle synapse. The latter may possibly be an intrinsic contact. This strategic situation of round vesicle terminals and the axo-axo-axonic complex at the axon hillock or initial segment has major implications relevant to the overall output of these neurons.

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.

Morphology of single vestibulospinal collaterals in the upper cervical spinal cord of the cat: I. Collaterals originating from axons in the ventromedial funiculus contralateral to their cells of origin

Vestibulospinal neurons in the medial and descending vestibular nuclei have widespread bilateral terminations in the upper cervical spinal cord. These terminations arise from axons travelling in several funiculi, including the ventromedial, ventrolateral, lateral, and dorsolateral funiculi in addition to the dorsal columns. The purpose of the present study was to examine the morphology of single vestibulospinal collaterals which terminate in the upper cervical spinal cord and which originate from axons located in one of these funicular pathways, the ventromedial funiculus, contralateral (cVMF) to their cells of origin in the vestibular nuclei. The 32 collaterals described were selected from two separate sets of experiments which took advantage of different techniques. Nineteen of the collaterals were labelled following Phaseolus vulgaris leucoagglutinin (PHA-L) injections into the medial vestibular nucleus and medial regions of the descending vestibular nucleus. The remaining 13 collaterals originated from physiologically identified vestibulospinal axons that were stained after intra-axonal injections of horseradish peroxidase (HRP). The combined projection of all cVMF axon collaterals spread from laminae V to IX, and included the central cervical nucleus. There was a high degree of variability in the pattern of terminations of individual collaterals. This variability was more pronounced among PHA-L-labelled collaterals than HRP-labelled collaterals whose terminations were restricted to laminae VIII and IX. Some PHA-L-labelled collaterals had terminations which were focused within a single lamina, whereas others had termination zones spanning as many as four laminae. The differences between collaterals were compounded when the characteristics of branching patterns were considered. Some collaterals which occupied similar termination zones had different branching structures.(ABSTRACT TRUNCATED AT 250 WORDS)

Connections from the lateral vestibular nucleus to the upper cervical spinal cord of the cat: a study with the anterograde tracer PHA-L

The projections of neurons in the lateral vestibular nucleus (LVN) to the upper cervical spinal cord of the cat were investigated by means of the anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L). At the junction of C1 and C2, axons were distributed bilaterally in the ventromedial funiculi, and ipsilaterally in the ventrolateral and lateral funiculi. The majority of boutons were found ipsilateral to the injection sites and most of these boutons were found at the base of the ventral horn and throughout the medial two-thirds of lamina VIII. A more modest termination zone was found along the ventral border of lamina VII and a small number of boutons were scattered in the dorsal horn. Contralateral termination zones were similar to the ipsilateral projections. There were significant changes in the distribution of vestibulospinal axons and density of boutons at the junction of C3 and C4. At this level, most vestibulospinal axons travelled ipsilaterally and were found along the medial border of the ventromedial funiculus and the ventral margin of the ventrolateral funiculus. The overall distribution of boutons near the border of C3 and C4 was similar to the pattern seen at the junction of C1 and C2. However, bouton density fell by a factor of three. Large zones of the grey matter were devoid of boutons in individual experiments. These results demonstrate that the projections of neurons in the LVN to the upper cervical spinal cord are densest in the regions containing motoneurons supplying suboccipital muscles. This result suggests that monosynaptic connections to those motoneurons may be an important part of the neural circuitry responsible for vestibulocollic reflexes. However, the large number of boutons found in regions dorsal to motoneuron nuclei in all upper cervical segments indicates that the primary path from vestibulospinal axons to neck motoneurons may be indirect and involve relays via spinal interneurons.

Olfactory bulb organization and development in Monodelphis domestica (grey short-tailed opossum)

The olfactory bulbs of adult and developing Monodelphis domestica were examined with a number of techniques. Golgi, Nissl, and Timm stains as well as acetylcholinesterase histochemistry revealed a high degree of order within the adult bulb. All major cell classes characteristic of most mammalian species were observed. Tufted cells appeared to be restricted to the superficial portion of the external plexiform layer. Developing Monodelphis pups were examined with Nissl-stained semithin sections and with immunocytochemistry for tyrosine hydroxylase, microtubule-associated protein 2, vimentin, and glial fibrillary acidic protein. Newborn pups are extremely immature, with few postmitotic cells present in the forebrain. Considerable maturation occurs over the first four postnatal weeks, and by postnatal day 30, the bulb assumes an adult-like organization. The extreme immaturity of the bulb at birth, coupled with its strict organization, suggest that Monodelphis is a particularly appropriate species for experimental examinations of olfactory system development.

Projections of the tectospinal tract to the upper cervical spinal cord of the cat: a study with the anterograde tracer PHA-L

The goal of the present experiments was to re-examine the spinal projections of neurons in the superior colliculus (SC) of the cat by taking advantage of the high sensitivity of the anterograde tracer, phaseolus vulgaris leucoagglutinin (PHA-L). In seven experiments, multiple injections of PHA-L into different regions of the SC labelled a total of 172 axons in the predorsal bundle; yet only 11 tectospinal tract (TST) axons were found in the upper cervical spinal cord. Collaterals emerging from these axons were rare and arose exclusively from TST axons with a diameter of less than 1 micron. Individual collaterals had different termination zones: some terminated in the lateral part of lamina V and VI after taking a dorsolateral course through lamina VII and VIII; others terminated in the medial part of lamina VII. One collateral terminated within lamina IX and the ventral part of lamina VIII. The combined termination of all collaterals was densest in lamina VII and dorsal lamina VIII. A small number of boutons were also found in the lateral parts of laminae V and VI, and in lamina IX and immediately adjacent regions in lamina VIII. Compared to axons belonging to other spinal descending systems, individual TST axons give rise to much simpler intraspinal collaterals with relatively few boutons. This feature, together with the relative paucity of TST axons, suggests that direct connections from the SC to neurons in the upper cervical spinal cord are sparse. Furthermore, our results are consistent with electrophysiological studies that show that few, if any, neck motoneurons receive monosynaptic connections from TST neurons. Projections to neck motoneurons must therefore involve a relay, either through other descending pathways, such as the reticulospinal system, or via local segmental interneurons.

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.

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.

Piriform cortex brain slices: techniques for isolation of synaptic inputs

Methods are described for preparation of 3 different slices of piriform cortex which allow convenient study of pyramidal neurons and segregation of synaptic inputs. In slices cut parallel to the pyramidal neurons (perpendicular to the brain surface) one can study chemosensitivity of the various parts of the dendritic tree and the soma. By selected division of this slice the population postsynaptic response to activation of the lateral olfactory tract can be studied without action potential generation. Alternatively the superficial lateral olfactory tract can be removed. Stimulation of deeper regions of the slice under these circumstances elicits a pharmacologically different excitation which appears to be that of association fibers.

Immunocytochemical localization of glutamate decarboxylase in the rat basolateral amygdaloid nucleus, with special reference to GABAergic innervation of amygdalostriatal projection neurons

Glutamate decarboxylase (GAD) immunohistochemistry was employed at the light and electron microscopic levels to localize GABAergic structures in the basolateral amygdaloid nucleus (BL). The GAD-immunoreactive (GAD-IR) staining pattern consisted of punctate structures and a morphologically diverse group of GAD-IR neurons. At the electron microscopic level many of these punctate structures were found to make symmetrical synaptic contacts with cell bodies as well as distal parts of unlabeled, presumably projection and nonprojection, neurons. In addition, GAD-immunoreactive neurons were identified in the BL, and they had the ultrastructural characteristics of local circuit or intrinsic neurons and were not retrogradely labeled with HRP following ventral striatal injections. Some of these GAD-immunoreactive neurons were contacted by GABAergic boutons, forming symmetrical synaptic contacts. GABAergic innervation of amygdaloid projection neurons in the BL was identified by combining GAD immunohistochemistry with Golgi impregnation and retrograde tracing of horseradish peroxidase (HRP) following injections of the tracer in the olfactory-tubercle-related parts of the ventral striatum. Amygdalostriatal projection neurons in the BL were observed to be in continuity with neurons in the piriform cortex which project to the ventral striatum. The results provide direct evidence for the presence of GAD-IR boutons in the BL making synaptic contacts with identified amygdalostriatal projection neurons. The present study provides direct anatomical evidence for the physiological observation that GABA exhibits a powerful regulation of the amygdaloid projection neurons in the BL and lends further support to the concept of a corticallike functional organization of the basolateral amygdala.

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)