The pyramidal neuron of the cerebral cortex: morphological and chemical characteristics of the synaptic inputs

Glutamate decarboxylase immunoreactivity in corticocortical projecting neurons of rat somatic sensory cortex

Combined retrograde tracing and immunocytochemical experiments were carried out on rats to ascertain whether corticocortical projecting neurons in the somatic sensory areas are immunoreactive to an antiserum against glutamate decarboxylase. Injections of a retrograde tracer (colloidal gold-labelled wheat germ agglutinin conjugated to enzymatically inactive horseradish peroxidase) in the first somatic sensory area labelled neurons in the injected area, in the second somatic sensory area, and in the parietoventral area of the ipsilateral hemisphere. The topographical and laminar distribution of these retrogradely-labelled corticocortical neurons in the first and second somatic sensory areas and in the parietoventral area was in line with a previous description (Fabri M. and Burton H. (1991b) J. comp. Neurol. 311, 405-424). In sections processed for the simultaneous visualization of the retrograde tracer and glutamate decarboxylase immunoreactivity, a number of neurons were double-labelled. Double-labelled neurons were most numerous in the first somatic sensory cortex, where they accounted for 5% of all retrogradely-labelled neurons. Outside this region, double-labelled cells were observed in the second somatic sensory cortex and in the parietoventral cortex, where they amounted respectively to 2.8% and 2.3% of all corticocortical neurons labelled in these two areas. Glutamate decarboxylase-immunopositive corticocortical neurons were mainly concentrated in the infragranular layers (73.8% of all double-labelled neurons in the first somatic sensory area, 81.7% in the second somatic sensory area, and 76.5% in the parietoventral area). The results indicate the presence of a small but significant contingent of GABAergic inhibitory neurons in the associative connections of the somatic sensory areas.

Pyramidal neurons in rat prefrontal cortex show a complex synaptic response to single electrical stimulation of the locus coeruleus region: evidence for antidromic activation and GABAergic inhibition using in vivo intracellular recording and electron microscopy

Cognition and acquisition of novel motor skills and responses to emotional stimuli are thought to involve complex networking between pyramidal and local GABAergic neurons in the prefrontal cortex. There is increasing evidence for the involvement of cortical norepinephrine (NE) deriving from the nucleus locus coeruleus (LC) in these processes, with possible reciprocal influence via descending projections from the prefrontal cortex to the region of the LC. We used in vivo intracellular recording in rat prefrontal cortex to determine the synaptic responses of individual neurons to single electrical stimulation of the mesencephalic region including the nucleus LC. The most common response consisted of a late-IPSP alone or preceded by an EPSP. The presence of an early-IPSP following the EPSP was sometimes detected. Analysis of the voltage dependence revealed that the late-IPSP and early-IPSP were putative K(+)- and Cl- dependent, respectively. Synaptic events occurred following short delays and were inconsistent with the previously reported time for electrical activation of unmyelinated LC fibers. Moreover, systemic injection of the adrenergic antagonists propranolol (beta receptors), or prazosin (alpha 1 receptors), did not block synaptic responses to stimulation of the LC region. Finally, certain neurons were antidromically activated following electrical stimulation of this region of the dorsal pontine tegmentum. Taken together, these results suggest that the complex synaptic events in pyramidal neurons of the prefrontal cortex that are elicited by single electrical stimulation of the LC area are mainly due to antidromic activation of cortical efferents. Further insight into the chemical circuitry underlying these complex synaptic responses was provided by electron microscopic immunocytochemical analysis of the relations between the physiologically characterized neurons and either 1) GABA or 2) dopamine-beta-hydroxylase (DBH), a marker for noradrenergic terminals. GABA-immunoreactive terminals formed numerous direct symmetric synapses on somata and dendrites of pyramidal cells recorded and filled with lucifer yellow (LY). In contrast, in single sections, noradrenergic terminals immunoreactive for DBH rarely contacted LY-filled somata and dendrites. These results support the conclusion that IPSPs observed following single electrical stimulation of the LC region are mediated by GABA, with little involvement of NE. These IPSPs, arising from antidromic invasion of mPFC cells innervating the LC, may improve the signal-to-noise ratio and favor a better responsiveness of neighboring neurons to NE released in the mPFC.

Parvalbumin-like immunoreactivity of layer V pyramidal cells in the motor and somatosensory cortex of adult primates

Most previous immunocytochemical studies have indicated that the calcium-binding protein parvalbumin is present only in non-pyramidal neurons of the adult cerebral cortex. Using nickel and cobalt to enhance the diaminobenzidine reaction product, we observed large layer V pyramidal cells with parvalbumin-like immunoreactivity in the primary motor cortex (area 4) and somatosensory cortex of adult macaque monkeys and galagos, including giant Betz cells in area 4.

Comparative morphology of three types of projection-identified pyramidal neurons in the superficial layers of cat visual cortex

The morphology and dendritic organization of corticocortical neurons in the superficial layers of area 18 that project to area 17 were studied by intracellular injection of lucifer yellow in the fixed-slice preparation. This corticocortical population contains primarily standard pyramidal cells, but occasional nonpyramidal, modified, fusiform, star, and inverted pyramidal cells were also seen. All cell types were present throughout layer 2 and in the upper and middle parts of layer 3. Standard pyramidal cells were found exclusively in lower layer 3. The mean somatic area of the area 17 projecting neurons was 251 microns 2. The width of basal dendritic fields was correlated to cell size for standard pyramidal cells but not for the other cell types. Next, the morphology and dendritic organization of the area 17 projecting neurons were compared to the pyramidal cells of the local horizontal patch networks and of the callosal system. The depth profile of the area 17 projecting and callosal pyramidal groups was virtually identical, peaking at 400 microns from the pial surface, whereas the local patch pyramidal group peaked at 281 microns. The local patch, area 17 projecting, and callosal pyramidal cells displayed increasingly larger mean somatic areas and basilar dendritic field width measurements. The number of basal dendritic branch points was greatest for callosal cells, and it was indistinguishable between local patch and area 17 projecting neurons. In the tangential plane, circular dendritic fields were observed on all callosal cells, but they were found on only approximately half of the local patch and area 17 projecting neurons. The remaining local patch and area 17 projecting neurons displayed mediolaterally and anteroposteriorly elongated basal dendritic fields, respectively.

Distribution of parvalbumin immunoreactivity in the neocortex of hypothyroid adult rats

Early hypothyroidism produces a generalized damage in the brain and in particular, changes in the connectivity of neocortical sensory areas. In this paper, the potential alterations in local neocortical circuits have been explored using immunocytochemistry for parvalbumin (PV) in normal and hypothyroid adult rats. The number and radial distribution of PV-positive cells were similar in both groups, but in hypothyroid rats, the density of PV-positive terminal-like puncta and processes was dramatically reduced, especially in layers II-III and the lower part of layers IV-V and VI. These results suggest that thyroid hormones are necessary for normal development of cortical circuits in which PV-positive cells are involved.

The alpha 2-adrenergic antagonist idazoxan enhances the frequency selectivity and increases the threshold of auditory cortex neurons

Idazoxan (IDA), an alpha 2 antagonist of adrenoceptors, has been shown to increase cortical release of norepinephrine (NE) by an action mediated primarily by the alpha 2 autoreceptors located on the NE terminals. In the present experiment, IDA application was used to increase the cortial concentration of NE. Single unit activity (n = 107) was recorded in the rat auditory cortex, and the neurons' frequency receptive fields (FRF) were determined before and after systemic (intraperitoneal or intravenous) or local application of IDA. In the whole population (n = 107) there was a decrease in spontaneous activity and/or evoked activity for 84% of the recordings (90/107 cells). Decreased tone-evoked responses were obtained after systemic injections (n = 39), as well as after local applications (n = 68) of IDA. These effects were not observed after either systemic injections (n = 13) or local applications (n = 9) of saline. The signal-to-noise ratio (the mean evoked responses divided by the spontaneous activity) was slightly decreased after systemic injections and slightly increased after local applications. However, after both systemic and local injections the frequency selectivity of the neuronal responses was increased. For a group of neurons (n = 27), testing the FRF at three intensities indicated that this increased selectivity can be expressed at high or middle range intensity but not at low intensity. For 37 cells, the intensity function was tested at the best frequency before and after IDA application, and the threshold for excitatory responses was determined in 28 cases. An increased threshold was observed in 16 of 28 cases after IDA application. Thus, using a pharmacological procedure to increase the extracellular concentration of NE, the dominant inhibitory effect on the auditory cortex neurons led to an enhancement of the frequency selectivity, but also an increase in the threshold of these neurons.

Innervation of burst firing spiny interneurons by pyramidal cells in deep layers of rat somatomotor cortex: paired intracellular recordings with biocytin filling

Intracellular recordings were obtained from a class of neuron defined electrophysiologically as burst firing interneurons in layers V and VI in slices of adult rat somatomotor cortex. Four of these cells were recovered histologically. These four cells had resting membrane potentials between -68 and -80 mV, a mean input resistance of 77 +/- 16.2 M omega (measured from the voltage deflection produced by a 100 ms, 0.5 nA hyperpolarizing pulse delivered from a membrane potential of -80 mV) and responded to injections of depolarizing current from membrane potentials negative of -70 to -75 mV with an initial burst of action potentials followed by a complex afterhyperpolarization. In response to injection of larger (0.5-1.5 nA) hyperpolarizing current pulses from membrane potentials between -60 and -70 mV, 15 of 20 burst firing cells (three of four recovered histologically) that were tested displayed delayed inward rectification, and in all 20 cells of this type, responses to large negative current pulses were followed by a rebound depolarization that could initiate action potentials. Filling of four of these cells with biocytin and subsequent histological processing revealed that they were bitufted with sparsely to medium spiny dendrites and extensive local axon ramifications. These neurons are similar to low threshold spiking cells [Kawaguchi (1993) J. Neurophysiol. 69, 416-431]. Ultrastructural examination of the axons of three cells revealed that of 53 labelled terminals studied, the majority formed synaptic contacts with dendritic shafts. Filling neurons with biocytin during paired intracellular recordings resulted in three well labelled interneurons, each of which was postsynaptic to a simultaneously recorded pyramidal neuron. In these pairs both cells were identified, but the presynaptic axon was poorly labelled in one. In one of the two pairs in which the pre- and postsynaptic neurons were fully recovered, light microscopic assessment indicated that the axon of the presynaptic pyramid formed 12 close appositions with dendrites of the postsynaptic interneuron. Six of these appositions were examined at the electron microscopic level and were identified as possible synaptic contacts. In the other pair three of six close appositions observed at the light level were verified as possible synaptic connections at the ultrastructural level. These correlated electrophysiological and anatomical studies provide the first evidence for connections from pyramid to burst firing interneurons in the neocortex and indicate that these connections can be mediated by multiple synaptic contacts. The accompanying paper describes the functional properties of these connections.

Intra-amygdaloid projections of the basolateral and basomedial nuclei in the cat: Phaseolus vulgaris-leucoagglutinin anterograde tracing at the light and electron microscopic level

The amygdaloid complex plays an essential role in auditory fear conditioning of the Pavlovian type. The available evidence suggests that the lateral nucleus is the input station of the amygdala for auditory conditioned stimuli, whereas the central medial nucleus is the output for conditioned fear responses. However, the intrinsic pathway transmitting auditory information about the conditioned stimulus from the lateral to the central medial nuclei is unknown as there are no direct projections between these nuclei. The present study was undertaken to determine if the main intra-amygdaloid targets of the lateral nucleus, namely the basomedial and basolateral nuclei, project to the central medial nucleus. To this end, iontophoretic injections of the anterograde tracer Phaseolus vulgaris-leucoagglutinin were performed in these nuclei. To rule out the possibility that the anterograde labeling reflected passing fibers merging with the major fiber bundles that course in and around the central medial nucleus, labeled terminals and varicosities were observed in the electron microscope. It was determined that the basolateral and basomedial nuclei have partially overlapping intraamygdaloid targets. They both project to the central medial nucleus, nucleus of the lateral olfactory tract and peri-amygdaloid cortex, but have limited projections to each other. Small Phaseolus vulgaris-leucoagglutinin injections in both nuclei gave rise to prominent intranuclear projections but only the basomedial nucleus was found to project to the lateral and anterior cortical nuclei. At the electron microscopic level, all labeled axon terminals and varicosities formed asymmetric synapses (n = 245) with dendritic spines (83%) or with dendritic shafts (17%). This is the first unambiguous demonstration that the basolateral and basomedial nuclei project to the central medial nucleus. Since these nuclei constitute the main intra-amygdaloid targets of the lateral nucleus, they represent likely candidates for the transmission of auditory conditioned stimuli to the central medial nucleus in auditory fear conditioning.

Laminar pattern of termination of the ipsilateral cortical projection from SII to SI in cats

The present light and electron microscopic experiments were carried out on the first somatic sensory area (SI) of cats to determine the laminar distribution of axon terminals from the ipsilateral second somatic sensory area (SII) and to identify the types of synapses between these terminals and the neuronal elements of SI. Phaseolus vulgaris-leucoagglutinin (PHA-L) was iontophoretically injected into multiple sites and at different cortical depths of the forepaw representation zone of SII. Fixed brain blocks containing the injected SII and ipsilateral SI were cut into slices and processed immunocytochemically to stain PHA-L-filled fibers and terminals. Light microscopic examination of SI revealed patches of anterograde labeling in the forepaw representation zone, concentrated mainly in supragranular layers. In these layers, thin immunolabeled fibers branched extensively and formed a dense plexus that was more prominent in layers II and I. Conversely, the infragranular layers contained fragments of vertically oriented thick fibers that rarely emitted axon collaterals. PHA-L-labeled axons had numerous swellings along their course, interpreted as boutons en passant, and stalked boutons. Of 19,661 labeled terminals (17,833 beads and 1,828 stalked boutons), 84.74% were observed in supragranular layers, with the highest concentration in layer II (33.15%) and lower in layers I (26.27%) and III (25.30%). The proportion of terminals was lower in layers IV (6.49%) and V (5.45%) and lowest in layer VI (3.32%). These counts also showed that boutons en passant were the majority (90.70%) and stalked boutons, the minority (9.30%). The ratio of these two types of presynaptic specializations was similar (9:1) in all six layers. Electron microscopic examination of the labeled regions of SI showed that both axon swellings and stalked boutons formed synapses of the asymmetric type with SI neuronal elements. The majority (85.37%) of a sample of 130 labeled terminals synapsed on SI neurons in layers I-III. The identified postsynaptic profiles were dendritic spines (61.11%) or medium-sized and small dendrites (38.89%). These results are discussed in relation to those of a companion study on the laminar pattern of the projection from SI to SII of cats (P. Barbaresi, A. Minelli, and T. Manzoni, 1994, J. Comp. Neurol. 343:582-596). Based on the anatomical organization of these reciprocal connections, there seems to be no clear hierarchicalal relationship between SI and SII in cats.

Spatial profile of dendritic calcium transients evoked by action potentials in rat neocortical pyramidal neurones

1. Simultaneous measurements of intracellular free calcium concentration ([Ca2+]i) and intrasomatic and intradendritic membrane potential (Vm) were performed using fura-2 fluorimetry and whole-cell recording in neocortical layer V pyramidal neurones in rat brain slices. 2. Back-propagating action potentials (APs) evoked [Ca2+]i transients in the entire neurone including the soma, the axon initial segment, the apical dendrite up to the distal tuft branches, and the oblique and basal dendrites, indicating that following suprathreshold activation the entire dendritic tree is depolarized sufficiently to open voltage-dependent calcium channels (VDCCs). 3. The [Ca2+]i transient peak evoked by APs showed large differences between various compartments of the neurone. Following a single AP, up to 6-fold differences were measured, ranging from 43 +/- 14 nM in the soma to 267 +/- 109 nM in the basal dendrites. 4. Along the main apical dendrite, the [Ca2+]i transients evoked by single APs or trains of APs had the largest amplitude and the fastest decay in the proximal region; the [Ca2+]i transient peak and decay time constant following a single AP were 128 +/- 25 nM and 420 +/- 150 ms, respectively, and following a train of five APs (at 10-12 Hz), 710 +/- 214 nM and 390 +/- 150 ms, respectively. The [Ca2+]i transients gradually decreased in amplitude and broadened in more distal portions of the apical dendrite up to the main bifurcation. 5. In the apical tuft branches, the profile of the [Ca2+]i transients was dependent on AP frequency.(ABSTRACT TRUNCATED AT 250 WORDS)

A light and electron microscopic study of calbindin D-28k immunoreactive double bouquet cells in the human temporal cortex

Correlative light and electron microscopic methods were used to examine the morphology, distribution and synaptic connections of double bouquet cells immunoreactive for the calcium-binding protein calbindin D-28k in the human temporal neocortex. Double bouquet cells form symmetric synapses with small dendritic shafts and dendritic spines. The distribution and proportion of synapses found in the present work are very similar to those found in previous studies on the synaptic connectivity of double bouquet cells in the monkey cerebral cortex. Thus, double bouquet cells are probably involved in similar synaptic circuits in monkeys and humans.

GABA and glycine in the central auditory system of the mustache bat: structural substrates for inhibitory neuronal organization

The distribution and morphology of neurons and axonal endings (puncta) immunostained with antibodies to gamma-aminobutyric acid (GABA) and glycine (Gly) were analyzed in auditory brainstem, thalamic, and cortical centers in the mustache bat. The goals of the study were (1) to compare and contrast the location of GABAergic and glycinergic neurons and puncta, (2) to determine whether nuclei containing immunoreactive neurons likewise have a similar concentration of puncta, (3) to assess the uniformity of immunostaining within a nucleus and to consider regional differences that were related to or independent of cytoarchitecture, and (4) to compare the patterns recognized in this bat with those in other mammals. There are nine major conclusions. (1) Glycinergic immunostaining is most pronounced in the hindbrain. (2) In the forebrain, GABA alone is present. (3) Some nuclei have GABAergic or glycinergic neurons exclusively; a few have neither. (4) Although there is sometimes a close relationship between the relative number of immunopositive neurons and the density of the puncta, just as often there is no particular correlation between them; this reflects the fact that many GABAergic and glycinergic neurons project beyond their nucleus of origin. (5) Even nuclei devoid of or with few GABAergic or glycinergic neurons contain relatively abundant numbers of puncta; some neurons receive axosomatic terminals of each type. (6) In a few nuclei there are physiological subregions with specific local patterns of immunostaining. (7) The patterns of immunostaining resemble those in other mammals; the principal exceptions are in nuclei that, in the bat, are hypertrophied (such as those of the lateral lemniscus) and in the medial geniculate body. (8) Cellular colocalization of GABA and Gly is specific to only a few nuclei. (9) GABA and glutamic acid decarboxylase (GAD) immunostaining have virtually identical distributions in each nucleus. Several implications follow. First, the arrangements of GABA and Gly in the central auditory system represent all possible patterns, ranging from mutually exclusive to overlapping within a nucleus to convergence of both types of synaptic endings on single neurons. Second, although both transmitters are present in the hindbrain, glycine appears to be dominant, and it is often associated with circuitry in which precise temporal control of aspects of neuronal discharge is critical. Third, the auditory system, especially at or below the level of the midbrain, contains significant numbers of GABAergic or glycinergic projection neurons. The latter feature distinguishes it from the central visual and somatic sensory pathways.

Morphology of individual axons projecting from area V2 to MT in the macaque

Efferent axons from area V2 to the middle temporal area (MT) were anterogradely labeled by Phaseolus vulgaris-leucoagglutinin (PHA-L) or biocytin and analyzed in serial reconstructions. Five of seven reconstructed axons had three arbors (each < or = 200 microns in diameter) in layers 3-4, separated by 200-600 microns. Two axons terminated in what was apparently a single focus in layers 3-4. Of 15 additional single arbors analyzed, 12 were concentrated in layers 3-4, and measured 200-250 microns across at their widest point. Three of these arbors were more columnar in shape (about 400 microns in diameter), and extended from layer 4 toward layer 1. This system differs in several features from MT-projecting axons originating from V1. Namely, V2 axons terminating in MT are thinner (approximately 1.0 microns vs. 3.0 microns), their terminal specializations are more delicate, and their arbors are concentrated in layer 4 and overlying layer 3, with no collaterals to layer 6. These differences may reflect the distinctive neuronal populations giving rise to these two connectional systems (different sizes of pyramidal neurons in layer 3 of V2, and a mix of pyramidal and spiny stellate cells in area V1). Differences may have implications for timing factors; that is, impulses from V1, subserved by large-caliber axons, may arrive in MT coincidentally with indirect connections via V2 to MT. Another consideration may be the functional architecture of MT. Regularly spaced clusters of neurons have been described in MT which have similar directionality preferences. The interarbor spacing of cortical efferents is consistent with a columnar organization, but the laminar specificity may indicate recruitment of different combinations of postsynaptic populations by V1 or V2 terminations.

Migrating neurons in the developing cerebral cortex of the mouse send callosal axons

The presence of migrating callosal neurons during the development of the murine cerebral cortex was studied using biocytin and the lipophilic dye, 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate as retrograde tracers. After injections of biocytin in the presumptive somatosensory cortex of newborn mice which were analysed one day later, many anterogradely labelled fibres coursed towards the contralateral hemisphere through the corpus callosum. Retrogradely labelled callosal cells were also observed. Most callosal neurons corresponded to immature pyramidal cells. In addition, a few biocytin-labelled callosal neurons displayed extremely fusiform shapes, vertical orientation and a short, single process emerging from the apical side of the perikaryon. At the electron microscopic level, these cells had features identical to those described for migrating callosal neurons. Twenty-four hours after birth, these migrating neurons were almost exclusively observed in the upper, dense aspect of the cortical plate (presumptive layers II-III) and only very exceptionally in the infragranular layers. No retrogradely labelled cell resembling migrating neurons were noticed after injections on postnatal days 2 or 5. To study migrating callosal neurons at embryonic stages, crystals of the lipophilic dye were injected in the corpus callosum or the contralateral white matter in embryos aged 17, 18 and 19 days, corresponding to the initial development of the corpus callosum in mice. Whereas callosal migrating neurons were not detected at embryonic days 17 and 18, injections of the lipophilic dye on embryonic day 19 revealed the presence of labelled migrating neurons in the infragranular layers. To corroborate further that these cells are migrating neurons, [3H]thymidine was administered on embryonic days 16 and 17, and labelled mice were injected with biocytin on embryonic day 19 or the first postnatal day. Retrogradely labelled callosal neurons resembling migrating neurons were autoradiographically labelled. These results indicate that the specification of certain neuronal types and the emergence of their cell type-specific characteristics occur shortly after postmitotic neurons leave the ventricular zone, before being positioned within the cortical plate.

Spatial synaptic integration in Purkinje cell dendrites

Synaptic integration occurs within a framework of synaptic connections, and cell type-specific, intrinsic and transmitter-gated ion channels. These components are differentially distributed over the somato-dendritic membrane. Recent results from Purkinje cells and pyramidal cells exemplify some of these mechanisms of spatial synaptic integration. This paper focusses on the cerebellar Purkinje cell. In these neurons, the amplitude and distribution of single climbing fibre and parallel fibre EPSP-evoked Ca2+ influx were regulated by the transient outward, IA-like current in the distal (spiny) dendrites. The synaptically evoked Ca2+ influx was graded from a local response involving only a few terminal spiny dendrites to a propagated Ca2+ spike. The climbing fibre-evoked Ca2+ influx in the spiny dendrites was finely graded by parallel fibre-induced depolarization. Climbing fibre and parallel fibre-evoked Ca2+ influx elicited a short lasting afterhyperpolarization that affected subsequent dendritic Ca2+ influx. In addition, inhibitory synaptic input controlled dendritic Ca2+ influx. Interaction between information from different sources along the dendrites is thus controlled by intrinsic potassium conductances and IPSPs. Different electrophysiological properties are found in the cerebellar neurons. Thus, Golgi cells, stellate cells and granule cells seem to integrate on a shorter intrinsic timescale than do Purkinje cells, the output neuron of the cerebellar cortex. The specific mechanisms by which different types of presynaptic neurons specifically innervate a given dendritic compartment remain to be elucidated, but recent results provide some experimental evidence of a differential distribution of cell adhesion molecules between the axonal and the somato-dendritic membrane, suggesting one mechanism contributing to the ordered distribution of synapses during synaptogenesis.

Cellular localization and laminar distribution of AMPA glutamate receptor subunits mRNAs and proteins in the rat cerebral cortex

The cellular and laminar distributions of the alpha-amino-3-hydroxy-5- methyl-4-isoxazole propionate (AMPA) receptor subunits GluR1-4 have been investigated in the cerebral cortex of adult rats by in situ hybridization with 35S-labeled cRNA probes and by immunocytochemistry with subunit-specific antibodies. In sections incubated with the GluR1-4 antisense probes, specific hybridization signal was observed in many but not all cortical cells. Experiments with in situ hybridization and antibodies to glial fibrillary acidic protein (GFAP) showed that percentages of GFAP-immunoreactive cells labeled by the GluR1-4 probes were 20%, 9.4%, 8.2%, and 57.3%, respectively. A semiquantitative evaluation revealed that about 56% of cortical neurons contained the GluR1 subunit, 80% the GluR2, 63% the GluR3, and 44% the GluR4. The number of grains associated with every neuron was determined from sections exposed for 15 days, the background level was subtracted, and labeled neurons were divided into four groups: A (< or = 10 grains), B (11-20 grains), C (21-30 grains), and D (> 30 grains). The number of neurons belonging to each of these groups was then evaluated for their occurrence in each cortical layer. Immunocytochemistry with subunit-specific antibodies showed that 1) GluR1-immunoreactive neurons were mostly layers V and VI nonpyramidal neurons; 2) GluR2/3-immunoreactive neurons were more numerous in layers II-III and V-VI, and most of them were pyramidal; and 3) GluR4-positive cells were the least numerous, and they were either neurons (pyramidal and nonpyramidal) or astrocytes. These observations indicate that cortical neurons exhibit a remarkable degree of heterogeneity with regard to both the composition and the number of AMPA receptors and suggest that this diversity might be correlated with the functional attributes of neurons receiving glutamatergic afferents and with the physiological features of corticifugal neurons.

Cellular and synaptic localization of NMDA and non-NMDA receptor subunits in neocortex: organizational features related to cortical circuitry, function and disease

Excitatory amino acid (EAA) receptors are an important component of neocortical circuitry as a result of their role as the principal mediators of excitatory synaptic activity, as well as their involvement in use-dependent modifications of synaptic efficacy, excitoxicity and cell death. The diversity in the effects generated by EAA-receptor activation can be attributed to multiple receptor subtypes, each of which is composed of multimeric assemblies of functionally distinct receptor subunits. The use of subunit-specific antibodies and molecular probes now makes it feasible to localize individual receptor subunits anatomically with a high level of cellular and synaptic resolution. Initial studies of the distribution of immunocytochemically localized EAA-receptor subunits suggest that particular subunit combinations exhibit a differential cellular, laminar and regional distribution in the neocortex. While such patterns might indicate that the functional heterogeneity of EAA-receptor-linked circuits, and the cell types in which they operate, are based partly on differential subunit parcellation, a definitive integration of these anatomical details into current schemes of cortical circuitry and organization awaits many further studies. Ideally, such studies should link a high level of molecular precision regarding subunit localization with synaptic details of identified connections and neurochemical features of neocortical cells.

The neocortex. An overview of its evolutionary development, structural organization and synaptology

By way of introduction, an outline is presented of the origin and evolutionary development of the neocortex. A cortical formation is lacking in amphibians, but a simple three-layered cortex is present throughout the pallium of reptiles. In mammals, two three-layered cortical structures, i.e. the prepiriform cortex and the hippocampus, are separated from each other by a six-layered neocortex. Still small in marsupials and insectivores, this "new" structure attains amazing dimensions in anthropoids and cetaceans. Neocortical neurons can be allocated to one of two basic categories: pyramidal and nonpyramidal cells. The pyramidal neurons form the principal elements in neocortical circuitry, accounting for at least 70% of the total neocortical population. The evolutionary development of the pyramidal neurons can be traced from simple, "extraverted" neurons in the amphibian pallium, via pyramid-like neurons in the reptilian cortex to the fully developed neocortical elements designated by Cajal as "psychic cells". Typical mammalian pyramidal neurons have the following eight features in common: (1) spiny dendrites, (2) a stout radially oriented apical dendrite, forming (3) a terminal bouquet in the most superficial cortical layer, (4) a set of basal dendrites, (5) an axon descending to the subcortical white matter, (6) a number of intracortical axon collaterals, (7) terminals establishing synaptic contacts of the round vesicle/asymmetric variety, and (8) the use of the excitatory aminoacids glutamate and/or aspartate as their neurotransmitter. The pyramidal neurons constitute the sole output and the largest input system of the neocortex. They form the principal targets of the axon collaterals of other pyramidal neurons, as well as of the endings of the main axons of cortico-cortical neurons. Indeed, the pyramidal neurons constitute together a continuous network extending over the entire neocortex, justifying the generalization: the neocortex communicates first and foremost within itself. The typical pyramidal neurons represent the end stage of a progressive evolutionary process. During further development many of these elements have become transformed by reduction into various kinds of atypical or aberrant pyramidal neurons. Interestingly, none of the six morphological characteristics, mentioned above under 1-6, has appeared to be unassailable; pyramidal neurons lacking spines, apical dendrites, long axons and intracortical axon collaterals etc. have all been described. From an evolutionary point of view the typical pyramidal neurons represent not only the principal neocortical elements, but also the source of various excitatory local circuit neurons. The spiny stellate cells, which are abundant in highly specialized primary sensory areas, form a remarkable case in point.(ABSTRACT TRUNCATED AT 400 WORDS)

Microzonal decreases in the immunostaining for non-NMDA ionotropic excitatory amino acid receptor subunits GluR 2/3 and GluR 5/6/7 in the human epileptogenic neocortex

Potential alterations in glutamate-utilizing excitatory circuits in resected human epileptogenic frontal and temporal neocortex were investigated by using immunocytochemical methods to visualize receptor subunits which comprise the AMPA/kainate (GluR2/3) and kainate (GluR5/6/7) receptor subtypes. Examination of the patterns of immunostaining in regions of neocortex that were identified as spiking and non-spiking based on intraoperative electrocorticography revealed dramatic, microzonal decreases in immunoreactivity for the receptor subunits examined. The patches of decreased immunostaining for GluR2/3 and for GluR5/6/7 were often coincident with respect to each other. However, such abnormal regions were not necessarily correlated with any particular electrocorticographically defined regions nor any overtly abnormal cytoarchitectural features in adjacent Nissl-stained sections. Moreover in many but not all cases, the focal regions of decreased receptor subunit immunoreactivity coincided with small patches of decreased parvalbumin immunoreactivity a calcium-binding protein which labels a subpopulation of powerful inhibitory GABAergic interneurons. These results indicate that in the human epileptogenic neocortex there may be alterations in particular excitatory and/or inhibitory synaptic systems at small, multiple neocortical foci, and that these alterations are found mostly in the same regions. We suggest that these alterations may contribute to the initiation and/or propagation of seizure activity.

The plasma membrane Ca(2+)-ATPase mRNA isoform PMCA 4 is expressed at high levels in neurons of rat piriform cortex and neocortex

Ca2+ transport mediated by the plasma membrane Ca(2+)-ATPase (PMCA) serves an important role in regulation of cytosolic-free Ca2+ in a variety of cells. Isoform PMCA4 mRNA distribution in rat brain was studied by in situ hybridization using 33P-labeled antisense oligodeoxynucleotide probes. Very high levels of hybridization were found in piriform cortex with high levels in amygdaloid nucleus and laminae 2 and 6 of cerebral cortex. Significantly lower levels were found in hypothalamic nuclei and very low or undetectable levels were found in cerebellum, habenula, olfactory bulb, thalamus, choroid plexus of the third and fourth ventricles and in CA1 and CA3 cells of the hippocampus. These results suggest that PMCA4 is not a housekeeping form of the Ca(2+)-ATPase.

Relationships between morphology and physiology of pyramid-pyramid single axon connections in rat neocortex in vitro

1. Double intracellular recordings were made from 1163 pairs of pyramidal neurones in layer V-VI of the rat somatomotor cortex in vitro using sharp electrodes filled with biocytin. Monosynaptically connected pairs of cells were identified when an action potential in one could elicit a constant latency excitatory postsynaptic potential (EPSP) in the other and the cells were filled with biocytin. Labelled cells were subsequently identified histologically with avidin-horseradish peroxidase. 2. Thirty-four pairs of cells were found to be monosynaptically connected. Fifteen of these pairs were sufficiently stable for electrophysiological recordings and three of these were recovered sufficiently to permit full morphological reconstruction. 3. The EPSP recorded between the first pair of pyramids varied in amplitude between 0 and 3 mV (mean 1.33 +/- 1.06 mV) and fluctuated considerably (coefficient of variation, 0.796). This was largely due to a high incidence of apparent failures of transmission. On reconstruction two boutons from the presynaptic pyramid axon were in close apposition to the proximal portions of basal dendrites of the postsynaptic cell. 4. In the second pair of pyramids the EPSP had a mean amplitude of 1.06 mV, and displayed a 10-90% rise time of 2.8 ms and a width at half-amplitude of 23 ms. This EPSP did not alter significantly with changes in membrane potential at the soma. The presynaptic axon closely apposed the distal apical dendrite of the postsynaptic cell in eight places. 5. In the third pair of pyramids, the EPSPs, recorded at a relatively depolarized membrane potential, were long lasting and could elicit slow dendritic spikes with long and variable latencies. These slow spikes suggested that the postsynaptic recording site was dendritic and on reconstruction a possible location was identified on the apical dendrite. A total of five presynaptic boutons closely apposed three separate, proximal branches of the postsynaptic apical dendrite. 6. These results provide the first illustration of a morphological basis for variations in functional properties of pyramid-pyramid connections in the neocortex.

Acetylcholinesterase provides deeper insights into Alzheimer's disease

This article reviews the considerable evidence which rejects the cholinergic hypothesis of Alzheimer's disease (AD) and proposes that it is the AChE system of which the lightly stained neurons are located in the entorhinal cortex, the CA1/subiculum of the hippocampus and the amygdala which are the most vulnerable and are the earliest affected in the pathological processes of AD. Changes then spread out to the intermediately stained neurons of the association cortex, until they affect the heavily stained cells of the motor cortex. In general, senile plaque, a hallmark of AD, may be formed from the terminals of AChE-containing neurons. Neurofibrillary tangle, another hallmark of AD, may be formed in the perikarya of AChE-containing cells and bring about the demise of the neuron, thus leading to dementia.

Laminar distribution and neuronal targets of GABAergic axon terminals in cat primary auditory cortex (AI)

The form, density, and neuronal targets of presumptive axon terminals (puncta) that were immunoreactive for gamma-aminobutyric acid (GABA) or its synthesizing enzyme, glutamic acid decarboxylase (GAD), were studied in cat primary auditory cortex (AI) in the light microscope. High-resolution, plastic-embedded material and frozen sections were used. The chief results were: 1) There was a three-tiered numerical distribution of puncta, with the highest density in layer Ia, an intermediate number in layers Ib-IVb, and the lowest concentration in layers V and VI, respectively. 2) Each layer had a particular arrangement: layer I puncta were fine and granular (less than 1 micron in diameter), endings in layers II-IV were coarser and more globular (larger than 1 micron), and layer V and VI puncta were mixed in size and predominantly small. 3) The form and density of puncta in every layer were distinctive. 4) Immunonegative neurons received, in general, many more axosomatic puncta than immunopositive cells, with the exception of the large multipolar (presumptive basket) cells, which invariably had many puncta in layers II-VI. 5) The number of puncta on the perikarya of GABAergic neurons was sometimes related to the number of puncta in the layer, and in other instances it was independent of the layer. Thus, while layer V had a proportion of GABAergic neurons similar to layer IV, it had only a fraction of the number of puncta; perhaps the intrinsic projections of supragranular GABAergic cells are directed toward layer IV, as those of infragranular GABAergic neurons may be. Since puncta are believed to be the light microscopic correlate of synaptic terminals, they can suggest how inhibitory circuits are organized. Even within an area, the laminar puncta patterns may reflect different inhibitory arrangements. Thus, in layer I the fine, granular endings could contact preferentially the distal dendrites of pyramidal cells in deeper layers. The remoteness of such terminals from the spike initiation zone contrasts with the many puncta on all pyramidal cell perikarya and the large globular endings on basket cell somata. Basket cells might receive feed-forward disinhibition, pyramidal cells feed-forward inhibition, and GABAergic non-basket cells would be the target of only sparse inhibitory axosomatic input. Such arrangements imply that the actions of GABA on AI neurons are neither singular nor simple and that the architectonic locus, laminar position, and morphological identity of a particular neuron must be integrated for a more refined view of its role in cortical circuitry.

Topographical relations between ipsilateral cortical afferents and callosal neurons in the second somatic sensory area of cats

Experiments were carried out on the second somatic sensory area (SII) of cats to study 1) the laminar distribution of axon terminals from the ipsilateral first somatic sensory cortex (SI); and 2) the topographical relations between their terminal field and the callosal neurons projecting to the contralateral homotopic cortex. To label simultaneously in SII both ipsilateral cortical afferents and callosal cells, cats were given iontophoretic injections of Phaseolus vulgaris-leucoagglutinin (PHA-L) in the forepaw zone of ipsilateral SI, and pressure injections of horseradish peroxidase (HRP) in the same zone of contralateral SII. The possibility that ipsilateral cortical axon terminals synapse callosal neurons was investigated with the electron microscope by combining lesion-induced degeneration with retrograde HRP labelling. Fibers and terminations immunolabelled with PHA-L from ipsilateral SI were distributed in SII in a typical patchy pattern and were mostly concentrated in supragranular layers. Labelled fibers formed a very dense plexus in layer III and ramified densely also in layers I and II. Labelled axon terminals were both en passant and single-stalked boutons. Counts of 8,303 PHA-L-labelled terminals of either type showed that 82.40% were in supragranular layers. The highest concentration was in layer III (43.99%), followed by layers II (30.32%) and I (8.09%). The remaining terminals were distributed among layers IV (6.96%), V (4.93%), and VI (5.68%). The same region of SII containing anterogradely labelled axons and terminals also contained numerous neurons retrogradely labelled with HRP from contralateral SII. Callosal projection neurons were pyramidal, dwelt mainly in layer III, and were distributed tangentially in periodic patches. Patches of anterograde and retrograde labelling either interdigitated or overlapped both areally and laminarly. In the zones of overlap, numerous PHA-L-labelled axon terminals were seen in close apposition to HRP-labelled pyramidal cell dendrites. Combined HRP-electron microscopic degeneration experiments showed that in SII axon terminals from ipsilateral SI form asymmetric synapses with HRP-labelled dendrites and dendritic spines pertaining to callosal projection neurons. These results are discussed in relation to the layering and function of the SI to SII projection, and to the evidence that SII neurons projecting to the homotopic area of the contralateral hemisphere have direct access to the sensory information transmitted from ipsilateral SI.

A study of SMI 32-stained pyramidal cells, parvalbumin-immunoreactive chandelier cells, and presumptive thalamocortical axons in the human temporal neocortex

Immunocytochemical studies in the primate neocortex have shown that particular populations of pyramidal cells can be identified by antibody SMI 32 that recognizes a nonphosphorylated epitope of neurofilament protein, while chandelier cells (a powerful type of cortical inhibitory interneuron) and presumptive thalamocortical axons can be identified by antibodies directed against the calcium-binding protein parvalbumin (PV). We used these antibodies in correlative light and electron microscopic immunocytochemical studies to analyze certain aspects of the synaptic circuitry of human temporal neocortex. In sections cut in the tangential plane, many PV-immunoreactive chandelier cell axon terminals and apical dendrites of SMI 32-stained pyramidal cells were distributed in small clusters. Combination of immunocytochemistry for PV and SMI 32 revealed four subpopulations of pyramidal cells with regard to the immunocytochemical staining by SMI 32 and the innervation of their axon initial segments by PV-positive or -negative chandelier cell axon terminals, but there were differences in the concentration and proportion of these subpopulations by layers. Furthermore, we present electron microscopic evidence suggesting that the characteristic layer III dense band of PV-immunoreactive puncta is made up mainly of presumptive thalamocortical axon terminals. Besides, coincidence was found between the dense PV-immunoreactive band and the dendritic plexus formed by the SMI 32-stained pyramidal cells in the lower half of layer III, which leads us to think that they are probably a major target of PV-immunoreactive thalamic terminations.

Intra-amygdaloid projections of the lateral nucleus in the cat: PHA-L anterograde labeling combined with postembedding GABA and glutamate immunocytochemistry

Research on the implication of the amygdala in classical fear conditioning suggests that the central amygdaloid nucleus is the output station of the amygdala for conditioned fear responses, while the lateral nucleus acts as the input nucleus, at least for auditory conditioned stimuli. However, the nature and locus of the plastic changes taking place between these two nuclei are unknown partly because the neurotransmitter(s) used by intra-amygdaloid projections of the lateral nucleus has not been identified. To address this issue in cats, anterograde tracing with Phaseolus vulgaris-leucoagglutinin (PHA-L) was combined with postembedding immunocytochemistry for gamma-aminobutyric acid (GABA) and glutamate. Two sectors can be recognized in the lateral nucleus of the cat: a shell located laterally along the external capsule, and a core. Iontophoretic injections of PHA-L in these two sectors revealed that they have nonoverlapping intra-amygdaloid targets with the exception of a common projection to the central lateral nucleus. The core projects mainly to itself and to the basomedial nucleus, whereas the shell contributes a massive projection to the basolateral nucleus. No projection of the lateral nucleus to the central medial nucleus was found. Electron microscopically, PHA-L-labeled axon terminals in the lateral, basomedial, basolateral, and central lateral nuclei as well as in the perirhinal and insular cortices formed asymmetric synapses (100%; n = 289) with dendritic spines (77-100%). Moreover, postembedding immunocytochemistry revealed that PHA-L-labeled axon terminals are immunoreactive for glutamate but not GABA. Since most amygdaloid projections to the brainstem originate in the central medial nucleus, these results suggest that intra-amygdaloid targets of the lateral nucleus are involved in the transmission of auditory conditioned stimuli to the central medial nucleus. Moreover, these findings imply that intra-amygdaloid projections of the lateral nucleus use glutamate but not GABA as a neurotransmitter.

A comparison of synapses onto the somata of intrinsically bursting and regular spiking neurons in layer V of rat SmI cortex

Regular spiking (RS) and intrinsically bursting (IB) neurons show distinct differences in their inhibitory responses. Under various conditions, the synaptic responses of RS cells display marked inhibitory postsynaptic potentials (IPSPs), whereas the responses of most IB cells do not (Silva et al: Soc Neurosci Abstr 14:883, 1988; Chagnac-Amitai and Connors: J Neurophysiol 61:747, 62:1149, 1989; Connors and Gutnick: TINS 13:99, 1990). This investigation is designed to determine if differences in the inhibitory responses of RS versus IB cells are reflected in differences in the concentration of inhibitory synapses onto their somata. RS and IB neurons in rat somatosensory cortex were identified by using intracellular recording and labeling, examined with the light microscope, and then serial thin-sectioned prior to examination with the electron microscope. Axonal terminals presynaptic to their somata and proximal dendrites were identified and classified according to criteria described by Peters and coworkers (Peters et al: J Neurocytol 19:584, 1990; Peters and Harriman: J Neurocytol 19:154, 1990; 21:679, 1992). The locations of these boutons were displayed on the surfaces of 3-D reconstructions of the somata and proximal dendrites. The reconstructions were produced directly from the serial thin sections by using a novel, electron microscopic, image-processing computer resource. Our analysis showed no significant difference in the types and concentration of boutons presynaptic to the cell bodies and proximal dendrites of intrinsically bursting versus regular spiking neurons. We conclude that the differences observed in the inhibitory responses of intrinsically bursting versus regular spiking neurons cannot be explained by differences in the concentrations of synapses onto their somata.

A comparison of the muscarinic response and morphological properties of identified cells in the guinea-pig olfactory cortex in vitro

The electrophysiological and morphological characteristics of neurons in the guinea-pig olfactory cortex brain slice were investigated using a combined intracellular recording and neurobiotin-dye filling technique, in an attempt to show whether a clear relation existed between cell morphology and excitatory muscarinic response profile. Out of 46 sampled neurons, 25 (termed type 1), responded to bath-application of the muscarinic agonist oxotremorine-M (10 microM, 2-3 min) with a strong and persistent excitation coupled with the appearance of a slow depolarizing afterpotential (10-20 mV amplitude) following a large depolarizing stimulus. These neurons were identified as deep pyramidal cells located in cortical layer III, with characteristic pyramidal/ovoid shaped cell bodies, prominent apical dendrites with branches extending to the surface, and extensive basal dendritic trees. The cells showed a regular spiking pattern in response to injected depolarizing current, with no evidence of bursting behaviour. Nine cells (termed type 2), were strongly excited by oxotremorine-M, but only generated a weak depolarizing afterpotential (< 5 mV) following stimulation. These neurons (located in layer III or at layer II-III border) had a variable, non-pyramidal morphology with either a fusiform/tripolar, stellate/multipolar or bipolar/bi-tufted appearance, respectively. Apart from a more prominent post-spike afterhyperpolarization observed in some type 2 cells, their resting membrane properties and firing patterns were indistinguishable from those of type 1 responding cells. Twelve cells (termed type 3) showed little or no excitatory response to oxotremorine-M, and never generated a post-stimulus slow afterdepolarization. These cells (within compact layer II) had the morphological features of superficial pyramidal cells, typified by their short apical trunks and well-developed apical dendritic trees. They could be distinguished electrophysiologically by their ability to show spike fractionation during injection of large depolarizing current pulses. The morphology and laminar position of neurobiotin-filled cells was also compared with those of cells stained by the Golgi-Cox method. Some factors that may have contributed to the observed differences in muscarinic response profile are discussed. It is proposed that the selective muscarinic induction of the slow depolarizing afterpotential phenomenon in deep pyramidal cells may be important in olfactory cortical learning and memory processes.

The release and uptake of excitatory amino acids in rat brain: effect of aging and oxidative stress

The excitatory amino acids (EAAs) L-aspartate and L-glutamate constitute the major neurotransmitters in the mammalian brain. This study established the influence of aging and oxidative stress on the release and uptake of EAAs. The high affinity uptake of D-[3H]aspartate in synaptosomal fractions of the neostriatum, hippocampus, and neocortex was not significantly different in Fisher 344/Norwegian Brown hybrid rats aged 3, 12, 24, and 37 months. Similarly, the K(+)-evoked efflux of endogenous aspartate and glutamate from neocortical minislices was also unaffected by age. To examine the possibility that EAA nerve terminals become more vulnerable to oxidative stress with age, the influence of an inhibitor of the electron transport chain (sodium cyanide) on EAA uptake and release was determined. Although cyanide inhibited D-[3H]aspartate uptake and potentiated the potassium-evoked efflux of aspartate and glutamate in a Ca(2+)-independent fashion, neither of these changes were influenced by age. Thus, the functional integrity of EAA nerve terminals and their vulnerability to oxidative stress are both preserved in normal aging. The potency of cyanide to inhibit D-[3H]aspartate uptake did, however, display regional variability: hippocampus > neocortex > neostriatum (IC50 = 1.2 +/- 0.2 mM, 1.9 +/- 0.1 mM and 2.7 +/- 0.2 mM, respectively), suggesting that EAA nerve terminals in the hippocampus may be selectively vulnerable to oxidative stress.

Quantitative morphology and shape classification of neurons by computerized image analysis

We describe a new image processing method for semiautomatic quantitative analysis of neuronal morphology. It has been developed in a specific image analysis environment (IBAS 2.0), but the algorithms and the methods can be employed elsewhere. The program is versatile and allows the analysis of histological preparations of different quality on the basis of different levels of evaluation and image extraction. Some significant algorithms have been implemented (i.e. one for multiple focus image acquisition and one for automatic cell body shape recognition and classification). A wide set of specific morphological parameters has been defined to allow a better mathematical characterization of neuronal morphology as regards both dendrite trees and cell bodies. Cell bodies' shapes can be classified automatically, defining different neuronal populations. This is done by evaluating the number of main dendrites and perikarya shapes through a multi-valued-decision-tree based method, tested on somatostatin-positive cells in mouse brain. The methods presented have been applied to analysis of neurons, but they can well be used for any quantitative morphological study of other cell populations.