gamma-Aminobutyric acid (GABA) immunoreactivity in mouse and rat first somatosensory (SI) cortex: description and comparison

Intravital Imaging of Neocortical Heterotopia Reveals Aberrant Axonal Pathfinding and Myelination around Ectopic Neurons

Neocortical heterotopia consist of ectopic neuronal clusters that are frequently found in individuals with cognitive disability and epilepsy. However, their pathogenesis remains poorly understood due in part to a lack of tractable animal models. We have developed an inducible model of focal cortical heterotopia that enables their precise spatiotemporal control and high-resolution optical imaging in live mice. Here, we report that heterotopia are associated with striking patterns of circumferentially projecting axons and increased myelination around neuronal clusters. Despite their aberrant axonal patterns, in vivo calcium imaging revealed that heterotopic neurons remain functionally connected to other brain regions, highlighting their potential to influence global neural networks. These aberrant patterns only form when heterotopia are induced during a critical embryonic temporal window, but not in early postnatal development. Our model provides a new way to investigate heterotopia formation in vivo and reveals features suggesting the existence of developmentally modulated, neuron-derived axon guidance and myelination factors.

Inhibitory interneurons in a cortical column form hot zones of inhibition in layers 2 and 5A

Although physiological data on microcircuits involving a few inhibitory neurons in the mammalian cerebral cortex are available, data on the quantitative relation between inhibition and excitation in cortical circuits involving thousands of neurons are largely missing. Because the distribution of neurons is very inhomogeneous in the cerebral cortex, it is critical to map all neurons in a given volume rather than to rely on sparse sampling methods. Here, we report the comprehensive mapping of interneurons (INs) in cortical columns of rat somatosensory cortex, immunolabeled for neuron-specific nuclear protein and glutamate decarboxylase. We found that a column contains ~2,200 INs (11.5% of ~19,000 neurons), almost a factor of 2 less than previously estimated. The density of GABAergic neurons was inhomogeneous between layers, with peaks in the upper third of L2/3 and in L5A. IN density therefore defines a distinct layer 2 in the sensory neocortex. In addition, immunohistochemical markers of IN subtypes were layer-specific. The "hot zones" of inhibition in L2 and L5A match the reported low stimulus-evoked spiking rates of excitatory neurons in these layers, suggesting that these inhibitory hot zones substantially suppress activity in the neocortex.

GAD67-positive puncta: contributors to learning-dependent plasticity in the barrel cortex of adult mice

We have previously shown that a classical aversive conditioning paradigm involving stimulation of a row of facial vibrissae (whiskers) in the mouse produced expansion of the cortical representation of the activated vibrissae ("trained row"). This was demonstrated by labeling with 2-deoxyglucose (2DG) in layer IV of the barrel cortex. We have also shown that functional reorganization of the S1 cortex is accompanied by increases in the density of small GABAergic cells, and in GAD67 mRNA in the hollows of barrels representing the "trained row". The aim of this study was to determine whether GAD67-positive puncta (boutons) are affected by learning. Unbiased optical disector counting was applied to sections from the mouse barrel cortex that had been immunostained using a polyclonal antibody against GAD67. Quantification of the numerical density of GAD67-positive boutons was performed for four groups of mice: those that had been given aversive conditioning, pseudoconditioned mice with random application of the unconditioned stimulus, mice that had received only whisker stimulation, and naive animals. This study is the first to demonstrate that learning-dependent modification of mature somatosensory cortex is associated with a 50% increase in GAD67-positive boutons in the hollows of "trained" barrels compared with those of control barrels. Sensory learning seems to mobilize the activity of the inhibitory transmission system in the cortical region where plastic changes were previously detected by 2DG labeling.

Short-term sensory learning does not alter parvalbumin neurons in the barrel cortex of adult mice: A double-labeling study

We have previously reported that a classical conditioning paradigm involving stimulation of a row of facial vibrissae produced expansion of the cortical representation of the activated vibrissae ("trained row"), this was demonstrated by labeling with 2-deoxyglucose in layer IV of the barrel cortex. We have also shown that functional reorganization of the primary somatosensory cortex is accompanied by an increase in the density of small GABAergic cells and glutamate decarboxylase 67-positive neurons in the hollows of barrels representing the "trained row." GABA neurons of the rat neocortex co-localize with calcium-binding proteins [parvalbumin, carletinin, calbindin D28k] and neuropeptides (vasoactive intestinal polypeptide, somatostatin). In the present study we have examined GABAergic parvalbumin-positive, interneurons in the cortical representation of "trained" facial vibrissae after short-term aversive training, in order to determine whether the observed changes in glutamate decarboxylase 67-positive neurons are accompanied by changes in parvalbumin-positive neurons. Using double immunofluorescent staining, it was found that (i) all parvalbumin-positive neurons in the barrel hollows were glutamate decarboxylase 67-positive, (ii) following aversive training density of glutamate decarboxylase 67-positive neurons in barrel hollows increased significantly compared with controls and (iii) density glutamate decarboxylase 67-positive/parvalbumin-positive neurons in "trained" barrel hollows did not change compared with controls. This study is the first to demonstrate that the density of double-labeled glutamate decarboxylase 67-positive/parvalbumin-positive neurons does not alter during cortical plasticity, thus suggesting that some other population (i.e. parvalbumin negative) of GABAergic interneurons is involved in learning-dependent changes in layer IV of the barrel cortex.

Mediodorsal thalamic afferents to layer III of the rat prefrontal cortex: synaptic relationships to subclasses of interneurons

The mediodorsal nucleus of the thalamus (MD) represents the main subcortical structure that projects to the prefrontal cortex (PFC) and it regulates key aspects of the cognitive functions of this region. Within the PFC, GABA local circuit neurons shape the activity patterns and hence the "memory fields" of pyramidal cells. Although the connections between the MD and PFC are well established, the ultrastructural relationships between projecting fibers from the MD and different subclasses of GABA cells in the PFC are not known. In order to address this issue in the rat, we examined MD axons labeled by tract-tracing in combination with immunogold-silver to identify different calcium-binding proteins localized within separate populations of interneurons. Electron micrographic examination of PFC sections from these animals revealed that MD terminals made primarily asymmetric synapses onto dendritic spines and less commonly onto dendritic shafts. Most of the dendrites receiving MD synaptic input were immunoreactive for parvalbumin (ParV), whereas MD synapses onto dendrites labeled for calretinin or calbindin were less frequent. We also observed that some MD terminals were themselves immunoreactive for calcium-binding proteins, again more commonly for ParV. These results suggest that the MD exerts a dual influence on PFC pyramidal cells: direct inputs onto spines and an indirect influence mediated via synapses onto each subclass of interneurons. The apparent preferential input to ParV cells endows MD afferents with a strong indirect inhibitory influence on pyramidal neuron activity by virtue of ParV cell synapses onto soma, proximal dendrites, and axon initial segments.

Local GABA Receptor Blockade Reveals Hindlimb Responses in the SI Forelimb-Stump Representation of Neonatally Amputated Rats

In adult rats that sustained forelimb amputation on the day of birth, there are numerous multi-unit recording sites in the forelimb-stump representation of primary somatosensory cortex (SI) that also respond to cutaneous stimulation of the hindlimb when cortical receptors for GABA are blocked. These normally suppressed hindlimb inputs originate in the SI hindlimb representation and synapse in the dysgranular cortex before exciting SI forelimb-stump neurons. In our previous studies, GABA (A + B) receptor blockade was achieved by topically applying a bicuculline methiodide/saclofen solution (BMI/SAC) to the cortical surface. This treatment blocks receptors throughout SI and does not allow determination of where along the above circuit the GABA-mediated suppression of hindlimb information occurs. In this study, focal injections of BMI/SAC were delivered to three distinct cortical regions that are involved in the hindlimb-to-forelimb-stump pathway. Blocking GABA receptors in the SI hindlimb representation and in the dysgranular cortex was largely ineffective in revealing hindlimb inputs (∼10% of hindlimb inputs were revealed in both cases). In contrast, when the blockade was targeted at forelimb-stump recording sites, >80% of hindlimb inputs were revealed. Thus GABAergic interneurons within the forelimb-stump representation suppress the expression of reorganized hindlimb inputs to the region. A circuit model incorporating these and previous observations is presented and discussed.

Enhancement of laminar FosB expression in frontal cortex of rats receiving long chronic clozapine administration

The frontal cortex (FrC) and cingulate cortex (CgC) are critical sites for normal cognitive function, as well as cognitive dysfunction in schizophrenia. Thus, modulation of synaptic transmission within these cortical areas may, in part, account for the therapeutic actions of antipsychotic drugs such as haloperidol and clozapine. FosB and DeltaFosB are immediate-early gene (IEG) products sensitive to changes in response to chronic neuroleptic drug administration. We quantitatively examine whether there are light microscopic regional and/or laminar variations in FosB or DeltaFosB in the FrC or CgC of normal adult rats, or animals receiving 6 months administration of either drinking water clozapine, or depot haloperidol. Only animals receiving chronic haloperidol developed vacuous chewing movements, the equivalent of tardive dyskinesia in humans. In control animals, the deep and superficial layers of the FrC showed a higher area density of FosB, but not DeltaFosB immunoreactive cells than the medial layers of FrC or any of the CgC layers. In animals receiving clozapine, but not haloperidol there was increase in the area density of FosB immunoreactive neurons in all FrC layers, but the major increase occurs in medial layers. These findings suggest that FosB expression identifies those FrC neurons that are most active during normal waking behaviors and are further activated following chronic administration of atypical neuroleptics without motor side effects. The results also indicate that the actions of clozapine are attributed in large part to modulation of the output of frontal cortical pyramidal neurons residing in the medial layers.

Changes in GABAergic neuron distribution in situ and in neuron cultures in ovine (OCL6) Batten disease

The neuronal ceroid lipofuscinoses (NCLs) are a group of inherited human and animal diseases characterized by progressive brain atrophy. A form in sheep is syntenic to the human CLN6 disease. Cell type specific neurodegeneration in these sheep was indicated by the distribution of GABAergic interneurons in coronal sections of normal and CLN6 affected sheep brains. A reduction of parvalbumin immunoreactive neurons in NCL cerebral cortex was the most striking feature. This was most pronounced in parietal cortex where very few positive cells remained. Calretinin immunoreactive somata in infragranular layers of the neocortex were also reduced while the number of calbindin positive cells was similar in affected and normal brains. There were fewer GAD immunoreactive neurons in the deeper layers of all NCL cortical areas examined. The parietal lobe was relatively more affected than frontal or temporal lobes while the cerebellum and the basal ganglia showed no signs of selective neuron loss. Since horizontally extending basket cells are mainly labelled by parvalbumin, the loss of these interneurons in the neocortex may render pyramidal neurons more excitable and compromise their co-ordinated output. In vitro, cultures of control and affected neurons from 60 to 70-day-old fetal brain hemispheres were examined for the presence of GABAergic and glutamatergic neurons. Different neurons developed distinct immunoreactivity to glutamate or GABA but the overall distribution was similar in normal and affected cultures. This culture system may provide a useful model to compare GABAergic cell function of normal and NCL affected neurons.

Differential expression of GABA(A) receptor subunit mRNAs and ligand binding sites in rat brain following phencyclidine administration

Recent biochemical observations have suggested the abnormalities in the gamma-amino-butyric acid (GABA)ergic system in schizophrenic brains. In the present study, we investigated the subunits gene expressions and ligand binding of the GABA(A) receptor following acute and chronic administration of phencyclidine (PCP), which induces schizophrenia-like symptoms, in rats using in situ hybridization and in vitro quantitative autoradiography. PCP i.p. administration at a daily dose of 7.5 mg/kg resulted in a significant decrease in expression of alpha 1 subunit mRNA in cerebral cortices (cingulate (-13%) and temporal cortex (-6%)) and hippocampal formation (CA1 (-11%), CA2 (-10%), CA3 (-11%) and dentate gyrus (-12%)) 1 h after a single treatment. In the repeated PCP administrations for 14 days, the expression of beta 2 mRNA in the cerebellum (-10%) and of beta 3 mRNA in the cerebral cortices (cingulate (-12%), parietal (-16%) and temporal cortex (-16%), caudate putamen (-18%), inferior colliculus (-18%), and cerebellum (-15%) were significantly decreased. In addition, [(35)S]t-butylbicyclophosphorothionate (TBPS) binding was also reduced in layer IV of the frontoparietal cortex (-14%), inferior colliculus (-17%), and cerebellum (-12%) following chronic PCP treatment, while no changes were observed following acute PCP treatment. These results indicate that single and repeated administrations of PCP independently regulate the expression of GABA(A)/benzodiazepine (BZD) receptor subunits mRNA and its receptor binding in the brain.

Direct inhibition evoked by whisker stimulation in somatic sensory (SI) barrel field cortex of the awake rat

Whisker deflection typically evokes a transient volley of action potentials in rat somatic sensory (SI) barrel cortex. Postexcitatory inhibition is thought to quickly terminate the cortical cell response to whisker deflection. Using dual electrode extracellular recording in awake rats, we describe an infrequent type of cell response in which stimulation of single hairs consistently blocks the ongoing discharge of neurons without prior excitation (I-only inhibition). Reconstruction of the recording sites indicates that I-only inhibition occurs most frequently when the recording site is clearly in the septum or at the barrel-septum junction. The same cells that respond with I-only inhibition to one whisker can show an excitatory discharge to other whiskers, usually followed by inhibition. Stimulation of either nose hairs or the large mystacial vibrissa can evoke I-only inhibition in SI cortex. I-only inhibition is most commonly observed at low stimulus frequencies ( approximately 1 Hz). At stimulus frequencies of >6 Hz, I-only inhibition typically converts to excitation. We conclude that single whisker low-frequency stimulation can selectively block the spontaneous discharge of neurons in SI barrel field septa. The observation that this cell response is found most often in or at the edge of septa and at relatively long latencies supports the idea that I-only inhibition is mediated through cortical circuits. We propose that in these cells inhibition alone or a combination of inhibition and disfacilitation play a role in suppressing neuronal discharge occasioned by low frequency contact of the whiskers with the environment.

GABA immunoreactivity in mouse barrel field after aversive and appetitive classical conditioning training involving facial vibrissae

We have previously reported that a classical conditioning paradigm involving stimulation of a row of facial vibrissae produced an expansion of the cortical representation of the "trained row", labeled with 2-deoxyglucose (2DG), in layer IV of the barrel field. The present study has examined the pattern of GABA immunoreactivity (GABA-IR) in the cortical representation of row B of the facial vibrissae after (i) 3 days of aversive training, and (ii) 2 months of appetitive training, where stimulation of row B of vibrissae on one side of the snout was used as a conditioned stimulus. The most notable observation was a greater density of GABA-IR cells concentrated in the hollows of the "trained row" B barrels compared to the hollows in the barrel field of the opposite hemisphere in the same mouse. After aversive training, we noted a 2-fold increase in the density of GABA-IR neurons in the hollows of row B; after reward training, the increase amounted to 49%. In contrast, GABA-IR was unchanged in the control groups, which received only stimulation of vibrissae without the unconditioned stimulus. The classification of labeled neurons according to size revealed that the increase in density of GABA-IR neurons was confined to the small (12-15 microm) diameter group. We concluded that the GABAergic system undergoes up-regulation, after both associative learning paradigms, and that the population of small, GABAergic neurons plays an active role in use-dependent plasticity.

Modular distribution of vasoactive intestinal polypeptide in the rat barrel cortex: changes induced by neonatal removal of vibrissae

The distribution of vasoactive intestinal polypeptide-immunoreactive neuronal structures in the barrel cortex (posteromedial barrel subfield) of adult rats was analysed after unilateral removal of the vibrissal follicles of row C in neonatal rats. The hypothesis was tested whether the distribution of vasoactive intestinal polypeptide-immunoreactive structures depends on the normal anatomical organization of the specific sensory input. After three months survival the distribution of the vasoactive intestinal polypeptide-immunoreactive structures was morphometrically evaluated. This approach revealed alterations in the contralateral posteromedial barrel subfield, where the disappearance of barrel row C and a substantial increase in size mainly of barrel row D, but also of other rows could be detected. Increase in row D included both barrels and the interspace (septal segments between barrels in one row). As vasoactive intestinal polypeptide immunoreactivity of the barrel field was found previously to be localized in synaptic boutons involved in symmetric synapses, our present findings suggest that (i) the interspace is enriched in inhibitory vasoactive intestinal polypeptide-immunoreactive synapses as opposed to the excitatory thalamocortical input reaching the barrel hollow, (ii) the spatial distribution of the vasoactive intestinal polypeptide system in the barrel cortex is closely associated with the neuronal organization of the sensory input and reacts with a considerable plasticity to lesion-induced changes of the input, and (iii) the compensatory barrel hypertrophy in a row neighbouring the deafferented row involves an increasing number of vasoactive intestinal polypeptide-immunoreactive synapses per barrel.

Co-localization of vasoactive intestinal polypeptide, gamma-aminobutyric acid and choline acetyltransferase in neocortical interneurons of the adult rat

Interneurons immunoreactive for vasoactive intestinal polypeptide (VIP) are integral elements of columnar organization patterns in the rat cerebral cortex. By application of the sensitive mirror technique, the co-localization of VIP with the classical inhibitory neurotransmitter gamma-aminobutyric acid (GABA) and the acetylcholine-synthesizing enzyme, choline acetyltransferase (ChAT), was investigated in neocortical neurons. Furthermore, the frequency of co-localization of ChAT with GABA was determined. In a sample of 118 VIP-immunoreactive neurons, mostly from the primary somatosensory cortex, it was demonstrated that virtually all of them reveal immunoreactivity for GABA and, therefore, are to be GABAergic. Moreover, 34% of mostly bipolar, VIP-positive neurons contained ChAT and are, thus, supposedly cholinergic as well. Co-localization of VIP and ChAT varied according to cortical laminae. Finally, 88% of a total of 60 ChAT-immunoreactive neurons were also immunostained for GABA. It is concluded that almost all VIP-immunoreactive neurons and most of the cholinergic neurons in rat neocortex represent partly overlapping subpopulations of inhibitory interneurons utilizing GABA.

Distribution of GABAergic elements postsynaptic to ventroposteromedial thalamic projections in layer IV of rat barrel cortex

The spatial synaptic pattern formed by boutons, originating in the ventroposteromedial thalamic nucleus, with GABAergic neurons in the rat barrel cortex was mapped. The aim was to shed light on the structural basis by which inhibitory circuits may be activated at the first stage of cortical information processing. The thalamic afferent projection was labelled by anterograde transport of Phaseolus vulgaris leucoagglutinin (PHA-L), whereas the GABAergic targets in layer IV of the rat barrel cortex was visualized by postembedding GABA immunogold-labelling or by pre-embedding parvalbumin immunocytochemistry. In the first set of experiments, we mapped barrels, contained in single ultrathin sections, by means of a computer-controlled electron microscope stage in their entire layer IV representation. From a total of 1199 asymmetric PHA-L-labelled synapses, only 98 were on GABAergic elements, mainly on dendritic shafts. This corresponded to 8.2% of all synapses counted. These synapses on GABAergic targets were essentially homogeneously distributed without a reliable relationship to barrel subdivisions, i.e., hollow versus wall; or layer IVa versus layer IVb. In the second part of the study, we demonstrated that parvalbumin-containing neurons represent the major GABAergic cell type targetted by thalamic afferents in layer IV of the barrel cortex, since all parvalbumin-positive cells investigated received multiple synaptic contacts (up to eight synapses per neuron) from the ventroposteromedial thalamic nucleus. These results imply that interneurons responsible for perisomatic inhibition (basket and chandelier cells known to contain parvalbumin) are likely to be strongly excited by thalamic afferents, despite the relatively low proportion of thalamic synapses on GABAergic elements compared to spines of principal cells, and participate in the early stages of cortical sensory information processing.

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.

Differential effect of whisker trimming on excitatory and inhibitory transmission in primary somatosensory cortex of the adult rat in vivo

The effects of sensory deprivation on excitatory and inhibitory activity in the primary somatosensory cortex were studied in the adult rat. Excitatory and inhibitory transmission generated by whisker stimulation, and neuronal responsiveness to iontophoretically applied excitatory amino acids were recorded. Whisker input deprivation, through whisker trimming for a median of 24 days, resulted in a significant decrease in excitatory transmission to surround whisker stimulation. In contrast, the response magnitude to principal whisker stimulation remained unchanged. However, the response latencies to principal whisker and surround whisker stimulation were significantly reduced, which led to altered temporal response distributions in deprived cells. Neurons deprived of sensory input were significantly less responsive to glutamate, N-methyl-D-aspartate, alpha-amino-3-hydroxy-5-methyl-4- isoxazolepropionate and kainate. Following deprivation, no change was observed in cortical inhibitory transmission measured 30-200 ms post-stimulus. These results show that excitatory transmission (including excitatory amino acid receptor function) is altered by adult whisker deprivation.

Developmental role of fatty acid-binding proteins in mouse brain

While the functions of the cytoplasmic fatty acid-binding proteins (FABPs) are not well defined, one possibility in neural tissue is in establishing and maintaining the high levels of polyunsaturated fatty acids in membrane lipids characteristic of this tissue and thought essential for normal function. We investigated the reactivity of a protein in developing mouse brain to antiserum prepared against rat heart (H)-FABP. By immunoblot analysis, levels of H-FABP in brain were nearly undetectable until fetal day 17-19, after which levels increased until at least postnatal day 14. Levels of H-FABP were lower in the adult mouse brain, suggesting a function for the protein during differentiation of neural tissue. In immunohistochemical studies with postnatal day 14 mouse brain, the most intensely stained area was the choroid plexus. H-FABP also localized to regions of the somatosensory cortex and to the spinal trigeminal nucleus. In addition, H-FABP was present in the thalamus, entorhinal and piriform cortex, and throughout the pontine and medullary nuclei. Tracts related to the auditory system, including ventral cochlear nucleus and lateral lemniscus, also were H-FABP-positive. In cerebellum, the molecular layer was heavily labeled in cells and processes; in the granule cell layer, there was punctate staining suggestive of mossy fiber terminals. Small cells adjacent to Purkinje cells were intensely stained, while the Purkinje cells were negative. We conclude that H-FABP in brain participates in neurite formation and synapse maturation, and may be related to the similar pattern of expression of GABA related markers.

The development of parvalbumin-immunoreactivity in the neocortex of the mouse

In the present study the postnatal development of parvalbumin-immunoreactivity was examined in the neocortex of the mouse. Postnatal mice were processed at different developmental stages using a well-characterized monoclonal antibody against parvalbumin, and immunocytochemistry. The first immunoreactive neurons appeared in the first parietal and retrosplenial cortices at postnatal day 10 (P10). From P11 to P12, immunoreactivity emerged in the second parietal, cingular, frontal, hindlimb-forelimb, first temporal, primary and secondary occipital and gustatory cortices, and at P14, parvalbumin-positive cells were present in the remaining regions. In general, parvalbumin-immunoreactivity appeared first in the primary sensory/motor areas, and then in second sensory/motor or associative areas. The maturation of parvalbumin-immunoreactivity, however, was a long-lasting process, which was not completed until adult stages. In all cortical regions, parvalbumin-immunoreactive cells were present first in layer V, from which immunoreactivity expanded to the upper and inner cortical layers at subsequent developmental stages. This pattern of maturation differed from the usual 'inside-out' gradient of neocortical neurogenesis and maturation. At the cellular level, parvalbumin-immunoreactivity appeared first in cell somata, and staining of dendrites and boutons was apparent two days later. From the second postnatal week onwards, an immunoreactive axonal system was observed in the neocortical white matter and the corpus callosum. We conclude that the emergence and maturation of parvalbumin-immunoreactivity in the mouse neocortex shows marked area-specific differences, but proceeds following a similar center-to-outside radial gradient. These features may reflect the acquisition of certain physiological properties by a subset of GABAergic inhibitory neurons.

Loss of glutamate immunoreactivity from mouse first somatosensory (SI) cortex following neonatal vibrissal lesion

Whisker follicles were surgically ablated (lesioned) on two entire rows (B and C) of the left snout of two groups of Swiss mice, in the first 2 days after birth (neonatally lesioned) with the animals being allowed to survive for 4 weeks. In the second group at 8 weeks of age (adults), the whisker follicles of rows B and C were similarly lesioned and a survival period of 3 days allowed. Glutamate-immunoreactivity (Glu-IR) was examined in tangential sections of the first somatosensory (SI) 'barrel' cortex of these two groups (at which time it was also confirmed that the follicles had not regrown). In the neonatally lesioned mice, barrel rows B and C were more poorly defined in the right (experimental) hemisphere sections and a semi-quantitative study showed that there was a decrease in Glu-IR cell number (up to 41%) in rows B and C of the right hemisphere compared to the spared barrel rows (A and D). The loss appears to occur over almost the entire area of each deprived barrel rather than being confined to the sides or hollows. In contrast to neonatally lesioned animals, the barrels of the adult-lesioned mice appeared intact and visually similar in both the experimental (right) and control hemispheres (left). The only notable change in Glu-IR observed in the adult-lesioned animals was a decrease of 38% in the number of Glu-IR cells in the sides between the two deafferented rows of barrels (B and C), compared to the cell number between the unaffected barrels (D and E), a change also seen in the neonatally lesioned mice.(ABSTRACT TRUNCATED AT 250 WORDS)

Vasoactive intestinal polypeptide immunoreactive structures in the mouse barrel field

Immunohistochemistry for vasoactive intestinal polypeptide was carried out in tangentially cut vibratome sections of the barrel cortex in adult mice. Sections through layer IV have revealed an association between the cytoarchitectonically visible modular organization of barrels and the distribution of immunoreactive axon terminals. These terminals are preferentially localized in the side region of a barrel, whereas the hollow shows a relative scarcity of these structures as shown with image analysis. This finding is the first direct demonstration of a modular distribution of vasoactive intestinal polypeptide-containing axon terminals in the neocortex.

JUN, FOS, KROX, and CREB transcription factor proteins in the rat cortex: basal expression and induction by spreading depression and epileptic seizures

The expression of the nuclear c-JUN, JUN B, JUN D, c-FOS, FOS B, KROX-24, and CREB transcription factors was investigated in the cortex of adult rats by immunocytochemistry. The expression patterns were studied in untreated rats and up to 24 hours following topical application of 1 M KCl to the cortical surface (KCl) or i.v. injection of bicuculline (BIC). Topical KCl induced cortical spreading depression and systemic injection of bicuculline evoked generalized tonic-clonic seizures. In untreated rats, JUN B, c-FOS, and FOS B were expressed in a small number of neurons in the piriform, perirhinal, entorhinal, and insular cortex and in layers II, III, and VI of all neocortical areas. In contrast, c-JUN, JUN D, and KROX-24 were expressed in all cortical layers but with different intensities of immunoreactivity (IR): c-JUN-IR was generally weak and predominantly present in layers II, III, and VI. JUN D-IR was equally strong in all layers. KROX-24 showed a prominent expression in layers II, IV, and VI. The CREB protein exhibited a slight preponderance in layer II and piriform cortex. Following KCl or BIC, a strong induction was seen for c-FOS, JUN B, and KROX-24, whereas c-JUN, JUN D, and FOS B showed only a moderate increase compared to basal levels. Changes of CREB-IR could not be detected. The localization of induced JUN, FOS, and KROX proteins reflected the pattern of labelling in untreated animals but demonstrated a higher intensity of labelling and an increased number of immunoreactive nuclei. The intensity and persistence of IR as well as the number of labelled cells following BIC exceeded those following KCl. Following BIC, increased levels of FOS B and JUN D were still present after 24 hours. Counterstaining with cresyl-violet and GFAP, a marker for astrocytes, revealed that JUN, FOS, and KROX proteins were expressed in neurons but not in glial cell populations. The present data demonstrate that CREB, JUN, FOS, and KROX transcription factors exhibit a layer-specific expression in the cerebral cortex with only slight area-specific differences both in untreated rats and following stimulation with KCl and BIC. The expression of transcription factor proteins indicate complex molecular genetic changes in cortical neurons due to pathophysiological events such as seizure activity and spreading depression.

Numerical data on neocortical neurons in adult rat, with special reference to the GABA population

The disector method was used to estimate the numerical density of neurons (number per unit volume) and their actual number per column (number under a given area of pial surface), in the occipital (monocular segment of the primary visual area, Oc1M), the parietal (somatosensory barrelfield area, Par1) and the frontal cortex (primary motor area, Fr1) of adult rat. Values were first obtained for all neurons in each layer, and then for GABA neurons as identified with postembedding immunocytochemistry on semithin sections. The numerical density of neurons in the frontal cortex (34,000/mm3) was significantly lower than in the two other neocortical areas (occipital: 52,000; parietal: 48,000/mm3). The GABA population showed a similar difference and consequently represented an equivalent proportion of total (15%) in the three cortical areas. Across layers, there was an alternate distribution of low and high density of neurons from layers II-III to VI in the three cortical areas, with the highest density in layer IV of the two sensory areas. The laminar changes in density of the GABA neurons were not as pronounced as those of the overall population. Consequently, the layers with the highest overall neuronal densities tended to have a lower proportion of GABA neurons and vice versa. There were more neurons under 1 mm2 of surface in the parietal (90,000) than the occipital or the frontal cortex (71,000), which was also true of the GABA neurons. The greater number of neurons per column in the parietal cortex was mostly imputable to layer IV, the main recipient of thalamic axons.(ABSTRACT TRUNCATED AT 250 WORDS)

Immunoreactivity for GAD and three peptides in somatosensory cortex and thalamus of the raccoon

Immunocytochemical methods were used to determine the distributions of glutamic acid decarboxylase (GAD), vasoactive intestinal polypeptide (VIP), cholecystokinin (CCK), and somatostatin (SOM) in the primary somatosensory cortex and somatosensory thalamus of adult raccoons. The cortex showed extensive immunoreactivity for GAD, revealing a large population of GABAergic neurons. GAD-labeled cells were numerous in all cortical layers, but were most concentrated in laminae II-IV. The cells were nonpyramidal and of varying morphology, typically with somata of small or medium size. GAD-immunoreactive puncta, presumably synaptic terminals, were widespread and often appeared to end on both GAD-negative and GAD-positive neurons. Immunoreactivity for the peptides was much less extensive than that for GAD, with the number of labeled neurons for VIP > CCK > SOM. Peptidergic cells were preferentially located in the upper and middle cortical layers, especially laminae II and III. The cells were nonpyramidal, often bitufted or bipolar in morphology, and small to medium in size. Their processes formed diffuse plexuses of fibers with terminal-like varicosities that occasionally surrounded nonpeptidergic neurons. The thalamus showed a clearly differentiated pattern of immunoreactivity for GAD, but little or no labeling for the three peptides. Nuclei adjoining the ventral posterior lateral (VPL)/ventral posterior medial (VPM) complex--including the reticular nucleus--contained many GAD-positive neurons and fibers. In contrast, the VPL and VPM nuclei displayed considerably less GAD immunoreactivity, somewhat surprising given the raccoon's highly developed somatosensory system. However, the ventral posterior inferior (VPI) nucleus revealed rather dense GAD labeling, perhaps related to a specialized role in sensory information processing. Thus, the primary somatosensory cortex of the raccoon showed patterns of immunoreactivity for GAD and peptides that were similar to those of other species; the somatosensory thalamus revealed a distinctive profile of GAD immunoreactivity, with labeling that was light to moderate in the VPL/VPM complex and relatively extensive in VPL.

High-affinity uptake of GABA and glutamate decarboxylase activity in rat primary somatosensory cortex after sciatic nerve injury

We have studied the changes in the GABAergic system in the rat somatosensory cortex 1-14 d after sensory deprivation of the hind-limb representation area. Glutamate decarboxylase (GAD) activity was measured in the individual cortical layers using serial sections cut on a freezing microtome parallel to the cortical surface. Gamma-aminobutyric acid (GABA) high-affinity uptake was studied in cortical homogenates of the hind-limb representation area. There was a less than or equal to 13% decrease in GAD activity in layer II-IV in both cortical hemispheres 3 d after sciatic nerve injury. In contrast, we found that high-affinity uptake of GABA is not affected. The data mirror only small changes in GABAergic transmission, probably as a result of the methods employed. These changes correspond to electrophysiological studies suggesting that peripheral manipulation of the somatosensory system, e.g., nerve transection, is accompanied by changes in GABAergic transmission.

Loss of gamma-aminobutyric acid (GABA) immunoreactivity from mouse first somatosensory (SI) cortex following neonatal, but not adult, denervation

Whisker follicles were surgically ablated (lesioned) on two entire rows (B and C) of the left snout of two groups of Swiss mice, in the first 2 days after birth, and in the second group at 8 weeks of age (adults). Two months after surgery GABA-immunoreactivity (GABA-IR) was examined in tangential sections of the first somatosensory (SI) 'barrel' cortex of these two groups (at which time it was also confirmed that the follicles had not regrown). In the adult-lesioned mice all of the barrels appeared intact and visually similar in both the experimental (right) and control (left) hemispheres. The staining pattern of immunopositive cells and puncta was qualitatively similar to that which we have described previously. However, in the neonatally-lesioned mice barrel rows B and C were not visible in the right hemisphere sections and there was a marked reduction in GABA-IR, with fewer immunopositive cells. Many of those that did show GABA-IR stained only weakly and the puncta were also fewer in number than in the left (control) hemisphere where the GABA-IR pattern and staining intensity was normal.

Modulation of vibrissa-evoked cortical potentials after infraorbital nerve crush in rats

Cortical potentials evoked by unilateral stimulation of the major vibrissae were recorded in 12 rats subjected to unilateral crush of the infraorbital nerve. Immediately after nerve crushing, the latency of the initial positive potential evoked at contralateral scalp sites by stimulating the vibrissae of the nerve-crushed side was increased. In contrast, the latency of the ipsilaterally evoked potential was shortened. The relative amplitude of the negative component to the positive one of the evoked potentials tended, immediately after the nerve crush, to be smaller on the contralateral cortex (N/P-contra) and greater on the ipsilateral cortex (N/P-ipsi). These changes disappeared largely by the 2nd post-operative week. It is suggested that reduction of the tactile signals transmitted through the crossed pathway is responsible for the prolonged latency and the smaller N/P-contra. Shortening of the ipsilateral latency and the enhanced N/P-ipsi may be due to liberation of the ipsilateral sensory system from inhibition by the contralateral one.

Triads: a synaptic network component in the cerebral cortex

The objective of this study was to examine synaptic relationships among 3 neuronal elements in the cerebral cortex: thalamocortical afferents (TC), corticothalamic projection cells (CT), and GABAergic neurons. TC axon terminals in the barrel cortex of the mouse were labeled by lesion induced degeneration; local axon collaterals belonging to CT cells were labeled by the retrograde transport of horseradish peroxidase; and GABAergic neurons were identified using immunocytochemistry. CT and GABAergic neurons form synapses with each other and both receive synapses from TC afferents. These findings indicate the existence in the cerebral cortex of a triadic circuit involving afferent input both to projection and to local inhibitory neurons, and reciprocal synaptic interactions among these neuronal populations.