Area-specific morphological and neurochemical maturation of non-pyramidal neurons in the rat hippocampus as revealed by parvalbumin immunocytochemistry

Distinct in vivo dynamics of excitatory synapses onto cortical pyramidal neurons and parvalbumin-positive interneurons

Cortical function relies on the balanced activation of excitatory and inhibitory neurons. However, little is known about the organization and dynamics of shaft excitatory synapses onto cortical inhibitory interneurons. Here, we use the excitatory postsynaptic marker PSD-95, fluorescently labeled at endogenous levels, as a proxy for excitatory synapses onto layer 2/3 pyramidal neurons and parvalbumin-positive (PV⁺) interneurons in the barrel cortex of adult mice. Longitudinal in vivo imaging under baseline conditions reveals that, although synaptic weights in both neuronal types are log-normally distributed, synapses onto PV⁺ neurons are less heterogeneous and more stable. Markov model analyses suggest that the synaptic weight distribution is set intrinsically by ongoing cell-type-specific dynamics, and substantial changes are due to accumulated gradual changes. Synaptic weight dynamics are multiplicative, i.e., changes scale with weights, although PV⁺ synapses also exhibit an additive component. These results reveal that cell-type-specific processes govern cortical synaptic strengths and dynamics.

Melander et al. use a genetic strategy to visualize excitatory neuronal connections that cannot be inferred from morphology, and they monitor how the connections change over weeks in mice. They find distinct characteristics between synapses onto cells that “suppress” brain activity and those onto cells that “excite” brain activity.

The Neuroprotective Effects of 17β-Estradiol Pretreatment in a Model of Neonatal Hippocampal Injury Induced by Trimethyltin

Hippocampal dysfunction plays a central role in neurodevelopmental disorders, resulting in severe impairment of cognitive abilities, including memory and learning. On this basis, developmental studies represent an important tool both to understanding the cellular and molecular phenomena underlying early hippocampal damage and to study possible therapeutic interventions, that may modify the progression of neuronal death. Given the modulatory role played by 17β-estradiol (E2) on hippocampal functions and its neuroprotective properties, the present study investigates the effects of pretreatment with E2 in a model of neonatal hippocampal injury obtained by trimethyltin (TMT) administration, characterized by neuronal loss in CA1 and CA3 subfields and astroglial and microglial activation. At post-natal days (P)5 and P6 animals received E2 administration (0.2 mg/kg/die i.p.) or vehicle. At P7 they received a single dose of TMT (6.5 mg/kg i.p.) and were sacrificed 72 h (P10) or 7 days after TMT treatment (P14). Our findings indicate that pretreatment with E2 exerts a protective effect against hippocampal damage induced by TMT administration early in development, reducing the extent of neuronal death in the CA1 subfield, inducing the activation of genes involved in neuroprotection, lowering the neuroinflammatory response and restoring neuropeptide Y- and parvalbumin- expression, which is impaired in the early phases of TMT-induced damage. Our data support the efficacy of estrogen-based neuroprotective approaches to counteract early occurring hippocampal damage in the developing hippocampus.

Live imaging of endogenous PSD-95 using ENABLED: a conditional strategy to fluorescently label endogenous proteins

Stoichiometric labeling of endogenous synaptic proteins for high-contrast live-cell imaging in brain tissue remains challenging. Here, we describe a conditional mouse genetic strategy termed endogenous labeling via exon duplication (ENABLED), which can be used to fluorescently label endogenous proteins with near ideal properties in all neurons, a sparse subset of neurons, or specific neuronal subtypes. We used this method to label the postsynaptic density protein PSD-95 with mVenus without overexpression side effects. We demonstrated that mVenus-tagged PSD-95 is functionally equivalent to wild-type PSD-95 and that PSD-95 is present in nearly all dendritic spines in CA1 neurons. Within spines, while PSD-95 exhibited low mobility under basal conditions, its levels could be regulated by chronic changes in neuronal activity. Notably, labeled PSD-95 also allowed us to visualize and unambiguously examine otherwise-unidentifiable excitatory shaft synapses in aspiny neurons, such as parvalbumin-positive interneurons and dopaminergic neurons. Our results demonstrate that the ENABLED strategy provides a valuable new approach to study the dynamics of endogenous synaptic proteins in vivo.

Amelioration of improper differentiation of somatostatin-positive interneurons by triiodothyronine in a growth-retarded hypothyroid mouse strain

Thyroid hormone (TH) plays an important role in brain development, and TH deficiency during pregnancy or early postnatal periods leads to neurological disorders such as cretinism. Hypothyroidism reduces the number of parvalbumin (PV)-positive interneurons in the neocortex and hippocampus. Here we used a mouse strain (growth-retarded; grt) that shows growth retardation and hypothyroidism to examine whether somatostatin (Sst)-positive interneurons that are generated from the same pool of neural progenitor cells as PV-positive cells are also altered by TH deficiency. The number of PV-positive interneurons was significantly decreased in the neocortex and hippocampus of grt mice as compared with normal control mice. In contrast to the decrease in the number of PV neurons, the number of Sst-positive interneurons in grt mice was increased in the stratum oriens of the hippocampus and the hilus of the dentate gyrus, although their number was unchanged in the neocortex. These changes were reversed by triiodothyronine administration from postnatal day (PD) 0 to 20. TH supplementation that was initiated after PD21 did not, however, affect the number of PV- or Sst-positive cells. These results suggest that during the first three postnatal weeks, TH may be critical for the generation of subpopulations of interneurons.

Chronic fluoxetine treatment reduces parvalbumin expression and perineuronal nets in gamma-aminobutyric acidergic interneurons of the frontal cortex in adult mice

BACKGROUND

The selective serotonin reuptake inhibitor fluoxetine (FLX) is widely used to treat depression and anxiety disorders, but cellular mechanisms underlying the antidepressant effect of FLX remain largely unknown. The generally accepted effect of chronic FLX treatment is increased adult neurogenesis in the hippocampal dentate gyrus. It was recently demonstrated that FLX treatments can reverse the established neuronal maturation of granule cells in the hippocampal dentate gyrus and of gamma-aminobutyric acidergic (GABAergic) interneurons in the basolateral amygdala. However, it is not clear whether this dematuration effect of FLX occurs in other brain regions. Thus, in this study, we used immunohistological analysis to assess the effect of FLX treatment on GABAergic interneurons in the medial frontal cortex (mFC) and reticular thalamic nucleus (RTN).

RESULTS

Immunofluorescence analysis for perineuronal nets (PNNs), which is a marker of neuronal maturation, and for parvalbumin, calretinin, and somatostatin, which are markers for specific GABAergic interneuron type, showed lower number of parvalbumin-positive (+) cells and PNN+/parvalbumin+ cells in the mFC of FLX-treated mice compared to vehicle-treated mice. However, FLX treatment had no effect on the number of cells expressing calretinin and somatostatin in the mFC. In the RTN, the number of PNN+ cells and parvalbumin+ cells was unaltered by FLX treatments. Furthermore, the number of total GABA+ cells and apoptotic cells in the mFC was similar between vehicle- and FLX-treated mice, suggesting that FLX treatment did not induce cell death in this region. Rather, our findings suggest that the decreased number of parvalbumin+ cells in the mFC was due to a decreased expression of parvalbumin proteins in the interneurons.

CONCLUSIONS

This study indicates that FLX decreases the levels of parvalbumin, a mature marker of fast-spiking interneurons, and PNNs in parvalbumin+ interneurons in the mFC, suggesting that FLX treatment induces a dematuration of this type of neurons. Induction of a juvenile-like state in fast-spiking inhibitory interneurons in these regions might be involved in the therapeutic mechanism of this antidepressant drug and/or some of its adverse effects.

Early-life iron deficiency anemia alters the development and long-term expression of parvalbumin and perineuronal nets in the rat hippocampus

Early-life iron deficiency anemia (IDA) alters the expression of critical genes involved in neuronal dendritic structural plasticity of the hippocampus, thus contributing to delayed maturation of electrophysiology, and learning and memory behavior in rats. Structural maturity in multiple cortical regions is characterized by the appearance of parvalbumin-positive (PV(+)) GABAergic interneurons and perineuronal nets (PNNs). Appearance of PV(+) interneurons and PNNs can serve as cellular markers for the beginning and end of a critical developmental period, respectively. During this period, the system progresses from an immature yet highly plastic condition, to a more mature and efficient state that is however less flexible and may exhibit poorer potential for recovery from injury. To test if fetal-neonatal IDA alters parvalbumin (PV) mRNA expression, protein levels, and the number of PV(+) interneurons and PNNs in the male rat hippocampus, pregnant dams were given an iron-deficient (ID) diet (3 mg iron/kg chow) from gestational day 2 to postnatal day (P) 7 and then placed on an iron-sufficient (IS) diet (198 mg/kg) for the remainder of the experiment. On this regimen, formerly ID animals become fully iron-replete by P56. Minimal levels of PV (mRNA and protein), PV(+) interneurons, and PNNs were found in IS and ID P7 rats. By P15, and continuing through P30 and P65, ID rats had reduced PV mRNA expression and protein levels compared to IS controls. While there were no differences in the number of PV(+) neurons at either P30 or P65, the percentage of PV(+) cells surrounded by PNNs was slightly greater in ID rats as compared to IS controls. The lower levels of these acknowledged critical period biomarkers in the ID group are consistent with studies that demonstrate later maturation of the acutely ID hippocampus and lower plasticity in the adult formerly ID hippocampus. The findings provide additional potential cellular bases for previously described electrophysiologic and behavioral abnormalities found during and following early-life IDA.

Neuroprotective role of taurine during aging

Aging of the brain is characterized by several neurochemical modifications involving structural proteins, neurotransmitters, neuropeptides and related receptors. Alterations of neurochemical indices of synaptic function are indicators of age-related impairment of central functions, such as locomotion, memory and sensory performances. Several studies demonstrate that ionotropic GABA receptors, glutamate decarboxylase (GAD), and somatostatinergic subpopulations of GABAergic neurons are markedly decreased in experimental animal brains during aging. Additionally, levels of several neuropeptides co-expressed with GAD decrease during aging. Thus, the age-related decline in cognitive functions could be attributable, at least in part, to decrements in GABA inhibitory neurotransmission. In this study, we showed that chronic supplementation of taurine to aged mice significantly ameliorated the age-dependent decline in spatial memory acquisition and retention. We also demonstrated that concomitant with the amelioration in cognitive function, taurine caused significant alterations in the GABAergic and somatostatinergic system. These changes included (1) increased levels of the neurotransmitters GABA and glutamate, (2) increased expression of both isoforms of GAD (65 and 67) and the neuropeptide somatostatin, (3) decreased hippocampal expression of the β3 subunits of the GABAA receptor, (4) increased expression in the number of somatostatin-positive neurons, (5) increased amplitude and duration of population spikes recorded from CA1 in response to Schaefer collateral stimulation and (6) enhanced paired pulse facilitation in the hippocampus. These specific alterations of the inhibitory system caused by taurine treatment oppose those naturally occurring in the aging brain, suggesting a protective role of taurine in this process. An increased understanding of age-related neurochemical changes in the GABAergic system will be important in elucidating the underpinnings of the functional changes of aging. Taurine supplementation might help forestall the age-related decline in cognitive functions through interaction with the GABAergic system.

Nav1.1 haploinsufficiency in excitatory neurons ameliorates seizure-associated sudden death in a mouse model of Dravet syndrome

Dravet syndrome is a severe epileptic encephalopathy mainly caused by heterozygous mutations in the SCN1A gene encoding a voltage-gated sodium channel Nav1.1. We previously reported dense localization of Nav1.1 in parvalbumin (PV)-positive inhibitory interneurons in mice and abnormal firing of those neurons in Nav1.1-deficient mice. In the present study, we investigated the physiologic consequence of selective Nav1.1 deletion in mouse global inhibitory neurons, forebrain excitatory neurons or PV cells, using vesicular GABA transporter (VGAT)-Cre, empty spiracles homolog 1 (Emx1)-Cre or PV-Cre recombinase drivers. We show that selective Nav1.1 deletion using VGAT-Cre causes epileptic seizures and premature death that are unexpectedly more severe than those observed in constitutive Nav1.1-deficient mice. Nav1.1 deletion using Emx1-Cre does not cause any noticeable abnormalities in mice; however, the severe lethality observed with VGAT-Cre-driven Nav1.1 deletion is rescued by additional Nav1.1 deletion using Emx1-Cre. In addition to predominant expression in PV interneurons, we detected Nav1.1 in subpopulations of excitatory neurons, including entorhino-hippocampal projection neurons, a subpopulation of neocortical layer V excitatory neurons, and thalamo-cortical projection neurons. We further show that even minimal selective Nav1.1 deletion, using PV-Cre, is sufficient to cause spontaneous epileptic seizures and ataxia in mice. Overall, our results indicate that functional impairment of PV inhibitory neurons with Nav1.1 haploinsufficiency contributes to the epileptic pathology of Dravet syndrome, and show for the first time that Nav1.1 haploinsufficiency in excitatory neurons has an ameliorating effect on the pathology.

Neuroendocrine alterations in the fragile X mouse

The expression of GABA(A) receptors in the fragile X mouse brain is significantly downregulated. We additionally found that the expression of somatostatin and voltage-sensitive calcium channels (VSCCs) is also reduced. GABA(A) and the VSCCs, through a synergistic interaction, perform a critical role in mediating activity-dependent developmental processes. In the developing brain, GABA is excitatory and its actions are mediated through GABA(A) receptors. Subsequent to GABA-mediated depolarization, the VSCCs are activated and intracellular calcium is increased, which mediates gene transcription and other cellular events. GABAergic excitation mediated through GABA(A) receptors and the subsequent activation of the VSCCs are critically important for the establishment of neuronal connectivity within immature neuronal networks. Data from our laboratories suggest that there is a dysregulation of axonal pathfinding during development in the fragile X mouse brain and that this is likely due to a dysregulation of the synergistic interactions of GABA and VSCC. Thus, we hypothesize that the altered expression of these critical channels in the early stages of brain development leads to altered activity-dependent gene expression that may potentially lead to the developmental delay characteristic of the fragile X syndrome.

Multiple origins of the cortical γ rhythm

Gamma rhythms (30-80 Hz) are a near-ubiquitous feature of neuronal population activity in mammalian cortices. Their dynamic properties permit the synchronization of neuronal responses to sensory input within spatially distributed networks, transient formation of local neuronal "cell assemblies," and coherent response patterns essential for intercortical regional communication. Each of these phenomena form part of a working hypothesis for cognitive function in cortex. All forms of physiological gamma rhythm are inhibition based, being characterized by rhythmic trains of inhibitory postsynaptic potentials in populations of principal neurons. It is these repeating periods of relative enhancement and attenuation of the responsivity of major cell groups in cortex that provides a temporal structure shared across many millions of neurons. However, when considering the origins of these repeating trains of inhibitory events considerable divergence is seen depending on cortical region studied and mode of activation of gamma rhythm generating networks. Here, we review the evidence for involvement of multiple subtypes of interneuron and focus on different modes of activation of these cells. We conclude that most massively parallel brain regions have different mechanisms of gamma rhythm generation, that different mechanisms have distinct functional correlates, and that switching between different local modes of gamma generation may be an effective way to direct cortical communication streams. Finally, we suggest that developmental disruption of the endophenotype for certain subsets of gamma-generating interneuron may underlie cognitive deficit in psychiatric illness.

Effects of alcohol administration during adulthood on parvalbumin and glial fibrillary acidic protein immunoreactivity in the rat cerebral cortex

The pathology of brain atrophy mediated by alcohol was investigated in all parts of the cerebral cortex (the frontal, parietal, temporal lobes and occipital cortex) by using two markers: parvalbumin (PV) and glial fibrillary acidic protein (GFAP). Three-month old male Wistar rats were divided into control (C) and alcohol-exposed groups. The control group received distilled water, whereas the alcohol-exposed groups received either a low dose (2g/kg body wt) or a high dose (5g/kg) of ethanol for periods of 21 days, 3 or 6 months. The brains of the animals were processed for immunohistochemistry using anti-parvalbumin and anti-GFAP antibodies and the number of PV immunoreactive (PV-ir) neurons and GFAP immunoreactive (GFAP-ir) astrocytes were counted per unit area. Results showed that all groups exposed to ethanol had significantly reduced numbers of PV-ir neurons in all parts of the cerebral cortex compared to those of the control group (p<0.05). In contrast, the numbers of GFAP-ir astrocytes were increased in all parts of the cerebral cortex following the exposure to a high dose of ethanol after 21-days (but not a low dose) and both high and low doses of ethanol after 3-months or 6-months treatment compared to those of age-matched control groups (p<0.05). This indicated that in young rats (21-days), PV-ir neurons in all cerebral cortex areas seemed to be more sensitive to alcohol than GFAP-ir astrocytes. Moreover, the change in densities of both PV-ir neurons and GFAP-ir astrocytes became more apparent after exposure to prolonged and high doses of ethanol. The decrease of PV-ir neurons and the increase of GFAP-ir astrocytes indicated that alcohol may induce pathology in broad areas of the cerebral cortex. This may explain the underlying mechanism of brain atrophy and other impairments found in alcoholics. For investigations of the effects of alcohol on mediating brain pathology, we recommend the use of the two markers (PV and GFAP).

Immunohistochemical characterization of substance P receptor (NK(1)R)-expressing interneurons in the entorhinal cortex

It has been reported that application of substance P (SP) to the medial portion of the entorhinal cortex (EC) induces a powerful antiepileptic effect (Maubach et al. [1998] Neuroscience 83:1047-1062). This effect is presumably mediated via inhibitory interneurons expressing the neurokinin-1 receptor (NK(1)R), but the existence of NK(1)R-expressing inhibitory interneurons in the EC has not yet been reported. The present immunohistochemical study was performed in the rat to examine the existence and distribution of NK(1)R-expressing neurons in the EC as well as any co-expression of other neurotransmitters/neuromodulators known to be associated with inhibitory interneurons: gamma-aminobutyric acid (GABA), parvalbumin (PARV), calretinin (CT), calbindin (CB), somatostatin (SST), and neuropeptide Y (NPY). Our results indicated that NK(1)R-positive neurons were distributed rather sparsely (especially in the medial EC), primarily in layers II, V, and VI. The results of our double-immunohistochemical staining indicated that the vast majority of NK(1)R-expressing neurons also expressed GABA, SST, and NPY. In addition, CT was co-expressed in a weakly stained subgroup of NK(1)R-expressing neurons, and CB was co-expressed very rarely in the lateral EC, but not in the medial EC. In contrast, SP-immunopositive axons with fine varicosities were distributed diffusely throughout all layers of the EC, appearing to radiate from the angular bundle. SP may be released in a paracrine manner to activate a group of NK(1)R-expressing entorhinal neurons that co-express GABA, SST, and NPY, exerting a profound inhibitory influence on synchronized network activity in the EC.

The development of hippocampal interneurons in rodents

Interneurons are GABAergic neurons responsible for inhibitory activity in the adult hippocampus, thereby controlling the activity of principal excitatory cells through the activation of postsynaptic GABAA receptors. Subgroups of GABAergic neurons innervate specific parts of excitatory neurons. This specificity indicates that particular interneuron subgroups are able to recognize molecules segregated on the membrane of the pyramidal neuron. Once these specific connections are established, a quantitative regulation of their strength must be performed to achieve the proper balance of excitation and inhibition. We will review when and where interneurons are generated. We will then detail their migration toward and within the hippocampus, and the maturation of their morphological and neurochemical characteristics. We will finally review potential mechanisms underlying the development of GABAergic interneurons.

Region-specific reduction in entorhinal gamma oscillations and parvalbumin-immunoreactive neurons in animal models of psychiatric illness

Psychiatric illnesses, particularly schizophrenia, are associated with disrupted markers for interneuronal function and interneuron-mediated brain rhythms such as gamma frequency oscillations. Here we investigate a possible link between these two observations in the entorhinal cortex and hippocampus by using a genetic and an acute model of psychiatric illness. Lysophosphatidic acid 1 receptor-deficient (LPA1-deficient) mice show psychomotor-gating deficits and neurochemical changes resembling those seen in postmortem schizophrenia studies. Similar deficits are seen acutely with antagonism of the NMDA subtype of glutamate receptor. Neither model induced any change in power or frequency of gamma rhythms generated by kainate in hippocampal slices. In contrast, a dramatic decrease in the power of gamma oscillations was seen in superficial, but not deep, medial entorhinal cortex layers in both models. Immunolabeling for GABA, parvalbumin, and calretinin in medial entorhinal cortex from LPA1-deficient mice showed an approximately 40% reduction in total GABA- and parvalbumin-containing neurons, but no change in the number of calretinin-positive neurons. This deficit was specific for layer II (LII). No change in the number of neurons positive for these markers was seen in the hippocampus. Acute NMDA receptor blockade, which selectively reduces synaptic drive to LII entorhinal interneurons, also disrupted gamma rhythms in a similar manner in superficial entorhinal cortex, but not in hippocampus. These data demonstrate an area-specific deficit in gamma rhythmogenesis in animal models of psychiatric illness and suggest that loss, or reduction in function, of interneurons having a large NMDA receptor expression may underlie the network dysfunction that is seen.

A role for L-type calcium channels in the maturation of parvalbumin-containing hippocampal interneurons

While inhibitory interneurons are well recognized to play critical roles in the brain, relatively little is know about the molecular events that regulate their growth and differentiation. Calcium ions are thought to be important in neuronal development and L-type voltage gated Ca(+2) channels have been implicated in activity-dependent mechanisms of early-life. However, few studies have examined the role of these channels in the maturation of interneurons. The studies reported here were conducted in hippocampal slice cultures and indicate that the L-type Ca(+2) channel agonists and antagonists accelerate and suppress respectively the growth of parvalbumin-containing interneurons. The effects of channel blockade were reversible suggesting they are not the result of interneuronal cell death. Results from immunoblotting showed that these drugs have similar effects on the expression of the GABA synthetic enzymes, glutamic acid decarboxylase65, glutamic acid decarboxylase67 and the vesicular GABA transporter. This suggests that L-type Ca(+2) channels regulate not only parvalbumin expression but also interneuron development. These effects are likely mediated by actions on the interneurons themselves since the alpha subunits of L-type channels, voltage-gated calcium channel subunit 1.2 and voltage-gated calcium channel subunit 1.3 were found to be highly expressed in neonatal mouse hippocampus and co-localized with parvalbumin in interneurons. Results also showed that while these interneurons can contain either subunit, voltage-gated calcium channel subunit 1.3 was more widely expressed. Taken together results suggest that an important subset of developing interneurons expresses L-type Ca(+2) channels alpha subunits, voltage-gated calcium channel subunit 1.2 and especially voltage-gated calcium channel subunit 1.3 and that these channels likely regulate the development of these interneurons in an activity-dependent manner.

Analysis of SNAP-25 immunoreactivity in hippocampal inhibitory neurons during development in culture and in situ

Synaptosomal associated protein of 25 kDa (SNAP-25) is a component of the soluble N-ethylmaleimide-sensitive fusion protein (NSF) attachment protein receptor (SNARE) complex which plays a central role in synaptic vesicle exocytosis. We have previously demonstrated that adult rat hippocampal GABAergic synapses, both in culture and in brain, are virtually devoid of SNAP-25 immunoreactivity and are less sensitive to the action of botulinum toxin type A, which cleaves this SNARE protein [Neuron 41 (2004) 599]. In the present study, we extend our findings to the adult mouse hippocampus and we also provide demonstration that hippocampal inhibitory synapses lacking SNAP-25 labeling belong to parvalbumin-, calretinin- and cholecystokinin-positive interneurons. A partial colocalization between SNAP-25 and glutamic acid decarboxylase is instead detectable in developing mouse hippocampus at P0 and, at a lesser extent, at P5. In rat embryonic hippocampal cultures at early developmental stages, SNAP-25 immunoreactivity is detectable in a percentage of GABAergic neurons, which progressively reduces with time in culture. Consistent with the presence of the substrate, botulinum toxin type A is partially effective in inhibiting synaptic vesicle recycling in immature GABAergic neurons. Since SNAP-25, beside its role as a SNARE protein, is involved in additional processes, such as neurite outgrowth and regulation of calcium dynamics, the presence of higher levels of the protein at specific stages of neuronal differentiation may have implications for the construction and for the functional properties of brain circuits.

Developmental expression of potassium-channel subunit Kv3.2 within subpopulations of mouse hippocampal inhibitory interneurons

The developmental expression of the voltage-gated potassium channel subunit, Kv3.2, and its localization within specific mouse hippocampal inhibitory interneuron populations were determined using immunoblotting and immunohistochemical techniques. Using immunoblotting techniques, the Kv3.2 protein was weakly detected at postnatal age day 7 (P7), and full expression was attained at P21 in tissue extracts from homogenized hippocampal preparations. A similar developmental profile was observed using immunohistochemical techniques in hippocampal tissue sections. Kv3.2 protein expression was clustered on the somata and proximal dendrites of presumed inhibitory interneurons. Using double immunofluorescence, Kv3.2 subunit expression was detected on subpopulations of GABAergic inhibitory interneurons. Kv3.2 was detected in approximately 100% of parvalbumin-positive interneurons, 86% of interneurons expressing nitric oxide synthase, and approximately 50% of somatostatin-immunoreactive cells. Kv3.2 expression was absent from both calbindin- and calretinin-containing interneurons. Using immunoprecipitation, we further demonstrate that Kv3.2 and its related subunit Kv3.1b are coexpressed within the same protein complexes in the hippocampus. These data demonstrate that potassium channel subunit Kv3.2 expression is developmentally regulated in a specific set of interneurons. The vast majority of these interneuron subpopulations possess a "fast-spiking" phenotype, consistent with a role for currents through Kv3.2 containing channels in determining action potential kinetics in these cells.

Schizophrenia and viral infection during neurodevelopment: a focus on mechanisms

The task of defining schizophrenia pathogenesis has fascinated and frustrated researchers for nearly a century. In recent years, unprecedented advances from diverse fields of study have given credence to both viral and developmental theories. This review considers possible mechanisms by which viral and developmental processes may interact to engender schizophrenia. Many of the current controversies in schizophrenia pathogenesis are reviewed in light of the viral hypothesis, including: epidemiological findings and the role of a genetic diathesis, phenotype heterogeneity, abnormalities in excitatory and inhibitory neurotransmitter systems, anomalous cerebral latereralization, and static vs progressive disease. The importance of animal models in elucidating the impact of viral infections on developing neurons is illustrated by recent studies in which neonatal rats are infected with lymphocytic choriomeningitis virus in order to examine alterations in hippocampal circuitry. Finally, consideration is given to a new hypothesis that some cases of schizophrenia could be instigated by a viral infection that disrupts developing inhibitory circuits, consequently unleashing glutamatergic neurotransmission leading to selective excitotoxicity, and a degenerative disease course.

GABAergic neurons that pioneer hippocampal area CA1 of the mouse: morphologic features and multiple fates

Dramatic changes occur in the expression of glutamic acid decarboxylase (GAD67) immunoreactivity in mouse hippocampus during postnatal development. Most striking is the presence of a dense population of immunopositive cells in stratum radiatum and stratum oriens in area CA1 during the first postnatal week. Between days 5 and 10, these cells disappear and the GAD67 immunoreactivity begins to resemble that of adulthood. These neurons are considered pioneer cells, and studies were undertaken to determine their fate. Between days 5 and 50, area CA1 doubles in size; however, the loss of cells expressing GAD67 mRNA cannot be explained solely by dilution resulting from hippocampal growth. In stratum radiatum, cell loss is particularly dramatic. Although between days 5 and 15, many cells seem to migrate from stratum radiatum to its border with stratum lacunosum-moleculare, both fate maps of pioneer cells labeled with bromodeoxyuridine (BrdU) on embryonic day 13 (E13) and in situ DNA end-labeling studies suggest that some cells die by means of programmed cell death. However, not all pioneer cells die, because many cells labeled with BrdU on E13 are present in adulthood and express markers for and have anatomic features of hippocampal interneurons. In conclusion, events that underlie the age-dependent disappearance of gamma-aminobutyric acid (GABA) -ergic pioneer cells are complex and cannot be completely explained by dilution in an expanding neuropile. Although some GABAergic pioneer cells likely undergo programmed cell death during the first postnatal weeks, others relocate within hippocampal laminae and terminally differentiate into the interneurons of adulthood.

Viral infection of developing GABAergic neurons in a model of hippocampal disinhibition

Mechanisms by which perinatal viral infections can disrupt hippocampal development and cause selective neuronal death may have implications for temporal lobe epilepsy and schizophrenia. Despite abnormalities of inhibitory interneurons in these diseases, the causal relationships between such neurotransmitter changes and viral infections remain unclear. This relationship was examined in a model in which rats, infected with lymphocytic choriomeningitis virus (LCMV) as neonates, manifest a gradual loss of hippocampal dentate granule cells and neuronal hyperexcitability. The current data demonstrate that GABAergic interneurons are dual immunostained for LCMV antigens prior to the loss of dentate granule cells, supporting the hypothesis that LCMV may disrupt developing inhibitory circuits causing unbalanced excitatory neurotransmission and the eventual death of dentate granule cells due to excitotoxicity.

Postnatal development of parvalbumin and calbindin D-28k immunoreactivities in the canine hippocampus

The calcium-binding proteins, parvalbumin and calbindin D-28k, are markers of different classes of GABAergic interneurons and display different functions. The present study was attempted to determine immunoreactivities and colocalization of the parvalbumin and calbindin D-28k in the developing canine hippocampus by immunohistochemistry. The calcium-binding protein-containing neurons showed different developmental patterns. The first appearance of parvalbumin immunoreactive nonpyramidal cells was observed at P7. Parvalbumin immunoreactivity was elicited by the sequence from CA3 to CA1 to reach an adult-like distribution pattern, which was reached at P60, while calbindin D-28k immunoreactivity appeared from P0, including pyramidal and nonpyramidal cells. The characteristic distribution of calbindin D-28k immunoreactive pyramidal cells was clarified by P28, and an adult-like distribution pattern was reached by the end of the second postnatal month. Double-labeled nonpyramidal cells were frequently seen in the subareas, CA3 of P14/CA1-CA2 of P28, where parvalbumin immunoreactive nonpyramidal cells were emerging. These data suggest that the colocalization of the two calcium-binding proteins during development is related closely to the area-specific maturation of parvalbumin expression, although either prenatal expression of calbindin D-28k or parvalbumin was not determined.

Novel hippocampal interneuronal subtypes identified using transgenic mice that express green fluorescent protein in GABAergic interneurons

The chief inhibitory neurons of the mammalian brain, GABAergic neurons, are comprised of a myriad of diverse neuronal subtypes. To facilitate the study of these neurons, transgenic mice were generated that express enhanced green fluorescent protein (EGFP) in subpopulations of GABAergic neurons. In one of the resulting transgenic lines, called GIN (GFP-expressing Inhibitory Neurons), EGFP was found to be expressed in a subpopulation of somatostatin-containing GABAergic interneurons in the hippocampus and neocortex. In both live and fixed brain preparations from these mice, detailed microanatomical features of EGFP-expressing interneurons were readily observed. In stratum oriens of the hippocampus, EGFP-expressing interneurons were comprised almost exclusively of oriens/alveus interneurons with lacunosum-moleculare axon arborization (O-LM cells). In the neocortex, the somata of EGFP-expressing interneurons were largely restricted to layers II-IV and upper layer V. In hippocampal area CA1, two previously uncharacterized subtypes of interneurons were identified using the GIN mice: stratum pyramidale interneurons with lacunosum-moleculare axon arborization (P-LM cells) and stratum radiatum interneurons with lacunosum-moleculare axon arborization (R-LM cells). These newly identified interneuronal subtypes appeared to be closely related to O-LM cell, as they selectively innervate stratum lacunosum-moleculare. Whole-cell patch-clamp recordings revealed that these cells were fast-spiking and showed virtually no spike frequency accommodation. The microanatomical features of these cells suggest that they function primarily as "input-biasing" neurons, in that synaptic volleys in stratum radiatum would lead to their activation, which in turn would result in selective suppression of excitatory input from the entorhinal cortex onto CA1 pyramidal cells.

Neuronal subpopulations of developing rat hippocampus containing different calcium-binding proteins behave distinctively in trimethyltin-induced neurodegeneration

The present study investigates, by immunocytochemistry, the behavior of different neuronal subpopulations of the developing rat hippocampus, selectively labeled by the calcium-binding proteins calbindin D28-k (CB), parvalbumin (PV), and calretinin (CR), in neurodegenerative processes induced by the neurotoxicant trimethyltin (TMT). Previous studies on adult rats indicated that CB-immunoreactive (IR) neurons were affected by TMT, while PV- and CR-IR neurons were selectively spared. The present findings show that only CR-IR neurons are spared in developing rats, and in addition the number of CR-IR neurons are significantly higher in the DG of treated animals. On the contrary, PV-IR neurons, spared in adult rats, were affected by TMT during development. CB-IR neurons were affected also in developing rats, as in adults. The different postnatal time-courses of calcium-binding protein expression in relationship to the time of TMT administration (presence of CR but absence of PV) could have a role in the different behavior of CR- and PV-IR cells in developing rats.

First one in, last one out: the role of gabaergic transmission in generation and degeneration

This paper is the result of discussions between scientists working in widely separated areas, united by an interest in the hippocampus. The discussions focused on the possible role of GABA in the development and maturation of the hippocampus and in neurodegeneration in Alzheimer's disease (AD). GABA neurons are among the first to differentiate in the hippocampus and the properties of GABA neurotransmission in the developing hippocampus are distinct from those in the adult. GABAergic transmission may play a role in the clustering and maturation of GABA receptors, as well as of receptors for other neurotransmitters. The development and maturation of synaptic connections involves changes in the organization of the cytoskeleton, and mechanical force generation is probably required to establish appropriate points of contact. This generation of force may require coupling of specific receptors to the cytoskeleton through specialized proteins. In AD, much of the developmental process is progressively unraveled in the hippocampus, as afferent fibers, most notably from entorhinal excitatory neurons and from basal forebrain cholinergic cells, degenerate. This denervation undoubtedly has consequences for receptor systems, dendritic morphology and the underlying cytoskeleton. GABA neurons remain in the AD hippocampus, and may actually contribute to abnormal firing and degeneration of remaining pyramidal neurons. This attempt to bring together data from different areas of research has allowed the development of a scheme which identifies significant specific gaps in our knowledge, which could be readily filled by focused experimental work.

Expression of calretinin in diverse neuronal populations during development of rat hippocampus

The prenatal and postnatal expression of calretinin was studied in hippocampus of the rat using immunohistochemical procedures. Calretinin was detected as early as embryonic day 15 in the primordial hippocampus where calretinin-containing neurons and fibres were localized to the primitive plexiform layer. Upon emergence of the hippocampal plate (the prospective stratum pyramidale), large numbers of immunopositive multipolar cells were observed in the marginal zone. Fewer cells with fusiform cell bodies were observed bordering the hippocampal plate and subplate. During the perinatal period (embryonic day 20 to postnatal day 0), large numbers of immunoreactive pyramidal-like neurons were observed at the margin of the hippocampal plate with the subplate. At this same time, many calretinin-containing neurons with irregularly shaped dendrites were observed in stratum radiatum. Soon after birth (postnatal day 3), the calretinin immunoreactivity of both these later cell types rapidly declined and a new population of calretinin-immunopositive cells emerged, the Cajal-Retzius cells of stratum lacunosum-moleculare and the dentate gyrus. The Cajal-Retzius cells rapidly matured but disappeared by the second postnatal week. During the second postnatal week, calretinin interneurons of the adult hippocampal formation began to appear. Their immunoreactivity increased by postnatal day 15, when the number of calretinin-immunopositive interneurons in area CA1 and stratum radiatum of CA3 exceeded that of the adult. At this time, the soma and proximal dendrites of many calretinin interneurons were found to contact each other. The frequency of such cellular appositions decreased in adulthood. The results presented here show that calretinin immunohistochemistry can be very useful in recording the development of subpopulations of hippocampal neurons that are present during distinct embryonic and postnatal periods. Although some neuronal types may exist only briefly during hippocampal development, others appear to express calretinin transiently during restricted phases of neuronal differentiation. Surprisingly, this includes some hippocampal pyramidal cells. However, even as the adult pattern of immunostaining emerges in week 2, morphological refinement of interneurons continues to take place, which eventually leads to the population of calretinin-containing interneurons of the mature hippocampus.

Differential expression of calretinin, calbindin D28K and parvalbumin in the developing human cerebellum

Three calcium-binding proteins, calretinin, calbindin D28K and parvalbumin, were immunohistochemically localized in the human cerebellum at different developmental stages. Cells positive for calretinin were not detected during early development of the cerebellum until 21 weeks of gestation at which stage weak staining was found in Purkinje and basket cells of the cortex and in neurons of the dentate nucleus. Both the number of positive cells and the intensity of immunoreactivities were found to increase as the cerebellum became more mature. Calbindin D28K immunoreactivity was, however, detected early in development at 14 weeks of gestation. Positive cells were found in Purkinje, basket, stellate and granule cells of the cerebellar cortex and in neurons of fastigial, globose, emboliform and dentate nuclei. The number of positive cells and the staining intensity for calbindin in both the cerebellar cortex and deep nuclei decreased at more advanced developmental stages. At 21-31 weeks of gestation, positive staining was restricted to Purkinje and basket cells of the cortex. Parvalbumin immunoreactivity was also observed early in development at 14 weeks of gestation. Positivity was found in Purkinje, basket and stellate cells of the cerebellar cortex and in neurons of all the deep nuclei, with the highest number of positive cells in the fastigial nucleus followed by emboliform, globose and dentate nuclei. As the cerebellum became more mature, both the number of positive cells and the staining intensity for parvalbumin decreased in the cortex and deep nuclei. The results of the present study showed that among the three calcium-binding proteins examined, strong immunoreactivities for calbindin D28K and parvalbumin were found inthe human cerebellum early in development at 14 weeks of gestation, but there was a decrease in both the intensity and number of positive cells at more advanced stages. In contrast, calretinin positive cells were not detected until 21 weeks of gestation and the immunoreactivity increased as the cerebellum became more mature. A possible correlation between the developmentally regulated expression of the calcium-binding proteins and expression of different neurotransmitters during development is discussed.

Neuronal changes in the basolateral complex during development of the amygdala of the rat

Neuronal changes in the amygdala basolateral complex were studied during development and maturation in fetal and postnatal rat brains using morphometrical methods. Forty brains of animals of various ages were fixed in formalin, frozen and cut into 25 microm thick sections and stained with cresyl violet or haematoxylin and eosin (H&E). In cresyl violet preparations, the complex appeared for the first time on embryonic day (E)17 and was composed of two homogeneous nuclei lateral and basolateral. On about the seventh postnatal day, each of these nuclei was divided into two parts the first one into the dorsolateral and ventromedial and the second one into the anterior and posterior. Morphometric investigations showed a different increase of the neuronal and nuclear size in various parts of the basolateral complex up to postnatal day (P)14; after that time these parameters did not change significantly. The neuronal density and the total number of neurons stabilized at P7 in all parts of this complex, except for the dorsolateral part of the lateral nucleus in which a 30% decrease of the total number of cells was observed. From P14, in all nuclei under study, the total number of neurons did not change significantly.

The expression pattern of somatostatin and calretinin by postnatal hippocampal interneurons is regulated by activity-dependent and -independent determinants

Hippocampal interneurons form distinct populations identified on the basis of their projection pattern and neurochemical characteristics, which includes the expression of specific neuropeptides and/or calcium-binding proteins. The neurochemical maturation of hippocampal interneurons is largely a postnatal event, and factors which govern this maturation are presently unknown. Using slice cultures, we have investigated the role of neuronal activity in regulating the expression of somatostatin and calretinin during the postnatal maturation of hippocampal interneurons. Blocking inhibitory activity with bicuculline, or excitatory activity with 6,7-dinitroquinoxaline-2,3-dione, for 14 days in slice cultures from seven-day-old rat increased and decreased, respectively, the number of somatostatin-immunoreactive neurons. Withdrawal of the blocking agents resulted in a reversal of the effects on somatostatin immunoreactivity. In addition, bicuculline slightly increased the number of calretinin-positive neurons, while 6,7-dinitroquinoxaline-2,3-dione exerted no effect. However, bicuculline and 6,7-dinitroquinoxaline-2,3-dione markedly increased and decreased, respectively, the number of calretinin-labelled axons. Despite activity-linked modifications of immunoreactivity levels, no change in the organotypic location of somatostatin-labelled neurons was observed, whatever the treatment. Double labelling studies demonstrated that somatostatin and calretinin were expressed by different neurons, even when the number of labelled cells was highly increased. These results show that the levels of expression of somatostatin and calretinin in maturing hippocampal interneurons are tuned to the endogenous balance of excitatory and inhibitory activity. In contrast, the neurochemical specificity of each subtype of interneurons does not depend upon variations in neuronal activity.

Vicia villosa agglutinin labels a subset of neurons coexpressing both the mu opioid receptor and parvalbumin in the developing rat subiculum

Vicia villosa agglutinin (VVA), anti-parvalbumin antiserum and an affinity-purified anti-mu opioid receptor antibody were used to triple-label neurons in the postnatal rat subiculum. VVA labeled a subset of mu opioid receptor-positive neurons that were also immunoreactive for parvalbumin. The morphology of the triple-labeled neurons was heterogeneous, and included multipolar, ovoid and pyramidal-shaped neurons. Neurons single-labeled for the mu opioid receptor, VVA or parvalbumin were also morphologically heterogeneous. The postnatal development of mu opioid receptor immunoreactivity (IR), parvalbumin-IR and VVA binding was investigated using triple-labeling immunocytochemistry. Mu opioid receptor-IR appeared first and was present at postnatal day 1 (P1). Parvalbumin-IR was first observed in somata at P10, followed by proximal and distal dendrites at P15 and P20 respectively. Faint VVA labeling was seen first at P10 and surrounded a limited number of neurons. The intensity of labeling and the number of neurons labeled with VVA increased between P10 and P20; however, both measures remained below adult levels at P20. This study further illustrates the neurochemical heterogeneity of interneurons in the hippocampal formation and shows the developmentally early appearance of mu opioid receptor-IR compared to the late appearance of VVA binding and parvalbumin-IR.

Neurochemical development of the hippocampal region in the fetal rhesus monkey, III: calbindin-D28K, calretinin and parvalbumin with special mention of cajal-retzius cells and the retrosplenial cortex

In spite of continuing controversy on the precise function of the calcium-binding proteins expressed in the hippocampal formation, nothing is known about their prenatal development in primates. In this study, calbindin-D28K, calretinin, and parvalbumin were localized in the hippocampal formation of seven rhesus monkey fetuses aged E47 to E90 (term 165 days). All of the three markers were expressed during the first half of gestation in distinct subsets of nonpyramidal neurons: calretinin-containing cells were the most numerous and relatively differentiated contrasting with a more restricted, less mature, parvalbumin-labeled population and a poor calbindin-positive nonpyramidal contingent. The granule cells and pyramidal neurons were calbindin-positive, including the pyramids of CA3 and the subicular complex, in contrast to the situation found in the adult monkey. The presubiculum and retrosplenial cortex, whose merging formed the caudal pole of the hippocampal formation, also expressed precociously the three calcium-binding proteins. A heterogeneous population of Cajal-Retzius-like cells was demonstrated in the marginal zone of the ventral hippocampal formation. The majority co-expressed calbindin-D28K and calretinin and displayed acetylcholinesterase activity but no GABA-like immunoreactivity. Major intrinsic and extrinsic pathways of the hippocampal system (mossy fiber system, alveus, fimbria, angular, and cingular bundles) were immunoreactive for calretinin and/or calbindin. The distinct developmental time course and regional pattern of distribution of calbindin-D28K, calretinin, and parvalbumin in the nonprincipal neurons suggests a precocious but asynchronous prenatal development of different inhibitory circuits in the hippocampal formation of primates. The labeling of several fiber systems in keeping with comparable early events in the entorhinal cortex (Berger et al.: Hippocampus 3:279-305, 1993), suggests the possibility of earlier functional circuits than hitherto inferred from the observations available in rodents, a hypothesis that deserves further investigation.

Development of parvalbumin immunoreactivity in the chick Edinger Westphal nucleus

To determine when the calcium-binding protein parvalbumin appears during development, neurons in the chick Edinger Westphal nucleus were examined for parvalbumin immunoreactivity at a variety of embryonic stages. Parvalbumin immunoreactivity appeared on embryonic day 14 (E14, Hamburger and Hamilton stage 40) in predominantly lateral Edinger Westphal neurons. Cytochrome oxidase activity within the nucleus was examined throughout development, as an indicator of physiological activity, and expression of cytochrome oxidase was compared with that of parvalbumin. Cytochrome oxidase activity was found to be uniformly high in all parts of the Edinger Westphal nucleus throughout development. Either the Edinger Westphal nucleus in physiologically active quite early in its development or other energy demands mask the correlation of cytochrome oxidase with electrical activity. Cytochrome oxidase was expressed well before parvalbumin immunoreactivity appeared. Voltage-activated calcium currents were characterized in E12 Edinger Westphal neurons. In both amplitude and composition, E12 calcium currents resemble those of E16 neurons, excluding the possibility that calcium currents appear de novo during or just prior to the appearance of parvalbumin. Both cytochrome oxidase activity and calcium currents are observed in Edinger Westphal neurons well before the appearance of parvalbumin during development. These findings do not exclude the possibility that physiological activity affects the expression of parvalbumin since other factors such as changing patterns of synaptic activity or the appearance of calcium conducting NMDA receptors have yet to be examined. However, they raise the possibility that additional factors such as an intrinsic developmental program or a change in the neuron's basal intracellular calcium requirements may also be involved.

Developmental expression of parvalbumin mRNA in the cerebral cortex and hippocampus of the rat

Parvalbumin (PARV) belongs to the family of calcium-binding proteins bearing the EF hand domain. Immunocytochemical studies in the cerebral cortex have demonstrated that neurons containing PARV include two types of GABAergic interneurons, namely, basket and axo-axonic chandelier cells. The present study examines the onset and pattern of PARV mRNA expression during the development of rat neocortex and hippocampus by means of 'in situ' hybridization with an oligonucleotide probe corresponding to rat PARV cDNA. In animals aged P0-P6 no signal was detected above background in neocortex or hippocampus. At P8, a few cortical cells displayed a number of silver grains just above background levels. By P10 PARV mRNA-expressing cells in the neocortex were detected almost exclusively in layer V of somatosensory, frontal and cingulate cortices. At P12 PARV mRNA was mainly detected in layers IV, V and VIa. By P14 there was a marked overall increase in the entire neocortex, including layer II-III, both in the number of cells and in their intensity of labelling. Further maturation in the pattern of PARV mRNA concentration was observed between P16 and P21. In the hippocampus low hybridization was observed at P10-P12. In subsequent stages both the number of positive cells and the intensity of labelling increased steadily. No clear-cut radial gradients for the expression of PARV mRNA were observed in the hippocampal region. Our results show that the developmental radial gradient followed by PARV mRNA expression in the neocortex does not follow an 'inside-out' gradient, consistent with previous immunocytochemical findings. Taken together, these data indicate that the developmental sequence followed by the PARV protein directly reflects mRNA abundance and suggest that PARV mRNA expression correlates with the functional maturation of cortical interneurons.

Igf1 gene disruption results in reduced brain size, CNS hypomyelination, and loss of hippocampal granule and striatal parvalbumin-containing neurons

Homozygous Igf1-/- mice at 2 months of age had reduced brain weights, with reductions evenly affecting all major brain areas. The gross morphology of the CNS was normal, but the size of white matter structures in brain and spinal cord was strongly reduced, owing to decreased numbers of axons and oligodendrocytes. Myelinated axons were more strongly reduced in number than unmyelinated axons. The volume of the dentate gyrus granule cell layer was reduced in excess of the decrease in brain weight. Among populations of calcium-binding protein-containing neurons, there was a selective reduction in the number of striatal parvalbumin-containing cells. Numbers of mesencephalic dopaminergic neurons, striatal and basal forebrain cholinergic neurons, and spinal cord motoneurons were unaffected. Cerebellar morphology was unaltered. Our findings suggest cell type- and region-specific functions for IGF-I and emphasize prominent roles in axon growth and maturation in CNS myelination.

Postnatal development of parvalbumin immunoreactivity in the cerebral cortex of the cat

Parvalbumin immunoreactivity in the developing neocortex of the cat progresses following specific laminar, areal, and, in a particular area, roughly anteroposterior gradients. Parvalbumin immunoreactivity first occurs in basket cells and later in chandelier neurons. Pyramid-like immunoreactive neurons are also transitorily observed from the second to the third week in layer V of the auditory association-related areas. Parvalbumin-immunoreactive neurons first appear in the primary somatosensory cortex and primary auditory and visual areas, followed by the primary motor and polysensory association areas and, finally, the auditory association areas and cortical areas related to the limbic system. In addition to cortical neurons, three fiber systems are immunolabeled with antiparvalbumin antibodies: thalamocortical, callosal, and ipsilateral corticocortical. Parvalbumin-immunoreactive thalamocortical fibers appear during the first month of postnatal life. Parvalbumin-immunoreactive callosal and ipsilateral corticocortical fibers are seen from the fourth postnatal week onward. Because all parvalbumin-immunoreactive cortical neurons in adulthood are nonpyramidal inhibitory cells, the present findings suggest that a number of ipsilateral corticocortical and callosal connections may be inhibitory.

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.

Changes in GABA and parvalbumin immunoreactivities in the cerebral cortex of lizards after narine occlusion

Olfactory deprivation produced by narine occlusion has been suggested to reduce the activity in the cerebral cortex of lizards. Here we analyzed the short-term changes in GABA and parvalbumin (PV) immunoreactivities in the cerebral cortex of lizards after narine occlusion. The number and distribution of GABA- and parvalbumin-immunoreactive (IR) cells have been studied by immunocytochemistry in the cerebral cortex of control and olfactory-deprived lizards. The distribution of GABA-IR cells as well as that of PV-IR cells was similar in control and deprived animals, and PV-IR cells were GABA-IR in all cases. However, significant changes were observed in the absolute number of GABA- and PV-IR cells. GABA-IR cells were more abundant in deprived animals than in control ones. In contrast, the number of PV-IR cells decreased significantly and PV immunoreactivity in dendrites and boutons was lower in deprived animals. These results suggest that the reduction in the number of PV-IR cells in olfactory-deprived lizards occurs without loss of GABA cells, and that PV expression is under the control of olfactory activity and remains plastic in the cerebral cortex of adult lizards.

Poor correlation between delayed neuronal death induced by transient forebrain ischemia, and immunoreactivity for parvalbumin and calbindin D-28k in developing gerbil hippocampus

In the normal developing hippocampus of the gerbil, parvalbumin-immunoreactive neurons first appear in the stratum pyramidale of CA3 at postnatal day 15 (P15), and in CA2 and hilus of the dentate gyrus from P21 onwards. Immunoreactive terminals also follow the same sequence from CA3 to CA1 to reach adult patterns by the end of the 1st month. Calbindin D-28k immunoreactivity is seen in the external part of the upper blade of the dentate gyrus at P5, and progresses to the granule cell and molecular layers of the whole gyrus by P15, except for a thin band of immature cells located at the base of the granule cell layer which are calbindin negative. Calbindin immunoreactivity in mossy fibers progresses from the external to the hilar region of CA3 during the same period. A few immunoreactive cells are also found in the stratum radiatum/lacunare of the CA3, but no calbindin-immunoreactive cells are observed in the CA1 and CA2 subfields. The adult pattern of calbindin immunoreactivity is reached at P21. Vulnerability following transient forebrain ischemia for 20 min was examined in the hippocampal formation of gerbils during postnatal development. No cellular damage was seen in animals aged 7 days. Dying cells were observed at the base of the granule cell layer of the dentate gyrus in animals aged 15, 21 and 30 days. Pyramidal cells in the CA3 subfield were also sensitive to ischemia in gerbils aged 15 days, and less frequently in animals aged 21 days. The adult pattern of cellular damage, characterized by selective vulnerability of the CA1 subfield, was seen from day 30 onwards.(ABSTRACT TRUNCATED AT 250 WORDS)

Neurochemical development of the hippocampal region in the fetal rhesus monkey. II. Immunocytochemistry of peptides, calcium-binding proteins, DARPP-32, and monoamine innervation in the entorhinal cortex by the end of gestation

Material for the study came from one 126 day-old rhesus monkey fetus and two 3 day-old neonates. The immunocytochemical detection of somatostatin, neurotensin (NT), parvalbumin, calbindin D-28K, DARPP-32 as well as tyrosine hydroxylase (TH), dopamine-beta-hydroxylase and serotonin (5-HT), was carried out on serial cryostat sections of the entorhinal cortex. The authors reported in a previous paper the precocious differentiation of the entorhinal cortex in rhesus monkey fetuses and featured the conspicuous expression of calbindin D-28K, somatostatin, neurotensin, and the monoaminergic innervation during the first half of gestation. The present study shows distinct temporal profiles of neurochemical development during the second half of gestation: the dense neuropeptidergic innervation remained a constant feature; the three aminergic systems gradually increased in density; parvalbumin, unlike calbindin D-28K, was primarily expressed during the last quarter of gestation. Three other prominent features of the last quarter of gestation are illustrated: the refinement of the modular neurochemical organization of the lamina principalis externa, the delayed chemoanatomical development of the rhinal sulcus area, and the establishment of a distinct rostrocaudal pattern of neurochemical distribution. In correspondence with the cluster-like organization of the lamina principalis externa, the authors observed in the olfactory, rostral, and intermediate fields of the neonate monkey entorhinal cortex, a particular subset of pyramidal-shaped neurons: located in layer III, they were characterized by fasciculated apical dendrites ascending between the cellular islands of the discontinuous layer II and the coexpression of calbindin D-28K and DARPP-32. Besides, most of the other chemical systems displayed a distinct, area-specific, patchy distribution, except for the homogeneously distributed noradrenergic innervation. In the olfactory and rostral fields, TH positive dopaminergic fibers accumulated on the neuronal islands of layers II-III, and parvalbumin labeled fibers on those of layer III, whereas patches of 5-HT and NT-like reactive terminals were segregated between the cellular islands, overlapping the DARPP-32/calbindin D-28 K labeled dendritic bundles. At the opposite, in the intermediate field, 5-HT positive terminals overlapped the cellular islands of layer II and thin fascicles of dopaminergic fibers ran in the inter island spaces. The somatostatin-LIR innervation was apparently too dense to reveal a patchy distribution that existed at earlier developmental stages. In the caudal field, the patchy pattern was replaced by a predominant bilaminar type of distribution of NT, 5-HT, and TH-like positive afferents.(ABSTRACT TRUNCATED AT 400 WORDS)

Distribution of cytochrome oxidase and parvalbumin in the primary visual cortex of the adult and neonate monkey, Callithrix jacchus

The anatomical distributions of the mitochondrial enzyme cytochrome oxidase (CO) and of the calcium binding protein parvalbumin (PV) were studied in the striate cortex of adult and neonate New World monkeys (Callithrix jacchus). In the adult marmoset, both proteins were found in laminar arrangements similar to those described for the macaque monkey, with prominent bands of PV-like immunoreactive (PV-LI) puncta in layers IV and IIIb, and fairly evenly distributed PV-LI nonpyramidal neurons. Furthermore, the pattern of CO activity in area 17 of the neonate marmoset was almost identical to the CO pattern described in neonate macaque and squirrel monkeys. It came, therefore, as a surprise to find that the adult pattern of PV-like immunoreactivity (PV-LI) in the marmoset striate cortex arises from a neonatal pattern strikingly different from that seen in any developmental stage of the macaque, or in any other mammal studied so far. In the deep layers IV through VI of the neonate marmoset, a large number of PV-LI neurons was stained in bandlike patterns, their number in layers IV and V exceeding the number of PV-LI neurons present in these layers of the adult marmoset area 17. Staining of layers IV and VI was restricted to area 17 and involved nonpyramidal cells and their processes. The stained band of layer V, in contrast, continued throughout most of the neocortex. In area 17, an estimated 10 to 20% of the stained cells in layer V exhibited pyramidal shapes. The findings show that the expression of PV by visual cortical cells occurs before birth and suggest that the comparatively early onset of PV expression is not dependent on the onset of textured vision. The exuberant number of stained cells in some layers, and particularly the staining of pyramidal cells, in the neonate marmoset, suggest that a considerable number of cells possesses the stainability for PV-LI only transiently, i.e., in the marmoset, these cells have a specific demand for parvalbumin during this phase of their development.

Postnatal increase of GABA- and PV-IR cells in the cerebral cortex of the lizard Podarcis hispanica

The number and distribution of GABA- and parvalbumin (PV)-immunoreactive (IR) cells have been studied by immunocytochemistry in the cerebral cortex of newborn and adult lizards. The distribution of GABA-IR cells as well as that of PV-IR cells were similar in newborn and adult lizards, and PV-IR cells were GABA-IR in all cases. However, the absolute number of GABA- and PV-IR cells increased significantly during development. In addition, the rate of of GABA-IR cells also displaying PV immunoreactivity also increased after birth. Moreover, dendrites were rarely found to be PV-IR in newborn lizards, whereas they appeared stained in a Golgi-like manner in adult animals. These results suggest that the GABAergic neuronal population of the cerebral cortex of lizards experiments a significant increment in number and neurochemical maturation after birth.

Developmental pattern and subcellular localization of parvalbumin in the rat tooth germ

The EF-hand calcium-binding protein parvalbumin has been extensively studied in nerve and muscle cells. Its possible role in biomineralization during tooth development was here investigated by determining its subcellular localization by immunogold cytochemistry. The developmental sequences of amelogenesis and dentinogenesis were studied in rat molars, and in continuously growing rat incisors. The findings confirm that parvalbumin is a nuclear and a cytosolic protein, not associated with any particular intracellular organelle. Epithelial and mesenchymal undifferentiated cells contained no specific parvalbumin immunolabelling. In differentiated ameloblasts, secretory-pole (Tomes' process) formation was associated with a proximal-distal gradient of parvalbumin labelling. But after the Tomes' process had formed, parvalbumin was evenly distributed throughout the cell. The parvalbumin contents of ruffle-ended and smooth-ended ameloblasts appeared to be very different. Differentiated odontoblasts were less heavily labelled than ameloblasts, and the label was restricted to the cell body during the whole of dentinogenesis. These data suggest that parvalbumin could contribute to membrane plasticity during differentiation, as shown during dendritic growth in the nervous cells. Moreover, as may occur in excitable cells, parvalbumin could buffer calcium specifically in the cells producing mineralized enamel and dentine during the later stages of tooth development.

Parvalbumin immunoreactivity in the hippocampus of the gerbil after transient forebrain ischaemia: a qualitative and quantitative sequential study

Parvalbumin immunoreactivity is examined in the hippocampus of the Mongolian gerbil (Meriones unguiculatus) in controls and in animals subjected to 20 min of forebrain ischaemia produced by bilateral clipping of the carotids. In comparison with other species, the hippocampus of the gerbil is characterized by strong immunoreactivity of the (presumably excitatory) perforant pathway, and weak immunoreactivity (low numbers of neurons and scarce dendritic arbors) in nonpyramidal nerve cells (inhibitory neurons) of the CA1 area. These properties may play some role in the development and maintenance of seizures in this susceptible species. Parvalbumin immunoreactivity is rapidly and ephemerally increased in the hippocampus 15 min after reperfusion. Later on, there is a transitory decrease of parvalbumin immunoreactivity which is followed by an increase 6 h later in the stratum granulare hilus and CA3 area, and not until the first and second days in the CA1 area. This increase significantly surpasses the number of immunoreactive neurons in control animals in CA1 and CA3 from 48 h after reperfusion onwards. The effect is similar using different anaesthetics and does not occur in sham-operated animals. In contrast with these findings, the number of parvalbumin-immunoreactive neurons in the somatosensory cortex is not affected in our model of forebrain ischaemia. On the other hand, GABA-immunoreactive neurons in CA1 are preserved during the first week after reperfusion, although an increase in the number of these cells occurs at the end of this period. Delayed neuronal death occurs in the CA1 area 48 h after ischaemia, and marked reduction in the number of CA1 neurons is found by the end of the first week. Eighty per cent of the remaining cells in CA1 at day 7, and 83% at day 15, are parvalbumin-immunoreactive nonpyramidal neurons in contrast to 3% parvalbumin-immunoreactive cells in control animals. These findings indicate that GABAergic neurons in CA1 are preserved after forebrain ischaemia, and that parvalbumin in CA1 neurons is associated with survival.

Neurochemical development of the hippocampal region in the fetal rhesus monkey. I. Early appearance of peptides, calcium-binding proteins, DARPP-32, and monoamine innervation in the entorhinal cortex during the first half of gestation (E47 to E90)

Although the entorhinal cortex is a key structure connecting the hippocampal formation with the rest of the cerebral cortex, little is known about its early chemoanatomical development in primates. In the present study, a cytoarchitectonic analysis and immunocytochemical detection of somatostatin, neurotensin, parvalbumin, calbindin-D 28K, DARPP-32, as well as tyrosine hydroxylase, dopamine-beta-hydroxylase, and serotonin, were carried out on serial sections of the entorhinal cortex of six rhesus monkey fetuses aged E47 to E90 (gestation period 165 days). At E56 the cortical plate of the entorhinal cortex already exhibited a sublamination; at E64 the lamina dissecans was partly formed, allowing the emergence of the lamina principalis externa and interna, and at E83 most of the regional and laminar subdivisions characteristic of the adult cortex could be identified, except for the rhinal sulcus restricted to a small dimple. The neurochemical development paralleled the early cytoarchitectonic differentiation, both largely preceding that of the neighboring cortical areas. The somatostatin-like immunoreactive innervation, first detected at E56, was very dense as early as E64 and displayed by E83 a laminar distribution similar to that found in the adult. Labeled neurons indicated an intrinsic origin for this innervation but an extrinsic connection might be present as labeled fibers in the subplate of the entorhinal cortex were in continuity with positive fibers in the intermediate zone of the hippocampal formation. A faint neurotensin-like immunoreactivity first detected at E64 became prominent at E83 in the entorhinal cortex but stopped abruptly at the anlage of the rhinal sulcus. The lack of neurotensin-labeled neurons contrasted with their presence in other parts of the hippocampal region and suggested a precocious extrinsic connection. Only rare parvalbumin-LIR neurons were detected at midgestation, whereas calbindin-D 28K was expressed from E47 on in Cajal-Retzius cells and from E56 on in various types of neurons in the cortical plate and subplate. Most characteristic was a category of medium-sized, deeply stained calbindin-LIR neurons, present only in the lamina principalis externa and possibly corresponding to the population of large neurons described by Kostovic et al. (1990, Soc Neurosci Abstr 16:846) in early developing entorhinal cortex of human fetuses. These and probably other neurons were also DARPP-32-positive, suggesting the possibility of an early dopaminergic regulation. Indeed, the monoaminergic innervation of the entorhinal cortex was detected from E56 on and gradually increased in density, displaying areal and laminar differences in the distribution of the dopaminergic, noradrenergic, and serotoninergic afferents.(ABSTRACT TRUNCATED AT 400 WORDS)