Perisomatic inhibition and cortical circuit dysfunction in schizophrenia

Deficits of cognitive control in schizophrenia are associated with altered gamma oscillations in the prefrontal cortex. Paralbumin basket interneurons, which innervate the perisomatic region of pyramidal neurons, appear to play a key role in generating cortical gamma oscillations. In the prefrontal cortex of subjects with schizophrenia, alterations are present in both pre- and post-synaptic markers of the strength of GABA inputs from parvalbumin basket neurons to pyramidal neurons. These alterations may contribute to the neural substrate for impaired gamma oscillations in schizophrenia.     

Non-associative Potentiation of Perisomatic Inhibition Alters the Temporal Coding of Neocortical Layer 5 Pyramidal Neurons

In the neocortex, the coexistence of temporally locked excitation and inhibition governs complex network activity underlying cognitive functions, and is believed to be altered in several brain diseases. Here we show that this equilibrium can be unlocked by increased activity of layer 5 pyramidal neurons of the mouse neocortex. Somatic depolarization or short bursts of action potentials of layer 5 pyramidal neurons induced a selective long-term potentiation of GABAergic synapses (LTPi) without affecting glutamatergic inputs. Remarkably, LTPi was selective for perisomatic inhibition from parvalbumin basket cells, leaving dendritic inhibition intact. It relied on retrograde signaling of nitric oxide, which persistently altered presynaptic GABA release and diffused to inhibitory synapses impinging on adjacent pyramidal neurons. LTPi reduced the time window of synaptic summation and increased the temporal precision of spike generation. Thus, increases in single cortical pyramidal neuron activity can induce an interneuron-selective GABAergic plasticity effectively altering the computation of temporally coded information.     

Dendritic GABA release depresses excitatory transmission between layer 2/3 pyramidal and bitufted neurons in rat neocortex

GABAergic, somatostatin-containing bitufted interneurons in layer 2/3 of rat neocortex are excited via glutamatergic excitatory postsynaptic potentials (EPSPs) by pyramidal neurons located in the same cortical layer. Pair recordings showed that short bursts of backpropagating dendritic action potentials (APs) reduced the amplitude of unitary EPSPs. EPSP depression was dependent on a rise in dendritic [Ca2+]. The effect was blocked by the GABA(B) receptor (GABA(B)-R) antagonist CGP55845A and was mimicked by the GABA(B)-R agonist baclofen. As presynaptic GABA(B)-Rs were activated neither by somatostatin nor by GABA released from axon collaterals of the bitufted cell, we conclude that GABA(B)-Rs were activated by a retrograde messenger, most likely GABA, released from the dendrite. Because synaptic depression was prevented by loading bitufted neurons with GDP-beta-S, it is likely to be caused by exocytotic GABA release from dendrites.     

Changes in serotonin2A and GABA(A) receptors in schizophrenia: studies on the human dorsolateral prefrontal cortex

Having shown a decrease in serotonin2A receptors in the dorsolateral prefrontal cortex (DLPFC) from schizophrenic subjects, we have now determined if this change was reflective of widespread changes in neurochemical markers in DLPFC in schizophrenia. In Brodmann's area (BA) 9 from 19 schizophrenic and 19 control subjects, we confirmed a decrease in the density of [3H]ketanserin binding to serotonin2A receptors in tissue from the schizophrenic subjects [39 +/- 3.3 vs. 60 +/- 3.6 fmol/mg estimated tissue equivalents (ETE); p < 0.005]. In addition, the density of [3H]muscimol binding to GABA(A) receptors was increased in the schizophrenic subjects (526 +/- 19 vs. 444 +/- 28 fmol/mg ETE; p < 0.02). [3H]YM-09151-2, N-[1-(2-thienyl)cyclohexyl]-3,4-[3H]piperidine, [3H]SCH 23390, [3H]mazindol, and N(G)-nitro-L-[3H]arginine binding to BA 9 did not differ between groups, and there was no specific binding of [3H]raclopride or 7-hydroxy-2-(di-n-[3H]propylamino)tetralin to BA 9 from either cohort of subjects. This suggests the density of dopamine D1-like and NMDA receptors, the dopamine transporter, and nitric oxide synthase activity are not altered in BA 9 from schizophrenic subjects. The selective nature of the changes in serotonin2A and GABA(A) receptors in DLPFC could indicate that these changes are involved in the pathology of schizophrenia.     

Glutamate, GABA and acetylcholine signaling components in the lamina of the Drosophila visual system

Synaptic connections of neurons in the Drosophila lamina, the most peripheral synaptic region of the visual system, have been comprehensively described. Although the lamina has been used extensively as a model for the development and plasticity of synaptic connections, the neurotransmitters in these circuits are still poorly known. Thus, to unravel possible neurotransmitter circuits in the lamina of Drosophila we combined Gal4 driven green fluorescent protein in specific lamina neurons with antisera to gamma-aminobutyric acid (GABA), glutamic acid decarboxylase, a GABA(B) type of receptor, L-glutamate, a vesicular glutamate transporter (vGluT), ionotropic and metabotropic glutamate receptors, choline acetyltransferase and a vesicular acetylcholine transporter. We suggest that acetylcholine may be used as a neurotransmitter in both L4 monopolar neurons and a previously unreported type of wide-field tangential neuron (Cha-Tan). GABA is the likely transmitter of centrifugal neurons C2 and C3 and GABA(B) receptor immunoreactivity is seen on these neurons as well as the Cha-Tan neurons. Based on an rdl-Gal4 line, the ionotropic GABA(A) receptor subunit RDL may be expressed by L4 neurons and a type of tangential neuron (rdl-Tan). Strong vGluT immunoreactivity was detected in alpha-processes of amacrine neurons and possibly in the large monopolar neurons L1 and L2. These neurons also express glutamate-like immunoreactivity. However, antisera to ionotropic and metabotropic glutamate receptors did not produce distinct immunosignals in the lamina. In summary, this paper describes novel features of two distinct types of tangential neurons in the Drosophila lamina and assigns putative neurotransmitters and some receptors to a few identified neuron types.     

Examining neocortical circuits: Some background and facts

The first definitive studies of where afferents to cerebral cortex terminate were made possible by the finding that as they degenerate axon terminals become electron dense. Gold toning of Golgi impregnated neurons allowed the postsynaptic targets of these afferents to be identified by electron microscopy and also allowed the termination sites of axons from a variety of types of cortical neurons to be ascertained, while the development of antibodies to GAD and to GABA made it possible to determine which types of cortical neurons are inhibitory. Subsequently the use of gold toned, Golgi impregnated material to examine neuronal connectivity was made redundant by the development of techniques that allowed the physiological properties of cortical neurons to be evaluated in neurons filled intracellularly with markers. Intracellular filling showed the axonal trees of cortical neurons are much more widespread than had been revealed by Golgi impregnations. As a result of numerous studies of the axons of identified neurons, we know a great deal about where most of the different types of neurons in cerebral cortex form their synapses, but on the other side of the picture there is a dearth of information about the origins of the inputs that specific types of cortical neurons receive. However, it is evident that each cortical neuron is the focus of input from many other neurons, and on the basis of the available data it is estimated that a single pyramidal cell in cortex receives its input from as many as 1,000 other excitatory neurons and as many as 75 inhibitory neurons.     

Signal processing in the axon initial segment

The axon initial segment (AIS) is a specialized membrane region in the axon of neurons where action potentials are initiated. Crucial to the function of the AIS is the presence of specific voltage-gated channels clustered at high densities, giving the AIS unique electrical properties. Here we review recent data on the physiology of the AIS. These data indicate that the role of the AIS is far richer than originally thought, leading to the idea that it represents a dynamic signal processing unit within neurons, regulating the integration of synaptic inputs, intrinsic excitability, and transmitter release. Furthermore, these observations point to?a critical role of the AIS in disease.     

Rats with Malformations of Cortical Development Exhibit Decreased Length of AIS and Hypersensitivity to Pilocarpine-Induced Status Epilepticus

Malformations of cortical development (MCD) are critical brain development disorders associated with varied abnormalities in both anatomic structures and neural functioning. It is also a very common etiology to the epilepsy, in which the alteration on excitability of cortical neurons is hypothesized as one of important causes to the epileptic seizures. Due to the key role in regulating neuron firing properties, the plasticity of axon initial segment (AIS) was investigated in present study to further determine the relation between MCD and epilepsy. Our results showed a prolonged decrease in the length of AIS occurred in MCD animal models. Besides, the AIS was also found greatly shortened in MCD models during the acute, but not chronic phase of status epileptics compared with intact controls. Our findings of identification of AIS plasticity in MCD animal models and its hypersensitivity to status epilepsy are significant in furthering our understanding of the pathophysiological mechanisms involved in this disorder.     

Axon initial segment Kv1 channels control axonal action potential waveform and synaptic efficacy

Action potentials are binary signals that transmit information via their rate and temporal pattern. In this context, the axon is thought of as a transmission line, devoid of a role in neuronal computation. Here, we show a highly localized role of axonal Kv1 potassium channels in shaping the action potential waveform in the axon initial segment (AIS) of layer 5 pyramidal neurons independent of the soma. Cell-attached recordings revealed a 10-fold increase in Kv1 channel density over the first 50 microm of the AIS. Inactivation of AIS and proximal axonal Kv1 channels, as occurs during slow subthreshold somatodendritic depolarizations, led to a distance-dependent broadening of axonal action potentials, as well as an increase in synaptic strength at proximal axonal terminals. Thus, Kv1 channels are strategically positioned to integrate slow subthreshold signals, providing control of the presynaptic action potential waveform and synaptic coupling in local cortical circuits.     

Anatomical and functional differentiation of glutamatergic synaptic innervation in the neocortex.

Pyramidal neurons are the principal neurons of the neocortex and their excitatory impact on other pyramidal neurons and interneurons is central to neocortical dynamics. A fundamental principal that has emerged which governs pyramidal neuron excitation of other neurons in the local circuitry of neocortical columns is differential anatomical and physiological properties of the synaptic innervation via the same axon depending on the type of neuron targeted. In this study we derive anatomical principles for divergent innervation of pyramidal neurons of the same type within the local microcircuit. We also review data providing circumstantial and direct evidence for differential synaptic transmission via the same axon from neocortical pyramidal neurons and derive some principles for differential synaptic innervation of pyramidal neurons of the same type, of pyramidal neurons and interneurons and of different types of interneurons. We conclude that differential anatomical and physiological differentiation is a fundamental property of glutamatergic axons of pyramidal neurons in the neocortex.     

Electrophysiological Heterogeneity of Fast-Spiking Interneurons: Chandelier versus Basket Cells

In the prefrontal cortex, parvalbumin-positive inhibitory neurons play a prominent role in the neural circuitry that subserves working memory, and alterations in these neurons contribute to the pathophysiology of schizophrenia. Two morphologically distinct classes of parvalbumin neurons that target the perisomatic region of pyramidal neurons, chandelier cells (ChCs) and basket cells (BCs), are generally thought to have the same “fast-spiking” phenotype, which is characterized by a short action potential and high frequency firing without adaptation. However, findings from studies in different species suggest that certain electrophysiological membrane properties might differ between these two cell classes. In this study, we assessed the physiological heterogeneity of fast-spiking interneurons as a function of two factors: species (macaque monkey vs. rat) and morphology (chandelier vs. basket). We showed previously that electrophysiological membrane properties of BCs differ between these two species. Here, for the first time, we report differences in ChCs membrane properties between monkey and rat. We also found that a number of membrane properties differentiate ChCs from BCs. Some of these differences were species-independent (e.g., fast and medium afterhyperpolarization, firing frequency, and depolarizing sag), whereas the differences in the first spike latency between ChCs and BCs were species-specific. Our findings indicate that different combinations of electrophysiological membrane properties distinguish ChCs from BCs in rodents and primates. Such electrophysiological differences between ChCs and BCs likely contribute to their distinctive roles in cortical circuitry in each species.     

Of mice and men, and chandeliers

How does the human neocortex reliably propagate information through neural circuits? One mechanism appears to involve relying on strong connections from pyramidal neurons to interneurons and a depolarizing action of cortical chandelier cells.     

Concurrent acetylcholinesterase staining and gamma-aminobutyric acid uptake of cortical neurons in culture

Gamma-aminobutyric acid (GABA) uptake and acetylcholinesterase (AChE) content were demonstrated concurrently in cortical neurons grown in tissue culture. Positive reactions either for GABA uptake or for AChE content were encountered in pyramidal and stellate, as well as spindle-shaped neurons. Neither reaction was confined to a specific morphological subtype. Nearly half the neurons were negative for either reaction. Most of the remaining neurons were positive only for GABA or only for AChE. However, a subpopulation of neurons showed not only a high AChE content, but also an avid GABA uptake. Thus, four types of neurons could be identified on the basis of these two reactions. The high AChE content in some of the cortical neurons that also showed GABA uptake indicates that there are at least two distinct types of GABAergic neurons.     

Parvalbumin-expressing interneurons linearly transform cortical responses to visual stimuli

The response of cortical neurons to a sensory stimulus is shaped by the network in which they are embedded. Here we establish a role of parvalbumin (PV)-expressing cells, a large class of inhibitory neurons that target the soma and perisomatic compartments of pyramidal cells, in controlling cortical responses. By bidirectionally manipulating PV cell activity in visual cortex we show that these neurons strongly modulate layer 2/3 pyramidal cell spiking responses to visual stimuli while only modestly affecting their tuning properties. PV cells' impact on pyramidal cells is captured by a linear transformation, both additive and multiplicative, with a threshold. These results indicate that PV cells are ideally suited to modulate cortical gain and establish a causal relationship between a select neuron type and specific computations performed by the cortex during sensory processing.     

Characteristics of human cortical pyramidal neuron development during the second gestational trimester

Prenatal ontogeny of the human neocortex exhibits specific characteristics that make its organization unique. Therefore, experimental data obtained on animal models cannot be extrapolated to human cortex morphogenesis during the middle and late gestational periods. Characteristics of the development of cortical pyramidal neurons of the human brain were studied in the brains of eight fetuses at gestational ages between 16 and 26 weeks. Immunohistochemical labeling of neurons was performed using antibodies against microtubule associated protein 2 (MAP2), a structural protein of microtubules. Expression of this protein marks the beginning of dendrogenesis. MAP2 is mainly located in the neuron body and dendrites, which allowed the neuron morphotype and location in specific cortical layers to be determined. It was shown that MAP2-immunopositive neurons were identifiable in embryonic cortical layer eV as early as the 18th gestational week. By the 25th gestational week, two populations of pyramidal neurons were discernible in the cortical plate, one of them located in layer eV and the other, in layer eIII, which developed later. Since differentiating neurons are known to be more vulnerable than neuroblasts and mature neurons, these results suggest that critical periods for corticofugal and corticocortical populations of pyramidal cells occur at different stages of the second gestational trimester.     

The morphology and distribution of 'Kolmer-Agduhr cells', a class of cerebrospinal-fluid-contacting neurons revealed in the frog embryo spinal cord by GABA immunocytochemistry

An immunocytochemical method that localizes GABA in glutaraldehyde-fixed tissue has been applied to the study of the Xenopus embryo spinal cord. This procedure stained an anatomical class of neuron, which had somata forming two more or less continuous rows, one on either side of the central canal, in the ventral part of the spinal cord. The total number of stained neurons in the stage 37-38 embryo spinal cord was about 300. The medial surface on the soma protruded into the central canal and had a brush border which electron microscope studies showed to consist of many microvilli or stereocilia and one or two cilia. The external end of the neuron soma had an ipsilateral ascending axon. The axon of many of these neurons had a growth cone which was also clearly stained. We propose calling these neurons 'Kolmer-Agduhr cells' after W. Kolmer and E. Agduhr who described them in the spinal cords of many vertebrate classes. Their early embryonic origin, GABA-like immunoreactivity, axonal projections and distribution as a whole population have not previously been known.     

A light and electron microscopic study of betaine/GABA transporter distribution in the monkey cerebral neocortex and hippocampus

The present study aimed to elucidate the distribution of betaine/γ-aminobutyric acid (GABA) transporter-1 (BGT-1) in the normal monkey cerebral neocortex and hippocampus by immunoperoxidase and Immunogold labelling. BGT-1 was observed in pyramidal neurons in the cerebral neocortex and the CA fields of the hippocampus. Large numbers of small diameter dendrites or dendritic spines were observed in the neuropil. These made asymmetrical synaptic contacts with unlabelled axon terminals containing small round vesicles, characteristic of glutamatergic terminals. BGT-1 label was observed in an extra-perisynaptic region, away from the post-synaptic density. Immunoreactivity was not observed in portions of dendrites that formed symmetrical synapses, axon terminals, or glial cells. The distribution of BGT-1 on dendritic spines, rather than at GABAergic axon terminals, suggests that the transporter is unlikely to play a major role in terminating the action of GABA at a synapse. Instead, the osmolyte betaine is more likely to be the physiological substrate of BGT-1 in the brain, and the presence of the transporter in pyramidal neurons suggests that these neurons utilize betaine to maintain osmolarity.     

Ultrastructural localization of nitric oxide synthase immunoreactivity in the cat ventrobasal complex

This study describes the ultrastructural localization of nitric oxide synthase (NOS) immunoreactivity in the cat ventrobasal complex. NOS immunoreactivity was found in the cell bodies and dendrites of local circuit neurons and in vesicle-containing profiles. The vesicle-containing profiles could be divided into two classes, those of dendritic origin (presynaptic dendrite boutons) and those of axonal origin. The NOS labelled axon terminals varied in size and packing density and were principally located in the extra-glomerular neuropil. These boutons presented a range of morphologies and it was not possible to determine the probable source based on morphological criteria. The NOS immunoreactive presynaptic dendrite boutons were found both within and outside glomeruli and established both pre- and post-synaptic relationships with other elements. Post-embedding GABA immunocytochemistry showed that some NOS immunoreactive axonal boutons and presynaptic dendrites were also immunopositive for GABA. This finding suggests that some of the NOS labelled axonal boutons are of local circuit neuron origin. These results suggest that local circuit neurons in the cat ventrobasal complex might be involved in specific, short range interactions using GABA and longer, more global interactions using nitric oxide.     

Depolarization recruits DCC to the plasma membrane of embryonic cortical neurons and enhances axon extension in response to netrin-1

The netrin-1 receptor Deleted in Colorectal Cancer (DCC) is required for the formation of major axonal projections by embryonic cortical neurons, including the corpus callosum, hippocampal commissure, and cortico-thalamic tracts. The presentation of DCC by axonal growth cones is tightly regulated, but the mechanisms regulating DCC trafficking within neurons are not well understood. Here, we investigated the mechanisms regulating DCC recruitment to the plasma membrane of embryonic cortical neurons. In embryonic spinal commissural neurons, protein kinase A (PKA) activation recruits DCC to the plasma membrane and enhances axon chemoattraction to netrin-1. We demonstrate that PKA activation similarly recruits DCC and increases embryonic cortical neuron axon extension, which, like spinal commissural neurons, respond to netrin-1 as a chemoattractant. We then determined if depolarization might recruit DCC to the plasma membrane. Neither netrin-1 induced axon extension, nor levels of plasma membrane DCC, were altered by depolarizing embryonic spinal commissural neurons with elevated levels of KCl. In contrast, depolarizing embryonic cortical neurons increased the amount of plasma membrane DCC, including at the growth cone, and increased axon outgrowth evoked by netrin-1. Inhibition of PKA, phosphatidylinositol-3-kinase, protein kinase C, or exocytosis blocked the depolarization-induced recruitment of DCC and suppressed axon outgrowth. Inhibiting protein synthesis did not affect DCC recruitment, nor were the distributions of trkB or neural cell adhesion molecule (NCAM) influenced by depolarization, consistent with selective mobilization of DCC. These findings identify a role for membrane depolarization modulating the response of axons to netrin-1 by regulating DCC recruitment to the plasma membrane.