Whole cell recordings from brain of adult Drosophila

In this video, we demonstrate the procedure for isolating whole brains from adult Drosophila in preparation for recording from single neurons. We begin by describing the dissecting solution and capture of the adult females used in our studies. The procedure for removing the whole brain intact, including both optic lobes, is illustrated. Dissection of the overlying trachea is also shown. The isolated brain is not only small but needs special care in handling at this stage to prevent damage to the neurons, many of which are close to the outer surface of the tissue. We show how a special holder we developed is used to stabilize the brain in the recording chamber. A standard electrophysiology set up is used for recording from single neurons or pairs of neurons. A fluorescent image, viewed through the recording microscope, from a GAL4 line driving GFP expression (GH146) illustrates how projection neurons (PNs) are identified in the live brain. A high power Nomarski image shows a view of a single neuron that is being targeted for whole cell recording. When the brain is successfully removed without damage, the majority of the neurons are spontaneously active, firing action potentials and/or exhibiting spontaneous synaptic input. This in situ preparation, in which whole cell recording of identified neurons in the whole brain can be combined with genetic and pharmacological manipulations, is a useful model for exploring cellular physiology and plasticity in the adult CNS.

Preparation of neuronal cultures from midgastrula stage Drosophila embryos

This video illustrates the procedure for making primary neuronal cultures from midgastrula stage Drosophila embryos. The methods for collecting embryos and their dechorionation using bleach are demonstrated. Using a glass pipet attached to a mouth suction tube, we illustrate the removal of all cells from single embryos. The method for dispersing cells from each embryo into a small (5 l) drop of medium on an uncoated glass coverslip is demonstrated. A view through the microscope at 1 hour after plating illustrates the preferred cell density. Most of the cells that survive when grown in defined medium are neuroblasts that divide one or more times in culture before extending neuritic processes by 12-24 hours. A view through the microscope illustrates the level of neurite outgrowth and branching expected in a healthy culture at 2 days in vitro. The cultures are grown in a simple bicarbonate based defined medium, in a 5% CO(2) incubator at 22-24 degrees C. Neuritic processes continue to elaborate over the first week in culture and when they make contact with neurites from neighboring cells they often form functional synaptic connections. Neurons in these cultures express voltage-gated sodium, calcium, and potassium channels and are electrically excitable. This culture system is useful for studying molecular genetic and environmental factors that regulate neuronal differentiation, excitability, and synapse formation/function.

Mutant ubiquitin found in Alzheimer's disease causes neuritic beading of mitochondria in association with neuronal degeneration

A dinucleotide deletion in human ubiquitin (Ub) B messenger RNA leads to formation of polyubiquitin (UbB)+1, which has been implicated in neuronal cell death in Alzheimer's and other neurodegenerative diseases. Previous studies demonstrate that UbB+1 protein causes proteasome dysfunction. However, the molecular mechanism of UbB+1-mediated neuronal degeneration remains unknown. We now report that UbB+1 causes neuritic beading, impairment of mitochondrial movements, mitochondrial stress and neuronal degeneration in primary neurons. Transfection of UbB+1 induced a buildup of mitochondria in neurites and dysregulation of mitochondrial motor proteins, in particular, through detachment of P74, the dynein intermediate chain, from mitochondria and decreased mitochondria-microtubule interactions. Altered distribution of mitochondria was associated with activation of both the mitochondrial stress and p53 cell death pathways. These results support the hypothesis that neuritic clogging of mitochondria by UbB+1 triggers a cascade of events characterized by local activation of mitochondrial stress followed by global cell death. Furthermore, UbB+1 small interfering RNA efficiently blocked expression of UbB+1 protein, attenuated neuritic beading and preserved cellular morphology, suggesting a potential neuroprotective strategy for certain neurodegenerative disorders.

Primary neuronal cultures from the brains of late stage Drosophila pupae

In this video, we demonstrate the preparation of primary neuronal cultures from the brains of late stage Drosophila pupae. The procedure begins with the removal of brains from animals at 70-78 hrs after puparium formation. The isolated brains are shown after brief incubation in papain followed by several washes in serum-free growth medium. The process of mechanical dissociation of each brain in a 5 ul drop of media on a coverslip is illustrated. The axons and dendrites of the post-mitotic neurons are sheered off near the soma during dissociation but the neurons begin to regenerate processes within a few hours of plating. Images show live cultures at 2 days. Neurons continue to elaborate processes during the first week in culture. Specific neuronal populations can be identified in culture using GAL4 lines to drive tissue specific expression of fluorescent markers such as GFP or RFP. Whole cell recordings have demonstrated the cultured neurons form functional, spontaneously active cholinergic and GABAergic synapses. A short video segment illustrates calcium dynamics in the cultured neurons using Fura-2 as a calcium indicator dye to monitor spontaneous calcium transients and nicotine evoked calcium responses in a dish of cultured neurons. These pupal brain cultures are a useful model system in which genetic and pharmacological tools can be used to identify intrinsic and extrinsic factors that influence formation and function of central synapses.

nAChR-mediated calcium responses and plasticity inDrosophila Kenyon cells

In Drosophila, nicotinic acetylcholine receptors (nAChRs) mediate fast excitatory synaptic transmission in mushroom body Kenyon cells, a neuronal population involved in generation of complex behaviors, including responses to drugs of abuse. To determine whether activation of nAChRs can induce cellular changes that contribute to functional plasticity in these neurons, we examined nicotine-evoked responses in cells cultured from brains of late stage OK107-GAL4 pupae. Kenyon cells can be identified by expression of green fluorescent protein (GFP+). Nicotine activates alpha-bungarotoxin-sensitive nAChRs, causing a rapid increase in intracellular calcium levels in over 95% of the Kenyon cells. The nicotine-evoked calcium increase has a voltage-gated calcium channel (VGCC) dependent component and a VGCC-independent component that involves calcium influx directly through nAChRs. Thapsigargin treatment reduces the nicotine response consistent with amplification by calcium release from intracellular stores. The response to nicotine is experience-dependent: a short conditioning pulse of nicotine causes a transient 50% reduction in the magnitude of the response to a test pulse of nicotine when the interpulse interval is 4 h. This cellular plasticity is dependent on activation of the VGCC-component of the nicotine response and on cAMP-signaling, but not on protein synthesis. These data demonstrate that activation of nAChRs induces a calcium-dependent plasticity in Kenyon cells that could contribute to adult behaviors involving information processing in the mushroom bodies including responses to nicotine.

Alpha3Na+/K+-ATPase is a neuronal receptor for agrin

Agrin, through its interaction with the receptor tyrosine kinase MuSK, mediates accumulation of acetylcholine receptors (AChR) at the developing neuromuscular junction. Agrin has also been implicated in several functions in brain. However, the mechanism by which agrin exerts its effects in neural tissue is unknown. Here we present biochemical evidence that agrin binds to the alpha3 subunit of the Na+/K+-ATPase (NKA) in CNS neurons. Colocalization with agrin binding sites at synapses supports the hypothesis that the alpha3NKA is a neuronal agrin receptor. Agrin inhibition of alpha3NKA activity results in membrane depolarization and increased action potential frequency in cortical neurons in culture and acute slice. An agrin fragment that acts as a competitive antagonist depresses action potential frequency, showing that endogenous agrin regulates native alpha3NKA function. These data demonstrate that, through its interaction with the alpha3NKA, agrin regulates activity-dependent processes in neurons, providing a molecular framework for agrin action in the CNS.

Cholinergic synaptic transmission in adult Drosophila Kenyon cells in situ

Behavioral and genetic studies in Drosophila have contributed to our understanding of molecular mechanisms that underlie the complex processes of learning and memory. Use of this model organism for exploration of the cellular mechanisms of memory formation requires the ability to monitor synaptic activity in the underlying neural networks, a challenging task in the tiny adult fly. Here, we describe an isolated whole-brain preparation in which it is possible to obtain in situ whole-cell recordings from adult Kenyon cells, key members of a neural circuit essential for olfactory associative learning in Drosophila. The presence of sodium action potential (AP)-dependent synaptic potentials and synaptic currents in >50% of the Kenyon cells shows that these neurons are members of a spontaneously active neural circuit in the isolated brain. The majority of sodium AP-dependent synaptic transmission is blocked by curare and by alpha-bungarotoxin (alpha-BTX). This demonstrates that nicotinic acetylcholine receptors (nAChRs) are responsible for most of the spontaneous excitatory drive in this circuit in the absence of normal sensory input. Furthermore, analysis of sodium AP-independent synaptic currents provides the first direct demonstration that alpha-BTX-sensitive nAChRs mediate fast excitatory synaptic transmission in Kenyon cells in the adult Drosophila brain. This new preparation, in which whole-cell recordings and pharmacology can be combined with genetic approaches, will be critical in understanding the contribution of nAChR-mediated fast synaptic transmission to cellular plasticity in the neural circuits underlying olfactory associative learning.

DrosophilaMushroom Body Kenyon Cells Generate Spontaneous Calcium Transients Mediated by PLTX-Sensitive Calcium Channels

Spontaneous calcium oscillations in mushroom bodies of late stage pupal and adult Drosophila brains have been implicated in memory consolidation during olfactory associative learning. This study explores the cellular mechanisms regulating calcium dynamics in Kenyon cells, principal neurons in mushroom bodies. Fura-2 imaging shows that Kenyon cells cultured from late stage Drosophila pupae generate spontaneous calcium transients in a cell autonomous fashion, at a frequency similar to calcium oscillations in vivo (10–20/h). The expression of calcium transients is up regulated during pupal development. Although the ability to generate transients is a property intrinsic to Kenyon cells, transients can be modulated by bath application of nicotine and GABA. Calcium transients are blocked, and baseline calcium levels reduced, by removal of external calcium, addition of cobalt, or addition of Plectreurys toxin (PLTX), an insect-specific calcium channel antagonist. Transients do not require calcium release from intracellular stores. Whole cell recordings reveal that the majority of voltage-gated calcium channels in Kenyon cells are PLTX-sensitive. Together these data show that influx of calcium through PLTX-sensitive voltage-gated calcium channels mediates spontaneous calcium transients and regulates basal calcium levels in cultured Kenyon cells. The data also suggest that these calcium transients represent cellular events underlying calcium oscillations in the intact mushroom bodies. However, spontaneous calcium transients are not unique to Kenyon cells as they are present in approximately 60% of all cultured central brain neurons. This suggests the calcium transients play a more general role in maturation or function of adult brain neurons.

Isolation and characterization of neural progenitor cells from post-mortem human cortex

Post-mortem human brain tissue represents a vast potential source of neural progenitor cells for use in basic research as well as therapeutic applications. Here we describe five human neural progenitor cell cultures derived from cortical tissue harvested from premature infants. Time-lapse videomicrography of the passaged cultures revealed them to be highly dynamic, with high motility and extensive, evanescent intercellular contacts. Karyotyping revealed normal chromosomal complements. Prior to differentiation, most of the cells were nestin, Sox2, vimentin, and/or GFAP positive, and a subpopulation was doublecortin positive. Multilineage potential of these cells was demonstrated after differentiation, with some subpopulations of cells expressing the neuronal markers beta-tubulin, MAP2ab, NeuN, FMRP, and Tau and others expressing the oligodendroglial marker O1. Still other cells expressed the classic glial marker glial fibrillary acidic protein (GFAP). RT-PCR confirmed nestin, SOX2, GFAP, and doublecortin expression and also showed epidermal growth factor receptor and nucleostemin expression during the expansion phase. Flow cytometry showed high levels of the neural stem cell markers CD133, CD44, CD81, CD184, CD90, and CD29. CD133 markedly decreased in high-passage, lineage-restricted cultures. Electrophysiological analysis after differentiation demonstrated that the majority of cells with neuronal morphology expressed voltage-gated sodium and potassium currents. These data suggest that post-mortem human brain tissue is an important source of neural progenitor cells that will be useful for analysis of neural differentiation and for transplantation studies.

Fast synaptic currents in Drosophila mushroom body Kenyon cells are mediated by alpha-bungarotoxin-sensitive nicotinic acetylcholine receptors and picrotoxin-sensitive GABA receptors

The mushroom bodies, bilaterally symmetric regions in the insect brain, play a critical role in olfactory associative learning. Genetic studies in Drosophila suggest that plasticity underlying acquisition and storage of memory occurs at synapses on the dendrites of mushroom body Kenyon cells (Dubnau et al., 2001). Additional exploration of the mechanisms governing synaptic plasticity contributing to these aspects of olfactory associative learning requires identification of the receptors that mediate fast synaptic transmission in Kenyon cells. To this end, we developed a culture system that supports the formation of excitatory and inhibitory synaptic connections between neurons harvested from the central brain region of late-stage Drosophila pupae. Mushroom body Kenyon cells are identified as small-diameter, green fluorescent protein-positive (GFP+) neurons in cultures from OK107-GAL4;UAS-GFP pupae. In GFP+ Kenyon cells, fast EPSCs are mediated by alpha-bungarotoxin-sensitive nicotinic acetylcholine receptors (nAChRs). The miniature EPSCs have rapid rise and decay kinetics and a broad, positively skewed amplitude distribution. Fast IPSCs are mediated by picrotoxin-sensitive chloride conducting GABA receptors. The miniature IPSCs also have a rapid rate of rise and decay and a broad amplitude distribution. The vast majority of spontaneous synaptic currents in the cultured Kenyon cells are mediated byalpha-bungarotoxin-sensitive nAChRs or picrotoxin-sensitive GABA receptors. Therefore, these receptors are also likely to mediate synaptic transmission in Kenyon cells in vivo and to contribute to plasticity during olfactory associative learning.

GABA receptors containing Rdl subunits mediate fast inhibitory synaptic transmission in Drosophila neurons

GABAergic inhibition in Drosophila, as in other insects and mammals, is important for regulation of activity in the CNS. However, the functional properties of synaptic GABA receptors in Drosophila have not been described. Here, we report that spontaneous GABAergic postsynaptic currents (sPSCs) in cultured embryonic Drosophila neurons are mediated by picrotoxin-sensitive chloride-conducting receptors. A rapid increase in spontaneous firing in response to bath application of picrotoxin demonstrates that these GABA receptors mediate inhibition in the neuronal networks formed in culture. Many of the spontaneous GABAergic synaptic currents are sodium action potential independent [miniature IPSCs (mIPSCs)] but are regulated by external calcium levels. The large variation in mIPSC frequency, amplitude, and kinetics properties between neurons suggests heterogeneity in GABA receptor number, location, and/or subtype. A decrease in the mean mIPSC decay time constant between 2 and 5 d, in the absence of a correlated change in rise time, demonstrates that the functional properties of the synaptic GABA receptors are regulated during maturation in vitro. Finally, neurons from the GABA receptor subunit mutant Rdl exhibit reduced sensitivity to picrotoxin blockade of the mIPSCs and resistance to picrotoxin-induced increases in spontaneous firing frequency. This demonstrates that Rdl containing GABA receptors play a direct role in mediating synaptic inhibition in Drosophila neural circuits formed in culture.

Cellular bases of behavioral plasticity: establishing and modifying synaptic circuits in the Drosophila genetic system

Genetic malleability and amenability to behavioral assays make Drosophila an attractive model for dissecting the molecular mechanisms of complex behaviors, such as learning and memory. At a cellular level, Drosophila has contributed a wealth of information on the mechanisms regulating membrane excitability and synapse formation, function, and plasticity. Until recently, however, these studies have relied almost exclusively on analyses of the peripheral neuromuscular junction, with a smaller body of work on neurons grown in primary culture. These experimental systems are, by themselves, clearly inadequate for assessing neuronal function at the many levels necessary for an understanding of behavioral regulation. The pressing need is for access to physiologically relevant neuronal circuits as they develop and are modified throughout life. In the past few years, progress has been made in developing experimental approaches to examine functional properties of identified populations of Drosophila central neurons, both in cell culture and in vivo. This review focuses on these exciting developments, which promise to rapidly expand the frontiers of functional cellular neurobiology studies in Drosophila. We discuss here the technical advances that have begun to reveal the excitability and synaptic transmission properties of central neurons in flies, and discuss how these studies promise to substantially increase our understanding of neuronal mechanisms underlying behavioral plasticity.

tipE regulates Na+-dependent repetitive firing in Drosophila neurons

The tipE gene, originally identified by a temperature-sensitive paralytic mutation in Drosophila, encodes a transmembrane protein that dramatically influences sodium channel expression in Xenopus oocytes. There is evidence that tipE also modulates sodium channel expression in the fly; however, its role in regulating neuronal excitability remains unclear. Here we report that the majority of neurons in both wild-type and tipE mutant (tipE-) embryo cultures fire sodium-dependent action potentials in response to depolarizing current injection. However, the percentage of tipE- neurons capable of firing repetitively during a sustained depolarization is significantly reduced. Expression of a tipE+ transgene, in tipE- neurons, restores repetitive firing to wild-type levels. Analysis of underlying currents reveals a slower rate of repolarization-dependent recovery of voltage-gated sodium currents during repeated activation in tipE- neurons. This phenotype is also rescued by expression of the tipE+ transgene. These data demonstrate that tipE regulates sodium-dependent repetitive firing and recovery of sodium currents during repeated activation. Furthermore, the duration of the interstimulus interval necessary to fire a second full-sized action potential is significantly longer in single- versus multiple-spiking transgenic neurons, suggesting that a slow rate of recovery of sodium currents contributes to the decrease in repetitive firing in tipE- neurons.

Agrin regulates neuronal responses to excitatory neurotransmitters in vitro and in vivo

Agrin mediates motor neuron-induced differentiation of the postsynaptic apparatus of the neuromuscular junction but its function in brain remains unknown. Here we report that expression of c-fos, induced by activation of nicotinic or glutamatergic receptors, was significantly lower in cortical neurons cultured from agrin-deficient mutant mouse embryos compared to wildtype. Agrin-deficient neurons also exhibited increased resistance to excitotoxic injury. Treatment with recombinant agrin restored glutamate-induced c-fos expression and excitotoxicity of the agrin-deficient neurons to near wild-type levels, confirming the agrin dependence of the phenotype. The observation that c-fos induction by activation of voltage-gated Ca2+ channels is also reduced in agrin-deficient neurons raises the possibility that agrin may play a wider role by regulating responses to Ca(2+)-mediated signals. Consistent with the decline in response of cultured mutant neurons to glutamate, decreases in kainic acid-induced seizure and mortality were observed in adult agrin heterozygous mice. Together, these data demonstrate that agrin plays an important role in defining neuronal responses to excitatory neurotransmitters both in vitro and in vivo.

cAMP-dependent plasticity at excitatory cholinergic synapses in Drosophila neurons: alterations in the memory mutant dunce

It is well known that cAMP signaling plays a role in regulating functional plasticity at central glutamatergic synapses. However, in the Drosophila CNS, where acetylcholine is thought to be a primary excitatory neurotransmitter, cellular changes in neuronal communication mediated by cAMP remain unexplored. In this study we examined the effects of elevated cAMP levels on fast excitatory cholinergic synaptic transmission in cultured embryonic Drosophila neurons. We report that chronic elevation in neuronal cAMP (in dunce neurons or wild-type neurons grown in db-cAMP) results in an increase in the frequency of cholinergic miniature EPSCs (mEPSCs). The absence of alterations in mEPSC amplitude or kinetics suggests that the locus of action is presynaptic. Furthermore, a brief exposure to db-cAMP induces two distinct changes in transmission at established cholinergic synapses in wild-type neurons: a short-term increase in the frequency of spontaneous action potential-dependent synaptic currents and a long-lasting, protein synthesis-dependent increase in the mEPSC frequency. A more persistent increase in cholinergic mEPSC frequency induced by repetitive, spaced db-cAMP exposure in wild-type neurons is absent in neurons from the memory mutant dunce. These data demonstrate that interneuronal excitatory cholinergic synapses in Drosophila, like central excitatory glutamatergic synapses in other species, are sites of cAMP-dependent plasticity. In addition, the alterations in dunce neurons suggest that cAMP-dependent plasticity at cholinergic synapses could mediate changes in neuronal communication that contribute to memory formation.

GABA(A) receptor-mediated miniature postsynaptic currents and alpha-subunit expression in developing cortical neurons

Previous studies have described maturational changes in GABAergic inhibitory synaptic transmission in the rodent somatosensory cortex during the early postnatal period. To determine whether alterations in the functional properties of synaptically localized GABA(A) receptors (GABA(A)Rs) contribute to development of inhibitory transmission, we used the whole cell recording technique to examine GABAergic miniature postsynaptic currents (mPSCs) in developing cortical neurons. Neurons harvested from somatosensory cortices of newborn mice showed a progressive, eightfold increase in GABAergic mPSC frequency during the first 4 wk of development in dissociated cell culture. A twofold decrease in the decay time of the GABAergic mPSCs, between 1 and 4 wk, demonstrates a functional change in the properties of GABA(A)Rs mediating synaptic transmission in cortical neurons during development in culture. A similar maturational profile observed in GABAergic mPSC frequency and decay time in cortical neurons developing in vivo (assessed in slices), suggests that these changes in synaptically localized GABA(A)Rs contribute to development of inhibition in the rodent neocortex. Pharmacological and reverse transcription-polymerase chain reaction (RT-PCR) studies were conducted to determine whether changes in subunit expression might contribute to the observed developmental alterations in synaptic GABA(A)Rs. Zolpidem (300 nM), a subunit-selective benzodiazepine agonist with high affinity for alpha1-subunits, caused a reversible slowing of the mPSC decay kinetics in cultured cortical neurons. Development was characterized by an increase in the potency of zolpidem in modulating the mPSC decay, suggesting a maturational increase in percentage of functionally active GABA(A)Rs containing alpha1 subunits. The relative expression of alpha1 versus alpha5 GABA(A)R subunit mRNA in cortical tissue, both in vivo and in vitro, also increased during this same period. Furthermore, single-cell RT-multiplex PCR analysis revealed more rapidly decaying mPSCs in individual neurons in which alpha1 versus alpha5 mRNA was amplified. Together these data suggest that changes in alpha-subunit composition of GABA(A)Rs contribute to the maturation of GABAergic mPSCs mediating inhibition in developing cortical neurons.

Fast excitatory synaptic transmission mediated by nicotinic acetylcholine receptors in Drosophila neurons

Difficulty in recording from single neurons in vivo has precluded functional analyses of transmission at central synapses in Drosophila, where the neurotransmitters and receptors mediating fast synaptic transmission have yet to be identified. Here we demonstrate that spontaneously active synaptic connections form between cultured neurons prepared from wild-type embryos and provide the first direct evidence that both acetylcholine and GABA mediate fast interneuronal synaptic transmission in Drosophila. The predominant type of fast excitatory transmission between cultured neurons is mediated by nicotinic acetylcholine receptors (nAChRs). Detailed analysis of cholinergic transmission reveals that spontaneous EPSCs (sEPSCs) are composed of both evoked and action potential-independent [miniature EPSC (mEPSC)] components. The mEPSCs are characterized by a broad, positively skewed amplitude histogram in which the variance is likely to reflect differences in the currents induced by single quanta. Biophysical characteristics of the cholinergic mEPSCs include a rapid rise time (0.6 msec) and decay (tau = 2 msec). Regulation of mEPSC frequency by external calcium and cobalt suggests that calcium influx through voltage-gated channels influences the probability of ACh release. In addition, brief depolarization of the cultures with KCl can induce a calcium-dependent increase in sEPSC frequency that persists for up to 3 hr after termination of the stimulus, illustrating one form of plasticity at these cholinergic synapses. These data demonstrate that cultured embryonic neurons, amenable to both genetic and biochemical manipulations, present a unique opportunity to define genes/signal transduction cascades involved in functional regulation of fast excitatory transmission at interneuronal cholinergic synapses in Drosophila.

Formation of functional synaptic connections between cultured cortical neurons from agrin-deficient mice

Numerous studies suggest that the extracellular matrix protein agrin directs the formation of the postsynaptic apparatus at the neuromuscular junction (NMJ). Strong support for this hypothesis comes from the observation that the high density of acetylcholine receptors (AChR) normally present at the neuromuscular junction fails to form in muscle of embryonic agrin mutant mice. Agrin is expressed by many populations of neurons in the central nervous system (CNS), suggesting that this molecule may also play a role in neuron-neuron synapse formation. To test this hypothesis, we examined synapse formation between cultured cortical neurons isolated from agrin-deficient mouse embryos. Our data show that glutamate receptors accumulate at synaptic sites on agrin-deficient neurons. Moreover, electrophysiological analysis demonstrates that functional glutamatergic and gamma-aminobutyric acid (GABA)ergic synapses form between mutant neurons. The frequency and amplitude of miniature postsynaptic glutamatergic and GABAergic currents are similar in mutant and age-matched wild-type neurons during the first 3 weeks in culture. These results demonstrate that neuron-specific agrin is not required for formation and early development of functional synaptic contacts between CNS neurons, and suggest that mechanisms of interneuronal synaptogenesis are distinct from those regulating synapse formation at the neuromuscular junction.

Regulation of alpha7 nicotinic acetylcholine receptors in mouse somatosensory cortex following whisker removal at birth

Previous studies in postnatal mouse demonstrating high levels of alpha7 nicotinic acetylcholine receptors on layer IV somatosensory cortical neurons coincident with the onset of functional synaptic transmission led us to investigate whether the number and/or the localization of these receptors could be regulated by activity. Accordingly, we examined alpha-bungarotoxin binding in mouse somatosensory cortex following removal of all of the vibrissae on one side of the face, either by vibrissal follicle cauterization or daily plucking beginning on the day of birth. Following vibrissa plucking, the levels of [125I]alpha-bungarotoxin binding on postnatal day 6 were significantly higher (23 +/- 7%) in the denervated cortex (contralateral to the peripheral manipulation) than the intact cortex. Cauterization also resulted in significantly higher (14 +/- 3%) [125I]alpha-bungarotoxin binding in the contralateral vs. the ipsilateral cortex. In contrast, there was no difference in [125I]alpha-bungarotoxin binding in the left and right cortices of unoperated control animals. At postnatal day 14, levels of [125I]alpha-bungarotoxin binding in layer IV were very low in control animals as well as in animals subjected to whisker plucking or cautery. These findings suggest that reducing activity in the somatosensory pathway regulates the density of alpha7 nicotinic acetylcholine receptors during the first postnatal week. However, the normal decrease in receptor density that is seen during the second postnatal week of development proceeds despite altered sensory activity.

Heterogeneous expression of multiple putative patterning genes by single cells from the chick hindbrain

The metameric organization of the vertebrate hindbrain into rhombomeres appears to result from the patterned expression of several transcription factors and putative signaling molecules. We have applied a refined single-cell reverse transcription-polymerase chain reaction strategy to examine the molecular logic proposed to pattern the hindbrain at the single-cell level. This technique allows analysis of the concurrent expression of several genes within an individual cell at higher sensitivity than by in situ hybridization. Our results demonstrate that cells in rhombomere (r) 4 and r5 are heterogeneous in their expression of Hoxa-3, Hoxb-2, Sek-1, and Krox-20, suggesting that single cells are dynamically regulating their rhombomere-specific gene-expression profiles. Furthermore, the strong correlation between Sek-1 and Krox-20 expression at stage 12 was greatly diminished by stage 16, suggesting that the proposed interdependence of these two genes is present only at early stages of hindbrain development.

Agrin gene expression in mouse somatosensory cortical neurons during development in vivo and in cell culture

Agrin is an extracellular matrix protein involved in the formation of the postsynaptic apparatus of the neuromuscular junction. In addition to spinal motor neurons, agrin is expressed by many other neuronal populations throughout the nervous system. Agrin's role outside of the neuromuscular junction, however, is poorly understood. Here we use the polymerase chain reaction to examine expression and alternative splicing of agrin in mouse somatosensory cortex during early postnatal development in vivo and in dissociated cell culture. Peak levels of agrin gene expression in developing cortex coincide with ingrowth of thalamic afferent fibres and formation of thalamocortical and intracortical synapses. Analysis of alternatively spliced agrin messenger RNA variants shows that greater than 95% of all agrin in developing and adult somatosensory cortex originates in neurons, including isoforms that have little or no activity in acetylcholine receptor aggregation assays. The levels of expression of "active" and "inactive" isoforms, however, are regulated during development. A similar pattern of agrin gene expression is also observed during a period when new synapses are being formed between somatosensory neurons growing in dissociated cell culture. Changes in agrin gene expression, observed both in vivo and in vitro, are consistent with a role for agrin in synapse formation in the central nervous system.

Differential expression of K4-AP currents and Kv3.1 potassium channel transcripts in cortical neurons that develop distinct firing phenotypes

Maturation of electrical excitability during early postnatal development is critical to formation of functional neural circuitry in the mammalian neocortex. Little is known, however, about the changes in gene expression underlying the development of firing properties that characterize different classes of cortical neurons. Here we describe the development of cortical neurons with two distinct firing phenotypes, regular-spiking (RS) and fast-spiking (FS), that appear to emerge from a population of immature multiple-spiking (IMS) neurons during the first two postnatal weeks, both in vivo (within layer IV) and in vitro. We report the expression of a slowly inactivating, 4-AP-sensitive potassium current (K4-AP) at significantly higher density in FS compared with RS neurons. The same current is expressed at intermediate levels in IMS neurons. The kinetic, voltage-dependent, and pharmacological properties of the K4-AP current are similar to those observed by heterologous expression of Kv3.1 potassium channel mRNA. Single-cell RT-PCR analysis demonstrates that PCR products representing Kv3.1 transcripts are amplified more frequently from FS than RS neurons, with an intermediate frequency of Kv3.1 detection in neurons with immature firing properties. Taken together, these data suggest that the Kv3.1 gene encodes the K4-AP current and that expression of this gene is regulated in a cell-specific manner during development. Analysis of the effects of 4-AP on firing properties suggests that the K4-AP current is important for rapid action potential repolarization, fast after-hyperpolarization, brief refractory period, and high firing frequency characteristic of FS GABAergic interneurons.

Single-cell analysis of gene expression in the nervous system. Measurements at the edge of chaos

The characteristic functions of tissues and organs result from the integrated activity of individual cells. Nowhere is this more evident than in the nervous system, where the activities of single neurons communicating via electrical and chemical signals mediate complex functions, such as learning and memory. The past decade has seen an explosion in the identification of genes encoding proteins, such as voltage-gated channels and neurotransmitter receptors, responsible for neuronal excitability. These studies have highlighted the fact that even within a neuroanatomically defined region, the coexistence of multiple cell types makes it difficult, if not impossible, to correlate patterns of gene expression with function. The recent development of techniques sensitive enough to study gene expression at the single-cell level promises to break this bottleneck to our further understanding. Using examples taken from our own laboratories and the work of others, we review these techniques, their application, and discuss some of the difficulties associated with the interpretation of the data.

Functional GABAergic synaptic connection in neonatal mouse barrel cortex

Intracortical inhibition is crucial to proper functioning of the mature neocortex, yet, paradoxically, is reported to be rare or absent in the neonatal animal. We reexamined this issue by recording whole-cell postsynaptic currents (PSCs) of barrel cortex neurons in thalamocortical brain slices from neonatal mice. Monosynaptic, excitatory thalamocortical responses were elicited in layers V/VI neurons as early as postnatal day 0 (P0, the first 24 hr after birth) and in presumptive layer IV as early as P2. At very low stimulation frequencies, the monosynaptic response was invariably followed by a prolonged (up to 1 sec) synaptic barrage, which fatigued at stimulus repetition rates of 2/min or higher. This barrage consisted of postsynaptic responses to spiking activity in neighboring cortical cells, because (1) it could also be evoked by intracortical stimulation in coronal slices and (2) it was abolished by antagonists to NMDA receptors (NMDARs), even when NMDARs on the recorded cell were under a voltage-dependent block. Some of the larger polysynaptic events changed polarity at a negative reversal potential and were blocked by GABAA receptor (GABAAR) antagonists, with a concurrent enhancement of the extracellular field potential, indicating that they were GABAAR- mediated, CI-dependent inhibitory PSCs (IPSCs). We conclude that a network of functional intracortical GABAAR-mediated synaptic connections exists from the earliest postnatal ages, although it gives rise to responses that differ from mature IPSCs in reversal potential and latency.