Brain awareness week and beyond: encouraging the next generation

The field of neuroscience is generating increased public appetite for information about exciting brain research and discoveries. As stewards of the discipline, together with FUN and others, the Society for Neuroscience (SfN) embraces public outreach and education as essential to its mission of promoting understanding of the brain and nervous system. The Society looks to its members, particularly the younger generation of neuroscientists, to inspire, inform and engage citizens of all ages, and most importantly our youth, in this important endeavor. Here we review SfN programs and resources that support public outreach efforts to inform, educate and tell the story of neuroscience. We describe the important role the Brain Awareness campaign has played in achieving this goal and highlight opportunities for FUN members and students to contribute to this growing effort. We discuss specific programs that provide additional opportunities for neuroscientists to get involved with K-12 teachers and students in ways that inspire youth to pursue further studies and possible careers in science. We draw attention to SfN resources that support outreach to broader audiences. Through ongoing partnerships such as that between SfN and FUN, the neuroscience community is well positioned to pursue novel approaches and resources, including harnessing the power of the Internet. These efforts will increase science literacy among our citizens and garner more robust support for scientific research.

Illumination controls differentiation of dopamine neurons regulating behaviour

Specification of the appropriate neurotransmitter is a crucial step in neuronal differentiation because it enables signalling among populations of neurons. Experimental manipulations demonstrate that both autonomous and activity-dependent genetic programs contribute to this process during development, but whether natural environmental stimuli specify transmitter expression in a neuronal population is unknown. We investigated neurons of the ventral suprachiasmatic nucleus that regulate neuroendocrine pituitary function in response to light in teleosts, amphibia and primates. Here we show that altering light exposure, which changes the sensory input to the circuit controlling adaptation of skin pigmentation to background, changes the number of neurons expressing dopamine in larvae of the amphibian Xenopus laevis in a circuit-specific and activity-dependent manner. Neurons newly expressing dopamine then regulate changes in camouflage colouration in response to illumination. Thus, physiological activity alters the numbers of behaviourally relevant amine-transmitter-expressing neurons in the brain at postembryonic stages of development. The results may be pertinent to changes in cognitive states that are regulated by biogenic amines.

Implications of activity-dependent neurotransmitter-receptor matching

Electrical activity has numerous roles in early neuronal development. Calcium transients generated at low frequencies regulate neural induction and neuronal proliferation, migration and differentiation. Recent work demonstrates that these signals participate in specification of the transmitters expressed in different classes of neurons. Matching of postsynaptic receptor expression with the novel expression of transmitters ensues. These findings have intriguing implications for development, mature function and evolution of the nervous system.

Electrical activity in early neuronal development

The construction of the brain during embryonic development was thought to be largely independent of its electrical activity. In this view, proliferation, migration and differentiation of neurons are driven entirely by genetic programs and activity is important only at later stages in refinement of connections. However, recent findings demonstrate that activity plays essential roles in early development of the nervous system. Activity has similar roles in the incorporation of newly born neurons in the adult nervous system, suggesting that there are general rules underlying activity-dependent development. The extensive involvement of activity makes it likely that it is required at all developmental stages as a necessary partner with genetic programs.

Activity-dependent neurotransmitter-receptor matching at the neuromuscular junction

Signaling in the nervous system requires matching of neurotransmitter receptors with cognate neurotransmitters at synapses. The vertebrate neuromuscular junction is the best studied cholinergic synapse, but the mechanisms by which acetylcholine is matched with acetylcholine receptors are not fully understood. Because alterations in neuronal calcium spike activity alter transmitter specification in embryonic spinal neurons, we hypothesized that receptor expression in postsynaptic cells follows changes in transmitter expression to achieve this specific match. We find that embryonic vertebrate striated muscle cells normally express receptors for glutamate, GABA, and glycine as well as for acetylcholine. As maturation progresses, acetylcholine receptor expression prevails. Receptor selection is altered when early neuronal calcium-dependent activity is perturbed, and remaining receptor populations parallel changes in transmitter phenotype. In these cases, glutamatergic, GABAergic, and glycinergic synaptic currents are recorded from muscle cells, demonstrating that activity regulates matching of transmitters and their receptors in the assembly of functional synapses.

Second messenger pas de deux: the coordinated dance between calcium and cAMP

Dynamic calcium signaling is a well-established precept in biology. Different cell types exhibit spontaneous as well as stimulus-triggered transient changes in the concentration of intracellular calcium. Does this behavior extend to other second messengers? Optical dissection of various signal transduction pathways with fluorescent reporter molecules that enable visualization of changes in concentration of other second messengers is well under way. Recent research using technologically refined probes provides improved temporal and spatial resolution of adenosine 3',5'-monophosphate (cAMP) dynamics to generate insights into the bidirectional interplay between intracellular fluctuations of cAMP and calcium. cAMP oscillations are generated in response to hormones, and cells can recognize and differentially respond to transient versus sustained changes in this second messenger. Second messenger reporters are now available to track multiple players and so provide a dynamic picture of signaling networks.

Local calcium transients contribute to disappearance of pFAK, focal complex removal and deadhesion of neuronal growth cones and fibroblasts

Cell adhesion is crucial for migration of cells during development, and cell-substrate adhesion of motile cells is accomplished through the formation and removal of focal complexes that are sites of cell-substrate contact. Because Ca2+ signaling regulates the rate of axon outgrowth and growth cone turning, we investigated the potential role of Ca2+ in focal complex dynamics. We describe a novel class of localized, spontaneous transient elevations of cytosolic Ca2+ observed both in Xenopus neuronal growth cones and fibroblasts that are 2-6 mum in spatial extent and 2-4 s in duration. They are distributed throughout growth cone lamellipodia and at the periphery of fibroblast pseudopodia, which are regions of high motility. In both cell types, these Ca2+ transients lead to disappearance of phosphorylated focal adhesion kinase (pFAK) and deadhesion from the substrate as assessed by confocal and internal reflection microscopy, respectively. The loss of pFAK is inhibited by cyclosporin A, suggesting that these Ca2+ transients exert their effects via calcineurin. These results identify an intrinsic mechanism for local cell detachment that may be modulated by agents that regulate motility.

Homeostatic activity-dependent paradigm for neurotransmitter specification

Calcium-signaling plays a central role in specification of the chemical transmitters neurons express, adjusting the numbers of cells that express excitatory and inhibitory transmitters as if to achieve homeostatic regulation of excitability. Here we review the extent to which this activity-dependent regulation is observed for a range of different transmitters. Strikingly the homeostatic paradigm is observed both for classical and for peptide transmitters and in mature as well as in embryonic nervous systems. Transmitter homeostasis adds another dimension to homeostatic regulation of function in the nervous system that includes regulation of levels of voltage-gated ion channels, densities of neurotransmitter receptors, and synapse numbers and strength.

Orchestrating neuronal differentiation: patterns of Ca2+ spikes specify transmitter choice

Appropriate specification of neurotransmitters is a key feature of neuronal network assembly. There is much evidence that genetic programs are responsible for this aspect of cell fate and neuronal differentiation. Are there additional ways in which these processes are shaped? Recent findings demonstrate that altering patterned Ca(2+) spike activity that is spontaneously generated by different classes of embryonic spinal neurons in vivo changes expression of neurotransmitters in a homeostatic manner, as if to achieve a constant level of excitation. Activity-dependent changes in presynaptic transmitter expression pose a matching problem: are there corresponding changes in postsynaptic transmitter receptor expression, or are axons rerouted to novel targets with which functional synapses can be formed?

Activity-dependent homeostatic specification of transmitter expression in embryonic neurons

Neurotransmitters are essential for interneuronal signalling, and the specification of appropriate transmitters in differentiating neurons has been related to intrinsic neuronal identity and to extrinsic signalling proteins. Here we show that altering the distinct patterns of Ca2+ spike activity spontaneously generated by different classes of embryonic spinal neurons in vivo changes the transmitter that neurons express without affecting the expression of markers of cell identity. Regulation seems to be homeostatic: suppression of activity leads to an increased number of neurons expressing excitatory transmitters and a decreased number of neurons expressing inhibitory transmitters; the reverse occurs when activity is enhanced. The imposition of specific spike frequencies in vitro does not affect labels of cell identity but again specifies the expression of transmitters that are inappropriate for the markers they express, during an early critical period. The results identify a new role of patterned activity in development of the central nervous system.

Calcium transients regulate patterned actin assembly during myofibrillogenesis

The highly ordered arrangement of sarcomeric myosin during striated muscle development requires spontaneous calcium (Ca(2+)) transients. Here, we show that blocking transients also compromises patterned assembly of actin thin filaments, titin, and capZ. Because a conserved temporal assembly pattern has been described for these proteins, selective inhibitors of either thick or thin filament formation were used to determine their relative temporal interdependencies. For example, inhibition of myosin light chain kinase (MLCK) by application of a specific inhibitory peptide or phorbol myistate acetate (PMA) disrupts myosin assembly without significantly affecting formation of actin bands. The MLCK inhibitor ML-7, however, disrupted actin as well as myosin. Surprisingly, agents that interfere with actin dynamics, such as cytochalasin D, produced only minor organizational disruptions in actin, capZ, and titin staining. However, cytochalasin D and other actin disrupting compounds significantly perturbed myosin organization. The results indicate that (1) Ca(2+) transients regulate one or more of the earliest steps in sarcomere formation, (2) mature actin filaments can assemble independently of myosin band formation, and (3) myosin thick filament assembly is extremely sensitive to disruption of either the actin or titin filament systems.

Dynamic interactions of cyclic AMP transients and spontaneous Ca(2+) spikes

Transient increases of intracellular Ca(2+) drive many cellular processes, ranging from membrane channel kinetics to transcriptional regulation, and links of Ca(2+) to other second messengers should activate signalling networks. However, real-time kinetic interactions have been difficult to investigate. Here we report observations of spontaneous increases in concentration of cyclic AMP (cAMP) in embryonic spinal neurons, and their dynamic interactions with Ca(2+) oscillations. Blocking the production of these cAMP transients decreases the intrinsic frequency of spontaneous Ca(2+) spikes, whereas inducing cAMP increases causes spike frequency to increase. Transients of cAMP in turn are absent when Ca(2+) spikes are blocked, and are generated only in response to specific patterns of stimulated spikes that mimic endogenous Ca(2+) kinetics. We present a mathematical model of Ca(2+)-cAMP reciprocity that generates the slow cAMP oscillations and reproduces the dynamics of Ca(2+)-cAMP interactions observed experimentally. The model predicts that this module of coupled second messengers is tuned to optimize production of cAMP transients, and that simultaneous stimulation of Ca(2+) and cAMP systems produces distinct temporal patterns of oscillations of both messengers. Our findings may prove useful in the investigation of the regulation of gene expression by second-messenger transients.

Outside and in: development of neuronal excitability

Investigation of the development of excitability has revealed that cells are often specialized at early stages to generate Ca(2+) transients. Studies of excitability have converged on the central role of Ca(2+) and K(+) channels in the plasmalemma that regulate Ca(2+) influx and have identified critical functions for receptor-activated channels in the endoplasmic reticulum that allow efflux of Ca(2+) from intracellular stores. The parallels between excitability in these two locations motivate future work, because comparison of their properties identifies shared attributes.

Adaptation in the chemotactic guidance of nerve growth cones

Pathfinding by growing axons in the developing nervous system may be guided by gradients of extracellular guidance factors. Analogous to the process of chemotaxis in microorganisms, we found that axonal growth cones of cultured Xenopus spinal neurons exhibit adaptation during chemotactic migration, undergoing consecutive phases of desensitization and resensitization in the presence of increasing basal concentrations of the guidance factor netrin-1 or brain-derived neurotrophic factor. The desensitization is specific to the guidance factor and is accompanied by a reduction of Ca2+ signalling, whereas resensitization requires activation of mitogen-associated protein kinase and local protein synthesis. Such adaptive behaviour allows the growth cone to re-adjust its sensitivity over a wide range of concentrations of the guidance factor, an essential feature for long-range chemotaxis.

Activity-dependent neuronal differentiation prior to synapse formation: the functions of calcium transients

Spinal cord neurons become excitable prior to synapse formation, and generate spontaneous calcium transients that regulate aspects of their differentiation before neuronal networks are established. Calcium spikes, generated by calcium-dependent action potentials and calcium-induced calcium release (CICR), regulate transcription. Growth cone calcium transients, produced by calcium influx through unidentified channels that triggers CICR, control the rate of axon outgrowth in response to environmental cues. Filopodial calcium transients, generated by calcium influx through channels activated by beta1 integrins, signal information about the molecular identity of the substrate and regulate growth cone turning. All three classes of calcium transients appear to use a frequency code to implement their effects. Oscillations of second messengers in embryonic neurons and perhaps more generally in other differentiating cells may behave like a kinetic quilt, demonstrating patchy fluctuations in concentrations that orchestrate the complex processes of development.

Filopodial calcium transients promote substrate-dependent growth cone turning

Filopodia that extend from neuronal growth cones sample the environment for extracellular guidance cues, but the signals they transmit to growth cones are unknown. Filopodia were observed generating localized transient elevations of intracellular calcium ([Ca2+]i) that propagate back to the growth cone and stimulate global Ca2+ elevations. The frequency of filopodial Ca2+ transients was substrate-dependent and may be due in part to influx of Ca2+ through channels activated by integrin receptors. These transients slowed neurite outgrowth by reducing filopodial motility and promoted turning when stimulated differentially within filopodia on one side of the growth cone. These rapid signals appear to serve both as autonomous regulators of filopodial movement and as frequency-coded signals integrated within the growth cone and could be a common signaling process for many motile cells.

Differentiation of electrical excitability in motoneurons

Investigation of the differentiation of electrical properties of motoneurons has been stimulated by the importance of these neurons for embryonic behavior and facilitated by their experimental accessibility. In this review, we examine the development of different patterns of excitability and their functions, and discuss the emergence of repetitive firing and localization of ion channels in axons and dendrites. Finally, we summarize studies of the role of extrinsic factors in differentiation. These changes associated with differentiation of young motoneurons may presage those occurring later in the context of plasticity in the mature nervous system.

Specific frequencies of spontaneous Ca2+ transients upregulate GAD 67 transcripts in embryonic spinal neurons

Spontaneous Ca2+ transients expressed prior to synaptogenesis regulate the developmental appearance of GABA in cultured Xenopus spinal neurons. We find that glutamic acid decarboxylase (GAD) immunoreactivity is also Ca(2+)-dependent and parallels the appearance of GABA. We show that xGAD 67 transcripts first appear in the embryonic spinal cord during the period in which these Ca2+ spikes are generated, in a pattern that is temporally and spatially appropriate to account for differentiation of GABAergic interneurons. RNase protection and competitive quantitative RT-PCR demonstrate that transcript levels are approximately threefold greater when neurons are cultured in the presence of extracellular Ca2+ that permits generation of transients than when cultured in its absence. The frequency of spontaneous Ca2+ spikes plays a crucial role in the regulation of transcripts, since reimposition of Ca2+ transients at the frequency generated in cultured neurons rescues normal expression. We conclude that naturally occurring low frequencies of these Ca2+ transients regulate levels of xGAD 67 mRNA in differentiating neurons.

Coding of neuronal differentiation by calcium transients

Excitability has long been recognized as the basis for rapid signaling in the mature nervous system, but roles of channels and receptors in controlling slower processes of differentiation have been identified only more recently. Voltage-dependent and transmitter-activated channels are often expressed at early stages of development prior to synaptogenesis, and allow influx of Ca(2+). Here we examine the functions of spontaneous transient elevations of intracellular Ca(2+) in embryonic neurons. These Ca(2+) transients abruptly raise levels of Ca(2+) as much as tenfold, for brief periods, repeatedly, and can be highly localized. Like cloudbursts on the developing landscape, Ca(2+) transients modulate growth and stimulate differentiation, in a frequency-dependent manner, probably by changes in phosphorylation or proteolysis of regulatory and structural proteins in local regions. We review the mechanisms by which Ca(2+) transients are generated and their effects in regulating motility via the cytoskeleton and differentiation via transcription.