Formation and modulation of chemical connections: evoked acetylcholine release from growth cones and neurites of specific identified neurons

Uncovering the Cellular and Molecular Mechanisms of Synapse Formation and Functional Specificity Using Central Neurons of Lymnaea stagnalis

All functions of the nervous system are contingent upon the precise organization of neuronal connections that are initially patterned during development, and then continually modified throughout life. Determining the mechanisms that specify the formation and functional modulation of synaptic circuitry are critical to advancing both our fundamental understanding of the nervous system as well as the various neurodevelopmental, neurological, neuropsychiatric, and neurodegenerative disorders that are met in clinical practice when these processes go awry. Defining the cellular and molecular mechanisms underlying nervous system development, function, and pathology has proven challenging, due mainly to the complexity of the vertebrate brain. Simple model system approaches with invertebrate preparations, on the other hand, have played pivotal roles in elucidating the fundamental mechanisms underlying the formation and plasticity of individual synapses, and the contributions of individual neurons and their synaptic connections that underlie a variety of behaviors, and learning and memory. In this Review, we discuss the experimental utility of the invertebrate mollusc Lymnaea stagnalis, with a particular emphasis on in vitro cell culture, semi-intact and in vivo preparations, which enable molecular and electrophysiological identification of the cellular and molecular mechanisms governing the formation, plasticity, and specificity of individual synapses at a single-neuron or single-synapse resolution.

Acetylcholine elongates neuronal growth cone filopodia via activation of nicotinic acetylcholine receptors

In addition to acting as a classical neurotransmitter in synaptic transmission, acetylcholine (ACh) has been shown to play a role in axonal growth and growth cone guidance. What is not well understood is how ACh acts on growth cones to affect growth cone filopodia, structures known to be important for neuronal pathfinding. We addressed this question using an identified neuron (B5) from the buccal ganglion of the pond snail Helisoma trivolvis in cell culture. ACh treatment caused pronounced filopodial elongation within minutes, an effect that required calcium influx and resulted in the elevation of the intracellular calcium concentration ([Ca]i ). Whole-cell patch clamp recordings showed that ACh caused a reduction in input resistance, a depolarization of the membrane potential, and an increase in firing frequency in B5 neurons. These effects were mediated via the activation of nicotinic acetylcholine receptors (nAChRs), as the nAChR agonist dimethylphenylpiperazinium (DMPP) mimicked the effects of ACh on filopodial elongation, [Ca]i elevation, and changes in electrical activity. Moreover, the nAChR antagonist tubucurarine blocked all DMPP-induced effects. Lastly, ACh acted locally at the growth cone, because growth cones that were physically isolated from their parent neuron responded to ACh by filopodial elongation with a similar time course as growth cones that remained connected to their parent neuron. Our data revealed a critical role for ACh as a modulator of growth cone filopodial dynamics. ACh signaling was mediated via nAChRs and resulted in Ca influx, which, in turn, caused filopodial elongation.

Synaptic functions of invertebrate varicosities: what molecular mechanisms lie beneath

In mammalian brain, the cellular and molecular events occurring in both synapse formation and plasticity are difficult to study due to the large number of factors involved in these processes and because the contribution of each component is not well defined. Invertebrates, such as Drosophila, Aplysia, Helix, Lymnaea, and Helisoma, have proven to be useful models for studying synaptic assembly and elementary forms of learning. Simple nervous system, cellular accessibility, and genetic simplicity are some examples of the invertebrate advantages that allowed to improve our knowledge about evolutionary neuronal conserved mechanisms. In this paper, we present an overview of progresses that elucidates cellular and molecular mechanisms underlying synaptogenesis and synapse plasticity in invertebrate varicosities and their validation in vertebrates. In particular, the role of invertebrate synapsin in the formation of presynaptic terminals and the cell-to-cell interactions that induce specific structural and functional changes in their respective targets will be analyzed.

Molluscan neurons in culture: shedding light on synapse formation and plasticity

From genes to behaviour, the simple model system approach has played many pivotal roles in deciphering nervous system function in both invertebrates and vertebrates. However, with the advent of sophisticated imaging and recording techniques enabling the direct investigation of single vertebrate neurons, the utility of simple invertebrate organisms as model systems has been put to question. To address this subject meaningfully and comprehensively, we first review the contributions made by invertebrates in the field of neuroscience over the years, paving the way for similar breakthroughs in higher animals. In particular, we focus on molluscan (Lymnaea, Aplysia, and Helisoma) and leech (Hirudo) models and the pivotal roles they have played in elucidating mechanisms of synapse formation and plasticity. While the ultimate goal in neuroscience is to understand the workings of the human brain in both its normal and diseased states, the sheer complexity of most vertebrate models still makes it difficult to define the underlying principles of nervous system function. Investigators have thus turned to invertebrate models, which are unique with respect to their simple nervous systems that are endowed with a finite number of large, individually identifiable neurons of known function. We start off by discussing in vivo and semi-intact preparations, regarding their amenability to simple circuit analysis. Despite the 'simplicity' of invertebrate nervous systems however, it is still difficult to study individual synaptic connections in detail. We therefore emphasize in the next section, the utility of studying identified invertebrate neurons in vitro, to directly examine the development, specificity, and plasticity of synaptic connections in a well-defined environment, at a resolution that it is still unapproachable in the intact brain. We conclude with a discussion of the future of invertebrates in neuroscience in elucidating mechanisms of neurological disease and developing neuron-silicon interfaces.

Regulation and restoration of motoneuronal synaptic transmission during neuromuscular regeneration in the pulmonate snail Helisoma trivolvis

Regeneration of motor systems involves reestablishment of central control networks, reinnervation of muscle targets by motoneurons, and reconnection of neuromodulatory circuits. Still, how these processes are integrated as motor function is restored during regeneration remains ill defined. Here, we examined the mechanisms underlying motoneuronal regeneration of neuromuscular synapses related to feeding movements in the pulmonate snail Helisoma trivolvis. Neurons B19 and B110, although activated during different phases of the feeding pattern, innervate similar sets of muscles. However, the percentage of muscle fibers innervated, the efficacy of excitatory junction potentials, and the strength of muscle contractions were different for each cell's specific connections. After peripheral nerve crush, a sequence of transient electrical and chemical connections formed centrally within the buccal ganglia. Neuromuscular synapse regeneration involved a three-phase process: the emergence of spontaneous synaptic transmission (P1), the acquisition of evoked potentials of weak efficacy (P2), and the establishment of functional reinnervation (P3). Differential synaptic efficacy at muscle contacts was recapitulated in cell culture. Differences in motoneuronal presynaptic properties (i.e., quantal content) were the basis of disparate neuromuscular synapse function, suggesting a role for retrograde target influences. We propose a homeostatic model of molluscan motor system regeneration. This model has three restoration events: (1) transient central synaptogenesis during axonal outgrowth, (2) intermotoneuronal inhibitory synaptogenesis during initial neuromuscular synapse formation, and (3) target-dependent regulation of neuromuscular junction formation.

Diversity and abundance: the basic properties of neuropeptide action in molluscs

Neuropeptides, the most diverse group of signaling molecules, are responsible for regulating a variety of cellular and behavioral processes in all vertebrate and invertebrate animals. The role played by peptide signals in information processing is fundamentally different from that of conventional neurotransmitters. Neuropeptides may act as neurotransmitters or neuromodulators and are released at either synaptic or non-synaptic sites. Peptide signals control developmental processes, drive specific behaviors or contribute to the mechanisms of learning and memory storage. Co-transmission within or across peptide families, and between peptide and non-peptide signaling molecules, is common; this ensures the great versatility of their action. How these tasks are fulfilled when multiple neuropeptides are released has become an important topic for peptide research. Although our knowledge concerning the physiological and behavioral roles of most of the neuropeptides isolated from molluscs is incomplete, this article provides examples to address the complexity of peptide signaling.

A novel approach reveals temporal patterns of synaptogenesis between the isolated growth cones of Lymnaea neurons

All brain functions, ranging from motor behaviour to cognition, depend on precise developmental patterns of synapse formation between the growth cones of both pre- and postsynaptic neurons. While the molecular evidence for the presence of 'pre-assembled' elements of synaptic machinery prior to physical contact is beginning to emerge, the precise timing of functional synaptogenesis between the growth cones has not yet been defined. Moreover, it is unclear whether an initial assembly of various synaptic molecules located at the extrasomal regions (e.g. growth cones) can indeed result in fully mature and consolidated synapses in the absence of somata signalling. Such evidence is difficult to obtain both in vivo and in vitro because the extrasomal sites are often challenging, if not impossible, to access for electrophysiological analysis. Here we demonstrate a novel approach to precisely define various steps underlying synapse formation between the isolated growth cones of individually identifiable pre- and postsynaptic neurons from the mollusc Lymnaea stagnalis. We show for the first time that isolated growth cones transformed into 'growth balls' have an innate propensity to develop specific and multiple synapses within minutes of physical contact. We also demonstrate that a prior 'synaptic history' primes the presynaptic growth ball to form synapses quicker with subsequent partners. This is the first demonstration that isolated Lymnaea growth cones have the necessary machinery to form functional synapses.

Rapid assembly of functional presynaptic boutons triggered by adhesive contacts

CNS synapse assembly typically follows after stable contacts between "appropriate" axonal and dendritic membranes are made. We show that presynaptic boutons selectively form de novo following neuronal fiber adhesion to beads coated with poly-d-lysine (PDL), an artificial cationic polypeptide. As demonstrated by atomic force and live confocal microscopy, functional presynaptic boutons self-assemble as rapidly as 1 h after bead contact, and are found to contain a variety of proteins characteristic of presynaptic endings. Interestingly, presynaptic compartment assembly does not depend on the presence of a biological postsynaptic membrane surface. Rather, heparan sulfate proteoglycans, including syndecan-2, as well as others possibly adsorbed onto the bead matrix or expressed on the axon surface, are required for assembly to proceed by a mechanism dependent on the dynamic reorganization of F-actin. Our results indicate that certain (but not all) nonspecific cationic molecules like PDL, with presumably electrostatically mediated adhesive properties, can effectively bypass cognate and natural postsynaptic ligands to trigger presynaptic assembly in the absence of specific target recognition. In contrast, we find that postsynaptic compartment assembly depends on the prior presence of a mature presynaptic ending.

Synapse formation and plasticity: the roles of local protein synthesis

From simple reflexes in lower animals to complex motor patterns and learning and memory in higher animals, all nervous system functions hinge upon fundamental, albeit specialized, neuronal units termed synapses. The term synapse denotes the structural and functional building block upon which pivots the enormous information-processing capabilities of our brain. It is the neuronal communications through synapses that ultimately determine who we are and how we react and adapt to our ever-changing environment. Synapses are not only the epic center of our intellect, but they also control myriad traits of our personality, ranging from sinfulness to sainthood (see, e.g., Hamer 2004). Simply put-we are what our synapses deem us to be (LeDoux 2003)! Notwithstanding the reasoning that some aspects of the synaptic arrangement may be genetically hardwired, an overwhelming body of knowledge does nevertheless provide ample plausible evidence that synapses are highly plastic entities undergoing rapid adaptive changes throughout life. It is this adaptability that endows our brain with its "uncanny" powers.

Electrical synapse formation disrupts calcium-dependent exocytosis, but not vesicle mobilization

Electrical coupling exists prior to the onset of chemical connectivity at many developing and regenerating synapses. At cholinergic synapses in vitro, trophic factors facilitated the formation of electrical synapses and interfered with functional neurotransmitter release in response to photolytic elevations of intracellular calcium. In contrast, neurons lacking trophic factor induction and electrical coupling possessed flash-evoked transmitter release. Changes in cytosolic calcium and postsynaptic responsiveness to acetylcholine were not affected by electrical coupling. These data indicate that transient electrical synapse formation delayed chemical synaptic transmission by imposing a functional block between the accumulation of presynaptic calcium and synchronized, vesicular release. Despite the inability to release neurotransmitter, neurons that had possessed strong electrical coupling recruited secretory vesicles to sites of synaptic contact. These results suggest that the mechanism by which neurotransmission is disrupted during electrical synapse formation is downstream of both calcium influx and synaptic vesicle mobilization. Therefore, electrical synaptogenesis may inhibit synaptic vesicles from acquiring a readily releasable state. We hypothesize that gap junctions might negatively interact with exocytotic processes, thereby diminishing chemical neurotransmission.

Synaptogenesis in the CNS: an odyssey from wiring together to firing together

To acquire a better comprehension of nervous system function, it is imperative to understand how synapses are assembled during development and subsequently altered throughout life. Despite recent advances in the fields of neurodevelopment and synaptic plasticity, relatively little is known about the mechanisms that guide synapse formation in the central nervous system (CNS). Although many structural components of the synaptic machinery are pre-assembled prior to the arrival of growth cones at the site of their potential targets, innumerable changes, central to the proper wiring of the brain, must subsequently take place through contact-mediated cell-cell communications. Identification of such signalling molecules and a characterization of various events underlying synaptogenesis are pivotal to our understanding of how a brain cell completes its odyssey from "wiring together to firing together". Here we attempt to provide a comprehensive overview that pertains directly to the cellular and molecular mechanisms of selection, formation and refinement of synapses during the development of the CNS in both vertebrates and invertebrates.

The Role of Neurotransmitters in Neurite Outgrowth and Synapse Formation

Besides a well-established role in neuronal communication in the adult central nervous system, neurotransmitters have diverse tasks in the embryonic brain, ranging from early developmental functions in morphogenesis /13/, to later functions in target selection and synapse formation /87/. For example, growth cones of developing neurons are known to release transmitters /26,36,88,110,115/ and respond to transmitters released from other neurons /35,44,59, 61,70/. Moreover, depletion of transmitters during embryonic development results in developmental deficits of the brain /21,48,84,109/, suggesting that transmitters have crucial roles as morphogens and/or neurotrophic factors. Although recently the idea of neurotransmitters being important for neural development has been challenged /99/, there is a vast amount of literature that seems to support the hypothesis that neurotransmitter release in the developing central nervous system is crucial for proper brain development. In this review we focus on the roles that neurotransmitters play in neurite outgrowth, target selection and synapse formation, with particular emphasis on the effects of the transmitters serotonin and dopamine.

Intracellular injection of synapsin I induces neurotransmitter release in C1 neurons of Helix pomatia contacting a wrong target

The contact with the postsynaptic target induces structural and functional modifications in the serotonergic cell C1 of Helix pomatia. In previous studies we have found that the presence of a non-physiological target down-regulates the number of presynaptic varicosities formed by cultured C1 neurons and has a strong inhibitory effect on the action potential-evoked Ca(2+) influx and neurotransmitter release at C1 terminals. Since a large body of experimental evidence implicates the synapsins in the development and functional maturation of synaptic connections, we have investigated whether the injection of exogenous synapsin I into the presynaptic neuron C1 could affect the inhibitory effect of the wrong target on neurotransmitter release. C1 neurons were cultured with the wrong target neuron C3 for three to five days and then injected with either dephosphorylated or Ca(2+)/calmodulin-dependent protein kinase II-phosphorylated Cy3-labeled synapsin I. The subcellular distribution of exogenous synapsin I, followed by fluorescence videomicroscopy, revealed that only synapsin I phosphorylated by Ca(2+)/calmodulin-dependent protein kinase II diffused in the cytoplasm and reached the terminal arborizations of the axon, while the dephosphorylated form did not diffuse beyond the cell body. Evoked neurotransmitter release was measured during C1 stimulation using a freshly dissociated neuron B2 (sniffer) micromanipulated in close contact with the terminals of C1. A three-fold increase in the amplitude of the sniffer depolarization with respect to the pre-injection amplitude (190+/-29% increase, n=10, P<0.006) was found 5 min after injection of Ca(2+)/calmodulin-dependent protein kinase II-phosphorylated synapsin I that lasted for about 30 min. No significant change was observed after injection of buffer or dephosphorylated synapsin I. These data indicate that the presence of synapsin I induces a fast increase in neurotransmitter release that overcomes the inhibitory effect of the non-physiological target and suggest that the expression of synapsins may play a role in the modulation of synaptic strength and neural connectivity.

Influence of the target on distribution and functioning of the varicosities of Helix pomatia metacerebral cell C1 in dissociated cell culture

The serotonergic metacerebral giant cell (C1) of Helix pomatia was isolated with its bifurcate axon and plated in culture under five conditions: (i) with no target; (ii) with the appropriate target B2 near the stump of the bigger branch (CBC); (iii) with B2 near the stump of the smaller branch (CC); (iv) with a wrong target (C3) near the stump of the CBC branch and (v) with B2 and C3 positioned near the CBC and CC stump, respectively. The counting of anti-serotonin antibody-labelled varicosities of the C1 neuron showed that the presence of the appropriate target in either axonal domain both down-regulated the number of varicosities of the contralateral neuritic field, and increased their average size, whereas the wrong target induced an overall reduction of the number of C1 neuron varicosities, and inhibited the evoked transmitter release. The action potential-evoked calcium concentration increase in the neuritic terminals of the C1 neuron cultured alone, or in presence of the appropriate target, reached a value significantly higher than that reached in presence of the wrong target. These results provide evidence that the postsynaptic neuron regulates both morphological and functional development of presynaptic terminals.

Spontaneous acetylcholine secretion from developing growth cones of Drosophila central neurons in culture: effects of cAMP-pathway mutations

We describe a novel bioassay system that uses Xenopus embryonic myocytes (myoballs) to detect the release of acetylcholine from Drosophila CNS neurons. When a voltage-clamped Xenopus myoball was manipulated into contact with cultured Drosophila "giant" neurons, spontaneous synaptic current-like events were registered. These events were observed within seconds after contact and were blocked by curare and alpha-bungarotoxin, but not by TTX and Cd(2+), suggesting that they are caused by the spontaneous quantal release of acetylcholine (ACh). The secretion occurred not only at the growth cone, but also along the neurite and at the soma, with significantly different release parameters among various regions. The amplitude of these currents displayed a skewed distribution. These features are distinct from synaptic transmission at more mature synapses or autapses formed in this culture system and are reminiscent of the transmitter release process during early development in other preparations. The usefulness of this coculture system in studying presynaptic secretion mechanisms is illustrated by a series of studies on the cAMP pathway mutations, dunce (dnc) and PKA-RI, which disrupt a cAMP-specific phosphodiesterase and the regulatory subunit of cAMP-dependent protein kinase A, respectively. We found that these mutations affected the ACh current kinetics, but not the quantal ACh packet, and that the release frequency was greatly enhanced by repetitive neuronal activity in dnc, but not wild-type, growth cones. These results suggest that the cAMP pathway plays an important role in the activity-dependent regulation of transmitter release not only in mature synapses as previously shown, but also in developing nerve terminals before synaptogenesis.

Postnatal development of calretinin- and parvalbumin-positive interneurons in the rat neostriatum: an immunohistochemical study

On the basis of cytochemical and morphologic differences, two classes of gamma-aminobutyric acidergic (GABAergic) interneurons expressing calcium-binding proteins have been identified in the striatum of adult animals: neurons expressing either parvalbumin (PV) or calretinin (CR). The function of these calcium-binding proteins is not clear, however, they are associated with distinct classes of inhibitory interneurons within the adult neostriatum. By using immunocytochemical techniques, we analyzed the postnatal maturation and the spatiotemporal distribution of PV- and CR-positive neurons in the rat neostriatum compared with a third class of interneurons characterized by the expression of the acetylcholine-synthesizing enzyme, choline acetyltransferase (ChAT). PV-positive cells appeared initially on postnatal day 9 in the lateral region of the striatum. During postnatal weeks 2 and 3, the numbers of PV-positive neurons increased, and this cell population spread progressively in a lateromedial direction. In contrast, CR-expressing neurons were present at birth. During the first few days after birth, the number of CR-immunoreactive cells increased, reaching a peak on postnatal day 5 before declining during the following 2 weeks. A mediolateral gradient was evident temporarily. ChAT-containing neurons were detectable at birth in the lateral striatum. During postnatal weeks 1 and 2, the neurons matured along a lateral-to-medial gradient. The results indicate that the maturation of striatal interneurons is regulated differentially during postnatal development, resulting in a distinct spatiotemporal genesis of phenotypes. The sequential expression of CR and PV suggests a stage-dependent development of subsets of inhibitory interneurons and, hence, the stage-dependent maturation of functionally distinct inhibitory circuits within the neostriatum.

Retrograde signaling in the development and modification of synapses

Retrograde signaling from the postsynaptic cell to the presynaptic neuron is essential for the development, maintenance, and activity-dependent modification of synaptic connections. This review covers various forms of retrograde interactions at developing and mature synapses. First, we discuss evidence for early retrograde inductive events during synaptogenesis and how maturation of presynaptic structure and function is affected by signals from the postsynaptic cell. Second, we review the evidence that retrograde interactions are involved in activity-dependent synapse competition and elimination in developing nervous systems and in long-term potentiation and depression at mature synapses. Third, we review evidence for various forms of retrograde signaling via membrane-permeant factors, secreted factors, and membrane-bound factors. Finally, we discuss the evidence and physiological implications of the long-range propagation of retrograde signals to the cell body and other parts of the presynaptic neuron.

The glycine transporter GLYT2 is a reliable marker for glycine-immunoreactive neurons

The glycine transporter GLYT2 is present in neurons of the spinal cord, the brain stem and the cerebellum. This localization is similar to that of glycine immunoreactivity, suggesting a causal relationship between GLYT2 expression and glycine distribution. In this report, we analyzed if such a relationship does exist by using neuronal cultures derived from embryonic spinal cord. GLYT2 was synthesized in a small subpopulation of neurons where it was targeted both to dendrites and to axons, being the axonal content higher than the dendritic one. At early stages in the development of cultured spinal neurons, the highest GLYT2 levels were found in the axonal growth cones. As the culture matured, immunoreactivity extended to the axonal shaft. Double-immunofluorescence experiments indicated a perfect co-localization of GLYT2 and glycine immunoreactivity in cultured neurons. Moreover, the concentration of glycine into neurons expressing GLYT2 was proportional to the concentration of the transporter. This observation was reproduced in GLYT2-transfected COS cells. These evidences indicate that the high content of glycine observed in some neurons in culture is indeed achieved by the concentrative task performed by GLYT2, and that GLYT2 can be used as a reliable marker for identification of glycine-enriched neurons.

The 'sniffer-patch' technique for detection of neurotransmitter release

A wide variety of techniques have been employed for the detection and measurement of neurotransmitter release from biological preparations. Whilst many of these methods offer impressive levels of sensitivity, few are able to combine sensitivity with the necessary temporal and spatial resolution required to study quantal release from single cells. One detection method that is seeing a revival of interest and has the potential to fill this niche is the so-called 'sniffer-patch' technique. In this article, specific examples of the practical aspects of using this technique are discussed along with the procedures involved in calibrating these biosensors to extend their applications to provide quantitative, in addition to simple qualitative, measurements of quantal transmitter release.

Rapid development of synaptic connections and plasticity between sensory neurons and motor neurons of Aplysia in cell culture: implications for learning and regulation of synaptic strength

We describe here the time course of functional synapse formation and of the development of short-term synaptic plasticity at Aplysia sensorimotor synapses in cell culture, as well as the effects of blocking protein synthesis or postsynaptic receptors on the development of synaptic transmission and plasticity. We find that synaptic responses can be elicited in 50% of sensory neuron-motor neuron pairs by 1 h after cell contact and that short-term homosynaptic depression and synaptic augmentation and restoration by the endogenous facilitatory transmitter serotonin are present at the earliest stages of synapse formation. Neither block of protein synthesis with anisomycin nor block of two types of postsynaptic glutamate receptor has any effect on the development of synaptic transmission or synaptic plasticity. The rapidity of synapse formation and maturation and their independence of protein synthesis suggest that changes in the number of functional synapses could contribute to short- and intermediate-term forms of synaptic plasticity and learning.

Actions of FMRFamide-related peptides on the gCa2+ of the C1 neuron in Helix aspersa

The endogenous neuropeptides FMRFamide and FLRFamide (tetrapeptides) reversibly reduced a voltage-activated calcium current in the C1 neuron of Helix aspersa by an average of 20%. Two structurally related heptapeptides, pQDPFLRFamide and pQDPFLRIamide, both derived from another precursor protein in this species, did not reduce the current at all.

Morphologic and neurochemical target selectivity of regenerating adult photoreceptors in vitro

Regenerating adult central nervous system (CNS) neurons must re-establish synaptic circuits in an environment very different from that present during development. However, the complexity of CNS circuitry has made it extremely difficult to assess the selectivity and mechanisms of synaptic regeneration at the cellular level in vivo. The synaptic preferences of adult photoreceptors were examined by using a defined cell culture system known to support regenerative process growth, presynaptic varicosity formation, and establishment of functional synapses. Immunolabeling for synaptic vesicle protein 2 and ultrastructural analysis demonstrated that cell-cell contacts made by photoreceptors were synaptic in nature. Target selectivity was determined by quantitative analysis of contacts onto normal and novel target cell types in cultures in which opportunities to contact all retinal cell types were present. Target cells were identified by morphology and immunolabeling for the amino acid neurotransmitters glutamate, aspartate, gamma-aminobutyric acid (GABA), and glycine. Regenerating photoreceptors showed a strong preference for novel multipolar cell targets (amacrine and ganglion cells) over normal photoreceptor, horizontal, and bipolar cell targets. Additionally, photoreceptors were selective for targets containing the transmitter GABA. These results indicate first, that the normal synaptic partners for photoreceptors are not intrinsically the optimal targets for regenerative synapse formation, and second, that GABA may modulate synaptic targeting by adult photoreceptors.

Developmental expression and self-regulation of Ca2+ entry via AMPA/KA receptors in the embryonic chick retina

Excessive activation of glutamate receptors in the late embryonic and adult retina leads to excitotoxic cell death through an increase in intracellular calcium concentration. Here we use the cobalt-staining technique of Pruss et al. to investigate the developmental expression of Ca(2+)-permeable alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid/kainate (AMPA/KA) receptors in the embryonic chick retina, and the effects of AMPA/KA receptor activation on cell survival and AMPA/KA receptor expression. Ca(2+)-permeable AMPA/KA receptors are present in the retina as early as embryonic day 6 (E6). While sustained activation of these receptors with KA led to massive cell death in explant and dissociated cultures of the chick retina late in development, continuous application of high doses of KA from early times was not excitotoxic. Cell survival in KA is correlated with both a reduction in cobalt staining and the KA-evoked membrane current, and thus with a reduction in the Ca2+ entry into cells via AMPA/KA receptors. The effects of KA could be blocked by the non-N-methyl-D-aspartic acid (NMDA) receptor antagonist 6-cyano-7-nitro-quinoxaline-2,3-dione (CNQX), but not by the NMDA receptor antagonist D-2-amino-5-phosphonovalerate (AP5) nor the L-type Ca2+ channel blockers diltiazem and nifedipine. The action of AP5 was mimicked by exposure to glutamate but not by the metabotropic receptor agonist 1S,3R-1-aminocyclopentane-1,3-dicarboxylic acid. Thus exposure of retinal neurons to glutamate early in development may protect them from its excitotoxic actions later on.

Comparative ontogenic profile of cholinergic markers, including nicotinic and muscarinic receptors, in the rat brain

The ontogenic profiles of several cholinergic markers were assessed in the rat brain by using quantitative in vitro receptor autoradiography. Brain sections from animals at different stages of development were processed with [3H]AH5183 (vesamicol; vesicular acetylcholine transport sites), [3H]N-methylcarbamylcholine (alpha(4)beta(2) nicotinic receptor sites), [3H]hemicholinium-3 (high-affinity choline uptake sites), [3H]3-quinuclidinyl benzilate (total population of muscarinic receptor sites), [3H]4-DAMP (muscarinic M1/M3 receptor sites), [3H]pirenzepine (muscarinic M1 receptor sites), and [3H]AF-DX 116 and [3H]AF-DX 384 (muscarinic M2 receptor sites) as radiolabeled probes. The results revealed that, by the end of the prenatal period (embryonic day 20), the densities of nicotinic receptor and vesicular acetylcholine transport sites already represented a considerable proportion of those observed in adulthood (postnatal day 60) in different laminae of the frontal, parietal, and occipital cortices, in the layers of Ammon's horn fields and the dentate gyrus of the hippocampal formation, as well as in the amygdaloid body, the olfactory tubercle, and the striatum. In contrast, at that stage, the densities of total muscarinic, M1/M3, M1, and possibly M2 receptor and high-affinity choline uptake sites represent only a small proportion of levels seen in the adult. Differences were also observed in the postnatal ontogenic profiles of nicotinic, muscarinic, vesamicol, and high-affinity choline uptake sites. For example, between postnatal weeks 3 and 5, the levels of M1/M3 and M1 sites were at least as high as in the adult, whereas M2 and high-affinity choline uptake site densities appeared to be delayed and to reach adult values only after postnatal week 5. With regard to cholinergic innervation in the developing rat brain, the present findings suggest a temporal establishment of several components of the cholinergic systems. The first components are the vesicular acetylcholine transporter and nicotinic sites; these are followed by M1/M3 and M1 sites and, finally, by M2 and high-affinity choline uptake sites.

Inhibitory action of SKPYMRFamide on acetylcholine receptors of Helix aspersa neurons: role of second messengers

1. SKPYMRFamide, a novel FMRFamide-like endogenous peptide reversibly decreases excitatory responses (depolarization and inward current) evoked by local ionophoretic application of acetylcholine (ACh) onto the soma of identified neurons F1, F2, F4 and F5/6 of the land snail, Helix aspersa. 2. Threshold concentrations of SKPYMRFamide for an inhibitory action on ACh-induced responses are 0.5-1 mumoll-1. This modulatory action of peptide is dose- and time-dependent. 3. It is concluded that SKPYMRFamide inhibits ACh receptors through activation of specific binding sites on the plasma membrane. 4. The possible role of different second messengers in the modulatory influence of SKPYMRFamide on ACh receptors was tested using 13 modulators of different second messenger systems. 5. The results indicate that SKPYMRFamide may inhibit ACh receptors through activation of one or more of the following systems: phospholipases C, A2, NO-synthase, soluble guanylate cyclase and lipoxygenases which elevate basal intracellulal levels of NO, cGMP, arachidonic acid, acyclic eicosanoids, inositol-1,4,5-trisphosphate (I(1,4,5)P3), I(1,4,5)P3-dependent Ca(2+)-mobilization followed by activation of calmodulin and Ca2+/calmodulin-dependent protein kinase II. Protein kinases A, C and cyclic eicosanoids do not appear to participate in modulatory action of SKPYMRFamide.

From contact to connection: early events during synaptogenesis

When neuronal processes first come into contact, chemical synapses can form rapidly. Many neurons synthesize synaptic machinery through intrinsic programs before cell-cell interactions. During the formation of chemical synapses, contact with appropriate targets has been found to trigger intracellular signals that induce the assembly of pre-existing synaptic machinery. We propose that 'promiscuous' neurons secrete transmitter before contacting their targets, and form over-abundant synapses, which undergo additional activity-dependent refinement; 'selective' neurons, which retain their original connectivity, require concerted retrograde and anterograde signaling to ensure their correct matching.

Retrograde regulation of presynaptic development during synaptogenesis

Major advances are occurring in our understanding of the events leading to synapse formation. Contact between the growth cone and target tissue leads to intercellular signaling which controls both pre- and postsynaptic development of the synapse. The identity of retrograde signals that regulate presynaptic development are beginning to emerge, and the signal transduction cascades that are activated presynaptically are being characterized. Recent studies have shown that both the resting calcium level and activation of presynaptic protein kinase A are critical in the development of the presynaptic terminal. An understanding of these regulatory mechanisms is beginning to provide insight into the molecular control of synaptic specificity.

Selection of transmitter responses at sites of neurite contact during synapse formation between identified leech neurons

1. Pressure sensitive (P) neurons of the leech Hirudo medicinalis show both an inhibitory, Cl(-)-dependent response and a depolarizing, cationic response to pipette application of serotonin (5-HT). Serotonergic Retzius (R) neurons in culture reform inhibitory, Cl(-)-dependent synapses with P neurons but fail to elicit the extrasynaptic, depolarizing response to 5-HT. We have examined the localization of the selection of 5-HT responses by testing the sensitivity of P cell growth cones and neurites to 5-HT application. 2. As measured by intracellular recording at the P cell soma, synaptic release of 5-HT from R cell processes activated only the Cl(-)-dependent response in P cell neurites. Focal application of 5-HT from a micropipette depolarized uncontacted P cell growth cones and neurites. In contrast, processes from the same P cells that were contacted by R cells were rarely depolarized by 5-HT application unless the application pipette was moved along the neurites away from the sites of contact. 3. The channels underlying the depolarizing response to 5-HT were identified in patch clamp recordings from P cell growth cones. These cation channels showed rare, brief openings in the absence of 5-HT. Application of 5-HT in the bath (outside the patch pipette) increased channel activity in uncontacted P cell growth cones but not in growth cones of the same P cells contacted by R cells. 4. We conclude that the selection of transmitter responses during synapse formation was localized to discrete sites of contact between the synaptic partners.

Dendritic reduction in Passover, a Drosophila mutant with a defective giant fiber neuronal pathway

The jump response to a light-off startle stimulus in Drosophila melanogaster occurs when the Giant Fiber (GF), a neuron descending from the brain to the thorax, drives the jump (tergotrochanteral) muscle motorneuron (TTMn). Nonjumping mutants have been isolated in which this response is disrupted. Flies bearing the X-chromosome mutation Passover (Pas) fail to jump in response to a light-off stimulus, and electrical stimulation of the GF in the brain no longer elicits the normal response in the TTM. We have used retrograde HRP labelling to examine the TTMn motorneuron in wild-type flies and in a variety of newly identified Pas alleles. In wild type the medial branch (MB) of the TTMn has an extensive region of apposition with the GF. In Pas alleles, there is a general reduction in anterior-posterior (A-P) extent of the medial branch but not of the posterior branch. Nevertheless, Pas alleles usually leave the TTMn close enough to the GF so that contact would not be precluded. In flies carrying a particular deficiency of Pas, Df(1) 16-3-22, including Pas/Df(1) 16-3-22 heterozygotes, there can be extensive growth of the medial-branch including a contralateral projection; these heterozygotes have more than the normal amount of overlap between the GF and the TTMn. This phenotype, originally ascribed to Pas mutants, is associated with Df(1) 16-3-22, but not with other deletions of the Pas gene. The driving of the TTMn by the GF is defective in mutant genotypes with extensive medial branches as well as in mutants where GF-TTMn contact is reduced. The fact that the TTMn grows into its normal synaptic region in mutant genotypes, but the GF pathway functions abnormally suggests that pathfinding by the TTMn is not impaired. It is more likely that the Pas mutation disrupts cell recognition, synaptogenesis, or synaptic function in the TTMn or its presynaptic partners.

Synaptic target contact enhances presynaptic calcium influx by activating cAMP-dependent protein kinase during synaptogenesis

Individual dissociated supralateral radular tensor (SLT) muscle fibers were manipulated into contact with fura-2-filled neurites of presynaptic buccal motoneuron 19 from Helisoma in cell culture. Within 30 min of contact, action potential-evoked calcium accumulation was reversibly augmented from 228 +/- 82 nM to 803 +/- 212 nM, an action that was blocked by H-7 (40-100 microM). Calcium accumulation was not augmented when buccal motoneuron 19 contacted muscle or neuronal targets with which it does not form chemical synapses. Addition of pCPTcAMP (500 microM) to cultures reversibly enhanced calcium accumulation. Injection of IP20, a peptide inhibitor of cAMP-dependent protein kinase, prevented pCPTcAMP and SLT muscle from enhancing calcium accumulation. These data demonstrate that SLT muscle target retrogradely regulates calcium accumulation in presynaptic nerve terminals by locally activating presynaptic cAMP-dependent protein kinase.

Selective fasciculation as a mechanism for the formation of specific chemical connections between Aplysia neurons in vitro

Selective fasciculation of growth cones along preestablished axon pathways expressing matching or complementary adhesion molecules is thought to be an important strategy in axon guidance. Growth cone inhibiting factors also appear to influence pathfinding decisions. We have used identified Aplysia neurons in vitro to explore the hypothesis that similar mechanisms could be involved in target selection. Co-cultures of L10 neurons with RB neuron targets or R2 neurons with RUQ neuron targets reliably formed chemical connections. In contrast, co-cultures of L10 with RUQ targets usually failed to form detectable chemical connections unless cell-cell contact was forced during plating by intertwining the major axons. These data suggested that differences in the ability to form cell-cell contacts might underlie the observed synaptic specificity. This notion was supported when fluorescent dye fills of L10 and R2 revealed a positive correlation between the amount of target contact and the frequency of synapse formation: L10-RUQ cultures showed much less target contact than L10-RB or R2-RUQ cultures. To examine the cellular mechanisms of these differences in target contact, presynaptic growth cones were observed as they interacted with target processes. L10-RUQ cultures showed much less fasciculation and more avoidance behavior compared to L10-RB and R2-RUQ cultures. This initial specificity suggested that the differences in amount of target contact arose through selective fasciculation and avoidance rather than through selective elimination after indiscriminate fasciculation. Selective fasciculation and avoidance might, therefore, aid in target selection by regulating the amount of contact between presynaptic processes and potential target cells.

Reconstruction of neuronal networks in culture

Since the 1960s, the large neurones of some invertebrates have been exploited in attempts to define the neural circuits that underlie simple behaviours. Even in the relatively 'simple' nervous systems of these animals, it is often difficult to study individual synaptic connections in detail and to rule out involvement of unidentified neurones. These limitations have been overcome by reconstruction of partial circuits of identified neurones in cell culture. This approach has provided opportunities to examine the function of small neuronal circuits in a manner that is unapproachable in the intact nervous system.

Local increases in intracellular calcium elicit local filopodial responses in Helisoma neuronal growth cones

Highly localized changes in intracellular Ca2+ concentration ([Ca2+]i) can be evoked in neuronal growth cones; these are followed by local changes in filopodia. Focally applied electric fields evoked spatially restricted, high magnitude increases in growth cone [Ca2+]i. The earliest and greatest increases were localized to small regions within a growth cone. Such fields also produced characteristic changes in the disposition of filopodia: both filopodial length and number were significantly increased on the cathode side of growth cones. The requirement for extracellular Ca2+ and the strong correlation between the evoked rise in [Ca2+]i and the changes in filopodia (r = 0.98) indicate that cathode stimulation results in local Ca2+ influx, leading to locally increased [Ca2+]i and local changes in filopodial behavior.

Modulation of growth cone calcium current is mediated by a PTX-sensitive G protein

Growth cones of isolated neurons B5 of Helisoma were voltage clamped in the whole-cell configuration. Depolarization of growth cones to -20 mV or greater activated a high-voltage-activated (HVA) calcium current. Addition of the neuropeptide FMRFamide (1 microM), which causes a presynaptic inhibition of synaptic transmission, reversibly reduced the calcium current magnitude. This inhibitory effect is mediated by a pertussis toxin (PTX)-sensitive G protein. Dialysis with the non-hydrolyzable GTP analogs GTP gamma S and Gpp(NH)p caused FMRFamide's effect to become irreversible. Dialysis with GDP beta S or preincubation with PTX prevented FMRFamide from reducing the calcium current. Thus, one role of growth cone G proteins is to modulate ion channels in growth cone membrane which in turn may control growth cone motility.

Target contact regulates the calcium responsiveness of the secretory machinery during synaptogenesis

Neuron B19 of Helisoma is selective in synaptogenesis. Presynaptic mechanisms underlying this selectivity were tested. Acetylcholine-sensitive assay cells were micromanipulated into contact with B19 somata to assess its secretory state. Prior to appropriate muscle target contact, spontaneous synaptic currents were detected; however, action potential-evoked release of neurotransmitter was detected only following hours of muscle contact. Photolysis of a calcium cage, DM-nitrophen, accelerated the frequency of synaptic currents in muscle-contacted, but not novel neuron-contacted, B19 somata. These studies demonstrate that contact with appropriate target muscle enhances the responsiveness of this neuron's secretory machinery to internal calcium levels, thereby imparting the presynaptic cell with the ability to couple action potentials with neurotransmitter release.

Target-dependent induction of secretory capabilities in an identified motoneuron during synaptogenesis

Cholinergic neurons isolated from the buccal ganglia of Helisoma were plated into cell culture with a variety of defined target cells to study the specificity of synaptogenesis. Motoneuron B19 selectively formed chemical connections with single dissociated muscle fibers derived from its appropriate target, the supralateral radular tensor (SLT) muscle. B19 did not form such connections with novel neuronal targets. In contrast to neuron B19, cholinergic neuron B5 nonselectively formed chemical connections with novel muscle and neuronal targets. Target cells were micromanipulated into contact with presynaptic neurons to examine the latent period until the onset of functional synaptic transmission. Neuron B5 formed chemical connections within the first minutes of contact with ACh-sensitive neurons and muscle while B19 required sustained periods of muscle-specific contact to induce the acquisition of a functional excitation-secretion coupling mechanism. These different latent periods from the onset of target contact suggest that neuron B5 acquires presynaptic secretory function before target contact, while B19 must receive a specific signal(s) from its appropriate target to induce the transformation of its terminal into a secretory state.