A variety of studies support a trophic role for somatostatin in the developing nervous system, evidenced as stimulation of neurite outgrowth and axonal or neuronal migration in both in vivo and culture models. Cloning experiments have now demonstrated the existence of five subtypes of somatostatin receptor, differentially distributed in the nervous system, differentially linked to specific signal transduction systems and in certain cases differentially expressed during development. The combination of the differential and developmental regulation of expression of both the somatostatin peptides and their receptors thus provides great potential in terms of trophic effects. To substantiate trophic effects of somatostatin, data are presented from two different model systems, cultures of cerebellar granule cells as well as transgenic mice in which somatostatin is expressed under the control of the glial fibrillary acidic protein promoter. Finally, potential receptor subtypes and second messenger systems involved in these trophic effects are addressed.
GABA is formed primarily from decarboxylation of glutamate by a family of cytosolic and membrane-bound GAD enzymes. In the adult, GAD-derived GABA sustains the vitality of the central nervous system (CNS), since blockage of GAD rapidly leads to convulsions and death. In plants, cytosolic GAD synthesizes GABA in response to hormones and environmental stress. Since decarboxylation involves protonation, secretion of GABA serves to buffer cytosolic pH in plant cells. Families of GAD and GABAA receptor/Cl- channel transcripts and encoded proteins emerge early and seemingly everywhere during CNS development, with their abundance closely paralleling neurogenesis and peaking before birth. Micromolar GABA acts at receptor/Cl-channels to depolarize progenitor cells in the cortical neuroepithelium; it also elevates their cytosolic Ca2+ (Cac2+) levels. In some way, these effects decrease proliferation. GABA directs the migration of postmitotic neuroblasts at femtomolar concentrations and stimulates their random motility at micromolar concentrations via Ca2+ signaling mechanisms. Activation of GABAA receptors by micromolar GABA may limit motility via membrane depolarization and elevated Cac2+. These results indicate that in vitro GABA can affect embryogenesis of the CNS through effects on cell proliferation and migration. As neurons differentiate postnatally, Cl(-)-dependent depolarization disappears together with GABAergic Cac2+ signals. Physiologically occurring GABAergic signals at Cl-channels exist in tonic and transient forms. Since the former are found on progenitor cells while both are present in postmitotic neurons, mechanisms to generate transients differentiate in the latter. Surprisingly, tonic and transient forms of GABAergic signaling at Cl-channels are rapidly and smoothly interconvertible and seem to be derived from online GABA synthesis in a surface-accessible compartment of the membrane.
Current data on the role of classical neurotransmitters as multiple and multifunctional regulators of early embryogenesis are reviewed. It is shown that these developmental regulators are coupled with second messengers. Peculiarities of this prenervous coupling emphasized and are used as the basis for discussing the problem of the evolutionary origin of cell regulatory systems.
Recent studies have demonstrated that many of the mRNAs encoding GABA(A)-receptor subunits in the cerebellum exhibit distinct temporal profiles of expression. The levels of six of these subunit transcripts increase severalfold in the second week of postnatal ontogeny. Findings from a variety of experimental systems suggest that the onset and increases in subunit mRNA expression are mediated by the interaction of genetic and epigenetic programs. The initiation of subunit mRNA expression occurs relatively early in cellular maturation and may be directed by intrinsic mechanisms. However, the levels of expression attained in adult animals may be controlled by extrinsic signals received by neurons during the postnatal maturation process.
The correct establishment and function of synapses depend on a variety of factors, such as guidance of pre- and postsynaptic neurons as well as receptor development and localization. gamma-Aminobutyric acid (GABA) has a pronounced effect on these events and elicits differentiation of neurons; that is, GABA acts as a trophic signal. Accordingly, activating preexisting GABA receptors, a trophic GABA signal enhances the growth rate of neuronal processes, facilitates synapse formation, and promotes synthesis of specific proteins. Transcription and de novo synthesis are initiated by the GABA signal, but the intracellular link between GABA receptor activation and DNA transcription is largely unknown. GABA also controls the induction and development of functionally and pharmacologically different GABAA receptor subtypes. The induced receptors are likely to be inserted only into the synaptic membrane domain. However, this ability to target the induced GABAA receptors is probably coupled to the maturation of neurons and not to the action of GABA per se. The induced GABAA receptors apparently mediate a pronounced inhibition of neurotransmitter release, whereas other subtypes of GABAA receptors may be modulatory rather than inhibitory.
GABA is present in organisms belonging to at least four of the five kingdoms. It acts as a neurotransmitter, a paracrine signaling molecule, a metabolic intermediate, or a trophic factor. In mammals, GABA synthesis depends on two forms of the enzyme glutamic acid decarboxylase-GAD(65), and GAD(67)-that may serve distinctive functions within GABA-producing cells. The two GADs derive from two genes, which are differentially regulated, though nearly every GABA-producing cell contains both forms of GAD. GAD(67) predominates early in development and after neuronal injury, consistent with a possible role in producing GABA for trophic use. In the embryo, GAD(67) transcripts also undergo alternative splicing, which gives rise to truncated forms. In the mature neuron, GAD(67) is present in both terminals and the cell body, where it may subserve a nonsynaptic, intracellular GABA pool. In contrast, GAD(65) is usually expressed later in development and is primarily localized to nerve terminals. GAD(65) enzymatic activity is more subject to regulation by cofactor binding and neuronal activity, consistent with its involvement in the production of synaptic GABA. Thus, while both GAD(67) and GAD(65) mediate the synthesis of GABA, their unique distributions and expression patterns suggest divergent functional roles.
Gamma-aminobutyric acid (GABA) acts as an inhibitory neurotransmitter in the mature vertebrate retina, where it is localized predominantly in amacrine cells, and to a lesser extent in other cell types. During development, GABA is expressed transiently in additional cells, including retinal ganglion cells and horizontal cells. Elements of the GABA system, including GABA uptake and release mechanisms and GABA receptors, are also expressed early in retinal development, well in advance of the onset of visual function. The GABA transporter is a major component of the GABA system in the mature retina, and is most likely responsible for GABA release early in development, prior to the establishment of vesicular synaptic transmission. GABA, produced by amacrine cells and retinal ganglion cells, may serve a developmental role in the establishment of circuitry in the retinal inner plexiform layer and may also be involved in the formation of appropriate central connections by retinal ganglion cell axons.
Brain contains at least two pools of gamma-aminobutyric acid (GABA), the transmitter pool and the so-called metabolic pool. To a large extent these pools may reflect the presence of GABA in different intracellular compartments, as immunocytochemical studies show that GABA is not localized mainly in terminals but is distributed throughout neurons. An interesting issue is the extent to which the two major forms of glutamate decarboxylase (GAD(65) and GAD(67)) are specialized to synthesize GABA for these pools. Although GAD(65) and GAD(67) differ significantly in several characteristics, they also have substantial similarities and interactions, and the presence of individual forms of GAD in certain cell types is consistent with the idea that GAD(65) and GAD(67) can each synthesize GABA for both pools. Substantial progress has been made in understanding the regulatory properties of GAD, but the available data provide little indication of how differences between the forms might enable each to serve the demands for GABA synthesis in a specific pool.
It is well recognized that the neurotrophin family of factors as well as neurotransmitters play critical roles in the ontogeny of the brain. Moreover, a growing literature suggests that these environmental signals do not operate individually, but interact in critical ways to enhance maturation. This review focuses on three brain systems where this collaboration is particularly evident: the cerebellum, the basal forebrain-hippocampus and locus coeruleus-hippocampus. The material presented indicates that cross-talk between neurotransmitters and neurotrophins may be a mechanism common to the development of multiple neuronal groups throughout the central nervous system. Moreover, this cross-talk appears to involve the interaction of both neuronal and glial cell populations.
Non-NMDA ionotropic receptor channels were longtime thought to be impermeable to calcium. There is now increasing evidence that this is not a general feature. Neural cells bearing non-NMDA receptors permeable to divalent cations can be found not only in the adult CNS, but also at surprisingly early stages of development (well before the onset of synaptogenesis). Since modulation of cytosolic calcium is known to trigger numerous transcription, translation, and post-translation mechanisms, molecules acting at non-NMDA ionotropic receptors may profoundly affect the fate of individual cells, brain regions, and finally the whole nervous system. However, present knowledge of transduction mechanisms and possible roles (ranging from transcription of immediate early genes to regulation of cell survival) is still very fragmentary. These receptors can be activated and modulated by endogenous molecules, but also by exogenous naturally occurring or pharmaceutical substances. If significant amounts of these substances can pass the human placental barrier, their consumption or accidental intake during pregnancy may constitute a risk for the developing embryonic and foetal CNS.
Cholinergic afferents innervate cerebral cortex during the most dynamic period of neuronal differentiation and synapse formation, suggesting they play a possible regulatory role in these events. A number of in vivo studies have shown over the last decade that alterations in cholinergic innervation during early postnatal development can change various features of cortical ontogeny. In particular, neonatal lesions to basal forebrain cholinergic afferents result in delayed cortical neuronal development and permanently altered cortical cytoarchitecture and cognitive behaviors. Likewise, cholinergic manipulations affect morphological plasticity in cat visual cortex as well as in the somatosensory cortex of rodents. Furthermore, augmentation of cholinergic function by means of perinatal choline treatment enhances cognitive performance in a sex specific manner. Additional indications for a sexual dimorphism in cortical cholinergic innervation and resulting function are gathered from a variety of paradigms. Recent information about effects of NGF, BDNF and NTB-4/5 on cortical morphogenesis and plasticity reveals complex interactions between the cholinergic basal forebrain afferents and this neurotrophin family. Detailed studies on the expression of cholinergic receptor proteins in cortical development and their associated signal transduction pathways strongly point towards a morphogenetic function of muscarinic receptors, in particular. Transient receptor localization in thalamocortical terminal fields and on a variety of other non-cholinergic fiber bundles suggest a cholinergic role in target finding and/or synapse formation for cortical afferents and efferents. We propose a hypothesis regarding the mechanisms for cholinergic regulation of neuronal differentiation and synapse formation on the level of the individual growth cone and discuss possibilities for cholinergic interactions with differential gene expression. We conclude that understanding the precise role of the cholinergic system in cortical morphogenesis and its relationship to neurotrophin function will be of clinical relevance for a number of developmental brain disorders, including Down Syndrome and Rett Syndrome.
This review explores the role of individual opioid receptor types and signal transduction pathways on cell growth and differentiation. The findings reviewed herein provide suggestive evidence that while no single opioid receptor or peptide type exclusively regulates growth, depending on the cell type, the activation of all three (mu, delta, or kappa) opioid receptor types can affect maturation in a cell type-dependent manner. Specific developmental responses are determined primarily by how a particular opioid receptor type is coupled to intracellular signaling effectors. Moreover, the coupling of opioid receptors appears to be developmentally regulated, and these protein-protein interactions change during ontogeny. The diversity of opioid receptor types and intracellular effectors may be a mechanism by which individual cells discriminate among different opioid signals, and may permit diverse opioid signals to be translated into a unique developmental logic in distinct neuronal and glial subpopulations.
Neurite outgrowth and growth cone motility are among the many aspects of neuronal development that can be affected by specific neurotransmitters. This was first demonstrated in experiments on identified molluscan neurons that were isolated from mature ganglia and cultured under conditions that promote the regeneration of new neurites. The application of serotonin to a regenerating Helisoma neuron B19 produced an abrupt, reversible cessation of neurite outgrowth and growth cone motility. While this type of response would subsequently be demonstrated for other neurons and neurotransmitters in many different invertebrate and vertebrate species, experiments on Helisoma neurons have continued to play a pivotal role in advancing this field. In this paper, the mechanisms and sites of serotonin action and how these responses are manifested in vivo during embryonic development are discussed. Experiments primarily on neuron B19 have shown that serotonin acts on a novel serotonin receptor that is coupled to the elevation of cyclic AMP. This intracellular messenger directly activates a class of cyclic-nucleotide-gated sodium channels, leading to sodium influx, membrane depolarization, and activation of voltage-gated calcium channels. The resulting elevation of intracellular calcium acts through a calcium/calmodulin-dependent pathway to inhibit neurite outgrowth and growth cone motility. Although the final steps have yet to be completely resolved, they undoubtedly involve calcium-dependent regulation of cytoskeletal components. Regarding the sites of serotonin action, serotonin responses have been localized to growth cones and even filopodia in specific neurons. However, some studies suggest that neurite development may actually be regulated by serotonin in a paracrine, non-localized manner in a surprisingly large percentage of Helisoma neurons. Finally, experiments on Helisoma embryos have investigated how serotonin actually regulates the in vivo development of specific neurons. Pharmacological treatments that reduce the serotonin concentration in embryos affected the neurite morphology and synaptic efficacy of neuron B19 and the amount of neurite branching in embryonic neuron C1. All of these responses were consistent with the primary action of serotonin being the inhibition of neurite outgrowth, as predicted by the original cell culture studies.
It is now well established that neurotransmitters act as growth-regulatory signals for neuronal and non-neuronal cells of both primitive and higher organisms, where they control cell proliferation, motility, survival, growth, differentiation, and gene expression. Many of these actions are reminiscent of the actions of other growth-regulatory signals such as growth factors, neurotrophins, and proto-oncogenes. How, then, do neurotransmitters exert these effects? Although some information is available concerning second messengers activated by these neurotransmitters in developing cells, little is known about subsequent steps involving signal transduction cascades leading to their final outcomes. This review attempts to provide testable hypotheses regarding possible cellular and molecular mechanisms downstream of second messengers activated by neurotransmitters, based on recent insights into signal transduction cascades activated by classical growth-regulatory signals. In many cases, there are clear points of convergence between these pathways, raising the interesting possibility that neurotransmitters and other growth-regulatory signals may cooperate to regulate developmental functions of cells and tissues.
Neurotransmitters and their receptors appear early during nervous system development and are thought to play important roles in neurite outgrowth, growth cone motility, target cell selection and synaptogenesis. In vivo studies in both vertebrates and invertebrates have shown that the perturbations of embryonic transmitter expression result in abnormal morphological and synaptic development. In vitro studies have further revealed that transmitters are capable of affecting neurite outgrowth and growth cone behaviour. The precise cellular mechanisms by which neurotransmitters affect these developmental steps are, however, poorly defined. In vitro, a presynaptic neuron from the mollusc Lymnaea stagnalis releases dopamine, which induces both growth cone attraction and growth cone collapse of target and non-target cell growth cones, respectively. We propose that the ability of dopamine to differentially affect growth cone motility of two cell types results from a divergence of the dopamine receptor-activated second messenger pathways at the G-protein level. Such transmitter-receptor interactions between growth cones of specific neurons may not only induce changes in the growth cone motility, but may subsequently play an important role in target cell selection and specificity of synaptogenesis.
During development of the nervous system a common set of signal transduction pathways appear to regulate growth cone behaviors, synaptogenesis and natural cell death, three fundamental processes that comprise the "neurodevelopmental triad". Among the intercellular signals that coordinate the developmental triad in the mammalian brain are glutamate (the major excitatory neurotransmitter) and beta-amyloid precursor protein (beta APP). Localization of ionotropic glutamate receptors to dendritic compartments allows for selective regulation of dendrite growth cones and spine formation by glutamate released from axonal growth cones and presynaptic terminals. Expression of particular subtypes of glutamate receptors peaks during a developmental time window within which synaptogenesis and natural neuronal death occur. Calcium is the preeminent second messenger mediating both acute (rapid remodelling of the microtubule and actin cytoskeletal systems) and delayed (transcriptional regulation of growth-related proteins; e.g., neurotrophins) actions of glutamate. The expression of beta APP in brain is developmentally regulated and it is expressed ubiquitously in differentiated neurons. beta APP is axonally transported and secreted forms of beta APP (sAPPs) are released from neurons in an activity-driven manner. Secreted APPs modulate neuronal excitability, counteract effects of glutamate on growth cone behaviors, and increase synaptic complexity. Acute actions of sAPPs appear to be transduced by cyclic GMP which promotes activation of K+ channels and reduces [Ca2+]i. Delayed actions of sAPPs may involve regulation of gene expression by the transcription factor NF kappa B. Finally, the striking effects of glutamate, neurotrophic factors, and sAPPs on synaptogenesis and neuronal survival in cell culture systems and in vivo suggest that each of these signals plays major roles in the process of natural cell death. The same signalling mechanisms that mediate adapative regulation of neuroarchitecture during brain development appear to play prominent roles in maladaptive neurodegenerative processes in an array of disorders ranging from stroke to epilepsy to Alzheimer's disease.
Gamma-aminobutyric acid (GABA) is one of the principle inhibitory neurotransmitters in the mature spinal cord. It effectively suppresses synaptic transmission by mechanisms of postsynaptic and presynaptic inhibition. The function of GABA is less well understood early in spinal cord development, when the amino acid is transiently expressed in most neurons, and it depolarizes instead of hyperpolarizes neurons. This article reviews the possible physiological roles of GABA in modulating synaptic transmission, promoting neuronal development, and regulating neuronal pH during early stages of spinal cord differentiation. It is proposed that despite its depolarizing action, GABA acts as an inhibitory neurotransmitter that may also function as a neurotrophic agent.
Recent studies have demonstrated that many of the mRNAs encoding GABAA-receptor subunits in the cerebellum exhibit distinct temporal profiles of expression. The levels of six of these subunit transcripts increase severalfold in the second week of postnatal ontogeny. Findings from a variety of experimental systems suggest that the onset and increases in subunit mRNA expression are mediated by the interaction of genetic and epigenetic programs. The initiation of subunit mRNA expression occurs relatively early in cellular maturation and may be directed by intrinsic mechanisms. However, the levels of expression attained in adult animals may be controlled by extrinsic signals received by neurons during the postnatal maturation process.
Early and ubiquitous detection of GABA in the rat spinal cord before the occurrence of synaptogenesis has led to the concept of a neurotrophic role of GABA, in addition to a promoting effect on neurite extension and neurodevelopment. The aim of this study was to further establish, in vivo, evidence for a link between the maturation of spinal cord innervation and the regulation of several isoforms of the synthetic enzymes of GABA, the glutamic acid decarboxylases GAD65, GAD67, and EP10, the embryonic truncated form of GAD67. Neonatal capsaicin treatment was used to induce a specific loss of afferent fibers (unmyelinated C fibers, thin myelinated fibers A delta) to the dorsal horn. The regulation of various GAD mRNAs was investigated using sensitive techniques such as RT-PCR and in situ hybridization. The sensitivity of the methods was further enhanced by the use of a gaseous detector (beta-imager) to quantitate the mRNAs species. After neonatal capsaicin treatment, higher levels of GAD67 mRNA were detected transiently during the postnatal development of the rat spinal cord. A maximum two-fold increase of GAD67 mRNA was found on the day following the capsaicin injection and reached control values within 3 weeks. In contrast, GAD65 mRNA levels remained low and were unaffected by the treatment, and EP10 was not detected. In addition, we have found a similar upregulation, with the same time course, of the cytoskeletal protein beta-actin. The capsaicin-induction of mRNA synthesis was, however, two-fold greater for beta-actin than for GAD67. Moreover, since this upregulation of GAD67 mRNA coincides with the sprouting of unaffected afferent fibers and of 5HT axons, one can hypothesize that GAD67 participates in the structural plasticity occurring in reaction to the capsaicin-induced partial deafferentation.
