Developmental changes in the calcium dependency of gamma-aminobutyric acid release from isolated growth cones: correlation with growth cone morphology

The α1, α2, α3, and γ2 subunits of GABAA receptors show characteristic spatial and temporal expression patterns in rhombencephalic structures during normal human brain development

γ-Aminobutyric acid (GABA) is the most abundant inhibitory neurotransmitter in adult mammalian brain, mediating its actions chiefly via a pentameric chloride ion channel, the GABAA receptor. Nineteen different subunits (α1-6, β1-3, γ1-3, δ, ε, π, θ, ρ1-3) can give rise to multiple receptor subtypes that are the site of action of many clinically important drugs. In the developing brain, however, GABAA receptors mediate excitatory actions due to an increased chloride concentration within neurons and seem to control cell proliferation, migration, differentiation, synapse maturation, and cell death. Little is known about the distribution of single subunits in the human brain. Here we describe developmental changes in the immunohistochemical distribution of four subunits (α1, α2, α3, and γ2) in the human rhombencephalon. The γ2 was the most abundant subunit in all rhombencephalic structures during development and in adults, whereas α subunits showed a structure- and age-characteristic distribution. The α1 was expressed prenatally in the molecular and Purkinje cell layer, but only postnatally in the granule cell layer and the dentate nucleus. Expression was completely absent in the inferior olivary nucleus. The α2 gradually increased during development, showing some layer specificity in the cerebellar cortex. The α3-immunoreactivity in the cerebellar cortex was relatively weak, but it was abundantly observed in different cell populations in the subcortical cerebellar structures. Structure- and age-characteristic colocalization between subunits during development suggests differences in GABAA receptor composition. Interestingly, subunit expression in several instances differed between human and rodent brain, underlining the importance of immunohistochemical studies in humans.

GABA transporters in the mammalian cerebral cortex: localization, development and pathological implications

The extracellular levels of gamma-aminobutyric acid (GABA), the main inhibitory neurotransmitter in the mammalian cerebral cortex, are regulated by specific high-affinity, Na+/Cl- dependent transporters. Four distinct genes encoding GABA transporters (GATs), named GAT-1, GAT-2, GAT-3, and BGT-1 have been identified using molecular cloning. Of these, GAT-1 and -3 are expressed in the cerebral cortex. Studies of the cortical distribution, cellular localization, ontogeny and relationships of GATs with GABA-releasing elements using a variety of light and electron microscopic immunocytochemical techniques have shown that: (i) a fraction of GATs is strategically placed to mediate GABA uptake at fast inhibitory synapses, terminating GABA's action and shaping inhibitory postsynaptic responses; (ii) another fraction may participate in functions such as the regulation of GABA's diffusion to neighboring synapses and of GABA levels in cerebrospinal fluid; (iii) GATs may play a role in the complex processes regulating cortical maturation; and (iv) GATs may contribute to the dysregulation of neuronal excitability that accompanies at least two major human diseases: epilepsy and ischemia.

GABA and GABA receptors in the central nervous system and other organs

Gamma-aminobutyrate (GABA) is a major inhibitory neurotransmitter in the adult mammalian brain. GABA is also considered to be a multifunctional molecule that has different situational functions in the central nervous system, the peripheral nervous system, and in some nonneuronal tissues. GABA is synthesized primarily from glutamate by glutamate decarboxylase (GAD), but alternative pathways may be important under certain situations. Two types of GAD appear to have significant physiological roles. GABA functions appear to be triggered by binding of GABA to its ionotropic receptors, GABA(A) and GABA(C), which are ligand-gated chloride channels, and its metabotropic receptor, GABA(B). The physiological, pharmacological, and molecular characteristics of GABA(A) receptors are well documented, and diversity in the pharmacologic properties of the receptor subtypes is important clinically. In addition to its role in neural development, GABA appears to be involved in a wide variety of physiological functions in tissues and organs outside the brain.

GABA and development of the Xenopus optic projection

In the developing visual system of Xenopus laevis retinal ganglion cell (RGC) axons extend through the brain towards their major target in the midbrain, the optic tectum. Enroute, the axons are guided along their pathway by cues in the environment. In vitro, neurotransmitters have been shown to act chemotropically to influence the trajectory of extending axons and regulate the outgrowth of developing neurites, suggesting that they may act to guide or modulate the growth of axons in vivo. Previous work by Roberts and colleagues (1987) showed that populations of cells within the developing Xenopus diencephalon and mid-brain express the neurotransmitter gamma amino butyric acid (GABA). Here we show that Xenopus RGC axons in the midoptic tract grow alongside the GABAergic cells and cross their GABA immunopositive nerve processes. Moreover, RGC axons and growth cones express GABA-A and GABA-B receptors, and GABA and the GABA-B receptor agonist baclofen both stimulate RGC neurite outgrowth in culture. Finally, the GABA-B receptor antagonist CGP54626 applied to the developing optic projection in vivo causes a dose-dependent shortening of the optic projection. These data indicate that GABA may act in vivo to stimulate the outgrowth of Xenopus RGC axons along the optic tract.

The mechanism of the neurotransmitter release in growth cones

The growth cone is considered the precursor of the presynaptic terminal. To elucidate the minimal molecular machinery required for exocytosis, we examined the characteristics of alpha-latrotoxin-induced exocytosis in growth cones. In isolated growth cones (IGC), neurotransmitters were released in a SNARE-dependent manner, but rab3A cycling was blocked. By supplying rabphilin, a rab3A acceptor found in low levels in IGC, the IGC obtained as high an exocytotic efficiency as adult synaptosomes, and the complete GDP-GTP conversion of rab3A occurred on growth cone vesicles (GCV). GCVs bound SNAREs but not NSF or alpha-SNAP; whereas in the rabphilin-supplied IGC, GCVs recruited both NSF and alpha-SNAP, to form the SNARE-NSF-SNAP complex. These results suggest that rab3A cycling is dependent upon the accumulation of rabphilin and is completed later than the SNARE mechanism, and that rabphilin is involved in determining the efficiency of exocytosis by modifying the SNARE mechanism.

A shift in protein S-palmitoylation, with persistence of growth-associated substrates, marks a critical period for synaptic plasticity in developing brain

In the mammalian cortex, the initial formation of synaptic connections is followed by a prolonged period during which synaptic circuits are functional, but retain an elevated capacity for activity-dependent remodeling and functional plasticity. During this period, synaptic terminals appear fully mature, morphologically and physiologically. We show here, however, that synaptic terminals during this period are distinguished by their simultaneous accumulation of multiple growth-associated proteins at levels characteristic of axonal growth cones, and proteins involved in synaptic transmitter release at levels characteristic of adult synapses. We show further that newly formed synapses undergo a switch in the dynamic S-palmitoylation of proteins early in the critical period, which includes a large and specific decrease in the palmitoylation of GAP-43 and other major substrates characteristic of growth cones. Previous studies have shown that a similar reduction in ongoing palmitoylation of growth cone proteins is sufficient to stop advancing axons in vitro, suggesting that a developmental switch in protein S-palmitoylation serves to disengage the molecular machinery for axon extension in the absence of local triggers for remodeling during the critical period. Only much later does a decline in the availability of major growth cone components mark the molecular maturation of cortical synapses at the close of the critical period.

Cholinergic cell expression in the developing rat medial septal nucleus in vitro is differentially controlled by GABAA and GABAB receptors

The early appearance and relative abundance of GABAergic neurons in basal forebrain cholinergic nuclei like the medial septum suggest that the maturation of the later developing cholinergic neurons in these nuclei may be controlled by GABA. To examine this possibility, the effects of both exogenous GABA and specific GABA receptor agonists, as well as that of endogenous GABA on the phenotypic expression and survival of the cholinergic neurons in primary cultures from the fetal rat medial septum, were studied. Treatment of these cultures for six days with GABA significantly decreased the enzymatic activity of choline acetyltransferase (EC 2.3.1.6) (ChAT) in a dose-dependent manner. This response to exogenous GABA was blocked by bicuculline, mimicked by muscimol and slightly potentiated by saclofen. Consistent with this latter observation, the GABAB receptor agonist, baclofen, dose-dependently increased septal ChAT activity. However, while the effect of baclofen on cholinergic expression was lost in the absence of glia, the suppressive effects of GABA or muscimol were more marked. Acetylcholinesterase (EC 3.1.1.7) (AChE) expression in mixed neuronal-glial cultures, was, like ChAT activity, increased or decreased in intensity with the inclusion of baclofen or muscimol, respectively. Although the number of AChE positive neurons in muscimol-treated cultures was significantly lower than that in controls, no changes in neither neuronal nor general cell viability were noted. Finally, as GABAA or GABAB receptor antagonists bicuculline and picrotoxin or saclofen, when applied alone to mixed cultures, increased or decreased ChAT activity, respectively, it appears that endogenous GABA, tonically released in the developing septum, may, via specific receptor types, differentially control the biochemical maturation of the cholinergic neurons.

GABA(A) receptor subunit expression in intrastriatal striatal grafts comparison between normal developing striatum and developing striatal grafts

Expression of the alpha1, alpha2 and beta2/3 GABA(A) receptor subunits in maturing cell-suspension striatal grafts and in normal developing striatum was studied by immunocytochemistry. During normal postnatal development, the alpha1 subunit was present in the striatum only at very low density, while the alpha2 and beta2/3 subunits were present with a patchy distribution, in some patches at high density. Double-staining techniques indicated that DARPP-32 (a marker of striatal projection neurons) was not colocalized with alpha1, but was present in some beta2/3-positive areas and all alpha2-positive areas. In striatal grafts, alpha1 immunoreactivity was first detected 2 weeks post-grafting (p.g.), and by 3-10 weeks p.g. the pattern was similar to that observed in mature grafts (1 year p.g.), in which alpha1-immunopositive patches surrounding DARPP-32-positive (i.e. striatum-like) areas are observed. Alpha2 and beta2/3 immunoreactivity was observed within the first week p.g., and by 3-10 weeks p.g. was similar to that observed in mature grafts (i.e. immunoreactivity throughout the graft but with patches of different intensity). During graft maturation there was a marked decline in alpha2 immunoreactivity in DARPP-32-negative areas, as is observed during normal development of the globus pallidus and ventral pallidum. Interestingly, alpha1- and beta2/3-positive fibers (perhaps mostly dendrites) entered DARPP-32-positive patches from DARPP-32-negative areas. This study indicates that the time course of expression of GABA(A) receptor subunits in grafted striatal neurons, closely matches that of morphological maturation of the transplant, that of the development of functional synaptic activity and that of GABA(A) receptor subunit immunoreactivity in normal developing striatum. Our results also suggest that there are significant interactions between DARPP-32-positive and DARPP-32-negative areas with respect to the expression of GABA(A) receptors, and support the suggestion that miniature 'striatopallidal systems' may develop within grafts; such interactions may be important for the functional integration of striatal grafts with the host brain.

The soluble N-ethylmaleimide-sensitive factor attached protein receptor complex in growth cones: molecular aspects of the axon terminal development

Soluble N-ethylmaleimide-sensitive factor attached protein (SNAP) receptor (SNARE) mechanisms are thought to be involved in two important processes in axonal growth cones: (1) membrane expansion for axonal growth and (2) vesicular membrane fusion for mature synaptic transmission. We investigated the localization and interactions among the proteins involved in SNARE complex formation in isolated growth cone particles (GCP) from forebrain. We demonstrated that the SNARE complex is present in GCPs morphologically without synaptic vesicles (SVs) and associated with growth cone vesicles. However, the apparently SV-free GCP was lacking in the regulatory mechanisms inhibiting SNARE complex formation proposed in SV fusion, i.e., the association of synaptotagmin with the SNARE complex, and vesicle-associated membrane protein (VAMP)-synaptophysin complex formation. The core components of the SNARE complex (syntaxin, SNAP-25, and VAMP) accumulated for several days before postnatal day 7, when SVs first appeared, and preceded the accumulation of marker proteins such as synaptophysin, SV2, and V-ATPase. Our present results suggest that the SNARE mechanism for vesicular transmitter release is not fully functional in growth cones before the appearance of SVs, but the SNARE mechanism is working for membrane expansion in growth cones, which supports our recent report. We concluded that the regulation of the SNARE complex in growth cones is different from that in mature presynaptic terminals and that this switching may be one of the key steps in development from the growth cone to the presynaptic terminal.

Postnatal development of GABA-immunoreactive terminals in the reticular and ventrobasal nuclei of the rat thalamus: a light and electron microscopic study

The postnatal development of inhibitory GABAergic circuits in the thalamic reticular and ventrobasal nuclei was studied in rats ranging from the day of birth to the end of the third postnatal week by means of a postembedding immunogold staining procedure to visualize GABA. In the reticular nucleus, GABA labeling was present from birth in cell bodies, dendrites, growth cones and a few synaptic terminals, whereas in the ventrobasal nucleus it was exclusively in axonal processes identifiable as growth cones, vesicle-rich profiles and synaptic terminals. In both nuclei, GABA-labeled synaptic terminals were, however, very scarce and immature in neonatal animals and they became numerous and morphologically mature only after the end of the second postnatal week. These findings suggest that inhibitory synaptic responses in the somatosensory thalamus are not yet fully mature throughout the first two postnatal weeks and support the hypothesis that GABA may initially play trophic roles. The relatively late maturation of the thalamic GABAergic system may have important functional consequences, as the reticulothalamic circuits are responsible for the generation of spindle wave oscillations whose cellular mechanisms are also involved in the generation of spike-and-wave (absence) seizures in humans and in animal models.

Embryonic and postnatal expression of four gamma-aminobutyric acid transporter mRNAs in the mouse brain and leptomeninges

The distribution of gamma-aminobutyric acid (GABA) transporter mRNAs (mGATs) was studied in mouse brain during embryonic and postnatal development using in situ hybridization with radiolabeled oligonucleotide probes. Mouse GATs 1 and 4 were present in the ventricular and subventricular zones of the lateral ventricle from gestational day 13. During postnatal development, mGAT1 mRNA was distributed diffusely throughout the brain and spinal cord, with the highest expression present in the olfactory bulbs, hippocampus, and cerebellar cortex. The mGAT4 message was densely distributed throughout the central nervous system during postnatal week 1; however, the hybridization signal in the cerebral cortex and hippocampus decreased during postnatal weeks 2 and 3, and in adults, mGAT4 labeling was restricted largely to the olfactory bulbs, midbrain, deep cerebellar nuclei, medulla, and spinal cord. Mouse GAT2 mRNA was expressed only in proliferating and migrating cerebellar granule cells, whereas mGAT3 mRNA was absent from the brain and spinal cord throughout development. Each of the four mGATs was present to some degree in the leptomeninges. The expression of mGATs 2 and 3 was almost entirely restricted to the pia-arachnoid, whereas mGATs 1 and 4 were present only in specific regions of the membrane. Although mGATs 1 and 4 may subserve the classical purpose of terminating inhibitory GABAergic transmission through neuronal and glial uptake mechanisms, GABA transporters in the pia-arachnoid may help to regulate the amount of GABA available to proliferating and migrating neurons at the sub-pial surface during perinatal development.

Vesicle-associated proteins and calcium in nerve terminals of chick ciliary ganglia during development of facilitation

1. The developmental appearance of synaptic vesicle-associated proteins and nerve terminal calcium ([Ca2+]i) sequestering processes were determined for the chick ciliary ganglia in relation to the maturation of the different phase of increased efficacy of transmitter release following nerve impulses. The maturation phases studied were from stages 34-35, at the time of synapse formation, to stage 46 at hatching. 2. Western blots and immunohistochemical localization indicated that synaptotagmin 1 and synapsin IIa were detectable at stages 34-35 and were clearly localized at the nerve terminals by stage 37. Syntaxin was clearly localized at the nerve terminals at stage 34. 3. The relative size of the postganglionic compound action potential, used to measure the transmission efficacy through the ganglion, showed that the slope of the relationship between log efficacy and log extracellular calcium concentration ([Ca2+]o) in low [Ca2+]o was about 4 by stage 46. 4. A mature facilitatory mechanism for transmission was not present at stage 34 and did not emerge until stage 38. A mature augmentation was not present at stages 34 or 38 and was not established until stages 41-42. Post-tetanic potentiation (PTP) was not present at stage 34; it was evident at stages 37-38 and only reached maturity by stages 41-42. 5. The time course of calcium changes in the nerve terminals following trains of impulses that give rise to facilitation, augmentation and PTP was determined for different stages of development using the indicator Calcium Green-1 in the nerve terminal. The mature time course of the phases of calcium decline in the nerve terminal associated with facilitation and augmentation was observed as early as stage 38, whereas that of the PTP phase did not mature until after stage 42. 6. These results are discussed in terms of the maturation of the vesicle-associated proteins and calcium influx into the terminal following trains of impulses that give rise to the different components of increased synaptic efficacy.

Molecular characterization of the dendritic growth cone: regulated mRNA transport and local protein synthesis

The molecular mechanisms that regulate growth cone guidance of dendrite outgrowth remain to be elucidated. We hypothesized that mRNA localization in dendritic growth cones and their local protein synthesis may be important for growth cone functioning. The appearance of 23 of 31 growth cone mRNAs was developmentally regulated. Also, alteration of growth cone morphology affected the relative levels of three mRNAs. Finally, using single dendrite transfection, it was shown that local protein synthesis occurs in dendrites and growth cones. A heterogeneous population of mRNAs exists in dendritic growth cones of cultured hippocampal neurons whose relative abundances are developmentally regulated and can vary with changes in growth cone physiology. The demonstration of protein synthesis in growth cones suggests that translation of the localized mRNAs may contribute to regulation of growth cone motility and dendrite outgrowth.

Freeze-etched neuronal growth cones from rat cerebral cortex at birth: plasma membrane morphology in relation to synapse formation

Isolated growth cones from the cerebral cortex of newborn rats were studied using the freeze-etching technique. The intramembranous structure of their plasma membranes was examined in detail and synaptic sites were found. Their membrane morphology was compared with that of the synaptic sites in adult animals and several differences between them were established. The importance of the present results for understanding the formation and development of the synaptic sites in the cerebral cortex is outlined.

Synaptophysin and GAP-43 proteins in efferent fibers of the inner ear during postnatal development

A rearrangement of afferent and efferent fibers occurs in the postnatal development of the inner ear. Growth and synaptogenesis was explored during this critical period by immunohistochemically monitoring the expression of GAP-43 and synaptophysin. Both proteins were colocalized in efferent fibers beyond postnatal day 3 (pn3). Two distinct synaptophysin- and GAP-43-positive fibers innervated different parts of inner hair cells in the first and second postnatal weeks, respectively. GAP-43-positive efferents projecting to outer hair cells upregulated synaptophysin with base to apex gradient between postnatal day 5 and postnatal day 14. In efferents projecting to outer hair cells GAP-43 was downregulated about 6 days beyond synaptogenesis. In efferents projecting to inner hair cells, however, GAP-43 remained upregulated even beyond pn18, indicating continuous synapse replacement of this fiber type. Both proteins thus improved as excellent markers for growth and synaptogenesis of distinct postnatal efferent fibers.

Modulation of GABA-mediated synaptic transmission by endogenous zinc in the immature rat hippocampus in vitro

1. Intracellular recordings from postnatal 2- to 12-day-old (P2-12) rat hippocampal CA3 pyramidal neurones exhibited spontaneous synaptic potentials mediated by GABAA receptors. These potentials can be separated on the basis of amplitude into two classes which are referred to as small and large. 2. The large depolarizing potentials were reversibly inhibited by the Zn2+ chelator 1,2-diethyl-3-hydroxypyridin-4-one (CP94). The small inhibitory postsynaptic potentials. (IPSPs) were apparently unaffected. 3. Stimulation of the mossy fibre pathway evoked composite excitatory postsynaptic potentials (EPSPs) and IPSPs. Threshold stimulus-evoked synaptic potentials were mediated by GABAA receptors and were reversibly blocked by CP94. The responses evoked by suprathreshold stimulation and persisting in the presence of bicuculline or CP94 were partially inhibited by 2-amino-5-phosphonopropionic acid (AP5) and were completely blocked with 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). 4. L-Histidine, which preferentially forms complexes with Cu2+ > Zn2+ > Fe2+ > Mn2+, inhibited both naturally occurring spontaneous and evoked GABAA-mediated large synaptic potentials without affecting the neuronal resting membrane properties. Exogenously applied Zn2+ induced large spontaneous synaptic potentials and prolonged the duration of the evoked potentials. These effects were reversibly blocked by histidine. 5. The metal chelating agent diethyldithiocarbamate had little effect on the large amplitude synaptic potentials. 6. The transition metal divalent cations Fe2+ and Mn2+ did not initiate large synaptic potentials in CA3 neurones; however, Cu2+ depolarized the membrane and enhanced both excitatory and inhibitory synaptic transmission, resulting in a transient increase in the frequency of the large amplitude events. In comparison, zinc increased the frequency of the large potentials and also induced such events in neurons (P4-21) where innate potentials were absent. The postsynaptic response to ionophoretically applied GABA was either unaffected or slightly enhanced by Zn2+. 7. Under conditions favouring the activation of non-NMDA receptors, excitatory synaptic transmission was unaffected by CP94 but was depressed by Zn2+. Responses to ionophoretically applied glutamate were not inhibited by Zn2+, indicating that Zn2+ affects excitatory synaptic transmission via a presynaptic mechanism. 8. We conclude that the naturally occurring large synaptic potentials in young CA3 neurones are apparently induced by endogenous Zn2+ which can promote or synchronize the release of GABA in the immature hippocampus.

Ontogeny of gamma-aminobutyric acid in efferent fibers to the rat cochlea

Cochlear efferent innervation originates in two different groups of neurons located in the superior olivary complex. A first group of olivocochlear neurons (lateral efferent neurons) lies in the lateral superior olive. They send axons to the organ of Corti, where they synapse with radial afferent dendrites of primary auditory neurons, postsynaptic to the inner hair cells. The second group of neurons (medial efferent neurons) is found in medial subnuclei of the superior olivary complex and sends axons to synapse with outer hair cells. Subpopulations of both medial and lateral olivocochlear neurons probably use gamma-aminobutyric acid (GABA) as a neurotransmitter. We have used an immunoperoxidase technique to detect GABA-like immunoreactivity (GABA-LI) in postnatal maturing rat cochleas. The GABA-LI appeared in the inner hair cell region by P3 (P1 = birth) and reached a mature appearance by P15-P16. In the outer hair cell region, GABA-like immunoreactive fibers and terminals could not be identified until P9 and they were only found in the apical end of the cochlea. There was a dual gradient of maturation of GABA-LI in the cochlea. The GABA-LI appeared first at the cochlear base and then extended towards the apex. It also appeared earlier (about a week) in the inner hair cell region than in the outer hair cell region. This dual gradient of maturation is in close agreement with previous data concerning the maturation of the cochlea.

Calcium dependence of differentiation of GABA immunoreactivity in spinal neurons

The developmental regulation of neurotransmitter synthesis has been extensively studied and appears in many cases to depend on electrical activity. The central nervous system of the Xenopus embryo and young larva is an attractive subject for such studies, since action potentials first elicited from Xenopus spinal neurons at the time of closure of the neural tube are long in duration and calcium-dependent. Moreover, cells exhibit spontaneous elevations of intracellular calcium during this early period as a consequence of calcium influx through voltage-dependent channels, which induces calcium release from intracellular stores. Since the early differentiation of Xenopus spinal neurons in dissociated cell culture parallels development in vivo, we have examined the maturation of gamma-aminobutyric acid (GABA) immunoreactivity in cultured neurons and explored its dependence on spontaneous calcium influx at early stages of development. We find that specific GABA immunoreactivity develops in spinal neurons in dissociated cell culture with the same time course previously defined in vivo. Additionally, this process requires calcium influx that occurs spontaneously through voltage-dependent channels. The appearance of GABA immunoreactivity is blocked by transcriptional inhibitors. The early appearance of GABA raises the possibility that it may play additional roles at early stages of development.

Frequency modulation of transmitter release

In 1952 Fatt and Katz recorded at a frog neuromuscular junction while stimulating the nerve and found "... that successive endplate potential responses varied in a step-like manner, corresponding to units of miniature endplate potentials" (J Physiol 117, 109-128). This led them to propose that fast neuromuscular transmission is 'quantal'. Quantal release is now commonly ascribed to a vesicular form of neurosecretion since vesicles have routinely been visualized in presynaptic terminals. The vesicular hypothesis (Del Castillo and Katz, 1955) assumes that quanta, or 'transmitter packets of standard size', are assembled and stored in the numerous vesicles routinely identified in micrographs of virtually all central and peripheral presynaptic nerve terminals. Simply stated, this model predicts that each one of the miniature synaptic signals (MSSs) follows from the exocytosis of one vesicle's contents. However, the time required for membrane fusion preceding exocytosis (Almers and Tse, 1990) and the variability in MSS amplitude and time course (Vautrin et al, 1992a,b) cannot readily be reconciled by a simple, exocytotic model of quantal release from preloaded vesicles. These difficulties with the original model have led us to re-evaluate MSSs generated at the classical peripheral synapse, the cholinergic neuromuscular junction of the mouse diaphragm, as well as at central synapses between embryonic hippocampal neurons mediated by gamma-aminobutyric acid (GABA). At these synapses, the release of GABA is also assumed to have classical quantal properties like peripheral acetylcholine release (Edwards et al, 1990). Our results show that at both synapses, progressive alterations in elementary signal properties can be induced in a remarkably rapid manner. The original report of preferred amplitudes and intervals in the spontaneous miniature signals (Fatt and Katz, 1952) has repeatedly been confirmed and is here incorporated into a dynamic model of fast synaptic transmission. Although MSSs exhibit variable rise-times and peak amplitudes, they can both be described in terms of synchronization of transmitter release. We have reviewed many experimental findings, which together strongly suggest that the original interpretation of Fatt and Katz (1952) regarding MSSs as reflecting the non-propagated 'neurogenic' activity of 'terminal spots' may be a useful concept to pursue since it may help to explain part of the underlying molecular basis of quantal release.(ABSTRACT TRUNCATED AT 400 WORDS)

Widespread Distribution of Synaptophysin, a Synaptic Vesicle Glycoprotein, in Growing Neurites and Growth Cones

Synaptophysin, a 38-kD glycoprotein, is one of the most abundant of the integral membrane proteins of small synaptic vesicles. The protein is widely distributed at synapses throughout the nervous system, where it is believed to be involved in the exocytosis of stored neurotransmitter. We show here that synaptophysin is also widely expressed in growing neurites and growth cones both in vitro and in vivo. In dissociated rat cerebral cortical cultures anti-synaptophysin antiserum (G-95) stains growth cones punctately as soon as they emerge from the cell body. In early cultures all neurites are immunoreactive. Later, synaptophysin is redistributed to become concentrated in axonal varicosities. In developing rat embryos, synaptophysin is expressed in the growing axons of, for instance, the spinal commissural interneurons and the parallel fibres of the cerebellar granule cells long before these neurons have established synaptic connections. These observations suggest that synaptic vesicle proteins like synaptophysin are functionally important in neuronal development.

The ontogeny of the uptake systems for glutamate, GABA, and glycine in synaptic vesicles isolated from rat brain

The ontogeny of the uptake of glutamate, GABA and glycine into synaptic vesicles isolated from rat brain has been investigated. The vesicular uptake of the three amino acids increased with developmental age in parallel with synaptogenesis, indicating a functional role of uptake of the amino acids by synaptic vesicles in the nerve terminals. Uptake of the amino acids by plasma membrane particles (synaptosomes) in brain homogenate showed a somewhat different developmental profile. The uptake of glutamate increased markedly with developmental time, while the uptake of GABA showed only a slight increase. Uptake of glycine by plasma membrane particles was very low and therefore not registered. The observed developmental increase in uptake of glycine by synaptic vesicles isolated from brain, supports previous reports indicating that glycine can be taken up by vesicles from non-glycine terminals.

Biochemical characterization of nerve growth cones isolated from both fetal and neonatal rat forebrains: the growth cone particle fraction mainly consists of axonal growth cones in both stages

Nerve growth cones are responsible for the exact pathway finding, and for the establishment of neurocytoarchitecture. To elucidate the developmental changes of biochemical characteristics of nerve growth cones, growth cone particle (GCP) fractions were isolated biochemically from embryonal day 17 (E17) rat forebrain and from postnatal day 5 (P5). There were no significant differences in protein phosphorylation pattern in a Ca(2+)-dependent manner between E17-GCP fraction and that of P5. As for the membrane lipid composition, molar ratios of cholesterol to total phospholipids were well conserved during these ages. The immunoreactivity to anti-synaptophysin monoclonal antibody as a marker of mature synaptic elements could not be detected either in E17-GCP or P5-GCP fractions. To exclude the possibility of the contamination of dendritic elements, RNA contents and immunoreactivity to anti-high molecular weight microtubule-associated protein 2 (MAP2) monoclonal antibody were examined. RNA contents of the GCP fractions were extremely low compared to those of other subcellular fractions both in E17 and P5. No immunoreactivities to anti-MAP2 antibody were observed in either GCP fraction. Our results suggest that the GCP fractions, isolated from forebrains of E17 to P5 rat, are free from the contamination of the synaptic elements, and that the GCP fractions are mainly composed of axonal growth cones.

The ontogeny of the uptake systems for glycine, GABA and glutamate in synaptic vesicles isolated from rat spinal cord-medulla

Synaptic vesicles have been isolated from rat spinal cord-medulla at different postnatal ages, and the ontogeny of the uptake of glycine, gamma-aminobutyric acid (GABA) and glutamate has been investigated. The accumulation of the 3 amino acids increased with increasing time after birth reaching adult level at about postnatal day 30. This developmental increase probably parallels the synaptogenesis and suggests a functional role of the uptake of the amino acids into synaptic vesicles in the nerve terminals. The developmental time course for these vesicular uptake systems was totally different from those of the corresponding plasma membrane uptakes.

Growth-regulated proteins and neuronal plasticity. A commentary

Growth-regulated proteins (GRPs) of the neuron are synthesized during outgrowth and regeneration at an increased rate and enriched in nerve growth cones. Therefore, they can be used to some degree as markers of neurite growth. However, these proteins are not unique to the growing neuron, and their properties are not known sufficiently to assign them a functional and/or causal role in the mechanisms of outgrowth. During synaptogenesis, GRPs decrease in abundance, and growth cone functions of motility and organelle assembly are being replaced by junctional contact and transmitter release. However, there is a stage during which growth cone and synaptic properties overlap to some degree. We propose that it is this overlap and its continuation that allow for synaptic plasticity in developing and adult nervous systems. We also propose a hypothesis involving (a) trophic factor(s) that might explain the regulation of synaptic sizes and collateral sprouting. Some GRPs, especially GAP43/B50/pp46/F1, are more prominent in adult brain regions of high plasticity, and they undergo change, such as phosphorylation, during long-term potentiation (LTP). Without precise functional knowledge of GRPs, it is impossible to use changes in such proteins to explain the plasticity mechanism. However, changes in these "growth markers" are likely to be an indication of sprouting activity, which would explain well the various phenomena associated with plasticity and learning in the adult. Thus, plasticity and memory may be viewed as a continuation of the developmental process into adulthood.

Regulated plasmalemmal expansion in nerve growth cones

To study the mechanisms underlying plasmalemmal expansion in the nerve growth cone, a cell-free assay was developed to quantify membrane addition, using ligand binding and sealed growth cone particles isolated by subcellular fractionation from fetal rat brain. Exposed versus total binding sites of 125I-wheat germ agglutinin were measured in the absence or presence of saponin, respectively, after incubation with various agents. Ca2(+)-ionophore A23187 in the presence of Ca2+ increases the number of binding sites (Bmax) but does not change their affinity (KD), indicating that new receptors appear on the plasma membrane. Similarly, membrane depolarization by high K+ or veratridine significantly induces, in a Ca2(+)-dependent manner, the externalization of lectin binding sites from an internal pool. Morphometric analysis of isolated growth cones indicates that A23187 and high K+ treatment cause a significant reduction in a specific cytoplasmic membrane compartment, thus confirming the lectin labeling results and identifying the plasmalemmal precursor. The isolated growth cones take up gamma-amino-butyric acid and serotonin, but show no evidence for Ca2(+)-dependent transmitter release so that transmitter exocytosis is dissociated from plasmalemmal expansion. The data demonstrate that plasmalemmal expansion in the growth cone is a regulated process and identify an internal pool of precursor membrane.

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.

Muscarinic acetylcholine receptors are expressed and enriched in growth cone membranes isolated from fetal and neonatal rat forebrain: pharmacological demonstration and characterization

Nerve growth cones, the motile tips of growing neurites, are closely related to the exact pathway finding, and their roles for synaptogenesis have been proposed to be modified by some neurotransmitters. In the present study, to clarify the expression and the ontogeny of muscarinic acetylcholine receptors in growth cones, growth cone membranes from fetal and neonatal rat forebrain were isolated, and muscarinic receptors in growth cone membrane were pharmacologically characterized, by using the [3H]quinuclidinyl benzilate as a labeled ligand. The specific binding sites for [3H]quinuclidinyl benzilate had already been detected in growth cone membrane on embryonic day (E)17 (Bmax = 557 fmol/mg protein: KD = 19.7 pM) and gradually increased in amount without significant changes in the KD values from E17 to postnatal day (P)5. [3H]Quinuclidinyl benzilate binding sites in growth cone membrane were several times higher than that in the P2-fraction-derived membranes, and in perinuclear membranes. Competitive inhibition studies showed that the proportion of high-affinity sites for pirenzepine (M1-subtype) to total [3H]quinuclidinyl benzilate binding sites in growth cone membrane was significantly lower than that in adult synaptic plasma membranes. In contrast, the proportion of high-affinity sites for AF-DX 116 (M2-subtype) was significantly higher than that in adult synaptic plasma membranes (E17 growth cone membrane: M1, 29.5%; M2, 56.9%; adult synaptic plasma membrane: M1, 63.6%, M2, 5.9%). Electron micrographic examination revealed that there were no significant morphological differences among growth cone particle fractions at the developmental stages which we examined, and that mature synaptic elements did not contaminate the growth cone particle fractions. Biochemical examination by electrophoresis and the phosphorylation study of the growth cone particle fractions showed that the protein composition and the phosphoprotein pattern did not change markedly during these stages. Our results suggest that muscarinic receptors were expressed and more concentrated in growth cone membrane than in other membrane portions from perinatal rat forebrain, and that they may play some role in the axonal guidance in growth cone via receptor subtype-specific signal transduction mechanisms.

Calcium-independent gamma-aminobutyric acid release from growth cones: role of gamma-aminobutyric acid transport

Neuronal growth cones isolated in bulk from neonatal rat forebrain have uptake and K(+)-stimulated release mechanisms for gamma-aminobutyric acid (GABA). Up to and including postnatal day 5, the K(+)-stimulated release of [3H]GABA and endogenous GABA is Ca2+ independent. At these ages, isolated growth cones neither contain synaptic vesicles nor stain for synaptic vesicle antigens. Here we examined the possibility that the release mechanism underlying Ca2(+)-independent GABA release from isolated growth cones is by reversal of the plasma membrane GABA transporter. The effects of two GABA transporter inhibitors, nipecotic acid and an analogue of nipecotic acid, SKF 89976-A, on K(+)-stimulated release of [3H]GABA from superfused growth cones were examined. Nipecotic acid both stimulated basal [3H]GABA release and enhanced K(+)-stimulated release of [3H]GABA, which indicates that this agent can stimulate GABA release and is, therefore, not a useful inhibitor with which to test the role of the GABA transporter in K(+)-stimulated GABA release from growth cones. In contrast, SKF 89976-A profoundly depressed both basal and K(+)-stimulated [3H]GABA release. This occurred at similar concentrations at which uptake was blocked. These observations provide evidence for a major role of the GABA transporter in GABA release from neuronal growth cones.

GABAergic growth cones: release of endogenous gamma-aminobutyric acid precedes the expression of synaptic vesicle antigens

Growth cone fractions isolated from neonatal [postnatal day 3 (P3)] rat forebrain contain GABAergic growth cones as demonstrated by immunofluorescence staining with monospecific antibodies to gamma-aminobutyric acid (GABA). HPLC analysis shows that GABAergic growth cones release this endogenous GABA when stimulated with high K+. Endogenous GABA release is Ca2(+)-independent and, in this respect, similar to that seen previously with [3H]GABA. Isolated growth cone fractions also exhibit a K(+)-stimulated, Ca2(+)-independent release of endogenous taurine. None of the other amino acids shown to be present in isolated growth cone fractions were released, including glutamate, aspartate, and glycine. A population of dissociated cerebral cortical neurones prepared from P1 rat forebrain were GABA-immunoreactive after 1 day in culture. The cell body, neurites, and growth cones of these neurones were all stained with GABA antibodies. At this time in culture, neurones did not stain with either of two antibodies to synaptic vesicle antigens, i.e., p65 and synaptophysin. Growth cones isolated from P3 rat forebrain were also not immunoreactive with these antibodies. After about 8 days in culture, when neurones had established extensive networks of long, varicose axons and elaborately branched dendrites, many neurones and their neurites were immunoreactive for GABA antibodies. At this time in culture, p65 and synaptophysin antibodies did stain neuronal cell bodies and particularly their varicose axons. Dendrites were not stained with synaptic vesicle antibodies. These results suggest that GABAergic neurones synthesize GABA during neurite outgrowth and that GABA is present in, and can be released from, the growth cones of these neurones. The presence of GABA in GABAergic growth cones is not associated with synaptic vesicles, which explains the Ca2+ independency of both endogenous and [3H]GABA release from these growth cones.

Biochemical pharmacology of isolated neuronal growth cones: implications for synaptogenesis

The neuronal growth cone is critical to the establishment of neuronal polarity through its motile, pathfinding and target recognition properties exhibited during synaptogenesis. Subcellular fractionation procedures yielding milligram quantities of isolated growth cones has allowed for biochemical and pharmacological investigation of intrinsic growth cone components that are likely to be involved in regulation of growth cone function in neuronal development. These 'mapping' studies of growth cone components are prerequisites to elucidating the mechanisms by which extracellular factors influence the motility, adhesion and directed growth of the growth cone. For example, neurotransmitters and polypeptide growth factors which have been shown in other systems to modulate growth cone behavior are presumed to act through receptors on the growth cone, inducing second-messenger molecule formation and consequent modification and regulation of proteins effecting the response(s) of the growth cone (i.e. proteins involved in motility, adhesion and membrane turnover). In a relatively short period of time, work with the isolated growth cone preparation has identified, in independent studies, many of the elements involved in this proposed scheme of events, including transmitter receptors, second-messenger cascades, and second-messenger post-translational modifications. An obvious future goal will be to analyze in more detail the intracellular events, and the relationships between them, in the growth cone and how they transmit extracellular signals into responses such as motility and adhesivity which underly the growth cone's synaptogenic properties. It is to be expected that much of this information will come forth from experimentation with the isolated growth cone preparation.