Development of GABAergic function of dissociated hippocampal cultures from fetal mice

Synaptic glutamate release is modulated by the Na+ -driven Cl-/HCO₃⁻ exchanger Slc4a8

On the one hand, neuronal activity can cause changes in pH; on the other hand, changes in pH can modulate neuronal activity. Consequently, the pH of the brain is regulated at various levels. Here we show that steady-state pH and acid extrusion were diminished in cultured hippocampal neurons of mice with a targeted disruption of the Na(+)-driven Cl(-)/HCO(3)(-) exchanger Slc4a8. Because Slc4a8 was found to predominantly localize to presynaptic nerve endings, we hypothesize that Slc4a8 is a key regulator of presynaptic pH. Supporting this hypothesis, spontaneous glutamate release in the CA1 pyramidal layer was reduced but could be rescued by increasing the intracellular pH. The reduced excitability in vitro correlated with an increased seizure threshold in vivo. Together with the altered kinetics of stimulated synaptic vesicle release, these data suggest that Slc4a8 modulates glutamate release in a pH-dependent manner.

GAT-1 acts to limit a tonic GABAA current in rat CA3 pyramidal neurons at birth

Tonic activation of GABA(A) receptors takes place before the development of functional synapses in cortical structures. We studied whether inefficient GABA uptake might explain the presence of a tonic GABA(A)-mediated current (I(GABA-A)) in early postnatal hippocampal pyramidal neurons. The data show, however, that the tonic I(GABA-A) is enhanced by the specific blocker of GABA transporter-1 (GAT-1), NO-711 (1-[2-[[(Diphenylmethyleneimino]oxy]ethyl]-1,2,5,6-tetrahydro-3-pyridinecarboxylic acid hydrochloride), at birth in rat CA3 pyramidal neurons. NO-711 also prolonged the duration of GABA transients during endogenous hippocampal network events (known as giant depolarizing potentials) at postnatal day 0. The endogenous tonic I(GABA-A) was seen and it was enhanced by NO-711 in the presence of tetrodotoxin, which itself had only a minor effect on the holding current under control conditions. This indicates that the source of interstitial GABA is largely independent of action-potential activity. The tonic I(GABA-A) in neonatal CA3 pyramidal neurons was increased by zolpidem, indicating that at least a proportion of the underlying GABA(A) receptors contain gamma2 and alpha1-alpha3 subunits. The present data point to a significant role for GAT-1 in the control of the excitability of immature hippocampal neurons and networks.

A fast flexible ink-jet printing method for patterning dissociated neurons in culture

We present a new technique that uses a custom-built ink-jet printer to fabricate precise micropatterns of cell adhesion materials for neural cell culture. Other work in neural cell patterning has employed photolithography or "soft lithographic" techniques such as micro-stamping, but such approaches are limited by their use of an un-alterable master pattern such as a mask or stamp master and can be resource-intensive. In contrast, ink-jet printing, used in low-cost desktop printers, patterns material by depositing microscopic droplets under robotic control in a programmable and inexpensive manner. We report the use of ink-jet printing to fabricate neuron-adhesive patterns such as islands and other shapes using poly(ethylene) glycol as the cell-repulsive material and a collagen/poly-D-lysine (PDL) mixture as the cell-adhesive material. We show that dissociated rat hippocampal neurons and glia grown at low densities on such patterns retain strong pattern adherence for over 25 days. The patterned neurons are comparable to control, un-patterned cells in electrophysiological properties and in immunocytochemical measurements of synaptic density and inhibitory cell distributions. We suggest that an inexpensive desktop printer may be an accessible tool for making micro-island cultures and other basic patterns. We also suggest that ink-jet printing may be extended to a range of developmental neuroscience studies, given its ability to more easily layer materials, build substrate-bound gradients, construct out-of-plane structure, and deposit sources of diffusible factors.

Morphology of neurons cultured from subdivisions of the mouse cochlear nucleus

This study was designed to characterize the dendritic organization of cochlear nucleus (CN) cells grown in primary cell culture and to assess differences among cultures grown from different regions of CN. Cultures were prepared from postnatal mice and processed using microtubule-associated protein 2 (MAP2) or gamma-aminobutyric acid (GABA) immunohistochemistry. CN neurons were successfully cultured from preparations grown from either the anteroventral subdivision of the nucleus (AVCN), the posterior region [posteroventral (PVCN) and dorsal (DCN) subnuclei], or the whole CN, although the cultured neurons did not exhibit complex dendritic patterns characteristic of CN neurons in vivo. Neurons cultured from the entire nucleus exhibited an increased rate of survival compared to those cultured from either the anterior or posterior regions, although similar types of cells were observed in all preparations. The majority of cultured CN neurons were GABA-positive and had soma areas that were similar to the areas of immature GABAergic neurons measured in CN sections. Small cells (soma areas <or=60 microm(2)) with one to three symmetrically organized dendrites and large non-GABAergic cells (>or=120 microm(2)) were also present in significant numbers. Overall, CN cultures consisted of a heterogeneous population of neurons that had less elaborate dendritic organizations than cells of corresponding size that have been described in adult animals in vivo.

Characterization of GABAergic neurons in hippocampal cell cultures

The morphological characteristics of GABAergic neurons and the distribution of GABAergic synaptic terminals were examined in cultures of hippocampal neurons from 4-35 days in vitro. Neurons expressing GABA immunoreactivity represented about 6% of the total number of cultured neurons at all time points. Although the morphological characteristics of GABAergic cells suggested a heterogeneous population, GABAergic cells as a class were notably different from the non-GABAergic, presumably pyramidal cells. Most GABAergic cells had more fusiform or polygonal shaped somata, non-spiny and less tapering dendrites and appeared more phase-dense than nonGABAergic cells. Quantitative analysis revealed that GABAergic cells had fewer primary dendrites, more elongated dendritic arbors, and longer dendritic segments than non-GABAergic neurons-characteristics that are similar to GABAergic cells in situ. Double immunostaining revealed that GAD65-positive varicosities were also immunopositive for synapsin I, suggesting that GAD65-positive varicosities that contacted somata and dendrites represented presynaptic specializations. Confocal microscopy revealed the proportion of the synaptic specializations on the cell soma that were GAD65-positive was greater than on the dendrites, suggesting that somata and dendrites differ in their ability to induce the formation of presynaptic specializations by GABAergic axons. These data indicate that the GABAergic cells that develop in culture exhibit distinctive morphological characteristics and participate in different synaptic interactions that nonGABA cells. Thus many of the features that distinguish GABAergic neurons in culture are reminiscent of the characteristics that distinguish GABAergic neurons in situ.

Development of GABAergic synaptic connections in vivo and in cultures from the rat superior colliculus

Synaptic activity in the superficial (i.e. visual) layer of the superior colliculus was investigated with intracellular microelectrodes using a preparation of the isolated superfused tectum from neonatal rat. It was found that by postnatal day 9 (i.e. before eye opening) the majority of neurons in the superficial gray layer (SGS, stratum griseum superficiale) were already capable of generating Cl(-)-dependent inhibitory postsynaptic potentials (IPSPs) in response to intracollicular stimulation. Properties and development of GABAergic synaptic connections were further characterized in a dissociated cell culture from the SGS. The cultures were prepared from E21 rat embryos and studied between 1 and 38 days in vitro (DIV). gamma-[3H]aminobutyric acid ([3H]GABA) uptake served to identify GABAergic neurons and to estimate their relative density. Axon terminals were labeled by indirect immunostaining for glutamic acid decarboxylase (GAD) and examined with light (LM) and electron microscopy (EM). Responsiveness to exogenous and endogenous GABA was investigated by recording ionic currents with patch clamp techniques. [3H]GABA uptake-positive neurons constituted about 40% of the whole cellular population dissociated from the SGS of E21 rats. After 2 weeks in culture, [3H]GABA uptake was observed in 45-60% of the cells with neuronal features. The relative number of GAD-immunoreactive neuronal perikarya ranged from 28 to 39%, after 2 weeks in vitro. Responsiveness to exogenous GABA was found in all freshly plated neurons. Release of GABA could be demonstrated after 2 DIV by recording spontaneous bicuculline-sensitive Cl- currents. These currents had the characteristics of GABAA receptor-mediated synaptic currents. However, even as late as DIV 6, very few vesicle-containing axonal terminals apposing postsynaptic specializations were revealed with EM. GAD-labeled puncta became clearly visible only after DIV 10-12. Between DIV 14 and 21, the intensity of immunostaining and the density of GAD-labeled synaptic contacts increased, reaching a maximum around DIV 28. GAD-positive puncta covered both neurons and non-neuronal cells. At the level of EM, GAD-positive terminals were shown to establish synaptic contacts with neuronal somata and processes, forming in the majority of cases (22 out of 32 stained terminals) symmetrical contacts. It is concluded that in the SGS of the rat superior colliculus GABAergic neurons and GABAA receptors are present before birth. In dissociated cell cultures ionic currents can be generated in response to endogenous GABA before axonal terminals of GABAergic neurons fully mature. Finally, our experiments show that visual activity is not a prerequisite for the formation of GABAergic synapses between neurons of the SGS.

Expression of somatostatin and GABA immunoreactivity in cultures of rat hippocampus

Somatostatin (SOM) synthesis and release were studied with radioimmunoassay and immunocytochemical techniques in rat fetal hippocampal neurons maintained in monolayer tissue culture. SOM immunoreactivity increased from undetectable to over 4,000 pg/ml in media and over 2,500 pg/culture in neurons by 3 to 5 weeks. After 3 weeks, approximately 11% of the neurons stained for SOM. Gamma-aminobutyric (GABA) immunoreactivity was present in hippocampal neurons from 1 day to 5 weeks with 40-50% of the neurons staining for GABA by 5 weeks in vitro. Costaining neurons for SOM and GABA revealed that 63% which were positive for SOM also stained for GABA.

Development and selective neurodegeneration in cell cultures from different hippocampal regions

Previous studies have shown that pyramidal neurons in hippocampal regions CA1 and CA3 are selectively vulnerable in several neurodegenerative disorders and that a subpopulation of pyramidal neurons in cell cultures of embryonic hippocampus are sensitive to glutamate neurotoxicity. In order to determine whether the patterns of cell loss seen in situ correlate with intrinsic differences in neuronal sensitivities to glutamate-induced degeneration acquired during development, we characterized cultures established from different regions of postnatal rat hippocampus and then examined neuronal sensitivity to glutamate. Tissue corresponding to the dentate gyrus (DG) and regions CA1, CA2 and CA3 of Ammon's horn was removed by microdissection from transverse hippocampal slices and was used to establish cultures of dissociated cells. Cultures from all 4 regions contained 3 major morphological classes of neurons; pyramidal-like, bipolar and stellate. Pyramidal-like neurons comprised the majority of neurons in all cultures; these neurons extended one long and branching axon, and one or more short dendrites. Immunocytochemistry showed that all neurons possessed high levels of glutamate-like and gamma-aminobutyric acid (GABA)-like immunoreactivity when grown in isolation. In contrast, when bipolar and pyramidal neurons were cultured in contact with glial cells, glutamate and GABA immunoreactivity were selectively reduced in the bipolar and pyramidal cells, respectively, suggesting that cell interactions influence neurotransmitter phenotype. Subpopulations of hippocampal neurons from each hippocampal region were vulnerable to glutamate-induced neurotoxicity. Bipolar and stellate cells were resistant to glutamate, while pyramidal-like neurons showed varying degrees of sensitivity to glutamate depending upon which region they were taken from. Experiments with specific glutamate receptor agonists and antagonists demonstrated that both non N-methyl-D-aspartic acid (NMDA) receptors and NMDA receptors mediated glutamate-induced degeneration. There were clear differences in the vulnerability of the pyramidal-like neuron populations in cultures from the different hippocampal regions. The rank order of the vulnerability of pyramidal-like neurons to glutamate-induced neurodegeneration between regions in culture was: DG less than CA2 less than CA3 less than CA1. This pattern of selective vulnerability in cell culture corresponds directly to the pattern of selective cell loss seen in situ in Alzheimer's disease, epilepsy, and stroke suggesting that intrinsic neuronal differences in glutamate sensitivity may be involved in these disorders.

Anatomical and Physiological Properties of GABAergic Neurotransmission in Organotypic Slice Cultures of Rat Hippocampus

The anatomical and physiological properties of GABAergic inhibitory neurotransmission were investigated in organotypic slice cultures of rat hippocampus. Interneurons and terminal-like elements containing GABA-like immunoreactivity were numerous in tissue kept for 13 - 26 days in culture and showed a similar morphology and distribution to those known from investigations on the hippocampal formation in situ. Furthermore, after 8 - 30 days in culture, spontaneous and evoked IPSPs were observed in all CA3 pyramidal cells tested, resulting from an increase in chloride conductance, and were shown to be mediated by activation of GABA receptors. No functional decrement in the efficacy of GABAergic inhibitory synaptic transmission following chronic isolation and long-term maintenance in vitro was noticed. In particular, neither the magnitude of the synaptic conductance underlying the inhibitory postsynaptic currents nor its reversal potential varied with time in culture. Taken together, the present physiological and immunohistochemical data show that GABAergic inhibition is well expressed in organotypic hippocampal slice cultures and is maintained over periods of at least 4 weeks in vitro.

Gabaergic neurons of the hippocampus: development in homotopic grafts and in dissociated cell cultures

The hippocampus taken from E18-E19 rat embryos was dissociated into a cell suspension and was either grafted into the hippocampus of adult rats or cultured. The growth of GABAergic neurons was examined using a GABA directed antiserum. The implanted tissue was capable of survival and growth without exhibiting a laminar organization. Most of the various morphological neuronal types could be observed, establishing different types of synapses; however, granule neurons were rarely encountered. A substantial proportion of GABA-positive neurons was detected within the graft with profuse labelling of the neuropil. In cultures issued from the same cell suspension, GABA-immunoreactive neurons were numerous and had different morphologies. Altogether these data suggest that GABA neurons express a high potential for growth and sprouting in vitro and in vivo.

Development of gamma-aminobutyric acid (GABA)ergic neurons in cerebral cortical neurons in primary culture

The developmental patterns of gamma-aminobutyric acid (GABA)ergic neurons in primary culture obtained from the neopallium of 15-day-old fetus of mouse were investigated in terms of morphological features, GABA metabolism and GABA receptor binding. Morphological investigations revealed that these cells possessed typical features of neurons and the formation of synapses was detected at 10 days after the inoculation. During neuronal growth on polylysine surfaces, GABA contents and activity of GABA transaminase (GABA-T) showed a progressive increase in the time of culture. Similarly, L-glutamic acid decarboxylase (GAD) showed a progressive elevation during neuronal development in vitro, which corresponded well with the change in immunoreactivity to anti-GAD examined immunohistochemically. In addition, the high K+-evoked release of [3H]GABA also showed an enhancement during the growth in vitro. The numbers of binding sites (Bmax) for [3H]muscimol and [3H]flunitrazepam (FLN) also showed increases with the time of incubation, although affinity (Kd) to the labeled ligands did not show any noticeable changes. Moreover, it was observed that [3H]FLN binding was enhanced by GABA even in neurons cultured for 7 days. These results indicate that cerebral cortical neurons in primary culture possess GABA biosynthesizing and degrading systems including a high-affinity uptake mechanism for GABA. The present results also indicate that these cells possess synaptic contacts as well as GABAA receptors coupled with benzodiazepine receptor from a relatively early stage of cellular development.

GABAergic neurons in rat hippocampal culture

The experiments described here were designed to study biochemical and histological measures of gamma-aminobutyric acid (GABA) uptake and glutamic acid decarboxylase (GAD) in primary dissociated cell cultures prepared from 17-21-day fetal rat hippocampus. Preparations from all ages of animals, except 21-day fetuses, were enriched in GABAergic neurons, when compared to the adult hippocampus in situ. These cells comprise 30-50% of the large, phase-bright, process-bearing cells in hippocampal cultures as estimated by autoradiography of GABA uptake and GAD immunocytochemistry. Neurons concentrate GABA by a relatively slow but high-affinity process (Km = 2.6 microM) that has considerably higher maximum velocity than glial uptake (Vmax = 479 pmol/mg protein/min for neurons and 31 pmol/mg protein/min for glia). No low-affinity uptake process was noted in neurons or glia. GABA uptake into neurons was competitively inhibited by cis-4-OH-nipecotic acid (Ki = 39 +/- 11 microM). These cultures also possess considerable GAD activity, up to 6 nmol/mg protein/min in one-month-old cultures, which approximates that of the adult hippocampus. Both GABA uptake and GAD activity increased with time in culture. The enrichment of GABAergic markers indicates that this preparation may be useful for the detailed study of hippocampal GABAergic neurons.

Studies of neurotransmitter chemistry of central nervous system neurons in primary tissue culture

Primary tissue culture methods have been applied to various areas of the central nervous system, including cerebral cortex, spinal cord, cerebellum, hippocampus, hypothalamus, striatum, mesencephalon, lower brain stem and retina. Experimental studies in vitro involving central neurotransmission are discussed here. Information gleaned from such studies impacts on neurotransmitter identification, neuronal development, patterns of receptor distribution, peptidergic transmission, transmitter metabolism, synaptogenesis and the regulation of synaptic development.

Development of a high-affinity GABA uptake system in embryonic amphibian spinal neurons

High-affinity uptake systems for amino acid neurotransmitter precursors have been highly correlated with the use of the particular amino acid or its derivative as a transmitter. We have found interneurons in the Xenopus embryo spinal cord which accumulate GABA by a high-affinity uptake system. They originate near the end of gastrulation and their ability to accumulate GABA first appears at the early tail bud stage. By position and appearance they are comparable to some of the embryonic interneurons described by A. Roberts and J. D. W. Clarke (1982, Phil. Trans. R. Soc. London Ser. B 296, 195-212). GABA-accumulating neurons also develop in dissociated cell cultures made from the presumptive spinal cord of neural plate stage Xenopus embryos. GABA accumulation in cultured neurons, as in cells in vivo, occurs via a high-affinity uptake system; GABA-accumulating cells have the same time of origin as the cells in vivo, and the ability to accumulate GABA in the population of cultured neurons appears at a time equivalent to that observed in intact sibling embryos. Thus it seems likely that the population of GABA-accumulating neurons developing in cell culture corresponds to the GABA-accumulating interneurons in vivo. The development of these neurons in dissociated cell cultures permits perturbation experiments that would be difficult to perform in vivo. We have examined the development of high-affinity GABA uptake in conditions that permit no electrical impulse activity in the cultures. The onset and extent of development of GABA accumulation in the neuronal population are normal under these conditions.

Uptake of [3H]GABA by oligodendrocytes in dissociated brain cell culture: a combined autoradiographic and immunocytochemical study

Uptake of [3H]GABA by dissociated mixed cell cultures of fetal mouse brain was studied using light microscopic autoradiography. Major cell types in the cultures were identified and quantified by immunocytochemical localization of reliable cell type-specific antigenic markers. In 12 days in vitro (DIV) cultures [3H]GABA uptake was predominantly into neurons and oligodendrocytes, whilst at 28 DIV the only surface cells labeled were oligodendrocytes. This was confirmed by complement-dependent antibody-mediated cytotoxicity against galactocerebroside-positive oligodendrocytes. There was a moderate labeling of almost all flat cells, the majority of which were glial fibrillary acidic protein (GFAP)-positive astrocytes. Heavily labeled astrocytes were only occasionally observed. Oligodendrocytes accumulated [3H]GABA more rapidly than astrocytes but slower than neurons. Oligodendroglial labeling was predominantly over the cell body, whereas neuronal labeling was more uniformly distributed over cell body and processes. The uptake was inhibited by diaminobutyric acid (DABA) and nipecotic acid, but not by beta-alanine, and thus had similar characteristics to neuronal rather than astroglial uptake. Oligodendrocytes did not accumulate [3H]beta-alanine, which labeled only astrocytes. Oligodendroglial [3H]GABA uptake was Na+-dependent and sensitive to ouabain, but was only slightly enhanced by aminooxyacetic acid (AOAA), whereas astroglial uptake was not sensitive to ouabain but was markedly enhanced by AOAA. The results indicate that oligodendrocytes, in addition to astrocytes, may also be involved in the modification of neuronal function by the uptake and inactivation of neuroactive substances.

Alterations in uptake and release rates for GABA, glutamate, and glutamine during biochemical maturation of highly purified cultures of cerebral cortical neurons, a GABAergic preparation

This study demonstrates that virtually homogenous cultures of mouse cerebral neurons, obtained from 15-day-old embryos, differentiate at least as well as cultures which in addition contain astrocytes. This was indicated by glutamate decarboxylase activity which within 2 weeks rose from a negligible value to twice the level in the adult mouse cerebral cortex, and by a gamma-aminobutyric acid (GABA) uptake rate which quadrupled during the second week in culture and reached higher values than in brain slices. Within the same period, the GABA content increased four to five times to 75 nmol/mg protein, and a potassium-induced increase in [14C]GABA efflux became apparent. Although the development was faster than in vivo, optimum differentiation required maintenance of the cultures beyond the age of 1 week. Uptake and release rates for glutamate and glutamine underwent much less developmental alteration. At no time was there any potassium-induced release of radioactivity after exposure to [14C]glutamate, and the glutamate uptake was only slightly increased during the period of GABAergic development. This indicates that exogenous glutamate is not an important GABA precursor. Similarly, glutamine uptake was unaltered between days 7 and 14, although a small potassium-induced release of radioactivity after loading with glutamine suggests a partial conversion to GABA.

Selective uptake of tritiated glycine, GABA and D-aspartate by retinal cells in culture: a study using autoradiography and simultaneous immunofluorescence

Autoradiographic studies on the uptake of 3 tritiated amino acids by cell cultures of rat retina are reported. At 0.2 microM [3H]-GABA is taken up with great selectivity by a minority of medium to large multipolar cells. At 2 microM [3H]glycine is taken up with slightly less selectivity by a similar proportion of multipolar cells; the proportions of cells labeled by either amino acid are additive which indicates that they are separate populations. In simultaneous autoradiographical and indirect immunofluorescence experiments, both of these cell classes were found to bind tetanus toxin and are therefore presumably neurons. At 0.4 microM [3H]D-aspartate is preferentially taken up by virtually all small spheroid cells which lack long processes (putative immature photoreceptors), as well as by a minority of larger multipolar cells. All 3 tritiated amino acids also label flattened cells which include retinal glia. The results are discussed in relation to similar experiments performed on undissociated retina and we conclude that these experiments allow us to identify and quantify immature photoreceptors and certain sub-types of amacrine cells in these cultures.