Biochemical and morphological studies on GABA neurons in reaggregate culture

Characterization of three-dimensional rat central nervous system culture maturation, with applications to monitor cholinergic integrity

Studying age-related neuropathologies in vitro requires a three-dimensional (3D) culture system presenting mature phenotypes. In this study, we aimed to determine whether aged reaggregate cultures physiologically represent mature brain tissue. Results support that embryo-derived rat central nervous system (CNS) reaggregate cultures develop into mature-like tissues, comparable to in vivo maturation, including the following characteristics: (a) progressive reduction in cell proliferation (reduced anti-Ki-67 immunoreactivity), (b) progressive restriction of long neurite growth potential (as explant cultures), and (c) increased and sustained synaptic enzyme (acetylcholine esterase, AChE) activity. The acquisition of mature-like reaggregate cultures has allowed us to pursue the hypothesis that the physiological integrity of 3D CNS cultures may be monitored by synaptic enzyme activity. To assess this hypothesis, mature-like reaggregates were exposed to H₂ O₂ , glutamate, or amyloid β(1-42); each resulted in diminished AChE activity. H₂ O₂ exposure resulted in nuclear fragmentation. Glutamate and amyloid β(1-42) exposure resulted in acetylcholine content reduction. Simultaneous reduction of AChE activity and acetylcholine content verified diminished cholinergic integrity. This scheme exploiting synapse enzyme activity of mature-like 3D CNS tissue is therefore applicable to age-related neuropathology research including in vitro screening of conditions potentially affecting synapse integrity, including the promotion of dementia.

Immortalization of embryonic mesencephalic dopaminergic neurons by somatic cell fusion

To facilitate the study of trophic interactions between mesencephalic dopaminergic neurons and their target cells, clonal hybrid cell lines have been developed from rostral mesencephalic tegmentum (RMT) of the 14-day-old embryonic mouse employing somatic cell fusion techniques. Among the hybrid cell lines obtained, one contains a high level of dopamine (DA), another predominantly 3,4-dihydroxyphenylalanine (DOPA), and a third no detectable catecholamines. The hybrid nature of the cell lines is supported by karyotype analysis and by the expression of adhesion molecules as assessed by aggregation in rotation-mediated cell culture. The DA cell line shows neuronal properties including catecholamine-specific histofluorescence, neurite formation with immunoreactivity to neurofilament proteins, and large voltage-sensitive sodium currents with the generation of action potentials. In contrast to the pheochromocytoma cell line (PC12), the dopamine content of the DA hybrid cell line is depleted by low concentrations of N-methyl-4-phenylpyridinium ion (MPP+), the active metabolite of the neurotoxin N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP).

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.

Regulation by cAMP and vasoactive intestinal peptide of phosphorylation of specific proteins in striatal cells in culture

We have studied three low molecular weight phosphoproteins, ARPP-16, ARPP-19, and ARPP-21 (cAMP-regulated phosphoproteins of Mr 16,000, 19,000, and 21,000, respectively) in reaggregate cultures from various regions of fetal mouse brain. ARPP-16 and ARPP-21 were detected only in striatal and cortical cultures. In contrast, ARP-19, which is structurally related to ARPP-16, was also present in reaggregate cultures prepared from thalamus and ventral and dorsal mesencephalon, as well as in monolayer cultures of astroglial cells. In striatal aggregates cultured over a 3-week period, the relative levels of ARPP-16, ARPP-21, and synapsin I/protein IIIa (synaptic vesicle-associated phosphoproteins closely related to each other and treated as a single entity in the present study) increased with time, whereas the level of ARPP-19 decreased. Incubation of striatal aggregates with 8-Br-cAMP, forskolin, or vasoactive intestinal peptide increased the phosphorylation of all these proteins. We conclude that the state of phosphorylation of two proteins enriched in specific neurons (ARPP-16 and ARPP-21) and two more widely distributed proteins (ARPP-19 and synapsin I/protein IIIa) is regulated by cAMP and vasoactive intestinal peptide in striatal cells in culture. These phosphoproteins may therefore play a role in mediating some of the actions of vasoactive intestinal peptide in the caudate-putamen.