Glutamate decarboxylase in developing rat neocortex: does it correlate with the differentiation of GABAergic neurons and synapses?

CPP-115, a vigabatrin analogue, decreases spasms in the multiple-hit rat model of infantile spasms

OBJECTIVE

Infantile spasms (IS) have poor outcomes and limited treatment options, including vigabatrin, a γ-aminobutyric acid (GABA) aminotransferase inactivator. Vigabatrin has been associated with retinal toxicity. A high affinity vigabatrin analogue (CPP-115; Catalyst Pharmaceutical Partners, Inc., Coral Gables, FL, U.S.A.) has shown lower risk of retinal toxicity. Here, we test the efficacy of CPP-115 in reducing spasms and its tolerability in the multiple-hit rat model of IS, in which daily vigabatrin reduced spasms for only one day, but was not well tolerated.

METHODS

Male rats were treated with the protocol of the multiple-hit model of IS on postnatal day 3 (PN3). Using a randomized, blinded, vehicle-controlled, dose-response study design, CPP-115 (0.1, 1, or 5 mg/kg intraperitoneally [i.p.]) or vehicle was given daily (PN4-12) or as a single injection (PN7) after spasm onset. Intermittent video- or video-electroencephalography (EEG) monitoring was done. Secondary end points included the following: daily weights, survival, performance on open field activity, surface righting time, and negative geotaxis (PN3-20), horizontal bar (PN13-20), and Barnes maze (PN16-19). Statistics used a linear mixed model of raw or normalized log-transformed data, taking into account the repeated observations on each animal.

RESULTS

The lower CPP-115 doses (0.1-1 mg/kg/day, PN4-12) reduced spasms between PN6 and 7 without increasing mortality. CPP-115 at 5 mg/kg/day (PN4-12) reduced spasms earlier (PN5), but was eventually lethal. A single CPP-115 injection (1 mg/kg, i.p.) decreased electroclinical spasms acutely but transiently. CPP-115 transiently improved the probability to >50% reduction of spasms, but did not accelerate spasm cessation. CPP-115 did not alter neurodevelopmental outcomes or visuospatial learning.

SIGNIFICANCE

We provide proof-of-concept evidence that CPP-115, a vigabatrin analogue, decreases spasms in the multiple-hit rat model of IS at considerably lower and better tolerated doses than vigabatrin did in our previous studies. Further optimization of the treatment protocol is needed. CPP-115 may be a promising new candidate treatment for IS with better tolerability than vigabatrin.

Glutamatergic and GABAergic neurotransmitter cycling and energy metabolism in rat cerebral cortex during postnatal development

The contribution of glutamatergic and gamma-aminobutyric acid (GABA)ergic neurons to oxidative energy metabolism and neurotransmission in the developing brain is not known. Glutamatergic and GABAergic fluxes were assessed in neocortex of postnatal day 10 (P10) and 30 (P30) urethane-anesthetized rats infused intravenously with [1,6-(13)C(2)]glucose for different time intervals (time course) or with [2-(13)C]acetate for 2 to 3 h (steady state). Amino acid levels and (13)C enrichments were determined in tissue extracts ex vivo using (1)H-[(13)C]-NMR spectroscopy. Metabolic fluxes were estimated from the best fits of a three-compartment metabolic model (glutamatergic neurons, GABAergic neurons, and astroglia) to the (13)C-enrichment time courses of amino acids from [1,6-(13)C(2)]glucose, constrained by the ratios of neurotransmitter cycling (V(cyc))-to-tricarboxylic acid (TCA) cycle flux (V(TCAn)) calculated from the steady-state [2-(13)C]acetate enrichment data. From P10 to P30 increases in total neuronal (glutamate plus GABA) TCA cycle flux (3 x ; 0.24+/-0.05 versus 0.71+/-0.07 micromol per g per min, P<0.0001) and total neurotransmitter cycling flux (3.1 to 5 x ; 0.07 to 0.11 (+/-0.03) versus 0.34+/-0.03 micromol per g per min, P<0.0001) were approximately proportional. Incremental changes in total cycling (DeltaV(cyc(tot))) and neuronal TCA cycle flux (DeltaV(TCAn(tot))) between P10 and P30 were 0.23 to 0.27 and 0.47 micromol per g per min, respectively, similar to the approximately 1:2 relationship previously reported for adult cortex. For the individual neurons, increases in V(TCAn) and V(cyc) were similar in magnitude (glutamatergic neurons, 2.7 x versus 2.8 to 4.6 x ; GABAergic neurons, approximately 5 x versus approximately 7 x), although GABAergic flux changes were larger. The findings show that glutamate and GABA neurons undergo large and approximately proportional increases in neurotransmitter cycling and oxidative energy metabolism during this major postnatal growth spurt.

Brain-derived neurotrophic factor mediates activity-dependent dendritic growth in nonpyramidal neocortical interneurons in developing organotypic cultures

Brain-derived neurotrophic factor (BDNF) promotes postnatal maturation of GABAergic inhibition in the cerebral and cerebellar cortices, and its expression and release are enhanced by neuronal activity, suggesting that it acts in a feedback manner to maintain a balance between excitation and inhibition during development. BDNF promotes differentiation of cerebellar, hippocampal, and neostriatal inhibitory neurons, but its effects on the dendritic development of neocortical inhibitory interneurons remain unknown. Here, we show that BDNF mediates depolarization-induced dendritic growth and branching in neocortical interneurons. To visualize inhibitory interneurons, we biolistically transfected organotypic cortical slice cultures from neonatal mice with green fluorescent protein (GFP) driven by the glutamic acid decarboxylase (GAD)67 promoter. Nearly all GAD67-GFP-expressing neurons were nonpyramidal, many contained GABA, and some expressed markers of neurochemically defined GABAergic subtypes, indicating that GAD67-GFP-expressing neurons were GABAergic. We traced dendritic trees from confocal images of the same GAD67-GFP-expressing neurons before and after a 5 d growth period, and quantified the change in total dendritic length (TDL) and total dendritic branch points (TDBPs) for each neuron. GAD67-GFP-expressing neurons growing in control medium exhibited a 20% increase in TDL, but in 200 ng/ml BDNF or 10 mm KCl, this increase nearly doubled and was accompanied by a significant increase in TDBPs. Blocking action potentials with TTX did not prevent the BDNF-induced growth, but antibodies against BDNF blocked the growth-promoting effect of KCl. We conclude that BDNF, released by neocortical pyramidal neurons in response to depolarization, enhances dendritic growth and branching in nearby inhibitory interneurons.

Neonatal monosodium glutamate treatment modifies glutamic acid decarboxylase activity during rat brain postnatal development

Monosodium glutamate (MSG) produces neurodegeneration in several brain regions when it is administered to neonatal rats. From an early embryonic age to adulthood, GABA neurons appear to have functional glutamatergic receptors, which could convert them in an important target for excitotoxic neurodegeneration. Changes in the activity of the GABA synthesizing enzyme, glutamic acid decarboxylase (GAD), have been shown after different neuronal insults. Therefore, this work evaluates the effect of neonatal MSG treatment on GAD activity and kinetics in the cerebral cortex, striatum, hippocampus and cerebellum of the rat brain during postnatal development. Neonatal MSG treatment decreased GAD activity in the cerebral cortex at 21 and 60 postnatal days (PD), mainly due to a reduction in the enzyme affinity (K(m)). In striatum, the GAD activity and the enzyme maximum velocity (V(max)) were increased at PD 60 after neonatal MSG treatment. Finally, in the hippocampus and cerebellum, the GAD activity and V(max) were increased, but the K(m) was found to be lower in the experimental group. The results could be related to compensatory mechanisms from the surviving GABAergic neurons, and suggest a putative adjustment in the GAD isoform expression throughout the development of the postnatal brain, since this enzyme is regulated by the synaptic activity under physiological and/or pathophysiological conditions.

Maturation of anoxia-induced gasping in the rat: potential role for N-methyl-D-aspartate glutamate receptors

After anoxia-induced apnea, gasping remains the last operative mechanism for survival. In developing rats, the gasping response to anoxia exhibits triphasic characteristics. Because anoxia is associated with enhanced release of glutamate, we hypothesized that N-methyl-D-aspartate (NMDA) glutamate receptors may underlie components of the gasping response. Rat pups aged 2 d (n = 50), 5 d (n = 43), 10 d (n = 42), and 15 d (n = 45) underwent anoxic challenges with 100% N2 in a whole body plethysmograph, 30 min after intraperitoneal administration of MK801 [(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate; dizocilpine] (3 mg/kg), a noncompetitive NMDA glutamate receptor channel antagonist, or normal saline. In control pups, after primary apnea onset, a triphasic gasping pattern was apparent at all postnatal ages and included two distinct types of gasps (I and II). In 2- and 5-d MK801-treated animals, phase 1 and type I gasps were absent, leading to marked prolongations of the gasp latency and phase 2, the latter displaying type II gasps only. In addition, phase 3 duration was also prolonged with increased type II gasp frequencies. In contrast, in some 10-d-old (40%) and in all 15-d-old MK801-treated pups, although overall gasping duration was prolonged, the triphasic gasping pattern seen in matched controls was also present. We conclude that NMDA glutamate receptors mediate particular phasic components of the gasping response during early postnatal life but not at later stages of development. We speculate that developmental changes occur in both function and expression of NMDA and other neurotransmitters within brainstem regions underlying the neural substrate for gasp generation.

Expression pattern of glutamate decarboxylase (GAD) in the developing cortex of the embryonic chick brain

The development of the GABAergic system in the chick embryo telencephalon has been studied. Special emphasis was placed on the development of glutamate decarboxylase (GAD) between embryonic day 8 (E8) and E17. The GABA immunoreactivity and neuron-specific enolase expression was detected simultaneously in glutardialdehyde fixed sections, which confirmed that GABAergic cells exhibit neuronal phenotype. The GAD expression was studied by means of immunohistochemistry on cryo-sectioned material both at the light and electron microscopic levels. Furthermore, the presence and localization of GAD65 and GAD67 mRNAs were studied with an in situ hybridization technique with digoxigenin-labeled RNA probes. Protein expression as well as mRNA appearance mostly coincided both temporally and spatially. In the parahippocampal area, as well as in other regions of the developing cortex, GAD staining was seen from E8 onwards. The number of positive cells increased as did the intensity of staining up to E14. As observed in the electron microscope, the GAD protein was co-localized with GABA in most cases, although some GAD-positive cells devoid of GABA-staining also were observed. The pattern of GAD mRNA expression was in general similar to that of GAD immunostaining. Both GAD65 and GAD67 mRNA were detected during the entire period. Furthermore, GAD67 mRNA localization spatially was more correlated with GAD protein expression. The study provides evidence for the notion that development of the GABAergic system occurs rapidly during embryogenesis and, as suggested from mRNA data, that two forms of GAD with slight difference in distribution can contribute to this.

Morphological and GABA-immunoreactive development of the embryonic chick telencephalon

The development of neurons utilizing gamma-aminobutyric acid (GABAergic neurons) in prosencephalon and telencephalon from chicken embryonic days 4-14 (E4-E14) was studied by means of immunohistochemistry. Furthermore, routine histology and transmission electron microscopy. respectively, were performed in order to study the morphological development in the designated area. The main finding is that development of GABAergic neurons in the chick telencephalon is rapid; the GABA neurons are appearing in bulk at day 8, being "overexpressed" at days 10-11, decreasing in numbers thereafter and achieving mature morphology on day 14, which is considerably faster than in the rodent. Morphological analysis revealed that the prosencephalon mainly consisted of a thin layer of undifferentiated neuroblasts in the E4 embryo. By E6, the prosencephalon had increased in thickness and occasional cells outside the neuroepithelium showed a more mature morphology with a few cells weakly staining positive for GABA. At E8, the prospective granular and subventricular layers had developed. At E14, the appearance of the telencephalon is approximating that of the adult since both ependymal cells and morphologically mature neurons can be seen.

Glutamate decarboxylase solubilized from the rat cerebral cortex by two different concentrations of Triton X-100: effects of glutamate analogues and analysis by SDS-PAGE/western blotting using GAD6 and K2 antibodies

Analysis of two preparations (containing 0.1% and 0.5% Triton X-100) of glutamate decarboxylase (GAD) by Western blotting using GAD6 and K2 antibodies specifically recognizing two GAD isoenzymes, GAD65 and GAD67, respectively, indicated that the higher concentration of Triton X-100 at best only moderately favoured solubilization of GAD67. Several glutamate analogues were found to be either equally potent or equally inactive as inhibitors of glutamate decarboxylase activities in the two preparations. Among typical ligands for glutamate receptors and transporters, only quinolinic and L-cysteine sulphinic acids were weak inhibitors of GAD. Kainate, alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA), 3-((RS)-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP), L-threo-3-hydroxy-aspartate, L-trans-pyrrolidine-2,4-dicarboxylate, dihydrokainate, kynurenic acid and N-methyl-D-aspartate were inactive. Even though the activity of glutamate decarboxylase in homogenates of rat cerebral cortex is higher at 0.5% than at 0.1% Triton X-100, structural requirements of the enzyme active site appear to be independent of Triton X-100 concentration. Furthermore, since the less soluble component of the enzyme activity contains about the same ratio of GAD65 to GAD67 as the more soluble one, it does not seem that the fractionation with Triton X-100 can be easily used to separate the two isoenzymes from each other.

Modulation by GABA of neuroplasticity in the central and peripheral nervous system

Apart from being a prominent (inhibitory) neurotransmitter that is widely distributed in the central and peripheral nervous system, gamma-aminobutyric acid (GABA) has turned out to exert trophic actions. In this manner GABA may modulate the neuroplastic capacity of neurons and neuron-like cells under various conditions in situ and in vitro. In the superior cervical ganglion (SCG) of adult rat, GABA induces the formation of free postsynaptic-like densities on the dendrites of principal neurons and enables implanted foreign (cholinergic) nerves to establish functional synaptic contacts, even while preexisting connections of the preganglionic axons persist. Apart from postsynaptic effects, GABA inhibits acetylcholine release from preganglionic nerve terminals and changes, at least transiently, the neurochemical markers of cholinergic innervation (acetylcholinesterase and nicotinic receptors). In murine neuroblastoma cells in vitro, GABA induces electron microscopic changes, which are similar in principle to those seen in the SCG. Both neuroplastic effects of GABA, in situ and in vitro, could be mimicked by sodium bromide, a hyperpolarizing agent. In addition, evidence is available that GABA via A- and/or B-receptors may exert direct trophic actions. The regulation of both types of trophic actions (direct, receptor-mediated vs. indirect, bioelectric activity dependent) is discussed.