Studies on GABA and protein synthesis

Neurite outgrowth-regulating properties of GABA and the effect of serum on mouse spinal cord neurons in culture

Time-lapse photography was used to examine the effects of gamma-aminobutyric acid (GABA) on the outgrowth and motility of neurites in cultures from mouse spinal cord. GABA at concentrations of 100, 10 and 1 microM caused significant inhibition of neurite outgrowth and the motility of growth cones was significantly reduced by treatment with 100 and 10 microM GABA. This effect was mimicked by the GABA(B) receptor agonist baclofen, whereas the GABA(A) receptor agonist muscimol had no effect. The effect of GABA on outgrowth and motility seems to be dependent on the type of serum employed. The results reported here were obtained only when heat-inactivated serum was used and not when non heat-inactivated serum was added to the culture medium. They suggest that GABA has a role in the regulation of process outgrowth within the embryonic mouse spinal cord.

GABA attenuates the neurotoxic effects of ethanol in neuron-enriched cultures from 8-day-old chick embryo cerebral hemispheres

Neuron-enriched cultures were prepared from 8-day-old chick embryo cerebral hemispheres and exposed to ethanol (50 mM), GABA (10(-5) M) and ethanol (50 mM) + GABA (10(-5) M) from day 4 to 8 in culture. At day 8, control, ethanol, GABA and ethanol + GABA-treated cultures were examined morphologically and biochemically. Choline acetyltransferase (ChAT) and glutamic acid decarboxylase (GAD) activities were used as markers for cholinergic and GABAergic neuronal phenotypic expression, respectively. Control cultures showed more numerous and large neuronal aggregates as well as prominent neuritic bundles. Moreover, cultures treated with GABA depicted even more numerous neuronal aggregates with interconnecting neurites as compared to control. In contrast, ethanol-treated cultures exhibited smaller neuronal aggregates with less prominent neuritic bundles than control. However, cultures treated concomitantly with ethanol + GABA exhibited numerous and larger aggregates than cultures treated with ethanol alone. Neuritic bundles which were highly reduced in ethanol-treated cultures became prominent in the presence of GABA. As previously reported, ethanol alone enhanced ChAT and reduced GAD activities. GABA given alone enhanced the expression of both neuronal phenotypes. When GABA was given concomitantly with ethanol the decline in GAD and the rise in ChAT observed in ethanol-treated cultures was restored by GABA to almost control levels. Thus, ethanol-induced alterations in morphology and neuronal phenotypes were counteracted by the neurontrophic effect of GABA.

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.

The effects of various antidepressant drugs on the fine-structure of neurons of the cingulate cortex in culture

We have examined the effects of antidepressant drugs on the morphology and fine-structure of cingulate cortex neurons and synapses in vitro. Dissociated cell cultures from 16-day-old rat fetuses were maintained for up to three weeks in the presence of amitriptyline and desipramine (tricyclic antidepressants) mianserin and citalopram (non-tricyclic antidepressants) or without drugs as controls. Synapses in cultures exposed to amitriptyline and desipramine displayed increased numbers of presynaptic vesicles and more extensive membrane specializations than in control cultures, and increased numbers of coated vesicles both pre- and postsynaptically. In mianserin- and citalopram-treated cultures, synaptic specializations were increased in length, and aggregates of mitochondria at the base of neurites were observed. Our results indicate that long-term exposure to antidepressant drugs results in significant changes in intracellular structure, including changes in synaptic ultrastructure.

Effect of repeated treatment with a gamma-aminobutyric acid receptor agonist on postnatal neural development in rats

The effect of treatment with the gamma-aminobutyric acid (GABA) agonist tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP) on neural development was monitored in rats by following the expression of the neuron-specific proteins neural cell adhesion molecule (NCAM), D1, and D3 as well as the enzymes glutamate decarboxylase (GAD) and glutamate dehydrogenase (GLDH). As judged from the effect of the treatment on the expression of NCAM and GAD, GABA agonists have the capacity to accelerate and enhance neuronal development during the early postnatal period. However, as judged from the expression of D1- and D3-protein some adverse late effects may result from prolonged treatment with high doses of GABA agonists. The decrease in GLDH specific activity observed in THIP-treated rats during their late postnatal development possibly indicates a repression of glutamatergic neurons.

Gamma-aminobutyric acid affects the developmental expression of neuron-associated proteins in cerebellar granule cell cultures

The effect of gamma-aminobutyric acid (GABA) on the expression of the neuron-associated D2 and neuron-specific enolase (NSE) was studied during development in culture of cerebellar granule cells. It was found that the presence of GABA during culture development increased the overall protein content. D2 content was also increased but not above the general increase in protein whereas NSE increased above the general level of protein. The presence of GABA in the growth medium also appeared to accelerate the changes in molecular forms of D2 and NSE seen during neuronal development. This suggests that GABA promotes or accelerates the general maturation of neurons, as these two neuron-associated proteins otherwise differ from each other with respect to their subcellular localization and their physiological and biochemical properties.

Lack of effect of GABA on [3H]leucine incorporation into a rat oviduct ribosomal system

A ribosomal system for [3H]leucine incorporation was isolated from the rat oviduct in order to study the possible effect of GABA on [3H]leucine incorporation during the estrous cycle. The system showed an absolute requirement for Mg2+ and about 50% dependence on an energy source. Optimal [3H]leucine incorporation occurred under 3-6 mM Mg2+ and 100 mM K+ and was higher in diestrous-1 than in estrous or proestrous. GABA (10 mM) had no effect on [3H]leucine incorporation in any of the three estrous phases studied.

Transport of branched-chain amino acids in brain slices of developing and adult rats

The accumulation of labelled leucine, isoleucine and valine by cerebral slices of developing and adult rats was studied. The accumulation increased with age by 15-25%. It was strongly (from 52 to 86%) inhibited by a 100-fold excess of phenylalanine, tryptophan and another branched-chain amino acid, but moderately activated by GABA and glutamate. The inhibitions evoked by leucine and isoleucine were slightly stronger in young than in adult rats. The corresponding 2-oxoacid analogs of leucine, isoleucine and valine were also inhibitory but less effective. The 30-min accumulation of 3H-labelled branched-chain amino acids was ostensibly higher than the increase in their total concentrations in incubated slices, which apparently bespeaks lively homoexchange of endogenous intracellular and labelled extracellular amino acids.

Brain protein synthesis rates are not sensitive to elevated GABA, taurine, or glycine

The effects of elevated levels of GABA, glycine, or taurine on the rate of protein synthesis in plasma, brain, liver, and muscle of adult mice were measured in in vivo experiments after a flooding dose of labeled valine. Elevation of these amino acids caused hypothermia; keeping the animals in an incubator maintained physiological body temperature. The increase in GABA or glycine did not affect the rate of protein synthesis in these tissues to a significant degree. The increase in taurine levels caused inhibition of valine incorporation in plasma, liver, and muscle, while brain protein synthesis was unaffected. When glycine was increased in brain, the uptake of labeled free valine in the brain was greater.

The effect of chronic self-administration of d-amphetamine on food intake, locomotor activity, and C14-leucine incorporation into cerebral cortex protein

Rats had free access to 0.02% d-amphetamine solution instead of water for 23 days. The daily amphetamine consumption was found to increase from 16 mg/kg on day 1 up to 47 mg/kg on day 23. Tolerance to the anorexic effect of the drug was apparent on day 11. The initial depression in body weight persisted throughout the experiment. The hyperactivity of rats remained at the same level despite the daily increase in amphetamine intake. The incorporation of C14-leucine into cerebral cortex proteins was initially increased and returned to control level after 2 weeks of treatment. No direct correlation between hyperactivity and brain cortex protein synthesis was observed.

GABA metabolism and cerebral protein synthesis

Glutamate decarboxylase (GAD) activity and [14C)leucine incorporation into brain protein in vivo were measured in mice injected with L-glutamic acid-gamma-hydrazide and 60 min later with pyridoxal-5'-phosphate; this combined treatment produces a decrease in GAD activity and an increase in GABA levels. Under such conditions, protein synthesis was inhibited to the same extent as GAD activity. A parellelism between the decrease in GAD activity and that in protein synthesis was also observed in brain cortex slices treated with GAD inhibitors. This treatment did not affect leucine incorporation into protein in liver slices. The results support the previously suggested hypothesis that a certain pool of GABA may have a role in the regulation of protein synthesis in brain, and further suggest that the synthesis of GABA may be more important in this respect than its total concentration.