Loss of GABAergic neuronal phenotype in primary cerebellar cultures following blockade of glutamate reuptake

Analysis of membrane transport mechanisms of endogenous substrates using chromatographic techniques

Membrane transporters are expressed in various bodily tissues and play essential roles in the homeostasis of endogenous substances and the absortion, distribution and/or excretion of xenobiotics. For transporter assays, radioisotope-labeled compounds have been mainly used. However, commercially available radioisotope-labeled compounds are limited in number and relatively expensive. Chromatographic analyses such as high-performance liquid chromatography with ultraviolet absorptiometry and liquid chromatography with tandem mass spectrometry have also been applied for transport assays. To elucidate the transport properties of endogenous substrates, although there is no difficulty in performing assays using radioisotope-labeled probes, the endogenous background and the metabolism of the compound after its translocation across cell membranes must be considered when the intact compound is assayed. In this review, the current state of knowledge about the transport of endogenous substrates via membrane transporters as determined by chromatographic techniques is summarized. Chromatographic techniques have contributed to our understanding of the transport of endogenous substances including amino acids, catecholamines, bile acids, prostanoids and uremic toxins via membrane transporters.

Mechanism of GABA involvement in post-traumatic trigeminal neuropathic pain: activation of neuronal circuitry composed of PKCγ interneurons and pERK1/2 expressing neurons

BACKGROUND

GABA disinhibition within the spinal dorsal horn has been implicated in pain hypersensitivity on injury in different neuropathic models. However, GABA alteration has been explored in only one study on trigeminal neuropathic pain.

METHODS

The present study investigated the implication of GABA in trigeminal dynamic mechanical allodynia (DMA) obtained after chronic constriction of the infraorbital nerve (CCI-IoN), and explored the cellular and molecular mechanisms by which GABA dysfunction induced DMA.

RESULTS

Our data demonstrated a significant decrease in labelling in two GABA cell markers, glutamate acid decarboxylase (GAD67), and parvalbumin, in the medullary dorsal horn (MDH) of allodynic rats in comparison to sham rats. Increasing GABA by intracisternal injections of vigabatrin (VGB), a blocker of the catabolic enzyme GABA transaminase, alleviated pain behaviour and restored normal GABA cell marker expression in allodynic MDH. Interestingly, intracisternal VGB administration also significantly decreased PKCγ staining, i.e., of its phosphorylated active form and the number of pERK1/2 positive cells within the MDH. These two markers were highly expressed in allodynic MDH.

CONCLUSION

The circuitry composed of PKCγ and pERK1/2 cells is silent under physiological conditions but is activated after CCI-IoN, therefore, switching touch stimuli to pain sensation. The decrease of GABA transmission constituted a key factor in the activation of this neuronal circuitry, which opens the gate for non-noxious stimuli to reach nociceptive projection neurons in lamina I.

Multiple origins of the cortical γ rhythm

Gamma rhythms (30-80 Hz) are a near-ubiquitous feature of neuronal population activity in mammalian cortices. Their dynamic properties permit the synchronization of neuronal responses to sensory input within spatially distributed networks, transient formation of local neuronal "cell assemblies," and coherent response patterns essential for intercortical regional communication. Each of these phenomena form part of a working hypothesis for cognitive function in cortex. All forms of physiological gamma rhythm are inhibition based, being characterized by rhythmic trains of inhibitory postsynaptic potentials in populations of principal neurons. It is these repeating periods of relative enhancement and attenuation of the responsivity of major cell groups in cortex that provides a temporal structure shared across many millions of neurons. However, when considering the origins of these repeating trains of inhibitory events considerable divergence is seen depending on cortical region studied and mode of activation of gamma rhythm generating networks. Here, we review the evidence for involvement of multiple subtypes of interneuron and focus on different modes of activation of these cells. We conclude that most massively parallel brain regions have different mechanisms of gamma rhythm generation, that different mechanisms have distinct functional correlates, and that switching between different local modes of gamma generation may be an effective way to direct cortical communication streams. Finally, we suggest that developmental disruption of the endophenotype for certain subsets of gamma-generating interneuron may underlie cognitive deficit in psychiatric illness.

Glutamate is preferred over glutamine for intermediary metabolism in cultured cerebellar neurons

The glutamate-glutamine cycle is thought to be of paramount importance in the mature brain; however, its significance is likely to vary with regional differences in distance between astrocyte and synapse. The present study is aimed at evaluating the role of this cycle in cultures of cerebellar neurons, mainly consisting of glutamatergic granule cells. Cells were incubated in medium containing [U-13C]glutamate or [U-13C]glutamine in the presence and absence of unlabeled glutamine and glutamate, respectively. Cell extracts and media were analyzed using high-performance liquid chromatography (HPLC) and gas chromatography combined with mass spectrometry (GC/MS). Both [U-13C]glutamate and [U-13C]glutamine were shown to be excellent precursors for synthesis of neuroactive amino acids and tricarboxylic acid (TCA) cycle intermediates. Labeling from [U-13C]glutamate was higher than that from [U-13C]glutamine in all metabolites measured. The presence of [U-13C]glutamate plus unlabeled glutamine in the experimental medium led to labeling very similar to that from [U-13C]glutamate alone. However, incubation in medium containing [U-13C]glutamine in the presence of unlabeled glutamate almost abolished labeling of metabolites. Thus, it could be shown that glutamate is the preferred substrate for intermediary metabolism in cerebellar neurons. Label distribution indicating TCA cycle activity showed more prominent cycling from [U-13C]glutamine than from [U-13C]glutamate. Labeling of succinate was lower than that of the other TCA cycle intermediates, indicating an active role of the gamma-amino butyric acid shunt in these cultures. It can be concluded that the cerebellar neurons rely more on reuptake of glutamate than supply of glutamine from astrocytes for glutamate homeostasis.

Intrathecal midazolam regulates spinal AMPA receptor expression and function after nerve injury in rats

Spinal gamma-aminobutyric acid (GABA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors have been implicated in the mechanisms of neuropathic pain after nerve injury; however, how these two receptors interact at the spinal level remains unclear. Here we show that intrathecal midazolam through activation of spinal GABAA receptors attenuated the expression and function of spinal AMPA receptors in rats following peripheral nerve injury. Thermal hyperalgesia and mechanical allodynia induced by chronic constriction nerve injury (CCI) in rats were attenuated by the short-acting benzodiazepine midazolam (20=10>5 mug>vehicle) administered intrathecally once daily for 7 postoperative days. CCI-induced upregulation of AMPA receptors within the spinal cord dorsal horn was also significantly reduced by the intrathecal midazolam (10, 20 mug) treatment. The inhibitory effects of midazolam (10, 20 mug) on neuropathic pain behaviors and AMPA receptor expression were prevented by co-administration of midazolam with the GABAA receptor antagonist bicuculline (3 mug), whereas intrathecal treatment with bicuculline (1 or 3 mug) alone in naive rats induced the upregulation of spinal AMPA receptor expression and nociceptive responses, indicating a tonic regulatory effect from endogenous GABAergic activity on the AMPA receptor expression and spinal nociceptive processing. These results indicate that modulation of spinal AMPA receptor expression and function by the GABAergic activity may serve as a mechanism contributory to the spinal nociceptive processing.

Demonstration of extensive GABA synthesis in the small population of GAD positive neurons in cerebellar cultures by the use of pharmacological tools

Cultures of dissociated cerebella from 7-day-old mice were maintained in vitro for 1-13 days. GABA biosynthesis and degradation were studied during development in culture and pharmacological agents were used to identify the enzymes involved. The amount of GABA increased, whereas that of glutamate was unchanged during the first 5 days and both decreased thereafter. The presence of aminooxyacetic acid (AOAA, 10 microM) which inhibits transaminases and other pyridoxal phosphate dependent enzymes including GABA-transaminase (GABA-T), in the culture medium caused an increase in the intracellular amount of GABA and a decrease in glutamate. The GABA content was also increased following exposure to the specific GABA-T inhibitor gamma-vinyl GABA. From day 6 in culture (day 4 when cultured in the presence of AOAA) GABA levels in the medium were increased compared to that in medium from 1-day-old cultures. Synthesis of GABA during the first 3 days was demonstrated by the finding that incubation with either [1-(13)C]glucose or [U-(13)C]glutamine led to formation of labeled GABA. Synthesis of GABA after 1 week in culture, when the enzymatic machinery is considered to be at a more differentiated level, was shown by labeling from [U-(13)C]glutamine added on day 7. Altogether the findings show continuous GABA synthesis and degradation throughout the culture period in the cerebellar neurons. At 10 microM AOAA, GABA synthesis from [U-(13)C]glutamine was not affected, indicating that transaminases are not involved in GABA synthesis and thus excluding the putrescine pathway. At a concentration of 5 mM AOAA GABA labeling was, however, abolished, showing that glutamate decarboxylase, which is inhibited at this level of AOAA, is responsible for GABA synthesis in the cerebellar cultures. In conclusion, the present study shows that GABA synthesis is taking place via GAD in a subpopulation of the cerebellar neurons, throughout the culture period.

Excitotoxic death induced by released glutamate in depolarized primary cultures of mouse cerebellar granule cells is dependent on GABAAreceptors and niflumic acid-sensitive chloride channels

Excitotoxic neuronal death has been linked to neurological and neurodegenerative diseases. Several studies have sought to clarify the involvement of Cl(-) channels in neuronal excitotoxicity using either N-methyl-D-aspartic acid (NMDA) or alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate/kainic acid agonists. In this work we induced excitotoxic death in primary cultures of cerebellar granule cells by means of endogenously released glutamate. Excitotoxicity was provoked by exposure to high extracellular K(+) concentrations ([K(+)](o)) for 5 min. Under these conditions, a Ca(2+)-dependent release of glutamate was evoked. When extracellular glutamate concentration rose to between 2 and 4 microM, cell viability was significantly reduced by 30-40%. The NMDA receptor antagonists (MK-801 and D-2-amino-5-phosphonopentanoic acid) prevented cell death. Exposure to high [K(+)](o) produced a (36)Cl(-) influx which was significantly reduced by picrotoxinin. In addition, the GABA(A) receptor antagonists (bicuculline, picrotoxinin and SR 95531) protected cells from high [K(+)](o)-triggered excitotoxicity and reduced extracellular glutamate concentration. The Cl(-) channel blockers niflumic acid and 5-nitro-2-(3-phenylpropylamino)benzoic acid also exerted a neuroprotective effect and reduced extracellular glutamate concentration, even though they did not reduce high [K(+)](o)-induced (36)Cl(-) influx. Primary cultures of cerebellar granule cells also contain a population of GABAergic neurons that released GABA in response to high [K(+)](o). Chronic treatment of primary cultures with kainic acid abolished GABA release and rendered granule cells insensitive to high [K(+)](o) exposure, even though NMDA receptors were functional. Altogether, these results demonstrate that, under conditions of membrane depolarization, low micromolar concentrations of extracellular glutamate might induce an excitotoxic process through both NMDA and GABA(A) receptors and niflumic acid-sensitive Cl(-) channels.

Homeostasis of neuroactive amino acids in cultured cerebellar and neocortical neurons is influenced by environmental cues

Neuronal function is highly influenced by the extracellular environment. To study the effect of the milieu on neurons from cerebellum and neocortex, cells from these brain areas were cultured under different conditions. Two sets of cultures, one neocortical and one cerebellar neurons, were maintained in media containing [U-(13)C]glucose for 8 days at initial concentrations of 12 and 28 mM glucose, respectively. Other sets of cultures (8 days in vitro) maintained in a medium containing initially 12 mM glucose were incubated subsequently for 4 hr either by addition of [U-(13)C]glucose to the culture medium (final concentration 3 mM) or by changing to fresh medium containing [U-(13)C]glucose (3 mM) but without glutamine and fetal calf serum. (13)C Nuclear magnetic resonance (NMR) spectra revealed extensive gamma-aminobutyric acid (GABA) synthesis in both cultured neocortical and cerebellar neurons after maintenance in medium containing [U-(13)C]glucose for 8 days, whereas no aspartate labeling was observed in these spectra. Mass spectrometry analysis, however, revealed high labeling intensity of aspartate, which was equal in the two types of neurons. Addition of [U-(13)C]glucose (4 hr) on Day 8 in culture led to a similar extent of labeling of GABA in neocortical and in cerebellar cultures, but the cellular content of GABA was considerably higher in the neocortical neurons. The cellular content of alanine was similar regardless of culture type. Comparing the amount of labeling, however, cerebellar neurons exhibited a higher capacity for alanine synthesis. This is compatible with the fact that cerebellar neurons could ameliorate a low alanine content after culturing in low glucose (12 mM) by a 4-hr incubation in medium containing 3 mM glucose. A low glucose concentration during the culture period and a subsequent medium change were associated with decreases in glutathione and taurine contents. Moreover, glutamate and GABA contents were reduced in cerebellar cultures under either of these conditions. In neocortical neurons, the GABA content was decreased by simultaneous exposure to low glucose and change of medium. These conditions also led to an increase in the aspartate content in both types of cultures, although most pronounced in the neocortical neurons. Further experiments are needed to elucidate these phenomena that underline the impact of extracellular environment on amino acid homeostasis.