GABA A Receptor Activation Attenuates Excitotoxicity but Exacerbates Oxygen—Glucose Deprivation-Induced Neuronal Injury In Vitro

Abstract

We examined the effects of GABA receptor stimulation on the neuronal death induced by exogenously added excitatory amino acids or combined oxygen–glucose deprivation in mouse cortical cell cultures. Death induced by exposure to NMDA, AMPA, or kainate was attenuated by addition of GABA or the GABA_A receptor agonist, muscimol, but not by the GABA_B receptor agonist, baclofen. The antiexcitotoxic effect of GABA_A receptor agonists was blocked by bicuculline or Picrotoxin. In contrast, GABA or muscimol, but not baclofen, markedly increased the neuronal death induced by oxygen–glucose deprivation. Muscimol potentiation of neuronal death was associated with increased glutamate efflux to the bathing medium, and increased cellular ⁴⁵Ca²⁺ accumulation; it was blocked by MK-801, but not NBQX, suggesting mediation by NMDA receptors. Bicuculline only weakly attenuated muscimol potentiation of oxygen–glucose deprivation-induced neuronal death, probably because it itself increased this death. Present results raise a note of caution in the proposed use of GABA_A receptor stimulation to limit ischemic brain damage in vivo.

Keywords

Muscimol Baclofen Glutamate Ischemia Neurotoxicity

Excessive extracellular accumulation of the excitatory neurotransmitter, glutamate, and subsequent excitotoxic neuronal death have been implicated in the pathogenesis of hypoxia–ischemic neuronal death (Benveniste et al., 1984; Simon et al., 1984; Meldrum, 1985; Choi and Rothman, 1990; Albers et al., 1992). More recently, attention has also focused on the possibility that the inhibitory neurotransmitter, γ-aminobutyric acid (GABA), may serve as a brake on overexcitation and thus reduce the pathogenesis of ischemic brain damage. Extracellular GABA levels are also increased during hypoxia-ischemia in vivo (Hagberg et al., 1985; Globus et al., 1988). The idea that this increase may counteract the toxic effects of glutamate led Globus et al. (1991) to propose that an “excitotoxicity index,” calculated as [glutamate] × [glycine]/[GABA] in the extracellular space, might better predict ischemic brain damage than glutamate levels alone. GABA inhibition is mediated both by GABA_A receptors, which open membrane chloride channels and stabilize the membrane potential below firing threshold, and GABA_B receptors, which act via G proteins to reduce transmitter release from presynaptic terminals (Kaila, 1994).

Recent studies from several laboratories have examined enhancement of GABA receptor activation as a strategy for reducing brain damage in animal models of ischemia. Several such strategies reduce hippocampal CA1 neuronal loss following global ischemia in gerbils: (a) preischemic administration of muscimol, baclofen, or diazepam (Sternau et al. 1989); (b) pre- or postischemic administration of chlormethiazole (Cross et al., 1991); (c) pre- and postischemic intraventricular infusion of a GABA-transaminase inhibitor or muscimol (Shuaib et al., 1992, 1993; see also Abel and McCandless, 1992, which utilized preischemic administration); and (d) postischemic administration of diazepam (Schwartz et al., 1994). In rats, pre- and postischemic administration of GABA uptake inhibitor or diazepam (Johansen and Diemer, 1991) or postischemic administration of diazepam (Schwartz et al., 1995) reduced hippocampal CA1 cell loss following global ischemia. Neurological function following multifocal ischemia in rats was improved by postischemic administration of muscimol (Lyden and Hedges, 1992; Lyden and Lonzo, 1994), and chlormethiazole pretreatment reduced infarct volume in a rat model of transient focal ischemia (Sydserff et al., 1995). Protective effects of barbiturates have also been observed in both focal and global ischemia models (see review by Spetzler and Hadley, 1989), although the multiplicity of known barbiturate actions hinders interpretation of these observations strictly in context of GABA_A receptors.

In contrast to the consistent neuroprotective effects of GABAmimetic drugs in animal models of ischemia, both protective and injurious effects have been observed in studies of excitotoxicity in vitro. In our murine cortical cell cultures, neither GABA nor diazepam reduced the rapidly triggered excitotoxic neuronal death induced by a brief intense glutamate exposure (Koh and Choi, 1987b). Using a more prolonged and weaker N-methyl-d-aspartate (NMDA) exposure, Okuma et al. (1994) found that muscimol reduced NMDA neurotoxicity in cortical cultures. However, Erdo et al. (1991) observed that 1 mM GABA or 10–100 μM 1,2,3,4-tetrahydroisoxasolopyridinol (THIP), a GABA_A agonist, accelerated neuronal death in rat cortical cultures exposed continuously to NMDA or kainate.

While this last observation could simply reflect distortions induced by cell culture methods, it is also possible that cell culture isolates an important deleterious effect of GABA_A receptor activation that is masked in vivo by more powerful beneficial effects. Thus, we set out to characterize in detail the effects of both GABA_A and GABA_B receptor agonists on the neuronal death induced by excitotoxins or oxygen–glucose deprivation in cortical cell cultures. Abstracts have appeared elsewhere (Monyer et al., 1990; Muir et al., 1994; Muir and Choi, 1995).

MATERIALS AND METHODS

Cell culture

Primary cortical cell cultures containing both neuronal and glial elements were prepared from fetal mice at 15–16 days of gestation as previously described (Rose et al., 1993). Dissociated cortical cells were plated at approximately 2.75 hemispheres per 24-multiwell (15-mm) vessels (Primaria; Falcon) on a previously established glial bed (see below) in modified Eagle's minimal essential medium (MEM, Earle's salts, supplied glutamine-free) supplemented with 5% heat-inactivated horse serum, 5% fetal calf serum, glutamine (2 mM), and glucose (total 25 mM). Cultures were maintained at 37°C in a humidified incubator containing 5% CO₂. Nonneuronal cell division was halted after 5–7 days in vitro by a 1- to 3-day exposure to 10 μM cytosine arabinosine. Cultures were subsequently fed twice weekly with a medium identical to the plating medium except lacking fetal serum, and used for experiments between 13 and 16 days in vitro. Glial cultures were prepared similarly from 1- to 3-day-old mice neocortices plated at 0.5–1 hemisphere per 24-multiwell vessel in a plating medium similar to that described above, but containing 10% heat-inactivated horse serum, 10% fetal calf serum, and 10 ng/ml epidermal growth factor.

Excitatory amino acid exposure

Rapidly triggered excitotoxicity induced by a 5-min exposure to 100 μM NMDA was carried out at room temperature in a HEPES-buffered salt solution containing (millimolar): 120 NaCl, 5.4 KCl, 0.8 MgSO₄, 1.8 CaCl₂, 20 HEPES, 15 glucose, and 0.01 glycine (pH 7.4). GABA or related drugs were applied concurrently with NMDA exposure. Cultures were then returned to MEM containing 10 μM glycine. Slowly triggered excitotoxicity was induced by a 24-h exposure to NMDA (15 μM), α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA, 10 μM), or kainate (350 μM) in MEM supplemented with 10 μM glycine at 37°C. MK-801 (10 μM) was included with AMPA or kainate to block secondary activation of NMDA receptors.

Oxygen-glucose deprivation

Cultures were placed in an anaerobic chamber (Forma Scientific, Marietta, OH, U.S.A.) containing a gas mixture of 5% CO₂, 10% H₂ 85% N₂ (<0.2% O₂) (Goldberg and Choi, 1993). Culture medium was replaced by thorough exchange with a deoxygenated, glucose-free balanced salt solution (BSS) containing (millimolar): 116 NaCl, 5.4 KCl, 0.8 MgSO₄, 1 NaH₂PO₄, 26.2 NaHCO₃, 1.8 CaCl₂, and 0.01 glycine. Cultures were then placed in a 37°C humidified incubator within the chamber. Following 40–45 min oxygen–glucose deprivation, the exposure medium was exchanged with oxygenated MEM containing glucose and cultures returned to a normoxic incubator. Specified drugs were added during the period of oxygen–glucose deprivation only.

Cyanide exposure

Cultures were exposed to 1 mM potassium cyanide in oxygenated, glucose-free BSS at 37°C. Exposure was terminated by washout with oxygenated MEM containing glucose and cells returned to an incubator for 20–24 h. Specified drugs were added during the period of cyanide exposure only.

Assessment of neuronal injury

Neuronal cell death was estimated by examination of cultures under phase-contrast microscopy and quantitated by measurement of lactate dehydrogenase (LDH) released by damaged or destroyed cells in the bathing media 20–24 h after insult initiation (Koh and Choi, 1987a). LDH-mediated conversion of pyruvate to lactate was monitored by measuring levels of reduced nicotinamide adenine dinucleotide (NADH), by absorbance at 340 nm, using a spectrophotometric microplate reader (UVmax, Molecular Devices, Menlo Park, CA, U.S.A.). LDH was converted to units per milliliter by fitting the rate of change of absorbance of each sample to a LDH standard curve (Sigma Enzyme Control 2-E). The LDH signal corresponding to near-complete neuronal death without glial death was measured in sister cultures exposed to 300 μM NMDA for 24 h (total neuronal LDH). Since absolute total LDH levels may vary among plates obtained from different dissections, LDH values were normalized to the levels associated with control injury in sister cultures. Basal LDH levels determined in sister cultures subjected to sham wash (generally <15% of total LDH) were subtracted from values obtained in experiments to yield the signal specific to experimental injury.

45Ca2+ accumulation

Cultures were exposed to NMDA for 5 min or deprived of oxygen and glucose as described above with the inclusion of specified drugs and ⁴⁵Ca²⁺ (New England Nuclear, Wilmington, DE, U.S.A.; final activity, 2 μCi/ml) in the exposure medium (Hartley et al., 1993; Goldberg and Choi, 1993). Exposure was terminated by thoroughly washing cells of extracellular ⁴⁵Ca²⁺. Subsequently, the extracellular medium was removed, cells were lysed with 0.2% sodium dodecyl sulfate, and an aliquot was added to scintillation fluid for counting. Since absolute neuronal density may vary among plates obtained from different dissections, experimental values were normalized to the levels associated with control injury (= 100) in sister cultures. Previous study has determined that ⁴⁵Ca²⁺ accumulation is mostly neuronal (Hartley et al., 1993).

High-performance liquid chromatography (HPLC)

Samples of the bathing medium collected prior to the termination of oxygen–glucose deprivation were assayed for glutamate by using phenylisothiocyanate (PITC) derivatization, HPLC separation using a Hypersil-ODS reverse-phase column (Hewlett Packard, Wilmington, DE, U.S.A.), and ultraviolet detection at a wavelength of 254 nm (Cohen et al., 1986). A 200 μl of sample medium was derivatized with 100 μl of PITC, methanol, and triethylamine (2:7:4) and dried under vacuum. The samples were then reconstituted in solvent consisting of 0.14 μM sodium acetate, 0.05% triethylamine, 6% acetonitrile, and brought to pH 6.4 with glacial acetic acid, prior to the HPLC run. The above solvent was used as the mobile phase with the column being washed with 60% acetonitrile/40% water between each sample run. Media glutamate concentrations were calculated by normalizing to glutamate standards. Glutamate measurements were found to be linear over the range 0.1–10 μM, with 0.1 μM being the threshold for reliable detection.

Reagents

Culture medium was purchased from Gibco (Grand Island, NY, U.S.A.) as a 10× concentrated stock lacking bicarbonate and glutamine; serum was obtained from Hyclone Laboratories Inc. (Logan, UT, U.S.A.). NMDA, GABA, muscimol hydro-bromide, Picrotoxin, and phenylisothiocyanate were obtained from Sigma Chemical (St. Louis, MO, U.S.A.). (R)-(+)-Baclofen hydrochloride, guvacine hydrochloride, bicuculline methiodide, nipecotic acid, and MK-801 were purchased from Research Biochemicals Inc (Natick, MA, U.S.A.).

RESULTS

Rapidly triggered excitotoxicity

Consistent with an earlier study (Choi et al., 1988), exposure of mixed neuron/glial cortical cell cultures to 100 μM NMDA for 5 min resulted in degeneration of a majority (50–75%) of the neuronal population 20–24 h later. Concurrent application of either 100–1000 μM GABA (Fig. 1A), or 3–100 μM muscimol (Fig. 1B), a specific GABA_A receptor agonist, produced a partial, concentration-dependent reduction in NMDA-induced neuronal death. In contrast, the specific GABA_B receptor agonist, baclofen, had no effect on NMDA-induced neuronal death over the tested concentration range of 1–100 μM (Fig. 1C). Application of 1 mM GABA, muscimol, or baclofen alone for 24 h was not toxic.

Fig. 1.

GABA_A receptor agonists attenuate NMDA-mediated excitotoxicity. Cultures were exposed to 100 μM NMDA for 5 min in the absence or presence of increasing doses of GABA agonists. LDH was measured 20–24 h later. Data represent mean LDH ± SD scaled to the mean LDH measured in cultures exposed to 100 μl NMDA alone (=100). A: GABA, n = 8 cultures per condition pooled from two experiments; control NMDA injury = 68 ± 5% of the total neuronal population. B: Muscimol, n = 8–20 cultures per condition pooled from five experiments; control NMDA injury = 59 ± 19% of the total neuronal population. C: Baclofen, n = 16 cultures per condition pooled from four experiments; control NMDA injury = 68 ± 19% of the total neuronal population. An asterisk indicates values significantly different from NMDA alone by ANOVA followed by Dunnett's t test (p < 0.05).

NMDA-stimulated uptake of ⁴⁵Ca²⁺, which correlates with subsequent neuronal death (Hartley et al., 1993), was partially reduced by 10–100 μM muscimol (Table 1). ⁴⁵Ca²⁺ accumulation was completely blocked by the NMDA antagonist, MK-801 (10 μM).

Table 1.

Effect of muscimol on NMDA-stimulated ⁴⁵Ca accumulation^a

Treatment		NMDA-stimulated ⁴⁵Ca²⁺ Accumulation
NMDA		100 ± 9
+ Muscimol	1 μM	97 ± 8
	10 μM	82 ± 9^b
	100 μM	79 ± 15^b
+ MK-801	10 μM	8 ± 5^b

Cultures were exposed to 200 μM NMDA for 5 min in the presence of extracellular ⁴⁵Ca²⁺ and the indicated concentration of muscimol or MK-801. Following exposure, cells were washed and lysed to measure intracellular (neuronal) ⁴⁵Ca²⁺ accumulation. Data represent mean ± SD, n = 16–20 cultures per condition from five experiments, scaled to the mean values measured in cultures exposed to 200 μM NMDA alone (= 100).

Values are significantly different from NMDA alone by ANOVA followed by Dunnett's t test (p < 0.05).

To demonstrate that muscimol's protective action was mediated by GABA_A receptor activation, specific antagonists for the receptor were utilized. The competitive GABA_A receptor antagonist, bicuculline (10–100 μM), antagonized the protective effect of muscimol on NMDA-induced neuronal death (Fig. 2A). A similar antagonism was demonstrated with the noncompetitive GABA_A receptor antagonist, picrotoxin (10–100 μM, Fig. 2B). Neither antagonist increased NMDA-induced neuronal death when applied alone (data not shown).

Fig. 2.

GABA_A receptor antagonists block muscimol's protective action on NMDA excitotoxicity. Cultures were exposed to 100 μM NMDA for 5 min in the absence or presence of the indicated drugs. LDH was measured 20–24 h later. Data represent mean LDH ± SD scaled to the mean LDH measured in cultures exposed to 100 μM NMDA alone (=100). A: Antagonism by bicuculline, n = 20 cultures per condition pooled from five experiments; control NDMA injury = 62 ± 19% of the total neuronal population. B: Antagonism by Picrotoxin, n = 16 cultures per condition pooled from four experiments; control NMDA injury = 48 ± 12% of the total neuronal population. Data were analyzed by ANOVA followed by Student-Newman-Keuls multiple comparisons test. An asterisk indicates values significantly different (p < 0.05) from NMDA alone; a pound sign indicates values significantly different from NMDA plus muscimol.

Slowly triggered excitotoxicity

In addition to reducing rapidly triggered excitotoxicity, muscimol also reduced the slowly triggered excitotoxic euronal loss induced by a 24-h exposure to 15 μM NMDA, 10 μM AMPA, or 35–40 μM kainate (Table 2). A concentration-dependent reduction in neuronal death was observed with each agonist, although the protective effect against AMPA- or kainate-induced death was weaker than that against NMDA-induced death (Table 2, Fig. 1B). Reduction of NMDA-induced death was also observed with 1 mM guvacine or nipecotic acid, inhibitors of GABA uptake, but not against AMPA or kainate injury (Table 2). Addition of baclofen did not affect NMDA-, AMPA-, or kainate-induced neuronal death (data not shown).

Table 2.

Effect of muscimol or GABA uptake inhibitors on slowly triggered excitotoxicity by NMDA, kainate, or AMPA^a

Treatment		NMDA	Kainate % of control	AMPA
Muscimol	1 μM	86 ± 25	ND	ND
	10 μM	75 ± 29^b	96 ± 19	105 ± 8
	100 μM	59 ± 23^b	82 ± 18^b	83 ± 6^b
	1000 μM	44 ± 25^b	68 ± 17^b	71 ± 8^b
Guvacine	1 mM	36 ± 21^b	104 ± 19	96 ± 3
Nipecotic acid	1 mM	39 ± 20^b	100 ± 9	ND

Cultures were exposed to the indicated excitatory amino acids for 24 h in the absence or presence of increasing concentrations of muscimol or GABA uptake inhibitors. LDH was measured 20–24 h later. Data represent mean LDH ± SD scaled to mean LDH induced by individual excitotoxin exposure alone (= 100). NMDA (15 μM), n = 12–28 cultures per condition from four to seven experiments; control NMDA injury = 40–60% of the total neuronal population. Kainate (350 μM), n = 8–16 cultures per condition pooled from two to four experiments; control kainate injury = 35% of the total neuronal population. AMPA (10 μM), n = 8–24 cultures per condition pooled from two to six experiments; control AMPA injury = 80–85% of the total neuronal population. MK-801 at 10 μM was included in the kainate and AMPA experiments to block secondary activation of the NMDA receptor. ND = not determined.

Values are significantly different from indicated excitotoxin alone by ANOVA followed by Dunnett's t test (p < 0.05).

Oxygen-glucose deprivation

Consistent with earlier studies (Goldberg and Choi, 1993; Koh et al., 1995), a 40- to 45-min period of oxygen and glucose deprivation produced a low level (10–30%) of neuronal death 20–24 h later. In striking contrast to the protective effect observed for excitotoxicity, addition of 10–100 μM GABA or 1–100 μM muscimol (but not baclofen) during this oxygen–glucose deprivation period produced a concentration-dependent increase in immediate neuronal swelling and an exacerbation of neuronal death 20–24 h later (Fig. 3). With 100 μM GABA or muscimol, the same 40- to 45-min period of oxygen–glucose deprivation induced the death of a majority of neurons. This GABA- or muscimol-induced potentiation of neuronal death after oxygen–glucose deprivation was blocked by 10 μM MK-801 (Fig. 3). An antagonist of AMPA/kainate receptors, NBQX (30 μM) had no effect.

Fig. 3.

GABA_A receptor agonists potentiate neuronal injury induced by oxygen–glucose deprivation. Cultures were deprived of oxygen and glucose for 40–45 min in the absence or presence of the indicated drugs. LDH was measured 20–24 h later. Data represent mean LDH ± SD expressed as percentage of total neuronal LDH. A: GABA, n = 3–16 cultures per condition pooled from four experiments. B: Muscimol, n = 13–24 cultures per condition from six experiments. C: Baclofen, n = 10–14 cultures per condition from four experiments. Data were analyzed by ANOVA followed by Student-Newman-Keuls multiple comparisons test. An asterisk indicates values significantly different (p < 0.05) from 40- to 45-minute deprivation alone; a pound sign represents a significant decrease of the GABA_A receptor agonist-induced potentiation.

Addition of 1–100 μM muscimol also markedly increased accumulation of glutamate in the extracellular medium, and intracellular ⁴⁵Ca²⁺ uptake, measured immediately after a 40- to 45-min period of oxygen–glucose deprivation (Fig. 4). Muscimol itself did not alter intracellular ⁴⁵Ca²⁺ uptake in sham-washed control cells. MK-801 at 10 μM attenuated ⁴⁵Ca²⁺ accumulation but did not significantly decrease accumulation of extracellular glutamate (Fig. 4). Bicuculline (1–100 μM) only weakly reduced the muscimol-induced potentiation of neuronal death after oxygen–glucose deprivation (Fig. 5A), probably because it alone produced a concentration-dependent, MK-801-sensitive, increase in this neuronal death (Fig. 5B). Similar results were obtained using Picrotoxin (data not shown).

Fig. 4.

Effect of muscimol on glutamate release and ⁴⁵Ca²⁺ accumulation induced by oxygen–glucose deprivation. Cultures were deprived of oxygen and glucose for 40–45 min in the presence of the indicated concentration of muscimol. A: Data represent mean ± SD glutamate concentration in the bathing medium at the end of oxygen–glucose deprivation, n = 8–12 cultures per condition pooled from three experiments. Note that bath glutamate concentrations are much lower than observed in vivo (see for example, Benveniste et al., 1984) due to the larger dilution volume; however, local concentrations in synaptic regions are likely much higher. B: Data represent mean cpm ± SD scaled to the mean cpm measured in the condition without muscimol (=100); n = 10–18 cultures per condition pooled from five experiments. Note that ⁴⁵Ca²⁺ accumulation in the condition without muscimol was very low, similar to that found in cultures exposed to sham-wash alone. Data were analyzed by ANOVA followed by Student-Newman-Keuls multiple test. An asterisk indicates values significantly different (p < 0.05) from 40- to 45-min deprivation alone; a pound sign represents a significant decrease of the muscimol-induced potentiation.

Cyanide exposure

As an additional neuronal injury paradigm triggered by energy depletion, cultures were exposed to cyanide. GABA or muscimol addition also potentiated the neuronal death induced by energy-depletion due to a 40- to 45-min exposure to 1 mM potassium cyanide in glucose-free media (Fig. 6).

Fig. 5.

Effect of bicuculline on neuronal injury induced by oxygen–glucose deprivation in the presence and absence of muscimol. Cultures were deprived of oxygen and glucose for 40–45 min in the absence or presence of the indicated drugs. LDH was measured 20–24 h later. Data represent mean LDH ± SD expressed as percentage of total LDH. A: Effect of bicuculline on muscimol (10 μM) potentiation of neuronal injury, n = 8–11 cultures per condition from three experiments. B: Effect of bicuculline alone, n = 16 cultures per condition pooled from four experiments. Data were analyzed by ANOVA followed by Student-Newman-Keuls multiple comparisons test. An asterisk indicates values significantly greater (p < 0.05) than oxygen–glucose deprivation alone; a pound sign indicates values significantly different from (A) muscimol-induced potentiation of oxygen–glucose deprivation or (B) bicuculline-induced potentiation of oxygen–glucose deprivation.

Fig. 6.

GABA_A receptor agonists potentiate neuronal injury induced by cyanide exposure. Cultures were exposed to potassium cyanide for 40–45 min in the absence or presence of the indicated drugs. LDH was measured 20–24 h later. Data represent mean LDH ± SD expressed as percentage of total LDH; n = 16 cultures per condition pooled from four experiments. An asterisk indicates values significantly greater from cyanide exposure alone as determined by ANOVA followed by Dunnett's t test (p < 0.05).

DISCUSSION

Present data document an unexpected contrast between the protective effect of GABA_A agonists on the cortical neuronal death induced by exogenous NMDA addition and the exacerbating effect of the same drugs on the neuronal death induced by either oxygen–glucose deprivation or cyanide exposure. The latter is unexpected for two reasons: (a) there is convincing evidence that GABAmimetic drugs are neuroprotective in several animal models of global or focal ischemia (see above); and (b) prior study in our system has indicated that the death induced by short periods of oxygen–glucose deprivation (such as used here) is mostly mediated by NMDA receptors (Goldberg and Choi, 1993).

In earlier studies, neither we (Koh and Choi, 1987b; Monyer et al, 1990) nor Erdo and Michler (1990) observed a neuroprotective effect of GABA agonists against maximal levels of rapidly triggered, NMDA receptor-mediated excitotoxicity. Detection of a partial protective effect of GABA agonists in the present study probably reflects employment of lower NMDA concentrations, which produced submaximal neuronal death. Our observations are consistent with the finding by Ohkuma et al. (1994) that muscimol reduced the neuronal injury of cultured cortical neurons induced by low (10 μM) concentrations of NMDA. Taken together, available data suggest that GABA_A receptor stimulation produces a partial protective effect against submaximal NMDA receptor-mediated toxicity that can be overridden by intense NMDA receptor activation.

The protective effect of muscimol and GABA versus excitatory amino acid toxicity may be due to the ability of GABA_A receptor activation to drive the membrane potential towards E_c1, which is usually near the resting potential. This should increase the voltage-dependent Mg²⁺ block of the NMDA receptor-gated channel (Mayer et al., 1984; Nowak et al., 1984) and thereby reduce NMDA receptor-mediated toxic Ca²⁺ entry (Fig. 7). In addition, membrane hyperpolarization can be expected to reduce Ca²⁺ influx mediated via the Na⁺-Ca²⁺ exchanger or mediated via voltage-gated Ca²⁺ channels (Choi, 1988). Consistent with the idea that GABA_A receptor activation reduces NMDA-induced net neuronal Ca²⁺ influx, it decreased cellular accumulation of extracellular ⁴⁵Ca²⁺ induced by a 5-min exposure to 200 μM NMDA. Reduction of depolarization-induced Ca²⁺ influx may also explain the ability of GABA_A receptor activation to attenuate the slowly triggered excitotoxic neuronal death induced by a 24-h exposure to NMDA, AMPA or kainate.

Fig. 7.

Proposed effect of GABA_A receptor activation on NMDA receptor-mediated current (I) in energy-competent and energy-depleted neurons. This graph depicts a typical NMDA receptor I/V curve, exhibiting voltage-dependent Mg²⁺ block (modified from MacDermott and Dale, 1987). In energy-competent neurons (only partially depolarized by exposure to NMDA), the hyperpolarizing effect of GABA_A receptor activation may move the membrane potential from point 1 to point 2, thereby strengthening the Mg²⁺ block. However, in energy-depleted neurons, the same GABA_A receptor activation may move membrane potential from point 3 to point 4, producing no effect on the Mg²⁺ block, but still increasing the inward driving force for Ca²⁺.

In slowly triggered excitotoxicity paradigms, addition of either muscimol or drugs that inhibit GABA uptake produced a greater protective effect on NMDA-induced neuronal death than on AMPA- or kainate-induced death. Perhaps this is because the effect of membrane hyperpolarization on the Mg²⁺ block of NMDA receptor-gated channels is not shared with AMPA or kainate receptor-gated channels. The observed lack of protective effect of GABA uptake inhibitors against AMPA or kainate toxicity likely reflects lower levels of GABA_A receptor stimulation achieved via this route (compared to that achieved during NMDA exposure), perhaps because of less GABA release.

Further study will be required to elucidate the mechanisms underlying the unexpected potentiating effect of GABA_A receptor activation on oxygen–glucose deprivation-induced neuronal death. Data presented here indicate that this potentiated death is associated with enhanced Ca²⁺ accumulation and is still mediated by NMDA receptors. One intriguing possible explanation can be developed from the idea that membrane hyperpolarization has opposing effects on NMDA receptor-mediated Ca²⁺ influx: increasing the Mg²⁺ block of the NMDA receptor-gated channel, but also increasing the electrochemical gradient favoring Ca²⁺ entry. Thus, in energy-competent neurons with hyperpolarized membrane potentials (e.g., neurons exposed to exogenously added NMDA), GABA_A receptor activation may move the membrane potential towards E_c1 enough that the Mg²⁺ block is enhanced and Ca²⁺ influx through the NMDA receptor-gated channel falls (Fig. 7). However, in energy-depleted neurons with collapsed membrane potentials (e.g., neurons exposed to oxygen–glucose deprivation or cyanide; see Hansen, 1985), GABA_A receptor activation may hyperpolarize neuronal membranes enough to increase the inward Ca²⁺ driving force, but not enough to bring in the Mg²⁺ block (Fig. 7). Increased neuronal Ca²⁺ entry may also lead to greater glutamate release and help explain the observed increase in extracellular glutamate accumulation observed in cultures exposed to oxygen–glucose deprivation in the presence of muscimol. Increased extracellular glutamate itself would be expected to increase neuronal death.

Other mechanisms can also be considered. For instance, although GABA_A receptor stimulation usually inhibits central neuronal firing, it can, under some circumstances, be excitatory (Michelson and Wong, 1991; Kaila, 1994). In particular, while normally inhibitory, dendritic GABA_A receptors can become excitatory during intense activation due to an activity-dependent shift in the GABA_A reversal potential away from E_c1 and toward E_HCO3 (Staley et al, 1995). Furthermore, it is conceivable that prolonged exogenous application of GABA or muscimol desensitizes GABA_A receptors sufficiently that a paradoxical loss of inhibitory tone results. The apparent voltage-dependence of this desensitization (Frosch et al., 1992) raises the possibility that desensitization might be greater when the agonists are applied prior to membrane depolarization (as would be the case in the oxygen–glucose deprivation and cyanide paradigms), compared with when the agonists are applied together with a depolarizing stimulus (as would be the case in the excitotoxicity paradigms).

The weak antagonism of bicuculline or picrotoxin on the injury-potentiating effect of muscimol in cultures deprived of oxygen and glucose is well explained by the observed injury-potentiating effect exhibited by these GABA_A antagonist drugs when applied alone. This intrinsic injury-potentiating effect may reflect an effect of GABA_A receptor antagonists to increase circuit excitability, enhancing ATP depletion and glutamate release. In exogenous excitotoxicity induced by addition of NMDA, endogenous glutamate release is likely swamped by exogenously added NMDA.

In contrast to the observed modulatory effects of GABA_A receptor activation upon cortical neuronal death induced by exposure to excitotoxins or oxygen–gluocse deprivation, GABA_B receptor activation with baclofen lacked effect. The picture appears different in CNS white matter, which has few glutamate receptors and is not vulnerable to excitotoxic injury. Fern et al. (1995) recently reported that anoxia-induced injury of the rat optic nerve was attenuated by GABA or baclofen, but not GABA_A receptor agonists.

The present in vitro experiments should not be viewed as conflicting with existing in vivo data indicating that GABA_A agonists can reduce ischemic brain injury. The final weighting of different variables can only be accomplished in vivo, and nothing demonstrated here argues against a final predominance of beneficial effects. However, present results provide novel isolation of a potential deleterious effect of GABA_A receptor activation upon energy-depleted neurons. Strategies aimed at eliminating this deleterious action—for example, addition of NMDA receptor blockade (see for example, Lyden and Lonzo, 1994)—might further enhance the neuroprotective effects of GABA_A agonists in the ischemic brain.

Footnotes

Abbreviations used

Acknowledgment:

We thank Susan Adams and Aghdas Jafari for technical support and Dr. Sandra Hewett for helpful discussions. This work was supported by NIH research grant NS-32636 (D.W.C.).

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