Ethanol enhances GABAergic synaptic inhibition, and this interaction contributes to many of the behavioral and cognitive effects of this drug. Most studies suggest that ethanol enhances GABAergic neurotransmission via an allosteric potentiation of the postsynaptic GABAA receptors that mediate fast synaptic inhibition in the mammalian CNS. Despite widespread acceptance of this hypothesis, direct support for such a mechanism has been difficult to obtain. Ethanol does not enhance GABAA receptor function in all brain regions or under all experimental conditions, and factors responsible for this variability remain mostly unknown. Notably, blockade of GABAB receptors dramatically enhances ethanol potentiation of hippocampal GABAA IPSPs and IPSCs, suggesting that some unknown GABAB receptor mechanism limits the overall potentiating effect of ethanol on GABAergic synapses. In this study, we demonstrate that, at perisomatic synapses in the rat hippocampus, ethanol enhances presynaptic GABAB autoreceptor function and that this interaction reduces the overall potentiating effect of ethanol at these synapses. We further show that ethanol significantly elevates basal presynaptic GABAB receptor tone, possibly via an increase in spontaneous GABA release, and that pretreatment with a subthreshold concentration of the GABAB receptor agonist baclofen blocks ethanol but not flunitrazepam or pentobarbital potentiation of GABAA IPSCs. These data suggest that an interaction between ethanol and presynaptic GABAB autoreceptor activity regulates the ethanol sensitivity of GABAergic synapses. Given that the in vitro ethanol sensitivity of these synapses correlates with in vivo ethanol responsiveness in a number of rodent lines, our data further suggest that presynaptic GABAB receptor activity may play a role in regulating behavioral sensitivity to ethanol.
Alcohol addiction remains an imposing medical and socioeconomic concern for many nations (Volpicelli, 2001). For example, in the United States, alcoholism and alcohol abuse rank among the top three psychiatric illnesses (Kessler et al., 1994). In addition, alcohol-related disorders are responsible for >105,000 deaths annually in the United States (McGinnis and Foege, 1999) at a cost of >150 billion dollars. Despite these staggering statistics, the neurophysiological mechanisms that mediate alcohol intoxication, reinforcement, and dependence are not fully understood. A hypothesis that has received increasing support over the past 20 years is that alcohol interacts with a subset of neuronal proteins that control excitatory and inhibitory synaptic communication in the CNS (Deitrich et al., 1989; Faingold et al., 1998). In particular, much attention has focused on the acute potentiating effects of alcohol on inhibitory synaptic transmission mediated by GABAA receptors.
GABAA receptors mediate the majority of fast inhibitory synaptic transmission in the mammalian CNS (Krnjevic, 1991; Thompson, 1994). These receptors serve as the primary target for a variety of sedative and hypnotic drugs, such as barbiturates and benzodiazepines, which allosterically enhance GABAA receptor function (Macdonald and Olsen, 1994). There is also considerable evidence in support of the hypothesis that the behavioral and cognitive effects of ethanol are mediated, at least in part, via a potentiation of GABAA receptor-mediated synaptic inhibition (Grobin et al., 1998; Mihic, 1999). Despite the popularity of this hypothesis, considerable controversy remains. For example, studies that have examined the direct effects of ethanol on GABAergic synaptic transmission have reported potentiation (Proctor et al., 1992b; Weiner et al., 1994, 1997; Poelchen et al., 2000; Roberto et al., 2003), inhibition (Siggins et al., 1987), or no effect (Siggins et al., 1987; Soldo et al., 1994), and the reasons for these disparate results remain poorly understood. Interestingly, several studies have noted that blockade of GABAB receptors can dramatically enhance the acute potentiating effect of ethanol on GABAA receptor-mediated IPSCs in the rat hippocampus (Wan et al., 1996; Kang et al., 1998). These studies suggest that some unknown GABAB receptor-dependent process may play an important role in regulating the ethanol sensitivity of GABAergic synapses. Most GABAergic synapses contain both presynaptic and postsynaptic GABAB receptors. Presynaptic GABAB receptors function as autoreceptors, and their activation inhibits GABAA IPSCs, whereas postsynaptic GABAB receptors are primarily coupled to the activation of a potassium conductance (Misgeld et al., 1995). In this study, we demonstrate that ethanol exerts a novel facilitatory effect on presynaptic GABAB receptor function at GABAergic synapses in the rat CA1 region and that this interaction serves to actively limit the overall potentiating effect of ethanol at these synapses.
Materials and Methods
Slice preparation. Transverse hippocampal slices (400 μm) were prepared from 4- to 6-week-old male Sprague Dawley rats. Slices were maintained at ambient temperature for at least 2 hr in oxygenated artificial CSF (aCSF) containing (in mm): 124 NaCl, 3.3 KCl, 2.4 MgCl2, 2.5 CaCl2, 1.2 KH2PO4, 10 d-glucose, and 25 NaHCO3, saturated with 95% O2 and 5% CO2.
Electrophysiological recordings. Slices were transferred to a recording chamber maintained at ambient temperature and superfused with aerated aCSF at 2 ml/min. Recording electrodes were prepared from filamented borosilicate glass capillary tubes (inner diameter, 0.86 mm) using a horizontal micropipette puller (P-97; Sutter Instruments, Novato, CA). Patch-clamp recordings were made using a filling solution containing 130 mm K-gluconate, 10 mm KCl, 1 mm EGTA, 100 μm CaCl2, 2 mm Mg-ATP, 200 μm Tris-guanosine 5′-triphosphate, and 10 mm HEPES, pH adjusted with KOH, 275-280 mOsm. In most experiments, 5 mm N-(2,6-dimethyl-phenylcarbamoylmethyl)-triethylammonium bromide (QX-314) was included in the recording solution to block voltage-gated sodium currents and GABAB IPSCs in the CA1 neurons being recorded (Horn et al., 1980; Nathan et al., 1990). QX-314 was omitted from the filling solution in experiments measuring postsynaptic GABAB receptor function (see Fig. 3). Whole-cell patch-clamp recordings were made from CA1 pyramidal neurons voltage-clamped at -45 to -60 mV. Only cells with a stable access resistance of 5-20 MΩ were used in these experiments. Whole-cell currents were acquired using an Axoclamp 2B or Axopatch 200B amplifier, digitized (Digidata1200 or Digidata 1321A; Axon Instruments, Union City, CA), and analyzed on- and off-line using an IBM-compatible personal computer and pClamp 8.0 or 9.0 software (Axon Instruments). In one set of experiments, we recorded spontaneous GABAA IPSCs (sIPSCs) using a filling solution identical to that described above, except that 140 mm CsCl2 was substituted for K-gluconate and KCl. sIPSCs were digitized at 5-10 kHz in continuous 3 min epochs. Spontaneous events in each epoch were first identified using Clampfit event detection software (pClamp 9.0), and then all events were visually inspected to avoid inclusion of spurious responses in the data analysis (<2% of detected events were rejected). sIPSCs in each epoch were then averaged, and the amplitude and area of averaged traces were calculated using the Statistics function included in the Clampfit program (pClamp 9.0).
Pharmacological isolation of synaptic currents. GABAA IPSCs were evoked every 20 sec by electrical stimulation (0.2 msec duration) using a concentric bipolar stimulating electrode (FHC, Bowdoinham, ME) placed near the CA1 somatic layer [“proximal” stimulation (Weiner et al., 1997)]. Stimulation intensity was adjusted to evoke responses that were 10-20% of maximal currents (typically 50-200 pA). GABAA IPSCs were pharmacologically isolated using a mixture of 50 μm APV and 20 μm DNQX to block NMDA and AMPA/kainate receptors, respectively. GABAB IPSCs were recorded in the presence of a similar mixture that also included 20 μm bicuculline methiode to block GABAA receptors. Unless otherwise stated, all drugs used were purchased from Sigma (St. Louis, MO). Drugs were made up as 100- to 400-fold concentrates and applied to slices via calibrated syringe pumps (Razel Scientific Instruments, Stamford, CT). A 4 m ethanol solution was prepared immediately before each experiment from a 95% stock solution (Aaper Alcohol and Chemical, Shelbyville, KY) kept in a glass storage bottle.
Statistics. Drug effects were quantified as the percentage change in the area under the curve of synaptic currents relative to the mean of control and washout values. Statistical analyses of drug effects were performed using the two-tailed Student's paired or unpaired t tests or a one-way ANOVA followed by the Newman-Keuls post hoc test with a minimal level of significance of p < 0.05.
A GABAB receptor antagonist enhances ethanol potentiation of proximal GABAA IPSCs
Several studies have shown that GABAA IPSCs evoked by stimulation of perisomatic GABAergic synapses proximal to the CA1 pyramidal layer are reliably potentiated by pharmacologically relevant concentrations of ethanol, even in the absence of a GABAB receptor antagonist (Weiner et al., 1997; Poelchen et al., 2000; Proctor et al., 2003). Therefore, we first sought to determine whether a GABAB receptor antagonist enhanced ethanol potentiation of proximal hippocampal GABAA IPSCs, as had been shown for ethanol potentiation of IPSCs evoked by stimulation of the stratum radiatum in this brain region (Wan et al., 1996). Under our standard recording conditions, bath application of 80 mm ethanol significantly potentiated the area of proximal GABAA IPSCs (72.8 ± 7.8%; n = 14; p < 0.01) (Fig. 1). This enhancement was stable for the duration of the ethanol application and reversed on ethanol washout. Pretreating slices with the GABAB receptor antagonist SCH 50911 ((+)-5,5-dimethyl-2-morpholineacetic acid hydrochloride) had no effect on GABAA IPSCs (2.4 ± 5.9% potentiation; n = 14; p > 0.05). However, in the presence of SCH 50911, ethanol potentiation of proximal GABAA IPSCs was significantly enhanced (121.5 ± 12.7%; n = 14; p < 0.05) (Fig. 1). Because of the long duration required for these and many of the experiments in this study, recordings were performed at ambient temperature, which we found to increase the stability of patch clamp recordings in brain slices. However, we did examine the effect of SCH 50911 on ethanol potentiation of GABAA IPSCs in nine slices maintained at 32°C. In these slices, we observed no difference in the magnitude of the acute effect of 80 mm ethanol (62.5 ± 19.0% potentiation; n = 9; p < 0.05) or 20 μm SCH 50911 alone (3.8 ± 6.8% inhibition; n = 9; p > 0.05) or the effect of ethanol in the presence of SCH 50911 (127.7 ± 15.4; n = 9; p < 0.05).
Ethanol increases presynaptic GABAB receptor activity
The preceding experiments suggest that some unknown GABAB receptor-mediated process may actively limit ethanol potentiation of proximal GABAA IPSCs in the rat hippocampal CA1 region. Activation of presynaptic GABAB receptors at these and many other inhibitory synapses in the mammalian CNS produces a well characterized inhibition of GABA release along with an associated decrease in the size of evoked GABAA IPSCs (Bowery and Enna, 2000; Couve et al., 2000). We hypothesized that ethanol may enhance the activity of these presynaptic GABAB autoreceptors and that such an interaction could serve to reduce the overall potentiating effect of ethanol at these synapses. To test this hypothesis, we assayed the effect of ethanol on presynaptic GABAB receptor function at proximal GABAergic synapses. We determined the effect of a submaximal concentration of the GABAB receptor agonist baclofen on the area of proximal GABAA IPSCs in the absence and presence of 80 mm ethanol. These recordings were performed with 5 mm QX-314 in the patch pipette to block postsynaptic GABAB IPSCs (Nathan et al., 1990). Under these recording conditions, bath application of 1.25 μm baclofen reduced the area of GABAA IPSCs by 46.9 ± 5.0% (n = 23) (Fig. 2). Pretreating slices with 80 mm ethanol resulted in a significant potentiation of proximal GABAA IPSCs (Fig. 2), as seen in Figure 1 and previous studies (Weiner et al., 1997; Poelchen et al., 2000). However, once a new baseline was established in the presence of 80 mm ethanol, 1.25 μm baclofen had a significantly greater inhibitory effect on GABAA IPSCs, reducing their area by an average of 79.0 ± 2.7% (n = 9) (Fig. 2). In five cells in which ethanol was washed out for at least 20 min, baclofen inhibition of GABAA IPSCs returned to baseline levels (37.4 ± 9.5% inhibition).
We next examined the effect of a range of ethanol concentrations on baclofen inhibition of GABAA IPSCs. Ethanol significantly enhanced the inhibitory effect of baclofen at 40 and 80 but not 20 mm (Fig. 2B), and the potency of this interaction was identical to that of the overall potentiating effect of ethanol at these synapses.
One possible confound of the above experiment is that, because ethanol potentiates proximal GABAA IPSCs, it is possible that baclofen inhibition of proximal IPSCs is dependent on the initial size of these responses. To address this concern, we tested the effect of flunitrazepam (FLU), a well characterized benzodiazepine that potentiates GABAA receptor activity (Sieghart, 1995), on presynaptic GABAB receptor function. Bath application of 1 μm FLU increased the proximal GABAA IPSC area by 80.1 ± 13.5% (n = 8; p < 0.01). After a new baseline was established, we tested the effect of 1.25 μm baclofen on proximal IPSCs. Unlike ethanol, FLU pretreatment had no significant effect on baclofen inhibition of GABAA IPSCs (Fig. 2B).
Ethanol has no effect on postsynaptic GABAB receptor function
The preceding experiments provide evidence that ethanol potentiates presynaptic GABAB receptor function at proximal synapses in the rat hippocampus. Although this specific interaction has not been previously examined, several studies have characterized the acute effects of ethanol on postsynaptic GABAB receptor activity in the hippocampus and have reported no effect of ethanol at concentrations similar to those used in our studies (Frye et al., 1991; Frye and Fincher, 1996; Wan et al., 1996). We performed two separate experiments to evaluate possible effects of ethanol on postsynaptic GABAB receptor function under our recording conditions. First, we tested the effect of ethanol on GABAB IPSCs recorded from rat hippocampal CA1 pyramidal neurons. GABAB IPSCs were evoked by electrical stimulation of the stratum lacunosum in the presence of 50 μm APV and 20 μm DNQX, to block glutamatergic synaptic responses, and 20 μm bicuculline methiodide, to block GABAA IPSCs. The remaining synaptic current was completely blocked by 20 μm SCH 50911, indicating that it was mediated by the activation of GABAB receptors (Fig. 3A). Under these recording conditions, 80 mm ethanol had no effect on the area of GABAB IPSCs (4.3 ± 8.3% potentiation; n = 7; p > 0.05) (Fig. 3A).
In the second experiment, we tested the effect of ethanol on outward currents elicited by bath application of the GABAB receptor agonist baclofen in cells voltage-clamped at -50 mV. This protocol has been used in other studies to activate the G-protein coupled inwardly rectifying potassium channels that underlie the slow GABAB IPSCs recorded in Figure 3A (Newberry and Nicoll, 1984; Gahwiler and Brown, 1985; Sodickson and Bean, 1996; Liu and Leung, 2003). Under our recording conditions, bath application of 20 μm baclofen induced an outward current of 76.1 ± 14.8 pA (n = 5) (Fig 3B). Ethanol pretreatment (80 mm) had no effect on currents evoked by 20 μm baclofen (68.9 ± 8.6 pA; n = 5; p > 0.05). Although ethanol alone appeared to induce a small outward current in the example illustrated, this effect was not significant and has not been consistently observed in other studies (Frye and Fincher, 1996; Wan et al., 1996).
Ethanol enhances presynaptic GABAB receptor tone
The results of these studies suggest that ethanol selectively enhances presynaptic but not postsynaptic GABAB receptor activity at GABAergic synapses in the rat hippocampal CA1 region. Because this interaction occurs across the same ethanol concentration range that results in an overall potentiation of proximal GABAA IPSCs, this presynaptic interaction likely serves to reduce the overall potentiating effect of ethanol at these synapses. This interaction may thus account for the observation that a GABAB receptor antagonist can facilitate ethanol potentiation of GABAA IPSCs in this brain region. However, this interpretation implies that there must be some tonic presynaptic GABAB receptor activity in the presence of ethanol. To address this issue, we examined the effect of the GABAB receptor antagonist SCH 50911 on the area of proximal GABAA IPSCs in the absence and presence of 80 mm ethanol. As shown in Figure 1, bath application of 20 μm SCH 50911 had no effect on proximal GABAA IPSCs evoked every 20 sec (4.0 ± 8.0% inhibition; n = 9; p > 0.05). This concentration of SCH 50911 was, however, sufficient to completely block the inhibitory effect of baclofen on these responses (compare Figs. 2A, 4A). Notably, in the presence of 80 mm ethanol, SCH 50911 significantly potentiated proximal GABAA IPSCs (35.9 ± 4.1% potentiation; n = 11; p < 0.05) (Fig. 4). The results of this experiment demonstrate that, in the presence but not the absence of ethanol, presynaptic GABAB receptor activity tonically inhibits proximal GABAergic synapses in the rat hippocampal CA1 region.
Pretreatment with a GABAB receptor antagonist selectively enhances ethanol potentiation of sIPSC frequency
We performed one additional series of experiments to further address both the mechanism through which ethanol enhances GABAA receptor-mediated synaptic transmission in the CA1 region and the facilitation of this effect by blockade of GABAB receptors. In these experiments, CA1 pyramidal neurons were voltage-clamped at -70 mV, and GABAA sIPSCs were recorded in the presence of the glutamate receptor antagonist mixture used in the evoked GABAA IPSC experiments. Under these recording conditions, the majority of neurons exhibited spontaneous synaptic responses that reversed near 0 mV and were completely blocked by the GABAA receptor antagonist bicuculline methiodide (20 μm) (data not shown). In the first experiment, we tested the effect of 80 mm ethanol alone on the frequency, amplitude, and area of sIPSCs. Bath application of ethanol significantly and reversibly increased all three parameters (Fig. 5A,C) (frequency, 39.2 ± 6.6% potentiation; p < 0.001; amplitude, 16.7 ± 5.6% potentiation; p < 0.05; area, 21.9 ± 6.7% potentiation; p < 0.05; n = 10), with the increase in sIPSC frequency appearing to be the most robust effect (Fig. 5C). In the second experiment, we tested the effect of the GABAB receptor antagonist SCH 50911 (20 μm) and SCH 50911 plus 80 mm ethanol on sIPSCs. Pretreating slices with SCH 50911 alone had no effect on any of the sIPSC parameters measured (frequency, 1.4 ± 8.2% potentiation; p > 0.05; amplitude, 2.1 ± 5.2% potentiation; p > 0.05; area, 1.0 ± 4.9% inhibition; p > 0.05; n = 11) (Fig. 5B,C). However, in the presence of SCH 50911, ethanol enhancement of sIPSC frequency was significantly greater than that observed when ethanol was applied alone (99.8 ± 8.6% potentiation; p < 0.001) (Fig. 5B,C). In contrast, ethanol potentiation of sIPSC amplitude and area were not significantly altered by pretreatment with the GABAB receptor antagonist (amplitude, 18.7 ± 7.3% potentiation; area, 30.6 ± 8.5 potentiation; n = 11) (Fig. 5B,C).
A subthreshold concentration of baclofen selectively blocks ethanol potentiation of proximal GABAA IPSCs
Because ethanol appears to enhance tonic presynaptic GABAB receptor activity, it might be possible to antagonize ethanol potentiation of proximal GABAA IPSCs by pretreating slices with a low, subthreshold concentration of a GABAB receptor agonist. To test this hypothesis, we examined the effect of 80 mm ethanol on proximal GABAA IPSCs in the absence and presence of a subthreshold concentration of baclofen. Under our recording conditions, a concentration of 500 nm baclofen was determined to be just below the threshold to elicit a significant inhibition of proximal GABAA IPSCs (to 90.2 ± 5.1% of control; n = 21; p > 0.05). We therefore pretreated slices with 500 nm baclofen and tested the effect of 80 mm ethanol on proximal IPSCs (Figs. 6, 7). Although 80 mm ethanol produced a robust potentiation of proximal GABAA IPSCs under control conditions (Fig. 1), in the presence of 500 nm baclofen, 80 mm ethanol caused a modest but significant inhibition of proximal GABAA IPSCs (to 78.5 ± 6.1% of control; p < 0.05; n = 11). In a few cells, we were able to wash out the baclofen for at least 30 min and challenge slices again with the same concentration of ethanol. After baclofen washout, 80 mm ethanol enhanced proximal GABAA IPSCs (Fig. 6).
We next examined the effect of 500 nm baclofen pretreatment on flunitrazepam and pentobarbital potentiation of GABAA IPSCs. In contrast to the inhibitory effect of baclofen pretreatment on ethanol potentiation of proximal IPSCs, baclofen had no effect on flunitrazepam or pentobarbital potentiation of these responses. Bath application of 1 μm flunitrazepam potentiated GABAA IPSCs to 198.0 ± 19.2% under control conditions (n = 8) and to 209.7 ± 19.2% of control in the presence of 500 nm baclofen (n = 5). Similar results were observed with 50 μm pentobarbital (254 ± 31.3% of control in the absence of baclofen; n = 5; 228.1 ± 27.8% of control in the presence of baclofen; n = 5) (Fig. 7).
The results of this study suggest that acute ethanol exposure enhances presynaptic GABAB receptor function at proximal GABAergic synapses in the rat CA1 region. This interaction occurs across the same concentration range over which ethanol potentiates GABAA IPSCs at these synapses. Because presynaptic GABAB receptor activity reduces evoked GABA release (Misgeld et al., 1995), this interaction appears to actively limit the overall potentiating effect of ethanol at these synapses.
Although there is now compelling behavioral evidence that ethanol acts, in part, by potentiating GABAA receptor function (Grobin et al., 1998), direct evidence that ethanol actually enhances GABAA receptor-mediated synaptic inhibition has been somewhat difficult to demonstrate, particularly in the hippocampus. Our results suggest that variability in the level of presynaptic GABAB receptor activity, attributable to differences in the stimulation protocols used to evoke IPSCs, may have contributed to much of the disparity in the reported effects of ethanol on hippocampal GABAergic synapses. For example, within the CA1 region, ethanol has been reported to have little or no effect on compound IPSCs mediated by both GABAA and GABAB receptors (Siggins et al., 1987; Proctor et al., 1992a; Wan et al., 1996). Synaptic activation of GABAB receptors typically requires relatively intense stimulation or trains of stimuli (Thomson and Destexhe, 1999; Liu and Leung, 2003), possibly reflecting an extrasynaptic localization of these receptors (Fritschy et al., 1999). Synaptic GABAB IPSCs are also more readily evoked by stimulation of dendritic fields of the CA1 region (Williams and Lacaille, 1992; Thompson, 1994). In contrast, data presented in this study, as well as those of several other reports (Weiner et al., 1997; Poelchen et al., 2000; Crowder et al., 2002; Proctor et al., 2003), demonstrate that GABAA IPSCs evoked by minimal stimulation of perisomatic synapses in the CA1 region are reliably potentiated by intoxicating concentrations of ethanol. Our data suggest that the ethanol insensitivity of compound GABAergic synaptic responses may reflect a greater degree of presynaptic GABAB receptor modulation of such responses. In fact, presynaptic GABAB receptor activity inhibits dendritic IPSCs to a greater extent than those evoked by perisomatic stimulation (Lambert and Wilson, 1993b; Pearce et al., 1995). This hypothesis is further supported by the observation that blockade of GABAB receptors converts ethanol-insensitive compound IPSCs into ones that are reliably potentiated by ethanol (Wan et al., 1996) and by data presented in this study that a minimal elevation of presynaptic GABAB receptor tone, by pretreating slices with a subthreshold concentration of baclofen, was sufficient to convert ethanol-sensitive proximal GABAA IPSCs into responses that were actually inhibited by ethanol. Taken together, these data are consistent with the hypothesis that variation in the degree of presynaptic GABAB receptor activity can profoundly influence ethanol modulation of GABAA IPSCs in the hippocampus and that such variability may have contributed significantly to the disparity noted in earlier studies on ethanol modulation of GABAergic neurotransmission in this brain region.
It should be noted that, although proximal GABAA IPSCs were potentiated by ethanol under our standard recording conditions (i.e., in the absence of a GABAB receptor antagonist), pretreatment with a GABAB receptor antagonist significantly increased the ethanol sensitivity of these responses. Therefore, presynaptic GABAB receptor function can regulate the ethanol sensitivity of GABAergic synapses that are not under tonic regulation by these receptors. Interestingly, recent reports have demonstrated a strong correlation between the in vitro ethanol sensitivity of proximal GABAA IPSCs and behavioral sensitivity to some of the intoxicating effects of ethanol. This relationship has been found in lines of rats and mice bred for differences in ethanol-induced loss of the righting reflex (Poelchen et al., 2000) as well as in PKCγ or ϵ knock-out mice, which also exhibit alterations in a variety of ethanol-mediated behavioral effects (Harris et al., 1995; Hodge et al., 1999). In all of these rodent lines, proximal IPSCs from ethanol-sensitive strains were potentiated to a greater extent by ethanol than IPSCs recorded from slices prepared from the ethanol-resistant lines (Poelchen et al., 2000; Proctor et al., 2003). These findings suggest that factors that regulate the ethanol sensitivity of proximal GABAA IPSCs may also influence behavioral responsiveness to ethanol. Given that presynaptic GABAB receptor activity can profoundly influence the ethanol sensitivity of these synapses, it will be important in future studies to determine whether functional differences in presynaptic GABAB receptor activity contribute to these previously described in vitro and in vivo differences in ethanol sensitivity in these selected lines.
The importance of elucidating the physiological factor(s) that can influence behavioral sensitivity to ethanol is further underscored by epidemiological studies suggesting that lower initial sensitivity to ethanol may be an important risk factor associated with an elevated risk of developing ethanol-related problems later in life (Schuckit, 1994; Schuckit and Smith, 1996). Although much effort has been directed at identifying genetic differences that may contribute to the variance in behavioral responsiveness to ethanol consumption, the specific genes responsible for such differences remain mostly unknown. Our findings suggest that genes encoding for GABAB receptors, or any proteins that may modulate the activity of the cascade linking presynaptic GABAB receptor activation to a reduction in GABA release, represent genetic loci that could potentially be associated with an increased risk of alcoholism.
Another important question that remains to be fully addressed is the mechanism through which ethanol enhances presynaptic GABAB receptor function. One possibility is that ethanol interacts directly with presynaptic GABAB receptors. Our finding that ethanol potentiates baclofen inhibition of proximal GABAA IPSCs is consistent with such a mechanism. However, our data and those of others (Frye and Fincher, 1996; Wan et al., 1996) also suggest that ethanol does not enhance postsynaptic GABAB receptor function in the CA1 region. All GABAB receptors are heterodimers comprising GABAB1 and GABAB2 subunits (Bowery and Enna, 2000; Enna, 2001), and genetic deletion of the GABAB1 isoform completely abolishes presynaptic and postsynaptic GABAB receptor function (Prosser et al., 2001). These findings suggest a single, pharmacologically homogeneous distribution of GABAB receptors in the CNS. It therefore seems unlikely that ethanol could directly interact with presynaptic GABAB receptors but be devoid of activity at their postsynaptic counterparts. It should, however, be noted that ethanol has been shown to potentiate postsynaptic GABAB receptor function in regions other than the hippocampus [e.g., cerebellar granule neurons (Lewohl et al., 1999)]. In addition, our data do not rule out the possibility that ethanol acts directly on some element of the downstream cascade linking presynaptic GABAB receptor activation to the inhibition of GABA release because distinct coupling mechanisms are thought to mediate presynaptic and postsynaptic GABAB receptor signaling in the hippocampus (Thompson and Gahwiler, 1992; Lambert and Wilson, 1993a; Pitler and Alger, 1994).
An alternative hypothesis that is consistent with much of our data is that ethanol may actually enhance GABAergic synaptic transmission, in part, via an increase in GABA release. Such an effect might raise ambient GABA levels to a level sufficient to enhance presynaptic GABAB receptor function. In fact, GABAB receptors have a much lower functional EC50 than do GABAA receptors (Sodickson and Bean, 1996), and extracellular GABA levels have been estimated to be very near the threshold that we observed for activation of presynaptic GABAB receptors at proximal GABAergic synapses (∼0.5-1 μm) (Lerma et al., 1986; Tossman et al., 1986).
Direct evidence in support of this hypothesis stems from our finding that bath application of ethanol significantly increased the frequency of sIPSCs onto CA1 neurons. This finding is consistent with other recent studies demonstrating ethanol-mediated presynaptic enhancement of action potential-dependent(Carta et al., 2003) and -independent (Sanna et al., 2004) GABA release in the hippocampus and amygdala (Roberto et al., 2003; Nie et al., 2004) as well as action potential-dependent GABA release onto cerebellar granule cells (Carta et al., 2004). In addition, although ethanol significantly increased the amplitude and area of sIPSCs in this study, possibly reflecting postsynaptic actions of this drug, pretreatment with a GABAB receptor antagonist only facilitated the presynaptic ethanol enhancement of sIPSC frequency. An ethanol-mediated elevation of ambient GABA levels could also account for our observation that the GABAB receptor antagonist SCH 50911 potentiated GABAA IPSCs in the presence but not the absence of ethanol. This mechanism may also explain why the facilitatory effects of flunitrazepam and pentobarbital (allosteric potentiators of GABAA receptor activity that do not enhance GABA release) on proximal GABAA IPSCs are not subject to modulation by presynaptic GABAB receptor activity.
Additional studies are clearly needed to fully elucidate the complex mechanisms through which ethanol modulates GABAergic synaptic inhibition in the rat hippocampus and other brain regions. Nevertheless, our data clearly demonstrate that ethanol can enhance presynaptic GABAB receptor activity and that this interaction regulates the overall ethanol sensitivity of perisomatic hippocampal GABAergic synapses. These findings, coupled with the observation that ethanol can increase GABA release in the hippocampus and other brain regions, provide further evidence that ethanol modulation of GABAergic neurotransmission likely involves more than just a simple, allosteric interaction with the postsynaptic GABAA receptor complex.
This work was supported by National Institutes of Health Grants AA 013960 and AA 011997. We thank Drs. Mario Carta, Brian McCool, and Fernando Valenzuela for helpful comments on this manuscript.
Correspondence should be addressed to Dr. Jeff L. Weiner, Department of Physiology and Pharmacology, Wake Forest University School of Medicine, Medical Center Boulevard, Winston-Salem, NC 27157. E-mail:.
Copyright © 2004 Society for Neuroscience 0270-6474/04/2410679-08$15.00/0