Ethanol selectively attenuates NMDAR-mediated synaptic transmission in the prefrontal cortex

Abstract

Background: Brain imaging studies have revealed abnormal function in the prefrontal cortex (PFC) of alcoholics that may contribute to the impulsive behavior and lack of control over drinking that characterizes this disorder. Understanding how ethanol affects the physiology of PFC neurons may help explain this loss of control and lead to better treatments for alcohol addiction. In a previous study from this laboratory, we showed that ethanol inhibits complex patterns of persistent activity (known as "up-states") in medial PFC (mPFC) neurons in a reversible and concentration-dependent manner.

Methods: In the current study, whole-cell patch clamp recordings were used to directly examine the effects of ethanol on the glutamatergic and GABAergic components that underlie persistent activity.

Results: In deep-layer mPFC pyramidal neurons, ethanol reversibly attenuated electrically evoked N-methyl-D-aspartate-type glutamate receptor (NMDAR)-mediated EPSCs. Significant inhibition was observed at concentrations as low as 22 mM, equivalent to a blood ethanol concentration (0.1%) typically associated with legal limits for intoxication. In contrast to NMDA responses, neither evoked nor spontaneous EPSCs mediated by alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid-type glutamate receptor were affected by ethanol at concentrations as high as 88 mM, a concentration that can be fatal to non-tolerant individuals. At similar concentrations, ethanol also had little effect on spontaneous or evoked IPSCs mediated by a-type gamma-aminobutyric acid receptor. Finally, mPFC neurons showed little evidence of GABAR-mediated tonic current and this was unaffected by ethanol.

Conclusions: Together, these results suggest that NMDAR-mediated processes in the mPFC may be particularly susceptible to disruption following the acute ingestion of ethanol.

Fig. 1

Acute ethanol attenuates evoked NMDAR-mediated EPSCs (eEPSCs) in mPFC pyramidal neurons. (A) Representative averaged traces of NMDA eEPSCs before (black) and during (gray) application of acute ethanol (44 mM). (B) Normalized timecourse of the NMDA eEPSC area in response to sham solution exchange (open circles) or 44 mM ethanol application (black squares). Black bar indicates period of solution exchange. (C) Normalized response of NMDA eEPSC area during ethanol exposure (top) and following 10 to 15 minutes of ethanol washout (bottom). Values are expressed as averages ± SEM. *p < 0.001 compared with baseline values.

Fig. 2

Acute ethanol has no effect on evoked AMPAR-mediated EPSCs (eEPSCs) in mPFC pyramidal neurons. (A) Representative averaged traces of AMPA eEPSCs before (black) and during (gray) application of acute ethanol (44 mM). (B) Normalized timecourse of AMP eEPSC peak in response to sham solution exchange (open circles) or 44 mM ethanol application (black squares). Black bar indicates period of solution exchange. (C) Normalized response of AMPA eEPSC peak during ethanol exposure (top) and following 10 to 15 minutes of ethanol washout (bottom). Values are expressed as averages ± SEM.

Fig. 3

Acute ethanol has no effect on spontaneous AMPAR-mediated EPSCs (sEPSCs) in mPFC pyramidal neurons. (A) Representative traces containing sEPSCs from PFC pyramidal neurons in the presence of picrotoxin (100 μM) and DL-APV (100 μM) before (left) and after (right) acute exposure to ethanol (66 mM). Scale bar: 25 pA, 5 ms. (B) Averaged sEPSCs before (black) and after (gray) acute exposure to ethanol (66 mM). (C,D) Average sEPSC amplitude (C) and frequency (D) normalized to baseline during (dark gray) and 15 to 20 minutes following (light gray) sham solution exchange (n=7) or acute exposure to 66 mM ethanol (n=5). (E,F) sEPSC rise time (E) and decay time (F) before (white), during (dark gray), and 15 to 20 minutes following (light gray) sham solution exchange or acute exposure to 66 mM ethanol. Values are expressed as averages ± SEM.

Fig. 4

Acute ethanol has no effect on evoked GABAR-mediated IPSCs (eIPSCs) in mPFC pyramidal neurons. (A) Representative averaged traces of GABA eIPSCs before (black) and during (gray) application of acute ethanol (44 mM). (B) Normalized timecourse of the first GABA eIPSC peak in response to sham solution exchange (open circles) or 44 mM ethanol application (black squares). Black bar indicates period of solution exchange. (C) Normalized response of GABA eIPSC peak during ethanol exposure (top) and 10 to 15 minutes of ethanol washout (bottom). (D) Average paired-pulse ratio before (white bars), during (dark gray bars) and 10 to 15 minutes following (light gray bars) exposure to various doses of ethanol. Values are expressed as averages ± SEM.

Fig. 5

Acute ethanol has no effect on spontaneous GABAR-mediated IPSCs (sIPSCs) in mPFC pyramidal neurons. (A) Representative traces containing sIPSCs from PFC pyramidal neurons in the presence of NBQX (10 μM) and DL-APV (100 μM) before (left) and after (right) acute exposure to ethanol (88 mM). Scale bar: 25 pA, 5 ms. (B) Averaged sEPSCs before (black) and after (gray) acute exposure to ethanol (88 mM). (C,D) Average sIPSC amplitude (C) and frequency (D) normalized to baseline during (dark gray) and 15 to 20 minutes following (light gray) sham solution exchange (n=8) or exposure to 66 mM ethanol (n = 7), 88 mM ethanol (n = 8), or 50 μM pentobarbital (n=7). (E,F) sIPSC rise time (E) and decay time (F) before (white), during (dark gray), and 15 to 20 minutes following (light gray) sham solution exchange or exposure to 66 mM ethanol, 88 mM ethanol, or 50 μM pentobarbital. Values are expressed as averages ± SEM. *p < 0.001 compared with baseline values.

Fig. 6

Lack of tonic GABAergic current in mPFC neurons or modulation by ethanol. Graphs show holding currents (I_hold) of voltage-clamped PFC neurons before (y-axis) and during (x-axis) application of either bicuculline (30 μM) (A) or ethanol (B). Lines show best fit through the plotted points under each condition. All lines had slope values that were not significantly different from one indicating no effect on holding currents.