Elevated GABAergic neurotransmission prevents chronic intermittent ethanol induced hyperexcitability of intrinsic and extrinsic inputs to the ventral subiculum of female rats

Abstract

With the recent rise in the rate of alcohol use disorder (AUD) in women, the historical gap between men and women living with this condition is narrowing. While there are many commonalities in how men and women are impacted by AUD, an accumulating body of evidence is revealing sex-dependent adaptations that may require distinct therapeutic approaches. Preclinical rodent studies are beginning to shed light on sex differences in the effects of chronic alcohol exposure on synaptic activity in a number of brain regions. Prior studies from our laboratory revealed that, while withdrawal from chronic intermittent ethanol (CIE), a commonly used model of AUD, increased excitability in the ventral hippocampus (vHC) of male rats, this same treatment had the opposite effect in females. A follow-up study not only expanded on the synaptic mechanisms of these findings in male rats, but also established a CIE-dependent increase in the excitatory-inhibitory (E-I) balance of a glutamatergic projection from the basolateral amygdala to vHC (BLA-vHC). This pathway modulates anxiety-like behavior and could help explain the comorbid occurrence of anxiety disorders in individuals suffering from AUD. The present study sought to conduct a similar analysis of CIE effects on both synaptic mechanisms in the vHC and adaptations in the BLA-vHC pathway of female rats. Our findings indicate that CIE increases the strength of inhibitory neurotransmission in the vHC and that this sex-specific adaptation blocks, or at least delays, the increases in intrinsic vHC excitability and BLA-vHC synaptic transmission observed in males. Our findings establish the BLA-vHC pathway and the vHC as important circuitry to consider for future studies directed at identifying sex-dependent therapeutic approaches to AUD.

Keywords: Alcohol use disorder; Anxiety disorder; Chronic intermittent ethanol; Electrophysiology; GABAergic; Hyperexcitability.

Fig. 1

Poly-, but not monosynaptic, excitatory BLA inputs to the vSub are strengthened in response to CIE without impacting the E-I balance. A) Schematic illustration of recording configuration. Blue highlighted area signifies optical stimulation. B) Quantitative comparison of moEPSC amplitudes in Air and CIE rats. C) Representative moEPSC traces in the presence of TTX and 4 A P from an Air (top traces) and CIE (bottom traces) rat. In this and all other representative traces individual inputs are shown by thin gray lines while the average response amplitude is illustrated by a thicker overlaid black line. D-F) Quantitative comparison of oEPSC (D), oIPSC areas (E) and E/I area ratios (F). G) Representative traces of oEPSCs (upward deflecting traces) and oIPSCs (downward deflecting traces) obtained from Air (left) and CIE (right) rats. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Fig. 2

Presynaptic release probability of BLA input onto vSub neurons is increased in CIE rats. A) Quantitative graphical comparison of oEPSC PPrs at 50 ms ISI show no change in release probability between Air and CIE-treated rats B) Representative traces of oEPSC PPrs at 50 ms ISI. C and E) Quantitative graphical comparison of oEPSC PPrs at 100 (C) and 250 (E) ms ISI show an increase in release probability (decreased PPr) of CIE-treated rats. D and F) Representative traces of oEPSC PPrs at 100 (D) and 250 (F) ms ISI.

Fig. 3

Spontaneous inhibitory, but not excitatory, activity (sPSCs) is increased in CIE-treated rats. A) There is no CIE-dependent effect on sEPSC frequency (left graph) or amplitude (right graph). B) Representative sEPSC traces recorded from vSub neurons of Air (top two traces) and CIE (lower two traces) rats. C) Representative sIPSC traces recorded from vSub neurons of Air (top two traces) and CIE (lower two traces) rats. D) CIE elicits an increase in sIPSC frequency (left graph) but not amplitude (right graph). Blue line above upper traces indicate zoomed in section illustrated in lower traces. *p < 0.05 and errors are reported as ±SEM. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Fig. 4

Blocking GABA_A -mediated inhibition in the vSub shows a robust strengthening of BLA-driven inputs of vSub neurons from CIE-treated rats. A) Quantitative comparison of disinhibited (presence of PTX) BLA-driven inputs in Air and CIE-treated rats. B) Representative recordings of oEPSCs at −80 mV in the presence of PTX. **p < 0.01 and errors are reported as ±SEM.

Fig. 5

Blocking GABA_A -mediated inhibition in the vSub unmasks an increase in sEPSC activity. A) Representative recordings of oEPSCs at −80 mV in the presence of PTX. B) Quantitative comparison of disinhibited (presence of PTX) sEPSC frequency (top) and amplitude (bottom) in Air and CIE-treated rats. Blue line above upper traces indicate zoomed in section illustrated in lower traces. H ***p < 0.001 and errors are reported as ±SEM. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Fig. 6

AMPA/NMDA ratios are increased in vSub neurons of CIE rats. A) Quantitative comparison between Air and CIE rats show a strengthening of AMPA-mediated of BLA-driven inputs to vSub neurons in response to CIE. B) Representative recordings of oEPSCs at +40 mV in the presence of PTX before and after the application of the NMDA receptor antagonist AP-5. **p < 0.01 and errors are reported as ±SEM.