Chronic Intermittent Ethanol Exposure Selectively Increases Synaptic Excitability in the Ventral Domain of the Rat Hippocampus

Abstract

Many studies have implicated hippocampal dysregulation in the pathophysiology of alcohol use disorder (AUD). However, over the past twenty years, a growing body of evidence has revealed distinct functional roles of the dorsal (dHC) and ventral (vHC) hippocampal subregions, with the dHC being primarily involved in spatial learning and memory and the vHC regulating anxiety- and depressive-like behaviors. Notably, to our knowledge, no rodent studies have examined the effects of chronic ethanol exposure on synaptic transmission along the dorsal/ventral axis. To that end, we examined the effects of the chronic intermittent ethanol vapor exposure (CIE) model of AUD on dHC and vHC synaptic excitability. Adult male Long-Evans rats were exposed to CIE or AIR for 10 days (12 h/day; targeting blood ethanol levels of 175-225 mg%) and recordings were made 24 h into withdrawal. As expected, this protocol increased anxiety-like behaviors on the elevated plus-maze and successive alleys test. Extracellular recordings revealed marked CIE-associated increases in synaptic excitation in the CA1 region that were exclusively restricted to the ventral domain of the hippocampus. Western blot analysis of synaptoneurosomal fractions revealed that the expression of two proteins that regulate synaptic strength, GluA2 and SK2, were dysregulated in the vHC, but not the dHC, following CIE. Together, these findings suggest that the ventral CA1 region may be particularly sensitive to the maladaptive effects of chronic ethanol exposure and provide new insight into some of the neural substrates that may contribute to the negative affective state that develops during withdrawal.

Keywords: SK Calcium-activated potassium channels; anxiety; chronic intermittent ethanol; dorsal hippocampus; ventral hippocampus; withdrawal.

Figure 1

Chronic intermittent ethanol increases anxiety-like behavior in the elevated plus-maze. AIR exposed (N =12) rats exhibited less anxiety-like behavior on the elevated plus-maze than CIE (N =12) counterparts, as evidenced by more time spent in the open arms (A) and more open arm entries (B). AIR rats also exhibited a greater number of closed arm entries (C). Additionally, AIR and CIE rats do not exhibit differences in locomotor activity in the open field test (D; **, p<0.01, ***. p<0.001).

Figure 2

Chronic intermittent ethanol increases anxiety-like behavior in the successive alleys test. AIR exposed (N= 19) rats exhibited less anxiety-like behavior indicated by spending less time in zone 1 (A) relative to their CIE (N=18) counterparts. CIE rats spent significantly less time head dipping in zone 2 (B) relative to AIR rats. Additionally, AIR and CIE rats did not differ in the distance traveled (C) or velocity (D; *, p<0.05, **, p<0.01).

Figure 3

Illustrations of morphological differences in coronal slices from ventral and dorsal hippocampus. Representative images adapted from Paxinos and Watson (2005) depicting the ventral (A; Bregma −5.52mm) and dorsal hippocampus (B; Bregma −3.12mm) for extracellular field recordings.

Figure 4

Withdrawal from chronic intermittent ethanol increases synaptic excitability at vHC Schaffer collateral-CA1 synapses. Plots of stimulation intensity vs fiber volley amplitude (A) and fEPSP slope (B) in vHC slices from AIR and CIE rats. C) Plot of the relationship between fiber volley amplitude and fEPSP slope in vHC slices from AIR and CIE rats. Representative fEPSPs evoked at 200 μA from AIR and CIE slices. *, p<0.05, ***, p<0.001, AIR =6 slices, 5 rats; CIE n = 8 slices, 7 rats).

Figure 5

Withdrawal from chronic intermittent ethanol decreases synaptic excitability at dHC Schaffer collateral-CA1 synapses. Plots of stimulation intensity vs fiber volley amplitude (A) and fEPSP slope (B) in dHC slices from AIR and CIE rats. C) Plot of the relationship between fiber volley amplitude and fEPSP in dHC slices from AIR and CIE rats. Representative fEPSPs evoked at 200 μA from AIR and CIE slices (*, p<0.05, AIR n = 10 slices, 9 rats; CIE n = 9 slices, 8 rats).

Figure 6

Chronic intermittent ethanol has divergent effects on GluA2 and SK2 protein expression in the ventral and dorsal hippocampus. Representative Western blots of synaptoneurosomes isolated from AIR and CIE vHC (A) and dHC (B) tissue. Group data, normalized to actin, illustrating increased GluA2 protein expression in CIE vHC (C) and not in dHC (D). Group data, normalized to actin, illustrating decreased SK2 protein expression in CIE vHC (E) and not in dHC (F; *, p<0.05, AIR = 3 rats, CIE = 3 rats).

Figure 7

Chronic intermittent ethanol exposure does not alter GluN1 and GluN2B protein expression in the ventral or dorsal hippocampus. Representative Western blots of synaptoneurosomes isolated from AIR and CIE vHC (A) and dHC (B) tissue. Group data, normalized to actin, illustrating no changes in GluN1 protein expression in vHC (C) or dHC (D) of AIR and CIE exposed rats. Group data, normalized to actin, illustrating no changes in GluN2B protein expression in vHC (E) or dHC (F) of AIR and CIE exposed rats (AIR = 4 rats, CIE = 4 rats).