Alteration of synaptic plasticity in rat dorsal striatum induced by chronic ethanol intake and withdrawal via ERK pathway

Abstract

Aim: The dorsal striatum has been proposed to contribute to the formation of drug-seeking behaviors, leading to excessive and compulsive drug usage, such as addiction. The current study aimed to investigate the involvement of extracellular signal-regulated kinase (ERK) pathway in the modification of striatal synaptic plasticity.

Methods: Ethanol was administered to rats in drinking water at concentration of 6% (v/v) for 30 days. Rats were sacrificed on day 10, 20, or 30 during ethanol intake or on withdrawal day 1, 3, or 7 following 30-d ethanol intake. The striata were removed either for electrophysiological recording or for protein immuno-blot analysis. Extracellular recording technique was used to record population spikes (PS) induced by high-frequency stimulation (HFS) in the dorsolateral striatum (DLS).

Results: Corticostriatal long-term depression (LTD) was determined to be dependent upon ERK signaling. Chronic ethanol intake (CEI) attenuated ERK phosphorylation and LTD induction, whereas withdrawal for one day (W1D) potentiated ERK phosphorylation and LTD induction. These results showed that the impact of chronic ethanol intake and withdrawal on corticostriatal synaptic plasticity was associated with ethanol's effect on ERK phosphorylation. In particular, pharmacological inhibition of ERK hyper-phosphorylation by U0126 prevented LTD induction in the DLS and attenuated ethanol withdrawal syndrome as well.

Conclusion: In rat DLS, chronic ethanol intake and withdrawal altered LTD induction via ERK signaling pathway. Ethanol withdrawal syndrome is mediated, at least partly, by ERK hyper-phosphorylation in the DLS.

Figure 1

Effects of MEK inhibitor U0126 on striatal LTD induction and on ERK phosphorylation. (A) Thirty-minute bath application of 20 μmol/L U0126, but not 0.1% DMSO, inhibited ERK phosphorylation (n=4 in each group) in rat DLS. (B) Mean PS (population spike) amplitudes, expressed as a percentage of the mean baseline value, in 20 μmol/L U0126-treated slices (n=7), in 0.1% DMSO-treated slices (n=8), and in control (CTL) slices (n=8), respectively. U0126 or DMSO was present in the bath for 30 min before high-frequency stimulation (HFS). HFS was given at time 0 min. Upper panel: Sample traces of PS (5 min pre-HFS and 60 min post-HFS, respectively). Calibration: 2 ms, 0.2 mV. ^bP<0.05 vs control.

Figure 2

Chronic ethanol intake and withdrawal treatment changed phospho-ERK levels in rat DLS (n=5 in each group). The upper panel is a representative Western blot. CTL, control; CEI10, CEI20, and CEI30 represent chronic ethanol intake for 10, 20, and 30 days, respectively. WD1, WD3, and WD7 represent chronic ethanol withdrawal for 1 day, 3 days, and 7 days, respectively, after 30 days of ethanol intake. ^bP<0.05 vs control.

Figure 3

Ten days of chronic ethanol intake (CEI) decreased phospho-ERK levels and attenuated striatal LTD induction. MEK inhibitor U0126 enhanced the effects of CEI10 on both phospho-ERK levels and striatal LTD induction. (A) Phospho-ERK levels decreased in both CEI10 and CEI10+U0126 groups (n=5 in each group). (B) Induction of striatal LTD was attenuated in both CEI10 (n=11) and CEI10+U0126 groups (n=10; n=13 in CTL group). In the upper panel, the percentage change of PS amplitude is calculated by normalizing the mean response measured 50–60 min post-HFS to the mean response measured over the 10-min period immediately prior to HFS (baseline). In the lower panel, HFS was given at time 0 min. CTL, control; CEI10, 10 days of chronic ethanol intake. ^bP<0.05, ^cP< 0.01 vs control.

Figure 4

Effects of one-day withdrawal (WD1) and MEK inhibitor U0126 on both phospho-ERK levels and striatal LTD induction. (A) Phospho-ERK levels increased in WD1+DMSO group and decreased in WD1+U0126 group (n=5 in each group). (B) Striatal LTD induction was potentiated by WD1+DMSO treatment (n=10) and was blocked by WD1+U0126 treatment (n=11; n=14 in CTL group). In the lower panel, HFS was given at time 0 min. CTL, control; WD1, 1-day withdrawal after 30 days of ethanol intake. ^bP<0.05, ^cP<0.01 vs control; ^fP<0.01 vs WD1+DMSO.

Figure 5

Ethanol withdrawal syndrome was attenuated by infusion of U0126 (n=5 in each group). U0126 or vehicle (1% DMSO) was delivered through icv injection for 3 consecutive days prior to evaluation. ^bP<0.05 vs WD1+vehicle.