Alteration Of Synaptic Plasticity In Rat Dorsal Striatum Induced By Chronic Ethanol Intake And Withdrawal Via ERK Pathway

Addiction is defined as compulsive drug use despite negative consequences. The goals of the addicted person become narrowed to obtaining, using, and recovering from drugs, despite failure in life roles, medical illness, risk of incarceration, and other problems. Among the addicted drugs, alcohol/ethanol is the most widely used abused drug in human history. Compared to the effects of other addictive substances, such as cocaine and nicotine, the effects of ethanol are more diffuse, including diverse documented effects on various ligand-gated and voltage-gated channels. Consequently, the mechanisms underlying ethanol's actions on the central nervous system are difficult to pin down, and a greater understanding of these mechanisms might provide a foundation for new treatments for alcoholism.An important characteristic of addiction is its stubborn persistence. Although some individuals can stop compulsive use of tobacco, alcohol, or illegal drugs on their own, for a large number of individuals rendered vulnerable by both genetic and nongenetic factors, addiction proves to be a recalcitrant, chronic, and relapsing condition. The central problem in the treatment of addiction is that even after prolonged drug-free periods, well after the last withdrawal symptom has receded, the risk of relapse, often precipitated by drug-associated cues, remains very high.The specificity of drug cues and their relationship to specific behavioral sequences suggest that at least some of the mechanisms underlying addiction must be associative and synapse specific. Thus, addiction is increasingly viewed as a pathological process of learning, involving cell signaling and synaptic mechanisms similar to those implicated in neural models of learning and memory. Synaptic plasticity is required for neuroadaptations that result from a variety of environmental stimuli. The best-characterized candidate mechanisms for changing synaptic strength that are both associative and synapse specific are long-term potentiation and long-term depression. These mechanisms have been hypothesized to play critical roles in many forms of experience-dependent plasticity, in cluding various forms of learning and memory. Therefore, it is attractive to hypothesize that drug abuse causes long-term changes in behavior by altering synaptic function and plasticity in relevant brain circuits.The involvement of the ventral striatum, or nucleus accumbens (NAc), in mediating drug reward and reinforcement is well established. Recently, it has also been proposed that the dorsal striatum (caudate nucleus and putamen) is likely to be involved in advanced stages of addiction when drug use progresses toward a compulsive, habitual pathology. Previous work from our lab has shown that synaptic plasticity in the rat dorsal striatum was altered by chronic ethanol exposure and withdrawal. However, the relevant underlying molecular mechanism remains unclear.Intracellular signaling mechanisms that produce synaptic plasticity are attractive candidate mechanisms for addiction because they can convert drug-induced signals into long-term alterations in neural function and ultimately into the remodeling of neuronal circuits. It has been reported that extracellular signal-regulated kinase (ERK1/2. also known as p44 and p42 MAPK) signaling plays an important role in the induction of long-term potentiation (LTP). For example, stimuli inducing LTP in area CA1 of the hippocampus potently activate ERK, whereas pharmacological inhibition of MEK, an upstream activator of ERK, inhibits LTP. The participation of ERK in synaptic plasticity has also been found in other brain regions (e.g., LTD in the cerebellum and LTP in the visual cortex). Furthermore, several studies have suggested that the ERK signaling pathway is one of the targets of ethanol. For example, acute application of ethanol has been shown to attenuate MAPK activation in cultured cortical neurons and in mouse cerebral cortex in vivo. A reduction of MAPK activation by chronic intermittent ethanol treatment was observed in the amygdala, the cerebellum, the striatum and the hippocampus.Based on these results, we hypothesized that changes in ERK activation by ethanol may contribute to the modulation of corticostriatal plasticity following chronic ethanol intake (CEI) treatment. In the present study, we first investigated whether corticostriatal LTD, as indicated in other brain regions, was ERK dependent. We then compared the effects of CEI and withdrawal treatment on ERK activation and corticostriatal LTD induction. The results showed that the ERK signaling pathway was involved in both striatal LTD induction and the impairment of synaptic plasticity produced by CEI/withdrawal treatment.AIM:To establish method to induce LTD in DLS. To determine the relationship between striatal LTD and MAPK signal pathway.METHODS:Using extracellular recording technique to record the population spikes (PS) evoked by high-frequency stimulation (HFS), which are four bursts consisting of 100 pulses at 100 Hz delivered every 20 s, in DLS.RESULTS:Induction of LTD in DLS was blocked by 30-min pre-perfusion of 20μM U0126, an ERK activation inhibitor, but not affected by 30-min pre-perfusion of 20μM SP600125, a JNK activation inhibitor, and 10μM SB203580, a p38 activation inhibitor.CONCLUSION:LTD can be steadily induced by HFS in DLS. Induction of striatal LTD involved in activation of ERK/MAPK, not activation of JNK/MAPK and p38/MAPK. Besides, activation of ERK/MAPK is not neccessory for the expression of striatal LTD.AIM:According to the literature. ERK is one of the major intracellular targets of ethanol. So this part of the experiment is to elucidate the alteration of ERK phosphorylation in DLS upon chronic ethanol intake and withdrawal.METHODS:Male Sprague-Dawley rats weighing 150-170 g were housed in cages on a 12-h dark/light cycle (light on at 7:00 am) with ad libitum access to food and water. Rats were divided into seven groups (n=5 in each group):CTL (fed with tap water for 24 days). CEI10, CEI20, CEI30 (fed with 6%(v/v) ethanol for 10,20. 30 d, respectively). WD1, WD3, and WD7 (withdrawal for 1.3, and 7 d after 30 d of ethanol intake). Ethanol was administered in drinking water at a concentration of 6% (v/v). Both pERK and ERK levels in the DLS were assessed by Western-blot technique with specific antibodies.RESULTS:Western blot results showed that in DLS, chronic ethanol intake induced a decrease in ERK phosphorylation in the CEI10 (0.54±0.05. P<0.05), CEI20 (0.43±0.08, P<0.05) and CEI30 (0.76±0.07, P<0.05) groups, respectively, in comparison with the CTL group. In contrast, phospho-ERK levels increased in the WD1 group (1.32±0.08; P<0.05 vs CTL) and showed no statistically significant changes in either the WD3 group (0.96±0.10, n=5) or the WD7 group (0.88±0.12, n=5).CONCLUSION:These results indicate that in the DLS, chronic ethanol intake reduces ERK phosphorylation. whereas ethanol withdrawal enhances ERK phosphorylation transiently.AIM:To evaluate the probability that chronic ethanol intake and withdrawal alter LTD induction through ERK pathway. To test whether ERK could be a potential therapeutic target for treatment of alcohol abuse.METHODS:CEI10 (lower phospho-ERK) and WD1 (higher phospho-ERK) groups were selected to determine whether changes in corticostriatal LTD induction would coincide with ERK phosphorylation following chronic ethanol intake or withdrawal. U0126 or vehicle (1% DMSO) was delivered through i.c.v. injection for 3 consecutive days prior to decapitation to manipulate ERK activity to verify the relationship between corticostriatal synaptic plasticity and ERK phosphorylation. The pairfed rats, as the control group, received sham surgery. Both pERK and ERK levels in the DLS were assessed by Western-blot technique with specific antibodies. Using extracellular recording technique to record the PS evoked by HFS.RESULTS:In the CEI10+DMSO groups, the PS amplitudes post-HFS increased to 81.2%±4.4% of baseline (n=11, P<0.05 vs control), whereas phospho-ERK levels decreased to 0.58±0.11 (n=5, P<0.05 vs control). In the CEI10+U0126 group, the PS amplitudes post-HFS increased to 101.7%±7.8% of baseline (n=10, P<0.01 vs control) along with a marked decrease in phospho-ERK levels (0.31±0.08, n=5, P<0.01 vs control). In the WD1+DMSO group, phospho-ERK levels increased to 1.53±0.1 of control (n=5. P<0.05 vs control). when a weaker stimulation protocol was utilized, a greater depression of the PS amplitudes post-HFS in WD1+DMSO groups (34.5%±4.8% of baseline vs 58.5%±5.6% of baseline in CTL group, P<0.05) was observed.CONCLUSION:These results strongly indicated that chronic ethanol intake and withdrawal altered corticostriatal LTD induction via its effect on ERK phosphorylation in DLS. Preliminary studies also show the blockage of ERK activation reduced the amount of alcohol intake and attenuated the withdrawal syndrome.