Actin Dynamics in Development of Behavioral Sensitization After Withdrawal from Long-Term Ethanol Administration to Mice

Targeting the cytoskeleton as a therapeutic approach to substance use disorders

Substance use disorders (SUD) are chronic relapsing disorders governed by continually shifting cycles of positive drug reward experiences and drug withdrawal-induced negative experiences. A large body of research points to plasticity within systems regulating emotional, motivational, and cognitive processes as drivers of continued compulsive pursuit and consumption of substances despite negative consequences. This plasticity is observed at all levels of analysis from molecules to networks, providing multiple avenues for intervention in SUD. The cytoskeleton and its regulatory proteins within neurons and glia are fundamental to the structural and functional integrity of brain processes and are potentially the major drivers of the morphological and behavioral plasticity associated with substance use. In this review, we discuss preclinical studies that provide support for targeting the brain cytoskeleton as a therapeutic approach to SUD. We focus on the interplay between actin cytoskeleton dynamics and exposure to cocaine, methamphetamine, alcohol, opioids, and nicotine and highlight preclinical studies pointing to a wide range of potential therapeutic targets, such as nonmuscle myosin II, Rac1, cofilin, prosapip 1, and drebrin. These studies broaden our understanding of substance-induced plasticity driving behaviors associated with SUD and provide new research directions for the development of SUD therapeutics.

Proteomic Analysis of Brain Regions Reveals Brain Regional Differences and the Involvement of Multiple Keratins in Chronic Alcohol Neurotoxicity

AIMS

Alcohol abuse has attracted public attention and chronic alcohol exposure can result in irreversible structural changes in the brain. The molecular mechanisms underlying alcohol neurotoxicity are complex, mandating comprehensive mining of spatial protein expression profile.

METHODS

In this study, mice models of chronic alcohol intoxication were established after 95% alcohol vapor administration for 30 consecutive days. On Day 30, striatum (the dorsal and ventral striatum) and hippocampus, the two major brain regions responsible for learning and memorizing while being sensitive to alcohol toxicity, were collected. After that, isobaric tags for relative and absolute quantitation -based quantitative proteomic analysis were carried out for further exploration of the novel mechanisms underlying alcohol neurotoxicity.

RESULTS

Proteomic results showed that in the striatum, 29 proteins were significantly up-regulated and 17 proteins were significantly down-regulated. In the hippocampus, 72 proteins were significantly up-regulated, while 2 proteins were significantly down-regulated. Analysis of the overlay proteins revealed that a total of 102 proteins were consistently altered (P < 0.05) in both hippocampus and striatum regions, including multiple keratins such as Krt6a, Krt17 and Krt5. Ingenuity pathway analysis revealed that previously reported diseases/biofunctions such as dermatological diseases and developmental disorders were enriched in those proteins. Interestingly, the glucocorticoid receptor (GR) signaling was among the top enriched pathways in both brain regions, while multiple keratins from the GR signaling such as Krt1 and Krt17 exhibited significantly opposite expression patterns in the two brain nuclei. Moreover, there are several other involved pathways significantly differed between the hippocampus and striatum.

CONCLUSIONS

Our data revealed brain regional differences upon alcohol consumption and indicated the critical involvement of keratins from GR signaling in alcohol neurotoxicity. The differences in proteomic results between the striatum and hippocampus suggested a necessity of taking into consideration brain regional differences and intertwined signaling pathways rather than merely focusing on single nuclei or molecule during the study of drug-induced neurotoxicity in the future.

Changes in dendritic spine density in the nucleus accumbens do not underlie ethanol sensitization

Behavioral sensitization to various drugs of abuse has been shown to change dendritic spine density and/or morphology of nucleus accumbens (NAc) medium spiny neurons, an effect seen across drug classes. However, is it not known whether behavioral sensitization to ethanol (EtOH) is also associated with structural changes in this region. Here we compared dendritic spine density and morphology between mice showing High vs. Low levels of EtOH sensitization and found that high levels of EtOH sensitization were not associated with changes in dendritic spine density or spine type. Unexpectedly, however, a significant increase in the density of stubby-type spines was seen in mice that were resistant to sensitization. Since the presence of this spine type has been associated with long-term depression and cognitive/learning deficits this may explain why these mice fail to sensitize and why they show poor performance in conditioning tasks, as previously shown. A possible causal role for structural plasticity in behavioral sensitization to various drugs has been debated. In the case of EtOH sensitization, our results suggest that drug-induced changes in structural plasticity in the accumbens neurons may not be the cause of sensitized behavior.

Effect of chronic ethanol treatment on μ-opioid receptor function, interacting proteins and morphine-induced place preference

RATIONALE

Both the acute and chronic consumption of ethanol have been reported to modify several molecular events in the central nervous system, and the endogenous μ-opioid receptor system is involved in the reinforcing/rewarding effects of ethanol.

OBJECTIVES

The present study was designed to clarify the effects of chronic ethanol treatment on cellular processes involving μ-opioid receptor and the development of morphine-induced rewarding effects.

METHODS

Male C57BL/6J mice were continuously treated with a liquid diet containing 3.0 w/v ethanol. The direct involvement of μ-opioid receptor functions in the activation of G-proteins and changes in protein levels in the lower midbrain of mice after chronic treatment with ethanol were investigated by a [(35)S] GTPγS binding assay and Western blotting, respectively. The rewarding effects of morphine (5 mg/kg) under treatment with ethanol were measured by the conditioned place preference paradigm.

RESULTS

The function of μ-opioid receptor was increased by treatment with ethanol in the lower midbrain using [(35)S] GTPγS binding assay. Furthermore, the GRK2 protein level was significantly increased by treatment with ethanol. Chronic treatment with ethanol enhanced the rewarding effects of morphine. On the other hand, this enhancement of the rewarding effects of morphine by ethanol treatment was significantly inhibited by the GRK2 inhibitor β-adrenergic receptor kinase 1 inhibitor.

CONCLUSIONS

The present study demonstrated that chronic treatment with ethanol enhanced the rewarding effects of morphine by up-regulating functional changes in μ-opioid receptor, mediated by GRK2.

Secretion-related structures of hypothalamo-hypophysial terminals in the rat posterior pituitary

Hypothalamic terminals were investigated in the rat posterior pituitary (PP). Injection of wheat germ agglutinin conjugated horseradish peroxidase (WGA-HRP) and co-injection of WGA-HRP with Rab3A-siRNA were made into the hypothalamus, respectively. Additional injection of WGA-HRP was made into the hypothalamus in the animals exposed to ethanol. These injections resulted in heavy labeling of fibers exclusively confined to the PP. Ultrastructural observations showed terminals, fibers, pituicytes, capillaries and vascular spaces in the PP. Although the majority of terminals were observed to contain large dense core vesicles (LDCVs) and HRP-reaction products (HRP-RPs), exocytosis of LDCVs in close proximity to cell membrane was not found. Interestingly, a few terminals showed alteration of cell membrane called "apocrine-like structure" containing LDCV and RP. The narrow neck portion of the structure gave the appearance that it may have been in some stage of separating from terminals. Other remarkable feature was that terminals occasionally reveal the structure of "leakage" of RP discharged into vascular spaces crossing cell membrane. Such hormone-releasing mechanism might be involved in one of "diacrine-like secretion". In the present study secretion-related structures of hypothalamic terminals in the PP are quite different from normal vesicular exocytosis.

Morphological evidence of an altered process of synaptic transcytosis in adult rats exposed to ethanol

AIMS

The effects of ethanol exposure on synaptic structure were investigated in the nucleus of solitary tract (NST) in rats, using the horse-radish peroxidase (HRP) method.

METHODS

Eight-week-old experimental rats were allowed free access to a liquid diet containing ethanol for 3 weeks, while controls were given an isocaloric diet. Some of the control and experimental animals were given an injection of wheat germ agglutinin conjugated with HRP (WGA-HRP) into the vagus nerve toward the end of the treatment period. After the treatment, the neuropil region of the NST was examined under an electron microscope.

RESULTS

We observed that a few terminals were characterized by deep indentation of axodendritic membranes into the post-synaptic neurons. This appeared to be similar to that commonly seen in exocrine glands. Interestingly, the indented portion often contained various sizes of vacuoles and flattened cisternae. HRP-reaction product (RP) transported to terminals was recognized easily as an electron-dense lysosomal substance when lead citrate staining was omitted. Terminals containing HRP-RP also revealed quite a similar structure with indentation of axodendritic membranes as described earlier. The results are considered to confirm that terminals forming 'apocrine-like structures' observed in the ethanol-fed animals with no injection of WGA-HRP originate from afferent fibers of the vagus nerve.

CONCLUSION

The present study suggests the possibility that the alteration of the synaptic structure induced by ethanol exposure can lead to the neuronal transcytosis of materials including proteins which is different from the normal vesicular exocytosis involved in chemical synaptic transmission.