Rhythmic Regulation of DNA Methylation Factors and Core-Clock Genes in Brain Structures Activated by Cocaine or Sucrose: Potential Role of Chromatin Remodeling

Abstract

The circadian system interacts with the mesocorticolimbic reward system to modulate reward and memory in a time-of-day dependent manner. The circadian discrimination of reward, however, remains difficult to address between natural reinforcers and drugs of abuse. Circadian rhythms control cocaine sensitization and conversely cocaine causes long-term alteration in circadian periodicity in part through the serotonergic neurotransmission. Since neural circuits activated by cocaine and natural reinforcers do not completely overlap, we compared the effect of cocaine with that of sucrose, a strong reinforcer in rodents, by using passive chronic administration. The expression of fifteen genes playing a major role in DNA methylation (Dnmts, Tets), circadian rhythms (Clock, Bmal1, Per1/2, Cry1/2, Rev-Erbβ, Dbp1), appetite, and satiety (Orexin, Npy) was analyzed in dopamine projection areas like the prefrontal cortex, the caudate putamen, and the hypothalamus interconnected with the reward system. The corresponding proteins of two genes (Orexin, Per2) were examined by IHC. For many factors controlling biological and cognitive functions, striking opposite responses were found between the two reinforcers, notably for genes controlling DNA methylation/demethylation processes and in global DNA methylation involved in chromatin remodeling. The data are consistent with a repression of critical core-clock genes by cocaine, suggesting that, consequently, both agents differentially modulate day/night cycles. Whether observed cocaine and sucrose-induced changes in DNA methylation in a time dependent manner are long lasting or contribute to the establishment of addiction requires further neuroepigenetic investigation. Understanding the mechanisms dissociating drugs of abuse from natural reinforcers remains a prerequisite for the design of selective therapeutic tools for compulsive behaviors.

Epigenetic Regulation of Circadian Clocks and Its Involvement in Drug Addiction

“…A global DNA methylation analysis showed an increase in 5-mC by cocaine, consistent with repression of critical core-clock genes by cocaine. The DNA methylation state of some core-clock genes (clock, cry2) was consistent with an increase in global DNA methylation in response to cocaine (submitted article In a recent study, we found that cocaine and sucrose differentially regulate genes involved in DNA methylation and circadian gene expression in certain rat brain structures (PFC, CPu) (submitted article [203]). A global DNA methylation analysis showed an increase in 5-mC by cocaine, consistent with repression of critical core-clock genes by cocaine.…”

“…A global DNA methylation analysis showed an increase in 5-mC by cocaine, consistent with repression of critical core-clock genes by cocaine. The DNA methylation state of some core-clock genes (clock, cry2) was consistent with an increase in global DNA methylation in response to cocaine (submitted article [203]). Yet, additional gene-specific DNA methylation experiments are required to ascertain a definite link between the circadian timing system and its contribution to addiction.…”

“…On the other hand, TET1 was downregulated following cocaine administration [199]. Consistent with cocaine-induced DNA methylation changes, Dnmt and Tet genes are modulated by cocaine in various conditions in brain structures [196,[200][201][202], and in a time-dependent manner (submitted article [203]). Despite, in genome-wide studies, very few modifications in global brain DNA methylation upon cocaine exposure were detected [199,[204][205][206][207][208].…”

“…Although the aspects described and the common signalling cascades involved in the two systems underline the existence of a possible connection, still more focused and correlated studies are required to provide solid evidence (Figure 1). In a recent study, we found that cocaine and sucrose differentially regulate genes involved in DNA methylation and circadian gene expression in certain rat brain structures (PFC, CPu) (submitted article [203]). A global DNA methylation analysis showed an increase in 5-mC by cocaine, consistent with repression of critical core-clock genes by cocaine.…”

“…On the other hand, the functional role of DNA methylation in gene expression remains an important and challenging issue to address. While many genes involved in functions such as circadian rhythms, learning and memory processes, arousal and appetite or satiety are modulated by drugs of abuse and natural reinforcers, some of them, to some extent, are also Dnmts and/or Tets targets [194][195][196]202,203,300,301]. It therefore appears reasonable to hypothesise that some are regulated by DNA methylation, whereas others can be independent of it, notably through one-carbon metabolism.…”