Lentiviral and adeno-associated vectors are used as gene transfer systems to produce small interfering RNAs (siRNAs) to diminish or silence the expression of specific genes (Ryther et al. 2005), and they can also be used to express a cDNA coding for proteins or transcription factors like clock genes, lacking in the affected cells (Gene therapy clinical trial worldwide 2014). An important question is whether these methods can be employed in the CNS, without causing secondary damages, due to the vector, due to the method of administration, or by other effects on brain circuitry and functioning. To the best of our knowledge, there are few instances in the field of chronobiology that have used siRNA to silence genes; and in most of these experiments, liposomes were employed as the delivery system. For example, the expression of Bmal1 was downregulated using a siRNA, which reduced the levels of the transcription factor NF-YA (nuclear factor Y A), one of the activators of Bmal1 transcription (Xiao et al. 2013); there is another report using a siRNA to downregulate Rev-erb alpha in pancreatic cells (Vieira et al. 2013), and in luteinizing granulosa cells, Bmal1 was knocked down with an interference RNA (Chen et al. 2013c). Gene delivery is a crucial aspect that needs to be improved for gene therapy to be employed to regulate clock gene expression and activity.

Recent studies have demonstrated that microRNAs (miRNAs) which have key roles in regulating both synaptic plasticity and brain development can repress the expression of genes implicated in cellular mechanisms that regulate mood, cognition, and circadian functions (Hunsberger et al. 2009). For example, in bipolar disorders, the presence of miRNA-134 in circulating blood can be used as a peripheral marker that reflects acute manic episodes and that can be modulated by pharmacological mood stabilizers (Rong et al. 2011). Specifically, Clock and Bmal1 activate the transcription of miR-219, which attenuates the intracellular calcium response to NMDA activation in the SCN (Cheng et al. 2007), and Bmal1 is regulated by miR-142-3p (Shende et al. 2013). Also, miR-24, miR-29a, and miR-30a affect the circadian clock through the regulation of Per1 and Per2 in two ways, first in mRNA stability and second in protein translation (Chen et al. 2013a). In contrast, miR-132 increases calcium influx after depolarization (Cheng et al. 2007). These findings implicate that targeting clock gene-associated miRNAs may be a novel way to regulate clock gene expression.