siRNAs are a class of dsRNAs, 21-23 nucleotides in length, which are able to silence their target genes through enzymatic cleavage of target mRNA. The sequence-specific gene-silencing by siRNA can be used as a new therapeutic approach for treatment of a variety of diseases that are incurable by conventional drugs. Many efforts have been made to overcome the problems related to delivery, stability, off-target gene silencing and immunostimulatory effects of siRNA. Different studies have carried out done to improve in vitro and in vivo delivery of naked or formulated siRNAs. In this review, different aspects of using siRNA as a new class of drugs will be discussed.

• " Memory retention • " glycyrrhizin • " glycyrrhetinic acid • " protein kinase A • " Morris water maze • " Glycyrrhiza glabra • " Leguminosae

We have previously shown that infusion of the PKAII inhibitor H-89 in the CA1 area of the hippocampus impaired spatial memory retention. There is some evidence suggesting the neuroprotective effects of chronic lithium administration including its ability to attenuate a deleterious effect of chronic stress on spatial memory in rats. In the present study, we investigated whether chronic administration of lithium can improve memory as well as influence the inhibitory effect of H-89 on spatial memory retention. Male albino rats were treated systemically with lithium (600 mg/l) for 4 weeks and then trained for 4 days in the Morris water maze. Testing the animals 48 h later showed a significant reduction in escape latency (p < 0.05) and travel distance (p < 0.05) compared to the controls. In separate experiments, the rats were similarly treated with lithium for 4 weeks, followed by similar training for 4 days and then immediately infused bilaterally with vehicle or 5 µmol/l H-89 into the CA1 region of the hippocampus. Animals were then tested 48 h after H-89 infusion in order to assess their spatial memory retention. The lithium treatment caused a significant reduction in escape latency (p < 0.001) and travel distance (p < 0.001) compared to H-89-treated animals. The data suggest that lithium treatment for 4 weeks improved spatial memory retention and that lithium pretreatment prevented or reversed the H-89-induced spatial memory deficits.

Curcumin, the main part of the turmeric, could prevent the structural changes induced in the deep cerebellar nuclei by sodium metabisulfite, a preservative agent, in rats.