[Specific regulation of gene expression in brain by antisense oligodeoxynucleotides]

The synthetic oligodeoxynucleotide (ODN) complementary to the normal (sense) mRNA, so-called antisense ODN, has been used to regulate the gene expression in the brain. It has been reported to interfere with transcription, pre-mRNA splicing and translation through at least two mechanisms; i.e., its competition with transcription and protein synthesis machinery or induction of mRNA cleavage. The unmodified antisense ODN was shown to be the RNase activator when it hybridizes with at least four contiguous bases of mRNA. In contrast, the phosphorothioate ODN (S-ODN) is reported to be a less effective activator of RNas II and more resistant to the nuclease attack than unmodified ODN. Because of these properties, S-ODNs are preferentially employed in antisense ODN experiments. When the DNA sequence of the target gene is determined, we can design an antisense ODN that selectively hybridizes with the bases of a nucleic acid (DNA or RNA) related to the target gene. The initial sites of specific binding of most drugs are known to be proteins such as receptors and enzymes. Therefore, the specific modulation of target protein synthesis by the antisense ODN method is quite interesting to the pharmacologist. We have studied the change in the morphine-induced behaviors after the microinjection of antisense S-ODN directed against the m-opioid receptor (MOR) into the periaqueductal gray (PAG) or lateral ventricle of rat brain. We could detect the decrease of the MOR mRNA level in PAG by the RT-PCR method and that in whole brain by the Northern blot technique. Although the antisense ODN method seems to be quite useful for the modulation of a given gene expression, many problems still remain to be elucidated. These include the mechanism of the regulation of a target gene, pharmacokinetics of antisense ODN and toxicity of antisense ODN.