Effects of centrally administered antisense oligodeoxynucleotides on feeding behavior and hormone secretion

The effects of neurotransmitters and neuromodulators can be interrupted by either blockade or diminution in the amount of release by curtailing the availability of the neuropeptides in the nerve terminals. Theoretically, antisense oligodeoxynucleotides decrease the availability of signals by blocking the transcription process, thus offering an opportunity to dissect the relative roles of neurotransmitters that elicit similar biological responses. Both NPY and GAL stimulate feeding and LHRH secretion, but antisense oligodeoxynucleotides behaved differently in interrupting these two responses. Centrally administered antisense oligodeoxynucleotides were effective in blocking the stimulatory effects of NPY on LH release, thereby demonstrating that neuronal permeability, degradation, and toxicity of oligodeoxynucleotides are not limiting factors. Thus, for short-term studies the unmodified phosphodiester sequences can be successfully used. Because the attempts to block the behavioral effects of NPY yielded equivocal results, it is clear that newly synthesized NPY, critical for LH release, is relatively insignificant for feeding. Blockade of behavioral effects requires a longer period of effectiveness of oligodeoxynucleotides necessitating that the rate of oligodeoxynucleotide degradation be retarded. Effective protection from degradation in vivo can be achieved by phosphorothioating one or two terminal bases. This modification, unlike the earlier practice of phosphorothioate protection of each base, causes no toxicity and is well tolerated after central administration. Adequate controls, including vehicle and similarly modified missense or scrambled sequences, are essential to confirm specificity and to exclude toxicity. The site of administration is another important factor to be considered in the experimental design. Whereas i.c.v. injections (lateral ventricle, or IIIrd ventricle) have been largely effective in allowing access to multiple hypothalamic sites, direct injection into relevant hypothalamic nuclei may provide surgical precision to effect concentrated blockade at the site of synthesis. Earlier studies with centrally administered oligodeoxynucleotides were plagued by these limitations, resulting in inconsistent and equivocal results. However, more recent investigations, designed with these caveats in mind, have successfully used antisense oligodeoxynucleotides as exemplified by the studies to establish the role of the Y5R subtype in transducing the orexigenic NPY signal.