Retinoic acid signaling in the nervous system of adult vertebrates

The majority of the functions of vitamin A are carried out by its metabolite, retinoic acid (RA), a potent transcriptional activator acting through members of the nuclear receptor family of transcription factors. In the CNS, RA was first recognized to be essential for the control of patterning and differentiation in the developing embryo. It has recently come to light, however, that many of the same functions that RA directs in the embryo are involved in the regulation of plasticity and regeneration in the adult brain. The same intricate metabolic control system of synthetic and catabolic enzymes, combined with cytoplasmic binding proteins, is used in both embryo and adult to create regions of high and low RA to modulate gene transcription. This review summarizes some of the discoveries in the new field of retinoid neurobiology including its functions in neural plasticity and LTP in the hippocampus; its possible role in motor disorders such as Parkinson's disease, motoneuron disease, and Huntington's disease; its role in regeneration after sciatic nerve and spinal cord injury; and its possible involvement in psychiatric diseases such as depression.