In addition to their ability to change the electrical properties of neurones, evidence suggests that neurotransmitters are able to alter the cell's metabolism. Transmitter phenotype is labile and expression might be regulated, during development, by the cellular environment of neurones. The study of a key enzyme in the synthesis of catecholamines, tyrosine hydroxylase (TH), has provided clues about these adaptive responses. This enzyme has a large molecular diversity, resulting from the differential splicing of its mRNA, which is tissue-specific and might result in long-term changes in activity of the enzyme and, therefore, in the availability of neurotransmitter at various synapses. The presence of different DNA sequences at the TH locus confers susceptibility to various disorders of the brain, including manic-depressive illness and schizophrenia. Indeed, an association between a rare variant allele of the gene encoding TH and the occurrence of schizophrenia has been found in several populations. New techniques being developed to treat diseases such as Parkinson's disease involve various gene therapies, including a method of transferring genes directly into nerve cells using an adenovirus-based system.