Glutamic acid decarboxylase message isoforms in human suprachiasmatic nucleus.

The rat suprachiasmatic nucleus (SCN) is comprised of neurons that contain gamma-amino butyric acid (GABA) colocalized with one or more peptides. In the present study, the authors employed in situ hybridization histochemistry to determine whether the human SCN also contains GABA neurons using synthetic oligonucleotide probes complementary to sequences of two isoforms of the GABA-forming enzyme, glutamic acid decarboxylase (GAD), GAD65, and GAD67. Most, if not all, SCN neurons appear to express both GAD65 mRNA and GAD67 mRNA with the content of GAD67 greater than GAD65. Both isoforms also are expressed in some neurons of the anterior hypothalamic area, in small neurons of the paraventricular nucleus but not in the supraoptic nucleus. These data indicate that neurons in the human SCN, like those in rodents, use GABA as a neurotransmitter.