Sigma receptors are highly concentrated in the rat pineal gland.

The distribution of sigma (sigma) receptors in the rat brain was studied with autoradiography using [3H]1,3-di-ortho-tolyl-guanidine ([3H]DTG) as a ligand. The highest concentration of sigma receptors was seen in the pineal gland, an area which has not been previously studied. This result is of interest as both sigma receptors and the pineal gland have recently been shown to play a role not only in the nervous system but also in the immune and endocrine systems.