Immunohistochemical study of the glutamate transporter proteins GLT-1 and GLAST in rat and gerbil pineal gland.

Several investigations performed during this decade have led to the hypothesis that small secretory vesicles of pinealocytes (generally referred to as synaptic-like microvesicles, SLMVs) are components of a system for intercellular paracrine communication between pineal cells, which shares many features with the process of synaptic neurotransmission. According to a recent study, one parallel that can be drawn to synaptic signal transduction seems to be the presence of pineal re-uptake systems for messenger molecules released from SLMVs, i.e. for neuroactive amino acids such as L-glutamate. In order to further characterize these uptake mechanisms, we have carried out an immunohistochemical study to explore the presence and cellular localization of the glutamate transporters GLT-1 and GLAST in rat and gerbil pineal glands. GLT-1 and GLAST were always detected in a subpopulation of pineal parenchymal cells in both species. Using immunostaining of serial semithin sections with antibodies against marker proteins of pineal cell types, GLT-1- and GLAST-positive cells were identified as interstitial glial cells. In addition, some pinealocytes also displayed immunoreactivity for GLT-1. In contrast to current thinking, our findings show that GLT-1 is not the only glutamate transporter subtype expressed in the pineal gland. Moreover, our observations point to a significant participation of interstitial cells in the process of pineal glutamatergic communication, reminiscent of the role of glial cells during glutamatergic neurotransmission in the central nervous system.