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 immunhistochemical 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.