Recent evidence indicates the presence of a novel α(2D/A)-adrenergic receptor (α(2D/A)-AR) linked membrane guanylate cyclase signal transduction system in the pineal gland. This system operates via a Ca2+-driven rod outer segment membrane guanylate cyclase (ROS-GC). In the present study, this transduction system has been characterized via molecular, immunohistochemical, and biochemical approaches. The two main components of the system are ROS-GC1 and its Ca2+ regulator, S100B. Both components coexist in pinealocytes where the signaling component α(2D/A)-AR also resides. The presence of ROS-GC2 was not detected in the pineal gland. Thus, transduction components involved in processing α(2D/A)-AR-mediated signals are Ca2+, S100B, and ROS-GC1. During this investigation, an intriguing observation was made. In certain pinealocytes, ROS-GC1 coexisted with its other Ca2+ modulator, guanylate cyclase activating protein type 1 (GCAP1). In these pinealocytes, S100B was not present. The other GCAP protein, GCAP2, which is also a known modulator of ROS-GC in photoreceptors, was not present in the pineal gland. The results establish the identity of an α(2D/A)-AR- linked ROS-GC1 transduction system in pinealocytes. Furthermore, the findings show that ROS-GC1, in a separate subpopulation of pinealocytes, is associated with an opposite Ca2+ signaling pathway, which is similar to phototransduction in retina. Thus, like photoreceptors, pinealocytes sense both positive and negative Ca2+ signals, where ROS-GC1 plays a pivotal role; however, unlike photoreceptors, the pinealocyte is devoid of the ROS- GC2/GCAP2 signal transduction system.
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