ULTRASTRUCTURAL-LOCALIZATION OF SORCIN, A 22 KDA CALCIUM-BINDING PROTEIN, IN THE RAT CAUDATE-PUTAMEN NUCLEUS - ASSOCIATION WITH RYANODINE RECEPTORS AND INTRACELLULAR CALCIUM-RELEASE

Sorcin is a 22 kDa calcium binding protein that is widely distributed in mammalian tissues, including brain, and is associated with the ryan odine receptor (RyR) family of intracellular calcium-release channels in the heart. To determine the cellular sites for potential central fu nctions of sorcin, we examined the electron microscopic immunocytochem ical localization of antipeptide antisera against sorcin and against c ardiac and brain RyR in the rat caudate-putamen nucleus (CPN), one of the few regions expressing high levels of brain RyR. Sorcin-like immun oreactivity (S-LI) was detected in both neurons and glia by using immu noperoxidase and immunogold methods. Of 1,735 profiles containing immu nogold-silver labeling for sorcin, almost 50% were dendrites and many of these dendrites were spiny. The remainder were mainly small axons, axon terminals, and, more rarely, glia. Furthermore, analysis of duall y labeled tissue sections showed the presence of sorcin in many of the dendrites and some of the axonal and glial processes containing RyR. In dendrites, gold-silver deposits showing S-LI were prominently local ized to saccules of smooth endoplasmic reticulum and mitochondria, bot h of which are known to store calcium. These labeled structures were l ocated near the plasma membrane at sites postsynaptic to excitatory-ty pe asymmetric junctions, as well as non-synaptic portions of the plasm a membrane. In axons, S-LI was also often seen at extrasynaptic sites on, or near, the plasma membrane. We conclude that in the rat CPN, sor cin may act independently or, in conjunction with RyR, to modulate cyt oplasmic release of calcium, mainly from smooth endoplasmic reticulum and/or mitochondria in neurons. (C) 1997 Wiley-Liss, Inc.