Distribution of zincergic neurons in the mouse forebrain

Synaptically released zinc is thought to play an important role in neuronal signaling by modulating excitatory and inhibitory receptors and intracellular signaling proteins. Consequently, neurons that release zinc have been implicated in synaptic plasticity underlying learning and memory as well as neuropathological processes such as epilepsy, stroke, and Alzheimer's disease. To characterize the distribution of these neurons, investigators have relied on a technique that involves the retrograde transport of zinc-selenium crystals from axonal boutons to the cell bodies of origin. However, one major problem with this method is that labeling of cell bodies is obscured by high levels of staining in synaptic boutons, particularly within forebrain structures where this staining is most intense. Here, we used a modification of the retrograde labeling method that eliminates terminal staining for zinc, thereby enabling a clear and comprehensive description of these neurons. Zincergic neurons were found in all cerebral cortical regions and were arranged in a distinct laminar pattern, restricted to layers 2/3, 5, and 6 with no labeling in layer 4. In the hippocampus, labeling was present in CA1, CA3, and the dentate gyrus but not in CA2. Labeled cell bodies were also observed in most amygdaloid nuclei, anterior olfactory nuclei, claustrum, tenia tecta, endopiriform region, lateral ventricle, lateral septum, zona incerta, superior colliculus, and periaqueductal gray. Moreover, retrograde labeling was also noted in the dorsomedial and lateral hypothalamus, regions that previously were thought to be devoid of neurons with a zincergic phenotype. Collectively these data show that zincergic neurons comprise a large population of neurons in the murine forebrain and will provide an anatomical framework for understanding the functional importance of these neurons in the mammalian brain.