ZINC-ENRICHED CORTICAL MODULARITY AND PROJECTION SYSTEM
Kathleen S. Rockland and Noritake Ichinohe
Lab for Cortical Org. and Systematics; RIKEN Brain Science Institute, Saitama, Japan
The speakers in this symposium address several basic issues concerning cortical pyramidal neurons, including subtypes, and their organization within modular (Rockland; Ikegaya), laminar (Bannister), and local circuitry (Kubota, Kaneko).
Neuronal modules have long been associated with thalamocortical (TC) connections in layer 4 of sensory areas, or envisioned as layer 1-6 columnar structures. Other spatial arrangements, however, are known (e.g. orientation pinwheels); and multiple types of groupings are being distinguished in conjunction with spontaneous patterns of synaptic inputs. Several of these groupings are localized to the upper layers, consistent with other evidence that layers 1, 2 have an important role in cortical processing. This zone, we have now shown, corresponds to a small-scale honeycomb modularity in both rat (Ichinohe et al., 2003) and primate (Ichinohe and Rockland, 2004). In the rat visual cortex, where the components have been characterized in detail, glutamatergic zinc-positive (Zn+) and GABAergic parvalbumin-positive (PV+) terminals co-mingle in honeycomb walls, and surround hollows that are occupied by TC terminations (labeled by antibody against vescicular glutamate transporter 2). Subpopulations of pyramidal cell apical dendrites are distinguished in relation to the walls and hollows. Apical dendrites of layer 2 neurons are located preferentially in the Zn+/PV+ walls, and those of deeper neurons, in the TC hollows. One source of the Zn+ terminations may be Zn+ pyramidal neurons in layer 2. These are labeled by injections of sodium selenite. Sodium selenite forms crystals with synaptic zinc, which are transported retrogradely to cells of origin. In addition, labeled neurons occur near the injection in layer 6 and, to a lesser extent, in layers 3A and 5. As these are through-laminar injections, it is not clear whether there may be further laminar specificity; for example, at least a partially closed loop within layer 2 consisting of Zn+ neurons and Zn+ terminations. Another contribution may be from extrinsic cortical connections. These are labeled mainly in feedback- or ﾒlaterallyﾓ- projecting areas, rather than feedforward; and are particularly dense in layers 2, 3A and 6. Zinc is thought to be co-released with glutamate to modulate synaptic transmission.; and levels of synaptic zinc are rapidly and dynamically regulated in conditions of sensory deprivation. The localization of Zn+ neurons and terminations may thus contribute to laminar- and module ﾐspecific circuitry with differential potential for plasticity.