This report addresses the connectivity of the cortex occupying middle to dorsal levels of the anterior bank of the parieto-occipital sulcus in the macaque monkey. We have previously referred to this territory, whose perimeter is roughly circumscribed by the distribution of interhemispheric callosal fibres, as area V6, or the 'V6 complex'. Following injections of wheatgerm agglutinin conjugated to horseradish peroxidase (WGA-HRP) into this region, we examined the laminar organization of labelled cells and axonal terminals to attain indications of relative hierarchical status among the network of connected areas. A notable transition in the laminar patterns of the local, intrinsic connections prompted a sub-designation of the V6 complex itself into two separate areas, V6 and V6A, with area V6A lying dorsal, or dorsomedial to V6 proper. V6 receives ascending input from V2 and V3, ranks equal to V3A and V5, and provides an ascending input to V6A at the level above. V6A is not connected to area V2 and in general is less heavily linked to the earliest visual areas; in other respects, the two parts of the V6 complex share similar spheres of connectivity. These include regions of peripheral representation in prestriate areas V3, V3A and V5, parietal visual areas V5A/MST and 7a, other regions of visuo-somatosensory association cortex within the intraparietal sulcus and on the medial surface of the hemisphere, and the premotor cortex. Subcortical connections include the medial and lateral pulvinar, caudate nucleus, claustrum, middle and deep layers of the superior colliculus and pontine nuclei. From this pattern of connections, it is clear that the V6 complex is heavily engaged in sensory-motor integration. The specific somatotopic locations within sensorimotor cortex that receive this input suggest a role in controlling the trunk and limbs, and outward reaching arm movements. There is a secondary contribution to the brain's complex oculomotor circuitry. That the medial region of the cortex is devoted to tightly interconnected representations of the sensory periphery, both visual and somatotopic-which are routinely stimulated in concert-would appear to be an aspect of the global organization of the cortex which must facilitate multimodal integration.
Before synapses form in embryonic turtle cerebral cortex, an endogenous neurotransmitter activates N-methyl-D-aspartate (NMDA) channels on neurons in the cortical plate. Throughout cortical development, these channels exhibit voltage-dependent Mg2' blockade and are antagonized by D-2-amino-5-phosphonovaleric acid, a selective NMDA receptor antagonist. The activation in situ of these nonsynaptic NMDA channels demonstrates a potential physiological substrate for control of early neuronal differentiation.N-Methyl-D-aspartate (NMDA) receptors, in addition to their well-recognized role in synaptic plasticity (1-4), may play an equally important role in the control of neuronal growth and differentiation in the central nervous system (5, 6). Results from experiments using neurons growing in culture suggest that NMDA receptor activation can promote neuronal survival (7,8) and, by regulating intracellular Ca2l levels in growth cones during neurite outgrowth (9, 10) can influence neuronal form (7,8). If similar mechanisms are to operate during normal development, then NMDA receptors on young neurons must be activated by endogenously released neurotransmitter. The recent development of techniques for whole-cell voltage-clamp recording in intact embryonic brain (11) made it possible to test directly whether NMDA receptors are activated during early neuronal differentiation.The early anatomical differentiation of the cerebral cortex proceeds along common lines in all higher vertebrate species (12). Neurons are generated in a ventricular zone, migrate radially toward the pia, and collect in a layer to form the cortical plate. We chose turtles for our physiological studies because the intact embryonic turtle forebrain can be maintained in vitro, and whole-cell recordings can be obtained from cortical neurons (11), allowing analysis of the development and properties of functional NMDA receptors in situ. MATERIALS AND METHODSFertilized eggs of red-eared turtles (Pseudemys scripta, from Tangi Turtles, Ponchatoula, LA) were kept moist in an incubator at 30°C; embryos selected for use were staged according to the morphological criteria of Yntema (13). Embryonic turtles were removed from eggs and anesthetized with hypothermia according to Stanford University guidelines for the care and use of animals. Incisions were made in brain hemispheres at rostral and caudal levels and the midline, allowing the cortex to be flattened, ventricular side up.The preparation, secured on the stage of an upright microscope, was superfused with Ringer's solution (1.5 ml/min at 22-24°C) containing 96.5 mM NaCl, 2.6 mM KCI, 2 mM CaC12, 31.5 mM NaHCO3, and 10 mM dextrose. Bathing solution also contained 0.3-0.5 ,uM tetrodotoxin, 5-10 gM bicuculline methiodide, and in most experiments, the non-NMDA antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) at 4 uM. NMDA receptor antagonists D-2-amino-5-phosphonovaleric acid (D-APV; ref. 14) and Mg2+ were added to the bathing solution at 50-250 gM and 2 mM, respectively.Patch electrodes (5 Mfl) containe...
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