Functional neuroanatomy of visuo-spatial working memory in Turner syndrome.

Turner syndrome (TS), a genetic disorder characterized by the absence of an X chromosome in females, has been associated with cognitive and visuo-spatial processing impairments. We utilized functional MRI (fMRI) to investigate the neural substrates that underlie observed deficits in executive functioning and visuo-spatial processing. Eleven females with TS and 14 typically developing females (ages 7-20) underwent fMRI scanning while performing 1-back and 2-back versions of a standard visuo-spatial working memory (WM) task. On both tasks, TS subjects performed worse than control subjects. Compared with controls, TS subjects showed increased activation in the left and right supramarginal gyrus (SMG) during the 1-back task and decreased activation in these regions during the 2-back task. In addition, decreased activation in the left and right dorsolateral prefrontal cortex (DLPFC) and caudate nucleus was observed during the 2-back task in TS subjects. Activation differences localized to the SMG, in the inferior parietal lobe, may reflect deficits in visuo-spatial encoding and WM storage mechanisms in TS. In addition, deficits in the DLPFC and caudate may be related to deficits in executive function during WM performance. Together these findings point to deficits in frontal-striatal and frontal-parietal circuits subserving multiple WM functions in TS.

Brain imaging in neurogenetic conditions: realizing the potential of behavioral neurogenetics research.

Behavioral neurogenetics research is a new method of scientific inquiry that focuses on investigation of neurodevelopmental dysfunction associated with specific genetic conditions. This research method provides a powerful tool for scientific inquiry into human gene-brain-behavior linkages that complements more traditional research approaches. In particular, the use of specific genetic conditions as models of common behavioral and cognitive disorders occurring in the general population can reveal insights into neurodevelopmental pathways that might otherwise be obscured or diluted when investigating more heterogeneous, behaviorally defined subject groups. In this paper, we review five genetic conditions that commonly give rise to identifiable neurodevelopmental and neuropsychiatric disability in children: fragile X syndrome, velo-cardio-facial syndrome, Williams syndrome, Turner syndrome, and Klinefelter syndrome. While emphasis is placed on describing the brain morphology associated with these conditions as revealed by neuroimaging studies, we also include information pertaining to molecular genetic, postmortem, and neurobehavioral investigations to illustrate how behavioral neurogenetics research can contribute to an improved understanding of brain disorders in childhood.

Dye coupling in horizontal cells of developing rabbit retina.

In the mature rabbit retina, two classes of horizontal cells, A type and B type, provide lateral inhibition in the outer plexiform layer (OPL) and spatially modify the activation of bipolar cells by photoreceptors. Gap junctions connecting homologous horizontal cells determine the extent to which this inhibitory activity spreads laterally across the OPL. Little is currently known about the expression of gap junctions in horizontal cells during postnatal development or how cell-cell coupling might contribute to subsequent maturational events. We have examined the morphological attributes and coupling properties of developing A and B type horizontal cells in neonatal rabbit retina using intracellular injections of Lucifer Yellow and Neurobiotin. Prelabeling with DAPI permitted the targeting of horizontal cell bodies for intracellular injection in perfused preparations of isolated retina. A and B type horizontal cells were identifiable at birth although their dendritic field sizes had not reached adult proportions and their synaptic contacts in the OPL were minimal. Both cell types exhibited homologous dye coupling at birth. Similar to that seen in the adult, no heterologous coupling was observed, and homologous coupling among A type cells was stronger than that observed among B type cells. The spread of tracer compounds through gap junctions of morphologically immature horizontal cells suggests that ions and other small, bioactive compounds may likewise spread through coupled, horizontal networks to coordinate the subsequent maturational of emerging outer plexiform layer pathways.

Localization of nitric oxide synthase, NADPH diaphorase and soluble guanylyl cyclase in adult rabbit retina.

Nitric oxide (NO) acts as a neuronal messenger which activates soluble guanylyl cyclase (SGC) in neighboring cells and produces a wide range of physiological effects in the central nervous system (CNS). Using immunocytochemical and histochemical stains, we have characterized the NO/SGC system in the rabbit retina and to a lesser extent, in monkey retina. Based on staining patterns observed with an antibody to nitric oxide synthase (NOS) type I and a histochemical marker for NADPH diaphorase, a metabolic intermediate required for NOS activity, three major classes of neurons appear to generate NO in the rabbit retina. These include two subclasses of sparsely distributed wide field amacrine cells, rod and cone photoreceptors, and a subpopulation of ganglion cells. Equivalent cell populations were labeled in monkey retina. An antibody to SGC (tested only in rabbit retina), labeled large arrays of cone photoreceptors in the outer nuclear layer, both amacrine and bipolar cells in the inner nuclear layer (INL), as well as populations of neurons in the ganglion cell layer. These data suggest that the ability to generate NO is restricted to relatively few neurons in the inner retina and to photoreceptor cells in the outer retina; while presumptive target cells, containing pools of SGC, are widespread and form contiguous fields across the inner and outer nuclear layers (ONL) as well as the ganglion cell layer.

N-methyl-D-aspartate-mediated glutamate toxicity in the developing rabbit retina.

Extracellular levels of endogenous glutamate are relatively high in the developing rabbit retina but nonetheless appear to promote cell survival and developmental processes at concentrations considered toxic in the adult. We wished to examine the development of retinal susceptibility to glutamate toxicity as well as the protective effects of two N-methyl-D-aspartate (NMDA) antagonists, 2-amino-5-phosphono-5-valeric acid (APV) and dextromethorphan (Dex), and the nitric oxide synthase (NOS) inhibitor, NG-methyl-L-arginine (metARG). One day in vitro retinal explants of adult and neonatal rabbits were incubated with various agonists and antagonists, and stained with trypan blue to visualize necrotic cells. The density of the necrotic cells was analyzed using the Zeiss Videoplan 2. Immature neurons were approximately 10-fold less sensitive to NMDA toxicity compared to the adult. Although both NMDA antagonists and metARG provided marked protection for adult retinal neurons against glutamate toxicity, the modest susceptibility of the immature neuron was blocked only by Dex and not APV or metARG. At least two factors may contribute to the ability of the neonatal retina to survive in the presence of high levels of endogenous extracellular glutamate. First, the 10-fold developmental increase in NMDA toxicity occurs simultaneously with a 12-15-fold downregulation of extracellular glutamate, probably through the actions of maturing Muller cells. Second, the NMDA/NO excitotoxic pathway may not be active at birth since an NOS inhibitor had little effect at this stage and our previous morphological data demonstrate that NOS-containing cells are not present in their mature configuration until the second postnatal week.

High levels of extracellular glutamate are present in retina during neonatal development.

The three major classes of neurons which comprise the primary visual pathway in retina are glutamatergic. These cells are generated in two separate developmental stages, with one subclass of photoreceptors (cones) and ganglion cells generated before birth; and the other subclass of photoreceptors (rods) and bipolar cells generated during the first week after birth. Gas chromatography/mass spectroscopy analysis coupled with a new method for collecting small samples of extracellular fluids from retina were used to determine the levels of endogenous glutamate present during differentiation and synaptogenesis of these different cell types. As expected the total retinal content of glutamate increased during the postnatal period in synchrony with the generation and maturation of glutamatergic cells. However, a significant proportion of the endogenous pool was found extracellularly at birth. Intracellular glutamate is localized within cell bodies and growing processes of cones and ganglion cells at this time but few glutamatergic synapses are present. The extracellular concentration of glutamate actually declined during the most active period of synaptogenesis, reaching very low levels in the adult. The high concentrations of extracellular glutamate in neonatal retina could play an important role in a variety of developmental events such as dendritic pruning, programmed cell death and neurite sprouting.

Functional coupling of a Ca2+/calmodulin-dependent nitric oxide synthase and a soluble guanylyl cyclase in vertebrate photoreceptor cells.

Electrophysiological recordings on retinal rod cells, horizontal cells and on-bipolar cells indicate that exogenous nitric oxide (NO) has neuromodulatory effects in the vertebrate retina. We report here endogenous NO formation in mammalian photoreceptor cells. Photoreceptor NO synthase resembled the neuronal NOS type I from mammalian brain. NOS activity utilized the substrate L-arginine (Km = 4 microM) and the cofactors NADPH, FAD, FMN and tetrahydrobiopterin. The activity showed a complete dependence on the free calcium concentration ([Ca2+]) and was mediated by calmodulin. NO synthase activity was sufficient to activate an endogenous soluble guanylyl cyclase that copurified in photoreceptor preparations. This functional coupling was strictly controlled by the free [Ca2+] (EC50 = 0.84 microM). Activation of the soluble guanylyl cyclase by endogenous NO was up to 100% of the maximal activation of this enzyme observed with the exogenous NO donor compound sodium nitroprusside. This NO/cGMP pathway was predominantly localized in inner and not in outer segments of photoreceptors. Immunocytochemically, we localized NO synthase type I mainly in the ellipsoid region of the inner segments and a soluble guanylyl cyclase in cell bodies of cone photoreceptor cells. We conclude that in photoreceptors endogenous NO is functionally coupled to a soluble guanylyl cyclase and suggest that it has a neuromodulatory role in visual transduction and in synaptic transmission in the outer retina.