Functional selectivity of interhemispheric connections in cat visual cortex.

The functional specificity of callosal connections was investigated in visual areas 17 and 18 of adult cats, by combining in vivo optical imaging of intrinsic signals with labeling of callosal axons. Local injections of neuronal tracers were performed in one hemisphere and eight single callosal axons were reconstructed in the opposite hemisphere. The distributions of injection sites and callosal axon terminals were analyzed with respect to functional maps in both hemispheres. Typically, each callosal axon displayed 2 or 3 clusters of synaptic boutons in layer II/III and the upper part of layer IV. These clusters were preferentially distributed in regions representing the same orientation and the same visuotopic location as that at the corresponding injection sites in the opposite hemisphere. The spatial distribution of these clusters was elongated and its main axis correlated well with the preferred orientation at the injection site. These results demonstrate a specific organization of interhemispheric axons that link cortical regions representing the same orientation and the same location of visual stimuli. Visual callosal connections are thus likely involved in the processing of coherent information in terms of shape and position along the midline of the visual field, which may facilitate the fusion of both hemifields into the percept of a single visual scene.

Postnatal development of GFAP, connexin43 and connexin30 in cat visual cortex.

In cat visual cortex, neurons acquire progressively mature functional properties during the first postnatal months. The aim of this study was to analyze the development of astrocytes during this period. The patterns of expression of the glial fibrillary acidic protein (GFAP) as well as of two gap junction proteins expressed in astrocytes, connexin43 (Cx43) and connexin30 (Cx30), were investigated by immunohistochemistry and optical density measurements, in visual cortical areas 17 and 18 at four different ages: 2 weeks (postnatal days 12 to 15, P12-15), 1 month (P27-31), 2 months (P60-62) and beyond 1 year. Since visual experience is a key factor for neural development, the patterns of expression of these three proteins were studied both in normally-reared and monocularly deprived animals. Interestingly, the distribution of GFAP, Cx43 and Cx30 was found to change dramatically but independently of visual experience, during postnatal development, even beyond P60. During the first postnatal month, GFAP and Cx43 were mainly localized in the white matter underlying the visual cortical areas 17 and 18. Then, their distributions evolved similarly with a progressive decrease of their density in the white matter associated with an increase in the cortex. Connexin30 expression appeared only from the second postnatal month, strictly in the cortex and with a laminar distribution which was similar to that of Cx43 at the same age. In adults, a specific laminar distribution was observed, that was identical for GFAP, Cx43 and Cx30: their density was higher in layers II/III and V than in the other cortical layers.

Microglia and astrocytes may participate in the shaping of visual callosal projections during postnatal development.

In the adult cat, axons running through the corpus callosum interconnect the border between the visual cortical areas 17 and 18 (A17 and A18) of both hemispheres. This specific pattern emerges during postnatal development, under normal viewing conditions (NR), from the elimination of initially exuberant callosal projections. In contrast, if the postnatal visual experience is monocular from birth (MD), juvenile callosal projections are stabilised throughout A17 and A18. The present study aimed at using such a model in vivo to find indications of a contribution of glial cells in the shaping of projections in the developing CNS through interactions with neurones, both in normal and pathological conditions. As a first stage, the distribution and the morphology of microglial cells and astrocytes were investigated from 2 weeks to adulthood. Microglial cells, stained with isolectin-B4, were clustered in the white matter below A17 and A18. Until one month, these clustered cells displayed an ameboid morphology in NR group, while they were more ramified in MD animals. Their phenotype thus depends on the postnatal visual experience, which indicates that microglial cells may interact with axons of visual neurones. It also suggests that they may differentially contribute to the elimination and the stabilisation of juvenile exuberant callosal fibres in NR and MD animals respectively. Beyond one month, microglial cells were very ramified in both experimental groups. Astrocytes were labelled with a GFAP-antibody. The distributions of connexins 43 (Cx43) and 30 (Cx30), the main proteic components of gap junction channels in astrocytes, were also investigated using specific antibodies. Both in NR and MD groups, until 1 month, GFAP-positive astrocytes and Cx43 were mainly localised within the subcortical white matter. Then GFAP, Cx43 and Cx30 stainings progressively appeared within the cortex, throughout A17 and A18 but with a differential laminar expression according to the age. Thus, the distributions of both astrocytes and connexins changed with age; however, the monocular occlusion had no visible effect. This suggests that astrocytes may contribute to the postnatal development of neuronal projections to the primary visual cortex, including visual callosal projections.

A sustained increase in the endogenous level of cAMP reduces the retinoid concentration required for APL cell maturation to near physiological levels.

cAMP-dependent signal transduction co-operates with retinoids to induce acute promyelocytic leukaemia (APL) cell maturation. The rationale of this work was to determine whether signal cross-talk could be used to decrease the pharmacological doses of retinoids in the treatment of APL. When only the basal level of adenylate-cyclase (AC) activity is present in NB4 cells, up to 1 microM concentration of all-trans retinoic acid (RA) is required for full maturation (100%). In these conditions, with only 10 nM RA less than 20% of cells will differentiate. Although the use of membrane receptor agonists to activate AC has been proved to synergize with RA treatment, these agents were never as potent as cell permeant cAMP analogues. Analogues have disadvantages since cleavage by serum and cellular phosphodiesterases generates metabolites which interfere in cellular response. In the present study, we observed cell maturation by engrafting an autonomous Bordetella pertussis AC which steadily delivers natural cAMP into the cell. The enzyme alone had no effect on cell maturation. Importantly, cell maturation was increased in a dose-dependent manner when the bacterial AC (1 ng/ml to 1 microg/ml) was used to potentiate the effects of low doses RA (10 nM). More than 50% of cells matured with only 10 nM of RA and 200 ng/ml of B. pertussis AC. The maturation response was significantly increased when lower amounts of enzyme were repetitively added to the culture to compensate for enzymatic decay. These results indicate that a sustained AC activity enhanced cell maturation. We were able to reduce to 3 nM the RA requirement, provided that a minimal amount (20 ng/ml) of B. pertussis AC was added every 12 h in culture. Membrane signalling maintaining high the level of cAMP substantially improved the efficacy of APL cell maturation by retinoids. Therefore, therapeutic benefits are expected by lowering the concentration of RA towards physiological (nanomolar) levels, thus reducing the side-effects of the drug. cAMP-elevating drugs that act on a post-cyclase target (cyclic-nucleotide phosphodiesterases) or cell-targeted drug carriers (cAMP and RA loaded liposomes) should be evaluated as maturation therapies combining the activation of multiple signalling pathways.