Bidirectional control of CNS capillary diameter by pericytes.

Neural activity increases local blood flow in the central nervous system (CNS), which is the basis of BOLD (blood oxygen level dependent) and PET (positron emission tomography) functional imaging techniques. Blood flow is assumed to be regulated by precapillary arterioles, because capillaries lack smooth muscle. However, most (65%) noradrenergic innervation of CNS blood vessels terminates near capillaries rather than arterioles, and in muscle and brain a dilatory signal propagates from vessels near metabolically active cells to precapillary arterioles, suggesting that blood flow control is initiated in capillaries. Pericytes, which are apposed to CNS capillaries and contain contractile proteins, could initiate such signalling. Here we show that pericytes can control capillary diameter in whole retina and cerebellar slices. Electrical stimulation of retinal pericytes evoked a localized capillary constriction, which propagated at approximately 2 microm s(-1) to constrict distant pericytes. Superfused ATP in retina or noradrenaline in cerebellum resulted in constriction of capillaries by pericytes, and glutamate reversed the constriction produced by noradrenaline. Electrical stimulation or puffing GABA (gamma-amino butyric acid) receptor blockers in the inner retina also evoked pericyte constriction. In simulated ischaemia, some pericytes constricted capillaries. Pericytes are probably modulators of blood flow in response to changes in neural activity, which may contribute to functional imaging signals and to CNS vascular disease.

Differential effect of dopamine on mitosis in early postnatal albino and pigmented rat retinae.

Insufficient levels of L-DOPA, released from the retinal pigment epithelium (RPE), in albino animals are considered responsible for the abnormal development of the underlying neural retina. L-DOPA normalizes retinal neurogenesis by reducing levels of cell proliferation either by acting on the cells directly or by being converted into dopamine. Here we report the effects of dopamine on mitosis in early postnatal neural retinae from albino and pigmented rats, using 4D (x, y, z and time) confocal microscopy. Exogenous dopamine significantly prolongs mitosis in retinae from albino, but not pigmented, animals. As fewer cells move into and divide in the ventricular zone (VZ) in the presence of dopamine, we conclude that the overall cell cycle is affected. The D1 receptor blocker, SCH 23390, inhibits these effects. Thus, the differential effects of dopamine on neural retinae from pigmented and albino rats in vitro must result from the activation of D1 receptors, which are present in the retina from birth. Immunohistochemical labeling of D1 receptors shows that the pattern of their distribution is similar between pigmentation phenotypes, but levels of expression may be elevated in albinos. Labeling is most intense in the inner plexiform layer but is present throughout the neuroblastic layer. These findings are discussed in light of previous reports of reduced catecholamine levels in the albino retina.

Gap junctions modulate interkinetic nuclear movement in retinal progenitor cells.

During early retinal development, progenitor cells must divide repeatedly to expand the progenitor pool. During G(1) and G(2) of the cell cycle, progenitor cell nuclei migrate back-and-forth across the proliferative zone in a process termed interkinetic nuclear movement. Because division can only occur at the ventricular surface, factors that affect the speed of nuclear movement could modulate the duration of the cell cycle. Gap-junctional coupling and gap junction-dependent Ca(2+) activity are common features of proliferating cells in the immature nervous system. Furthermore, both gap-junctional coupling and changes in [Ca(2+)](i) have been shown to be positively correlated with the migration of a number of immature cell types. Using time-lapse confocal microscopy, we describe the nature and rate of progenitor cell interkinetic nuclear movement. We show that nuclear movement is usually, but not always, associated with Ca(2+) transients and that buffering of these transients with BAPTA slows movement. Furthermore, we show for the first time that gap-junctional communication is an important requirement for the maintenance of normal nuclear movement in retinal progenitor cells. Conventional blockers of gap junctions and transfection of cells with dominant-negative constructs of connexin 43 (Cx43) and Cx43-specific antisense oligodeoxynucleotides (asODNs) all act to slow interkinetic nuclear movement. The gap junction mimetic peptide Gap26 also acts to slow movement, an effect that we show may be attributable to the blockade of gap junction hemichannels.

ATP released via gap junction hemichannels from the pigment epithelium regulates neural retinal progenitor proliferation.

The retinal pigment epithelium (RPE) plays an essential role in the normal development of the underlying neural retina, but the mechanisms by which this regulation occurs are largely unknown. Ca2+ transients, induced by the neurotransmitter ATP acting on purinergic receptors, both increase proliferation and stimulate DNA synthesis in neural retinal progenitor cells. Here, we show that the RPE regulates proliferation in the underlying neural retina by the release of a soluble factor and identify that factor as ATP. Further, we show that this ATP is released by efflux through gap junction connexin 43 hemichannels, the opening of which is evoked by spontaneous elevations of Ca2+ in trigger cells in the RPE. This release mechanism is localized within the RPE cells to the membranes facing the neural retina, a location ideally positioned to influence neural retinal development. ATP released from RPE hemichannels speeds both cell division and proliferation in the neural retina.

Tonic excitation and inhibition of neurons: ambient transmitter sources and computational consequences.

Tonic activation of excitatory and inhibitory receptors, by the ambient concentration of neurotransmitters in the extracellular space of the brain, has been suggested to underlie phenomena as diverse as relapse to cocaine use by reward pathways in the striatum, sparse coding of motor information in the cerebellum, and control of the development of the cerebral and cerebellar cortices. Here we assess the mechanisms which may determine the ambient levels of excitatory and inhibitory neurotransmitters, and consider their likely effect on information processing.

Ca(2+) signalling and gap junction coupling within and between pigment epithelium and neural retina in the developing chick.

Development of the neural retina is controlled in part by the adjacent retinal pigment epithelium (RPE). To understand better the mechanisms involved, we investigated calcium signalling and gap junctional coupling within and between the RPE and the neural retina in embryonic day (E) 5 chick. We show that the RPE and the ventricular zone (VZ) of the neural retina display spontaneous Ca(2+) transients. In the RPE, these often spread as waves between neighbouring cells. In the VZ, the frequency of both Ca(2+) transients and waves was lower than in RPE, but increased two-fold in its presence. Ca(2+) signals occasionally crossed the boundary between the RPE and VZ in either direction. In both tissues, the frequency of propagating Ca(2+) waves, but not of individual cell transients, was reduced by gap junction blockers. Use of the gap junction permeant tracer Neurobiotin showed that neural retina cells are coupled into clusters that span the thickness of the retina, and that RPE cells are both coupled together and to clusters of cells in the neural retina. Immunolabelling for Cx43 showed this gap junction protein is present at the junction between the RPE and VZ and thus could potentially mediate the coupling of the two tissues. Immunolabelling for beta-tubulin and vimentin showed that clusters of coupled cells in the neural retina comprised mainly progenitor cells. We conclude that gap junctions between progenitor cells, and between these cells and the RPE, may orchestrate retinal proliferation/differentiation, via the propagation of Ca(2+) or other signalling molecules.

The orientation and dynamics of cell division within the plane of the developing vertebrate retina.

The orientation of a dividing cell within the plane of the tissue plays an essential role in regulating cell fate in a range of developing structures. To assess its potential role in the developing vertebrate retina we used standard confocal microscopy of fixed tissue and time-lapse confocal imaging of living tissue to examine the orientation of cell division and mitotic spindle rotation within the plane of the retinal neuroepithelium. Based on the study of three rat strains and chick, we report in contrast to recent findings that during the main phase of cell production (E18-P4 in the rat and E6-E11 in the chick) dividing cells are randomly orientated with respect to key anatomical landmarks as well as the orientation of their dividing neighbours. Results from live imaging of neonatal rat retinae support these findings and suggest that unlike the developing cortex, in which metaphase plates often rotate extensively before coming to rest in anaphase, retinal mitotic spindle rotations prior to cell division are minimal. Furthermore, the orientation of metaphase entry largely defines that which is finally adopted during anaphase. Hence, the dynamics of metaphase progression through to anaphase in the retina appear to differ markedly from the brain, and cell divisions within the plane of the tissue are randomly orientated. These results contribute to a growing body of evidence that suggests that the current paradigm with respect to asymmetric division derived from the study of invertebrates cannot be generalized to the developing vertebrate nervous system.

Changing patterns of ganglion cell coupling and connexin expression during chick retinal development.

We have used dye injection and immunolabeling to investigate the relationship between connexin (Cx) expression and dye coupling between ganglion cells (GCs) and other cells of the embryonic chick retina between embryonic days 5 and 14 (E5-14). At E5, GCs were usually coupled, via soma-somatic or dendro-somatic contacts, to only one or two other cells. Coupling increased with time until E11 when GCs were often coupled to more than a dozen other cells with somata in the ganglion cell layer (GCL) or inner nuclear layer (INL). These coupled clusters occupied large areas of the retina and coupling was via dendro-dendritic contacts. By E14, after the onset of synaptogenesis and at a time of marked cell death, dye coupling was markedly decreased with GCs coupled to three or four partners. At this time, coupling was usually to cells of the same morphology, whereas earlier coupling was heterogeneous. Between E5 and E11, GCs were sometimes coupled to cells of neuroepithelial morphology that spanned the thickness of the retina. The expression of Cx 26, 32, and 43 differed and their distribution changed during the period studied, showing correlation with events such as proliferation, migration, and synaptogenesis. These results suggest specific roles for gap junctions and Cx's during retinal development.

Purinergic and muscarinic modulation of the cell cycle and calcium signaling in the chick retinal ventricular zone.

Spontaneous calcium transients occur in the ventricular zone of the chick retina and result from the endogenous release of neurotransmitters in the absence of action potentials. Calcium transients resulting from the activation of purinergic and muscarinic receptors occur in a mixed population of interphase and mitotic cells, whereas those produced by ionotropic GABA and glutamate receptors are mostly restricted to the interphase population, the GABA responses primarily coming from cells that express the neuronal marker TuJ-1. Muscarinic and purinergic receptors can act respectively as a brake and an accelerator on mitosis, whereas GABA and glutamate receptors are without effect. Our results suggest that the balance between muscarinic and purinergic activation acts to control the rate of retinal proliferation in early development.