GABA_{B} Receptors Regulate Chick Retinal Calcium Waves

Correlated spiking activity and associated Ca²⁺ waves in the developing retina are important in determining the connectivity of the visual system. Here, we show that GABA, via GABA_{B} receptors, regulates the temporal characteristics of Ca²⁺ waves occurring before synapse formation in the embryonic chick retina. Blocking ionotropic GABA receptors did no affect these Ca²⁺ transients. However, when these receptors were blocked, GABA abolished the transients, as did the GABA_{B} agonist baclofen. The action of baclofen was prevented by the GABA_{B} antagonistp-3-aminopropyl-p-diethoxymethyl phosphoric acid (CGP35348). CGP35348 alone increased the duration of the transients, showing that GABA_{B} receptors are tonically activated by endogenous GABA. Blocking the GABA transporter GAT-1 with 1-(4,4-diphenyl-3-butenyl)-3-piperidine carboxylic acid (SKF89976A) reduced the frequency of the transients. This reduction was prevented by CGP35348 and thus resulted from activation of GABA_{B} receptors by an increase in external [GABA]. The effect of GABA_{B} receptor activation persisted in the presence of activators and blockers of the cAMP–PKA pathway. Immunocytochemistry showed GABA_{B} receptors and GAT-1 transporters on ganglion and amacrine cells from the earliest times when Ca²⁺ waves occur (embryonic day 8). Patch-clamp recordings showed that K⁺ channels on ganglion cell layer neurons are not modulated by GABA_{B} receptors, whereas Ca²⁺ channels are; however, Ca²⁺ channel blockade with ω-conotoxin-GVIA or nimodipine did not prevent Ca²⁺ waves. Thus, the regulation of Ca²⁺ waves by GABA_{B} receptors occurs independently of N- and L-type Ca²⁺ channels and does not involve K⁺ channels of the ganglion cell layer. GABA_{B} receptors are likely to be of key importance in regulating retinal development

Rapid onset of neuronal death induced by blockade of either axoplasmic transport or action potentials in afferent fibers during brain development.

We have investigated how neurons in the optic tecta of embryonic day 16 chick embryos depend for survival on their afferents from the retina. To distinguish between activity-mediated effects and other, "trophic," ones, we compared the effects on the tectal neurons of blocking intraocular axoplasmic transport (with colchicine) or action potentials (by means of TTX). Both interventions rapidly induced the appearance of dying (pyknotic) neurons in the tectum, with major increases in their number occurring within 13 hr post-colchicine and within 9 hr post-TTX. Following both drugs, the dying neurons were morphologically similar, and in both cases the cell death depended on protein synthesis. However, the effects of colchicine and of TTX could be dissociated, since the most superficial tectal neurons became pyknotic only in response to colchicine, and, with a sufficiently short survival time (9 hr), the deep cells of the stratum griseum centrale became pyknotic only in response to TTX. We hence argue that the survival of the tectal neurons depends on their ongoing maintenance by substances released from retinotectal axon terminals, the release being activity dependent in the case of the deep neurons but independent of activity in the case of the superficial ones

Nerve growth factor is expressed by postmitotic avian retinal horizontal cells and supports their survival during development in an autocrine mode of action.

Cell death in the developing retina is regulated, but so far little is known about what factors regulate the cell death. Several neurotrophic factors and receptors, including the neurotrophins and Trk receptors, are expressed during the critical time. We have studied the developing avian retina with respect to the role of nerve growth factor (NGF) in these processes. Our starting point for the work was that NGF and its receptor TrkA are expressed in a partially overlapping pattern in the inner nuclear layer of the developing retina. Our results show that TrkA and NGF-expressing cells are postmitotic. The first NGF-expressing cells were found on the vitreal side of the central region of E5.5–E6 retina. This pattern changed and NGF-expressing cells identified as horizontal cells were later confined to the external inner nuclear layer. We show that these horizontal cells co-express TrkA and NGF, unlike a subpopulation of amacrine cells that only expresses TrkA. In contrast to the horizontal cells, which survive, the majority of the TrkA-expressing amacrine cells die during a period of cell death in the inner nuclear layer. Intraocular injections of NGF protein rescued the dying amacrine cells and injection of antisense oligonucleotides for NGF that block its synthesis, caused death among the TrkA-expressing horizontal cells, which normally would survive. Our results suggest that NGF supports the survival of TrkA expressing avian horizontal cells in an autocrine mode of action in the retina of E10-E12 chicks. The cells co-express TrkA and NGF and the role for NGF is to maintain the TrkA-expressing horizontal cells. The TrkA-expressing amacrine cells are not supported by NGF and subsequently die. In addition to the effect on survival, our results suggest that NGF plays a role in horizontal cell plasticity

A centrifugally controlled circuit in the avian retina and its possible role in visual attention switching.

The isthmo-optic nucleus (ION) is the main source of efferents to the retina in birds. Isthmo-optic neurons project in topographical order on amacrine cells in the ventral parts of the retina, and a subclass of these known as proprioretinal neurons project onto the dorsal retina. We propose that, through the intermediary of the amacrine target cells, activity in the isthmo-optic pathway excites ganglion cells locally in the ventral retina but inhibits those in dorsal regions. This circuit would thereby mediate centrifugally controlled switches in attention between the dorsal retina, involved in feeding, and the more ventral parts, involved in scanning for predators. This hypothesis accounts for a wide range of disparate data from behavior, comparative anatomy, endocrinology, hodology, and neurophysiology

South African Dyslipidaemia Guideline Consensus Statement: A joint statement from the South African Heart Association (SA Heart) and the Lipid and Atherosclerosis Society of Southern Africa (LASSA)

The European Society of Cardiology together with the European Atherosclerosis Society published updated dyslipidaemia guidelines in 2011. SA Heart and the Lipid and Atherosclerosis Society of Southern Africa officially adopt these guidelines. This statement adapts aspects of the guidelines to the South African situation. Using the updated Framingham risk charts, interventional strategies are based according to the cardiovascular risk score and low-density lipoprotein cholesterol (LDL-C) levels. The Framingham risk score refers to the 10-year risk of any cardiovascular event, and includes four categories of risk. Treatment targets are those of the European guidelines. The LDL-C goal is 1.8 mmol/l for the very high-risk group (>30%), 2.5 mmol/l for the high-risk group (15 - 30%), and 3 mmol/l for those below 15% risk. Intensive management of dyslipidaemia in South Africa will significantly reduce the cardiovascular disease health burden

South African dyslipidaemia guideline consensus statement

The European Society of Cardiology together with the European Atherosclerosis Society published updated dyslipidaemia guidelines in 2011. SA Heart and the Lipid and Atherosclerosis Society of Southern Africa officially adopt these guidelines. This statement adapts aspects of the guidelines to the South African situation. Using the updated Framingham risk charts, interventional strategies are based according to the cardiovascular risk score and low-density lipoprotein cholesterol (LDL-C) levels. The Framingham risk score refers to the 10-year risk of any cardiovascular event, and includes four categories of risk. Treatment targets are those of the European guidelines. The LDL-C goal is 1.8mmol/l for the very high-risk group (>30%), 2.5mmol/l for the high-risk group (15 - 30%), and 3mmol/l for those below 15% risk. Intensive management of dyslipidaemia in South Africa will significantly reduce the cardiovascular disease health burden

GABA Maintains the Proliferation of Progenitors in the Developing Chick Ciliary Marginal Zone and Non-Pigmented Ciliary Epithelium

GABA is more than the main inhibitory neurotransmitter found in the adult CNS. Several studies have shown that GABA regulates the proliferation of progenitor and stem cells. This work examined the effects of the GABAA receptor system on the proliferation of retinal progenitors and non-pigmented ciliary epithelial (NPE) cells. qRT-PCR and whole-cell patch-clamp electrophysiology were used to characterize the GABAA receptor system. To quantify the effects on proliferation by GABAA receptor agonists and antagonists, incorporation of thymidine analogues was used. The results showed that the NPE cells express functional extrasynaptic GABAA receptors with tonic properties and that low concentration of GABA is required for a baseline level of proliferation. Antagonists of the GABAA receptors decreased the proliferation of dissociated E12 NPE cells. Bicuculline also had effects on progenitor cell proliferation in intact E8 and E12 developing retina. The NPE cells had low levels of the Cl–transporter KCC2 compared to the mature retina, suggesting a depolarising role for the GABAA receptors. Treatment with KCl, which is known to depolarise membranes, prevented some of the decreased proliferation caused by inhibition of the GABAA receptors. This supported the depolarising role for the GABAA receptors. Inhibition of L-type voltage-gated Ca2+ channels (VGCCs) reduced the proliferation in the same way as inhibition of the GABAA receptors. Inhibition of the channels increased the expression of the cyclin-dependent kinase inhibitor p27KIP1, along with the reduced proliferation. These results are consistent with that when the membrane potential indirectly regulates cell proliferation with hyperpolarisation of the membrane potential resulting in decreased cell division. The increased expression of p27KIP1 after inhibition of either the GABAA receptors or the L-type VGCCs suggests a link between the GABAA receptors, membrane potential, and intracellular Ca2+ in regulating the cell cycle

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