omega-agatoxin-iva and Hypoxia

Developing central white matter is subject to ischemic-type injury during the period that precedes myelination. At this stage in maturation, central axons initiate a program of radial expansion and ion channel redistribution. Here we test the hypothesis that during radial expansion axons display heightened ischemic sensitivity, when clusters of Ca(2+) channels decorate future node of Ranvier sites.. Functionality and morphology of central axons and glia were examined during and after a period of modeled ischemia. Pathological changes in axons undergoing radial expansion were probed using electrophysiological, quantitative ultrastructural, and morphometric analysis in neonatal rodent optic nerve and periventricular white matter axons studied under modeled ischemia in vitro or after hypoxia-ischemia in vivo.. Acute ischemic injury of central axons undergoing initial radial expansion was mediated by Ca(2+) influx through Ca(2+) channels expressed in axolemma clusters. This form of injury operated only in this axon population, which was more sensitive to injury than neighboring myelinated axons, smaller axons yet to initiate radial expansion, astrocytes, or oligodendroglia. A pharmacological strategy designed to protect both small and large diameter premyelinated axons proved 100% protective against acute ischemia studied under modeled ischemia in vitro or after hypoxia-ischemia in vivo.. Recent clinical data highlight the importance of axon pathology in developing white matter injury. The elevated susceptibility of early maturing axons to ischemic injury described here may significantly contribute to selective white matter pathology and places these axons alongside preoligodendrocytes as a potential primary target of both injury and therapeutics.

Topics: Age Factors; Animals; Animals, Newborn; Apoptosis; Astrocytes; Axons; Disease Models, Animal; Dizocilpine Maleate; Glucose; Green Fluorescent Proteins; Hypoxia; Hypoxia-Ischemia, Brain; Mice; Mice, Transgenic; Myelin Sheath; Nerve Degeneration; Nerve Fibers, Myelinated; Neuroprotective Agents; Oligodendroglia; omega-Agatoxin IVA; Optic Nerve; Organ Culture Techniques; Rats; Rats, Sprague-Dawley; Recovery of Function; Thy-1 Antigens

1. Amperometric recordings using polarized carbon fibre microelectrodes were used to detect exocytosis of catecholamines from rat phaeochromocytoma (PC12) cells in response to a reduction in pHo. 2. Exocytosis was detected at pHo levels of between 7.2 and 6.8. This was probably due to intracellular acidification, since acid-evoked secretion was enhanced by the Na+-H+ exchange blocker ethylisopropylamiloride (30 microM), and was mimicked by sodium propionate (10 mM), which causes selective intracellular acidosis. 3. Acid-evoked exocytosis was abolished by removal of Ca2+o or application of 200 microM Cd2+. It was unaffected by nifedipine, but significantly reduced by either omega-conotoxin GVIA (1 microM) or omega-agatoxin GIVA (200 nM). The two toxins applied together almost completely abolished (> 97 %) acid-evoked secretion. 4. Hypoxia-evoked catecholamine release was potentiated under acidic conditions and suppressed under alkaline conditions in a manner which indicated a greater than additive interaction of these two stimuli. 5. Our results indicate that, like carotid body arterial chemoreceptors, PC12 cells represent model chemoreceptor cells for both hypoxia and acidity and that the release of catecholamines in response to these physiological stimuli is dependent on Ca2+ influx through voltage-gated N- and P/Q-type Ca2+ channels.

Topics: Animals; Cadmium; Calcium; Catecholamines; Exocytosis; Hydrogen-Ion Concentration; Hypoxia; Nifedipine; omega-Agatoxin IVA; omega-Conotoxins; PC12 Cells; Rats