Global expression of NGF promotes sympathetic axonal growth in CNS white matter but does not alter its parallel orientation

Axonal regeneration is normally limited after injuries to CNS white matter. Infusion of neurotrophins has been successful in promoting regenerative growth through injured white matter but this growth generally fails to extend beyond the infusion site. These observations are consistent with a chemotropic effect of these factors on axonal growth and support the prevailing view that neurotrophin-induced axonal regeneration requires the use of gradients, i.e., gradually increasing neurotrophin levels along the target fiber tract. To examine the potential of global overexpression of neurotrophins to promote, and/or modify the orientation of, regenerative axonal growth within white matter, we grafted nerve growth factor (NGF) responsive neurons into the corpus callosum of transgenic mice overexpressing NGF throughout the CNS under control of the promoter for glial fibrillary acidic protein. One week later, glial fibrillary acidic protein and chondroitin sulfate proteoglycan immunoreactivity increased within injured white matter around the grafts. NGF levels were significantly higher in the brains of transgenic compared with non-transgenic mice and further elevated within injury sites compared with the homotypic region of the non-injured side. Although there was minimal outgrowth from neurons grafted into non-transgenic mice, extensive parallel axonal regeneration had occurred within the corpus callosum up to 1.5 mm beyond the astrogliotic scar (the site of maximum NGF expression) in transgenic mice. These results demonstrate that global overexpression of neurotrophins does not override the constraints limiting regenerative growth to parallel orientations and suggest that such factors need not be presented as positive gradients to promote axonal regeneration within white matter.     

Feeding enhances extracellular lactate of local origin in the rostromedial hypothalamus but not in the cerebellum

The use of brain microdialysis together with chronic vascular catheterization allowed us to assay extracellular fluid lactate (ECF_L) in both the ventromedial–paraventricular (VMH–PVN) area of the hypothalamus and the cerebellum, in parallel with measures of plasma levels, and in relation to food intake. A 45 min scheduled meal increased VMH–PVN ECF_L by 28%. This increase was not observed in the cerebellum. The prandial increase in plasma glucose (43%, from 4.74 to 6.77 mM) and lactate (84%, from 0.83 to 1.53 mM) showed a different temporal pattern and lasted longer than that of the ECF_L. Glucose delivery by reverse dialysis for 45 min into the VMH–PVN area increased ECF_L by 49%. When local glucose utilization was prevented by reverse dialysis-delivered 2-deoxy--glucose (2-DG), not only did VMH–PVN ECF_L drop, but the feed-related increase in ECF_L was blocked without affecting the normal rise in plasma glucose and in lactate. These results indicate that meal-related ECF_L production and variations are independent of circulating lactate, but may depend on substrate availability in these hypothalamic structures.     

Acute intranasal osteopontin treatment in male rats following TBI increases the number of activated microglia but does not alter lesion characteristics

Intranasal recombinant osteopontin (OPN) has been shown to be neuroprotective in different models of acquired brain injury but has never been tested after traumatic brain injury (TBI). We used a model of moderate-to-severe controlled cortical impact in male adult Sprague Dawley rats and tested our hypothesis that OPN treatment would improve neurological outcomes, lesion and brain tissue characteristics, neuroinflammation, and vascular characteristics at 1 day post-injury. Intranasal OPN administered 1 hr after the TBI did not improve neurological score, lesion volumes, blood–brain barrier, or vascular characteristics. When assessing neuroinflammation, we did not observe any effect of OPN on the astrocyte reactivity but discovered an increased number of activated microglia within the ipsilateral hemisphere. Moreover, we found a correlation between edema and heme oxygenase-1 (HO-1) expression which was decreased in OPN-treated animals, suggesting an effect of OPN on the HO-1 response to injury. Thus, OPN may increase or accelerate the microglial response after TBI, and early response of HO-1 in modulating edema formation may limit the secondary consequences of TBI at later time points. Additional experiments and at longer time points are needed to determine if intranasal OPN could potentially be used as a treatment after TBI where it might be beneficial by activating protective signaling pathways.     

Expression of corticotropin releasing factor, but not its binding protein, in CA1 pyramidal cells of gerbil hippocampus following transient ischemia

Recently, the ectopic expression of corticotropin-releasing factor (CRF) has been suggested as being associated with processes linked to neuronal injury and/or degeneration in response to a brain insult. However, there is little experimental data linking CRF directly to neuronal death induced by ischemia. Therefore, in the present study, we investigated the temporal and spatial alteration of CRF and its binding protein expressions in the hippocampus after transient ischemia. As a result, we found the selective increase of CRF immunoreactivity in the CA1 pyramidal cells and their processes at only 4 days post-ischemic insult. In contrast, CRF binding protein immunoreactivity was rarely detected in the CA1 region. These results suggest that transient ischemia may provoke selectively ectopic expression of CRF, but not of its binding protein, in vulnerable regions, and this enhancement of CRF may play important roles in the neurodegenerative process.     

Chemically induced long-term potentiation increases the number of perforated and complex postsynaptic densities but does not alter dendritic spine volume in CA1 of adult mouse hippocampal slices

Examination of the morphological correlates of long-term potentiation (LTP) in the hippocampus requires the analysis of both the presynaptic and postsynaptic elements. However, ultrastructural measurements of synapses and dendritic spines following LTP induced via tetanic stimulation presents the difficulty that not all synapses examined are necessarily activated. To overcome this limitation, and to ensure that a very large proportion of the synapses and spines examined have been potentiated, we induced LTP in acute hippocampal slices of adult mice by addition of tetraethylammonium (TEA) to a modified CSF containing an elevated concentration of Ca(2+) and no Mg(+). Quantitative electron microscope morphometric analyses and three-dimensional (3-D) reconstructions of both dendritic spines and postsynaptic densities (PSDs) in CA1 stratum radiatum were made on serial ultrathin sections. One hour after chemical LTP induction the proportion of macular (unperforated) synapses decreased (50%) whilst the number of synapses with simple perforated and complex PSDs (nonmacular) increased significantly (17%), without significant changes in volume and surface area of the PSD. In addition, the surface area of mushroom spines increased significantly (13%) whilst there were no volume differences in either mushroom or thin spines, or in surface area of thin spines. CA1 stratum radiatum contained multiple-synapse en passant axons as well as multiple-synapse spines, which were unaffected by chemical LTP. Our results suggest that chemical LTP induces active dendritic spine remodelling and correlates with a change in the weight and strength of synaptic transmission as shown by the increase in the proportion of nonmacular synapses.     

Cerebral blood flow does not mediate the effect of brain temperature on recovery of extracellular potassium ion activity after transient focal ischemia in the rat

Temperature plays an important role in determining outcome following both global and focal brain ischemia. After focal ischemia, the degree of infarction decreases with mild hypothermia and increases with mild hyperthermia. In this study, brain extracellular potassium ion activity and local cerebral blood flow were measured in cerebral cortex during 60 min of middle cerebral artery occlusion and 60 min of re-perfusion. Brain temperature was maintained at 32-34 degrees C (mild hypothermia), 35.5-36.5 degrees C (normothermia), or 37.5-38.5 degrees C (mild hyperthermia) throughout ischemia and re-perfusion. In normothermic animals and to a greater degree in hyperthermic animals, extracellular potassium ion activity showed delayed secondary elevation above pre-ischemia values within 40-60 min after re-perfusion. No secondary elevation of extracellular potassium ion activity was observed in hypothermic animals. There was no difference in cortical blood flow among groups with varying brain temperature, indicating that delayed deterioration of brain potassium ion homeostasis was not caused by temperature dependent alteration of cerebral blood flow. The data suggest that loss of potassium ion homeostasis during re-perfusion after focal cerebral ischemia is caused by cellular rather than vascular dysfunction and may reflect secondary inhibition of energy metabolism.