Blocking VEGF by Bevacizumab Compromises Electrophysiological and Morphological Properties of Hippocampal Neurons

One of the most remarkable aspects of an animal's behavior is the ability to modify that behavior by learning, an ability that reaches its highest form in human beings. For me, learning and memory have proven to be endlessly fascinating mental processes because they address one of the fundamental features of human activity: our ability to acquire new ideas from experience and to retain these ideas over time in memory. Moreover, unlike other mental processes such as thought, language, and consciousness, learning seemed from the outset to be readily accessible to cellular and molecular analysis. I, therefore, have been curious to know: What changes in the brain when we learn? And, once something is learned, how is that information retained in the brain? I have tried to address these questions through a reductionist approach that would allow me to investigate elementary forms of learning and memory at a cellular molecular level-as specific molecular activities within identified nerve cells.

Years of intensive investigation have yielded a sophisticated understanding of long-term potentiation (LTP) induced in hippocampal area CA1 by high-frequency stimulation (HFS). These efforts have been motivated by the belief that similar synaptic modifications occur during memory formation, but it has never been shown that learning actually induces LTP in CA1. We found that one-trial inhibitory avoidance learning in rats produced the same changes in hippocampal glutamate receptors as induction of LTP with HFS and caused a spatially restricted increase in the amplitude of evoked synaptic transmission in CA1 in vivo. Because the learning-induced synaptic potentiation occluded HFS-induced LTP, we conclude that inhibitory avoidance training induces LTP in CA1.

Vascular endothelial growth factor (VEGF) is an angiogenic protein with neurotrophic and neuroprotective effects. Because VEGF promotes the proliferation of vascular endothelial cells, we examined the possibility that it also stimulates the proliferation of neuronal precursors in murine cerebral cortical cultures and in adult rat brain in vivo. VEGF (>10 ng/ml) stimulated 5-bromo-2'-deoxyuridine (BrdUrd) incorporation into cells that expressed immature neuronal marker proteins and increased cell number in cultures by 20-30%. Cultured cells labeled by BrdUrd expressed VEGFR2/Flk-1, but not VEGFR1/Flt-1 receptors, and the effect of VEGF was blocked by the VEGFR2/Flk-1 receptor tyrosine kinase inhibitor SU1498. Intracerebroventricular administration of VEGF into rat brain increased BrdUrd labeling of cells in the subventricular zone (SVZ) and the subgranular zone (SGZ) of the hippocampal dentate gyrus (DG), where VEGFR2/Flk-1 was colocalized with the immature neuronal marker, doublecortin (Dcx). The increase in BrdUrd labeling after the administration of VEGF was caused by an increase in cell proliferation, rather than a decrease in cell death, because VEGF did not reduce caspase-3 cleavage in SVZ or SGZ. Cells labeled with BrdUrd after VEGF treatment in vivo include immature and mature neurons, astroglia, and endothelial cells. These findings implicate the angiogenesis factor VEGF in neurogenesis as well.