Anesthesia impairs developmental axon pruning and functional neuronal networks

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 DESCRIPTION (provided by applicant): General anesthesia (GA) allows for care of sick and premature children, but its use during critical stages of brain development (i.e. synaptogenesis) induces apoptotic death of immature neurons and long-term neurocognitive and behavioral impairments. The formation of functional neuronal networks during brain development depends on timely and proper neuronal axon selection and pruning, a process highly dependent on neuronal activity and homeostasis of neurotrophic factors, especially brain-derived neurotrophic factor (BDNF). Since GA is a potent inhibitor of neuronal activity and a potent modulator of BDNF homeostasis, our general hypothesis is that transient exposure to GA during critical stages of brain development causes long-term impairments of proper axon selection and pruning in neurons that survive the initial apoptotic `attack'. This, in turn, causes lasting impairments in formation of functional neuronal networks among the remaining (`normal') neurons. Our long-term goals are to understand how GA modulates the formation of functional neuronal networks responsible for long-term memory storage and consolidation and to establish the links between GA- induced BDNF-modulated axon pruning and synaptic neurotransmission. The rationale is that this understanding will enable pharmacological targeting of critical steps in axon pruning and formation of neuronal circuitries to prevent GA- induced impairment of synaptogenesis and memory/learning deficits. Using in vivo rat models of GA-induced developmental neurotoxicity we will focus on the infrapyramidal bundle (IPB) of mossy fiber projections in the hippocampus (from granular neurons in the dentate gyrus to pyramidal neurons in the CA3 region), a neuronal circuitry that plays a crucial role in learning and memory and is well established as a model of axon pruning. We will address specific hypothesis that GA impairs BDNF-p75NTR signaling, thereby hampering proper selection of axons and ultimately impairing synaptic neurotransmission. Specific Aim #1: will examine two intravenous anesthetics commonly used in children and having distinct mechanisms of action: the NMDA antagonist ketamine and the GABAA agonist propofol via morphometric analyses of the IPB and its synaptic contacts using light and electron microscopy. Our preliminary data suggest that ketamine impairs developmental IPB pruning that coincides with a significant down-regulation of BDNF and pro-BDNF. We will examine the importance of BDNF and p75NTR on GA-induced impairment in IPB pruning using BDNF+/- and Ngfr-/- (p75NTR-/-) mutant mice. The importance of the BDNF-p75NTR cascade will be examined using the p75NTR pharmacologic antagonist TAT-Pep5 to block p75NTR or the viral vector LV-syn-p75NTR to knock-in (`restore') p75NTR in dentate gyrus of p75NTR-/- mice. Specific Aim #2: will determine the functional link between GA-induced impairments in axon pruning and selection and the impairment of synaptic plasticity in IPB neuronal circuits. We will examine GA effects in p75NTR-/- and BDNF+/- mutant mice and their wild type (WT) counterparts using ex-vivo patch clamp slice recordings of synaptic neurotransmission in CA3 and dentate gyrus of the hippocampus. We will attempt to potentiate the effect of GA in WT mice on synaptic transmission using the p75NTR antagonist TAT-Pep5 and will attempt to block the effects of GA by knocking-in p75NTR in the dentate gyrus of p75NTR-/- mice.