BDNF upregulation rescues synaptic plasticity in middle-aged ovariectomized rats

Abstract

Brain-derived neurotrophic factor (BDNF) has emerged as a possible broad-spectrum treatment for the plasticity losses found in rodent models of human conditions associated with memory and cognitive deficits. We have tested this strategy in the particular case of ovariectomy. The actin polymerization in spines normally found after patterned afferent stimulation was greatly reduced, along with the stabilization of long-term potentiation, in hippocampal slices prepared from middle-aged ovariectomized rats. Both effects were fully restored by a 60-minute infusion of 2 nM BDNF. Comparable rescue results were obtained after elevating endogenous BDNF protein levels in hippocampus with 4 daily injections of a short half-life ampakine (positive modulator of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate [AMPA]-type glutamate receptors). These results provide the first evidence that minimally invasive, mechanism-based drug treatments can ameliorate defects in spine plasticity caused by depressed estrogen levels.

Introduction

Appropriately patterned afferent activity causes rapid reorganization of the dendritic spine cytoskeleton, as evidenced by intense actin polymerization (Kramár et al., 2006, Lin et al., 2005, Okamoto et al., 2004), along with marked changes in spine and synapse morphology in adult hippocampus (Chen et al., 2007, Honkura et al., 2008, Matsuzaki et al., 2004; Rex et al., 2009, Yang et al., 2008). Recent studies suggest that at least some of these effects occur during learning and are critical to the encoding of long-term memory (Fedulov et al., 2007). There are also reasons to suspect that defects in spine plasticity are important contributors to memory and cognitive problems in certain neuropsychiatric disorders (Kaufmann and Moser, 2000). Consonant with this idea, activity-driven polymerization of spine actin is disturbed in animal models of early-stage Huntington's disease (Lynch et al., 2007), a condition associated with memory impairments of varying degrees of severity. Deficits in spine cytoskeletal reorganization are also observed in ovariectomized (OVX) rats not given estrogen replacement (Kramár et al., 2009b). A substantial body of evidence links rapid changes in spine actin networks to long term potentiation (LTP) (Bramham, 2008, Fukazawa et al., 2003; Kramár et al., 2006, Krucker et al., 2000; Okamoto et al., 2004, Rex et al., 2009) and, as expected from this, the spine defects found in Huntington's disease and OVX rodents are accompanied by a failure of LTP consolidation (Kramár et al., 2009, Lynch et al., 2007).

The loss of plasticity in the OVX cases raised the possibility that estrogen engages actin signaling at synapses. We tested this idea and found that the steroid promotes rapid actin filament assembly within adult spines by stimulating the small GTPase RhoA, which then drives the phosphorylation, and thus inactivation, of the actin severing protein cofilin (Kramár et al., 2009b). Induction of LTP by theta pattern stimulation also phosphorylates cofilin via RhoA-dependent kinase (Rex et al., 2009), so it appears that estrogen regulates spine plasticity through actions on a learning-related enzyme cascade.

The above conclusion has significance with regard to the development of strategies for treating memory problems commonly reported to accompany the loss of estrogen (Devi et al., 2005, Phillips and Sherwin, 1992, Wegesin and Stern, 2007). Recent work indicates that the RhoA to cofilin signaling targeted by estrogen is also acutely modulated by factors released during the brief periods of repetitive afferent activity used to induce LTP (for review, Kramár et al., 2009a). Thus, it is conceivable that manipulation of these other factors, such as increasing TrkB signaling (Chen et al., 2010) can be used to offset the loss of estrogen's positive influence on spine cytoskeletal reorganization and LTP. We have explored this possibility by measuring the effects of brain-derived neurotrophic factor (BDNF), an established and potent positive modulator of LTP, on ovariectomy-induced defects in actin filament assembly and LTP consolidation. More directly relevant to the clinical problem of estrogen-related memory loss, we tested if elevating forebrain BDNF levels with the short half-life ampakine CX929 rescues spine plasticity in hippocampal slices from OVX rats. Ampakines, via positive modulation of central α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)-receptors, enhance excitatory transmission in the forebrain and thereby increase BDNF expression (Lauterborn et al., 2003). CX929 was chosen for the study because of its short half-life (<15 minutes) and past work showing that it upregulates BDNF and restores LTP in diverse rodent models (Rex et al., 2006, Simmons et al., 2009).