Here we show that IGF-1 mitigated SD susceptibility and decreased its associated OS. Furthermore, we show that OS-induced hyperexcitability and increased SD susceptibility, and that IGF-1 mitigated these effects. Finally, our results revealed that IGF-1 treatment lowered baseline levels of OS and simultaneously increased spontaneous activity of CA3 pyramidal neurons. These are the first results to indicate that a neuroprotective EE mimetic, IGF-1, prevents SD.
Evidence suggests that EE leads to a physiological increase in neuronal excitability that prevents SD. While SD increases aberrant hyperexcitability (Kruger et al., 1996
) that makes brain tissue more susceptible to future SD (Mitchell et al., 2010a
; Grinberg et al., 2011
), EE increases physiological neuronal excitability associated with improved learning and memory (Kumar et al., 2011
; Eckert & Abraham, 2010
). EE protects from the aberrant hyperexcitability of seizure (Young et al., 1999
; Kraig et al., 2010
), and has been shown to reduce SD (Guedes et al., 1996
). Furthermore, the neuroprotective effects of EE have also recently been shown to include migraine (Darabaneanu et al., 2011
). While various molecular mechanisms of how EE produces these neuroprotective effects have been characterized (Gagné et al., 1998
; van Praag et al., 2000
; Ekstrand et al., 2008
; Herring et al., 2010
; Kempermann et al., 2010
), the role of IGF-1 is particularly noteworthy.
IGF-1 mediates the neuroprotective effects of EE (Carro et al., 2001
), and improves learning and memory (Sonntag et al., 2000
). With EE, IGF-1 production is increased and its active uptake by the brain increases in an activity-dependent manner (Nishijima et al., 2010
). Once in brain, IGF-1 increases spontaneous hippocampal neuronal activity and improves hippocampus-dependent learning and memory test performance (Lupien et al., 2003
; Miltiadous et al., 2011
; Xing et al., 2007
). Mechanisms by which IGF-1 affects neuronal activity may include increasing ionic conductances (Blair & Marshall, 1997
; Kanzaki et al., 1999
), modulating neurotransmitter receptor activity (Gonzalez de la Vega et al., 2001
; Ramsey et al., 2005
), and decreasing generation of reactive oxygen species (Csiszar et al., 2008
; Pérez et al., 2008
). Important to our work here, increased neuronal activity enhances neural antioxidant production (Papadia et al., 2008
). Furthermore, IGF-1 has been shown to similarly increase antioxidant production in multiple peripheral tissues (Jallali et al., 2007
; Csiszar et al., 2008
). Here we show that IGF-1 reduced OS in control hippocampal slices, a preparation which shows spontaneous, physiological neuronal activity. In fact, this spontaneous activity increased after IGF-1 exposure, a phenomenon that should elevate metabolic activity and therefore the generation of reactive oxygen species. Despite this, we found that net OS significantly declined. We speculate that this results from neural activity-dependent signaling involving increased antioxidant production, as first shown by Papadia and coworkers (2008)
. While beyond the scope of our current report, future studies are designed to directly confirm that IGF-1 can lead to increased antioxidant production in brain.
We show that SD induces increased tissue OS, as other work has suggested (Viggiano et al., 2011
). OS increases hyperexcitability (Gulati et al., 2005
; Waldbaum & Patel, 2010
; Muller et al., 1993
). We confirm and extend these findings to show that OS-induced CA3 hyperexcitability can lead to SD. Furthermore, we show that IGF-1 mitigated the amount of OS generated by SD, SD susceptibility, OS-induced SD susceptibility, as well as the hyperexcitability of OS. Finally, the impact of IGF-1 protection from SD-induced OS increased with time. Together, these results suggest that IGF-1's effects on OS (and, therefore, SD susceptibility) may involve an adaptive response, consistent with physiological-conditioning hormesis (Radak et al., 2008
). These effects may help to entrain brain tissue away from the unfettered hyperexcitability needed for SD and toward physiological excitability and decreased OS.
Although we demonstrate the proof of principle that EE-mimetic IGF-1 decreases OS and SD susceptibility, a complete analysis of the optimal dose, duration, and frequency of treatment requires further study. As a first step, we chose to administer 7-day IGF-1 phasically to better mimic the inherently phasic effects of EE, as well as to avoid potentially harmful effects of prolonged tonic application of agents, such as those seen with corticosterone (de Kloet et al., 1999
; Zoladz & Diamond, 2009
We suspect that the 24-fold reduction in SD susceptibility seen with acute IGF-1 exposure that further increased to 75-fold at three days before settling to 22-fold at seven days reflects an adaptive, damped oscillatory response, commonly seen in biological systems (Stark et al., 2007
; Paszek et al., 2010
; Wang et al., 2012
). In contrast, the progressive reduction of OS from IGF-1 by 20, 30, and 73% at these time points suggests a maximal steady-state has not yet been reached. Thus, whether OS too would show a damped oscillatory or a more simple sigmoid response pattern remains unclear. However, collectively these results suggest that with optimal dosing, the ability of IGF-1 to protect against SD via reduced OS can be expected to be at least 20-fold.
While insulin is already recognized as an agent that increases learning and memory (Stockhorst et al., 2004
; Craft et al., 2011
), its ability to influence SD has not been previously examined. We show that insulin protects against SD and hypothesize that it does so, like IGF-1, via its actions as an EE mimetic (i.e., by increasing processes associated with improved learning and memory, such as CA3 bursting). However, while insulin mitigated SD susceptibility, we show that IGF-1 has this effect at a 15,500-fold smaller dose, suggesting insulin's effects may occur via cross-reactivity with the IGF-1 receptor. We thus conclude that neuroprotective EE mimetics are promising targets against SD, and by extension migraine and HFCM, with IGF-1 shown here to be a novel and potentially effective therapeutic.