Effects of chronic network hyperexcitability on the growth of hippocampal dendrites
Introduction
Neuronal activity influences the growth of dendrites during development and such activity-dependent morphological alterations are likely to have important consequences on the ultimate functioning of the central nervous system (Cline, 2001, Wong and Ghosh, 2002, Van and Cline, 2004). Indeed, dendrites appear to be anatomically plastic (Faherty et al., 2003, Chklovskii et al., 2004, Cooke and Woolley, 2005) and their adaptability is especially prominent during development (Rhin and Claiborne, 1990, Dalva et al., 1994, Lendavi et al., 2000, Hua and Smith, 2004). Dendritic abnormalities are also associated with developmental brain disorders (Purpura et al., 1982, Armstrong, 2005, Dierssen and Ramakers, 2006). This is true in epilepsy where a loss of dendritic branches and spines are commonly observed (Ward, 1969, Belichencko et al., 1992, Isokawa et al., 1993, Multani et al., 1994). Both glutamatergic and GABAergic synaptic transmission have been proposed to play important roles in regulating dendritic growth (Barbin et al., 1993, Rajan and Cline, 1998, Sin et al., 2002, Wayman et al., 2006). Although little is known about how neuronal activity regulates the formation of hippocampal circuitry, persistent disruptions in the balance of excitatory–inhibitory networks in hippocampus could result in alterations in dendritic growth which would be expected to impact synaptic connectivity and ultimately network operations.
Previous studies in animal models have reported that brief but recurrent seizures in early-life result in a loss of dendritic spines and branches (Jiang et al., 1998, Swann et al., 2000). Recurrent seizures have also been reported to produce a reduction in the expression of glutamatergic synaptic protein and postsynaptic scaffolding proteins (Swann et al., 2007a). We have observed similar alterations in synaptic proteins in response to prolonged periods of network hyperexcitability in developing hippocampal slice cultures (Swann et al., 2007b). Building on these observations, we report here that chronic network hyperexcitability in hippocampal cultures reduces the length and branching complexity of CA1 pyramidal cell dendrites. We also show for the first time, that epileptiform activity prevents normal dendrite growth and that this is an NMDA dependent process. Moreover, a number of laboratories have recently examined the molecular mechanisms that underlie activity-dependent growth of dendrites (Redmond et al., 2002, Vaillant et al., 2002, Yu and Malenka, 2003, Van and Cline, 2004, Wayman et al., 2006). Numerous signaling cascades have been implicated in these processes. One consistent finding is that activation of the transcription factor CREB appears to play a central role in mediating activity-dependent dendritic growth (Wong and Ghosh, 2002, Konur and Ghosh, 2005). Our results also show that CREB activation is reduced in neurons that have experienced prolonged periods of neuronal hyperexcitability — implicating alterations in an upstream signaling cascade as a possible contributor to the dendritic abnormalities observed.
Section snippets
Animals
Thy1YFP-H transgenic mice (Feng et al., 2000) were used in all experiments. The day of birth was designated as postnatal day 0 (P0) and transverse hippocampal slice cultures were prepared on P5 or P6. Maintenance of animals and surgical procedures were approved by the Baylor College of Medicine institutional animal care committee and were in keeping with guidelines established by the National Institutes of Health.
Hippocampal slice cultures
Slice cultures of hippocampus were made as previously described (Stoppini et al.,
Bicuculline induces epileptiform activity in slice cultures
In order to establish that neonatal mouse slice cultures treated with bicuculline develop spontaneous synchronized network activity we assessed the effect of acute bicuculline treatment on slice cultures grown in normal media. Slice cultures were made on P5 or 6 and following either 3 or 7 DIV, slice cultures were transferred to a recording chamber and perfused with normal ACSF followed by ACSF containing 50 μM bicuculline. Fig. 1 shows representative extracellular field recordings. Under
Discussion
The results reported here show that when hippocampal slice cultures are treated over a 4 day period with a GABAa receptor antagonist, a decrease in basilar dendritic length is observed. This effect is the result of a decrease in both branching complexity and dendritic segment length. These effects appear to result from a complete or near complete suppression of dendritic growth. Between 3 DIV and 7 DIV the dendrites of CA1 pyramidal cells double in length by gradually increasing segment length
Acknowledgments
This work was supported by NIH Grants NS18309, NS37171, and a grant from the Partnership for Pediatric Epilepsy Research (JWS), NIH Grant NS54882 (JO) and an Epilepsy Foundation Postdoctoral Fellowship (MN).
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