RAPID EYE MOVEMENT SLEEP DEPRIVATION DECREASES LONG-TERM POTENTIATION STABILITY AND AFFECTS SOME GLUTAMATERGIC SIGNALING PROTEINS DURING HIPPOCAMPAL DEVELOPMENT

The mammalian CNS relies on precise neuronal circuits to function properly. These circuits are assembled during development by formation and stabilization of synaptic connections between neurons. Refinement of neuronal connections in the developing CNS depends on synaptic plasticity mechanisms whereby a synapse, if its presynaptic terminal is active at the same time as the postsynaptic cell, is reinforced. In contrast, a synapse whose pre- and post-terminal firing is not coordinated is eliminated (Katz and Shatz, 1996) Long-term potentiation (LTP) is a synaptic plasticity model widely used to study synaptic changes underlying memory formation in adults and refinement of neuronal circuits in development (Malenka, 1994; Bear, 1996; Malenka and Nicoll, 1999). LTP instability in the developing hippocampus is associated with immature and unstable synapses. Reversal of LTP with low-frequency stimulation (LFS) in the hippocampus is developmentally regulated, with a greater susceptibility at immature (I) ages (postnatal day, (PN) 9-21) (Kramar and Lynch, 2003). Accordingly, LTP stability (resistance to reversal by LFS) can be regarded as a dependable measure of the developmental stage of synapse formation and refinement. Glutamatergic synapses mediate synaptic excitation in the hippocampus and changes in their signaling efficacy are the basis for several neural processes such as brain maturation, memory, and excitotoxicity (Lissin et al., 1998; Malenka and Bear, 2004; Choi, 1994). Synaptic composition of glutamatergic synapses (receptors and associated proteins) is regulated by activity during development and adulthood (Desai et al., 2002; Ehlers, 2003). To mention a few of those changes, functional N-methyl-d-aspartate receptors (NMDARs) consist of complexes containing two obligatory N-methyl-d-aspartate receptor subunit 1 (NR1) subunits and one or several NR2 (N-methyl-d-aspartate receptor subunit 2 A-D) subunits. NR2 subunits confer NMDARs with different pharmacological and functional properties and also couple them to distinct intracellular signaling machineries (Rubio and Wenthold, 1997). For instance, NR1/NR2B heteromers produce the largest currents, whereas NR1/NR2A heteromers are the least glutamate-sensitive receptors (Monyer et al., 1992, 1994). It has been shown that through the first 3 weeks of postnatal development in rats, a dramatic increase in NR2A subunit expression occurs that is accompanied by a substantial decrease of the NR2B subunit (Petralia et al., 2005; Sans et al., 2000). Another glutamatergic receptor, the AMPA receptor (AMPAR) is a tetramer composed of various combinations of four different subunits (AMPA receptor subunit 1 (GluR1)-GluR4). The GluR1 subunit is strongly regulated by the intensity of synaptic activity (Passafaro et al., 2001). Proteins associated with glutamatergic receptors are also modified by activity (Bolton et al., 2000; Perez-Otano and Ehlers, 2004). PSD-95 (postsynaptic density protein 95) is a scaffolding protein of the postsynaptic density matrix involved in anchoring and linking NMDARs to intracellular signaling pathways. Expression of PSD-95 is also regulated developmentally between PN10-15 (Sans et al., 2000). Furthermore, in vivo analysis has shown a large increase in calcium/calmodulin kinase II (CaMKII) expression during PN10-35. This some-what later expression of CaMKII correlates with spine production and spine stability (Petralia et al., 2005). Sleep in mammals is composed of two major stages, rapid eye movement sleep (REMS) and non-rapid eye movement sleep (NREMS). REMS is a state of high neuronal activity, more abundant in the young of almost every mammalian species than in the adult. This finding led to the hypothesis that REMS plays a role in CNS development (Roffwarg et al., 1966), a time when neuronal activity is recasting neuronal circuits to configure mature brain connectivity. The hippocampus is a limbic structure involved in learning and memory, and it has been the focus of intensive research aimed at understanding the function of REMS as it relates to memory formation. However, to date, no studies have been reported on the influence of REMS on hippocampal maturation. In this study, we carried out REMS deprivation in early life and assessed hippocampal development. We utilized LTP stability as an index of neuronal circuit maturation. LTP was induced with high frequency stimulation; its reversal was attempted at 30 min post-LTP induction (Kramar and Lynch, 2003). Additionally, we performed immunoblots on extracts of whole hippocampus to determine the effect of REMS deprivation on expression of some important glutamatergic signaling proteins involved in LTP induction and maintenance as well as in refinement of synapses.