The association of Kv3 channels with fast spiking interneurons (Lien and Jonas, 2003 and Rudy and McBain, 2001) does not preclude expression in CA3 pyramidal neurons, as is clear from in situ hybridization studies (Allen Brain Atlas; Supplemental Panobinostat price Experimental Procedures) and PCR experiments showing that Kv3.1/3.2/3.3 mRNA is present in CA3 pyramidal neurons (Perney et al., 1992 and Weiser et al., 1994), as confirmed by our PCR and immunohistochemistry data (Figure 4).

Kv2.1 is a prominent delayed rectifier of cortex and hippocampus (Du et al., 2000, Guan et al., 2007 and Murakoshi and Trimmer, 1999); Kv2.2 shows lower expression levels in cortical regions but is highly expressed in certain auditory nuclei (Johnston et al., 2008). Interestingly, both Kv2.1 and Kv2.2 show localization to the initial segment in native neurons ( Johnston et al., 2008 and Sarmiere

et al., 2008), suggesting a common role in regulating excitability; although clustering at INCB024360 ic50 cholinergic synapses and cell bodies is also important for other roles ( Misonou et al., 2004 and Muennich and Fyffe, 2004). CA3 pyramidal neurons in vivo show a majority of single spiking responses in awake animals (Tropp Sneider et al., 2006), with only 20% of events giving a burst firing response. Spontaneous firing rates are in the range of 0.2 Hz in urethane anesthetized mice (Hahn et al., 2007), but spike trains from freely moving rodents can range between 4 and 62 Hz (Fenton and Muller, 1998 and Klyachko and Stevens, 2006). As we demonstrate, potentiation of Kv2 favors single spiking (see Figure 2)

in the hippocampus and would contribute to activity-dependent suppression of after-depolarizing potentials observed in vitro (Brown and Randall, 2009). Indeed, the mediation of Kv2 potentiation by NMDAR/nitrergic signaling seen here suggests that the commissural associative pathways (DCG-IV insensitive EPSCs activated under our conditions, L-NAME HCl Figure S1C), which express high levels of NMDAR (Fukushima et al., 2009 and Rajji et al., 2006), may have a direct role in switching between CA3 pyramidal neuron single spiking and burst firing. This is consistent with increased CA3 pyramidal neuron excitability following genetic ablation of NMDAR (Fukushima et al., 2009) in the CA3 region. The dominant subunit of the MNTB Kv3 channel is Kv3.1b (Macica et al., 2003), which is basally phosphorylated (Song et al., 2005) and following moderate periods of activity, becomes dephosphorylated and active. Our observations extend the concept of activity-dependent regulation of K+ currents over longer time periods, to when Kv3 is inactivated and Kv2 channels dominate MNTB excitability.