In contrast, coherence between hippocampal LFP and the envelope of TGF-beta inhibition local gamma amplitude in different segments of the maze largely paralleled the power of the theta rhythm in the hippocampus (Figure 3A) and covaried more with the motoric aspects of the task than with the working-memory component. To determine the phase at which the 4 Hz rhythm modulated gamma power, we used troughs of the filtered gamma waves to construct LFP averages from epochs corresponding to different locations of the rat (Figure 3B). This analysis showed that the largest amplitude of gamma waves occurs on the ascending phase of the 4 Hz oscillation (preferred phase: 241.1° ± 11.8°). Moreover, the largest

amplitude and the most strongly modulated

average occurred in the central arm of the working-memory task, compared with the side arm and the averages obtained in the control task. To gain insight about the local impact of the 4 Hz and theta oscillations on unit firing, we examined their phase correlations with putative principal cells and interneurons (Figure 4A; Figure S4). A sizable fraction of neurons in both PFC and hippocampus was significantly modulated by the 4 Hz rhythm (PFC pyramidal cells: 17.7%; PFC interneurons: 51.6%; CA1 pyramidal TSA HDAC chemical structure cells: 17.8%; CA1 interneurons: 30.9%; p < 0.05; Rayleigh test was used for assessing uniformity). Large percentages of neurons were also phase locked to hippocampal theta oscillations (Figure 4A; PFC pyramidal cells: 36.4%; PFC interneurons: 55.9%; Siapas et al., 2005 and Sirota et al., 2008; CA1 pyramidal cells: 88.6%; CA1 interneurons: 96.8%; Sirota et al., 2008). In addition to spike modulation, spike transmission

efficacy between monosynaptically connected PFC neurons, as inferred from the short-term cross-correlograms between neuron pairs (Figure 4B; Fujisawa et al., 2008), was also phase modulated by both PFC 4 Hz and hippocampal theta oscillations in 42% and 22% of the pairs, respectively (Figure 4C). Neurons in the VTA were classified as putative dopaminergic either and putative GABAergic cells (Figures S4 and S5). Almost half (46.2%) of the putative dopaminergic and 37.5% of the putative GABAergic VTA neurons were significantly phase locked to the 4 Hz oscillation, as shown by their phase histograms and the significant peaks in their unit-LFP coherence spectra (Figure 4D). Approximately the same proportions of VTA neurons were also significantly phase locked to the hippocampal theta rhythm (putative dopaminergic: 43.6%; putative GABAergic cells: 39.4%). Phase modulation of neurons by 4 Hz and theta oscillations was also compared between the memory and nonmemory control tasks. For these comparisons, only the right-turn trials of the memory task were included, and the same neurons were compared in both tasks.