sation-induced facilitation of theta activity can play a causal role for the enhancement of gamma oscillations. In the dentate gyrus of the intact brain, the power of both theta and gamma activity is driven by and strongly depends upon entorhinal input, and is higher during exploratory behaviour (Bragin et al., 1995; Csicsvari et al., 2003). Furthermore, behav- ioural data indicating enhanced eyeblink condi- tioning in rabbits that received pairings of stimuli during epochs of prominent theta activity, when compared to those stimulated during non-theta periods (Seager et al., 2002), provide additional support for a facilitatory role of theta oscillations in learning. However, one should emphasise that the increase of theta power occurred in our study in the period after tetanisation. Thus, the differences in theta power that we observed, did not comprise an endogenous pre-condition for synaptic plasticity, but rather occurred as a consequence of perforant path stimulation. Accordingly, if natural theta rhythm is necessary for the acquisition or process- ing of sensory stimuli, one can presume that the artiﬁcial activation of sensory inputs (in the form of HFT) would require and/or induce an enhance- ment of oscillatory activity in the theta frequency range, in order to temporally organise neuronal activity and favour synaptic plasticity. The depolari- sation of dendritic compartments of dentate gyrus granule cells via activation of NMDA receptors and mGluRs, after strong activation of the glutamater- gic perforant path, is likely to contribute to such an enhancement. Perforant path stimulation of the dentate gyrus, in conjunction with subsequent ﬁring of mossy ﬁber collaterals, will also activate parvalbumin-expressing basket cells (Kneisler and Dingledine, 1995), which via ionotropic γ-amino- butyric acid (GABAA) receptors provide rhythmic inhibition of somata and perisomatic region of principal cells and play a pivotal role in the gen- eration of both theta (Buzsáki, 2002; Klausberger et al., 2003; Sik et al., 1997; Ylinen et al., 1995) and gamma (Bartos et al., 2001, 2007; Penttonen et al., 1998; Vida et al., 2006) oscillations. Additionally, activation of GABAA receptors has been reported to be critical also for the generation of transient tetan- ically induced gamma oscillations in vitro (Traub et al., 2004; Whittington et al., 1997). Hence, the combination of NMDAR-induced dendritic exci- tation and GABAAR-mediated somatic inhibition results in current ﬂow through distal dendrites, which is important for the generation and mainte- nance of extracellular theta currents (Buzsáki, 2002). This could also effectively trigger and/or enhance theta oscillations in post-HFT period. In turn, theta oscillations may dynamically modulate the prob- ability of NMDAR activation, which is highest on Hippocampal oscillations are linked to LTP the peak of the theta cycle and the lowest on the trough (Vertes, 2005). The increase of gamma power in the post-tetanisation period may rely on the activation of group I mGluRs, which was reported to induce GABAAR-dependent gamma oscillations in vitro (Whittington et al., 1995), and increase gamma power in vivo (Martin, 2001). We have found recently that prolonged inhibition of mGluR5 results in a signiﬁcant impairment of LTP associated with a marked suppression of gamma oscillations in the dentate gyrus of freely moving rats (Bikbaev et al., 2008). Taken together, corre- lated activation of granule cells and interneurons, via fast and slow glutamatergic excitation, not only can contribute to long-term synaptic potentiation, but, complemented with fast rhythmic GABAergic inhibition, can affect network oscillations on both short- and long-term time-scales (Figure 2). In this respect, pharmacological modulation of selected neurotransmitter systems, including the glutama- tergic, GABAergic and cholinergic systems, cou- pled with correlation analysis of network activity and long-term changes in synaptic transmission, would be necessary for a more conclusive support of this suggestion. For instance, such analysis after activation of the cholinergic system, which can facilitate oscillatory activity in the theta frequency band, and simultaneous inhibition of GABAAR- and/or mGluR-mediated signalling, which affects hippocampal gamma activity, could help to dissect the roles of theta and gamma oscillations in the shaping of synaptic plasticity. THE DIRECTION OF CHANGE IN SYNAPTIC WEIGHT CORRELATES WITH THE PHASE OF THETA CYCLE The apparent relationship between theta activity and certain types of behaviour, and the relative regular- ity and stability of this oscillatory pattern suggests that theta rhythm may serve as an internal timing mechanism for individual elements of spatially distributed neuronal ensembles (Buzsáki, 2005). In other words, enhanced theta oscillations may provide an appropriate time-window for the ﬁring of individual neurons within currently activated ensemble(s) and, therefore, optimise the encoding of the acquired signal. Indeed, the ﬁring rate of both principal cells and several classes of interneurons in the hippocampus has been found to be theta phase- locked, i.e. to depend upon the phase of the ongoing theta cycle (Csicsvari et al., 1999; Klausberger et al., 2003; O’Keefe and Recce, 1993). An important link between hippocampal theta oscillations and the probability of potentiation of synaptic transmission is provided by observations with regard to increases or decreases of synaptic efﬁcacy, which were caused Frontiers in Neuroscience July 2008 | Volume 2 | Issue 1 | 59