Bikbaev and Manahan-Vaughan ing animals, the amplitude of gamma oscillations is higher in the dentate gyrus than in other hip- pocampal regions expressing gamma oscillations (Bragin et al., 1995), and varies as a function of the theta cycle (Csicsvari et al., 2003; Penttonen et al., 1998). Hence, hippocampal theta and gamma oscil- lations comprise functionally-associated patterns of network activity that emerge as a consequence of the combination of intrinsic oscillatory properties of principal cells and interneurons, the rhythmic activation of which is driven by intra- and extrahip- pocampal connections (Bartos et al., 2007; Bragin et al., 1995; Csicsvari et al., 2003; Klausberger et al., 2003; Penttonen et al., 1998; Ylinen et al., 1995). The results of our study demonstrated that the successful potentiation of synaptic transmis- sion (LTP and STP groups) in freely moving rats, was preceded by a higher relative theta power in the period of 300 s after HFT, when compared with the pre-tetanisation period (Figure 1). In contrast, a gradual recovery of theta power to pre- tetanisation values, but not an increase, was found in the LTP-failure group. In other words, success- ful induction of potentiation (either in the form of STP or LTP), resulted in an enhancement of theta oscillatory activity after tetanisation, whereas no such enhancement occurred in rats that showed failure of LTP. The mean amplitude of a single theta cycle was also signiﬁcantly higher within the post- tetanisation period in the LTP and STP groups, when compared to the group of failed synaptic potentiation. Taken together, these data suggest that a transient increase of theta power is necessary for the proper handling of sensory input activation (mimicked here by HFT). This, in turn, may play a permissive role in LTP induction, and, in gen- eral, in the formation of memory trace(s) of newly acquired information. However, the differences between LTP, STP and failed potentiation groups were not limited in our study by theta oscillations. We found that within the ﬁrst 100 s after HFT the increase in theta power, and the mean amplitude of a single theta cycle, was accompanied by higher gamma power in the LTP group, and higher amplitude of gamma oscilla- tions within a single theta cycle in both the LTP and STP groups, when compared with cases of failed potentiation (Figure 1B). This indicates that not the increase of theta power per se, but a correlated increase of both theta and gamma power is impor- tant for enabling or mediating the plastic changes in synaptic weights. Furthermore, our results show that these enhancements of oscillatory activity should take place within a relatively short period of time for synaptic plasticity to occur. Taking into account that the amplitude of gamma oscillations in the dentate gyrus, but not in the CA3 or CA1 regions, is signiﬁcantly higher in the presence of theta oscillations than in non-theta states (Csicsvari et al., 2003), these ﬁndings suggest that the tetani- Figure 1 | Transient enhancement of theta and gamma power in the post-tetanisation period correlates with potentiation of synaptic transmission in the dentate gyrus of freely moving rats. (A) Examples of EEG epochs, which comprise 100 s long periods of tetanisation (200 Hz, 10 trains: ﬁrst train is marked by arrow) and following 300 s, recorded in rats that showed either LTP (left panel), STP (middle panel) or failure of potentiation (right panel), respectively. Asterisk denotes the period immediately after HFT, when the amplitude of network oscillations is higher in LTP and STP cases, in comparison with failed potentiation. Scale bar: 20 s. (B) Successful potentiation (LTP and STP) of synaptic transmission is associated with a prominent increase of the relative theta and gamma power par- ticularly in the period encompassing 100 s after tetanisation. The results were pooled for LTP, STP and failure groups and presented as Mean ± S.E.M. (modiﬁed from Bikbaev and Manahan-Vaughan, 2007). 58 | July 2008 | Volume 2 | Issue 1 www.frontiersin.org