transmitter systems acting via both ionotropic and metabotropic receptors. For the induction phase, several lines of evidence underlie the important role of the activation of N-methyl-D-aspartate (NMDA) receptors, at least for the hippocampal CA1 region and dentate gyrus (Abraham and Mason, 1988; Collingridge et al., 1983; Errington et al., 1987; Fox et al., 2006; Frey et al., 1996; Manahan-Vaughan, 1997; Niewoehner et al., 2007). For early LTP, activation of the metabotropic glutamate recep- tors (mGluRs), particularly group I mGluRs, plays a critical role (Aiba et al., 1994; Anwyl, 1999; Manahan-Vaughan, 1997; Manahan-Vaughan and Braunewell, 2005; Manahan-Vaughan and Reymann, 1996; Manahan-Vaughan et al., 1998; Richter-Levin et al., 1994). Late-LTP (maintenance phase) requires de novo protein synthesis (Frey et al., 1996; Krug et al., 1984; Otani and Abraham, 1989) and activation of several immediate-early genes and late-response genes (Abraham et al., 1991; Brakeman et al., 1997; Cole et al., 1989; Link et al., 1995; Messaoudi et al., 2007; Wisden et al., 1990; Yin et al., 2002). Additionally, various neu- romodulators can either increase or decrease the probability, magnitude and/or longevity of changes in synaptic transmission, hence affecting the out- comes of patterned synaptic activation (Granado et al., 2008; Kemp and Manahan-Vaughan, 2005, 2008; Kulla and Manahan-Vaughan, 2002; Lemon and Manahan-Vaughan, 2006; Li et al., 2003, 2007; Manahan-Vaughan and Kulla, 2003; Swanson-Park et al., 1999). Besides the different relative involvement of neurotransmitter systems, the induction and/or expression of LTP, as a reﬂection of central syn- apses to acquire, process and subsequently store new information, depends upon several other fac- tors, including environmental enrichment, vari- ous forms of stress, etc. (Artola et al., 2006; Avital et al., 2006; Bruel-Jungerman et al., 2005; Duffy et al., 2001; Foster et al., 2000; Kopp et al., 2006; Seidenbecher et al., 1997; van Praag et al., 1999; Xu et al., 1998), which couple the sensory input to a current behavioural state and provide the contex- tual frame for processing of sensory stimuli. Thus, taking into account the plethora of mechanisms underlying the phenomenon of LTP, it is not sur- prising that various factors and/or experimental manipulations can markedly affect its different phases. However, for most forms of LTP, and par- ticularly for electrically induced LTP, the state of the animal (synaptic and behavioural) during a short period of time, both during and immediately after tetanisation, appears decisive in determining the persistence of potentiation (Abraham, 2003). In our study (Bikbaev and Manahan-Vaughan, 2007), we applied high-frequency tetanisation Hippocampal oscillations are linked to LTP (HFT) in freely behaving adult rats and monitored ﬁeld excitatory synaptic potentials (fEPSPs) in par- allel with the intrahippocampal electroencephalo- gram over a period of 24 h. Despite the fact that an identical stimulation protocol was employed for all animals, substantial variation in the outcomes of tetanisation was found. In the ﬁrst group of ani- mals, a robust LTP occurred that was stable for at least 24 h (LTP group), in the second group short- term potentiation that endured for up to 3 h was induced (STP group), whereas in the third group a failure of potentiation occurred, i.e. no persistent increase in synaptic transmission was seen (failure group). Given this range of tetanisation results, we analysed them in the context of the changes in net- work activity that occurred during and within ﬁrst 300 s after tetanisation. TETANISATION-TRIGGERED FACILITATION OF THETA AND GAMMA ACTIVITY FAVOURS LTP Oscillatory activity is an integral part of functional neuronal networks, and signiﬁcant variability in patterns of oscillatory activity can be registered under certain conditions both in vivo and in vitro. Modiﬁcation of synaptic weights occurs very fast during the activated state of the hippocampus (Buzsáki, 1996), with entorhinal–hippocampal network oscillations at theta frequency playing a crucial role in this process (Kamondi et al., 1998). Particularly in the hippocampus, theta oscillations (theta rhythm, rhythmic slow activity) have been proposed to play a role in a wide variety of hip- pocampal functions (see for review: Buzsáki, 2005). In rodents, periods of prominent theta oscillations occur during exploratory behaviour and phases of rapid eye movement REM (sleep); conditions which are generally termed theta-associated behav- iour (Bland, 1986; Buzsáki, 2002, 2005; Vanderwolf, 1969). These behavioural states, and particu- larly exploratory behaviour, are characterised by neocortico-hippocampal transfer of new spatial information when acquisition and/or encoding of sensory information should be facilitated, whereas transfer of the information from the hippocam- pus to neocortex is associated with other patterns, such as sharp waves and ripples (Bragin et al., 1995; Buzsáki, 1996; Buzsáki et al., 2002; Chrobak et al., 2000). Gamma oscillatory activity repre- sents another pattern of network activity, which is characteristic for cortical locations, and is pro- posed to be involved in various cognitive functions (Buzsáki and Chrobak, 1995; Fell et al., 2001; Fries et al., 2001; Gray and Singer, 1989; Hermann et al., 2004; Hopﬁeld, 1995; Lisman, 1999; Lisman and Idiart, 1995; Montgomery and Buzsáki, 2007; Singer, 1993). In the hippocampus of freely mov- Frontiers in Neuroscience July 2008 | Volume 2 | Issue 1 | 57