Visual Cortex Plasticity Evokes Excitatory Alterations in the Hippocampus

The integration of episodic sequences in the hippocampus is believed to occur during theta rhythm episodes, when cortico-hippocampal dialog results in reconfiguration of neuronal assemblies. As the visual cortex (VC) is a major source of sensory information to the hippocampus, information processing in the cortex may affect hippocampal network oscillations, facilitating the induction of synaptic modifications. We investigated to what degree the field activity in the primary VC, elicited by sensory or electrical stimulation, correlates with hippocampal oscillatory and synaptic responsiveness, in freely behaving adult rats. We found that the spectral power of theta rhythm (4–10 Hz) in the dentate gyrus (DG), increases in parallel with high-frequency oscillations in layer 2/3 of the VC and that this correlation depends on the degree of exploratory activity. When we mimic robust thalamocortical activity by theta-burst application to dorsal lateral geniculate nucleus, a hippocampal theta increase occurs, followed by a persistent potentiation of the DG granule field population spike. Furthermore, the potentiation of DG neuronal excitability tightly correlates with the concurrently occurring VC plasticity. The concurrent enhancement of VC and DG activity is also combined with a highly negative synchronization between hippocampal and cortical low-frequency oscillations. Exploration of familiar environment decreases the degree of this synchrony. Our data propose that novel visual information can induce high-power fluctuations in intrinsic excitability for both VC and hippocampus, potent enough to induce experience-dependent modulation of cortico-hippocampal connections. This interaction may comprise one of the endogenous triggers for long-term synaptic plasticity in the hippocampus.


INTRODUCTION
The adult visual cortex (VC) possesses the remarkable ability to change its neuronal connectivity through experience (Sur et al., 2002). This remodeling of synapses involved in cortical activity is mediated by bidirectional changes in synaptic strength (Heynen et al., 2000;Liu et al., 2004) and these changes are related to structural plasticity through the loss and gain of synapses (Majewska et al., 2006). Experimental evidence derived from in vitro studies suggests that synaptic plasticity may be involved in information processing in the adult sensory cortex (Bear, 2003). Plasticity processes in the adult VC are also proposed to underlie different forms of visual perceptual learning (Hegde, 2006;Karmarkar and Dan, 2006). Recent data indicate that the adult VC serves as a synaptic network, where the direction of change in synaptic strength depends on the immediate visual experience (Tsanov and Manahan-Vaughan, 2007a,b). Thus, the VC dynamically modifi es its ability to process visual stimuli, and therefore may affect the processing of sensory information in related brain areas (Abbott et al., 1997). Importantly, synaptic plasticity occurring in the adult primary VC may infl uence the ability of the hippocampus to integrate spatial episodes in time (Ji and Wilson, 2007). This interaction may comprise a key element in cortico-hippocampal transfer of information and the subsequent formation of the spatial memories.
Visual cortex plasticity evokes excitatory alterations in the hippocampus believed to be critical for temporal coding/ decoding of active neuronal ensembles and the modifi cation of synaptic weights (Buzsáki, 2002). A major goal in our study therefore was to investigate the infl uence of behavioural and stimulation protocols of visual system on hippocampal theta oscillations. During theta behavioural states there is a prominent enhancement of another fi eld oscillation -at gamma frequency (40-100 Hz). A recent study has suggested that temporal coordination of neocortical gamma oscillations by hippocampal theta is a mechanism by which information contained in neocortical assemblies can be synchronously transferred to the associative networks of the hippocampus (Sirota et al., 2008). Thus, we focused our attention on the gamma rhythm particularly in the VC. High-frequency oscillations in neocortical structures are known to provide temporal windows that bind coherently cooperating neuronal assemblies for the representation, processing, sensory binding and storage of information (Singer, 1993;Buzsáki et al., 1994;Gray, 1994;Traub et al., 1999;Engel et al., 2001). Here we examined the concurrent changes in hippocampal theta-and cortical gamma spectral powers and compared their correlation during the presentation of novel and non-novel stimuli.
The entorhinal cortex provides the major cortical input to the hippocampus, with entorhinal-dentate gyrus interactions serving to fi lter the entry of sensory information and selectively gate learning-induced modifi cation of synaptic weights in the hippocampal formation (Moser and Moser, 1998;Lisman, 1999;Steffenach et al., 2005). As the dentate gyrus (DG) comprises the fi rst component of the polysynaptic pathway in the hippocampal formation, we investigated the effects of VC activity on the DG. The main hypothesis of the present study is that visually-driven novel exploration can induce synchronous alterations of cortical and hippocampal fi eld activity, which promote DG synaptic plasticity. Our data reveal that synaptic activity in the primary VC of adult, freely behaving rats, generates theta rhythm augmentation and subsequent fl uctuation of hippocampal excitability.

MATERIALS AND METHODS
All experimental and surgical procedures were carried out in accordance with the guidelines of the German Animal Protection Law and were approved by the North Rhine Westphalia State Authority.

SURGICAL PREPARATION
Under sodium pentobarbitone anesthesia (Nembutal; 40 mg/kg, i.p.), 7-to 9-week-old male Long Evans rats (Charles River Breeding Laboratories, Sulzfeld, Germany) underwent implantation of a monopolar recording and a bipolar stimulating electrode (made from 0.1 mm diameter Tefl on-coated stainless steel wire) in the visual system and also in the ipsilateral hippocampus.
For the recording electrode in the visual system, a drill hole was made, (1-mm in diameter), 7.1-7.3 mm posterior to bregma and 3.4-3.6 mm lateral to the midline, corresponding to the primary VC in the rat (Zilles, 1985). A second drill hole (3.8 mm posterior to bregma, 3.3-3.4 mm lateral to midline) was made for the bipolar stimulating electrode in a location that corresponds to dorsal lateral geniculate nucleus (dLGN). The dura was pierced through both holes using a needle. The recording electrode was lowered 200-250 µm from the dural surface to reach the supragranular layer 2/3. A stimulating electrode was positioned in the dLGN ipsilateral to the hemisphere from which VC recordings were obtained. The depth was 3.7-4.7 mm from the dural surface, corresponding with the position of the anterior pole where optic radiation fi bers are bundled in close proximity (Hughes, 1977;Molnar et al., 1998). Final positions of the stimulating and recording electrodes were then determined by maximizing the amplitude of the fi eld potential (FP) recorded in the VC in response to electrical stimulation of dLGN. Monopolar recordings from primary VC were made relative to ground and reference screws inserted into the contralateral parietal and frontal bones. For recordings from the hippocampus, a recording electrode was placed in the granule cell layer of the DG and a stimulating electrode was placed in the medial perforant path. A drill hole was made (1.0 mm diameter) for the recording electrode (3.1 mm posterior to bregma, 1.9 mm lateral to the midline) and a second drill hole (1.5 mm diameter, 6.9 mm posterior to bregma and 4.1 mm lateral to the midline) for the stimulating electrode. Electrodes were lowered until a characteristic positivegoing fi eld excitatory postsynaptic potential (fEPSP) upon which a negative going population spike (PS) was superimposed, was identifi ed and optimised.
The thalamic and hippocampal coordinates were chosen for the most distanced confi guration in order to prevent a current leak from the dLGN electrode to the perforant path or the DG. The lack of direct current stimulation of the DG via the dLGN electrode was evident with the absence of hippocampal fEPSP after thalamic test-pulse stimualtion. Once verifi cation of the location of the electrodes was complete, the entire assembly was sealed and fi xed to the skull with dental acrylic (Paladur; Heraeus Kulzer, GmbH, Wehrheim, Germany). The animals were allowed 10 days to recover from surgery before experiments were conducted. Throughout the experiments the animals could move freely within the recording chamber (40 cm × 40 cm × 40 cm), as the implanted electrodes were connected by a fl exible cable and swivel connector to a stimulation unit and amplifi er. Apart from the insertion of the connector cable at the start of the experiment, disturbance of the animals was kept to an absolute minimum. Throughout the experiments the electroencephalogram (EEG) of each animal was continuously monitored.

MEASUREMENT OF EVOKED POTENTIALS
The fEPSP slope was used as a measure of excitatory synaptic transmission in the VC and DG. To obtain these measurements, an evoked response was generated in the dLGN and medial perforant pathway (MPP), respectively, by stimulating at low frequency (0.025 Hz) with single biphasic square wave pulses of 0.1 ms duration per half wave, generated by a constant current isolation unit. For each time-point measured during the experiments, fi ve records of evoked responses were averaged. For hippocampal recordings both fEPSP slope and PS amplitude were monitored. The characteristic fi eld response of the DG granule cells to perforant path stimulation consisted of a positive-going fEPSP with a superimposed negative going PS. The fEPSP slope and PS amplitude were measured for each response. The slope function of the FP was taken as the slope between the fi rst minimum and the fi rst maximum of the potential. This value refl ects the granule cell depolarization. The PS amplitude was measured as the absolute value between the fi rst maximum and the following second minimum of the evoked potential in mV. This amplitude refl ects the number of granule cells fi ring, as well as the synchrony with which they fi re. For VC recordings, the slope and the amplitude of the stimulus-induced negative fEPSP, measured from the peak of the fi rst positive defl ection of the evoked potential to the peak of the following negative potential. By means of input-output curve determination, the maximum fEPSP was found, and during experiments all potentials used as baseline criteria were evoked at a stimulus intensity that produced 40% of this maximum (100-400 µA).
The animals were kept in a 12:12 h light-dark cycle of low (0.1-10 lux) and high (350-400 lux) illuminance conditions. Circadian time was based on Zeitgeber time (ZT), with ZT 0 denoting light on, and ZT 12 denoting light off. Twenty-four hours later, measurements were recorded at the same circadian time and conditions. Theta-burst stimulation (TBS) consisted of 10 bursts at 5 Hz, with each burst containing 5 pulses at 100 Hz, given four times with a 10-s interval between each train of 10 bursts. In all cases, the stimulus amplitude was the same as that used for recordings.

RECORDING ENVIRONMENT AND NOVEL OBJECTS
The recording chamber consisted of a grey Perspex box (40 cm × 40 cm × 40 cm) with a translucent red perspex door. Novel objects consisted of large objects (3 cm × 10 cm × 10 cm) with different colours (black, white, green) and shapes (cylindric or square). Rearing was considered as placement of animal's front limbs onto the surface of the object. Illuminance of objects and environments was assessed using a Luxmeter (MS-1300) with a sensitivity range of 0-50000 lux. The luxmeter was placed on the fl oor of the recording chamber and pointed at the light source. The sensor unit possessed a photosensitive surface, which recorded the total amount of falling light.

DATA ANALYSIS OF ELECTROPHYSIOLOGICAL DATA
The baseline fEPSP data for VC and DG were obtained by averaging fi ve sweeps at 40-s intervals, every 5 min over a period of 30 min after stimulation of the dLGN and of MPP, respectively. The LGN test-pulse-evoked EPSP in VC preceded MPP test-pulse-evoked EPSP in the DG by 10 s except for the application of TBS where the burst of pulses was delivered only to dLGN. Preliminary tests revealed no infl uence of the pairing interval (2, 4, 6, 8 and 10 s) and sequence on the amplitude of the evoked fEPSP. Electrophysiological data were then expressed as mean percentage of baseline fEPSP reading ± standard error of the mean (SEM). Statistical signifi cance was estimated by using ANOVA and post hoc Student's t-and LSDtests and by post hoc Student's t-tests. The probability level interpreted as statistically signifi cant was p < 0.05. For the correlation analyses we used Pearson's coeffi cient of comparison.

DATA ANALYSIS OF NETWORK ACTIVITY
An intracortical EEG was obtained by means of recordings obtained from the supragranular layer 2/3 of primary VC and from the granule cell layer in the DG. EEG was sampled at 0.5 kHz and stored on a hard disc for further off-line analysis. In order to evaluate delta (1-3.5 Hz), theta (4-10 Hz), alpha (10-13 Hz), beta1 (13.5-18 Hz), beta2 (18.5-30 Hz) and gamma (30-100 Hz) oscillatory activity during the course of experiment, 4-s long epochs, 1 s after each test-pulse were selected. Fourier analysis of artefact-free epochs was performed with Hanning window function using "Spike2" software (Cambridge Electronic Design). The absolute values of spectral power for each individual animal were transformed into relative ones (with mean value for baseline preexploration period -the fi rst six time points taken as 100%) that were used further for statistics. For each time point, the results of Fourier analysis of fi ve epochs were averaged. Theta activity was measured by the ratio between the relative values of theta and delta spectral powers (T-ratio). The statistical treatment and analysis of data included the calculation of descriptive statistics (mean, SEM) and analysis of variance. For the correlation analyses we used Pearson's coeffi cient of comparison. Power spectrograms are average of four 10 s-theta periods, which were evaluated and averaged per each animal. The statistical comparison between groups was based on paired student's t-test. EEG signal frequency analysis was done using MATLAB's Signal Processing Toolbox (MATLAB, Natick, MA, USA) where the power was calculated by extracting the short-time Fourier transform from a signal (hanning window of 2 s, overlap of 1 s, sampling frequency 100 Hz) and interpolated into a continuous power spectrum aligned to the haemodynamic traces. Information was displayed as the magnitude of the time-dependent Fourier transform versus time in a color gradient graph.

DATA ANALYSIS OF CROSS-CORRELOGRAMS
Electroencephalogram and FP waveforms were analyzed using the Spike2-software package and Matlab. Phase shifts in crosscorrelograms were calculated with respect to the deviation of the fi rst peak from zero (Seidenbecher et al., 2003). Twelve 10 s-theta periods each were evaluated and averaged for each group. The theta epochs for the novelty exploration-and re-exposure groups were taken immediately after the novel objects presentation. The theta epochs for the REM period-, non-exploration theta groups were taken from the highest theta spectral power epoch of each animal and each behavioral state. The REM epochs (non-aroused theta periods) were defi ned as theta epochs preceded and followed by large irregular activity, which is a characteristic feature of slow-wave sleep (SWS). Theta epochs unrelated to sleep (not linked to SWS) or object exploration were defi ned as baseline theta (aroused theta periods). Statistical analysis of electrophysiological and behavioral data was done with Student's unpaired T-test, keeping the criterion for signifi cance at p < 0.05.
Concurrently, to measure the stability of the theta rhythm, the peak-to-trough correlation coeffi cient was calculated from the cross-correlogram by subtracting the minimum negative value of the correlation coeffi cient from the maximum of the fi rst positive peak (Madriaga et al., 2004) over the fi rst 3 lags; each lag between 100 and 150 ms (theta range). Statistical comparisons between experimental conditions were made using a t-test or one-way ANOVA if the data were normally distributed and had equal variance.
An additional approach in analyzing the phase coherence in cross-correlograms was the estimation of the correlation coefficient between two signals recorded simultaneously (ventral root recordings), calculated at time t = 0 (Pearlstein et al., 2005). The mean correlation coeffi cient value is the average of all correlation coeffi cients observed in each experiment for a given experimental condition. Positive and negative values of the mean correlation coeffi cient were representative of synchronous and out-of-phase signals, respectively (Pearlstein et al., 2005).
In cross-correlation analysis, waveforms that alternate are out-of-phase from each other and will have a Negative relationship, whereas waveforms that are synchronous will be in-phase and have a Positive relationship. The strength of the relationship between two waveforms will be perfect at ±1 and will diminish to a minimum when approaching 0. A high degree of symmetry or stability along the x-axis indicates a stable relationship between the two waveforms. However, as the relationship between waveforms varies, therefore creating decreasing correlation values beyond zero lag, this would indicate less stability in the relationship.

POSTMORTEM VERIFICATION OF ELECTRODE SITE
At the end of the study, brains were removed for histological verifi cation of electrode localization. Brain sections (16 µm) were embedded in paraffi n, stained according to the Nissl method using 1% toluidine blue, and then examined using a light microscope. Brains in which an incorrect electrode localization was found were discarded from the study.

CORTICAL AND HIPPOCAMPAL OSCILLATIONS ARE CONCURRENTLY INTERLINKED DURING NOVELTY EXPLORATION
To examine naturally evoked frequency oscillations in the primary VC and DG in adult freely moving rats, animals underwent implantation of monopolar recording electrodes at the surface of the VC and in the granule cell layer of ipsilateral DG ( Figure 1A). The fi rst goal of our study was to clarify which neuronal oscillations concurrently change during active exploration in both regions. An intracortical EEG was recorded in parallel from the VC and DG of naive rats following placement in a recording box with low light conditions (0.1-10 lux). After the dark-light diurnal shift the baseline recording continued in high illuminance (350-400 lux) for 180 min (Figures 1B,D). Subsequently, novel objects were placed in the recording boxes and during their exploration, an increase in cortical gamma (30-100 Hz) spectral power was evident ( Figure 1B, black symbols). In order to distinguish the magnitude of cortical high-frequency oscillations associated with the acquisition of new sensory stimuli (Singer, 1993;Gray, 1994;Fries et al., 2001), the same novel objects were presented in the same confi guration 5 days later (re-exposure period) under the same recording protocol (Figure 1B, white symbols). The animals' behaviour was characterised by decreased exploratory activity refl ected by a lower amount of rearing activity per object (t-test, n = 4, p < 0.05; Figure 1C). Gamma spectral power during the re-exploration also showed signifi cantly diminished values (ANOVA, F = 3.27, p < 0.01, n = 4), compared to the novelty exposure experiment ( Figure 1B). Oscillatory activity in the ipsilateral hippocampus shared similar patterns during the novel exploration and the re-exposure. Concordantly, we observed increased values of theta (4-10 Hz) and gamma hippocampal rhythms (30-100 Hz). We found an increase, during the novelty exposure to the novel objects, in DG theta-ratio ( Figure 1D, black symbols), in parallel with an augmentation of VC gamma oscillations. Theta-ratio represents the ratio between the relative values of theta and delta spectral powers (Harris et al., 2002). The re-exposure to the same objects resulted in no increase of theta-ratio compared to the preceding novelty exposure (ANOVA, F = 2.59, p < 0.05, n = 4, white symbols). As the link between locomotion and hippocampal oscillations is well established (Bland, 1986;Sainsbury et al., 1987) the focus of our experiments was to investigate the link between hippocampal and cortical rhythms. In addition to the theta and gamma bands, the other measured frequencies: alpha (10-13 Hz), beta1 (13.5-18 Hz) and beta2 (18.5-30 Hz) also revealed a tendency towards a higher spectral power during the novel exposure compared to the re-exposure, but only cortical alpha and beta1 underwent a signifi cant increase (ANOVA, F = 3.24, p < 0.05, for alpha and F = 2.03, p < 0.05, n = 4, for beta1, data not shown).
Our EEG analyses demonstrated a signifi cant correlation between gamma spectral power in both VC and DG for the novelty exposure (Pearson, n = 4, p < 0.05, r = 0.108, data not shown) as well as for the re-exposure (Pearson, n = 4, p < 0.05, r = 0.109, data not shown). Most importantly, the correlation between VC gamma and DG theta-ratio was signifi cant only during the presentation of novel objects (Pearson, n = 4, p < 0.001, r = 0.2266; Figure 2A) but not during the subsequent re-exposure (Pearson, n = 4, p > 0.05, r = 0.0023; Figure 2B). This experiment proposes a direct functional relationship between high-frequency oscillatory activity in primary VC, and DG theta rhythm.
To demonstrate that the oscillatory changes of VC and hippocampus are predominantly interlinked during the processing of novel visual stimuli, we compared the synchronization in both regions during the novelty exploration and later during object re-exposure (Figure 2). The EEG recordings in VC ( Figure 1E) and hippocampus ( Figure 1F) suggest a negative relationship between the peaks and troughs of the slow spectral ranges in both regions ( Figure 3A). Cross-correlation analyses confi rmed a highly negative relationship between hippocampal and cortical oscillations ( Figure 3B). Re-exposure to the objects resulted in a non-signifi cant increase of the mean correlation coeffi cient calculated at time t = 0 and concurrently in a signifi cant decrease of peak-to-trough correlation coeffi cient (t-test, n = 4, p < 0.05; Figure 3B). Therefore, during the re-exposure, the stability of the synchronization in theta range is diminished and this observation is confi rmed by the drop of the absolute theta spectral powers in VC (t-test, n = 4, p < 0.05; Figure 3C) and DG (t-test, n = 4, p < 0.01; Figure 3D). The reduced theta/delta ratio in DG reveals a decline in exploration-related hippocampal oscillatory activity (t-test, n = 4, p < 0.05; Figure 3E). This decline can be visualized in a power spectrogram when compared to the novel exploration period (Figures 4A,C). The changes in theta range are less apparent for VC (Figures 4B,D). As a control measurement of the theta parameters in both regions, we analyzed EEG epochs during non-novelty-related theta periods in aroused-(baseline theta; Figure 5) and non-aroused state (REM sleep; Figure 6). The waveform synchronization during aroused behavioral state of freely behaving animals habituated with the recording box ( Figure 5A) has the same cross-correlation values as the novelty explorationinduced oscillations (Figure 5B). The difference in both groups is demonstrated by signifi cant changes in the amplitude of the low-spectrum oscillations recorded in VC (t-test, n = 4, p < 0.001; Figure 5C) and DG (t-test, n = 4, p < 0.001; Figure 5D). In addition a theta shift towards a higher frequency -9 Hz is evident in  4). During re-exposure to the objects a reduction in visual cortex gamma activity was observed (n = 4). To exclude the interference of diurnal fl uctuation of basal synaptic transmission in VC on the novelty protocol, baseline recordings were taken during the circadian illuminance shift. Higher levels of gamma frequencies were observed in VC during the dark phase (indicated with black rectangle) and in the beginning of the light phase (white rectangle) of the night:dark cycle; standard errors of the mean values were used for the error bars; *p < 0.05. (C) Behavioral arousal in response to novel object exploration (white) and re-exposure to the same objects 5 days later (grey) measured by number of rearings per object. (D) Comparison of theta-ratio in dentate gyrus during exploration of novel objects (n = 4) and the re-exposure (n = 4) to the objects in the same confi guration. The decrease of cortical gamma activity is paralleled by a drop in hippocampal theta-ratio. A transient increase in theta activity in the dentate gyrus also occurred as a consequence of the illuminance change; *p < 0.05. (E) Original traces of fi eld potential recordings in the VC during the novelty exploration-induced theta period (upper trace) and reexposure (lower trace). (F) Original traces of fi eld potential recordings in the DG during the novelty exploration-induced theta period (upper trace) and reexposure (lower trace).
the baseline theta group compared to the 8 Hz seen in the novelty theta group. Importantly the DG theta-ratio in both groups does not differ (t-test, n = 4, p < 0.001; Figure 5E), showing that both behavioural states share proportional changes in delta and theta frequencies. In order to clarify the specifi city of cortical and hippocampal oscillatory parameters during novelty exploration we also analyzed the theta periods of non-aroused REM state ( Figure 6A). Interestingly the profi le of the REM theta period is characterized with opposite changes in comparison to baseline theta group. The altered synchronization is demonstrated with a phase shift at time 0 ( Figure 6B). Although the peak-to-trough correlation coeffi cient is higher in REM theta periods (t-test, n = 4, p < 0.05; Figure 6B) the phase coherence is shifted and the synchronization between VC and DG is not locked in time. The mean correlation coeffi cient calculated at time t = 0 is higher for novelty group (t-test, n = 4, p < 0.05; Figure 6B). At the same time theta spectral value in VC is the same as for the novelty theta periods ( Figure 6C) and in DG is even higher (t-test, n = 4, p < 0.05; Figure 6D), paralleled by increased theta-ratio (t-test, n = 4, p < 0.05; Figure 6E).Altogether these data reveal that noveltyinduced theta periods underlie a unique combination of waveform synchronization, oscillatory spectral powers and theta-ratio, that are not found in either in the baseline-theta, or REM theta periods that occur during re-exposure. Furthermore, only the novelty theta group is characterized by simultaneously high values of EEG synchronization in VC and DG, high theta spectral powers in both regions, and high theta-ratio.

HIGH-FREQUENCY STIMULATION OF THE LATERAL GENICULATE NUCLEUS EVOKES A LONG-TERM ENHANCEMENT OF GRANULE CELL EXCITABILITY IN THE HIPPOCAMPUS
The next target of our investigation was the hippocampal synaptic response to novel stimuli and the ability to amplify the effect of neocortical input on hippocampal plasticity through a thalamocortical stimulation protocol. First, we examined the hippocampal fEPSP in relation to novelty exploration as a baseline comparison for the stimulation-induced plasticity alterations. Many confl icting reports exist that describe that exploration increases (Stanton and Sarvey, 1987;Kithigina et al., 1997;Davis et al., 2004) or decreases (Sharp et al., 1989;Green et al., 1990) the PS amplitude, and which discuss whether exploration also affects the fEPSP. These differences are probably related to variations in the behavioral protocols used, the time-scale of recordings and the fEPSP measurement parameters. Therefore, we fi rst examined the changes in the DG fi eld response to entorhinal stimulation during novelty exploration under our recording conditions. Investigations of the granule cell fi eld response ( Figure 7A) revealed a novelty-related fl uctuation of the fEPSP. The introduction of novel objects was characterized by a shortterm depression of the PS amplitude in the DG (t-test, p < 0.05, n = 6), that lasted for 20 min and was followed by a rebound tendency towards short-lasting potentiation. The PS fl uctuation in our experiments occurred in the absence of a comparable change in the fEPSP slope (Figure 7C), suggesting an occurrence of EPSP-spike (E-S) depression/potentiation. The depression period was paralleled by a transient increase of the DG T-ratio (ANOVA, F = 3.51, p < 0.01, n = 6; Figure 7A).
In order to amplify the impact neocortical high-frequency oscillations on hippocampal plasticity we activated a larger portion of VC neurons (than that elicited by visual exploration) through use of TBS of the dLGN. TBS of the dLGN induces LTP in the rat VC under anaesthesia (Heynen and Bear, 2001) and in awake animals (Tsanov Time  Medial perforant pathway-evoked fi eld responses were induced in the DG from freely behaving rats in parallel with ipsilateral thalamocortical recordings (Figure 1A). Baseline measurements, for 30 min, in both the VC and DG were followed by stimulation of the dLGN (Figure 7B). We found that TBS given to the dLGN results in a longlasting augmentation of DG population spike amplitude (ANOVA, F = 2.11, p < 0.05, n = 5; Figure 7B), preceded by a short-term period with a tendency towards depression. The fEPSP slope was unaffected, suggesting that E-S potentiation occurred (Figure 7C). The onset of the E-S potentiation appeared with about 90 min of delay after TBS to the dLGN. As the theta rhythm was already associated with a suppression of dentate excitability (Figure 7A), we analysed the T-ratio changes throughout the experiment. Notably, we found a signifi cant increase of theta oscillations in the DG induced by TBS stimulation of visual pathway (ANOVA, F = 3.12, p < 0.05, n = 5; Figure 7B). Furthermore, the theta augmentation temporally coincided with the delay in onset of E-S potentiation, confi rming the role of theta oscillations in the regulation of DG excitability.
Theta-burst stimulation to the dLGN also evokes slow-onset potentiation of the fEPSP amplitude in the VC (Tsanov and Manahan-Vaughan, 2007a). We compared the time-course of the potentiation in both the VC and DG during the parallel recordings. The augmentation of the cortical fEPSP response preceded the DG E-S potentiation (Figure 8A; n = 5). Furthermore, a similar magnitude of long-lasting plasticity occurred in both structures. In order to confi rm that these parallel potentiations were not independently occurring events, we compared each fEPSP response in the VC with the subsequent DG population spike amplitude ( Figure 8B) in every recording from all animals. The VC fEPSP and the subsequent hippocampal response correlated positively (Figure 8C; Pearson, n = 5, p < 0.0001, r = 0.3880), indicating that the degree of cortical plasticity was refl ected in the degree of changed DG excitability. The consequence of this parallel plasticity is prolonged DG excitability, and hence increased effectiveness of spatial information processing and encoding. Importantly, signifi cant correlation was also evident between VC gamma oscillations and DG theta-ratio ( Figure 8D). (Pearson, n = 4, p < 0.0001, r = 0.2812) Note the similarity of the TBS correlation profi le ( Figure 8D) and the novelty correlation (Figure 2A), but not the re-exposure correlation ( Figure 2B). This fi nding suggests a link between the sensory-evoked and electricallyinduced oscillatory activity in VC and hippocampus.

DISCUSSION
Despite signifi cant progress in understanding the cellular basis of cortical and hippocampal synaptic plasticity, the precise interaction between experience-dependent synaptic changes in these brain regions remains unknown. In our study, parallel fi eld recordings from the VC and hippocampus of freely behaving adult rats revealed that the interaction between cortical gamma oscillations and dentate theta rhythm is highly expressed during active visual exploration of novel objects. Theta-burst stimulation of the primary VC drove a long-term enhancement of granule cell excitability in the hippocampus that was proportional to the concurrent potentiation of the cortical response.
The entorhinal cortex provides the major cortical input to the hippocampus, and both structures have been implicated in memory processes (Moser and Moser, 1998). Entorhinal-dentate gyrus Short-term depression occurs in the course of novelty exploration (signifi ed by the grey rectangle), and is followed by a subtle potentiation of the population spike (PS) in the dentate gyrus (DG) (n = 6). The immediate response of theta oscillations, expressed by theta-ratio (T-ratio), occurs in parallel with the suppression of dentate gyrus excitability. (B) Theta-burst stimulation of lateral geniculate nucleus (LGN) results in a lateonset enhancement of the population spike response 90 min after the highfrequency protocol (n = 5). This potentiation continued until the end of the recording period. Immediately after LGN stimulation an increase in the thetaratio was observed in the dentate gyrus granule cell layer. (C) Analog traces represent an average of 5 fi eld potentials evoked at the points marked in the fi gures. The red traces reveal the fEPSP profi le of recorded potentials during the baseline period (marked with 1 and 4, respectively). Note the increase of PS paralleled by no change of the fEPSP slope. Horizontal bar: 5 ms, vertical bar: 5 mV; *p < 0.05 versus baseline 1.
and Manahan-Vaughan, 2007a). We thus investigated whether the induction of synaptic plasticity in the adult primary VC has any long-term effects on DG synaptic responsiveness.
interactions are believed to fi lter the entry of sensory information and selectively gate learning-induced modifi cation of synaptic weights in the hippocampal formation (Lisman, 1999). As the DG is the fi rst post of the trisynaptic pathway within the hippocampal formation, our attention was focused on entorhinal-dentate gyrus fi eld alterations in relation to VC activity. Cortico-hippocampal dialogue has been revealed during sleep with strong temporal correlations between primary VC spindles and hippocampal ripples (Ji and Wilson, 2007). Our data demonstrate for the fi rst time that visually-mediated learning triggers EEG synchronization in VC and DG, with a negative phase-shift correlation. In addition, increased theta spectral powers in both regions and high hippocampal thetaratio determined the specifi c characteristics of novelty-induced theta periods. Such combinations of waveform synchronization, oscillatory spectral powers and theta-ratio can not be evoked in other theta periods. As for the baseline periods, the oscillations of theta range have lower amplitude and a different frequency peak when compared to the novelty-induced theta periods. However, the REM periods are characterized by a high theta spectral power, but with a shifted out-of-phase synchronization between VC and DG. The re-exposure periods differ from the novelty periods with a signifi cantly lower degree of synchronization between VC and DG, lower theta amplitude in both regions and lower DG theta-ratio.
In the sensory cortices, behavioural arousal is characterised by a prominent enhancement of high-frequency oscillations in the gamma frequency range (40-100 Hz). High-frequency oscillations in cortical structures are believed to provide temporal windows that bind coherently cooperating neuronal assemblies for the representation, processing, storage, and retrieval of information (Buzsáki et al., 1994;Traub et al., 1999;Engel et al., 2001). Of the large family of brain oscillatory patterns, gamma oscillations have received special attention because of their postulated role in sensory binding (Gray, 1994;Singer and Gray, 1995), memory (Fell et al., 2001), attentional selection , and "conscious" experience (Varela et al., 2001). The gamma cycle is proposed to serve as a fundamental computational mechanism that enables fast processing and fl exible routing of activity, supporting fast selection and binding of distributed responses (Fries et al., 2007). A tight relationship exists between changes in gamma oscillatory activity and the direction of change in synaptic strength elicited in the VC (Tsanov and Manahan-Vaughan, 2007b). High-frequency oscillations underlie the processing along the visual pathway (Martinez-Conde et al., 2002) and at the same time are related to synaptic plasticity (Singer, 1993;Steriade, 1999;Wespatat et al., 2004). As the hippocampus processes episodic memory, and receives a large input from the VC for this purpose, a main focus of this study included the infl uence of cortical high-frequency oscillations on the hippocampal fi eld alterations. Our data support the hypothesis that gamma activity in the VC correlates with high-and low-frequency oscillations in the DG. Most importantly however, a signifi cant correlation between VC gamma activity and DG theta activity occurs only in the time-course of novel object exploration. In line with our results gamma oscillations in somatosensory cortex were recently shown to be phase locked to hippocampal theta oscillations (Sirota et al., 2008), thus suggesting that theta oscillation entrainment provides a mechanism by which activity in neocortical and hippocampal networks can be temporally coordinated. As a corollary of this, high-frequency stimulation of the dLGN that elicits LTP in the VC (Tsanov and Manahan-Vaughan, 2007a) generated a potentiation of gamma activity and an equivalent potentiation of DG theta activity. Furthermore, TBS to the dLGN resulted in a concurrent increase of cortical gamma and dentate theta in a similar manner to the novelty-evoked interregional oscillatory correlation. We propose that high-frequency oscillations in VC lead to alterations of excitability in the hippocampus, which in turn can facilitate the formation of visually-related synaptic memories in the hippocampus. This suggests that VC plasticity lowers the threshold for induction of hippocampal synaptic plasticity and may serve to prime the hippocampus for processing of visual sensory information in a spatial context. Our fi nding is particularly pertinent in light of reports that novel spatial object exploration elicits synaptic plasticity in the DG and other hippocampal subregions Manahan-Vaughan, 2007, 2008). Non-visual forms of sensory stimuli are also perceived during novel exploration and their impact on hippocampal plasticity (Mouly and Gervais, 2002;Truchet et al., 2002) confi rms the model of cortico-hippocampal dialogue in episodic memory formation. Associative olfactory tasks are known to involve a large subpopulation of synapses in the DG during the early stages of the learning process. The early increase of the monosynaptic response in the DG is observed immediately after the fi rst learning whereas the polysynaptic potential increases substantially when rats start to discriminate the learning cues (Mouly and Gervais, 2002;Truchet et al., 2002). Synaptic plasticity constitutes a cellular mechanism which is thought to underlie information storage in the brain and represents a use-dependent long-lasting increase or decrease in synaptic strength (Barnes, 1995;Bear, 2003). Sensory experience modifi es synapses, not only in the hippocampus, but also in the sensory cortex, and particularly in the primary VC (Bear, 2003;Tsanov and Manahan-Vaughan, 2007a,b). The adult VC is not simply a passive receiver of sensory inputs, but can dynamically amplify the processing of thalamic signals (Sur et al., 2002;Miyamoto and Hensch, 2003). Furthermore, plastic and oscillatory alterations are not simply a passive response of the VC to the light conditions, but crucially depend on visual activity of the animal: the levels of exploratory activity of animals to novel objects under low illuminance (dark phase) is the same for high-contrast and low contrast objects, but the gamma oscillations are signifi cantly higher for the group exposed to the high-contrast object (Tsanov and Manahan-Vaughan, 2007b). Therefore, non-visually guided exploratory activity is insuffi cient to produce the same amount of augmentation in the VC fi eld response. A central issue of our study was to establish whether the experience-dependent shift in neuronal responses in the visual cortical network is able to infl uence subsequent (e.g. downstream) stages of information processing. We demonstrate that long-term synaptic plasticity in VC correlates with an increased dentate PS and this increase is preceded by hippocampal theta augmentation. Our data are in agreement with recent fi ndings that confi rm the proposed link between theta rhythm and hippocampal plasticity (Bikbaev and Manahan-Vaughan, 2008;Tsanov and Manahan-Vaughan, 2009). We acknowledge that brain temperature could interfere with shortterm fEPSP fl uctuation in the DG during active exploration (Moser et al., 1994;Andersen and Moser, 1995), however, the arena in which exploration took place was (for reasons of its size) not conducive to large physical exertions that could change brain temperature. Theta-burst stimulation to the dLGN resulted in LTP of the visual cortical fEPSP, which was followed by a long-term, slow-onset E-S potentiation of entorhino-dentate transmission. Furthermore, the increased DG population response was closely correlated with the preceding cortical fEPSP augmentation. We observed a striking similarity between hippocampal fi eld changes during novelty exploration and those after TBS of dLGN. Both the post-stimulation period and the novelty exploration were followed by a tendency towards depression of the fEPSP combined with a rebound increase of the theta spectral power. The subsequent late-onset potentiation period was longer and more stable in the group of rats that underwent the stimulation protocol. Taken together, these data suggest that synaptic plasticity and network activity in the primary VC that occur as a consequence of active sensory processing, strongly infl uence DG excitability and may comprise one of the endogenous triggers for synaptic plasticity in the hippocampus.

OVERVIEW
Our data present evidence that experience-dependent plasticity of functional cortical circuits infl uences information processing at the cortico-hippocampal level. The long-term synaptic and excitatory alterations in hippocampus that we observed, as a consequence of the induction of synaptic plasticity in the adult primary VC, may provide an opportunity for network reconfi guration across temporally-spaced episodes: a mechanism by which memory formation is believed to be initiated. Our data indicate that synaptic plasticity in the primary VC that occurs as a consequence of active sensory processing may lower the threshold for induction of hippocampal synaptic plasticity and thus comprise one of the endogenous triggers for synaptic plasticity in the hippocampus.