Cell-specific synaptic plasticity induced by network oscillations

Gamma rhythms are known to contribute to the process of memory encoding. However, little is known about the underlying mechanisms at the molecular, cellular and network levels. Using local field potential recording in awake behaving mice and concomitant field potential and whole-cell recordings in slice preparations we found that gamma rhythms lead to activity-dependent modification of hippocampal networks, including alterations in sharp wave- ripple complexes. Network plasticity, expressed as long-lasting increases in sharp wave-associated synaptic currents, exhibits enhanced excitatory synaptic strength in pyramidal cells that is induced postsynaptically and depends on metabotropic glutamate receptor-5 activation. In sharp contrast, alteration of inhibitory synaptic strength is independent of postsynaptic activation and less pronounced. Further, we found a cell type-specific, directionally biased synaptic plasticity of two major types of GABAergic cells, parvalbumin- and cholecystokinin-expressing interneurons. Thus, we propose that gamma frequency oscillations represent a network state that introduces long-lasting synaptic plasticity in a cell-specific manner

Gamma-Oszillation-induziierte Plastizität in der CA3 Region des Hippokampus

Long-term potentiation (LTP) of synaptic transmission is crucial for learning and memory formation. However, most conventional LTP studies have been performed using high frequency stimulation of inputs while controlling postsynaptic cell activity allowing or preventing the generation of action potentials. The aim of this study was to investigate synaptic transmission changes induced during gamma frequency oscillations in vitro, which bear a close resemblance to the conditions under which LTP might occur in vivo. Gamma frequency oscillations are associated with exploratory activity and have an important role in memory encoding. Another memory-relevant network pattern, sharp-wave-ripples (SWRs), is implicated in consolidation of previously acquired information. Yet, the interaction and interdependence of these different network oscillatory states have not been fully elucidated. In our in vitro conditions, the neuronal network was capable of generating both SWRs and gamma rhythms allowing us to investigate synaptic properties and firing behavior of individual morphologically identified neurons during these rhythms and dynamic switching of their activity from one into another oscillatory state. We uncovered gamma rhythm-induced plasticity changes in the CA3 network, including alterations in subsequent SWR activity. After gamma frequency oscillations, we observed significantly increased SWR-associated reappeared excitatory postsynaptic currents (r-EPSC) in pyramidal cells (PCs). These changes were expressed postsynaptically and mediated by metabotropic glutamate receptor 5 (mGluR5) activation. In contrast to EPSC, SWR-associated reappeared inhibitory postsynaptic currents (r-IPSC) in PCs increased moderately and did not depend on postsynaptic activity. Consequently, gamma rhythm-induced changes in SWR- associated postsynaptic currents were reflected in a significantly increased EPSC/IPSC ratio in PCs obviously favoring their excitation. We further investigated gamma rhythm-induced synaptic plasticity in interneurons. Different interneuron types exhibited clear cell type- specific changes in their excitability, whereas parvalbumin-positive (PV+) interneurons showed raised excitability after gamma frequency oscillations, cholecystokinin-positive basket cells (CCK+ BC) demonstrated enhanced inhibition. These alterations were reflected in significant increase and decrease of the SWR-associated EPSC/IPSC ratio in PV+ and CCK+ interneurons, respectively. Thus, gamma rhythm led to activity-dependent long-lasting alterations in the CA3 network and induced postsynaptically mediated mGluR5-dependent LTP of excitatory postsynaptic currents in PCs. Cell type- specific contradirectional modifications of cell excitability in two distinct interneuron classes may account for a moderate increase in PC-IPSC, which, in turn, could serve the control of increased PC excitability. We propose that gamma rhythm-associated synchronization of network activity supports cell type-specific modifications of synaptic strength and may thereby lead to formation of memory traces.Die Langzeitpotenzierung (LTP) der synaptischen Signalübertragung ist grundlegend für Lernen und Gedächtnisbildung. Allerdings wurden die meisten konventionellen LTP-Studien mit einer hochfrequenten Stimulation des Eingangs bei gleichzeitiger Kontrolle der Aktivität der postsynaptsichen Zelle unter Ermöglichung oder Verhinderung der Generierung von Aktionspotentialen durchgeführt. Ziel der vorliegenden Studie war es, in vitro Veränderungen der synaptischen Übertragung induziert durch Netzwerkoszillationen im Gammafrequenzbereich zu untersuchen, die eine große Ähnlichkeit zu möglichen in vivo Bedingungen der LTP aufzeigen. Oszillationen im Gammafrequenzbereich sind mit Erkundungsverhalten verbunden und spielen eine wichtige Rolle für die Kodierung von Gedächtnisinhalten. Eine weitere für das Gedächtnis wichtige Netzwerkaktivität stellen die sogenannten „Sharp-wave-ripples“ (SWRs) dar, die an der Konsolidierung vorher erworbener Informationen beteiligt sind. Allerdings sind Wechselwirkung und Abhängigkeit dieser unterschiedlichen Aktivitätsmuster nicht vollständig geklärt. In unseren in vitro Bedingungen können beide Aktivitätsmuster, SWRs und Gamma Oszillationen, generiert werden, wodurch wir die synaptischen Eigenschaften und Entladungsmuster einzelner morphologisch identifizierten Neuronen während dieser Rhythmen untersuchen und deren dynamische Umschaltung zwischen den Rhythmen analysieren konnten. Wir fanden durch Gamma Oszillationen induzierte Aktivitäsänderung des CA3 Netzwerkes in nachfolgender SWRs. In Pyramidenzellen (PCs) beobachteten wir im Zusammenhang mit den wiederkehrenden SWRs signifikant erhöhte erregende postsynaptische Ströme (r-EPSC). Diese Änderungen wurden postsynaptisch generiert und durch die Aktivität des metabotropen Glutamatrezeptors vom Typ 5 (mGluR5) vermittelt. Im Gegensatz zu den EPSC, vergrößerten sich die mit den wiederkehrenden SWRs verbundenen hemmenden postsynaptischen Ströme (r- IPSC) in PC nur moderat und waren unabhängig von einer postsynaptischen Aktivität. Somit spiegeln die durch Gamma Oszillationen induzierten Veränderungen der SWR-assoziierten Ströme einen signifikanten Anstieg des PC EPSC/IPSC- Verhältnisses wider, wodurch offensichtlich die PC-Erregbarkeit gefördert wird. Zudem untersuchten wir Gamma induzierte synaptische Plastizität in Interneuronen. Verschiedene Interneuronengruppen wiesen dabei klare zelltypspezifische Veränderung der Erregbarkeit auf: Parvalbumin-positive (PV+) Interneurone zeigten nach dem Gamma-Rhythmus eine erhöhte Erregbarkeit, Cholecystokinin-enthaltenden Korbzellen (CCK+) eine verstärkte Hemmung. Diese Veränderungen zeigten sich in den entsprechend signifikant veränderten SWR- assoziierten EPSC/IPSC-Verhältnissen. Insgesamt führen Gamma Oszillationen zu aktivitätsabhängigen, lang anhaltenden Veränderungen im CA3 Netzwerk, inklusive einer postsynaptisch vermittelten mGluR5- abhängigen LTP der PC EPSC. Die entgegengesetzte Veränderung der Erregbarkeit zweier Interneurontypen kann verantwortlich für die insgesamt nur mäßige Verstärkung pyramidaler IPSC sein, was wiederum der Kontrollerhaltung über die erhöhte PC- Erregbarkeit dienen könnte. Unsere Ergebnisse legen nahe, dass die mit Gamma Oszillationen assoziierte Synchronisierung der Netzwerkaktivität zu zelltypspezifischer Veränderung der synaptischen Übertragungsstärke führen und dadurch zur Bildung von Gedächtnisspuren beitragen kann

Cell type-specific separation of subicular principal neurons during network activities.

The hippocampal output structure, the subiculum, expresses two major memory relevant network rhythms, sharp wave ripple and gamma frequency oscillations. To this date, it remains unclear how the two distinct types of subicular principal cells, intrinsically bursting and regular spiking neurons, participate in these two network rhythms. Using concomitant local field potential and intracellular recordings in an in vitro mouse model that allows the investigation of both network rhythms, we found a cell type-specific segregation of principal neurons into participating intrinsically bursting and non-participating regular spiking cells. However, if regular spiking cells were kept at a more depolarized level, they did participate in a specific manner, suggesting a potential bimodal working model dependent on the level of excitation. Furthermore, intrinsically bursting and regular spiking cells exhibited divergent intrinsic membrane and synaptic properties in the active network. Thus, our results suggest a cell-type-specific segregation of principal cells into two separate groups during network activities, supporting the idea of two parallel streams of information processing within the subiculum

Sharp wave and gamma network oscillations within the subiculum.

<p>(A) Spectrogram (top) with color-coded power spectral density (PSD) exemplifies the transition from spontaneously occurring sharp wave-ripples (SWR) to gamma frequency oscillations within the subiculum. The corresponding LFP recordings are displayed below. The application of kainic acid (KA, onset is marked by black line) abolishes the SWR rhythm and induces, after a brief transitory state, a stable oscillatory gamma rhythm. The recording interruptions of the top spectrograms and the underlying LFP traces are 12 s (middle) and 25 min (right). Red lines mark three examples that are illustrated below with higher temporal resolution (SWR, transition, gamma). (A, bottom, left) The SWR (filtered 2–300 Hz), the corresponding SPW (2–50 Hz) and the ripple components (100–300 Hz) supplemented by the color-coded power spectral density wavelet transform. (A, bottom, right) The boxplot depicts the distribution of the wavelet peak power spectral frequencies of 100 analyzed consecutive ripple events of the upper example trace. (B) Sharp waves of both polarities are exemplified on the left with each SWR trace (2–300 Hz, top), the ripple trace (100–300 Hz, middle) and the corresponding wavelet transform as color-coded power spectral density plot (bottom). The boxplot (right) illustrates the distribution of the mean SWP rates of all slices investigated (n = 42).</p

Temporal correlation of subicular RS cell activity to gamma frequency network oscillations.

<p>(A) Examples of simultaneous LFP (top) and intracellular (bottom) recordings during gamma frequency oscillations. In stark contrast to IB neurons, without current injection RS cells usually do not generate APs during gamma frequency oscillations and the data (n = 7) solely reveals a phase-locked mixed postsynaptic current (PSP, middle left). A depolarizing current injection initiates AP generation (middle right). Depolarized and spontaneous active RS cells (n = 9) show a distribution of AP generation (right) similar to the one observed in IB cells. (B) Example traces of synaptic potentials on the left. EPSP and IPSP were recorded at −80 mV and 0 mV respectively. Maximal cumulative postsynaptic potential peak deflections occur before the maximum of LFP gamma cycle. (C) There was no significant difference between the two classes of subicular PCs concerning the EPSP (IB: n = 8, RS: n = 11; <i>p</i> = 0.86) and IPSP amplitude (IB: n = 5, RS: n = 5; <i>p</i> = 0.56).</p

Behavior of subicular IB cells during spontaneous subicular SPWs.

<p>(A) Current-voltage relationship of a IB cell (left) with current injection steps of −300 pA, −100 pA, and +140 pA, respectively, displayed with the attached microphotograph of a biocytin-stained IB cell (right). These cells exhibit the typical pyramidal shaped cell body, prominent apical dendrites that travel through the molecular layer reaching the hippocampal fissure (hf), and basal dendrites that spread within the pyramidal cell layer. The axon leaves the subiculum (Sub) via the alveus. IB cells respond to a hyperpolarizing current injection with a sag in membrane potential whereas a positive current pulse leads to burst firing. (B) Example of simultaneous extracellular LFP (top trace) and intracellular (bottom trace) recordings at RMP is shown on the left. The intracellular recording reveals phase-locked synaptic responses as well as a full-blown AP (truncated for clarity) with respect to the LFP SPWs. The spike time histogram (n = 16 IB cells) on the right illustrates a clear peak of AP generation in close vicinity to the SPW peak. The vertical line marks the maximum mean SPW deflection as time point 0. (C) EPSPs and IPSPs are displayed in correlation to the LFP (left). The EPSPs and IPSPs were recorded at −80 mV and at 0 mV, respectively. (right) Accumulated mean EPSP/IPSP with respect to the maximum SPW peak deflection.</p

Gamma frequency network oscillations within the subiculum.

<p>(A) Gamma frequency oscillations were found in all of the examined subicular regions (middle, ventral, and dorsal). Sketches illustrate horizontal (middle, ventral and dorsal) slice preparation. The position of the scissors indicate the cuts made around the perimeters of the subicular region with the resulting subicular minislices marked by an asterisk. (A, right next to the sketch) Two example LFP recordings obtained from intact (grey, I, top trace) and isolated (black, II, bottom trace) middle (top), ventral (middle) and dorsal (bottom) slices are displayed together with the corresponding power spectra (A, middle column, color code according to the example traces). (A, right) The population data of the oscillatory frequency (top histogram) and spectral power (bottom histogram) exhibits no significant difference of the intact compared to the isolated subicular slices in network oscillatory gamma frequency as well as in spectral power (values and numbers in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0123636#pone.0123636.t001" target="_blank">Table 1</a>) except for the ventral subicular slices (<i>p</i> = 0.034, significance level indicated by asterisk). (B) Gamma frequency oscillations recorded from the medial (grey, I) and lateral (black, II) subiculum within the sagittal slice preparations. Same type of illustration as in (<i>A</i>), the subicular region is marked by an asterisk. The population histogram for frequency and power (values and numbers in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0123636#pone.0123636.t002" target="_blank">Table 2</a>) did not reveal a significant difference.</p

Electrophysiological properties of subicular PCs.

<p>Electrophysiological properties of subicular PCs.</p

Behavior of subicular RS cells during spontaneous subicular SPW activity.

<p>(A) Neuronal discharge pattern and microphotograph of an RS cell. RS cells do not fire bursts and show little or no sag potential. They respond to a depolarizing current injection with a train of single APs. RS cells show a typical pyramidal cell morphology. (A, right) The accommodation behavior (n = 19 IB cells; n = 16 RS cells) reveals a significant difference between subicular IB and RS cells (level of significance indicated by the asterisks, <i>p</i> < 0.0001). (B) Example of a RS cell at RMP (top left) and under the condition of depolarizing current injection (bottom left) during spontaneous SPW is given with the same type of illustration as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0123636#pone.0123636.g003" target="_blank">Fig 3</a>. Intracellular example recording of a silent RS cell at RMP (top left) depicting a SPW associated postsynaptic depolarization without AP generation. The population data (n = 7, right next) do not contain any APs, but aggregate a phase-locked mixed postsynaptic current (PSP) instead. When depolarized by current injection previously silent RS cells display a tonic AP firing mode (bottom left). In stark contrast to the IB cells, data of spontaneous active and depolarized RS cells reveal a prominent SPW peak correlated pause of AP generation (n = 11, bottom, middle left). EPSPs and IPSPs examples are displayed in correlation to the LFP (middle right). The EPSPs and IPSPs were recorded at −80 mV and at 0 mV, respectively. (right) Accumulated mean EPSP/IPSP with respect to the maximum SPW peak deflection. (C) EPSP (left) and IPSP (right) amplitudes for both cell classes. IB cells receive significantly higher synaptic excitation than RS neurons (indicated by the asterisk, <i>p</i> = 0.033; IB: n = 12; RS: n = 13), while there was no significant difference in IPSP (<i>p</i> = 0.52; IB: n = 3; RS: n = 4).</p

Electrophysiological properties of subicular PCs during gamma frequency oscillations.

<p>Electrophysiological properties of subicular PCs during gamma frequency oscillations.</p