Theta and gamma oscillations in the hippocampus have been hypothesized to play a role in the encoding and retrieval of memories. Recently, it was shown that an intrinsic fast gamma mechanism in medial entorhinal cortex can be recruited by optogenetic stimulation at theta frequencies, which can persist with fast excitatory synaptic transmission blocked, suggesting a contribution of interneuronal network gamma (ING). We calibrated the passive and active properties of a 100-neuron model network to capture the range of passive properties and frequency/current relationships of experimentally recorded PV+ neurons in the medial entorhinal cortex (mEC). The strength and probabilities of chemical and electrical synapses were also calibrated using paired recordings, as were the kinetics and short-term depression (STD) of the chemical synapses. Gap junctions that contribute a noticeable fraction of the input resistance were required for synchrony with hyperpolarizing inhibition; these networks exhibited theta-nested high frequency oscillations similar to the putative ING observed experimentally in the optogenetically-driven PV-ChR2 mice. With STD included in the model, the network desynchronized at frequencies above ~200 Hz, so for sufficiently strong drive, fast oscillations were only observed before the peak of the theta. Because hyperpolarizing synapses provide a synchronizing drive that contributes to robustness in the presence of heterogeneity, synchronization decreases as the hyperpolarizing inhibition becomes weaker. In contrast, networks with shunting inhibition required non-physiological levels of gap junctions to synchronize using conduction delays within the measured range.

Parvalbumin (Pvalb + )-and somatostatin (Sst + )-positive cells are the two largest subgroups of inhibitory interneurons. Studies in visual cortex indicate that synaptic connections between Pvalb + cells are common while connections between Sst + interneurons have not been observed. The inhibitory connectivity and kinetics of these two interneuron subpopulations, however, have not been characterized in medial entorhinal cortex (mEC). Using fluorescence-guided paired recordings in mouse brain slices from interneurons and excitatory cells in layer 2/3 mEC, we found that, unlike neocortical measures, Sst + cells inhibit each other, albeit with a lower probability than Pvalb + cells (18% versus 36% for unidirectional connections). Gap junction connections were also more frequent between Pvalb + cells than between Sst + cells. Pvalb + cells inhibited each other with larger conductances, smaller decay time constants and shorter delays. Similarly, synaptic connections between Pvalb + and excitatory cells were more likely and expressed faster decay times and shorter delays than those between Sst + and excitatory cells. Inhibitory cells exhibited smaller synaptic decay time constants between interneurons than on their excitatory targets. Inhibition between interneurons also depressed faster, and to a greater extent. Finally, inhibition onto layer 2 pyramidal and stellate cells originating from Pvalb + interneurons were very similar, with no significant differences in connection likelihood, inhibitory amplitude, and decay time. A model of short-term depression fitted to the data indicates that recovery time constants for refilling the available pool are in the range of 50-150 ms and that the fraction of the available pool released on each spike is in the range 0.2-0.5. SignificanceTwo large and distinct classes of interneurons in medial entorhinal cortex (mEC) include parvalbumin (Pvalb + )-and somatostatin (Sst + )-positive cells. Previous work has demonstrated unique functions with regards to spatial tuning and network oscillations for these two interneuron populations. Potential differences in kinetics of inhibition and likelihood of connection from these two interneuron groups, however, have not been quantified. Here, using fluorescence to guide intracellular recordings, we quantified the synaptic connections from both types of interneurons. We indicate that Sst + and Pvalb + express different synaptic kineticsare target-cell specific. In contrast to neocortical measures, we find substantial connections between Sst + interneurons.

We show that the inductive coupling between the quantum mechanical motion of a superconducting microcantilever and a flux-dependent microwave quantum circuit can attain the strong singlephoton nanomechanical coupling regime with feasible experimental parameters. We propose to use a superconducting strip, which is in the Meissner state, at the tip of a cantilever. A pick-up coil collects the flux generated by the sheet currents induced by an external quadrupole magnetic field centered at the strip location. The position-dependent magnetic response of the superconducting strip, enhanced by both diamagnetism and demagnetizing effects, leads to a strong magnetomechanical coupling to quantum circuits.In quantum nanomechanics, the strength of the radiation-pressure interaction between a single electromagnetic mode of frequency ω and a micromechanical mode of frequency Ω and effective mass M is denoted by g 0 , the so-called single-photon coupling rate [1]. This is the cavity frequency shift due to a zero-point motion displacement of the mechanical oscillator, given by1/2 , namely g 0 = z zp ∂ω/∂z| z=0 . The singlephoton coupling, being non-linear, could be exploited to observe non-Gaussian physics in micromechanical oscillators [2][3][4][5][6][7], a goal that would represent a milestone in the field [1]. However this is today experimentally very challenging. The mechanical mode (electromagnetic mode) suffers decoherence with a rate Γ (κ) whose origin depends on the particular experimental implementation. To fully exploit the non-Gaussian character of the single-photon nanomechanical coupling one would like to operate in the strong-coupling regime g 0 Γ, κ as well as in the resolved sideband regime Ω/κ 1. The latter is required to sideband cool the mechanical mode into the ground state [8][9][10]. While g 0 /Γ 1 and Ω/κ 1 has been achieved simultaneously in several experiments [1], the so-called single-photon strong coupling regime g 0 /κ 1 is much more challenging. Indeed, according to [1], the highest values of g 0 /κ obtained so far with solid mesoscopic objects are ∼ 10 −3 [11,12] (with cold gases one achieves g 0 /κ ∼ 1 [13, 14] but not in the resolved sideband regime).In this Letter we propose a microwave optomechanical scenario, see Fig. 1, where we show that the strong single-photon regime g 0 /κ 1 can be achieved in the resolved sideband regime with feasible experimental parameters. Contrary to most of the current experiments in microwave optomechanics [1,12], where the optomechanical coupling is implemented capacitively, here we motivate to use an inductive coupling to a flux-dependent quantum circuit as a way to obtain three orders of magnitude stronger couplings. Such a strong quantum magnetomechanical (MM) coupling is achieved via the magnetic response of a superconducting (SC) strip in an inhomogenous external field that is strengthened by the large FIG. 1: (Color online) Schematic illustration of the proposal (not to scale). A superconducting strip of length L and widthw is deposited on the tip of the c...

The transverse relaxation time T2 of protons in water suspensions of iron-oxide particles increases with the waiting time tw after the sample is inserted in the gap of the spectrometer magnet. Such a T2 increase becomes significant if the particles are aggregated into large clusters, for which field-induced formation of cluster-chains will occur and T2 should increase with increasing the length of chains. T2 increases with tw even for small particles, for which no chain formation may be induced, and for large clusters when tw is too small to form long enough chains. The T2 increase is accompanied by a significant echo-time dependence. All this is experimentally and theoretically studied.