Anatomical Organization and Spatiotemporal Firing Patterns of Layer 3 Neurons in the Rat Medial Entorhinal Cortex.

Layer 3 of the medial entorhinal cortex is a major gateway from the neocortex to the hippocampus. Here we addressed structure-function relationships in medial entorhinal cortex layer 3 by combining anatomical analysis with juxtacellular identification of single neurons in freely behaving rats. Anatomically, layer 3 appears as a relatively homogeneous cell sheet. Dual-retrograde neuronal tracing experiments indicate a large overlap between layer 3 pyramidal populations, which project to ipsilateral hippocampus, and the contralateral medial entorhinal cortex. These cells were intermingled within layer 3, and had similar morphological and intrinsic electrophysiological properties. Dendritic trees of layer 3 neurons largely avoided the calbindin-positive patches in layer 2. Identification of layer 3 neurons during spatial exploration (n = 17) and extracellular recordings (n = 52) pointed to homogeneous spatial discharge patterns. Layer 3 neurons showed only weak spiking theta rhythmicity and sparse head-direction selectivity. A majority of cells (50 of 69) showed no significant spatial modulation. All of the ∼28% of neurons that carried significant amounts of spatial information (19 of 69) discharged in irregular spatial patterns. Thus, layer 3 spatiotemporal firing properties are remarkably different from those of layer 2, where theta rhythmicity is prominent and spatially modulated cells often discharge in grid or border patterns. Significance statement: Neurons within the superficial layers of the medial entorhinal cortex (MEC) often discharge in border, head-direction, and theta-modulated grid patterns. It is still largely unknown how defined discharge patterns relate to cellular diversity in the superficial layers of the MEC. In the present study, we addressed this issue by combining anatomical analysis with juxtacellular identification of single layer 3 neurons in freely behaving rats. We provide evidence that the anatomical organization and spatiotemporal firing properties of layer 3 neurons are remarkably different from those in layer 2. Specifically, most layer 3 neurons discharged in spatially irregular firing patterns, with weak theta-modulation and head-directional selectivity. This work thus poses constraints on the spatiotemporal patterns reaching downstream targets, like the hippocampus.

Selective and brain-penetrant HCN1 inhibitors reveal links between synaptic integration, cortical function, and working memory.

Hyperpolarization-activated and cyclic-nucleotide-gated 1 (HCN1) ion channels are proposed to be critical for cognitive function through regulation of synaptic integration. However, resolving the precise role of HCN1 in neurophysiology and exploiting its therapeutic potential has been hampered by minimally selective antagonists with poor potency and limited in vivo efficiency. Using automated electrophysiology in a small-molecule library screen and chemical optimization, we identified a primary carboxamide series of potent and selective HCN1 inhibitors with a distinct mode of action. In cognition-relevant brain circuits, selective inhibition of native HCN1 produced on-target effects, including enhanced excitatory postsynaptic potential summation, while administration of a selective HCN1 inhibitor to rats recovered decrement working memory. Unlike prior non-selective HCN antagonists, selective HCN1 inhibition did not alter cardiac physiology in human atrial cardiomyocytes or in rats. Collectively, selective HCN1 inhibitors described herein unmask HCN1 as a potential target for the treatment of cognitive dysfunction in brain disorders.

Temporal compression mediated by short-term synaptic plasticity.

Time scales of cortical neuronal dynamics range from few milliseconds to hundreds of milliseconds. In contrast, behavior occurs on the time scale of seconds or longer. How can behavioral time then be neuronally represented in cortical networks? Here, using electrophysiology and modeling, we offer a hypothesis on how to bridge the gap between behavioral and cellular time scales. The core idea is to use a long time constant of decay of synaptic facilitation to translate slow behaviorally induced temporal correlations into a distribution of synaptic response amplitudes. These amplitudes can then be transferred to a sequence of action potentials in a population of neurons. These sequences provide temporal correlations on a millisecond time scale that are able to induce persistent synaptic changes. As a proof of concept, we provide simulations of a neuron that learns to discriminate temporal patterns on a time scale of seconds by synaptic learning rules with a millisecond memory buffer. We find that the conversion from synaptic amplitudes to millisecond correlations can be strongly facilitated by subthreshold oscillations both in terms of information transmission and success of learning.

Inactivity sets XL synapses in motion.

Neurons adapt their synaptic responses to the activity of the underlying network. In this issue of Neuron, Thiagarajan and colleagues report on specific subcellular mechanisms of homeostasis after prolonged neuronal inactivity. The results have important implications not only for neuronal homeostasis but also for further understanding of metaplasticity.

Retrograde signaling causes excitement.

Retrograde signaling is a powerful tool to shape synaptic transmission, typically inducing inhibition of transmitter release. A new study published in this issue of Neuron by Carta et al. (2014) now provides strong support for arachidonic acid as a potentiating retrograde messenger.

Inhibitory gradient along the dorsoventral axis in the medial entorhinal cortex.

Local inhibitory microcircuits in the medial entorhinal cortex (MEC) and their role in network activity are little investigated. Using a combination of electrophysiological, optical, and morphological circuit analysis tools, we find that layer II stellate cells are embedded in a dense local inhibitory microcircuit. Specifically, we report a gradient of inhibitory inputs along the dorsoventral axis of the MEC, with the majority of this local inhibition arising from parvalbumin positive (PV+) interneurons. Finally, the gradient of PV+ fibers is accompanied by a gradient in the power of extracellular network oscillations in the gamma range, measured both in vitro and in vivo. The reported differences in the inhibitory microcircuitry in layer II of the MEC may therefore have a profound functional impact on the computational working principles at different locations of the entorhinal network and influence the input pathways to the hippocampus.

Natural spike trains trigger short- and long-lasting dynamics at hippocampal mossy fiber synapses in rodents.

BACKGROUND: Synapses exhibit strikingly different forms of plasticity over a wide range of time scales, from milliseconds to hours. Studies on synaptic plasticity typically use constant-frequency stimulation to activate synapses, whereas in vivo activity of neurons is irregular. METHODOLOGY/PRINCIPAL FINDINGS: Using extracellular and whole-cell electrophysiological recordings, we have here studied the synaptic responses at hippocampal mossy fiber synapses in vitro to stimulus patterns obtained from in vivo recordings of place cell firing of dentate gyrus granule cells in behaving rodents. We find that synaptic strength is strongly modulated on short- and long-lasting time scales during the presentation of the natural stimulus trains. CONCLUSIONS/SIGNIFICANCE: We conclude that dynamic short- and long-term synaptic plasticity at the hippocampal mossy fiber synapse plays a prominent role in normal synaptic function.

Phase precession through synaptic facilitation.

Phase precession is a relational code that is thought to be important for episodic-like memory, for instance, the learning of a sequence of places. In the hippocampus, places are encoded through bursting activity of so-called place cells. The spikes in such a burst exhibit a precession of their firing phases relative to field potential theta oscillations (4-12 Hz); the theta phase of action potentials in successive theta cycles progressively decreases toward earlier phases. The mechanisms underlying the generation of phase precession are, however, unknown. In this letter, we show through mathematical analysis and numerical simulations that synaptic facilitation in combination with membrane potential oscillations of a neuron gives rise to phase precession. This biologically plausible model reproduces experimentally observed features of phase precession, such as (1) the progressive decrease of spike phases, (2) the nonlinear and often also bimodal relation between spike phases and the animal's place, (3) the range of phase precession being smaller than one theta cycle, and (4) the dependence of phase jitter on the animal's location within the place field. The model suggests that the peculiar features of the hippocampal mossy fiber synapse, such as its large efficacy, long-lasting and strong facilitation, and its phase-locked activation, are essential for phase precession in the CA3 region of the hippocampus.

Differential modulation of short-term synaptic dynamics by long-term potentiation at mouse hippocampal mossy fibre synapses.

Synapses continuously experience short- and long-lasting activity-dependent changes in synaptic strength. Long-term plasticity refers to persistent alterations in synaptic efficacy, whereas short-term plasticity (STP) reflects the instantaneous and reversible modulation of synaptic strength in response to varying presynaptic stimuli. The hippocampal mossy fibre synapse onto CA3 pyramidal cells is known to exhibit both a presynaptic, NMDA receptor-independent form of long-term potentiation (LTP) and a pronounced form of STP. A detailed description of their exact interdependence is, however, lacking. Here, using electrophysiological and computational techniques, we have developed a descriptive model of transmission dynamics to quantify plasticity at the mossy fibre synapse. STP at this synapse is best described by two facilitatory processes acting on time-scales of a few hundred milliseconds and about 10 s. We find that these distinct types of facilitation are differentially influenced by LTP such that the impact of the fast process is weakened as compared to that of the slow process. This attenuation is reflected by a selective decrease of not only the amplitude but also the time constant of the fast facilitation. We henceforth argue that LTP, involving a modulation of parameters determining both amplitude and time course of STP, serves as a mechanism to adapt the mossy fibre synapse to its temporal input.

Different regulation of Purkinje cell dendritic development in cerebellar slice cultures by protein kinase Calpha and -beta.

Activity of protein kinase C (PKC), and in particular the PKCgamma-isoform, has been shown to strongly affect and regulate Purkinje cell dendritic development, suggesting an important role for PKC in activity-dependent Purkinje cell maturation. In this study we have analyzed the role of two additional Ca(2+)-dependent PKC isoforms, PKCalpha and -beta, in Purkinje cell survival and dendritic morphology in slice cultures using mice deficient in the respective enzymes. Pharmacological PKC activation strongly reduced basal Purkinje cell dendritic growth in wild-type mice whereas PKC inhibition promoted branching. Purkinje cells from mice deficient in PKCbeta, which is expressed in two splice forms by granule but not Purkinje cells, did not yield measurable morphological differences compared to respective wild-type cells under either experimental condition. In contrast, Purkinje cell dendrites in cultures from PKCalpha-deficient mice were clearly protected from the negative effects on dendritic growth of pharmacological PKC activation and showed an increased branching response to PKC inhibition as compared to wild-type cells. Together with our previous work on the role of PKCgamma, these data support a model predicting that normal Purkinje cell dendritic growth is mainly regulated by the PKCgamma-isoform, which is highly activated by developmental processes. The PKCalpha isoform in this model forms a reserve pool, which only becomes activated upon strong stimulation and then contributes to the limitation of dendritic growth. The PKCbeta isoform appears to not be involved in the signaling cascades regulating Purkinje cell dendritic maturation in cerebellar slice cultures.