Interplay of Entorhinal Input and Local Inhibitory Network in the Hippocampus at the Origin of Slow Inhibition in Granule Cells.

Neuronal activity from the entorhinal cortex propagates through the perforant path (PP) to the molecular layer of the dentate gyrus (DG) where information is filtered and converted into sparse hippocampal code. Nearly simultaneous signaling to both granule cells (GC) and local interneurons (INs) engages network interactions that will modulate input integration and output generation. When triggered, GABA release from interneurons counteracts the glutamatergic signals of PP terminals, scaling down the overall DG activation. Inhibition occurs at fast or slow timescales depending on the activation of ionotropic GABAA-R or metabotropic GABAB-R. Although postsynaptic GABAA and GABAB-R differ in their location at the synapse, mixed GABAA/B-R IPSPs can also occur. Here we describe a slow inhibition mechanism in mouse GCs recorded from either sex, mediated by GABAA/B-R in combination with metabotropic glutamate receptors. Short burst PP stimulation in the gamma frequency range lead to a long-lasting hyperpolarization (LLH) of the GCs with a duration that exceeds GABAB-R IPSPs. As a result, LLH alters GC firing patterns and the responses to concomitant excitatory signals are also affected. Synaptic recruitment of feedforward inhibition and subsequent GABA release from interneurons, also successfully trigger mixed GABA responses in GCs. Together these results suggest that slow inhibition through LLH leads to reduced excitability of GCs during entorhinal input integration. The implication of LLH in regulation of neuronal excitability suggests it also contributes to the sparse population coding in DG.SIGNIFICANCE STATEMENT Our study describes a long-lasting hyperpolarization (LLH) in hippocampal granule cells. We used whole-cell patch-clamp recordings and an optogenetic approach to characterize this event. LLH is a slow inhibitory mechanism that occurs following the stimulation of the perforant pathway in the molecular layer of the dentate gyrus. We found that it is mediated via postsynaptic ionotropic and metabotropic GABA and metabotropic glutamate receptors. The duration of LLH exceeds previously described IPSPs mediated by any of these receptors. The activation of LLH requires presynaptic gamma frequency bursts and recruitment of the local feedforward inhibition. LLH defines prolonged periods of low excitability of GCs and a restrained neuronal discharge. Our results suggest that LLH can contribute to sparse activation of GCs.

Vesicular zinc regulates the Ca2+ sensitivity of a subpopulation of presynaptic vesicles at hippocampal mossy fiber terminals.

Synaptic vesicles segregate into functionally diverse subpopulations within presynaptic terminals, yet there is no information about how this may occur. Here we demonstrate that a distinct subgroup of vesicles within individual glutamatergic, mossy fiber terminals contain vesicular zinc that is critical for the rapid release of a subgroup of synaptic vesicles during increased activity in mice. In particular, vesicular zinc dictates the Ca(2+) sensitivity of release during high-frequency firing. Intense synaptic activity alters the subcellular distribution of zinc in presynaptic terminals and decreases the number of zinc-containing vesicles. Zinc staining also appears in endosomes, an observation that is consistent with the preferential replenishment of zinc-enriched vesicles by bulk endocytosis. We propose that functionally diverse vesicle pools with unique membrane protein composition support different modes of transmission and are generated via distinct recycling pathways.

Interneurons produced in adulthood are required for the normal functioning of the olfactory bulb network and for the execution of selected olfactory behaviors.

Olfactory bulb (OB) interneurons are continuously renewed throughout an animal's lifespan. Despite extensive investigation of this phenomenon, little is known about bulbar circuitry functioning and olfactory performances under conditions of ablated arrival of new neurons into the adult OB. To address this issue we performed morphological, electrophysiological, and behavioral analysis in mice with suppressed bulbar neurogenesis. Infusion of the antimitotic drug AraC to the lateral ventricle via 28 d osmotic minipumps abolished the arrival of newly born neurons into the adult OB without affecting the total number of granule cells. The number, dendritic arborization, and spine density of interneurons generated in adulthood, before pump installation, were also not affected by AraC treatment. As a result of ablated neurogenesis, mitral cells--the principal output neurons in the OB--receive fewer inhibitory synapses, display reduced frequency of spontaneous IPSCs, experience smaller dendrodendritic inhibition, and exhibit decreased synchronized activity. Consequently, short-term olfactory memory was drastically reduced in AraC-treated mice. In contrast, olfactory performances of AraC-treated animals were undistinguishable from those of control mice in other odor-associated tests, such as spontaneous odor discrimination and long-term odor-associative memory tasks. Altogether, our data highlight the importance of adult neurogenesis for the proper functioning of the OB network and imply that new bulbar interneurons are involved in some, but not all, odor-associated tasks.

Extracellular chelation of zinc does not affect hippocampal excitability and seizure-induced cell death in rats.

In the nervous system, zinc can influence synaptic responses and at extreme concentrations contributes to epileptic and ischaemic neuronal injury. Zinc can originate from synaptic vesicles, the extracellular space and from intracellular stores. In this study, we aimed to determine which of these zinc pools is responsible for the increased hippocampal excitability observed in zinc-depleted animals or following zinc chelation. Also, we investigated the source of intracellularly accumulating zinc in vulnerable neurons. Our data show that membrane-permeable and membrane-impermeable zinc chelators had little or no effect on seizure activity in the CA3 region. Furthermore, extracellular zinc chelation could not prevent the accumulation of lethal concentrations of zinc in dying neurons following epileptic seizures. At the electron microscopic level, zinc staining significantly increased at the presynaptic membrane of mossy fibre terminals in kainic acid-treated animals. These data indicate that intracellular but not extracellular zinc chelators could influence neuronal excitability and seizure-induced zinc accumulation observed in the cytosol of vulnerable neurons.

Cell type-specific action of seizure-induced intracellular zinc accumulation in the rat hippocampus.

Increased levels of intracellular zinc have been implicated in neuronal cell death in ischaemia, epilepsy and traumatic brain damage. However, decreases in zinc levels also lead to increased neuronal death and lowered seizure threshold. In the present study we investigated the physiological role of zinc in neurodegeneration and protection following epileptic seizures. Cells located in the strata oriens and lucidum of the CA3 region accumulated high concentrations of zinc and died. A decrease in zinc level could prevent the death of these neurones after seizures. Most of these cells were GABAergic interneurones. In contrast, neurones in the CA3 pyramidal cell layer accumulated moderate amounts of zinc and survived. Zinc chelation led to an increase in the mortality rate of these cells. Furthermore, in these cells low concentrations of intracellular zinc activated Akt (protein kinase B), thus providing protection against neurodegeneration. These results demonstrate that intracellularly accumulated zinc can be neurotoxic or neuroprotective depending on its concentration. This dual action is cell type specific.