Biology subjects

Nasrallah, K.

Publications and source records attributed to Nasrallah, K..

4 recordsLinked to original sources

bioRxiv2022

Heterosynaptic NMDA Receptor Plasticity in Hippocampal Dentate Granule Cells

The dentate gyrus is a key relay station that controls information transfer from the entorhinal cortex to the hippocampus proper. This process heavily relies on dendritic integration by dentate granule cells (GCs) of excitatory synaptic inputs from medial and lateral entorhinal cortex via medial and lateral perforant paths (MPP and LPP, respectively). N-methyl-D-aspartate receptors (NMDARs) can contribute significantly to the integrative properties of neurons. While early studies reported that excitatory inputs from entorhinal cortex onto GCs can undergo activity-dependent long-term plasticity of NMDAR-mediated transmission, the input-specificity of this plasticity along the dendritic axis remains unknown. Here, we examined the NMDAR plasticity rules at MPP-GC and LPP-GC synapses using physiologically relevant patterns of stimulation in acute rat hippocampal slices. We found that MPP-GC, but not LPP-GC synapses, expressed homosynaptic NMDAR-LTP. In addition, induction of NMDAR-LTP at MPP-GC synapses heterosynaptically potentiated distal LPP-GC NMDAR plasticity. The same stimulation protocol induced homosynaptic -amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR)-LTP at MPP-GC but heterosynaptic AMPAR-LTD at distal LPP synapses, demonstrating that NMDAR and AMPAR are governed by different plasticity rules. Remarkably, heterosynaptic but not homosynaptic NMDAR-LTP required Ca2+ release from intracellular, ryanodine-dependent Ca2+ stores. Lastly, the induction and maintenance of both homo- and heterosynaptic NMDAR-LTP were blocked by GluN2D antagonism, suggesting the recruitment of GluN2D-containing receptors to the synapse. Our findings uncover a mechanism by which distinct inputs to the dentate gyrus may interact functionally and contribute to hippocampal-dependent memory formation. Significance StatementNMDARs are key players in synaptic plasticity. In addition to their classical role as coincidence detectors and triggers of AMPAR plasticity, there is compelling evidence that NMDARs can undergo activity-dependent plasticity independent of AMPAR plasticity. However, whether NMDAR-plasticity is expressed heterosynaptically remains unclear. Here, in the dentate gyrus of the hippocampus, we show that the induction of burst timing-dependent LTP of NMDAR-mediated transmission at proximal medial perforant path synapses is accompanied by heterosynaptic NMDAR-LTP at lateral perforant path synapses. These findings provide the first evidence for heterosynaptic NMDAR plasticity, which may have important consequences on the dendritic integration of functionally distinct excitatory inputs by dentate granule cells.

neuroscience↗

bioRxiv2021

BDNF-induced BDNF release mediates presynaptic LTP and is regulated by cannabinoids

The brain-derived neurotrophic factor (BDNF) and its effector Tropomyosin receptor kinase B (TrkB) mediate diverse forms of activity-dependent synaptic plasticity implicated in learning, neural circuit refinement, and brain diseases, including epilepsy and mood disorders. Here, we report that activity-dependent release of presynaptic BDNF elicits the release of postsynaptic BDNF in a TrkB- and calcium-dependent manner. This BDNF-induced BDNF release was required for the induction of presynaptic long-term potentiation (LTP) of excitatory transmission in the mouse dentate gyrus. Tonic and phasic activity of presynaptic type-1 cannabinoid receptors suppressed BDNF release and dampened LTP, while exposure to enriched environment elicited BDNF-mediated LTP. In addition to mediating presynaptic plasticity, BDNF-induced BDNF release could be an important mechanism in synaptic stabilization during the maturation and refinement of neuronal connections. One-Sentence SummaryThe brain-derived neurotrophic factor induces its own release to mediate long-lasting increase in neurotransmitter release.

neuroscience↗

bioRxiv2021

Retrograde adenosine/A2A receptor signaling mediates presynaptic hippocampal LTP and facilitates epileptic seizures

Retrograde signaling at the synapse is a fundamental way by which neurons communicate and neuronal circuit function is fine-tuned upon activity. While long-term changes in neurotransmitter release commonly rely on retrograde signaling, the mechanisms remain poorly understood. Here, we identified adenosine/A2A receptor (A2AR) as a novel retrograde signaling pathway underlying presynaptic long-term potentiation (LTP) at a hippocampal excitatory circuit critically involved in memory and epilepsy. Transient burst activity of a single dentate granule cell induced LTP of mossy cell synaptic inputs, a BDNF/TrkB-dependent form of plasticity that facilitates seizures. Postsynaptic TrkB activation released adenosine from granule cells, uncovering a non-conventional BDNF/TrkB signaling mechanism. Moreover, presynaptic A2ARs were necessary and sufficient for LTP. Lastly, seizure induction released adenosine in a TrkB-dependent manner, while removing A2ARs or TrkB from the dentate gyrus had anti-convulsant effects. By mediating presynaptic LTP, adenosine/A2AR retrograde signaling may modulate dentate gyrus-dependent learning and promote epileptic activity. HighlightsO_LIPostsynaptic firing induces presynaptic LTP at mossy cell to granule cell synapses C_LIO_LIPostsynaptic TrkB activation induces adenosine release from granule cells C_LIO_LIPresynaptic adenosine A2A receptors are necessary and sufficient to induce LTP C_LIO_LIAdenosine/A2AR signaling within the dentate gyrus is pro-convulsant C_LI In BriefNasrallah et al. report a novel retrograde signaling pathway at hippocampal synapses that involves postsynaptic TrkB-dependent release of adenosine and the activation of presynaptic A2A receptors. This pathway mediates presynaptic long-term potentiation at a key hippocampal excitatory synapse and can also promote epileptic seizures.

neuroscience↗

bioRxiv2021

Activity-dependent LTP in the dentate gyrus promotes epileptic seizures

Epilepsy is a devastating brain disorder whose cellular mechanisms remain poorly understood. Excitatory mossy cells (MCs) in the dentate gyrus of the hippocampus are implicated in temporal lobe epilepsy, the most common form of focal epilepsy in adults. However, the role of MCs during initial seizures, before MC loss occurs, is unclear. Here, we show that initial seizures induced with kainic acid (KA) intraperitoneal injection in adult mice, a well-established model of experimental epilepsy, not only increased MC and granule cell (GC) activity in vivo, but also triggered a BDNF-dependent long-term potentiation at MC-GC synapses (MC-GC LTP). In vivo induction of MC-GC LTP worsened KA-induced seizures, whereas selective MC silencing and Bdnf genetic removal from GCs, which abolishes LTP, were both anti-epileptic. Thus, initial seizures strengthen MC-GC synaptic transmission, thereby promoting epileptic activity. Our findings reveal a potential mechanism of epileptogenesis that may help develop therapeutic strategies for early intervention.

neuroscience↗