Short-term inhibition of GABAergic IPSCs induced by association of pre- and postsynaptic activation in the neonatal hippocampus

Muscarinic Regulation of Spike Timing Dependent Synaptic Plasticity in the Hippocampus

Long-term changes in synaptic transmission between neurons in the brain are considered the cellular basis of learning and memory. Over the last few decades, many studies have revealed that the precise order and timing of activity between pre- and post-synaptic cells ("spike-timing-dependent plasticity; STDP") is crucial for the sign and magnitude of long-term changes at many central synapses. Acetylcholine (ACh) via the recruitment of diverse muscarinic receptors is known to influence STDP in a variety of ways, enabling flexibility and adaptability in brain network activity during complex behaviors. In this review, we will summarize and discuss different mechanistic aspects of muscarinic modulation of timing-dependent plasticity at both excitatory and inhibitory synapses in the hippocampus to shape learning and memory.

The Endocannabinoid System Contributes to Memory Deficits Induced by Rapid-eye-movement Sleep Deprivation in Adolescent Mice

Sleep loss or insomnia is among the contributing factors of cognitive deficit, the underlying mechanisms of which remain largely elusive. The endocannabinoid (eCB) system plays a role in sleep, while it is unknown if it is involved in the regulation of memory retrieval by sleep deprivation. In addition, it still controversial how rapid-eye-movement sleep deprivation (REMSD) affects the spatial memory of adolescent mice. Here, we found that 24-h REMSD impairs spatial memory retrieval of adolescent mice in an object-place recognition task, which was rescued by NESS0327, a neutral cannabinoid receptor 1 (CB1R) antagonist. Mechanistically, REMSD induced eCB-mediated short-term and long-term synaptic plasticity changing including depolarization-induced suppression of inhibition (DSI) in the pyramidal neurons of the hippocampus, in which long-term synaptic plasticity changing was rescued by NESS0327. REMSD downregulated monoacylglycerol lipase, a hydrolase for the endocannabinoid 2-arachidonoylglycerol (2-AG), suggesting the involvement of eCB accumulation and the consequent synaptic plasticity in REMSD-elicited memory impairment in adolescent mice. These findings shed light on the role of sleep disorders in learning and memory deficit of adolescents.

Metabotropic glutamate receptor 1-mediated calcium mobilization in the neonatal hippocampal marginal zone

The hippocampal marginal zone contains Cajal-Retzius (C-R) cells and participates in the regulation of cortical development. Two subtypes of group I metabotropic glutamate receptors (mGluRs), mGluR1 and mGluR5, are found in the central nervous system and are considered to regulate neuronal excitability. The release of Ca²⁺ from intracellular stores is thought to be a main consequence of activation of these receptor subtypes. In hippocampal C-R cells, the expression of mGluR1 has been showed using immunohistochemical techniques, but its function has not been elucidated. In this study, Ca²⁺ mobilization through mGluR1 activation was demonstrated in the neonatal rat hippocampus. In marginal zone C-R cells, intracellular Ca²⁺ elevation was detected by fluorescence imaging after the application of a group I mGluR-specific agonist. This response was prevented by application of an mGluR1 antagonist but was not changed by application of an mGluR5 antagonist. The intracellular Ca²⁺ elevation induced by mGluR1 activation was still observed in Ca²⁺ -free perfusate, indicating the release of Ca²⁺ from intracellular stores. γ-Aminobutyric acid and ionotropic glutamate receptor-mediated intracellular Ca²⁺ elevation was also detected in mGluR1-possessing neurons, although the former was much smaller than that mediated by mGluR1. These results indicate that mGluR1 is functionally expressed in C-R cells in the neonatal marginal zone and regulates cell function through the elevation of intracellular Ca²⁺ .