How development sculpts hippocampal circuits and function.

In mammals, the selective transformation of transient experience into stored memory occurs in the hippocampus, which develops representations of specific events in the context in which they occur. In this review, we focus on the development of hippocampal circuits and the self-organized dynamics embedded within them since the latter critically support the role of the hippocampus in learning and memory. We first discuss evidence that adult hippocampal cells and circuits are sculpted by development as early as during embryonic neurogenesis. We argue that these primary developmental programs provide a scaffold onto which later experience of the external world can be grafted. Next, we review the different sequences in the development of hippocampal cells and circuits at anatomical and functional levels. We cover a period extending from neurogenesis and migration to the appearance of phenotypic diversity within hippocampal cells and their wiring into functional networks. We describe the progressive emergence of network dynamics in the hippocampus, from sensorimotor-driven early sharp waves to sequences of place cells tracking relational information. We outline the critical turn points and discontinuities in that developmental journey, and close by formulating open questions. We propose that rewinding the process of hippocampal development helps understand the main organization principles of memory circuits.

GluK2 Is a Target for Gene Therapy in Drug-Resistant Temporal Lobe Epilepsy.

OBJECTIVE: Temporal lobe epilepsy (TLE) is characterized by recurrent seizures generated in the limbic system, particularly in the hippocampus. In TLE, recurrent mossy fiber sprouting from dentate gyrus granule cells (DGCs) crea an aberrant epileptogenic network between DGCs which operates via ectopically expressed GluK2/GluK5-containing kainate receptors (KARs). TLE patients are often resistant to anti-seizure medications and suffer significant comorbidities; hence, there is an urgent need for novel therapies. Previously, we have shown that GluK2 knockout mice are protected from seizures. This study aims at providing evidence that downregulating KARs in the hippocampus using gene therapy reduces chronic epileptic discharges in TLE. METHODS: We combined molecular biology and electrophysiology in rodent models of TLE and in hippocampal slices surgically resected from patients with drug-resistant TLE. RESULTS: Here, we confirmed the translational potential of KAR suppression using a non-selective KAR antagonist that markedly attenuated interictal-like epileptiform discharges (IEDs) in TLE patient-derived hippocampal slices. An adeno-associated virus (AAV) serotype-9 vector expressing anti-grik2 miRNA was engineered to specifically downregulate GluK2 expression. Direct delivery of AAV9-anti grik2 miRNA into the hippocampus of TLE mice led to a marked reduction in seizure activity. Transduction of TLE patient hippocampal slices reduced levels of GluK2 protein and, most importantly, significantly reduced IEDs. INTERPRETATION: Our gene silencing strategy to knock down aberrant GluK2 expression demonstrates inhibition of chronic seizure in a mouse TLE model and IEDs in cultured slices derived from TLE patients. These results provide proof-of-concept for a gene therapy approach targeting GluK2 KARs for drug-resistant TLE patients. ANN NEUROL 2023;94:745-761.

Molecular and Cellular Mechanisms of Epilepsy.

Despite the availability of a large number of antiepileptic drugs, about 30% of patients with epilepsy, especially temporal lobe epilepsy (TLE), continue to experience seizures [...].

Comparative Study of Terminal Cortical Potentials Using Iridium and Ag/AgCl Electrodes.

Brain ischemia induces slow voltage shifts in the cerebral cortex, including waves of spreading depolarization (SD) and negative ultraslow potentials (NUPs), which are considered as brain injury markers. However, different electrode materials and locations yield variable SD and NUP features. Here, we compared terminal cortical events during isoflurane or sevoflurane euthanasia using intracortical linear iridium electrode arrays and Ag/AgCl-based electrodes in the rat somatosensory cortex. Inhalation of anesthetics caused respiratory arrest, associated with hyperpolarization and followed by SD and NUP on both Ir and Ag electrodes. Ag-NUPs were bell shaped and waned within half an hour after death. Ir-NUPs were biphasic, with the early fast phase corresponding to Ag-NUP, and the late absent on Ag electrodes, phase of a progressive depolarizing voltage shift reaching -100 mV by two hours after death. In addition, late Ir-NUPs were more ample in the deep layers than at the cortical surface. Thus, intracortical Ag and Ir electrodes reliably assess early manifestations of terminal brain injury including hyperpolarization, SD and the early phase of NUP, while the late, giant amplitude phase of NUP, which is present only on Ir electrodes, is probably related to the sensitivity of Ir electrodes to a yet unidentified factor related to brain death.

Somatosensory-Evoked Early Sharp Waves in the Neonatal Rat Hippocampus.

The developing entorhinal-hippocampal system is embedded within a large-scale bottom-up network, where spontaneous myoclonic movements, presumably via somatosensory feedback, trigger hippocampal early sharp waves (eSPWs). The hypothesis, that somatosensory feedback links myoclonic movements with eSPWs, implies that direct somatosensory stimulation should also be capable of evoking eSPWs. In this study, we examined hippocampal responses to electrical stimulation of the somatosensory periphery in urethane-anesthetized, immobilized neonatal rat pups using silicone probe recordings. We found that somatosensory stimulation in ~33% of the trials evoked local field potential (LFP) and multiple unit activity (MUA) responses identical to spontaneous eSPWs. The somatosensory-evoked eSPWs were delayed from the stimulus, on average, by 188 ms. Both spontaneous and somatosensory-evoked eSPWs (i) had similar amplitude of ~0.5 mV and half-duration of ~40 ms, (ii) had similar current-source density (CSD) profiles, with current sinks in CA1 strata radiatum, lacunosum-moleculare and DG molecular layer and (iii) were associated with MUA increase in CA1 and DG. Our results indicate that eSPWs can be triggered by direct somatosensory stimulations and support the hypothesis that sensory feedback from movements is involved in the association of eSPWs with myoclonic movements in neonatal rats.

Early patterns of activity in the developing cortex: Focus on the sensorimotor system.

Early development of somatotopic cortical maps occurs during the fetal period in humans and during the postnatal period in rodents. During this period, the sensorimotor cortex expresses transient patterns of correlated neuronal activity including delta waves, gamma- and spindle-burst oscillations. These early activity patterns are largely driven by the thalamus and triggered, in a topographic manner, by sensory feedback resulting from spontaneous movements. Early cortical activities are instrumental for competitive interactions between sensory inputs for the cortical territories, they prevent cortical neurons from apoptosis and their alteration may lead to disturbances in cortical network development in a number of neurodevelopmental diseases.

Emergence of Coordinated Activity in the Developing Entorhinal-Hippocampal Network.

Correlated activity in the entorhinal-hippocampal neuronal networks, supported by oscillatory and intermittent population activity patterns is critical for learning and memory. However, when and how correlated activity emerges in these networks during development remains largely unknown. Here, we found that during the first postnatal week in non-anaesthetized head-restrained rats, activity in the superficial layers of the medial entorhinal cortex (MEC) and hippocampus was highly correlated, with intermittent population bursts in the MEC followed by early sharp waves (eSPWs) in the hippocampus. Neurons in the superficial MEC layers fired before neurons in the dentate gyrus, CA3 and CA1. eSPW current-source density profiles indicated that perforant/temporoammonic entorhinal inputs and intrinsic hippocampal connections are co-activated during entorhinal-hippocampal activity bursts. Finally, a majority of the entorhinal-hippocampal bursts were triggered by spontaneous myoclonic body movements, characteristic of the neonatal period. Thus, during the neonatal period, activity in the entorhinal cortex (EC) and hippocampus is highly synchronous, with the EC leading hippocampal activation. We propose that such correlated activity is embedded into a large-scale bottom-up circuit that processes somatosensory feedback resulting from neonatal movements, and that it is likely to instruct the development of connections between neocortex and hippocampus.

Segregation of seizures and spreading depolarization across cortical layers.

OBJECTIVE: Cortical spreading depolarization (SD) and seizures are often co-occurring electrophysiological phenomena. However, the cross-layer dynamics of SD during seizures and the effect of SD on epileptic activity across cortical layers remain largely unknown. METHODS: We explored the spatial-temporal dynamics of SD and epileptic activity across layers of the rat barrel cortex using direct current silicone probe recordings during flurothyl-induced seizures. RESULTS: SD occurred in half of the flurothyl-evoked seizures. SD always started from the superficial layers and spread downward either through all cortical layers or stopping at the L4/L5 border. In cases without SD, seizures were characterized by synchronized population firing across all cortical layers throughout the entire seizure. However, when SD occurred, epileptic activity was transiently silenced in layers involved with SD but persisted in deeper layers. During partial SD, epileptiform activity persisted in deep layers throughout the entire seizure, with positive signals at the cortical surface reflecting passive sources of population spikes generated in deeper cortical layers. During full SD, the initial phase of SD propagation through the superficial layers was similar to partial SD, with suppression of activity at the superficial layers and segregation of seizures to deep layers. Further propagation of SD to deep layers resulted in a wave of transient suppression of epileptic activity through the entire cortical column. Thus, vertical propagation of SD through the cortical column creates dynamic network states during which epileptiform activity is restricted to layers without SD. SIGNIFICANCE: Our results point to the importance of vertical SD spread in the SD-related depression of epileptiform activity across cortical layers.

Inhibition of Cortical Activity and Apoptosis Caused by Ethanol in Neonatal Rats In Vivo.

Alcohol consumption during pregnancy causes fetal alcohol spectrum disorder, which includes neuroapoptosis and neurobehavioral deficits. The neuroapoptotic effects of alcohol have been hypothesized to involve suppression of brain activity. However, in vitro studies suggest that ethanol acts as a potent stimulant of cortical activity. We explored the effects of alcohol (1-6 g/kg) on electrical activity in the rat somatosensory cortex in vivo at postnatal days P1-23 and compared them with its apoptotic actions. At P4-7, when the peak of alcohol-induced apoptosis was observed, alcohol strongly suppressed spontaneous gamma and spindle-bursts and almost completely silenced neurons in a dose-dependent manner. The dose-dependence of suppression of neuronal activity strongly correlated with the alcohol-induced neuroapoptosis. Alcohol also profoundly inhibited sensory-evoked bursts and suppressed motor activity, a physiological trigger of cortical activity bursts in newborns. The suppressive effects of ethanol on neuronal activity waned during the second and third postnatal weeks, when instead of silencing the cortex, alcohol evoked delta-wave electrographic activity. Thus, the effects of alcohol on brain activity are strongly age-dependent, and during the first postnatal week alcohol profoundly inhibits brain activity. Our findings suggest that the adverse effects of alcohol in the developing brain involve suppression of neuronal activity.

The Nature of the Sensory Input to the Neonatal Rat Barrel Cortex.

UNLABELLED: Sensory input plays critical roles in the development of the somatosensory cortex during the neonatal period. This early sensory input may involve: (1) stimulation arising from passive interactions with the mother and littermates and (2) sensory feedback arising from spontaneous infant movements. The relative contributions of these mechanisms under natural conditions remain largely unknown, however. Here, we show that, in the whisker-related barrel cortex of neonatal rats, spontaneous whisker movements and passive stimulation by the littermates cooperate, with comparable efficiency, in driving cortical activity. Both tactile signals arising from the littermate's movements under conditions simulating the littermates' position in the litter, and spontaneous whisker movements efficiently triggered bursts of activity in barrel cortex. Yet, whisker movements with touch were more efficient than free movements. Comparison of the various experimental conditions mimicking the natural environment showed that tactile signals arising from the whisker movements with touch and stimulation by the littermates, support: (1) a twofold higher level of cortical activity than in the isolated animal, and (2) a threefold higher level of activity than in the deafferented animal after the infraorbital nerve cut. Together, these results indicate that endogenous (self-generated movements) and exogenous (stimulation by the littermates) mechanisms cooperate in driving cortical activity in newborn rats and point to the importance of the environment in shaping cortical activity during the neonatal period. SIGNIFICANCE STATEMENT: Sensory input plays critical roles in the development of the somatosensory cortex during the neonatal period. However, the origins of sensory input to the neonatal somatosensory cortex in the natural environment remain largely unknown. Here, we show that in the whisker-related barrel cortex of neonatal rats, spontaneous whisker movements and passive stimulation by the littermates cooperate, with comparable efficiency, in driving cortical activity during the critical developmental period.