Biology subjects

Sullivan, B. J.

Publications and source records attributed to Sullivan, B. J..

7 recordsLinked to original sources

bioRxiv2025

Rapid neocortical network modifications via dendritic plateau potential induced plasticity

Learning in brains is associated with changes in neuronal network activity thought to be driven by synaptic plasticity. While recent work in the hippocampus has revealed some of the mechanisms involved there, less is known about how neocortical circuits adapt, especially during behavior. Here to determine if neocortical areas possess rapid plasticity mechanisms that could support online adaptations we used optical imaging and intracellular membrane potential (Vm) recordings to examine the activity of layer V neurons in a higher visual area of mice learning a task. The introduction of a novel rewarded stimulus resulted in a rapid modification of population activity that featured abrupt alterations in single neuron selectivity. Vm recordings revealed that both naturally occurring and experimentally-induced dendritic calcium plateau potentials (plateaus) rapidly alter the action potential (APs) output and Vm dynamics of neurons over many seconds of time around the plateau, in some cases from one trial to the next. Trains of high frequency APs had no effect. Finally, experimental inhibition during learning of the distal dendritic region responsible for initiating plateaus reduced the rate of population level adaptation. Our findings suggest that the deep layers of higher order visual cortex possess a rapid learning mechanism mediated by plateau-induced synaptic plasticity.

neuroscience↗

bioRxiv2023

Superior colliculus cell types bidirectionally modulate choice activity in frontal cortex

Action selection occurs through competition between potential choice options. Neural correlates of choice competition are observed across frontal cortex and downstream superior colliculus (SC) during decision-making, yet how these regions interact to mediate choice competition remains unresolved. Here we report that cell types within SC can bidirectionally modulate choice competition and drive choice activity in frontal cortex. In the mouse, topographically matched regions of frontal cortex and SC formed a descending motor pathway for directional licking and a re-entrant loop via the thalamus. During decision-making, distinct neuronal populations in both frontal cortex and SC encoded opposing lick directions and exhibited push-pull dynamics. SC GABAergic neurons encoded ipsilateral choice and glutamatergic neurons encoded contralateral choice, and activating or suppressing these cell types could bidirectionally drive push-pull choice activity in frontal cortex. These results thus identify SC as a major locus to modulate choice competition within the broader action selection network.

neuroscience↗

bioRxiv2020

GABAergic signaling promotes early-life seizures in epileptic SYNGAP1+/- mice

ObjectiveSYNGAP1 encephalopathy is a developmental and epileptic encephalopathy caused by pathogenic loss of function variants. Syngap1-heterozygous (Het+/-) mice demonstrate progressive epilepsy with multiple seizure phenotypes in adulthood. Here, we investigate early-life seizures in Het+/- pups and explore of Syngap1 encephalopathy during development. MethodsPost-natal day 7 (P7) and P12 mice were investigated by tethered video-electroencephalographic (vEEG). The effects of GABAergic drugs phenobarbital (PB) and pentylenetetrazol (PTZ) were investigated at P7 and P12, respectively. 24h tethered vEEG was performed at P24, and telemetric 24h vEEG with 6h sleep deprivation was performed at P35. The effect of perampanel (PMP), an AMPA receptor antagonist, was investigated at P24. ResultsHet+/- mice have spontaneous early-life seizures that lack an overt behavioral phenotype. These subclinical seizures are refractory to PB, but the GABAA receptor (GABAAR) antagonist PTZ significantly reduced seizure frequency suggesting that GABAergic signaling may promote seizure generation in Het+/- pups. At juvenile ages, Het+/- pups recapitulated the early emergence of high gamma (35-50Hz) during NREM and disruption of behavioral-state gamma homeostasis. This biomarker was significantly exacerbated in Het+/- pups after increasing sleep pressure with sleep deprivation. SignificanceSeizures during development have adverse effects on cognitive function. Therefore, an improved understanding of the SYNGAP1 epilepsy during developmental ages is necessary to delineate the deleterious interactions between aberrant synaptic function and recurrent seizures. The development of evidence-based therapies for early-life intervention will benefit from these insights.

neuroscience↗

bioRxiv2020

Targeting ischemia-induced KCC2 hypofunction rescues refractory neonatal seizures and mitigates epileptogenesis in a mouse model

Neonatal seizures pose a clinical challenge for their early detection, acute management, and mitigation of long-term comorbidities. A major cause of neonatal seizures is hypoxic-ischemic encephalopathy that results in seizures that are frequently refractory to the first-line anti-seizure medication phenobarbital (PB). One proposed mechanism for PB-inefficacy during neonatal seizures is the reduced expression and function of the neuron-specific K+/Cl- cotransporter 2 (KCC2), the main neuronal Cl- extruder that maintains chloride homeostasis and influences the efficacy of GABAergic inhibition. To determine if PB-refractoriness after ischemic neonatal seizures is dependent upon KCC2 hypofunction and can be rescued by KCC2 functional enhancement, we investigated the recently developed KCC2 functional enhancer CLP290 in a CD-1 mouse model of refractory ischemic neonatal seizures quantified with vEEG. We report that acute CLP290 intervention can rescue PB-resistance, KCC2 expression, and the development of epileptogenesis after ischemic neonatal seizures. KCC2 phosphorylation sites have a strong influence over KCC2 activity and seizure susceptibility in adult experimental epilepsy models. Therefore, we investigated seizure susceptibility in two different knock-in mice in which either phosphorylation of S940 or T906/T1007 was prevented. We report that KCC2 phosphorylation regulates both neonatal seizure susceptibility and CLP290-mediated KCC2 functional enhancement. Our results validate KCC2 as a clinically relevant target for refractory neonatal seizures and provide insights for future KCC2 drug development.

neuroscience↗

bioRxiv2019

TrkB agonists prevent post-ischemic BDNF-TrkB mediated emergence of refractory neonatal seizures in CD-1 pups.

Refractory neonatal seizures do not respond to first-line anti-seizure medications (ASMs) like phenobarbital (PB), a positive allosteric modulator for GABAA receptors, the most widely used ASM to treat neonatal seizures. GABAA receptor-mediated inhibition is dependent upon neuronal chloride regulation. The electroneutral cation-chloride transporter KCC2 mediates neuronal chloride extrusion; an age-dependent increase of KCC2 expression enables the shift of GABAergic signaling from depolarizing to hyperpolarizing. BDNF-TrkB activation following excitotoxic injury recruits downstream targets like PLC{gamma}1, leading to KCC2 hypofunction. This study investigated the efficacy of partial and full TrkB agonists; LM22A-4 (LM), HIOC and Deoxygedunin (DG) respectively, on PB-refractory seizures, post-ischemic TrkB-pathway activation, and KCC2 membrane stability in a P7 CD-1 mouse model of refractory neonatal seizures. Anti-seizure efficacy was determined by quantifying seizure burdens with continuous video-EEG. LM rescued PB-refractory seizures in a sexually dimorphic manner. LM anti-seizure efficacy was associated with a significant reduction in the post-ischemic phosphorylation of TrkB at Y816, a site known to mediate post-ischemic KCC2 hypofunction via PLC{gamma}1 activation. LM additionally rescued ischemia-induced pKCC2-S940 dephosphorylation preserving its membrane stability. HIOC and DG, two novel full TrkB agonists, also rescued PB-refractoriness and post-ischemic TrkB-PLC{gamma}1 pathway activation. Additionally, chemogenetic inactivation of TrkB significantly reduced post-ischemic neonatal seizure burdens at P7. Developmental expression profiles of TrkB and KCC2 in naive pups identified developmental differences that may underlie the sex-dependent variance in anti-seizure efficacy. These results support a novel role for the TrkB receptor in the emergence of age-dependent refractory neonatal seizures.

neuroscience↗

bioRxiv2019

Perampanel rescues cortical gamma dysregulation associated with parvalbumin interneuron GluA2 upregulation in epileptic Syngap1+/- mice

Loss-of-function SYNGAP1 mutations cause a neurodevelopmental disorder characterized by intellectual disability and epilepsy. SYNGAP1 is a Ras-GTPase-activating protein that underlies the formation and experience-dependent regulation of postsynaptic densities. The mechanisms that contribute to this proposed monogenic cause of intellectual disability and epilepsy remain unresolved. Here, we establish the phenotype of the epileptogenesis in a Syngap1+/- mouse model using 24h video electroencephalogram/electromyogram (vEEG/EMG) recordings at advancing ages. A progressive worsening of clinically-similar seizure phenotypes, interictal spike frequency, sleep dysfunction, and hyperactivity was identified in Syngap1+/- mice. Interictal spikes emerged predominantly during NREM in 24h vEEG of Syngap1+/- mice. Myoclonic seizures occurred at behavioral-state transitions both in Syngap1+/- mice and during an overnight EEG from a child with SYNGAP1 haploinsufficiency. In Syngap1+/- mice, EEG spectral power analyses identified a significant loss of cortical gamma homeostasis during behavioral-state transitions from NREM to Wake and NREM to REM. The loss of gamma homeostasis was associated with a region- and location-specific significant increase of GluA2 AMPA receptor subunit expression in the somas of parvalbumin-positive (PV+) interneurons. Acute dosing with Perampanel, an FDA approved AMPA antagonist significantly rescued cortical gamma homeostasis, identifying a novel mechanism implicating Ca2+ impermeable AMPARs on PV+ interneurons underlying circuit dysfunction in SYNGAP1 haploinsufficiency.

neuroscience↗

bioRxiv2018

Transcranial photoacoustic imaging of NMDA-evoked focal circuit dynamics in rat forebrain

Transcranial functional photoacoustic (fPA) voltage-sensitive dye (VSD) imaging promises to overcome current temporal and spatial limitations of current neuroimaging modalities. The technique previously distinguished global seizure activity from control neural activity in groups of rats. To validate the focal specificity of transcranial fPA neuroimaging in vivo, we now present proofs-of-concept that the results differentiate between low- and high-dose N-methyl-D-aspartate (NMDA) evoked neural activity in rat hippocampus. Concurrent quantitative EEG (qEEG) and microdialysis recorded real-time circuit dynamics and glutamate concentration change, respectively. We hypothesized that location-specific fPA VSD contrast would identify the neural dynamics in hippocampus with the correlation to NMDA evoked focal glutamate release and time-specific EEG signals. To test the hypothesis, we infused 0.3 to 3.0 mM NMDA at 2 l/min over 60 min via an implanted microdialysis probe. The dialysate samples collected every 20 min during the infusion were analyzed for focal changes in extracellular glutamate release. Transcranial fPA VSD imaging provided NMDA-evoked VSD responses with positive correlation to extracellular glutamate concentration change at the contralateral side of the microdialysis probe. The graded response represents the all-or-none gating system of the dentate gyrus (DG) in hippocampus. Quantitative EEG (qEEG) successfully confirmed induction of focal seizure activity during NMDA infusion. We conclude that transcranial fPA VSD imaging distinguished graded DG gatekeeping functions, based on the VSD redistribution mechanism sensitive to electrophysiologic membrane potential. The results suggest the potential future use of this emerging technology in clinics and science as an innovative and significant functional neuroimaging modality.

neuroscience↗