Abstracts

Rationale: Chronic activation of astrocytes in the rodent thalamus has been observed after cerebrocortical stroke and brain injury and is associated with epileptogenesis (Ref. 1). Healthy astrocytes are well-positioned to mediate normal rhythmogenic properties of thalamic circuits (Ref. 2), especially by regulating GABAergic dynamics. Given that abnormal thalamic oscillatory activity is associated with seizures, reactive astrocytes may contribute to maladaptive changes underlying epileptogenesis. Yet, comorbidities such as inflammation and cell death have prevented a detailed investigation of the role of astrogliosis on neural circuits.  Methods: We determined the role of reactive astrocytes on the rhythmogenic properties of thalamic and thalamocortical circuits in vitro and in vivo, by selectively activating astrocytes in the mouse somatosensory thalamus (Ref. 3). We performed whole-cell patch-clamp recordings of thalamic neurons, multi-array recordings of thalamic microcircuits in acute brain slices, and electrocorticographic and multi-unit thalamic recordings in awake freely behaving mice. We also performed RNA sequencing of reactive astrocytes isolated from the mouse thalamus and immunohistochemical analyses of post-mortem human thalamic tissue.  Results: We deconstructed the effects of thalamic astrogliosis on molecular, cellular, circuit, and behavioral levels. Thalamic astrogliosis decreased mRNA and protein expression of the GABA transporters, GAT-1 and GAT-3, in reactive astrocytes, and led to enhanced cellular excitability of neighboring thalamocortical relay neurons by increasing extrasynaptic GABA(A)R-mediated tonic inhibitory currents (p<0.003). GAT reduction was corroborated by immunohistochemical data from post-mortem thalamic tissue obtained from patients with brain injury or stroke (p<0.005). Mice with thalamic astrogliosis displayed a higher susceptibility to pentylenetetrazol-induced spike-wave discharge events compared to control mice (p<0.05). Targeted, conditional deletion of the delta subunit of the GABA(A) receptor in the thalamus counteracted the increase in tonic inhibition (p<0.005) and in seizure susceptibility following astrogliosis. Finally, overexpression of GAT-3 in thalamic reactive astrocytes also counteracted the increased seizure susceptibility following astrogliosis (p<0.05).  Conclusions: Selective activation of astrocytes in the mouse somatosensory thalamus is sufficient to enhance intrathalamic and thalamocortical circuit excitability. We found that this enhanced excitability is mediated by alterations in astrocytic regulation of GABA and a subsequent increase in tonic inhibition. Importantly, increased seizure susceptibility was rescued by counteracting the increase in tonic inhibition, as well as by selectively overexpressing astrocytic GABA transporters. In ongoing studies, we are investigating the therapeutic potential of GAT overexpression in mouse models of post-traumatic epilepsies. Our study uncovers a link between reactive astrocytes and thalamic neuronal excitability, pinpointing the disruption of astrocytic GABA metabolism as a potential target for preventing epileptogenesis.  Funding: F.S.C. is supported by a F31 Predoctoral Fellowship from NINDS, a Graduate Fellowship from the NSF and the UCSF Discovery Fellowship.J.T.P is supported by NINDS (R01NS096369) and Department of Defense.