Abstracts

Rationale: Robust and chronic activation of astrocytes in the rodent thalamus has been observed after cerebrocortical stroke and brain injury, and has been associated with epileptogenesis (Ref. 1). Healthy astrocytes are well-positioned to regulate the physiological rhythmogenic properties of thalamic circuits (Ref. 2). Given that abnormal thalamic oscillatory activity is associated with focal and generalized seizures, reactive astrocytes may contribute to maladaptive changes in the thalamus that underlie epileptogenesis. However, the role of astrogliosis on neural circuits remains poorly understood and has been confounded by comorbidities such as inflammation and cell death. 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 downregulated GABA transporter GAT-1 mRNA and protein in reactive astrocytes, and enhanced the cellular excitability of neighboring thalamocortical relay neurons by increasing extrasynaptic GABA(A)-mediated tonic inhibitory currents (p<0.003). GAT-1 reduction was corroborated by immunohistochemical data from post-mortem thalamic tissue obtained from patients with brain injury or stroke (p<0.005). Targeted, conditional deletion of the delta subunit of the GABA(A) receptor in the thalamus counteracted the increase in tonic inhibition following astrogliosis (p<0.005).Acute brain slices from mice with thalamic astrogliosis displayed more evoked bursts and longer durations of high-frequency oscillatory activity compared to slices from control mice (p=0.006). Mice with thalamic astrogliosis displayed a higher susceptibility to pentylenetetrazol-induced spike-wave discharge events compared to control mice (p<0.05). Conclusions: Selective activation of astrocytes in the mouse somatosensory thalamus is sufficient to enhance intrathalamic and thalamocortical circuit excitability. This enhanced excitability is mediated by alterations in astrocytic regulation of GABA and a subsequent increase in tonic inhibition. Targeted deletion of the extrasynaptic GABA(A) receptor is sufficient to counteract the increase in tonic inhibition, suggesting that such an approach may counteract the enhanced circuit excitability and therefore seizure susceptibility following thalamic astrogliosis. 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.ReferencesPaz et al, Nat Neurosci. 2013;16(1).Khakh and Sofroniew. Nat Neurosci. 2015;18(7).Ortinski et al. Nat Neurosci. 2010;13(5). Funding: F.S.C. is supported by the NSF Graduate and the UCSF Discovery fellowships. J.T.P is supported by NINDS (R01NS096369).