Developmental and cell type-specific contributions of thalamic serotonin 2A receptors to absence seizures

Serotonin 2A receptors (5-HT_2ARs) play a complex role in focal and generalized seizures due to their diverse cellular and regional distribution. Although systemic activation of 5-HT_2ARs suppresses absence seizures (ASs) in Genetic Absence Epilepsy Rats From Strasbourg (GAERS) rats, the contribution of thalamic receptors and their cell-type specificity remains unclear. Here, we performed a developmental immunohistochemical analysis in the nucleus reticularis thalami (NRT) and the ventrobasal thalamic nucleus (VB) of GAERS rats to assess developmental alterations in 5-HT_2AR expression and used genetic manipulation to determine whether the antiabsence effect of systemic 5-HT_2AR activation depends on thalamocortical (TC) neurons or astrocytes.

Double-immunofluorescence labeling of 5-HT_2ARs with either γ-aminobutyric acid (GABA) or glial fibrillary acidic protein in adult GAERS rats was used to investigate the neuronal and astrocytic distribution of these receptors in the NRT and VB. In addition, [³H]GABA uptake and its modulation by 5-HT_2AR activation were assessed in thalamic slices. Electroencephalographic and video recordings in freely moving GAERS were used to evaluate the effects of VB microinjection of TCB-2, a 5-HT_2AR agonist, on ASs. Finally, the cellular mechanisms underlying these effects were investigated using selective shRNA-mediated knockdown of 5-HT_2ARs in either TC neurons or astrocytes in the VB.

In the VB, at postnatal day (P) 25, 5-HT_2ARs were mainly expressed in TC neurons and in the majority of the few GABAergic interneurons, whereas by P90 they were exclusively localized to TC neurons. In the NRT, neuronal expression increased from ~60% to nearly 100% over development. Astrocytic 5-HT_2AR expression increased developmentally in the NRT but remained unchanged in the VB. GABA uptake was decreased in GAERS compared to Wistar rats and was not modified by 5-HT_2AR activation. In vivo, intra-VB injection of TCB-2 reduced ASs; this effect was abolished by shRNA knockdown of 5-HT_2ARs in TC neurons, but not in astrocytes.

The developmental reorganization of thalamic 5-HT_2AR signaling coincides with the expression of ASs, suggesting a contributory role. Our findings indicate that neuronal, but not astrocytic, thalamic 5-HT_2ARs drive seizure modulation, identifying a potential therapeutic target.