# Dysregulated Plasticity in Serotonin, Galanin, and Opioid Systems Contributes to Limbic Seizure Recruitment in Wistar Audiogenic Rat

## TL;DR

A genetic rat model shows that chronic loud noise leads to limbic seizures due to disrupted brain signaling in serotonin, galanin, and opioid systems.

## Contribution

The study reveals that limbic epilepsy progression in WAR rats is due to widespread failure of adaptive plasticity across multiple neuromodulatory systems.

## Key findings

- WAR rats failed to upregulate presynaptic serotonergic components in the DRN in response to chronic stimulation.
- Limbic seizure severity was strongly correlated with reduced prodynorphin (Pdyn) expression.
- Increased mu-opioid receptor expression in WARs contributes to pro-convulsant effects.

## Abstract

The Wistar Audiogenic Rat (WAR) strain is a genetically selected model of reflex epilepsy, susceptible to mesencephalic and, following chronic stimulation, limbic seizures. In this study, we examined the molecular underpinnings of this seizure progression by assessing gene expression profiles of pre‐synaptic serotonergic components in Dorsal Raphe Nucleus (DRN) and post‐synaptic receptors in the Basolateral Amygdala (BLA), Central Amygdala (CeA), and Hippocampus (HIP). Concurrently, we evaluated mRNA expression of Galanin (Gal) and Prodynorphin (Pdyn) in the Supraoptic Nucleus (SON) and their respective receptors in the BLA, CeA, and HIP. WARs and control Wistar rats underwent a ten‐day audiogenic kindling (AK) protocol, involving twice‐daily exposure to a high‐intensity acoustic stimulus to induce seizures. WARs were sub‐grouped based on their behavioral phenotype (seizure scales) into limbic‐recruited seizures (LiR) and non‐limbic‐recruited (n‐LiR). Quantitative PCR analysis of brain micropunches revealed a significant failure of adaptive plasticity in WARs. Unlike control rats, which showed a robust upregulation of serotonergic (5‐HT‐ergic) components in the DRN in response to the chronic stress of the kindling protocol, WARs had a significantly blunted pre‐synaptic response. Rats that did not show limbic seizures showed compensatory upregulation of amygdala 5‐HT receptors, a mechanism that failed in rats that developed chronic seizures. Furthermore, WARs showed elevated hypothalamic galanin but reduced limbic receptor expression. The opioid system was also imbalanced, with an increase in the pro‐convulsant mu‐opioid receptor. Critically, Pdyn expression was strongly and negatively correlated with limbic seizure severity. Collectively, these findings suggest that the progression to limbic epilepsy, already demonstrated in behavioral and EEG protocols in this model, is driven by a widespread failure of plasticity across interconnected neuromodulatory networks, rather than a single molecular defect, highlighting novel targets for therapeutic intervention.

To find molecular reasons why a genetic rat model (Wistar Audiogenic Rat, WAR) moves from sound‐triggered brainstem seizures to longer‐lasting limbic (temporal‐lobe) epilepsy after repeated loud‐noise exposure. WAR and normal Wistar rats received a 10‐day audiogenic kindling (twice‐daily loud noise). Brains were sampled to measure mRNA for serotonin‐related presynaptic components (dorsal raphe nucleus—DRN), serotonin receptors (basolateral and central amygdala—BLA and CeA—and hippocampus—HIP), galanin (GAL) and prodynorphin (PDYN) in the hypothalamus (SON) and their receptors in limbic areas. WARs were classified by behavior into limbic‐recruited (LiR) and non‐limbic (n‐LiR) groups. Normal rats showed strong upregulation of presynaptic serotonin genes in the dorsal raphe after repeated stimulation; WARs failed to mount this adaptive response. n‐LiR WARs increased amygdala serotonin receptors (a compensatory protective change); LiR WARs did not. WARs had increased hypothalamic galanin but reduced galanin receptor expression in limbic regions, limiting galanin's protective action. The opioid system was imbalanced in WARs: higher mu‐opioid receptor expression (pro‐convulsant) and lower prodynorphin (Pdyn); lower Pdyn correlated strongly with worse limbic seizures. Progression to limbic epilepsy in WARs is not due to a single defect but to a widespread failure of adaptive plasticity across multiple neuromodulatory systems (serotonin, galanin, dynorphin/opioids) and their limbic connections. Preventing or treating this epilepsy progression likely requires restoring network‐wide adaptive signaling rather than targeting one molecule.