Abstracts

Rationale: Recently, we developed a new model of idiopathic infantile spasms in the brain compromised by the prenatal exposure to betamethasone on gestational day 15 (G15). This treatment sensitizes infant offspring on postnatal day 15 (P15) for accelerated development of NMDA-induced spasms and moreover, these spasms become responsive to ACTH treatment. Brain regions activated during NMDA-induced spasms include hypothalamic arcuate and medial tuberal nuclei as well as medial amygdala (Velisek et al., Ann Neurol, 61:109-119; 2007). In this study we focused on identification of changes elicited by prenatal betamethasone exposure in these target structures. We first determined expression and density of glucocorticoid receptors (GR) since they represent direct betamethasone target. Further, since the introduction of exogenous corticosteroids disturbs the HPA control systems, we determined expression and distribution of corticotropin releasing hormone (CRH). Indeed in infant rats, CRH has strong convulsant effects (Baram and Schultz, Dev Brain Res. 61: 91-101; 1991).Methods: Pregnant Sprague-Dawley rats were injected with two doses of betamethasone (0.4 mg/kg per dose) or saline (controls) on G15 (10 hours apart). Offspring was anesthetized on P15 and transcardially perfused with paraformaldehyde. Brains were removed, cryoprotected and sectioned at 40 µm. Alternate sections were processed with anti-GR and anti-CRH antibodies and visualized with biotinylated secondary antibody and DAB. Numbers of GR and CRH immunopositive neurons were counted in 5 matching sections in each structure. Results were compared to GR and CRH immunopopsitive neurons in saline-injected controls.Results: The number of GR-immunopositive cells was significantly decreased in the medial amygdala of betamethasone-exposed rats compared to saline controls. There were no differences in GR expression in the hypothalamus between betamethasone- and saline-exposed rats. On the other hand, we found dramatic increases in numbers of CRH-immunopositive neurons in the arcuate (+50%) and medial tuberal (+30%) nuclei of the hypothalamus in betamethasone-exposed rats compared to controls while there were no differences in amygdala.Conclusions: Results indicate that prenatal betamethasone exposure elevates convulsant CRH in those hypothalamic areas, which are activated during NMDA-triggered spasms. This may contribute to enhanced sensitivity to NMDA. Although we do not have information about ACTH levels, this model of infantile spasms associates alterations in the HPA axis hormones and spastic seizures mimicking a situation occurring in children with infantile spasms. (Supported by NS-41366 from the NIH and by the Albert Einstein College of Medicine).