Abstracts

Rationale: Infantile spasms, one of the catastrophic epilepsy syndromes of childhood, are refractory to conventional antiepileptic drugs, while adrenocorticotropic hormone (ACTH) is a drug of choice. We developed a model of cryptogenic infantile spasms based on prenatal exposure to betamethasone combined with postnatal administration of N-methyl-D-aspartic Acid (NMDA). The prenatal treatment impairs the hypothalamic pituitary adrenal (HPA) axis in a way similar to the findings in children with infantile spasms. Postnatal NMDA administration triggers age-dependent spasms with involvement of hypothalamus, limbic areas, and the brainstem. In this model, acute treatment with ACTH is effective as in humans. Here we investigate mechanisms of the ACTH effects against the spasms in our model. Our hypothesis proposes that the melanocortin 4 (MC4) receptor, which binds ACTH and is found throughout the brain, is responsible for the ACTH effects. Methods: Rats prenatally exposed to betamethasone or saline were used on postnatal day (P) 15. By using immunohistochemistry, we determined the MC4 receptor expression in the brain structures critical for development of spasms in our model (arcuate nucleus [Arc]; medial tuberal Nucleus [MeTu], medial ventral nuclei of amygdala [MePV]; and in the pedunculopontine tegmental nucleus [PPTg]). Additional rats on P13 had a cannula implanted in the lateral ventricle for delivery of MC4 receptor antagonist HS014. These rats were treated with HS014 (1 µg/0.25 µl of HS014; or saline) i.c.v. along with the i.p. administration of 0.1 mg/kg of ACTH (water solution, full molecule) on P15. After 60 minutes, NMDA was administered intraperitoneally (15 mg/kg) to trigger the spasms. The latency to onset of NMDA-induced phenomena, especially to the onset of partial and full spasms as well as the number of full spasms was evaluated. Results: Prenatal betamethasone exposure significantly increased the MC4 receptor expression in two critical structures PPTg and MePV compared to control group prenatally exposed to saline (p <0.05, ANOVA). I.c.v pretreatment with MC4 receptor antagonist reversed anticonvulsant effects of ACTH in this model (p <0.05, Student’s t-test). Conclusions: MC4 receptors are upregulated by prenatal betamethasone exposure in brain structures (brainstem and limbic areas) critical for development of spasms in our model. Thus, the increase in the receptor expression improves chances of the ACTH to exert its effects. We can speculate that increases in MC4 receptor expression after prenatal betamethasone exposure in our model is a substrate for ACTH effects. The ACTH effects are absent in prenatally saline-exposed rats. This finding may represent an important pathophysiological mechanism in the development of infantile spasms in humans, as prenatal/perinatal stress may be one of the conditions for the occurrence of infantile spasms, yet the associated MC4 receptor upregulation also creates a substrate for therapeutic effects of ACTH.