Post-doctoral research associate The University of Sydney Camperdown, New South Wales, Australia
Rationale: Dravet syndrome (DS) is an early onset epileptic encephalopathy that typically begins with febrile seizures, progresses into spontaneous afebrile seizures that are poorly managed by anti-epileptic drugs. Cannabidiol (CBD) has been FDA-approved for treatment of intractable childhood epilepsies, DS and Lennox-Gastaut syndrome. While the mechanism underlying the anti-seizure action of CBD is unknown, it activates and rapidly desensitizes transient receptor potential vanilloid type 1 (Trpv1) cation channels that depolarize neurons. Despite low expression in the brain, Trpv1 inhibition has been shown to be anticonvulsant against PTZ, 6-Hz, and MES-induced seizures in mice. Thus, we sought to determine whether heterozygous deletion or pharmacological blockade of Trpv1 would be anticonvulsant in the Scn1a+/- mouse model of DS. Methods: Scn1a+/- mice were crossed with Trpv1+/- mice to generate mice that were heterozygous for both Scn1a and Trpv1 (Scn1a+/−,Trpv1+/−). We evaluated the effect of heterozygous Trpv1 deletion on hyperthermia-induced seizures, spontaneous seizures, and survival of Scn1a+/- mice. Additionally, we examined the effect of selective Trpv1 antagonist, SB-705498, on hyperthermia-induced seizures of Scn1a+/- mice. Hyperthermia was induced by increasing the body temperature of postnatal (P) 14-16 mice with a heat lamp and measured using a rectal probe. Onset of the first clonic seizure with loss of posture was noted as the seizure threshold temperature. Mice that reached 42.5 °C without clonic seizure for 3 min were considered seizure free. For spontaneous seizures, P18 mice were first subjected to hyperthermia and immediately cooled following the first GTCS to terminate the seizure. After recovery mice were individually housed and video monitored for three days. For pharmacological assessment Scn1a+/- mice were challenged with hyperthermia-induced seizures 15 minutes following i.p. injection of 10 and 20 mg/kg of selective Trpv1 antagonist SB-705498 via i.p. injection. Results: Heterozygous deletion of Trpv1 in Scn1a+/- mice (Scn1a+/−,Trpv1+/−) had no effect on the temperature threshold for hyperthermia-induced seizures compared to that of Scn1a+/- mice at postnatal day (P) 14-16. Survival and frequency of spontaneous seizures was not different between Scn1a+/−,Trpv1+/− mice and Scn1a+/- mice. Severity of spontaneous seizures measured by the proportion of seizures progressing to full hindlimb extension, however, was significantly reduced in Scn1a+/−,Trpv1+/− mice compared to Scn1a+/- mice (n=16-18, p = 0.0003). Interestingly, when mice were primed with a hyperthermia-induced seizure at P18, heterozygous deletion of Trpv1 in Scn1a+/- mice was pro-convulsant. Scn1a+/−,Trpv1+/− mice had a significantly lower temperature threshold for hyperthermia-induced seizures than Scn1a+/- mice (n=18-20, p = 0.007) at P18. Additionally, acute administration of SB-705498 (10 or 20 mg/kg) did not affect temperature threshold of hyperthermia-induced seizures in Scn1a+/- mice compared to vehicle-treated controls. Conclusions: While antagonism of Trpv1 has anticonvulsant properties in several mouse seizure models, targeting Trpv1 either genetically or pharmacologically does not appear to be very effective in the Scn1a+/- mouse model of DS. Furthermore, heterozygous deletion of Trpv1 in Scn1a+/- mice was even pro-convulsant against febrile seizures at P18. Antagonism of Trpv1 appears not to be a versatile anticonvulsant target in the Scn1a+/− mouse model of DS. Funding: Please list any funding that was received in support of this abstract.: Lambert Initiative for Cannabinoid Therapeutics and the National Health and Medical Research Council grant (GNT1161571) to JCA.