Infantile spasms are a catastrophic form of pediatric epilepsy with inadequate treatment. In patients, mutation of ARX, a transcription factor selectively expressed in neuronal precursors and adult inhibitory interneurons, impairs cell migration and causes a major inherited subtype of the disease X-linked infantile spasms syndrome. Using an animal model, the Arx(GCG)10+7 mouse, we determined that brief estradiol (E2) administration during early postnatal development prevented spasms in infancy and seizures in adult mutants. E2 was ineffective when delivered after puberty or 30 days after birth. Early E2 treatment altered mRNA levels of three downstream targets of Arx (Shox2, Ebf3, and Lgi1) and restored depleted interneuron populations without increasing GABAergic synaptic density. Postnatal E2 treatment may induce lasting transcriptional changes that lead to enduring disease modification and could potentially serve as a therapy for inherited interneuronopathies.

X-linked Infantile Spasms Syndrome (ISSX) is a catastrophic epilepsy of early childhood with intractable seizures, intellectual disability, and poor prognosis. A spectrum of mutations in the Aristaless-Related Homeobox gene (ARX) has been linked to ISSX, and downstream targets of this interneuron-expressed transcription factor are being defined. Recent advances combining in vitro and in vivo methods have unveiled complex interactions between Arx and its binding partners and their effects on cell migration and maturation that can help explain the diversity of ARX phenotypes. New mutant mouse models of Arx-induced pathology, including a recent human triplet-repeat expansion mutation with a phenotype of infantile spasms and electrographic seizures, provide valuable tools for exploring the pathophysiology of Arx and substrates for testing novel therapies.

The ClC protein family includes voltage-gated chloride channels and chloride/proton exchangers. In eukaryotes, ClC proteins regulate membrane potential of excitable cells, contribute to epithelial transport, and aid in lysosomal acidification. Although structure/function studies of ClC proteins have been aided greatly by the available crystal structures of a bacterial ClC chloride/proton exchanger, the availability of useful pharmacological tools, such as peptide toxin inhibitors, has lagged far behind that of their cation channel counterparts. Here we report the isolation, from Leiurus quinquestriatus hebraeus venom, of a peptide toxin inhibitor of the ClC-2 chloride channel. This toxin, GaTx2, inhibits ClC-2 channels with a voltage-dependent apparent K D of ϳ20 pM, making it the highest affinity inhibitor of any chloride channel. GaTx2 slows ClC-2 activation by increasing the latency to first opening by nearly 8-fold but is unable to inhibit open channels, suggesting that this toxin inhibits channel activation gating. Finally, GaTx2 specifically inhibits ClC-2 channels, showing no inhibitory effect on a battery of other major classes of chloride channels and voltagegated potassium channels. GaTx2 is the first peptide toxin inhibitor of any ClC protein. The high affinity and specificity displayed by this toxin will make it a very powerful pharmacological tool to probe ClC-2 structure/function.

ClC chloride channels play essential roles in membrane excitability and maintenance of osmotic balance. Despite the recent crystallization of two bacterial ClC-like proteins, the gating mechanism for these channels remains unclear. In this study we tested scorpion venom for the presence of novel peptide inhibitors of ClC channels, which might be useful tools for dissecting the mechanisms underlying ClC channel gating. Recently, it has been shown that a peptide component of venom from the scorpion L. quinquestriatus hebraeus inhibits the CFTR chloride channel from the intracellular side. Using two-electrode voltage clamp we studied the effect of scorpion venom on ClC-0, -1, and -2, and found both dose- and voltage-dependent inhibition only of ClC-2. Comparison of voltage-dependence of inhibition by venom to that of known pore blockers revealed opposite voltage dependencies, suggesting different mechanisms of inhibition. Kinetic data show that venom induced slower activation kinetics compared to pre-venom records, suggesting that the active component(s) of venom may function as a gating modifier at ClC-2. Trypsinization abolished the inhibitory activity of venom, suggesting that the component(s) of scorpion venom that inhibits ClC-2 is a peptide.

Impaired motivation has been a long recognized negative symptom of schizophrenia, as well as a common feature of non-psychotic psychiatric disorders, responsible for a significant share of functional burden, and with limited treatment options. The striatum and dopamine signalling system play a central role in extracting motivationally relevant information from the environment, selecting which behavioural direction the animal should follow, and the vigour with which to engage it. Much of this function relies on striatal projection neurons, known as medium spiny neurons (MSNs) expressing dopamine D2 receptors (D2Rs), or D2-MSNs. However, determining the precise nature of D2-MSNs in regulating motivated behaviour in both healthy individuals and experimental manipulations of D2-MSN function has at times yielded a somewhat confusing picture since their activity has been linked to either enhancement or dampening of motivation in animal models. In this MiniReview, we describe the latest data from rodent studies that investigated how D2Rs exert their modulatory effect on motivated behaviour by regulating striatal indirect pathway neuronal activity. We will include a discussion about how functional selectivity of D2Rs towards G protein-dependent or β-arrestin-dependent signalling differentially affects motivated behaviour. Lastly, we will describe a recent preclinical attempt to improve motivation by exploiting serotonin receptor-mediated modulation of dopamine transmission in the striatum. K E Y W O R D SD2 receptors, dopamine, indirect pathway, motivation, striatum