Therapeutic doses of theophylline exert proconvulsant effects in developing mice

Zebrafish Embryo as an In Vivo Model for Behavioral and Pharmacological Characterization of Methylxanthine Drugs

Zebrafish embryo is emerging as an important tool for behavior analysis as well as toxicity testing. In this study, we compared the effect of nine different methylxanthine drugs using zebrafish embryo as a model. We performed behavioral analysis, biochemical assay and Fish Embryo Toxicity (FET) test in zebrafish embryos after treatment with methylxanthines. Each drug appeared to behave in different ways and showed a distinct pattern of results. Embryos treated with seven out of nine methylxanthines exhibited epileptic-like pattern of movements, the severity of which varied with drugs and doses used. Cyclic AMP measurement showed that, despite of a significant increase in cAMP with some compounds, it was unrelated to the observed movement behavior changes. FET test showed a different pattern of toxicity with different methylxanthines. Each drug could be distinguished from the other based on its effect on mortality, morphological defects and teratogenic effects. In addition, there was a strong positive correlation between the toxic doses (TC₅₀) calculated in zebrafish embryos and lethal doses (LD₅₀) in rodents obtained from TOXNET database. Taken together, all these findings elucidate the potentiality of zebrafish embryos as an in vivo model for behavioral and toxicity testing of methylxanthines and other related compounds.

Histamine H3 receptor antagonists in relation to epilepsy and neurodegeneration: a systemic consideration of recent progress and perspectives

The central histaminergic actions are mediated by H(1) , H(2) , H(3) and H(4) receptors. The histamine H(3) receptor regulates the release of histamine and a number of other neurotransmitters and thereby plays a role in cognitive and homeostatic processes. Elevated histamine levels suppress seizure activities and appear to confer neuroprotection. The H(3) receptors have a number of enigmatic features like constitutive activity, interspecies variation, distinct ligand binding affinities and differential distribution of prototypic splice variants in the CNS. Furthermore, this Gi/Go-protein-coupled receptor modulates several intracellular signalling pathways whose involvement in epilepsy and neurotoxicity are yet to be ascertained and hence represent an attractive target in the search for new anti-epileptogenic drugs. So far, H(3) receptor antagonists/inverse agonists have garnered a great deal of interest in view of their promising therapeutic properties in various CNS disorders including epilepsy and related neurotoxicity. However, a number of experiments have yielded opposing effects. This article reviews recent works that have provided evidence for diverse mechanisms of antiepileptic and neuroprotective effects that were observed in various experimental models both in vitro and in vivo. The likely reasons for the apparent disparities arising from the literature are also discussed with the aim of establishing a more reliable basis for the future use of H(3) receptor antagonists, thus improving their utility in epilepsy and associated neurotoxicity.

Histamine 1 receptor knock out mice show age-dependent susceptibility to status epilepticus and consequent neuronal damage

The central histaminergic neuron system is an important regulator of activity stages such as arousal and sleep. In several epilepsy models, histamine has been shown to modulate epileptic activity and histamine 1 (H1) receptors seem to play a key role in this process. However, little is known about the H1 receptor-mediated seizure regulation during the early postnatal development, and therefore we examined differences in severity of kainic acid (KA)-induced status epilepticus (SE) and consequent neuronal damage in H1 receptor knock out (KO) and wild type (WT) mice at postnatal days 14, 21, and 60 (P14, P21, and P60). Our results show that in P14 H1 receptor KO mice, SE severity and neuronal damage were comparable to those of WT mice, whereas P21 KO mice had significantly decreased survival, more severe seizures, and enhanced neuronal damage in various brain regions, which were observed only in males. In P60 mice, SE severity did not differ between the genotypes, but in KO group, neuronal damage was significantly increased. Our results suggest that H1 receptors could contribute to regulation of seizures and neuronal damage age-dependently thus making the histaminergic system as a challenging target for novel drug design in epilepsy.

Theophylline-induced changes in mouse electroencephalograms

Theophylline can induce life-threatening seizures in humans, especially in infants, but the mechanism of induction remains unknown. We investigated the effects of orally administered theophylline on mouse electroencephalograms (EEGs). ddY mice, which are generally completely free of seizures, were used for the experiments. While EEGs, used as controls, showed no paroxysmal spike discharges, theophylline induced clear spike discharges. This study demonstrated that theophylline administered at doses that achieve low serum concentrations can cause spike discharges in mouse EEGs even without causing clinical seizures, indicating that theophylline plays a potent role in subclinical epileptogenicity.

First-line therapy for theophylline-associated seizures

Theophylline-associated seizures (TAS) are considered a neurologic emergency, as they can sometimes be intractable and difficult to stop with standard treatments such as intravenous administration of diazepam. As a consequence, a proportion of patients who experience status epilepticus while receiving theophylline will require endotracheal intubation. The optimal first-line therapy for TAS has not yet been fully investigated. We compared 54 cases of TAS with 779 cases of non-TAS, that had presented at a single institution between 1991 and 2002. Among the 54 cases of TAS, 36 experienced generalized tonic-clonic seizures, with the remainder experiencing partial seizures. TAS occurred mainly in children under 3 years of age, and serum theophylline levels were within the therapeutic range in 78% of the cases. The duration of TAS tended to be longer than for non-TAS, and intravenous administration of diazepam was less effective in controlling TAS (45%), compared with non-TAS (68%). Many cases required repeated injections of diazepam, and 15 cases (27%) eventually required endotracheal intubation. Reports concerning the therapy for TAS were also reviewed. Theophylline is known to antagonize the effects of benzodiazepines, and this may explain why drugs such as diazepam are relatively ineffective in treating TAS. In TAS, the prompt use of barbiturates is recommended when diazepam is not effective, to avoid potential brain injury secondary to status epilepticus.

Theophylline impairs memory/learning in developing mice

We studied the relationship between theophylline and memory/learning using an elevated plus-maze test and measuring spontaneous locomotor activity in developing mice. There were no significant differences in transfer latency (TL) in 21-, 30- and 42-day-old mice in the acquisition trial, but theophylline significantly prolonged TL in comparison with the control group in 21- and 30-day-old mice in the retention trial (n = 20; 21-day-old mice, P = 0.0009; 30-day-old mice, P = 0.03). The dose-dependent stimulant effects of theophylline were shown in spontaneous locomotor activity (n = 16; 21-day-old mice, P = 0.0005; 30-day-old mice, P = 0.0007; 42-day-old mice, P = 0.0003) in all groups and in rearing in 21-day-old mice (P = 0.02) 1 h after drug administration, but there were no significant effects 24 h after drug administration in all groups. Our study showed that even a therapeutic dose of theophylline impaired memory/learning only in developing mice, and theophylline increased locomotor activity both in developing and adolescent mice. These observations are consistent with several clinical studies, which reported the relationships between theophylline use and memory/learning and learning-related behaviors in children, such as difficulty in studying and hyperactivity. Theophylline possibly induces memory/learning disabilities and hyperactivity in infants and young children.

Changes in motor cortical excitability in humans following orally administered theophylline

The effects of theophylline on human corticospinal excitability were studied using transcranial magnetic stimulation (TMS) before and after double-blind oral administration of theophylline or placebo in 20 healthy volunteers. TMS measurements included resting and active motor threshold, silent period, intracortical inhibition (ICI), and intracortical facilitation. F-wave and compound muscle action potential (CMAP) were also measured. Theophylline produces a reduction in ICI, while other parameters of corticospinal excitability remained unaffected. Since ICI is thought to depend on GABAA intracortical inhibitory mechanisms, our data suggest that the increase of human motor cortex excitability is the result of a decrease in GABAergic transmission. Our results further support the hypothesis that theophylline might induce convulsions by inhibiting GABAA receptor binding.

Toluene exposure increases aminophylline-induced seizure susceptibility in mice

The effects of toluene on the sensitivity to seizures induced by aminophylline were investigated. Mice were pretreated with an ip injection of corn oil or toluene (100-500 mg/kg) followed by a timed intravenous infusion of aminophylline at various time intervals to assess the seizure thresholds and lethal doses. Toluene increased seizure susceptibility to aminophylline in a dose- and time-dependent manner. Toluene-induced enhancement of seizure susceptibility to aminophylline occurred as early as 30 min and persisted for at least 3 days after a single administration of toluene (500 mg/kg). Treatment of benzaldehyde, one of toluene's metabolites, also showed an increase in the susceptibility to aminophylline. The enhancing effect was also observed in caffeine-induced seizures 1 h, but not 1 day after toluene treatment. These results suggest that individuals with toluene exposure may increase the risk for convulsive and even lethal complications associated with the therapeutic use of aminophylline.

Genomic organization and expression of the mouse equilibrative, nitrobenzylthioinosine-sensitive nucleoside transporter 1 (ENT1) gene

We have cloned and characterized the genomic structure of the mouse gene for the NBMPR-sensitive equilibrative nucleoside transporter (mENT1), which is located on chromosome 17C. About 12-kb of genomic DNA was sequenced including the promoter region, 12 exons, 11 introns, and the 3'-untranslated region. All exon-intron junction sequences conform to the GT/AG rule. Primer extension analysis demonstrated a transcription initiation site located 252 bp upstream of the translation start site. Analysis of the 2.5-kb 5'-flanking sequence shows putative binding sites for several transcription factors, including GATA-1, IRF-2, Pit-1, myogenin, CREB, Sp-1, Ap-2, MAZ, and GR. We demonstrated that mouse ENT1 mRNA was highly expressed in heart, spleen, lung, liver, and testis. Lower levels of expression were detected in brain and kidney. Functional analysis of the 5'-flanking region showed that the nucleotide sequence from -652 to -111 contains cis-regulatory elements that promote gene expression. We found two Sp-1 binding sites (-296/-303, -307/-313) and two MAZ binding sites (-353/-359, -522/-528) in this region. Luciferase assay results suggest that MAZ and Sp-1 transcription factors are important positive regulators of transcription for the ENT1 gene in NG108-15 cells.

Apnea of prematurity: diagnosis, implications for care, and pharmacologic management

Apnea is a disorder of respiratory control commonly seen in premature infants. Several mechanisms have been proposed to explain apnea, and many clinical conditions have been associated with its development. Apnea of prematurity is seen in infants less than 37 weeks gestation, with the incidence increasing as gestational age decreases. Expert and consistent nursing care is essential for management of premature infants with apnea. This article reviews the differential diagnosis, pathogenesis, and implications for care of apnea of prematurity.