Ih Blockers Have a Potential of Antiepileptic Effects

Abstract

Summary:Purpose: The h current (Ih) is an inwardly mixed cationic conductance activated by membrane hyperpolarization and distributed predominantly in the apical dendrites of hippocampal pyramidal neurons. To verify a hypothesis that an anomalous hyperpolarization generates an abnormal excitation by way of Ih channels, we examined the effects of Ih blockers (CsCl and ZD7288) on electrically induced paroxysmal discharges (PADs).Methods: Fifty-three adult male rabbits were used. We measured the PAD threshold elicited by stimulation to the apical dendritic layer of the hippocampal CA1 region before and after injecting 50 l of each Ih blocker or saline extracellularly into the same region.Results: In Ih blocker injection groups (n ‫ס‬ 26), we obtained a significant increase in PAD threshold (1 mM CsCl: 163%, p < 0.01; 10 mM CsCl: 265%, p < 0.01; 100 mM CsCl: 199%, p < 0.01; 100 M ZD7288: 192%, p < 0.05; 1 mM ZD7288: 246%, p < 0.05). Conversely, we did not obtain the increase in PAD threshold in a saline injection group (n ‫ס‬ 10, 107%). The magnitude as well as duration of the effect had a tendency to depend on concentration of Ih blockers, although a saturated or declining tendency was observed with the 100 mM CsCl injection.Conclusions: We concluded that Ih channels might contribute to hippocampal epileptiform discharges in vivo. Our hypothesis for epileptogenesis demonstrated in the present experiment offers an idea to develop a new type of antiepileptic drug based on Ih blockers for the treatment of epileptic disorders refractory to current medications. Key Words: EpilepsyHippocampus-Paroxysmal discharge-Hyperpolarizationactivated current-In vivo physiology.Multiple ionic channels regulate neuronal activity. In the hippocampal CA1 region, Na + and Ca 2+ channels activate and inactivate neuronal membrane depolarizations and contribute to the generation of excitatory postsynaptic potentials (1-3). Before as well as after the membrane excitation, the release of ␥-aminobutyric acid from inhibitory interneurons induces inhibitory postsynaptic potentials (IPSPs) and suppresses/terminates the depolarizing potentials (4-6). Previous hypotheses for epileptogenesis were based on imbalance between such excitatory and inhibitory currents (7).Recently the h current (Ih) has been investigated (8-24), and the following characteristics of Ih were reported: (a) Ih is a voltage-gated nonselective cationic conductance activated by membrane hyperpolarization, not by depolarization (8,14); (b) Ih is blocked by cesium chloride (CsCl) or 4-ethylphenylamino-1,2-dimethyl-6-methylaminopyrimidinium chloride (ZD7288) instead of tetrodotoxin (8,13,15); and (c) Ih channels occur in higher density in the apical dendrites than in the soma (8,14). In spite of many in vitro reports on Ih channels (8-24) as well as in vivo studies on the pacemaking function of Ih (25-28), any contributions of Ih to epileptic activity are unknown in vivo, except for the one that showed the long-lasting enhancement of the widely expressed intrinsic Ih that converts...

Effects of the Ih Blockers CsCl and ZD7288 on Inherited Epilepsy in Mongolian Gerbils

“…We previously demonstrated that CsCl and ZD7288 increased the threshold of electrically induced paroxysmal discharge in the rabbit hippocampus CA1 region [19]. In the present study, oral administration of CsCl or ZD7288 to Mongolian gerbils decreased not only the frequency but also the severity of their inherited epilepsy.…”

“…However, studies indicate that HCN1 and HCN2 are most prevalent in the cortex and hippocampus of rats [29], that Ih channel density is highest in the apical dendritic layer of the rat hippocampus CA1 [42], and that HCN1 mRNA is expressed at high levels in the mouse hippocampus CA1 region [37]. In a previous study in which the Ih blockers CsCl and ZD7288 were injected into the stratum radiatum or lacunosum-moleculare in the hippocampus CA1, we found that CsCl and ZD7288 increased the threshold of electrically induced paroxysmal discharge [19]. These findings suggest that Ih channels are present in the hippocampus of Mongolian gerbils.…”

“…Profound loss of interneuronal and principal cell populations and consequent reduced inhibition may induce a compensatory increase in HCN1 mRNA and protein expression [2]. The Yang (=excitatory) aspects include blockade of Ih channels by CsCl or ZD7288, which has been found to raise the afterdischarge threshold in an in vivo model of stimulus-evoked seizures [19]. It has been reported that blockade of Ih by ZD7288 in vitro did not produce epileptiform discharges [43].…”

“…We previously demonstrated that the Ih blockers CsCl and ZD7288 exert potential antiepileptic effects by significantly increasing the threshold of electrically induced paroxysmal discharge in rabbit hippocampus region CA1 [19]. Because there have been no reported studies of the contribution of Ih channels to seizures in Mongolian gerbils, the present study was designed to reveal whether Ih blockers can affect inherited seizures in Mongolian gerbils.…”

“…In a recent study using genetically modified mice in which HCN2 was deleted, the mice had spontaneous absence seizures, suggesting that a lack of HCN2 has a strongly proconvulsive effect [24]. However, in other studies, blockade of Ih channels with CsCl or ZD7288 raised the afterdischarge threshold in an in vivo model of stimulus-evoked seizures [19], and blockade of Ih by ZD7288 in vitro did not produce epileptiform discharges [43].…”