Alterations in hyperpolarization-activated cyclic nucleotidegated cation channel (HCN) expression in the hippocampus following pilocarpine-induced status epilepticus

Abstract

To understand the effects of HCN as potential mediators in the pathogenesis of epilepsy that evoke long-term impaired excitability; the present study was designed to elucidate whether the alterations of HCN expression induced by status epilepticus (SE) is responsible for epileptogenesis. Although HCN1 immunoreactivity was observed in the hippocampus, its immunoreactivities were enhanced at 12 hrs following SE. Although, HCN1 immunoreactivities were reduced in all the hippocampi at 2 weeks, a re-increase in the expression at 2-3 months following SE was observed. In contrast to HCN1, HCN 4 expressions were un-changed, although HCN2 immunoreactive neurons exhibited some changes following SE. Taken together, our findings suggest that altered expressions of HCN1 following SE may be mainly involved in the imbalances of neurotransmissions to hippocampal circuits; thus, it is proposed that HCN1 may play an important role in the epileptogenic period as a compensatory response.

Fig. 1.. The HCN1 expressions in the hippocampus following pilocarpine-induced SE. HCN1 immunoreactivity is selectively detected in the stratum lacunosum-molecule and some hilar neurons (arrows in panel A1 and A4). However, HCN1 immunoreactivities at 30 min-12 hrs following SE are significantly enhanced in CA1-3 and the stratum lacunosum-molecule (B1-B3 and C1-C3). In addition, HCN1-positive interneurons and hilar neurons are increased (arrows and open arrows in panel B2, B4, C2, and C4). Nevertheless, at 2 weeks following SE, HCN1 immunoreactivities are down-regulated to levels similar to the control (D1-D4). At 11 weeks after SE, HCN1 expression in all hippocampal regions and its immunoreactive interneurons is re-enhanced, similar to 12 hrs following SE (E1-E4). Bar = 280 μm (panels A1, B1, C1, D1, and E1), 50 μm (panels A2-A4, B2-B4, C2-C4, D2-D4, and E2-E4).

Fig. 2.. Quantitative analyses of HCN1 immunoreactive interneurons (A) and immunodensity (B) in normal and epileptic hippocampi following pilocarpine-induced SE (mean ± S.E.M). Significant differences from the control group, *P ＜ 0.05, **P ＜ 0.01.

Fig. 3.. The HCN2 immunoreactivity in the hippocampus in the normal and epileptic animal models following SE. HCN2 expressions are detected in some interneurons of CA1-3 and the dentate gyrus (arrows in panel A2-A4), whereas its immunoreactivity is observed in the granule cell layer of the dentate gyrus (A1 and A4). At 12 hrs following SE, HCN2 immunoreactivities in interneuronal populations and the granule cell layer are reduced compared to the control (arrows in panel B2-B4). These expressions are enhanced in CA1 and the dentate gyrus at day 7 following SE (C1-C4). After this time window, HCN2 immunoreactive interneurons are re-declined until 5 weeks following SE (D1-D4). Bar = 280 μm (panels A1, B1, C1, and D1), 50 μm (panels A2-A4, B2-B4, C2-C4, and D2-D4). Quantitative analyses of HCN2 immunoreactivity in the normal and epileptic hippocampi following SE (E, mean ± S.E.M). Significant differences from the control group, *P ＜ 0.05, **P ＜ 0.01.

Fig. 4.. The HCN4 expressions in the hippocampus of the normal and epileptic animal groups following SE. HCN4 immunoreactivities are rarely detected in all hippocampal regions, whereas its expression is observed in some interneurons (arrows in panels A2-A4). However, the immunoreactivities of HCN4 in the hippocampus following SE are unchanged depending on the time course after pilocarpine treatment (B1-D4). Bar = 280 μm (panels A1, B1, C1, and D1), 50 μm (panels A2-A4, B2-B4, C2-C4, and D2-D4). Quantitative analyses of HCN4 immunoreactivity in the normal and epileptic hippocampi following SE (E, mean ± S.E.M).