Comparison of neuronal death and expression of TNF‑α and MCT4 in the gerbil hippocampal CA1 region induced by ischemia/reperfusion under hyperthermia to those under normothermia

Hyperthermia accelerates neuronal loss differently between the hippocampal CA1 and CA2/3 through different HIF‑1α expression after transient ischemia in gerbils

The hippocampus has a different vulnerability to ischemia according to the subfields CA1 to CA3 (initials of cornu ammonis). It has been reported that body temperature changes during ischemia affect the degree of neuronal death following transient ischemia. Hypoxia-inducible factor 1α (HIF-1α) plays a key role in regulating cellular adaptation to low oxygen conditions. In the present study, we investigated the pattern of neuronal death (loss) in CA1 and CA2/3 following 5 min transient forebrain ischemia (TFI) under hyperthermia (39.5±0.2°C) and the relationship between neuronal death and changes in HIF-1α expression using western blot analysis and immunohistochemistry in gerbils. Normothermia or hyperthermia was induced for 30 min before and during the TFI, and neuronal death and HIF-1α expression were observed at 0, 3, 6 and 12 h, 1, 2 and 5 days after TFI. Under normothermia, TFI-induced neuronal death of CA1 pyramidal neurons occurred on day 5 after TFI, but CA2/3 pyramidal neurons did not die. In contrast, under hyperthermia, the death of CA1 and CA2/3 pyramidal neurons was observed on day 2 after TFI. Under normothermia, HIF-1α expression was significantly elevated in both CA1 and CA2/3 pyramidal neurons at 12 h and 1 day after TFI, and the increased HIF-1α immunoreactivity in CA1 was dramatically reduced from 2 days after TFI, but not in CA2/3 pyramidal neurons. Under hyperthermia, the basal expression of HIF-1α in the sham group was significantly higher in both CA1 and CA2/3 pyramidal neurons at 0 h after TFI than in the normothermia group. HIF-1 expression was continuously higher, peaked at 12 h after TFI, and then significantly decreased from 1 day after TFI. Overall, the present results indicate that resistance to ischemia in CA2/3 pyramidal neurons is closely associated with the persistence of increased expression of HIF-1α after ischemic insults and that hyperthermia-induced exacerbation of death of pyramidal neurons is closely related to decreased HIF-1α expression after ischemic insults.

Differences in TNF‑α and TNF‑R1 expression in damaged neurons and activated astrocytes of the hippocampal CA1 region between young and adult gerbils following transient forebrain ischemia

Tumor necrosis factor (TNF)-α and TNF receptor 1 (TNF-R1) play diverse roles in modulating the neuronal damage induced by cerebral ischemia. The present study compared the time-dependent changes of TNF-α and TNF-R1 protein expression levels in the hippocampal subfield cornu ammonis 1 (CA1) between adult and young gerbils following transient forebrain ischemia (tFI), via western blot and immunohistochemistry analyses. In adult gerbils, delayed neuronal death of pyramidal neurons, the principal neurons in CA1, was recorded 4 days after tFI; however, in young gerbils, delayed neuronal death was recorded 7 days after tFI. TNF-α protein expression levels gradually increased in both groups following tFI; however, TNF-α expression was higher in young gerbils compared with adult gerbils. TNF-R1 protein expression levels markedly increased in both groups 1 day after tFI. Subsequently, TNF-R1 expression gradually decreased in young gerbils, whereas TNF-R1 expression levels were irregularly altered in adult gerbils following tFI. Notably, TNF-α immunoreactivity significantly increased in pyramidal neurons in both groups 1 day after tFI; however, the patterns altered between both groups. In adult gerbils, TNF-α immunoreactivity was rarely exhibited in pyramidal neurons 4 days after tFI due to neuronal death, suggesting that TNF-α immunoreactivity was newly expressed in astrocytes. In young gerbils, TNF-α immunoreactivity increased in pyramidal neurons 4 days after tFI, and TNF-α immunoreactivity was newly expressed in astrocytes. In addition, TNF-R1 immunoreactivity was exhibited in pyramidal cells of both sham groups, and significantly increased 1 day after tFI; however, the patterns altered between both groups. In adult gerbils, TNF-R1 immunoreactivity was rarely exhibited 4 days after tFI, and astrocytes newly expressed TNF-R1 immunoreactivity. In young gerbils, TNF-R1 immunoreactivity increased in pyramidal neurons 4 days after tFI; however, TNF-R1 immunoreactivity was not reported in pyramidal neurons and astrocytes thereafter. Taken together, the results of the present study suggest that different expression levels of TNF-α and TNF-R1 in ischemic CA1 between adult and young gerbils may be due to age-dependent differences of tFI-induced neuronal death.

Ischemia-Reperfusion under Hyperthermia Increases Heme Oxygenase-1 in Pyramidal Neurons and Astrocytes with Accelerating Neuronal Loss in Gerbil Hippocampus

It has been studied that the damage or death of neurons in the hippocampus is different according to hippocampal subregions, cornu ammonis 1–3 (CA1–3), after transient ischemia in the forebrain, showing that pyramidal neurons located in the subfield CA1 (CA1) are most vulnerable to this ischemia. Hyperthermia is a proven risk factor for brain ischemia and can develop more severe and extensive brain damage related with mortality rate. It is well known that heme oxygenase-1 (HO-1) activity and expression is increased by various stimuli in the brain, including hyperthermia. HO-1 can be either protective or deleterious in the central nervous system, and its roles depend on the expression levels of enzymes. In this study, we investigated the effects of hyperthermia during ischemia on HO-1 expression and neuronal damage/death in the hippocampus to examine the relationship between HO-1 and neuronal damage/death following 5-min transient ischemia in the forebrain using gerbils. Gerbils were assigned to four groups: (1) sham-operated gerbils with normothermia (Normo + sham group); (2) ischemia-operated gerbils with normothermia (Normo + ischemia group); (3) sham-operated gerbils with hyperthermia (39.5 ± 0.2 °C) during ischemia (Hyper + sham group); and (4) ischemia-operated gerbils with hyperthermia during ischemia (Hyper + ischemia group). HO-1 expression levels in CA1–3 of the Hyper + ischemia group were significantly higher than those in the Normo + ischemia group. HO-1 immunoreactivity in the Hyper + ischemia group was significantly increased in pyramidal neurons and astrocytes with time after ischemia, and the immunoreactivity was significantly higher than that in the Normo + ischemia group. In the Normo + Ischemia group, neuronal death was shown in pyramidal neurons located only in CA1 at 5 days after ischemia. However, in the Hyper + ischemia group, pyramidal neuronal death occurred in CA1–3 at 2 days after ischemia. Taken together, our findings showed that brain ischemic insult during hyperthermic condition brings up earlier and severer neuronal damage/death in the hippocampus, showing that HO-1 expression in neurons and astrocytes is different according to brain subregions and temperature condition. Based on these findings, we suggest that hyperthermia in patients with ischemic stroke must be taken into the consideration in the therapy.