Although multiple CK isoforms are expressed in brain [41–44], little is known regarding the localization of these isoforms of the functional role for CK in brain. Indeed, it has been reported that expressions of CKs in the brain are detected in astrocytes , in neurons and astrocytes , and in nuclei of glial cells . In the present study, uMtCK immunoreactivity was observed in CA1-3 pyramidal cells, dentate granule cells and hilar neurons. In contrast, astrocytes and a small subpopulation of hilar neurons showed BCK immunoreactivity. In addition, CRT immunoreactivity was observed in dentate granule cells and CA3 pyramidal cells. These findings are agreement with a previous study demonstrating the localization of CKs and CRT in the rat hippocampus [48, 49].
Seizure activity is one of the acute high-energy demand situations showing an increase in local cerebral glucose utilization , a higher CK rate constant and a high ATP turnover . Therefore, Cr and CKs would play a role in providing a large amount of energy to be required for seizure progression. In the present study, BCK and CRT immunoreactivities were decreased following acute seizure, while uMtCK immunoreactivity was unaltered. Considering a slower chemically induced seizure development in BCK KO mice , these findings are simply indicated that reduced BCK immunoreactivity would be a compensatory response for seizure activity. However, the present study showed that BCK expression was mainly detected in hilar interneurons, which are vulnerable to seizure-induced insults . It has been reported that PILO injection induces astroglial degeneration in the molecular layer of the dentate gyrus [34, 35, 50]. Furthermore, these alterations are accompanied by neuronal excitability [38, 50]. Therefore, our findings reveal that reduced BCK immunoreactivity induced by PILO may result in dysfunction of hilar interneurons and astrocyte, or at least indicate decreases in their activity. The fast-spiking capability play a role in the responsiveness of inhibitory neurons [51–56]. Therefore, it is conceivable that reduced BCK expression/activity may result in a loss of fast-firing capability to allow the development of uncontrolled discharges. Furthermore, it is noteworthy that astroglial activity can change electrophysiological properties in a synaptic transmission independent manner: enhanced astroglial glutamate release, reduced glutamate reuptake, reductions of glutamine synthase and glutamate dehydrogenase, and impaired K+ buffering in response to seizure activity [38, 57–60]. Taken together, these reports led us to speculate on a possible enhanced seizure activity induced by reduced BCK expression/activity in hilar interneurons and astrocytes. In the present study, indeed Cr or tat-BCK treatment reduced PILO-induced acute seizure susceptibility. In addition, GPA treatment induced epileptiform discharges without PILO application. With respect to suppressing seizure activity and increasing neuronal survival by Cr (Cr-like compound) feeding [20, 21], our findings suggest that reduced BCK immunoreactivity may be a practical cause of abnormal discharge rather than an indicative of a compensatory consequence of seizure activity, and that maintenance of Cr-PCr/CK circuit by BCK may play an important role in increasing acute/initial seizure threshold.
In the present study, uMtCK immunoreactivity was markedly reduced in CA1-3 pyramidal cells and hilar neurons due to massive neuronal loss. In contrast, BCK immunoreactivity was similar to that observed in control animals. Furthermore, Cr-, GPA and tat-BCK treatment could not change EEG patterns or RMS values. Vielhaber et al.  reported that Cr feeding has deleterious effects on pyramidal cell survival in the PILO model of temporal lobe epilepsy, since mitochondrial enzyme activities are decreased in epileptic rats and Cr feeding induces more significant decrease in mitochondrial enzyme activities. With respect to this previous study, the present data demonstrating the ineffectiveness of Cr-, GPA and tat-BCK on EEG in epileptic rats are not surprising. Although the exact biological mechanism is unclear, furthermore, the phenomenon may be a consequence from loss of mitochondria or defective oxidative phosphorylation in epileptic hippocampus . Further detailed studies are needed to elucidate the roles of Cr and its metabolism in spontaneous seizure activity of chronic epileptic rats.
CRT expresses in neurons and oligodendroglia in physiological condition. Under physiological conditions, therefore Cr can cross from blood to brain through the blood-brain barrier (BBB) , but with a low permeability , partly because astrocytes lining the BBB do not express CRT [62–64]. However, Acosta et al.  reported the localization of CRT in perivascular astrocytes within mouse retina. Furthermore, Braissant et al.  have recently reported that NH4Cl induced CRT expression in astrocytes, including the swollen astrocytes that develop during NH4
+ exposure. Therefore, they suggested that the Cr level-dependent CNS gene regulation for CRT depends on the cell types considered and the pathological state of the brain. Similarly, the present study revealed that CRT expression was observed in reactive hypertrophic astrocytes in the hippocampi of epileptic rats, while CRT immunoreactivity was markedly reduced in CA3 pyramidal cells. Therefore, our findings support that CRT may express in astrocytes in pathological condition, although CRT expression in astrocytes is rarely detected in the brain under physiological condition. Indeed, the properties of reactive astrocyte are different from those of naive astrocyte [34, 35, 38, 50] Taken together with preservation of BCK expression in reactive astrocytes, our findings suggest that maintenance of Cr-PCr/CK circuit in reactive astrocytes may be involved in migration, proliferation and differentiation of reactive astrocytes in epileptic hippocampus described in previous studies [34, 38, 50].