Nitric oxide (NO) is postulated to be involved in the pathophysiology of different epilepsy models. NO is syn- thesized from L-arginine by three isoforms of nitric oxide synthase (NOS) and the citrulline formed as a co-product may be used for arginine production by the action of argininosuccinate synthetase (AS) and argini- nosuccinate lyase (AL) through citrulline-NO cycle [1] . The induction of AS, cationic amino acid transporter-2, and NOS in activated murine microglial cells [2] and induction of inducible NOS, AS and AL in cyto- kine-stimulated PC12 cells and high production of NO were earlier reported by Zhang et al. [1] . Kainic acid (KA) is a powerful excitotoxin, and produces acute and sub-acute epileptic activity, ultimately resulting in widespread irreversible neuropathological changes [3] . The mechanisms contributing to high concentration of NO in epilepsy are not well understood. It was shown that NOS knockout mice were more severely affected by epileptic activity than controls and the response to NO during epilepsy depends on its concentration and NO may be regarded as an anticonvulsant and proconvulsant substance in relation to convulsions induced by penty- lenetetrazole (PTZ) [4] . In the CNS increased excitatory glutamate receptors, activation is considered as an im- portant mechanism in neurodegenerative disorders and the conversion of glutamate to glutamine by glutamine synthetase (GS), that takes place within the astrocytes, represents a key mechanism in the regulation of excita- tory neurotransmission under physiological as well as pathological conditions in brain [5] . Earlier studies re- ported the nitration and inhibition of GS during PTZ induced seizure model at repeated PTZ seizure induction, but there was no change in protein concentration [6] .

However, there were gene expression studies indicating decreased GS expression in chronic phase of epilepsy induced by KA [7] . It is also reported that haploinsufficiency of GS increases susceptibility to experimental fe- brile seizures [8] . It is earlier reported the decreased activity of GS and increased activities of NOS, AS, AL in acute and chronic groups of KA mediated epilepsy [9] . Therefore the present study was conducted to analyze immunohistochemically the expression of nNOS, iNOS, AS, AL and GS in cerebral cortex (CC), cerebellum (CB) and brain stem (BS) of rats in acute and chronic groups of KA mediated epilepsy.

The immunoreactivity of AS, AL, nNOS, iNOS and GS in different regions of rat brain in control, acute and chronic groups of KA mediated epilepsy were given in Figures 1-5 respectively and their immunoreactivity analysis was given in Figure 6. In acute group, AS and iNOS immunoreactivity (Figure 1, Figure 4 and Figure 6) were increased in CC and CB, nNOS immunoreactivity (Figure 3 and Figure 6) increased in CC and BS, and there was no change in AL immunoreactivity (Figure 2 and Figure 6) compared to control group. In chronic group AS, AL, nNOS and iNOS immunoreactivity was increased when compared to control in all the brain re- gions tested (Figures 1-6). GS immunoreactivity (Figure 5 and Figure 6) was decreased in all the three brain regions in chronic group and CB in acute group.

Neuronal NO generation is implicated in the pathogenesis of both direct and secondary excitotoxic neuronal in- juries in vivo. NMDA receptors may be contributing critically to neuronal injury in various acute conditions, but several studies support the hypothesis that AMPA/KA receptors may be of greater importance to the neurode- generative process [14] . Earlier studies reported a high production of NO in cortex than in amygdala and hippo- campus, eventhogh there are most kainate receptors in the hippocampus [10] . The increased levels of NO in brain in epilepsy indicate the involvement of NO in pathophysiology of excitotoxicity [9] [10] [15] [16] . The in- creased production of NO in acute as well as in chronic group indicates the continuous increased production of NO and its deleterious effects in CNS in chronic conditions of epilepsy [9] . The increased iNOS immunoreac- tivity observed in all brain regions in chronic group in this study supports the increased activity of NOS and in- creased iNOS mRNA expression reported [9] [17] . The increased iNOS immunoreactivity along with AS and AL immunoreactivity in chronic group of this study indicates the co-expression of these enzymes in epilepsy. Such a co-induction of iNOS, AS and AL were reported earlier in cytokine-stimulated PC12 cells [1] .

In the CNS increased excitatory glutamate receptors, activation is considered as an important mechanism in neurodegenerative disorders [18] [19] . The conversion of glutamate to glutamine by GS, that takes place within the astrocytes, represents a key mechanism in the regulation of excitatory neurotransmission under physiological as well as pathological conditions in brain [5] . The GS is present in all regions of brain and it is high in cerebral cortex, cerebellum and hippocampus [20] [21] . The modulation of GS activity is important and its impairment or saturation can cause pathological consequences in brain [22] . The GS activity was decreased in acute and chronic groups of epilepsy [9] . The mechanisms of inhibition of GS by NO are not clear, but it is reported to be as a covalent modification due to nitrosylation or nitration of tyrosine in GS [21] [23] . Earlier studies reported the nitration and inhibition of GS during PTZ induced seizure model at repeated PTZ seizure induction, but there was no change in protein concentration [6] , however, there were gene expression studies indicating de- creased GS expression in chronic phase of epilepsy induced by KA [7] [17] . The decreased GS immunoreactivty observed in all the brain regions of chronic group in this study support the earlier report of decreased GS ex- pression [7] [17] . The results of GS immunoreactivty in acute group did not show any change in CC and BS, hence the reported decrease of GS activity [15] may be due to its nitration by NO in this condition. It is also re- ported that haploinsufficiency of GS increases susceptibility to experimental febrile seizures [8] . The decreased GS immunoreactivty observed in this study and decreased activity and mRNA expression reported earlier may contribute for prolonged availability of glutamate for excitotoxicity [9] [16] [17] . The increased formation of NO along with increased activities and mRNA expression of NOS, AS, and AL reported earlier [9] [16] [17] and the immunoreactivty results presented in this study are in agreement with increased formation of NO in KA me- diated epilepsy and NO involvement in modulation of GS. It is indicated that the decreased GS may provide sustained availability of glutamate, which may contribute to excitotoxicity in chronic epilepsy.

The study clearly shown the increased immunoreactivty of nNOS and iNOS in all the brain regions studied and indicate a complimentary condition for increased NO synthesis in chronic epilepsy. The increased immunoreac- tivty of AS and AL in all the three brain regions studied in chronic group may contribute for increased recycling of citrulline to arginine which may support the increased NO generation. Decreased GS immunoreactivty ob- served in this study provides a reason for the decreased GS activity in addition to the nitration of GS which was reported earlier. The decreased GS immunoreactivty observed in this study and decreased activity and mRNA expression (reported earlier) may contribute for prolonged availability of glutamate for excitotoxicity.

This study received financial support from Universiti Sains Malaysia—Research University grant (A/C No: 1001/PSKBP/8120196). The parts of the study were presented in 36^th Annual Conference of the Malaysian So- ciety for Biochemistry & Molecular Biology, 27^th - 28^th July 2011, Selangor, Malaysia, 22^nd Malaysian Associa- tion of Clinical Biochemists Conference, 2^nd - 3^rd July 2012, Kula Lumpur, and International Conference on Natural Products 2013, 4^th - 6^th March 2013, SACC, Selangor, Malaysia.