peroxidase activity in the brain of diabetic rats after piperine and the 35% decrease in
concentration of hepatic GSSG in piperine-treated diabetic rats as compared to
untreated diabetic rats appear unrelated to the effects of diabetes. In the study, the
lipophilic piperine was more likely to interact with membranes, influencing lipid
peroxidation and GSH transport. Superoxide dismutase and catalase, on the other
hand, are part of the defense system in the hydrophilic compartment of the cell, and
were less likely to be modulated by piperine treatment.
22.9 Effect on the CNS
After exposure to piperine for 96 h, the concentration toxicity relationships of
piperine on hippocampal neurons and astrocytes displayed marked differences in
terms of cellular vulnerability to piperine. Cultured astrocytes showed only a
moderate increase in LDH release without any significant change in morphology at
100 piperine. On the contrary, cultured hippocampal neurons showed marked
injuries both biochemically and morphologically. In the time course study, piperine
induced delayed neuronal injury in which significant damage was apparent after
48 h and increased gradually up to 96 h of incubation, whereas the same concentrations did not affect astrocytes at any of the times studied (Unchern et al. 1997).
Their data suggest that piperine is selectively cytotoxic to cultured neurons
compared with cultured astrocytes. It was notable that MTT reduction of
piperine-treated astrocytes increased at all concentrations used in spite of the
unchanged protein content. Mitochondrial dehydrogenases in viable cells are
mainly responsible for MTT reduction activity. These findings imply that the
apparent increase in MTT reduction in piperine-treated astrocytes may be due to
enhanced mitochondrial enzymatic activity rather than increased cell proliferation.
In addition, the fact that astrocyte cultures were grown to confluence before being
used in the experiment does not support piperine-induced astrocyte proliferation.
The relationship between increased mitochondrial function and the tolerance of
astrocytes to piperine toxicity is still unknown.
Several lines of evidence indicate that astrocytes possess more defences, particularly the glutathione system, against free radicals than neurons. In neurons, the
level of glutathione is much lower than in astroglial cells (Bolaños et al. 1995;
Makar et al. 1994). This is likely because the activity of c-glutamyl cysteine
synthetase, a key enzyme in glutathione synthesis, is approximately eightfold
higher in astrocytes compared with neurons. Therefore, cultured neurons seem to be
more vulnerable to reactive compounds such as peroxynitrite (ONOO–) than cultured astroglial cells. In this connection, it is possible that the differences in sensitivity of cultured neurons and cultured astrocytes to piperine may be explained by
their different cellular content of glutathione. Piperine neurotoxicity was prevented
by free radical scavengers thereby suggesting the involvement of lipid peroxidation
and/or free radicals in its toxic action. The reason for the failure of SOD and
catalase to protect hippocampal neurons may be because these agents cannot cross
22 Piperine: Sources, Properties, Applications …
765
concentration of hepatic GSSG in piperine-treated diabetic rats as compared to
untreated diabetic rats appear unrelated to the effects of diabetes. In the study, the
lipophilic piperine was more likely to interact with membranes, influencing lipid
peroxidation and GSH transport. Superoxide dismutase and catalase, on the other
hand, are part of the defense system in the hydrophilic compartment of the cell, and
were less likely to be modulated by piperine treatment.
22.9 Effect on the CNS
After exposure to piperine for 96 h, the concentration toxicity relationships of
piperine on hippocampal neurons and astrocytes displayed marked differences in
terms of cellular vulnerability to piperine. Cultured astrocytes showed only a
moderate increase in LDH release without any significant change in morphology at
100 piperine. On the contrary, cultured hippocampal neurons showed marked
injuries both biochemically and morphologically. In the time course study, piperine
induced delayed neuronal injury in which significant damage was apparent after
48 h and increased gradually up to 96 h of incubation, whereas the same concentrations did not affect astrocytes at any of the times studied (Unchern et al. 1997).
Their data suggest that piperine is selectively cytotoxic to cultured neurons
compared with cultured astrocytes. It was notable that MTT reduction of
piperine-treated astrocytes increased at all concentrations used in spite of the
unchanged protein content. Mitochondrial dehydrogenases in viable cells are
mainly responsible for MTT reduction activity. These findings imply that the
apparent increase in MTT reduction in piperine-treated astrocytes may be due to
enhanced mitochondrial enzymatic activity rather than increased cell proliferation.
In addition, the fact that astrocyte cultures were grown to confluence before being
used in the experiment does not support piperine-induced astrocyte proliferation.
The relationship between increased mitochondrial function and the tolerance of
astrocytes to piperine toxicity is still unknown.
Several lines of evidence indicate that astrocytes possess more defences, particularly the glutathione system, against free radicals than neurons. In neurons, the
level of glutathione is much lower than in astroglial cells (Bolaños et al. 1995;
Makar et al. 1994). This is likely because the activity of c-glutamyl cysteine
synthetase, a key enzyme in glutathione synthesis, is approximately eightfold
higher in astrocytes compared with neurons. Therefore, cultured neurons seem to be
more vulnerable to reactive compounds such as peroxynitrite (ONOO–) than cultured astroglial cells. In this connection, it is possible that the differences in sensitivity of cultured neurons and cultured astrocytes to piperine may be explained by
their different cellular content of glutathione. Piperine neurotoxicity was prevented
by free radical scavengers thereby suggesting the involvement of lipid peroxidation
and/or free radicals in its toxic action. The reason for the failure of SOD and
catalase to protect hippocampal neurons may be because these agents cannot cross
22 Piperine: Sources, Properties, Applications …
765