The p53 protein is activated by cellular stress and DNA damage and, depending on the severity of stress and the specific cell type, can contribute to adaptive responses to stress or can trigger cell cycle arrest or its apoptosis 10. When normal proliferating cells are DNA damaged, they may react in one of the two ways: cell cycle arrest r apoptosis, and p53 is engaged in both of those processes 11.
The p53 protein is an important component in the neuron apoptosis, for example, after ischemic event or excitotoxicity 12. An increase in number of such neurons has been demonstrated in ischemia, traumatic brain injuries 13. Some research studies have proved that p53 is part of the biochemical processes in the cell caused by the activation of NMDA- receptors (N-methyl-D-aspartate) and finally resulting to apoptosis 14.
The mechanisms of proapoptotic action are assumed to be realized through the induction of the TP53 gene (protein p53) expression, the regulation of which inhibits the passage of the cell cycle from G1 to S-phase, which blocks the division of cancer cells and tumor growth 15. Except to the widely studied role of p53 as a regulator of apoptosis triggering, its neuroprotective role has also been demonstrated 16. The main goal of neuroprotection is to prevent the death of neurons in the ischemic area, where apoptosis is one of the mechanisms of neuronal death. Bioenergetic processes are slowed down in the penumbra and neurons which have not died yet, remaining in them. In this regard, the absence of neurons in the hippocampus with obvious morphological signs of apoptosis in the analysis of the slices obtained by us, may indicate the presence of neuroprotective properties of the p53 protein. It has been shown that posttranslational modifications of p53 can contribute to the differentiation of neurons, as well as to the growth and regeneration of axons 17.
It was shown that p53 is a neuroprotector in an in vivo model of taupathy 18. By analyzing the chromatin immunoprecipitation chip, it was determined, that p53 controls the transcription of a group of genes participated in providing synaptic function. Genetic manipulation of these genes changed the neurotoxicity of the tau- protein. The authors have found, that both in mice’s neurons and in the human brain, transcriptional control of these synaptic genes is maintained due to p53. Thus, it has been suggested, that the provision of synaptic function, as a manifestation of neuroprotection, can be performed by p53 protein 18.
In addition to activating the TP53 gene, NMDA receptors participate in caspase-dependent apoptosis, increasing the level of calcium ions, boosting the enzyme caspase-3 activity. This enzyme, in turn, start the formation of dark neurons and their next degeneration. When inhibiting caspases with the pancaspase inhibitor FK011, a decrease in the changes characteristic of dark neurons has been achieved 19.
Previously, it was also shown, that dark neurons can both restore their morpho-functional state by increasing the cisterns of the granular endoplasmic reticulum with the formation of membrane curls, the transition of this process to astrocytic processes and, as a consequence, with a subsequent decline in the degree of structural compaction of the cell 7, and be a sign of the final necrotic decay of the cell regardless of the cause of neuron death, including various biochemical cascades of apoptosis 8. There is an opinion, that dark neurons- are the result of oxidative stress. Thus, it has been shown, that the use of luteolin after brain injury in vivo reduces the number of dark neurons and oxidative stress in them in the hippocampus 20. It has been shown, that the presence of regenerating dark neurons in the case of animal studies indicates the vulnerability of neuroprotective properties of neuroglia 21. The presence of such phenomena, as cytoplasmic shrinkage and surface reduction in DN is compared by some authors with the manifestation of neuroplasticity characteristics 22, 23. In fact, plasticity refers to the unique ability of the brain (neuron) to change and reorganize in response to changes in the environment. This property of neurons contributes to their viability and, consequently, the organism’s survival 24. The most well-known examples of neuronal plasticity are the formation of new synapses, the proliferation of dendritic spines, the retraction and simplification of dendrites, and the reduction of dendritic spines under stressful conditions. Some studies have shown that endogenous or exogenous stressors are associated with a decrease in the surface and dendritic spike of neurons 25.
The high determination coefficients found in this study confirm the theory that presence of dark neurons in the hippocampus and dentate gyrus is most likely closely related to the expression of the p53 protein during surgical stress caused by septoplasty simulation in rats. This is probably due to the activation of NMDS receptors in neurons under the influence of surgical stress, as it has been shown that stress leads to an increase in the content of NMDA receptors in the dendritic spike apparatus 26. In addition, exposure to a large amount of glutamate leads to functional changes in neurons and subsequent launch of the apoptosis program 27. Stress is known to result in degeneration of hippocampal neuron dendrites 28. In dark neurons, the dendrites are poorly developed or practically absent as a result of modulation of NMDA receptors. This was shown by the example of CA3 subfield neurons in the hippocampus 29. In addition, chronic stress has been reported to cause atrophy of the pyramidal layer of the subfield CA1 30, 31, decrease the long-term potentiation of neurons of the hippocampal CA1 subfield 32 and cause apoptosis of neurons, as well as a decrease in the density of dendritic spikes in neurons in the CA1 region of the hippocampus 33. Thus, it can be assumed that there is a common mechanism that result to the start of two processes, discussed in this article – the expression of the p53 protein in the cytoplasm and the formation of dark neurons. The trigger of these pathways is probably the activation of NMDA receptors of neurons.