Effect of p53 Inhibition by Pifithrin-Alpha on Functional Activity of Vasopressin Neurons in Rat Hypothalamus

Abstract

Introduction. The transcription factor and tumor suppressor p53 participates in the regulation of transcription, cell cycle, apoptosis, as well as differentiation and cellular lifespan [11,27,40]. The wide range of reports made on p53 knockout animals demonstrates the differential effects of p53 deficiency. It was shown that part of p53 knockout embryos die during development, other surviving mice develop exencephaly, but mainly the p53 knockout mice develop normally [15,33]. However, Armstrong and coauthors mentioned increased neuronal apoptosis in the spinal cord of p53 knockout mice [1], which can mean another role of p53 under certain conditions [33]. Such differential effects of p53 deficiency and, on the other hands, high percentages of surviving animals suggests that p53 dependent effects in neural development and neural differentiation can be compensated by the other members of the p53 family or other regulatory mechanisms [15].Accumulating data support processes other than apoptosis in which p53 can participate, such as the regulation of neuronal proliferation and differentiation, the control of axonal outgrowth, and even, regeneration after nerve injury [31,33]. Expression of p53 was demonstrated in different parts of the central and peripheral nervous systems such as the cerebellum, hindbrain, hypothalamus, thalamus, the cortex, and spinal cord [34]. In hippocampal neurons, p53 was also detected in synapses where its content and phosphorylation level were increased after exposure of the cells to the DNA-damaging agent, and such increased levels of p53 correlated with dramatically decrease of synapsin I immunoreactivity in the cell terminals [9]. Immobilization stress, which is not associated with direct cellular damage, leads to increased expression of p53 mRNA in the hippocampus and In the present work, we investigated the effects of p53 inhibition by pifithrin-alpha (PFT) in vitro and in vivo on functioning vasopressinergic magnocellular neurons of rat hypothalamus. In vivo treatments with PFT were done by intrahypothalamic microinjections or by intraperitoneal injections. In in vitro experiments hypothalamic slices containing supraoptic nuclei and intact pituitary were incubated with or without PFT. In all experiments, we observed accumulation of vasopressin (VP) in the cell perikarya after PFT injections; however, expression of VP mRNA was not changed. Analysis of VP content in the posterior pituitary demonstrated that the amount of VP was significantly decreased after PFT treatments. Additionally, long-term inhibition of p53 in experiments with intrahypothalamic injections of PFT resulted in an increased diuresis rate. The obtained results demonstrated that in all experiments PFT treatments inhibited VP anterograde transport from the cells of supraoptic nuclei. Moreover, analysis of MEK/ERK activities revealed that phosphorylation levels of MEK1/2 and ERK1/2 were decreased after PFT treatments. Our findings provide new evidence that p53 could be involved in the control of VP secretion from the hyp...