Neuroprotective and neurotherapeuticeffects of dihydrotestosterone,17β-estradiol, and pueraria mirifica extract on cognitive impairment in androgen deficient rats

Abstract

This study aims to understand the mechanism of action of androgen deficiency, which usually occurs in elderly men, and the neuroprotective and neurotherapeutic effects of dihydrotestosterone (DHT), 17β-estradiol (E2) and Pueraria mirifica extract (PME) on cognitive impairment in male rats. Four series of experiments were conducted. Experiment I was to understand the mechanism of action of androgen deficiency on cognitive impairment. Rats at the age of 4 months old were orchidectomized (ODX) or sham-operated (SH), kept for 3 days, 2, 4, 6, and 8 months (M0, M2, M4, M6 and M8, respectively), determined spatial learning behavior and memory capacity, and examined transcriptional and translational levels of genes associated with synaptic plasticity (Syn, GluN1, a7-nAChR, M1-mAChR and Bdnf mRNA levels, and SYN and PSD95 immunoreactivity levels), neurofibrillary tangles (Tau3 and Tau4 mRNA levels, and total tau and phosphorylated tau protein levels), and amyloid plaque (App, Bace1, and Adam10 mRNA levels) in hippocampus. With advancing in age, serum testosterone levels were decreased gradually and significantly in 12-month old SH-M8 rats. A cognitive impairment was first detected in 8-month old SH-M4 rats while it had no changes in any hippocampal marker genes. An abrupt androgen deficiency in the ODX rats accelerated the onset of cognitive impairment as early as 2 months after orchidectomy, and became worsen after 8 months in the ODX-M8 rats. Transcriptional and translational levels of genes associated with synaptic structure and function, and neurofibrillary tangles were deteriorated in comparison with the age-match SH rats, and the degrees of change were exacerbated in 12-month old ODX-M8 rats. Experiment II was to detect the early onset of molecular changes of synaptic plasticity and neurofibrillary tangles. Rats were ODX, kept for 1 - 9 days, and examined mRNA and protein expression levels of genes mentioned in Exp. I. The onset of changes was detected as early as 1 day after orchidectomy for mRNA levels of genes associated with synaptic function, and the chronological changes were detected at 6 and 9 days for neurofibrillary tangle and synaptic structure, respectively. Experiment III was to examine the neuroprotective effects of DHT, E2, and PME. Rats were ODX, kept recovery for 1 day, and fed daily with distilled water or 100 mg/kg BW of PME, or subcutaneously injected with 1 mg/kg BW of DHT or 80 µg/kg BW of E2 for 2 months. DHT, E2 and PME could prevent the cognitive impairment and changes of mRNA levels of genes associated with synaptic structure and function, and mRNA and protein levels of genes associated with neurofibrillary tangle. Among 3 treatments, DHT showed the strongest effects. Experiment IV was to examine the neurotherapeutic effects of DHT, E2, and PME. Rats were ODX, kept for 2 months, and treated with DHT, E2, and PME as mentioned in Exp. III for another 2 months. Only DHT could rescue the cognitive impairment by increasing Syn and Bdnf and decreasing α7-nAChR and M1-mAChR mRNA levels. In conclusion, androgen deficiency is a key factor in accelerating and exacerbating the age-associated cognitive impairment in males through a sequential deterioration of synaptic function and structure, and formation of neurofibrillary tangles. It can be prevented by DHT > E2 ≥ PME, while only DHT can cure the symptoms. Taken together, the preventive approach is suggested for male cognitive impairment, and DHT should be a major treatment, while E2 is an ancillary agent. The present study also encourages the development of natural product, P. mirifica, as an alternative treatment for cognitive impairment in androgen deficient men.