Prostate-Confined Radiation Decreased Pelvic Ganglia Neuronal Survival and Outgrowth
Authors: Powers S. A. , Odom M. R. , Pak E. S. , Moomaw M. A. , Ashcraft K. A. , Koontz B. F. , Hannan J. L. 
Published: January 2019

Abstract

Background
Erectile dysfunction (ED) is common following radiation therapy (RT) for prostate cancer. Although the cause of RT-induced ED is unknown, damage to both the neuronal and vascular components supporting erections are often implicated.

Aim
To determine the effects of prostatic RT on erections, penile vascular physiology, and major pelvic ganglia (MPG) neuron growth and survival in a rat model.

Methods
Male rats underwent 0 Gy or 22 Gy single fraction of prostate-confined, conformal RT. At 2 weeks or 10 weeks post-RT (n = 10/group), cavernous nerve stimulation was performed and erections were assessed. Tissue bath experiments were performed to assess both penile artery and internal pudendal artery (IPA) function. MPGs were dissociated and neurons grown in culture for 72 hours. Immunofluorescence staining was done to quantify neuron survival (terminal deoxynucleotidyl transferase nick-end labeling), outgrowth (beta-tubulin III), type (nitric oxide synthase [nNOS] and tyrosine hydroxylase [TH]), and nerve injury markers (small GTPase Rac1 and ninjurin-1 [Ninj-1]). Whole MPG real-time quantitative polymerase chain reaction (qPCR) was performed to measure expression of genes related to nerve type, neuron injury, repair, and myelination, such as Ninj-1, Rac1, ATF3, GAP43, GFAP, SOX10, and KROX20.

Outcomes
Intracavernosal pressure (ICP) to mean arterial pressure (MAP) ratio, smooth muscle contractility and relaxation, gene expression, neuritogenesis, and apoptosis.

Results
Following RT, ICP/MAP was unchanged at 2 weeks or 10 weeks. Nerve-mediated penile contraction was increased at 2 weeks, whereas adrenergic contraction was reduced at 10 weeks. Penile relaxation and IPA vasoreactivity were unchanged. Neuronal apoptosis was more than doubled both early and late post-RT. RT caused a progressive decrease in neurite branching but an early increase and then late decrease in neurite lengthening. RT reduced the numbers of nNOS-positive neurons both early and late and also decreased MPG nitrergic gene expression. TH neurons and gene expression were unchanged at 2 weeks; however, both were decreased after 10 weeks. Although most markers of gene injury and repair were unaffected early post-RT, MPG expression of Ninj1 and GFAP increased. After 10 weeks, Ninj1 and GFAP remained elevated while markers of neuron injury (ATF3), outgrowth (GAP43 and Rac1), and myelin regulation (SOX10) were decreased.

Clinical Translation
RT-induced ED may result from damage to the ganglia controlling erections.

Strengths & Limitations
This study used a clinically relevant, prostate-confined model to examine neurovascular structures not accessible in human studies. Unfortunately, rats did not exhibit ED at this time point.

Conclusion
This is the first study to demonstrate impaired health and regeneration potential of dissociated MPG neurons following RT. Neuronal injury was apparent early post-RT and persisted or increased over time but was insufficient to cause ED at the time points examined.