D.J. Bochinski, MD, Department of Urology, University of California, San Francisco, CA, 94143–0738, USA. e-mail: email@example.com
To test the hypothesis that an intracavernosal injection with brain-derived neurotrophin factor (BDNF) and vascular endothelial growth factor (VEGF) can facilitate nerve regeneration and recovery of erectile function after cavernosal nerve injury.
MATERIALS AND METHODS
The study included 25 Sprague-Dawley rats; four had a sham operation, seven bilateral nerve crushing with no further intervention, and 14 bilateral nerve crushing with either an immediate (seven) or delayed for 1 month (seven) intracavernosal injection with BDNF+VEGF. Erectile function was assessed by cavernosal nerve electrostimulation at 3 months, and neural regeneration by NADPH-diaphorase staining and tyrosine hydroxylase (TH) staining of penile tissue and major pelvic ganglia (MPG).
After nerve crushing, the functional evaluation at 3 months showed a lower mean (sd) intracavernosal pressure (ICP) with cavernosal nerve stimulation, at 33.9 (15.3) cmH2O, than in the sham group, at 107.8 (18.1) cmH2O. With an immediate injection with BDNF+VEGF the ICP was significantly higher than in the controls, at 67.8 (38.5) cmH2O. Even delayed injection with BDNF+VEGF improved the ICP, to 78.0 (21.8) cmH2O. Histological analysis of specimens stained for NADPH and TH showed a significant change in the morphology of terminal branches of the cavernosal and dorsal nerves, and the staining quality of the neurones in the MPG. The number of positively stained nerve fibres tended to revert to normal after treatment with BDNF+VEGF.
An intracavernosal injection with BDNF+VEGF appears to both prevent degeneration and facilitate regeneration of neurones containing neuronal nitric oxide synthase in the MPG, dorsal nerve and intracavernosal tissue. Therefore it might have therapeutic potential for enhancing the recovery of erectile function after radical pelvic surgery.
The cavernosal nerves of the penis run posterolaterally to the prostate and thus render them vulnerable to injury during radical prostatectomy or cystoprostatectomy. Various techniques have been used to try to decrease the incidence of erectile dysfunction (ED) after prostatectomy. Walsh  developed a nerve-sparing technique which has reduced the incidence of ED after surgery. Despite these advances ED still affects a significant proportion of patients and many of them do not recover their erectile capacity.
In many instances during nerve-sparing prostatectomy the cavernosal nerves may have been inadvertently damaged by manipulation, with substantial axonal damage. The recovery of erectile function may depend on re-growth of nerves from the remaining neural tissue . In our previous report we showed that injections with systemic growth hormone enhanced cavernosal nerve regeneration in a nerve-injury rat model . However, there are obvious concerns about the use of systemic growth hormones and their inadvertent side-effects.
Brain-derived neurotrophic factor (BDNF) is a member of the neurotrophin family of growth factors that include nerve growth factor, neurotrophin-3 and -4. BDNF has been shown to enhance the survival and differentiation of several classes of neurones in vitro, and is expressed within peripheral ganglia and not restricted to neuronal target fields. It may thus act in an autocrine or paracrine fashion on neurones . Neurotrophins have a significant role in the regeneration of neurones. After transection or focal crush injury there is a rapid influx of nerve growth factors to the segment distal from the site of injury . Interestingly, BDNF is only induced in distal segments 2 weeks after injury . However, it is reasonable to assume that these neurotrophins and their corresponding receptors are important in promoting neuronal survival and differentiation around the time of peripheral nerve injury.
While vascular endothelial growth factor (VEGF)-A has been extensively studied for its mitogenic effects on vascular endothelium, it also has significant musculotrophic and neurotrophic properties [8,9]. It is through these pathways that VEGF-A exerts its protective effects on the cavernosal and neural tissue. VEGF-A also induces the endothelial and inducible forms of nitric oxide synthase (NOS) in the penis of rats ; these synthases are responsible for producing NO, which results in vascular smooth muscle relaxation and penile erection.
In the present study we injected penile tissue with a combination of neurotrophic factors (VEGF+BDNF) after a nerve crush injury, using functional studies and immunohistochemical analyses to assess the tissue responses to treatment.
MATERIALS AND METHODS
The study included 25 male Sprague Dawley rats (3 months old, 250–300 g), divided into four groups: group 1, four rats that had a sham operation comprising a midline incision and dissection of the cavernosal nerves with no further surgical manipulation; group 2, seven control rats that had the cavernosal nerves dissected and a subsequent intentional 2-min crush injury with a haemostatic clamp; group 3, seven rats treated as group 2, but with an immediate intracavernosal injection with VEGF+BDNF; and group 4, as group 3 but with the injection delayed for 1 month.
For the surgical procedure, the rats were anaesthetized by initial induction using isoflurane inhalation, followed by an intraperitoneal injection with sodium pentobarbital (40 mg/kg). The rats were kept isothermic by placing them on a heating pad at 37 °C. A lower midline abdominal incision was made after the abdomen was shaved and prepared with an iodine-based solution. The prostate gland was exposed and the cavernosal nerves tracking posterolaterally identified and isolated. The major pelvic ganglion (MPG) was also identified more proximally along the course of the cavernosal nerve. In group 1 there was no further surgical manipulation; in the remaining groups the cavernosal nerves were isolated and a crush injury applied using a haemostat clamp for 2 min, after which in group 2 and group 4 the abdomen was closed. In group 3, recombinant VEGF (4000 ng/rat) and BDNF (600 ng/rat) were injected into the corpus cavernosum. The abdominal was closed in all rats in two layers. In addition, before the rats were killed, samples of the MPG and penile tissue were collected for NADPH-diaphorase and tyrosine hydroxylase (TH) staining.
The erectile response was assessed in all rats after 3 months by electrostimulation of the cavernosal nerves, and by measuring intracavernosal pressure (ICP). Through a repeat midline abdominal incision the cavernosal nerves were exposed and isolated. The skin overlying the penis was incised and the crura of the penis identified. A 23 G scalp-vein needle filled with 250 U/mL of heparin solution was connected to polyethylene-50 tubing and inserted into the right crus body to measure the ICP. A bipolar stainless steel electrode was used to directly stimulate the cavernosal nerve (probes 2 mm in diameter and separated by 1 mm). Monophasic rectangular pulses were generated by a computer with a custom-built constant current amplifier. The stimulus parameters were 1.5 mA, 20 Hz, pulse width 0.2 ms and duration 50 s. The ICPs were recorded in all rats ( Fig. 1) using a computer program and appropriate software. After the functional evaluation a mid-shaft corporal and MPG sample were obtained for immunostaining.
The BDNF used was a human recombinant form expressed in SF21 insect cells; the contents were reconstituted using 0.2 µm filtered PBS containing 0.1% human serum albumen or BSA, to ≥ 10 µg/mL. Similarly, a human recombinant VEGF expressed from SF21 insect cells was used, and reconstituted as above to ≥ 1 µg/mL.
Four rats selected at random from each group provided samples for the NADPH-diaphorase and TH staining. Tissues were fixed for 4 h in phosphate buffer containing 0.002% picric acid and 2% formaldehyde, then transferred to 30% sucrose before freezing. Serial cryosections (10 µm) were adhered to charged slides (Superfrost Plus, Fisher Scientific, Pittsburgh, PA). After air drying for 5 min, the sections were incubated with 0.1 mmol/L NADPH, 0.2 mmol/L nitroblue tetrazolium, 0.2% Triton X-100 (Sigma-Aldrich, St. Louis, MO) in buffer with constant microscopic monitoring for colour development. When a deep blue stain was detected for NADPH-diaphorase positive nerves, the slides were rinsed in buffer to terminate the reaction. For TH staining, endogenous peroxidase activity was quenched by incubating slides in 3% H2O2/methanol for 10 min; after rinsing, the slides were incubated in blocking serum, followed by overnight incubation with primary antibody (1 : 100 TH monoclonal antibody, NovoCastra, Belmont, CA). After washing with buffer, sections were immunostained using the avidin-biotin-peroxidase method (Elite ABC, Vector Laboratories, Burlingame, CA), with diaminobenzidine as the chromogen.
All sections were photographed and some images analysed using Image-Pro Plus software (Media Cybernetics, Silver Spring, MD). The presence of NADPH diaphorase-positive nerves is obvious as a highly localized, densely blue region. The staining pattern was assessed by counting the number of the NADPH-positive nerve fibres present at × 400 in each corpus cavernosum (endothelium staining was not included in the count).
The results were assessed statistically by comparing groups using a Student-Newman-Keuls test, with differences considered to be significant at P < 0.05.
There was a significant difference in ICP (Table 1) between group 1 (Fig. 1) and all other groups (P < 0.05), and a significant difference between group 2 and groups 3 and 4 (P < 0.05) but no significant differences between groups 3 and 4; only six rats were assessed in group 4, one died prematurely and no data were collected.
Table 1. The mean ICP in response to electrostimulation, and the staining of the cavernosal and dorsal nerves with NADPH and TH
Mean (sd) variable
all comparisons P < 0.05, except 3 vs 4;
† number of nerve fibres positive per specimen;
ratio of NADPH/TH stained fibres. For %NADPH in the cavernosal samples, P= 0.07 for 1 vs 2 and > 0.1 for all others; for the dorsal nerve, P = 0.07 for 1 vs 2, 0.01 for 1 vs 3, 0.03 for 1 vs 4 and > 0.01 for the others.
Immunohistochemical staining showed a distinct pattern; the number of NADPH-diaphorase-positive nerve fibres in the corpora cavernosa and dorsal nerve of group 2 was significantly less than in group 1 (P < 0.05) (Table 1) but there were no significant differences in TH staining or the NADPH ratio (Table 1). The percentage NADPH tended to increase with treatment, but there were no significant differences among the groups. There were several significant changes histologically. In group 2 the nerves in the corpora cavernosa appeared attenuated, thinned and beaded (Fig. 1b), in contrast to those in group 1, 3 and 4 (Fig. 1a,c,d), where the nerves had more normal morphological features. The findings were similar in the dorsal nerve and MPG (Figs 2 and 3).
Several cavernosal nerve injury models have been reported; one group of researchers used either a crush model with a predetermined weight or a nerve-cutting model with re-approximation of the cut ends [11,12]. In our previous studies we used bilateral nerve-freezing to be certain that there would be minimal or no nerve recovery after 3 months . In the present study, we tried a new approach with bilateral nerves crushed using a haemostat for 2 min, again to ensure that there would be minimal recovery of nerve function after 3 months. The degree of recovery of erectile function in the present model was similar to that in the freezing-injured model. The major advantage of the present methods is its simplicity and reproducibility among researchers involved in these experiments.
We previously reported the beneficial effect of adenovirus-mediated BDNF gene transfer in the freeze-injured model . Although there was an improvement in erectile function after gene transfer the approach was not considered suitable for clinical application. First, there was only partial recovery of erectile function. Second, the use of gene transfer via viral vectors remains a major concern to the public and regulatory agencies. Third, the efficiency and duration of gene transfer is unknown. Therefore, we used BDNF protein in the current experiments. As we did not know the optimal frequency and duration of BDNF protein therapy, we assessed the effects of one injection immediately after nerve injury (group 3) or 1 month afterward (group 4). These two treatments enabled us to evaluate whether BDNF had both preventive and restorative effects on the injured cavernosal nerve.
Intracavernosal injection was used rather than intravenous or intraperitoneal, to minimize systemic side-effects and undesirable angiogenesis or neural growth from VEGF or BDNF on other tissues, e.g. the retina. The retrograde transport of BDNF from axons to the neuronal cell body was reported in other tissues . Therefore, intracavernosal injection theoretically could provide the best chance for the axons of the cavernosal nerve to accumulated the injected neurotrophin and transport it to the cell body in the MPG, to enhance neural regeneration.
VEGF has also been termed vascular permeability factor because of its effect in enhancing the permeability of the endothelium. Others have taken advantage of this and were able to show the additive angiogenic effect of giving platelet-derived growth factor and VEGF together . In earlier in vitro studies (data not shown) VEGF had a neurotrophic effect on the cultured MPG of rats. When both VEGF and BDNF were added to the culture medium, there was an additive effect on neural growth. Therefore, we injected both VEGF and BDNF proteins into the corpus cavernosum assuming that VEGF would not only enhance the diffusion of BDNF into the penile tissue to be absorbed by the axons, but also would have an additive effect on neural regeneration. A pilot in vivo study confirmed the hypothesis that VEGF+BDNF produces better recovery of erectile function than VEGF or BDNF alone (data not shown).
Another interesting finding was the decrease in the ratio of NOS/TH nerves after crush injury of the cavernosal nerves. Our previous studies showed a clear decrease in NOS-containing nerves after cutting or freezing injury. The present study is unique because double-staining with NADPH-diaphorase and TH clearly showed the ‘competitive sprouting’ of the TH-containing sympathetic nerves when the NOS-containing nerves degenerate after cavernosal nerve injury. Previous reports suggested that the NOS-containing parasympathetic nerve fibres of the penis originate from the dorsal caudal portion of the MPG and the cavernosal nerve, while TH-containing sympathetic nerve fibres originate from the sympathetic chain and enter the penis via the pudendal nerve and cavernosal artery [16–18]. Theoretically, the overgrowth of TH-containing sympathetic nerve fibres might cause excessive contraction of penile smooth muscles and result in ‘shrunken penis’, a frequent complaint of patients after radical prostatectomy which selectively damages the cavernosal nerves.
Besides the detection of a changed ratio of NOS/TH nerve fibres after cavernosal nerve injury, this study also showed that intracavernosal injection of VEGF+BDNF protein enhanced the recovery of NOS nerve fibres and thus restored the ratio of NOS/TH nerve fibres. Although the difference was not statistically significant, because there were too few animals in each group, the qualitative comparison of nerve fibres between the treated and untreated groups clearly showed that VEGF+BDNF-treated rats have longer and thicker nerve fibres.
In conclusion, crush injury of the cavernosal nerve with a haemostat for 2 min produces a reliable and reproducible model of neurogenic ED in rats. Injuring the cavernosal nerve results in poor erectile function and a change in NOS/TH nerve ratio, which may be responsible for the complaint of ‘shrunken penis’ after radical prostatectomy. A bolus intracavernosal injection with VEGF+BDNF protein immediately and 1 month after nerve injury causes a significant recovery in erectile function, and the morphology of cavernosal nerve fibres in the corpus cavernosum. This VEGF-enhanced neurotrophin therapy might be useful for preserving and recovering erectile function after radical pelvic surgery.