Spontaneous regeneration of the seriously injured sympathetic pathway projecting to the prostate over a long period in the dog

Authors

  • T. Kobayashi,

    1. Department of Urology and Reproductive Medicine, and Allied Health Sciences, Graduate School, Tokyo Medical and Dental University, Tokyo, Japan
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  • K. Kihara,

    Corresponding author
    1. Department of Urology and Reproductive Medicine, and Allied Health Sciences, Graduate School, Tokyo Medical and Dental University, Tokyo, Japan
    • K. Kihara, Department of Urology, School of Medicine, Tokyo Medical and Dental University, 1-5-45 Yushima, Bunkyo-ku, Tokyo 113–8519, Japan.
      e-mail: k-kihara.uro@med.tmd.ac.jp

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  • N. Hyochi,

    1. Department of Urology and Reproductive Medicine, and Allied Health Sciences, Graduate School, Tokyo Medical and Dental University, Tokyo, Japan
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  • H. Masuda,

    1. Department of Urology and Reproductive Medicine, and Allied Health Sciences, Graduate School, Tokyo Medical and Dental University, Tokyo, Japan
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  • K. Sato

    1. Department of Urology and Reproductive Medicine, and Allied Health Sciences, Graduate School, Tokyo Medical and Dental University, Tokyo, Japan
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Abstract

OBJECTIVES

To explore the spontaneous regeneration, over a long period, of the seriously injured sympathetic pathway controlling the prostate.

MATERIALS AND METHODS

The hypogastric nerve (HGN), which is part of the sympathetic pathway from the spinal cord to the prostate, was partly removed over half of its length on both sides in six dogs. Four years after surgery the responses of the prostate to electrical stimulation of the lumbar splanchnic nerve (LSN) or the HGN proximal to the site removed, were assessed.

RESULTS

In six dogs, 10 of the 17 LSNs (second to fourth) and four of the 10 HGNs stimulated elicited prostatic contraction. The pathways via the ipsilateral HGN and/or the contralateral HGN from the LSNs to the prostate were identified as having regenerated in four of six dogs.

CONCLUSION

These results indicate that the sympathetic pathways via the HGN to the canine prostate can regenerate spontaneously over a long period after serious injury, and that their cross-innervation system can also be repaired.

INTRODUCTION

The outgrowth of regenerating axons after injury has been well investigated in peripheral motor nerves, and reattachment of the nerve is a well-established technique in plastic surgery. Although spontaneous regeneration of autonomic nerves has been reported in various organs of animals and humans [1–3], regeneration of the nerve controlling the prostate after serious injury has not been assessed in detail.

Currently, many cross-innervation mechanisms from the spinal cord via the hypogastric nerve (HGN) to the prostate have been reported [4]. It is unclear whether spontaneous regeneration occurs when a long section of the pathway is removed, which usually happens during surgery, and whether the cross-innervation mechanisms can also regenerate.

In the present study, spontaneous regeneration of the canine HGN 4 years after removing half its length was investigated. As 4 years in the dog lifespan corresponds to> 20 years in humans, the results could reflect the maximum rate of spontaneous repair of the human HGN after serious injury. The aims of the study were: (i) to examine the occurrence of regeneration of the pathway from the spinal cord to the prostate; (ii) to identify recovery of the cross-innervation system; and (iii) to compare the regeneration rate with that in HGN-HGN reattachment, previously reported [5].

MATERIALS AND METHODS

The experiment comprised removal of half of the HGN bilaterally and 4 years later assessing the response of the prostate to electrical stimulation of the lumbar splanchnic nerve (LSN) or the HGN proximal to the site removed. The study included 10 male mongrel dogs (14–19 kg); six had part of the HGN removed bilaterally and the remaining four were used to confirm the completeness of partial removal of the HGN. The dogs were handled in the laboratory according to the institutional guideline for the care of laboratory animals and the guidelines and principles of the American Physiological Society, under the approval of the Institutional Animal Care and Use Committee.

To expose the sympathetic nerve and prostate, the dog was anaesthetized with ketamine hydrochloride (10 mg/kg body weight) intramuscularly, placed supine, a vein in the forefoot cannulated and Ringer lactate solution administered throughout the experiment. Just after intravenous injection with pentobarbital sodium (20 mg/kg body weight) the dog was intubated and ventilated with a Harvard respirator. Surgical anaesthesia was maintained by repeated intravenous injections with pentobarbital (5 mg/kg body weight, bolus) as necessary. After the abdominal cavity was opened by a midline incision, the caudal mesenteric plexus around the caudal mesenteric artery, LSN, HGN and prostate were exposed as previously described [6] (Fig. 1a).

Figure 1.

Diagram showing innervation of the canine prostate (a) and signals from each LSN to the prostate (b) [4]. Each of the lumbar splanchnic nerves (second to fourth) send signals to the prostate, and the signals cross to the other side at the level of the CMP. Am, ampula; CMA, caudal mesenteric artery; CN, colonic nerve; CP, celiac plexus; CrMP, cranial mesenteric plexus; IMP, intermesenteric plexus; PN, pelvic nerve; PP, pelvic plexus; Pro, prostate; RA, renal artery; SC, sympathetic chain; SN, spermatic nerve.

The HGN identified near the ureter was dissected with surrounding connective tissues for 4 cm in the middle portion and raised using a 1/0 silk thread. Half of the total length of the HGN (mean 2.5 cm) was removed with surrounding connective tissues. The procedure was bilateral; muscle, fascia and skin were then sutured successively in layers. The animals recovered from anaesthesia under supervision and were assessed regularly for several hours. The completeness of the partial removal of the HGN was confirmed in four dogs and the response of the prostate induced by electrical stimulation of the proximal HGN, as described below, confirmed to be completely eliminated by partial removal of the HGN in all eight HGNs.

As previous experiments showed that the LSNs from vertebral levels L2 to L4 mainly elicit responses in the prostate of the dog [4], these LSNs were assessed. Each of the LSNs (L2–L4) and each of the HGNs on both sides were stimulated, respectively, at a point ≈ 10 mm apart from the sympathetic chain and from the site of removal. Before electrical stimulation the nerve was transected centrally to acutely decentralize the peripheral autonomic pathway and thereby eliminate reflex input from the spinal cord. The stimulus parameters were 8 V, 2 ms and 10 Hz, provided by a stimulator (model DPS-06, Daiya Medical Systems, Tokyo, Japan) and applied to the distal end of the severed nerve with a pair of wire electrodes, as previously described [7]; 2 ms was selected as it provided maximum contraction of the canine vas deferens and prostate [4,8]. To measure contraction of the prostate the stimulation was continued until responses reached a plateau, or for 30 s if it did not. The parameters used were confirmed to be enough to cause contraction of the prostate by stimulating the distal end of the severed LSN, but not by stimulating the proximal end.

The contraction of the prostate was measured by suturing a force transducer (Star Medical Corp., Tokyo, Japan) to the surface of the prostate, as described previously [4]. The upper and lower edges of the force transducers were attached by suturing them with 5/0 nylon thread to the mid-anterior surface of the prostate, in parallel with the longitudinal axis. A reduction in tension was recorded when the site where the transducer was sutured elicited stronger contraction than the surrounding area, and an increase in tension when a stronger contraction was generated at a site apart from the suture.

After electrical stimulation the dogs were killed with a lethal dose of pentobarbital sodium and the relevant tissues fixed in 10% formalin to assess by dissection the relationship of the LSN to the caudal mesenteric plexus (CMP).

RESULTS

The CMP, which lies around the caudal mesenteric artery, comprises the LSN (L2–4) and inter-mesenteric plexus, which connects the celiac and cranial mesenteric plexuses with the CMP (Fig. 1a). The LSNs sometimes merged together before merging with the CMP. From the CMP, the colonic nerve projects to the descending colon and rectum, and the right and left HGNs run caudally to the ipsilateral pelvic plexus, which is near the lateral surface of the rectum. The nerve branches originating from the plexus innervate the prostate. When the LSNs merged together, electrical stimulation was applied at the merge site; some LSNs were too fine to stimulate.

Of the 17 LSNs stimulated 10 elicited contraction of the prostate (Fig. 2). In dog no. 1, all three of the LSNs stimulated elicited contraction of the prostate (Fig. 2Aa). There was a reduction or increase in tension in two responses and one response, respectively. Stimulating the right HGN generated prostatic contraction, recorded as a reduction in tension; stimulating the left elicited no response. These results indicate that three nerve pathways via the ipsilateral or the contralateral HGN from the LSNs to the prostate had regenerated (Fig. 2Ab)

Figure 2.

Figure 2.

Diagrams and traces of contraction of the prostate caused by electrical stimulation of the LSN in four dogs (A–D) in which half the HGN was removed (a). The signals reconstructed are shown in (b). Bars under the traces indicate the times of stimulation. Vertical calibration, 1 g; horizontal calibration, 60 s. (See Fig. 1 for abbreviations.)

Figure 2.

Figure 2.

Diagrams and traces of contraction of the prostate caused by electrical stimulation of the LSN in four dogs (A–D) in which half the HGN was removed (a). The signals reconstructed are shown in (b). Bars under the traces indicate the times of stimulation. Vertical calibration, 1 g; horizontal calibration, 60 s. (See Fig. 1 for abbreviations.)

Figure 2.

Figure 2.

Diagrams and traces of contraction of the prostate caused by electrical stimulation of the LSN in four dogs (A–D) in which half the HGN was removed (a). The signals reconstructed are shown in (b). Bars under the traces indicate the times of stimulation. Vertical calibration, 1 g; horizontal calibration, 60 s. (See Fig. 1 for abbreviations.)

Figure 2.

Figure 2.

Diagrams and traces of contraction of the prostate caused by electrical stimulation of the LSN in four dogs (A–D) in which half the HGN was removed (a). The signals reconstructed are shown in (b). Bars under the traces indicate the times of stimulation. Vertical calibration, 1 g; horizontal calibration, 60 s. (See Fig. 1 for abbreviations.)

In dog nos 2, 3 and 4 (Fig. 2B–D), the responses indicated that, respectively, four, one and two nerve pathways via the ipsilateral or the contralateral HGN from the LSNs to the prostate had regenerated. In dog no. 5, none of the three LSNs stimulated elicited a response in the prostate, and the response of the prostate in dog no. 6 could not be evaluated because the force transducer malfunctioned during stimulation.

DISCUSSION

These results indicate that the pathway from the LSN to the prostate can regenerate spontaneously over a long period after serious injury, and that various pathways from the LSN via the ipsilateral and/or the contralateral HGN can regenerate spontaneously. The rate of regeneration was lower than when the nerve was reattached; in our previous study [5], 15 (65%) of 23 LSNs stimulated elicited a response in the prostate 18 months after surgery in six dogs, showing that surgical reattachment is effective. However, considering the risk of mismatched repair when a long section of the nerve is removed [9], the rate of regeneration of the original pathway via the HGN over a long period was relatively high.

The processes of regenerating transected axons include resealing the ruptured cell membrane of the axon [10] and then re-growth of the proximal stump, which is guided by the Schwann cell processes [11,12]. Nerve growth factor secreted from the distal stump accelerates axonal re-growth of the proximal stump [13,14]. Four years after removing half of the total length of the HGN, both the proximal and distal stumps approached each other, by fibrous change in the surrounding tissue. Nerve growth factor and this fibrous change may support the regeneration of the HGN. The reason for the remarkable differences in regeneration among the dogs is unclear; one cause might be that both stumps failed to align correctly, and in the dog in which there was no regeneration, both stumps were pointing in opposite directions when the LSN was stimulated.

The terminal sites of the axons require further study; it is possible that regenerating axons make synapses with sympathetic postganglionic neurones innervating the prostate in the pelvic plexus, as the HGN is composed primarily of preganglionic sympathetic neurones [15–17]. The second possibility is that they may terminate in the smooth muscle of the target organs.

Clinically, after interrupting the abdominal or pelvic sympathetic pathways to the vas deferens and prostate by trauma or surgery, dry ejaculation (including loss of emission) is common [18–21]. However, some patients spontaneously recover these functions with time [22,23], suggesting that there are reconstructive mechanisms in the injured nerve pathway. In the present study, various pathways spontaneously regenerated from the spinal cord to the prostate, including crossed ones.

Not only reductions but also increases in tension were recorded on the anterior surface of the prostate in the present dogs. Little is known about contractions in different parts of the prostate, and this remains a problem for future investigation.

In conclusion, various sympathetic nerve pathways from the spinal cord via the HGN to the canine prostate were identified as having regenerated spontaneously over a long period, including crossed pathways.

ACKNOWLEDGEMENTS

This research was supported by Grant-in-Aid 09470341 for Scientific Research (B) and 09671611 for Scientific Research (C), and 09630003 for Scientific Research (C) from the Ministry of Education, Science, Sports and Culture, Japan

Abbreviations
HGN

hypogastric nerve

LSN

lumbar splanchnic nerve

CMP

caudal mesenteric plexus.

Ancillary