| Spinal cord injury(SCI)results not only in motor and sensory deficits but also in autonomic dysfunctions,such as disordered micturition.An injury rostral to the lumbosacral interrupts the connection between higher micturition centers and the spinal cord and causes areflexic bladder immediately.Although involuntary micturition reflex can be reestablished to facilitate urinary void based on reorganization of intraspinal neuronal circuits,the partial functional recovery is inefficient for elimination.The occurrence of bladder hyperactivity and detrusor-sphincter dyssynergia often gives rise to difficulties in urine empty or continence,which severely affects the quality of life in patients with SCI.Currently,intermittent catheterization is the only method to drain urine out.There is no effective drug therapy for the disease.Dopamine(DA)is a crucial neurotransmitter in the central nervous system(CNS)and plays important roles in a variety of biological functions.With traditional concepts,DA-ergic neurons are restricted to the brain in the CNS and DA in the spinal cord is originated from supraspinal transportation.However,we recently reported a population of DA-ergic neurons exists within the lower spinal cord in female rats,and the concentration of DA sustains at a certain level even after the complete thoracic spinal cord transection;this group of neurons undergo plasticity following SCI and regulates the spontaneous micturition reflex.In consideration of profound sex differences in anatomical structure and physiological behavior of urogenital system,it is necessary to determine if the spinal-derived DA-ergic mechanisms exert the same or similar regulation of the micturition function on the males.Thus,the present study has examined the neuronal machinery by means of histological and physiological techniques in male rats.The results may provide guidance for targeting spinal endogenous DA-ergic signaling to improve micturition function after SCI in both males and females.Part ? The presence of DA-ergic neurons in the male rat spinal cord and their plasticity after SCIDA is synthesized within DA-ergic neurons and this process requires two essential enzymes,tyrosine hydroxylase(TH)and dopamine decarboxylase(DDC).In this pathway,TH converts L-tyrosine to L-DOPA and DDC converts L-DOPA to DA.DA also serves as the precursor in synthesis of noradrenaline(NA)in NA-ergic neurons,catalyzed by dopamine-?-hydroxylase(DBH),so DA-ergic neurons do not express DBH.For a long time,DA-ergic neurons was considered to be confined to several areas in the brain,and the DA in the spinal cord is assumed to originate from the A11 cell groups.However,previous studies reported that TH+ cells are located in the lumbosacral part of rat spinal cord.Our recent studies in female rats demonstrated that these spinal TH+ cells are DA-ergic.Following SCI,these TH+cells undergo plasticity and contribute to a low DA level in the lumbosacral spinal cord.Though this phenomenon is prominent in female rats,whether it is also true in males still remains unknown.In the present study,we used multiple immunostaining to show the DA-ergic neurons in the male rat spinal cord.We also performed fast scan cyclic voltammetry(FSCV)to detect DA release in the injured spinal cord.The results revealed that TH+ cells are present in the gray matter of lumbosacral spinal cord in both naive and SCI male rats,and the majority of these cells reside in the L6-S3 spinal segments.In coronal sections,TH+ cells mainly distribute into the lateral parasympathetic region and the superficial dorsal horn,indicating their relationship with autonomic function.None of these TH+ cells are co-labeled with DBH,suggesting they are DA-ergic neurons.After thoracic spinal cord transection,the number of TH+ cells increased significantly.Furthermore,FSCV detected DA release under electrical stimulation within the lumbosacral parasympathetic region in the injured spinal cord slice.Collectively,DA-ergic neurons exist in male rat spinal cord and their plasticity is induced after SCI.The distribution of these DA-ergic neurons indicates the possibility of their involvement in the pelvic function.Part ? Pharmacological manipulation of spinal DA signaling influences the spinal micturition reflex in male rats with SCIDA signaling regulates lower urinary tract(LUT)activity.Micturition dysfunction often occurs in patients with DA signaling disruption like Parkinson's disease(PD).The regulation mainly takes place by acting on pontine micturition center(PMC).However,DA receptors are found in the lower spinal cord and indicate that DA in the spinal cord may help regulate the micturition function.Recently,spinal DA-ergic mechanisms were shown to be involved in the recovered micturition reflex in female rats after SCI.Now that DA-ergic neurons are located in the lumbosacral spinal cord in male rats,they may play a similar role in LUT activity as in female rats.To investigate their effects on the micturition reflex,we employed complete transection at the 10th thoracic(T10)spinal cord level in male rats.Three weeks after injury,bladder cystometry and external urethral sphincter(EUS)electromyogram(EMG)were performed to examine reflexic micturition function.During the physiological assessments,we intravenously(i.v.)administered different doses of Di-like receptor agonist SKF 38393,Di-like receptor antagonist SCH 23390,D2-like receptor agonist quinpirole,or D2-like receptor antagonist remoxipride,respectively,to evaluate the effect on the bladder and sphincter reflexes.The results showed that stimulating Di-like receptors did not significantly influence the micturition reflex,while blocking Di-like receptors increased bladder pressure and EUS tonic activity.Manipulating D2-like receptors showed very different results.Specifically,blocking D2-like receptors did not cause obvious change,whereas stimulation facilitated micturition activity and increased voiding.To exclude the possible peripheral effects of i.v.administration,intrathecal(i.t.)drug administration was also performed.The results are very similar to those obtained from i.v.delivery,indicating the effect is indeed from the spinal cord.It suggests that spinal DA signaling regulates the LUT function in male rats.Likewise,spinal Di-like receptors are more related to urine continence while stimulating D2-like receptors facilitates voiding.Part ? Stimulating specific spinal DA-ergic circuits enhance spontaneous micturition function in male rats after SCITraumatic SCI results in the loss of voluntary urinary function due to the disruption of spinobulbospinal micturition reflex pathways.Although the spontaneous voiding could partially recover after a period of time post-injury,the function is still very limited.As described above,we have observed that stimulating D2-like receptors facilitates urine voiding in the experimental micturition reflex assay in male rats.In this part,we examined if this intervention could improve spontaneous micturition function following SCI.Adult male rats underwent complete spinal cord transection at T10 level.Three weeks later,the reestablished spontaneous voiding function was evaluated with metabolic cages.Drugs were administrated subcutaneously(s.c.)and the spontaneous voiding volume was recorded constantly for 6 hours.The results showed that administration of D2-like receptor agonist quinpirole significantly increased the total voiding volume and frequency,indicating stimulating spinal D2-like receptors significantly improves spontaneous voiding.In contrast,administration of L-DOPA,the precursor of DA,or apomorphine,the non-selective dopamine agonist,did not induce meaningful changes.Since both L-DOPA and apomorphine activate DA circuits non-specifically,the interaction of D1 and D2 machinery may balance their effects on voiding and continence.These results demonstrated that stimulating spinal D2-like receptors enhances spontaneous voiding in males with SCI.It suggests that spinal DA-ergic mechanisms could be a novel therapeutic target to improve micturition functional recovery after SCI without sex difference. |
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