Spinal interneurons and preganglionic neurons in sacral autonomic reflex pathways

Stabilization of V1 interneuron-motor neuron connectivity ameliorates motor phenotype in a mouse model of ALS

Loss of connectivity between spinal V1 inhibitory interneurons and motor neurons is found early in disease in the SOD1G93A mice. Such changes in premotor inputs can contribute to homeostatic imbalance of motor neurons. Here, we show that the Extended Synaptotagmin 1 (Esyt1) presynaptic organizer is downregulated in V1 interneurons. V1 restricted overexpression of Esyt1 rescues inhibitory synapses, increases motor neuron survival, and ameliorates motor phenotypes. Two gene therapy approaches overexpressing ESYT1 were investigated; one for local intraspinal delivery, and the other for systemic administration using an AAV-PHP.eB vector delivered intravenously. Improvement of motor functions is observed in both approaches, however systemic administration appears to significantly reduce onset of motor impairment in the SOD1G93A mice in absence of side effects. Altogether, we show that stabilization of V1 synapses by ESYT1 overexpression has the potential to improve motor functions in ALS, demonstrating that interneurons can be a target to attenuate ALS symptoms.

An Open-source Computational Model of Neurostimulation of the Spinal Pudendo-Vesical Reflex for the Recovery of Bladder Control After Spinal Cord Injury

Spinal cord stimulation (SCS) could be used to restore control of the bladder after spinal cord injury, but substantial development is still required to tailor this technology for bladder function. Computational models could be utilized to accelerate these efforts enabling in-silico optimization of stimulation parameters. However, no model of the spinal pudendo-vesical reflex can simulate the effect of stimulation amplitude on neuron recruitment. This limitation hinders accurate prediction of bladder pressure changes for different stimulation configurations. Here., we implemented an open-source realistic spiking neural network model of the pudendo-vesical reflex enabling exploration of the impact of stimulation amplitude and frequency on bladder pressure changes. We used the o2S2 PARC platform to design a parallel implementation of the bladder reflex circuits with NEURON. Our model successfully reproduced and expanded previous studies., producing a decrease in bladder pressure at low stimulation frequency (10 Hz) and excitation at high stimulation frequency (≥33 Hz) in isovolumetric experiments. We then explored the effect of mixed nerve recruitment., simulating a common case of poorly selective spinal cord stimulation. We found that high recruitments of pudendal nerve axons are necessary to maintain this bi-modal behavior., regardless of stimulation specificity. Our framework is fully open-source and can be used to simulate any type of axon stimulations such as SCS and peripheral nerve stimulation.

Sophisticated regulation of micturition: review of basic neurourology

The neurological regulation of the lower urinary tract can be viewed separately from the perspective of sensory neurons and motor neurons. First, in the receptors of the bladder and urethra of sensory nerves, sensations are transmitted through the periaqueductal gray matter of the midbrain to the cerebral cortex, and the cerebrum goes through the process of decision-making. Motor neurons are divided into upper motor neurons (UMNs) and lower motor neurons (LMNs). UMNs coordinate storage and micturition in the brain stem so that synergic voiding can occur. LMNs facilitate muscle contractions in the spinal cord. The muscles involved in urinary storage and micturition are innervated by the somatic branches of sympathetic, parasympathetic, and peripheral nerves. Sympathetic nerves are responsible for contractions of urethral smooth muscles, while parasympathetic nerves originate from S2–S4 and are in charge of contractions of the bladder muscle. Somatic nerves originate from the motor neurons in Onuf’s nucleus, which is a specific part of somatic nerves. In this review, we will investigate the structures of the nervous systems related to the lower urinary tract and the regulatory system of innervation for the urinary storage and micturition and discuss the clinical significance and future prospects of neurourological research.

Sex differences in c-Fos and EGR-1/Zif268 activity maps of rat sacral spinal cord following cystometry-induced micturition

Storage and voiding of urine from the lower urinary tract (LUT) must be timed precisely to occur in appropriate behavioral contexts. A major part of the CNS circuit that coordinates this activity is found in the lumbosacral spinal cord. Immediate early gene (IEG) activity mapping has been widely used to investigate the lumbosacral LUT-related circuit, but most reports focus on the effects of noxious stimulation in anesthetized female rats. Here we use c-Fos and EGR-1 (Zif268) activity mapping of lumbosacral spinal cord to investigate cystometry-induced micturition in awake female and male rats. In females, after cystometry c-Fos neurons in spinal cord segments L5-S2 were concentrated in the sacral parasympathetic nucleus (SPN), dorsal horn laminae II-IV, and dorsal commissural nucleus (SDCom). Comparisons of cystometry and control groups in male and female revealed sex differences. Activity mapping suggested dorsal horn laminae II-IV was activated in females but showed net inhibition in males. However, inhibition in male rats was not detected by EGR-1 activity mapping, which showed low coexpression with c-Fos. A class of catecholamine neurons in SPN and SDCom neurons were also more strongly activated by micturition in females. In both sexes, most c-Fos neurons were identified as excitatory by their absence of Pax2 expression. In conclusion, IEG mapping in awake male and female rats has extended our understanding of the functional molecular anatomy of the LUT-related circuit in spinal cord. Using this approach, we have identified sex differences that were not detected by previous studies in anesthetized rats.

Sacral neuromodulation treating chronic pelvic pain: a meta-analysis and systematic review of the literature

INTRODUCTION AND HYPOTHESIS

Sacral neuromodulation (SNM) is gaining popularity as a treatment option for chronic pelvic pain (CPP). Our hypothesis is that SNM is effective in improving CPP.

METHODS

A systematic search was conducted through September 2018. Peer-reviewed studies using pre- and postpain intensity scores were selected. The primary outcome was pain improvement on a 10-point visual analog scale (VAS) (adjusted or de novo) in patients with CPP. Secondary outcomes included comparing SNM approaches and etiologies and evaluating lower urinary tract symptoms (LUTS).

RESULTS

Fourteen of 2175 studies, evaluating 210 patients, were eligible for further analysis. The overall VAS pain score improvement was significant [weighted mean difference (WMD) -4.34, 95% confidence interval (CI) = -5.22, to-3.64, p < 0.0001)]. Regarding SNM approach, both standard and caudal approaches had significant reduction in pain scores: WMD -4.32, CI 95% = -5.32, to -3.31 (p < 0.001) for the standard approach, compared with WMD -4.63, 95% CI = -6.57 to -2.69 (P < 0.001), for the caudal approach (p = 0.75). While significant improvement in pain was observed both in patients with and without interstitial cystitis/bladder pain syndrome (IC/BPS), the observed improvement was lower in patients with (WMD -4.13, CI 95% -5.36 to -2.90 versus without (WMD -5.72, CI 95% = -6.18, to-5.27) IC/BPS (p = 0.02). SNM was effective in treating voiding symptoms (frequency, urgency, nocturia) associated with IC/BPS (all p < 0.01).

CONCLUSIONS

SNM is an effective therapy for CPP in both IC/BSP and non-IC/BSP patients, with better results in non-IC/BSP patients. Outcomes of the antegrade caudal approach were comparable with the standard retrograde approach.

Bladder sensory physiology: neuroactive compounds and receptors, sensory transducers, and target-derived growth factors as targets to improve function

Urinary bladder dysfunction presents a major problem in the clinical management of patients suffering from pathological conditions and neurological injuries or disorders. Currently, the etiology underlying altered visceral sensations from the urinary bladder that accompany the chronic pain syndrome, bladder pain syndrome (BPS)/interstitial cystitis (IC), is not known. Bladder irritation and inflammation are histopathological features that may underlie BPS/IC that can change the properties of lower urinary tract sensory pathways (e.g., peripheral and central sensitization, neurochemical plasticity) and contribute to exaggerated responses of peripheral bladder sensory pathways. Among the potential mediators of peripheral nociceptor sensitization and urinary bladder dysfunction are neuroactive compounds (e.g., purinergic and neuropeptide and receptor pathways), sensory transducers (e.g., transient receptor potential channels) and target-derived growth factors (e.g., nerve growth factor). We review studies related to the organization of the afferent limb of the micturition reflex and discuss neuroplasticity in an animal model of urinary bladder inflammation to increase the understanding of functional bladder disorders and to identify potential novel targets for development of therapeutic interventions. Given the heterogeneity of BPS/IC and the lack of consistent treatment benefits, it is unlikely that a single treatment directed at a single target in micturition reflex pathways will have a mass benefit. Thus, the identification of multiple targets is a prudent approach, and use of cocktail treatments directed at multiple targets should be considered.

Neuropeptides in lower urinary tract function

Numerous neuropeptide/receptor systems including vasoactive intestinal polypeptide, pituitary adenylate cyclase-activating polypeptide, calcitonin gene-related peptide, substance P, neurokinin A, bradykinin, and endothelin-1 are expressed in the lower urinary tract (LUT) in both neural and nonneural (e.g., urothelium) components. LUT neuropeptide immunoreactivity is present in afferent and autonomic efferent neurons innervating the bladder and urethra and in the urothelium of the urinary bladder. Neuropeptides have tissue-specific distributions and functions in the LUT and exhibit neuroplastic changes in expression and function with LUT dysfunction following neural injury, inflammation, and disease. LUT dysfunction with abnormal voiding, including urinary urgency, increased voiding frequency, nocturia, urinary incontinence, and pain, may reflect a change in the balance of neuropeptides in bladder reflex pathways. LUT neuropeptide/receptor systems may represent potential targets for therapeutic intervention.

Basic bladder neurophysiology

Maintenance of normal lower urinary tract function is a complex process that requires coordination between the central nervous system and the autonomic and somatic components of the peripheral nervous system. This article provides an overview of the basic principles that are recognized to regulate normal urine storage and micturition, including bladder biomechanics, relevant neuroanatomy, neural control of lower urinary tract function, and the pharmacologic processes that translate the neural signals into functional results. Finally, the emerging role of the urothelium as a sensory structure is discussed.

The neural control of micturition

Micturition, or urination, occurs involuntarily in infants and young children until the age of 3 to 5 years, after which it is regulated voluntarily. The neural circuitry that controls this process is complex and highly distributed: it involves pathways at many levels of the brain, the spinal cord and the peripheral nervous system and is mediated by multiple neurotransmitters. Diseases or injuries of the nervous system in adults can cause the re-emergence of involuntary or reflex micturition, leading to urinary incontinence. This is a major health problem, especially in those with neurological impairment. Here we review the neural control of micturition and how disruption of this control leads to abnormal storage and release of urine.

Recurrent inhibition of the bladder C fibre reflex in the cat and its response to naloxone

Recurrent inhibition of the bladder C fibre reflex was studied in adult female cats anaesthetized with alpha-chloralose. Test reflexes were evoked by electrical stimulation of bladder Adelta and C afferents in the right pelvic nerve and were recorded from the proximal end of a small ipsilateral pelvic nerve branch, transected close to the bladder. Such test reflexes were consistently depressed by repetitive electrical stimulation of the contralateral bladder pelvic nerve (20 Hz, 20 s) at intensities sufficient to recruit axons of bladder preganglionic neurones. The inhibition could be evoked after transection of the left dorsal roots S1-S4 and the sympathetic supply to the bladder but was abolished by transection of the pelvic nerve central to the site of stimulation. Hence, it most likely involved central recurrent collaterals of antidromically activated bladder preganglionic neurones. The reflex suppression was quite considerable - maximal C fibre reflexes were reduced to a group mean of 25% (+/- 9% confidence interval) of their control size. The effect had a slow onset, requiring a few seconds of conditioning stimulation to be revealed, and was very long lasting (minutes). Naloxone (0.01-0.5 mg kg(-1) i.v.) abolished the recurrent inhibition of both the C fibre and Adelta bladder reflexes, while inhibition from afferents in the dorsal clitoris nerve remained unchanged. It is concluded that the segmental bladder C fibre reflex and the spino-ponto-spinal Adelta micturition reflex are both targets of recurrent inhibition from bladder parasympathetic preganglionic neurones and that the effect involves an enkephalinergic mechanism.

Control of urinary bladder function with devices: successes and failures

The management of urinary tract dysfunction is crucial for the health and well-being of people with spinal cord injury. Devices, specifically catheters, play an important role in the daily regime of bladder management for most people with spinal cord injury. However, the high incidence of complications associated with the use of catheters, and the fact that the spinal segments involved in lower urinary tract control remain intact in most cord-injured people, continue to motivate research into devices that could harness the nervous system to provide greater control over lower urinary tract function. Mechanical devices discussed in this review include catheters, artificial urethral sphincters, urethral stents and intraurethral pumps. Additionally, many attempts to restore control of the lower urinary tract with electrical stimulation have been made. Stimulation sites have included: inside the bladder, bladder wall, thigh, pelvic floor, dorsal penile nerve, pelvic nerve, tibial nerve, sacral roots, sacral nerves and spinal cord. Catheters and sacral root stimulators are two techniques whose efficacy is well established. Some approaches have proven less successful and others are still in the development stage. Modifications to sacral root stimulation including posterior root stimulation, anodal blockade and high-frequency blockade as well as new techniques including intraspinal microstimulation, urethral afferent stimulation and injectable microstimulators are also discussed. No single device has yet restored the control and function of the lower urinary tract to the pre-injury state, but new techniques are bringing this possibility closer to reality.

Sacral neuromodulation for intractable urge incontinence: Are there factors associated with cure?

OBJECTIVES

To determine the variables that affect the cure rate in patients with urge incontinence treated with sacral neuromodulation.

METHODS

This prospective analysis of patients with refractory urinary urge incontinence who underwent placement of a neuromodulator lead and generator was undertaken between October 2000 and December 2003. Quantitative assessment of the severity of their urinary leakage was assessed by preoperative and postoperative 3-day bladder diaries documenting leakage episodes, number of pads used per day, and a 24-hour pad weight assessment. Cure was defined as no daily leakage episodes after permanent implantation. Subjective outcome was assessed using the Incontinence Impact Questionnaire. Two-sample independent t tests, two-way chi-square tests, and tests of two proportions were performed when appropriate, with P < 0.05 considered significant.

RESULTS

The mean postimplantation follow-up was 29 months, and the average age was 60 years (range 29 to 83). The cure rate was associated with age, with individuals younger than 55 years having a statistically significant greater cure rate (65% versus 37% for older individuals; P < 0.05). Having three or more chronic conditions was associated with a lower cure rate in both younger and older individuals. Patients with a neurologic condition also had a lower cure rate, but no specific neurologic condition was associated.

CONCLUSIONS

Age older than 55 years and more than three chronic conditions were independent factors associated with a lower cure rate in patients implanted with a sacral neuromodulator for refractory urge incontinence. A neurologic condition may be associated with a decrease in the cure rate.

Plateau potentials and membrane oscillations in parasympathetic preganglionic neurones and intermediolateral neurones in the rat lumbosacral spinal cord

Whole-cell patch recordings were made from parasympathetic preganglionic neurones (P-PGNs) and unidentified intermediolateral (IML) neurones in thick slices of the lower lumbar and sacral spinal cord of 14- to 21-day-old rats. The P-PGNs and IML neurones examined were similar in terms of soma sizes, input resistance and capacitance, and displayed a sag conductance as well as rebound firing. In the absence of drugs, the neurones responded with either tonic or adapting firing to depolarizing current steps. However, in the presence of the group I metabotropic glutamate receptor agonist (RS)-3,5-dihydroxyphenylglycine (DHPG), almost half of the neurones displayed accelerating firing rates during the constant current injection, followed by a sustained after-discharge. In the presence of TTX, plateau potentials were observed. The firing changes and plateaux were blocked by nifedipine, an L-type Ca2+ channel blocker, and (S)-(-)-Bay K8644 was able to produce these firing changes and plateaux in the absence of DHPG, demonstrating the involvement of an L-type Ca2+ conductance. Ca2+-activated nonspecific cationic conductances also appear to contribute to the firing changes. A few neurones displayed membrane oscillations and burst firing in the presence of DHPG. The results suggest that the firing characteristics of both P-PGNs and other neurones likely to be involved in caudal spinal reflex control are not static but, rather, quite dynamic and under metabotropic glutamate receptor modulatory control. Such changes in firing patterns may be involved in normal pelvic parasympathetic reflex function during micturition, defaecation and sexual reflexes, and may contribute to the abnormal output patterns seen with loss of descending brainstem input and visceral or perineal sensory disturbances.

Bladder and urethral sphincter responses evoked by microstimulation of S2 sacral spinal cord in spinal cord intact and chronic spinal cord injured cats

Urinary bladder and urethral sphincter responses evoked by bladder distention, ventral root stimulation, or microstimulation of S2 segment of the sacral spinal cord were investigated under alpha-chloralose anesthesia in cats with an intact spinal cord and in chronic spinal cord injured (SCI) cats 6-8 weeks after spinal cord transection at T9-T10 spinal segment. Both SCI and normal cats exhibited large amplitude reflex bladder contractions when the bladder was fully distended. SCI cats also exhibited hyperreflexic bladder contractions during filling and detrusor-sphincter dyssynergia during voiding, neither was observed in normal cats. Electrical stimulation of the ventral roots revealed that the S2 sacral spinal cord was the most effective segment for evoking large amplitude bladder contractions or voiding in both types of cats. Microstimulation with a stimulus intensity of 100 microA and duration of 30-60 s via a single microelectrode in the S2 lateral ventral horn or ventral funiculus evoked large amplitude bladder contractions with small urethral contractions in both normal and SCI cats. However, this stimulation evoked incomplete voiding due to either co-activation of the urethral sphincter or detrusor-sphincter dyssynergia. Stimulation in the S2 dorsal horn evoked large amplitude sphincter responses. The effectiveness of spinal cord microstimulation with a single electrode to induce prominent bladder and urethral sphincter responses in SCI animals demonstrates the potential for using microstimulation techniques to modulate lower urinary tract function in patients with neurogenic voiding dysfunctions.

Artificial autonomic reflexes: using functional electrical stimulation to mimic bladder reflexes after injury or disease

Autonomic reflexes controlling bladder storage (continence) and emptying (micturition) involve spinal and supraspinal nerve pathways, with complex mechanisms coordinating smooth muscle activity of the lower urinary tract with voluntary muscle activity of the external urethral sphincter (EUS). These reflexes can be severely disrupted by various diseases and by neurotrauma, particularly spinal cord injury (SCI). Functional electrical stimulation (FES) refers to a group of techniques that involve application of low levels of electrical current to artificially induce or modify nerve activation or muscle contraction, in order to restore function, improve health or rectify physiological dysfunction. Various types of FES have been developed specifically for improving bladder function and while successful for many urological patients, still require substantial refinement for use after spinal cord injury. Improved knowledge of the neural circuitry and physiology of human bladder reflexes, and the mechanisms by which various types of FES alter spinal outflow, is urgently required. Following spinal cord injury, physical and chemical changes occur within peripheral, spinal and supraspinal components of bladder reflex circuitry. Better understanding of this plasticity may determine the most suitable methods of FES at particular times after injury, or may lead to new FES approaches that exploit this remodeling or perhaps even influence the plasticity. Advances in studies of the neuroanatomy, neurophysiology and plasticity of lumbosacral nerve circuits will provide many further opportunities to improve FES approaches, and will provide "artificial autonomic reflexes" that much more closely resemble the original, healthy neuronal regulatory mechanisms.

Spinal neuronal responses to urinary bladder stimulation in rats with corticosterone or aldosterone onto the amygdala

Elevating glucocorticoids in the amygdala produces colorectal hypersensitivity through activation of lumbosacral spinal neurons. The aim of this study was to determine if descending modulation from the amygdala affects spinal processing of input from urinary bladder afferents. Fischer-344 rats received cholesterol (inactive control)-, corticosterone-, or aldosterone-containing micropellets placed stereotaxically on the dorsal margin of the left and right amygdala (n = 10 for each group). Seven days after amygdaloid implantation, extracellular potentials of single L6-S1 spinal neurons were examined for the responses to graded (0.5-2.0 ml, 20 s) urinary bladder distension (UBD). Spontaneous activity of neurons with excitatory responses to UBD in aldosterone-implanted rats [11.0 +/- 1.7 (SE) imp/s], but not in corticosterone-implanted rats, was higher than in the cholesterol-implanted group (6.6 +/- 1.1 imp/s, P < 0.05). Noxious UBD (1.5 ml) produced a greater excitatory response (21.6 +/- 2.6 imp/s) in aldosterone-implanted rats compared with cholesterol- or corticosterone-implanted rats (15.1 +/- 1.5 and 13.6 +/- 1.4 imp/s; P < 0.05). In contrast, the duration of excitatory responses to UBD in corticosterone-implanted rats (38.5 +/- 3.4 imp/s) was significantly longer than those in the aldosterone or control groups (26.8 +/- 1.8 and 24.7 +/- 1.5 imp/s). Neurons with low thresholds for excitatory responses to UBD were seen more frequently in aldosterone-implanted rats than in corticosterone or cholesterol treated rats (74 vs. 44% and 39%, P < 0.05). No difference in somatic field properties of spinal neurons responsive or nonresponsive to UBD was found among the three groups. These findings suggest that both mineralocorticoid- and glucocorticoid-mediated mechanisms in the amygdala are involved in descending modulation to lumbosacral spinal neurons receiving inputs from the urinary bladder; and this mechanism may play a role in the activation and maintenance of primary central sensitization to noxious visceral stimuli.

Sacral dorsal horn neurone activity during micturition in the cat

The excitability of two groups of neurones located in different parts of the sacral spinal cord were examined during micturition in decerebrate adult cats. One group of cells, characterized by their activation by pudendal cutaneous afferents, was located in the dorsal commissure of the first and second sacral spinal segments. The second group, located in the dorsal horn of the first sacral spinal segment, was excited by group II muscle and cutaneous afferents. Micturition was evoked by distension of the urinary bladder or by electrical stimulation of the pontine micturition centre. Tonic firing was induced in the neurones by ejection of DL-homocysteic acid from the recording extracellular micropipette. The instantaneous firing frequency of 11/17 sacral dorsal grey commissure neurones was decreased from 7 to 100 % during micturition, and on average was about half of the prevoid firing frequency. It is hypothesized that these sacral neurones are interposed in polysynaptic excitatory pathways from sacral perineal afferents to sphincter motoneurones and that they are subject to direct postsynaptic inhibition during micturition. One other cell showed no change in firing during micturition, two displayed complex patterns of modulation, while 3/17 of the dorsal grey commissure neurones increased their firing rate 30 to 1000 % during micturition. It is hypothesized that the excited neurones may be part of the inhibitory pathways mediating postsynaptic inhibition of sphincter motoneurones or sacral primary afferent depolarization during micturition. Alternatively, they may be part of the excitatory urethral-bladder reflex circuitry. A small (5-15 %) but significant decrease in firing was observed in 4/5 of the group II rostral sacral neurones examined; the firing of a fifth neurone was unchanged. The depression of group II neurones may serve to suppress unwanted hindlimb reflexes that could disrupt micturition.

Ectopic sympathetic preganglionic neurons maintain proper connectivity in the reeler mutant mouse

The location of sympathetic preganglionic neurons (SPN) in the spinal cord of the reeler mouse mutant is abnormal. Instead of their normal location in the intermediolateral column, the majority of SPN in the reeler cluster around the central canal. To determine whether ectopically located SPN in the reeler form appropriate synaptic connections with their pre- and postsynaptic partners, we examined 1). whether the axons of descending neural pathways that normally terminate on SPN follow them to their ectopic location, and 2). whether the central autonomic neural circuit that controls sympathetic output to the kidney is organized normally in the reeler. Using antibodies against tyrosine hydroxylase, serotonin, neuropeptide Y, substance P and calcitonin gene-related peptide as markers for adrenergic, serotonergic and peptidergic terminals, we found that axons which normally innervate SPN follow these neurons to their ectopic spinal location in the reeler. Injection of pseudorabies virus into the kidney of wild type and reeler mutant mice revealed similar patterns of renal sympathetic and pre-sympathetic control circuits in the spinal cord, brainstem and forebrain. These results indicate that the presynaptic inputs and postsynaptic targets of SPN in the reeler are normal, despite the ectopic spinal location of their cell bodies.

Spinal cord neural organization controlling the urinary bladder and striated sphincter

The storage and elimination of urine requires the coordination of activity between the autonomic nervous system (thoracolumbar sympathetic and sacral parasympathetic divisions) controlling the urinary bladder and urethra and the lumbosacral somatic motoneurons innervating the striated sphincter and pelvic floor muscles. These three efferent systems involved in the control of lower urinary tract function receive segmental sensory information from various visceral organs and the perineum, as well as inputs from supraspinal regions. Ascending and descending connections between the various spinal segments levels and supraspinal regions provide the reflex substrates participating in normal bladder continence and micturition reflexes. Many of the actions of descending and segmental reflexes are mediated by excitatory and inhibitory sacral spinal interneurons located within the region of the parasympathetic preganglionic autonomic neurons and the sphincter ventral horn motoneurons. This review will: (1) discuss the basic organization and spinal elements of the reflex pathways subserving continence and micturition; (2) describe features of the identified sacral interneuronal circuitry contributing to the control of the bladder and sphincter function; and (3) discuss how changes in the control of these reflex pathways and neurons may contribute to abnormal patterns of bladder and sphincter function commonly observed following spinal cord injury.

Nicotinic mechanisms in the autonomic control of organ systems

Most visceral organs are under the control of the autonomic nervous system (ANS). Information on the state and function of these organs is constantly relayed to the central nervous system (CNS) by sensory afferent fibers. The CNS integrates the sensory inputs and sends neural commands back to the organ through the ANS. The autonomic ganglia are the final site for the integration of the message traveling from the CNS. Nicotinic acetylcholine receptors (nAChRs) are the main mediators of fast synaptic transmission in ganglia, and therefore, are key molecules for the processing of neural information in the ANS. This review focuses on the role of nAChRs in the control of organ systems such as heart, gut, and bladder. The autonomic control of these organ systems is discussed in the light of the results obtained from the analysis of mice carrying mutations targeted to nAChR subunits expressed in the ANS.

Bladder contractility-related neurons in Barrington's nucleus: axonal projections to the spinal cord in the cat

Barrington's nucleus projects directly to the sacral parasympathetic nucleus. The purpose of this study was to clarify whether neurons in Barrington's nucleus that increase their firing during bladder contractions project to the spinal cord and, if so, to which level(s) the axon reaches. Single units were recorded in Barrington's nucleus of cat with glass microelectrodes, and the termination level of descending axons was determined by antidromic stimulation of the spinal cord. Thirty-nine neurons projecting to the spinal cord were located in rostral parts of the dorsolateral pontine tegmentum, medial and ventral to the mesencephalic trigeminal tract. This finding is consistent with previous neuronal tracing studies. All neurons increased their firing rates during contraction associated with micturition. In 19 examined neurons, the most caudal level of the descending axon distributed between the L7 and the S3 level. Stimulation of the axon at this most caudal level resulted in antidromic spike latencies ranging between 19.5 msec and 45.0 msec. These antidromic latencies were much smaller than previously reported orthodromic conduction times between neurons in Barrington's nucleus and sacral preganglionic neurons innervating the bladder. The mean conduction velocity of the descending axon from the cell body to the border between Th13 and the L1 ranged between 7.2 m/sec and 27.7 m/sec. The decrease of the mean conduction velocity was observed at the lumbar as well as at the sacral segments, suggesting that axons issue collaterals to the lumbar level as well as to the sacral level.

Electrical stimulation for the treatment of bladder dysfunction: current status and future possibilities

Electrical stimulation of peripheral nerves can be used to cause muscle contraction, to activate reflexes, and to modulate some functions of the central nervous system (neuromodulation). If applied to the spinal cord or nerves controlling the lower urinary tract, electrical stimulation can produce bladder or sphincter contraction, produce micturition, and can be applied as a medical treatment in cases of incontinence and urinary retention. This article first reviews the history of electrical stimulation applied for treatment of bladder dysfunction and then focuses on the implantable Finetech-Brindley stimulator to produce bladder emptying, and on external and implantable neuromodulation systems for treatment of incontinence. We conclude by summarizing some recent research efforts including: (a) combined sacral posterior and anterior sacral root stimulator implant (SPARSI), (b) selective stimulation of nerve fibers for selective detrusor activation by sacral ventral root stimulation, (c) microstimulation of the spinal cord, and (d) a newly proposed closed-loop bladder neuroprosthesis to treat incontinence caused by bladder overactivity.

Neural control of the urethra

Coordination between the urinary bladder and the urethra is mediated by multiple reflex pathways organized in the brain and spinal cord. Some reflexes promote urine storage; whereas other reflexes facilitate voiding. During bladder filling, activation of mechanoreceptor afferent nerves in the bladder wall triggers firing in the cholinergic efferent pathways to the external urethral sphincter (EUS) and in sympathetic adrenergic pathways to the urethral smooth muscle. These storage reflexes are dependent upon interneuronal circuitry in the spinal cord. During voiding the spinal storage reflexes are inhibited by supraspinal mechanisms which originate in the pontine micturition center. Glutamatergic, serotonergic and alpha, adrenergic excitatory transmission as well as GABAergic/glycinergic inhibitory transmission have been implicated in the central control of sphincter reflexes. During voiding, a parasympathetic nitrergic inhibitory input to the urethral smooth is activated. This reflex mechanism which is triggered by bladder afferents persists in paraplegic rats and therefore must be mediated at least in part by spinal interneuronal circuitry. In female rats, the parasympathetic nitrergic pathway is prominent; but in male rats it is obscured by a dominant parasympathetic cholinergic excitatory input to the urethral smooth muscle. The function of the cholinergic pathway in voiding is uncertain. Stimulation of urethral afferents can also influence bladder activity. Contraction of the external urethral sphincter activates afferents that inhibit reflex bladder contractions; whereas infusion of fluid through the urethra facilitates bladder contractions. These reflexes are also organized in the spinal cord and presumably play a role in urine storage and elimination. Alterations in primitive bladder-to-urethra and urethra-to-bladder reflex mechanisms may contribute to neurogenic bladder dysfunction.

Evidence for two populations of rat spinal dorsal horn neurons excited by urinary bladder distension

Spinal L6-S2 dorsal horn neurons of cervical spinal cord-transected, decerebrate female rats were characterized using urinary bladder distension (UBD) as a visceral stimulus. Constant pressure, phasic, graded (20-80 mm Hg, 20 s) air UBD was delivered via a transurethral catether and extracellular single-unit recordings obtained from all neurons excited by UBD. Responses to graded UBD and noxious/non-noxious cutaneous stimuli were determined in 258 neurons which could be stratified into two groups based on their effect of a counterirritation stimulus: Type I neurons (n=112) were inhibited by noxious pinch presented in a non-segmental field; Type II neurons (n=146) were not similarly inhibited. Both Types of neurons were identified in both superficial and deep recording sites and demonstrated graded responses to graded UBD. All UBD-excited neurons had convergent cutaneous receptive fields (RFs) excited by non-noxious and/or noxious stimuli. As a group, Type I neurons had a period of decreased activity following termination of the distending stimulus whereas Type II neurons typically had a sustained afterdischarge. UBD-evoked activity in Type II neurons was inhibited more than similar activity in Type I neurons by both intravenous morphine and lidocaine. These results support the assertion that at least two different populations of spinal dorsal horn neurons exist which encode for a stimulus of urinary bladder distension. These populations are an analogue to previously characterized, similar neuronal populations excited by colorectal distension and suggest that they are representative of the overall phenomenon of visceral sensory processing, a component of which is nociception.

Depletion of corticotrophin-releasing factor neurons in the pontine micturition area in multiple system atrophy

We sought to determine whether the putative pontine micturition center in the human dorsal pons contains corticotrophin-releasing factor (CRF) neurons, and whether these neurons are depleted in patients with multiple system atrophy and bladder dysfunction. Brains were obtained at autopsy from 4 control subjects and 4 patients with clinical diagnosis of multiple system atrophy, confirmed neuropathologically. Serial 50 microm cryostat sections were obtained throughout the rostral half of the pons, and every eighth section was processed for CRF immunocytochemistry (rabbit polyclonal antibody). Consecutive sections were stained for nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d) to identify neurons of the laterodorsal tegmental nucleus or for both CRF and NADPH. Locus ceruleus neurons were identified by their neuromelanin content. Abundant CRF immunoreactive neurons were identified in the dorsal pontine tegmentum just ventral to the locus ceruleus. CRF neurons were intermingled with, but distinct from, the NADPH-d-reactive neurons of the laterodorsal tegmental nucleus. In all multiple system atrophy cases, there was a severe depletion of these CRF-immunoreactive neurons (26.6 +/- 3 neurons/section in patients; 73.7 +/- 4 neurons/section in controls). Our results suggest that depletion of CRF neurons in the putative pontine micturition center may contribute to the severe bladder dysfunction that characterizes multiple system atrophy.

Correlation between electrophysiology and morphology of three groups of neuron in the dorsal commissural nucleus of lumbosacral spinal cord of mature rats studied in vitro

The dorsal commissural nucleus (DCN) in the lumbosacral spinal cord receives afferent inputs from the pelvic organs via pudendal and pelvic nerves. Electrophysiological and morphological properties of neurons in the DCN of L6-S1 were examined using whole-cell recordings with biocytin-filled electrodes in transverse slices of mature rat spinal cord. Neurons were categorized into three groups according to their discharge in response to suprathreshold depolarizing pulses; neurons with tonic (19/42) and phasic (13/42) firing patterns, and neurons (10/42) that fired in bursts arising from a Ca(2+)-dependent hump. The predominantly fusiform somata of neurons labeled during recording (n = 31) had on average 3.1 primary dendrites, 7.5 terminating dendritic branches, 3.1 axon collaterals, and 14.2 axon terminations per neuron. The groups were morphologically distinct on the basis of their dendritic branching patterns. Phasic neurons (n = 10) had the most elaborate dendritic branching and the largest numbers of axon collaterals. All tonic neurons (n = 11) had axons/collaterals projecting to the intermediolateral area but none to the funiculi, suggesting that they function as interneurons in local autonomic reflexes. Many axons/collaterals of all phasic neurons lay within the DCN, suggesting that they integrate segmental and descending inputs. Seven of 10 neurons with Ca(2+)-dependent humps had axons/collaterals extending into one of the funiculi, suggesting that they project intersegmentally or to the brain. Ca(2+) hump neurons also had more axons/collaterals within the DCN and fewer in the intermediolateral area than tonic neurons. This correlation between firing pattern and morphology is an important step toward defining the cellular pathways regulating pelvic function.

Sacral spinal interneurones and the control of urinary bladder and urethral striated sphincter muscle function

Normally, during bladder filling (continence) and expulsion (micturition) there is a reciprocity between the pattern of activity in the urinary bladder sacral parasympathetic efferents and the somatic motoneurones innervating the striated external urethral sphincter muscle. The co-ordination of this pattern of reciprocal activity appears to be determined by excitatory and inhibitory actions of a variety of segmental afferents and descending systems with sacral spinal actions. These actions may in part be mediated through lower lumbar and sacral excitatory and inhibitory spinal interneurones. Over the past 30 years, both neuroanatomical and electrophysiological approaches have been used to reveal an ever-increasing richness in the neuronal network in the lower spinal cord related to the bladder and striated external urethral sphincter muscle. The purpose of this review is to present an overview of the identified excitatory and inhibitory spinal interneurones hypothesized to be involved in the central networks controlling the sacral bladder parasympathetic preganglionic neurones and striated urethral sphincter motoneurones during continence and micturition.

Estrogen receptor-alpha immunoreactivity in parasympathetic preganglionic neurons innervating the bladder in the adult ovariectomized cat

Estrogen affects autonomic functions such as micturition. The sacral cord is important in the control of micturition and contains numerous estrogen receptor-alpha immnoreactive (ER-alpha IR) neurons. Therefore, the present double labeling study examines whether sacral parasympathetic preganglionic neurons innervating the bladder are immunoreactive for ER-alpha. In the sacral cord of seven female ovariectomized cats, the distribution of ER-alpha IR neurons was studied using the H222 and 1D5 antibodies. Choleratoxin subunit b (CTb) was injected into the bladder wall to visualize its preganglionic neurons. ER-alpha IR was present in the nuclei of cells in laminae I, II, V, VII, and X, and in nuclei and cytoplasm of neurons in the sacral parasympathetic nucleus. The vast majority of CTb labeled neurons contained ER-alpha IR nuclei, indicating that preganglionic neurons innervating the bladder express ER-alpha. The results suggest that estrogen modulates micturition in the cat via ER-alpha in bladder preganglionic neurons.

Treatment of overactive bladder: other drug mechanisms

The well-known side effects of antimuscarinic drugs have focused interest on other ways of treating overactive bladder. Targets for pharmacologic intervention may be found in the central nervous system (CNS) or peripherally. Several CNS transmitter systems can modulate voiding, but few drugs with a defined CNS site of action have been demonstrated to be clinically useful. The mechanism of action of imipramine, which may be effective in the treatment of overactive bladder, has not yet been clarified. Like imipramine, duloxetine is an inhibitor of serotonin (5-HT) and norepinephrine reuptake. Duloxetine has shown some promise in the treatment of urinary incontinence, but, as with the selective serotonin reuptake inhibitors, its effectiveness in the treatment of overactive bladder has not been proven. Drugs affecting norepinephrine, dopamine or gamma-aminobutyric acid (GABA) receptors and mechanisms may be useful. There is also evidence for a central site of action of alpha(1)-adrenoceptor (AR) antagonists. Traditionally, drugs for the treatment of overactive bladder have had a peripheral site of action. Drugs acting on ARs or membrane channels, as well as prostaglandin synthase inhibitors and several other agents, have been used with moderate success. However, recent developments may lead to more effective drugs, including potassium channel openers, prostaglandins, selective and nonselective inhibitors of cyclooxygenase and those acting on the beta(3)-ARs in the human detrusor. Drugs that reduce afferent activity represent an attractive therapeutic approach, and transmitters of afferent nerves and their receptors are possible targets for pharmacologic interventions.

Nicotinic acetylcholine receptors in the autonomic control of bladder function

Micturition is achieved through complex neurological mechanisms involving somatic, autonomic and central components. This article briefly reviews recent findings on the autonomic control of urinary bladder function. Neuronal nicotinic acetylcholine receptors mediate fast synaptic transmission in autonomic ganglia, and activation of nicotinic receptors in parasympathetic bladder neurons produces contraction of the detrusor muscle. Autonomic ganglia contain transcripts for the alpha(3), alpha(4), alpha(5), alpha(7), beta(2) and beta(4) nicotinic subunits, which can assemble to form multiple nicotinic receptor subtypes, but the exact nicotinic receptor subunit composition in bladder ganglia is unknown. Mutant mice lacking the alpha(3) or the beta(2) and the beta(4) nicotinic subunits have enlarged bladders with dribbling urination and develop urinary infection and bladder stones. Bladder strips from alpha(3) null mice do not respond to nicotine but contract when stimulated with a muscarinic agonist or electric field stimulation. Mice lacking the beta(2) subunit have no overt bladder phenotype, and their bladders contract in response to nicotine. Surprisingly, bladder strips from beta(4) mutant mice do not respond to nicotine despite the absence of major bladder dysfunction in vivo. These findings suggest that nicotinic receptors containing the alpha(3) and the beta(4) subunits are necessary for normal bladder function.

Dynamics of intrinsic electrophysiological properties in spinal cord neurones

The spinal cord is engaged in a wide variety of functions including generation of motor acts, coding of sensory information and autonomic control. The intrinsic electrophysiological properties of spinal neurones represent a fundamental building block of the spinal circuits executing these tasks. The intrinsic response properties of spinal neurones--determined by the particular set and distribution of voltage sensitive channels and their dynamic non-linear interactions--show a high degree of functional specialisation as reflected by the differences of intrinsic response patterns in different cell types. Specialised, cell specific electrophysiological phenotypes gradually differentiate during development and are continuously adjusted in the adult animal by metabotropic synaptic interactions and activity-dependent plasticity to meet a broad range of functional demands.

Bladder and urethral pressures evoked by microstimulation of the sacral spinal cord in cats

Experiments were conducted to measure the bladder and urethral pressures evoked by intraspinal microstimulation of the sacral segments (S1-S2) in neurologically intact, chloralose anesthetized adult male cats. The bladder pressure was measured with a superpubic catheter and the urethral pressure was measured simultaneously at the level of the urethral sphincter and at the level of the penis using a two-element micromanometer. Intraspinal stimuli (typically 1 s, 20 Hz, 100 microA, 100 microseconds) were applied with activated iridium microwire electrodes in ipsilateral segments and intersegmental boundaries with a 250 micrometer mediolateral resolution and a 200 micrometer dorsoventral resolution. Increases in bladder pressures were generated by microstimulation in the intermediolateral region, in the lateral and ventrolateral ventral horn, and around the central canal. Simultaneous increases in urethral pressure were evoked by microstimulation in the ventrolateral ventral horn, but not at the other locations. Small reductions in urethral pressure (<10 cm H(2)O) were evoked at locations in the intermediate laminae and around the central canal. The magnitude of these pressure reductions was weakly dependent on the stimulus parameters. Stimulation around the central canal produced bladder contractions with either no change or a reduction in urethral pressure and voiding of small amounts of fluid. These results demonstrate that regions are present in the spinal intact anesthetized cat where microstimulation generates selective contraction of the bladder without increases in urethral pressure and that regions are present where microstimulation generates small reductions in urethral pressure.

Increased c-fos expression in spinal lumbosacral projection neurons and preganglionic neurons after irritation of the lower urinary tract in the rat

Chemical irritation of the lower urinary tract (LUT) induces c-fos expression in neurons in the lumbosacral (L(6) and S(1)) spinal cord. This study used axonal tracing with fluorescent dyes to identify the types of spinal neurons expressing Fos immunoreactivity (IR) after LUT irritation in the rat. Fos-IR was detected in lateral and medial superficial dorsal horn, the sacral parasympathetic nucleus (SPN) and lamina X around the central canal. Fos-IR was detected in spinal neurons projecting to supraspinal sites (brainstem and hypothalamus), in preganglionic neurons (PGN) and in unlabeled segmental interneurons. A substantial percentage (20%) of dye labeled PGN exhibited Fos-IR after LUT irritation; and a larger percentage (36%) exhibited Fos-IR after electrical stimulation of the pelvic nerve which contains afferent pathways from all of the pelvic organs. The majority (average 55%) of Fos-positive neurons projecting to supraspinal sites were also located in the region of the SPN. A selective distribution of different types of neurons was detected in this region: PGN were located ventral to the spinal projection neurons which in turn were located ventral to the majority of unidentified Fos-positive neurons. The distribution of Fos-positive PGN and projection neurons was similar in spinal intact and spinal transected animals indicating that c-fos expression was mediated by monosynaptic afferent input or input from segmental interneurons and was not due to activation of supraspinal micturition reflex pathways.

Protracted elevation of neuronal nitric oxide synthase immunoreactivity in axotomised adult pudendal motor neurons

Neuronal nitric oxide synthase immunoreactivity (NOS1-ir) in sacral motor neurons of normal adult cats was compared with that in cats surviving 1-10 wk after unilateral transection and ligation of the pudendal nerve. Levels of immunostaining were measured by microdensitometry. In nonoperated cats 60% of motor neurons in the ventrolateral nucleus (VL) and Onuf's nucleus (ON) showed high levels of NOS1-ir with lower NOS1-ir in 40%. Following axotomy, motor neurons in ON on both sides of the cord showed an acute rise in mean level of NOS1-ir at 1 wk, with a further increase at 2 wk. Mean levels of NOS1-ir in the ipsilateral and contralateral ON remained elevated at 10 wk after axotomy. Elevation of NOS1-ir occurred in the VL with a similar time-course to that in ON, implying a wider response in motor nuclei synaptically coupled to ON. Measurements of neuronal size in ON and VL revealed an increase in neuronal size in ON but not VL, indicating increased NOSI-ir in ON was not an artifact of neuronal atrophy. The proportion of motor neurons in ON and VL possessing higher levels of NOS1-ir increased from 60% in controls to 100% at 2-3 wk postaxotomy. The proportion slightly declined by 8 wk due to re-emergence of motor neurons exhibiting low NOS1-ir, but remained greater than normal at 10 wk in both nuclei. Based on evidence from related analyses of synaptology, we argue that acute axotomy induced alterations in presynaptic complement which increased overall Ca2+ influx and thereby stimulated NOS1-ir.

Identification of the spinal neural network involved in coordination of micturition in the male cat

In these studies, we used the expression of Fos protein to identify cells within the spinal cord that regulate micturition in male cats. The immediate early gene c-fos that encodes the Fos protein can be induced rapidly and transiently in neurons by increased electrical activity. Animals were anesthetized with alpha-chloralose, and received one of four stimulus protocols: electrical stimulation of the pelvic nerve, electrical stimulation of the pudendal nerve, a period of isometric micturition (induced by ligating the proximal urethra and infusing saline into the bladder), or electrical stimulation of Barrington's nucleus. After the period of stimulation, the animals were perfused and neurons expressing Fos-like immunoreactivity (FLI) were visualized with immunocytochemical methods. Stimulation with each protocol resulted in a substantially larger number of neurons expressing FLI than in operated but unstimulated controls, which exhibited few Fos-positive neurons localized to the superficial dorsal horn. In animals undergoing isometric micturition or stimulation of Barrington's nucleus, neurons exhibiting FLI were found bilaterally in the sacral (S1-S3) spinal cord and were localized to the lateral portion of the superficial dorsal horn (laminae I and II), in the intermediolateral region (lateral laminae V-VII), and around the central canal (lamina X and medial laminae V-VII). The intermediolateral region appeared to contain two populations of cells exhibiting FLI: a group of large multipolar cells and a group of small round cells. Few Fos-immunoreactive nuclei were observed in the medial portion of the superficial dorsal horn, and FLI was not observed in ventral horn neurons. Electrical stimulation of the pudendal or pelvic nerves resulted in fewer numbers of cells exhibiting FLI, with a less widespread spatial distribution. These results identify spinal neurons that are active during the micturition cycle, and demonstrate that a behaviorally relevant stimulus (isometric micturition) generated more widespread and greater intensity of Fos expression than repetitive electrical stimulation of the component peripheral nerves.

The overactive bladder: pharmacologic basis of drug treatment

OBJECTIVES

To provide an overview of the basis for drug treatment of the overactive bladder.

METHODS

Published information is evaluated.

RESULTS

The causes of bladder overactivity are not known, but theoretically, increased afferent activity, decreased inhibitory control in the central nervous system (CNS) or peripheral ganglia, and increased sensitivity of the detrusor to efferent stimulation may be involved. Several CNS transmitters can modulate voiding, but few useful drugs with a defined CNS site of action have been developed. Drugs that stimulate gamma-aminobutyric acid receptors are used clinically. Potentially, drugs affecting opioid, 5-hydroxytryptamine, norepinephrine, dopamine, and glutamatergic receptors and mechanisms can be developed, but a selective action on the lower urinary tract may be difficult to obtain. Traditionally, drugs used for treatment of bladder overactivity have had a peripheral site of action, mainly efferent neurotransmission or the detrusor itself. Antimuscarinic drugs, beta-adrenoceptor agonists, alpha-adrenoceptor antagonists, drugs affecting membrane channels, prostaglandin synthetase inhibitors, and several other agents have been used with limited success. New information on the alpha-adrenoceptor and muscarinic receptor subtypes in the human detrusor has emerged and may be the basis for the development of new compounds with effects on bladder overactivity. Decreasing afferent activity seems an attractive therapeutic approach, and drugs affecting afferent nerves by causing release of tachykinins, such as capsaicin and analogs, as well as agents blocking tachykinin receptors, may be of therapeutic interest.

CONCLUSIONS

New drugs, specifically designed for the treatment of bladder overactivity, are desirable.

Developmental synaptic depression underlying reorganization of visceral reflex pathways in the spinal cord

During development, neuronal connectivity has a remarkable plasticity. Synaptic refinement in the spinal autonomic nucleus might be involved in the elimination of primitive segmental reflexes and the emergence of mature spinobulbospinal reflexes, which occurs a few weeks after birth. To address this possibility, we examined the postnatal changes of segmental excitatory synaptic transmission by applying the whole-cell recording technique to parasympathetic preganglionic neurons in slice preparations of the rat lumbosacral spinal cord. The mean magnitude of unitary excitatory synaptic currents evoked in preganglionic neurons by stimulation of single interneurons remained unchanged during the first two postnatal weeks but was reduced by 50% during the third postnatal week. This reduction in synaptic efficacy was associated with a decrease in the amount of transmitter release from interneurons. Moreover, this developmental depression of segmental synaptic transmission was prevented by spinal cord transection at the thoracic level on postnatal day 14. Thus, developmental modification of excitatory synapses on preganglionic neurons appears to be attributable to competition between segmental interneuronal and descending bulbospinal inputs, which results in the developmental reorganization of parasympathetic excretory reflex pathways.