Mechanisms of reflex bladder activation by pudendal afferents.

Activation of pudendal afferents can evoke bladder contraction or relaxation dependent on the frequency of stimulation, but the mechanisms of reflex bladder excitation evoked by pudendal afferent stimulation are unknown. The objective of this study was to determine the contributions of sympathetic and parasympathetic mechanisms to bladder contractions evoked by stimulation of the dorsal nerve of the penis (DNP) in α-chloralose anesthetized adult male cats. Bladder contractions were evoked by DNP stimulation only above a bladder volume threshold equal to 73 ± 12% of the distension-evoked reflex contraction volume threshold. Bilateral hypogastric nerve transection (to eliminate sympathetic innervation of the bladder) or administration of propranolol (a ß-adrenergic antagonist) decreased the stimulation-evoked and distension-evoked volume thresholds by -25% to -39%. Neither hypogastric nerve transection nor propranolol affected contraction magnitude, and robust bladder contractions were still evoked by stimulation at volume thresholds below the distension-evoked volume threshold. As well, inhibition of distention-evoked reflex bladder contractions by 10 Hz stimulation of the DNP was preserved following bilateral hypogastric nerve transection. Administration of phentolamine (an α-adrenergic antagonist) increased stimulation-evoked and distension-evoked volume thresholds by 18%, but again, robust contractions were still evoked by stimulation at volumes below the distension-evoked threshold. These results indicate that sympathetic mechanisms contribute to establishing the volume dependence of reflex contractions but are not critical to the excitatory pudendal to bladder reflex. A strong correlation between the magnitude of stimulation-evoked bladder contractions and bladder volume supports that convergence of pelvic afferents and pudendal afferents is responsible for bladder excitation evoked by pudendal afferents. Further, abolition of stimulation-evoked bladder contractions following administration of hexamethonium bromide confirmed that contractions were generated by pelvic efferent activation via the pelvic ganglion. These findings indicate that pudendal afferent stimulation evokes bladder contractions through convergence with pelvic afferents to increase pelvic efferent activity.

Finite element modeling and in vivo analysis of electrode configurations for selective stimulation of pudendal afferent fibers.

BACKGROUND: Intraurethral electrical stimulation (IES) of pudendal afferent nerve fibers can evoke both excitatory and inhibitory bladder reflexes in cats. These pudendovesical reflexes are a potential substrate for restoring bladder function in persons with spinal cord injury or other neurological disorders. However, the complex distribution of pudendal afferent fibers along the lower urinary tract presents a challenge when trying to determine the optimal geometry and position of IES electrodes for evoking these reflexes. This study aimed to determine the optimal intraurethral electrode configuration(s) and locations for selectively activating targeted pudendal afferents to aid future preclinical and clinical investigations. METHODS: A finite element model (FEM) of the male cat urethra and surrounding structures was generated to simulate IES with a variety of electrode configurations and locations. The activating functions (AFs) along pudendal afferent branches innervating the cat urethra were determined. Additionally, the thresholds for activation of pudendal afferent branches were measured in alpha-chloralose anesthetized cats. RESULTS: Maximum AFs evoked by intraurethral stimulation in the FEM and in vivo threshold intensities were dependent on stimulation location and electrode configuration. CONCLUSIONS: A ring electrode configuration is ideal for IES. Stimulation near the urethral meatus or prostate can activate the pudendal afferent fibers at the lowest intensities, and allowed selective activation of the dorsal penile nerve or cranial sensory nerve, respectively. Electrode location was a more important factor than electrode configuration for determining stimulation threshold intensity and nerve selectivity.

Efficiency analysis of waveform shape for electrical excitation of nerve fibers.

Stimulation efficiency is an important consideration in the stimulation parameters of implantable neural stimulators. The objective of this study was to analyze the effects of waveform shape and duration on the charge, power, and energy efficiency of neural stimulation. Using a population model of mammalian axons and in vivo experiments on cat sciatic nerve, we analyzed the stimulation efficiency of four waveform shapes: square, rising exponential, decaying exponential, and rising ramp. No waveform was simultaneously energy-, charge-, and power-optimal, and differences in efficiency among waveform shapes varied with pulse width (PW). For short PWs (< or = 0.1 ms), square waveforms were no less energy-efficient than exponential waveforms, and the most charge-efficient shape was the ramp. For long PW s (> or = 0.5 ms), the square was the least energy-efficient and charge-efficient shape, but across most PW s, the square was the most power-efficient shape. Rising exponentials provided no practical gains in efficiency over the other shapes, and our results refute previous claims that the rising exponential is the energy-optimal shape. An improved understanding of how stimulation parameters affect stimulation efficiency will help improve the design and programming of implantable stimulators to minimize tissue damage and extend battery life.

Intraurethral stimulation evokes bladder responses via 2 distinct reflex pathways.

PURPOSE: Recent animal studies have shown that selective activation of pudendal nerve branches can evoke bladder responses through 2 distinct reflex pathways. We examined intraurethral electrical stimulation as a minimally invasive means of selectively activating these pathways in the cat. MATERIALS AND METHODS: Bladder responses evoked by intraurethral electrical stimulation were measured in alpha-chloralose anesthetized male cats at different stimulation frequencies, stimulation intensities and intraurethral locations. RESULTS: Intraurethral electrical stimulation evoked inhibitory and excitatory bladder reflexes depending on stimulation frequency and location. Stimulation in the penile urethra 0 to 3 cm from the urethral meatus at 33 Hz evoked bladder contraction and at 10 Hz it evoked bladder relaxation. These responses were abolished after bilateral transection of the dorsal penile nerves. Stimulation in the membranous urethra 5 to 7 cm from the urethral meatus at 2, 10 and 33 Hz evoked bladder contractions. These responses were abolished after bilateral transection of the cranial sensory nerves. Following acute spinal cord transection bladder contractions were still evoked by 33 Hz stimulation in the penile urethra but not by stimulation at any frequency in the membranous urethra. CONCLUSIONS: Intraurethral electrical stimulation selectively evoked bladder responses by activating 2 distinct pudendal afferent pathways. Responses depended on stimulation frequency and location. Intraurethral electrical stimulation is a valid means of determining the pathways involved in bladder responses evoked by pudendal nerve stimulation.

Somatic innervation of the feline lower urinary tract.

Electrical stimulation of pudendal nerve sensory pathways can evoke excitatory bladder reflexes. However, the precise peripheral innervation pattern of these somatic fibers remains unclear. In adult male cats, we investigated pudendal nerve innervation of the lower urinary tract (LUT) by employing anatomical (Sihler's stain) and electrophysiological (selective electrical nerve stimulation) techniques. The stained specimens revealed differential innervation of the proximal and distal urethrae by fibers derived from the sensory branch of the pudendal nerve. Cranial sensory branch fibers penetrated the prostate to terminate along the intraluminal surface of the urethra, whereas the dorsal nerve of the penis primarily innervated the glans penis. Further examination of the proximal urethra showed a separate pathway (deep perineal nerve) that inserted directly into the external urethral sphincter. These observations were confirmed electrophysiologically by the measured urethral sphincter activity evoked in response to selective nerve stimulation. Electrical activation of the sensory pathway evoked only reflex (latency=8.9+/-1.1 ms) contractions of the urethral muscle, whereas stimulation of the perineal pathway elicited direct (latency=1.3+/-0.1 ms) responses. Our findings identify specific pudendal nerve sensory pathways that can be used potentially to restore bladder function in persons with spinal cord injury and also treat LUT symptoms such as urinary retention.

Bladder activation by selective stimulation of pudendal nerve afferents in the cat.

Bladder contractions evoked by pudendal nerve stimulation in both spinal intact and spinal transected cats support the possibility of restoring urinary function in persons with chronic spinal cord injury (SCI). However, electrically evoked bladder responses in persons with SCI were limited to transient contractions at relatively low pressures. This prompted the present study, which presents a detailed quantification of the responses evoked by selective stimulation of individual branches of the pudendal nerve at different stimulation frequencies. In spinal intact cats anesthetized with alpha-chloralose, selective frequency-dependent electrical activation of the sensory (2 Hzor=20 Hz) and rectal perineal (f

Activation and inhibition of the micturition reflex by penile afferents in the cat.

Coordination of the urinary bladder and the external urethral sphincter is controlled by descending projections from the pons and is also subject to modulation by segmental afferents. We quantified the effects on the micturition reflex of sensory inputs from genital afferents traveling in the penile component of the somatic pudendal nerve by electrical stimulation of the dorsal nerve of the penis (DNP) in alpha-chloralose anesthetized male cats. Depending on the frequency of stimulation (range, 1-40 Hz), activation of penile afferents either inhibited contractions of the bladder and promoted urine storage or activated the bladder and produced micturition. Stimulation of the DNP at 5-10 Hz inhibited distension-evoked contractions and increased the maximum bladder capacity before incontinence. Conversely, stimulation at 33 and 40 Hz augmented distension-evoked contractions. When the bladder was filled above a threshold volume (70% of the volume necessary for distension-evoked contractions), stimulation at 20-40 Hz activated de novo the micturition reflex and elicited detrusor contractions that increased voiding efficiency compared with distension-evoked voiding. Electrical stimulation of the DNP with a cuff electrode or percutaneous wire electrode produced similar results. The ability to evoke detrusor contractions by activation of the DNP was preserved following acute spinal cord transection. These results demonstrate a clear role of genital afferents in modulating the micturition reflex and suggest the DNP as a potential target for functional restoration of bladder control using electrical stimulation.

Intraspinal application of endothelin results in focal ischemic injury of spinal gray matter and restricts the differentiation of engrafted neural stem cells.

Previous data have shown that pluripotent stem cells engrafted into the contused spinal cord differentiate only along an astrocytic lineage. The unknown restrictive cues appear to be quite rigid as even neuronal-restricted precursors fail to differentiate to the mature potential they exhibit in vitro after similar grafting into the contused spinal cord. It has been hypothesized that this potent lineage restriction is, in part, the result of the significant loss of both gray and white matter observed following spinal contusion, which elicits a massive acute inflammatory response and is manifested chronically by dramatic cystic cavitation. To evaluate the gray matter component, we developed a clinically relevant model of focal gray matter ischemic injury using the potent vasoconstrictor endothelin (ET-1) and characterized the differentiation of pluripotent stem cells transplanted into this atraumatic vascular SCI. Results demonstrate that low dose ET-1 microinjection into cervical spinal gray matter results in an inflammatory response that is temporally comparable to that observed following traumatic SCI, as well as chronic gray matter loss, but without significant cystic cavitation or white matter degeneration. However, despite the preservation of host spinal parenchyma, no elaboration of neuronal phenotypes was observed from engrafted stem or precursor cells. These results suggest that a common pathologic component responsible for this lineage restriction exists between contusive SCI and ET-1 mediated focal ischemic SCI.

Consequences of noggin expression by neural stem, glial, and neuronal precursor cells engrafted into the injured spinal cord.

Bone morphogenetic proteins (BMPs) are a large class of secreted factors, which serve as modulators of development in multiple organ systems, including the CNS. Studies investigating the potential of stem cell transplantation for restoration of function and cellular replacement following traumatic spinal cord injury (SCI) have demonstrated that the injured adult spinal cord is not conducive to neurogenesis or oligodendrogenesis of engrafted CNS precursors. In light of recent findings that BMP expression is modulated by SCI, we hypothesized that they may play a role in lineage restriction of multipotent grafts. To test this hypothesis, neural stem or precursor cells were engineered to express noggin, an endogenous antagonist of BMP action, prior to transplantation or in vitro challenge with recombinant BMPs. Adult rats were subjected to both contusion and focal ischemic SCI. One week following injury, the animals were transplanted with either EGFP- or noggin-expressing neural stem or precursor cells. Results demonstrate that noggin expression does not antagonize terminal astroglial differentiation in the engrafted stem cells. Furthermore, neutralizing endogenous BMP in the injured spinal cord significantly increased both the lesion volume and the number of infiltrating macrophages in injured spinal cords receiving noggin-expressing stem cell grafts compared with EGFP controls. These data strongly suggest that endogenous factors in the injured spinal microenvironment other than the BMPs restrict the differentiation of engrafted pluripotent neural stem cells as well as suggest other roles for BMPs in tissue protection in the injured CNS.