Central control of visceral pain and urinary tract function

Afferent input from Aδ and C-fibres innervating the urinary bladder are processed differently by the brain, and have different roles in signaling bladder sensation. Aδ fibres that signal bladder filling activate a spino-bulbo-spinal loop, which relays in the midbrain periaqueductal grey (PAG) and pontine micturition centre (PMC). The excitability of this circuitry is regulated by tonic GABAergic inhibitory processes. In humans and socialised animals micturition is normally under volitional control and influenced by a host of psychosocial factors. Higher nervous decision-making in a social context to 'go now' or 'do not go' probably resides in frontal cortical areas, which act as a central control switch for micturition. Exposure to psychosocial stress can have profoundly disruptive influence on the process and lead to maladaptive changes in the bladder. During sleeping the voiding reflex threshold appears to be reset to a higher level to promote urinary continence.Under physiological conditions C-fibre bladder afferents are normally silent but are activated in inflammatory bladder states and by intense distending pressure. Following prolonged stimulation visceral nociceptors sensitise, leading to a lowered threshold and heightened sensitivity. In addition, sensitization may occur within the central pain processing circuitry, which outlasts the original nociceptive insult. Visceral nociception may also be influenced by genetic and environmental influences. A period of chronic stress can produce increased sensitivity to visceral pain that lasts for months. Adverse early life events can produce even longer lasting epigenetic changes, which increase the individual's susceptibility to developing visceral pain states in adulthood

Modeling co-operative volume signaling in a plexus of nitric oxide synthase-expressing neurons

In vertebrate and invertebrate brains, nitric oxide (NO) synthase (NOS) is frequently expressed in extensive meshworks (plexuses) of exceedingly fine fibers. In this paper, we investigate the functional implications of this morphology by modeling NO diffusion in fiber systems of varying fineness and dispersal. Because size severely limits the signaling ability of an NO-producing fiber, the predominance of fine fibers seems paradoxical. Our modeling reveals, however, that cooperation between many fibers of low individual efficacy can generate an extensive and strong volume signal. Importantly, the signal produced by such a system of cooperating dispersed fibers is significantly more homogeneous in both space and time than that produced by fewer larger sources. Signals generated by plexuses of fine fibers are also better centered on the active region and less dependent on their particular branching morphology. We conclude that an ultrafine plexus is configured to target a volume of the brain with a homogeneous volume signal. Moreover, by translating only persistent regional activity into an effective NO volume signal, dispersed sources integrate neural activity over both space and time. In the mammalian cerebral cortex, for example, the NOS plexus would preferentially translate persistent regional increases in neural activity into a signal that targets blood vessels residing in the same region of the cortex, resulting in an increased regional blood flow. We propose that the fineness-dependent properties of volume signals may in part account for the presence of similar NOS plexus morphologies in distantly related animals

High Frequency Stimulation of the Pelvic Nerve Inhibits Urinary Voiding in Anesthetized Rats

Urge Urinary Incontinence: “a sudden and uncontrollable desire to void which is impossible to defer” is extremely common and considered the most bothersome of lower urinary tract conditions. Current treatments rely on pharmacological, neuromodulatory, and neurotoxicological approaches to manage the disorder, by reducing the excitability of the bladder muscle. However, some patients remain refractory to treatment. An alternative approach would be to temporarily suppress activity of the micturition control circuitry at the time of need i.e., urgency. In this study we investigated, in a rat model, the utility of high frequency pelvic nerve stimulation to produce a rapid onset, reversible suppression of voiding. In urethane-anesthetized rats periodic voiding was induced by continuous infusion of saline into the bladder whilst recording bladder pressure and electrical activity from the external urethral sphincter (EUS). High frequency (1–3 kHz), sinusoidal pelvic nerve stimulation initiated at the onset of the sharp rise in bladder pressure signaling an imminent void aborted the detrusor contraction. Urine output was suppressed and tone in the EUS increased. Stimulating the right or left nerve was equally effective. The effect was rapid in onset, reversible, and reproducible and evoked only minimal “off target” side effects on blood pressure, heart rate, respiration, uterine pressure, or rectal pressure. Transient contraction of abdominal wall was observed in some animals. Stimulation applied during the filling phase evoked a small, transient rise in bladder pressure and increased tonic activity in the EUS, but no urine output. Suppression of micturition persisted after section of the contralateral pelvic nerve or after ligation of the nerve distal to the electrode cuff on the ipsilateral side. We conclude that high frequency pelvic nerve stimulation initiated at the onset of an imminent void provides a potential means to control urinary continence

Cardiac effects of repeated focal seizures in rats induced by intrahippocampal tetanus toxin:bradyarrhythmias, tachycardias and prolonged interictal QT interval

Objective To determine electrical changes in the heart in a chronic, nonstatus model of epilepsy. Methods Electrocorticography (ECoG) and electrocardiography (ECG) of nine animals (five made epileptic by intrahippocampal injection of tetanus neurotoxin (TeNT) and four controls), are monitored continuously by radiotelemetry for up to 7 weeks. Results Epileptic animals develop a median of 168 seizures, with postictal tachycardias reaching a mean of 487 beats/min and lasting a mean of 661 seconds. Ictal changes in heart rate include tachycardia and in the case of convulsive seizures, bradyarrhythmias resembling Mobitz type 1 second‐degree atrioventricular block; notably the P‐R interval increased before block. Postictally, the amplitude of T wave increases. Interictally, QT dependence on RR is modest and conventional QT corrections prove ineffective. Interictal QT intervals, measured at a heart rate of 400 bpm, increased from 65 to 75 ms, an increase dependent on seizure incidence over the preceding 10‐14 days. Significance Repeated seizures induce a sustained tachycardia and increase in QT interval of the ECG and evoke arrhythmias including periods of atrioventricular block during Racine type 4 and 5 seizures. These changes in cardiac function may predispose to development in fatal arrhythmias and sudden death in humans with epilepsy.</p

Suppression of Urinary Voiding by Conditional High Frequency Stimulation of the Pelvic Nerve in Conscious Rats:Pelvic nerve stimulation suppresses urinary voiding

Female Wistar rats were instrumented to record bladder pressure and to stimulate the left pelvic nerve. Repeated voids were induced by continuous infusion of saline into the bladder (11.2 ml/h) via a T-piece in the line to the bladder catheter. In each animal tested (n = 6) high frequency pelvic nerve stimulation (1–3 kHz, 1–2 mA sinusoidal waveform for 60 s) applied within 2 s of the onset of a sharp rise in bladder pressure signaling an imminent void was able to inhibit micturition. Voiding was modulated in three ways: (1) Suppression of voiding (four rats, n = 13 trials). No fluid output or a very small volume of fluid expelled (&lt;15% of the volume expected based on the mean of the previous 2 or 3 voids). Voiding suppressed for the entirety of the stimulation period (60 s) and resumed within 37 s of stopping stimulation. (2) Void deferred (four rats, n = 6 trials). The imminent void was suppressed (no fluid expelled) but a void occurred later in the stimulation period (12–44 s, mean 24.5 ± 5.2 s after the onset of the stimulation). (3) Reduction in voided volume (five rats, n = 20 trials). Voiding took place but the volume of fluid voided was 15–80% (range 21.8–77.8%, mean 45.3 ± 3.6%) of the volume expected from the mean of the preceding two or three voids. Spontaneous voiding resumed within 5 min of stopping stimulation. Stimulation during the filling phase in between voids had no effect. The experiments demonstrate that conditional high frequency stimulation of the pelvic nerve started at the onset of an imminent void can inhibit voiding. The effect was rapidly reversible and was not accompanied by any adverse behavioral side effects