Responses of feline raphespinal neurons to urinary bladder distension

Visceral pain: the ins and outs, the ups and downs

PURPOSE OF REVIEW

Visceral pain represents a major clinical problem, yet far less is known about its mechanisms compared with somatic pains, for example, from cutaneous and muscular structures.

RECENT FINDINGS

In this review, we describe the neuroanatomical bases of visceral pain signalling in the peripheral and central nervous system, comparing to somatic pains and also the channels and receptors involved in these events. We include an overview of potential new targets in the context of mechanisms of visceral pain and hypersensitivity.

SUMMARY

This review should inform on the recognition of what occurs in patients with visceral pain, why comorbidities are common and how analgesic treatments work.

Pregabalin modulation of spinal and brainstem visceral nociceptive processing

Brainstem and spinal mechanisms mediating visceral nociception are investigated here using electrophysiology and immunohistochemistry techniques in a model of acute visceral pain. Colorectal distension (CRD) produced graded visceromotor responses (VMR) in normal rats, and these were facilitated by intracolonic mustard oil (MO) that generated acute visceral hyperalgesia. The neuropathic pain drug pregabalin (PGB) is thought to have state-dependent effects in attenuating neuropathic, but not acute somatic pain, likely by impairing calcium-channel trafficking. We found that systemic PGB produced antinociceptive effects on CRD-evoked VMRs in naïve rats lacking pathophysiology and in MO-pretreated rats. Systemic PGB also significantly reduced Fos labelling in lumbosacral spinal cords of rats given noxious repetitive CRD; however, PGB did not alter this measure of neural activity in the brainstem. Differential brainstem processing of noxious somatic and visceral stimuli may underlie the unique lack of state-dependent actions of PGB in this visceral pain model. Single-unit recordings in the rostral ventromedial medulla (RVM) verify that brainstem processing of somatic and visceral stimuli differs. The effects of CRD on RVM cells classed as ON, OFF, or NEUTRAL were independent of their somatic responses, with surprising changes in RVM cell activity to innocuous visceral stimulation. PGB also markedly reduced the visceral responses of RVM ON-cells to noxious CRD. These results illustrate clear differences in the central processing of visceral and somatic stimuli, yet a common role for descending modulation by brainstem activity in mediating evoked pain measures.

Rostral ventral medulla modulation of the visceromotor reflex evoked by urinary bladder distension in female rats

The present studies examined the involvement of the rostral ventral medulla (RVM) in modulating the visceromotor response (VMR) evoked by urinary bladder distension (UBD) in adult female rats. The VMR was indexed by electromyographic (EMG) responses of the abdominal external oblique muscle to UBD. Experiment 1 showed that the predominant effect of electrical stimulation of the RVM in normal rats was to produce intensity-dependent inhibition of the VMR (54% of sites sampled). Facilitatory, biphasic, or no effects were obtained at the remaining sites. Experiment 2 showed that RVM-induced inhibition of the VMR was significantly attenuated by intraperitoneal (i.p.) administration of naloxone but not saline vehicle. In experiment 3, we examined the effect of lesions of the RVM in rats with inflamed bladders because previous research has shown that an endogenous opioid inhibitory system is engaged by bladder inflammation. Electrolytic lesions of the RVM but not sham lesions of the RVM significantly increased the VMR to graded UBD in rats with augmented VMRs induced by prior inflammation of the bladder. The present data suggest that the RVM can inhibit the VMR to UBD, acting in part via an opioid-inhibitory system, and that bladder inflammation can recruit the RVM to produce a net inhibitory effect on the VMR to UBD.

PERSPECTIVE

Stimulation of the RVM resulted in inhibitory, facilitatory, and biphasic modulation of the visceromotor reflex to urinary bladder distension. Inhibitory effects of stimulation were attenuated by naloxone, and lesions of the RVM enhanced the VMR in rats with inflamed bladders. These data indicate an important role of the RVM in modulating bladder pain.

Effects of electrical stimulation of the raphe area on the micturition reflex in cats

The raphe nucleus has a variety of physiological functions, including emotion, regulation of skeletal muscle motoneurons, spinal transmission of nociceptive signals, sleep, respiration, gastric motility, and cardiovascular function. Recent evidence has shown that centrally administered serotonin has modulatory effects on micturition function, and that decreased brain serotonin might underlie depression and an overactive bladder. We applied high-frequency stimulation (HFS; 0.2-ms duration, 100 Hz) in the raphe nucleus and the adjacent midline area in 20 supracollicular decerebrate cats, which mostly elicited inhibition of the micturition reflex. The effective amplitude of the electrical stimulation for evoking inhibitory responses was less than 50 muA. We also examined single neuronal activities in the raphe nucleus in response to isovolumetric spontaneous micturition reflexes. In total, 79 neurons were recorded in the raphe nucleus that were related to urinary storage/micturition cycles. Of the neurons recorded, the most common were tonic storage neurons (48%), followed by tonic micturition neurons (28%), phasic storage neurons (18%), and phasic micturition neurons (6%). In addition to the tonic/phasic as well as storage/micturition classification, the neurons showed diverse discharge patterns: augmenting, constant and decrementing, with the constant discharge pattern being most common. Among neurons in the raphe nucleus, the neurons with a decrementing discharge pattern were concentrated in the rostral portion, whereas the augmenting and constant neurons existed diffusely. The storage and micturition neurons were intermingled in the rostral portion, whereas they were separate in the caudal portion. In conclusion, the results of the present study indicate that HFS of the raphe area inhibits the micturition reflex and that there are micturition-related neuronal firings in the raphe area in cats, suggesting that the raphe nucleus is involved in neural control of micturition.

Roles for pain modulatory cells during micturition and continence

We studied how the nervous system selects between noxious stimulus-evoked withdrawals and micturition, movements that are necessary for survival but use overlapping muscles and therefore cannot occur simultaneously. In lightly anesthetized rats, micturition was favored, because noxious stimulation never interrupted micturition, whereas withdrawals were suppressed during voiding. Neurons in the ventromedial medulla (VMM) are a major source of descending antinociceptive signals. To test whether VMM neurons support withdrawal suppression during micturition, the discharge of VMM neurons was recorded during continence and micturition. VMM cells that were inhibited (M-inh) or excited (M-exc) during micturition were observed. M-inh cells were excited by noxious cutaneous stimulation and thus are likely nociception facilitating, whereas M-exc cells were inhibited by noxious heat and are likely nociception inhibiting. The excitation of nociception-inhibiting M-exc and inhibition of nociception-facilitating M-inh cells predicts suppression of withdrawals during micturition. M-exc cells were typically silent before micturition, whereas most M-inh cells fired before micturition, suggesting that these cells may also play a preparatory role for micturition. To test this idea, we examined manipulations that either advanced or delayed the onset of micturition. Hypothalamic stimulation and noxious paw heat advanced micturition while exciting M-inh cells and inhibiting M-exc cells. In contrast, colorectal distension, a stimulus that delays micturition, inhibited M-inh cells and excited M-exc cells. These results suggest a model in which, during continence, VMM M-inh cells facilitate and M-exc cells inhibit bladder afferents, advancing micturition onset when M-inh cells are activated and delaying onset when M-exc cells are activated.

Targeting serotonin and norepinephrine receptors in stress urinary incontinence

Stress urinary incontinence (SUI) in women is prevalent, and there are no globally developed or widely approved drugs for the disease. One strategy for improving urinary continence is to augment the function of the urethral rhabdosphincter through neuropharmacology. The present review describes the innervation of the urethra, and the role of the central nervous system in controlling nerve activity. Targeting serotonin and norepinephrine (or noradrenaline) receptors in Onuf's nucleus is shown to augment the function of the urethral rhabdosphincter by increasing pudendal nerve efferent activity. It is proposed that the ability of serotonin and norepinephrine to enhance the effects of glutamate (the primary excitatory neurotransmitter for pudendal sphincter motor neurons) while having no direct effects of their own, allow facilitation of rhabdosphincter activity during urine storage while allowing complete relaxation during micturition. Duloxetine, a potent and balanced dual serotonin (5-HT)-norepinephrine reuptake inhibitor (SNRI), potentiates these physiological effects of endogenous serotonin and norepinephrine (by inhibiting the reuptake of these neurotransmitters in the pre-synaptic element) and thereby enhances the central nervous system's natural continence control mechanisms.

Raphe magnus neurons respond to noxious colorectal distension

Physiological studies of neurons in raphe magnus (RM) and the adjacent nucleus reticularis magnocellularis (NRMC) have demonstrated that the response to noxious cutaneous stimulation predicts the response to opioid administration and therefore a cell's functional role in nociceptive modulation. Although visceral stimulation, like opioids, elicits antinociception, little is known about how RM and NRMC cells respond to visceral stimulation. Therefore RM and NRMC cells were tested for their responses to both colorectal distension (CRD) and noxious cutaneous heat in halothane-anesthetized rats. Less than a third of serotonergic cells responded to CRD with small increases or decreases in discharge rate. In contrast, almost two-thirds of nonserotonergic cells responded to CRD stimulation with either excitatory (35%) or inhibitory (30%) responses to CRD. The response to heat did not predict the response to CRD with nearly equal proportions of heat-excited, -inhibited, and -unaffected cells being excited, inhibited, or unaffected by CRD. The dissociation between the responses to cutaneous heat and CRD demonstrates that cell classes based on the response to noxious heat are not homogeneous and may play multiple functional roles.

Differential effects of urinary bladder distension on high cervical projection neurons in primates

Projection neurons located in high cervical segments of primates are generally excited instead of inhibited by cardiopulmonary spinal inputs, which enter thoracic dorsal roots. Thus, high cervical neurons with axons that either ascend to the thalamus or descend to thoracolumbar spinal segments can process and transmit excitatory cardiac information. The purpose of this study was to determine whether the excitatory effects observed to cardiopulmonary afferent stimulation are a universal response in high cervical projection neurons to spinal visceral inputs. Urinary bladder distension (UBD) was used to stimulate visceral afferent inputs that enter lumbosacral dorsal roots. Effects were determined on extracellular activity of either spinothalamic tract (STT) neurons or descending propriospinal neurons that were recorded in high cervical segments of anesthetized monkeys. Results showed that 17/34 STT neurons were inhibited by UBD and 3/34 STT neurons were excited. Widespread visceral inputs, therefore, can excite high cervical STT neurons but the majority of responsive STT neurons were inhibited by UBD. Effects of UBD on high cervical descending propriospinal neurons were significantly different from responses in STT neurons. Extracellular activity of fewer propriospinal neurons was affected by UBD and responses were more variable; 3/26 neurons were inhibited, 5/26 neurons were excited and one neuron was excited/inhibited by UBD. These results showed that the generally excitatory responses of high cervical projection neurons to cardiopulmonary inputs were not duplicated by stimulation of sensory input from the urinary bladder. Furthermore, results of this study indicated that effects of sensory inputs on spinal neurons might vary depending on axonal projections of the neurons examined.

Characterization of neurons in the area of the medullary lateral reticular nucleus responsive to noxious visceral and cutaneous stimuli

In halothane-anesthetized rats, 283 caudal medullary neurons responsive to colorectal distension (CRD) were characterized using extracellular electrodes. Neurons inhibited by CRD (n = 82) were in the area dorsal to the lateral reticular nucleus (LRN). Most neurons excited by CRD (n = 130) were located within or immediately adjacent to the LRN, were excited by noxious heat and/or noxious pinch of at least half the body surface and were called bilateral nociceptive specific (bNS) neurons. bNS neurons had accelerating responses to graded CRD (threshold: 20 +/- 2 mmHg). Ten of twelve bNS neurons tested could be antidromically activated by electrical stimulation of the midline cerebellum. Other neurons excited by CRD (n = 71) had mixed responses to cutaneous stimuli and were generally located in the area dorsal to the LRN. Increases in blood pressure due to intravenous phenylephrine did not significantly alter the spontaneous activity of neurons excited by CRD, but altered spontaneous activity (12 excited, four inhibited) in all neurons tested which were inhibited by CRD. Decreases in blood pressure produced by intravenous nitroprusside produced a reciprocal response in most neurons inhibited by CRD and had a delayed onset (20-30 s after bolus administration) excitatory effect on 21 of 27 units excited by CRD. Combined with other studies, these data suggest a role for neurons within and adjacent to the LRN in the modulation of visceral nociception. They also implicate a role for the cerebellum in visceral nociceptive processing.