Functional characterization of preganglionic neurons projecting in the lumbar splanchnic nerves: neurons regulating motility

Lumbar preganglionic neurons, which projected in the lumbar splanchnic nerves and were probably involved in regulating motility of colon and pelvic organs (motility-regulating, MR neurons), were analyzed for their discharge patterns. The responses of the neurons to the following stimuli were tested: stimulation of arterial baro- and chemoreceptors and of afferents from the urinary bladder, colon, mucosal skin of the anus and perianal hairy skin. The following findings were made: a total of 131 preganglionic neurons were classified as MR neurons; these reacted to natural stimulation of at least one of the afferent inputs from the urinary bladder, colon and anal and perianal skin. The ongoing activity of these neurons did not correlate with the cardiac cycle or the cycle of the artificial ventilation. Most of them did not respond to an increase of blood pressure produced by i.v. injection of adrenaline or noradrenaline; some showed a weak depression or weak excitation which, in the time course, was untypical for visceral vasoconstrictor neurons. Stimulation of arterial chemoreceptors either did not influence MR neurons or produced only a secondary response owing to contraction of the urinary bladder. Ninety-seven preganglionic MR neurons could be subclassified: MR1 neurons were excited by distension and contraction of the urinary bladder and/or inhibited by distension and contraction of the colon (n = 61), a few were excited from both organs (n = 4); MR2 neurons were inhibited by distension and contraction of the urinary bladder and/or excited by distension and contraction of the colon (n = 32). Ninety-five out of 121 MR neurons (78.5%) were excited, 10 (8%) were inhibited and 16 (13%) not influenced by mechanical shearing stimuli applied to the mucosal skin of the anus. Most neurons which were excited by anal stimulation were not influenced by mechanical stimulation of the perianal (perigenital) skin. Twenty-eight per cent of the MR neurons (18 out of 64) were excited or inhibited upon stimulation of perianal skin. A few of these (7 out of 64 neurons, 11%) were involved in reflex responses which were different from those elicited from anal skin. At present no further consistent subclassification of MR1 and MR2 neurons appears possible on the basis of the excitatory and inhibitory anal and perianal reflexes. The results show that the population of visceral preganglionic neurons, which are probably involved in regulation of motility of colon and pelvic organs, is not homogeneous and probably consists of several subpopulations.     

Secondary functional properties of lumbar visceral preganglionic neurons

Preganglionic visceral vasoconstrictor (VVC) neurons and motility-regulating (MR) neurons and other visceral preganglionic neurons, which project in the lumbar splanchnic nerves, were analyzed for their segmental distribution, the conduction velocity of their axons, ongoing activity and reflexes elicited by electrical stimulation of visceral afferents in white rami and of somatic afferents in spinal nerves. Identified preganglionic neurons and neurons without ongoing and reflex activity were distributed over segments L1-L5. VVC neurons were distributed over segments L1-L4 and MR neurons over segments L3-L5. VVC axons conducted at 2.8 +/- 2.5 m/s (mean +/- 1 S.D., n = 49), MR axons at 8.1 +/- 4.7 m/s (n = 131). The ongoing activity of VVC neurons was 1.6 +/- 0.7 imp/s (n = 46), that of MR neurons 0.8 +/- 0.7 imp/s (n = 91). There was no correlation between the conduction velocity of preganglionic axons and the rate of ongoing activity for VVC and MR neurons. (4) Electrical stimulation of visceral afferents in white rami and of somatic afferents in spinal nerves elicited short-latency (less than 50 ms) and long-latency (greater than 50 ms) reflexes in practically all VVC neurons, but preferentially short-latency reflexes in only 50 to 60% of the MR neurons. These results show that VVC and MR neurons are not only different in their reflex patterns, elicited by stimulation of visceral receptors and of arterial baro- and chemoreceptors, but also in the 4 properties analyzed in this paper.     

Functional characterization of preganglionic neurons projecting in the lumbar splanchnic nerves: vasoconstrictor neurons

Lumbar preganglionic neurons, which project in the lumbar splanchnic nerves and which probably have a vasoconstrictor function (visceral vasoconstrictor, VVC neurons), were analyzed for their discharge patterns. The responses of these neurons to the following natural stimuli were tested: stimulation of arterial baroreceptors, arterial chemoreceptors and visceral afferents from the urinary bladder, the colon and the mucosal skin of the anus. Forty-nine preganglionic neurons were classified as VVC neurons. They showed the following characteristics: the ongoing activity of the VVC neurons exhibited pronounced cardiac rhythmicity and correlated with the cycle of the artificial ventilation. Stimulation of arterial baroreceptors, produced by increase of blood pressure or by increase of pressure in an isolated carotid blind sac, led to inhibition of activity in VVC neurons. Unloading of arterial baroreceptors, produced by decrease of blood pressure, led to an increase in VVC neuron activity. Stimulation of arterial chemoreceptors by bolus injections of CO2-enriched saline solution, close to a carotid glomus, led to a weak excitation of VVC neurons. Stimulation of arterial chemoreceptors by systemic hypoxia led to weak excitation and/or to depression of activity in VVC neurons. Stimulation of visceral afferents from urinary bladder and colon by isovolumetric contractions and distensions of the organs had no effect on most VVC neurons. Anal stimulation also did not induce reflexes in the majority of the VVC neurons. Some 14% of the VVC neurons (7 from 49) were excited by at least one of the visceral stimuli in the same manner as the motility-regulating (MR) neurons. This investigation shows that preganglionic neurons, probably involved in regulation of vascular resistance in colon and pelvic organs, are functionally a distinct population of neurons with some interesting functional overlap with the motility-regulating neurons.     

Role of vesical afferent nerve fibres involved in the control of internal anal sphincter motility

The neural pathways involved in the interactions between urinary bladder and internal anal sphincter (IAS) were studied in anaesthetized spinal cats. Activation of vesical afferents produced in the IAS a reflex increase in the electrical activity and a reflex inhibition of the excitatory responses evoked by stimulation of one hypogastric nerve. Both reflexes are achieved partly in the lumbar spinal cord and partly within the inferior mesenteric ganglion.     

Activity in sympathetic neurons supplying skin and skeletal muscle in spinal cats

In anesthetized cats with intact neuraxis, vasoconstrictor neurons supplying skeletal muscle (MVC) and hairy and hairless skin (CVC), and sudomotor neurons innervating sweat glands (SM), exhibit distinct reflex patterns. MVC and SM are largely under excitatory, CVC under inhibitory control of various afferent input systems from the body surface and from the viscera. In chronic spinal animals all 3 types of sympathetic neurons exhibit some resting activity without cardiac and respiratory modulation. Sixty to 150 days after isolation of the neural circuits within the sympathetic systems within the spinal cord from their descending control systems by spinalization, these reflex patterns are very similar to those in animals with intact neuraxis. Important changes which do occur after spinalization are the following: CVC neurons are excited by stimulation of visceral afferents in spinal animals but inhibited in animals with intact neuraxis; noxious stimulation of skin leads to long-lasting after-effects in CVC and SM neurons in spinal animals. Comparison of reflexes among spinal animals and animals with intact neuraxis indicates that spinal circuits are probably important for the functioning of the sympathetic systems. It is possible that these circuits determine the typical reaction patterns seen in the sympathetic systems by integrating multisensory information from primary afferents and information from spinal descending fiber tracts.     

Viscero-sympathetic reflex responses to mechanical stimulation of pelvic viscera in the cat.

Viscero-sympathetic reflex responses to mechanical stimulation of urinary bladder and colon were studied in cutaneous vasoconstrictor (CVC) neurones supplying hairy skin, in muscle vasoconstrictor (MVC) neurones supplying skeletal muscle and in sudomotor (SM) neurones supplying the sweat glands of the central paw pad of the cat hindlimb. The cats were anaesthetized, paralysed and artificially ventilated. The vasoconstrictor activity was recorded from the axons of the postganglionic fibres that were isolated in filaments from the respective peripheral hindlimb nerves. The activity in the sudomotor neurones was monitored by recording the fast skin potential changes occurring on the surface of the central paw pad. Afferents from the urinary bladder and from the colon were stimulated by isotonic distension and isovolumetric contraction of the organs. Most CVC neurones with ongoing activity were inhibited by these stimuli; only a few CVC neurones were excited. The MVC and SM neurones were generally excited by the visceral stimuli, yet the size of the evoked skin potential changes was variable. The reflex responses elicited in the sympathetic outflow to the cat hindlimb by stimulation of visceral afferents from the pelvic organs are uniform with respect to the different types of afferent input system but differentiated with respect to the efferent output systems. Graded stimulation of the visceral afferents from the urinary bladder by isotonic pressure steps elicited graded reflex responses in CVC (threshold less than 30 mmHg) and MVC neurones (threshold less than 20 mmHg) and a graded increase of the arterial blood pressure (threshold less than 20 mmHg). These graded reflex responses are closely related to the quantitative activation of sacral afferent neurones with thin myelinated axons innervating the urinary bladder that are also responsible for eliciting the micturition reflex, but not to the quantitative activation of sacral afferent neurones with unmyelinated axons. The latter have thresholds of 40-50 mmHg intravesical pressure at which the size of the vesico-sympathetic reflexes in the vasoconstrictor neurones was about 50% of maximal size. This does not exclude the fact that activation of unmyelinated vesical afferents contributes to the vesico-sympathetic reflexes.     

Viscerovisceral convergence of urinary bladder and colorectal inputs to lumbosacral spinal neurons in rats

Extracellular potentials of single L6-S2 spinal neurons were recorded in pentobarbital anesthetized male rats. The urinary bladder was catheterized through the fundus and filled with warm saline for urinary bladder distension (UBD, 0.5-2.0 ml, 20 s). Colorectal distension (CRD) was produced by distending a latex balloon with air (20-80 mmHg, 20 s). Of 171 deeper neurons examined for responses to UBD and CRD, 49 (29%) neurons responded to both UBD and CRD; whereas 6/42 (14%) superficial neurons (depth < 0.3 mm) responded to both organs. Of 55 viscerovisceral convergent neurons, 25 (45%) neurons were excited by both UBD and CRD; the remainder exhibited multiple patterns of excitation and inhibition. In conclusion, responses of superficial and deeper lumbosacral spinal neurons to convergent inputs from urinary bladder and colon suggested that these neurons might contribute to the cross-talk that occurs between visceral organs.     

Activation of adrenal preganglionic neurons during autonomic dysreflexia in the chronic spinal cord-injured rat

Autonomic dysreflexia (AD) occurs in a majority of high paraplegic and quadriplegic patients and is particularly characterized by a paroxysmal hypertension elicited by somatic or visceral stimuli. We have previously shown that plasma adrenaline and noradrenaline levels were significantly increased during episodes of AD in the 30-day spinal cord-injured (SCI) rats, suggesting the participation of adrenal catecholamines in the cardiovascular changes associated to AD. Thus, adrenal sympathetic preganglionic neurons (SPN) could be activated by visceral afferences leading to AD. The aim of this study was then to demonstrate whether visceral stimulation that induces AD activates adrenal SPN in chronic SCI rats. To this end, a retrograde tracer, the cholera toxin B subunit (CTB), was combined with the immunocytochemical detection of Fos protein after visceral stimulation. Chronic SCI rats received a CTB injection into the adrenal gland and, 3 days later, were stimulated by repetitive distension of the colon. Results showed that this stimulation elicited typical hypertensive episodes of AD and a significant increase in the number of double-labeled neurons (CTB/Fos immunoreactive neurons) in the thoracic spinal cord below the level of injury (T4 segment) when compared to the stimulated non-SCI rats. In conclusion, visceral stimulations in the chronic SCI rats activate adrenal SPN, which could induce release of catecholamines by the adrenal medulla. The present study brings new data on the spinal mechanisms of AD cardiovascular dysfunctions.     

Reflex changes in circular muscle activity elicited by stroking the mucosa: an electrophysiological analysis in the isolated guinea-pig ileum

A preparation of isolated small intestine from the guinea-pig was studied in which reflex responses of the circular muscle were recorded intracellularly when sensory receptors in the mucosa were stimulated mechanically. This preparation was used to examine the properties of mucosa to muscle reflexes that involve non-cholinergic motor neurons innervating the circular muscle. Reproducible stimulation of the mucosa was achieved by stroking with a motor-driven brush. Gentle brush-strokes applied to the mucosa typically evoked inhibitory junction potentials anal to the stimulus and excitatory junction potentials at recording sites oral to the stimulus. Both events were rapid in onset and up to 25 mV in amplitude. The reflexes were blocked by tetrodotoxin (0.5 microM). Junction potentials declined in amplitude with distance from the stimulus, the amplitude of the excitation 15 mm oral to the stimulus was half that at 5 mm from the stimulus, whereas the amplitude of the inhibitory potential at 40-45 mm was about 60% of that at 5-10 mm anal to the stimulus. Hexamethonium (100-200 microM) blocked the ascending excitation but only slightly reduced the descending inhibition. Ascending excitation was blocked by antagonists for substance P receptors in the muscle, and inhibition was substantially reduced by apamin (0.2 microM), both before and after hexamethonium. Both responses were abolished by removal of the mucosa from the stimulus site and when lesions were made through the myenteric plexus between the stimulation and recording sites, but persisted when similar lesions were made through the submucous plexus. It is concluded that there are neurons with mechanoreceptive nerve endings in the mucosa. Stimulation of such sensory neurons leads to activation of pathways in the myenteric plexus that excite motor neurons to the muscle both oral and anal to the stimulation site. The demonstration that mucosa to muscle reflexes can be consistently evoked in the small intestine in vitro provides an opportunity for close analysis of the reflex pathways. Such analysis is not so readily achieved when reflexes are initiated by distension that, by moving the intestine, can dislodge the recording electrode.     

Role of Barrington’s nucleus in the activation of rat locus coeruleus neurons by colonic distension

The locus coeruleus (LC)-noradrenergic system, which has been implicated in arousal and attention, is activated by visceral stimuli such as colon and bladder distension. Neurons of Barrington's nucleus (the pontine micturition center) have been identified which project to both the LC and preganglionic column of the lumbosacral spinal cord. Thus, Barrington's nucleus is positioned to coordinate brain noradrenergic activity with pelvic visceral functions. The aim of this study was to determine whether LC activation by colonic distension was mediated by projections from Barrington's nucleus to the LC in the rat. Lesions of Barrington's nucleus were performed unilaterally by local injection of ibotenic acid (microg/microl, 90 nl) 10 days prior to recording: (i) ipsilateral spontaneous LC discharge rate; (ii) LC responses to colonic distension; and (iii) LC responses to sciatic nerve stimulation. In some rats LC activation by hypotensive challenge was also examined. Lesions of Barrington's nucleus significantly reduced LC activation by colon distension from a magnitude of 26.6+/-6% increase in discharge rate (n=8) to 6.9+/-3% (n=6), while having no effect on basal LC discharge rate. In contrast, LC responses to sciatic nerve stimulation were not altered in rats with lesions of Barrington's nucleus and LC neurons were still activated by hypotensive challenge. These results support the hypothesis that Barrington's nucleus selectively relays input from pelvic visceral afferents to the LC. This may serve as a limb in a circuit designed to coordinate central and peripheral responses to pelvic visceral stimuli.     

Prenatal development of peptidergic primary afferent projections to mouse lumbosacral autonomic preganglionic cell columns

To examine the prenatal development of spinal visceral reflexes, primary sensory nerve fibers immunoreactive for calcitonin gene-related peptide (CGRP) were examined in the spinal cord, particularly in the autonomic preganglionic nuclei of mouse embryos. On embryonic day 16 (E16), CGRP-immunoreactive fibers were first observed in the sacral intermediolateral nucleus (IML) of the parasympathetic division as well as in the lumbar central autonomic nucleus (CA) of the sympathetic division, where they appeared in proximity to preganglionic neuronal perikarya immunoreactive for choline acetyltransferase or nitric oxide synthase. Most of the CGRP-immunoreactive varicosities were negative for substance P. Substance P-immunoreactive varicosities were scattered in these nuclei, but no appositions were seen on the preganglionic neuronal perikarya. On E18, CGRP-immunoreactive fibers were more abundant in the sacral IML and the lumbar CA. Co-expression of substance P and CGRP was frequently observed in the varicosities very close to the preganglionic neuronal perikarya on E18. CGRP-immunoreactive fibers were also observed in the lumbar IML on E18, although significantly fewer were found in this nucleus compared with the sacral IML. In contrast to the upper lumbar level, no fibers immunoreactive for CGRP were observed in the IML at the thoracic level. These results suggest that peptidergic primary sensory fibers grow to project to the selective targets of autonomic preganglionic neurons during the embryonic period. The potential direct connections between the peptidergic primary sensory fibers and preganglionic neurons innervating the pelvic viscera might provide a circuit for spinal visceral reflexes active in embryos.     

Reflex patterns in preganglionic sympathetic neurons projecting to the superior cervical ganglion in the rat

Reflex patterns in preganglionic neurons projecting in the cervical sympathetic trunk (CST) were analyzed in response to stimulation of various afferent systems. We focused on the question whether these preganglionic neurons can be classified into functionally distinct subpopulations. Reflex responses were elicited by stimulation of trigeminal and spinal nociceptive, thermoreceptive as well as baroreceptor and chemoreceptor afferents. Multi- and single fiber preparations were studied in baroreceptor intact and sino-aortically denervated animals. Spontaneous activity of 36 preganglionic single neurons ranged from 0.2 to 3.5 imp/s (median= 1.11 imp/s). The degree of cardiac rhythmicity (CR) in the activity of sympathetic neurons was 69.5+/-13% (mean+/-S.D.; N=52; range=39-95%). Noxious stimulation of acral skin activated the majority (67%) of sympathetic preparations by 37+/-25% (N=35) above pre-stimulus activity; 15% were inhibited. In these neurons the response to noxious stimulation of acral skin was significantly correlated with the degree of CR (P<0.001, N=52) in that neurons showing the strongest excitation to noxious stimulation displayed the strongest CR. Noxious mechanical stimulation of body trunk skin (N=60) inhibited the majority (80%) of fiber preparations tested (by 34+/-18% of pre-stimulus activity, N=48); an activation was not observed. Cold stimulation of acral (N=9) and body trunk skin (N=42) activated most fiber preparations. Trigeminal stimulation evoked a uniform reflex activation of preganglionic neurons (+79+/-73% of pre-stimulus activity, N=32). Chemoreceptor stimulation by systemic hypercapnia elicited inhibitory (-31+/-19%, N=8) as well as excitatory (+59+/-5%, N=4) responses. These results show that preganglionic sympathetic neurons projecting to target organs in the head exhibit distinct reflex patterns to stimulation of various afferent systems; however, a clear classification into different functional subgroups did not emerge. Furthermore, reflex patterns showed a segmental organization to noxious cutaneous stimulation of acral parts and body trunk reflecting a differential central integration of spinal afferent input. Compared with the cat the reflex organization of sympathetic neurons projecting to the head seems to be less differentiated in the anesthetized rat.     

Influence of spinalization on spinal withdrawal reflex responses varies depending on the submodality of the test stimulus and the experimental pathophysiological condition in the rat

The influence of midthoracic spinalization on thermally and mechanically induced spinal withdrawal reflex responses was studied in the rat. There were three experimental groups of rats: healthy controls, rats with a spinal nerve ligation-induced unilateral neuropathy, and rats with a carrageenan-induced inflammation of one hindpaw. Tail flick response was induced by radiant heat. Hindlimb withdrawal was induced by radiant heat, ice water, and innocuous or noxious mechanical stimulation of the paw. Prior to spinalization, spinal nerve ligated and carrageenan-treated animals had a marked unilateral allodynia and hyperalgesia. Spinalization tended to induce a facilitation of noxious heat-evoked reflexes. This spinalization-induced facilitation was stronger on tail than hindlimb withdrawal. Spinalization-induced skin temperature change did not explain the facilitation of noxious heat-evoked reflexes. In contrast, spinal withdrawal responses induced by noxious cold or mechanical stimulation were significantly suppressed following spinalization. The spinalization-induced facilitatory effects as well as inhibitory ones on spinal reflexes were enhanced in inflamed/neuropathic animals. The results indicate that the tonic descending control of spinal nocifensive responses varies depending on the submodality of the test stimulus, the segmental level of the reflex (tail vs. hindlimb), and on the pathophysiological condition.     

Principles of Sacral Nerve Stimulation (SNS) for the Treatment of Bladder and Urethral Sphincter Dysfunctions

Objectives. Sacral nerve stimulation (SNS) (Medtronic, Inc., Minneapolis, MN) is an exciting new treatment for refractory voiding disorders including urinary incontinence, retention, and voiding dysfunction. It is known that both voiding and continence reflex mechanisms are organized in the sacral spinal cord and that pathologic conditions can alter the balance between these two opposing mechanisms. Methods. The background and surgical technique of SNS will be presented. This will be followed by a discussion of hypotheses on how SNS works. Results. The beneficial effects of SNS are most reasonably attributed to activation of somatic afferent axons in the sacral spinal roots. This evoked afferent activity in turn modulates sensory processing and micturition reflex pathways in the spinal cord. Hyperactive voiding can be suppressed by direct inhibition of bladder preganglionic neurons as well as inhibition of interneuroneal transmission in the afferent limb of the micturition reflex. On the other hand, voiding in patients with urinary retention can be facilitated by inhibition of reflex pathways to the urethral outlet (guarding reflexes). Conclusions. SNS, a nonablative, minimally invasive technique for urologists, holds great promise for a large number of patients who suffer debilitating and refractory urinary symptoms.     

Spinalization unmasks clonidine's alpha 1-adrenergic mediated excitation of the flexor reflex in rats

Clonidine exerts alpha 2-adrenergic mediated depressant effects on most behaviors measured in a normal animal. However, in the spinal-transected (spinalized) animal, clonidine apparently facilitates the flexor reflex through a stimulation of spinal alpha 1-adrenoceptors. The purpose of the present study was to determine if spinalization per se causes the shift in clonidine's profile from an alpha 2- to an alpha 1-adrenergic agonist. The hindlimb flexor reflex was elicited by electrical pulses delivered through electrodes implanted subcutaneously in the hindpaw and was measured with a force transducer and polygraph. In contrast to an alpha 2-adrenergic mediated inhibition of the flexor reflex in intact rats, clonidine produced an alpha 1-adrenergic mediated increase in flexor reflex amplitude in spinalized rats. Because decerebration did not alter the depression due to clonidine, and intraventricular (but not intrathecal) administration of oxymetazoline mimicked the effect of clonidine, the depressant effects of alpha 2-adrenergic agonists are mediated through alpha 2-adrenergic receptors localized in the brainstem. Alternate methods for inducing a functional spinal transection (spinal block with intrathecal procaine; spinal ligation) indicated that the shift in clonidine's effect from inhibition of the flexor reflex to excitation occurred immediately following spinalization. Spinal ligation did not produce alpha 1-adrenergic supersensitivity at 15 min or 2 hr after transection, as measured by alterations in [3H]prazosin receptor binding or behavioral responses to clonidine. Thus, the shift in clonidine's effects from alpha 2-adrenergic mediated inhibition of the flexor reflex in intact rats to alpha 1-adrenergic mediated excitation in spinalized rats results because spinal transection unmasks clonidine's alpha 1-adrenergic stimulatory effect. Other conditions under which clonidine exerts alpha 1-adrenoceptor mediated excitatory effects on behavior are discussed.     

Recent advances in enteric neurobiology: mechanosensitive interneurons

Until recently, it was generally assumed that the only intrinsic sensory neuron, or primary afferent neuron, in the gut was the after-hyperpolarizing AH/Type II neuron. AH neurons excited by local chemical and mechanical stimulation of the mucosa appear to be necessary for activating the peristaltic reflex (oral excitation and anal inhibition of the muscle layers) and anally propagating ring like contractions (peristaltic waves) that depend upon smooth muscle tone. However, our recent findings in the guinea-pig distal colon suggest that different neurochemical classes of interneuron in the colon are also mechanosensitive in that they respond directly to changes in muscle length, rather than muscle tone or tension. These interneurons have electrophysiological properties consistent with myenteric S-neurons. Ascending and descending interneurons respond directly to circumferential stretch by generating an ongoing polarized peristaltic reflex activity (oral excitatory and anal inhibitory junction potentials) in the muscle for as long as the stimulus is maintained. Some descending (nitric oxide synthase +ve) interneurons, on the other hand, appear to respond directly to longitudinal stretch and are involved in accommodation and slow transit of faecal pellets down the colon. This review will present recent evidence that suggests some myenteric S interneurons, in addition to AH neurons, behave as intrinsic sensory neurons.     

Tonic descending inhibition of the spinal cardio-sympathetic reflex in the cat

Electrical stimulation of the left inferior cardiac nerve elicited a two-component reflex potential (spinal and supraspinal reflexes) in the ipsilateral white ramus T3 from which recordings were made in chloralose-anaesthetised cats. Reversible interruption of all spinal pathways achieved by cooling the spinal cord at C2/C3 produced an enhancement of the spinal reflex and abolished the supraspinal reflex, the latter usually being the more prominent reflex potential prior to spinal cord block. The spinal cord block-induced increase in the amplitude of the spinal reflex was, however, less than the increase observed during stimulation of the somatic intercostal nerve T4. Recordings of the afferent volley following cardiac nerve stimulation and analysis of the stimulus-reflex response relationship in neuraxis-blocked cats indicated that the spinal reflex as determined here was activated by A delta afferent fibres. However, if stimulus strength was raised above C-fibre threshold, spinal cord block revealed in addition a late spinal reflex response. In some cases, the appearance of this late potential was accompanied by a secondary decline of the earlier spinal reflex potential, possibly indicating C-fibre-mediated afferent inhibition. Neither baroreceptor activation nor denervation had any effect on spinal reflex amplitudes. Pharmacologically, clonidine given i.v. to cats with a blocked neuraxis reduced the spinal reflex amplitudes to pre-block values, an action which could be antagonised by the subsequent administration of the alpha 2-adrenoceptor antagonist rauwolscine. When given to non-pretreated cats with intact neuraxis, however, neither rauwolscine nor its analog yohimbine were capable of inducing a persistent release from tonic inhibition. The results suggest that both purely visceral and somato-visceral reflexes are subject to tonic descending inhibition, but they do not support the hypothesis that a catecholamine is the responsible transmitter mediating this inhibition.     

Identification of spinal neurons involved in the urethrogenital reflex in the female rat

The urethrogenital (UG) reflex is a spinal sexual reflex that consists of autonomic and somatic nerve activity and vaginal, uterine, and anal sphincter contractions. The UG reflex is under tonic descending inhibition by neurons in the region of the nucleus paragigantocellularis (nPGi). The location of spinal neurons activated by the UG reflex was examined in the female rat using the immediate early gene, c-fos. In addition, the descending inputs from the nPGi onto fos-activated neurons was examined using the anterograde tracer biotin dextran amine injected into the nPGi. The UG reflex resulted in a significant increase in fos-positive nuclei in segments T12-S1, compared with experimental controls in which the UG reflex was not activated. Spinal circuits involved in the UG reflex include neurons relaying afferent information from the pudendal sensory nerve, in the dorsal horn and medial cord of L5-S1. Efferent output includes preganglionic neurons located in the lateral gray of L5-S1 and lateral and medial gray of T13-L2. Spinal interneurons involved in the UG reflex were found close to the preganglionic neurons and in the dorsal horn and intermediate and medial gray of T12-S1. NPGi inputs were found primarily on the autonomic efferents and interneurons in the medial and intermediate gray. These studies demonstrate multisegmental spinal circuits activated with the UG reflex and demonstrate that the descending inhibition from the nPGi is by means of preganglionic and somatic efferents and spinal interneurons closely associated with the efferent output.     

Effect of capsaicin on micturition and associated reflexes in chronic spinal rats

The role of capsaicin-sensitive bladder afferents in micturition was studied in unanesthetized chronic spinal rats. Reflex voiding in response to tactile stimulation of the perigenital region appeared 5–9 days after spinal cord injury (SCI) whereas voiding induced by bladder distension occurred 2–3 weeks after SCI. The frequency and amplitude of reflex bladder contractions recorded under isovolumetric conditions were similar in chronic spinal and urethane-anesthetized CNS-intact rats. However, cystometrograms (CMGs) performed 6–8 weeks after SCI revealed that the chronic spinal rats had larger bladder capacities (1.86 ml) than CNS-intact rats (0.48 ml) and also exhibited multiple, small-amplitude, nonvoiding bladder contractions that were not detected in CNS-intact rats. Administration of capsaicin (50 mg/kg s.c.) acutely (onset 14–40 min) suppressed reflex bladder activity induced by bladder distension or by perigenital stimulation in chronic spinal animals. However, pretreatment of chronic spinal rats with capsaicin (125 mg/kg s.c.) 4 days before the experiment did not depress voiding reflexes or change bladder capacity but did eliminate the nonvoiding contractions. Inhibition of reflex bladder contractions by mechanical stimulation of rectoanal canal or the uterine cervix-vagina was not altered by pretreatment with capsaicin. These data indicate that capsaicin-sensitive bladder afferents are not essential for the initiation of reflex micturition in chronic spinal rats. However, these afferents do contribute to hyperactivity of the bladder during the filling phase of the CMG. Thus, capsaicin-sensitive bladder afferents should be evaluated as possible targets for the pharmacological treatment of bladder hyperreflexia in patients with SCI.     

Plasticity of urinary bladder reflexes evoked by stimulation of pudendal afferent nerves after chronic spinal cord injury in cats

Bladder reflexes evoked by stimulation of pudendal afferent nerves (PudA-to-Bladder reflex) were studied in normal and chronic spinal cord injured (SCI) adult cats to examine the reflex plasticity. Physiological activation of pudendal afferent nerves by tactile stimulation of the perigenital skin elicits an inhibitory PudA-to-Bladder reflex in normal cats, but activates an excitatory reflex in chronic SCI cats. However, in both normal and chronic SCI cats electrical stimulation applied to the perigenital skin or directly to the pudendal nerve induces either inhibitory or excitatory PudA-to-Bladder reflexes depending on stimulation frequency. An inhibitory response occurs at 3–10 Hz stimulation, but becomes excitatory at 20–30 Hz. The inhibitory reflex activated by electrical stimulation significantly (P < 0.05) increases the bladder capacity to about 180% of control capacity in normal and chronic SCI cats. The excitatory reflex significantly (P < 0.05) reduces bladder capacity to about 40% of control capacity in chronic SCI cats, but does not change bladder capacity in normal cats. Electrical stimulation of pudendal afferent nerves during slow bladder filling elicits a large amplitude bladder contraction comparable to the contraction induced by distension alone. A bladder volume about 60% of bladder capacity was required to elicit this excitatory reflex in normal cats; however, in chronic SCI cats a volume less than 20% of bladder capacity was sufficient to unmask an excitatory response. This study revealed the co-existence of both inhibitory and excitatory PudA-to-Bladder reflex pathways in cats before and after chronic SCI. However our data combined with published electrophysiological data strongly indicates that the spinal circuitry for both the excitatory and inhibitory PudA-to-Bladder reflexes undergoes a marked reorganization after SCI.