Discharge patterns of motility-regulating neurons projecting in the lumbar splanchnic nerves to visceral stimuli in spinal cats

Reflexes in visceral preganglionic motility-regulating (MR) neurons which project in the lumbar splanchnic nerves were investigated in acutely spinalized cats. Some neurons were analyzed before and after spinalization. The stimuli used were mechanical stimulation of mucosal skin of the anus and of perianal (perigenital) hairy skin, and distension and contraction of urinary bladder and colon. Most MR neurons exhibited a reflex pattern which consists of the following components: excitation upon bladder distension, inhibition or no effect upon colon distension and excitation (or, rarely, no effect) upon anal stimulation. This is the reflex pattern of MR1 neurons. Some neurons were excited by anal stimulation but not affected from the colon and urinary bladder. Some were inhibited by anal and perianal stimulation but otherwise exhibited the reflex patterns of the MR1 neurons. Analysis of the reflexes before and after spinalization showed that, in particular, inhibition elicited by anal, perianal and bladder stimulation was abolished; inhibition elicited from the colon was enhanced after spinalization. It is concluded that the reflexes elicited in preganglionic lumbar visceral neurons by the natural stimuli probably use spinal pathways, with the afferent input occurring at the sacral spinal cord. These spinal reflex pathways are probably controlled by descending inhibitory and excitatory spinal systems from the supraspinal neuraxis.     

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.     

Responses of neurons in ventroposterolateral nucleus of primate thalamus to urinary bladder distension.

The purpose of this study was to examine effects of a noxious visceral stimulus, urinary bladder distension (UBD), on cells in the ventroposterolateral (VPL) nucleus of anesthetized monkeys. We hypothesized that processing of visceral information in the VPL nucleus of the thalamus is similar to spinothalamic tract (STT) organization of visceral afferent input. Urinary bladder distension excites sacral and upper-lumbar STT cells that have somatic input from proximal somatic fields; whereas, thoracic STT cells are inhibited by UBD. Extracellular action potentials of 67 neurons were recorded in VPL nucleus. Urinary bladder distension excited 22 cells, inhibited 9 cells, and did not affect activity of 36 cells. Seventeen of 22 cells excited by UBD also received convergent somatic input from noxious squeeze of the hip, groin, or perineal regions. No cells activated only by innocuous somatic stimuli were excited by UBD. Five of 9 cells inhibited by UBD had upper-body somatic fields. There was a significant tendency for VPL neurons excited by UBD to have proximal lower-body somatic fields that were excited by noxious stimulation of skin and underlying muscle (P less than 0.001). Antidromic activation of 4 thalamic neurons affected by UBD showed that visceral input stimulated by UBD reached the primary somatosensory (SI) cortex.     

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.     

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.     

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.     

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.     

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.     

Responses of neurons in the lateral thalamus of the cat to stimulation of urinary bladder, colon, esophagus, and skin

In seven female α-chloralose-anesthetized cats, 52 lateral thalamic neurons were tested with noxious distension of the urinary bladder, the distal colon and the lower esophagus. In addition, the neurons were characterized with innocuous and noxious mechanical stimulation of the skin and deep structures. Of the 52 neurons tested, 32 (62%) were visceroceptive. Of these visceroceptive neurons, 20 (63%) were located in the periphery of the ventral posterolateral nucleus (VPL_p), 10 (31%) in the adjacent posterior complex (PO), and two (6%) in the ventrolateral nucleus (VL). No differences were found with respect to location between neurons responsive or unresponsive to visceral stimulation. Ten neurons (31%) received input from more than one viscus and, therefore, showed viscerovisceral convergence. Excitatory or “inhibitory” responses were elicited by stimulation of the esophagus in 21 neurons, of the colon in 13, and of the urinary bladder in 11 neurons. No indications were found for a segregation of neurons responsive to a certain viscus and their location in VPL_p or PO. Of 51 neurons, for which a somatic receptive field was determined, 44 (86%) exhibited low threshold type (LT), and seven (14%) wide dynamic range type (WDR) responses. The data indicate that there might exist a somatovisceral coregistration, because many neurons (69%) had homosegmental receptive fields, and bladder stimulation was the most successful stimulus. It is concluded that VPL_p and the adjacent PO in the cat play a role in the perception and localization of painful events originating from thoracic and pelvic organs.     

Reflex patterns in postganglionic vasoconstrictor neurons following chronic nerve lesions

Lesions of limb nerves in man may be associated with a variety of painful disorders with trophic changes described by the generic term 'reflex sympathetic dystrophy'. Our hypothesis is that pain and trophic changes are produced by an abnormal discharge pattern in postganglionic neurons supplying the limb (see refs. 3,24). In relation to this hypothesis, reflex patterns in postganglionic vasoconstrictor neurons supplying the skin (CVC) and the skeletal muscle (MVC) of the cat hindlimb were investigated at various times after a peripheral nerve lesion had been produced. These reflex patterns were compared with those in animals without nerve lesions (control preparations). The following lesions were made: cutting and ligating the superficial peroneal nerve (skin nerve) with subsequent neuroma formation, suturing the central stump of the superficial peroneal nerve to the peripheral stumps of muscle branches of the deep peroneal nerve, suturing the central stumps of muscle branches of the deep peroneal nerve to the peripheral stump of the superficial peroneal nerve, cutting and resuturing the superficial peroneal nerve, deafferentation of the whole hindlimb. The responses of vasoconstrictor neurons to stimulation of arterial chemoreceptors, arterial baroreceptors (cardiac rhythmicity of postganglionic activity) and cutaneous nociceptors were tested. In the animals with nerve lesions, the following groups of postganglionic vasoconstrictor neurons were analyzed: neurons projecting to the lesioned nerve, neurons projecting to hairy skin through an intact skin nerve (sural nerve) and neurons projecting to skeletal muscle through intact muscle nerves. In control preparations without nerve lesions, MVC neurons were excited by stimulation of arterial chemoreceptors and cutaneous nociceptors and inhibited by stimulation of arterial baroreceptors. Most CVC neurons were inhibited by stimulation of chemoreceptors and nociceptors and weakly inhibited by stimulation of baroreceptors. In animals with nerve lesions a and b, many CVC neurons in the lesioned nerves, as well as in the non-lesioned cutaneous nerve nearby, behaved in the same manner as MVC neurons. With respect to the control, this difference proved to be statistically significant. In preparations with lesions a, b and c, MVC neurons did not change their reflex patterns. After nerve lesions d and e, no major changes of reflex patterns were observed in CVC and MVC neurons. The inhibitory influence of arterial baroreceptors on CVC activity decreased in deafferented preparations (lesion e).(ABSTRACT TRUNCATED AT 400 WORDS)     

Firing of putative cholinergic neurons and micturition center neurons in the rat laterodorsal tegmentum during distention and contraction of urinary bladder

The relation between unit activity in the laterodorsal tegmental (LDT) area and the state of the urinary bladder was examined in urethane-anesthetized rats. Neurons in the LDT area can be classified into two populations: broad-spike (possibly cholinergic) and brief-spike (non-cholinergic). When the rats showed cortical electroencephalographic activity with large amplitude lower frequency, indicative of deep anesthesia, more than 40% of the broad-spike neurons was excited and about 10% was inhibited by infusion of saline into the bladder. The response was followed by decrease in amplitude and slight increase in frequency of the cortical activity, i.e., lightening of anesthesia. During light anesthesia, excitation was observed only in less than 10% of the units, while 17% was inhibited. In the brief-spike neurons, a similar proportion (about 20%) was excited and less than 10% was inhibited by the distention during either state of anesthesia. About 10% of the broad-spike neurons in the LDT area and 30% of the brief-spike neurons examined were discharged prior to the bladder contraction. Such neurons of the brief-spike category were encountered frequently outside of the central gray; lateral, caudal and ventral to the main mass of cholinergic neurons in the LDT area. These results suggest the possible involvement of the broad-spike (cholinergic) neurons in the elevation of vigilance level caused by bladder distention. The brief-spike (non-cholinergic) neurons firing with relation to bladder contraction may be part of the micturition reflex center.     

A gentle mechanical skin stimulation technique for inhibition of micturition contractions of the urinary bladder

Effects of gentle skin stimulation of various segmental areas on the micturition contractions of the urinary bladder were examined in anesthetized male rats. The bladder was expanded by infusing saline via urethral cannula until the bladder produced rhythmic micturition contractions as a consequence of rhythmic burst discharges of vesical pelvic efferent nerves. Gentle stimulation was applied for 1 min by slowly rolling on top of skin with an elastomer "roller". Rolling on the perineal area inhibited both micturition contractions and pelvic efferent discharges during and after stimulation. Stimulation of the hindlimb, abdomen and forelimb inhibited micturition contractions after stimulation ended, in this order of effectiveness. During stimulation of the perineal skin, the reflex increase in pelvic efferent discharges in response to bladder distension to a constant pressure was also inhibited up to 45% of its control response. The inhibition of the micturition contractions induced by perineal stimulation was abolished, to a large extent by the opioid receptor antagonist naloxone and completely by severing cutaneous nerves innervating the perineal skin. We recorded unitary afferent activity from cutaneous branches of the pudendal nerve and found that the fibers excited by stimulation were low-threshold mechanoreceptive Aβ, Aδ and C fibers. Discharge rates of afferent C fibers (7.9 Hz) were significantly higher than those of Aβ (2.2 Hz) and Aδ (2.9 Hz) afferents. The results suggest that low frequency excitation of low threshold cutaneous mechanoreceptive myelinated and unmyelinated fibers inhibits a vesico-pelvic parasympathetic reflex, mainly via release of opioids, leading to inhibition of micturition contraction.     

Inhibitory effects of phrenic afferent fibers on primate lumbosacral spinothalamic tract neurons.

Studies were conducted to determine if electrical or mechanical stimulation of phrenic afferent fibers (PHR) would inhibit the activity of lumbosacral spinothalamic tract (STT) neurons. Twelve monkeys were anesthetized, paralyzed, and artificially ventilated. Extracellular action potentials were recorded from 78 STT neurons located in L2-S3 spinal segments. Electrical stimulation of PHR reduced the activity of 65%, did not affect 33%, and excited 1% of STT neurons. Mechanical stimulation of the diaphragm reduced the activity of 63%, did not effect 34%, and excited 1% of lumbosacral STT neurons. Distention of the urinary bladder (UBD) inhibited 52%, did not affect 23%, excited 23%, and elicited a biphasic response in 1% of STT neurons. However, there was no correlation between the effect of PHR and UBD or somatic classification of the neurons. We conclude that electrical or mechanical stimulation of PHR can produce a generalized inhibition of lumbosacral STT neurons. This inhibitory effect of PHR is similar to inhibitory effects reported for a variety of other afferent systems.     

Effects of urinary bladder distension on activity of T3-T4 spinal neurons receiving cardiac and somatic noxious inputs in rats

The aims of this study were to examine effects of urinary bladder distension (UBD) on T(3)-T(4) spinal neurons receiving cardiac and somatic noxious inputs and to determine the pathway involved in transmitting urinary bladder inputs to thoracic spinal segments. Extracellular potentials of single T(3)-T(4) neurons were recorded in pentobarbital anesthetized male rats. Either bradykinin solution (10(-5) M) or an allogenic mixture (adenosine 10(-3) M, bradykinin, histamine, serotonin, prostaglandin E2 10(-5) M each) was administered intrapericardially. UBD was produced by saline inflation (0.5-2.0 ml, 20 s). Of 487 neurons tested for responses to UBD, 70 were inhibited and 37 were excited. Seventy-six out of 336 neurons received convergent input from UBD and heart; 69/76 viscerovisceral convergent neurons had somatic fields. Spinal transection at rostral C(1) abolished UBD inhibition in 5/9 neurons; whereas transections at L(1)-L(2) abolished UBD inhibition in 3/3 cells tested. Results showed that T(3)-T(4) spinal neurons processing cardiac and somatic nociceptive information were primarily inhibited by input from the urinary bladder through either supraspinal structures or direct intraspinal pathways.     

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.     

Characterization of superficial T13-L2 dorsal horn neurons encoding for colorectal distension in the rat: comparison with neurons in deep laminae

Fifty-five neurons responsive to colorectal distension located in the superficial spinal dorsal horn (0.0-0.3 mm ventral to the cord dorsum) of the T13-L2 spinal segments of pentobarbital-anesthetized, physiologically intact or spinalized (C1 transection), decerebrate rats were characterized. These neurons could be separated into three groups based upon their response to an 80 mm Hg, 20 s colorectal distension: (1) short latency-abrupt (SL-A) neurons (n = 22) that were excited by colorectal distension at a short latency (less than 1 s) and abruptly terminated responses following the termination of the distending stimulus; (2) short latency-sustained (SL-S) neurons (n = 26) that were excited by colorectal distension at a short latency (less than 1 s) and demonstrated sustained responses (greater than 4 s) following termination of the distending stimulus; and (3) INHIB neurons (n = 7) that were spontaneously active and were inhibited by colorectal distension. All 55 neurons had convergent cutaneous receptive fields (i.e. were 'viscerosomatic'), exhibiting excitatory responses to noxious (pinch/heat) and/or non-noxious (brush) stimuli. Neurons excited by colorectal distension also demonstrated monotonic, accelerating responses to graded colorectal distension, were excited by the intraarterial administration of bradykinin, could be antidromically activated by electrical stimulation in the caudal ventrolateral medulla and were subject to tonic descending inhibitory modulation as evidenced by more vigorous responses to distension when rats were reversibly spinalized using a cold block.(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

Role of cervical neurons in propriospinal inhibition of thoracic dorsal horn neurons.

We previously reported that electrical or glutamate stimulation of the cervical spinal cord elicits a 40-60% decrease in renal sympathetic nerve activity (RSA) in the anesthetized rats. This sympatho-inhibition was possible, however, only after transection of the spinal cord at C1 or GABAergic inhibition of neurons in the rostral ventrolateral medulla. We postulated that cervical neurons inhibit RSA by inhibiting the activity of spinal interneurons that are antecedent to sympathetic preganglionic neurons (SPNs), and that these interneurons may be, in turn, excited by afferent signals. In this study, we tested the hypothesis that cervical neurons can inhibit visceroceptive thoracic spinal neurons. We recorded the spontaneous and evoked activity of 45 dorsal horn neurons responsive to splanchnic stimulation before, during, and after chemical or electrical stimulation of the cervical spinal cord in chloralose-anesthetized spinal rats. Cervical spinal stimulation that inhibited RSA also inhibited the spontaneous and/or evoked activity of 44 dorsal horn neurons. In addition to inhibiting splanchnic-evoked neuronal responses, cervical stimulation also inhibited responses, in the same neurons, evoked by noxious heat or light brushing of receptive dermatomes. We concluded that cervical neurons participate in propriospinal inhibition of afferent transmission and that this inhibitory system may be involved in controlling the access of afferent information to SPNs.     

Actions of carotid chemoreceptors on subretrofacial bulbospinal neurons in the cat.

Thirty-one barosensitive bulbospinal neurons were recorded from the subretrofacial (SRF) nucleus in eight chloralose-anaesthetised, paralysed cats. Close arterial injections of CO2-saturated saline were used to stimulate carotid body chemoreceptors. Seven neurons were abruptly excited and six neurons abruptly inhibited by chemoreceptor stimuli: these were primary responses, independent of changes in blood pressure or central respiratory drive (monitored from the phrenic nerve). A further eight neurons responded only indirectly to chemoreceptor stimuli, showing enhanced modulation of their activity coupled to the enhanced central respiratory drive. The distinction between primary and secondary responses was shown more clearly when central respiratory drive was inhibited by superior laryngeal nerve stimulation. The remaining ten neurons showed no clear response to chemoreceptor stimuli. SRF bulbospinal neurons thus show the same range of responses to chemoreceptor stimuli as the sympathetic neurons they are believed to drive.     

Characteristics of caudal ventrolateral medullary neurons antidromically activated from rostral ventrolateral medulla in the rabbit.

We made extracellular recordings from 107 spontaneously active neurons in the caudal ventrolateral medulla, after identifying the cells by antidromically activating them from the rostral ventrolateral medulla, in urethane-anesthetized rabbits. We tested the response of these neurons to inputs from baroreceptors and chemoreceptors. The median conduction velocity for antidromically activated neurons was 0.84 m/s. Raising blood pressure with intravenous noradrenaline excited 22% of 96 neurons tested, inhibited 61%, and had no effect on the remaining 17%. The spontaneous discharge rate of neurons excited by an increase in blood pressure was 1.6 +/- 0.3 spikes/s, lower than the discharge rate of neurons inhibited by this procedure (4.9 +/- 0.5 spikes/s). Excitation of chemoreceptors by hypoxia increased the discharge rate of 14/16 neurons tested in the group excited by baroreceptor inputs. In the group inhibited by baroreceptor inputs 21/35 neurons tested were excited and 12/35 neurons were inhibited by chemoreceptor inputs. Neurons excited by an increase in blood pressure were located in the previously defined caudal vasodepressor region and in a region just rostral to the obex, intermediate between the vasodepressor region and the rostral sympathoexcitatory region. These neurons may form part of the central inhibitory link in the baroreceptor-vasomotor pathway. Other antidromically activated neurons in the vasodepressor region may be inhibitory vasomotor cells with a function relatively independent of baroreceptor inputs, or they may be A1 catecholamine neurons, with axons passing through the rostral medulla en route to the forebrain.