Rostroventrolateral medulla neurons preferentially project to target-specified sympathetic preganglionic neurons

Revisiting differential control of sympathetic outflow by the rostral ventrolateral medulla

The rostral ventrolateral medulla (RVLM) is an important brain region involved in both resting and reflex regulation of the sympathetic nervous system. Anatomical evidence suggests that as a bilateral structure, each RVLM innervates sympathetic preganglionic neurons on both sides of the spinal cord. However, the functional importance of ipsilateral versus contralateral projections from the RVLM is lacking. Similarly, during hypotension, the RVLM is believed to rely primarily on withdrawal of tonic gamma aminobutyric acid (GABA) inhibition to increase sympathetic outflow but whether GABA withdrawal mediates increased activity of functionally different sympathetic nerves is unknown. We sought to test the hypothesis that activation of the ipsilateral versus contralateral RVLM produces differential increases in splanchnic versus adrenal sympathetic nerve activities, as representative examples of functionally different sympathetic nerves. We also tested whether GABA withdrawal is responsible for hypotension-induced increases in splanchnic and adrenal sympathetic nerve activity. To test our hypothesis, we measured splanchnic and adrenal sympathetic nerve activity simultaneously in Inactin-anesthetized, male Sprague-Dawley rats during ipsilateral or contralateral glutamatergic activation of the RVLM. We also produced hypotension (sodium nitroprusside, i.v.) before and after bilateral blockade of GABA_A receptors in the RVLM (bicuculline, 5 mM 90 nL). Glutamate (100 mM, 30 nL) injected into the ipsilateral or contralateral RVLM produced equivalent increases in splanchnic sympathetic nerve activity, but increased adrenal sympathetic nerve activity by more than double with ipsilateral injections versus contralateral injections (p < 0.05; n = 6). In response to hypotension, increases in adrenal sympathetic nerve activity were similar after bicuculline (p > 0.05), but splanchnic sympathetic nerve activity responses were eliminated (p < 0.05; n = 5). These results provide the first functional evidence that the RVLM has predominantly ipsilateral innervation of adrenal nerves. In addition, baroreflex-mediated increases in splanchnic but not adrenal sympathetic nerve activity are mediated by GABA_A receptors in the RVLM. Our studies provide a deeper understanding of neural control of sympathetic regulation and insight towards novel treatments for cardiovascular disease involving sympathetic nervous system dysregulation.

Epididymal Fat-Derived Sympathoexcitatory Signals Exacerbate Neurogenic Hypertension in Obese Male Mice Exposed to Early Life Stress

[Figure: see text].

Increased neuronal activation in sympathoregulatory regions of the brain and spinal cord in type 2 diabetic rats

Increased cardiac sympathetic nerve activity in type 2 diabetes mellitus (DM) suggests impaired autonomic control of the heart. However, the central regions that contribute to the autonomic cardiac pathologies in type 2 DM are unknown. Therefore, we tested the hypothesis that neuronal activation would be increased in central sympathoregulatory areas in a pre-clinical type 2 DM animal model. Immunohistochemistry in 20-week-old male Zucker diabetic fatty (ZDF) rats revealed an increased number of neurones expressing ΔFosB (a marker of chronic neuronal activation) in the intermediolateral column (IML) of the spinal cord in DM compared to non-diabetic (non-DM) rats (P < 0.05). Rostral ventrolateral medulla (RVLM) neurones activate IML neurones and receive inputs from the hypothalamic paraventricular nucleus (PVN), as well as the nucleus tractus solitarius (NTS) and area postrema (AP), in the brainstem. We observed more ΔFosB-positive noradrenergic RVLM neurones (P < 0.001) and corticotrophin-releasing hormone PVN neurones (P < 0.05) in DM compared to non-DM rats. More ΔFosB-positive neurones were also observed in the NTS (P < 0.05) and AP (P < 0.01) of DM rats compared to non-DM rats. Finally, because DM ZDF rats are obese, we also expected increased activation of pro-opiomelanocortin (POMC) arcuate nucleus (ARC) neurones in DM rats; however, fewer ΔFosB-positive POMC ARC neurones were observed in DM compared to non-DM rats (P < 0.01). In conclusion, increased neuronal activation in the IML of type 2 DM ZDF rats might be driven by RVLM neurones that are possibly activated by PVN, NTS and AP inputs. Elucidating the contribution of central sympathoexcitatory drive in type 2 DM might improve the effectiveness of pharmacotherapies for diabetic heart disease.

The abnormalities of adrenomedullary hormonal system in genetic hypertension: Their contribution to altered regulation of blood pressure

It is widely accepted that sympathetic nervous system plays a crucial role in the development of hypertension. On the other hand, the role of adrenal medulla (the adrenomedullary component of the sympathoadrenal system) in the development and maintenance of high blood pressure in man as well as in experimental models of hypertension is still controversial. Spontaneously hypertensive rats (SHR) are the most widely used animal model of human essential hypertension characterized by sympathetic hyperactivity. However, the persistence of moderately elevated blood pressure in SHR subjected to sympathectomy neonatally as well as the resistance of adult SHR to the treatment by sympatholytic drugs suggests that other factors (including enhanced activity of the adrenomedullary hormonal system) are involved in the pathogenesis of hypertension of SHR. This review describes abnormalities in adrenomedullary hormonal system of SHR rats starting with the hyperactivity of brain centers regulating sympathetic outflow, through the exaggerated activation of sympathoadrenal preganglionic neurons, to the local changes in chromaffin cells of adrenal medulla. All the above alterations might contribute to the enhanced release of epinephrine and/or norepinephrine from adrenal medulla. Special attention is paid to the alterations in the expression of genes involved in catecholamine biosynthesis, storage, release, reuptake, degradation and adrenergic receptors in chromaffin cells of SHR. The contribution of the adrenomedullary hormonal system to the development and maintenance of hypertension as well as its importance during stressful conditions is also discussed.

Cervical Decompression Surgery for Cervical Spondylotic Myelopathy and Concomitant Hypertension: A Multicenter Prospective Cohort Study

STUDY DESIGN

We performed decompression surgery or conservative treatments on 135 cervical spondylotic myelopathy (CSM) patients with concomitant hypertension and did follow-up assessments up to 1 year to examine the change of blood pressure, spinal cord function, and cervical pain.

OBJECTIVE

The aim of this study was to determine whether concomitant hypertension is relieved after decompression surgery, and whether it is related to the improvement of spinal cord function or cervical pain.

SUMMARY OF BACKGROUND DATA

In clinical practice, we often found that some patients with CSM have concomitant hypertension. Interestingly, after CSM was treated successfully by decompression surgery, some patients' high blood pressure returned to normal range even without oral medications.

METHODS

We enrolled 135 CSM patients with hypertension, 103 of whom received decompression surgery, and remaining 32 patients accepted conservative treatments. We did follow-up assessments at 3, 6, and 12 months. The primary endpoints were changes of blood pressure, and secondary endpoints were changes of modified Japanese Orthopedic Association (mJOA) score and cervical pain visual analogue scale (VAS). Spearman correlation coefficients were calculated between changes in systolic blood pressure (SBP) and mJOA scores, VAS scores.

RESULTS

In patients with decompression surgery, the significant decrease in both SBP and diastolic blood pressure (DBP) that was seen 3 months and sustained through subsequent visit at 12 months. Paired-samples t test showed that both SBP and DBP were significantly lower than baseline blood pressure at all time points after procedure (P < 0.001). Significant correlation was found between the improvement rates of mJOA score and changes in SBP (r = -0.579, P < 0.001). But the correlation between changes in VAS score and changes in SBP was not significant (r = 0.58, P = 0.571).

CONCLUSION

Cervical decompression surgery could reduce concomitant high blood pressure in CSM patients, indicating a significant association between the decrease in blood pressure and the improvement of spinal cord function.

LEVEL OF EVIDENCE

2.

Neural Control of Non-vasomotor Organs in Hypertension

Hypertension affects over 25 % of the population with the incidence continuing to rise, due in part to the growing obesity epidemic. Chronic elevations in sympathetic nerve activity (SNA) are a hallmark of the disease and contribute to elevations in blood pressure through influences on the vasculature, kidney, and heart (i.e., neurogenic hypertension). In this regard, a number of central nervous system mechanisms and neural pathways have emerged as crucial in chronically elevating SNA. However, it is important to consider that "sympathetic signatures" are present, with differential increases in SNA to regional organs that are dependent upon the disease progression. Here, we discuss recent findings on the central nervous system mechanisms and autonomic regulatory networks involved in neurogenic hypertension, in both non-obesity- and obesity-associated hypertension, with an emphasis on angiotensin-II, salt, oxidative and endoplasmic reticulum stress, inflammation, and the adipokine leptin.

Glutamatergic activities in neonatal rat spinal cord heterogeneously regulate single-fiber splanchnic nerve discharge

Kynurenic acid (KYN) is a metabolite of tryptophan and is involved in various neurological disorders. Using whole-bundle nerve recording techniques, we previously observed that applications of KYN to block endogenous ionotropic glutamate receptor activities in neonatal rat spinal cords in vitro cause a reversible fluctuation of splanchnic sympathetic nerve discharge (SND). We hypothesized that the SND fluctuation was due to a heterogeneous single-fiber response. To detail individual fiber activities, we used the so-called 'oligofiber recordings'. Spontaneous single-fiber activities were recorded from the collagenase-dissociated splanchnic nerve fascicles. Applications of KYN increased, decreased or did not change firing rates. The heterogeneous responses in spontaneous spiking activities were confirmed by applications of APV or CNQX, suggesting an effect mediated by endogenous NMDA- or non-NMDA receptor activities. In addition to changes in firing rates, apparent drug-induced changes in firing patterns were also observed in some fiber activities. Using the oligofiber recording techniques, we confirmed a differential role of endogenous ionotropic glutamate receptor activities in regulating sympathetic outflows from the spinal cord of neonatal rats. Fine-tuning of ionotropic glutamate receptor activities in the spinal cord may serve as a simple way for heterogeneous regulation of various sympathetic-targeting tissues.

Cerebral aqueduct block attenuates cardio-renal injuries in post-DOCA-NaCl-hypertensive Dahl R rats

The systemic and/or local effects of the hydrocephalic brain were investigated in DOCA-NaCl-hypertensive Dahl R rats induced by 250 mg kg(-1) DOCA in silicone and 1% saline water. After a 1-week recovery with 0.3% NaCl chow and tap water, one group had the aqueduct of Sylvius blocked with silicone and epoxy materials with a control sham group matching mean blood pressure (BP) and body weight. The 4-week-postsurgery BP on the 0.3% NaCl diet averaged 161±3.2 in the sham group and 146±2.3 mm Hg in the blocked group (P<0.0001). Both groups were then given an 8% NaCl diet and after 4 weeks, the sham group's BP was increased further with markedly increased mortality: 186 mm Hg vs. 154 mm Hg (P<0.0001); 12 sham rats died after 11 weeks, while all the blocked rats survived (P<0.0001). A transient change in plasma Na levels was observed in the blocked group after 48 h on the 8% NaCl diet. At 14 weeks, 0 sham rats survived, compared with 10 out of 16 blocked rats (P<0.0001). After 11 weeks on 8% NaCl, the average tail venous pressure in the sham group was significantly higher than that of the blocked rats (P<0.0001) indicating the end stage of renal and heart failure. The hearts and kidneys weighed significantly more in the sham vs. the blocked rats (P<0.0001 for both groups). These results indicate that the aqueduct block prevents post-DOCA hypertension and cardio-renal injuries, suggesting that centralized third ventricular brain signaling has a role in salt-genetic hypertension.

C1 catecholamine neurons form local circuit synaptic connections within the rostroventrolateral medulla of rat

C1 catecholamine neurons reside within the rostroventrolateral medulla (RVLM), an area that plays an integral role in blood pressure regulation through reticulospinal projections to sympathetic preganglionic neurons in the thoracic spinal cord. In a previous investigation we mapped the efferent projections of C1 neurons, documenting supraspinal projections to cell groups in the preautonomic network that contribute to the control of cardiovascular function. Light microscopic study also revealed putative local circuit connections within RVLM. In this investigation we tested the hypothesis that RVLM C1 neurons elaborate a local circuit synaptic network that permits communication between C1 neurons giving rise to supraspinal and reticulospinal projections. A replication defective lentivirus vector that expresses enhanced green fluorescent protein (EGFP) under the control of a synthetic dopamine beta hydroxylase (DβH) promoter was used to label C1 neurons and their processes. Confocal fluorescence microscopy demonstrated thin varicose axons immunopositive for EGFP and tyrosine hydroxylase that formed close appositions to C1 somata and dendrites throughout the rostrocaudal extent of the C1 area. Dual-labeled electron microscopic analysis revealed axosomatic, axodendritic and axospinous synaptic contacts with C1 and non-C1 neurons with a distribution recapitulating that observed in the light microscopic analysis. Labeled boutons were large, contained light axoplasm, lucent spherical vesicles, and formed asymmetric synaptic contacts. Collectively these data demonstrate that C1 neurons form a synaptic network within the C1 area that may function to coordinate activity among projection-specific subpopulations of neurons. The data also suggest that the boundaries of RVLM should be defined on the basis of function criteria rather than the C1 phenotype of neurons.

In vivo assessment of neurocardiovascular regulation in the mouse: principles, progress, and prospects

A growing body of evidence indicates that a number of common complex diseases, including hypertension, heart failure, and obesity, are characterized by alterations in central neurocardiovascular regulation. However, our understanding of how changes within the central nervous system contribute to the development and progression of these and other diseases remains unclear. As with many areas of cardiovascular research, the mouse has emerged as a key species for investigations of neuroregulatory processes because of its amenability to highly specific genetic manipulations. In parallel with the development of increasingly sophisticated murine models has come the miniaturization and advancement in methodologies for in vivo assessment of neurocardiovascular end points in the mouse. The following brief review will focus on a number of key direct and indirect experimental approaches currently in use, including measurement of arterial blood pressure, assessment of cardiovascular autonomic control, and evaluation of arterial baroreflex function. The advantages and limitations of each methodology are highlighted to allow for a critical evaluation by the reader when considering these approaches.

Differential activation of adrenal, renal, and lumbar sympathetic nerves following stimulation of the rostral ventrolateral medulla of the rat

Under acute and chronic conditions, the sympathetic nervous system can be activated in a differential and even selective manner. Activation of the rostral ventrolateral medulla (RVLM) has been implicated in differential control of sympathetic outputs based on evidence primarily in the cat. Although several studies indicate that differential control of sympathetic outflow occurs in other species, only a few studies have addressed whether the RVLM is capable of producing varying patterns of sympathetic activation in the rat. Therefore, the purpose of the present study was to determine whether activation of the RVLM results in simultaneous and differential increases in preganglionic adrenal (pre-ASNA), renal (RSNA), and lumbar (LSNA) sympathetic nerve activities. In urethane-chloralose anesthetized rats, pre-ASNA, RSNA, and LSNA were recorded simultaneously in all animals. Microinjections of selected concentrations and volumes of glutamate increased pre-ASNA, RSNA, and LSNA concurrently and differentially. Pre-ASNA and RSNA (in most cases) exhibited greater increases compared with LSNA on a percentage basis. By varying the volume or location of the glutamate microinjections, we also identified individual examples of differential and selective activation of these nerves. Decreases in arterial pressure or bilateral blockade of RVLM GABA(A) receptors also revealed differential activation, with the latter having a 3- to 4-fold greater effect on sympathetic activity. Our data provide evidence that activation of the rat RVLM increases renal, lumbar, and preganglionic adrenal sympathetic nerve activities concurrently, differentially, and, in some cases, selectively.

Sympathetic nervous system overactivity and its role in the development of cardiovascular disease

This review examines how the sympathetic nervous system plays a major role in the regulation of cardiovascular function over multiple time scales. This is achieved through differential regulation of sympathetic outflow to a variety of organs. This differential control is a product of the topographical organization of the central nervous system and a myriad of afferent inputs. Together this organization produces sympathetic responses tailored to match stimuli. The long-term control of sympathetic nerve activity (SNA) is an area of considerable interest and involves a variety of mediators acting in a quite distinct fashion. These mediators include arterial baroreflexes, angiotensin II, blood volume and osmolarity, and a host of humoral factors. A key feature of many cardiovascular diseases is increased SNA. However, rather than there being a generalized increase in SNA, it is organ specific, in particular to the heart and kidneys. These increases in regional SNA are associated with increased mortality. Understanding the regulation of organ-specific SNA is likely to offer new targets for drug therapy. There is a need for the research community to develop better animal models and technologies that reflect the disease progression seen in humans. A particular focus is required on models in which SNA is chronically elevated.

Neuropathology of the intermediolateral nucleus of the spinal cord in sudden unexplained perinatal and infant death

Experimental studies have demonstrated that breathing activity in rats is generated early in embryonic stages in rostral spinal cord, precisely in the intermediolateral nucleus, then establishing a spinal cord-brainstem network. In this study we aimed to individuate and to define the developmental steps of the intermediolateral nucleus, still inadequately known in humans, in the thoracic spinal cord of a large series of perinatal and infant death victims, aged from 17 gestational weeks to 10 months of life. Besides we investigated a possible link between alterations of this nucleus and sudden unexplained perinatal and infant death. The normal developmental pattern of the human intermediolateral nucleus consists of a progressive maturation of its neurons, that change from a round to a polygonal shape with long axons and significantly decrease in number. Various degrees of intermediolateral nucleus hypodevelopment (neuronal immaturity in a normal structure/hypoplasia/agenesis) were found almost exclusively in unexplained fetal and infant death victims. Besides, a significant correlation was found between maternal smoking in pregnancy and the neuropathological results. In conclusion this work underlines the negative effects of prenatal nicotine exposure on the development of autonomic nervous centers checking the vital functions, already in early gestational stages, when the integrity of the intermediolateral nucleus is indispensable for the first breathing bursts.

Adrenergic stimulation mediates visceral hypersensitivity to colorectal distension following heterotypic chronic stress

BACKGROUND & AIMS

Chronic stress exacerbates or causes relapse of symptoms such as abdominal pain and cramping in patients with irritable bowel syndrome. We investigated whether chronic stress increases plasma norepinephrine and sensitizes colon-specific dorsal root ganglion (DRG) neurons by increasing expression of nerve growth factor (NGF) in the colon wall.

METHODS

Heterotypic chronic stress (HeCS) was applied to male Wistar rats and neurologic and molecular responses were analyzed. Tissues were analyzed for NGF expression.

RESULTS

HeCS significantly increased visceromoter response to colorectal distension; expression of NGF increased in colonic muscularis externa and mucosa/submucosa. Rheobase decreased, resting membrane potential was depolarized, and electrogenesis of action potentials increased in colon-specific thoracolumbar DRG neurons. Luminal administration of resiniferatoxin in distal colon, systemic administration of anti-NGF antibody, or inhibition of the NGF receptor trkA by k252a or antisense oligonucleotides in thoracolumbar DRG blocked the chronic stress-induced visceral hypersensitivity to colorectal distension. Blockade of alpha1/alpha2- and beta1/beta2-adrenergic receptors prevented the stress-induced visceral hypersensitivity and increased expression of NGF in the colon wall. HeCS did not induce any inflammatory response in the colon wall.

CONCLUSIONS

The peripheral stress mediator norepinephrine induces visceral hypersensitivity to colorectal distension in response to HeCS by increasing the expression of NGF in the colon wall, which sensitizes primary afferents in the absence of an inflammatory response.

Catecholaminergic systems in stress: structural and molecular genetic approaches

Stressful stimuli evoke complex endocrine, autonomic, and behavioral responses that are extremely variable and specific depending on the type and nature of the stressors. We first provide a short overview of physiology, biochemistry, and molecular genetics of sympatho-adrenomedullary, sympatho-neural, and brain catecholaminergic systems. Important processes of catecholamine biosynthesis, storage, release, secretion, uptake, reuptake, degradation, and transporters in acutely or chronically stressed organisms are described. We emphasize the structural variability of catecholamine systems and the molecular genetics of enzymes involved in biosynthesis and degradation of catecholamines and transporters. Characterization of enzyme gene promoters, transcriptional and posttranscriptional mechanisms, transcription factors, gene expression and protein translation, as well as different phases of stress-activated transcription and quantitative determination of mRNA levels in stressed organisms are discussed. Data from catecholamine enzyme gene knockout mice are shown. Interaction of catecholaminergic systems with other neurotransmitter and hormonal systems are discussed. We describe the effects of homotypic and heterotypic stressors, adaptation and maladaptation of the organism, and the specificity of stressors (physical, emotional, metabolic, etc.) on activation of catecholaminergic systems at all levels from plasma catecholamines to gene expression of catecholamine enzymes. We also discuss cross-adaptation and the effect of novel heterotypic stressors on organisms adapted to long-term monotypic stressors. The extra-adrenal nonneuronal adrenergic system is described. Stress-related central neuronal regulatory circuits and central organization of responses to various stressors are presented with selected examples of regulatory molecular mechanisms. Data summarized here indicate that catecholaminergic systems are activated in different ways following exposure to distinct stressful stimuli.

Cross over of forebrain and brainstem neuronal projections to spinal cord sympathetic preganglionic neurons in the rat

Neuronal inputs from the forebrain and the brainstem to sympathetic preganglionic neurons in the spinal cord were investigated by the transneuronal retrograde tracing technique using pseudorabies virus in intact and brainstem-lesioned rats. After unilateral subcutaneous viral inoculations into the hind limb of intact rats, infected neurons were then visualized by immunostaining. At 3.5 days after inoculation, infected neurons appeared in the thoracic (T10) intermediolateral (IML) cell column. On the 4th day, infected neurons were present in the C1, A5, A6, A7 catecholamine cell groups and the rostral ventromedial medulla (RVMM). On the 5th day, viral labeling was seen in the hypothalamic paraventricular and arcuate nuclei and the lateral hypothalamic area. In all of these nuclei, the infected cells appeared bilaterally. However, the appearance of virus-labeled cells in these nuclei was unilateral following unilateral coronal sections between the medulla and the spinal cord (depending on the side of hemisection, but not on the site of virus inoculation). Midsagittal sections throughout the entire medulla oblongata did not alter the topographical pattern of virus-infected neurons in the forebrain or the brainstem. These findings indicate that descending fibers to the spinal neurons may not cross over in the lower brainstem but that they decussate within the spinal cord.

Landmarks in understanding the central nervous control of the cardiovascular system

In this Paton Lecture I have tried to trace the key experiments that have developed ideas on how the brain regulates the cardiovascular system. It is a personal view and inevitably, owing to constraints on space and time, I have not been able to cover areas such as the nucleus tractus solitarius and cardiac vagal neurones, although I acknowledge that some may consider the story is incomplete without them. Starting with the crucial discovery of vasomotor nerves and 'vasomotor tone', the patterns of activity in sympathetic nerves which led to the important idea of central oscillating networks of neurones are described. I discuss how this knowledge has informed current controversies on the origin of vasomotor activity in presympathetic neurones in the ventral medulla, which identify intrinsic pacemaker activity or synaptic input from multiple oscillators as prime mechanisms. I present an emerging view that the role of other regions of the brain, in particular supramedullary sites, has been underplayed. These regions are pivotal for the non-uniform distribution of cardiac output that is unique to each reflex and behavioural state. I discuss the most recent evidence for 'central command' neurones that offers a plausible explanation for how these patterns of sympathetic activity are achieved. Finally, I stress the importance of these current ideas to the understanding of pathological changes in sympathetic activity in cardiovascular diseases such as hypertension or congestive heart failure.

Increased close appositions between corticospinal tract axons and spinal sympathetic neurons after spinal cord injury in rats

Treatments for spinal cord injury may promote new spinal cord synapses. However, the potential for new synapses between descending somatomotor and spinal sympathetic neurons has not been investigated. We studied rats with intact spinal cords and rats after a chronic, bilateral, dorsal spinal hemisection. We identified sympathetically related spinal neurons by transynaptic, retrograde transport of renally injected pseudorabies virus. We counted retrogradely labeled sympathetic preganglionic neurons (SPN) and putative sympathetic interneurons (IN) that, under light microscopy, appeared closely apposed by anterogradely labeled axons of the corticospinal tract (CST) and by axons descending from the well-established sympathetic regulatory region in the rostral ventrolateral medulla (RVLM). Spinal sympathetic neurons that were closely apposed by CST axons were significantly more numerous in lesioned rats than in unlesioned rats. CST axons closely apposed 5.4% of SPN and 10.3% of IN in rats with intact spinal cords, and 38.0% of SPN and 37.3% of IN in rats with chronically lesioned spinal cords. Further, CST appositions in SCI rats consisted of many more varicosities than those in uninjured rats. SPN and IN closely apposed by axons from the RVLM were not more numerous in lesioned rats. However, RVLM axons apposed many more SPN than IN in both control and lesioned rats. Therefore, RVLM sympathoexcitation may be mediated largely by direct synapses on SPN. Although we have not determined the functional significance of close appositions between the CST and spinal sympathetic neurons, we suggest that future studies of spinal cord repair and regeneration include an evaluation of potential, new, somatic-autonomic interactions.

GABAergic neurons in the central region of the spinal cord: a novel substrate for sympathetic inhibition

Homeostatic maintenance of widespread functions is critically dependent on the activity of the sympathetic nervous system. This activity is generated by the CNS acting on the sole output cells in the spinal cord, sympathetic preganglionic neurons (SPNs). SPNs are subject to control from both supraspinal and spinal inputs that exert effects through activation of direct or indirect pathways. A high proportion of indirect control is attributable to activation of spinal interneurons in a number of locations. However, little is known about the different groups of interneurons with respect to their neurochemistry or function. In this study, we report on a novel group of GABAergic interneurons located in the spinal central autonomic area (CAA) that directly inhibit SPN activity. In situ hybridization studies demonstrated a group of neurons that contained mRNA for glutamic acid decarboxylase (GAD)65 and GAD67 within the CAA. Combining in situ hybridization with trans-synaptic labeling from the adrenal gland using pseudorabies virus identified presympathetic GABAergic neurons in the CAA. Electrical stimulation of the CAA elicited monosynaptic IPSPs in SPNs located laterally in the intermediolateral cell column. IPSPs were GABAergic, because they reversed at the chloride reversal potential and were blocked by bicuculline. Chemical activation of neurons in the CAA hyperpolarized SPNs, an effect that was also bicuculline sensitive. We conclude that the CAA contains GABAergic interneurons that impinge directly onto SPNs to inhibit their activity and suggest that these newly identified interneurons may play an essential role in the regulation of sympathetic activity and thus homeostasis.

Visceral hypersensitivity

Visceral hypersensitivity is considered one of the causes of functional gastrointestinal disorders. The objectives of this review are to provide a practical description of neuroanatomy and physiology of gut sensation, to describe the diverse tests of visceral sensation and the potential role of brain imaging to further our understanding of visceral sensitivity in health and disease. Changes in motor function in the gut may influence sensory levels, eg, during contractions or as a result of changes in viscus compliance. New insights on sensory end organs, such as intraganglionic laminar endings, and basic neurophysiologic studies showing afferent firing during changes in stretch rather than tension illustrate the importance of different types of stimuli, not just tension, to stimulate afferent sensation. These insights provide the basis for understanding visceral sensation in health and disease, which will be extensively discussed in subsequent articles.

Phenotypic identification of rat rostroventrolateral medullary presympathetic vasomotor neurons inhibited by exogenous cholecystokinin

Systemic administration of the gastrointestinal hormone cholecystokinin (CCK) selectively inhibits splanchnic sympathetic vasomotor discharge and differentially affects presympathetic vasomotor neurons of the rostroventrolateral medulla (RVLM). Stimulation of the sympathoexcitatory region of the periaqueductal grey (PAG) produces profound mesenteric vasoconstriction. In this study, our aim was to identify phenotypically different populations of RVLM presympathetic vasomotor neurons using juxtacellular neuronal labelling and immunohistochemical detection of the adrenergic neuronal marker phenylethanolamine-N-methyl transferase (PNMT) and to determine whether the PAG provides functional excitatory input to these neurons. Fifty-eight percent (36/62) of RVLM presympathetic neurons were inhibited by systemic administration of CCK. These cells had conduction velocities (3.6 +/- 0.2 m/sec) in the non-C-fiber range consistent with neurons possessing lightly myelinated spinal axons. Of these, 79% (22/28) were excited by PAG stimulation, and 59% (10/17) were not immunoreactive for PNMT. Conversely, 42% (26/62) of RVLM presympathetic neurons were either unaffected or activated by CCK administration and had slower conduction velocities (1.4 +/- 0.3 m/sec) than cells inhibited by CCK. Fifty percent (11/22) of these cells were driven by PAG stimulation, and most (11/14 or 79%) were PNMT-positive. These results suggest that cardiovascular responses elicited by PAG stimulation occur via activation of non-C1 and C1 RVLM presympathetic neurons. RVLM neurons inhibited by CCK were more likely to be driven by PAG stimulation and may be a subset of neurons responsible for driving gastrointestinal sympathetic vasomotor tone. CCK-induced inhibition of a subpopulation of RVLM presympathetic neurons may be implicated in postprandial hyperemia and postprandial hypotension.

Motor cortical control of cardiovascular bulbar neurones projecting to spinal autonomic areas

There is evidence that the motor cortex is involved in cardiovascular adjustments associated with somatic motor activity, as it has functional connections with the ventrolateral medulla, a brainstem region critically involved in the control of blood pressure and the regulation of plasma catecholamine levels. The ventrolateral medulla sends projections to the spinal intermediolateral nucleus, where preganglionic neurones controlling heart and blood vessels (T2 segment) and adrenal medulla (T8 segment) are found. The aim of the present study was to determine whether electrical stimulation of the rat motor cortex induces cardiovascular responses and Fos expression in ventrolateral medulla neurones projecting to the T2 and T8 segments. After a set of experiments designed to record cardiovascular parameters (blood pressure and plasma catecholamine levels), injections of retrograde tracer (Fluorogold) were performed in the intermediolateral nucleus of two groups of rats, at the T2 or at the T8 segmental levels. Five days later, the motor cortex was stimulated in order to induce Fos expression in the ventrolateral medulla. Stimulation of the motor cortex induced: (1). hypotension and a significant decrease in plasma noradrenaline levels, and (2). a significant increase in the number of the double-labelled neurones in the rostral ventrolateral medulla projecting to T2. These data demonstrate that cardiovascular adjustments, preparatory to, or concomitant with, motor activity may be initiated in the motor cortex and transmitted to cardiac and vasomotor spinal preganglionic neurones, via the ventrolateral medulla.

Opioid circuits originating from the nucleus paragigantocellularis and their potential role in opiate withdrawal

Neurons in the rat nucleus paragigantocellularis (PGi), located in the ventrolateral medulla, send collateral projections to the locus coeruleus (LC) and to the nucleus of the solitary tract (NTS). The present study examined whether neurons in the PGi that project to both the LC and NTS contain leucine(5)-enkephalin (ENK), and also whether opioid-containing neurons in the PGi are activated following withdrawal from opiates. Retrograde transport of Fluoro-Gold (FG) from the LC and transport of a protein-gold tracer from the NTS was combined with detection of an antibody directed against ENK in the PGi. Using fluorescence and brightfield microscopy, it was established that more than half of the neurons containing both FG and the protein-gold tracer, also exhibited immunolabeling for ENK. The most frequent location of triply labeled neurons was the retrofacial portion of the PGi. In a separate series, rats were chronically implanted with morphine or placebo pellets and, on the fifth day, were subjected to an intraperitoneal injection of naltrexone. Two hours following initiation of withdrawal, rat brains were obtained and processed for detection of c-fos and in situ hybridization labeling of preproenkephalin (PPE) mRNA. Naltrexone injections into morphine-dependent rats caused a dramatic increase in c-fos as compared to control rats. Approximately 66% of the c-fos-labeled neurons exhibited labeling for PPE mRNA. These were also enriched in the retrofacial portion of the PGi. Taken together, the present data indicate that withdrawal from opiates engages opioid neurons in the PGi, some of which may coordinate activity of neurons in both the NTS and the LC.

NADPH-diaphorase activity and c-fos expression in medullary neurons after fatiguing stimulation of hindlimb muscles in the rat

In anaesthetised rats, Fos-immunoreactive and NADPH-diaphorase-positive neurons in the medulla and, for comparison, in the spinal cord were studied after fatiguing stimulation of the hindlimb muscles. Following both direct muscle stimulation and L5 ventral root stimulation, fatigue-related c-fos gene expression was most prominent in the dorsal horn of the ipsilateral L2-L5 segments and within the ipsilateral nucleus tractus solitarius, the caudoventrolateral and rostroventrolateral reticular nuclei, and the intermediate reticular nucleus at levels of -14.3 and -13.8 mm, and contralaterally at -13.2 mm caudal to the bregma. The order of intensity of c-fos expression was as follows: nucleus tractus solitarius>caudoventrolateral and rostroventrolateral reticular nuclei>intermediate reticular nucleus>lateral paragigantocellular nucleus. NADPH-diaphorase reactivity was changed in the following sequence: NTS>intermediate reticular nucleus lateral paragigantocellular nucleus>rostroventrolateral reticular nucleus. Fos-immunoreactive neurons were codistributed with NADPH-diaphorase-reactive cells within the dorsomedial and ventrolateral medulla, and double-staining neurons were found in the nucleus tractus solitarius, intermediate reticular nucleus and lateral paragigantocellular nucleus. The patterns of distribution of c-fos expression and NADPH-diaphorase reactivity show that afferent signals arising from fatiguing muscles may activate spinal and medullary neurons which are involved in nociceptive and cardiovascular reflex pathways. The functional role of nitric oxide (NO) in the generation of cardiovascular and somatosensory responses in the medulla during fatigue of skeletal muscles is discussed.

Lateralisation of projections from the rostral ventrolateral medulla to sympathetic preganglionic neurons in the rat

Spinally projecting sympathoexcitatory neurons in the rostral ventrolateral medulla (RVLM), synapse with sympathetic preganglionic neurons (SPN) and regulate the activity of sympathetic nerves that control the heart, blood pressure and the adrenal medulla (AM). However, the degree of lateralization of the bulbospinal projections to SPN innervating specific targets is poorly understood. Three approaches were used in this study. Anterograde tracer was iontophoresed into a pressor site in the RVLM (left or right) and retrograde tracer injected into the superior cervical ganglion (SCG, right) and the AM (left). Close appositions between anterogradely labelled axons and retrogradely labelled SCG- or AM-SPN were counted. Projections to the SCG were bilateral. Projections to the AM were markedly ipsilateral. In the second part, retrograde tracers were injected unilaterally into the region of the intermediolateral cell column at spinal segment T2 or T8 on one side and the number of labelled neurons in the RVLM counted. The results from each level of injection were similar showing that approximately 63-64% of the neurons were ipsilateral. Responses to glutamate microinjection into the RVLM on adrenal nerve (left) and superior cervical nerve (left) activity were measured. The ratio of the nerve responses was the same even when different sides of the RVLM were injected. The anterograde data strongly suggest that the RVLM projections to AM-SPN are predominantly ipsilateral. Although other experimental approaches also attempted to investigate lateralization, the retrograde data target different and functionally heterogeneous pools of SPN that may mask the ipsilateral projection to the AM. Similarly, chemical stimulation of the RVLM will excite not only monosynaptic projections but also polysynaptic projections that may also mask the predominant ipsilateral monosynaptic projection to AM.

Spinal interneurons play a minor role in generating ongoing renal sympathetic nerve activity in spinally intact rats

The purpose of the present study was to determine whether spinal interneurons play a role in the regulation of sympathetic activity in spinally intact rats. In acutely spinally transected rats, we have described a population of spinal interneurons that, by virtue of correlations between their ongoing firing rates and the magnitude of ongoing renal sympathetic nerve activity (RSNA), are candidates for generators of sympathetic activity. Further evidence for a sympathetic role for these neurons comes from our observation that cervical spinal stimulation that reduces RSNA also reduces their discharge rates. In chloralose-anesthetized, spinally intact and spinally transected rats, we recorded ongoing RSNA and the ongoing activities of T(10) dorsal horn and intermediate zone interneurons, and we determined the incidence of sympathetically related neurons in these rats by cross-correlating their activities with RSNA. The incidence of correlated neurons was much smaller in spinally intact than in spinally transected rats. We stimulated the dorsolateral, C(2-3) spinal cord before and after acute C(1) spinal transection. Dorsolateral cervical stimulation in spinally transected rats reduced both RSNA and the activities of most T(10) interneurons, but stimulation in spinally intact rats increased RSNA while still reducing the activities of most T(10) interneurons. Both the low incidence of sympathetically correlated spinal neurons in intact rats and the dissociation between the effects of cervical stimulation on RSNA and the discharge rates of spinal interneurons argue against these neurons playing a major role in regulating sympathetic activity in intact rats.

Microinjection of bombesin into the ventrolateral reticular formation inhibits peripherally stimulated gastric acid secretion through spinal pathways in rats

Bombesin injected into the cisterna magna potently inhibits gastric acid secretion stimulated by intravenous infusion of pentagastrin. Sites in the medulla oblongata where bombesin acts to suppress gastric acid secretion were investigated in urethane-anesthetized rats with gastric cannula. Bombesin or vehicle was injected into the medullary parenchyma or intracisternally (i.c.) 60 min after the start of an intravenous pentagastrin infusion; gastric acid secretion was monitored every 10 min for 20 min before and 150 min after the start of pentagastrin. Bombesin (0.2, 0.6 or 6.2 pmol) microinjected into the ventrolateral reticular formation (VLRF) inhibited dose-dependently the net acid response to pentagastrin by 40.8+/-11.1, 75.4+/-12.8 and 96.7+/-19.4%, respectively, at the 40-50 min period after microinjection compared with the vehicle group. Bombesin action in the VLRF was long lasting (96% inhibition still observed at 90 min after 6.2 pmol), and completely abolished by cervical spinal cord transection at the C6 level. By contrast, bombesin injected i.c. at 0.2 or 0.6 pmol had no effect while at 6.2 pmol, there was a 79.0+/-3.9% peak inhibition of pentagastrin-stimulated acid secretion. Bombesin (6.2 pmol) injected into the dorsal motor nucleus reduced the acid response to pentagastrin by 29%. The parvicellular and gigantocellular reticular nuclei were not responsive to bombesin. These results indicate that bombesin acts in the VLRF to inhibit pentagastrin-stimulated gastric acid secretion through spinal pathways, suggesting a potential role of medullary VLRF area in the sympathetic control of gastric acid secretion.

Tracing functionally identified neurones in a multisynaptic pathway in the hamster and rat using herpes simplex virus expressing green fluorescent protein

Using a genetically modified herpes simplex virus encoding green fluorescent protein we sought to establish if this viral modification could be used in transneuronal tracing studies of the sympathetic nervous system. The herpes simplex virus encoding green fluorescent protein was injected into the adrenal medulla of three hamsters and six rats. After a suitable survival period, neurones in the sympathetic intermediolateral cell column of the thoracolumbar spinal cord, rostral ventral medulla and paraventricular nucleus of the hypothalamus were clearly identified by the presence of a green fluorescence in the cytoplasm of the neurones of both species. Thus, herpes simplex virus encoding green fluorescent protein labelled chains of sympathetic neurones in the hamster and rat and therefore has the potential to be used in transneuronal tracing studies of autonomic pathways in these species.

Activation of alpha(2)-receptors in the rostral ventrolateral medulla evokes natriuresis by a renal nerve mechanism

The contribution of alpha(2)-receptor mechanisms in the rostral ventrolateral medulla (RVLM) in mediating the enhanced renal excretory responses evoked by the intravenous infusion of the alpha(2)-receptor agonist xylazine was examined in ketamine-anesthetized rats. In ketamine-anesthetized rats, the bilateral microinjection of the alpha(2)-receptor antagonist yohimbine into the RVLM significantly reduced the enhanced levels of urine flow rate (V) and urinary sodium excretion (UNaV) produced by xylazine. In contrast, microinjection of yohimbine into the RVLM of chronically bilaterally renal-denervated rats significantly reduced the xylazine-evoked diuretic, but not natriuretic, response. In separate ketamine-anesthetized rats, intravenous xylazine infusion produced a near complete inhibition of renal sympathetic nerve activity (RSNA). The subsequent microinjection of yohimbine into the RVLM reversed this neural response and concurrently decreased V and UNaV. Together, these results indicate that during intravenous infusion, xylazine activates alpha(2)-receptor mechanisms in the RVLM to selectively promote urinary sodium excretion by a renal nerve-dependent pathway. In contrast, activation of alpha(2)-receptor in the RVLM affects the renal handling of water by a pathway independent of the renal nerves. This latter pathway may involve an interaction with other brain regions involved in antidiuretic hormone release (e.g., paraventricular nucleus of the hypothalamus).

T (tail)-rhythm oscillators: principles of nervous control of the vasculature revealed?

This review focuses on the nervous control of the caudal ventral artery of the rat tail, and aims to convince the reader that sympathetic control of the vasculature can be mediated via neural oscillators intrinsic to the sympathetic nervous system. The definitive functional significance of these oscillators is unknown at present. However, it is expected that through dynamic relationships with modulating and driving inputs, such oscillators would permit graded vascular responses.

Activation of ventrolateral medullary neurons projecting to spinal autonomic areas after chemical stimulation of the central nucleus of amygdala: a neuroanatomical study in the rat

Several studies have shown that the central nucleus of amygdala is involved in cardiovascular regulation. The control of this function may be mediated by activation of the ventrolateral medulla neurons that project to preganglionic neurons located in the intermediolateral nucleus of the spinal cord. The aim of the present study was to examine whether stimulation of the central nucleus of amygdala activated ventrolateral medulla neurons projecting to the intermediolateral nucleus. For this purpose, the injection of a retrograde tracer, the cholera toxin b subunit (CTb), into the intermediolateral nucleus of the T2 segment was combined with immunohistochemical detection of Fos protein following chemical stimulation of the central nucleus of amygdala. Results showed that retrogradely labeled neurons were found throughout the ventrolateral medulla. Moreover, chemical stimulation of the central nucleus of amygdala induced: (1) a decrease of arterial blood pressure; (2) an expression of Fos protein mainly in sub-populations of neurons located in the intermediate and caudal parts of the ventrolateral medulla; (3) a significantly higher number of double labeled neurons (CTb-immunoreactive/Fos-immunoreactive) in the rostral part of the ventrolateral medulla than in the other parts of this region. These results show that the central nucleus of amygdala influences the activity of brainstem neurons projecting to the intermediolateral nucleus. Data were discussed in terms of descending amygdalofugal pathways involved in the hypotension.

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.

N-methyl-D-aspartate-type glutamate receptors are found in post-synaptic targets of adrenergic terminals in the thoracic spinal cord

Adrenergic (C1) neurons in the rostral ventrolateral medulla (RVL) are sympathoexcitatory and project directly to sympathetic preganglionic neurons (SPNs) in the thoracic spinal cord. C1 neurons also contain glutamate which may mediate the excitatory effects of RVL stimulation on SPNs through the N-methyl-D-aspartate (NMDA)-type receptor. Dual-labeling immunocytochemistry, combined with electron microscopy, was used to determine if NMDA receptors are located post-synaptic to adrenergic terminals in the spinal cord. Adrenergic terminals were labeled using an antibody directed against phenylethanolamine-N-methyl transferase (PNMT) and the NMDA receptor was identified using an antibody directed against the R1 subunit of the receptor (NMDAR1). NMDAR1 was found primarily in large and small dendrites and a few perikarya. The presence of NMDAR1 in the dendritic targets of PNMT-containing terminals was quantified in spinal cords sectioned either horizontally or coronally. PNMT-labeled terminals formed asymmetric synapses on dendrites containing immunogold labeling for NMDAR1, but NMDAR1 was more often detected in the targets of PNMT terminals when spinal cords were sectioned horizontally (59%) rather than coronally (28%). This difference in prevalence of NMDAR1 in targets of PNMT terminals is likely due to the preferential orientation of SPN dendrites in the horizontal plane, since longer dendritic shafts were visible in horizontal sections. When NMDAR1 was present in the dendritic targets of many adrenergic terminals, it was usually located at sites distal to the adrenergic input. We conclude that NMDA receptor ligands are likely to modulate the activity of dendritic targets of adrenergic terminals in the spinal cord, but are not closely associated with adrenergic synaptic input.

Expression of Fos protein in adrenal preganglionic neurons following chemical stimulation of the rostral ventrolateral medulla of the rat

The ventrolateral medulla is known to be involved in the regulation of arterial blood pressure, especially via its connections with sympathetic preganglionic neurons (SPNs) mainly located in the intermediolateral nucleus of the spinal cord. It has been shown that stimulation of the rostral part of the ventrolateral medulla (RVLM) elicits a release of catecholamines from the adrenal medulla. The aim of the present study was to demonstrate the existence of a functional pathway between the RVLM and adrenal SPNs using the combination of a retrograde tract tracing technique (cholera toxin B subunit) with the immunohistochemical detection of Fos protein following the chemical stimulation of RVLM. The data obtained showed that: (1) chemical stimulation of the RVLM induced Fos immunoreactivity in the intermediolateral nucleus and particularly in SPNs projecting to the adrenal medulla; (2) along the thoracic segments T2-T12, 26.1% of retrogradely identified adrenal SPNs were Fos-immunoreactive with the greatest percentage (30.9%) in the T8 segment. These results favored a functional control of the RVLM on adrenal SPNs which may contribute to a substantial activation of the cardiovascular system via the release of adrenal catecholamines.

Central control of the cardiovascular and respiratory systems and their interactions in vertebrates

This review explores the fundamental neuranatomical and functional bases for integration of the respiratory and cardiovascular systems in vertebrates and traces their evolution through the vertebrate groups, from primarily water-breathing fish and larval amphibians to facultative air-breathers such as lungfish and some adult amphibians and finally obligate air-breathers among the reptiles, birds, and mammals. A comparative account of respiratory rhythm generation leads to consideration of the changing roles in cardiorespiratory integration for central and peripheral chemoreceptors and mechanoreceptors and their central projections. We review evidence of a developing role in the control of cardiorespiratory interactions for the partial relocation from the dorsal motor nucleus of the vagus into the nucleus ambiguus of vagal preganglionic neurons, and in particular those innervating the heart, and for the existence of a functional topography of specific groups of sympathetic preganglionic neurons in the spinal cord. Finally, we consider the mechanisms generating temporal modulation of heart rate, vasomotor tone, and control of the airways in mammals; cardiorespiratory synchrony in fish; and integration of the cardiorespiratory system during intermittent breathing in amphibians, reptiles, and diving birds. Concluding comments suggest areas for further productive research.

Induction of c-Fos-like protein in bulbar catecholaminergic neurones by electrical stimulation of the sensorimotor cortex in the rat

The sensorimotor cortex (SMC) establishes a functional connectivity with the nucleus tractus solitarius (NTS) and the rostral ventrolateral medulla (RVLM). These bulbar nuclei are known to contain catecholaminergic neurones involved in the cardiovascular control. The aim of the present study was to establish the proportion of catecholaminergic neurones activated by electrical stimulation of SMC. For this purpose, double immunocytochemical procedures were used to reveal the distribution of Fos protein and tyrosine hydroxylase (TH). The results showed that, in the NTS, 7% of the neurones immunoreactive for TH expressed Fos-protein, versus 34% in the RVLM. These data provide evidence that the SMC activated preferentially catecholaminergic neurones of the RVLM which are known to be involved in cardiovascular control via spinal preganglionic neurones.