Patterns of single unit activity in sympathetic postganglionic nerves

Low-Frequency Oscillations in Cardiac Sympathetic Neuronal Activity

Sudden cardiac death caused by ventricular arrhythmias is among the leading causes of mortality, with approximately half of all deaths attributed to heart disease worldwide. Periodic repolarization dynamics (PRD) is a novel marker of repolarization instability and strong predictor of death in patients post-myocardial infarction that is believed to occur in association with low-frequency oscillations in sympathetic nerve activity. However, this hypothesis is based on associations of PRD with indices of sympathetic activity that are not directly linked to cardiac function, such as muscle vasoconstrictor activity and the variability of cardiovascular autospectra. In this review article, we critically evaluate existing scientific evidence obtained primarily in experimental animal models, with the aim of identifying the neuronal networks responsible for the generation of low-frequency sympathetic rhythms along the neurocardiac axis. We discuss the functional significance of rhythmic sympathetic activity on neurotransmission efficacy and explore its role in the pathogenesis of ventricular repolarization instability. Most importantly, we discuss important gaps in our knowledge that require further investigation in order to confirm the hypothesis that low frequency cardiac sympathetic oscillations play a causative role in the generation of PRD.

Reinnervation of the Canine Posterior Cricoarytenoid Muscle with Sympathetic Preganglionic Neurons

This experiment investigated the reinnervation of the canine posterior cricoarytenoid (PCA) muscle with preganglionic neurons of the sympathetic nervous system. Six dogs had their right recurrent laryngeal nerve (RLN) sectioned. Four of these dogs had the sympathetic cervical trunk (SCT) implanted into the right PCA muscle, and the two remaining dogs served as denervated controls. Four months later all dogs underwent videolaryngoscopy, electromyography, and electrical stimulation of the SCT. The PCA muscles were excised, sectioned, and stained for glycogen and ATPase. All four experimental PCA muscles demonstrated electrically evoked abduction and tonic electromyographic activity. In two of the specimens, staining (ATPase and PAS) revealed areas of reinnervation with fiber type grouping and glycogen depletion. These results are consistent with the successful reinnervation of the PCA muscle. Further refinement of this technique could be of benefit to patients with bilateral vocal cord paralysis.

Sympathetic tales: subdivisons of the autonomic nervous system and the impact of developmental studies

Remarkable progress in a range of biomedical disciplines has promoted the understanding of the cellular components of the autonomic nervous system and their differentiation during development to a critical level. Characterization of the gene expression fingerprints of individual neurons and identification of the key regulators of autonomic neuron differentiation enables us to comprehend the development of different sets of autonomic neurons. Their individual functional properties emerge as a consequence of differential gene expression initiated by the action of specific developmental regulators. In this review, we delineate the anatomical and physiological observations that led to the subdivision into sympathetic and parasympathetic domains and analyze how the recent molecular insights melt into and challenge the classical description of the autonomic nervous system.

Modulation of synchronous sympathetic firing behaviors by endogenous GABA(A) and glycine receptor-mediated activities in the neonatal rat spinal cord in vitro

Delivering effective commands in the nervous systems require a temporal integration of neural activities such as synchronous firing. Although sympathetic nerve discharges are characterized by synchronous firing, its temporal structures and how it is modulated are largely unknown. This study used a collagenase-dissociated splanchnic sympathetic nerve-thoracic spinal cord preparation of neonatal rats in vitro as an experimental model. Several single-fiber activities were recorded simultaneously and verified by rigorous computational algorithms. Among 3763 fiber pairs that had spontaneous fiber activities, 382 fiber pairs had firing positively correlated. Their temporal relationship was quantitatively evaluated by cross-correlogram. On average, correlated firing in a fiber pair occurred in scales of ∼40ms lasting for ∼11ms. The relative frequency distribution curves of correlogram parametrical values pertinent to the temporal features were best described by trimodal Gaussians, suggesting a correlated firing originated from three or less sources. Applications of bicuculline or gabazine (noncompetitive or competitive GABA(A) receptor antagonist) and/or strychnine (noncompetitive glycine receptor antagonist) increased, decreased, or did not change individual fiber activities. Antagonist-induced enhancement and attenuation of correlated firing were demonstrated by a respective increase and decrease of the peak probability of the cross-correlograms. Heterogeneity in antagonistic responses suggests that the inhibitory neurotransmission mediated by GABA(A) and glycine receptors is not essential for but can serve as a neural substrate to modulate synchronous firing behaviors. Plausible neural mechanisms were proposed to explain the temporal structures of correlated firing between sympathetic fibers.

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.

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.

Glycine produced pressor responses when microinjected in the pressor areas of pons and medulla in cats

In 24 cats under chloralose/urethane anesthesia changes of systemic arterial pressure (SAP) and sympathetic vertebral nerve activities (VNA) were induced by microinjection of glycine (Gly, 1.0 M, 50 nl) into the pressor areas of the rostral pons, i.e., locus coeruleus-parabrachial nucleus (LC-PBN), nucleus of gigantocellular tegmental field-lateral tegmental field (FTG-FTL), and dorsomedial (DM) and ventrolateral (VLM) medulla. The effects were compared with those induced by microinjection of sodium glutamate (Glu, 0.25 M, 50 nl) into the same sites. In about 60% of the injections Gly produced increases in SAP and VNA similar to that of Glu. The increase in SAP was greater in VLM, while the increase in VNA was more marked in DM. In the rest of microinjections Gly and Glu produced changes of SAP and VNA in different combinations. The latency of Gly-induced increases in SAP and VNA was 1 to 3 s longer than that induced by Glu. Our findings show that although Gly is classified as an inhibitory transmitter, it often produced excitation of the pressor neurons in the pons and medulla similar to that of Glu. Whether Gly acts through the same cardiovascular neurons that respond to Glu or through activation of different kinds of neurons remains to be elucidated.

Angiotensin II activates pressor and depressor sites of the pontomedulla that react to glutamate

1. In cats anaesthetized with a mixture of alpha-chloralose (40 mg/kg) and urethane (400 mg/kg) and in rats anaesthetized with a mixture of alpha-chloralose (60 mg/kg) and urethane (800 mg/kg), changes in systemic arterial pressure (SAP), heart rate (HR) and sympathetic activities of vertebral (VNA) and renal (RNA) nerves were determined following the microinjection of angiotensin II (AngII; 0.16 mmol/L; 50 nL) into the pressor and depressor sites of the pontomedulla previously reacted to a microinjection of monosodium L-glutamate (Glu; 0.1 mol/L; 50 nL). Pressor sites included gigantocellular tegmental field (FTG) and dorsal medulla (DM) and rostral ventrolateral medulla (VLM). The depressor site was the caudal VLM (CVLM). The effects of losartan (1 mmol/L; 50 nL), a specific AT1 receptor non-peptide antagonist for AngII, on responses induced by AngII in the VLM, DM and CVLM were also determined. 2. In 30% of pressor sites in the FTG, 55% in the VLM and 67% in the DM and in 76% of depressor sites in the CVLM previously exposed to Glu, microinjection of AngII to the same site produced pressor or depressor responses similar to that of Glu, but smaller in magnitude, particularly in the pressor VLM. Changes in both VNA and RNA induced by AngII were also smaller than those induced by Glu, particularly RNA from DM activation. 3. In the dorsal motor nucleus of the vagus, AngII, as Glu, produced marked bradycardia, but again this was smaller in magnitude than the bradycardia produced by Glu. 4. In rats, in the DM near or around the nucleus of the solitary tract where Glu increased SAP, microinjection of AngII (0.8 mmol/L; 60 nL) produced a depressor response, while the microinjection of 1.6 mmol/L (60 nL) AngII produced a pressor response. 5. Losartan blocked the increase in SAP induced by AngII in the VLM and DM. Decreases in SAP induced by AngII in the CVLM, however, were only slightly decreased by losartan. 6. Our data suggest that a significant portion of pressor and depressor sites of the pontomedulla contain neurons responsive to both AngII and Glu. In neurons in the VLM and DM, AngII produced pressor responses that were primarily mediated through AT1 receptors, while the depressor actions of AngII in the CVLM were not mediated by AT1 receptors.

The amplitude of synchronized cardiac sympathetic nerve activity reflects the number of activated pre- and postganglionic fibers in anesthetized cats

In order to obtain information regarding the number of pre- and postganglionic fibers that are firing, we measured cardiac sympathetic nerve activity (CSNA) before and after the successive sectioning of T1-T5 thoracic rami in anesthetized cats. Total activity from the area was measured under the mean CSNA curve. Peak amplitude, width and periodicity of the synchronized discharge was analyzed from the CSNA curve by the method we developed. Total CSNA decreased to 91 +/- 6%, 63 +/- 6%, 27 +/- 10%, 8 +/- 6% and < 1% of the control due to successive section of the T5, T4, T3, T2 and T1 rami, respectively. The peak amplitude of synchronized CSNA decreased to 95 +/- 6%, 73 +/- 8%, 40 +/- 5% and < 10% of the control value, due to section of the T5, T4, T3 and T2 rami, respectively. The control width was 107 +/- 8 ms and decreased to 106 +/- 1 ms, 92 +/- 6 ms and 68 +/- 5 ms by successive section of the respective T5, T4 and T3 rami. However, periodicities of 80-120 ms (Tc rhythm) and 140-500 ms (Tb rhythm) of synchronized CSNA remained unchanged after section of the T3-T5 rami. The total CSNA decreased gradually due to decreases in the peak amplitude and width of synchronized CSNA with the successive section of preganglionic fibers. These results indicate that the peak amplitude of synchronized CSNA reflects the number of pre- and post-ganglionic fibers that are firing and suggest that the number of preganglionic neurons which activate the cardiac fibers naturally was largest in the T3 segment.(ABSTRACT TRUNCATED AT 250 WORDS)

Presence of neuronal cell bodies in the sympathetic pressor areas of dorsal and ventrolateral medulla inhibiting phrenic nerve discharge in cats

To examine whether neuronal cell bodies (perikarya) in the pressor areas of dorsal medulla or ventrolateral medulla also modulate respiratory function, phrenic nerve activity was monitored and correlated with the pressor response in chloralose-urethane anaesthetized cats. The animals were paralyzed and artificially ventilated maintaining the end-tidal fractional concentration of CO2 at 0.04-0.05. The same pressor point in the dorsal or ventrolateral medulla was stimulated electrically (rectangular pulse of 20-200 microA, 80 Hz and 0.5 ms) and then chemically (0.25-0.5 M sodium glutamate, 80-200 nl). Areas producing pressor effects in either the dorsal or ventrolateral medulla produced a current-dependent decrease of phrenic discharge. The decrease in Pna was significant when the electrical current reached 50 microA or above. It occurred immediately before the onset of increase in blood pressure. Injection of glutamate to the same pressor point in either the dorsal or ventrolateral medulla produced a similar decrease in phrenic nerve activity. The results suggest that in the pressor areas of dorsal and ventrolateral medulla there are neuronal perikarya that can increase systemic arterial pressure and inhibit phrenic nerve activity. Whether the same or separate neurones are responsible for these responses is to be determined.

Properties of postganglionic sympathetic neurons with axons in the right thoracic vagus

The resting and reflex-evoked activities of single postganglionic sympathetic neurons with axons in the right thoracic vagus were tested in chloralose-anaesthetized cats. The properties of a majority of neurons were found to be similar. Cardiac- and inspiration-related rhythmicities were present in the resting activity of sympathetic neurons. Their resting activity was not affected by hyperventilation which abolished phrenic nerve discharges. Systemic hypoxia (2 min; 8% O2 in N2) increased the activity of the neurons more effectively in the deafferented state than when both sinus nerves remained intact. Injection of 0.1 ml 1 M sodium bicarbonate saturated with CO2, which activates peripheral chemoreceptors in the right or left carotid sinus, usually evoked a decrease in sympathetic activity in animals with both sinus nerves intact. We concluded that activation of peripheral chemoreceptors may inhibit the activity of the sympathetic neurons with axons in the right thoracic vagus. We suggest that the described sympathetic neurons may be a functionally homogeneous population which may innervate the conducting system of the heart. The close localization of sympathetic and parasympathetic axons in the vagus nerve may facilitate sympathetic-parasympathetic interaction at the level of their endings in the heart.

Neuroendocrine and beta-adrenoceptor response to chronic ethanol and aggression in rats

Male rats were administered either ethanol (6-8 g/kg/day) or dextrin-maltose, an isocaloric equivalent, for two weeks prior to a 24-hour resident-intruder test. After the first 20 minutes of the aggression test residents showed a greater increase in norepinephrine than intruders (216% vs. 97%), while intruders showed a greater increase in epinephrine (394% vs. 51%) and corticosterone (338% vs. 129%) than residents. Ethanol administration increased the initial epinephrine response of intruders almost two-fold compared to dextrin-maltose intruders. After 24 hours of aggression testing plasma norepinephrine was still elevated in residents (92%) and intruders (71%), however, only intruders continued to show an elevation in plasma corticosterone (98%) and epinephrine (107%). Using a cumulative dose-response technique, the dose of isoproterenol required to produce 50% of the maximal heart rate response (ED50) increased in intruders by 108% following aggression testing with ethanol intruders showing significantly smaller mean change. The increase in ED50 was related to drug type, behavior, and plasma corticosterone and epinephrine levels. Rats treated with ethanol had a greater beta-adrenoceptor density than control rats. However, no relationship was found between receptor density and the other measures in this study.

Multi- and single-fibre mesenteric and renal sympathetic responses to chemical stimulation of intestinal receptors in cats

1. In cats anaesthetized with alpha-chloralose and artificially respired, stimulation of intestinal receptors with bradykinin caused greater reflex excitation of mesenteric than of renal efferent multifibre nerve activity and significant pressor responses. 2. Activity of all nerve bundles used in this study was inhibited by stimulation of pressoreceptors. Increases in systemic arterial pressure caused inhibition of activity of renal nerves which was significantly greater than that of mesenteric nerves. 3. Spinal transection caused significant decreases in tonic renal nerve activity without altering the ongoing discharge rate of mesenteric nerves. Stimulation of intestinal receptors in spinal cats still caused significant increases is discharge of mesenteric and renal nerves, indicating that this reflex contains a spinal component. 4. Recordings of activity of individual fibres within mesenteric (21) and renal (23) nerves provided information regarding the basis for the multifibre responses to stimulation of intestinal receptors. The same proportion of fibres from both nerves was excited, but the increase in activity of mesenteric fibres was significantly greater than that of renal fibres. 5. Mesenteric fibres could be classified into two groups, based on their sensitivity to pressoreceptor influences. Fibres that exhibited pressoreceptor-independent discharge had the greatest responses to stimulation of intestinal receptors. 6. Following spinal transection the majority of mesenteric fibres continued to fire, whereas most renal fibres became quiescent. 7. The non-uniform pattern of neuronal excitation to chemical stimulation of intestinal receptors was manifest after spinal transection, demonstrating that exclusively spinal pathways can mediate this differential response pattern. 8. These results support the hypothesis that viscero-sympathetic reflexes may be organized to cause preferential excitation of neural activity directed to the organ from which the reflex originates.

The influence of the sympathetic impulse pattern on contractile responses of rat mesenteric arteries and veins

Contractile responses to electrical field stimulation of excised small mesenteric arteries and veins of the rat were compared when stimuli were delivered in irregular bursts or at regular intervals. Spontaneously occurring skin vasoconstrictor impulses in a few-unit median nerve recording in man were stored on tape and used to trigger a stimulator. Two irregular stimulation sequences at average frequencies of 1.6 and 1.8 Hz, respectively, were used. In the arteries, average contractile responses were significantly greater at an irregular than at an even stimulation frequency, but in the veins, similar degrees of contraction were obtained with the two modes of stimulation. The frequency-response relationships to continuous regular stimulation showed the artery to respond less than the vein at low frequencies. This apparently explains the differences in behaviour between the vessels to irregular stimulation. The results show that not only the number of impulses, but also their pattern of occurrence, may influence the degree of vasoconstriction. Thus, the normal irregular sympathetic discharge pattern in itself has a bearing on the physiology of neuro-effector control mechanisms.

Effects of alpha- and beta-adrenoceptor blockade on the neurally evoked overflow of endogenous noradrenaline from the rat isolated heart

beta-Adrenoceptor blockade by propranolol (4 X 10(-6)M) was without effect on the overflow of endogenous noradrenaline from the isolated heart of the rat induced by stimulation of sympathetic nerves for 1 min at 1 and at 4 Hz. The increase in heart rate in response to such stimulation was abolished by propranolol treatment. alpha 2-Adrenoceptor blockade by yohimbine (10(-6)M) induced approximately a two fold increase in the overflow of endogenous noradrenaline induced by stimulation of sympathetic nerves for 1 min at 1 and at 4 Hz. A combination of yohimbine (10(-6)M) and desipramine (10(-7)M) induced a more than 3 fold increase in the overflow of endogenous noradrenaline produced by sympathetic nerve stimulation at 1 and at 4 Hz. Heart rate increases produced by such stimulation were intensified. These results provide no evidence for the feedback stimulation of presynaptic beta-adrenoceptors in this preparation. The action of alpha-2-blockade was equipotent at stimulation frequencies of 1 and 4 Hz.

Single-unit and multiunit analyses of renorenal reflexes elicited by stimulation of renal chemoreceptors in the rat

Stimulation of renal chemoreceptors, induced by backflow of concentrated urine into the renal pelvis, elicited a reflex increase in the rate of multiunit efferent renal nerve activity in rats. Ipsilateral stimulation increased the rate by 17.1 +/- 4.5%; contralateral stimulation increased the rate by 20.8 +/- 4.5%. Efferent activity returned toward control levels within the first minute following removal of the stimulus. The reflex responses were quantitatively similar in rats anesthetized with sodium pentobarbital or alpha-chloralose, but increased following complete transection of the spinal cord at C3. Studies of single efferent units indicated that responses to the stimuli were nonuniform. The majority of single units exhibited an increase in firing frequency during the stimulus; other units showed no change or, less frequently, a reduction in activity during the stimulus. Some postganglionic neurons projecting to the kidney were activated by stimuli applied to either kidney. It was concluded that stimulation of intrarenal chemoreceptors provokes bilateral excitatory renorenal reflexes in the rat. The observation of nonuniform responses among single units suggests that renal nerves contain a mixture of efferent fibers which may contact different targets in the kidney.

Cardiac rhythmicity of skin sympathetic activity recorded from peripheral nerves in man

In previous microelectrode recordings of sympathetic impulse activity in human peripheral nerves a marked cardiac rhythmicity has been found in the spontaneous firing of vasoconstrictor neurones supplying the vascular bed of skeletal muscles. Evidence has been presented that this rhythmicity depends on a potent baroreflex control of these neurones which are significantly involved in blood pressure regulation. In contrast, no cardiac rhythmicity has previously been seen in the spontaneous firing of sympathetic fibres supplying vessels and sweat glands in the human skin. The present study shows that when strong sudomotor activity is induced in skin nerves by a rise in ambient temperature, the sudomotor impulses tend to occur in volleys time-locked to the cardiac cycle. A similar cardiac rhythmicity is not exhibited by the skin vasoconstrictor fibres which can be activated by lowering of the ambient temperature. Induced falls in blood pressure do not produce any baroreflex modulations of the sudomotor outflow, suggesting that the cardiac rhythmicity of the sudomotor impulses is mot dependent on the action of this reflex.