The effects of catechol on various membrane conductances in lumbar sympathetic postganglionic neurones of the guinea-pig.

The effects of catechol on membrane properties in lumbar sympathetic postganglionic neurones isolated from guinea-pigs were studied in vitro in current and voltage clamp using single intracellular microelectrodes. Neurones with properties characteristic of two previously described classes of neurone (phasic and tonic) were studied. Catechol (3-12 mM) produced a few mV depolarization and a dose-dependent increase in membrane resistance which were both larger in tonic than in phasic neurones. In the presence of catechol, both phasic and tonic neurones fired only a single action potential at the beginning of a maintained depolarizing current step. In both neurone types, catechol reduced action potential amplitude and slowed its time course. The peak of the afterhyperpolarization became delayed and reduced in amplitude, particularly in tonic neurones. The time constant of inactivation of IA was reduced by catechol without change in the voltage sensitivity of activation or inactivation: IC50 was 3 mM in phasic and 4 mM in tonic neurones. Catechol also blocked a slow voltage-activated K+ current (resembling ID) that was present in many tonic neurones. Catechol did not modify the slow calcium-activated potassium current (gKCa1) or the anomalous rectifier; neither did it appear to affect the fast calcium-activated potassium current (IC) or the delayed rectifier. Catechol did not change the overall rate of spontaneous synaptic activity nor enhance the release of quanta of ACh from preganglionic terminals evoked by nerve stimulation. We conclude that, in addition to blocking IA, catechol blocks the slow ID-like current in sympathetic neurones. It also has a profound effect on the action potential probably by increasing inactivation of voltage-dependent Na+ channels. The change from tonic to phasic discharge in tonic neurones cannot be attributed solely to its effects on IA.

Relation between electrophysiological class and neuropeptide content of guinea pig sympathetic prevertebral neurons.

1. Sympathetic neurons in superior mesenteric ganglion and inferior mesenteric ganglion (IMG) isolated from guinea pigs were classified as tonic, phasic, or long after hyperpolarizing (LAH) on the basis of their discharge characteristics and the different types of potassium currents recorded from them with the soma under single-microelectrode voltage clamp. 2. Passive electrical properties showed a progressive increase in input resistance across the prevertebral ganglia in the rostrocaudal direction when compared with those previously reported for the same classes of neurons in celiac ganglia (CG). 3. The proportions of tonic, phasic, and LAH neurons changed markedly in a rostrocaudal progression from 37, 14, and 49%, respectively, in the CG to 80, 18, and 2%, respectively, overall in the IMG. 4. Three populations of neurons distinguished immunohistochemically by their content of somatostatin (SOM), neuropeptide Y (NPY), or neither neuropeptide were present in different proportions in each prevertebral ganglion. The proportions of SOM, NPY, and no peptide neurons changed from 27, 34, and 39%, respectively, in the CG to 45, 20, and 35%, respectively, in the IMG. There was no significant difference in these distributions between the sexes. 5. Individual electrophysiologically characterized neurons were filled with biocytin and later examined for SOM immunoreactivity. All SOM-positive neurons (9/9) in the CG but only 7/10 in the IMG were tonic, whereas SOM-negative neurons were classified in all electrophysiological classes. 6. Other than this one group of sympathetic neurons (constituting nearly 30% of neurons in both CG and IMG), the three electrophysiological classes do not correlate directly with the three neurochemical types so far identified. This is consistent with the existence of more than three functional groups of sympathetic neurons in the prevertebral ganglia. The findings also suggest that a major inhibitory effect on mucosal secretion, as well as on motility, is mediated by peripheral reflex pathways along much of the length of the gastrointestinal tract.

Selective uptake of C-fragment of tetanus toxin by sympathetic preganglionic nerve terminals.

Immunohistochemical techniques have shown that the C-fragment of tetanus toxin injected into medial gastrocnemius muscle in the guinea-pig and rat, in addition to its retrograde transport in the axons of the somatomotor, sympathetic and sensory neurons supplying this muscle, is taken up and concentrated by terminal varicosities within pre- and paravertebral sympathetic ganglia at all thoracolumbar levels. Staining was absent in chronically denervated ganglia, demonstrating the specific association of the antigen with preganglionic varicosities. Preganglionic varicosities at all levels were also labelled after C-fragment injection into the peritoneal cavity or into denervated medial gastrocnemius; both of these procedures failed to label somatomotor or sensory neurons. Although retrograde trans-synaptic transport could be demonstrated, sympathetic labelling was sparse and non-specific, so that the C-fragment of tetanus toxin is unsuitable for the identification of sympathetic pathways. The selective and widespread uptake of tetanus toxin by sympathetic preganglionic terminals could explain the diverse autonomic symptoms observed in tetanus intoxication.

Characteristics of synaptic input to three classes of sympathetic neurone in the coeliac ganglion of the guinea-pig.

1. Intracellular recordings from sympathetic neurones in the isolated coeliac ganglion of guinea-pigs have been used to define the synaptic input to three subtypes of neurone, classified on the basis of their discharge during maintained depolarizing current as phasic neurones, neurones with prolonged after-hyperpolarizations (LAH), and tonic neurones. 2. The three classes of neurone were distributed characteristically in different parts of the ganglion. 3. Passive membrane properties differed between the three neurone types. Mean input resistance was highest in phasic neurones and was inversely related to the size of the prolonged calcium-activated potassium conductance in LAH neurones. Mean input time constant was highest in tonic neurones, because of significantly higher cell capacitance. 4. Phasic and LAH neurones usually received one suprathreshold ('strong') as well as several subthreshold excitatory synaptic potentials (ESPs) from the ipsilateral splanchnic nerve. In general, the amplitude and number of splanchnic inputs were greater, and the occurrence of two strong inputs more common, in phasic than in LAH neurones. The input to tonic neurones was small and usually subthreshold, even with supramaximal splanchnic stimulation. In a few (mostly tonic) neurones lying close to the midline, small ESPs were evoked by contralateral splanchnic stimulation. 5. Antidromic action potentials were evoked in more than half of all neurones by high voltage coeliac nerve stimulation. In addition, multiple small subthreshold ESPs were recorded in virtually all tonic neurones (99%) on coeliac nerve stimulation. In contrast, coeliac stimulation rarely evoked a few very small ESPs in LAH neurones (9%), but no synaptic response in phasic neurones. 6. In about half of the tonic neurones tested (but no phasic or LAH neurones), small ESPs were evoked by stimulation of the intermesenteric nerve. 7. Slow depolarization elicited by repetitive activation of splanchnic and coeliac nerve trunks, at voltages supramaximal for the fast cholinergic responses, were recorded from about half of both phasic and tonic neurones, but only one of twenty-four LAH neurones. These responses commonly faded during subsequent trials, so that it was difficult to characterize them. 8. The data indicate that the three broad groups of coeliac neurone, classified on the basis of their voltage- and calcium-dependent potassium conductances, receive different patterns of synaptic input. The differences may be related to the three major functions of vasoconstriction, motility and mucosal secretion in the small intestine.

Characteristics of ongoing and reflex discharge of renal postganglionic neurons.

Characteristics of basal and reflex firing of single renal postganglionic fibers are reviewed to ascertain whether subpopulations of renal neurons can be distinguished. Moreover, characteristics of renal neurons are contrasted with those of sympathetic neurons innervating vascular and nonvascular tissues of the spleen and small intestine. Attempts to distinguish functional subtypes of renal neurons based on their responses to excitatory or inhibitory influences led to no clear conclusions. Responses of the renal population of neurons differed from those of the splenic and mesenteric populations of postganglionic neurons. Our findings provided no concrete answers about the neural mechanisms by which different functions of the kidney may be regulated selectively. This question continues to require careful and insightful investigation.

Persistent firing of splenic and renal nerves after acute decentralization but failure to produce ganglionic reflexes.

Experiments were done to evaluate the contribution of peripheral neural circuits to generation of ongoing splenic and renal sympathetic discharge as well as to the reflex alteration of this discharge by chemical stimulation of receptors of intestinal afferent nerves. After decentralization of the celiac and superior mesenteric ganglia, low amplitude spikes with low discharge rates still were observed in both nerves. Stimulation of intestinal receptors with bradykinin or capsaicin did not alter this residual firing. Cholinergic blockade eliminated most of this discharge. The source of the residual firing and its contribution to basal discharge of splenic and renal nerves remains to be determined.

Axons of peripheral origin preferentially synapse with tonic neurones in the guinea pig coeliac ganglion.

Intracellular recordings from a population of guinea pig coeliac neurones projecting in the coeliac nerves were used to define their central and peripheral synaptic input. The neurones were classified as 'phasic' or 'tonic' by their discharge in response to depolarizing current. Stimuli applied to the greater splanchnic nerves evoked suprathreshold responses in 89% of phasic neurones, but in less than half the tonic neurones. In contrast, coeliac nerve stimulation evoked only small subthreshold responses in 11% of phasic neurones, but multiple synaptic potentials in 92% of tonic neurones. This suggests that peripheral intestinal reflexes involve only one sympathetic neurone type.

Characteristics of ongoing and reflex discharge of single splenic and renal sympathetic postganglionic fibres in cats.

1. Electrical discharge of thirty-nine single splenic and renal postganglionic nerve fibres was recorded in artificially respired, chloralose-anaesthetized cats. 2. Ongoing discharge rates, averaged over 10 s periods, did not differ between renal and splenic fibres. All neurones of both groups had irregular discharge frequencies. 3. Half of the splenic population and all renal fibres had cardiac-related discharge patterns. Of those tested for respiratory-related firing, 30% of the splenic fibres and 69% of the renal fibres exhibited this pattern. 4. Firing of splenic fibres was less inhibited than that of renal fibres by stimulation of pressoreceptors with phenylephrine-induced increases in blood pressure. Firing of splenic fibres also was less excited than that of renal fibres by unloading pressoreceptors with depressor doses of sodium nitroprusside. 5. Chemical stimulation of splenic afferent nerves with bradykinin consistently elicited greater increases in splenic than renal nerve discharge by causing large increases in firing of all splenic fibres and smaller excitatory responses in 75% of the renal fibres. 6. Application of bradykinin to the intestinal serosa produced greater increases in renal than splenic nerve discharge by consistently causing increased firing of renal fibres and by causing excitation, inhibition, or no change in splenic fibre discharge. 7. Responses of splenic and renal fibres to stimulation of splenic and intestinal afferent nerves after spinal cord transection were similar to those responses elicited when the neuraxis was intact. 8. In conclusion, the differential reflex responses of splenic and renal neuronal populations can be due to the heterogeneity or to the intensity of responses within a neuronal population.

Splenic, renal, and cardiac nerves have unequal dependence upon tonic supraspinal inputs.

Stimulation of visceral receptors can lead to unequal reflex responses in splenic, renal and cardiac sympathetic nerves. Activity of splenic nerves is often more excited or less inhibited than that of cardiac or renal nerves. This study was undertaken to determine potential differences in resting discharge among these 3 nerves. Dependence upon supraspinal drive was evaluated by comparing the relative decrease in activity of these nerves in chloralose-anesthetized cats 30 min to 2 h following high cervical spinal cord transection. After this transection, discharge rates of cardiac and renal nerves were significantly depressed to less than 50% of initial values. In contrast, splenic nerve activity was not significantly affected. To determine if this sustained splenic nerve activity resulted from greater responsiveness to potential external sources of excitation, splenic, renal and cardiac neural responses to factors known to affect sympathetic discharge in spinal animals were compared. Neither increased arterial pressure, decreased arterial pressure, systemic hypercapnia and acidosis, nor thoracolumbar dorsal rhizotomy revealed specific inputs responsible for the preferential maintenance of splenic nerve activity in spinal cats. It was concluded that ongoing activity of splenic nerves is less dependent upon supraspinal sources of excitation than is activity of renal or cardiac nerves. The cause of this difference among these 3 components of sympathetic outflow remains to be determined.

Comparison of the distributions of renal and splenic neurons in sympathetic ganglia.

Postganglionic neurons in different sympathetic ganglia are innervated selectively by preganglionic neurons originating from different segments of the spinal cord. These selective connections between pre- and postganglionic neurons may determine the specificity with which postganglionic nerves participate in differential reflex reactions. Because specificity of renal and splenic nerve responses to stimulation of visceral afferent nerves may depend on the distribution of postganglionic neurons in sympathetic ganglia, retrograde axonal transport of horseradish peroxidase was employed in this study to identify the ganglionic distribution of cell bodies of postganglionic neurons supplying the kidney and spleen in cats. Superior mesenteric, left and right celiac ganglia usually are fused together into a complex ganglion (solar plexus) in the cat. Most labeled cell bodies of renal nerves were clustered in groups within the solar plexus, but some cell bodies of renal neurons were observed in upper lumbar (L1-L3) and lower thoracic (T12-T13) paravertebral sympathetic ganglia. In contrast, 90% of labeled splenic neurons were scattered randomly throughout the left and right celiac poles of the solar plexus. In conclusion, the disparate distribution of renal and splenic neurons in sympathetic ganglia provides an anatomical basis for differential reflex responses in the two populations of nerves.

Differential renal and splenic nerve responses to vagal and spinal afferent inputs.

Specific contributions of the kidney and spleen to cardiovascular homeostasis may be determined partially by differential sympathetic influences on each organ. This investigation was a comparison of renal and splenic sympathetic responses to stimulation of vagally innervated cardiac receptors and receptors of abdominal visceral and muscle spinal afferent nerves. Experiments were performed in anesthetized, sinoaortic-denervated cats in which upper thoracic sympathetic ganglia had been removed. Left atrial injections of veratridine depressed renal and splenic nerve discharges by 68 and 44%, respectively. In contrast, injections of bradykinin into the abdominal aorta caused 18 and 104% excitation of renal and splenic nerves, respectively. Visceral ischemia and mesenteric stretch by snare occlusion of the celiac artery caused 20 and 64% excitation of renal and splenic nerves, respectively. Left atrial injections of bradykinin caused biphasic renal nerve responses (range, 200% excitation to complete inhibition) and 246% excitation of splenic nerves; after vagotomy both renal and splenic nerves were excited (21 and 117%, respectively). In conclusion, sympathetic control of the kidney and spleen can be selective, illustrating significant potential discreteness of sympathetic outflow to the viscera.

Multisegmental spinal sympathetic reflexes originating from the heart.

Activation of cardiac sympathetic afferent nerves can initiate excitatory cardiocardiac reflexes through pathways that are exclusively spinal. In addition, stimulation of the same nerves also causes lower thoracic and lumbar sympathetic excitation, but the contribution of spinal pathways to these reflexes is unknown. Therefore experiments were performed to compare cardiac, splenic, and renal sympathetic responses to cardiac sympathetic afferent stimulation before and after cervical spinal cord transection in anesthetized, vagotomized, sinoaortic-denervated cats. Electrical stimulation of afferent cardiac sympathetic nerves produced excitatory responses in cardiac and renal nerves before transection but only cardiac nerve responses after transection. In contrast, afferent stimulation by epicardially applied bradykinin excited cardiac, renal, and splenic nerves before and after cord transection. Splenic nerve responses were greater than renal nerve responses in intact and spinal cats. These results demonstrate that spinal reflexes initiated by activation of cardiac sympathetic afferent nerves are not limited to cardiocardiac pathways. The similarity of patterns of responses in intact and spinal cats suggests that spinal pathways contribute significantly to the reflex excitation observed in intact animals.

Widespread neural excitation initiated from cardiac spinal afferent nerves.

Reflex effects of cardiac sympathetic afferent nerve excitation on cardiac and renal components of sympathetic outflow have been well investigated, but the extent of these cardiac afferent influences on other sympathetic nerves or on respiratory or cortical neural systems is unknown. Therefore such influences were investigated electrophysiologically in anesthetized, vagotomized, baroreceptor-denervated cats. Stimulation of cardiac sympathetic spinal afferent neurons by the noxious substance bradykinin caused excitation of cardiac, renal, splenic, gastrohepatic, adrenal, and deep peroneal (muscle) vasoconstrictor sympathetic nerves, as well as excitation of phrenic nerves and concomitant desynchronization of the electroencephalogram. Possible supraspinal pathways mediating these responses were investigated. Sympathetic reflexes caused by cardiac afferent stimulation were unchanged after decerebration, thereby demonstrating that supramedullary mediation was not essential to the sympathetic responses. Potential contributions of the medulla to the observed sympathetic or other responses were demonstrated by recording from medullary neurons responsive to electrical and chemical stimulation of the afferent nerves. In summary, noxious stimulation of cardiac sympathetic afferent neurons leads to widespread neural excitation which may contribute to sensory, visceral, and somatic responses caused by cardiac pain or which occur during activated states such as exercise or emotional stress.

Excitation of cardiac sympathetic afferent nerves: effects on renal function.

Cardiac sympathetic afferent nerves can reflexly alter renal efferent nerve activity during myocardial ischemia and in response to mechanical or chemical stimulation of cardiac receptors. They also may influence renal excretion of water and electrolytes; however, this potential influence on renal function has not been determined. Therefore, receptors of cardiac sympathetic afferent nerves were chemically stimulated by epicardial application of bradykinin to determine effects on renal function. Experiments were performed in anesthetized dogs in which cervical vagosympathetic trunks were severed and common carotid arteries were tied to diminish influences of arterial baroreceptors and vagal afferent nerves. Chemical stimulation of cardiac afferent neurons excited renal nerve activity and produced decreases in urine flow rate, glomerular filtration rate, and excretion of sodium and potassium. In contrast, no consistent changes in renal function were observed in control dogs, which did not undergo cardiac afferent stimulation. These data provide evidence that activation of cardiac sympathetic afferent neurons can lead to alterations in excretion of water and electrolytes as well as changes in renal nerve activity.

Contrasting reflex influences of cardiac afferent nerves during coronary occlusion.

Afferent neurons contained within cardiac sympathetic nerves may have important influences on the circulation when activated during myocardial ischemia. Although such activation is known to reflexly excite upper thoracic sympathetic efferent neurons, effects on other components of sympathetic outflow are unknown. Therefore, cardiac sympathetic afferent nerves were stimulated by occlusion of coronary arteries to investigate their reflex influences on renal sympathetic nerve activity and systemic arterial blood pressure. Responses were observed in anesthetized cats in which sympathetic and/or vagal cardiac afferent nerves remained intact and arterial baroreceptors remained intact or had been denervated. Stimulating sympathetic afferent neurons caused excitation of renal nerve activity, which was accompanied by variable changes in arterial pressure. Stimulation of vagal afferents by coronary occlusion consistently produced inhibition of renal nerve activity and marked depressor responses. When both components of cardiac innervation remained intact, increases or decreases in renal nerve activity and blood pressure were elicited by coronary artery occlusion in the presence or absence of arterial baroreceptors. These results illustrate that cardiac sympathetic afferent nerves can contribute significantly to cardiovascular control during myocardial ischemia.