Enhancement of resting activity in postganglionic vasoconstrictor neurones following short-lasting repetitive activation of preganglionic axons

Resting activity in postganglionic neurones supplying the cat hindlimb was enhanced after activation of preganglionic axons in the lumbar sympathetic trunk with short trains of stimuli (50 stimuli at 25 Hz). The characteristics of this enhancement were as follows: 1) It has values of 120 to 600% and lasts for 4 to more than 40 min. 2) It can be elicited heterosynaptically by repetitive stimulation of the peripheral end of a cut white ramus. 3) It is dependent on the activation of thin, probably unmyelinated, preganglionic axons. 4) It is probably produced by non-nicotinic (muscarinic and non-cholinergic) synaptic mechanisms in the sympathetic chain ganglia. 5) Vasoconstrictor neurones can be activated in this way, but not sudomotor neurones. The results argue that nicotinic transmission of activity from pre- to postganglionic vasoconstrictor neurones can be modulated heterosynaptically by slow non-nicotinic synaptic processes.     

Cardiac and respiratory rhythmicities in cutaneous and muscle vasoconstrictor neurones to the cat's hindlimb

Summary Cardiac and respiratory rhythmicities have been investigated quantitatively in postganglionic vasoconstrictor neurones supplying skeletal muscle and skin of the hindlimb in chloralose anaesthetized, immobilized cats. Both rhythmicities are largest in muscle vasoconstrictor neurones, smaller in vasoconstrictor neurones supplying hairy skin, and smallest in vasoconstrictor neurones supplying hairless skin. The magnitude of the cardiac rhythmicity in the vasoconstrictor neurones is positively correlated with the quantitative reaction to systemic hypoxia.     

The convergence of sympathetic preganglionic fibres on vasomotor neurones of the ganglia in lumbar sympathetic chain

By means of intracellular recordings from the neurones of isolated L₃–L₆ ganglia it was shown that B₁-, B₂- and C-preganglionic fibres widely converge on these neurones. Besides, a part of the ganglionic neurones is activated by slow-conducting C-fibres coming into the ganglia through the grey rami. Identified vasomotor neurones have the distinctive feature of preganglionic fibre convergence. Only B₂- and C-preganglionic fibres were able to activate them, but the efficacy of the B₂-fibre excitatory action is considerably higher.     

Activation and inhibition of muscle and cutaneous postganglionic neurones to hindlimb during hypothalamically induced vasoconstriction and atropine-sensitive vasodilation

Summary Subcutaneous adipose tissue blood flow (ATBF) was examined in 8 subjects during 6 h exercise on a bicycle ergometer. The initial work load was 118 W corresponding to about 50% of maximal work capacity. The oxygen uptake increased from 0.26 l ·min^–1 at rest to about 1.6l·min^–1 during work. In 7 subjects ATBF increased, in 1 it remained constant. After 3 h exercise ATBF at an average reached values 3–4 times the control value. This increase was maintained for the remaining work periods. The increase was significant at the 5% level. Plasma free fatty acids increased 7-, plasma glycerol 10-fold during work.     

Reflex activation of postganglionic vasoconstrictor neurones supplying skeletal muscle by stimulation of arterial chemoreceptors via non-nicotinic synaptic mechanisms in sympathetic ganglia

Postganglionic sympathetic neurones supplying skeletal muscle and skin can be activated from the preganglionic site via cholinergic nicotinic, muscarinic and noncholinergic synaptic mechanisms. The experiments described in this paper were designed in order to show that postganglionic vasoconstrictor neurones supplying skeletal muscle can be activated by the naturally occurring discharge pattern in preganglionic axons when the nicotinic transmission is blocked. For this purpose, the activity was recorded simultaneously from postganglionic vasoconstrictor axons supplying skeletal muscle and vasoconstrictor axons supplying hairy skin. The preganglionic neurones were driven reflexly by stimulation of the arterial chemoreceptors. 1) During blockade of nicotinic transmission muscle vasoconstrictor neurones were activated via the CNS during stimulation of arterial chemoreceptors. This activation is either generated by muscarinic action of released acetylcholine or by a noncholinergic synaptic mechanism. 2) Postganglionic cutaneous vasoconstrictor neurones were inhibited during stimulation of arterial chemoreceptors. During blockade of cholinergic nicotinic transmission these neurones were not activated reflexly by stimulation of arterial chemoreceptors although they received inputs via cholinergic muscarinic and noncholinergic synaptic mechanisms. 3) The results illustrate that postganglionic vasoconstrictor neurones supplying skeletal muscle can not only be activated via non-nicotinic synaptic mechanisms through synchronous repetitive electrical stimulation of preganglionic axons but also by the discharge pattern produced in preganglionic neurones during stimulation of arterial chemoreceptors.     

Reflex changes in post- and preganglionic sympathetic adrenal nerve activity and postganglionic sympathetic renal nerve activity upon arterial baroreceptor activation and during severe haemorrhage in the rat

Carlsson , S., Skarphedinsson , J. O., Delle , M., Hoffman , P. & Thorén , P. 1992. Reflex changes in post- and preganglionic sympathetic nerve activity and postganglionic sympathetic renal nerve activity upon arterial baroreceptor activation and during severe haemorrhage in the rat. Acta Physiol Scand 144 , 317–323. Received 19 August 1 991 , accepted 18 November 1991. ISSN 0001–6772. Department of Physiology, University of Göteborg, Sweden and Department of Physiology, University of Iceland, Reykjavik, Iceland. The aim of the study was to compare pre- (pre-aSNA) and postganglionic adrenal sympathetic nerve activity (post-aSNA) and postganglionic renal sympathetic nerve activity (rSNA) in rats during arterial baroreceptor activation and haemorrhage. Adrenal multifibre nerve activity was recorded in chloralose-anaesthetized Wistar rats. To test for pre-aSNA or post-aSNA in adrenal nerves, a ganglionic blocker, trimethaphan (10 mg kg^-1), was administered i.v. If the nerve activity in the adrenal nerve decreased or increased the nerve was considered to contain predominantly post-or preganglionic fibres, respectively. In contrast, the renal nerves exhibit an almost pure postganglionic activity. Baroreceptor activity was tested by activation of baroreceptors, with an a-receptor agonist, phenylephrine, which was slowly infused (0.5–2 mUg kg^-1 min-^l), and to deactivate the baroreceptors the rats were bled down to 50 mmHg for 8 min. The experiments showed that all tested nerve types were baroreceptor dependent. There were no significant differences between the slopes relating nerve activity inhibition to increase in blood pressure (infusion of phenylephrine). During maximal inhibition there was a difference between the rSNA and pre-aSNA, 87 ± 4%, n= 6 , and 68±6%, n = 10 (P < 0.01) of the control value, respectively. The maximal inhibition of post-aSNA was 80 ± 3%, n= 7 , of the control value. During haemorrhage there was a difference between the nerve populations. Pre-aSNA responded with a marked increase within 1.5 rnin (159 ± 29% of control, n= 7) and was then maintained at that level until retransfusion. Post-aSNA responded with i transient increase, reaching 158 ± 19% of control after 1.5min but then decreased slowly during the next 3 min, reaching a value of 89 8 76 at 7.5 rnin of haemorrhage. Finally, rSNA (n = 7) responded with an initial increase (peak value 1.5 min 138 ± 16%) followed by an inhibition to a minimum of 74±12% of control. Our conclusion is that the postganglionic adrenal nerves respond in a similar way to postganglionic renal nerves during baroreceptor activation and also show the same pattern of response during haemorrhage. This might indicate that postganglionic fibres to the adrenal gland and postganglionic renal fibres to the kidney have the same target structures, i.e. blood vessels. and postganglionic adrenal fibres might therefore be of importance in adrenal blood flow regulation.     

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.     

The effect of previous transection on quantitative estimates of the preganglionic neurones projecting in the cervical sympathetic trunk of the guinea-pig and the cat made by retrograde labelling of damaged axons by horseradish peroxidase

The numbers of sympathetic preganglionic neurones in the upper thoracic spinal cord retrogradely labelled after application of horseradish peroxidase to the severed axons of the cervical sympathetic trunk have been determined in guinea-pigs and in cats. Neuronal counts were made on both sides of spinal segments C8 to T10 in control animals and in others in which the enzyme was applied 5–8 days after the left cervical sympathetic trunk had been transected. Labelling was not significantly different between sides of unoperated animals, but after previous cervical trunk section labelling was always less, by from 5 to 100% of that on the control side. The results were not significantly different if the application of horseradish peroxidase was made close to the original lesion or 1 cm more proximal along the nerve trunk. Analysis of the dimensions of labelled cells suggested that the deficit in labelling was greatest for small preganglionic neurones.These findings do not support an earlier report¹⁴ that retrograde labelling from regenerating axons is enhanced at this stage after axotomy, and the possible reasons for the discrepancy in results are discussed. It is suggested that labelling of cell bodies may not be restricted if intact axon sprouts are exposed to horseradish peroxidase. Reduced labelling from axons at the time of a second transection might, at least in part, be due to axon atrophy, emphasizing limitations to labelling when horseradish peroxidase is applied to severed axons of fine diameter.The high numbers and reproducibility of our control data enabled estimates to be made of the segmental distribution of the cell bodies of origin of the axons of the cervical sympathetic trunk in both species.     

Arborization pattern of sympathetic preganglionic axons in the rat superior cervical and stellate ganglia

Anterograde labeling technique with Phaseolus Vulgaris leucoagglutinin (PHA-L) was employed to observe how a single preganglionic axon arborizes in the superior cervical ganglion (SCG) and stellate ganglion (STG) of rats. PHA-L was injected into the intermediolateral nucleus of the spinal cord at the middle point between segments T1 and T2, and labeled axons were detected immunohistochemically in serial sections. We traced and drew three preganglionic axons over their full length in the SCG and STG. In SCG, the labeled axons bifurcated repeatedly and extended to a length of 600–700 μm in the rostrocaudal direction, and about 200 μm in the transverse direction. These three preganglionic axons made 11, 14 and 11 dense terminal plexus regions along their trajectory. The pattern of the most dense terminal plexus corresponded to the pericellular type dendritic plexus, one of the plexus patterns of dendritic collaterals of SCG neurons. In the STG, the extent of axonal arborization was more variable than that in the SCG, ranging from 400 to 800 μm in the rostrocaudal direction and about 400 μm in the transverse direction. The three analyzed axons made 21, 19 and 20 dense terminal plexus regions along their trajectory, with a similar pattern to those in SCG. These results indicated that there might be a columnar or ellipsoidal organization of postganglionic neurons which are innervated by single preganglionic axons.     

Reflex changes in post- and preganglionic sympathetic adrenal nerve activity and postganglionic sympathetic renal nerve activity upon arterial baroreceptor activation and during severe haemorrhage in the rat.

The aim of the study was to compare pre- (pre-aSNA) and postganglionic adrenal sympathetic nerve activity (post-aSNA) and postganglionic renal sympathetic nerve activity (rSNA) in rats during arterial baroreceptor activation and haemorrhage. Adrenal multifibre nerve activity was recorded in chloralose-anaesthetized Wistar rats. To test for pre-aSNA or post-aSNA in adrenal nerves, a ganglionic blocker, trimethaphan (10 mg kg-1), was administered i.v. If the nerve activity in the adrenal nerve decreased or increased the nerve was considered to contain predominantly post- or preganglionic fibres, respectively. In contrast, the renal nerves exhibit an almost pure postganglionic activity. Baroreceptor activity was tested by activation of baroreceptors, with an alpha-receptor agonist, phenylephrine, which was slowly infused (0.5-2 micrograms kg-1 min-1), and to deactivate the baroreceptors the rats were bled down to 50 mmHg for 8 min. The experiments showed that all tested nerve types were baroreceptor dependent. There were no significant differences between the slopes relating nerve activity inhibition to increase in blood pressure (infusion of phenylephrine). During maximal inhibition there was a difference between the rSNA and pre-aSNA, 87 +/- 4%, n = 6, and 68 +/- 6%, n = 10 (P less than 0.01) of the control value, respectively. The maximal inhibition of post-aSNA was 80 +/- 3%, n = 7, of the control value. During haemorrhage there was a difference between the nerve populations. Pre-aSNA responded with a marked increase within 1.5 min (159 +/- 29% of control, n = 7) and was then maintained at that level until retransfusion.(ABSTRACT TRUNCATED AT 250 WORDS)     

Respiratory modulation of pre- and postganglionic lumbar vasomotor sympathetic neurons in the rat

The respiratory contribution to the activity of lumbar vasomotor pre- and postganglionic neurons was compared with previously reported respiratory patterns in the activity of neurons in the rostral ventrolateral medulla (RVLM-SE). Two patterns of respiratory modulation were observed characterized by (1) a depression of activity during inspiration and a postinspiratory peak (n = 19), and (2) a peak of activity during inspiration (n = 3). These patterns were similar to those previously reported in RVLM-SE neurons. The latency from the phrenic burst to the onset of respiratory modulation was consistent with the conduction time from the RVLM to the sympathetic chain. This suggests that respiratory modulation observed in lumbar sympathetic activity originates, in part, in RVLM-SE neurons.