The mechanisms of differential control in the sympathetic system studied by hypothalamic stimulation

Differential control of sympathetic outflow

With advances in experimental techniques, the early views of the sympathetic nervous system as a monolithic effector activated globally in situations requiring a rapid and aggressive response to life-threatening danger have been eclipsed by an organizational model featuring an extensive array of functionally specific output channels that can be simultaneously activated or inhibited in combinations that result in the patterns of autonomic activity supporting behavior and mediating homeostatic reflexes. With this perspective, the defense response is but one of the many activational states of the central autonomic network. This review summarizes evidence for the existence of tissue-specific sympathetic output pathways, which are likely to include distinct populations of premotor neurons whose target specificity could be assessed using the functional fingerprints developed from characterizations of postganglionic efferents to known targets. The differential responses in sympathetic outflows to stimulation of reflex inputs suggest that the circuits regulating the activity of sympathetic premotor neurons must have parallel access to groups of premotor neurons controlling different functions but that these connections vary in their ability to influence different sympathetic outputs. Understanding the structural and physiological substrates antecedent to premotor neurons that mediate the differential control of sympathetic outflows, including those to noncardiovascular targets, represents a challenge to our current technical and analytic approaches.

Sympathetic nerve and cardiovascular responses to chemical activation of the midbrain defense region

The changes in mean arterial pressure (MAP), renal (RBF) and femoral (FBF) blood flows, and inferior cardiac (CN) and vertebral nerve (VN) sympathetic nerve discharges (SND) produced by chemical activation (d,l-homocysteic acid) of the midbrain periaqueductal gray (PAG) were compared in baroreceptor-denervated and -innervated cats anesthetized with urethan. Defenselike cardiovascular responses in both states were similar in magnitude and consisted of increased MAP and FBF and decreased RBF; however, the nerve responses differed. In baroreceptor-denervated cats, PAG activation increased CN 10-Hz activity, decreased VN 10-Hz activity, and lengthened the CN-VN phase angle. In baroreceptor-innervated cats in which the rhythm in SND was cardiac related, PAG activation increased CN activity, but VN activity and the CN-VN phase angle were unchanged. These results demonstrate that chemical activation of PAG neurons induces differential patterns of sympathetic outflow generally consistent with accompanying defenselike cardiovascular responses. However, the mechanisms responsible for the changes in 10-Hz and cardiac-related SND appear to be different.

Differential patterns of spinal sympathetic outflow involving a 10-Hz rhythm

Time and frequency domain analyses were used to examine the changes in the relationships between the discharges of the inferior cardiac (CN) and vertebral (VN) postganglionic sympathetic nerves produced by electrical activation of the midbrain periaqueductal gray (PAG) in urethan-anesthetized, baroreceptor-denervated cats. CN-VN coherence and phase angle in the 10-Hz band served as measures of the coupling of the central oscillators controlling these nerves. The 10-Hz rhythm in CN and VN discharges was entrained 1:1 to electrical stimuli applied to the PAG at frequencies between 7 and 12 Hz. CN 10-Hz discharges were increased, and VN 10-Hz discharges were decreased when the frequency of PAG stimulation was equal to or above that of the free-running rhythm. In contrast, stimulation of the same PAG sites at lower frequencies increased, albeit disproportionately, the 10-Hz discharges of both nerves. In either case, PAG stimulation significantly increased the phase angle between the two signals (VN 10-Hz activity lagged CN activity); coherence values relating their discharges were little affected. However, the increase in phase angle was significantly more pronounced when the 10-Hz discharges of the two nerves were reciprocally affected. Importantly, partialization of the phase spectrum using the PAG stimuli did not reverse the change in CN-VN phase angle. This observation suggests that the increase in the CN-VN phase angle reflected changes in the phase relations between coupled oscillators in the brain stem rather than the difference in conduction times to the two nerves from the site of PAG stimulation. In contrast to the effects elicited by PAG stimulation, stimulation of the medullary lateral tegmental field induced uniform increases in the 10-Hz discharges of the two nerves and no change in the CN-VN phase angle. Our results demonstrate that changes in the phase relations among coupled brain stem 10-Hz oscillators are accompanied by differential patterns of spinal sympathetic outflow. The reciprocal changes in CN and VN discharges produced by PAG stimulation are consistent with the pattern of spinal sympathetic outflow expected during the defense reaction.

Organization of the sympathetic innervation of the forelimb resistance vessels in the cat

Detailed information on the outflow pathway of sympathetic vasoconstrictor fibers to the upper extremity is lacking. We studied the organization of the sympathetic innervation of the forelimb resistance vessels and of the sinoatrial (SA) node in the decerebrated, artificially respirated cat. The distal portion of sectioned individual rami T1-8 and the sympathetic chain immediately caudal to T8 on the right side were electrically stimulated while the right forelimb perfusion pressure (forelimb perfused at constant flow) and heart rate were recorded. Increases in perfusion pressure were evoked by stimulation of T2-8 (maximal response T7: 55 +/- 2.3 mm Hg). Responses were still evoked by stimulation of the sympathetic chain immediately caudal to T8 (44 +/- 15 mm Hg). Increases in heart rate were evoked by the stimulation of more rostral rami (T1-5; maximal response T3: 55.2 +/- 8 bpm). These vasoconstrictor and cardioacceleratory responses were blocked by the cholinergic antagonists hexamethonium and scopolamine. Sectioning of the vertebral nerve and the T1 ramus abolished the vasoconstrictor response. Stimulation of the vertebral nerve and of the proximal portion of the sectioned T1 ramus increased perfusion pressure (69 +/- 9 and 34 +/- 14 mm Hg, respectively), which was unaffected by ganglionic cholinergic block. These data suggest that forelimb resistance vessel control is subserved by sympathetic preganglionic neurons located mainly in the middle to caudal thoracic spinal segments. Some of the postganglionic axons subserving vasomotor function course through the T1 ramus, in addition to the vertebral nerve.

IMPLICATIONS

Forelimb vasculature is controlled by sympathetic preganglionic neurons located in middle to caudal thoracic spinal segments and by postganglionic axons carried in the T1 ramus and vertebral nerve. This helps to provide the anatomical substrate of interruption of sympathetic outflow to the upper extremity produced by major conduction anesthesia of the stellate ganglion or spinal cord.

Organ and development related difference in tissue norepinephrine concentrations in Dahl rats

To determine organ and development related differences in tissue norepinephrine concentration (tNE) in Dahl salt-sensitive (S) and -resistant (R) rats, we measured the tNE of 16 organs, including the heart (left ventricle), kidney, cerebrum, brain stem, stomach, jejunum, ileum, colon, spleen, pancreas, liver, aorta, lung, bone, salivary gland, and muscle, at 1, 3, 5, 7, 9, 11 weeks old. Large differences were found in tNE among the organs of both S and R rats, ranging from 4.0 +/- 1.1 ng/g tissue (the bone of S rats) to 1234.8 +/- 32.5 ng/g tissue (the salivary gland of R rats). tNE in R rats increased development-dependently in 12 of 16 organs, but did not significantly change in the other three organs, and decreased in the bone. On the other hand, the development-dependent increase in tNE was suppressed in S rats, and the tNE values of S rats were significantly lower than those of R rats in 14 of 16 organs. To eliminate the baroreflexive effects on tNE, another group of 5-week-old S and R rats were subjected to sinoaortic denervation (SAD) or the sham operation. The tNE was measured in 10 organs in these animals at 9 weeks old. SAD did not alter the tNE in most of the organs in both S and R rats. There was no significant differences in mean arterial pressure (MAP) between S and R rats with baroreceptor intact at 9 weeks old. SAD slightly but significantly increased MAP in S rats, whereas not in R rats. There was no significant differences in plasma NE concentration (pNE) between S and R rats with the baroreceptor intact. SAD did not alter pNE in S or R rats. These results demonstrate that variations of the tNE were dependent on the organ and development. Many organs of S rats had lower tNE than those of R rats. The developmental-dependent increases in tNE in S rats were suppressed, compared with those in R rats. These tNE behaviors in S rats may not be related to blood pressure or baroreflex sensitivity, but might be involved in an abnormal sympathetic nerve activity.

Increased firing of neurons in the posterior hypothalamus which precede classically conditioned pupillary dilations

Paralyzed cats were used as subjects in a classical conditioning experiment where each subject was exposed to 40 explicitly unpaired 1-s bursts of white noise and 0.5-s paw shocks. This training was followed by 60 trials of the two stimuli paired, where the white noise immediately preceded the paw shock. Following this training, the subjects were re-exposed to 40 trials of the explicitly unpaired procedure. The pupil was monitored as the behavior and electrodes implanted in the thalamus, the dorsal hypothalamus and the posterior hypothalamus recorded the activity of clusters of cells. Only the cells in the posterior hypothalamus showed robust changes in firing rates that preceded the pupillary behavior, both (a) on any particular trial and (b) as the learned association was being demonstrated behaviorally across trials.

Changes in tissue blood flow and sympathetic activities to various organs during prolonged hemorrhagic hypotension in monkeys

This experiment was designed to determine whether prolonged hemorrhagic hypotension in anesthetized monkeys produces differential control of tissue blood flow and sympathetic nerve activities to various organs (heart, kidney, liver, spleen, and hind-limb). We performed simultaneous multifiber recording of sympathetic nerve activity to the kidney (RNA), heart (CNA), spleen (SpNA), liver (HNA), and hind-limb (LNA) during sustained hemorrhagic hypotension at a mean blood pressure of 40 mmHg for 2 h. Immediately after bleeding, all of the sympathetic nerve activities increased significantly (Stage I) and then gradually decreased towards the prebleeding levels (Stage II). Thereafter, the secondary sympathetic excitation was observed (Stage III), followed by a gradual decrease in sympathetic activities below the prebleeding levels (Stage IV). The shed blood started to return to the animals at this final stage. Time course of changes in sympathetic nerve activities did not differ among organs innervated. However, tissue blood flow of the renal cortex, liver, skeletal muscle and spleen significantly decreased at Stage I and remained at low levels until the end of the experimental period. In contrast, blood flow of the renal medulla and heart was preserved until Stage III and Stage IV, respectively. These results indicate that although the sympathetic response to prolonged hemorrhagic hypotension of 40 mmHg did not differ among organs, changes in tissue blood flow were variable and blood flow to the heart and renal medulla was maintained at a steady level until a late stage of hemorrhage.

Reduced heart rate variability after right-sided stroke

BACKGROUND AND PURPOSE

Recently, asymmetries have been demonstrated in skin sudomotor and vasomotor function after unilateral cerebral lesions. The present study was performed to determine whether other bedside tests reflecting sympathetic and parasympathetic cardiovascular functions would reveal differences with respect to the side of cerebrovascular lesions.

METHODS

Heart rate variability during deep breathing as well as blood pressure and heart rate changes during tilt and isometric handgrip was measured in a group of patients with a monofocal stroke and compared with similar data from age-matched patients with transient ischemic attack and healthy control subjects.

RESULTS

Compared with left-sided stroke and with the control subjects, stroke location on the right side was associated with a reduced respiratory heart rate variability (P > .01), a reflex mainly under parasympathetic control. In contrast, reflexes mainly reflecting peripheral sympathetic function were equal for right- and left-sided lesions.

CONCLUSIONS

Since an imbalance in cardiac autonomic innervation may be crucial for the generation of cardiac arrhythmias and since reduced heart rate variability has been associated with increased mortality, the findings suggest that the risk of sudden death may be correlated with lateralization and location of the brain infarct after stroke.

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.

Peripheral venous catecholamines versus adrenal secretory rates after brain stem stimulation in cats

The relationship between adrenal catecholamine secretion and peripheral venous catecholamine concentration was assessed in samples collected before and after brief (15-s) periods of brain stem electrical stimulation in cats anesthetized with alpha-chloralose-urethan. Adrenal blood flow from the left lumboadrenal vein averaged 0.75 +/- 0.05 ml/min (means +/- SE, n = 56). Peripheral norepinephrine (NE) concentration (1.19 +/- 0.07 ng/ml) was not well correlated with adrenal NE secretion (38.18 +/- 3.99 ng/min) before stimulation (r = 0.334, P less than 0.025). Although brain stem sites that evoked large increases in adrenal NE secretion often caused an increase in peripheral NE, sites that evoked significant decreases in adrenal NE secretion were not accompanied by decreases in peripheral NE. Peripheral epinephrine (E) concentration (0.11 +/- 0.01 ng/ml) was well correlated with adrenal E secretion (23.72 +/- 2.33 ng/min) before stimulation (r = 0.468, P less than 0.001). Brain stem stimulation-evoked changes in adrenal E secretion were generally reflected in changes of peripheral E concentration, although many exceptions were seen. Peripheral dopamine (DA) concentration (0.23 +/- 0.02 ng/ml) was well correlated with adrenal DA secretion (0.94 +/- 0.11 ng/min) before stimulation (r = 0.505, P less than 0.001). However, stimulus-evoked changes in adrenal DA secretion were not reflected in changes of peripheral DA concentration. The data indicate that, whereas brief periods of brain stem stimulation evoke a wide range of adrenal secretory responses, peripheral catecholamine responses best reflected adrenal secretion only when large increases in adrenal secretion were evoked and were poor indicators of decreased adrenal catecholamine secretion.(ABSTRACT TRUNCATED AT 250 WORDS)

Plasma catecholamines associated with hypothalamically-elicited defense behavior

Sympatho-adrenal (SA) activation was determined by measuring levels of norepinephrine (NE) and epinephrine (E) in bilateral adrenal venous and peripheral venous plasma of 20 anesthetized cats following stimulation of medial hypothalamic sites. Hypothalamic sites were selected that elicited affective defense behavior in the freely moving cat. Fifty-eight percent of these hypothalamic sites elicited a bilateral increase greater than or equal to 10 ng/min in the output of both adrenal catecholamines (CAs); these increases were greater from the gland ipsilateral to the side of stimulation. Other SA responses included both preferential increases or decreases in either NE or E. Under baseline conditions, an average of 67% of the NE in the peripheral venous plasma was contributed by the sympathetic noradrenergic nerves; hypothalamic stimulation at "defense" sites increased the contribution to 75%. The data suggest that hypothalamic regions that elicit defense behaviour may overlap with regions that activate the adrenal medullary and sympathetic nervous systems.

Specificity of spinal projections from hypothalamic and brainstem areas which innervate sympathetic preganglionic neurons

The specificity and topographic organization of afferent projections to the intermediolateral column (IML) were examined using retrograde transport of fluorescent tracers injected into pairs of thoracic spinal segments. Neurons within the hypothalamus (parvocellular paraventricular nucleus, dorsomedial nucleus and lateral hypothalamus), pons (Kolliker-Fuse and A5 nuclei) and medulla (ventrolateral nucleus of the solitary tract and rostral ventrolateral medulla) each appeared to innervate only a single spinal segment. Neurons in each cell group projecting to different spinal segments were intermixed and showed no evidence of topographic organization. These results provide a potential anatomical substrate for organ-specific autonomic responses to physiological and psychological stimuli.

Control of reciprocal and non-reciprocal action of vagal and sympathetic efferents: study of centrally induced reactions

The mechanism of control of reciprocal and non-reciprocal action between parasympathetic and sympathetic nerves was investigated. Simultaneous recordings were made from both vagal and sympathetic nerves innervating the heart of responses evoked by hypothalamic stimulation in chloralose-anesthetized dogs. Four patterns of responses could be elicited by repetitive stimulations of various sites within the hypothalamus: (1) A reciprocal pattern of cardiac vagal and sympathetic discharges accompanied by a rise in blood pressure, heart rate and a 2 to 8-fold increase in muscle blood flow. The vagal activity completely ceased, while sympathetic discharges were greatly augmented. These changes occurred quickly and often lasted throughout the stimulation, preventing baroreceptor reflex from breaking through. This pattern is similar to the cardiovascular component of the "defense reaction". (2) A reciprocal pattern of discharges accompanied by a depressor response and bradycardia; the vagal discharges increased while those of the sympathetic efferents decreased. (3) A non-reciprocal pattern of response in which activity of the two efferents increased. The blood pressure was elevated and heart rate decreased. (4) A non-reciprocal pattern of action of the two efferents in which discharges of both nerves were depressed; the blood pressure decreased and heart rate increased. Factors affecting these responses patterns were found to be: (a) secondarily occurring reflex responses due to baroreceptor activation or deactivation. These baroreceptor-related reactions always showed reciprocal changes between vagal and sympathetic nerve activity; (b) the level of tonic activity of the autonomic nerves, e.g. during a high level of tonic activity inhibitory action became less effective while the excitatory effect was greater. Thus central states maintaining influenced the pattern of reaction and the relationship between activity in these two efferent nerves. Single pulse stimulations of the hypothalamic regions from which different patterns were evoked, when stimulated repetitively, yielded patterns which were basically reciprocal and biphasic; in vagus efferents an inhibitory phase was followed by an excitation of tonic activity, while in sympathetic an excitation was followed by an inhibition. The degree and duration of these excitatory and inhibitory phases differed, depending on the site of stimulation, but only to a minor degree, and the basic pattern remained unchanged. The implication of these findings is that the hypothalamus can organized both reciprocal and non-reciprocal patterns of activity in the two autonomic limbs in response to varied afferent stimuli. Probably only subtle changes in the central states or influences on neurons of the autonomic system suffice to result in different response pattern. No pattern of response is fixed within the central control complex.