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

Differential Sympathetic Vasomotor Activation Induced by Liver Cirrhosis in Rats

We tested the hypothesis that there is a topographical sympathetic activation in rats submitted to experimental cirrhosis. Baseline renal (rSNA) and splanchnic (sSNA) sympathetic nerve activities were evaluated in anesthetized rats. In addition, we evaluated main arterial pressure (MAP), heart rate (HR), and baroreceptor reflex sensitivity (BRS). Cirrhotic Wistar rats were obtained by bile duct ligation (BDL). MAP and HR were measured in conscious rats, and cardiac BRS was assessed by changes in blood pressure induced by increasing doses of phenylephrine or sodium nitroprusside. The BRS and baseline for the control of sSNA and rSNA were also evaluated in urethane-anesthetized rats. Cirrhotic rats had increased baseline sSNA (BDL, 102 vs control, 58 spikes/s; p<0.05), but no baseline changes in the rSNA compared to controls. These data were accompanied by increased splanchnic BRS (p<0.05) and decreased cardiac (p<0.05) and renal BRS (p<0.05). Furthermore, BDL rats had reduced basal MAP (BDL, 93 vs control, 101 mmHg; p<0.05) accompanied by increased HR (BDL, 378 vs control, 356; p<0.05). Our data have shown topographical sympathetic activation in rats submitted to experimental cirrhosis. The BDL group had increased baseline sSNA, independent of dysfunction in the BRS and no changes in baseline rSNA. However, an impairment of rSNA and HR control by arterial baroreceptor was noted. We suggest that arterial baroreceptor impairment of rSNA and HR is an early marker of cardiovascular dysfunction related to liver cirrhosis and probably a major mechanism leading to sympathoexcitation in decompensated phase.

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.

Methods of analysis and physiological relevance of rhythms in sympathetic nerve discharge

1. Like virtually all other physiological control systems, the sympathetic nervous system controlling cardiovascular function is characterized by the presence of rhythmic activity. Despite the prevalence of rhythms, their function is often not obvious, which leads to the question, what can one learn about the neural control of autonomic function by studying sympathetic nervous system rhythms? 2. Sympathetic nerve discharge (SND) is characterized by a mixture of periodicities ranging between approximately 0.04 and 10 Hz, depending on the physiological conditions, type of nerve being analysed and the species. The present article illustrates why frequency domain (power density spectral) analysis is more suitable than time domain (autocorrelation) analysis to quantify a complex signal (i.e. one with multiple frequency components) such as SND. 3. The present article entertains the possibilities that rhythmic activity may lead to more effective activation of sympathetic neurons than randomly occurring activity, that rhythmicity is important for coordinating activity in different sympathetic nerves and in formulating complex cardiovascular response patterns and that sympathetic rhythmicity may help maintain homeostasis.

Role of ventrolateral medulla in generating the 10-Hz rhythm in sympathetic nerve discharge

We recorded changes in right inferior cardiac and either left inferior cardiac or left vertebral sympathetic nerve discharge (SND) produced by unilateral microinjections of GABA-A and excitatory amino acid (EAA) receptor antagonists into the ventrolateral medulla (VLM) of urethane-anesthetized, baroreceptor-denervated cats. Unilateral microinjections of GABA-A receptor antagonists, SR-95531 or bicuculline, into single tracks in VLM anywhere between 1 and 5 mm rostral to the obex eliminated or markedly reduced 10-Hz power in SND on both sides of the body. Low-frequency components (<6 Hz) of SND were unaffected. Complete blockade of the 10-Hz rhythm occurred with a dose of SR-95531 as low as 6.25 pmol in a 50-nl volume. Unilateral microinjections of the nonselective EAA receptor antagonist, kynurenate (KYN; 7.5 nmol), into the caudal or rostral VLM significantly reduced, but did not eliminate, 10-Hz SND ipsilateral to the injection sites, while 10-Hz SND contralateral to the injection sites was not significantly changed. These observations suggest that 1) GABAergic transmission in VLM is critical for generation of the 10-Hz rhythm, 2) the caudal and rostral portions of VLM act together to generate the 10-Hz rhythm, and 3) 10-Hz rhythm generation depends, at least in part, on tonic or phasic excitatory drive to GABAergic interneurons in caudal VLM and presympathetic neurons in rostral VLM. The data also suggest that pathways interconnecting the two halves of the brain stem play an important role in promoting 10-Hz rhythm generation.

Cardiovascular effects of noradrenaline microinjected into the dorsal periaqueductal gray area of unanaesthetized rats

The periaqueductal grey area (PAG) is a mesencephalic region that is involved in the modulation of cardiovascular changes associated with behavioural responses. Among the neurotransmitters present in the PAG, noradrenaline (NA) is also known to be involved in central nervous system cardiovascular regulation. In the present study we report the cardiovascular effects of the microinjection of NA into the dorsal portion of the PAG (dPAG) of unanaesthetized rats and the peripheral mechanism involved in their mediation. Injection of NA in the dPAG of unanaesthetized rats evoked a dose-dependent pressor response accompanied by bradycardia. The magnitude of the pressor responses was higher at more rostral sites in the dPAG and decreased when NA was injected into the caudal portion of the dPAG. The responses to NA were markedly reduced in urethane-anaesthetized rats. The pressor response was potentiated by i.v. pretreatment with the ganglion blocker pentolinium and blocked by i.v. pretreatment with the vasopressin antagonist dTyr(CH2)5(Me)AVP. The results suggest that activation of noradrenergic receptors within the dPAG can evoke pressor responses, which are mediated by acute vasopressin release.

An unusual form of phase walk in a system of coupled oscillators

The authors describe an unusual form of phase walk (i.e., a progressive change in phase angle between coupled oscillators) using the 10-Hz rhythmic discharges of the inferior cardiac and vertebral postganglionic sympathetic nerves (CN and VN, respectively) in hypercapnic, baroreceptor-denervated, and vagotomized cats anesthetized with urethane. Unlike phase walk ascribable to weakened coupling (desynchronization of oscillators), the phase walk of VN 10- Hz activity relative to CN10-Hz activity 1) recurred on the time scale of the respiratory cycle, 2) was bidirectional with CN-VN phase angle increasing during expiration and decreasing during inspiration, and 3) occurred over a range equivalent to one-half the period of the 10-Hz rhythm rather than a full cycle. Moreover, this form of phase walk occurred during strong coupling of the 10-Hz oscillators, as reflected by CN-VN coherence values approaching 1.0. The authors propose that the bidirectional phase walk reflects a state of strong coupling of the 10-Hz oscillators controlling the CN and VN, the angle of which is reset from cycle to cycle by the continuously changing level of activity in their respiratory inputs. In addition, the data demonstrate that frequency and amplitude modulation of sympathetic nerve discharge can be independently regulated by respiratory inputs.

Cardiovascular variability after arousal from sleep: time-varying spectral analysis

We performed time-varying spectral analyses of heart rate variability (HRV) and blood pressure variability (BPV) recorded from 16 normal humans during acoustically induced arousals from sleep. Time-varying autoregressive modeling was employed to estimate the time courses of high-frequency HRV power, low-frequency HRV power, the ratio between low-frequency and high-frequency HRV power, and low-frequency power of systolic BPV. To delineate the influence of respiration on HRV, we also computed respiratory airflow high-frequency power, the modified ratio of low-frequency to high-frequency HRV power, and the average transfer gain between respiration and heart rate. During cortical arousal, muscle sympathetic nerve activity and heart rate increased and returned rapidly to baseline, but systolic blood pressure, the ratio between low-frequency and high-frequency HRV power, low-frequency HRV power, the modified ratio of low-frequency to high-frequency HRV power, and low-frequency power of systolic BPV displayed increases that remained above baseline up to 40 s after arousal. High-frequency HRV power and airflow high-frequency power showed concommitant decreases to levels below baseline, whereas the average transfer gain between respiration and heart rate remained unchanged. These findings suggest that 1) arousal-induced changes in parasympathetic activity are strongly coupled to respiratory pattern and 2) the sympathoexcitatory cardiovascular effects of arousal are relatively long lasting and may accumulate if repetitive arousals occur in close succession.