Cardiovascular functions of central noradrenergic neurons in rabbits

Altered central catecholaminergic transmission and cardiovascular disease

Numerous studies, some of which date back more than three decades, have established a link between disorders of the cardiovascular system and the catecholaminergic system of the brain. Central noradrenergic (and putative adrenergic) neurones are involved in numerous brain functions, and there appears to be more than one mechanism via which a dysfunction of central nor/adrenergic signalling may be detrimental to the cardiovascular system. Moreover, in some cases, such as essential hypertension, altered noradrenergic transmission could play a causative role. Numerous controversies are evident throughout the literature, which are very difficult to explain without much better understanding of the basic physiology of central noradrenergic transmission. Recently, using a combination of novel molecular, electrochemical and imaging techniques, we have started to unravel how noradrenergic neurones in the brain store and release their transmitter. Targeted long-term modulation of specific noradrenergic cell groups in defined brain areas using viral gene transfer is helping to clarify the links between central catecholamines and cardiovascular control in health and disease. These studies may reveal new therapeutic strategies for various cardiovascular diseases which are accompanied by heightened sympathetic nerve activity.

Sympathetic overactivity in active ulcerative colitis: effects of clonidine

Previous reports suggest that inflammatory bowel diseases may be accompanied by abnormalities in the neural autonomic profile. We tested the hypotheses that 1) an exaggerated sympathetic activity characterizes active ulcerative colitis (UC) and 2) a reduction of sympathetic activity by clonidine would be associated with clinical changes of UC. In 23 patients with UC and 20 controls, muscle sympathetic nerve activity (MSNA), ECG, blood pressure, and respiration were continuously recorded, and plasma catecholamine was evaluated both at rest and during a 75 degrees head-up tilt. Autonomic profile was assessed by MSNA, norepinephrine, epinephrine, spectral markers of low-frequency (LF) cardiac sympathetic (LF(RR); normalized units) and high-frequency (HF) parasympathetic (HF(RR); normalized units) modulation and sympathetic vasomotor control (LF systolic arterial pressure; LF(SAP)), obtained by spectrum analysis of the R-R interval and systolic pressure variability. Among UC patients, 16 agreed to be randomly assigned to 8-wk transdermal clonidine (15 mg/wk, 9 subjects), or placebo (7 patients). An autonomic profile, Disease Activity Index (DAI), and endoscopic pattern were compared before and after clonidine/placebo. At rest, MSNA, heart rate (HR), LF(RR), LF/HF, and LF(SAP) were higher and HF(RR) was lower in patients than in controls. Tilt decreased HF(RR) and increased MSNA and LF(RR) less in patients than in controls. Clonidine decreased HR, MSNA, epinephrine, LF(RR), and increased HF(RR), whereas placebo had no effects. Changes of the autonomic profile after clonidine were associated with reduction of DAI score. An overall increase of sympathetic activity characterized active UC. Normalization of the autonomic profile by clonidine was accompanied by an improvement of the disease.

α-2 Adrenergic Transmission and Human Baroreflex Regulation

We observed earlier that central α-2 adrenoceptor stimulation in mice greatly augments parasympathetic tone. To test the effects in humans, we assessed autonomic vasomotor tone and baroreflex regulation in 9 normal young adults on 2 occasions, once with and once without clonidine. We determined heart rate (HR), beat-by-beat blood pressure (BP), and muscle sympathetic nerve activity. HR variability was analyzed in the time and frequency domain. Pharmacological baroreflex slopes were determined using incremental phenylephrine and nitroprusside infusions. Clonidine lowered resting BP (122±4/73±3 versus 100±7/55±3 mm Hg, P <0.01), muscle sympathetic nerve activity (18±3 versus 4±2 bursts/min, P <0.01), and HR (62±3 versus 56±3 bpm, P <0.05). The baroreflex heart rate curve was reset to much lower HR values and showed no saturation at low HR. HR variability profoundly increased during clonidine plus phenylephrine (total power: 3224±843 versus 8943±2329 ms 2 , P <0.05). High-frequency power was 1451±520 at baseline and 6720±2475 ms 2 during baroreceptor loading ( P <0.05). The low-frequency/high-frequency ratio decreased (1.94±0.41 versus 0.69±0.10, P <0.05). In contrast, clonidine reduced resting sympathetic vasomotor tone and shifted the operating point of the sympathetic baroreflex to a flat part of the sympathetic baroreflex curve. The shift decreased the ability of the baroreflex to withdraw sympathetic vasomotor tone during baroreflex loading. These baroreflex changes were associated with a moderate increase in phenylephrine responsiveness. We conclude that α-2 adrenoceptor stimulation has a differential effect on baroreflex HR and vasomotor regulation. α-2 Adrenoceptor stimulation greatly augments baroreflex-mediated bradycardia, most likely by parasympathetic activation.

Sympathoinhibitory effects of rilmenidine may be mediated by sites located below the brainstem

1. To determine the site of action of rilmenidine, we examined its effets on arterial blood pressure (BP), heart rate (HR) and postganglionic renal sympathetic nerve activity (RSNA) after intracerebroventricular (i.c.v.) administration (300 micrograms kg-1), in groups (all n = 6) of conscious and freely moving, pentobarbitone-anaesthetized and pentobarbitone-anaesthetized and spinally transected, fifteen week-old male spontaneously hypertensive rats (SHRs). 2. In conscious SHRs, which exhibited a low sympathetic nerve activity (RSNA: 3.4 +/- 0.9 muV), rilmenidine was inactive on systolic BP (SBP), diastolic BP (DBP), HR and RSNA. 3. In intact pentobarbitone-anaesthetized SHRs, which exhibited an elevated sympathetic nerve activity (RSNA: 10.6 +/- 0.9 muV), rilmenidine exerted potent antihypertensive (delta SBP: -37 +/- 4%; delta DBP: -43 +/- 6%), bradycardic (delta HR: -32 +/- 3%) and sympathoinhibitory (delta RSNA: -68 +/- 9%) activities. 4. In pentobarbitone-anaesthetized SHRs with cervical spinal cord transection, BP was markedly decreased and sympathetic nerve activity (RSNA: 10.3 +/- 3.1 muV) returned to the level observed in conscious SHRs (RSNA: 3.6 +/- 0.5 muV). In these conditions, rilmenidine remained sympathoinhibitory (delta RSNA: -74 +/- 5%). 5. In conclusion, we have shown that pentobarbitone-anaesthesia enhances the peripheral sympathetic tone by a central action, as the spinal cord transection allows RSNA to return to normal levels and that, spinal or ganglionic structures could be a major site of the sympathoinhibitory action of rilmenidine.