Norepinephrine transporter variant A457P knock-in mice display key features of human postural orthostatic tachycardia syndrome

Postural orthostatic tachycardia syndrome (POTS) is a common autonomic disorder of largely unknown etiology that presents with sustained tachycardia on standing, syncope and elevated norepinephrine spillover. Some individuals with POTS experience anxiety, depression and cognitive dysfunction. Previously, we identified a mutation, A457P, in the norepinephrine (NE; also known as noradrenaline) transporter (NET; encoded by SLC6A2) in POTS patients. NET is expressed at presynaptic sites in NE neurons and plays a crucial role in regulating NE signaling and homeostasis through NE reuptake into noradrenergic nerve terminals. Our in vitro studies demonstrate that A457P reduces both NET surface trafficking and NE transport and exerts a dominant-negative impact on wild-type NET proteins. Here we report the generation and characterization of NET A457P mice, demonstrating the ability of A457P to drive the POTS phenotype and behaviors that are consistent with reported comorbidities. Mice carrying one A457P allele (NET(+/P)) exhibited reduced brain and sympathetic NE transport levels compared with wild-type (NET(+/+)) mice, whereas transport activity in mice carrying two A457P alleles (NET(P/P)) was nearly abolished. NET(+/P) and NET(P/P) mice exhibited elevations in plasma and urine NE levels, reduced 3,4-dihydroxyphenylglycol (DHPG), and reduced DHPG:NE ratios, consistent with a decrease in sympathetic nerve terminal NE reuptake. Radiotelemetry in unanesthetized mice revealed tachycardia in NET(+/P) mice without a change in blood pressure or baroreceptor sensitivity, consistent with studies of human NET A457P carriers. NET(+/P) mice also demonstrated behavioral changes consistent with CNS NET dysfunction. Our findings support that NET dysfunction is sufficient to produce a POTS phenotype and introduces the first genetic model suitable for more detailed mechanistic studies of the disorder and its comorbidities.

Abstract 351: Temperature Dependency of Blood Pressure and Heart Rate in Norepinephrine-Deficient Mice

Dopamine beta-hydroxylase (Dbh) knockout mice lack the sympathetic neurotransmitter, norepinephrine, but have intact sympathetic neurons and adrenergic receptors. These mice offer an excellent model for assessing the role of the sympathetic nervous system in physiological and pathophysiological processes. Mouse blood pressure (BP) and heart rate (HR) increase as ambient temperature drops below the thermoneutral temperature (30° C), apparently due to sympathetic nervous system activation. The present study tested the hypothesis that BP and HR are not sensitive to changes in ambient temperature in Dbh knockout (KO) mice lacking sympathetic function. BP, HR and activity data were obtained at least one week after BP telemeters were implanted in the carotid artery. Eight Dbh wild type (WT) and 7 KO mice were initially maintained at 23° C, singly housed in temperature-controlled brooders with a 12-hour light-dark cycle. With 3° C temperature changes every 3-4 days, chambers were warmed to 32° C, followed by cooling back to 23° C. Twenty-four hour averaged BP and HR were significantly lower in KO than WT mice at all temperatures, although activity levels were no different. HR, averaged over 24hr, were significantly higher at 23° C after cooling from 32° C for both genotypes (mean±SD; KO: 376±24 to 462±53 beats/min, P=0.016; WT: 411±22 to 542±46 beats/min, P=0.008). In contrast, BP increased with the drop in temperature only in the WT mice (KO: 83±7 to 87±10 mmHg, P=0.109; WT: 94±9 to 107±9 mmHg, P=0.008), and BP differences between 32° and 23° C were greater in WT mice (delta BP 23 ° -32 ° averaged 12.2±5.4 mmHg for WT and 4.2±5.0 mmHg for KO, P=0.011). Activity did not change as ambient temperature was lowered. These data indicate that effects of ambient temperature on HR occur independently of sympathetic activation, but our results confirm that an intact sympathetic nervous system is required for the pressor effect of a cooler environmental temperature.

Tachycardia, reduced vagal capacity, and age-dependent ventricular dysfunction arising from diminished expression of the presynaptic choline transporter

Healthy cardiovascular function relies on a balanced and responsive integration of noradrenergic and cholinergic innervation of the heart. High-affinity choline uptake by cholinergic terminals is pivotal for efficient ACh production and release. To date, the cardiovascular impact of diminished choline transporter (CHT) expression has not been directly examined, largely due to the transporter's inaccessibility in vivo. Here, we describe findings from cardiovascular experiments using transgenic mice that bear a CHT genetic deficiency. Whereas CHT knockout (CHT(-/-)) mice exhibit early postnatal lethality, CHT heterozygous (CHT(+/-)) mice survive, grow, and reproduce normally and exhibit normal spontaneous behaviors. However, the CHT(+/-) mouse heart displays significantly reduced levels of high-affinity choline uptake accompanied by significantly reduced levels of ACh. Telemeterized recordings of cardiovascular function in these mice revealed tachycardia and hypertension at rest. After treadmill exercise, CHT(+/-) mice exhibited slower heart rate recovery, consistent with a diminished cholinergic reserve, a contention validated through direct vagal nerve stimulation. Echocardiographic and histological experiments revealed an age-dependent decrease in fractional shortening, increased left ventricular dimensions, and increased ventricular fibrosis, consistent with ventricular dysfunction. These cardiovascular phenotypes of CHT(+/-) mice encourage an evaluation of humans bearing reduced CHT expression for their resiliency in maintaining proper heart function as well as risk for cardiovascular disease.

Portal osmopressor mechanism linked to transient receptor potential vanilloid 4 and blood pressure control

Human subjects with impaired baroreflex function cannot buffer rises or falls in blood pressure (BP), thus allowing BP effects of endogenous or environmental stimuli that previously escaped detection to emerge dramatically. Studies in these patients led us to discover that water ingestion induced a robust increase in BP and vascular resistance. Here, using a mouse model of baroreflex impairment, we show that the increase in blood pressure after water ingestion is mediated through sympathetic nervous system activation and that the osmosensitive transient receptor potential vanilloid 4 channel (Trpv4) is an essential component of the response. Although portal osmolality decreases after water ingestion in both wild-type and Trpv4(-/-) mice, only the wild-type animals show a pressor response. The same volume of physiological saline does not elicit an increase in BP, suggesting osmolality as the stimulus. The osmopressor response to water, and Trpv4 thus represent new factors now implicated in the physiology of BP regulation.

Wavelet methods for spike detection in mouse renal sympathetic nerve activity

Abnormal autonomic nerve traffic has been associated with a number of peripheral neuropathies and cardiovascular disorders prompting the development of genetically altered mice to study the genetic and molecular components of these diseases. Autonomic function in mice can be assessed by directly recording sympathetic nerve activity. However, murine sympathetic spikes are typically detected using a manually adjusted voltage threshold and no unsupervised detection methods have been developed for the mouse. Therefore, we tested the performance of several unsupervised spike detection algorithms on simulated murine renal sympathetic nerve recordings, including an automated amplitude discriminator and wavelet-based detection methods which used both the discrete wavelet transform (DWT) and the stationary wavelet transform (SWT) and several wavelet threshold rules. The parameters of the wavelet methods were optimized by comparing basal sympathetic activity to postmortem recordings and recordings made during pharmacological suppression and enhancement of sympathetic activity. In general, SWT methods were found to outperform amplitude discriminators and DWT methods with similar wavelet coefficient thresholding algorithms when presented with simulations with varied mean spike rates and signal-to-noise ratios. A SWT method which estimates the noise level using a "noise-only" wavelet scale and then selectively thresholds scales containing the physiologically important signal information was found to have the most robust spike detection. The proposed noise-level estimation method was also successfully validated during pharmacological interventions.

Norepinephrine transporter-deficient mice respond to anxiety producing and fearful environments with bradycardia and hypotension

The study of anxiety and fear involves complex interrelationships between psychiatry and the autonomic nervous system. Altered noradrenergic signaling is linked to certain types of depression and anxiety disorders, and treatment often includes specific transporter blockade. The norepinephrine transporter is crucial in limiting catecholaminergic signaling. Norepinephrine transporter-deficient mice have increased circulating catecholamines and elevated heart rate and blood pressure. We hypothesized, therefore, that reduced norepinephrine clearance would heighten the autonomic cardiovascular response to anxiety and fear. In separate experiments, norepinephrine transporter-deficient (norepinephrine transporter-/-) mice underwent tactile startle and trace fear conditioning to measure hemodynamic responses. A dramatic tachycardia was observed in norepinephrine transporter-/- mice compared with controls following both airpuff or footshock stimuli, and pressure changes were also greater. Interestingly, in contrast to normally elevated home cage levels in norepinephrine transporter-deficient mice, prestimulus heart rate and blood pressure were actually higher in norepinephrine transporter+/+ animals throughout behavioral testing. Upon placement in the behavioral chamber, norepinephrine transporter-deficient mice demonstrated a notable bradycardia and depressor effect that was more pronounced in females. Power spectral analysis indicated an increase in low frequency oscillations of heart rate variability; in mice, suggesting increased parasympathetic tone. Finally, norepinephrine transporter-/- mice exhibited sexual dimorphism in freeze behavior, which was greatest in females. Therefore, while reduced catecholamine clearance amplifies immediate cardiovascular responses to anxiety- or fear-inducing stimuli in norepinephrine transporter-/- mice, norepinephrine transporter deficiency apparently prevents protracted hemodynamic escalation in a fearful environment. Conceivably, chronic norepinephrine transporter blockade with transporter-specific drugs might attenuate recognition of autonomic and somatic distress signals in individuals with anxiety disorders, possibly lessening their behavioral reactivity, and reducing the cardiovascular risk factors associated with persistent emotional arousal.

Norepinephrine transporter-deficient mice exhibit excessive tachycardia and elevated blood pressure with wakefulness and activity

BACKGROUND

Norepinephrine (NE) is a primary neurotransmitter of central autonomic regulation and sympathetic nerve conduction, and the norepinephrine transporter (NET) is crucial in limiting catecholaminergic signaling. NET is sensitive to antidepressants, cocaine, and amphetamine. NET blockade often is associated with cardiovascular side effects, and NET deficiency is linked to tachycardia in familial orthostatic intolerance.

METHODS AND RESULTS

We telemetrically monitored NET-deficient (NET(-/-)) mice to determine the cardiovascular effects of reduced NE reuptake. Mean arterial pressure was elevated in resting NET(-/-) mice compared with NET(+/+) controls (103+/-0.6 versus 99+/-0.4 mm Hg; P<0.01), and corresponding pressures increased to 122+/-0.3 and 116+/-0.3 mm Hg (P<0.0001) with activity. Heart rate was also greater in resting NET(-/-) mice (565+/-5 versus 551+/-3 bpm; P<0.05), and genotypic differences were highly significant during the active phase (640+/-5 versus 607+/-3 bpm; P<0.0001). Conversely, the respiratory rate of resting NET(-/-) mice was dramatically reduced, whereas increases after the day/night shift surpassed those of controls. Plasma catecholamines in NET(-/-) and NET(+/+) mice were as follows: NE, 69+/-8 and 32+/-7; dihydroxyphenylglycol, 2+0.4 and 17+/-3; epinephrine, 15+/-3 and 4+/-0.6; and dopamine, 13+/-4 and 4+/-1 pmol/mL. Catechols in urine, brain, and heart also were determined.

CONCLUSIONS

Resting mean arterial pressure and heart rate are maintained at nearly normal levels in NET-deficient mice, most likely as a result of increased central sympathoinhibition. However, sympathetic activation with wakefulness and activity apparently overwhelms central modulation, amplifying peripheral catecholaminergic signaling, particularly in the heart.

Animal model of neuropathic tachycardia syndrome

Clinically relevant autonomic dysfunction can result from either complete or partial loss of sympathetic outflow to effector organs. Reported animal models of autonomic neuropathy have aimed to achieve complete lesions of sympathetic nerves, but incomplete lesions might be more relevant to certain clinical entities. We hypothesized that loss of sympathetic innervation would result in a predicted decrease in arterial pressure and a compensatory increase in heart rate. Increased heart rate due to loss of sympathetic innervation is seemingly paradoxical, but it provides a mechanistic explanation for clinical autonomic syndromes such as neuropathic postural tachycardia syndrome. Partially dysautonomic animals were generated by selectively lesioning postganglionic sympathetic neurons with 150 mg/kg 6-hydroxydopamine hydrobromide in male Sprague-Dawley rats. Blood pressure and heart rate were monitored using radiotelemetry. Systolic blood pressure decreased within hours postlesion (Delta>20 mm Hg). Within 4 days postlesion, heart rate rose and remained elevated above control levels. The severity of the lesion was determined functionally and pharmacologically by spectral analysis and responsiveness to tyramine. Low-frequency spectral power of systolic blood pressure was reduced postlesion and correlated with the diminished tyramine responsiveness (r=0.9572, P=0.0053). The tachycardia was abolished by treatment with the beta-antagonist propranolol, demonstrating that it was mediated by catecholamines acting on cardiac beta-receptors. Partial lesions of the autonomic nervous system have been hypothesized to underlie many disorders, including neuropathic postural tachycardia syndrome. This animal model may help us better understand the pathophysiology of autonomic dysfunction and lead to development of therapeutic interventions.

Modulatory effects of endothelin on baroreflex activation in the nucleus of the solitary tract

In this study, we determine the effects of endogenous endothelin on baroreflex activation. After control baroreflex slopes were obtained, the animals received bilateral intra-nucleus tractus solitarii microinjections of saline, or equimolar doses (4 pmol/60 nl) of the endothelin ETA receptor antagonist cyclo (D-Trp-D-Asp-Pro-Val-Leu (BQ-123), Homopiperinidinyl-CO-Leu-D-Trp(CHO)-D-Trp-OH (BQ-610), or the endothelin ETB receptor antagonist N-cis-2,6-dimethylpiperidinocarbonyl-L-gamma-MeLeu-D-Trp( COOCH3)-D-Nle (BQ-788). Intra-nucleus tractus solitarii administration of BQ-123 resulted in a brief initial pressor effect followed by hypotension which resolved by 15 min. The baroreflex slope was significantly enhanced when tested 15 min after BQ-123 treatment (from 2.4 +/- 0.5 ms/mmHg to 3.5 +/- 0.4 ms/mmHg). Similar effects were observed with the other endothelin ETA receptor antagonist, except that the hypertensive and hypotensive responses were more pronounced while the baroreflex slope was similarly increased. In contrast, the endothelin ETB receptor antagonist did not evoke appreciable changes in hemodynamics or in baroreflex slopes. Our results support the concept that endothelin prominently affects reflex cardiovascular function through the endothelin ETA receptor subtype.

Inhibition by ethanol of the cardiovascular effects of glutamate in the nucleus of the solitary tract

This study investigates whether ethanol affects the cardiovascular changes evoked by the excitatory amino acid glutamate in the nucleus of the solitary tract (NTS). Male Sprague-Dawley rats were anesthetized with urethane and instrumented for microinjection of drugs into the NTS. In 28 animals, an initial dose-response curve for glutamate (37, 74, and 148 pmol/60 nL) was obtained. In eight animals (control group), the dose-response curve was unchanged when it was tested after microinjection of 60 nL of saline into the NTS. In contrast, the prior intra-NTS administration of ethanol (25 or 50 mM) consistently inhibited the hypotensive and bradycardic effects of glutamate. The specificity of this inhibitory response was corroborated by the inability of intra-NTS administrations of ethanol (50 mM) to affect the hypotensive and bradycardic responses to nicotine (922 pmol. 1.84 nmol, and 3.96 nmol). These results indicate that the cardiovascular effects of glutamate can be antagonized by ethanol in the NTS. This inhibitory effect, in turn, may provide a basis for the hypertensinogenic action of ethanol in humans.

Effects of nicotine on brain stem mechanisms of cardiovascular control

The role of the central nervous system in the pressor effect of nicotine is not well understood. In this study, we evaluated the cardiovascular effects of nicotine in the lower brainstem of normotensive and hypertensive rats. Microinjection of nicotine (0.012-3696 pmol/60 nl) into the nucleus of the solitary tract and area postrema of Sprague-Dawley (SD), Wistar Kyoto (WKY) and spontaneously hypertensive rats (SHR) decreased blood pressure and heart rate. In contrast, administration of similar doses of nicotine within the rostral ventrolateral medulla (RVLM) evoked a long-lasting pressor and tachycardic effect. This pressor effect was completely abolished by prior microinjection of hexamethonium. In SHR the depressor and bradycardic responses in the nucleus of the solitary tract and area postrema were similar to those of normotensive animals. The pressor effect in the RVLM, however, was more pronounced in the SHR than in WKY or SD rats. In additional experiments, the changes produced by intra-RVLM administration of nicotine on renal sympathetic nerve activity, blood pressure and heart rate were evaluated before and after equidepressor intravenous doses of either clonidine, labetalol or prazosin. The prior administration of labetalol antagonized the pressor effect of nicotine in the three strains of rats (SHR, 82 +/- 6%; SD, 96 +/- 4%; WKY, 83 +/- 9%). Prazosin inhibited the nicotine pressor response by 69% in SHR, by 44% in SD and by 70% in WKY. Clonidine had no effect on nicotine response in the three groups of rats. In conclusion, nicotine administration within the RVLM increases renal sympathetic nerve activity and blood pressure.(ABSTRACT TRUNCATED AT 250 WORDS)

Endothelin as a neuropeptide. Cardiovascular effects in the brainstem of normotensive rats

The relevance of endothelin in central cardiovascular function was studied in urethane-anesthetized Sprague-Dawley rats. Blood pressure (BP) was monitored intra-arterially, and cerebrospinal fluid (CSF) was collected through an intracisternal catheter for radioimmunoassay of endothelin-1 (ET-1). Endothelin levels in the CSF were significantly higher (39 +/- 3 pg/ml) than in plasma (10 +/- 3 pg/ml, n = 11). ET-1 in CSF or plasma was not affected by systemic infusion of saline, but its levels significantly decreased when a sustained increase in BP was elicited with phenylephrine (14 +/- 7 pg/ml in the CSF and 6 +/- 4 pg/ml in plasma, n = 5). In sinoaortic-denervated animals, phenylephrine failed to reduce CSF endothelin levels. In different experiments, intracisternal administration of ET-1 (10 pmol) evoked an initial decrease in BP and heart rate (HR), followed by pronounced hypertension, bradycardia, and, in 70% of the animals, death from cardiorespiratory failure. Intracisternal administration of endothelin-3 (ET-3, 80 pmol, n = 11) evoked only a modest hypotensive and bradycardic response without cardiorespiratory impairment. Microinjection of ET-1 (0.5, 1, 2, 4, and 6 pmol/60 nl) into the nucleus of the solitary tract or area postrema produced a decrease in BP and HR. On the other hand, injection of low concentrations of ET-3 into the nucleus of the solitary tract increased BP and HR (at 2 pmol, 17 +/- 3 mm Hg, 14 +/- 6 beats per minute, n = 7), whereas ET-3 in the area postrema produced a prominent dose-related decrease in BP and HR. In the rostroventrolateral medulla, the lowest doses of ET-1 first modestly increased BP and renal sympathetic nerve activity. These effects were followed by hypotension, bradycardia, increase in respiratory frequency, and further enhancement of sympathetic nerve traffic. In 29% of the animals, these effects were followed by cardiorespiratory arrest. The specificity of the cardiovascular response to endothelin was demonstrated by the inhibitory effects of the receptor antagonist BQ-123. These results demonstrate that endothelin has specific cardiovascular effects in the brainstem of the rat and support a role for endothelin in cardiovascular regulation.

A Na pump inhibitor from bovine posterior pituitary: purification, structure determination and its cardiovascular effect in rat

We examined the hypothesis that hypothalamo-hypophysial tissue contains an endogenous Na pump inhibitor. From bovine posterior pituitary, we purified a substance which inhibits Rb uptake by human erythrocytes. This inhibitory activity was found in the eluate of 10% acetonitrile from a C18 flash column and purified by subsequent three steps of reversed-phase high-performance liquid chromatography (HPLC). Sequence analysis revealed that this substance was identical to joining peptide, one of the major products of proopiomelanocortin (POMC). This peptide had hypertensive and tachycardiac effects in spontaneously hypertensive rats (SHR) after central administration, with weak Na,K-ATPase inhibitory activity (IC50 = 0.5 mM).

Cardiovascular excitatory effects of adenosine in the nucleus of the solitary tract

Adenosine is an inhibitory neuromodulator in several brain regions. In the nucleus tractus solitarius (NTS), however, adenosine exerts excitatory cardiovascular effects. The purpose of the present study was to elucidate the involvement of other endogenous mechanisms that could contribute to the final hemodynamic response to adenosine in this nucleus. In normotensive Sprague-Dawley rats, intra-NTS microinjection of adenosine (2.3 nmol/60 nl) decreased blood pressure and heart rate. These effects were blocked by prior administration of the specific adenosine receptor antagonist 1,3-dipropyl-8-p-sulfophenylxanthine (0.92 nmol) and by the two glutamate receptor antagonists kynurenic acid and glutamic diethylester. The specificity of the adenosine-glutamate interaction in the NTS was demonstrated with adrenergic and angiotensin receptor antagonists that did not affect the adenosine response and by experiments with glutamate receptor antagonists that did not affect nicotine actions in the NTS. Furthermore, an increase in glutamate levels was demonstrated during perfusion of adenosine through a microdialysis probe in the NTS of anesthetized rabbits. These findings indicate that adenosine increases the release of glutamate in the NTS and, thus, are at variance with the concept of a "universal" inhibitory effect of adenosine in the central nervous system.

Administration of carbamazepine in the nucleus of the solitary tract inhibits the antihypertensive effect of clonidine

The effects of carbamazepine administration into the nucleus tractus solitarii (NTS) on central alpha 2-adrenergic cardiovascular function were studied in normotensive Wistar Kyoto (WKY) and spontaneously hypertensive rats (SHR). Stereotaxic microinjections of carbamazepine (288 ng/60 nl) into the NTS of SHR transiently increased blood pressure, heart rate, and renal sympathetic nerve activity. These effects were followed by a modest depressor and bradycardic effect, with no change in sympathetic nerve activity. In addition, previous intra-NTS administration of carbamazepine attenuated the antihypertensive effect of the centrally acting alpha 2-adrenoceptor agonist, clonidine. Similar carbamazepine effects were observed in the WKY rats. These results suggest that some of the cardiovascular changes observed during carbamazepine treatment involve antagonism of central noradrenergic mechanisms.

Cardiovascular effects of microinjection of angiotensin II in the brainstem of renal hypertensive rats

The cardiovascular effects of microinjection of angiotensin II (AII) into the area postrema (AP), nucleus of the solitary tract (NTS) and rostroventrolateral medulla were studied in urethane anesthetized sham-normotensive (NT) and two-kidney, one-clip renal hypertensive rats. Microinjection of AII (2-2000 ng) in the AP of renal hypertensive rats elicited a dose-dependent decrease in blood pressure, heart rate and renal sympathetic nerve activity. Similar effects were observed in the NTS. In the NT rats, low doses of AII (2 and 20 ng), either in the AP or NTS, were also depressor. High doses of AII (200-2000 ng) were needed to observe a modest pressor effect in the NT animals. A decrease in heart rate and renal sympathetic activity was observed with the pressor effect. The AII-antagonist, [Sar1,Val5,Ala8]-AII, into the NTS or AP increased blood pressure and heart rate and inhibited the cardiovascular effects of low doses of AII in both group of rats. In contrast, [Sar1,Val5,Ala8]AII did not affect the pressor action of high doses of AII in the NT group. While the microinjection of AII into the rostroventrolateral medulla did not produce any significant cardiovascular effect in the renal hypertensive group, it resulted in a modest pressor effect in the NT rats. These results indicate that acute activation of AII receptors in the AP or NTS does not contribute to the pressor effect of AII in renal hypertensive rats.

Modulatory effects of adenosine on baroreflex activation in the brainstem of normotensive rats

The effects of the adenosine antagonists, 1,3-dipropyl-8-p-sulphenylxanthine (DPSPX) and caffeine, on baroreflex activity were tested in normotensive Sprague-Dawley rats. The microinjection of DPSPX (0.92 nmol) into the nucleus tractus solitarii (NTS) of urethane-anesthetized animals did not modify basal blood pressure or heart rate but inhibited the reflex bradycardia elicited by phenylephrine. Similar inhibitory effects on baroreflex activation were observed after intracisternal administration of caffeine to conscious or anesthetized animals. These results suggest that central endogenous adenosine is involved in the medullary regulation of blood pressure and that adenosine antagonists such as caffeine can inhibit baroreflex activation.

Cardiovascular effects of neuropeptide Y in rat brainstem nuclei

Central catecholaminergic neurons are involved in cardiovascular regulation. Neuropeptide Y (NPY) coexists with adrenaline and noradrenaline in the rat brain, and interactions among these substances have been studied. The purpose of this study was to investigate the possible role of NPY in central cardiovascular control. Male Sprague-Dawley rats were anesthetized with urethane, and blood pressure was monitored intra-arterially. Intramedullary microinjection (60 nl) of NPY (0, 46.5 fmol, 465 fmol, 1.5 pmol, and 4.65 pmol) was made into the nucleus tractus solitarii (NTS), into the area postrema, and into the C1 area in the rostroventrolateral medulla. Injection site was identified by L-glutamate administration and confirmed histologically. Unilateral injection of NPY into the NTS produced a prominent dose-related decrease in heart rate and systolic and diastolic blood pressure (-106 +/- 8 beats/min, -56 +/- 2 mm Hg, and -33 +/- 2 mm Hg, respectively after 4.65 pmol NPY, n = 7, p less than 0.001). Maximal changes occurred at 30 seconds and recovered in 10 minutes for blood pressure and 20 minutes for heart rate. Injection into the area postrema produced an initial increase in heart rate and mean blood pressure (+23 +/- 2 beats/min and +18 +/- 2 mm Hg) followed by a prolonged decrease in heart rate and mean blood pressure (-14 +/- 4 beats/min and -15 +/- 2 mm Hg, respectively, n = 7, p less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Purinergic receptors in the brainstem mediate hypotension and bradycardia

Adenosine acts at many sites to modulate neuronal activity. The purpose of this study was to investigate a possible role for adenosine as a neuromodulator of brainstem cardiovascular control. Microinjections of adenosine (0-2.3 nmol) were made stereotaxically into various brainstem sites. Injection of adenosine into the nucleus tractus solitarii (NTS) produced dose-related decreases in heart rate and systolic and diastolic blood pressures. Maximal changes occurred 90 seconds after injection. Injection into the area postrema also produced decreased heart rate and systolic and diastolic blood pressures. No significant effect occurred following injection into the C1 area. Adenosine 5'-triphosphate and its analogue, beta, gamma-methylene adenosine 5'-triphosphate also produced dose-related and potent vasodepressor and bradycardia effects in the NTS. Injection of 1,3-dipropyl-8-p-sulfophenylxanthine (0.92 nmol), a potent adenosine receptor antagonist, produced no effect itself, but abolished for 45 minutes the actions of further injections of adenosine and adenosine 5'-triphosphate (but not L-glutamate) in both the NTS and area postrema. Thus, NTS and area postrema injections of adenosine decrease blood pressure and heart rate in anesthetized normotensive rats through adenosine receptors located in these areas. These findings support a role for endogenous adenosine as a central modulator in cardiovascular control.

Smoking and mechanisms of cardiovascular control

In humans short-term administration of nicotine, whether by smoking or intravenous injection, will typically raise blood pressure by 5 to 10 mm Hg and heart rate by 10 to 25 bpm. Smoking causes reduced myocardial contractility and left ventricular function in patients with angina pectoris or heart failure. Nicotine's mechanism of action is more complex than the classic concept of nicotinic ganglionic stimulation can account for. Nicotine exerts a potent pressor effect in the ventral lateral medulla (C-1 area). Little current data are available documenting the efficacy of centrally acting antihypertensive agents and converting-enzyme inhibitors with regard to preventing nicotine's acute cardiovascular effects.