Importance of spinal noradrenergic pathways in cardiovascular reflexes and central actions of clonidine and alpha-methyldopa in the rabbit

Cervical Decompression Surgery for Cervical Spondylotic Myelopathy and Concomitant Hypertension: A Multicenter Prospective Cohort Study

STUDY DESIGN

We performed decompression surgery or conservative treatments on 135 cervical spondylotic myelopathy (CSM) patients with concomitant hypertension and did follow-up assessments up to 1 year to examine the change of blood pressure, spinal cord function, and cervical pain.

OBJECTIVE

The aim of this study was to determine whether concomitant hypertension is relieved after decompression surgery, and whether it is related to the improvement of spinal cord function or cervical pain.

SUMMARY OF BACKGROUND DATA

In clinical practice, we often found that some patients with CSM have concomitant hypertension. Interestingly, after CSM was treated successfully by decompression surgery, some patients' high blood pressure returned to normal range even without oral medications.

METHODS

We enrolled 135 CSM patients with hypertension, 103 of whom received decompression surgery, and remaining 32 patients accepted conservative treatments. We did follow-up assessments at 3, 6, and 12 months. The primary endpoints were changes of blood pressure, and secondary endpoints were changes of modified Japanese Orthopedic Association (mJOA) score and cervical pain visual analogue scale (VAS). Spearman correlation coefficients were calculated between changes in systolic blood pressure (SBP) and mJOA scores, VAS scores.

RESULTS

In patients with decompression surgery, the significant decrease in both SBP and diastolic blood pressure (DBP) that was seen 3 months and sustained through subsequent visit at 12 months. Paired-samples t test showed that both SBP and DBP were significantly lower than baseline blood pressure at all time points after procedure (P < 0.001). Significant correlation was found between the improvement rates of mJOA score and changes in SBP (r = -0.579, P < 0.001). But the correlation between changes in VAS score and changes in SBP was not significant (r = 0.58, P = 0.571).

CONCLUSION

Cervical decompression surgery could reduce concomitant high blood pressure in CSM patients, indicating a significant association between the decrease in blood pressure and the improvement of spinal cord function.

LEVEL OF EVIDENCE

2.

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.

Yohimbine Attenuates Baroreflex-Mediated Bradycardia in Humans

α-2 Adrenoreceptor stimulation profoundly augments baroreflex-mediated bradycardia presumably through parasympathetic activation. We tested the hypothesis that endogenous α-2 adrenergic tone mediates a similar response. In 10 healthy men (age: 33±3 years; body mass index: 24±1.3 kg/m 2 ), we determined baroreflex control of heart rate and sympathetic traffic after ingestion of the selective α-2 adrenoceptor antagonist yohimbine (20 mg) or placebo. Testing was conducted in a randomized, double-blind, crossover fashion. We measured heart rate, brachial and finger blood pressure, and muscle sympathetic nerve activity. Sympathetic and parasympathetic baroreflex curves were determined using incremental phenylephrine and nitroprusside infusions (0.3, 0.6, 0.9, 1.2, and 1.5 μg/kg per minute). Plasma norepinephrine increased with yohimbine (50±38 ng/L; P <0.05) and was unchanged with placebo (2.2±7.6 ng/L). Blood pressure increased 13±4/8±1 mm Hg with yohimbine and 6±2/3±1 mm Hg with placebo ( P <0.01). HR increased 5±1 bpm with yohimbine but did not change with placebo ( P <0.01). Ninety minutes after drug ingestion, resting muscle sympathetic nerve activity was similar with yohimbine and with placebo. Baroreflex control of heart rate was decreased with yohimbine (6 ms/mm Hg versus 10 ms/mm Hg; P <0.01) and reset to higher blood pressure and heart rate values. In contrast, yohimbine did not alter the sympathetic baroreflex curve. Yohimbine selectively attenuates baroreflex heart rate control in normotensive young men possibly through parasympathetic mechanisms.

Imidazoline receptors associated with noradrenergic terminals in the rostral ventrolateral medulla mediate the hypotensive responses of moxonidine but not clonidine

We determined whether the cardiovascular actions of central anti-hypertensive agents clonidine and moxonidine are dependent on noradrenergic or serotonergic innervation of the rostral ventrolateral medulla (RVLM) in conscious rabbits. 6-Hydroxydopamine (6-OHDA) or 5,6-dihydroxytriptamine (5,6-DHT) was injected into the RVLM to deplete noradrenergic and serotonergic terminals respectively. One, 2 and 4 weeks later, responses to fourth ventricular (4V) clonidine (0.65 microg/kg) and moxonidine (0.44 microg/kg) were examined. Destruction of noradrenergic pathways in the RVLM by 6-OHDA reduced the hypotensive response to 4V moxonidine to 62%, 47% and 60% of that observed in vehicle treated rabbits at weeks 1, 2 and 4 respectively. The moxonidine induced bradycardia was similarly attenuated (to 46% of vehicle). Conversely, 6-OHDA had no effect on the hypotensive or bradycardic effects of 4V clonidine. Efaroxan (I(1)-imidazoline receptor/alpha(2)-adrenoceptor antagonist; 3.5, 11, 35 microg/kg) and 2-methoxyidazoxan (alpha(2)-adrenoceptor antagonist; 0.3, 0.9, 3 microg/kg) equally reversed the hypotension to 4V clonidine, suggesting a mainly alpha(2)-adrenoceptor mechanism. Efaroxan preferentially reversed responses to moxonidine in both vehicle and 5,6-DHT groups and in the 1st week after 6-OHDA, suggesting a mechanism involving mainly I(1)-imidazoline receptors. This selectivity was subsequently lost in the 2nd and 4th weeks when the remaining hypotension was mainly mediated by alpha(2)-adrenoceptors. Depletion of serotonergic terminals did not alter the responses to either agonist nor did it change the relative effectiveness of the antagonists. Western blots of RVLM tissues probed with imidazoline and alpha(2)-adrenoceptor antisera showed a pattern of bands close to that reported in other species. The main effect of 6-OHDA was an 18% lower level of the 42 kDa imidazoline protein (P<0.05). We conclude that the hypotensive and bradycardic actions of moxonidine but not clonidine are mediated through imidazoline receptors and are dependent on intact noradrenergic pathways within the RVLM. Furthermore, the noradrenergic innervation may be associated with a 42 kDa imidazoline receptor protein.

The rostral ventrolateral medulla mediates sympathetic baroreflex responses to intraventricular angiotensin II in rabbits

The present study examined the role of the rostral ventrolateral medulla (RVLM) in mediating the pressor and renal sympathetic baroreflex effects of intraventricularly administered angiotensin II (Ang II) in urethane anaesthetised rabbits. Microinjection of Ang II over a wide range of medullary sites showed that pressor responses were observed only in the RVLM. Ang II was particularly potent in producing a transient pressor response at this site with a half maximal dose of 9 fmol. The administration of the Ang II antagonist Sar(1)-Ile(8)-Ang II (10 pmol) bilaterally into the RVLM inhibited the pressor response to local and fourth ventricular Ang II, but not the pressor response to RVLM applied glutamate. To determine the contribution of the RVLM to the renal sympathetic baroreflex effects of Ang II, blood pressure-renal sympathetic nerve activity (RSNA) curves were constructed with intravenous infusion of phenylephrine or nitroprusside before and after Ang II, vehicle or glutamate infusions into the RVLM. Ang II infusion of 4 pmol/min into the RVLM increased blood pressure by 8+/-3 mm Hg and shifted the renal sympathetic baroreflex curve to the right. The maximum RSNA evoked by lowering blood pressure increased by 36+/-6%, similar to the effect seen with fourth ventricular Ang II and RVLM glutamate. These studies suggest that the major medullary pressor site of action of Ang II when injected into the hindbrain cerebro-spinal fluid of anaesthetized rabbits is the RVLM where it facilitates baroreflex control of RSNA.

[Baroreflex failure syndrome: an uncommon case of an extreme blood pressure variability]

INTRODUCTION

The arterial baroreflex operates in physiological conditions. It induces sympathetic and vagal activity modulation resulting in arterial tone and heart rate changes. These appropriate responses limit blood pressure fluctuations and blood pressure is therefore regulated since the baroreflex constantly buffers the changes.

EXEGESIS

Bilateral carotid body tumor excision resulted in excessive fluctuations of blood pressure. Indices of spontaneous baroreflex activity were markedly altered in the patient described herein.

CONCLUSION

The excessive fluctuations of blood pressure due to the sinoaortic denervation demonstrate how powerful is this negative feedback control mechanism in control conditions.

Role of presympathetic C1 neurons in the sympatholytic and hypotensive effects of clonidine in rats

The rostral ventrolateral medulla (RVLM) may play an important role in the sympatholytic and hypotensive effects of clonidine. The present study examined which type of presympathetic RVLM neuron is inhibited by clonidine, and whether the adrenergic presympathetic RVLM neurons are essential for clonidine-induced sympathoinhibition. In chloralose-anesthetized and ventilated rats, clonidine (10 microg/kg iv) decreased arterial pressure (116 +/- 6 to 84 +/- 2 mmHg) and splanchnic nerve activity (93 +/- 3% from baseline). Extracellular recording and juxtacellular labeling of barosensitive bulbospinal RVLM neurons revealed that most cells were inhibited by clonidine (26/28) regardless of phenotype [tyrosine hydroxylase (TH)-immunoreactive cells: 48 +/- 7%; non-TH-immunoreactive cells: 42 +/- 5%], although the inhibition of most neurons was modest compared with the observed sympathoinhibition. Depletion of most bulbospinal catecholaminergic neurons, including 76 +/- 5% of the rostral C1 cells, by microinjection of saporin anti-dopamine beta-hydroxylase into the thoracic spinal cord (levels T2 and T4, 42 ng. 200 nl(-1). side(-1)) did not alter the sympatholytic or hypotensive effects of clonidine. These data show that although clonidine inhibits presympathetic C1 neurons, bulbospinal catecholaminergic neurons do not appear to be essential for the sympatholytic and hypotensive effects of systemically administered clonidine. Instead, the sympatholytic effect of clonidine is likely the result of a combination of effects on multiple cell types both within and outside the RVLM.

A five-parameter logistic equation for investigating asymmetry of curvature in baroreflex studies

Baroreceptor reflex curves are usually analyzed using a symmetric four-parameter function. We wished to ascertain the validity of assuming symmetry in the baroreflex curve and also of constraining the curves to pass through the resting blood pressure and heart rate (HR) values. Therefore, we have investigated the suitability of a new five-parameter asymmetric logistic model for analysis of baroreflex curves from rabbits and dogs. The five-parameter model is an extension of the usual four-parameter model and reduces to that model when the fitted data are symmetrical. Using 30 data sets of blood pressure versus renal sympathetic nerve activity (RSNA) and HR from six conscious rabbits, we compared the five-parameter curves with the four-parameter model. We also tested the effect of forcing these baroreflex curves through the resting point. We found that the five-parameter model reduced the unexplained variation and gave small but important improvements to the estimates of plateaus for RSNA and HR and the HR gain. Although forcing the HR curves through the resting values had little effect, this procedure, when applied to RSNA, produced a worse curve fit by increasing the unexplained variation with alteration to most of the estimated curve parameters. The mean arterial pressure-HR baroreflex relationship from six conscious dogs was also analyzed and showed clear evidence of systematic asymmetry. We conclude that the asymmetric model is a valuable extension to the symmetric logistic model when examining baroreceptor reflexes, giving improved estimates of the parameters and a new approach to examining the mechanisms contributing to baroreflex curve asymmetry. Furthermore, forcing the curves through the resting value is a statistically questionable practice when analyzing RSNA, because it affects the parameter estimates.

Role of central catecholaminergic pathways in the actions of endogenous ANG II on sympathetic reflexes

In the present study, we examined the effect of blockade of the brain stem renin-angiotensin system on renal sympathetic baroreflexes and chemoreflexes in conscious rabbits and examined the role of central catecholaminergic pathways in these responses. Eleven rabbits underwent preliminary surgical instrumentation and pretreatment with central 6-hydroxydopamine (6-OHDA, 500 micrograms/kg) or ascorbic acid 6 wk before the commencement of the experiments. Baroreflex curves were determined under conditions of normoxia and hypoxia (10% O2 + 3% CO2) before and after central administration of either Ringer solution, the ANG II receptor antagonist losartan (10 micrograms), or the angiotensin-converting enzyme inhibitor enalaprilat (500 ng) on separate days. Losartan increased the upper plateau and the range of the mean arterial pressure (MAP)-renal sympathetic nerve activity (RSNA) curve (79 and 78%, respectively) in intact rabbits, whereas this effect was not observed in 6-OHDA-pretreated rabbits. Hypoxia elicited an increase in resting RSNA (111% in intact rabbits and 74% in 6-OHDA-injected rabbits) and elevated the upper plateau of the RSNA-MAP curve in both groups (89% in intact rabbits and 114% in 6-OHDA-injected rabbits). During hypoxia, losartan and enalaprilat increased the RSNA upper plateau in intact rabbits but had no effect in 6-OHDA-pretreated rabbits. No effects on the MAP-heart rate baroreflex curves were observed. Thus the effect of losartan to increase RSNA, particularly during hypoxia and baroreceptor unloading, being abolished by central noradrenergic depletion suggests that the endogenous ANG II which normally causes an inhibition of renal sympathetic motoneurons is dependent on the integrity of central catecholaminergic pathways.

Contribution of alpha 2-adrenoceptors in caudal ventrolateral medulla to cardiovascular regulation in rat

The inhibitory action of alpha 2-agonists on the cardiovascular neurons has been elucidated in the rostral ventrolateral medulla (RVLM) but not in the caudal ventrolateral medulla (CVLM). Our study aimed to clarify whether microinjection of clonidine into the CVLM elicits any cardiovascular effect and whether endogenous alpha 2-adrenoceptor-mediated mechanisms contribute to the tonic activity of the CVLM neurons. In male Sprague-Dawley rats (7-9 wk old, 270-320 g) anesthetized with urethan, unilateral microinjection of 8 nmol of clonidine into the CVLM (n = 10) increased mean arterial pressure (MAP) and renal sympathetic nerve activity (RSNA) by 12.1 +/- 1.8 mmHg (mean +/- SE, P < 0.01) and 25.8 +/- 4.8% (P < 0.01), while heart rate (HR) remained unaltered. Unilateral microinjection of 2 nmol of SKF-86466, a selective blocker of the alpha 2-adrenoceptors, into the CVLM (n = 10) decreased MAP, HR, and RSNA (-11.6 +/- 2.6 mmHg, -26 +/- 7 beats/min, and -15.3 +/- 1.7%, respectively, P < 0.01 for each). Artificial cerebrospinal fluid caused neither a cardiovascular effect nor a sympathetic response. Prior injection of SKF-86466 into the ipsilateral CVLM attenuated the effects of clonidine. Bilateral microinjection of muscimol into the RVLM abolished the effects of both clonidine and SKF-86466 injected into the CVLM. The pressor and sympathoexcitatory effects of clonidine injected into the CVLM suggest a neuroinhibitory action of the drug on the CVLM neurons. In addition, the depressor and sympathoinhibitory effects of SKF-86466 injected into the CVLM indicated that activation of alpha 2-adrenoceptors by endogenous ligand inhibits CVLM neurons. The effects of clonidine and the alpha 2-adrenoceptor antagonist in the CVLM require the integrity of the RVLM.

Hypotensive and hypertensive effects of catecholamines intrathecally injected in anesthetized rats

The cardiovascular effects of catecholamines intrathecally (i.t.) injected at the T12-L1 level were analyzed in pentobarbital anesthetized rats. Volumes of injection were not greater than 3 microliters. Noradrenaline in doses ranging from 0.03 to 0.3 micrograms (i.t.) did not alter the mean blood pressure (MBP) while higher doses (1, 3 and 10 micrograms, i.t.) caused a dose-dependent increase in MBP. Adrenaline induced hypotensive effects at low doses (0.03-0.3 micrograms i.t.) and pressor effects at high doses (3 and 10 micrograms, i.t.). Neither adrenaline nor noradrenaline modified the heart rate. The pressor responses to both catecholamines were antagonized by the alpha 1-adrenoceptor blocker prazosin (0.05-1 microgram, i.t.) and by the selective alpha 1A-adrenoceptor antagonist 5-methyl urapidil (10 and 15 micrograms, i.t.). In contrast, these pressor effects were not modified by the alpha 1B-adrenoceptor antagonist chloroethylclonidine (90 micrograms i.t.). In animals pretreated with 1 microgram prazosin (i.t.), low doses of noradrenaline (0.03 and 0.1 microgram, i.t.) caused a hypotensive effect. Prazosin (1 microgram i.t.) failed to alter the hypotension caused by 0.1 microgram adrenaline. The hypotensive response induced by either 0.1 microgram noradrenaline (in the presence of prazosin) or 0.1 microgram adrenaline was blocked by the alpha 2-adrenoceptor antagonist yohimbine (1 mg/kg, i.v.), by the GABA-A antagonists bicuculline (3.2 micrograms, i.t.) and picrotoxin (2.7 micrograms, i.t.), and by the GABA-B antagonist 2-hydroxy saclofen (30 micrograms, i.t.). The glycine-receptor antagonist strychnine (25 micrograms, i.t.) did not modify the hypotension induced by either noradrenaline (in the presence of prazosin) or adrenaline. These findings suggest that in the low thoracolumbar spinal cord of pentobarbital-anesthetized rats, noradrenaline and adrenaline have excitatory as well as inhibitory effects on the control of the BP. The pressor responses of high doses of i.t. injected catecholamines could be mediated by the activation of spinal alpha 1A-adrenoceptors, although the participation of alpha 1B-adrenoceptors cannot be rule out entirely. The hypotensive responses induced by low doses of i.t. injected catecholamines seem to involve the activation of spinal alpha 2A-adrenoceptors and the stimulation of an inhibitory GABAergic neuron in the spinal cord.

Central monoamine systems and new antihypertensive agents

This review describes the relationship between central monoamine pathways and centrally acting antihypertensive agents. By using antagonists with affinity for alpha 2-adrenoceptors and also imidazoline receptors we have found that the first generation agents clonidine and alpha-methyldopa activate alpha 2-adrenoceptors while the newer second generation antihypertensive agents rilmenidine and moxonidine activate imidazoline receptors. Despite the difference in receptors activated, the hypotension produced by central administration of all agents was attenuated after chemical lesioning of the brainstem noradrenergic or serotonergic pathways suggesting a similar dependence on central monoamine pathways. Since the acute 6-hydroxydopamine-induced release of noradrenaline in the brainstem produces hypotension it suggests that these agents normally mimic brainstem noradrenergic function. By contrast the pressor response shortly following 5,6-dihydroxytryptamine suggests serotonergic neurones in the brainstem are pressor and that part of the anti-hypertensive action of centrally acting antihypertensive agents is mediated by inhibition of bulbar serotonergic pathways. We suggest that the similar haemodynamic and baroreflex effects of the two generations of agents can be explained by the alpha 2-adrenoceptors and the imidazoline receptors being in series along the noradrenergic and serotonergic pathways.

Spatially and temporally differentiated patterns of c-fos expression in brainstem catecholaminergic cell groups induced by cardiovascular challenges in the rat

Brainstem catecholaminergic neurons have been implicated as mediating adaptive autonomic and neuroendocrine responses to cardiovascular challenges. To clarify the nature of this involvement, immuno- and hybridization histochemical methods were used to follow c-fos expression in these neurons in response to acute stimuli that differentially affect blood pressure and volume. From low basal levels, hypotensive hemorrhage (15%) provoked a progressive increase in the number and distribution of Fos-immunoreactive (ir) nuclei in the nucleus of the solitary tract (NTS), the A1 and C1 cell groups of the ventrolateral medulla, and in the pontine A5, locus coeruleus, and lateral parabrachial cell groups peaking at 2.0-2.5 hours after the challenge. Fos-ir ventrolateral medullary neurons, subsets of which were identified as projecting to the paraventricular hypothalamic nucleus or spinal cord, were predominantly aminergic, whereas most of those in the NTS were not. Infusion of an angiotensin II antagonist blunted hemorrhage-induced Fos expression in the area postrema, and attenuated that seen elsewhere in the medulla and pons. Nitroprusside-induced isovolemic hypotension yielded a pattern of c-fos induction similar to that seen following hemorrhage, except in the area postrema and the A1 cell group, where the response was muted or lacking. Phenylephrine-induced hypertension stimulated a restricted pattern of c-fos expression, largely limited to induced hypertension stimulated a restricted pattern of c-fos expression, largely limited to non-aminergic neurons, whose distribution in the NTS conformed to the termination patterns of primary baroreceptor afferents, and in the ventrolateral medulla overlapped in part with those of vagal cardiomotor and depressor neurons. These findings underscore the importance of brainstem catecholaminergic neurons in effecting integrated homeostatic responses to cardiovascular challenges and their ability to responding strategically to specific modalities of cardiovascular information. They also foster testable predictions as to effector neuron populations that might be recruited to respond to perturbations in individual circulatory parameters.

Cardiac baroreflexes and hypertension

1. The role of cardiac reflexes in baroreflex control mechanisms and the changes that occur in chronic hypertension is reviewed. The rapid resetting properties of the arterial baroreceptors ensures its role in short-term rather than long-term control of blood pressure. 2. In hypertensive humans and animals, the baroreceptor-heart rate reflex has diminished sensitivity due mainly to reduced maximum capacity of the cardiac vagal component rather than a change to the sympathetic. 3. The development of this vagal deficit in spontaneously hypertensive rats (SHR) coincides with the onset of cardiac hypertrophy rather than vascular hypertrophy. A combination of chronic perindopril treatment regimens in young and older SHR showed that the vagal deficit was better correlated with the degree of cardiac hypertrophy than with other variables such as blood pressure, hypertension or indices of vascular hypertrophy. Similar results have been shown for renovascular hypertension in rats. 4. The bradycardia produced by electrical stimulation of the vagus in urethane anaesthetized young SHR was enhanced compared to Wistar-Kyoto rats (WKY) while responses observed in adult SHR and WKY were similar, suggesting that the vagal deficit in hypertensive rats is not due to a defect in the efferent arm of the baroreflex. 5. The association of the vagal deficit with cardiac hypertrophy, but not with the vagal efferent pathways, suggests an important role for cardiac afferents in hypertension in mediating the baroreflex deficit. 6. A diminished baroreflex and also a reduced heart rate variability is an independent risk factor for sudden death following myocardial infarction. Thus, antihypertensive therapy, which more effectively reduces cardiac hypertrophy, should have a desirable effect on baroreceptor reflexes and therefore in reducing blood pressure variability.

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.

Contribution of forebrain noradrenaline innervation to the central circulatory effects of alpha-methyldopa and 6-hydroxydopamine

We studied the circulatory effects of chronic lesions of the ascending noradrenergic (NA) projections to the forebrain on the acute effects of intracisternal (i.c.) alpha-methyldopa (alpha-MD) and 6-hydroxydopamine (6-OHDA) on mean arterial pressure (MAP) and heart rate (HR) in conscious rabbits with arterial baroreceptors either intact or denervated (sinoaortic denervation, SAD). Both drugs acutely release neurotransmitter from central NA neurons. I.c. 6-OHDA produced acute hypertension and bradycardia while i.c. alpha-MD produced acute hypotension and bradycardia. The responses are qualitatively similar in SAD rabbits except that after 6-OHDA, HR increased. In another group we studied the effects of the drugs 3-4 weeks after localised injections of 6-OHDA in the midbrain dorsal and ventral NA bundles. Local 6-OHDA depleted forebrain regions of NA by 44-76%, and had no effects on basal values of MAP or HR. The pressor and depressor effects, of 6-OHDA and alpha-MD respectively, were little affected by the lesions in either intact or SAD rabbits. By contrast, in rabbits with intact baroreceptors, the lesion abolished the bradycardia produced by i.c. alpha-MD and 6-OHDA. The latter drug now produced a late tachycardia. In SAD rabbits, however, there was no effect on the alpha-MD-induced bradycardia, but the 6-OHDA tachycardia was enhanced. Since the major effects of the lesions were confined to the rabbits with intact baroreceptor afferents, it suggests that the ascending NA pathways are important for the cardiac responses dependent on baroreceptor input. In intact animals, both drugs produce bradycardia through facilitation of the vagal component of the baroreceptor-heart rate reflex. In SAD rabbits, almost all the changes to HR are mediated through the cardiac sympathetic and the lesions have little effect on HR.

Cardiovascular functions of central noradrenergic neurons in rabbits

1. The cardiovascular actions of central noradrenergic (NA) neurons was examined using the acute neurotransmitter releasing actions of intracisternal 6-hydroxydopamine in conscious rabbits. 2. The predominant actions of NA pathways in the brain-stem are to inhibit vasomotor and cardiac sympathetic activity and to facilitate cardiac vagal responses. 3. NA projections to the spinal cord tonically inhibit vasoconstrictor tone and form part of a high gain control system for responding to changes in specific afferent information. 4. The forebrain NA projections influence heart rate through modulation of the baroreflex gain of the cardiac vagus. 5. Central antihypertensive drugs such as clonidine and alpha-methyldopa mimic most of the brainstem actions of the central NA neurons. They also utilize the ascending NA pathways influencing the baroreflex vagal gain to minimize the baroreflex effects of reducing mean arterial pressure. 6. Thus, NA neurons influence the circulatory system through actions at all levels of the central nervous system and provide a comprehensive framework for integrating cardiovascular information from multiple inputs. This provides a number of targets for future development of useful pharmacological agents.

Effects of combined serotonin depletion and lesions of the nucleus basalis magnocellularis on acquisition of a complex spatial discrimination task in the rat

The purpose of the present experiment was to determine the effects of lesions of cholinergic neurons originating from the nucleus basalis magnocellularis (NBM), alone or in combination with central serotonin depletion, on learning and memory in rats trained in the Stone 14-unit T-maze--a complex, positively-reinforced spatial discrimination task. Lesion of cholinergic neurons within the NBM was accomplished by bilateral infusion of ibotenic acid. Serotonin depletion was accomplished by the systemic administration of p-chloroamphetamine (PCA). The results show that PCA-induced serotonin depletion enhanced learning. This effect was completely prevented by NBM lesions, despite the fact that NBM lesions alone did not affect the performance of rats in this task. The results of this study support the view that the cholinergic and serotonergic systems may functionally interact in learning and memory processes. The significance of this interaction in the etiology and treatment of dementia should be further investigated.

Region-specific noradrenaline depletion by neonatal DSP-4: functional consequences and effect on a coexisting neurotransmitter

Noradrenaline (NA) depletion following neonatal treatment of rats with DSP-4 (N-(2-chloroethyl)-N-ethyl-2-bromobenzyl-amine) was not accompanied by any changes in neuropeptide tyrosine (NPY) levels. After 85-95% NA depletion in the spinal cord of DSP-4-treated rats, the reaction time in the tail withdrawal test was shorter than in the controls but the endogenous activation of the sympathetic nervous system in the baroreceptor reflex remained unchanged. These results suggest that the NA neurons in the spinal cord have an inhibitory function the processing of incoming sensory information.