Central adrenoceptors and cholinoceptors in cardiovascular control

Effects of physical exercise on baroreflex sensitivity and renal sympathetic nerve activity in chronic nicotine-treated rats

Chronic nicotine exposure may increase cardiovascular risk by impairing the cardiac autonomic function. Besides, physical exercise (PE) has shown to improve cardiovascular health. Thus, we aimed to investigate the effects of PE on baroreflex sensitivity (BRS), heart rate variability (HRV), and sympathetic nerve activity (SNA) in chronically nicotine-exposed rats. Male Wistar rats were assigned to four independent groups: Control (treated with saline solution), Control+Ex (treated with saline and submitted to treadmill training), Nicotine (treated with Nicotine), and Nicotine+Ex (treated with nicotine and submitted to treadmill training). Nicotine (1 mg·kg-1) was administered daily for 28 consecutive days. PE consisted of running exercise (60%-70% of maximal aerobic capacity) for 45 min, 5 days per week, for 4 weeks. At the end of the protocol, cardiac BRS, HRV, renal SNA (rSNA), and renal BRS were assessed. Nicotine treatment decreased absolute values of HRV indexes, increased low frequency/high frequency ratio of HRV, reduced the bradycardic and sympatho-inhibitory baroreceptor reflex responses, and reduced the rSNA. PE effectively restored time-domain HRV indexes, the bradycardic and sympatho-inhibitory reflex responses, and the rSNA in chronic nicotine-treated rats. PE was effective in preventing the deterioration of time-domain parameters of HRV, arterial baroreceptor dysfunction, and the rSNA after nicotine treatment.

β adrenergic receptor modulation of neurotransmission to cardiac vagal neurons in the nucleus ambiguus

β-adrenergic receptors are a class of G protein-coupled receptors that have essential roles in regulating heart rate, blood pressure, and other cardiorespiratory functions. Although the role of β adrenergic receptors in the peripheral nervous system is well characterized, very little is known about their role in the central nervous system despite being localized in many brain regions involved in autonomic activity and regulation. Since parasympathetic activity to the heart is dominated by cardiac vagal neurons (CVNs) originating in the nucleus ambiguus (NA), β adrenergic receptors localized in the NA represent a potential target for modulating cardiac vagal activity and heart rate. This study tests the hypothesis that activation of β adrenergic receptors alters the membrane properties and synaptic neurotransmission to CVNs. CVNs were identified in brainstem slices, and membrane properties and synaptic events were recorded using the whole-cell voltage-clamp technique. The nonselective β agonist isoproterenol significantly decreased inhibitory GABAergic and glycinergic as well as excitatory glutamatergic neurotransmission to CVNs. In addition, the β(1)-selective receptor agonist dobutamine, but not β(2) or β(3) receptor agonists, significantly decreased inhibitory GABAergic and glycinergic and excitatory glutamatergic neurotransmission to CVNs. These decreases in neurotransmission to CVNs persisted in the presence of tetrodotoxin (TTX). These results provide a mechanism by which activation of adrenergic receptors in the brainstem can alter parasympathetic activity to the heart. Likely physiological roles for this adrenergic receptor activation are coordination of parasympathetic-sympathetic activity and β receptor-mediated increases in heart rate upon arousal.

The hypothalamus and hypertension

Most forms of hypertension are associated with a wide variety of functional changes in the hypothalamus. Alterations in the following substances are discussed: catecholamines, acetylcholine, angiotensin II, natriuretic peptides, vasopressin, nitric oxide, serotonin, GABA, ouabain, neuropeptide Y, opioids, bradykinin, thyrotropin-releasing factor, vasoactive intestinal polypeptide, tachykinins, histamine, and corticotropin-releasing factor. Functional changes in these substances occur throughout the hypothalamus but are particularly prominent rostrally; most lead to an increase in sympathetic nervous activity which is responsible for the rise in arterial pressure. A few appear to be depressor compensatory changes. The majority of the hypothalamic changes begin as the pressure rises and are particularly prominent in the young rat; subsequently they tend to fluctuate and overall to diminish with age. It is proposed that, with the possible exception of the Dahl salt-sensitive rat, the hypothalamic changes associated with hypertension are caused by renal and intrathoracic cardiopulmonary afferent stimulation. Renal afferent stimulation occurs as a result of renal ischemia and trauma as in the reduced renal mass rat. It is suggested that afferents from the chest arise, at least in part, from the observed increase in left auricular pressure which, it is submitted, is due to the associated documented impaired ability to excrete sodium. It is proposed, therefore, that the hypothalamic changes in hypertension are a link in an integrated compensatory natriuretic response to the kidney's impaired ability to excrete sodium.

Intracerebroventricular injection of prostaglandin E(1) changes concentrations of biogenic amines in the posterior hypothalamus of the rat

Since the posterior hypothalamus (PH) plays a key role in the control of body temperature, the aim of this study was to evaluate the changes in adrenaline, noradrenaline and dopamine levels in the PH during the hyperthermia induced by prostaglandin E(1) (PGE(1)). The concentration of adrenaline, noradrenaline and dopamine in the PH, the firing rate of the sympathetic nerves innervating interscapular brown adipose tissue (IBAT), IBAT and colonic temperatures (T(IBAT) and T(C)) were monitored in 12 urethane-anaesthetized male Sprague-Dawley rats before and after an intracerebroventricular injection of 500 ng PGE(1) dissolved in 2 microl of 0.9% NaCl saline solution or only saline. The catecholamines were collected using a microdialysis probe and quantified by HPLC. The results showed that PGE(1) caused a significant increment in the concentration of adrenaline from 15. 83+/-2.69 to 34.95+/-3.9 ng ml(-1) and of dopamine from 35.15+/-4.48 to 55.68+/-6.21 ng ml(-1). A significant decrease in the level of noradrenaline from 18.75+/-2.05 to 8.56+/-2.26 ng ml(-1) was registered. The firing rate of sympathetic nerves to IBAT was increased from 100+/-0% to 204.83+/-15.22% by PGE(1). T(IBAT) and T(C) rose respectively from 36.91+/-0.15 degrees C to 38.88+/-0.29 degrees C, and from 36.7+/-0.15 degrees C to 38.13+/-0.36 degrees C after the injection of PGE(1). The changes in adrenaline and noradrenaline occurred during the first 20 min as did the changes in temperature and firing rate, while the change in dopamine was delayed until 21-60 min after the PGE(1) injection. No significant change of analyzed variables was found in the control rats. These findings suggest that these biogenic amines of the PH are involved in the control of the sympathetic and thermogenic changes induced by PGE(1).

Interaction between cholinergic and adrenergic pathways of the hypothalamic ventromedial nucleus on cardiovascular regulation

In this work we studied the possible interaction between cholinergic (muscarinic and nicotinic) and adrenergic (alpha 1- and beta-adrenergic) pathways of the hypothalamic ventromedial nucleus on the regulation of arterial pressure and heart rate in conscious normotensive rats. Mean arterial pressure and heart rate were recorded in rats with cerebral chronic stainless steel cannulae implanted directly into the ventromedial nucleus. The changes in arterial pressure and heart rate produced by the injection of the cholinergic agonist (carbachol or nicotine) into the ventromedial nucleus were studied before and after the injection of prazosin (an alpha 1-adrenergic antagonist) or propranolol (a beta-adrenergic antagonist) into this same area. The injection of carbachol (2 nmol) or nicotine (40 mmol) into the ventromedial nucleus induced pressor and tachycardia responses. Previous treatment with prazosin or propranolol blocked the pressor response to carbachol and nicotine. Propranolol also abolished the tachycardic response to carbachol or nicotine, but prazosin reduced only the tachycardia produced by carbachol into the ventromedial nucleus. These results show an interaction between cholinergic and adrenergic pathways of the ventromedial nucleus affecting cardiovascular regulation and suggest that the alpha 1- and beta-adrenoceptors of this nucleus are involved in these responses.

Tolerance to nicotine-induced sympathoadrenal stimulation and cross-tolerance to stress: differential central and peripheral mechanisms in rats

Nicotine stimulates the secretion of catecholamines from sympathetic nerve endings and adrenal medulla by acting on peripheral nicotinic cholinergic receptors. Nicotine is also a potent stimulant in the central nervous system but the significance of nicotinic receptors in brain in mediating cardiovascular and sympathoadrenal responses to nicotine is unclear. The responses of resting plasma catecholamines, blood pressure and heart rate were compared in rats receiving nicotine, administered either systemically or intracerebroventricularly (i.c.v.). Sympathoadrenal stress responses were also studied in rats rendered tolerant to nicotine from repeated systemic or intraventricular injections. Nicotine, given either intraventricularly or systemically, produced dose-related increases in the concentration of epinephrine in plasma. Little effect on norepinephrine in plasma was observed with nicotine given intraventricularly, indicating predominant stimulation of adrenomedullary pathways. In contrast, nicotine, given systemically, produced comparable increases in both epinephrine and norepinephrine. Blood pressure increased and heart rate fell in response to either intraventricular or systemic administration of nicotine. Rats exhibited tolerance to nicotine 24 hr after a single intraventricular injection; however, tolerance was not detected with systemically injected nicotine unless the injections were given at least every 30 min. Whereas rats rendered tolerant to systemic administration of nicotine were cross-tolerant to stress, with respect to sympathoadrenal stimulation, cross-tolerance with stress was not detected in rats treated with nicotine repeatedly by the intraventricular route. These results indicate that nicotinic receptors in brain modulate the central sympathetic outflow and adapt readily to nicotine stimulation with prolonged tolerance, but are probably not involved in sympathoadrenal stress responses. Peripheral nicotinic receptors, regulating sympathoadrenal secretion of catecholamines, displayed much shorter-lasting tolerance.

Cardiovascular and muscle tone changes produced by microinjection of cholinergic and glutamatergic agonists in dorsolateral pons and medial medulla

Cardiovascular and muscle responses to L-glutamic acid (Glut) and cholinergic agonists injected into the dorsolateral pontine tegmentum and medial medullary reticular formation (MMRF) were examined in unanesthetized, decerebrated cats. Glut, or cholinergic agonists acetylcholine (ACh) or carbachol (Carb), were injected into pons and MMRF at sites from which electrical stimulation produced bilateral suppression of muscle tone. Glut injection in MMRF produced hypotension without change in heart rate at doses as low as 1 mM. At higher doses (0.1-0.4 M), Glut induced hypotension with bradycardia in 23 out of 40 injections in both pons and MMRF. High concentrations of microinjected Glut decreased muscle tone or produced complete atonia in pons and rostral MMRF. Both N-methyl-D-aspartic acid (NMDA) and non-NMDA receptor blockers attenuated or completely blocked the cardiovascular response, while only non-NMDA antagonists blocked muscle inhibition to Glut injection. Microinjection of cholinergic agonists produced consistent hypotension in all of the injections in pons and MMRF, however, the heart rate response was variable with increase (27/42), decrease (2/42), or no change (13/42) in rate seen. Cholinergic injection produced muscle atonia in pons and caudal MMRF but not in rostral MMRF. Both muscle and cardiovascular responses were blocked by atropine but not by hexamethonium. The time course of muscle atonia and cardiovascular change differed in most of the experiments. We conclude that muscle tone suppression and cardiovascular response to Glut or cholinergic agonists use different receptor mechanisms and possibly different neurons. However, the co-localization of these mechanisms suggests that neuronal networks in the medial medulla and dorsolateral pons coordinate motor and cardiovascular responses.

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.

Brain nicotine cholinoceptor binding in spontaneous hypertension

To study the role of central cholinergic mechanisms in hypertension, we have determined nicotinic and muscarinic agonist binding sites in the brain regions of stroke-prone spontaneously hypertensive rats (SHRSP), using [3H]nicotine and [3H]cismethyldioxolane (CD). There was a significant decrease in specific [3H]nicotine binding in the cerebral cortex, thalamus, midbrain, cerebellum and medulla oblongata of SHRSP at 16-24 weeks of age compared to that of age-matched Wistar Kyoto rats (WKY). Scatchard analysis revealed 35% decrease in the Bmax value for [3H]nicotine binding in the SHRSP medulla oblongata without a change in the Kd value, suggesting a change in the receptor density. Similar reduction of nicotinic cholinoceptor binding sites was also observed in the discrete brain regions of young (5-week-old) SHRSP. In contrast, there was no alteration in specific [3H]CD binding in the SHRSP brains regions, except the hypothalamus which showed a significant increase. The SHRSP medulla oblongata showed no change in the ChAT activity. Thus, the present study suggests an important role for medullary nicotinic cholinoceptors in the pathogenesis of spontaneous hypertension.

Cardiovascular effects of perfusion of the rostral rat hypothalamus with clonidine: differential interactions with prazosin and yohimbine

The present studies examined the role of alpha 1- versus alpha 2-adrenoceptors in the cardiovascular effects of clonidine administered into the anterior hypothalamic/pre-optic (AH/PO) region of the forebrain by the push-pull perfusion technique. Push-pull cannulas were placed bilaterally into the AH/PO region of anesthetized, paralyzed and ventilated rats. Perfusion of this area with artificial CSF (0.015 ml/min), yohimbine (5 or 50 microM) for 30 min did not affect mean arterial blood pressure or heart rate. Perfusion of the AH/PO region with clonidine (0.55-5.50 mM) resulted in a concentration-dependent reduction of mean arterial pressure and heart rate. The hypotensive effects of clonidine were found to be greater than the bradycardic effects, when expressed as a percent of pre-infusion baseline values. Co-perfusion with yohimbine (5, 50 microM) significantly attenuated the hypotensive, but not the bradycardic, effects of a single concentration (1.75 mM) of clonidine; this selective antagonism of the hypotensive effect of clonidine by yohimbine was concentration dependent. In contrast to yohimbine, co-perfusion with 5 microM prazosin did not significantly affect either the clonidine-induced hypotension or bradycardia. Co-perfusion with the higher concentration (50 microM) of prazosin significantly reversed the bradycardic, but not the hypotensive, effects of 1.75 mM clonidine. These results suggest that AH/PO clonidine perfusion depresses both mean arterial pressure and heart rate, and that the clonidine-induced hypotension is due to alpha 2-adrenoceptor activation, while the clonidine-induced bradycardia is due to alpha 1-adrenoceptor activation.

Alterations in blood pressure and REM sleep after pontine carbachol microinfusion

Infusions of cholinomimetics, either systemically in normal humans, or directly into the brain stem of unanesthetized cats and rats, readily induce rapid eye-movement (REM) sleep. In anesthetized cats and rats, infusions of cholinomimetics have been shown to produce both increases and decreases in arterial blood pressure. We determined the relation of these blood pressure changes to REM sleep, by examining both blood pressure and sleep states after injecting carbachol at midbrain, pontine, and medullary sites in unanesthetized cats. In the pons, carbachol infusions produced an early decrease in blood pressure followed by a sustained hypertensive effect. The early blood pressure decrease was associated with the occurrence of REM sleep; however, higher values were associated with later REM sleep episodes. In other brain stem sites carbachol did not produce REM sleep or its associated reductions in blood pressure. Instead it produced a hypertensive response that increased throughout the 1-h observation period. We hypothesize that pontine muscarinic mechanisms trigger REM sleep and a REM sleep-associated decrease in blood pressure. Thereafter, nicotinic receptors mediating the blood pressure increase override the muscarinic-induced decrease.

Role of medullary cholinoceptors in baroreflex bradycardia

Cholinergic receptors present in three medullary nuclei namely, the nucleus tractus solitarii (NTS), nucleus ambiguous (AMB) and the dorsal motor nucleus of the vagus nerve (DMV) have been studied with regard to their role in regulation of heart rate (HR), blood pressure (BP) and baroreceptor reflex activation induced bradycardia in cats. Microinjection of carbachol into NTS was without effect while administration of carbachol or pilocarpine into AMB and DMV elicited dose related decrease in HR without affecting BP. These effects were completely antagonized by ethylbenztropine. Bilateral muscarinic cholinoceptor blockade of either AMB or DMV, with ethylbenztropine, produced a partial inhibition of the baroreflex bradycardia while intracisternal ethylbenztropine completely abolished this reflex response. Involvement of muscarinic cholinoceptors of AMB or DMV in baroreflex mediated adjustments of HR is therefore suggested.

An analysis of the alpha-adrenoceptor modulation of vasomotor tone at the level of lateral medullary pressor area (LMPA)

Microinjection of noradrenaline and clonidine into lateral medullary pressor area (LMPA) of chloralose anaesthetized cats produced dose dependent decrease in blood pressure without affecting heart rate, while phenylephrine did not elicit any cardiovascular response. Selective alpha 2-adrenoceptor antagonists idazoxan and piperoxan, microinjected locally, blocked the effects of the agonists but prazosin and phenoxybenzamine, which are relatively selective for alpha 1-adrenoceptors, failed to do so. Clonidine did not elicit any response in guanethidine pretreated cats but noradrenaline microinjected into LMPA of these animals induced a pressor response which was blocked by prazosin pretreatment. It is concluded that catecholaminergic fibres impinging upon this area inhibit the activity of the inhibitory second order baroreceptor neurone by activating alpha 1-adrenoceptors while alpha 2-adrenoceptors situated presynaptically on these inhibitory catecholaminergic nerve terminals are responsible for the manifestation of the hypotensive effect of clonidine and exogenously administered noradrenaline.

Nucleus locus coeruleus: evidence for alpha 1-adrenoceptor mediated hypotension in the cat

Microinjection of noradrenaline or phenylephrine into the nucleus locus coeruleus of cats induced a dose dependent and long lasting hypotension. Clonidine was required in a dose of 1 microgram for eliciting a significant hypotension while its lower doses (up to 500 ng) failed to elicit any significant cardiovascular alteration. The effects on heart rate evoked by these agents were insignificant. Microinjection of alpha-adrenoceptor antagonists prazosin, piperoxan and RX 781094 per se did not evoke any significant cardiovascular effects and only prazosin pretreatment showed dose dependent antagonism of the hypotensive effect of clonidine. Piperoxan was required in four times higher dose (20 micrograms) to partially antagonize the clonidine induced hypotension. RX 781094, a selective alpha 2-adrenoceptor antagonist, however, even up to a dose of 20 micrograms (four times that of prazosin) did not alter the effect of clonidine. Similar pattern of antagonism was also seen for noradrenaline and phenylephrine. The results demonstrate the presence of alpha 1-adrenoceptors in the nucleus locus coeruleus, the activation of which leads to a fall in blood pressure.

Involvement of central dopamine receptors in the hypotensive action of pergolide

The involvement of central dopamine receptors in the hypotensive action of the dopaminergic ergoline, pergolide was determined in anesthetized rats. Intravenous (i.v.) or intracerebroventricular (i.v.t.) administration of pergolide (12.5, 25 and 50 micrograms/kg) produced dose-dependent decreases in blood pressure. The magnitude of hypotension seen following either i.v. or i.v.t. administration of pergolide was similar. However, while both sulpiride (1 mg/kg, i.v.) as well as phentolamine (1 mg/kg, i.v.) antagonized the hypotensive action of i.v. pergolide, only sulpiride (1 mg/kg, i.v.t.) was able to antagonize the hypotension seen following i.v.t. administration of pergolide. Phentolamine (1 mg/kg, i.v.t.) did not alter the central hypotensive action of pergolide. In a separate group of rats, clonidine (25 micrograms/kg, i.v.t.) also produced a decrease in blood pressure. While phentolamine (i.v.t.) antagonized the central hypotensive action of clonidine, sulpiride (i.v.t.) did not have any effect on the action of clonidine. These results show that selective activation of central dopamine receptors was responsible for the hypotensive action of centrally-administered pergolide. In a separate group of rats greater splanchnic sympathetic nerve activity was measured. Intravenous pergolide produced similar hypotensive response as seen in previous groups, and this was accompanied by a concomitant decrease in the sympathetic nerve activity. The maximum fall in blood pressure (26 +/- 6 mm Hg) was correlated with a 40% reduction in sympathetic nerve activity. The return of blood pressure to control levels occurred after 60-70 min and was also associated with the return of sympathetic nerve activity to control levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Adrenergic influences on the spinal cardiovascular neurones

In cervical spinal cord transected and bilaterally vagotomized dogs, intrathecal (i.t.) injection of clonidine decreased the resting heart rate and mean arterial pressure (MAP) while isoprenaline increased the heart rate. The clonidine induced bradycardia and hypotension were antagonized by piperoxan pretreatment. Similarly, the isoprenaline induced tachycardia was antagonized by pretreatment (i.t.) with atenolol. Intrathecal atenolol per se decreased the heart rate and MAP while piperoxan had no effect. The post-coronary artery ligation cardiac arrhythmia, in the cervical spinal cord transected and bilaterally vagotomized dogs, was inhibited by clonidine, phenylephrine and methoxamine and facilitated by isoprenaline. beta-Adrenoreceptor antagonists propranolol, atenolol and acebutolol, given intrathecally, inhibited while piperoxan facilitated the cardiac arrhythmia. The clonidine and isoprenaline induced changes in the arrhythmia were antagonized by i.t. pretreatment with piperoxan and atenolol respectively. It appears that the alpha-adrenoreceptors are inhibitory while beta-adrenoreceptors are facilitatory for spinal control of heart rate and blood pressure and for post-coronary artery ligation cardiac arrhythmia.

Evidence for involvement of alpha 2-adrenoceptors in the nucleus ambiguous in baroreflex-mediated bradycardia

Microinjection of noradrenaline and clonidine into the nucleus ambiguus elicited dose-dependent bradycardia with insignificant alteration of blood pressure. Phenylephrine failed to elicit any cardiovascular effect. The bradycardic effects of noradrenaline and clonidine were antagonized by piperoxan but not by phenoxybenzamine. Adrenergic neurone blockade with local guanethidine pretreatment also abolished the response to clonidine. No significant cardiovascular effect of clonidine microinjection into the nucleus ambiguus was observed in bilaterally vagotomized animals. The baroreflex bradycardia induced by volume loading as abolished by yohimbine and piperoxan but not by phenoxybenzamine, microinjected bilaterally into the nucleus ambiguus. These results demonstrate the presence of cardioinhibitory, presynaptic alpha 2-adrenoceptors in the nucleus ambiguus and their involvement in baroreflex bradycardia.

The regulation of cardiovascular functions by monoamine neurotransmitters in the brain

The complexity of the central nervous system's influence on cardiovascular integration is underscored by the extensive neural interconnections between the various higher and lower brain structures shown by electrophysiological studies to have an influence on cardiovascular function. Furthermore, complex neural connections exist within each level of cardiovascular integration (for instance, the medullary reticular formation, or the limbic system). The electrophysiological analysis of the integrative aspects of central cardiovascular regulation has been the subject of several reviews (Reis, 1972; Smith, 1974; Calaresu et al., 1975; Oberg, 1976). In recent years, the methodology of analyzing the anatomical neural pathways of central cardiovascular regulation by lesion-degeneration, evoked potential, and fluorescent histochemical procedures, has been improved with the introduction of anterograde and retrograde tracer techniques. The result is the delineation of previously undetected neural interconnections among various brain areas, long known as cardiovascular centres, such as the NTS, the parabrachial nucleus, the paraventricular nucleus of the hypothalamus, the central nucleus of the amygdala, and the bed nucleus of the stria terminalis (Loewy & McKellar, 1980). Undoubtedly the next few years will see major advances in the understanding of the neuroanatomical pathways involved in central cardiovascular control.

Cholinergic influences on the spinal cardiovascular neurones

1 In bilaterally vagotomized and spinal cord (C1) transected dogs, intrathecal (i.t.) injection of pilocarpine decreased while DMPP increased the blood pressure. These responses were antagonized by i.t. pretreatment with ethylbenztropine and chlorisondamine respectively. 2 No change in the resting heart rate occurred after intrathecal injection of cholinergic drugs in these animals. 3 The post-coronary artery ligation cardiac arrhythmia, in bilaterally vagotomized and spinal (C1) transected dogs, was inhibited by i.t. pilocarpine and chlorisondamine and facilitated by ethylbenztropine and DMPP. 4 The effects of muscarinic and nicotinic receptor agonists on the arrhythmia were blocked by their respective blockers. 5 It appears that the muscarinic receptors are inhibitory while nicotinic receptors are facilitatory for spinal control of vasomotor tone and the post-coronary artery ligation cardiac arrhythmia. However, the spinal muscarinic and nicotinic receptors do not appear to have a significant role in the regulation of normal heart rate.

Involvement of alpha2-adrenoceptors of nucleus tractus solitarius in baroreflex mediated bradycardia

Microinjections of noradrenaline and clonidine into nucleus tractus solitarius produced dose dependent bradycardia without significant decrease in blood pressure in chloralose anaesthetized cats. Phenylephrine failed to produce any significant alteration of heart rate or blood pressure. Piperoxan microinjection into nucleus tractus solitarius elicited a mild but significant tachycardia and could also block the noradrenaline and clonidine responses. Phenoxybenzamine however neither affected resting heart rate and blood pressure nor antagonized the responses of noradrenaline and clonidine. Guanethidine pretreatment of nucleus tractus solitarius also abolished the clonidine response. Baroreceptor reflex activation induced bradycardia was inhibited by yohimbine or piperoxane injected into the cisterna magna or microinjected bilaterally into the nucleus tractus solitarius. Pretreatment of nucleus tractus solitarius with phenoxybenzamine by either route, did not affect the reflex bradycardia. It is concluded that the alpha-adrenoceptors of nucleus tractus solitarius involved in the decrease in heart rate during baroreceptor activation are alpha2 in nature.

Cardiovascular dopamine receptors: physiological, pharmacological and therapeutic implications

Dopamine receptor activation can lead to pronounced changes in cardiovascular function. The myriad of effects produced by dopamine receptor agonists results from the activation of dopamine receptors located at different anatomical sites in the cardiovascular system. Further basic research is required to better characterize these dopamine receptors so as to allow the development of more specific dopamine receptor agonists. Endogenous dopamine may be involved in the physiological control of fluid and electrolyte balance and continuing research efforts in this area should provide for a better understanding of the role of cardiovascular dopamine receptors in the maintenance of overall circulatory homeostasis. Cardiovascular dopamine receptor stimulation represents an important and promising approach for the development of novel therapeutic agents for the treatment of hypertension, angina pectoris, congestive heart failure and acute renal failure.

The influence of physostigmine on respiratory and circulatory changes caused by overdoses of orphenadrine or imipramine in the rat

Orphenadrine or imipramine were given intravenously as an infusion to spontaneously breathing, anaesthetized rats until respiratory arrest, the primary cause of death for both drugs. Intravenous injections of physostigmine did not prolong survival. Artificial ventilation prolonged survival for orphenadrine and imipramine by about a factor 3 and the rats died from cardiogenic shock. The cardiotoxic properties of orphenadrine and imipramine express themselves as a progressing disturbance in stimulus formation and conduction, a decrease in dP/dt max and increase in left ventricular end-diastolic pressure and a decrease in cardiac output caused by the progressing decrease of heart rate. An intravenous injection of physostigmine did not prolong survival and had no favourable effect on the cardiotoxicity caused by orphenadrine and imipramine. Although physostigmine may be useful in the treatment of the anticholinergic syndrome it has, at least in the rat, no favourable effect on the respiratory insufficiency, due to overdoses of orphenadrine and imipramine. Moreover it is not effective in antagonizing the cardiotoxic effects of orphenadrine or imipramine.

Central effects of paraoxon on haemodynamics in the cat

Application of paraoxon into the left vertebral artery (8--80 micrograms) or both the left and right vertebral artery (4--8 micrograms) of the anaesthetized cat evoked dose-dependent depressor effects, whereas heart rate was not influenced significantly. Also after systemic administration of paraoxon (150--825 micrograms . kg-1), while peripheral muscarinic receptors were blocked, depressor effects were still observed. Dose-response curves for the depressor response to paraoxon were established. Infusion of low doses of dexetimide via the vertebral artery prevented the hypotensive action of paraoxon. The distribution of this antimuscarinic drug in the brain was investigated. The depressor effect of paraoxon can be attributed to both a decrease in peripheral resistance and cardiac output. Decerebration and midcollicular transection were carried out in order to elucidate the site and mechanism of action. The depressor effect of paraoxon seems to be mediated by a central mechanism of action located within the lower brain stem. It is concluded that stimulation of muscarinic receptors in the pontomedullary region gives rise to the observed changes in haemodynamic parameters. Muscarinic receptors in the hypothalamus seem to be of minor importance for the hypotensive action of paraoxon.

Central cardiovascular effects of nylidrin (buphenine)

The injection of small doses of nylidrin into the vertebral artery lowered blood pressure in chloralose-anesthetized cats. Cerebroventricular perfusion of nylidrin also decreased blood pressure and inhibited or modified cardiovascular reflexes. There was inhibition of the reflex bradycardia evoked by i.v. noradrenaline or angiotensin. Likewise the pressure increase due to bilateral carotid occlusion was diminished. The blood pressure decrease elicited by stimulation of the central stump of a cut vagus nerve was inhibited or reversed. The pattern of central cardiovascular effects of nylidrin seemed not compatible with its well known beta-sympathomimetic properties. Indirect evidence points to an interasction with central alpha-receptors.