Cardiac sympathetic nerve activity: changes induced by ouabain and propranolol.

A study of the effects of ouabain and propranolol on the spontaneous activity in the preganglionic sympathetic nerves to the cat heart showed that ouabain can produce both an inhibition and a stimulation of the spontaneous activity in sympathetic nerves. The inhibition appears to be reflex in nature and is not present when the buffer nerves are sectioned. The stimulation is correlated with the development of cardiac arrhythmias and is antagonized by propranolol.

Failure of beta-adrenergic receptor blockade to prevent arrhythmias induced by sympathetic nerve stimulation.

Cardiac arrhythmias produced by electrical stimulation of the ventrolateral cardiac sympathetic nerve in dogs were not blocked by the combined administration of propranolol and practolol in amounts that completely blocked cardiac beta-adrenergic receptors. Blockade of cardiac alpha-adrenergic receptors, as well as cardiac cholinergic receptors, also had no influence on the arrhythmias. These results suggest that the adrenergic neuroeffector junction is fundamentally different from any hitherto described, differing perhaps in the neurotransmitter involved or in the nature of the receptor.

GABA receptor control of parasympathetic outflow to heart: characterization and brainstem localization.

Blockade of gamma-aminobutyric acid (GABA) receptor function by direct microinjection of the GABA receptor antagonist bicuculline into the nucleus ambiguus of the brainstem produced a marked, dose-related depression of heart rate and blood pressure which was mediated by the vagus nerve. This effect was not obtained in other regions of the brainstem and was reversed by the GABA receptor agonist muscimol. These data indicate that the nucleus ambiguus may be the site of a GABA receptor-mediated inhibition of vagal outflow.

Hindbrain GABA receptors influence parasympathetic outflow to the stomach.

Blockade of gamma-aminobutyric acid receptor function by direct microinjection of bicuculline into the nucleus ambiguous in cats produced a marked increase in gastric motility which was mediated by the vagus nerve. This effect was reversed by muscimol. These data indicate that the nucleus ambiguous may be an important brain site influencing gastric function and that the neurotransmitter controlling parasympathetic overflow from this nucleus to the stomach is gamma-aminobutyric acid.

Central nervous system site of action for the respiratory depressant effect of diacetylmorphine (heroin) in the cat.

The purpose of our study was to identify central nervous system sites involved in the respiratory depressant effect of drugs that stimulate opioid receptors. Diacetylmorphine (heroin) was administered into several cerebroventricular regions of chloralose-anesthetized cats, while monitoring pulmonary ventilation with a Fleisch pneumotachograph. Administration of heroin (17, 50, 150, and 450 micrograms) into the forebrain ventricles, which was restricted to these ventricles, resulted in no significant respiratory effects. In contrast, administration of heroin into either the fourth ventricle or the cisterna magna resulted in a significant (P less than 0.05) decrease in respiratory minute volume (VE). In the fourth ventricle this was because of a decrease in frequency (f) and in the cisterna magna, to a decrease in tidal volume (VT). Intravenous administration of heroin in the same dose-range produced a decrease in VE, which was primarily due to a decrease in f. Bilateral application of heroin (70 micrograms/side) to each of three ventral medullary surface sites (Mitchell's, Schlaefke's, and Loeschcke's areas) known to influence respiration elicited a decrease in VE only at Mitchell's area. This decrease was due to decreases in f and VT. The role of this site in the action of intravenously administered heroin was tested by topical application of naloxone to this area in animals with respiratory depression evoked by intravenous heroin. Bilateral application of naloxone (15 micrograms/side) to Mitchell's area restored breathing to normal. These results lead us to suggest that the site of heroin-induced respiratory depression is a specific area (Mitchell's area) on the ventral surface of the medulla.

Basic fibroblast growth factor increases long-term survival of spinal motor neurons and improves respiratory function after experimental spinal cord injury.

Acute focal injection of basic fibroblast growth factor (FGF2) protects ventral horn (VH) neurons from death after experimental contusive spinal cord injury (SCI) at T8. Because these neurons innervate respiratory muscles, we hypothesized that respiratory deficits resulting from SCI would be attenuated by FGF2 treatment. To test this hypothesis we used a head-out plethysmograph system to evaluate respiratory parameters in conscious rats before and at 24 hr and 7, 28, and 35 d after SCI. Two groups of rats (n = 8 per group) received either FGF2 (3 microg) beginning 5 min after injury or vehicle (VEH) solution alone. We found significantly increased respiratory rate and decreased tidal volume at 24 hr and 7 d after SCI in the VEH-treated group. Ventilatory response to breathing 5 or 7% CO(2) was also significantly reduced. Recovery took place over time. Respiration remained normal in the FGF2-treated group. At 35 d after injury, histological analyses were used to compare long-term neuron survival. FGF2 treatment doubled the survival of VH neurons adjacent to the injury site. Because the number of surviving VH neurons rostral to the injury epicenter was significantly correlated to the ventilatory response to CO(2), it is likely that the absence of respiratory deficits in FGF2-treated rats was caused by its neuroprotective effect. Our results demonstrate that FGF2 treatment prevents the respiratory deficits produced by thoracic SCI. Because FGF2 also reduced the loss of preganglionic sympathetic motoneurons after injury, this neurotrophic factor may have broad therapeutic potential for SCI.

Effect of cocaine on the coronary circulation and systemic hemodynamics in dogs.

This study investigated the effect of intravenous cocaine (0.5 to 2 mg/kg body weight) on the coronary circulation and systemic hemodynamics in closed chest sedated dogs. The role of alpha- and beta-adrenoceptor stimulation in mediating these effects was also investigated. Cocaine produced dose-dependent increases in mean arterial pressure and rate-pressure product. Although the lower doses of cocaine had no significant effect on the coronary circulation, the 2 mg/kg dose produced a 55 +/- 14% increase in coronary vascular resistance (p less than 0.05 versus baseline) and a 19 +/- 3% reduction in diameter of the left anterior descending coronary artery (p less than 0.05 versus baseline). Despite these potentially deleterious effects on the coronary circulation (occurring at a time of markedly increased myocardial oxygen demand), the electrocardiogram did not demonstrate ischemic changes and there was no myocardial lactate production. Cocaine-induced coronary vasoconstriction was abolished by pretreatment with the alpha-adrenoceptor antagonist phentolamine, but not by pretreatment with the beta-adrenoceptor antagonist propranolol. The findings that cocaine did not change systemic vascular resistance in dogs without adrenergic blockade, reduced systemic vascular resistance in dogs after alpha-blockade (p less than 0.05) and increased systemic vascular resistance in dogs after beta-blockade (p = 0.06) suggest that epinephrine (rather than norepinephrine) is primarily responsible for the peripheral vascular actions of cocaine. Thus, in this canine preparation with normal coronary arteries, cocaine produced vasoconstriction of both epicardial and coronary resistance vessels that was not associated with evidence of myocardial ischemia. The pharmacologic mechanism for the effect of cocaine on the coronary circulation is alpha-adrenoceptor stimulation, whereas systemic hemodynamic effects are mediated by combined alpha- and beta-adrenoceptor stimulation.

Exacerbation of coronary occlusion induced ventricular arrhythmias by the vagolytic effect of procainamide.

The influence of the vagolytic effect of procainamide on the early ventricular arrhythmias induced by left anterior descending coronary (LAD) occlusion was studied in chloraloseanaesthetised cats. All control animals developed a ventricular arrhythmia (1119 +/- 166 PVCs per hour), with a consistent onset time, duration, and overall mortality due to ventricular fibrillation (ie 20%). In 18 animals pretreated with procainamide (0.5 mg.kg-1.min-1 for 50 min), there was no effect on the ventricular arrhythmia in terms of ectopic frequency (1020 +/- 180 PVCs per hour), time to onset of arrhythmia, duration of arrhythmia, and mortality incidence (ie 16.7%). However, subdividing the data according to whether or not vagal blockade had been produced by procainamide revealed that animals exhibiting complete vagal blockade demonstrated significantly more ectopic beats (1606 +/- 310 PVCs per hour) and 33% developed ventricular fibrillation. Treated animals without complete vagal blockade exhibited an ectopic frequency rate of 620 +/- 98 PVCs per hour and none of the animals developed ventricular fibrillation. The haemodynamic parameters were similar between both procainamide treated subgroups. These results suggest that an important factor in response of the ischaemic heart to the cardiac rhythm effects of procainamide is the degree of vagal blockade induced by this agent.

Influence of vagotomy on changes in feline plasma catecholamine levels induced by occlusion of either the left or right coronary vessel.

The purpose of this study was two-fold: (1) to determine the effect of occlusion of the left anterior descending branch (LAD) of the left coronary artery or the right coronary (RC) artery on plasma catecholamine levels, and (2) to determine whether bilateral vagotomy has an effect on changes in plasma catecholamine levels evoked by coronary occlusion. Chloralose anaesthetised cats subjected to LAD occlusion exhibited increases in plasma noradrenaline and adrenaline at 3 min post-occlusion. The increases in noradrenaline and adrenaline were unrelated to the hypotension that occurred at this time. Bilateral vagotomy did not appear to alter the effect of LAD occlusion on catecholamine release into the circulation but did unmask a significant correlation between the degree of hypotension and the magnitude of increase in plasma catecholamines. Right coronary occlusion in animals with intact and sectioned vagus nerves evoked noradrenaline and adrenaline release that was significantly correlated with a fall in arterial pressure. Bilateral vagotomy per se caused an increase in baseline plasma catecholamine levels. Pretreatment with atropine mimicked the increase in baseline catecholamine levels seen with vagotomy. These results indicate that occlusion of the LAD and RC arteries increase the release of catecholamines into the circulation. The role of the vagus nerves in this response was observed only with LAD occlusion and consisted of altering the relationship between the degree of hypotension and the magnitude of increase in plasma catecholamines. That is, after vagotomy, the decrease in blood pressure following LAD occlusion was effective in causing release of catecholamines, presumably because of the hypotension causing a decrease in baroreceptor stimulation. Finally, it appeared that vagotomy increases the release of noradrenaline into the circulation by removing efferent vagal tone that inhibits noradrenaline release. This inhibitory action is mediated by activation of muscarinic receptors.

Adrenaline-synthesizing neurons in the medulla of the cat.

In this study, the distribution of neurons containing the adrenaline-synthesizing enzyme phenylethanolamine-N-methyltransferase (PNMT) was mapped in the medulla of the cat. Data from recent studies in the rat suggest that the anatomical structure responsible for cardiorespiratory changes that occur following application of neurotransmitters and drugs to Schlaefke's area on the ventral medullary surface is the nucleus reticularis rostroventrolateralis (RVL), which is distinguished from adjacent regions of the reticular formation, in part, by the presence of adrenaline-synthesizing neurons. To determine whether an equivalent adrenergic population is present in the RVL of the cat, we used antibodies raised against bovine adrenal PNMT to map the distribution of adrenaline-synthesizing neurons in the reticular formation. In the ventrolateral medulla, we found that labeled cells extended from the level of the retrofacial nucleus to the calamus scriptorius. The majority of labeled cells were seen in a nucleus designated RVL at the level of the rostral one-third of the inferior olive. In the dorsomedial medulla, cells were labeled in the caudal aspect of the nucleus tractus solitarii (NTS) and were especially dense in the subnucleus gelatinosus and commissural nucleus of the vagus. A few lightly labeled cells were also present in the rostral pole of the area postrema (AP). In contrast to the rat, few or no immunoreactive cells were found in the rostral NTS, medial longitudinal fasciculus, nucleus paragigantocellularis dorsalis, or periventricular gray. Our results are consistent with the notion that an area of the RVL containing adrenergic perikarya is the anatomical structure responsible for cardiovascular changes that occur when chemicals are applied to Schlaefke's area.

Comparative effects of urapidil, prazosin, and clonidine on ligand binding to central nervous system receptors, arterial pressure, and heart rate in experimental animals.

We studied the effects of urapidil, clonidine, and prazosin on ligand binding to central nervous system receptors in rats and on arterial pressure and heart rate in chloralose-anesthetized cats. Ligand binding studies indicated that urapidil had 90 times greater affinity for alpha 1 than for alpha 2 adrenergic receptors. Administration of urapidil (129 micrograms) into the cerebroventricles of cats revealed no effect after lateral ventricle injection, a decrease of 9.7 +/- 3.0 mm Hg after fourth ventricle injection, and an increase of 10.8 +/- 2.2 mm Hg after restriction of the drug in the forebrain ventricles. Clonidine (30 micrograms) produced hypotension and bradycardia after injection into the lateral ventricle. Prazosin was ineffective after cerebroventricular injection. Intravenous administration of 0.22, 0.67, and 2.00 mg/kg urapidil produced dose-dependent decreases in arterial pressure that were associated with blockade of alpha 1 adrenergic receptors. Intravenous administration of prazosin elicited the same response. Clonidine (10 micrograms/kg, intravenously produced an initial increase in arterial pressure that was unaffected by pretreatment with urapidil or prazosin. These results suggest that urapidil produces hypotension by an action on the peripheral vasculature and in the hindbrain. The peripheral effect involves blockade of alpha 1 adrenoceptors.

Increases in coronary vascular resistance initiated by blockade of CNS GABAergic tone occurs in the hindbrain.

The purpose of this study was to determine the site of action in the CNS responsible for producing picrotoxin-induced sympathetic mediated increase in coronary vascular resistance. To do this, picrotoxin, was administered either into the lateral cerebral ventricle, with the perfusion restricted to the forebrain, or into the fourth ventricle, to perfuse only the hindbrain and spinal cord, in chloralose-anesthetized cats, while monitoring coronary blood flow from the anterior descending branch of the left coronary artery, arterial pressure, heart rate and electrocardiogram (ECG). There was no difference between administration into the forebrain and hind-brain in terms of changes in coronary vascular resistance, ECG, arterial pressure and sinus rate when 600 micrograms of picrotoxin was used. Administration into either area elicited significant increases in coronary vascular resistance, arterial pressure and sinus rate, as well as changes in the ST segment and occasional ventricular tachyarrhythmias. However, a separation of effects was noted between the forebrain and the hindbrain when 200 micrograms of picrotoxin was administered. Administration of this dose into the forebrain did not significantly alter coronary vascular resistance or the ST segment, although significant increases in arterial pressure and sinus rate occurred in these animals. In contrast, administration of this dose into the hindbrain elicited significant increases in coronary vascular resistance, ST segment, arterial pressure and sinus rate. These results indicate that the most sensitive site for eliciting picrotoxin-induced increase in coronary vascular lies in the hindbrain.

Cardiorespiratory effects produced by injecting drugs that affect GABA receptors into nuclei associated with the ventral surface of the medulla.

It has recently been shown that L-glutamic acid induced stimulation of cell bodies in a circumscribed area of the rostral ventrolateral medulla (RVLM) in the cat, produced increases in arterial pressure (AP), decreases in heart rate (HR) and transient apnea (Gatti, Norman, DaSilva and Gillis, 1986). The purpose of the present study was to determine if this same area was sensitive to GABA receptor agonists and antagonists. Injection of the GABA agonist muscimol (200 ng), into the rostral ventrolateral medulla of cats anesthetized with chloralose produced a precipitous and immediate fall in arterial pressure (-95 +/- 4.6) and heart rate (-31 +/- 5.9; n = 4, P less than 0.05). Maximal cardiovascular effects could only be achieved if muscimol was injected bilaterally. These effects of muscimol on arterial pressure were dose-dependent. Time-action curves for the effects of muscimol on arterial pressure and respiration were different. Hypotension occurred first and was followed later in time by a decrease in minute ventilation. Within 30 min all animals were apneic after the 200 ng dose. The cardiovascular effects of muscimol were reversed by the injection of the GABA receptor antagonist bicuculline. These data indicate that stimulation of GABA receptors in the rostral ventrolateral medulla produced selective cardiovascular effects and that respiratory neurons sensitive to GABA are apparently not localized with these cardiovascular neurons.

Blockade of bicuculline-induced pressor and tachycardic responses by forebrain administration of muscimol.

To determine whether i.v. administered bicuculline acts in the forebrain to increase arterial blood pressure and heart rate, this agent was administered i.v. to chloralose anesthetized cats that had muscimol injected into and restricted to the forebrain ventricles. Bicuculline (0.5 mg/kg i.v.) given alone increased arterial pressure by 56 +/- 8 mm Hg and heart rate by 45 +/- 11 beats/min. Bicuculline given to animals exposed to muscimol exhibited no increase in either of these parameters. Muscimol localized to the forebrain did not alter the pressor response to a non-GABA antagonist agent, strychnine, indicating a specific interaction of the drugs with GABA receptors in the forebrain.

Central cardiovascular effects produced by the GABA receptor agonist drug, THIP.

In chloralose-anesthetized cats, injection of THIP (30-1000 microgram) into the fourth ventricle produced dose-dependent decreases in blood pressure and heart rate. Administration of similar doses into either the lateral and third ventricle or administration of the largest dose intravenously did not produce these effects. Microinjection of THIP (1.95 microgram) into nucleus ambiguus reversed bradycardia evoked by microinjection of bicuculline methiodide (160 ng) into this nucleus. These results confirm previous findings indicating that activation of GABA receptors in the hindbrain produces hypotension and bradycardia as well as reversal of bicuculline-induced activation of cardiac parasympathetic nerves.

Respiratory depression produced by glycine injected into the cisterna magna of cats.

Glycine (0.8-64.8 mumol) was administered into the cisterna magna of alpha-chloralose anesthetized cats to determine its effect on ventilation. Glycine caused a dose-related decrease in respiratory minute volume with the highest dose resulting in a decrease from 454 +/- 35 to 159 +/- 44 ml/min (p less than 0.05). This decrease was due primarily to a reduction in tidal volume which decreased from 31 +/- 2 to 12 +/- 2 ml (p less than 0.05). The two largest doses tested (21.6 and 64.8 mumol) also produced a decrease in respiratory rate from 14 +/- 1 to 11 +/- 1 and from 15 +/- 1 to 12 +/- 1 breaths/min (p less than 0.05), respectively. Apnea occurred in 2 of 7 animals given 64.8 mumol of glycine. No consistent dose-related changes in inspiratory and expiratory durations were observed. Intravenous administration of glycine (64.8 mumol) did not affect respiratory activity. These results indicate that glycine causes respiratory depression by an action in the central nervous system.

Stress-induced changes in the function of the parasympathetic nervous system are mimicked by blocking GABA in the CNS of the cat.

The purpose of this study was to determine the effect of blockade of receptors for gamma-aminobutyric acid (GABA) in the forebrain, on vagal activity to the stomach and heart. This was done by injecting bicuculline (50 micrograms) into the lateral ventricle of the brain and restricting the drug to the forebrain ventricles by cannulating the cerebral aqueduct. Studies were performed in chloralose-anesthetized cats and gastric motility was monitored using extraluminal force transducers, sutured to the antrum and pylorus. Cardiac vagal activity was determined by noting the sinus bradycardia that developed from activation of the baroreceptor reflex induced by phenylephrine. Administration of bicuculline into the lateral ventricle of 7 animals produced increases in the minute motility index of 5.3 +/- 0.8 (antrum) and 13.9 +/- 2.1 (pylorus). This was associated with inhibition of baroreceptor-induced vagal bradycardia (i.e. -38 +/- 6.4 beats/min before bicuculline and -7.7 +/- 5.7 beats/min after bicuculline). These data indicate that a GABAergic mechanism in the forebrain may be important for controlling vagal outflow to both the stomach and the heart.

Nicotinic receptor mediates spontaneous GABA release in the rat dorsal motor nucleus of the vagus.

Spontaneous postsynaptic currents were investigated in neurons of the caudal portion of the dorsal motor nucleus of the vagus using the patch-clamp technique to study the effect of neuronal nicotinic acetylcholine receptor activation on synaptic transmission. In voltage-clamped neurons, bath application of nicotine (1-30 microM) elicited a concentration-dependent increase in the frequency of the spontaneous synaptic currents. The effect was also observed with application of the nicotinic receptor agonists epibatidine (10 nM) and cytisine (10 microM). Mecamylamine (20 microM) and curare (50 microM), two nicotinic receptor antagonists, both decreased the effect of 3 microM nicotine on the frequency of the spontaneous postsynaptic currents. This effect of 3 microM nicotine was also blocked by 20 microM bicuculline, a competitive antagonist of the GABA(A) receptor; in contrast, it was not affected by 1 mM kynurenic acid, an antagonist of the ionotropic glutamate receptor. In the presence of 1 microM tetrodotoxin, 3 microM nicotine was unable to affect the synaptic activity. Our findings suggest the existence of nicotinic receptors on GABAergic axons projecting to the vagal motoneurons. Because the effect is completely abolished by 1 microM tetrodotoxin, the nicotinic receptors are not localized on the presynaptic nerve terminal and their action on the GABA release requires the propagation of an action potential from their location to the synaptic terminal. This effect of nicotinic receptor activation on spontaneous GABA release in the dorsal motor nucleus of the vagus may have an important role in the regulation of gastrointestinal motility.

Superiority of practolol versus propranolol in protection against ventricular fibrillation induced by coronary occlusion.

The ability of practolol and propranolol of prevent ventricular fibrillation in experimental anterior myocardial infarction was compared in dogs subjected to ligation of the left anterior descending and first septal coronary arteries. This procedure, which causes ventricular fibrillation in 90 percent of animals within 30 minutes, was performed in control dogs and in dogs pretreated with propranolol (0.5 mg/kg body weight) or with practolol (1.5 to 2.5 mg/kg). These doses produced nearly equivalent shifts in isoproterenol-induced chronotropic dose-response curves, indicating equivalent degrees of beta adrenergic blockade. In 21 dogs with confirmed ligation, cardiogenic shock did not develop. Six of seven control dogs died with ventricular fibrillation. Six of seven dogs pretreated with propranolol also had fibrillation, whereas only one of the seven dogs pretreated with practolol manifested ventricular fibrillation during the 45 minute postligation observation period. Practolol afforded significant protection compared with no treatment or treatment with propranolol (P less than 0.05).

Experimental studies on the neurocardiovascular effects of urapidil.

The major purpose of our study was to determine whether urapidil acts in the central nervous system (CNS) to lower arterial blood pressure. Once demonstrating a CNS antihypertensive action of urapidil we further set out to determine: (1) the relative role of a CNS antihypertensive action to the total antihypertensive effect of urapidil; (2) the brain site of action for the antihypertensive effect of urapidil; and, (3) the receptor mechanism whereby urapidil acts in the CNS to lower arterial blood pressure. Studies were conducted in chloralose-anaesthetised cats, and arterial blood pressure and heart rate were monitored. Drugs were administered intravenously (IV), into the cerebral ventricles (ICV), topically by application to the ventral surface of the medulla and by microinjection into specific nuclei. Receptor binding studies were also conducted using rat cerebral cortex homogenates. We found that injection of urapidil into the fourth ventricle decreased arterial pressure. Local application of urapidil to the ventral medullary surface also decreased arterial blood pressure. Microinjection of urapidil into one of the nuclei associated with the ventral surface of the medulla, the rostral part of the nucleus reticularis lateralis (rLRN), produced a similar degree of antihypertensive effect. The effect of urapidil was not altered by alpha 1-receptor blockade. Instead, the urapidil effect resembled that produced by drugs that stimulate serotonin (5-hydroxytryptamine)-1A receptors (B695-40 and 8-OH-DPAT). Furthermore, urapidil was found to have the highest potency for binding to serotonin-1A receptor sites (as compared to alpha 1- and alpha 2-receptor sites). Urapidil administered IV was shown to lower arterial blood pressure in part by blocking peripheral alpha 1-adrenoceptors but also, in high doses, by acting in the CNS to decrease central sympathetic outflow. These data indicate that urapidil is a unique drug, possessing both peripheral and CNS actions which contribute to its antihypertensive effect. Urapidil may also be unique in that its central action may involve activation of serotonin-1A receptors.