Opiate analogs, substance P, and baroreceptor reflexes in the rabbit

Neuroactive substances in the dorsal vagal complex of the medulla oblongata: nucleus of the tractus solitarius, area postrema, and dorsal motor nucleus of the vagus

The distributions of classical and putative neurotransmitters within somata and fibres of the dorsal vagal complex are reviewed. The occurrence within the dorsal medulla oblongata of receptors specific for some of these substances is examined, and possible functional correlations of the specific neurochemicals with respect to their distribution within the dorsal vagal complex are discussed. Many of the known transmitters and putative transmitters are represented in the dorsal vagal complex, particularly within various subnuclei of the nucleus of the solitary tract, the main vagal afferent nucleus. In a few cases, some of these have been examined in detail, particularly with respect to their possible mediation of cardiovascular or gastrointestinal functions. For example, the catecholamines, substance P and angiotensin II in the nucleus of the solitary tract have all been strongly implicated as playing a role in the central control of cardiovascular function. Other neurotransmitters or putative transmitters may be involved as well, but probably to a lesser extent. Similarly, the roles in the dorsal vagal complex of dopamine, the endorphins and cholecystokinin in control of the gut have been studied in some detail. Future investigations of the distributions of and electrophysiological parameters of neurotransmitters at the cellular level should provide much needed clues to advance our knowledge of the correlations between anatomical distributions of specific neurochemicals and physiological functions mediated by them.

Viscerosensory-cardiovascular reflexes: altered baroreflex sensitivity in irritable bowel syndrome

Animal studies have demonstrated that visceral afferent stimulation alters autonomic cardiovascular reflexes. This mechanism might play an important role in the pathophysiology of conditions associated with visceral hypersensitivity, such as irritable bowel syndrome (IBS). As such, studies in humans are lacking, we measured viscerosensory-cardiovascular reflex interactions in IBS patients and healthy controls. Systolic blood pressure (SBP), heart rate (HR), and arterial baroreflex sensitivity (BRS) were studied in 87 IBS patients and 36 healthy controls under baseline conditions and during mild (15 mmHg) and intense (35 mmHg) visceral stimulation by rectal balloon distension. BRS was computed from continuous ECG and arterial blood pressure signals (Finapres-method) during 5-min periods of 15-min metronome respiration. Baseline SBP and HR were not different between patients and controls. In both groups, SBP increased similarly during rectal stimulation, whereas HR decreased during mild and increased intense stimulation. BRS was significantly higher in patients compared with controls at baseline (7.9 +/- 5.4 vs. 5.7 +/- 3.7 ms/mmHg, P = 0.03) and increased significantly in both groups during mild stimulation. This increase persisted in controls during intense stimulation, but BRS returned to baseline in patients. BRS was not significantly different between groups during rectal distension. This study demonstrates the presence of a viscerosensory-cardiovascular reflex in healthy individuals and in IBS patients. The increased BRS in IBS patients at baseline may either be a training-effect (frequent challenging of the reflex) or reflects altered viscerosensory processing at the nucleus tractus solitarii.

Substance P- and tyrosine hydroxylase-containing neurons in the human dorsal motor nucleus of the vagus nerve

The aim of this study was to provide a comprehensive account of the topography, morphology, and frequencies of the substance P- and tyrosine hydroxylase-containing neurons in the human dorsal motor nucleus of the vagus nerve. The morphology of immunoreactive neurons was studied and the variations of the cell distributions were presented by three-dimensional computer reconstructions. Three types of substance P-like immunoreactive neurons were identified. They were predominantly located in the dorsointermediate, centrointermediate, caudointermediate, and caudal division of the dorsal motor nucleus of the vagus nerve. The morphology of substance P-like immunoreactive neurons varied according to the subnuclei in which they were found. Three types of tyrosine hydroxylase-like immunoreactive neurons were identified, mainly in the periphery of the dorsal motor nucleus of the vagus nerve, including the medial fringe, ventrointermediate, and dorsointermediate subnuclei of the 10. Many cells throughout the ventrointermediate subnucleus of the dorsal motor nucleus of the vagus nerve are seen ventrally to intermingle with the tyrosine hydroxylase neurons of the intermediate reticular zone. Computer reconstructions provided a three-dimensional view of the positions of substance P- and tyrosine hydroxylase-like immunoreactive neurons within the subdivisions of the dorsal motor nucleus of the vagus nerve. The uneven distribution of substance P- and tyrosine hydroxylase-like immunoreactive neurons within the subdivisions suggests an involvement of these substances in some, but not all, autonomic functions of the dorsal motor nucleus of the vagus nerve.

Effects of central substance P on baroreflex regulation

The effects of intracerebroventricularly (i.c.v.) administered Substance P (100 pg, 1 microgram, 10 micrograms) on mean blood pressure (MBP), inter-beat interval (IBI) and the baroreceptor heart reflex (BHR) were studied in conscious Wistar rats. The BHR was induced by intravenous injection of both phenylephrine (1 microgram) and sodium nitroprusside (5 micrograms) before and after SP administration (3 and 15 min). The dose of 100 pg SP was without effect on the resting values of both MBP and IBI but enhanced the BHR sensitivity by about + 0.5 ms/mm Hg in the phenylephrine test 3 min after i.c.v. SP. 1 microgram and 10 micrograms SP caused a long-lasting dose dependent increase in MBP and changed the IBI. In contrast to the results obtained with 100 pg, the BHR sensitivity was impaired-1 microgram SP: -0.2 ms/mm Hg (phenylephrine) and -0.45 ms/mm Hg (nitroprusside), 10 micrograms SP: -0.35 ms/mm Hg (phenylephrine). These changes in BHR sensitivity were only recognised 3 min, but not 15 min, after i.c.v. treatment even hough changes in the resting values of MBP and IBI were still present at 15 min. These data suggest that SP through the cerebrospinal fluid may participate in central cardiovascular control and, moreover, it may influence the baroreflex regulation.

Inhibition of sympathetic neurotransmission by the opioid delta-receptor agonist DAMA in the pithed rat

1. The effects of [D-Ala2,Met5]enkephalinamide (DAMA), an analogue of [Met5]-enkephalin that acts selectively on opioid receptors of the delta-subtype, were studied on pressor responses elicited by sympathetic stimulation in pithed rats. 2. Intravenous injections of bolus doses of 0.1 mg/kg and 0.3 mg/kg of DAMA did not affect either the basal blood pressure or pressor responses to noradrenaline. 3. Pressor responses elicited either by electrical stimulation of the spinal sympathetic outflow or by stimulation of sympathetic ganglion cells with the muscarinic agonist McN-A-343 were reduced by DAMA. 4. Naloxone (1 mg/kg + 0.5 mg/kg per h) had no significant effect on the basal blood pressure or on pressor responses to spinal sympathetic stimulation, but antagonised the inhibitory effect of DAMA. 5. These results indicate that activation of opioid delta-receptors on sympathetic vasomotor nerve terminals can inhibit noradrenergic neurotransmission.

Carotid baroreflex sensitivity at rest and during exercise is not influenced by opioid receptor antagonism

Physical effort involves, along with an increase in the plasma concentration of beta-endorphin, profound cardiovascular adaptations. The aim of the present study was to investigate with the use of the variable neck chamber technique, the influence of the endogenous opioids on the carotid baroreflex control of blood pressure and heart rate at rest as well as during exercise. Ten normal volunteers exercised in the supine position up to 33% and 66% of their maximal exercise capacity and received, in a randomized double-blind cross-over protocol, either saline or naloxone (10 mg intravenously, followed by a continuous infusion of 10 mg.h-1). During exercise a progressive attenuation of the carotid baroreceptor reflex control of blood pressure and heart rate was noted. However, neither at rest nor during exercise, did opioid antagonism influence the carotid baroreceptor control of blood pressure and heart rate. Intra-arterial pressure and heart rate also remained unaffected. In contrast, both at rest and during exercise, naloxone administration produced a significant increase in the plasma concentration of cortisol. The latter suggests that in vivo the opioid receptors were effectively antagonized. In conclusion the present study confirms that opioids play only a minor role in cardiovascular homeostasis at rest. In addition, this study demonstrates that they are not involved in the cardiovascular adaptation to exercise, nor in the exercise-related attenuation of the carotid baroreceptor control of pressure and heart rate.

Can central neuropeptides be implicated in carotid sinus reflex hypersensitivity?

Carotid sinus reflex hypersensitivity involves profound and intermittent changes in heart rate and blood pressure associated with symptoms of dizziness and syncope. This involves a reflex arc in which the main defect is believed to lie within the central nervous system. The discovery of classical and peptidergic neurotransmitters within the same neurone, and the presence of these peptides within the central nervous system raises the possibility that carotid sinus reflex hypersensitivity may be related to an abnormality of peptide distribution or function.

An enkephalin-containing pathway from nucleus tractus solitarius to the pressor area of the rostral ventrolateral medulla of the rabbit

A technique combining retrograde tracing of wheat germ-conjugated gold particles with immunocytochemical demonstration of enkephalin-containing neurons was used to study intramedullary enkephalin-containing pathways to the pressor area of the rostral ventrolateral medulla in rabbits. The rostral ventrolateral medulla represents a main source of bulbospinal sympathoexcitatory neurons, and is critical to the tonic and reflex control of blood pressure. Firstly, the distribution of enkephalin-positive neurons and terminal fibres in rabbit medulla were described, with special reference to a moderately dense terminal plexus in the rostral ventrolateral medulla. Then, retrograde tracing studies were conducted; the rostral ventrolateral medullary pressor region was first localized by injection of L-glutamate (25 nmol in 50 nl). Slow (30-min) injections of wheat germ-gold (1.00 microliter) were then made at the same coordinates, resulting in a restricted injection site corresponding to the C1 pressor area, verified by the presence of tyrosine hydroxylase- and neuropeptide Y-containing neurons. Transported gold was revealed by silver reduction, and enkephalin immunoreactive cells were revealed by immunocytochemistry. Enkephalin-positive gold-containing neurons were found primarily in the nucleus tractus solitarius, especially in the commissural and medial intermediate subnuclei. Cells in the nucleus tractus solitarius containing other transmitters (substance P. galanin, neuropeptide Y and catecholamines) did not show the same degree or pattern of double-labelling, suggesting that the transport was not due to non-specific silver reduction or spread from the pipette track. The potential importance of this endogenous intramedullary opiate system is discussed in terms of medullary control of the cardiovascular system. It is hypothesized that this opiate projection from the nucleus tractus solitarius to the rostral ventrolateral medulla could play an important modulatory function, influencing baroreceptor or other cardiopulmonary reflex pathways involved in the primary regulation of the cardiovascular system. Furthermore, this pathway could represent a central substrate underlying opiate effects on the cardiovascular system during such conditions as hemorrhagic shock, stress or opiate intoxication.

Antifibrillatory action of the narcotic agonist fentanyl

Morphine, an opiate alkaloid with mixed mu- and delta-agonist properties, raises the ventricular fibrillation threshold in anesthetized dogs by altering autonomic tone. To elucidate further underlying structure-activity relationships, the effect of fentanyl, a nonalkaloid, mu-selective agonist in wide clinical use, was studied. Fentanyl (30 micrograms/kg) was given intravenously to 27 chloralose-anesthetized dogs, and ventricular fibrillation threshold was measured by means of the single-stimulus technique. In the baseline state fentanyl raised the ventricular fibrillation threshold by 14%. When the dogs were subjected to hemorrhagic stress, this effect was amplified to 29% (p less than 0.0001). Bilateral cervical vagotomy abolished fentanyl's antifibrillatory effect, but neither atropine sulfate (0.4 mg/kg/hr) nor atropine methylnitrate (0.5 mg/kg/hr) did so. Fentanyl's influence on the fibrillation threshold during hemorrhage was significantly reduced by bilateral stellate ganglionectomy (p less than 0.005). It is concluded that fentanyl raises the ventricular fibrillation threshold by its known sympathoinhibitory action rather than by its vagal efferent activating effect. The facts that an intact vagus is required and that hemorrhage amplifies the effect suggest that the antifibrillatory effect of fentanyl is mediated through the afferent component of the baroreflex arc.

Cardiovascular effects of dynorphin A (1-13) in conscious rats and its modulation of morphine bradycardia over time

The short-term cardiovascular effects of dynorphin A (1-13), as well as its effects upon morphine bradycardia were investigated. In unanesthetized, unrestrained rats, intracerebroventricular (ICV) dynorphin A (1-13) injections (10-20 micrograms) produced a dose-related pressor effect, whereas intravenous (IV) dynorphin A (1-13) (1.0 mg/kg) produced a depressor effect; these responses persisted less than five min. Heart rate was not significantly altered by these doses or routes of administration. Dynorphin A (1-13) also produced behavioral effects in the unanesthetized animals, such as wet dog shakes in response to IV administration and wet dog shakes accompanied by barrel rolling in response to ICV administration. To evaluate the effects of dynorphin A (1-13) pretreatment on the bradycardic response to IV morphine, rats were pretreated with 10 micrograms dynorphin A (1-13) ICV four, six or eight hours prior to challenge with morphine sulfate (0.1 mg/kg IV). Four hour pretreatment with dynorphin A (1-13) (tested at 14:00 hr) resulted in a potention of morphine bradycardia, with six hours pretreatment (tested at 16:00 hr) no effect was observed, and eight hours following dynorphin A (1-13) pretreatment (tested at 18:00 hr) morphine bradycardia was attenuated. Additionally, the bradycardic response to IV morphine alone became more exaggerated as rats approached their nocturnal activity cycle. These data further establish that dynorphin A (1-13) exerts a potent, long lasting modulatory effect on morphine bradycardia and emphasize the importance of circadian variables in altering the magnitude of cardiovascular responses to opioid agonists.

Cardiovascular effects of substance P and capsaicin microinjected into the nucleus tractus solitarii of the rat

This report deals with the effect of substance P (SP) and capsaicin on blood pressure and heart rate after administration into different sites of the nucleus tractus solitarii (NTS) of urethane-anesthetized rats. Microinjection of SP at 6 different coordinates throughout the NTS showed 3 sites where SP administration evoked changes in blood pressure and heart rate. The most sensitive sites where application of SP into the NTS evoked dose-dependent hypotension and bradycardia were at the level of the posterior tip of the area postrema (zero level) and at the level of the obex. Capsaicin evoked dose-dependent hypotension and bradycardia at the same sites. These results further support the possibility that SP may be a neurotransmitter or neuromodulator of baroreceptor afferents in the NTS.

Effects of bestatin on the central cardiovascular regulatory mechanisms in the rat

We evaluated the effects of bestatin, the specific aminopeptidase-B and leucine aminopeptidase inhibitor, on the central cardiovascular regulatory mechanisms in Sprague-Dawley rats anesthetized with pentobarbital sodium (40 mg/kg, i.p.). Intracerebroventricular injection of bestatin (100 or 200 nmol/5 microliters) consistently elevated the basal systemic arterial pressure and heart rate. At the same time, this degradative enzyme blocker increased the sensitivity of the baroreceptor reflex responses as well as the efficacy of the modulatory actions of the medullary nucleus reticularis gigantocellularis on these reflexes. We speculate that enhancing the tonic activities of the endogenous neuropeptides in the brain by protecting them from their catabolic enzymes may affect the central cardiovascular regulatory machinery by modifying the operations of the baroreceptor feedback controls and their modulatory mechanisms.

Cardio-respiratory actions of substance P, TRH and 5-HT in the nucleus tractus solitarius of rats: evidence for functional interactions of neuropeptides and amine neurotransmitters

The cardiovascular and respiratory effects of Substance P (SP) and Thyrotrophin-Releasing Hormone (TRH) microinjections into the nucleus tractus solitarius (NTS) of urethane anaesthetized rats have been investigated. Dual injections of the peptides with serotonin (5-HT) were given to investigate possible functional interactions. In addition, SP and TRH were injected into rats in which 5-HT in the NTS area had been depleted by prior treatment with 5,7-Dihydroxytryptamine (5,7-DHT). SP (65pmol) did not elicit significant effects on blood pressure (BP) or heart rate (HR), but produced a marked, acute reduction in respiration rate (RR). TRH (110pmol) elicited a small but significant reduction in mean arterial pressure (MAP), whereas 5-HT (15nmol) caused a rise in MAP. Neither TRH nor 5-HT modified RR when given alone. A dual injection of SP (6.5pmol, ineffective alone) and 5-HT (15nmol) resulted in a rise in MAP which was insignificantly different from the effect of 5-HT alone. However, a prolonged fall in RR, unlike the effect of SP alone was also observed. A dual injection of TRH (11pmol, ineffective alone) and 5-HT (15nmol) resulted in a profound fall of RR but no significant changes in MAP or HR. SP elicited similar effects in 5,7-DHT lesioned animals as in sham operated controls. In contrast, TRH microinjections in lesioned rats were associated with a profound fall in RR, and a blood pressure response significantly different to that observed in the corresponding sham group. The results are discussed in relation to other evidence suggesting functional interactions between neuropeptides and amine neurotransmitters in the mammalian brainstem.

Substance P-immunoreactive neurons in the brainstem of the cat related to cardiovascular centers

The distribution of substance P-immunoreactivity (SP-IR) in the brainstem and spinal cord of normal and colchicine-pretreated cats was analysed using the peroxidase-antiperoxidase (PAP) technique. Numerous SP-IR fibers are present in the nucleus solitarius, nucleus dorsalis nervi vagi and nucleus spinalis nervi trigemini, various parts of the formatio reticularis, substantia grisea centralis mesencephali, locus coeruleus and nucleus parabrachialis. SP-IR perikarya occur in the substantiae gelatinosa and intermedia of the spinal cord, the nucleus spinalis nervi trigemini-pars caudalis, the nucleus dorsalis nervi vagi, and the nucleus solitarius, as well as in the adjacent formatio reticularis and the medullary nuclei of the raphe. In addition, SP-IR cell bodies are located in the nuclei raphe magnus and incertus, ventral and dorsal to the nucleus tegmentalis dorsalis (Gudden), nucleus raphe dorsalis, substantia grisea centralis mensencephali, locus coeruleus, nucleus parabrachialis and colliculus superior. The results indicate that SP-IR neurons may be involved in the regulation of cardiovascular functions both at the central and peripheral level. A peripheral afferent portion seems to terminate in the nucleus solitarius and an efferent part is postulated to originate from the nucleus dorsalis nervi vagi and from the area of the nuclei retroambiguus, ambiguus and retrofacialis.

Cardiovascular interactions between methionine-enkephalin and substance P in the conscious dog

Interactions between the undecapeptide Substance P (SP) and the pentapeptide methionine-enkephalin (Met5-ENK) have been described in isolated organ systems and in baroreceptor reflex mechanisms. Previous studies in our laboratory have demonstrated that systemically injected Met5-ENK simultaneously increases mean systemic arterial pressure (MAP) and heart rate (HR) in the conscious, chronically instrumented dog. We have now evaluated cardiovascular interactions between SP and Met5-ENK. In this model, SP injected intravenously produces a rapid and transient decrease in MAP and increase in HR over the dose range from 1.0 to 10.0 ng/kg. SP does not appear to appreciably alter subsequent responses to Met5-ENK. At SP doses of 1.0 ng/kg, the peak hemodynamic response to SP and Met5-ENK (35 micrograms/kg) given together appears to represent a simple summation effect of both drugs on HR and MAP. However, at higher SP doses (5.0 ng/kg), the SP response predominates and is little altered by the presence of Met5-ENK. Thus, Met5-ENK does appear capable of modulating the hemodynamic responses to SP over certain dose ranges.

Bombesin and substance P modulate peripheral sympathetic and cardiovascular activity

The vascular and cardiac effects of bombesin and substance P were evaluated in a "pithed rat preparation" in order to study the cardiovascular effects of these neuropeptides in the absence of the central nervous system control mechanisms. Bombesin (0.7, 7.0 and 70 nmol/kg) produced dose-dependent increases in mean arterial blood pressure (MAP): +6 +/- 1, +11 +/- 2 (p less than 0.05) and +22 +/- 2 mm Hg (p less than 0.01), respectively, when injected IV into the pithed rat. Heart rate (HR) responses to bombesin (0.7-70 nmol/kg) were also increased in a dose-dependent manner: +14 +/- 6, +28 +/- 5 (p less than 0.05) and +45 +/- 3 (p less than 0.05) beats/min (bpm), respectively. The blood pressure (BP) and HR responses to bombesin were diminished but not completely abolished by pretreatment with 1 mg/kg of either the beta-adrenergic receptor blocker propranolol or the H1-histamine receptor antagonist pyrilamine. Substance P similarly caused a dose-dependent increase in MAP: +7 +/- 1 and +38 +/- 7 mm Hg (p less than 0.001), by 0.7 and 700 nmol/kg, respectively, without altering HR. BP responses to sympathetic stimulation were also examined in the pithed rat utilizing electrical stimulation of the spinal cord (1 Hz, 50 V, 1 msec duration for 30 sec). These responses were potentiated by the low doses (0.7 and 7.0 nmol/kg) of bombesin (p less than 0.01) but tended to be suppressed by the highest dose. The HR response to stimulation was significantly reduced: -25 +/- 8 bpm (p less than 0.05) by the highest dose of bombesin (70 nmol/kg) but not with lower doses.(ABSTRACT TRUNCATED AT 250 WORDS)

Naloxone and methylprednisolone sodium succinate enhance sympathomedullary discharge in patients with septic shock

Naloxone and methylprednisolone sodium succinate (MPSS) may act in synergy to improve hemodynamics in patients with septic shock by enhancement of sympathomedullary discharge. This randomized double-blind study describes the effect of various dosing regimens of naloxone and MPSS upon hemodynamics and plasma catecholamines in patients with septic shock (n = 57). Consecutive bolus doses of naloxone were given 30 minutes apart (10 micrograms/kg;-100 micrograms/kg) and a single dose of MPSS (30 mg/kg); bolus doses of 5% dextrose in water solution plus single dose of MPSS as above; bolus dose of naloxone (30 micrograms/kg) followed by continuous infusion (30 micrograms/kg/hr for 1 hour) with single dose of MPSS as above; a bolus and continuous infusion of naloxone as above without MPSS; MPSS alone and standard therapy alone. In patients treated with bolus doses of naloxone in combination with MPSS, plasma levels of epinephrine and norepinephrine were increased approximately five-to tenfold. In patients treated with bolus plus continuous infusion of naloxone given with or without MPSS, only plasma epinephrine levels were increased. Systolic blood pressure and left ventricular stroke work index were improved within 15 minutes in groups which received naloxone and corticosteroids regardless of dose. In those groups, there were no changes in heart rate or filling pressure. Systemic vascular resistance improved significantly only in the group which received low dose bolus and continuous infusion of naloxone and MPSS. Naloxone and MPSS quickly improved cardiac function in patients with septic shock by enhanced sympathomedullary discharge and may be useful as an adjunct in the therapy of this disorder.

Effects of fentanyl on a bradykinin-induced excitatory cardiac reflex in the anaesthetized rat

A reflex increase in arterial blood pressure and heart rate was elicited by application of epicardial bradykinin (4 X 10(-8), 2 X 10(-7), 10(-6) g/ml) in pentobarbital-anaesthetized, thoracotomized rats. The influence of the opioid fentanyl and the tranquilizer diazepam on this increase was then investigated. When fentanyl (2.2 micrograms/(kg X min] was infused i.v. during periods of bradykinin application, a highly significant suppression of the excitatory reflex resulted. Epicardial bradykinin (10(-6) g/ml) induced an increase in mean arterial blood pressure of 25.3 +/- 3.5 mm Hg during the control period and of 6.8 +/- 1.4 mm Hg during fentanyl infusion. The corresponding changes in heart rate were +20.8 +/- 4.7 beats/min during the control period and +1.9 +/- 1.3 beats/min during fentanyl infusion. When cardioacceleration was induced by the carotid occlusion reflex, fentanyl antagonized this effect too. The effect of fentanyl was entirely neutralized by injecting naloxone (30 micrograms/kg i.v.) prior to application of the former. Diazepam had no influence on the bradykinin-induced reflex. The results support the view that morphinoceptors may modulate the excitatory cardiac reflex induced by epicardial application of the algesic bradykinin in the rat.

Evidence that L-glutamic acid mediates baroreceptor function in the cat

The possible role of L-glutamic acid (L-glu) as a neurotransmitter of baroreceptor afferent neurons was investigated in the cat by monitoring the changes in three indices of baroreceptor function seen with the L-glu antagonists L-glutamic acid diethyl ester (GDEE) and 1-hydroxy-3-amino-pyrrolidone-2-(HA-966). Baroreceptor function was determined from a) the computer summed inhibition of sympathetic nerve discharge (SND) evoked by electrical stimulation of vagal baroreceptor afferent pathways, b) the locking of SND to the cardiac cycle, and c) the sympathoinhibitory response to i.v. pressor doses of phenylephrine. Direct bilateral microinjections of GDEE (20 micrograms) and HA-966 (4 micrograms) into the region of the nucleus tractus solitarii (NTS) resulted in immediate, marked reductions in the SND inhibitory response to vagal stimulation, a loss in SND locking to the cardiac cycle, a shift in the arterial pulse/SND phase relation, and a diminished sympathoinhibitory response to phenylephrine. Control microinjections of isotonic saline (1 mu 1/NTS) were devoid of these effects. The vagal induced sympathoinhibitory response was restored after NTS microinjections of GDEE by increasing the intensity of the vagal stimulus, or by directly stimulating the NTS injection site, suggesting that the impairment in baroreceptor function seen with this L-glu antagonist was independent of mechanical or local anesthetic effects. These data strongly suggest that L-glu may act as a neurotransmitter of baroreceptor afferent neurons in the NTS of the cat.

Physiological and pharmacological properties of the three subsystems constituting the aortic nerve-renal sympathetic reflex in rabbits

Electrical stimulation of the aortic nerve of anesthetized rabbits reflexly evoked both excitation and inhibition of renal nerve activity. The excitatory component of the reflex, observed in about 75% of the animals, was elicited by activation of aortic C-fibers. It was selectively suppressed by chronic treatment of the animal with capsaicin. Intracisternal injection of either [D-ala2]-met-enkephalinamide or beta-endorphin markedly attenuated this excitatory component, although neither affected the excitatory component mediated by chemoreceptor fibers in response to stimulation of the carotid sinus nerve. It seems most likely that nociceptive C-fibers of the rabbit's aortic nerve were responsible for the excitatory component. On the other hand, the inhibitory component was reflexly elicited by stimulation of the aortic A- or C-fiber group activated separately or in combination. In agreement with previous reports, the sympatho-inhibitory action of C-fibers was more powerful and longer-lasting than that of A-fibers. We found that the inhibitory component induced by C-fibers was markedly attenuated by the two opioid peptides mentioned above, but was resistant to pentobarbital. On the contrary, the component mediated by A-fibers was suppressed by pentobarbital but was relatively resistant to the opioid peptides. Thus, the rabbit's aortic nerve-renal sympathetic reflex consists of the following 3 subsystems characterized by different physiological and pharmacological properties: sympatho-inhibitory systems activated by barosensory A- or C-fibers and a sympatho-excitatory system attributable to C-fibers probably of nociceptive modality.

Development of cerebral methionine-enkephalinergic neurons in rats: some difference in Wistar-Kyoto rats and spontaneously hypertensive rats

Development of methionine-enkephalin (ME) and ME receptor binding in the embryonic and neonatal rat cerebral nuclei was defined quantitatively by immunocytofluorescent and microautoradiographic methods. On embryonic days 14 and 18, ME was localized in 10 of 83 nuclei particularly in the n. amygdaloideus centralis, n. periventricularis, n. supraopticus, n. interpeduncularis, n. suprachiasmaticus, n. arcuatus and n. ambiguus. ME receptor binding was distributed in the former 4 nuclei on embryonic day 14 and additional 5 nuclei on embryonic day 18. At the day of birth both levels markedly increased in these nuclei (2-3 times) and abruptly emerged in 65 out of 73 nuclei in which ME neurons were not detectable in embryonic nuclei. The abrupt appearance in most nuclei at birth probably regulates nociceptive and non-nociceptive stimuli at birth and in the neonatal period. Both levels attained their maximum at the postnatal day 7 and gradually declined thereafter. Among the perinatal period examined, ME immunoreactivity in the tractus spinalis nervi trigemini and ME receptor binding in the n. tractus spinalis nervi trigemini were markedly lower in both SHR neonates than in corresponding Wistar-Kyoto rats. Lowered levels in both areas of neonatal SHR may be involved in central hyperreactivity and preganglionic sympathetic activation of young SHR.

Effect of naloxone on baroreflex, sympathetic tone and blood pressure in the cat

Studies were designed to determine the effect of naloxone on sympathetic nerve activity and blood pressure and, also to investigate the central effect of naloxone in relation to the baroreflex system in alpha-chloralose-anesthetized cats. Following intravenous injection of naloxone (2 mg/kg), preganglionic splanchnic nerve (PSN) activity significantly increased in parallel to the increase in blood pressure. Set gain (mmHg-1) of carotid sinus baroreflex together with the operational range (mmHg) was increased after naloxone. Intraventricular (4th) injection of naloxone (0.2 mg/kg) produced identical responses in blood pressure and PSN activity as well as an altered set gain and range of the baroreflex. Additionally, the pressor response to carotid hypotension and medullary pressor area stimulation were augmented during naloxone activation but not the pressor response to posterior hypothalamus stimulation. The depressor responses to stimulation of both carotid sinus nerve and medullary depressor region were facilitated by naloxone. These data suggest that naloxone has an effect on the central autonomic blood pressure regulatory circuits which participate in the carotid sinus baroreflex system.

Cardiovascular responses to opioid agonists injected into the nucleus of tractus solitarius of anesthetized cats

It has been proposed that various opiate receptor subtypes mediate different cardiovascular responses to centrally administered opioids. We evaluated this hypothesis in chloralose-urethane anesthetized cats by monitoring the cardiovascular and respiratory responses to relative mu [morphine, morphiceptin, D-Ala2, MePhe4, Gly-ol5 enkephalin (DAGO)] and delta [D-Ala2, D-Leu5enkephalin (DADL)] agonists microinjected (0.5 ul/kg) into the caudal region of the Nucleus of Tractus Solitarius (NTS). Dynorphin (1-13), an endogenous opioid which exhibits selective affinity towards the kappa receptor, was also tested. Dynorphin at a dose of 50 nMol/kg did not alter cardiovascular or respiratory variables. Morphine (10-54 nMol/kg) and DAGO (50 nMol/kg) had no effect on blood pressure, heart rate or respiratory rate; morphiceptin (100-320 nMol/kg) caused tachycardia only at the highest dose. DADL (10-100 nMol/kg) elicited a dose-dependent depression of blood pressure. High doses of DADL depressed heart rate and respiratory rate. The depressor effects of DADL were reversed by low doses of naloxone (0.1 mg/kg). This dose of naloxone also elicited pressor responses in cats treated with the other opioids and reversed the morphiceptin-induced tachycardia. These data indicate that opioid agonists differ with regard to their cardiovascular and respiratory effects following microinjection into the NTS of anesthetized cats, with the delta agonist DADL showing greatest activity.

Pyrimido-pyrimidine derivative, RA642, a central pressor agent in cat endotoxin shock

The effects of a pyrimido-pyrimidine derivative, RA642, in endotoxin shock were studied. Blood pressure and preganglionic splanchnic nerve (PSN) activity were measured in alpha-chloralosed cats. Within 30 min of intravenous administration of E. coli endotoxin (1 mg/kg), blood pressure and PSN activity were 68 +/- 8% and 47 +/- 10% of control and by 60 min were depressed by 54 +/- 8% and 42 +/- 8%. RA642 (0.25 mg/kg) injected i.v. 30 min after endotoxin, caused blood pressure to recover significantly to 83 +/- 6% of control within 5 min and be maintained at that level. PSN activity was similarly increased to and maintained at 62 +/- 9% of control. The efficacy of RA642 in reversing the lethal consequences of endotoxin shock were dramatic; all treated animals survived whereas the mortality rate of non-treated animals was 83%. This strongly suggests that the central pressor agent, RA642, may have important therapeutic applications in the management of endotoxin shock.

Methionine enkephalinergic neuronal activity in cerebral nuclei of spontaneously hypertensive rats

The role of methionine enkephalin (ME) neurons in the development of genetic hypertension in SHR is the subject of this study. Methionine enkephalin-like immunoreactivity (MELI) and ME receptor binding (MERB) levels were assayed quantitatively by microdensitometry of fluorescence micrographs and autoradiographs of 85 cerebral nuclei and areas of both young and adult spontaneously hypertensive rats (SHR). Normotensive Wistar Kyoto rats (WKY) were used as controls. In young SHR, both MELI and MERB levels were markedly higher in the n. dorsalis nervi vagi, n. amygdaloideus medialis, and group of stria terminal nuclei than in those of young WKY, while both levels were lower in the n. reticularis lateralis, n. corporis mamillaris lateralis, and n. arcuatus. MELI levels in the tractus spinalis nervi trigemini and MERB in the n. tractus spinalis nervi trigemini and median eminence were also lower in young SHR, whereas MERB in the n. amygdaloideus centralis was higher. Alteration in these nuclei was no longer detectable in adult SHR. Whereas in adult SHR, both MELI and MERB levels in the n. reticularis medialis were higher than those of adult WKY, and MELI in the n. accumbens septi and MERB in the n. caudatus were also higher, while MELI in the area lateralis hypothalami was lower than that in adult WKY. The findings indicate that activation of ME neurons in the n. dorsalis nervi vagi and limbic area and also a decrease in ME neuronal activity in the area spinalis nervi trigemini, n. reticularis lateralis, and n. arcuatus may be casually related to the development of hypertension and hyperreactivity in SHR.

Neuroanatomical localization of substance p: implications for central cardiovascular control

The presence of substance P (SP) neurons in pathways known to be involved in the central control of the cardiovascular system has been studied with neuroanatomical and neurochemical techniques. SP-immunoreactive (SP-I) neurons are found in afferent baro- and chemoreceptor pathways which transmit information from peripheral receptors to the nucleus tractus solitarius. In addition, SP-I neurons located in the nucleus interfascicularis hypoglossi of the ventral medulla innervate the intermediolateral cell column, the site of origin of preganglionic sympthetic nerves. The role of these SP-I neurons in cardiovascular control remains to be determined.