Neuroanatomy of central cardiovascular control. Nucleus tractus solitarii: afferent and efferent neuronal connections in relation to the baroreceptor reflex arc

Immunoreactive pro-enkephalin and prodynorphin products are differentially distributed within the nucleus of the solitary tract of the rat

In this study we examined the distribution of two different endogenous opioid peptides in the nucleus of the solitary tract of the rat medulla. As a marker for immunoreactive enkephalin, we used an antiserum directed against one of the proenkephalin products, methionine enkephalin-arg-gly-leu (m-Enk). To identify immunoreactive dynorphin we used an antiserum directed against the prodynorphin product, dynorphin B (Dyn B). The PAP method was used on both colchicine and normal animals. Caudal to the obex, within the commissural nucleus, there is extensive overlap of both immunoreactive m-Enk and Dyn B terminals and cells. While the cells are morphologically similar, the immunoreactive dynorphin cells are somewhat larger. Rostral to the obex, there is a marked difference in the distribution of the two compounds. Immunoreactive m-Enk terminals are concentrated medial to the solitary tract; there is minimal staining laterally. In contrast, immunoreactive Dyn B terminals are concentrated lateral to the solitary tract. The rostral cellular distribution of the two opioid peptides follows a similar pattern. The morphology of the medially located m-Enk and laterally located Dyn B cells is also readily distinguished. The former are small, round cells with minimal dendritic labelling; the latter are larger, pyramidal neurons with prominent apical and basal dendrites. Since the medial and lateral nuclei of the solitary tract have been associated with cardiovascular and respiratory control, respectively, these data suggest that different endorphin families have different functional actions within the nucleus of the solitary tract.

Baroreflex impairment precedes hypertension during chronic cerebroventricular infusion of hypertonic sodium chloride in rats

Osmotic minipumps were implanted chronically for continuous 11-d infusion of hypertonic sodium chloride (NaCl) into the third cerebral ventricle (ICV) of awake rats to determine whether baroreflex sensitivity would be altered. Systolic and mean pressures, recorded from aortic catheters on day 11 while the rats were anesthetized with alpha-chloralose, were significantly higher in rats infused with artificial cerebrospinal fluid (CSF) containing hypertonic NaCl than in controls similarly infused with artificial CSF alone. Reflex changes in heart rate produced by subsequent intravenous infusions of either phenylephrine or sodium nitroprusside were inhibited, but reflex changes in renal nerve activity were unaltered. Magnitude of reflex bradycardia during pressor responses to phenylephrine, as well as of reflex tachycardia during depressor responses to sodium nitroprusside, was consistently smaller in NaCl-infused than in control rats. By contrast, group differences in attendant renal nerve firing were not significant. After sinoaortic denervation, drug-induced blood pressure effects persisted, but reflex responses in heart rate and renal nerve firing were abolished or markedly diminished. Peripheral effects produced by hypertonic NaCl leakage from the infusion site were considered unlikely because after 11 d of ICV infusion, sodium concentration, though appreciably elevated in CSF samples collected from the cisterna magna, was unaffected in corresponding serum samples. When cardiovascular responses to phenylephrine were recorded while chronic ICV infusions were in progress, awake rats receiving hypertonic NaCl were still normotensive on day 2 yet reflex bradycardia was already attenuated. In showing that baroreflex impairment preceded the development of hypertension, our results suggest that by depressing the anterior hypothalamus, chronic ICV infusion of hypertonic NaCl reduces sympatho-inhibition, and the ensuing baroreflex impairment then elevates blood pressure. However, other mechanisms could also be involved.

Catecholamine synapses and contents in the paraventricular hypothalamic nucleus and nucleus tractus solitarius of DOCA-salt hypertensive rats

Central catecholamine (CA) neurons in the nucleus tractus solitarius (NTS) and paraventricular hypothalamic nucleus (PVN) were studied in Wistar rats that had been unilaterally nephrectomized. The experimental animals were then treated with deoxycorticosterone acetate (DOCA) and salt water. The control animals were treated with the vehicle and tap water. Blood pressure of animals 4 weeks after DOCA/salt treatment was significantly elevated when compared to control rats. Morphologically, CA terminals showed no noticeable changes in the DOCA/salt hypertensive rats. Furthermore, the density of CA terminals either in the NTS or in the PVN of the DOCA/salt hypertensive rats was not statistically different from that of normotensive controls, suggesting that salt does not cause lesions or destruction of CA terminals. However, an extensive electron-microscopic morphometric analysis indicated that there was an enhancement of CA synaptogenesis (expressed by increased synaptic frequency among all CA boutons labeled with 5-hydroxydopamine) in the PVN, but not in the NTS of DOCA/salt hypertensive rats. In addition, the high-performance liquid chromatography revealed decreased CA contents in the PVN, but not in the NTS, of DOCA/salt hypertensive animals. Since synapses are primary sites for neurotransmitter release, the above results collectively suggest that more CA synapses formed in the PVN may reflect a net CA release from CA terminals resulting in the decreased CA content in the axonal terminals. Such an increased CA release and enhanced CA synaptogenesis may consequently enhance CA function in the PVN of hypertensive rats 4 weeks after DOCA/salt treatment, and relate to the development and/or maintenance of hypertension in the DOCA/salt rats.

Ultrastructural studies on catecholaminergic terminals and GABAergic neurons in nucleus tractus solitarius of the rat medulla oblongata

Synaptogenesis of catecholamine (CA) boutons in the nucleus tractus solitarius (NTS) was compared between spontaneously hypertensive (SHR) rats and Wistar-Kyoto (WKY) rats at different ages. On the average, there were about 32 CA varicosities per 2200 microns2 area of the NTS in both SHR and WKY rats as revealed by glyoxylic acid fluorescence microscopic (FM) morphometric study. The FM analysis indicated that there were no significant changes in the CA varicosity density between SHR and WKY rats. The CA boutons were labeled with 5-hydroxydopamine and appeared to contain small granular vesicles at the electron microscopic (EM) level. A total of 1402 CA boutons were studied in a 540,000 micron2 area of the NTS. The number of CA boutons involved in synaptic contacts vs the number of total CA boutons was used to obtain synaptic frequency which was taken as an index for synaptogenesis. A reduction of approximately 18% and 14% of synaptogenesis of CA boutons was observed in the NTS of SHR rats at 4 weeks (prehypertensive stage) and 12 weeks (early hypertensive stage) of age respectively, as compared to age-matched WKY rats. No significant difference of synaptogenesis of CA neurons was found between SHR and WKY rats at 16 weeks of age, a stage in which hypertension is well established and maintained in SHR rats. These results suggest that CA neurons with fewer synaptic contacts in the NTS may play a more important role in the initiation than in the maintenance of hypertension in the SHR rats. In addition to CA terminals, there were numerous GABAergic cell bodies in the NTS which were identified by immunocytochemistry using antibodies to the GABA synthesizing enzyme, L-glutamate decarboxylase (GAD). GABAergic dendrites with GAD-positive reaction were often seen to receive several GAD-negative synapses at EM random profiles. In the text, a viewpoint is thus discussed that emphasizes that a synaptic abnormality of CA terminals with fewer synaptic contacts affecting GABAergic neurons may participate in the pathogenesis of hypertension. However, it remains to be determined as to whether or not there is a direct contact between CA boutons and GABAergic dendrites.

Neuropeptides are present in projection neurones at all levels in visceral and taste pathways: from periphery to sensory cortex

Using combined immuno-staining and retrograde tracing techniques many of the ascending visceral and taste pathways within the rat central nervous system have been shown to be composed of a variety of neuropeptide and catecholamine synthesizing enzyme containing neurones. The pathway we examined extended from the periphery to sensory cortex and included: the nodose ganglion (periphery)----solitary nucleus (medulla)----parabrachial nucleus (pons)----ventral posterior medial nucleus (thalamus)----visceral and taste sensory areas (cortex). In the solitary nucleus of the medulla many neuronal cell bodies could be shown to be both immuno-positive for one of 6 neuropeptides including avian pancreatic peptide (APP), cholecystokinin (CCK), enkephalin (ENK), neurotensin (NT), somatostatin (SOM) and substance P (SP) or the catecholamine synthesizing enzyme tyrosine hydroxylase (TOH) and to have a projection to the parabrachial nucleus of the pons. In the parabrachial nucleus of the pons many neuronal cell bodies could be shown to be immuno-positive for one of 5 neuropeptides (CCK, ENK, NT, SOM, SP) and have a projection to the ventral posterior medial nucleus of the thalamus. In the ventral posterior medial nucleus of the thalamus several neuronal cell bodies were shown to be immuno-positive for one of 3 neuropeptides (CCK, ENK, SOM) and project to the visceral and taste sensory cortex. This is the first report of neuropeptides being present in the projection neurones of any sensory system in the central nervous system and for the first time describes an entire set of putative neurotransmitters which extends from the periphery to the sensory cortex. From previous studies it also appears that in all cases examined the relevant receptors are present in these visceral and taste relay nuclei in order for the neuropeptide or catecholamine to produce an effect upon release. Comparisons between rat and other animals suggest that a similar organization of these visceral and taste pathways may also be present in other mammals including man. Functionally these neuropeptides containing projection neurones appear to be primarily involved in relaying visceral information rather than taste information. In this capacity activation of these neurones may produce such visceral sensations as malaise, well being, hunger, satiety or thirst.

Ascending noradrenergic projections from the brainstem: evidence for a major role in the regulation of blood pressure and vasopressin secretion

The role of projections from the lateral tegmental (A1, A2) and coeruleal (A6) noradrenergic cell groups in the control of arginine vasopressin (AVP) secretion was studied following lesions to the ventral (VNAB) and dorsal (DNAB) noradrenergic bundles by 6-hydroxydopamine. These lesions were associated with the expected, large reductions in cortical (DNAB) and hypothalamic (VNAB) noradrenaline concentrations. Vehicle injected, control animals and VNAB lesioned animals showed a similar AVP secretory response to haemorrhage, whilst the DNAB group showed a markedly diminished release of AVP in response to this challenge. Following Clonidine injection, both controls and VNAB animals showed major reductions in plasma AVP concentrations, but again the DNAB group behaved in a different manner, with a marked attenuation of the inhibitory effect of Clonidine on AVP secretion. In addition, the DNAB group had a significantly lower basal blood pressure, a greater initial agonist response to Clonidine and a loss of the hypotensive response to Clonidine in comparison to sham and VNAB lesioned groups. All three groups showed a similar AVP response to intravenous nicotine. These data suggest that noradrenergic projections originating in the locus coeruleus, or in the lateral tegmental NA groups but which ascend together with coeruleal axons in the DNAB, modulate the vasopressin response to visceral stimuli and to Clonidine, and that they also play an important role in mediating the hypotensive effect of Clonidine.

Distribution of alpha 2 agonist binding sites in the rat and human central nervous system: analysis of some functional, anatomic correlates of the pharmacologic effects of clonidine and related adrenergic agents

Using [3H]para-aminoclonidine, alpha 2 adrenergic binding sites have been mapped in the rat and human CNS using in vitro labeling autoradiographic techniques. In both the rat and human thoracic spinal cord, high densities of alpha 2 binding sites were associated with the substantia gelatinosa and the intermediolateral cell column. In the rat medulla, high binding site density was observed in the medial nucleus of the solitary tract, dorsal motor nucleus of the vagus, raphe pallidus and the substantia gelatinosa of the trigeminal nucleus, while lower levels of specific binding were found in the lateral and ventrolateral medulla. In the human, a similar distribution was observed. However, significantly lower levels of specific binding were seen in the medial nts as opposed to the dmv. In the rat, high levels of specific binding were seen at pontine and midbrain levels in the locus coeruleus, parabrachial nucleus and periaqueductal gray. In the forebrain, several hypothalmic and limbic regions, including the paraventricular and arcuate nuclei of the hypothalamus, the central, medial and basal nuclei of the amygdala, lateral septum and bed nucleus of the stria terminalis and pyriform, entorhinal and insular cortex were labeled. Each of these regions are involved in either modulating autonomic functions directly or integrating somatosensory and/or affective function with autonomic mechanisms. Further, these regions are interrelated by reciprocal connections, and neurons that utilize noradrenaline or adrenaline as their neurotransmitter form a vital part of these connections. Thus, these functional, anatomical and neurochemical correlates of the alpha 2 binding site distribution establish a neurological basis for the complex pharmacological effects of centrally acting alpha 2 agonists.

Studies on the origin and release of somatostatin-immunoreactivity in the nucleus of the solitary tract

The somatostatin content of the nucleus of the solitary tract (NTS) was regionally distributed within the nucleus and a calcium-dependent release of the neuropeptide was evoked by potassium-induced depolarization in vitro. The origin of the somatostatin in the NTS was studied with various denervation procedures. Unilateral NTS deafferentation decreased the concentration of somatostatin in the NTS. However, neither midbrain hemisection nor nodose ganglionectomy reduced the somatostatin content of the NTS. In addition, the nodose ganglion contained very low quantities of the peptide. These results suggest that (1) somatostatin may function as a neurotransmitter in the NTS and (2) the somatostatin-containing innervation of the NTS is at least in part from extrinsic neurons located in the lower brainstem and not from vagal afferent or forebrain neurons.

TRH: cardiovascular and sympathetic modulation in brain nuclei of the rat

The cardiovascular and sympathetic effects of TRH in discrete cardiovascular-related brain nuclei were studied. Microinjections of TRH were made into the nucleus preopticus medialis (POM) of conscious rats and the nucleus tractus solitarius (NTS) of pentobarbitone-anesthetized, artificially respired rats. POM injections (1 microliter, 0.8-80 nM) elicited dose dependent pressor and tachycardic responses which were accompanied by increased levels of norepinephrine (NE) and epinephrine (EPI) in the plasma. These pressor/tachycardic effects of TRH were also elicited in adrenal demedullated (ADM-x) rats, but completely abolished in ADM-x rats pretreated with bretylium (30 mg/kg, IA). NTS injections (0.1 microliter, 30 and 150 nM) had a short depressor effect on blood pressure (BP) and a delayed increase in heart rate (HR). From these findings we suggest that the POM, a central nucleus in the AV3V region, may be an important forebrain site for autonomic regulation by TRH, mediated through the sympathetic nervous system.

gamma-Aminobutyric acid and cardiovascular function

Evidence supporting the hypothesis that GABA-ergic mechanisms are involved in controlling mammalian cardiovascular function has been reviewed and analyzed. In vivo and in vitro studies with GABA-agonists and GABA-antagonists have revealed that activation of GABA-receptors is involved in the control of blood pressure and heart rate. Further studies conducted with agents that modify central and/or peripheral GABA-ergic systems could lead to the discovery of drugs that might be useful for treating certain cardiovascular disorders in man, such as hypertension and stroke, and should increase our understanding of the pathophysiological bases of such disorders.

Effects of clonidine on the rate of noradrenaline turnover in discrete areas of the rat central nervous system

Turnover of noradrenaline in various regions of the rat brain was estimated by the decrease in noradrenaline content and/or formaldehyde-induced catecholamine fluorescence after inhibition of noradrenaline biosynthesis with alpha-methyl-p-tyrosine. Clonidine (0.1 and 0.3 mg/kg p.o.) decelerated the decrease in noradrenaline content of the locus coeruleus, the nucleus of the solitary tract, the intermediolateral cell column and the ventral horn of the thoracic spinal cord, as measured in tissue punches of the respective regions with a sensitive radioenzymatic method. In all these central regions the clonidine-induced decrease in noradrenaline turnover was antagonized by yohimbine, but not by phenoxybenzamine, indicating mediation through central alpha 2-adrenoceptors, similar to the cardiovascular effects of clonidine. When given alone, both yohimbine and phenoxybenzamine accelerated the disappearance of noradrenaline after inhibition of its biosynthesis. The combined results of radioenzymatic assay and fluorescence histochemistry determinations demonstrated that clonidine markedly reduced noradrenaline turnover in central noradrenaline-containing nerve terminals, but had no effect on the cell bodies of the A1 and A2 cell groups. Noradrenaline turnover was, however, decreased in projection areas of the A1 and A2 cell groups, namely the intermediolateral cell column of the spinal cord and nucleus of the solitary tract, respectively. This observation argues against the existence of a neuronal feedback loop running from the projection areas to the cell bodies of the A1 and A2 cell groups and mediating inhibition of noradrenaline turnover. The effect of clonidine on noradrenaline turnover is, therefore, most likely the result of a local feedback inhibition through presynaptic alpha-adrenoceptors.(ABSTRACT TRUNCATED AT 250 WORDS)

The nucleus tractus solitarii of the cat: a comparison of Golgi impregnated neurons with methionine-enkephalin- and substance P-immunoreactive neurons

Golgi-impregnated and methionine-enkephalin (ME)- and substance P (SP)-immunoreactive neurons were studied throughout the feline nucleus tractus solitarii. The majority of Golgi-impregnated neurons in the NTS range in size from 5 to 18 micron. A noticeable exception is the large (15-30 micron) neurons of the ventrolateral subdivision. The Golgi-impregnated neurons possess dendritic trees which remain within the nucleus and even at times within the particular subdivisions. Golgi-impregnated neurons had a variety of spine forms: pedunculated, sessile, filiform, and complex. A number of neurons exhibited axons originating from the cell and they could be followed for distances up to 100 micron. ME- and SP-immunoreactive neurons were found in commissural, medial, lateral, and parvocellular subdivisions while ME-immunoreactive neurons were situated additionally in the intermediate and ventrolateral subdivisions. Both types of immunostained neurons were similar in size (6-20 micron) and shape of dendritic arbor. One population of ME-immunoreactive neurons resembled the large ventrolateral neurons of the Golgi impregnations. Neither type of immunostained neuron possessed the extensive dendritic arbor, numbers of spines, or axons of the Golgi-impregnated neurons. The presence of ME- and SP-immunoreactive neurons in regions which are associated with autonomic regulation suggests that these two peptides are involved in this process.

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.

The identification of specific serotonergic nuclei inhibited by cardiac vagal afferents during acute myocardial ischemia in the rat

Cardio-cardiac autonomic reflexes mediated by the cardiac vagus during acute myocardial infarction play an important role in determining post-infarction hemodynamic function and the susceptibility of the infarcting heart to lethal arrythmias. In an earlier study we had demonstrated that serotonin-containing neurons in the brain participate in the mediation of these reflexes following left coronary artery ligation in the rat. In this study we identify brain serotonin nuclei involved. The accumulation of serotonin was measured in 19 brain nuclei from rats treated with pargyline, a monoamine oxidase inhibitor. The rats were subjected to either left coronary artery ligation or sham operation with or without bilateral cervical vagotomy or lidocaine applied topically only to the left ventricle. Serotonin accumulation was markedly reduced in the nucleus hypothalamicus posterior, nucleus raphe magnus and nuclei-raphe medianus-centralis superior in the rats subjected to coronary artery ligation as opposed to sham operation; no other brain regions were affected. The topical application of lidocaine to the left ventricle or vagotomy completely obviated the ligation-induced decrease in serotonin turnover. We conclude that there is an inhibition of serotonergic activity in the nucleus hypothalamicus posterior, nucleus raphe magnus and nuclei raphe medianus-centralis superior following left ventricular myocardial infarction in the rat. The afferent signal arises from receptors in the left ventricle and is conducted by the vagus.

Evidence for an increase in [3H]spiperone binding in hypothalamic nuclei during the development of spontaneous hypertension in the rat

The in vivo binding of [3H]spiperone was measured in discrete areas of the hypothalamus in 7, 9 and 16 weeks old spontaneously hypertensive rats (SHR) and age matched normotensive Wistar Kyoto (WKY) controls. The specific binding of [3H]spiperone was significantly higher in the four different hypothalamic regions (H1, H2, H3, H4) that we have tested in 7 or 9 weeks old SHR than in age matched WKY controls. At 16 weeks a significant increase was only present in H3. These results suggest that dopaminergic hypothalamic neurons might be implicated in the onset of hypertension in the rat.

Stress and high sodium effects on blood pressure and brain catecholamines in spontaneously hypertensive rats

The following experiments were designed to determine if territorial stress, dietary sodium (Na), or the combination of stress and Na effect the rate of development of hypertension in the spontaneously hypertensive rat (SHR 4-18 wks) and if central catecholamines (C) were altered by these treatments. BP was significantly elevated from 2-8 weeks of stress treatment as compared to SHR controls. Norepinephrine (NE) levels in the nucleus tractus solitarius and amygdala (A), and dopamine (D) levels in the hippocampus and A showed significant elevations in the stressed group. High Na (3%) treatment combined with stress treatment produced an even further BP increase and elevated D levels in the amygdala, and elevated NE levels in the area postrema as compared to control SHR's. Selected brain C variables were able to correctly classify animals into high and low BP groups with 90-100% accuracy. Our data support the concept that there are important stress and Na effects upon brain neurochemistry which influence the development of hypertension in the SHR.

Cholecystokinin in the nucleus of the solitary tract of the rat: evidence for its vagal origin

Nerve fibers and terminals immunoreactive for cholecystokinin (CCK) were demonstrated in the nucleus of the solitary tract (NTS) of the rat using light and electron microscopic immunocytochemistry. The following morphological and biochemical evidence suggests that CCK in the NTS seems to be of extrinsic, most probably vagal, origin: (1) axon fragments of the intracranial vagus were identified by immunostaining on their way to the solitary tract: (2) CCK-immunostaining could be localized in nerve terminals in the nucleus of the solitary tract, where only a very few immunopositive dendrites or cell bodies were present; and (3) transecting the major neuronal afferents (via solitary tract and/or more laterally) resulted in a complete disappearance of radioimmunoassayable CCK from the nucleus of the solitary tract.

Modification of the baroreceptor reflex by superfusion of the canine posterior hypothalamus with tetrodotoxin

The role of the posterior hypothalamus in the baroreceptor reflex was investigated in dogs. The posterior hypothalamus was superfused bilaterally through push-pull cannulae with artificial cerebrospinal fluid. After bilateral superfusion with tetrodotoxin (10(-5) M) for 120 min, the increases in mean arterial pressure and heart rate induced by the electrical stimulation of the posterior hypothalamus were almost abolished, and the resting mean arterial pressure was significantly decreased. Under these circumstances the increases in mean arterial pressure and heart rate in response to bilateral carotid occlusion were significantly reduced. The decrease in mean arterial pressure induced by intravenous injection of sodium nitroprusside (5 micrograms/kg) was significantly augmented while the associated increase in heart rate was suppressed. These results suggest that the posterior hypothalamus plays an important role in maintaining the resting arterial pressure and in the baroreceptor reflex to increase the arterial pressure and heart rate.

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.

mu Receptors and opioid cardiovascular effects in the NTS of rat

In order to assess the potential role of mu (mu) and delta (delta) opiate receptors in the central regulation of the cardiovascular and respiratory systems, the cardiovascular and respiratory effects of the relatively selective mu-opioid agonist D-Ala2, MePhe4, Gly-ol5 enkephalin (DAGO) and relatively selective delta-agonist D-Ala2-D-Leu5 enkephalin (DADL) were compared following microinjection of these compounds into the nucleus tractus solitarius of pentobarbital-anesthetized rats. Both opioid agonists produced dose dependent increases in systolic and diastolic blood pressure as well as heart rate; but DAGO was nearly ten times more potent in eliciting these changes. Respiratory rate was increased by DADL and by lower doses of DAGO, but was depressed by higher doses of DAGO. Tidal volume was depressed by both peptides. These data support the concept that the cardiovascular pressor responses and tachycardia as well as the respiratory effects of opioids in the rat NTS are mediated by mu receptors.

Delta versus mu receptors: cardiovascular and respiratory effects of opiate agonists microinjected into nucleus tractus solitarius of cats

The cardiovascular and respiratory responses to relatively specific mu or delta agonists microinjected (0.5 microliter/kg) into the region of the nucleus of tractus solitarius (NTS) were examined in anesthetized cats. Blood pressure, heart rate, and respiratory rate were monitored for 30 min after the microinjection of opioid compounds or saline vehicle. The delta agonist, (D-Ala2,D-Leu5)-enkephalin (10-100 nmol/kg) elicited dose-dependent decreases in blood pressure, heart rate, and respiratory rate which were naloxone reversible. In contrast the mu agonists, morphine (10-54 nmol/kg) and morphiceptin (100-320 nmol/kg) had no effect on blood pressure or respiratory rate; yet, naloxone elicited pressor responses in animals pretreated with these mu agonists. A receptor-binding assay demonstrated a predominance of mu sites in the NTS. These data show that the delta opiate agonist is more effective than mu agonists in modifying cardiovascular variables in the NTS; we suggest caution in relating specific cardiovascular function to receptor subtypes defined by binding assays.

Influence of neonatally administered capsaicin on baroreceptor and chemoreceptor reflexes in the adult rat

1 Baroreceptor and chemoreceptor reflex activity was studied in anaesthetized adult rats which had been treated neonatally with a single injection of capsaicin (50 mg/kg s.c.). 2 Pressor responses to bilateral carotid artery occlusion were significantly lower in capsaicin-treated rats compared with vehicle-treated controls. Pressor responses to intravenously injected noradrenaline were similar in the two groups of rats. 3 Resting respiratory minute volume and tidal volume were lower in anaesthetized capsaicin-treated animals than in vehicle-treated controls, but there was no significant difference in respiratory frequency. 4 The increases in respiration evoked by intravenous administration of the peripheral arterial chemoreceptor stimulant, sodium cyanide, or by breathing a hypoxic gas mixture, were significantly lower in capsaicin-treated rats compared with the controls. 5 It is concluded that baroreceptor and chemoreceptor reflex activity are significantly reduced in anaesthetized adult rats which had been treated neonatally with capsaicin, and that this is likely to result from the destruction of unmyelinated baro- and chemoreceptor afferent fibres.

Comparative microdistribution of the activity of catecholamine-synthesizing enzymes in horizontal sections of the rat lower brainstem

The activities of the three major catecholamine-synthesizing enzymes were determined in brain tissue pellets dissected from 500-micrometers thick horizontal sections of rat lower brainstem. The rostrocaudal distributions of the three enzymatic activities were generally not parallel, suggesting differences in the respective localization of the noradrenergic and adrenergic neurons. The difference was most important in the A2-C2 region where the maximal activity of phenylethanolamine-N-methyltransferase (EC 2.1.1.28) was located 1.5 mm more rostrally than the maximal activities of the tyrosine hydroxylase (EC 1.14.16.2) and dopamine beta-hydroxylase (EC1.14.17.1). This result indicates that a more specific dissection of the adrenergic and noradrenergic neurons could be performed in the A2-C2 area of the rat brainstem.

Cardiovascular effects of beta-endorphin after microinjection into the nucleus tractus solitarii of the anaesthetised rat

The cardiovascular effects of beta-endorphin after administration directly into the nucleus tractus solitari (NTS) of urethane-anaesthetised rats were investigated. Unilateral injection resulted in a U-shaped dose-response relationship with a fall in mean arterial pressure and heart rate occurring at low doses (less than 10 ng). No change in respiratory frequency was observed at any of the doses examined. The hypotensive effects of beta-endorphin were anatomically specific and restricted to the NTS. The depressor response was prevented and bradycardia reduced by naloxone (1 mg/kg s.c. or 10 ng injected into the NTS) and also by beta-endorphin antiserum (1:50 dilution) but not by antiserum to [Met5]enkephalin (1:50 dilution) applied locally into the NTS. The beta-endorphin antiserum caused a rise in blood pressure when administered alone. Conversely microinjection of antiserum to [Met 5]enkephalin resulted in a brief depressor response. Doses of beta-endorphin larger than 10 ng induced a rise in blood pressure accompanied by variable effects on heart rate. Similarly unilateral administration of Des-tyr-endorphin (100 pg) resulted in a blood pressure increase and [D-Ala2,Met5]enkephalin produced a dose-related pressor response and tachycardia. The results indicate that at least two separate endorphin systems are involved in cardiovascular control at the level of NTS, one being depressor in nature (beta-endorphin-like) and the other pressor ([met5]enkephalin-like).

Fluorescence microscopy used in conjunction with horseradish peroxidase localization and electron microscopy for studying sympathetic nuclei of the rat spinal cord

After application of horseradish peroxidase (HRP) to the transected cervical sympathetic trunk, labeled sympathetic neurons were seen in the intermediolateral nucleus (IML), central autonomic nucleus (CAN) and intercalated nucleus of rat spinal cords. As studied by glyoxylic acid-induced histofluorescence microscopy (FM), catecholaminergic (CA) terminals were most densely packed in the IML. There were 60 +/- 2 CA varicosities per 2,200 mu2 area in 20 mu thick sections through the IML. A combined FM/HRP method confirmed that preganglionic sympathetic neurons are heavily innervated by CA terminals. CA boutons were tagged with 5-hydroxydopamine for EM identification, and in the IML, 56% of CA boutons were seen to make synaptic contacts as compared to 60% in the CAN. It seems likely that virtually all CA boutons may form synapses but serial sections through boutons were not studied. It is inferred that preganglionic sympathetic neurons of both IML and CAN are served by CA inputs, and electron microscopy has revealed that most or all of these CA terminals transmit their signals via synapses; which may permit more precise structural sorting than non-synaptic transmission.

Evidence for a neuromodulatory role of GABA at the first synapse of the baroreceptor reflex pathway. Effects of GABA derivatives injected into the NTS

Microinjections of GABA and of the specific agonist of GABA receptors, muscimol, in the intermediate nucleus tractus solitarii (NTS) of pentobarbitone anaesthetized cats produced hypertension and tachycardia. The GABA receptor antagonist, bicuculline, had opposite effects and prevented those of muscimol. Therefore, a GABAergic system appears to modulate the cardiovascular regulation within the NTS. d,l-Baclofen also increased blood pressure and heart rate when injected into the same region, but this effect was not antagonized by bicuculline. The mechanism of this action of baclofen is discussed.

Alterations in local cerebral glucose utilization during hemorrhagic hypotension in the rat

The alterations in local cerebral glucose utilization in 58 anatomically discrete regions which occur during a period of hemorrhagic hypotension have been investigated in conscious rats, using the quantitative autoradiographic 14C-deoxyglucose technique. Hemorrhagic hypotension (mean arterial pressure reduced by approximately 50 mm Hg) effected significant increases in glucose utilization in eight areas of the central nervous system, namely, the nucleus of the tractus solitarius (glucose utilization increased by 38%), the dorsal motor nucleus of the vagus (by 36%), locus coeruleus (by 38%), lateral habenular nucleus (by 40%), periventricular nucleus of the hypothalamus (by 41%), paraventricular nucleus of the hypothalamus (by 97%), supraoptic nucleus (by 86%), and the interstitial nucleus of the stria terminalis (by 84%). In five of these eight areas (nucleus of the tractus solitarius, dorsal motor nucleus of the vagus, paraventricular and supraoptic nuclei, and the interstitial nucleus of the stria terminalis), a significant relationship could be demonstrated between the level of glucose utilization and mean arterial blood pressure. In the majority of the CNS regions examined (neocortex, hippocampus, thalamus, extrapyramidal and motor areas), hemorrhagic hypotension was without significant effect upon local cerebral glucose utilization. The results provide direct evidence of the functional involvement of specific brain areas of conscious rats (thus obviating complicating anesthetic influences) in the response of the CNS to hemorrhagic hypotension.

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.

Potassium evoked catecholamine release from the nucleus tractus solitarius in vitro

The release of endogenous catecholamines (CA) from rat brain slices containing the nucleus tractus solitarius (NTS) was measured using a sensitive radioenzymatic assay. KCl (35 to 75 mM) induced a dose-related increase in norepinephrine (NE) release. Dopamine (DA) release was maximal with 50 mM KCl. An increase in epinephrine (E) release was only observed with 75 mM CKl. NE and E release was totally calcium-dependent whereas DA release was only partially calcium-dependent. Subsequent administrations of KCl released less CA. The calcium dependency of the KCl induced released of E, NE, and DA suggests a neurotransmitter function in the NTS for these CA. A difference in storage sites and/or mechanisms may be responsible for the observed differences in sensitivity to KCl and to extracellular calcium.

Distribution of angiotensin-converting enzyme activity in specific areas of the rat brain stem

Angiotensin-converting enzyme (ACE) activity was measured by a radiochemical assay in 30 specific areas of the rat brain stem. ACE activity is unevenly distributed, with a 60-fold difference between the lowest and the highest activity. The area postrema exhibits the highest activity. The substantia nigra (pars reticulata), the locus coeruleus, the areas A1 and A2, the nuclei commissuralis, and tractus solitarii have a substantial ACE activity, whereas the lowest activity is found in the raphe nuclei and the nuclei of the reticular formation.

Regional distribution of noradrenaline and dopamine-beta-hydroxylase in the brain of spontaneously hypertensive rats

The catecholamine concentration and dopamine-beta-hydroxylase activity were determined in several nuclei of the brain of spontaneously hypertensive rats (SHR) compared with Wistar Kyoto (WKY) controls. Catecholamines were measured by using liquid chromatography coupled with electrochemical detection. The threshold of detection was 5 X 10(-14) mole. Dopamine-beta-hydroxylase (DBH) was assayed by a sensitive radioenzymatic assay using tyramine as the substrate. The limit of detection was 5 pmoles of octopamine per sample. Significantly lower noradrenaline content was observed in 4 week-old SHR in some medullary and hypothalamic areas which are involved in cardiovascular regulation. This abnormally was no longer detectable in 12 week-old rats. The changes in catecholamine levels observed in young rats were not observed during the development of deoxycorticosterone-salt hypertension and therefore probably do not represent a compensatory mechanism tending to limit the progressive rise blood pressure. No significant difference in DBH activity was observed between the young SHR and WKY in any brain region measured. The altered relationship between noradrenaline content and DBH activity observed in young SHR when compared to WKY suggests a change in noradrenergic neurones activity and/or structure which could correspond to a genetically transmitted neurochemical abnormality associated with the initiation of hypertension in the SHR.

Biochemical evidence that L-glutamate is a neurotransmitter of primary vagal afferent nerve fibers

To determine in rat if vagal afferent fibers projecting into the intermediate one third of the nucleus tractus solitarius (NTS), the site of termination of baroafferents, utilize glutamate as a neurotransmitter, the high-affinity uptake of [3H]L-glutamate and content of glutamate were analyzed in micropunches of rat brain stem. The intermediate NTS contains a high-affinity synaptosomal uptake system for [3H]L-glutamate that is greater in capacity than that in areas adjacent to the NTS; it is almost two-fold higher than uptake in medial septum and nucleus accumbens and equal to that of hippocampal regions purportedly containing a rich glutamatergic innervation. Unilateral ablation of the nodose ganglion (i.e. cells of origin of vagal afferents) resulted, within 24 h in a prolonged significant reduction, to 56% of control, of [3H]L-glutamate uptake, bilaterally in the NTS. The reduction of Na+-dependent synaptosomal uptake of [3H]L-glutamate, resulted from a decrease in Vmax without change in the Km of the process, was anatomically restricted to the intermediate NTS, and was not associated with changes in [3H]GABA uptake. The content of glutamate in the NTS was significantly (P less than 0.01) decreased by 30% 7 days following unilateral extirpation of the nodose ganglion without changes in the concentrations of aspartate, glycine, glutamine, or GABA. A population of vagal afferent fibers projecting to NTS are glutamatergic. The results are consistent with the hypothesis obtained by physiological and pharmacological techniques that glutamate is a neurotransmitter of baroafferents.

Phylogenic and neurologic aspects of the vomiting process

Hindbrain morphology is compared in a variety of animals representing species that vomit and those that do not. Special attention is given to the nucleus of the tractus solitarius (NTS) and its synaptic connections with surrounding structures. The role of the area postrema is examined from the standpoints of containing the chemoreceptor trigger zone (CTZ) for vomiting, of its functional relationship to the blood-brain barrier, and of its significance as part of the circumventricular organ system. A survey is made of the manifold behavioral contributions of NTS to cardiovascular and respiratory regulation, appetite control, arousal, water balance, and digestive and neuroendocrine functions. An overview is presented of neurochemical distribution in the brain stem with respect to aminergic, cholinergic, and peptidergic substances. Finally, causes and prevention of vomiting are discussed in light of the above considerations.

Acute hypertension after the local injection of kainic acid into the nucleus tractus solitarii of rats

Kainic acid (KA), an analogue of L-glutamate, was microinjected in 0.1 microliter of saline into the nucleus tractus solitarii (NTS) of adult rats. In rats anesthetized with halothane or alpha-chloralose, KA injected unilaterally elicited hypotension, bradycardia, and apnea. The threshold dose was 0.1-0.2 ng (10(-13) mol). Doses greater than 0.2 ng blocked responses to subsequent injections for at least 30 minutes. Doses of KA greater than 15 ng reduced the reflex bradycardia elicited by raising the arterial pressure with phenylephrine and produced arterial hypertension in rats anesthetized with alpha-chloralose or in other rats within 15 minutes of terminating halothane anesthesia. Bilateral injection of KA in doses greater than 15 ng completely blocked baroreflexes and resulted in a dose-dependent elevation of arterial pressure (167 +/- 9.4; P less than 0.001) both in alpha-chloralose-anesthetized rats and in awake rats after the termination of halothane anesthesia. The hypertension rapidly led to pulmonary edema and death. Procaine microinjected also elicited fulminating hypertension; vehicle did not. Doses of KA producing hypertension caused no histological or biochemical evidence of neuronal death. The cardiovascular responses to KA were restricted to sites in the intermediate one-third of NTS and could not be elicited by injection into adjacent sites in brainstem. The results indicate that, in low doses, KA injected into NTS stimulates neurons which mediate the baroreflex, whereas, in higher doses, it produces baroreflex blockade and neurogenic hypertension. The results suggest that fulminating hypertension can be produced by nondestructive perturbations of neurochemical transmission in brain. Since the cardiovascular responses of KA are similar to those produced by microinjection into NTS of the amino acid neurotransmitter glutamic acid, the study adds further support to the hypothesis that L-glutamate is the neurotransmitter released by baroreceptor afferent nerves.

Altered cyclic AMP levels in specific cardiovascular brain centers of spontaneously hypertensive rats (SHR)

The possible participation of cAMP in central regulation of arterial blood pressure was investigated in spontaneously hypertensive rats (SHR). Cyclic AMP concentrations of 36 microdissected individual brain areas-including primary and higher cardiovascular centers-were measured in adult SHR and compared with those of normotensive control rats of the same strain and age. In the adult SHR, elevated cAMP concentrations were found in brain areas which are in close connection with the central regulation of blood pressure: nucleus tractus solitarii, A1- and A2- catecholaminergic cell groups in the medulla oblongata, locus coeruleus, central grey matter (subnucleus medialis), and certain cortical areas (especially cingulate cortex and hippocampus). On the other hand, hypothalamic cell groups which have been also suggested to control blood pressure, such as paraventricular, anterior, ventromedial, dorsomedial and posterior hypothalamic nuclei, show lower concentrations of cAMP in the SHR than in normotensive controls. Cyclic AMP levels in 19 other brain areas of SHR which seem to be not involved in mechanisms of central blood pressure regulation practically do not differ from the values of normotensive rats. The results suggest that cAMP-dependent processes are involved in the regulatory mechanisms of central blood pressure control. The brain areas which show alterations of cAMP-levels are also distinguished by close neuronal connections to the baroreceptor reflex arc. It is supposed that these areas represent modulatory higher centers capable to affect baroreceptor reflex function.

Brain stem projections of sensory and motor components of the vagus complex in the cat: I. The cervical vagus and nodose ganglion

The motor and sensory connections of the cervical vagus nerve and of its inferior ganglion (nodose ganglion) have been traced in the medulla oblongata of 32 adult cats with the neuroanatomical methods of horseradish peroxidase (HRP) histochemistry and amino acid autoradiography (ARG). In 14 of these subjects, an aqueous solution of HRP was applied unilaterally to the central end of the severed cervical vagus nerve. In 13 other cases, HRP was injected directly into the nodose ganglion. Three of these 13 subjects had undergone infranodose vagotomy 6 weeks prior to the HRP injection. A mixture of tritiated amino acid was injected into the nodose ganglion in five additional cats. The retrograde transport of HRP yielded reaction product in nerve fibers and perikarya of parasympathetic and somatic motoneurons in the medulla oblongata. Furthermore, a tetramethyl benzidine (TMB) method for visualizing HRP enabled the demonstration of anterograde and transganglionic transport, so that central sensory connections of the nodose ganglion and of the vagus nerve could also be traced. The central distribution of silver grain following injections of tritiated amino acids in the nodose ganglion corresponded closely with the distribution of sensory projections demonstrated with HRP, thus confirming the validity of HRP histochemistry as a method for tracing these projections. The histochemical and autoradiographic experiments showed that the vagus nerve enters the medulla from its lateral aspect in multiple fascicles and that it contains three major components--axons of preganglionic parasympathetic neurones, axons of skeletal motoneurons, and central processes of the sensory neurons in the nodose ganglion. Retrogradely labeled neurons were seen in the dorsal motor nucleus of X(dmnX), the nucleus ambiguus (nA), the nucleus retroambigualis (nRA), the nucleus dorsomedialis (ndm) and the spinal nucleus of the accessory nerve (nspA). The axons arising from motoneurons in the nA did not traverse the medulla directly laterally; rather, all of these axons were initially directed dorsomedially toward the dmnX, where they formed a hairpin loop and then accompanied the axons of dmnX neurons to their points of exit. Afferent fibers in the vagus nerve reached most of the subnuclei of the nTS bilaterally, with the more intense labeling being found on the ipsilateral side. Labeling of sensory vagal projections was also found in the area postrema of both sides and around neurons of the dmnX. These direct sensory projections terminating within the dmnX may provide an anatomical substrate for vagally mediated monosynpatic reflexes. Following deefferentiation by infranodose vagotomy 6 weeks prior to HRP injections into the nodose ganglion, a number of neurons in the dmnX were still intensely labeled with the HRP reaction product. The axons of these HRP-labeled perikarya may constitute the bulbar component of the accessory nerve.