Adrenergic responses of baroreceptive cells in the nucleus tractus solitarii of the rat: a microiontophoretic study

Oxidant Mechanisms in Renal Injury and Disease

SIGNIFICANCE

A common link between all forms of acute and chronic kidney injuries, regardless of species, is enhanced generation of reactive oxygen species (ROS) and reactive nitrogen species (RNS) during injury/disease progression. While low levels of ROS and RNS are required for prosurvival signaling, cell proliferation and growth, and vasoreactivity regulation, an imbalance of ROS and RNS generation and elimination leads to inflammation, cell death, tissue damage, and disease/injury progression.

RECENT ADVANCES

Many aspects of renal oxidative stress still require investigation, including clarification of the mechanisms which prompt ROS/RNS generation and subsequent renal damage. However, we currently have a basic understanding of the major features of oxidative stress pathology and its link to kidney injury/disease, which this review summarizes.

CRITICAL ISSUES

The review summarizes the critical sources of oxidative stress in the kidney during injury/disease, including generation of ROS and RNS from mitochondria, NADPH oxidase, and inducible nitric oxide synthase. The review next summarizes the renal antioxidant systems that protect against oxidative stress, including superoxide dismutase and catalase, the glutathione and thioredoxin systems, and others. Next, we describe how oxidative stress affects kidney function and promotes damage in every nephron segment, including the renal vessels, glomeruli, and tubules.

FUTURE DIRECTIONS

Despite the limited success associated with the application of antioxidants for treatment of kidney injury/disease thus far, preventing the generation and accumulation of ROS and RNS provides an ideal target for potential therapeutic treatments. The review discusses the shortcomings of antioxidant treatments previously used and the potential promise of new ones. Antioxid. Redox Signal. 25, 119-146.

Anatomical and functional connections between the locus coeruleus and the nucleus tractus solitarius in neonatal rats

This study was designed to investigate brain connections among chemosensitive areas in newborn rats. Rhodamine beads were injected unilaterally into the locus coeruleus (LC) or into the caudal part of the nucleus tractus solitarius (cNTS) in Sprague-Dawley rat pups (P7-P10). Rhodamine-labeled neurons were patched in brainstem slices to study their electrophysiological responses to hypercapnia and to determine if chemosensitive neurons are communicating between LC and cNTS regions. After 7-10 days, retrograde labeling was observed in numerous areas of the brainstem, including many chemosensitive regions, such as the contralateral LC, cNTS and medullary raphe. Whole-cell patch clamp was done in cNTS. In 4 of 5 retrogradely labeled cNTS neurons that projected to the LC, firing rate increased in response to hypercapnic acidosis (15% CO2), even in synaptic blockade medium (SNB) (high Mg(2+)/low Ca(2+)). In contrast, 2 of 3 retrogradely labeled LC neurons that projected to cNTS had reduced firing rate in response to hypercapnic acidosis, both in the presence and absence of SNB. Extensive anatomical connections among chemosensitive brainstem regions in newborn rats were found and at least for the LC and cNTS, the connections involve some CO2-sensitive neurons. Such anatomical and functional coupling suggests a complex central respiratory control network, such as seen in adult rats, is already largely present in neonatal rats by at least day P7-P10. Since the NTS and the LC play a major role in memory consolidation, our results may also contribute to the understanding of the development of memory consolidation.

Chronic sustained hypoxia enhances both evoked EPSCs and norepinephrine inhibition of glutamatergic afferent inputs in the nucleus of the solitary tract

The nucleus of the solitary tract (NTS) receives inputs from both arterial chemoreceptors and central noradrenergic neural structures activated during hypoxia. We investigated norepinephrine (NE) modulation of chemoreceptor afferent integration after a chronic exposure to sustained hypoxia (CSH) (7-8 d at 10% FIO(2)). Whole-cell recordings of NTS second-order neurons identified by DiA (1,1'-dilinoleyl-3,3,3',3'-tetra-methylindocarbocyanine, 4-chlorobenzenesulphonate) labeling of carotid bodies were obtained in a brain slice. Electrical stimulation of the solitary tract was used to evoke EPSCs. CSH exposure increased evoked EPSC (eEPSC) amplitude via both presynaptic and postsynaptic mechanisms. NE dose dependently decreased the amplitude of eEPSCs. NE increased the paired-pulse ratio of eEPSCs and reduced the frequency of miniature EPSCs, suggesting a presynaptic mechanism. EC(50) of NE inhibition of eEPSCs was lower in CSH cells (3.0 +/- 0.9 microM; n = 5) than in normoxic (NORM) cells (7.6 +/- 1.0 microM; n = 7; p < 0.01). NE (10 microM) elicited greater inhibition of eEPSCs in CSH cells (63 +/- 2%; n = 16) than NORM cells (45 +/- 3%; n = 21; p < 0.01). The alpha-adrenoreceptor antagonist phentolamine abolished NE inhibition of eEPSCs. CSH enhanced the alpha2-adrenoreceptor agonist clonidine-mediated inhibition (3 microM; NORM, 23 +/- 2%, n = 5 vs CSH, 44 +/- 5%, n = 4; p < 0.05) but attenuated alpha1-adrenoreceptor agonist phenylephrine-mediated inhibition (40 microM; NORM, 36 +/- 2%, n = 11 vs CSH, 26 +/- 4%, n = 6; p < 0.05). The alpha2-adrenoreceptor antagonist yohimbine abolished CSH-induced enhancement of NE inhibition of eEPSCs. These results demonstrate that CSH increases evoked excitatory inputs to NTS neurons receiving arterial chemoreceptor inputs. CSH also enhances NE inhibition of glutamate release from inputs to these neurons via presynaptic alpha2-adrenoreceptors. These changes represent central neural adaptations to CSH.

d,l-sotalol enhances baroreflex sensitivity in conscious rats surviving acute myocardial infarction

The aim of the present study was to characterize the influence of D,L-sotalol on arterial baroreflex sensitivity (BRS) which is generally considered as an estimate of vagal activity and has prognostic value in patients after myocardial infarction (MI). Conscious rats were studied 3 days after left anterior descending coronary artery ligation (n= 5) or sham-operation (SH, n= 6). BRS was determined by linear regression analysis of the RR-interval (interval between heart beats) and mean arterial pressure changes evoked by i.v. bolus injections of methoxamine (inducing reflex bradycardia, RB) and nitroprusside (inducing reflex tachycardia, RT). In MI-rats heart rate and mean arterial pressure were not significantly different from values in SH-rats, left ventricular end-diastolic pressure was increased and contractility was depressed. The BRS (RB: MI: 0.48 +/- 0.04(*), SH: 0.79 +/- 0.08; RT: MI: 0.41 +/- 0.05(*), SH: 0.86 +/- 0.08 ms mmHg(-1)) ((*)P< 0.05 vs SH) was markedly reduced. d, l -Sotalol (1.5 mg kg(-1)i.v.) reduced heart rate (MI: -11 +/- 3 %(*), SH: -11 +/- 3 %(*)) and mean arterial pressure only moderately [MI: -6 +/- 4 %(n.s.), SH: -7 +/- 2 %(*)], while BRS depression in MI-rats was completely neutralized [RB: MI: 1.08 +/- 0.14(*), SH: 1.19 +/- 0.11(*); RT: MI: 0.84 +/- 0.08(*), SH: 0.88 +/- 0.12 (n.s.) ms mmHg(-1)] ((*)P< 0.05 vs pretreatment). The BRS is reduced in rats early after MI, indicating a depressed reflex vagal activity. Treatment with D,L-sotalol at a dose with little effect on heart rate and mean arterial pressure markedly enhances and, thus, restores BRS in MI-rats. These data suggest that D,L-sotalol has both peripheral and central effects leading to an increase of reflex vagal control of heart rate in rats.

The effects of noradrenergic activation of the nucleus tractus solitarius on memory and in potentiating norepinephrine release in the amygdala

Although it is known that norepinephrine (NE) modulates memory by acting on limbic areas, few studies describe how structures supplying NE to the limbic system, such as the nucleus tractus solitarius (NTS) contribute to this process. The present study examined the effects on memory of activating the NE pathway between the NTS and the amygdala (AMYG). Rats received buffer or the beta-noradrenergic agonist clenbuterol (CLN; 10, 50, or 100 ng/0.5 microl) into the NTS after footshock training in a Y-maze discrimination task. Infusion of 100 ng CLN significantly improved memory when retention was tested in the absence or presence of cues associated with the footshock. Experiment 2 used in vivo microdialysis to determine whether the mnemonic effects of CLN are mediated by influencing NE output in the AMYG. Subjects were given an intra-NTS infusion of CLN or phosphate buffered saline, footshock (0.8 mA, 1 s), and injected with epinephrine (EPI; 0.3 mg/kg ip) or saline. CLN or EPI injection produced a significant increase in NE sampled from the AMYG. These findings indicate that activation of NTS neurons that project to and release NE in the AMYG modulates memory storage processing.

Responses of aortic depressor nerve-evoked neurones in rat nucleus of the solitary tract to changes in blood pressure

Using electrophysiological techniques, the discharge of neurones in the nucleus of the solitary tract (NTS) receiving aortic depressor nerve (ADN) inputs was examined during blood pressure changes induced by I.V. phenylephrine or nitroprusside in anaesthetized, paralysed and artificially ventilated rats. Various changes in discharge rate were observed during phenylephrine-induced blood pressure elevations: an increase (n = 38), a decrease (n = 5), an increase followed by a decrease (n = 4) and no response (n = 11). In cells receiving a monosynaptic ADN input (MSNs), the peak discharge frequency response was correlated to the rate of increase in mean arterial pressure (P < 0.01) but was not correlated to the absolute increase in blood pressure. The peak discharge frequency response of cells receiving a polysynaptic ADN input (PSNs) was correlated to neither the absolute increase in blood pressure nor the rate of increase in mean arterial pressure. Diverse changes in discharge rate were observed during nitroprusside-induced reductions in blood pressure: an increase (n = 3), a decrease (n = 10), an increase followed by a decrease (n = 3) and no response (n = 6). Reductions in pressure of 64 +/- 2 mmHg produced weak reductions in spontaneous discharge of 1.3 +/- 0.9 Hz and only totally abolished spontaneous discharge in one neurone. These response patterns of NTS neurones during changes in arterial pressure suggest that baroreceptor inputs are integrated differently in MSNs compared to PSNs. The sensitivity of MSNs to the rate of change of pressure provides a mechanism for the rapid regulation of cardiovascular function. The lack of sensitivity to the mean level of a pressure increase in both MSNs and PSNs suggests that steady-state changes in pressure are encoded by the number of active neurones and not graded changes in the discharge of individual neurones. Both MSNs and PSNs receive tonic excitatory inputs from the arterial baroreceptors; however, these tonic inputs appear to be insufficient to totally account for their spontaneous discharge.

Autonomic ganglionic neurones in rabbits with differing resistance to emotional stress

The aim of the present study was to evaluate the reactions of autonomic neurones of the nodose ganglion of the vagus nerve, and the stellate and superior cervical ganglia in rabbits under emotional stress, and to correlate these reactions with the individual variations in responses to the stressor. Emotional stress was induced in immobilized adult male Chinchilla rabbits by electrical stimulation of the ventromedial hypothalamus and skin. During the experiment (3 hours) arterial blood pressure (BP) was recorded. Metabolic activities of the stellate and superior cervical sympathetic ganglia and nodose ganglion were measured as contents of biogenic amines and their synthesizing and degrading enzymes, neuronal size and dry mass and total RNA; these corresponded to the changes in BP. One group of rabbits showed small fluctuations of BP throughout the experiment and were defined as resistant to stress, whereas in the other group (predisposed to stress) BP progressively decreased. In the former, there was a smaller increase of sympathetic and nodose ganglia metabolic activity than in the latter, in which changes included reduced neuronal dry mass, increased RNA content compared with controls, and reduced tyrosine hydroxylase activity and increased norepinephrine content compare with controls and stress- resistant rabbits. The predisposed rabbits showed earlier and greater increases in circulating norepinephrine concentrations than the resistant rabbits, indicating sustained sympathetic activation. The data indicate that the ganglia of the sympathetic nervous system are part of a major mechanism of BP regulation under acute experimental emotional/painful stress. The nodose ganglion participates in the maintenance of stable cardiovascular function in extreme conditions.

Adrenergic activation of the nucleus tractus solitarius potentiates amygdala norepinephrine release and enhances retention performance in emotionally arousing and spatial memory tasks

It is well documented that noradrenergic systems in the amygdala modulate memory formation, however, less research has examined how sources of limbic norepinephrine contribute to this process. The amygdala receives a dense supply of norepinephrine from neurons in the nucleus of the solitary tract (NTS). The present experiments examined whether adrenergic activation of these NTS neurons affects memory in learning tasks that are sensitive to amygdala norepinephrine release. Separate groups of male Sprague-Dawley rats were trained in either an emotionally arousing or spatial memory task. They then received vehicle or the adrenergic agonist epinephrine (50, 125, or 250 ng/0.5 microl) into the NTS. Rats given the 125 ng dose had significantly longer retention latencies on a 48 h inhibitory avoidance retention test and made a significantly higher percentage of correct responses on an 18 h delayed radial maze retention test. A third experiment using in vivo microdialysis and high performance liquid chromatography (HPLC) demonstrated that intra-NTS infusion of a memory-enhancing dose of epinephrine potentiated amygdala norepinephrine release. Collectively, these results suggest that stimulation of the NTS contributes to memory processing by influencing noradrenergic systems in the amygdala.

Role of glutamate receptors in transmission of vagal cardiac input to neurones in the nucleus tractus solitarii in dogs

1. Vagal afferent input from cardiac mechanoreceptors excites neurones in the nucleus tractus solitarii (NTS), but discharge patterns evoked by physiological activation of pressure-sensitive cardiac mechanoreceptors have not been studied in vivo. The role of glutamate receptor subtypes in transmission of afferent activity to the NTS neurones has not been determined. The present study therefore has two aims: first, to characterise the discharge patterns of neurones in the NTS that receive pressure-sensitive vagal cardiac receptor input and second, to determine the roles of ionotropic glutamate receptor subtypes in the transmission of this putative cardiac mechanoreceptor-related activity to NTS neurones. 2. Pulse-synchronous activity of neurones in the NTS evoked by vagal afferent input was recorded extracellularly in an anaesthetised dog model using multibarrel glass electrodes, which allowed picoejection of the glutamate receptor antagonists NBQX or AP5 to block either non-NMDA or NMDA receptors, respectively, during the neuronal recording. Pressure sensitivity of the recorded neurones was examined by monitoring their response to a small increase in arterial blood pressure. Selective pressure activation of carotid sinus baroreceptors in an isolated sinus or selective denervation of aortic baroreceptors were used to test for convergent excitation of the neurones by arterial baroreceptors. 3. Pulse-synchronous cardiac-related neuronal activity recorded from neurones in both the right and left NTS was eliminated following section of the left (n = 17) or right (n = 1) vagus nerves. No spontaneous, non-pulsatile activity was observed in these neurones before or after vagotomy. Activity transmitted via left vagal afferents was found to be sensitive to changes in arterial blood pressure. In these neurones, activity was blocked in 13 of 17 neurones by picoejection of NBQX, with the remainder requiring both NBQX and AP5. None of the cardiac-related neurones responded to activation of carotid baroreceptors or denervation of aortic baroreceptors, indicating no convergence of activity from carotid baroreceptors or aortic baroreceptors with pressure thresholds of approximately 130 mmHg or less. 4. The results suggest that vagal pressure-sensitive afferent input from cardiac mechanoreceptors is transmitted primarily by left vagal afferent fibres via non-NMDA receptors to neurones in both the ipsilateral and contralateral NTS. NMDA receptors were also found to have a role in the activation of a small subpopulation of neurones.

Elevated Fos expression in the nucleus tractus solitarii is associated with reduced baroreflex response in spontaneously hypertensive rats

We delineated the functional role of Fos protein at the nucleus tractus solitarii in the manifestation of reduced baroreceptor reflex control of heart rate during hypertension, using spontaneously hypertensive rats (SHR), stroke-prone SHR, Wistar-Kyoto rats, or Sprague-Dawley rats. Microinjection into the bilateral nucleus tractus solitarii of an antisense oligonucleotide that targets against the initiation codon of c-fos mRNA significantly potentiated the baroreceptor reflex in response to 30 minutes of sustained increase in blood pressure. Of particular note was the restoration of both the impaired sensitivity and capacity of baroreceptor reflex in SHR and stroke-prone SHR to levels comparable to those in normotensive rats. Likewise, the number of Fos-immunoreactive nuclei evoked by the sustained increase in blood pressure in the caudal nucleus tractus solitarii of SHR and stroke-prone SHR was reduced, after this antisense c-fos treatment, to the basal level exhibited by the normotensive animals. Control treatment with the corresponding sense oligonucleotide, an antisense oligonucleotide that targets against a different portion of the coding sequence of the c-fos mRNA or artificial cerebrospinal fluid, on the other hand, elicited no discernible effect on either the baroreceptor reflex response or the induced expression of Fos protein in the nucleus tractus solitarii by baroreceptor activation. We also found that the basal level of Fos expression in the caudal nucleus tractus solitarii was significantly elevated in the SHR and stroke-prone SHR. Together, these novel findings suggest that an elevated expression of basal Fos protein in the NTS during hypertension may be associated with the dysfunction in baroreceptor reflex control of heart rate.

Catecholamines in rabbit nodose ganglion following exposure to an acute emotional stressor

The aim of the present investigation was to examine catecholamine content and the activities of catecholamine synthesizing and degrading enzymes in the nodose ganglia of rabbits with different patterns of arterial blood pressure during exposure to an acute emotional stressor. The stress protocol involved exposure of immobilized adult male rabbits to electrical stimulation of the ventromedial hypothalamus and the skin for 3 hours. Stress-resistant rabbits that had small fluctuations in arterial pressure during exposure to the stressor had significant reductions in levels ofnorepinephrine (NE) in the nodose ganglion during the 3 hours of stress exposure. In contrast, stress-sensitive rabbits that had progressive decreases in arterial pressure exhibited significant elevations in nodose ganglion content of NE, dopamine (DA) and dihydroxyphenylacetic acid (DOPAC) throughout the period of stress. Tyrosine hydroxylase (TH) activity was changed during the course of the experiment while monoamine oxidase (MAO) activity was unaffected by stress exposure. The changes in nodose ganglion catecholamine content of stress-sensitive and stress-resistant rabbits suggest that the nodose ganglion plays an important role in the maintenance of cardiovascular homeostasis during exposure of animals to an acute emotional stressor.

Differences in the intrinsic membrane characteristics of parabrachial nucleus neurons processing gustatory and visceral information

Whole-cell current-clamp recordings were made from neurons in the rat parabrachial nucleus (PBN) in three rostro-caudal brain slices. During recording the neurons were located in one of four quadrants of the PBN. Successful recordings were obtained from neurons in three of these quadrants termed the dorsolateral (DL), dorsomedial (DM) and ventromedial (VM) quadrants. Recordings were made of the intrinsic membrane properties and repetitive discharge characteristics of 58 neurons in the DL, 60 neurons in the DM, and 54 neurons in the VM-quadrants. The input resistance of the neurons in the DL quadrant was significantly lower and the membrane time constant significantly shorter than that of the neurons in the DM- and VM-quadrants. The mean action potential duration of the VM-quadrant neurons was significantly longer than that of both DL- and DM-quadrant neurons. The discharge frequency in response to a 1500 ms 100 pA current pulse of the DL quadrant neurons was significantly lower than that of the neurons in the other two quadrants. The latency of action potential initiation following a 100 pA depolarizing current pulse was significantly longer for DL quadrant neurons compared to neurons in the other two quadrants. Neurons were divided into groups based on their response to a long depolarizing current pulse immediately preceded by a hyperpolarizing current pulse. In all three rostro-caudal slices of the PBN, the largest populations of neurons were in Group II and Group III. The results demonstrate that neurons in different locations in the PBN have different membrane and repetitive discharge properties. These different PBN locations receive inputs from the visceral and gustatory regions of the NST. It is possible therefore that the differences in properties of the PBN neurons may relate to the type of sensory information that they process.

In vivo effects of 5-hydroxytryptamine receptor activation on rat nucleus tractus solitarius neurones excited by vagal C-fibre afferents

The effects of ionophoretically applied 5-hydroxytryptamine (5-HT) and 5-HT receptor agonists were studied on rat nucleus tractus solitarius (NTS) neurones receiving unmyelinated vagal afferent input. 5-HT excited 15 of 34 neurones (44%), inhibited 10 (29%) and had no effect on nine. 8-Hydroxy-2-(di-N-propylamino)tetralin HBr (8-OH-DPAT) excited 23 of 53 neurones (43%), inhibited 24 (45%) and had no effect on six neurones and (+/-)-2,5-dimethoxy-4-iodoamphetamine HCl activated 18 of 37 neurones (49%), inhibited nine (24%) and had no effect on 10. These results demonstrate that activation of 5-HT1A and 5-HT2 receptors can excite or inhibit populations of NTS neurones. Phenylbiguanide, however, excited 20 of 23 neurones (87%), inhibited only one (4%) and had no effect on two indicating that 5-HT3 receptor activation has an excitatory action. NTS neurones receiving cardiac vagal afferent input were more likely to be excited by 5-HT (five of five, 100%) or 8-OH-DPAT (four of five. 80%) than the population as a whole. In conclusion, the data demonstrate that 5-HT1A, 5-HT2, and 5-HT3 receptor subtypes are functionally present on NTS neurones receiving excitatory vagal afferent input. Further, the subpopulation of NTS neurones receiving input from cardiac afferents are excited by 5-HT, possibly by an action on 5-HT1A or 5-HT3 receptors.

Markers of adenosine removal in normotensive and hypertensive rat nervous tissue

Adenosine mechanisms are altered in brain stem nuclei associated with cardiovascular control in spontaneously hypertensive rats (SHR). Therefore, in the present study we used a number of techniques to compare the binding of the adenosine transport inhibitor [3H]nitrobenzylthioinosine ([3H]NBMPR) as well as adenosine deaminase immunoreactivity (ADA-IR) in brain stems and nodose ganglia of SHR and age-matched normotensive Donryu rats (DRY). Saturation binding revealed a single class of [3H]NBMPR binding sites in the dorsal brain stem of both strains, with Kd and Bmax values of 65 +/- 9 pmol/L and 282 +/- 31 fmol/mg protein, respectively, in SHR and 129 +/- 2 pmol/L and 217 +/- 23 fmol/mg protein in DRY. The Kd for [3H]NBMPR was significantly lower in SHR than in DRY. In competition assays, NBMPR, dilazep, dipyridamole, and adenosine displaced [3H]NBMPR binding, with Kd values of 0.21 +/- 0.04, 57.16 +/- 16.20, 1340 +/- 100, and 87000 +/- 12500 nmol/L, respectively, in DRY and 0.17 +/- 0.04, 28.24 +/- 3.60, 621 +/- 100, and 32000 +/- 6820 in SHR. Kd values for all displacers were lower in SHR; however, only values for dipyridamole and adenosine reached statistical significance. Autoradiography of adenosine transport sites with [3H]NBMPR revealed that unilateral nodose ganglionectomy reduced [3H]NBMPR binding on the denervated side of the nucleus tractus solitarius by 20.6 +/- 1.1% in DRY and 18.7 +/- 2.3% in SHR. The density of [3H]NBMPR binding in nodose ganglia was significantly lower in SHR (0.99 +/- 0.06 Bq/mm2) than in DRY (1.25 +/- 0.08). Immunohistochemical studies demonstrated ADA-IR in the dorsal vagal complex, associated with both nerve cells and fibers. Measurement of ADA-IR in the dorsal vagal complex with an 125I-labeled secondary antibody revealed a significantly higher level of ADA-IR in SHR (122%) than in DRY. In the nodose ganglia, ADA-IR was associated with a population of vagal perikarya. The present study helps provide a molecular explanation for the previously reported impaired cardiovascular responses to intra-nucleus tractus solitarius microinjection of adenosine in hypertensive rats.

Discharge patterns of baroreceptor-modulated neurons in the nucleus tractus solitarius

Activity of baroreceptor-modulated neurons in the nucleus tractus solitarius (NTS) was recorded extracellularly during selective pressure stimulation of carotid baroreceptors, using an isolated carotid sinus preparation in anesthetized dogs. One of two different patterns of activity was recorded from individual baro-sensitive neurons in response to slow ramp increases in carotid sinus pressure. The cause of these two distinct firing patterns is not known but preliminary results indicate that it may be due in part to input from different functional types of baroreceptors. These results suggest that some differentiation in blood pressure control may be encoded in the responses of central baro-sensitive neurons in the NTS.

Synaptic morphology of substance P terminals on catecholamine neurons in the commissural subnucleus of the nucleus tractus solitarii in the rat

The ultrastructure of substance P-containing nerve terminals synapsing on catecholamine neurons in the rat commissural subnucleus of the nucleus tractus solitarii (NTScom) was studied using a double immunocytochemical labeling technique. Although there were numerous tyrosine hydroxylase-immunoreactive (TH-I) somata present, substance P immunoreactive (SP-I) cell bodies were only occasionally found in the NTScom. At the light microscopic level, many SP-I terminals were seen closely associated with TH-I dendrites and somata. At the electron microscopic level, SP-I terminals synapsing on TH-I structures were also readily encountered. SP-I terminals contained small, clear, and predominantly spherical vesicles (32 +/- 4 nm diameter), as well as large dense-cored vesicles approximately 100 nm in diameter. Postsynaptic TH-I dendritic profiles of various calibers and somata were encountered. These postsynaptic TH-I structures often showed postsynaptic densities. The morphological features of the SP-TH synapses in the present study, that is, the size of synaptic vesicles and the presence of postsynaptic densities, are quite different from those of central carotid sinus afferent synapses reported in our previous study [Chen et al. (1992), J. Neurocytol., 21:137-147]. Therefore, most of the SP terminals of the SP-TH synapses in the NTScom appear not to originate from the carotid sinus afferents. SP-I second-order neurons of the carotid sinus afferent pathway [Chen et al. (1991), J. Auton. Nerv. Syst., 33:97-98] may be one of the possible sources of such terminals.

Expression of Fos-like protein in brain following sustained hypertension and hypotension in conscious rabbits

The purpose of this study was to examine comprehensively and quantitatively the effects of sustained hypertension and hypotension on neuronal expression of Fos, the protein product of the proto-oncogene c-fos, in the brain of conscious rabbits. Hypertension or hypotension was produced by continuous intravenous infusion of phenylephrine or nitroprusside, at a rate sufficient to increase or decrease, respectively, arterial pressure by 20-30 mmHg, maintained for a period of 60 min. In comparison with a sham control group of rabbits that were infused with the vehicle solution alone, hypertension induced a significant increase in Fos immunoreactivity in the area postrema, the nucleus tractus solitarii, the caudal and intermediate ventrolateral medulla, the lateral parabrachial nucleus and the central nucleus of the amygdala. Double-labelling for tyrosine hydroxylase and Fos immunoreactivity showed that few (approximately 5%) of the Fos-positive neurons in the caudal and intermediate ventrolateral medulla in this group of animals were also positive for tyrosine hydroxylase. Hypotension also produced a significant increase in Fos immunoreactivity in the above regions, as well as in the rostral ventrolateral medulla, the A5 area, the locus coeruleus and subcoeruleus, the paraventricular nucleus, the supraoptic nucleus, the arcuate nucleus and the medial preoptic area. Approximately 65% of neurons in the rostral, intermediate and caudal ventrolateral medulla that expressed Fos following hypotension were also positive for tyrosine hydroxylase. Similarly, in the pons, approximately 75% of Fos-positive cells in the locus coeruleus, subcoeruleus and A5 area were positive for tyrosine hydroxylase. In the hypothalamus, 92% of Fos-positive neurons in the supraoptic nucleus, and 37% of Fos-positive neurons in the paraventricular nucleus, were immunoreactive for vasopressin. Our results demonstrate that hypertension and hypotension induce reproducible and specific patterns of Fos expression in the brainstem and forebrain. The distribution patterns and chemical characteristics of Fos-positive neurons following sustained hypertension or hypotension are significantly different. In particular, hypotension, but not hypertension, caused Fos expression in many tyrosine hydroxylase-positive cells within all pontomedullary catecholamine cell groups.

Noradrenaline contributes to modulation of the carotid sinus baroreflex in the nucleus tractus solitarii area in the rabbit

The authors examined the contribution of the noradrenergic system to central cardiovascular regulation in the dorso-medial region of the medulla oblongata, including the nucleus tractus solitarii (NTS) area, in anaesthetized rabbits. First, to determine the modulation of the carotid sinus baroreflex system by alpha-blocker, which blocks the action of noradrenaline, the open loop relationship between isolated carotid sinus pressure (CSP) and renal nerve activity (RNA) was examined before and after injection of alpha-blocker (phentolamine, 10 micrograms, bilateral). alpha-Blocker injected to the NTS area caused a decrease in RNA at low CSP level (40-50 mmHg), and RNA response remained higher at high CSP levels (130 mmHg), in comparison with control. Baroreflex sensitivity was attenuated significantly from 10.9 +/- 2.2 to 5.0 +/- 0.5% (mmHg-1). The data show that the NTS area is one of the important sites in central modulation of the carotid sinus baroreflex function. It is suggested that the noradrenergic system in the NTS area contributed to facilitating carotid sinus baroreflex function. Second, using the dialysis technique, a dialysis probe was implanted in the dorso-medial region of the medulla oblongata, including the NTS area, and perfused with Ringer's solution. Dialysates were collected from this region and noradrenaline (NA) output was determined by high-performance liquid chromatography/electrochemical methods. Drug-induced hypotension caused a 30% decrease in arterial pressure and an increase in dialysate NA levels from 6.8 +/- 0.2 to 9.0 +/- 0.4 pg sample-1. The results show that noradrenaline in the NTS area is involved in the regulation of arterial pressure through the central modulation of arterial baroreflex function.

GABA receptor subtypes involved in the neuronal mechanisms of baroreceptor reflex in the nucleus tractus solitarii of rabbits

The role of GABAA and GABAB receptors in neuronal mechanisms of the baroreceptor reflex in the nucleus tractus solitarii (NTS) of the anesthetized and immobilized rabbits were investigated using a microiontophoretic technique. Baroreceptive neurons (BRNs) activated or depressed by baroreceptor stimulation (phenylephrine, 10 micrograms/kg, i.v.) were identified in the NTS (activated BRN (A-BRN) and depressed BRN (D-BRN), respectively). The GABAA antagonist bicuculline (40-80 nA) increased spontaneous activities of these neurons, but the GABAB antagonist phaclofen (80-160 nA) did not. Evoked responses of A-BRNs were potentiated by bicuculline and phaclofen, while the responses of D-BRNs were not clearly affected by these drugs. These results suggest that most of A- and D-BRNs are tonically inhibited by endogenous GABA acting on GABAA receptors, but not on GABAB receptors, and that GABAergic mechanisms suppressively modulate the baroreceptor reflex acting on GABAA and GABAB receptors of A-BRNs, but not of D-BRNs.

Central action of α-adrenoceptor agents on the baroreceptor reflex

In chloralose-anaesthetized cats the effects of intravenous application of the alpha 1- and alpha 2-adrenoceptor agonistic and antagonistic agents methoxamine, prazosin, B-HT 933 and rauwolscine were tested on baroreceptor reflex, sympathetic background activity and blood pressure. Sympathetic activity was recorded from the renal nerve and the efficacy of the central transmission of the baroreceptor reflex was measured by the duration of the complete inhibition of renal nerve activity during electrical stimulation of the left carotid sinus nerve. All baroreceptors were denervated by sectioning both carotid sinus and vagal nerves. The alpha 1-agonist methoxamine increased baroreceptor-induced sympatho-inhibition, sympathetic background activity and blood pressure. The alpha 1-antagonist prazosin had the opposite effects. The alpha 2-agonist B-HT 933 was most effective in augmenting the inhibitory response in sympathetic activity to baroreceptor stimulation; sympathetic background activity and blood pressure were also decreased. At low doses (50 micrograms/kg) the alpha 2-antagonist rauwolscine reduced the baroreceptor sympathetic reflex inhibition and increased sympathetic activity and blood pressure. The effect of B-HT 933 upon the baroreceptor reflex could be completely antagonized by rauwolscine. These findings demonstrate a very effective facilitation of the baroreceptor reflex transmission by stimulation of central alpha 2-adrenoceptors. Through such humoral-neuronal interaction circulating catecholamines are likely to modulate cardiovascular control.

C-terminal tail of beta-adrenergic receptors: immunocytochemical localization within astrocytes and their relation to catecholaminergic neurons in N. tractus solitarii and area postrema

beta-Adrenergic receptors (beta AR) in the medial nuclei of tractus solitarii (m-NTS) and area postrema (AP) may bind to catecholamines released from neurons, whereas only the AP has fenestrated capillaries allowing access to circulating catecholamines. Since varied autonomic responses are seen following beta AR activation of the dorsal vagal complex, including the m-NTS and AP, we hypothesized that there might be a cellular basis for varied responses to beta AR stimulation that depends on the differential access to circulating catecholamines. Therefore, we comparatively examined the ultrastructural localization of the beta AR in relation to catecholaminergic neurons in these regions. An antibody directed against the C-terminal tail (amino acids 404-418) of hamster beta-adrenergic receptor (beta AR404) was used in this study. The localization of beta AR404 was achieved by the avidin-biotin peroxidase complex (ABC) technique in combination with a pre-embed immunogold labeling method to localize tyrosine hydroxylase (TH), the catecholamine-synthesizing enzyme. Within m-NTS and at subpostremal border, labeling for beta AR404 was evident along the intracellular surface of plasma membranes of small, apparently distal, astrocytic processes. Astrocytic processes with beta AR404-immunoreactivity formed multiple, thin lamellae around TH-labeled and non-TH neuronal cell bodies and dendrites. beta AR404-immunoreactive astrocytes also extended end-feet around blood vessels and surrounded groups of axon terminals that were directly juxtaposed to each other. Some, but not all, of these axons demonstrated TH-immunoreactivity. Fewer beta AR404-immunoreactive astrocytes were detected in AP, regardless of their proximity to catecholaminergic processes or blood vessels. The present astrocytic localization of beta AR404, together with the earlier, neuronal localization of beta AR's third intracellular loop, suggest that the beta AR may be substantially different between neurons and astrocytes. The regional difference in the prevalence of beta AR404-immunoreactive astrocytes suggests that these receptive sites may either: (i) be preferentially activated by catecholamines released from terminals rather than circulating catecholamines; or (ii) be down-regulated in AP due to blood-born substances, such as catecholamines. The extensive localization of beta AR in the border between m-NTS and AP also suggests that catecholaminergic activation of these astrocytes may dictate the degree of diffusion of catecholamines which are of neuronal or vascular origin. The specific localization of beta AR404-immunoreactivity to the more distal portions of astrocytes suggests the possibility that astrocytes have restrictive distributions of beta AR and that the beta-adrenergic activation lead to morphological or chemical changes that are also localized to the distal portions of astrocytes.(ABSTRACT TRUNCATED AT 400 WORDS)

Stimulation of α2-adrenergic receptors in nucleus tractus solitarius is required for the baroreceptor reflex

Bilateral injection into the nucleus tractus solitarius (NTS) of the alpha 2-adrenergic receptor antagonist yohimbine produced a dose-related (10-500 pmol) increase in arterial pressure, with a maximal response of approximately 60 mm Hg. Idazoxan, also an alpha 2-adrenergic receptor antagonist, produced a similar response although idazoxan was less potent than yohimbine. The pressor response elicited by these drugs was attenuated by stimulation of adrenergic receptors in the NTS by local administration of either clonidine or tyramine. Doses of yohimbine (200 pmol) or idazoxan (5 nmol) that maximally increased arterial pressure also completely inhibited the depressor and bradycardic responses to electrical stimulation of the aortic depressor nerve. These results indicate that tonic stimulation of alpha 2-adrenergic receptors in the NTS is required for baroreceptor reflex function.

Organization of amygdaloid projections to brainstem dopaminergic, noradrenergic, and adrenergic cell groups in the rat

The distribution of amygdaloid axons in the various brainstem dopaminergic, noradrenergic, and adrenergic cell groups was examined. This was accomplished by means of the Phaseolus vulgaris leucoagglutinin lectin (PHA-L) anterograde tracing technique combined with glucose-oxidase immunocytochemistry to catecholamine markers (i.e., tyrosine hydroxylase, dopamine beta hydroxylase, and phenylethanolamine N-methyltransferase). Injections of PHA-L in the medial part of the central amygdaloid nucleus resulted in axonal and terminal labeling in most catecholamine cell groups in the brainstem. Amygdaloid terminals appeared to contract catecholaminergic cells in several brainstem regions. The most heavily innervated catecholaminergic cells were the A9 (lateral) and A8 dopaminergic cell groups and the C2/A2 adrenergic/noradrenergic cell groups in the nucleus of the solitary tract. The medial part of the A9 and adjacent A10 dopaminergic cell groups was moderately innervated. A moderate innervation by amygdaloid terminals was observed on rostral locus coeruleus noradrenergic cells (A6 rostral) and adrenergic cells of the rostral ventrolateral medulla (C1). Noradrenergic cells of the A5, main body of the locus coeruleus (A6), A7, and subcoeruleus were sparsely innervated. Amygdaloid axons were not observed on noradrenergic neurons of the A4 cell group, area postrema, and A1 cells of the ventrolateral medulla. The results demonstrate that the amygdala primarily innervates the dopaminergic cells of midbrain (i.e., A8 and lateral A9 cells) and the adrenergic cells (C2) and noradrenergic (A2) cells in the nucleus of the solitary tract. The possible functional significance of amygdaloid innervation of catecholaminergic cells is discussed.

Effect of low amphetamine doses on cardiac responses to emotional stress in aged rats

In young Wistar rats conditioned emotional stress can be characterized by a learned bradycardiac response to an inescapable footshock. In aged rats this bradycardiac response is attenuated and accompanied by suppressed behavioral arousal in response to novelty. In the present study, cardiac responses to emotional stress and behavioral reactivity to a novel experience in an open field were tested in aged and young rats under the influence of a low dose of d-amphetamine (AMPH, 0.5 mg/kg IP). AMPH administration in 27-month-old rats reinstated the bradycardiac response to emotional stress, while it failed to influence the resting heart rate in the home cage. Age-associated differences in open-field ambulation, present in drug-free conditions, were antagonized by low doses of AMPH (0.25-1.0 mg/kg). It is concluded that enhanced arousal by aminergic stimulation with AMPH in the aged rat invoked cardiac and behavioral response patterns resembling those at younger ages.

Alpha 2-adrenergic stimulation within the nucleus tractus solitarius attenuates vasopressin release induced by depletion of cardiovascular volume

The functional role of the nucleus tractus solitarius (NTS) in the regulation of arginine-vasopressin (AVP) release mediated by baroreceptor activation was investigated by examining the effects induced by the presynaptic alpha-adrenergic agonist clonidine. The present data show that microinjection of clonidine into NTS resulted in a significant attenuation of AVP secretion induced by hypovolemia in the rat. This effect produced by NTS injection of 8 and 10 nmol clonidine was prevented by NTS pretreatment with the alpha 2-adrenoceptor blocker, yohimbine (10 nmol), indicating alpha 2-adrenergic receptors were required for the biological response. These findings suggest that catecholaminergic projections from NTS to hypothalamic vasopressinergic neurons play a facilitatory role in controlling AVP secretion.

Role of neurotransmitters in the central regulation of the cardiovascular system

The last decade has seen tremendous progress in determining the nature of the neurotransmitters which regulate central nervous system pathways involved in the regulation of blood pressure. Investigations are now pursuing the identity and functional importance of neurotransmitters contained within pathways shown to be important in cardiovascular regulation. In addition, several key components of the brain stem networks involved in the control of sympathetic activity have been identified. For example, numerous studies indicate the importance of neurons located in the rostral ventrolateral medulla in the regulation of SPN. Indeed, this area contains medullospinal sympathoexcitatory neurons which represent the final site of integration of many brain stem and reflex pathways involved in the regulation of sympathetic nerve activity. The neurotransmitter which is utilized by this medullospinal pathway remains unknown. Epinephrine, substance P and glutamate have all been hypothesized as primary chemical mediators in the descending pathway from the brain stem to SPN. Interestingly, lesions of, or antagonists to, epinephrine, substance P, glutamate and 5-HT neurons all abolish sympathetic activity and reduce blood pressure to a level similar to that in a spinal animal. Clearly, not all these transmitters are primary mediators of sympathetic information carried from the brain stem to the spinal cord. It is likely that monoamines and neuropeptides act in the IML, as in other area of the central nervous system, as neuromodulators to set the level of excitability of SPN rather than relaying sympathetic information over a functionally specific medullospinal pathway. This conclusion is supported by the observation that midline medullary 5-HT neurons provide a tonic excitatory input to SPN, but receive no afferent inputs from other central sympathetic or baroreceptor pathways. However, the firing of 5-HT neurons appears to relate to the state of vigilance of the animal. This suggests that 5-HT neurons may lower the threshold of SPN to sympathetic inputs during states of wakefulness. In addition, the time course of the norepinephrine-mediated slow EPSPs and IPSPs in SPN is consistent with a gain-setting function. By analogy, epinephrine is likely to act as a neuromodulator in the IML rather than to serve as the primary mediator of sympathetic information descending from the rostral ventrolateral medulla.(ABSTRACT TRUNCATED AT 400 WORDS)

Cellular substrates for interactions between neurons containing phenylethanolamine N-methyltransferase and GABA in the nuclei of the solitary tracts

Adrenaline and gamma-aminobutyric acid (GABA) have been implicated in autonomic functions involving the intermediate and caudal portions of the medial nuclei of the solitary tracts (m-NTS). We sought to determine whether there was a cellular basis for direct intracellular or synaptic interactions between these transmitters in neurons in the m-NTS of rat brain by using dual-labeling immunocytochemical methods. Light microscopy revealed immunoautoradiographic labeling for the adrenaline-synthesizing enzyme phenylethanolamine N-methyltransferase (PNMT) in perikarya and processes in close proximity to cells demonstrating peroxidase reaction product for GABA. Electron microscopy of the intermediate m-NTS at the level of the area postrema further established the localization of immunoautoradiographic and peroxidase labels for PNMT and GABA in common as well as separate perikarya and dendrites. All axon terminals were labeled separately for PNMT and GABA. The PNMT-labeled terminals formed both symmetric and asymmetric synapses, whereas the GABA-labeled terminals formed exclusively symmetric synapses. Twenty-four percent (n = 42) of the PNMT- and 39% (n = 128) of the GABA-labeled terminals formed synaptic junctions on unlabeled soma and dendrites. Occasionally both types of terminals converged on a common unlabeled dendrite and on GABA-labeled dendrites. Only 3% of the PNMT- and 12% of the GABA-containing terminals formed synapses on PNMT-labeled soma and dendrites, whereas 7% of each type formed synapses with GABA-labeled profiles. The remaining labeled terminals lacked synaptic relations within the sections examined. The autoradiographic results were confirmed and extended by means of immunogold labeling for PNMT in combination with peroxidase-antiperoxidase localization of the GABA-synthesizing enzyme glutamic acid decarboxylase (GAD). GAD-labeled terminals formed symmetric synapses with dendrites that were either unlabeled or contained low levels of PNMT (gold particles) or PNMT and GAD. We conclude that in caudal, more cardiovascular portions of the NTS, adrenaline and GABA may coexist, but they are more commonly detected in separate populations of neurons having receptive sites for both transmitters and innervating certain common target neurons.

Cytoplasmic loop of beta-adrenergic receptors: synaptic and intracellular localization and relation to catecholaminergic neurons in the nuclei of the solitary tracts

Pharmacological studies suggest that beta-adrenergic receptors (beta AR) in the medial nuclei of the solitary tracts (m-NTS) facilitate presynaptic release of catecholamines and also function at postsynaptic sites. We have localized the antigenic sites for a monoclonal antibody against a peptide corresponding to amino acids 226-239 of beta AR in the m-NTS of rat brain. By light microscopy, immunoperoxidase labeling for this antibody was detected in somata and proximal processes of many small cells that were distributed throughout the rostrocaudal extent of the m-NTS. Electron microscopy confirmed the cytoplasmic localization of beta AR in perikarya and proximal dendrites of neurons. Immunoreactivity occurred as discrete patches associated with cytoplasmic surfaces of plasma membrane and with irregularly-shaped saccules with clear lumen in the immediate vicinity. Select regions of nuclear envelopes, mitochondrial membranes, and rough endoplasmic reticulum were also immunoreactive along their cytoplasmic surfaces. In contrast, the Golgi apparatus was labeled, but infrequently. Immunoreactivity was also detected at numerous post- and occasional presynaptic membrane specializations of select axodendritic junctions. Dual labeling for the beta AR-antibody by the immunoperoxidase method and for a rabbit antiserum against the catecholamine-synthesizing enzyme, tyrosine hydroxylase (TH), by the immunoautoradiographic method within the same sections, further established the precise cellular relations between beta AR and catecholaminergic neurons. Immunoreactivity for beta AR was detected in numerous perikarya and proximal dendrites that did not show detectable levels of TH. However, a few cells were dually labeled for both antigens, as seen by both light and electron microscopy. The TH-labeled terminals formed synapses at junctions both with and without beta AR-like immunoreactivity. These results from the single and dual labeling studies: (1) confirm biochemical predictions that amino acids 226-239 of beta AR protein reside intracellularly; (2) provide the first ultrastructural evidence for beta AR localization within both pre- and postsynaptic membrane specializations of a subset of catecholaminergic synapses; and (3) suggest select intracellular sites that may be involved with synthesis and/or internalization and degradation of the receptor protein.

The amygdalo-brainstem pathway: selective innervation of dopaminergic, noradrenergic and adrenergic cells in the rat

The present study investigated the organization and distribution of amygdaloid axons within the various brainstem dopaminergic, noradrenergic and adrenergic cell groups. This was accomplished via Phaseolus vulgaris leucoagglutinin lectin (PHA-L) anterograde tracing technique combined with glucose-oxidase immunocytochemistry to catecholamine markers (i.e. tyrosine hydroxylase, dopamine beta-hydroxylase, and phenylethanolamine N-methyltransferase). Injections of PHA-L within the medial part of the central amygdaloid nucleus resulted in axonal labeling within most catecholamine containing cell groups within the brainstem. The most heavily innervated catecholaminergic groups were the A9 (lateral) cells of the substantia nigra, the A8 dopaminergic cells of the retrorubral field and the C2 adrenergic cells of nucleus of the solitary tract. Amygdaloid terminals frequently contacted cells within these regions. A moderate amount of amygdaloid terminals were located within the rostral A6 (locus coeruleus) and A2 (nucleus of the solitary tract) groups. Amygdaloid terminal contacts were apparent on the majority of the rostral A6 and A2 neurons. Light or no amygdaloid terminal labeling was observed within the other brainstem catecholaminergic cell groups. Thus, the amygdala mainly innervates the A8 and lateral A9 dopaminergic cells of midbrain, rostral locus coeruleus (A6) noradrenergic neurons and the adrenergic (C2) and noradrenergic (A2) cells within the nucleus of the solitary tract. Selective innervation of these brainstem catecholaminergic systems may be important for integration of amygdaloid-mediated defensive and stress-induced behaviors.

Alpha 2-adrenergic modulation of synaptic excitability in the rat nucleus tractus solitarius

These studies examined the effects of the alpha-adrenergic agonist clonidine on spontaneous and synaptically evoked activity in the solitary tract nucleus in superfused rat brain slices. In one group of neurons which showed no spontaneous spike activity (n = 27), clonidine superfusion induced a dose-dependent increase of postsynaptic responsiveness to input from the ipsilateral solitary tract. In another group of neurons which were spontaneously active but unresponsive to tract input (n = 20), clonidine induced a dose-dependent depression of spontaneous discharge. A third group of neurons which were both spontaneously active and responsive to tract input (n = 11) showed primarily a depression of both activities. The neuronal responses to clonidine in all 3 groups were selectively blocked by the alpha 2-adrenoceptor antagonist yohimbine but not by the alpha 1-antagonist prazosin. These results provide insight into the possible actions of endogenous adrenergic systems in the synaptic processing of afferent sensory information within the solitary tract nucleus.