Hypertensive effect of yohimbine following selective injection into the nucleus tractus solitarii of normotensive rats

We administered yohimbine (0.5 nmol in 0.2 microliter) directly into the nucleus tractus solitarii (NTS) of urethane-anesthetized normotensive male rats. We noted a prompt rise in blood pressure which lasted 30 min. These data suggest that the NTS is a central site in which the hypertensive effects of yohimbine can originate.     

Baroreflex actions of substance P microinjected into the nucleus tractus solitarii in rat: a consequence of local distortion

Biologically active substance P (SP) (1000 ng in 0.1 μl saline) microinjected into the nucleus tractus solitarii (NTS) of 25 rats did not affect arterial pressure, heart rate, or the baroreceptor reflex. However, microinjection of saline alone in volumes greater than 0.3 μl consistently elicited hypotension and bradycardia followed occasionally by transient hypertension. These data suggest that previously reported cardiovascular effects of SP microinjected into the NTS resulted from local distortion.     

Cardiovascular effects produced by bradykinin microinjection into the nucleus tractus solitarii of anesthetized rats

In this study, we characterized the cardiovascular effects produced by microinjection of doses in the femtomole range of bradykinin (BK) into the nucleus tractus solitarii of male Wistar rats (230–280 g, n = 120) anesthetized with urethane (1.2 g/kg, i.p.). Microinjections of BK (1, 10, 100 fmol, and 1 and 10 pmol, in 50 nl) or vehicle (NaCl, 0.9%) were made by using a triple-barreled glass micropipette into the medial nTS (0.4 mm anterior, 0.3 mm lateral to the obex and 0.3 mm deep from the dorsal surface). Microinjection of BK produced a shallow dose-dependent decrease in mean arterial pressure and heart rate reaching −18 ± 6 mmHg and −21 ± 5 beats/min, with the dose of 10 pmol. The peripheral mechanism of these effects, tested in animals treated with methylatropine (2 mg/kg, i.V.), or propranolol (2 mg/kg, i.v.) or prazosin (30 μg/kg, i.v.), was shown to be mainly dependent on an increase in vagal efferent activity for bradycardia and a decrease in sympathetic activity for hypotension. In order to investigate the receptor subtype involved in these effects, BK was microinjected into the nTS before and after the injection of the B₁ receptor antagonist, Des-Arg⁹-Leu⁸-BK (DALBK) (11.5 pmol) or before and after the B₂ receptor antagonist, HOE-140 (7.7 pmol). The cardiovascular effects of BK were significantly attenuated by the microinjection of HOE-140 and DALBK into the nTS. The effect of BK microinjected into the nTS on the baroreflex modulation was also investigated. While BK produced a significant facilitation of the baroreflex, HOE-140 and DALBK produced a significant attenuation of the baroreceptor control of heart rate. Taken together, the data presented in this study indicate the nTS as a site, in the central nervous system, for the modulatory effect of BK on the central cardiovascular control.     

Evidence for a differential modulation of the alpha-2 adrenoceptors by angiotensin II in the nucleus tractus solitarii of the spontaneously hypertensive and the Wistar-Kyoto normotensive rats

An interaction between angiotensin II (Ang II) receptors and α₂-adrenoceptors was evaluated in the nucleus tractus solitarii (NTS) of the normotensive Wistar-Kyoto rat (WKY) and of the spontaneously hypertensive rat (SHR) using quantitative receptor autoradiography and cardiovascular analysis. In the WKY rat, Ang II promoted a dose-dependent increase in the IC₅₀ value ofl-noradrenaline when competing for [³H]p-aminoclonidine ([³H]PAC) binding sites, which reached a maximum of 400% with 10 nM of Ang II and was associated with a small decrease in theB₀ value (20%). In the SHR Ang II (0.1 nM) had an opposite effect leading to a decrease in the IC₅₀ value of about 57%, and no change was observed in theB₀ value. Saturation analysis also showed that Ang II (0.1 nM) increased theK_D value of [³H]PAC in the WKY strain but in contrast decreased theK_D value of [³H]PAC in the SHR. TheB_max value was not significantly changed neither in the WKY rat nor in the SHR. The cardiovascular analysis showed that a threshold dose of Ang II (0.05 pmol) counteracted the vasodepressor effect produced byl-noradrenaline coinjected in the NTS of the WKY rat. No effect was observed in heart rate. In the SHR no counteraction of thel-noradrenaline-induced vasodepressor effect was found, and in contrast a slight increase of the vasodepressor effect associated with a significant increase in the bradycardiac response was observed. The results give evidence for an antagonistic Ang II/α₂ receptor interaction in the cardiovascular part of the NTS of the WKY rat as previously observed in the Sprague-Dawley rat. However, this interaction is altered in the SHR, so that in this strain the Ang II/α₂ receptor interaction enhances α₂ affinity and possibly α₂ receptor function. This opposite effect observed in the SHR may represent one compensatory mechanism to counteract the development of high blood pressure in the SHR.     

Cardiovascular roles of tachykinin peptides in the nucleus tractus solitarii of rats

Unilateral removal of the afferent fibers of the IXth and Xth cranial nerve (nodose ganglionectomy) caused significant decrease in the content of substance P-like immunoreactivity (SP-LI) and neurokinin A-like immunoreactivity (NKA-LI) in the nucleus tractus solitarii (NTS) of rats. Microinjection of SP (1 ng) or NKA (10-100 ng) into the NTS caused prompt, transient hypotension and bradycardia, suggesting that SP and NKA may be neurotransmitters of the baroreceptor reflex in the NTS. NKB-like immunoreactivity (NKB-LI) was also detected in the NTS of rats by radioimmunoassay, but its content in the NTS was not affected by unilateral nodose ganglionectomy. The microinjection of 1-10 ng of suc-[Asp5, Me-Phe8]-SP(6-11) (senktide, a selective neurokinin B receptor peptide) into the NTS caused long-lasting hypertension and tachycardia. These results indicate that NKB may also be a neuromodulator on cardiovascular responses in the NTS.     

The ultrastructural localization of enkephalin and substance P immunoreactivities in the nucleus tractus solitarii of the cat

In the medial and commissural subdivisions of the nucleus tractus solitarii enkephalin and substance P immunoreactivities were localized within synaptic terminals, unmyelinated axons, and neuronal cell bodies. Both enkephalin and substance P immunoreactivities were contained within synaptic terminals which had a mixture of small clear vesicles and dense core vesicles. The presence of dense core vesicles within both the enkephalin- and substance P-immunoreactive terminals was a consistent feature, although they were not associated with the actual synaptic junction. While enkephalin- and substance P-immunoreactive terminals shared a similar morphology, their respective distributions along the dendritic tree were quite distinct. Enkephalin-immunoreactive terminals contacted mainly the cell body and proximal portions of the dendritic tree. In contrast, substance P-immunoreactive terminals synapsed predominantly with spines and shafts of small to medium-sized dendrites. Few substance P-immunoreactive terminals contacted proximal dendrites and they were never presynaptic to the neuronal cell body. This apparent segregation of synaptic terminals on neurons suggests that enkephalin synapses have a more pronounced effect than substance P terminals.     

Glycine elicits release of acetylcholine from the nucleus tractus solitarii in rat

Previous studies have suggested that cardiovascular responses elicited by injection of glycine into the nucleus tractus solitarii (NTS) depend upon interactions between glycinergic and cholinergic neuronal elements in NTS. Release of acetylcholine in response to glycine is one such interaction that has been shown in slices of hippocampus and striatum. In this study we sought to test the hypothesis that glycine causes release of acetylcholine from neurotransmitter stores in NTS. We compared release from NTS with that from adjacent hypoglossal nucleus and from caudate nucleus. Release of radiolabeled acetylcholine was determinedin vitro after incubating NTS with [³H]choline. Exposure of NTS and caudate nucleus, but not hypoglossal nucleus, to glycine caused release of acetylcholine in a calcium-dependent manner that varied with concentration of glycine in the incubation medium. The maximally effective concentration (1 mM) of glycine elicited 136% increases over basal levels. Glycine did not elicit release of [³H]acetylcholine from tissue when calcium ion had been removed from the bath. Acetylcholine also was not released if tissue was incubated with either strychnine (10 μM) or hemicholinium-3 (1 mM) prior to exposure to glycine (1 mM). Thus, glycine, acting at strychnine-sensitive receptors in NTS, elicits release of acetylcholine from a portion of locally synthetized neurotransmitter stores.     

Reciprocal connections between nucleus tractus solitarii and rostral ventrolateral medulla

Experiments were done to test the hypothesis that there are reciprocal connections between the nucleus of the tractus solitarius (NTS) and the rostral ventrolateral medulla (RVLM). Spontaneous activity was recorded from units in the right RVLM or NTS of urethan-anesthetized and artificially ventilated rats. Twenty-four of 42 RVLM and 12 of 21 NTS units were classified as cardiovascular because they were inhibited by baroreceptor activation and displayed a cardiac rhythm. Electrical stimulation of depressor sites in the NTS inhibited 14 and excited 10 RVLM units. Stimulation of pressor sites in the RVLM excited 10 and inhibited 2 NTS units. None of the units in the NTS or in the EVLM could be activated antidromically. These results provide evidence that there are reciprocal excitatory and inhibitory connections between NTS and RVLM and that these connections are not monosynaptic.     

Vasopressin-induced pressor responses in rats to bilateral electrolytic lesioning of the caudal portion of the nucleus tractus solitarii

Bilateral electrolytic lesioning of the nucleus tractus solitarii elicited a pressor response in rats with spinal transection. This response was abolished by a vasopressin pressor antagonist and plasma vasopressin was increased during the response. This evidence suggests that the nucleus tractus solitarii is involved in inhibiting vasopressin release and that an impaired function would lead to an enhancement of vasopressin release, to the extent of eliciting a pressor response.     

Antagonistic oxytocin/alpha2-adrenoreceptor interactions in the nucleus tractus solitarii: relevance for central cardiovascular control

The modulation of the central cardiovascular effects of alpha2-adrenoceptor activation by oxytocin in the nucleus tractus solitarii has been evaluated by cardiovascular analysis and by quantitative receptor autoradiography. Microinjections in the nucleus tractus solitarii of a threshold dose of oxytocin effectively and significantly counteracted the vasodepressor and bradycardic actions of an ED50 dose of the alpha2-adrenoceptor agonist clonidine. The coinjection of a threshold dose of oxytocin with a threshold dose of clonidine did not produce any changes in the mean arterial pressure but a tachycardic response was observed. Receptor autoradiographical experiments showed that oxytocin (3 nM) significantly increased the Kd and Bmax values of [3H]p-aminoclonidine binding sites in the nucleus tractus solitarii compatible with a possible antagonistic interaction with the alpha2-adrenoceptors, and this effect was blocked by the presence of the specific oxytocin receptor antagonist 1-deamino-2-D-Tyr-(OEt)-4-Thr-8-Orn-oxytocin. These findings suggest the existence of an antagonistic oxytocin/alpha2-adrenoceptor interaction in nucleus tractus solitarii that may be of relevance for the demonstrated modulation of alpha2-adrenoceptor induced cardiovascular responses by oxytocin.     

Distribution of neurophysin II immunoreactive nerve fibers within the subnuclei of the nucleus of the tractus solitarius of the rat

The location of neurophysin II immunoreactive nerve fibers and preterminal processes has been examined in various functionally distinct subnuclei of the nucleus of the tractus solitarius (nTS) using the indirect immunofluorescence method for immunocytochemistry combined with cytoarchitectonic identification. The nTS is responsible for integrating respiratory and autonomic reflex activity: the vlnTS, vnTS, ni and nI are associated with respiratory activity; the dlnTS and dnTS are important sites for the integration of baroreceptor and chemoreceptor activity; the ncom, dnTS and dlnTS integrate cardiac afferent activity and the mnTS mediates both cardiovascular and gastrointestinal effects. At levels caudal to the obex, the ncom contained the largest number of neurophysin II immunoreactive nerve fibers and the mnTS and dmnX contained moderate neurophysin II immunoreactivity. At levels rostral to the obex the region of the dorsal medulla adjacent to the mnTS and dnTS (PVR and dPSR) showed the densest immunoreactivity and the mnTS, dmnX and vPSR showed moderate immunoreactivity. At the rostral pole of the nTS, neurophysin II immunoreactive nerve terminals were seen in the dendritic regions of cells in dmnX and mnTS. This selective distribution of neurophysin II immunoreactive nerve terminals in the cardiovascular and gastrointestinal subnuclei of the nTS implicates a direct, descending, hypothalamic, oxytocin-neurophysin II containing pathway interacting with these nTS functions. These results confirm the hypothesis (Sawchenko and Swanson) that descending neurophysin II immunoreactive pathways represent an important neuronal system for the hypothalamic regulation of cardiovascular (vasomotor) and gastrointestinal nuclei in the brainstem.     

Adenosine A<Subscript>1</Subscript> receptor distribution in the nucleus tractus solitarii of normotensive and spontaneously hypertensive rats

Summary. Adenosine acts at many sites to modulate neuronal activity. The nucleus tractus solitarii (NTS) is a major brain site in cardiovascular control. The present study was undertaken for a detailed analysis of the distribution of A₁ adenosine receptor (A₁R) in the NTS of spontaneously hypertensive rats (SHR) and Wistar Kyoto (WKY), using in vitro autoradiography with [³H]DPCPX. The density of [³H]DPCPX in the whole NTS decreased according to the rostral-caudal levels. This high level of [³H]DPCPX binding at rostral sites is due to an specific label of the dorsomedial/dorsolateral subnuclei. On the other hand, analysis of subpostremal subnucleus, showed opposite results. The density of [³H]DPCPX binding in the subpostremal NTS increased according to the rostral-caudal levels. Furthermore, it was observed an increased [³H]DPCPX binding in the SHR compared with WKY. The results show a complex pattern of A₁R distribution in the NTS, which highlight the powerful modulatory actions mediated by adenosine in the NTS barosensitive neurons.     

Denervation supersensitivity to glutamate in the nucleus tractus solitarii after removal of the nodose ganglion

Ifl-glutamate (l-Glu) is the transmitter released from cardiovascular vagal afferent nerve fibers in the nucleus tractus solitarii (NTS), then interruption of those afferents should lead to denervation supersensitivity tol-Glu in the NTS. Therefore, we sought to determine if dose-related changes in arterial pressure (AP) and heart rate (HR) elicited byl-Glu microinjected into the NTS evolve after removal of the left nodose ganglion in rats. Twelve rats served as unoperated controls; and eight were studied 5 days, eleven 10 days, and nine 15 days after ganglionectomy. Each rat was anesthetized with halothane (1.5–2.0%) and cannulated for recording AP and HR. After exposure of the brainstem, vehicle orl-Glu (3, 30, 75, 150, 300 and 1500 pmol/50 nl) was microinjected alternately into the right and left dorsomedial NTS. In control animals, and in animals 5 days and 15 days after ganglionectomy, the lowest dose ofl-Glu that produced a significant fall of mean AP (−10±2, −5±2, −6±1 mmHg, respectively) was 30 pmoles. The threshold dose with injections on the lesioned side 10 days after ganglionectomy was 3 pmol (−8±2 mmHg). In rats studied at 10 days, but not in control, 5 or 15 day rats, the dose-responses for injections made on the left were shifted significantly to the left of those made on the right (P < 0.05). In control, 5, and 15 day rats there were no significant differences between dose-related responses elicited from right and left NTS. Responses of HR did not differ between groups of animals or sides of injection. These results suggest that supersensitivity tol-Glu evolves after nodose ganglionectomy and further support a role forl-Glu as a transmitter of cardiovascular afferents in the NTS.     

Central noradrenergic activity is responsible for nitroglycerin-induced cardiovascular effects in the nucleus tractus solitarii

The studies show that unilateral microinjection of nitroglycerin (NTG) into nucleus tractus solitarii (NTS) produce dose-dependent decreases in mean arterial pressure (MAP) and heart rate, but injection of sodium nitroprusside (SNP) into the area induced slight effects. Hypotensive responses to NTG injected into the NTS showed that the compound was 20 times more potent than after i.v. administration. Responses to NTG injected into the NTS were abolished in an additive fashion by either rauwolscine, an alpha 2-adrenoceptor antagonist, or guanethidine which inhibits release of norepinephrine (NE). Injection of prazosin, an alpha 1-adrenoceptor antagonist, into the NTS reduced the hypotensive responses of NTG, but did not alter the bradycardia induced by the drug. Tachycardic responses following i.v. administration of either NTG or SNP were attenuated by bilateral injection of rauwolscine into the NTS, whereas only hypotensive responses to i.v. NTG were reduced by the pretreatment. NTG produced a dose-dependent increase in concentrations of 3,4-dihydroxyphenylalanine in media bathing medulla-pons, which were quantified using high-performance liquid chromatography with electrochemical detection. NE and 3,4-dihydroxyphenylglycol concentrations in media of incubated medulla-pons slices were simultaneously increased following higher concentrations of NTG. The results suggest that NTG in the NTS induces hypotensive and bradycardiac responses, and an increase in turnover of NE may stimulate alpha 2-adrenoceptors and be responsible for the effects of the drug. The NTS may contribute a component of action to the cardiovascular effects of intravenous NTG. The cardiovascular responses of intravenous SNP appear to involve peripheral action.     

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

Anatomical and pharmacological evidence suggests a role for catecholamines (CAs) in the modulation of the baroreceptor reflex within the nucleus tractus solitarii (NTS). Single neurons in the NTS of the rat were studied for their responses to activation of the baroreceptor reflex and to iontophoretic administration of dopamine, norepinephrine (NE), and epinerphrine (EPI) to determine the relationship between the effects of baroreflex activation and CA application on baroreceptive neurons in the vagal sensory nucleus. Of 269 cells studied, 104 (38.7%) exhibited decreases and 41 cells (15.2%) showed increases in firing rate in response to baroreflex activation, while the remaining 124 neurons showed no response. All 3 CAs inhibited spike activity in the majority (68.5%) of NTS cells. These inhibitory effects on spontaneous firing were observed regardless of the response profile of a particular neuron to baroreflex activation. The inhibitory effects of NE and EPI on NTS neuronal activity were specifically blocked by the -adrenergic receptor antagonist tolazoline, but not by the β-adrenergic antagonist sotalol. These results indicate that CAs may interact at several sites within the NTS to influence baroreflex integration, and that the effects of NE and EPI on neuronal activity are mediated by an -adrenergic receptor.     

Reduced dipsogenic response induced by angiotensin II activation of subfornical organ projections to the median preoptic nucleus in estrogen-treated rats

The present study was carried out to investigate whether estrogen modulates the drinking response induced by activation of angiotensinergic neural pathways from the subfornical organ (SFO) to the median preoptic nucleus (MnPO). Microinjection of angiotensin II (ANG II, 10(-10) M, 0.2 microl) into the SFO elicited drinking in ovariectomized (OVX) female rats that were treated with either propylene glycol (PG) vehicle or estrogen benzoate (EB). The amount of water intake induced by the ANG II injection was significantly greater in the PG-treated than in the EB-treated animals. In both groups of female rats, previous injections of saralasin (Sar, 10(-10) M, 0.2 microl), a specific ANG II antagonist, into the MnPO resulted in the significant attenuation of the drinking response to ANG II, showing that the ANG II-induced drinking response may be mediated in part by the angiotensinergic SFO projections to the MnPO. Injections of ANG II (10(-10) M, 0.2 microl) into the MnPO caused drinking in both groups, while no significant difference was found between the groups in the amount of water intake. These results suggest that increases in the circulating level of estrogen may attenuate the drinking response induced by ANG II activation of the SFO projections to the MnPO.