大鼠CVLM中NMDA和非NMDA受体在介导动脉压力感受器反射中的作用

在氨基甲酸乙酯麻醉、制动的大鼠 ,研究尾端延髓腹外侧区 (CVLM)中 NMDA和非 NMDA受体在介导动脉压力感受器反射 (ABR)中的作用。双侧 CVLM微量注射选择性 NMDA受体拮抗剂氯胺酮 (50mmol/L ,1 0 0 nl)或非 NMDA受体拮抗剂 kynurenic acid(KYA,50 mmol/L ,1 0 0 nl)后平均动脉压 (MAP)和心率 (HR)均明显升高 (P<0 .0 5) ,同时观察到 CVLM微量注射氯胺酮或 KYA后电刺激主动脉神经(AN)导致的血压下降比对照有明显的减少 ,双侧 CVLM微量注射氯胺酮和 KYA的混合物 (均为 50mmol/L,50 nl)后能完全阻断电刺激 AN后导致的降压反应。本研究结果提示 CVLM中 NMDA和非 NM-DA受体在紧张性维持交感神经的兴奋活动和介导 ABR中起十分重要的作用     

Tonic stimulation of GABAB receptors in the nucleus tractus solitarius modulates the baroreceptor reflex.

Previous studies have indicated that tonic stimulation of GABAB receptors in the nucleus tractus solitarius (NTS) contributes to the regulation of arterial blood pressure (AP). The present studies examined the hypotheses that (1) tonic stimulation of GABAB receptors in the NTS provides a tonic attenuation of the baroreceptor reflex and (2) enhanced stimulation of these GABAB receptors markedly attenuates the baroreceptor reflex resulting in an increase in AP. In chloralose-anesthetized rats electrical stimulation of the aortic depressor nerve elicited frequency-dependent decreases in AP and heart rate (HR). These responses were markedly attenuated, but not eliminated, by injection of the GABAB receptor agonist baclofen into the ipsilateral NTS. In contrast, the GABAA receptor agonist muscimol completely inhibited aortic depressor nerve-evoked responses. Blockade of GABAB receptors in the NTS by local injection of CGP-35348 elicited a dose-dependent decrease in AP, and a dose-dependent blockade of the pressor response elicited by injection of baclofen into the NTS. These results support the hypothesis that GABA acts tonically on GABAB receptors in the NTS to attenuate the baroreceptor reflex, thereby contributing to the regulation of AP.     

Tonic stimulation of GABAB receptors in the nucleus tractus solitarius modulates the baroreceptor reflex

Previous studies have indicated that tonic stimulation of GABA_B receptors in the nucleus tractus solitarius (NTS) contributes to the regulation of arterial blood pressure (AP). The present studies examined the hypotheses that (1) tonic stimulation of GABA_B receptors in the NTS provides a tonic attenuation of the baroreceptor reflex and (2) enhanced stimulation of these GABA_B receptors markedly attenuates the baroreceptor reflex resulting in an increase in AP. In chloralose-anesthetized rats electrical stimulation of the aortic depressor nerve elicited frequency-dependent decreases in AP and heart rate (HR). These responses were markedly attenuated, but not eliminated, by injection of the GABA_B receptor agonist baclofen into the ipsilateral NTS. In contrast, the GABA_A receptor agonist muscimol completely inhibited aortic depressor nerve-evoked responses. Blockade of GABA_B receptors in the NTS by local injection of CGP-35348 elicited a dose-dependent decrease in AP, and a dose-dependent blockade of the pressor response elicited by injection of baclofen into the NTS. These results support the hypothesis that GABA acts tonically on GABA_B receptors in the NTS to attenuate the baroreceptor reflex, thereby contributing to the regulation of AP.     

Somatic nociception activates NK1 receptors in the nucleus tractus solitarii to attenuate the baroreceptor cardiac reflex

There is limited information regarding the integration of visceral and somatic afferents within the nucleus of the solitary tract (NTS). We studied the interaction of nociceptive and baroreceptive inputs in this nucleus in an in situ arterially perfused, un-anaesthetized decerebrate preparation of rat. At the systemic level, the gain of the cardiac component of the baroreceptor reflex was attenuated significantly by noxious mechanical stimulation of a forepaw. This baroreceptor reflex depression was mimicked by NTS microinjection of substance P and antagonized by microinjection of either bicuculline (a GABAA receptor antagonist) or a neurokinin type 1 (NK1) receptor antagonist (CP-99994). The substance P effect was also blocked by a bilateral microinjection of bicuculline, at a dose that was without effect on basal baroreceptor reflex gain. Baroreceptive NTS neurons were defined by their excitatory response following increases in pressure within the ipsilateral carotid sinus. In 27 of 34 neurons the number of evoked spikes from baroreceptor stimulation was reduced significantly by concomitant electrical stimulation of the brachial nerve (P < 0.01). Furthermore, the attenuation of baroreceptor inputs to NTS neurons by brachial nerve stimulation was prevented by pressure-ejection of bicuculline from a multi-barrelled microelectrode (n = 8). In a separate population of 17 of 45 cells tested, brachial nerve stimulation evoked an excitatory response that was antagonized by blockade of NK1 receptors. We conclude that nociceptive afferents activate NK1 receptors, which in turn excite GABAergic interneurons impinging on cells mediating the cardiac component of the baroreceptor reflex.     

Cardiorespiratory responses to glutamatergic antagonists in the caudal ventrolateral medulla of rats.

The role of caudal ventrolateral medullary (CVLM) depressor neurons in influencing arterial pressure and ventilation as well as the baroreflex control of arterial pressure was investigated, and the part played by excitatory N-methyl-D-aspartate (NMDA) and non-NMDA receptors in mediating the responses was determined. In urethane-anesthetized, spontaneously breathing rats unilateral microinjections into the caudal depressor area of the broad-band glutamatergic antagonist kynurenic acid (KYN, 5 nmol or 1.58 nmol), or NMDA antagonist 2-amino-5-phosphonovaleric acid (2-APV, 2.7 nmol), or the non-NMDA antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 0.257 nmol) caused a respiratory arrest within 4 min and the animals had to be artificially ventilated. Respiratory frequency increased on injecting KYN and CNQX while it did not change significantly with 2-APV. Apnea resulted from progressive decrease in tidal volume. During the apnea ventilation with 5% CO2 did not revive breathing. Mean arterial pressure (MAP) increased significantly with KYN and 2-APV injections but not with CNQX. The baroreflex decrease of MAP, elicited by left or right aortic depressor nerve stimulation, was significantly reduced or abolished after bilateral microinjections of all 3 antagonists. Ventilation as well as the baroreflex usually recovered after 1-1.5 h. Microinjections of the same doses of antagonists into the facial nucleus, as well as application of KYN (25 nmol) to the ventral medullary surface above the hypoglossal rootlets, had no significant effect. The results support previous findings that the CVLM neurons of the rat inhibit sympathetic neurons providing the vasomotor tone, and that an intact CVLM is obligatory for mediating the baroreflex decrease of arterial pressure. The results also indicate that: (1) the CVLM is essential for sustaining ventilation in the rat; (2) only NMDA receptors are involved in maintaining baseline blood pressure while both NMDA and non-NMDA receptors mediate the baroreceptor depressor reflex; and (3) both NMDA and non-NMDA receptor activation is necessary to sustain ventilation.     

Membrane and synaptic properties of nucleus tractus solitarius neurons projecting to the caudal ventrolateral medulla

Projections from the nucleus of tractus solitarius (NTS) to the caudal ventrolateral medulla (CVLM) are important in mediating autonomic reflexes. However, little is known about the cellular properties of the CVLM-projecting NTS neurons. In this study, the CVLM-projecting NTS neurons were retrogradely labeled by fluorescent microspheres injected into the CVLM. Whole cell voltage- and current-clamp recordings were performed on labeled NTS neurons in coronal brainstem slices. Compared with unlabeled neurons, the labeled NTS neurons had more depolarized resting membrane potentials, larger input resistance, and higher firing activity in response to depolarizing currents. Bath application of an ionotropic glutamate receptor antagonist kynurenic acid and a non-NMDA receptor antagonist CNQX significantly decreased the firing activity in the majority of labeled NTS neurons. In contrast, an NMDA receptor antagonist AP5 failed to alter the firing activity in labeled neurons tested. While the glycine receptor antagonist strychnine had no effect on the firing activity, blockade of GABA(A)receptors with bicuculline significantly increased the firing rate in the majority of labeled NTS neurons. Furthermore, CNQX blocked the majority of spontaneous excitatory postsynaptic currents (EPSCs) and evoked EPSCs elicited by stimulation of the tractus solitarius. The residual spontaneous and evoked EPSCs were abolished by the nicotinic receptor antagonist mecamylamine and the purinergic P2X receptor antagonist iso-PPADS. Finally, while bicuculline completely blocked the miniature inhibitory postsynaptic currents (IPSCs), the spontaneous and evoked IPSCs were abolished by a combination of bicuculline and strychnine in labeled NTS neurons. Collectively, these data suggest that the CVLM-projecting neurons are a population of neurons with distinctive membrane properties.     

Purinergic mechanisms of the nucleus of the solitary tract and neural cardiovascular control

OBJECTIVES: This review addresses the role of central purinergic receptors in the operation of the cardiovascular reflexes. METHODS: Potential physiological role of purinergic receptors operating in the nucleus of the solitary tract (NTS) was assessed via comparison of the regional patterns of hemodynamic and sympathetic responses evoked by selective stimulation/inhibition of NTS purinergic receptor subtypes, with the patterns evoked by stimulation and unloading of arterial baroreceptors, and other known patterns of autonomic responses. The effects of sino-aortic denervation plus vagotomy and ionotropic glutamatergic blockade of NTS mechanisms on the patterns of the responses were also considered. RESULTS: Selective stimulation of NTS A1 receptors with CPA evoked a pattern of regional autonomic responses consistent with inhibition of baroreflex mechanisms and facilitation/ disinhibition of chemoreflex mechanisms. Selective stimulation of NTS A(2a) receptors with CGS 21680-evoked pattern of the responses different than that evoked by stimulation of baroreflex afferents what remains in contrast to previous reports suggesting that NTS A2a receptors facilitate baroreflex transmission. The pattern of the responses was similar to that observed during hypotensive hemorrhage. Preferential, b -adrenergic iliac vasodilation evoked by stimulation of adenosine A2a receptors and preferential activation of sympathetic output to the adrenal medulla by both adenosine A1 and A2a receptors are consistent with contribution of these receptors to the defense response, stress and exercise. These observations support previous findings that NTS A1 receptors contribute to the hypothalamic defense response. The effects of stimulation and blockade of NTS P2x receptors with alpha, beta-methylene ATP and suramin, respectively, suggested that neuronally-released ATP operating via P2x receptors may be a crucial co-transmitter with glutamate in mediating baroreflex responses. DISCUSSION: The above observations strongly suggest that purinergic receptor subtypes operating in NTS circuitry are linked to specific afferent and descending mechanisms primarily integrated in the NTS.     

Distribution of neurotensin-immunoreactivity within baroreceptive portions of the nucleus of the tractus solitarius and the dorsal vagal nucleus of the rat

We have examined the distribution of neurotensin immunoreactivity within subnuclear regions of the nucleus of the tractus solitarius (NTS) and the dorsal motor nucleus of the vagus nerve (DVN) in the rat. In order to determine which regions of the NTS were involved in the regulation of baroreceptor reflexes, we mapped the central distribution of the aortic branch of the vagus nerve using transganglionic transport of horseradish peroxidase. Comparison of the pattern of aortic nerve innervation with that of the distribution of neurotensin-immunoreactive cells and fibers shows the dorsomedial nucleus of the NTS both to be the primary site of aortic baroreceptor termination and to contain the highest concentration of neurotensin-immunoreactive elements within the NTS. Neurotensin-immunoreactive fibers are also present in medial regions of the NTS adjacent to the area postrema where they may be involved in the modulation of vagal gastric afferents. Double-label experiments, in which, on the same tissue sections, neurotensin immunohistochemistry was combined with retrograde horseradish peroxidase labeling of DVN neurons, reveal a topographic innervation of vagal preganglionic motoneurons by neurotensin-immunoreactive fibers. The heaviest innervation is of lateral portions of the DVN and adjacent ventral portions of the NTS at the level of the obex, an area which may contain cardiac motoneurons. In this region neurotensin-immunoreactive fibers can be observed in close proximity to retrogradely labeled cells. The concentration of neurotensin elements in a region of the NTS which is involved in the control of baroreceptor reflexes provides a morphological basis for the cardiovascular effects produced by central administration of the peptide. Additional control may be exerted at the level of the motoneuron, as evidenced by apparent neurotensin fiber innervation of presumptive cardiac preganglionic neurons. Similarly, the distribution of neurotensin fibers suggests that the peptide may be acting in gastric regulatory areas of the NTS or on vagal secretomotor neurons to regulate gastric acid secretion.     

Effect of atropine injected into the nucleus tractus solitarius on the regulation of blood pressure

Previous experiments have demonstrated that stimulation of muscarinic cholinergi receptors in the nucleus tractus solitarius (NTS) of the rat decreases arterial blood pressure and heart rate. The present studies were designed to examine the role of cholinergic mechanisms in the NTS in the tonic regulation of arterial pressure and the baroreceptor reflex. Atropine injected into the NTS of chloralose-anesthetized rats produced a dose-dependent inhibition of cardiovascular responses elicited by injection of acetylcholine into the same site; 240 pmol atropine eliminated acetylcholine-evoked responses. Atropine also increased arterial blood pressure but only at higher doses. Even larger doses of atropine were required to alter cardiovascular responses elicited by electrical stimulation of the aortic depressor nerve. Methylatropine injected into the NTS also blocked acetylcholine-evoked responses but, in contrast to the actions of atropine, did not increase arterial pressure in the dose range required to block acetylcholine-evoked responses. Furthermore, a dose of methylatropine (1 nmol) capable of blocking acetylcholine-evoked cardiovascular responses did not alter aortic depressor nerve-evoked cardiovascular responses. This lack of an effect of methylatropine on arterial pressure and aortic depressor nerve-evoked responses was not due to limited diffusion of the drug within the NTS since 1 nmol methylatropine completely blocked acetylcholine-evoked responses even when injected 0.5 mm distant from the site of acetylcholine injection. These results suggest that cholinergic mechanisms in the NTS are not involved in the tonic regulation of cardiovascular function or the baroreceptor reflex. Furthermore, these results highlight the importance of characterizing doses of drugs used in microinjection studies.     

Stimulation of alpha 2-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.     

Glutamate and NO mediation of the pressor response to 5-HT3 receptor stimulation in the nucleus tractus solitarii

The possible participation of glutamate and NO/cGMP in the pressor response to 5-HT3 receptor activation in the nucleus tractus solitarii (NTS) was investigated using selective antagonists in urethane-anaesthetized rats. Intra-NTS administration of NMDA and non-NMDA receptor antagonists, but not metabotropic glutamate receptor antagonists, markedly reduced (70%) the increase in blood pressure caused by local application of the potent 5-HT3 receptor agonist, 1-(m-chlorophenyl)-biguanide. The 5-HT3 receptor-mediated pressor response was also significantly attenuated by the local blockade of nitric oxide synthase and soluble guanylyl cyclase. These data suggest that ionotropic glutamate receptors and the associated NO/cGMP transduction mechanism contribute downstream to the pressor effect elicited by 5-HT3 receptor stimulation in the NTS.     

Purinergic mechanisms of the nucleus of the solitary tract and neural cardiovascular control

Abstract

Objectives: This review addresses the role of central purinergic receptors in the operation of the cardiovascular reflexes.

Methods: Potential physiological role of purinergic receptors operating in the nucleus of the solitary tract (NTS) was assessed via comparison of the regional patterns of hemodynamic and sympathetic responses evoked by selective stimulation/inhibition of NTS purinergic receptor subtypes, with the patterns evoked by stimulation and unloading of arterial baroreceptors, and other known patterns of autonomic responses. The effects of sino-aortic denervation plus vagotomy and ionotropic glutamatergic blockade of NTS mechanisms on the patterns of the responses were also considered.

Results: Selective stimulation of NTS A1 receptors with CPA evoked a pattern of regional autonomic responses consistent with inhibition of baroreflex mechanisms and facilitation/ disinhibition of chemoreflex mechanisms. Selective stimulation of NTS A_2a receptors with CGS 21680-evoked pattern of the responses different than that evoked by stimulation of baroreflex afferents what remains in contrast to previous reports suggesting that NTS A2a receptors facilitate baroreflex transmission. The pattern of the responses was similar to that observed during hypotensive hemorrhage. Preferential, b -adrenergic iliac vasodilation evoked by stimulation of adenosine A2a receptors and preferential activation of sympathetic output to the adrenal medulla by both adenosine A1 and A2a receptors are consistent with contribution of these receptors to the defense response, stress and exercise. These observations support previous findings that NTS A1 receptors contribute to the hypothalamic defense response. The effects of stimulation and blockade of NTS P2x receptors with α, β-methylene ATP and suramin, respectively, suggested that neuronally-released ATP operating via P2x receptors may be a crucial co-transmitter with glutamate in mediating baroreflex responses.

Discussion: The above observations strongly suggest that purinergic receptor subtypes operating in NTS circuitry are linked to specific afferent and descending mechanisms primarily integrated in the NTS.     

Role of excitatory amino acids in rat vagal and sympathetic baroreflexes

Vagal baroreflexes were studied by measuring the atropine-sensitive cardioinhibition produced by raising arterial pressure with phenylephrine in anesthetized rats pretreated with the beta-adrenergic receptor antagonist nadolol. Sympathetic baroreflexes were determined in halothane-anesthetized rats by measuring the inhibition of lumbar sympathetic discharge produced by elevating arterial pressure with gradual aortic constriction. Both reflexes were drastically reduced by bilateral injections of 2.2 nmol of the glutamate receptor antagonist kynurenic acid (KYN) into either the nucleus of the solitary tract (NTS) or the ventrolateral medulla between 0 and 1 mm posterior to the level of the obex. Injections of KYN elsewhere in the medulla were generally ineffective and injections of 8-OH kynurenate (an inactive analog) into the ventrolateral medulla or NTS were also without effect. KYN injections (2.2 nmol) into the intermediate portion of the NTS produced small increases in mean arterial pressure (0-15 mm Hg) and no change in heart rate while injections of similar amounts into the ventrolateral medulla at obex level were followed by large (35-116 mm Hg) increases in pressure and bradycardia. Both types of injections produced a similar degree of blockade of vagal and sympathetic baroreflexes. These results support previous evidence that baroreceptor primary afferents may release a glutamate-like transmitter in the NTS and indicate that a similar type of excitatory transmitter is involved at the level of the ventrolateral medulla in mediating or modulating both vagal and sympathetic baroreflexes. Finally the bradycardia and hypertension produced by blocking amino acid receptors in the ventrolateral medulla appear largely unrelated to the disruption of peripheral baroreceptor inputs.     

The role of excitatory amino acids and substance P in the mediation of the cough reflex within the nucleus tractus solitarii of the rabbit

We hypothesized that cough evoked by mechanical stimulation of the tracheobronchial tree in the rabbit is primarily mediated by glutamatergic neurotransmission at the level of the caudal portions of the medial subnucleus of the nucleus tractus solitarii (NTS) and the lateral commissural NTS where cough-related afferents terminate, and that this reflex is potentiated by local release of substance P. To test our hypothesis, we performed bilateral microinjections (30-50 nl) of ionotropic glutamate receptor antagonists or substance P into these locations in pentobarbitone anaesthetized, spontaneously breathing rabbits. Blockade of NMDA and non-NMDA receptors by 50mM kynurenic acid abolished the cough reflex without affecting the Breuer-Hering inflation reflex or the pulmonary chemoreflex. Blockade of non-NMDA receptors using 10mM CNQX or 5mM NBQX caused identical effects. Blockade of NMDA receptors by 10mM D-AP5 strongly reduced, but did not abolish cough responses. Microinjections of 1mM substance P increased peak and rate of rise of abdominal muscle activity as well as cough number. These results are the first to provide evidence that ionotropic glutamate receptors, especially non-NMDA receptors, located within specific regions of NTS are primarily involved in the mediation of cough evoked by mechanical stimulation of the tracheobronchial tree in the rabbit. Present findings on substance P cough-enhancing effects extend previous observations and are relevant to the tachykinin-mediated central sensitization of the cough reflex. They also may provide hints for further studies on centrally acting antitussive drugs.     

Frequency dependence of synaptic vesicle exocytosis in aortic baroreceptor neurons and the role of group III mGluRs

Synaptic transmission between baroreceptor afferents and the nucleus tractus solitarius (NTS) is essential for reflex regulation of blood pressure. High frequency stimulation of the afferents in vivo leads to a decrease in synaptic strength and is generally attributed to reduction in presynaptic neurotransmitter release. It has been hypothesized that during high frequency stimulation glutamate a major neurotransmitter at the baroreceptor afferent terminals inhibits its own release via presynaptic group III metabotropic glutamate receptors (mGluRs). A key player in modulation of presynaptic release is vesicle exocytosis. The present study utilized cultured aortic baroreceptor neurons and the styryl dye FM2-10 to characterize (1) the dependence of exocytosis at these afferent nerve terminals on the frequency of neuronal activation, (2) the effect of duration of stimulation on the rate of exocytosis and (3) the role of mGluRs in the frequency-dependent modulation of exocytosis. Destaining in the FM2-10 loaded boutons during 3 min of stimulation, a measure of exocytosis, progressively decreased with increasing frequency (0.5, 1.0 and 10 Hz). Blockade of group III mGluRs with 300 microM (RS)-cyclopropyl-4-phosphonophenylglycine (CPPG) facilitated exocytosis evoked by 10 Hz stimulation but not at 0.5 Hz. The data suggest that aortic baroreceptor terminals exhibit frequency-dependent depression of exocytosis and support a role for group III mGluRs in the frequency-dependent modulation of exocytosis.     

Importance of neurokinin‐1 receptors in the nucleus tractus solitarii of mice for the integration of cardiac vagal inputs

Unmyelinated vagal afferents from the heart terminate within the nucleus tractus solitarii (NTS) located in the dorsomedial medulla. The neurotransmitter and postsynaptic receptors mediating information from cardiac vagal receptors to the NTS are unknown. This study determined the effects of neurokinin-1 (NK₁) receptor blockade on: (i) the reflex response evoked following aortic root injection of either veratridine (1–3 μg/kg) or bradykinin (80–300 ng/kg) to stimulate cardiac receptors in in vivo anaesthetized mice; and (ii) the evoked synaptic response of cardioreceptive NTS neurons following both intraleft-ventricular injection of veratridine or bradykinin, and electrical stimulation of the ipsilateral vagus nerve in an arterially perfused working heart-brainstem preparation of mouse. Administration of CP-99,994 (0.75–1.5 mg/kg i.v.), a specific NK₁ antagonist, attenuated significantly the evoked reflex bradycardia and depressor response following cardiac receptor (n = 6), but not pulmonary chemoreflex stimulation in vivo. From extracellular recordings of cardioreceptive NTS neurons, CP-99,994 reduced reversibly the total number of evoked spikes, peak firing frequency and response duration evoked by intraventricular injections of veratridine (n = 5) or bradykinin (n = 5). The number of evoked action potentials following electrical stimulation of the vagus nerve was also reduced. In five whole cell recordings of NTS neurons, both the evoked depolarization following cardiac receptor stimulation, and the peak amplitude and duration of vagus nerve-evoked EPSPs were reduced by CP-99 994; synaptic inputs from both peripheral chemoreceptors or pulmonary C-fibres were unaffected. These data support a selective involvement of NK₁ receptors in the transmission of cardiac vagal afferent inputs to NTS neurons integrating cardiorespiratory information.     

Capsaicin treatment attenuates the reflex excitation of sympathetic activity caused by chemical stimulation of intestinal afferent nerves

Sympathetic reflexes elicited by stimulation of visceral receptors have been well investigated, but the central neurotransmitters mediating these reflexes are largely unknown. Therefore, experiments were done to evaluate the role of substance P in the central transmission of a sympathoexcitatory reflex elicited by stimulation of intestinal receptors. Activity of mesenteric and renal nerves was recorded electrophysiologically in chloralose/urethane-anesthetized rats. Stimulation of intestinal receptors by serosal application of 0.5-1.0 microgram bradykinin increased mesenteric nerve activity by 100 +/- 21%, renal nerve discharge by 33 +/- 9%, and systemic arterial pressure by 10 mm Hg. Chronic capsaicin treatment (cumulative dose 950 mg/kg) caused a 52% depletion of substance P-like immunoreactivity from dorsal root ganglia and a significant attenuation of these reflexes. Mesenteric nerve activity increased by 48 +/- 6% in the capsaicin-treated rats. Bradykinin did not cause significant changes in renal nerve activity or systemic arterial pressure in these rats. The excitation of mesenteric nerve activity was significantly greater than the increase in renal nerve activity int he untreated and capsaicin-treated rats; capsaicin treatment affected responses of both nerves similarly. Capsaicin treatment did not have generalized effects on sympathetic reflexes, as mesenteric and renal nerve activities were decreased by baroreceptor activation similarly in the untreated and capsaicin-treated rats. These results suggest that the central transmission of the reflex response to intestinal receptor stimulation is mediated in part by substance P or other capsaicin-sensitive peptides.     

Origin of tonic GABAergic inputs to vasopressor neurons in the subretrofacial nucleus of the rabbit

The aims of this study were to determine (1) whether the vasomotor effects reflexly elicited by baroreceptor stimulation are dependent upon gamma-aminobutyric acid (GABA) receptors in the subretrofacial (SRF) nucleus in the rostral ventrolateral medulla; (2) the extent to which inputs other than those arising from peripheral baroreceptors, or transmitted via the nucleus tractus solitarius (NTS), contribute to the tonic GABAergic inhibition of SRF vasopressor cells. Following bilateral injection of a mixture of the GABA antagonist bicuculline methiodide (500 pmol) and GABA agonist muscimol (500 pmol) into the SRF nucleus, the sympathoinhibitory response normally evoked by a rise in arterial pressure (induced by inflating an aortic cuff) was abolished in 4 out of 8 rabbits and reduced in the remainder. For the whole group, the mean reduction in this response was 71%. In other experiments, the pressor response produced by injection of bicuculline methiodide into the SRF nucleus was still present after (1) destruction of the intermediate portion of the NTS, and (2) complete removal of the brain rostral to the pons. We conclude that (1) an inhibitory GABAergic input into the SRF nucleus is an important component of the central pathways mediating baroreceptor inhibition of sympathetic vasomotor tone; (2) the SRF nucleus also receives tonic GABAergic inputs that are intrinsic to the lower brainstem and are independent of baroreceptor or other cardiovascular inputs relayed by the NTS.     

Involvement of I(1)-imidazoline receptors in baroreceptor reflex in the caudal ventrolateral medulla of rats

There is ample evidence to show the existence of center I(1)-imidazoline receptors that are involved in the regulation of cardiovascular activities. The purpose of this study was to examine the possible role of I(1)-imidazoline receptors and alpha(2)-adrenoceptors within the caudal ventrolateral medulla (CVLM) in mediating the baroreceptor reflex in anesthetized rats. Unilateral microinjection of idazoxan (2 nmol in 50 nl), a mixed antagonist of I(1)-imidazoline receptors and alpha(2)-adrenoceptors, into the CVLM significantly (P<0.01) decreased blood pressure (BP), heart rate (HR), and the firing rate of presympathetic neurons in the rostral ventrolateral medulla (RVLM) by 21+/-6 mmHg, 25+/-5 beats per min and 3.5+/-0.9 spikes/s, respectively. Moreover, unilateral injection of idazoxan into the CVLM significantly (P<0.01) reduced the inhibitory responses of the ipsilateral RVLM presympathetic neurons evoked by stimulation of aortic nerve and elevation of BP, and partially inhibited the neuronal cardiac cycle-related rhythm. Depressor responses evoked by aortic nerve stimulation were significantly (P<0.01) attenuated 10 and 20 min after bilateral microinjection of idazoxan (2 nmol in 50 nl for each side) into the CVLM (-20+/-4 and -30+/-4 vs. -40+/-1 mmHg). However, injection of yohimbine (500 pmol in 50 nl), a selective alpha(2)-adrenoceptor antagonist, into the CVLM did not affect the resting cardiovascular activities and baroreceptor reflex. It is concluded that the CVLM I(1)-imidazoline receptors are involved in maintenance of tonic cardiovascular activities and transmission of the baroreceptor reflex.     

Evidence for an excitatory amino acid pathway in the brainstem and for its involvement in cardiovascular control

The source and possible role of excitatory amino acid projections to areas of the ventrolateral medulla (VLM) involved in cardiovascular control were studied. Following the injection of [3H]D-aspartate ([3H]D-Asp), a selective tracer for excitatory amino acid pathways, into vasopressor or vasodepressor areas of the VLM in rats, more than 90% of retrogradely labelled neurones were found in the nucleus of the solitary tract (NTS). Very few of the [3H]D-Asp-labelled cells were immunoreactive for tyrosine hydroxylase, none for phenylethanolamine-N-methyltransferase or gamma-aminobutyric acid. The density of labelled cells in the NTS was similar to that obtained with the non-selective tracers wheat germ agglutinin-horseradish peroxidase (WGA-HRP) and WGA-colloidal gold, but these tracers also labelled other cell groups in the medulla. Furthermore, the decrease in blood pressure, caused by pharmacological activation of neurones in the NTS of rats, or by electrical stimulation of the aortic depressor nerve in rabbits could be blocked by the selective N-methyl-D-aspartate (NMDA) receptor antagonist 2-amino-5-phosphonovalerate injected into the caudal vasodepressor area of the VLM. This area corresponds to the termination of [3H]D-Asp transporting NTS neurones. These results provide evidence that a population of NTS neurones projecting to the VLM use excitatory amino acids as transmitters. Among other possible functions, this pathway may mediate tonic and reflex control of blood pressure via NMDA receptors in the VLM.