Intrathecal kynurenate but not benextramine blocks hypoxic sympathoexcitation in chemodenervated anesthetized rats

Catestatin has an unexpected effect on the intrathecal actions of PACAP dramatically reducing blood pressure

This study focuses on presympathetic neurons of the rostral ventrolateral medulla (RVLM) that regulate sympathetic vasomotor tone. Many neurotransmitters are colocalized in RVLM neurons and are released under specific conditions to modulate efferent homeostatic responses. Of particular interest here are two peptides colocalized in catecholaminergic RVLM neurons: catestatin and pituitary adenylate cyclase-activating polypeptide (PACAP). Chromogranin A-derived catestatin is a potent endogenous noncompetitive nicotinic and adrenoreceptor antagonist. Catestatin impairs adenylate cyclase and phospholipase C action: mechanisms engaged by PACAP. Although PACAP and catestatin are likely coreleased, the possible effects of this are unknown. We aimed to determine whether catestatin affects the normal sympathoexcitatory but isotensive responses to intrathecal PACAP. Urethane-anesthetized, vagotomized, ventilated Sprague-Dawley rats (n = 22) were given an intrathecal injection of catestatin at different times prior to intrathecal administration of PACAP-38. Arterial pressure, splanchnic sympathetic nerve activity, heart rate, and reflex responses to baroreceptor and chemoreceptor activation were recorded. The key findings of this study are that pretreatment with catestatin time dependently enhances the PACAP-38 effect on mean arterial pressure and enhances sympathetic barosensitivity and chemosensitivity. The time-scale of the effect of catestatin on the response to PACAP-38 strongly suggests that catestatin is either causing changes in gene expression to exert its effects, or modifying intracellular mechanisms normally engaged by PAC(1) receptors. The ability of catestatin pretreatment to enhance barosensitivity and chemosensitivity after PACAP-38 injection supports the hypothesis that catestatin manipulates the intracellular environment within sympathetic neurons in a way that increases responses to PACAP.

Orexin A in rat rostral ventrolateral medulla is pressor, sympatho-excitatory, increases barosensitivity and attenuates the somato-sympathetic reflex

BACKGROUND AND PURPOSE

The rostral ventrolateral medulla (RVLM) maintains sympathetic nerve activity (SNA), and integrates adaptive reflexes. Orexin A-immunoreactive neurones in the lateral hypothalamus project to the RVLM. Microinjection of orexin A into RVLM increases blood pressure and heart rate. However, the expression of orexin receptors, and effects of orexin A in the RVLM on splanchnic SNA (sSNA), respiration and adaptive reflexes are unknown.

EXPERIMENTAL APPROACH

The effect of orexin A on baseline cardio-respiratory variables as well as the somato-sympathetic, baroreceptor and chemoreceptor reflexes in RVLM were investigated in urethane-anaesthetized, vagotomized and artificially ventilated male Sprague-Dawley rats (n= 50). orexin A and its receptors were detected with fluorescence immunohistochemistry.

KEY RESULTS

Tyrosine hydroxylase-immunoreactive neurones in the RVLM were frequently co-localized with orexin 1 (OX(1) ) and orexin 2 (OX(2) ) receptors and closely apposed to orexin A-immunoreactive terminals. Orexin A injected into the RVLM was pressor and sympatho-excitatory. Peak effects were observed at 50 pmol with increased mean arterial pressure (42 mmHg) and SNA (45%). Responses to orexin A (50 pmol) were attenuated by the OX(1) receptor antagonist, SB334867, and reproduced by the OX(2) receptor agonist, [Ala(11) , D-Leu(15) ]orexin B. Orexin A attenuated the somato-sympathetic reflex but increased baroreflex sensitivity. Orexin A increased or reduced sympatho-excitation following hypoxia or hypercapnia respectively.

CONCLUSIONS AND IMPLICATIONS

Although central cardio-respiratory control mechanisms at rest do not rely on orexin, responses to adaptive stimuli are dramatically affected by the functional state of orexin receptors.

Intrathecal neuromedin U induces biphasic effects on sympathetic vasomotor tone, increases respiratory drive and attenuates sympathetic reflexes in rat

BACKGROUND

Neuromedin U (NMU) is a brain-gut peptide that plays regulatory roles in feeding, anxiety, smooth muscle contraction, blood flow, pain and adrenocortical function via two receptors, the NMU receptor 1 and NMU receptor 2. NMU has several known functions in the periphery, but its role in central cardiorespiratory regulation remains poorly understood.

EXPERIMENTAL APPROACH

Experiments were conducted on urethane-anaesthetized, vagotomized and artificially ventilated male Sprague-Dawley rats (n= 42) to determine if NMU modulates sympathetic vasomotor output at the spinal level or modulates baro-, chemo- and somato-sympathetic reflexes.

KEY RESULTS

Intrathecal (i.t.) injections of NMU (2.5-20 nmol) caused a dose-dependent biphasic response, initially a brief period of hypertension and sympatho-excitation followed by prolonged hypotension and sympatho-inhibition. Peak excitatory as well as inhibitory responses were observed at 20 nmol. NMU (20 nmol) initially increased mean arterial pressure and splanchnic sympathetic nerve activity by 24 mmHg and 27% and then reduced these by 37 mmHg and 47%, respectively. NMU also dose-dependently increased respiratory drive, as indicated by a rise in phrenic nerve amplitude, an increase in neural minute ventilation and a shortening of the inspiratory period. Both sympatho-excitatory peaks of the somato-sympathetic reflex were abolished by i.t. NMU. Pressor, sympatho-excitatory and tachycardiac responses to chemoreceptor activation (100% N₂) were blocked or significantly reduced following i.t. NMU. NMU also reduced barosensitivity.

CONCLUSIONS

The data demonstrate that NMU, acting in the spinal cord, differentially contributes to the control of sympathetic tone and adaptive sympathetic reflexes.

Vasostatin I (CgA17-76) vasoconstricts rat splanchnic vascular bed but does not affect central cardiovascular function

Vasostatin I (CgA(1-76)) is a naturally occurring biologically active peptide derived from chromogranin A (CgA), and is so named for its inhibitory effects on vascular tension. CgA mRNA is expressed abundantly in sympathoexcitatory catecholaminergic neurons of the rostral ventrolateral medulla (RVLM). CgA microinjection into the RVLM decreases blood pressure (BP), heart rate (HR) and sympathetic nerve activity (SNA). Proteolytic fragments of CgA are thought to be responsible for the cardiovascular effects observed. We hypothesised that vasostatin I is one of the fragments responsible for the central effects of CgA. We examined the role of a vasostatin I fragment, CgA(17-76) (VS-I((CgA17-76))), containing the portion important for biological effects. The effects of VS-I((CgA17-76)) delivered by intrathecal injection, or microinjection into the RVLM, on cardio-respiratory function in urethane anaesthetised, vagotomised, mechanically ventilated Sprague-Dawley rats (n=21) were evaluated. The effects of intrathecal VS-I((CgA17-76)) on the somato-sympathetic, baroreceptor and peripheral chemoreceptor reflexes were also examined. At the concentrations used (10, 100 or 200 μM, intrathecal; or 5 μM, RVLM microinjection) VS-I((CgA17-76)) produced no change in mean arterial pressure, HR, splanchnic SNA, phrenic nerve amplitude or phrenic nerve frequency. All reflexes examined were unchanged following intrathecal VS-I((CgA17-76)). In the periphery, VS-I((CgA17-76)) potentiated the contractile effects of noradrenaline on rat mesenteric arteries (n=6), with a significant left-shift in the dose response curve to noradrenaline (3.7×10(-7) vs 7.7×10(-7)). Our results indicate that VS-I((CgA17-76)) is active in the periphery but not centrally, and is not a central modulator of cardiorespiratory function and physiological reflexes.

Intrathecal orexin A increases sympathetic outflow and respiratory drive, enhances baroreflex sensitivity and blocks the somato-sympathetic reflex

BACKGROUND

Intrathecal (i.t.) injection of orexin A (OX-A) increases blood pressure and heart rate (HR), but the effects of OX-A on sympathetic and phrenic, nerve activity, and the baroreflex(es), somato-sympathetic and hypoxic chemoreflex(es) are unknown.

EXPERIMENTAL APPROACH

Urethane-anaesthetized, vagotomized and artificially ventilated male Sprague-Dawley rats were examined in this study. The effects of i.t. OX-A (20 nmol 10 µL⁻¹) on cardiorespiratory parameters, and responses to stimulation of the sciatic nerve (electrical), arterial baroreceptors (phenylephrine hydrochloride, 0.01 mg kg⁻¹ i.v.) and peripheral (hypoxia) chemoreceptors were also investigated.

KEY RESULTS

i.t. OX-A caused a prolonged dose-dependent sympathoexcitation, pressor response and tachycardia. The peak effect was observed at 20 nmol with increases in mean arterial pressure, HR and splanchnic sympathetic nerve activity (sSNA) of 32 mmHg, 52 beats per minute and 100% from baseline respectively. OX-A also dose-dependently increased respiratory drive, as indicated by a rise in phrenic nerve amplitude and a fall in phrenic nerve frequency, an increase in neural minute ventilation, a lengthening of the expiratory period, and a shortening of the inspiratory period. All effects of OX-A (20 nmol) were attenuated by the orexin receptor 1 antagonist SB 334867. OX-A significantly reduced both sympathoexcitatory peaks of somato-sympathetic reflex while increasing baroreflex sensitivity. OX-A increased the amplitude of the pressor response and markedly amplified the effect of hypoxia on sSNA.

CONCLUSIONS

Thus, activation of OX receptors in rat spinal cord alters cardiorespiratory function and differentially modulates sympathetic reflexes.

VGLUT1 and VGLUT2 innervation in autonomic regions of intact and transected rat spinal cord

Fast excitatory neurotransmission to sympathetic and parasympathetic preganglionic neurons (SPN and PPN) is glutamatergic. To characterize this innervation in spinal autonomic regions, we localized immunoreactivity for vesicular glutamate transporters (VGLUTs) 1 and 2 in intact cords and after upper thoracic complete transections. Preganglionic neurons were retrogradely labeled by intraperitoneal Fluoro-Gold or with cholera toxin B (CTB) from superior cervical, celiac, or major pelvic ganglia or adrenal medulla. Glutamatergic somata were localized with in situ hybridization for VGLUT mRNA. In intact cords, all autonomic areas contained abundant VGLUT2-immunoreactive axons and synapses. CTB-immunoreactive SPN and PPN received many close appositions from VGLUT2-immunoreactive axons. VGLUT2-immunoreactive synapses occurred on Fluoro-Gold-labeled SPN. Somata with VGLUT2 mRNA occurred throughout the spinal gray matter. VGLUT2 immunoreactivity was not noticeably affected caudal to a transection. In contrast, in intact cords, VGLUT1-immunoreactive axons were sparse in the intermediolateral cell column (IML) and lumbosacral parasympathetic nucleus but moderately dense above the central canal. VGLUT1-immunoreactive close appositions were rare on SPN in the IML and the central autonomic area and on PPN. Transection reduced the density of VGLUT1-immunoreactive axons in sympathetic subnuclei but increased their density in the parasympathetic nucleus. Neuronal cell bodies with VGLUT1 mRNA occurred only in Clarke's column. These data indicate that SPN and PPN are densely innervated by VGLUT2-immunoreactive axons, some of which arise from spinal neurons. In contrast, the VGLUT1-immunoreactive innervation of spinal preganglionic neurons is sparse, and some may arise from supraspinal sources. Increased VGLUT1 immunoreactivity after transection may correlate with increased glutamatergic transmission to PPN.

Preprotachykinin A mRNA is colocalized with tyrosine hydroxylase-immunoreactivity in bulbospinal neurons

Previous studies have generated controversy about the extent of co-localization between substance P- and catecholamine-containing neurons that project to the spinal cord. In earlier studies, estimates using immunofluorescence after colchicine have ranged from almost all, to almost none. We sought to resolve this issue by combining in situ hybridization and immunofluorescence. Catecholamine (A1 to A7, C1 to C3; tyrosine hydroxylase immunoreactive) neurons in the rat brainstem were examined to determine their content of mRNA for the preprotachykinin-A gene. In the A1 to A7 and the C1 to C3 cell groups, preprotachykinin-A mRNA was found only in substantial amounts in the C1-C3 cell groups. On average 20.9+/-0.9% (234/1120, n=3) of rostral C1 neurons contained preprotachykinin-A mRNA. Co-localization was also observed in C2 and C3 neurons to a similar extent. Retrograde tract-tracing with cholera toxin B subunit was used to identify bulbospinal neurons and 17.9+/-2.7% (96/529 cells) of the bulbospinal tyrosine hydroxylase-containing neurons of the rostral C1 cell group were found to contain preprotachykinin-A mRNA. In addition a new population of non-catecholaminergic bulbospinal preprotachykinin-A neurons is described in an area corresponding to the recently described caudal pressor area. To confirm that the preprotachykinin-A mRNA observed in cells in the medulla was converted to protein, dual immunofluorescence for fiber labeling at the confocal level was carried out. This confirmed colocalization of substance P and tyrosine hydroxylase in the intermediolateral cell column, but nowhere else, in a small number of cases. The results provide evidence for a much larger population of substance P/neurokinin A containing neurons in the brainstem than was previously suspected. Furthermore, many of these neurons are catecholaminergic and spinally projecting. The specific sympathetic outflow that these neurons influence remains to be determined.

Intraspinal amino acid neurotransmitter activities are involved in the generation of rhythmic sympathetic nerve discharge in newborn rat spinal cord

Endogenous neurotransmitter activities underlying the sympathetic nerve discharge (SND) generated by newborn rat spinal cord in vitro were investigated using glutamatergic, glycinergic, and GABAergic antagonists. Under control conditions, the SND power spectrum had two major frequency components: synchronous bursting SND (bSND) with power dominant at < 0.1 Hz and quasiperiodic SND (qSND) oscillating at 1-2 Hz. Using high Mg2+ solution (12-24 mM) to block Ca2+-dependent synaptic transmission reversibly abolished SND. An interruption of glutamatergic neurotransmission by CNQX (non-NMDA receptor blocker) or L-AP4 (reducing the synaptic release of glutamate) failed to affect qSND, but consistently reduced bSND. Application of kynurenate, a broad-spectrum ionotropic glutamate receptor blocker, only caused an unstable SND but did not reduce SND. In contrast, strychnine (Stry, glycine receptor antagonist) consistently reduced qSND in a dose-dependent manner. Bicuculline (Bic, GABA(A) receptor antagonist) induced a synchronous bSND of irregular rhythm, which could be further regularized by adding Stry. Bic-induced bSND was reversibly abolished by CNQX or L-AP4. In conclusion, intraspinal glycinergic, GABAergic, and glutamatergic activities are involved in the generation of the spinal cord-derived SND in newborn rats. Intraspinal GABAergic interneurons may tonically inhibit the glutamatergic bursting neurons that generate a synchronous bSND. Activities of these glutamatergic bursting neurons may also be modulated by intraspinal glycinergic interneurons.

Cuneiform nucleus stimulation-induced sympathoexcitation: role of adrenoceptors, excitatory amino acid and serotonin receptors in rat spinal cord

Stimulation of the midbrain cuneiform nucleus has previously been shown to produce increases in arterial blood pressure and lumbar sympathetic nerve activity. While this sympathoexcitatory effect is, in part, due to excitation of premotor sympathoexcitatory neurons in the rostral ventrolateral medulla, the specific spinal neurotransmitter systems recruited by cuneiform nucleus stimulation remains to be elucidated. In this study, mean arterial pressure, resting and cuneiform nucleus stimulation-evoked lumbar sympathetic nerve activity were analysed following intrathecal injections of an excitatory amino acid antagonist (kynurenic acid), alpha1-adrenoceptor antagonist (prazosin) and a serotonin receptor antagonist (methiothepin) in anesthetized, paralysed male Sprague-Dawley rats. Mean arterial pressure and resting sympathetic nerve discharge were decreased by all treatments (n = 6/group) compared to the vehicle control group. Intermittent electrical stimulation of the cuneiform nucleus produced a bimodal sympathoexcitatory response, of which the short latency peak was significantly attenuated (43% reduction) by intrathecal kynurenate whereas the long latency peak was reduced by intrathecal prazosin (decrease of 21%) and methiothepin (38% attenuation). These results are consistent with the significant roles of excitatory amino acid, alpha1-adrenergic and serotonin receptors in modulating the activity of sympathetic vasomotor preganglionic neurons supplying the lumbar sympathetic nerve trunk, and suggest the existence of at least three neuronal groups and/or pathways associated with the sympathoexcitatory response to cuneiform nucleus stimulation.

Urethane directly inhibits chemoreflex excitation of medullary vasomotor neurons in rats

In anesthetized rats, stimulation of the arterial chemoreceptors excites reticulospinal vasomotor neurons of the nucleus rostroventrolateral reticularis of the medulla oblongata, via activating the neuronal NMDA receptors. Excitation of these neurons is responsible for reflex increases in sympathetic neuronal activity and arterial pressure. Additional doses (0.12-0.24 g/kg i.v.) of urethane dose-dependently reduced the elevations in the discharge rate of reticulospinal vasomotor and sympathetic neurons and of arterial pressure, elicited by stimulating the carotid chemoreceptors with intra-carotid injections of sodium cyanide (100 nmol/10 microl), without affecting discharges of the carotid chemoafferents. Microiontophoresis of urethane onto the reticulospinal vasomotor neurons reversibly inhibited the excitation elicited by stimulation of the chemoreceptors and by iontophoretically applied L-glutamate. The results indicate that the suppression of chemoreflexes by excessive amount of urethane is central and one of its blocking excitatory amino acid transmission onto these vasomotor neurons.

A medullary dorsal horn relay for the cardiorespiratory responses evoked by stimulation of the nasal mucosa in the muskrat Ondatra zibethicus: evidence for excitatory amino acid transmission

Stimulation of the upper respiratory tract, including the nasal mucosa, with water, vaporous irritants, or gases, induces a collation of several cardiorespiratory responses including an apnea and bradycardia and often some change in arterial blood pressure. Since the nasal mucosa is innervated by branches of the trigeminal nerve, it implies that some part of the trigeminal system within the central nervous system mediates the autonomic responses induced by nasal stimulation. In the present study, respirations, heart rate and arterial pressure were monitored in muskrats anesthetized with a mixture of chloralose-urethane. We induced a bradycardia and apnea by stimulating the nasal mucosa of muskrats with brief (5 s) transnasal application of vapors of ammonia hydroxide. In an effort to determine the central site where the trigeminal mediation of the cardiorespiratory responses occurs, small nanoliter injections of 2% lidocaine were made bilaterally into the subnucleus caudalis of the spinal trigeminal nucleus (referred to as the medullary dorsal horn) to determine if the responses could be blocked. The responses could be blocked when the lidocaine injections on both sides were placed in the rostral, ventral parts of the medullary dorsal horn, but persisted when the injections were placed elsewhere. Since lidocaine blocks both neurons and fibers of passage, nanoliter injections of kynurenate, a general excitatory amino acid antagonist, were used in a similar paradigm to circumvent the problem of blocking only fibers of passage.(ABSTRACT TRUNCATED AT 250 WORDS)

NMDA receptor-mediated sympathetic chemoreflex excitation of RVL-spinal vasomotor neurones in rats

1. We investigated the role of N-methyl-D-aspartic acid (NMDA) receptors in mediating hypoxic excitation of the reticulospinal vasomotor neurones of the rostroventrolateral reticular nucleus (RVL) of the medulla oblongata in paralysed ventilated rats. 2. Unilateral close arterial injection of sodium cyanide (100 nmol) into the carotid sinus region or ventilation with 100% N2 for 12 s rapidly, reversibly and reproducibly excited the RVL-spinal vasomotor neurones, followed about 1-2 s later by increases in sympathetic nerve activity and arterial pressure, effects abolished by denervation of the ipsilateral carotid sinus nerve. 3. Ionophoresis onto the RVL-spinal vasomotor neurones of kynurenate (a wide-spectrum antagonist of the excitatory amino acid receptors) or of 2-amino-5-monophosphovaleric acid (APV; a selective NMDA receptor antagonist), but not of xanthurenate (an inactive analogue of kynurenate), blocked the excitation elicited by intracarotid cyanide or 12 s of hypoxia. Kynurenate completely and APV partially blocked the excitatory responses to ionophoretically applied L-glutamate. APV, however, did not alter the excitatory responses of the vasomotor neurones to ionophoreses of kainate and quisqualate. 4. Bilateral microinjection of kynurenate (10 nmol, 50 nl per site) or APV (5 nmol, 50 nl per site) into the RVL blocked the increases in arterial pressure elicited by intracarotid cyanide or 12 s of 100% N2 ventilation. 5. Twenty seconds of intratracheal administration of 100% N2 resulted in complex and prolonged elevations of arterial pressure, the late component of which was affected neither by sinus denervation nor by microinjections of kynurenate or APV into the RVL. 6. We conclude that the sympathetic and cardiovascular responses to stimulation of arterial chemoreceptors result from excitation of RVL-spinal vasomotor neurones via activation of the NMDA subtypes of the excitatory amino acid receptors of the neurones. In contrast, the failure of these antagonists to influence the delayed excitation of the RVL-spinal vasomotor neurons by more prolonged exposure to N2 inhalation further supports the view that these neurones are directly stimulated by hypoxia.