Cholinergic mechanisms responsible for blood pressure regulation on sympathoexcitatory neurons in the rostral ventrolateral medulla of the rat

Cardiovascular responses induced by the activation of muscarinic receptors of the pedunculopontine tegmental nucleus in anesthetized rats

BACKGROUND

The cardiovascular effects of nicotinic receptors of cholinergic system in the pedunculopontine tegmental nucleus (PPT) were shown.

OBJECTIVE

In the following, the cardiovascular effects of the muscarinic receptor, another receptor in this system, were examined.

METHODS

Rats were divided into eight groups: 1) control; 2 and 3) Ach (acetylcholine, an agonist) 90 and 150 nmol; 4 and 5) Atr (atropine; a muscarinic antagonist) 3 and 9 nmol; 6) Atr 3 + Ach 150; 7) Atr 9 + Ach 150; and 8) Atr 3 + hexamethonium (Hexa; 300 nmol) + Ach 150. After anesthesia, cannulation of the femoral artery was performed, and then the mean arterial pressure (MAP), systolic blood pressure (SBP), and heart rate (HR) were recorded using a power lab apparatus.

RESULTS

Following drug microinjection, the maximum change (Δ) in MAP, SBP, and HR was calculated and analyzed. Both doses of Ach (90 and 150) significantly decreased ΔMAP and ΔSBP but could not change ΔHR. Neither of the doses of Atr significantly affected ΔMAP, ΔSBP, and ΔHR. Co-injection of Atr 3 + Ach 150 only increased ΔHR, but Atr 9 + Ach 150 decreased ΔMAP and ΔSBP than Ach 150 alone. The effect of the co-injection of Atr 9 + Hexa 300 + Ach 150 was also the same as the Atr 9 + Ach 150 group.

CONCLUSION

The present results revealed that cholinergic muscarinic receptors in the PPT have an inhibitory effect on MAP and SBP with no important effect on HR.

Another controller system for arterial pressure. AngII-vasopressin neural network of the parvocellular paraventricular nucleus may regulate arterial pressure during hypotension

Angiotensin II (AngII) immunoreactive cells, fibers and receptors, were found in the parvocelluar region of paraventricular nucleus (PVNp) and AngII receptors are present on vasopressinergic neurons. However, the mechanism by which vasopressin (AVP) and AngII may interact to regulate arterial pressure is not known. Thus, we tested the cardiovascular effects of blockade of the AngII receptors on AVP neurons and blockade of vasopressin V1a receptors on AngII neurons. We also explored whether the PVNp vasopressin plays a regulatory role during hypotension in anesthetized rat or not. Hypovolemic-hypotension was induced by gradual bleeding from femoral venous catheter. Either AngII or AVP injected into the PVNp produced pressor and tachycardia responses. The responses to AngII were blocked by V1a receptor antagonist. The responses to AVP were partially attenuated by AT1 antagonist and greatly attenuated by AT2 antagonist. Hemorrhage augmented the pressor response to AVP, indicating that during hemorrhage, sensitivity of PVNp to vasopressin was increased. By hemorrhagic-hypotension and bilateral blockade of V1a receptors of the PVNp, we found that vasopressinergic neurons of the PVNp regulate arterial pressure towards normal during hypotension. Taken together these findings and our previous findings about angII (Khanmoradi and Nasimi, 2017a) for the first time, we found that a mutual cooperative system of angiotensinergic and vasopressinergic neurons in the PVNp is a major regulatory controller of the cardiovascular system during hypotension.

Angiotensin-Converting Enzyme 2 in the Rostral Ventrolateral Medulla Regulates Cholinergic Signaling and Cardiovascular and Sympathetic Responses in Hypertensive Rats

The rostral ventrolateral medulla (RVLM) is a key region in cardiovascular regulation. It has been demonstrated that cholinergic synaptic transmission in the RVLM is enhanced in hypertensive rats. Angiotensin-converting enzyme 2 (ACE2) in the brain plays beneficial roles in cardiovascular function in hypertension. The purpose of this study was to determine the effect of ACE2 overexpression in the RVLM on cholinergic synaptic transmission in spontaneously hypertensive rats (SHRs). Four weeks after injecting lentiviral particles containing enhanced green fluorescent protein and ACE2 bilaterally into the RVLM, the blood pressure and heart rate were notably decreased. ACE2 overexpression significantly reduced the concentration of acetylcholine in microdialysis fluid from the RVLM and blunted the decrease in blood pressure evoked by bilateral injection of atropine into the RVLM in SHRs. In conclusion, we suggest that ACE2 overexpression in the RVLM attenuates the enhanced cholinergic synaptic transmission in SHRs.

Pharmacological study of cholinergic system on cardiovascular regulation in the cuneiform nucleus of rat

In the present study the effect of cholinergic system of Cuneiform nucleus (CnF) on central regulation of cardiovascular system was investigated. Two doses of acetylcholine (Ach; 90 and 150 nmol), atropine (3 and 9 nmol) and hexamethonium (Hexa; 100 and 300 nmol) were microinjected into the CnF. The maximum changes of MAP and HR were compared with control group (independent t-test). Both doses of Ach significantly decreased MAP but had no significant effect on HR. Administration of atropine and Hexa by themselves did not alter the MAP or HR. However, both doses of atropine and higher dose of Hexa significantly attenuated the hypotensive effect of Ach with no significant effect on HR. Our results suggest the involvement of CnF cholinergic system only on central blood pressure regulation that strongly mediated by muscarinic receptors.

Differential muscarinic receptor gene expression levels in the ventral medulla of spontaneously hypertensive and Wistar-Kyoto rats: role in sympathetic baroreflex function

OBJECTIVE

We demonstrated previously that central muscarinic cholinergic receptor (mAChR) activation increased splanchnic sympathetic nerve activity and sympathetic baroreflex function via activation of mAChR in the rostral ventrolateral medulla (RVLM), and we found that some RVLM bulbospinal neurons contain muscarinic M2R mRNA. Here, we examined the gene expression, cellular distribution and functional role of muscarinic receptors in the RVLM in spontaneously hypertensive rats (SHR) compared with Wistar-Kyoto (WKY) rats.

METHOD AND RESULTS

Using the sensitive technique of quantitative real time reverse transcriptase-PCR, M2R mRNA level was elevated two-fold (P<0.05) and M4R mRNA was downregulated two-fold (P<0.001), with all other receptors expressed at similar levels, in the rostral ventral medulla of SHR compared with WKY. Bulbospinal, but not catecholaminergic neurons, in the RVLM expressed M2R mRNA (M2RR), and similar numbers were found in the RVLM of SHR and WKY. Could elevated M2R within individual neurons or enhanced presynaptic activity reflects enhanced cholinergic effects in the RVLM? Activation of central mAChR using oxotremorine evoked a larger increase in mean arterial pressure in SHR compared with WKY (P<0.01); however, oxotremorine-induced increases in splanchnic sympathetic nerve activity, and sympathetic baroreflex function were similar in SHR and WKY.

CONCLUSION

These data indicate that enhanced pressor responses in SHR, following centrally mediated mAChR activation, are not associated with RVLM-mediated constriction of the splanchnic circulation or effects on the sympathetic baroreflex, but could reflect modified mAChR gene expression elsewhere. RVLM-dependent splanchnic sympathetic nerve activity effects, evoked by mAChR activation, are not mediated by the differential M2/M4 receptor mRNA levels identified in SHR compared with WKY.

Transcriptional up-regulation of nitric oxide synthase II by nuclear factor-kappaB at rostral ventrolateral medulla in a rat mevinphos intoxication model of brain stem death

As the origin of a 'life-and-death' signal that reflects central cardiovascular regulatory failure during brain stem death, the rostral ventrolateral medulla (RVLM) is a suitable neural substrate for mechanistic delineation of this vital phenomenon. Using a clinically relevant animal model that employed the organophosphate pesticide mevinphos (Mev) as the experimental insult, we evaluated the hypothesis that transcriptional up-regulation of nitric oxide synthase I or II (NOS I or II) gene expression by nuclear factor-kappaB (NF-kappaB) on activation of muscarinic receptors in the RVLM underlies brain stem death. In Sprague-Dawley rats maintained under propofol anaesthesia, co-microinjection of muscarinic M2R (methoctramine) or M4R (tropicamide), but not M1R (pirenzepine) or M3R (4-diphenylacetoxy-N-dimethylpiperidinium) antagonist significantly reduced the enhanced NOS I-protein kinase G signalling ('pro-life' phase) or augmented NOS II-peroxynitrite cascade ('pro-death' phase) in ventrolateral medulla, blunted the biphasic increase and decrease in baroreceptor reflex-mediated sympathetic vasomotor tone that reflect the transition from life to death, and diminished the elevated DNA binding activity or nucleus-bound translocation of NF-kappaB in RVLM neurons induced by microinjection of Mev into the bilateral RVLM. However, NF-kappaB inhibitors (diethyldithiocarbamate or pyrrolidine dithiocarbamate) or double-stranded kappaB decoy DNA preferentially antagonized the augmented NOS II-peroxynitrite cascade and the associated cardiovascular depression exhibited during the 'pro-death' phase. We conclude that transcriptional up-regulation of NOS II gene expression by activation of NF-kappaB on selective stimulation of muscarinic M2 or M4 subtype receptors in the RVLM underlies the elicited cardiovascular depression during the 'pro-death' phase in our Mev intoxication model of brain stem death.

[Mechanisms of hypertension in the central nervous system]

This article reviews studies by the author on central mechanisms of hypertension. Spontaneously hypertensive rats (SHR) have been developed as a rat model of genetic hypertension, and central acetylcholine has been implicated in hypertension in SHR. The rostral ventrolateral medulla (RVL), a major source of efferent sympathetic activity, has cholinergic pressor systems. The release of acetylcholine is enhanced in the RVL of SHR, leading to hypertension. The alteration of the RVL cholinergic system in SHR results from enhanced angiotensin systems in the anterior hypothalamic area (AHA). Angiotensin II-sensitive neurons are present in the AHA and they are tonically activated by endogenous angiotensins. The basal activity of AHA angiotensin II-sensitive neurons is enhanced in SHR, mainly due to enhanced sensitivity of AHA neurons to angiotensin II. The AHA angiotensin system is also responsible for hypertension induced by emotional stress and central Na(+) increases. These findings suggest that the AHA angiotensin system may play a critical role in the development of hypertension.

Central cholinergic modulation of blood pressure short-term variability

The role of neurally born acetylcholine in the central modulation of cardiovascular short-term variability was assessed using a pharmacological probe physostigmine, a cholinesterase inhibitor that can act centrally also. Experiments were performed in instrumented conscious rats. Equidistant sampling at 20 Hz of systolic arterial pressure (SAP), diastolic arterial pressure (DAP) and heart rate (HR) allowed direct spectral analysis. Spectra were analysed in the whole, very-low frequency (VLF), low-frequency (LF) and high-frequency (HF) domains. Physostigmine, but not neostigmine, increased SAP, LF SAP and HF SAP variability while neostigmine, but not physostigmine, decreased HR without affecting HR variability. Atropine methyl nitrate prevented neostigmine-induced bradycardia and potentiated the effects of physostigmine on DAP, LF DAP and HF DAP variability. Atropine sulphate, hexamethonium, phentolamine and metoprolol inhibited physostigmine-induced increase of SAP and LF SAP. Pre-treatment of rats by quinapril prevented physostigmine-induced increase of SAP, but not of LF SAP, while the V(1a) antagonist prevented the increase of HF SAP. The results suggest that central cholinergic neurons facilitate but do not create LF SAP and HF SAP variability. The effect of physostigmine on LF SAP seems to be mediated via central muscarinic sites and the peripheral sympathetic system, while non-muscarinic central sites and vasopressin pathways subserve the increase of HF SAP.

Heart rate recovery after maximal exercise is associated with acetylcholine receptor M2 (CHRM2) gene polymorphism

The determinants of heart rate (HR) recovery after exercise are not well known, although attenuated HR recovery is associated with an increased risk of cardiovascular mortality. Because acetylcholine receptor subtype M2 (CHRM2) plays a key role in the cardiac chronotropic response, we tested the hypothesis that, in healthy individuals, the CHRM2 gene polymorphisms might be associated with HR recovery 1 min after the termination of a maximal exercise test, both before and after endurance training. The study population consisted of sedentary men and women (n = 95, 42 +/- 5 yr) assigned to a training (n = 80) or control group (n = 15). The study subjects underwent a 2-wk laboratory-controlled endurance training program, which included five 40-min sessions/wk at 70-80% of maximal HR. HR recovery differed between the intron 5 rs324640 genotypes at baseline (C/C, -33 +/- 10; C/T, -33 +/- 7; and T/T, -40 +/- 11 beats/min, P = 0.008). Endurance training further strengthened the association: the less common C/C homozygotes showed 6 and 12 beats/min lower HR recovery than the C/T heterozygotes or the T/T homozygotes (P = 0.001), respectively. A similar association was found between A/T transversion at the 3'-untranslated region of the CHRM2 gene and HR recovery at baseline (P = 0.025) and after endurance training (P = 0.005). These data suggest that DNA sequence variation at the CHRM2 locus is a potential modifier of HR recovery in the sedentary state and after short-term endurance training in healthy individuals.

Activities of hypothalamic angiotensin II-sensitive neurons are greately enhanced even in prehypertensive spontaneously hypertensive rats

We have previously demonstrated that some neurons in the anterior hypothalamic area (AHA) of rats are tonically activated by endogenous angiotensins and that reactivities of these neurons to angiotensin II are enhanced in 15- to 16-week-old spontaneously hypertensive rats (SHR). To investigate whether the enhanced reactivity of SHR AHA neurons to angiotensin II is secondary to raised blood pressure, we examined whether the enhanced reactivity to angiotensin II also occurs in prehypertensive SHR. We also examined whether reactivities of AHA angiotensin II-sensitive neurons to intracerebroventricular hypertonic saline are enhanced in prehypertensive SHR, since intracerebroventricular injection of hypertonic saline increases the firing rate of AHA neurons via release of angiotensins at AHA neuron levels. Male 4-week-old SHR and age-matched Wistar Kyoto rats (WKY) were used in this study. There was no difference in systolic blood pressure between both rats. They were anesthetized and artificially ventilated. Extracellular potentials were recorded from single neurons in the AHA. Pressure application of angiotensin II onto some AHA neurons increased their firing rate. The basal firing rate of angiotensin II-sensitive neurons was increased in SHR as compared with WKY. The increase of unit firing by angiotenisn II was enhanced in SHR as compared with WKY. Intracerebroventricular injection of hypertonic saline increased the firing rate of AHA angiotensin II-sensitive neurons. The average threshold sodium concentration for the saline-induced increase of neural firing was lower in SHR than in WKY. These findings demonstrate that basal activities and responsiveness to angiotensin II in AHA angiotensin II-sensitive neurons are enhanced in prehypertensive SHR as compared with age-matched WKY. In addition, these findings indicate that central saline-induced activation of AHA angiotensin II-sensitive neurons is also enhanced in SHR. It appears that the enhanced reactivity of SHR AHA neurons to angiotensin II occurs primarily in nature but not secondarily to raised blood pressure in SHR.

Central injection of hypertonic saline activates angiotensin II-sensitive neurons in the anterior hypothalamic area of rats

We have previously reported that microinjection of angiotensin II into the anterior hypothalamic area (AHA) produces pressor responses and that angiotensin II-sensitive neurons in the AHA are tonically activated by endogenous angiotensins in rats. Central injection of hypertonic saline causes pressor responses via release of angiotensins in brain. In this study, we examined whether angiotensin II-sensitive neurons in the AHA are responsive to intracerebroventricular injection of hypertonic saline and whether endogenous angiotensins in the AHA are involved in the central hypertonic saline-induced pressor response. Male Wistar rats were anesthetized and artificially ventilated. Extracellular potentials were recorded from single neurons in the AHA. Intraventricular injection of hypertonic saline increased the neural activity of angiotensin II-sensitive neurons, whereas pressure application of hypertonic saline onto angiotensin II-sensitive neurons themselves did not affect their neural activities. The intraventricular hypertonic saline-induced increase of unit activity of AHA neurons was inhibited by pressure application of the angiotensin AT1 receptor antagonist losartan onto the same neurons. The hypertonic saline-induced increase of unit firing was also blocked by intraventricular injection of the amiloride-sensitive sodium channel blocker benzamil. In conscious rats, intraventricular injection of hypertonic saline produced pressor responses, and the hypertonic saline-induced pressor response was inhibited by bilateral microinjection of losartan into the AHA. Repeated intraventricular injection of hypertonic saline caused an increase in the release of angiotensins in the AHA of anesthetized rats. These findings indicate that intracerebroventricular injection of hypertonic saline increases neural activity of angiotensin II-sensitive neurons trans-synaptically via endogenous angiotensins in the AHA. In addition, these findings also indicate that the intracerebroventricular injection of hypertonic saline produces a pressor response at least partly via release of angiotensins in the AHA.

Protein kinase C activation-induced increases of neural activity are enhanced in the hypothalamus of spontaneously hypertensive rats

We have previously reported that some neurons in the anterior hypothalamic area (AHA) are tonically activated by endogenous angiotensins in rats and that activities of these angiotensin II-sensitive neurons in the AHA are enhanced in spontaneously hypertensive rats (SHR). In addition, neural activations induced by both angiotensin II and glutamate were enhanced in the AHA of SHR. In this study, we examined whether intracellular neural activation mechanisms via protein kinase C (PKC) and a potassium channel are altered in angiotensin II-sensitive neurons in the AHA of SHR. Male 15- to 16-week-old SHR and age-matched Wistar-Kyoto rats (WKY) and Wistar rats were anesthetized and artificially ventilated. Extracellular potentials were recorded from single neurons in the AHA. Pressure application of the PKC activator phorbol 12-myristate 13-acetate (PMA) onto angiotensin II-sensitive neurons in the AHA of Wistar rats increased their firing rate. The increase of unit activity by PMA was inhibited by the potent inhibitor of PKC, 1-(5-isoquinolinesulfonyl)-2-methylpiperazine dihydrochloride (H-7), but not by the weak PKC inhibitor, N-(2-guanidinoethyl)-5-isoquinolinesulfonamide hydrochloride (HA1004). The increase of unit firing by PMA was enhanced in SHR as compared with WKY. Pressure application of H-7 alone decreased the basal firing activity of angiotensin II-sensitive neurons in SHR but not in WKY. HA1004 did not affect the basal firing activity of angiotensin II-sensitive neurons in SHR. Angiotensin II-induced increases of firing rate in AHA neurons were inhibited by H-7 and the inhibition by H-7 was enhanced in SHR as compared with WKY. Pressure application of 4-aminopyridine, a blocker of the transient potassium current, onto angiotensin II-sensitive neurons increased their firing rate and the increase of unit firing rate was almost the same in WKY and SHR. These findings indicate that activation of PKC increases neural activity in angiotensin II-sensitive neurons in the AHA and that this PKC activation-induced increase of neural activity is enhanced in the AHA of SHR. It seems likely that the enhanced PKC activation effect is responsible for the enhanced basal neural activity seen in the AHA of SHR.

Enhanced activity of angiotensin II-sensitive neurons in the anterior hypothalamic area of spontaneously hypertensive rats

We have previously reported that an angiotensin system in the anterior hypothalamic area (AHA) is enhanced in spontaneously hypertensive rats (SHRs) and that this enhancement is involved in hypertension in this strain. In addition, we have reported that some neurons in the AHA are tonically activated by endogenous angiotensins in rats. In this study, we examined whether activities of neurons receiving tonic angiotensinergic inputs in the AHA are enhanced in SHR as compared with those of Wistar Kyoto rats (WKY). Male 15- to 16- or 6-week-old SHR and age-matched WKY were anesthetized and artificially ventilated. Extracellular potentials were recorded from single neurons in the AHA. Pressure application of angiotensin II onto some neurons in the AHA increased their firing rate. The basal firing rate of angiotensin II-sensitive neurons was increased in both 15- to 16- and 6-week-old SHR than in age-matched WKY. The increase of unit firing by angiotenisn II was enhanced in both 15- to 16- and 6-week-old SHR as compared with age-matched WKY. Pressure application of losartan, an angiotensin type 1 (AT1) receptor antagonist, alone decreased the basal firing rate of angiotensin II-sensitive neurons in 15- to 16-week-old SHR and WKY. The decrease of unit firing by losartan was also enhanced in SHR as compared with WKY. Pressure application of glutamate onto angiotensin II-sensitive neurons increased their firing rate and the increase of unit firing by glutamate was enhanced in 15- to 16-week-old SHR as compared with age-matched WKY. These findings suggest that activities of angiotensin II-sensitive neurons in the AHA are enhanced in SHR as compared with WKY. It is possible that the enhanced activity of angiotensin II-sensitive neurons in the AHA of SHR is partly due to enhanced neuronal reactivity to angiotensin II.

Differential contributions of NOS isoforms in the rostral ventrolateral medulla to cardiovascular responses associated with mevinphos intoxication in the rat

The organophosphate poison mevinphos (Mev) elicits cardiovascular responses via nitric oxide (NO) produced on activation of M2 muscarinic receptors (M2R) in the rostral ventrolateral medulla (RVLM), where sympathetic vasomotor tone originates. This study further evaluated the contribution of nitric oxide synthase (NOS) isoforms at the RVLM to this process, using adult Sprague-Dawley rats. Bilateral co-microinjection into the RVLM of the selective NOS I inhibitor (250 pmol), 7-nitroindazole or N(omega)-propyl-L-arginine antagonized the initial sympathoexcitatory cardiovascular responses to Mev (10 nmol). Co-administration of a selective NOS II inhibitor, N6-(1-iminoethyl)-L-lysine (250 or 500 pmol) further enhanced these cardiovascular responses and reversed the secondary sympathoinhibitory actions of Mev. A potent NOS III inhibitor, N5-(1-iminoethyl)-L-ornithine (46 or 92 nmol) was ineffective. We also found that M2R co-localized only with NOS I- or NOS II-immunoreactive RVLM neurons. Furthermore, only NOS I or II in the ventrolateral medulla exhibited an elevation in mRNA or protein levels during the sympathoexcitatory phase, with further up-regulated synthesis of NOS II during the sympathoinhibitory phase of Mev intoxication. We conclude that whereas NOS III is not engaged, NO produced by NOS I and II in the RVLM plays, respectively, a sympathoexcitatory and sympathoinhibitory role in the cardiovascular responses during Mev intoxication.

Tonic angiotensinergic inputs to neurons in the anterior hypothalamic area of rats

We have previously reported that microinjection of angiotensin II into the anterior hypothalamic area (AHA) produces a pressor response in rats and that the angiotensin AT1 receptor antagonist, losartan, similarly injected causes a depressor response in hypertensive rats. In this study, we examined whether endogenous angiotensins are involved in activation of neurons in the AHA. Male Wistar rats were anesthetized and artificially ventilated. Extracellular potentials were recorded from single neurons in the AHA. Pressure-ejected application of angiotensin II and glutamate onto some neurons in the AHA increased their firing rate. The increase of unit firing induced by angiotensin II but not by glutamate was inhibited by losartan. Application of losartan alone inhibited the basal firing rate of angiotensin II-sensitive neurons in a concentration-dependent manner. Application of the angiotensin AT2 receptor antagonist, PD123319, did not affect the increase of unit firing induced by angiotensin II and the basal firing rate of angiotensin II-sensitive neurons. Pressure application of angiotensin I onto angiotensin II-sensitive neurons also increased firing rate and the increase of unit firing by angiotensin I was inhibited by the angiotensin converting enzyme inhibitor, captopril. Captopril alone inhibited the basal firing rate of angitensin II-sensitive neurons. Acetylcholine did not affect unit firing of angiotensin II-sensitive neurons, whereas it increased the firing rate of some angiotensin II-insensitive neurons in the AHA. Increases of blood pressure by intravenous phenylephrine completely inhibited the basal firing rate of angiotensin II-sensitive neurons. These findings suggest that some neurons in the AHA are tonically activated by endogenous angiotensins. It seems likely that newly synthesized angiotensins are used for the angiotensinergic transmission in the AHA.

Characterization of muscarinic receptor subtypes in the rostral ventrolateral medulla and effects on morphine-induced antinociception in rats

The present study investigated the role of muscarinic receptor subtypes in the nucleus reticularis gigantocellularis/nucleus reticularis gigantocellularis alpha of the rat rostral ventrolateral medulla in morphine-induced antinociception. The antinociceptive effects of morphine were evoked by systemic administration or microinjection into the nucleus reticularis gigantocellularis/nucleus reticularis gigantocellularis alpha. Administration of morphine produced antinociception for hot plate and tail immersion responses to noxious heat stimuli. These effects were antagonized by prior exposure to naloxone and inhibited by mecamylamine pretreatment. Morphine-induced antinociception was significantly inhibited by atropine in a dose-dependent manner. Muscarinic toxin-1 and pirenzepine inhibited morphine-induced antinociception for both the hot plate and tail immersion tests. At a dose of 5 nmol/site, 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP) also inhibited morphine-induced antinociception, although low doses of this drug did not significantly affect hot plate test response latency when morphine was systemically administered. These results suggest that the antinociceptive effects induced by morphine in part involve the muscarinic M(1) and M(3) receptors of the rat nucleus reticularis gigantocellularis/nucleus reticularis gigantocellularis alpha.

Effects of microinjected carbachol on the antinociceptive response to noxious heat stimuli

Injecting muscarinic receptor agonists into a specific area of the brainstem produces an antinociceptive response. The present study investigates whether direct injections of the cholinergic agonist, carbachol, into the rat nucleus reticularis gigantocellularis (NRGC)/nucleus reticularis gigantocellularis alpha (NRGCalpha) of the rostral ventrolateral medulla evokes antinociception, and then examines the interference action of cholinergic antagonists in rats. Microinjections of carbachol (0.75, 1.5, 3 micro g/site) prolonged hot plate (HP) and tail flick (TF) responses to noxious heat stimuli in a dose-dependent manner. The level of carbachol-induced antinociception during the HP and TF tests reached a maximum at 5-15 min after carbachol administration in all groups. Thereafter, the peak level progressively decreased and reached the baseline by the end of the experiment. Antinociception induced by carbachol at 3 micro g/site was attenuated by the prior administration of the muscarinic receptor antagonist, atropine (200, 500 ng/site). On the other hand, the nicotinic autonomic ganglion blocker, mecamylamine (1, 3 micro g/site), did not affect subsequent carbachol-induced antinociception. These results suggest that the antinociceptive effects induced by a microinjection of carbachol depend on muscarinic, but not nicotinic, mechanisms within the rat NRGC/NRGCalpha.

Engagement of inducible nitric oxide synthase at the rostral ventrolateral medulla during mevinphos intoxication in the rat

We evaluated the relationship between the toxicity induced by the organophosphate mevinphos (Mev) and inducible nitric oxide synthase (iNOS) in the rostral ventrolateral medulla (RVLM), the medullary origin of sympathetic neurogenic vasomotor tone. Adult Sprague-Dawley rats that were anesthetized and maintained with propofol were used. Laser scanning confocal microscopic analysis revealed colocalization of the M2 subtype of muscarinic receptors (M(2)R) and iNOS immunoreactivity in RVLM neurons. Comicroinjection bilaterally of Mev (10 nmol) and artificial cerebrospinal fluid (aCSF) into the RVLM elicited a progressive decline in systemic arterial pressure (SAP) and heart rate. This was accompanied during phase 1 Mev intoxication by an increase in the power density of the very high-frequency (VHF; 5-9 Hz), high-frequency (HF; 0.8-2.4 Hz), low-frequency (LF; 0.25- 0.8 Hz) and very low-frequency (VLF; 0-0.25 Hz) components of SAP signals. Phase 2 exhibited a reversal of the VHF and VLF power to control levels and a further reduction in the power density of both HF and LF components to below baseline. Hypotension and bradycardia promoted by Mev were significantly blunted on coadministration into the RVLM of the selective iNOS inhibitors S-methylisothiourea (250 pmol) or aminoguanidine (250 pmol). Not only was the augmented power density of HF and LF components during phase 1 Mev intoxication further enhanced, the reduced power of these two spectral components during phase 2 was appreciably antagonized. On the other hand, the temporal changes in VHF and VLF power were essentially the same as with coadministration of Mev and aCSF. We conclude that, as a cholinesterase inhibitor, Mev may induce toxicity via nitric oxide produced by iNOS on activation of the M(2)R by the accumulated acetylcholine in the RVLM.

Cholinergic inputs to rostral ventrolateral medulla pressor neurons from hypothalamus

The rostral ventrolateral medulla (RVLM) has cholinergic mechanisms responsible for pressor responses. Stimulation of the hypothalamic paraventricular nucleus (PVN) causes an increase of arterial pressure via activation of neurons in the RVLM. In this study, we examined whether PVN stimulation causes a pressor response via activation of cholinergic mechanisms in the RVLM. Male Wistar rats were used and they were anesthetized, paralyzed and artificially ventilated. Electrical stimulation of the PVN produced a pressor response. Microinjection of the muscarinic receptor antagonist scopolamine and the cholinesterase inhibitor physostigmine into the RVLM inhibited and potentiated, respectively, the pressor response induced by PVN stimulation. PVN stimulation also increased the firing rate of RVLM barosensitive neurons and the increase in the firing rate was inhibited and potentiated by scopolamine and physostigmine, respectively, iontophoretically applied on neurons. Microinjection of L-glutamate into the PVN produced a release of ACh in the RVLM. The inhibitory amino acid gamma-aminobutyric acid injected into the lateral parabrachial nucleus (LPBN) inhibited the pressor response induced by PVN stimulation. These results suggest that PVN stimulation causes an increase in arterial pressure via activation of cholinergic inputs in the RVLM. It appears that the pressor response is mediated, at least in part, via cholinergic inputs from the LPBN.

The role of cholinergic and noncholinergic mechanisms in the cardiorespiratory failure produced by N-methylcarbamate cholinesterase inhibitors in rabbits

We investigated the relative contribution of several cardiorespiratory components to acute lethality produced by N-methylcarbamate cholinesterase (ChE) inhibitors physostigmine, 2-sec-butylphenyl methylcarbamate (BPMC), and 2-isopropoxyphenyl methylcarbamate (PHC) in halothane-anesthetized rabbits. Intravenous injection of these compounds produced dose-dependent pressor and/or depressor responses related to each compound. A lethal dose of physostigmine resulted in cardiovascular collapse after a pressor response. That of PHC produced cardiovascular collapse after biphasic effects on blood pressure, a transient decrease followed by an increase. Unlike these compounds, BPMC elicited a rapidly developing depressor response followed by cardiovascular collapse. Artificial ventilation prevented cardiovascular collapse and lethal actions to physostigmine and PHC, but not BPMC. A degree of acute lethality to physostigmine and PHC depended on their anti-ChE activity, whereas BPMC exhibited a low degree of lethality relative to its anti-ChE activity. While the pressor response to physostigmine and PHC was ascribed to an atropine-sensitive increase in cardiac contractility, the depressor response to PHC and BPMC was attributed to an atropine-insensitive decrease in cardiac contractility and/or vascular resistance. Similar to the order for eliciting the depressor response in vivo, all three compounds inhibited contraction of the isolated cardiac and aortic smooth muscles with the order of their inhibition in terms of anti-ChE activity, i.e., BPMC > PHC > physostigmine. Thus, the primary cause of death with physostigmine and PHC is respiratory arrest subsequent to ChE inhibition, whereas BPMC exhibiting the low degree of lethality causes cardiovascular collapse mediated through direct inhibitory effects on cardiac and vascular smooth muscle contraction.

Midbrain central gray is involved in mediation of cholinergic inputs to the rostral ventrolateral medulla of the rat

There are cholinergic inputs responsible for pressor responses in the rostral ventrolateral medulla (RVLM) and stimulation of midbrain central gray (CG) increases arterial pressure via activation of neurons in the RVLM. In this study, we examined whether the CG was involved in mediation of the cholinergic inputs to the RVLM. Male Wistar rats were anesthetized, paralyzed, and artificially ventilated. Unilateral microinjection of L-glutamate into the CG produced a pressor response. Microinjection of the muscarinic receptor antagonist scopolamine into the unilateral RVLM inhibited the pressor response to L-glutamate injected ipsilaterally into the CG, whereas microinjection of the cholinesterase inhibitor physostigmine into the RVLM enhanced it. CG stimulation also enhanced the firing rate of RVLM barosensitive neurons and the enhancement of the firing rate was inhibited by scopolamine iontophoretically applied on neurons. CG injection of L-glutamate produced a release of acetylcholine in the RVLM. Unilateral microinjection of L-glutamate into the pedunculopontine tegmental nucleus (PPT) also produced a pressor response, but the pressor response to L-glutamate was not affected by scopolamine injected ipsilaterally into the RVLM. These results provide evidence that the CG but not the PPT is involved in mediation of cholinergic inputs responsible for pressor responses in the RVLM.

Spontaneously hypertensive rats cholinergic hyper-responsiveness: central and peripheral pharmacological mechanisms

1. The mechanisms and the subtypes of muscarinic receptors implicated in the cardiovascular effects of physostigmine were investigated in conscious normotensive and spontaneously hypertensive rats. 2. Intravenous (i.v.) physostigmine (50 microg kg-1) induced in both strains a long pressor response, accompanied by a bradycardia. This pressor response was larger in spontaneously hypertensive (+41+/-6 mmHg) than in Wistar-Kyoto (+25+/-2 mmHg) rats (P<0.05). 3. Pretreatment with atropine sulphate (0.4 mg kg-1 i.v.), completely abolished the physostigmine-induced pressor response in both normotensive and hypertensive rats. In both strains, the physostigmine pressor response was significantly reduced by the systemic administration of either an alpha1-adrenoceptor antagonist (prazosin, 1 mg kg-1) or a V1A-vasopressin receptor antagonist (AVPX, 20 microg kg-1). This physostigmine pressor effect was completely abolished in both strains when both antagonists were administered concomitantly. 4. In WKY rats, the pressor response to physostigmine (50 microg kg-1 i.v.) was inhibited in a dose-dependent manner by i. c.v. administration of atropine (ID50=3.70 nmoles), the M1 receptor antagonist pirenzepine (ID50=10.71 nmoles), the M2 receptor antagonist methoctramine (ID50=4.31 nmoles), the M3 receptor antagonist p-F-HHSiD (ID50=60.52 nmoles) and the M4 receptor antagonist tropicamide (ID50=214.20 nmoles). In the hypertensive strain, the ID50 were found to be significantly higher for atropine (7.34 nmoles), pirenzepine (21.60 nmoles) and p-F-HHSiD (139.50 nmoles) (P<0.05). 5. The present results indicate that physostigmine acts in normotensive and spontaneously hypertensive rats, through stimulation of both central M2 and M1 cholinoceptors to induce a rise in blood pressure mediated by an increase in plasma vasopressin and sympathetic outflow. Moreover, our results suggest that some modifications of the M1 receptor subtypes in terms of expression or affinity could be responsible for the hyper-responsiveness of the hypertensive strain to cholinomimetic agents.

Cholinergic mechanism and blood pressure regulation in the central nervous system

Cholinergic neurons in numerous brain regions have been implicated in blood pressure regulation. One of the most important brain regions where cholinergic neurons play a role in the pathogenesis of hypertension is the rostral ventrolateral medulla (RVL), an essential source of efferent sympathetic activity. Pharmacological and biochemical studies have revealed that acetylcholine release in the RVL is increased in experimental hypertension regardless of its etiology and that this enhanced release of acetylcholine leads to hypertension. The lateral parabrachial nucleus, another important hindbrain area involved in blood pressure regulation, is responsible for the enhanced release of acetylcholine in the RVL of hypertensive animals. Moreover, recent studies have demonstrated the involvement of the hypothalamic defence area, an area believed to be involved in the hypertension induced by chronic stress, in the release of acetylcholine in the RVL and also have demonstrated the existence of direct projections from the hypothalamic structures to the lateral parabrachial nucleus. More studies about mechanisms of the enhanced release of acetylcholine in the RVL of experimentally hypertensive animals will provide important information for central mechanisms of hypertension.

Evidence for involvement of the lateral parabrachial nucleus in mediation of cholinergic inputs to neurons in the rostral ventrolateral medulla of the rat

We examined whether sites in the lateral parabrachial nucleus (PBN) where L-glutamate produced increases in arterial pressure were involved in mediation of cholinergic inputs to neurons in the rostral ventrolateral medulla (RVLM). Male Wistar rats were anesthetized, paralyzed and artificially ventilated. Unilateral microinjection of L-glutamate into the lateral PBN produced a pressor response. Microinjection of the muscarinic receptor antagonist scopolamine into the unilateral RVLM inhibited the pressor response to L-glutamate injected ipsilaterally into the lateral PBN, whereas microinjection of the cholinesterase inhibitor physostigmine into the RVLM enhanced it. PBN microinjection of L-glutamate also enhanced the firing rate of RVLM sympathoexcitatory neurons and the enhancement of the firing rate was inhibited by scopolamine iontophoretically applied on neurons. PBN injection of L-glutamate produced a tetrodotoxin (TTX)-sensitive release of ACh in the RVLM. Unilateral microinjection of TTX into the lateral PBN inhibited the pressor response induced by RVLM microinjection of physostigmine. These results provide evidence that neurons in the pressor sites of the lateral PBN are involved in mediation of cholinergic inputs responsible for pressor responses in the RVLM.

Altered cholinergic mechanisms and blood pressure regulation in the rostral ventrolateral medulla of DOCA-salt hypertensive rats

We examined whether cholinergic transmission in the rostral ventrolateral medulla (RVLM) of deoxycorticosterone acetate-salt hypertensive rats (DHR) is enhanced and the enhancement is involved in the maintenance of hypertension in DHR, and whether cholineacetyltransferase (ChAT) activities and ChAT mRNA expression are enhanced in neurons intrinsic to the RVLM of DHR. Rats were anesthetized, paralyzed, and artificially ventilated. Unilateral microinjection of cholinergic agents into the RVLM produced a pressor response. The pressor response to physostigmine was greater in DHR than in control rats, whereas the response to carbachol was the same in both sets of rats. Bilateral microinjection of scopolamine into the RVLM produced a decrease in blood pressure. The depressor response was greater in DHR than in control rats. The number of ChAT-activity-detected neurons in the RVLM was greater in DHR than in control rats. The number of ChAT mRNA-expressing neurons in the RVLM was also clearly greater in DHR than in control rats. These results demonstrate that cholinergic transmission in the RVLM is enhanced in DHR, and this enhancement may play a role in the maintenance of hypertension in DHR. It is probable that enhanced activity of cholinergic neurons intrinsic to the RVLM is at least in part, responsible for the enhanced cholinergic transmission in the RVLM of DHR.

Baroreceptor activation causes release of acetylcholine in the rostral ventrolateral medulla of the rat

We examined whether baroreceptor activation causes a release of acetylcholine (ACh) in the rostral ventrolateral medulla (RVLM) of the rat, in order to investigate a possible connection between RVLM cholinergic systems and cardiovascular baroreflexes. Male Wistar rats were anesthetized, paralyzed and artificially ventilated. Either electrical stimulation of aortic nerve or baroreceptor activation by intravenous phenylephrine produced an increase of the release of ACh in the RVLM, whereas baroreceptor denervation and tetrodotoxin (TTX) microinfusion in the RVLM inhibited the increase in ACh release induced by phenylephrine. TTX injected in the caudal ventrolateral medulla (CVLM) inhibited the phenylephrine-induced increase of ACh release. The excitatory amino acid L-glutamate microinfused in the CVLM produced an release in ACh release in the RVLM. These results suggest that there is a connection between RVLM cholinergic systems and cardiovascular baroreflexes. It is probable that neurons in the CVLM are involved in mediating the release of ACh in the RVLM.

Central amino acids mediate cardiovascular response to angiotensin II in the rat

To elucidate the role of the rostral ventrolateral medulla (RVLM) in cardiovascular control through the release of central amino acid neurotransmitters, experiments were performed in Sprague-Dawley (normotensive) rats and spontaneously hypertensive rats (SHR) anesthetized with urethane by using microdialysis sampling from the RVLM for determination of amino acid neurotransmitters. The baseline release of the excitatory amino acid neurotransmitter, glutamate (GLU) from the RVLM in SHR was higher and those of the inhibitory amino acid neurotransmitters, glycine (GLY), taurine (TAU), and gamma-aminobutyric acid (GABA), were lower than in normotensive rats. Microinjection of angiotensin II (ANG II) into the RVLM caused a dose-dependent increase in mean arterial pressure (MAP) and heart rate (HR), accompanied by increased release of GLU in the RVLM. In contrast, microinjection of the ANG II type 1 receptor (AT1) antagonist CV 11974 into the RVLM reduced MAP and HR, accompanied by increased release of GLY, TAU and GABA. These changes in MAP and HR after administration of ANG II or AT1 antagonist were partially blocked by the use of the corresponding antagonist of each amino acid neurotransmitter. Furthermore, these effects were more prominently seen in SHR than in normotensive rats. These results suggest that the release of amino acid neurotransmitters mediate the cardiovascular effects of the angiotensin system in the RVLM, which may be involved in the generation of hypertension in SHR.