Synthesis, release and receptor binding of acetylcholine in the C1 area of the rostral ventrolateral medulla: contributions in regulating arterial pressure

Cholinergic activation of neurons in the medulla oblongata changes urinary bladder activity by plasma vasopressin release in female rats

The central control of the micturition is dependent on cortical areas and other ascending and descending pathways in the brain stem. The descendent pathways from the pons to the urinary bladder (UB) can be direct or indirect through medullary neurons (MN). Chemical stimulation with l-glutamate of MN known for their involvement in cardiovascular regulation evokes changes in pelvic nerves activities, which innervate the urinary bladder. Different neurotransmitters have been found in medullary areas; nevertheless, their involvement in UB control is few understood. We focused to investigate if cholinergic activation of neurons in the medulla oblongata changes the urinary bladder activity. Carbachol (cholinergic agonist) or atropine (cholinergic antagonist) was injected into the 4thV in anesthetized female Wistar rats and the intravesical pressure (IP), mean arterial pressure (MAP), heart rate (HR) and renal conductance (RC) were recorded for 30 min. Carbachol injection into the 4thV increased IP with peak response at 30 min after carbachol and yielded no changes in MAP, HR and RC. Atropine injection into the 4thV decreased IP and elicited no changes in MAP, HR and RC. Plasma vasopressin levels evaluated by ELISA kit assay increased after carbachol into the 4th V. Intravenous blockade of V1 receptors prior to carbachol into the 4thV abolished the increase in IP evoked by carbachol. Therefore, our findings suggest that cholinergic activation of neurons in the medulla oblongata by carbachol injections into the 4thV increases IP due to plasma vasopressin release, which acts in V1 receptors in the UB.

Cardiovascular responses to injections of angiotensin II or carbachol into the rostral ventrolateral medulla in rats with AV3V lesions

Injection of l-glutamate (GLU) into the rostral ventrolateral medulla (RVLM) produces sympathetically-mediated pressor responses that depend on the integrity of the tissue surrounding the anteroventral third ventricle (AV3V region). The injection of angiotensin II (ANG II) or the cholinergic agonist carbachol into the RVLM also produces pressor responses. In the present study, we investigated if the lesion of the AV3V region affects the pressor responses to ANG II or carbachol injected into the RVLM in unanesthetized rats. Male Holtzman rats with sham or electrolytic AV3V lesions and a stainless steel cannula implanted into the RVLM were used. The pressor responses to ANG II (200ng/100nl) injected into the RVLM were reduced by acute (1 day) (12±3 vs. sham lesions: 26±4mmHg) or chronic (15 days) AV3V lesions (12±5 vs. sham lesions: 27±4mmHg), whereas acute or chronic AV3V lesions did not affect the pressor responses to carbachol (1nmol/100nl) injected into the RVLM. The present results suggest that the AV3V region modulates the excitability of the RVLM neurons involved with the pressor response produced by the activation of angiotensinergic mechanisms in this area.

Central mechanisms involved in pilocarpine-induced pressor response

Pilocarpine (cholinergic muscarinic agonist) injected peripherally may act centrally to produce pressor responses; in the present study, using c-fos immunoreactive expression, we investigated the forebrain and brainstem areas activated by pressor doses of intravenous (i.v.) pilocarpine. In addition, the importance of vasopressin secretion and/or sympathetic activation and the effects of lesions in the anteroventral third ventricle (AV3V) region in awake rats were also investigated. In male Holtzman rats, pilocarpine (0.04 to 4μmol/kg b.w.) i.v. induced transitory hypotension followed by long lasting hypertension. Sympathetic blockade with prazosin (1mg/kg b.w.) i.v. or AV3V lesions (1 day) almost abolished the pressor response to i.v. pilocarpine (2μmol/kg b.w.), whereas the vasopressin antagonist (10μg/kg b.w.) i.v. reduced the response to pilocarpine. Pilocarpine (2 and 4μmol/kg b.w.) i.v. increased the number of c-fos immunoreactive cells in the subfornical organ, paraventricular and supraoptic nuclei of the hypothalamus, organ vasculosum of the lamina terminalis, median preoptic nucleus, nucleus of the solitary tract and caudal and rostral ventrolateral medulla. These data suggest that i.v. pilocarpine activates specific forebrain and brainstem mechanisms increasing sympathetic activity and vasopressin secretion to induce pressor response.

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.

Central cholinergic blockade reduces the pressor response to L-glutamate into the rostral ventrolateral medullary pressor area

Injections of the excitatory amino acid l-glutamate (l-glu) into the rostral ventrolateral medulla (RVLM) directly activate the sympathetic nervous system and increase mean arterial pressure (MAP). A previous study showed that lesions of the anteroventral third ventricle region in the forebrain reduced the pressor response to l-glu into the RVLM. In the present study we investigated the effects produced by injections of atropine (cholinergic antagonist) into the lateral ventricle (LV) on the pressor responses produced by l-glu into the RVLM. Male Holtzman rats (280-320 g, n=5 to 12/group) with stainless steel cannulas implanted into the RVLM, LV or 4th ventricle (4th V) were used. MAP and heart rate (HR) were recorded in unanesthetized rats. After saline into the LV, injections of l-glu (5 nmol/100 nl) into the RVLM increased MAP (51+/-4 mm Hg) without changes in HR. Atropine (4 nmol/1 microl) injected into the LV reduced the pressor responses to l-glu into the RVLM (36+/-5 mm Hg). However, atropine at the same dose into the 4th V or directly into the RVLM did not modify the pressor responses to l-glu into the RVLM (45+/-2 and 49+/-4 mm Hg, respectively, vs. control: 50+/-4 mm Hg). Central cholinergic blockade did not affect baro and chemoreflex nor the basal MAP and HR. The results suggest that cholinergic mechanisms probably from forebrain facilitate or modulate the pressor responses to l-glu into the RVLM. The mechanism is activated by acetylcholine in the forebrain, however, the neurotransmitter released in the RVLM to facilitate the effects of glutamate is not acetylcholine.

Muscarinic receptors and alpha2-adrenoceptors interact to modulate the respiratory rhythm in mouse neonates

The respiratory rhythm generator (RRG) is modulated by several endogenous substances, including acetylcholine (ACh) and noradrenaline (NA) that interact in several modulatory processes. To know whether ACh and NA interacted to modulate the RRG activity, we used medullary "en bloc" and slice preparations from neonatal mice where the RRG has been shown to receive a facilitatory modulation from A1/C1 neurons, via a continuous release of endogenous NA and activation of alpha2 adrenoceptors. Applying ACh at 25 microM activated the RRG but ACh had no effects at 50 microM. Applying the ACh receptor agonists nicotine and muscarine facilitated and depressed the RRG, respectively. After yohimbine pre-treatment that blocked the alpha2 facilitation, the nicotinic facilitation was not altered, the muscarinic depression was reversed and ACh 50 microM significantly facilitated the RRG. After L-tyrosine pre-treatment that potentiated the alpha2 facilitation, the muscarinic depression was enhanced. Thus, ACh regulates the RRG activity via nicotinic and muscarinic receptors, the muscarinic receptors interacting with alpha2 adrenoceptors.

[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.

Impaired cardiac and sympathetic autonomic control in rats differing in acetylcholine receptor sensitivity

Acetylcholine receptors (AChR) are important in premotor and efferent control of autonomic function; however, the extent to which cardiovascular function is affected by genetic variations in AChR sensitivity is unknown. We assessed heart rate variability (HRV) and baroreflex sensitivity (BRS) in rats bred for resistance (FRL) or sensitivity (FSL) to cholinergic agents compared with Sprague-Dawley rats (SD), confirmed by using hypothermic responses evoked by the muscarinic agonist oxotremorine (0.2 mg/kg ip) ( n ≥ 9 rats/group). Arterial pressure, ECG, and splanchnic sympathetic (SNA) and phrenic (PNA) nerve activities were acquired under anesthesia (urethane 1.3 g/kg ip). HRV was assessed in time and frequency domains from short-term R-R interval data, and spontaneous heart rate BRS was obtained by using a sequence method at rest and after administration of atropine methylnitrate (mATR, 2 mg/kg iv). Heart rate and SNA baroreflex gains were assessed by using conventional pharmacological methods. FRL and FSL were normotensive but displayed elevated heart rates, reduced HRV and HF power, and spontaneous BRS compared with SD. mATR had no effect on these parameters in FRL or FSL, indicating reduced cardiovagal tone. FSL exhibited reduced PNA frequency, longer baroreflex latency, and reduced baroreflex gain of heart rate and SNA compared with FRL and SD, indicating in FSL dual impairment of cardiac and circulatory baroreflexes. These findings show that AChR resistance results in reduced cardiac muscarinic receptor function leading to cardiovagal insufficiency. In contrast, AChR sensitivity results in autonomic and respiratory abnormalities arising from alterations in central muscarinic and or other neurotransmitter receptors.

Imidazoleacetic acid-ribotide: an endogenous ligand that stimulates imidazol(in)e receptors

We identified the previously unknown structures of ribosylated imidazoleacetic acids in rat, bovine, and human tissues to be imidazole-4-acetic acid-ribotide (IAA-RP) and its metabolite, imidazole-4-acetic acid-riboside. We also found that IAA-RP has physicochemical properties similar to those of an unidentified substance(s) extracted from mammalian tissues that interacts with imidazol(in)e receptors (I-Rs). ["Imidazoline," by consensus (International Union of Pharmacology), includes imidazole, imidazoline, and related compounds. We demonstrate that the imidazole IAA-RP acts at I-Rs, and because few (if any) imidazolines exist in vivo, we have adopted the term "imidazol(in)e-Rs."] The latter regulate multiple functions in the CNS and periphery. We now show that IAA-RP (i) is present in brain and tissue extracts that exhibit I-R activity; (ii) is present in neurons of brainstem areas, including the rostroventrolateral medulla, a region where drugs active at I-Rs are known to modulate blood pressure; (iii) is present within synaptosome-enriched fractions of brain where its release is Ca(2+)-dependent, consistent with transmitter function; (iv) produces I-R-linked effects in vitro (e.g., arachidonic acid and insulin release) that are blocked by relevant antagonists; and (v) produces hypertension when microinjected into the rostroventrolateral medulla. Our data also suggest that IAA-RP may interact with a novel imidazol(in)e-like receptor at this site. We propose that IAA-RP is a neuroregulator acting via I-Rs.

Spontaneously hypertensive rats exhibit altered cardiovascular and neuronal responses to muscle contraction

We examined the cardiovascular and ventrolateral medullary neuronal responses to muscle contraction in the spontaneously hypertensive rat (SHR) and normotensive Wistar-Kyoto rat (WKY) control. Cardiovascular, respiratory and ventrolateral medullary neuronal responses to muscle contraction evoked by tibial nerve stimulation were recorded. SHRs exhibited significantly larger drops in arterial pressure compared to WKYs in response to muscle contraction (P < 0.05). Basal ventrolateral medulla neuronal discharge rates were similar between the SHR and the WKY groups. A majority of neurons recorded responded to muscle contraction in both the WKY (77 %; n = 53) and the SHR groups (68 %; n = 62). There was no difference in the percentage of neurons that responded with an increase (approximately 60 %) or decrease (approximately 40 %) in firing rate between hypertensive and normotensive rats. Pulse wave-triggered averaging techniques showed that most neurons that responded to muscle contraction also possessed a basal firing rhythm temporally related to the cardiac cycle (85 % in WKYs, 83 % in SHRs). However, decreases in neuronal firing rates in response to muscle contraction were significantly greater in SHRs than WKYs. Therefore, we conclude that muscle contraction unmasks a hyperexcitability of neurons in the ventrolateral medulla of SHRs that parallels the heightened blood pressure responses.

Excitatory inputs to the RVLM in the context of the baroreceptor reflex

The central neural circuit mediating baroreceptor control of sympathetic vasomotor outflow involves an excitatory projection from arterial baroreceptors to nucleus tractus solitarius, an excitatory projection from nucleus tractus solitarius to the caudal ventrolateral medulla, an inhibitory projection from the caudal ventrolateral medulla to the rostral ventrolateral medulla (RVLM), and an excitatory projection from the RVLM to sympathetic preganglionic neurons in the spinal cord. For this circuit to be operational, the relevant neurons in the RVLM must be tonically active. Indeed, numerous studies have demonstrated that RVLM vasomotor neurons are tonically active; however, little is known regarding the nature of the tonic excitatory drive to these neurons. We present a model in which RVLM vasomotor neurons are tonically excited by inputs to the RVLM that can be blocked by the excitatory amino acid receptor antagonist, kynurenic acid, as well as an input from the caudal ventrolateral medulla that is not sensitive to kynurenic acid.

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.

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.

Prevention of morphine-induced muscarinic (M2) receptor adaptation suppresses the expression of withdrawal symptoms

Treatment of opiate addiction is generally directed at the suppression of withdrawal symptoms through maintenance of the 'addicted' state with methadone. Yet relatively little is known regarding the neural substrates that contribute to, and maintain the prolonged state of withdrawal experienced by addicts. Opiates can profoundly alter the dynamics of brain and peripheral cholinergic systems, and central administration of anticholinergic drugs in dependent rats has been shown to decrease the expression of precipitated withdrawal symptoms. The purpose of this study was to determine whether the adaptive changes to M2 muscarinic receptors in autonomic centers are linked to the expression of withdrawal phenomena. During the peak period of withdrawal, there was a significant increase in both the expression of M2 muscarinic receptors and its corresponding mRNA within the rostral ventrolateral medulla, a primary vasomotor region. That most of these changes in receptor expression were adaptive in nature was suggested by the fact that when the acetylcholinesterase inhibitor DFP was co-administered with morphine, both the increased mRNA expression and the appearance of withdrawal symptoms were inhibited. Thus, interference with morphine-induced M2 muscarinic receptor adaptation in critical brain regions was correlated with a reduction in the development of physical dependence.

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.

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.

Autoradiographic comparison of muscarinic M1 and M2 binding sites in the CNS of spontaneously hypertensive and normotensive rats

Spontaneously hypertensive rats (SHR) respond with exaggerated pressor responses of central origin in response to pharmacologic stimulation of brain muscarinic receptors when compared with those to normotensive Wistar Kyoto (WKY) rats. At least part of the enhanced response to central muscarinic stimulation may be due to alterations in the expression of one or more of the five subtypes of muscarinic receptors. SHR are also known to exhibit regional alterations in the levels of mRNA encoding the M1, M2 and M4 receptors. In this study, we estimated the number of specific muscarinic receptor binding sites in 12-week-old SHR and WKY by measuring the binding of M1- and M2-selective ligands. Using standard autoradiographic techniques, coronal sections obtained from 12-week-old SHR and WKY were incubated with [3H]pirenzepine or [3H]AFDX 384 to label M1 and M2 receptors, respectively. Although both strains exhibited similar distribution patterns for both binding sites, sections derived from SHR expressed a significant increase in the number of [3H]pirenzepine binding sites compared to normotensive WKY in caudate putamen, CA3 region of the hippocampus, cingulate cortex, substantia nigra, posterior hypothalamic area and tuberomammillary nucleus. An increased number of [3H]AFDX 384 binding sites in SHR were observed in the olfactory tubercle, nucleus accumbens, basolateral amygdaloid nucleus, rostroventrolateral medulla and nucleus paragigantocellularis. Decreases in the number of [3H]AFDX 384 binding sites in SHR were also observed in the parietal cortex, medial geniculate, and lateral hypothalamic area. Statistically significant site-selective differences in binding densities between strains ranged from 4.0% to 35.5% of WKY means. These alterations in the expression of M1 and M2 binding sites in cardiovascular regions may contribute to the strain's hyper-responsiveness to cholinergic drugs and possibly to the appearance of other autonomic or behavioral phenotypes exhibited by SHR, including the hypertensive state itself.

Role of ouabain-like compound in the rostral ventrolateral medulla in rats

To determine whether ouabain-like compound (OLC) exerts modulatory influences on the activity of vasomotor neurons in the rostral ventrolateral medulla (RVLM), we examined the effects of microinjecting ouabain, digoxin-specific antibody Fab fragments, and mAb against ouabain on the rat RVLM. Microinjection of ouabain into the unilateral RVLM of anesthetized normotensive rats elicited dose-dependent increases in mean arterial pressure (MAP) and renal sympathetic nerve activity (RSNA). The pressor and sympathoexcitatory effects of ouabain in the RVLM were reversed by microinjections of an M2 muscarinic antagonist, gallamine, or digoxin-specific antibody Fab fragments. Furthermore, a prior microinjection in the RVLM of gallamine, digoxinspecific antibody Fab fragments, or kainic acid or intravenous injection of hexamethonium all prevented the pressor and sympathoexcitatory effects induced by a subsequent microinjection of ouabain. Microinjections of either digoxinspecific antibody Fab fragments or gallamine per se significantly decreased baseline MAP and RSNA. Injection of digoxin-specific antibody Fab fragments attenuated the effects of a subsequent injection of gallamine. Microinjection of mAb against ouabain, but not nonspecific IgG, also significantly decreased baseline MAP and RSNA. These results suggest that OLC in the RVLM contributes to the tonic activity of vasomotor neurons in anesthetized normotensive rats, and the action of OLC in the RVLM is at least partly mediated by M2 muscarinic mechanisms.

Increased expression of M2 muscarinic receptor mRNA and binding sites in the rostral ventrolateral medulla of spontaneously hypertensive rats

A significant body of evidence suggests that the development and maintenance of elevated blood pressure in the spontaneously hypertensive rat (SHR), a genetic model for essential hypertension, is due at least partly to a central hyper-cholinergic state. For example, this strain responds with an exaggerated pressor response to pharmacological stimulation of central muscarinic receptors in certain brain regions compared to normotensive Wistar Kyoto rats (WKY). At least part of the enhanced response to central muscarinic receptor stimulation in SHR is due to the altered expression of post-synaptic receptors. In the present study, the reverse transcriptase-polymerase chain reaction and autoradiographic techniques were used to estimate the relative levels of mRNA and density of receptor binding sites for the five subtypes of muscarinic receptors within the rostral ventrolateral medulla (RVL) of SHR and WKY. Adult (12-week-old) SHR exhibited an increase in the levels of both M2 muscarinic mRNA, and M2 receptor binding sites in RVL compared to age-matched normotensive WKY. Similarly, 4-week-old pre-hypertensive SHR exhibited increased levels of M2 mRNA in whole medulla oblongata, and an increase in the number of binding sites for M2 receptors in the RVL. Since the RVL is known to integrate tonic cholinergic sympathoexcitatory input, these results suggest that the increased expression of M2 muscarinic receptors in this region represents one neurochemical correlate for the maintenance of excessive central efferent sympathetic nervous activity in the SHR. Since the neurochemical change precedes the development of hypertension, the altered medullary M2 receptor expression may play a role as an initiating or predisposing factor for the development of hypertension in SHR.

Influence of exercise and ethanol on cholinesterase activity and lipid peroxidation in blood and brain regions of rat

1. This study elucidates the interaction of acute exercise and single ethanol intake on cholinergic enzyme and its relationship to lipid peroxidation in the blood and brain regions of the rat. 2. Butyrylcholinesterase (BuChE) in plasma and acetylcholinesterase (AChE) in brain regions as well as lipid peroxidation (MDA) were assayed in 1) sedentary control rats; 2) after acute exercise (100% VO2max); 3) ethanol 20% (1.6 gm/kg, p.o.); 4) exercise and then ethanol 20% (1.6 gm/kg, p.o.). 3. Acute exercise significantly increased BuChE activity (155% of control) in plasma and decreased AChE activity (60% of control) in the corpus striatum with a significant increase in the striatal MDA level (254% of control). Ethanol significantly decreased AChE activity only in striatum (86% of control) with a significant increase in striatal MDA level (132% of control). 4. The combination of exercise and ethanol 20% (1.6 gm/kg, p.o.) significantly increased BuChE activity (123% of control) in plasma, and decreased AChE activity (76% of control) in striatum with significant increase in striatal MDA level (147% of control). 5. Acute exercise, single ethanol 20% (1.6 gm/kg, p.o.) intake and the combination selectively inhibited striatal AChE, and the inhibition was correlated with increased lipid peroxidation indicating perturbation of motor function. The combination reduced the peripheral stress response caused by exhaustive exercise.

The central respiratory effects of acetylcholine vary with CSF pH

Hydrogen ion concentration [H+] centrally is a major determinant of ventilation. Its action involves central cholinergic mechanisms. The point(s) where increased [H+] induces its changes in the cholinergic system is unclear. If H+ acts presynaptically by increasing endogenous ACh synthesis and release, its effect should be absent when ACh is supplied exogenously. If H+ acts postsynaptically by changing ACh degradation or ACh receptor sensitivity, its effect should persist in the presence of exogenous ACh. We perfused the brain ventricular system in spontaneously breathing anesthetized dogs with progressively higher concentrations of ACh (0-52.8 mM) in cerebrospinal fluid (CSF) at pH 7.4 and CSF pH 7.1. Increasing concentrations of ACh increased ventilation > 4-fold in a linear manner in the presence of non-acidic and acidic CSF. With acidic CSF the ACh ventilatory response line was shifted to a higher y-intercept, resulting in a higher ventilation at any [ACh]. These findings are consistent with the hypothesis that central acidosis augments ventilation by postsynaptic cholinergic events.

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

We examined whether reticulospinal sympathoexcitatory neurons in the rostral ventrolateral medulla (RVLM) have muscarinic receptors and ACh inputs, and whether these cholinergic mechanisms on RVLM neurons are involved in the pressor response induced by peripheral administration of physostigmine. Microiontophoretic application of ACh and carbachol enhanced the firing rate of RVLM sympathoexcitatory neurons and the enhancement of RVLM neurons by these cholinoceptor agonists was abolished by the nonselective muscarinic receptor antagonist scopolamine and/or by the M2 muscarinic receptor antagonist methoctramine. Physostigmine and the ACh releaser 3,4-diaminopyridine also enhanced the firing rate of RVLM neurons. Intravenous administration of physostigmine enhanced RVLM sympathoexcitatory neuronal activity and the physostigmine-induced response was reversed by iontophoretic application of scopolamine onto the neurons. These results are consistent with the hypothesis that M2 muscarinic receptors responsible for blood pressure regulation are present on RVLM sympathoexcitatory neurons and these receptors receive ACh inputs. Physostigmine injected systemically may exert a portion of its hypertensive effect through a direct enhancement of cholinergic mechanisms on RVLM sympathoexcitatory neurons.

Enhanced release of acetylcholine in the rostral ventrolateral medulla of spontaneously hypertensive rats

We examined whether the altered rostral ventrolateral medulla (RVLM) cholinergic function in spontaneously hypertensive rats (SHR) results from enhanced presynaptic cholinergic tone. Male 12- to 16-week-old SHR and age-matched Wistar Kyoto rats (WKY) 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 SHR than that of WKY whereas the response to ACh and carbachol was the same in WKY and SHR. Bilateral microinjection of scopolamine produced a decrease in blood pressure. The depressor response was greater in SHR than that of WKY. When a microdialysis probe was placed in the RVLM, ACh release in the RVLM was greater in SHR than that of WKY. Choline acetyltransferase (CAT) activity was increased only in the rostro-ventral part of the medulla, which contained the RVLM, but not in other parts of the medulla oblongata. Physostigmine (0.5 mg/kg, i.p.)-induced increases in ACh content were also enhanced only in the rostro-ventral part of the medulla. These results provide direct evidence that ACh release in the RVLM is enhanced in SHR. It appears that the enhanced cholinergic activity in the RVLM of SHR results from an increase in cholinergic impulse flow in the RVLM of SHR. This abnormality may play a role in the maintenance of hypertension in SHR.

Cardiovascular regulation by cholinergic mechanisms in rostral ventrolateral medulla of spontaneously hypertensive rats

This study aimed to demonstrate the role of acetylcholine receptors in the rostral ventrolateral medulla (RVL) in the central regulation of the cardiovascular system in normotensive Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR). The effects of cholinergic drugs, microinjected into the rostral ventrolateral medullary vasopressor area, on blood pressure and heart rate in anesthetized and artificially ventilated rats were investigated. Unilateral microinjection of carbachol (1 nmol/site), physostigmine (300 pmol/site) or 3,4-diaminopyridine (500 pmol/site) into the RVL elicited a pressor and tachycardiac response, of which only the pressor response was significantly greater in SHR than in WKY. Bilateral microinjection of atropine (1 nmol/site) caused a depressor and bradycardiac response. The depressor response produced by atropine injected in the RVL was also significantly greater in SHR than in WKY. These results suggest that there are tonic cholinergic mechanisms in the RVL of the rats, which exert an excitatory cardiovascular action, and that the enhanced responsiveness to acetylcholine receptor stimulation in the RVL may contribute to the sustained elevation of blood pressure in the SHR.

Effect of cholinesterase inhibitor and exercise on choline acetyltransferase and acetylcholinesterase activities in rat brain regions

This study sought to determine whether the choline acetyltransferase (ChAT) and acetylcholinesterase (AChE) enzymes in the brain were affected in a regionally selective manner by chemical and physical stressors: 1) subacute administration of physostigmine (Phy); 2) exercise; and 3) the combination of these two stressors. ChAT and AChE activities in corpus striatum were significantly decreased due to Phy as well as Phy + exercise. This suggests that corpus striatum is affected by chemical stressors but more so by the combination of chemical and physical stressors. The brainstem is the only region which showed inhibition of ChAT activity due to exercise. Subacute Phy also inhibited brainstem ChAT activity. The hippocampus showed significant decrease in ChAT activity due to Phy + exercise but not due to Phy alone. These results suggest that the brain regions involved with control of motor, autonomic and cognitive functions were affected by subacute Phy and exercise. These data are consistent with the hypothesis that the responsiveness of these brain regions to different stressors is a function of the level of ongoing cholinergic activity and that elevations in ACh levels due to AChE inhibition may have long-term effects on the regulation of ChAT and AChE activities through a negative feedback mechanism.

Evidence for clonidine presynaptically modulating amino acid release in the rostral ventral medulla: role in hypertension

Reports suggested that the predominant site of action for the antihypertensive effects of clonidine is the rostral ventrolateral medulla (RVL), the presumed tonic vasomotor center. This study examined whether clonidine directly interacts with nerve terminal alpha 2-adrenergic receptors in the RVL to inhibit the release of sympathoexcitatory transmitters like glutamate (Glu) and aspartate (Asp), and/or facilitate the release of sympathoinhibitory transmitters like gamma-aminobutyric acid (GABA). Release of GABA and Glu was measured from synaptosomes prepared from the rostral ventral medulla of spontaneously hypertensive rats (SHR), a genetic model of hypertension, and normotensive Wistar-Kyoto rats (WKY). Quantification of neurotransmitter release was performed by high-performance liquid chromatography. Depolarization with 35 mM K+ significantly increased by 58-110% the release of GABA, Glu and Asp; however, no strain differences were observed. In contrast, spontaneous release of GABA and Asp was significantly lower in SHR than that of WKY (-36 and -41%, respectively); this effect was not observed for Glu. Clonidine (1 and 10 microM) enhanced the spontaneous release of GABA (+44%), Asp (+50%) and Glu (+70%) in SHR, but not WKY; this effect was prevented by yohimbine (1 microM). These data, together with previous findings, support the presence of facilitory alpha 2-adrenergic receptors on nerve terminals of GABAergic, glutamatergic and aspartatergic neurons in the rostral ventral medulla. These findings also suggest the existence of another inhibitory transmitter that may mediate the actions of clonidine to decrease sympathetic outflow from the RVL.