Control of cardiac rate, contractility, and atrioventricular conduction by medullary raphe neurons in anesthetized rats

Toward accurate cerebral blood flow estimation in mice after accounting for anesthesia

Purpose: To improve the accuracy of cerebral blood flow (CBF) measurement in mice by accounting for the anesthesia effects. Methods: The dependence of CBF on anesthesia dose and time was investigated by simultaneously measuring respiration rate (RR) and heart rate (HR) under four different anesthetic regimens. Quantitative CBF was measured by a phase-contrast (PC) MRI technique. RR was evaluated with a mouse monitoring system (MouseOX) while HR was determined using an ultrashort-TE MRI sequence. CBF, RR, and HR were recorded dynamically with a temporal resolution of 1 min in a total of 19 mice. Linear regression models were used to investigate the relationships among CBF, anesthesia dose, RR, and HR. Results: CBF, RR, and HR all showed a significant dependence on anesthesia dose (p < 0.0001). However, the dose in itself was insufficient to account for the variations in physiological parameters, in that they showed a time-dependent change even for a constant dose. RR and HR together can explain 52.6% of the variations in CBF measurements, which is greater than the amount of variance explained by anesthesia dose (32.4%). Based on the multi-parametric regression results, a model was proposed to correct the anesthesia effects in mouse CBF measurements, specificallyC B F c o r r e c t e d = C B F + 0.58 R R - 0.41 H R - 32.66 D o s e . We also reported awake-state CBF in mice to be 142.0 ± 8.8 mL/100 g/min, which is consistent with the model-predicted value. Conclusion: The accuracy of CBF measurement in mice can be improved by using a correction model that accounts for respiration rate, heart rate, and anesthesia dose.

Atrioventricular Conduction in Mesial Temporal Lobe Seizures

Purpose: Asymmetric cerebral representation of autonomic function could help to stratify cardiac complications in people with epilepsy, as some seizures are associated with potentially deleterious arrhythmias including bradycardia and atrioventricular (AV) conduction block. We investigated seizure-related changes in AV conduction and ascertained whether these alterations depend on the hemisphere in mesial temporal lobe epilepsy (mTLE).

Methods: EEG and ECG data of people with pharmacoresistant mTLE undergoing pre-surgical video-EEG telemetry with seizures independently arising from both hippocampi, as determined by intracranial depths electrodes were reviewed. RR and PR intervals were measured using one-lead ECG. Statistics were done with paired student's t-tests and linear regression analysis. Data are given as mean ± SD.

Results: Fifty-six seizures of 14 patients (5 men, age 34.7 ± 9.8 years) were included (2 seizures per hemisphere and patient). There were no differences of absolute PR intervals and HR before and during unilateral ictal activity between left- and right-sided hippocampal seizures. Peri-ictal modulation of AV conduction, however, appeared greater with left-sided seizures, as the slope of the PR/HR correlations was significantly steeper with seizures originating in the left hippocampus. PR lengthening >200 ms or full block did not occur in any seizure.

Conclusions: Our data show that on average, PR intervals shortens with mesial temporal lobe seizures with more prominent effects in seizures with left-sided onset, supporting the notion of lateralized cerebral control of cardiac function. The clinical relevance of this subtle finding is unclear but may indicate a lateralized susceptibility to seizure-related AV node dysfunction in mTLE.

Ventromedial medullary pathway mediating cardiac responses evoked from periaqueductal gray

Periaqueductal gray (PAG) is a midbrain region that projects to areas controlling behavioral and autonomic outputs and is involved in the behavioral and physiological components of defense reactions. Since Raphe Pallidus (RPa) is a medial medullary region comprising sympathetic premotor neurons governing heart function, it is worth considering the PAG-RPa path. We assessed: i) whether PAG projects to RPa; ii) the amplitude of cardiac responses evoked from PAG; iii) whether cardiovascular responses evoked from PAG rely on RPa. Experiments conducted in Wistar rats (±300 g) were approved by Ethics Committee CEUA-UFG (092/18). Firstly, (n = 3), monosynaptic retrograde tracer Retrobeads was injected into RPa; PAG slices were analyzed. Other two groups (n = 6 each) were anesthetized with urethane (1.4 g/kg) and chloralose (120 mg/kg) and underwent craniotomy, tracheostomy, catheterization of femoral artery and vein and of cardiac left ventricle. In one group, we injected the GABA_A receptor antagonist, bicuculline methiodide (BMI - 40 pmol/100 nL) into lateral/dorsolateral PAG. Another group was injected (100 nL) with the GABA_A receptor agonist muscimol (20 mM) into RPa, 20 min before BMI into PAG. The results were: i) retrogradely labelled neurons were found in PAG; ii) PAG activation by BMI caused positive chronotropism and inotropism, which were accompanied by afterload increases; iii) RPa inhibition with Muscimol reduced heart rate, arterial and ventricular pressures; iv) the subsequent PAG activation still increased arterial pressure, cardiac chronotropy and inotropy, but these responses were significantly attenuated. In conclusion, PAG activation increases cardiac chronotropy and inotropy, and these responses seem to rely on a direct pathway reaching ventromedial medullary RPa neurons.

elPBN neurons regulate rVLM activity through elPBN-rVLM projections during activation of cardiac sympathetic afferent nerves

The external lateral parabrachial nucleus (elPBN) within the pons and rostral ventrolateral medulla (rVLM) contributes to central processing of excitatory cardiovascular reflexes during stimulation of cardiac sympathetic afferent nerves (CSAN). However, the importance of elPBN cardiovascular neurons in regulation of rVLM activity during CSAN activation remains unclear. We hypothesized that CSAN stimulation excites the elPBN cardiovascular neurons and, in turn, increases rVLM activity through elPBN-rVLM projections. Compared with controls, in rats subjected to microinjection of retrograde tracer into the rVLM, the numbers of elPBN neurons double-labeled with c-Fos (an immediate early gene) and the tracer were increased after CSAN stimulation (P < 0.05). The majority of these elPBN neurons contain vesicular glutamate transporter 3. In cats, epicardial bradykinin and electrical stimulation of CSAN increased the activity of elPBN cardiovascular neurons, which was attenuated (n = 6, P < 0.05) after blockade of glutamate receptors with iontophoresis of kynurenic acid (Kyn, 25 mM). In separate cats, microinjection of Kyn (1.25 nmol/50 nl) into the elPBN reduced rVLM activity evoked by both bradykinin and electrical stimulation (n = 5, P < 0.05). Excitation of the elPBN with microinjection of dl-homocysteic acid (2 nmol/50 nl) significantly increased basal and CSAN-evoked rVLM activity. However, the enhanced rVLM activity induced by dl-homocysteic acid injected into the elPBN was reversed following iontophoresis of Kyn into the rVLM (n = 7, P < 0.05). These data suggest that cardiac sympathetic afferent stimulation activates cardiovascular neurons in the elPBN and rVLM sequentially through a monosynaptic (glutamatergic) excitatory elPBN-rVLM pathway.

The rostral ventromedial medulla control of cutaneous vasomotion of paws and tail in the rat: implication for pain studies

Thermal neutrality in rodents is achieved by large cyclic variations of the sympathetic drive of the vasomotion of the tail and paws, the most widely used target organs in current acute or chronic animal models of pain. Given the pivotal functional role of rostral ventromedial medulla (RVM) in nociception and rostral medullary raphe (rMR) in thermoregulation, two largely overlapping brain regions, we aimed at circumscribing the brainstem regions that are the source of premotor afferents to sympathetic preganglionic neurons that control the vasomotor tone of the tail and hind paws. A thermometric infrared camera recorded indirectly the vasomotor tone of the tail and hind paws. During the control period, the rat was maintained in vasoconstriction by preserving a stable, homogeneous, and constant surrounding temperature, slightly below the core temperature. The functional blockade of the RVM/rMR by the GABAA receptor agonist muscimol (0.5 nmol, 50 nl) elicited an extensive increase of the temperature of the paws and tail, associated with a slight decrease of blood pressure and heart rate. Both the increased heat loss through vasodilatation and the decrease heart-induced heat production elicited a remarkable reduction of the central temperature. The effective zones were circumscribed to the parts of the RVM/rMR facing the facial nucleus. They match very exactly the brain regions often described as specifically devoted to the control of nociception. Our data support and urge on the highest cautiousness regarding the interpretation of results aimed at studying the effects of any pharmacological manipulations of RVM/rMR with the usual tests of pain.

Asymmetry in the control of cardiac performance by dorsomedial hypothalamus

Dorsomedial hypothalamus (DMH) plays a key role in integrating cardiovascular responses to stress. We have recently reported greater heart rate responses following disinhibition of the right side of the DMH (R-DMH) in anesthetized rats and greater suppression of stress-induced tachycardia following inhibition of the R-DMH in conscious rats [both compared with similar intervention in the left DMH (L-DMH)], suggesting existence of right/left side asymmetry in controlling cardiac chronotropic responses by the DMH. The aim of the present study was to determine whether similar asymmetry is present for controlling cardiac contractility. In anesthetized rats, microinjections of the GABAA antagonist bicuculline methiodide (BMI; 40 pmol/100 nl) into the DMH-evoked increases in heart rate (HR), left ventricular pressure (LVP), myocardial contractility (LVdP/dt), arterial pressure, and respiratory rate. DMH disinhibition also precipitated multiple ventricular and supraventricular ectopic beats. DMH-induced increases in HR, LVP, LVdP/dt, and in the number of ectopic beats dependent on the side of stimulation, with R-DMH provoking larger responses. In contrast, pressor and respiratory responses did not depend on the side of stimulation. Newly described DMH-induced inotropic responses were rate-, preload- and (largely) afterload-independent; they were mediated by sympathetic cardiac pathway, as revealed by their sensitivity to β-adrenergic blockade. We conclude that recruitment of DMH neurons causes sympathetically mediated positive chronotropic and inotropic effects, and that there is an asymmetry, at the level of the DMH, in the potency to elicit these effects, with R-DMH > L-DMH.

Glucoprivation in the ventrolateral medulla decreases brown adipose tissue sympathetic nerve activity by decreasing the activity of neurons in raphe pallidus

In urethane/α-chloralose anesthetized rats, cold exposure increased brown adipose tissue sympathetic nerve activity (BAT SNA: +699 ± 104% control). Intravenous administration of 2-deoxy-D-glucose (2-DG; 200 mg·ml(-1)·kg(-1)) reversed the cold-evoked activation of BAT SNA (nadir: 139 ± 36% of control) and decreased BAT temperature (-1.1 ± 0.2°C), expired CO(2) (-0.4 ± 0.1%), and core temperature (-0.5 ± 0.0). Similarly, unilateral nanoinjection of the glucoprivic agent 5-thioglucose (5-TG; 12 μg/100 nl) in the ventrolateral medulla (VLM) completely reversed the cold-evoked increase in BAT SNA (nadir: 104 ± 7% of control), and decreased T(BAT) (-1.4 ± 0.3°C), expired CO(2) (-0.2 ± 0.0%), and heart rate (-35 ± 10 beats/min). The percentage of rostral raphé pallidus (RPa)-projecting neurons in the dorsal hypothalamic area/dorsomedial hypothalamus that expressed Fos in response to cold exposure (ambient temperature: 4-10°C) did not differ between saline (28 ± 6%) and 2-DG (30 ± 5%) pretreated rats, whereas the percentage of spinally projecting neurons in the RPa/raphé magnus that expressed Fos in response to cold exposure was lower in 2-DG- compared with saline-pretreated rats (22 ± 6% vs. 42 ± 5%, respectively). The increases in BAT SNA evoked by nanoinjection of bicuculline in the RPa or by transection of the neuraxis at the pontomedullary border were resistant to inhibition by glucoprivation. These results suggest that neurons within the VLM play a role in the glucoprivic inhibition of BAT SNA and metabolism, that this inhibition requires neural structures rostral to the pontomedullary border, and that this inhibition is mediated by a GABAergic input to the RPa.

Control of cardiac contractility in the rat working heart-brainstem preparation

A great deal of knowledge exists regarding neural control of myocardial function in the rat. Most of the studies addressing this issue were conducted either under general anaesthesia or in isolated hearts in vitro. Our principal aim was to provide a detailed quantitative description of mechanisms controlling cardiac contractility in the rat, in an anaesthetic-free preparation with a preserved functional brainstem. Furthermore, while vagally mediated negative inotropy is a well-known phenomenon, at present there is no direct evidence for its presence in the rat; we searched for such evidence. To this end, in the arterially perfused working heart-brainstem preparation of the rat, we measured left ventricular pressure (LVP) and computed its first derivative (LVdP/dt). We made the following new observations. (i) Zatebradine (cardiac sodium pacemaker current blocker) caused a bradycardia associated with increases in LVP and LVdP/dt; the latter effect was via a frequency-dependent mechanism. (ii) We confirmed that in the rat, the force-frequency relationship (dependence of contractility on heart rate) is positive over a low range of heart rates, and negative and linear at physiological levels of heart rate, and provided its quantitative description. (iii) The increase in systemic pressure caused a rise in contractility, and vagal blockade or destruction of the central nervous system did not alter this inotropic effect, suggesting that it was mediated by intrinsic cardiac mechanisms. (iv) Vagal stimulation caused complex polyphasic changes in LVdP/dt and LVP in unpaced preparations; during pacing, it caused slowly developing falls in LVdP/dt that could be prevented by atropine. We conclude that control of ventricular contractility in the rat heart differs from that in other mammals not only by its negative frequency dependence, but also in the potent influence of aortic pressure on LVdP/dt. At the level of autonomic neural control, our newly found, vagally mediated negative inotropic effect adds to the accumulating body of data regarding both the presence and the functional importance of parasympathetic innervation of the ventricular myocardium.

Brainstem control of cerebral blood flow and application to acute vasospasm following experimental subarachnoid hemorrhage

Symptomatic ischemia following aneurysmal subarachnoid hemorrhage (SAH) is common but poorly understood and inadequately treated. Severe constriction of the major arteries at the base of the brain, termed vasospasm, traditionally has been thought to be a proximal event underlying these ischemias, although microvascular changes also have been described. The vast majority of studies aimed at understanding the pathogenesis of ischemic deficits, and vasospasm have focused on the interaction of the "spasmogen" of the extravasated blood with the smooth muscle and endothelium of the arteries. This has led to a comparative neglect of the contribution of the CNS to the maintenance of cerebral perfusion. In the present study, we focused on the role of the rostral ventromedial medulla (RVM) in modulating cerebral perfusion at rest and following an experimental SAH in the rat. Changes in cerebral blood flow (CBF) were measured using laser-Doppler flowmetry and three-dimensional optical microangiography. Focal application of a GABA(A) receptor agonist and antagonist was used to respectively inactivate and activate the RVM. We show here that the RVM modulates cerebral blood flow under resting conditions, and further, contributes to restoration of cerebral perfusion following a high-grade SAH. Failure of this brainstem compensatory mechanism could be significant for acute perfusion deficits seen in patients following subarachnoid hemorrhage.

Oestrogen receptors in the central nervous system and evidence for their role in the control of cardiovascular function

Oestrogen is considered beneficial to cardiovascular health through protective effects not only on the heart and vasculature, but also on the autonomic nervous system via actions on oestrogen receptors. A plethora of evidence supports a role for the hormone within the central nervous system in modulating the pathways regulating cardiovascular function. A complex interaction of several brainstem, spinal and forebrain nuclei is required to receive, integrate and co-ordinate inputs that contribute appropriate autonomic reflex responses to changes in blood pressure and other cardiovascular parameters. Central effects of oestrogen and oestrogen receptors have already been demonstrated in many of these areas. In addition to the classical nuclear oestrogen receptors (ERalpha and ERbeta) a recently discovered G-protein coupled receptor, GPR30, has been shown to be a novel mediator of oestrogenic action. Many anatomical and molecular studies have described a considerable overlap in the regional expression of these receptors; however, the receptors do exhibit specific characteristics and subtype specific expression is found in many autonomic brain areas, for example ERbeta appears to predominate in the hypothalamic paraventricular nucleus, whilst ERalpha is important in the nucleus of the solitary tract. This review provides an overview of the available information on the localisation of oestrogen receptor subtypes and their multitude of possible modulatory actions in different groups of neurochemically and functionally defined neurones in autonomic-related areas of the brain.