Autonomic Brain Centers and Pathophysiology of COVID-19

Accumulating data have now shown strong evidence that COVID-19 infection leads to the occurrence of neurological signs with different injury severity. Anosmia and agueusia are now well documented and included in the criteria list for diagnosis, and specialists have stressed that doctors screen COVID-19 patients for these two signs. The eventual brainstem dysregulation, due to the invasion of SARS CoV-2, as a cause of respiratory problems linked to COVID-19, has also been extensively discussed. All these findings lead to an implication of the central nervous system in the pathophysiology of COVID-19. Here we provide additional elements that could explain other described signs like appetite loss, vomiting, and nausea. For this, we investigated the role of brainstem structures located in the medulla oblongata involved in food intake and vomiting control. We also discussed the possible pathways the virus uses to reach the brainstem, i.e., neurotropic and hematogenous (with its two variants) routes.

Effects of intraperitoneally administered L-histidine on food intake, taste, and visceral sensation in rats

To evaluate relative factors for anorectic effects of L-histidine, we performed behavioral experiments for measuring food and fluid intake, conditioned taste aversion (CTA), taste disturbance, and c-Fos immunoreactive (Fos-ir) cells before and after i.p. injection with L-histidine in rats. Animals were injected with saline (9 ml/kg, i.p.) for a control group, and saline (9 ml/kg, i.p.) containing L-histidine (0.75, 1.5, 2.0 g/kg) for a L-histidine group. Injection of L-histidine decreased the average value of food intake, and statistically significant anorectic effects were found in animals injected with 1.5 or 2.0 g/kg L-histidine but not with 0.75 g/kg L-histidine. Taste abnormalities were not detected in any of the groups. Animals injected with 2.0 g/kg L-histidine were revealed to present with nausea by the measurement of CTA. In this group, a significant increase in the number of Fos-ir cells was detected both in the area postrema and the nucleus tractus solitarius (NTS). In the 0.75 g/kg L-histidine group, a significant increase in the number of Fos-ir cells was detected only in the NTS. When the ventral gastric branch vagotomy was performed, recovery from anorexia became faster than the sham-operated group, however, vagotomized rats injected with 2.0 g/kg L-histidine still acquired CTA. These data indicate that acute anorectic effects induced by highly concentrated L-histidine are partly caused by induction of nausea and/or visceral discomfort accompanied by neuronal activities in the NTS and the area postrema. We suggest that acute and potent effects of L-histidine on food intake require substantial amount of L-histidine in the diet.

The time-course of hindbrain neuronal activity varies according to location during either intraperitoneal or subcutaneous tumor growth in rats: single Fos and dual Fos/dopamine β-hydroxylase immunohistochemistry

Neuronal activity in the nucleus of the solitary tract, ventrolateral medulla, area postrema, and parabrachial nucleus was studied in rats with intraperitoneal or subcutaneous tumors on the 7th, 14th, 21st, and 28th day after injection of fibrosarcoma tumor cells. We found that the number of Fos and dopamine β-hydroxylase immunopositive neurons differs between animals with intraperitoneal and subcutaneous tumors and also between tumor-bearing rats at different times following injection. Our data indicate that responses of the brainstem structures to peripheral tumor growth depend on the localization as well as the stage of the tumor growth.

Neuropathology of the area postrema in sudden intrauterine and infant death syndromes related to tobacco smoke exposure

The area postrema is a densely vascularized small protuberance at the inferoposterior limit of the fourth ventricle, outside of the blood-brain barrier. This structure, besides to induce emetic reflex in the presence of noxious chemical stimulation, has a multifunctional integrative capacity to send major and minor efferents to a variety of brain centers particularly involved in autonomic control of the cardiovascular and respiratory activities. In this study we aimed to focus on the area postrema, which is so far little studied in humans, in a large sample of subjects aged from 25 gestational weeks to 10 postnatal months, who died of unknown (sudden unexplained perinatal and infant deaths) and known causes (controls). Besides we investigated a possible link between alterations of this structure, sudden unexplained fetal and infant deaths and maternal smoking. By the application of morphological and immunohistochemical methods, we observed a significantly high incidence of alterations of the area postrema in fetal and infant victims of sudden death as compared with age-matched controls. These pathological findings, including hypoplasia, lack of vascularization, cystic formations and reactive gliosis, were related to maternal smoking. We hypothesize that components from maternal cigarette smoke, particularly in pregnancy, could affect neurons of the area postrema connected with specific nervous centers involved in the control of vital functions. In conclusion, we suggest that the area postrema should be in depth examined particularly in victims of sudden fetal or infant death with smoker mothers.

How T-cell-dependent and -independent challenges access the brain: vascular and neural responses to bacterial lipopolysaccharide and staphylococcal enterotoxin B

Bacterial lipopolysaccharide (LPS) is widely used to study immune influences on the CNS, and cerebrovascular prostaglandin (PG) synthesis is implicated in mediating LPS influences on some acute phase responses. Other bacterial products, such as staphylococcal enterotoxin B (SEB), impact target tissues differently in that their effects are T-lymphocyte-dependent, yet both LPS and SEB recruit a partially overlapping set of subcortical central autonomic cell groups. We sought to compare neurovascular responses to the two pathogens, and the mechanisms by which they may access the brain. Rats received iv injections of LPS (2 microg/kg), SEB (1mg/kg) or vehicle and were sacrificed 0.5-3h later. Both challenges engaged vascular cells as early 0.5h, as evidenced by induced expression of the vascular early response gene (Verge), and the immediate-early gene, NGFI-B. Cyclooxygenase-2 (COX-2) expression was detected in both endothelial and perivascular cells (PVCs) in response to LPS, but only in PVCs of SEB-challenged animals. The non-selective COX inhibitor, indomethacin (1mg/kg, iv), blocked LPS-induced activation in a subset of central autonomic structures, but failed to alter SEB-driven responses. Liposome mediated ablation of PVCs modulated the CNS response to LPS, did not affect the SEB-induced activational profile. By contrast, disruptions of interoceptive signaling by area postrema lesions or vagotomy (complete or hepatic) markedly attenuated SEB-, but not LPS-, stimulated central activational responses. Despite partial overlap in their neuronal and vascular response profiles, LPS and SEB appear to use distinct mechanisms to access the brain.

ANG II-induced hypertension and the role of the area postrema during normal and increased dietary salt

It has been shown that the area postrema (AP) plays a role in the development of certain types of chronic angiotensin II (ANG II)-induced hypertension in the rat but is not of great importance in the salt sensitivity of arterial pressure. It has recently been proposed, however, that elevated sodium levels may exacerbate the hypertensive effects of ANG II, which by itself dramatically affects salt sensitivity, by acting at sodium-sensing neurons in certain circumventricular organs of the brain. Thus the interactions of ANG II, sodium, and the central nervous system remain to be fully understood. The purpose of this study was to examine the role of the AP in ANG II-induced hypertension during periods of normal and elevated dietary salt. We hypothesized that an intact AP was necessary for the full development of hypertension under chronic ANG II infusion and that its role would be pronounced during periods of increased dietary sodium. To test this, male Sprague-Dawley rats underwent ablation of the area postrema (APx, n = 6) or sham operation (sham, n = 6). After 3 wk of recovery, rats were instrumented with radiotelemetry transducers for constant blood pressure and heart rate monitoring and venous catheters for vehicle infusion. After a 3-day control period of 0.9% saline infusion (7 ml/day) and 0.4% dietary sodium, a 10-day period of ANG II infusion (10 ng·kg−1·min−1) was begun, immediately followed by a second 10-day period during which rats were fed a 4.0% sodium diet. By day 6 of ANG II infusion, mean arterial pressure (MAP) in APx rats had increased to 139 ± 4 mmHg, whereas MAP in sham rats had increased to 126 ± 3 mmHg. This difference was found to be significant and continued through day 1 of the high-salt period, after which MAP of the two groups had risen to similar levels. On day 9 of high salt, MAP was again observed to be significantly higher (162 ± 1 mmHg) in APx rats when compared with sham rats (147 ± 4 mmHg.) These results do not support the hypothesis that the AP is necessary for the full development of ANG II-induced hypertension at normal or elevated levels of dietary sodium.

Pharmacological MRI in awake rats reveals neural activity in area postrema and nucleus tractus solitarius: relevance as a potential biomarker for detecting drug-induced emesis

Drug-induced vomiting (emesis) is a major concern in patient care and a significant hurdle in the development of novel therapeutics. With respect to the latter, rodents, such as the rat and mouse, are typically used in efficacy and safety studies; however, drug-induced emesis cannot be readily observed in these species due to the lack of an emetic reflex. It is known that emesis can be triggered by neural activity in brain regions including area postrema (AP) and nucleus tractus solitarius (NTS). In this study, using pharmacological magnetic resonance imaging (phMRI) and a blood-pool contrast agent, we imaged the hemodynamic consequences of brain activity in awake rats initiated by the administration of compounds (apomorphine 0.1, 0.3 micromol/kg i.v. and ABT-594 0.03, 0.1, 0.3 micromol/kg i.v.) that elicit emesis in other species. Regional drug-induced relative cerebral blood volume (rCBV) changes and percent activated area within the AP and NTS were calculated, in which a dose-dependent relationship was evident for both apomorphine and ABT-594. Additionally, to correlate with behavioral readouts, it was found that the activation of AP and NTS was observed at plasma concentrations consistent with those that induced emesis in ferrets for both drugs. Our data thus suggest that phMRI in awake rats may be a useful tool for predicting emetic liability of CNS-acting drugs and may provide insights into depicting the underlying emetic neural pathways in vivo.

Modulation of emetic response by carotid baro- and chemoreceptor activations

We hypothesized that baroreceptor or chemoreceptor activation might be involved in the emetic, and prodromal cardiovascular and respiratory responses. To test this hypothesis, we induced the emetic responses by gastric distension in anesthetized Suncus murinus (house musk shrew), that had intact and absent baroreceptor and chemoreceptor afferents. Secondly, we stimulated the aortic depressor nerve (ADN) and the carotid sinus nerve (CSN) with or without gastric distension. Internal carotid artery ligation in the bifurcation area, which abolished reflex bradycardia by baroreceptor activation, and abolition of chemoreceptor reflex bradycardia and hyperventilation, by carotid body denervation, suppressed the emetic response but did not abolish it. ADN denervation, which produced no significant effects on the baroreceptor or chemoreceptor reflex bradycardia, had no effect on the emetic response, including the prodromal phase. CSN stimulation with gastric distension elicited retching accompanied by reflex bradycardia and hypotension during or just after stimulation, whereas ADN stimulation with gastric distension did not induce the cardiovascular reflex, and had no effects on the emetic response. These results indicate that carotid, rather than aortic, baroreceptor or chemoreceptor activation plays an important role in the augmentation of cardiac parasympathetic activity and the development of emetic response. In conclusion, carotid baroreceptor or chemoreceptor activation, which is non-emetic stimulation, acts as a modulator in the central mechanisms of emesis.

Essential hypertension seems to result from melatonin-induced epigenetic modifications in area postrema

Essential hypertension is a complex multifactorial disorder with epigenetic and environmental factors contributing to its prevalence. Epigenetic system is a genetic regulatory mechanism that allows humans to maintain extraordinarily stable patterns of gene expression over many generations. Sympathetic nervous system plays a major role in the maintenance of hypertension and the rostral ventrolateral medulla is the main source of this sympathetic activation. A possible mechanism to explain the sympathetic hyperactivity in the rostral ventrolateral medulla is an action of the area postrema. Area postrema seems to be the region where a shift of the set-point to a higher operating pressure occurs resulting in hypertension. But, how can a shift occur in the area postrema. We propose that melatonin-induced epigenetic modifications in the neurons of area postrema plays a role in this shift. Area postrema is reported to contain high levels of melatonin receptors that play a role in the epigenetic modifications in certain cells. Environmental stressors cause epigenetic modifications in the neurons of area postrema via the pineal hormone melatonin and these changes lead to a shift in the set-point to a higher operating pressure. This signal is then sent via efferent projections to key medullary sympathetic nuclei in rostral ventrolateral medulla resulting in increases in sympathetic nerve activity. This model may explain the long-term alterations in sympathetic activity in essential hypertension.

Intractable hiccup and nausea with periaqueductal lesions in neuromyelitis optica

Intractable hiccup and nausea (IHN) was found in eight of 47 cases of relapsing neuromyelitis optica (NMO) (17%) but in none of 130 cases of multiple sclerosis (MS). IHN resolved with methylprednisolone. In six cases, MRI detected linear medullary lesions involving the pericanal region, the area postrema, and the nucleus tractus solitarius. Like long and centrally located myelitis, a linear medullary lesion causing IHN may distinguish NMO from MS.

Energy expenditure by intravenous administration of glucagon-like peptide-1 mediated by the lower brainstem and sympathoadrenal system

Glucagon-like peptide-1 (GLP-1) is released from the gut in response to nutrient ingestion. Intravenous (iv) administration of GLP-1 (50 pmol-20 nmol) elicited dose-dependent increases in the rate of whole-body O2 consumption (VO2), an index of energy expenditure, and heart rate of urethane-anesthetized rats. The body core (colonic) temperature increased up to 0.3 degrees C without accompanying alteration of tail skin temperature. Intracerebroventricular (icv) administration of GLP-1 induced a slower and smaller increase in VO2 than the intravenous administration. The injection of glucagon-like peptide-2 (iv or icv) had no effect on VO2, body temperatures, or heart rate. Decerebration had no effect on the thermogenic responses induced by the iv administration of GLP-1, suggesting that the forebrain is not essential for these responses. However, cervical spinal transection greatly attenuated the responses, suggesting the critical involvement of the lower brainstem. Adrenalectomy or pretreatment with an autonomic ganglion blocker, hexamethonium, or a beta-adrenergic blocker, propranolol, also significantly attenuated the thermogenic response. However, subdiaphragmatic vagotomy or celiac-superior mesenteric ganglionectomy had no effect. Rats made insulin-deficient by pretreatment with streptozotocin also exhibited the normal thermogenic response to GLP-1. These results suggest the involvement of the GLP-1 in postprandial energy expenditure, mediated by the lower brainstem and sympathoadrenal system.

Area postrema-lesions increase operant responding to sucrose in rats

Rats with lesions of the area postrema (APX) are known to exhibit an enhanced intake of highly palatable foods such as sweetened condensed milk and cookies. These observations suggest the possibility that APX rats find these foods more rewarding and will work harder to obtain these foods. Sham and APX rats were tested on fixed ratio (FR) and progressive ratio (PR) schedules. APX rats consistently pressed more times to receive sucrose solution and attained both FR 3 and FR 5 criteria significantly faster than sham-lesioned control rats. Furthermore, rats with APX had significantly higher break points than sham-lesioned control rats on a progressive ratio schedule. These results support the hypothesis that rats with lesions of the area postrema will consistently work harder to obtain a highly palatable food reward.

Area postrema lesions elevate NPY levels and decrease anxiety-related behavior in rats

Intracerebroventricular administration of neuropeptide Y (NPY) has been shown to reduce anxiety-like effects in rodents and also profoundly alter feeding. The area postrema-lesioned (APX) rat model of food motivated behavior overconsumes palatable foods and expresses significantly higher levels of NPY in the hypothalamus than sham-lesioned control rats. For this reason, we examined APX rats in the open field test, which is a standard measure of anxiety- or fear-related behavior and also investigated NPY mRNA levels in the hippocampus, amygdala and hypothalamus. We found that APX rats display reduced anxiety-like behavior in the open field test as indicated by spending increased time in the center of the field as opposed to the perimeter of the field. NPY mRNA levels were also found to be significantly elevated in the amygdala, hippocampus and arcuate nucleus of the hypothalamus of APX rats when compared to sham-lesioned rats. These results support the action of limbic NPY to reduce anxiety-like behavior in a rodent model that appears to express higher than normal NPY levels.