Cardiovascular changes following acute and chronic chemical lesions of the dorsal periaqueductal gray in conscious rats

Baroreflex Curve Fitting Using a WYSIWYG Boltzmann Sigmoidal Equation

Arterial baroreflex assessment using vasoactive substances enables investigators to collect data pairs over a wide range of blood pressures and reflex reactions. These data pairs relate intervals between heartbeats or sympathetic neural activity to blood pressure values. In an X-Y plot the data points scatter around a sigmoidal curve. After fitting the parameters of a sigmoidal function to the data, the graph’s characteristics represent a rather comprehensive quantitative reflex description. Variants of the 4-parameter Boltzmann sigmoidal equation are widely used for curve fitting. Unfortunately, their ‘slope parameters’ do not correspond to the graph’s actual slope which complicates the analysis and bears the risk of misreporting. We propose a modified Boltzmann sigmoidal function with preserved goodness of fit whose parameters are one-to-one equivalent to the sigmoidal curve’s characteristics.

Periaqueductal gray and emotions: the complexity of the problem and the light at the end of the tunnel, the magnetic resonance imaging

The periaqueductal gray (PAG) is less referred in relationship with emotions than other parts of the brain (e.g. cortex, thalamus, amygdala), most probably because of the difficulty to reach and manipulate this small and deeply lying structure. After defining how to evaluate emotions, we have reviewed the literature and summarized data of the PAG contribution to the feeling of emotions focusing on the behavioral and neurochemical considerations. In humans, emotions can be characterized by three main domains: the physiological changes, the communicative expressions, and the subjective experiences. In animals, the physiological changes can mainly be studied. Indeed, early studies have considered the PAG as an important center of the emotions-related autonomic and motoric processes. However, in vivo imaging have changed our view by highlighting the PAG as a significant player in emotions-related cognitive processes. The PAG lies on the crossroad of networks important in the regulation of emotions and therefore it should not be neglected. In vivo imaging represents a good tool for studying this structure in living organism and may reveal new information about its role beyond its importance in the neurovegetative regulation.

Neuroreflex control of cardiovascular function is impaired after acute poisoning with chlorpyrifos, an organophosphorus insecticide: Possible short and long term clinical implications

Although it is well-established that severe poisoning by organophosphorus (OP) compounds strongly affects the cardiorespiratory system, the effects of sub-lethal exposure to these compounds on the neural control of cardiovascular function are poorly explored. The aim of this study was to evaluate the effects of acute sub-lethal exposure to chlorpyrifos (CPF), a commonly used OP insecticide, on three basic reflex mechanisms involved in blood pressure regulation, the peripheral chemoreflex, the baroreflex and the Bezold-Jarisch reflex. Adult male Wistar rats were injected intraperitoneally with a single dose of CPF (30 mg/kg) or saline (0.9%). 24 h after injections, cardiovascular reflexes were tested in awake rats. Potassium cyanide (KCN) and phenylbiguanide (PBG) were injected intravenously to activate the chemoreflex and the Bezold-Jarisch reflex, respectively. The baroreflex was activated by phenylephrine and sodium nitroprusside infusions. Blood samples were taken for measurements of butyrylcholinesterase (BChE) activity while acetylcholinesterase (AChE) activity was measured in brainstem samples. Animals treated with CPF presented signs of intoxication such as ataxia, tremor, lacrimation, salivation, tetany, urination and defecation. The hypertensive and the bradycardic responses of the chemoreflex as well as the hypotensive and bradycardic responses of the Bezold-Jarisch reflex were attenuated in CPF treated animals (P < 0.05). Concerning the baroreflex responses, CPF treatment reduced the bradycardia plateau, the range and the gain of the reflex (P < 0.05). Plasma BChE and brainstem AChE were both reduced significantly after CPF treatment (P < 0.05). Our results showed that acute sub-lethal exposure to CPF impairs the cardiovascular responses of homeostatic and defensive cardiovascular reflexes. These effects are associated with a marked inhibition of plasma BChE and brainstem AChE.

Components of the cannabinoid system in the dorsal periaqueductal gray are related to resting heart rate

The present study was undertaken to examine whether variations in endocannabinoid signaling in the dorsal periaqueductal gray (dPAG) are associated with baseline autonomic nerve activity, heart rate, and blood pressure. Blood pressure was recorded telemetrically in rats, and heart rate and power spectral analysis of heart rate variability were determined. Natural variations from animal to animal provided a range of baseline values for analysis. Transcript levels of endocannabinoid signaling components in the dPAG were analyzed, and endocannabinoid content and catabolic enzyme activity were measured. Higher baseline heart rate was associated with increased anandamide content and with decreased activity of the anandamide-hydrolyzing enzyme, fatty acid amide hydrolase (FAAH), and it was negatively correlated with transcript levels of both FAAH and monoacylglycerol lipase (MAGL), a catabolic enzyme for 2-arachidonoylglycerol (2-AG). Autonomic tone and heart rate, but not blood pressure, were correlated to levels of FAAH mRNA. In accordance with these data, exogenous anandamide in the dPAG of anesthetized rats increased heart rate. These data indicate that in the dPAG, anandamide, a FAAH-regulated lipid, contributes to regulation of baseline heart rate through influences on autonomic outflow.

A resting-state functional connectivity study in patients at high risk for sudden unexpected death in epilepsy

OBJECTIVE

Seizure-related respiratory and cardiac dysfunctions were once thought to be the direct cause of sudden unexpected death in epilepsy (SUDEP), but both may be secondary to postictal cerebral inhibition. An important issue that has not been explored to date is the neural network basis of cerebral inhibition. Our aim was to investigate the features of neural networks in patients at high risk for SUDEP using a blood oxygen level-dependent (BOLD) resting-state functional connectivity (FC) approach.

SUBJECTS AND METHODS

Resting-state functional magnetic resonance imaging (Rs-fMRI) data were recorded from 13 patients at high risk for SUDEP and 12 patients at low risk for SUDEP. Thirteen cerebral regions that are closely related to cardiorespiratory activity were selected as regions of interest (ROIs). The ROI-wise resting-state FC analysis was compared between the two groups.

RESULTS

Compared with patients at low risk for SUDEP, patients at high risk exhibited significant reductions in the resting-state FC between the pons and the right thalamus, the midbrain and the right thalamus, the bilateral anterior cingulate cortex (ACC) and the right thalamus, and the left thalamus and the right thalamus.

CONCLUSIONS

This investigation is the first to use neuroimaging methods in research on the mechanism of SUDEP and demonstrates the abnormally decreased resting-state FC in the ACC-thalamus-brainstem circuit in patients at high risk for SUDEP. These findings highlight the need to understand the fundamental neural network dysfunction in SUDEP, which may fill the missing link between seizure-related cardiorespiratory dysfunction and SUDEP, and provide a promising neuroimaging biomarker for risk prediction of SUDEP.

Cardiac-autonomic imbalance and baroreflex dysfunction in the renovascular Angiotensin-dependent hypertensive mouse

Mouse models provide powerful tools for studying the mechanisms underlying the dysfunction of the autonomic reflex control of cardiovascular function and those involved in cardiovascular diseases. The established murine model of two-kidney, one-clip (2K1C) angiotensin II-dependent hypertension represents a useful tool for studying the neural control of cardiovascular function. In this paper, we discuss the main contributions from our laboratory and others regarding cardiac-autonomic imbalance and baroreflex dysfunction. We show recent data from the angiotensin-dependent hypertensive mouse demonstrating DNA damage and oxidative stress using the comet assay and flow cytometry, respectively. Finally, we highlight the relationships between angiotensin and peripheral and central nervous system areas of cardiovascular control and oxidative stress in the 2K1C hypertensive mouse.

Could sudden cardiac death in epilepsy be related to the occurrence of thalamic dysfunction or anatomic change?

Sudden unexpected death in epilepsy (SUDEP) is the most important direct epilepsy-related cause of death in people with chronic epilepsy. Its physiopathology is still unknown; however, the most commonly suggested potential mechanisms involve cardiac or respiratory abnormalities. As the anatomical substrate of epileptic activity in the central nervous system (CNS) shows a direct relationship with cardiovascular alterations, this may suggests that patients with epilepsy associated with focal CNS lesions may be at particular risk of SUDEP. Currently, experimental and clinical data support an important role for thalamic nuclei in the behavioural manifestations, initiation and propagation of seizures. In view of the above findings, we purpose that SUDEP, at least in some cases, could be related to the occurrence of thalamic dysfunction or anatomic change.

Dorsal periaqueductal gray area synapses modulate baroreflex in unanesthetized rats

The dorsal portion of the periaqueductal gray area (dPAG) is involved in behavioral and cardiovascular control. We report the effect of acute and reversible dPAG blockade by local microinjection of either lidocaine or CoCl2 on the baroreflex response of unanesthetized rats. Acute and reversible blockade evoked by lidocaine microinjection into the dPAG did not affect the bradycardic response to mean arterial pressure (MAP) increases evoked by i.v. infusion of phenylephrine. However, lidocaine increased baroreflex gain and tachycardic reflex in response to MAP decreases evoked by i.v. infusion of sodium nitroprusside, thus suggesting an action on the sympathetic component of the baroreflex. The effects of dPAG synapses blockade caused by CoCl2 were similar to those observed after lidocaine microinjection. CoCl2 microinjection also increased baroreflex gain and tachycardiac responses to MAP decreases without affecting the parasympathetic baroreflex component. In conclusion, our data point to a dPAG tonic inhibitory involvement in baroreflex control, specifically modulating the sympathetic baroreflex component. Temporary dPAG ablation by local microinjection of lidocaine increased the sympathetic baroreflex component. Because CoCl2 microinjection had similar effects on the baroreflex, this modulation involves local synaptic neurotransmission within the dPAG.

Cardiovascular effects of noradrenaline microinjected into the dorsal periaqueductal gray area of unanaesthetized rats

The periaqueductal grey area (PAG) is a mesencephalic region that is involved in the modulation of cardiovascular changes associated with behavioural responses. Among the neurotransmitters present in the PAG, noradrenaline (NA) is also known to be involved in central nervous system cardiovascular regulation. In the present study we report the cardiovascular effects of the microinjection of NA into the dorsal portion of the PAG (dPAG) of unanaesthetized rats and the peripheral mechanism involved in their mediation. Injection of NA in the dPAG of unanaesthetized rats evoked a dose-dependent pressor response accompanied by bradycardia. The magnitude of the pressor responses was higher at more rostral sites in the dPAG and decreased when NA was injected into the caudal portion of the dPAG. The responses to NA were markedly reduced in urethane-anaesthetized rats. The pressor response was potentiated by i.v. pretreatment with the ganglion blocker pentolinium and blocked by i.v. pretreatment with the vasopressin antagonist dTyr(CH2)5(Me)AVP. The results suggest that activation of noradrenergic receptors within the dPAG can evoke pressor responses, which are mediated by acute vasopressin release.

Effect of dorsal periaqueductal gray lesion on cardiovascular and behavioural responses to contextual conditioned fear in rats

Contextual conditioned fear in the rat is characterized by a freezing immobility associated with a marked increase in blood pressure, a slow increase in heart rate, and ultrasonic vocalizations. A previous Fos study also revealed a marked activation of the ventrolateral part of the periaqueductal gray (VLPAG) and a much smaller activation of its dorsal part (DPAG). Recent chemical blockade experiments indicate that the main role of the VLPAG in the response is to impose the immobility necessary for the expression of the freezing component. We now test the role of the DPAG to see if its small activation (as revealed by Fos) is of any functional significance in the contextual fear response. Large N-methyl-D-aspartate (NMDA) excitotoxic lesions that destroyed most of the DPAG were made in 10 rats. Another group of 10 rats had sham lesions with saline. The animals were then implanted with blood pressure telemetric probes, fear conditioned, and finally tested. There was no significant difference in the amount of freezing and in the blood pressure response between the two groups. However, there was a complete abolition of ultrasonic vocalizations and a significantly greater increase in heart rate in the DPAG-lesioned group. The effect on vocalization and heart rate may be explained by lesion of adjacent structures: the lateral PAG and the superior colliculus (baroreflex alteration), respectively. Thus, most of DPAG appears to play little role in the expression of the contextual fear response.

c-Fos expression in the midbrain periaqueductal gray after chemoreceptor and baroreceptor activation

The pattern of Fos-like immunoreactivity (FLI) in the periaqueductal gray (PAG) associated with activation of arterial chemoreceptors versus baroreceptor afferents was examined in urethane-anesthetized rats. Chemoreflex responses elicited by repeat intravenous injections of potassium cyanide (KCN; 90 microg/kg) significantly increased FLI in all columns of the PAG relative to saline-injected animals. Pressor responses elicited by intravenous phenylephrine (PE) produced a similar pattern of increased FLI throughout the PAG except in the dorsomedial and lateral columns of the caudal PAG, where FLI was minimal. Chemoreflex responses were unaltered by blockade of excitatory amino acid receptors in the dorsomedial PAG, and < 10% of the neurons of the caudal PAG that expressed FLI after KCN stimulation were retrogradely labeled from the A5 region of the caudal ventrolateral pons. These results indicate that integration of chemoreceptor inputs occurs primarily in the dorsal and lateral columns of the caudal PAG, but these neurons have little direct descending influence over lower brain stem regions integral to the central arterial chemoreflex arc.

Cardiovascular neural reflexes in L-NAME-induced hypertension in mice

The mouse is the most used animal for studying the genetic basis of cardiovascular diseases. However, the mechanisms of regulation of cardiovascular function in this animal are not yet well understood. The goal of this study was to evaluate the baroreflex, the Bezold-Jarisch cardiopulmonary reflex (BJR), and the chemoreflex in mice with hypertension induced by inhibition of NO using Nomega-nitro-L-arginine-methyl ester (L-NAME). Basal mean arterial pressure (MAP) measured under anesthesia (urethane, 1 mg/g IP) was significantly higher in L-NAME (400 microgram/g IP for 7 days)-treated (HT) mice (n=7) compared with vehicle-treated (NT; n=10) animals (126+/-9 versus 79+/-2 mm Hg) without differences in heart rate (HR). Baroreflex sensitivity, evaluated using phenylephrine (1 microgram/g IV) was enhanced in HT mice compared with NT mice (-9.8+/-1.4 versus -4.9+/-0.5 bpm/mm Hg). The BJR, induced by phenylbiguanide (40 ng/g IV), was significantly attenuated in HT animals (MAP, -13+/-5%; HR, -39+/-6%) compared with NT animals (MAP, -38+/-5%; HR, -66+/-2%). The chemoreflex, induced by potassium cyanide (0.26 microgram/g IV), was significantly attenuated in HT animals (MAP, +14+/-4%; HR, -8+/-2%) compared with NT animals (MAP, +29+/-4%; HR, -15+/-4%). As has been observed in rats, chronic inhibition of NO synthase in mice results in arterial hypertension. Enhancement of baroreflex sensitivity and attenuation of BJR and chemoreflex seem to be mainly caused by inhibition of NO synthesis because individual analyses did not show positive correlation between changes in these reflexes and MAP levels in the HT group.