Baroreceptor reflex is suppressed in rats that develop hyperalgesia behavior after nerve injury

Probiotic and high-fat diet: effects on pain assessment, body composition, and cytokines in male and female adolescent and adult rats

Obesity in adolescence is increasing in frequency and is associated with elevated proinflammatory cytokines and chronic pain in a sex-dependent manner. Dietary probiotics may mitigate these detrimental effects of obesity. Using a Long-Evans adolescent and adult rat model of overweight (high-fat diet (HFD) - 45% kcal from fat from weaning), we determined the effect of a single-strain dietary probiotic [Lactiplantibacillus plantarum 299v (Lp299v) from weaning] on the theoretically increased neuropathic injury-induced pain phenotype and inflammatory cytokines. We found that although HFD increased fat mass, it did not markedly affect pain phenotype, particularly in adolescence, but there were subtle differences in pain in adult male versus female rats. The combination of HFD and Lp299v augmented the increase in leptin in adolescent females. There were many noninteracting main effects of age, diet, and probiotic on an array of cytokines and adipokines with adults being higher than adolescents, HFD higher than the control diet, and a decrease with probiotic compared with placebo. Of particular interest were the probiotic-induced increases in IL12p70 in female adolescents on an HFD. We conclude that a more striking pain phenotype could require a higher and longer duration caloric diet or a different etiology of pain. A major strength of our study was that a single-strain probiotic had a wide range of inhibiting effects on most proinflammatory cytokines. The positive effect of the probiotic on leptin in female adolescent rats is intriguing and worthy of exploration.NEW & NOTEWORTHY A single-strain probiotic (Lp299v) had a wide range of inhibiting effects on most proinflammatory cytokines (especially IL12p70) measured in this high-fat diet rat model of mild obesity. The positive effect of probiotic on leptin in female adolescent rats is intriguing and worthy of exploration.

Low pulse pressure and high serum complement C1q are risk factors for hemodialysis headache: A case-control study

BACKGROUND AND OBJECTIVE

Hemodialysis headache (HDH) is a common complication of dialysis that negatively affects the patient's quality of life. The etiology and triggering factors of HDH are not fully understood. This study aims to assess the prevalence and characteristics of HDH among patients undergoing hemodialysis across multiple centers in China. Furthermore, we conducted a case-control study at one hospital to identify risk factors associated with HDH.

METHODS

The study consisted of two phases including a cross-sectional observational study and a case-control study. Participants underwent neurological examinations and interviews. Demographic and medical information were collected from both medical records and patient files. Serum creatinine, uric acid, urea, estimated glomerular filtration rate (eGFR), plasma osmolarity, glucose, C1q, and a variety of electrolytes including potassium, sodium, chloride, calcium, magnesium, and phosphorus were measured before and after dialysis. Blood pressure variables including systolic blood pressure, diastolic blood pressure, pulse pressure (PP), and heart rate were monitored hourly. Serum levels of inflammatory factors, including tumor necrosis factor α (TNF-α), interleukin (IL)-1β, IL-4, IL-6, and IL-10 were quantified using a double-antibody sandwich enzyme-linked immunosorbent assay (ELISA).

RESULTS

The prevalence of HDH was 37.7% (183/485). HDH was characterized by a bilateral tightening headache of moderate intensity and duration of <2 h, occurring in different locations. The case-control study included 50 patients with HDH and 84 control patients, pre-dialysis PP was found to be lower in the HDH group than in the control group (mean ± standard deviation 51.5 ± 18.2 vs. 67.9 ± 14.9, p = 0.027). Furthermore, the pre-dialysis serum complement C1q level was significantly higher for the HDH group than the control group (median and interquartile range 201.5 [179.0-231.5] vs. 189.0 [168.9-209.0], p = 0.021). Pre-dialysis PP was associated with 5.1% decreased odds of HDH (odds ratio [OR] = 0.96; 95% confidence interval [CI], 0.93-0.99, p = 0.026), body weight was associated with a 5.4% decreased risk of HDH (OR = 0.95; 95% CI, 0.91-0.99, p = 0.013), and pre-dialysis C1q levels increased the odds of HDH by 1.9% (OR = 1.02; 95% CI, 1.01-1.03, p = 0.005).

CONCLUSION

Low PP, low body weight, and high blood complement C1q may be potential risk factors associated with HDH.

The medial prefrontal cortex and the cardiac baroreflex activity: physiological and pathological implications

The cardiac baroreflex is an autonomic neural mechanism involved in the modulation of the cardiovascular system. It influences the heart rate and peripheral vascular resistance to preserve arterial blood pressure within a narrow variation range. This mechanism is mainly controlled by medullary nuclei located in the brain stem. However, supramedullary areas, such as the ventral portion of medial prefrontal cortex (vMPFC), are also involved. Particularly, the glutamatergic NMDA/NO pathway in the vMPFC can facilitate baroreflex bradycardic and tachycardic responses. In addition, cannabinoid receptors in this same area can reduce or increase those cardiac responses, possibly through alteration in glutamate release. This vMPFC network has been associated to cardiovascular responses during stressful situations. Recent results showed an involvement of glutamatergic, nitrergic, and endocannabinoid systems in the blood pressure and heart rate increases in animals after aversive conditioning. Consequently, baroreflex could be modified by the vMPFC neurotransmission during stressful situations, allowing necessary cardiovascular adjustments. Remarkably, some mental, neurological and neurodegenerative disorders can involve damage in the vMPFC, such as posttraumatic stress disorder, major depressive disorder, Alzheimer's disease, and neuropathic pain. These pathologies are also associated with alterations in glutamate/NO release and endocannabinoid functions along with baroreflex impairment. Thus, the vMPFC seems to play a crucial role on the baroreflex control, either during pathological or physiological stress-related responses. The study of baroreflex mechanism under such pathological view may be helpful to establish causality mechanisms for the autonomic and cardiovascular imbalance found in those conditions. It can explain in the future the reasons of the high cardiovascular risk some neurological and neurodegenerative disease patients undergo. Additionally, the present work offers insights on the possible contributions of vMPFC dysfunction on baroreflex alterations, which, in turn, may raise questions in what extent other brain areas may play a role in autonomic deregulation under such pathological situations.

Fatty acid amide hydrolase activity in the dorsal periaqueductal gray attenuates neuropathic pain and associated dysautonomia

The complexity of neuropathic pain and its associated comorbidities, including dysautonomia, make it difficult to treat. Overlap of anatomical regions and pharmacology of sympathosensory systems in the central nervous system (CNS) provide targets for novel treatment strategies. The dorsal periaqueductal gray (dPAG) is an integral component of both the descending pain modulation system and the acute stress response and is critically involved in both analgesia and the regulation of sympathetic activity. Local manipulation of the endocannabinoid signaling system holds great promise to provide analgesia without excessive adverse effects and also influence autonomic output. Inhibition of fatty acid amide hydrolase (FAAH) increases brain concentrations of the endocannabinoid N-arachidonoylethanolamine (AEA) and reduces pain-related behaviors in neuropathic pain models. Neuropathic hyperalgesia and reduced sympathetic tone are associated with increased FAAH activity in the dPAG, which suggests the hypothesis that inhibition of FAAH in the dPAG will normalize pain sensation and autonomic function in neuropathic pain. To test this hypothesis, the effects of systemic or intra-dPAG FAAH inhibition on hyperalgesia and dysautonomia developed after spared nerve injury (SNI) were assessed in male and female rats. Administration of the FAAH inhibitor PF-3845 into the dPAG reduces hyperalgesia behavior and the decrease in sympathetic tone induced by SNI. Prior administration of the CB1 receptor antagonist AM281, attenuated the antihyperalgesic and sympathetic effects of FAAH inhibition. No sex differences were identified. These data support an integrative role for AEA/CB1 receptor signaling in the dPAG contributing to the regulation of both hyperalgesia behavior and altered sympathetic tone in neuropathic pain.

Heterogeneity in patterns of pain development after nerve injury in rats and the influence of sex

The genesis of neuropathic pain is complex, as sensory abnormalities may differ between patients with different or similar etiologies, suggesting mechanistic heterogeneity, a concept that is largely unexplored. Yet, data are usually grouped for analysis based on the assumption that they share the same underlying pathogenesis. Sex is a factor that may contribute to differences in pain responses. Neuropathic pain is more prevalent in female patients, but pre-clinical studies that can examine pain development in a controlled environment have typically failed to include female subjects. This study explored patterns of development of hyperalgesia-like behavior (HLB) induced by noxious mechanical stimulation in a neuropathic pain model (spared nerve injury, SNI) in both male and female rats, and autonomic dysfunction that is associated with chronic pain. HLB was analyzed across time, using both discrete mixture modeling and rules-based longitudinal clustering. Both methods identified similar groupings of hyperalgesia trajectories after SNI that were not evident when data were combined into groups by sex only. Within the same hyperalgesia development group, mixed models showed that development of HLB in females was delayed relative to males and reached a magnitude similar to or higher than males. The data also indicate that sympathetic tone (as indicated by heart rate variability) drops below pre-SNI level before or at the onset of development of HLB. This study classifies heterogeneity in individual development of HLB and identifies sexual dimorphism in the time course of development of neuropathic pain after nerve injury. Future studies addressing mechanisms underlying these differences could facilitate appropriate pain treatments.

Baroreceptor Modulation of the Cardiovascular System, Pain, Consciousness, and Cognition

Baroreceptors are mechanosensitive elements of the peripheral nervous system that maintain cardiovascular homeostasis by coordinating the responses to external and internal environmental stressors. While it is well known that carotid and cardiopulmonary baroreceptors modulate sympathetic vasomotor and parasympathetic cardiac neural autonomic drive, to avoid excessive fluctuations in vascular tone and maintain intravascular volume, there is increasing recognition that baroreceptors also modulate a wide range of non-cardiovascular physiological responses via projections from the nucleus of the solitary tract to regions of the central nervous system, including the spinal cord. These projections regulate pain perception, sleep, consciousness, and cognition. In this article, we summarize the physiology of baroreceptor pathways and responses to baroreceptor activation with an emphasis on the mechanisms influencing cardiovascular function, pain perception, consciousness, and cognition. Understanding baroreceptor-mediated effects on cardiac and extra-cardiac autonomic activities will further our understanding of the pathophysiology of multiple common clinical conditions, such as chronic pain, disorders of consciousness (e.g., abnormalities in sleep-wake), and cognitive impairment, which may result in the identification and implementation of novel treatment modalities. © 2021 American Physiological Society. Compr Physiol 11:1373-1423, 2021.

Changes in cardiovascular parameters in rats exposed to chronic widespread mechanical allodynia induced by hind limb cast immobilization

To elucidate the relationship between chronic pain conditions with cast immobilization and autonomic function, we investigated the functional changes of the autonomic nervous system in conscious rats with chronic post-cast pain (CPCP) induced by a two-week cast immobilization of one hind limb. We telemetrically examined the time courses of systolic arterial blood pressure (SBP), heart rate (HR), and the middle-frequency (MF) component obtained from the power spectral analysis of SBP variability as a vasomotor sympathetic index. We also investigated the baroreflex sensitivity to phentolamine, an α-adrenoceptor antagonist, and the SBP and HR responses to a low ambient temperature (LT; 9.0 ± 0.2°C) exposure, a sympathetic stimulant. Rats exposed to cast immobilization exhibited mechanical allodynia lasting for at least 10 weeks after cast removal in the calf area (skin and muscle) of the bilateral hind limbs. Under resting conditions, the SBP, HR, and MF components were significantly increased during cast immobilization (all p < 0.001). Following cast removal, these parameters gradually decreased and within 1 week reached lower than baseline levels, lasting for over 10 weeks. Phentolamine administration (10 mg/kg, intraperitoneally) significantly decreased the SBP before and during cast immobilization (before, p < 0.001; during, p = 0.001) but did not lower the SBP after cast removal. The baroreflex gain after phentolamine administration, calculated as the HR increase divided by the SBP reduction, was significantly increased after cast removal (p = 0.002). The SBP increase on LT exposure was significantly greater after cast removal than that before cast immobilization, suggesting hypersensitivity to sympathetic neurotransmitters. These results revealed that, in the CPCP model, sympathetic activation was augmented during cast immobilization, which then decreased after cast removal and remained below normal levels with persisting pain behaviors. Additionally, the responsiveness of the autonomic nervous system was impaired in the CPCP model.

Use of dual-flow bioreactor to develop a simplified model of nervous-cardiovascular systems crosstalk: A preliminary assessment

Chronic conditions requiring long-term rehabilitation therapies, such as hypertension, stroke, or cancer, involve complex interactions between various systems/organs of the body and mutual influences, thus implicating a multiorgan approach. The dual-flow IVTech LiveBox2 bioreactor is a recently developed inter-connected dynamic cell culture model able to mimic organ crosstalk, since cells belonging to different organs can be connected and grown under flow conditions in a more physiological environment. This study aims to setup for the first time a 2-way connected culture of human neuroblastoma cells, SH-SY5Y, and Human Coronary Artery Smooth Muscle Cells, HCASMC through a dual-flow IVTech LiveBox2 bioreactor, in order to represent a simplified model of nervous-cardiovascular systems crosstalk, possibly relevant for the above-mentioned diseases. The system was tested by treating the cells with 10nM angiotensin II (AngII) inducing PKCβII/HuR/VEGF pathway activation, since AngII and PKCβII/HuR/VEGF pathway are relevant in cardiovascular and neuroscience research. Three different conditions were applied: 1- HCASMC and SH-SY5Y separately seeded in petri dishes (static condition); 2- the two cell lines separately seeded under flow (dynamic condition); 3- the two lines, seeded in dynamic conditions, connected, each maintaining its own medium, with a membrane as interface for biohumoral changes between the two mediums, and then treated. We detected that only in condition 3 there was a synergic AngII-dependent VEGF production in SH-SY5Y cells coupled to an AngII-dependent PKCβII/HuR/VEGF pathway activation in HCASMC, consistent with the observed physiological response in vivo. HCASMC response to AngII seems therefore to be generated by/derived from the reciprocal cell crosstalk under the dynamic inter-connection ensured by the dual flow LiveBox 2 bioreactor. This system can represent a useful tool for studying the crosstalk between organs, helpful for instance in rehabilitation research or when investigating chronic diseases; further, it offers the advantageous opportunity of cultivating each cell line in its own medium, thus mimicking, at least in part, distinct tissue milieu.

A refinement approach in a mouse model of rehabilitation research. Analgesia strategy, reduction approach and infrared thermography in spinal cord injury

The principles of Refinement, Replacement and Reduction (3R’s) should be taken into account when animals must be used for scientific purpose. Here, a Reduction / Refinement approach was applied to the procedure of spinal cord injury (SCI), an animal model used in rehabilitation medicine research, in order to improve the quality of experiments, avoiding unnecessary suffering. The aims of this investigation were 1- to assess acute surgical pain in mice subjected to SCI, 2- to compare the efficacy of commonly used analgesia (three buprenorphine subcutaneous injection in 48 hours, 0,15 mg/kg each) with a combination of opioid and NSAID (one subcutaneous injection of 5 mg/kg carprofen before surgery followed by three buprenorphine subcutaneous injection in 48 hours, 0,15 mg/kg each) and 3- to test if Infrared Thermography (IRT) could be a potential new Refinement method to easily assess thermoregulation, an important metabolic parameter. Finally, we aimed to achieve these goals without recruiting animals on purpose, but using mice already scheduled for studies on SCI. By using behaviours analysis, we found that, despite being commonly used, buprenorphine does not completely relieve acute surgical pain, whereas the combination of buprenorphine and carprofen significantly decreases pain signs by 80%. IRT technology turned out to be a very useful Refinement tool being a non invasive methods to measure animal temperature, particularly useful when rectal probe cannot be used, as in the case of SCI. We could find that temperatures constantly and significantly increased until 7 days after surgery and then slowly decreased and, finally, we could observe that in the buprenorphine and carprofen treated group, temperatures were statistically lower than in the buprenorphine-alone treated mice. To our knowledge this is the first work providing an analgesic Refinement and a description of thermoregulatory response using the IRT technology, in mice subjected to SCI.

Post-ictal Modulation of Baroreflex Sensitivity in Patients With Intractable Epilepsy

Objective: Seizure-related autonomic dysregulation occurs in epilepsy patients and may contribute to Sudden Unexpected Death in Epilepsy (SUDEP). We tested how different types of seizures affect baroreflex sensitivity (BRS) and heart rate variability (HRV). We hypothesized that BRS and HRV would be reduced after bilateral convulsive seizures (BCS).

Methods: We recorded blood pressure (BP), electrocardiogram (ECG) and oxygen saturation continuously in patients (n = 18) with intractable epilepsy undergoing video-EEG monitoring. A total of 23 seizures, either focal seizures (FS, n = 14) or BCS (n = 9), were analyzed from these patients. We used 5 different HRV measurements in both the time and frequency domains to study HRV in pre- and post-ictal states. We used the average frequency domain gain, computed as the average of the magnitude ratio between the systolic BP (BPsys) and the RR-interval time series, in the low-frequency (LF) band as frequency domain index of BRS in addition to the instantaneous slope between systolic BP and RR-interval satisfying spontaneous BRS criteria as a time domain index of BRS.

Results: Overall, the post-ictal modulation of HRV varied across the subjects but not specifically by the type of seizures. Comparing pre- to post-ictal epochs, the LF power of BRS decreased in 8 of 9 seizures for patients with BCS; whereas following 12 of 14 FS, BRS increased. Similarly, spontaneous BRS decreased following 7 of 9 BCS. The presence or absence of oxygen desaturation was not consistent with the changes in BRS following seizures, and the HRV does not appear to be correlated with the BRS changes. These data suggest that a transient decrease in BRS and temporary loss of cardiovascular homeostatic control can follow BCS but is unlikely following FS.

Significance: These findings indicate significant post-ictal autonomic dysregulation in patients with epilepsy following BCS. Further, reduced BRS following BCS, if confirmed in future studies on SUDEP cases, may indicate one quantifiable risk marker of SUDEP.

Medial prefrontal cortex TRPV1 and CB1 receptors modulate cardiac baroreflex activity by regulating the NMDA receptor/nitric oxide pathway

The ventral medial prefrontal cortex (vMPFC) facilitates the cardiac baroreflex response through N-methyl-D-aspartate (NMDA) receptor activation and nitric oxide (NO) formation by neuronal NO synthase (nNOS) and soluble guanylate cyclase (sGC) triggering. Glutamatergic transmission is modulated by the cannabinoid receptor type 1 (CB₁) and transient receptor potential vanilloid type 1 (TRPV₁) receptors, which may inhibit or stimulate glutamate release in the brain, respectively. Interestingly, vMPFC CB₁ receptors decrease cardiac baroreflex responses, while TRPV₁ channels facilitate them. Therefore, the hypothesis of the present study is that the vMPFC NMDA/NO pathway is regulated by both CB₁ and TRPV₁ receptors in the modulation of cardiac baroreflex activity. In order to test this assumption, we used male Wistar rats that had stainless steel guide cannulae bilaterally implanted in the vMPFC. Subsequently, a catheter was inserted into the femoral artery, for cardiovascular recordings, and into the femoral vein for assessing baroreflex activation. The increase in tachycardic and bradycardic responses observed after the microinjection of a CB₁ receptors antagonist into the vMPFC was prevented by an NMDA antagonist as well as by the nNOS and sGC inhibition. NO extracellular scavenging also abolished these responses. These same pharmacological manipulations inhibited cardiac reflex enhancement induced by TRPV₁ agonist injection into the area. Based on these results, we conclude that vMPFC CB₁ and TRPV₁ receptors inhibit or facilitate the cardiac baroreflex activity by stimulating or blocking the NMDA activation and NO synthesis.

Effect of type 2 diabetes, surgical incision, and volatile anesthesia on hemodynamics in the rat

Diabetic patients have increased cardiac complications during surgery, possibly due to impaired autonomic regulation. Anesthesia lowers blood pressure and heart rate (HR), whereas surgical intervention has opposing effects. The interaction of anesthesia and surgical intervention on hemodynamics in diabetes is unknown, despite being a potential perioperative risk factor. We aimed to determine the effect of diabetes on the integrative interaction between hemodynamics, anesthesia, and surgical incision. Zucker type 2 diabetic rats (DM) and their nondiabetic littermates (ND) were implanted with an intravenous port for drug delivery, and a radiotelemeter to measure mean arterial blood pressure (MAP) and derive HR (total n = 50). Hemodynamic pharmacological responses were assessed under conscious, isoflurane anesthesia (~2-2.5%), and anesthesia-surgical conditions; the latter performed as a laparotomy. MAP was not different between groups under conscious conditions (ND 120 ± 6 vs. DM 131 ± 4 mmHg, P > 0.05). Anesthesia reduced MAP, but not differently in DM (ND -30 ± 6 vs. DM -38 ± 4 ΔmmHg, P > 0.05). Despite adequate anesthesia, surgical incision increased MAP, which tended to be less in DM (ND +21 ± 4 vs. DM +13 ± 2 ΔmmHg, P = 0.052). Anesthesia disrupted central baroreflex HR responses to sympathetic activation (sodium nitroprusside 10 μg·kg^-1, ND conscious 83 ± 13 vs. anesthetized 16 ± 5 Δbpm; P < 0.05) or to sympathetic withdrawal (phenylephrine 10 μg·kg^-1, ND conscious -168 ± 37 vs. anesthetized -20 ± 6 Δbpm; P < 0.05) with no additional changes observed after surgical incision or during diabetes. During perioperative conditions, type 2 diabetes did not impact on short-term hemodynamic regulation. Anesthesia had the largest hemodynamic impact, whereas surgical effects were limited to modulation of baseline blood pressure.

Neuropathic pain attenuates ischemia reperfusion injury through β2-adrenergic pathway

AIMS

The relationship between neuropathic pain and myocardial infarction (MI) was uncertain because of some medication or underlying diseases. This study investigated the impact of neuropathic pain on ischemia reperfusion injury using isolated rat hearts and cardiomyocytes.

MAIN METHODS

Male Sprague-Dawley rats were assigned to the control and allodynia (AL) groups, with the latter subjected to the fifth lumbar spinal-nerve ligation. First, isolated hearts underwent 25-min ischemia and 90-min reperfusion to assess hemodynamic changes and MI area. Second, isolated cardiomyocytes underwent 10-min laser illumination to assess the opening of mitochondrial permeability transition pore (mPTP) and cellular hypercontraction. Lastly, expression of pro-survival kinases was measured in another cardiomyocytes using flow cytometry. AL-treated hearts were concomitantly examined regarding the involvement of β-adrenergic pathways by esmolol (ESM), β1-blocker (100μM, AL+ESM), and ICI118551 (ICI), β2-blocker (50nM, AL+ICI).

KEY FINDINGS

All hemodynamic variables did not change significantly in between-group comparisons except at 30min of reperfusion. MI area decreased remarkably in the AL and AL+ESM groups after 90-min reperfusion. The AL+ICI group significantly increased it as compared with the AL and AL+ESM groups. Similarly, the AL and AL+ESM groups significantly inhibited mPTP opening and cellular hypercontraction, whereas the AL+ICI group reversed these effects. Enhanced expression of pro-survival kinases was observed in the AL and AL+ESM groups, but the AL+ICI group abolished this enhancement.

SIGNIFICANCE

Our findings suggested that neuropathic pain possessed cardioprotective effects through inhibiting mPTP opening. The underlying mechanisms were possibly regulated by β2-adrenergic activation and pro-survival kinase expression in cardiomyocytes.

Upregulation of fatty acid amide hydrolase in the dorsal periaqueductal gray is associated with neuropathic pain and reduced heart rate in rats

Nerve damage can induce a heightened pain response to noxious stimulation, which is termed hyperalgesia. Pain itself acts as a stressor, initiating autonomic and sensory effects through the dorsal periaqueductal gray (dPAG) to induce both sympathoexcitation and analgesia, which prior studies have shown to be affected by endocannabinoid signaling. The present study addressed the hypothesis that neuropathic pain disrupts autonomic and analgesic regulation by endocannabinoid signaling in the dPAG. Endocannabinoid contents, transcript levels of endocannabinoid signaling components, and catabolic enzyme activity were analyzed in the dPAG of rats at 21 days after painful nerve injury. The responses to two nerve injury models were similar, with two-thirds of animals developing hyperalgesia that was maintained throughout the postinjury period, whereas no sustained change in sensory function was observed in the remaining rats. Anandamide content was lower in the dPAG of rats that developed sustained hyperalgesia, and activity of the catabolic enzyme fatty acid amide hydrolase (FAAH) was higher. Intensity of hyperalgesia was correlated to transcript levels of FAAH and negatively correlated to heart rate and sympathovagal balance. These data suggest that maladaptive endocannabinoid signaling in the dPAG after nerve injury could contribute to chronic neuropathic pain and associated autonomic dysregulation. This study demonstrates that reduced anandamide content and upregulation of FAAH in the dPAG are associated with hyperalgesia and reduced heart rate sustained weeks after nerve injury. These data provide support for the evaluation of FAAH inhibitors for the treatment of chronic neuropathic pain.

Ventrolateral periaqueductal grey matter neurotransmission modulates cardiac baroreflex activity

Baroreflex activity is a neural mechanism responsible for short-term adjustments in blood pressure (BP). Several supramedullary areas, which send projections to the medulla, are able to control this reflex. In this context, the ventrolateral part of the periaqueductal grey matter (vlPAG), which is a mesencephalic structure, has been suggested to regulate the cardiovascular system. However, its involvement in baroreflex control has never been addressed. Therefore, our hypothesis is that the vlPAG neurotransmission is involved in baroreflex cardiac activity. Male Wistar rats had stainless steel guide cannulae unilaterally or bilaterally implanted in the vlPAG. Afterward, a catheter was inserted into the femoral artery for BP and HR recording. A second catheter was implanted into the femoral vein for baroreflex activation. When the nonselective synaptic blocker cobalt chloride (CoCl₂ ) was unilaterally injected into the vlPAG, in either the left or the right hemisphere, it increased the tachycardic response to baroreflex activation. However, when CoCl₂ was bilaterally microinjected into the vlPAG it decreased the tachycardic response to baroreflex stimulation. This work shows that vlPAG neurotransmission is involved in modulation of the tachycardic response of the baroreflex. Moreover, we suggest that the interconnections between the vlPAG of both hemispheres are activated during baroreflex stimulation. In this way, our work helps to improve the understanding about brain-heart circuitry control, emphasizing the role of the autonomic nervous system in such modulation.

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.

Medial prefrontal cortex TRPV1 channels modulate the baroreflex cardiac activity in rats

BACKGROUND AND PURPOSE

The ventral portion of the medial prefrontal cortex (vMPFC) comprises the infralimbic (IL), prelimbic (PL) and dorsopenducular (DP) cortices. The IL and PL regions facilitate the baroreceptor reflex arc. This facilitatory effect on the baroreflex is thought to be mediated by vMPFC glutamatergic transmission, through NMDA receptors. The glutamatergic transmission can be modulated by other neurotransmitters, such as the endocannabinoids, which are agonists of the TRPV1 receptor. TRPV1 channels facilitate glutamatergic transmission in the brain. Thus, we hypothesized that TRPV1 receptors in the vMPFC enhance the cardiac baroreflex response.

EXPERIMENTAL APPROACH

Stainless steel guide cannulae were bilaterally implanted into the vMPFC of male Wistar rats. Afterwards, a catheter was inserted into the femoral artery, for recording MAP and HR, and into the femoral vein for assessing baroreflex activation.

KEY RESULTS

Microinjections of the TRPV1 receptor antagonists capsazepine and 6-iodo-nordihydrocapsaicin (6-IODO) into the vMPFC reduced the cardiac baroreflex activity in unanaesthetized rats. Capsaicin microinjected into the vMPFC increased the cardiac baroreflex activity in unanaesthetized rats. When an ineffective dose of the TRPV1 receptor antagonist 6-IODO was used, the capsaicin-induced increase in the cardiac baroreflex response was abolished. The higher doses of capsaicin administered into the vMPFC after the ineffective dose of 6-IODO displaced the dose-response curve of the baroreflex parameters to the right, with no alteration in the maximum effect of capsaicin.

CONCLUSIONS AND IMPLICATIONS

The results of the present study show that stimulation of the TRPV1 receptors in the vMPFC increases the cardiac baroreceptor reflex response.