Arterial Baroreflex Dysfunction Fails to Mimic Parkinson’s Disease in Rats

Using Rotenone to Model Parkinson's Disease in Mice: A Review of the Role of Pharmacokinetics

Rotenone is a naturally occurring toxin that inhibits complex I of the mitochondrial electron transport chain. Several epidemiological studies have shown an increased risk of Parkinson's disease (PD) in individuals exposed chronically to rotenone, and it has received great attention for its ability to reproduce many critical features of PD in animal models. Laboratory studies of rotenone have repeatedly shown that it induces in vivo substantia nigra dopaminergic cell loss, a hallmark of PD neuropathology. Additionally, rotenone induces in vivo aggregation of α-synuclein, the major component of Lewy bodies and Lewy neurites found in the brain of PD patients and another hallmark of PD neuropathology. Some in vivo rotenone models also reproduce peripheral signs of PD, such as reduced intestinal motility and peripheral α-synuclein aggregation, both of which are thought to precede classical signs of PD in humans, such as cogwheel rigidity, bradykinesia, and resting tremor. Nevertheless, variability has been noted in cohorts of animals exposed to the same rotenone exposure regimen and also between cohorts exposed to similar doses of rotenone. Low doses, administered chronically, may reproduce PD symptoms and neuropathology more faithfully than excessively high doses, but overlap between toxicity and parkinsonian motor phenotypes makes it difficult to separate if behavior is examined in isolation. Rotenone degrades when exposed to light or water, and choice of vehicle may affect outcome. Rotenone is metabolized extensively in vivo, and choice of route of exposure influences greatly the dose used. However, male rodents may be capable of greater metabolism of rotenone, which could therefore reduce their total body exposure when compared with female rodents. The pharmacokinetics of rotenone has been studied extensively, over many decades. Here, we review these pharmacokinetics and models of PD using this important piscicide.

Baroreflex dysfunction in Parkinson's disease: integration of central and peripheral mechanisms

The incidence of Parkinson's disease (PD) is increasing worldwide. Although the PD hallmark is the motor impairments, nonmotor dysfunctions are now becoming more recognized. Recently, studies have suggested that baroreflex dysfunction is one of the underlying mechanisms of cardiovascular dysregulation observed in patients with PD. However, the large body of literature on baroreflex function in PD is unclear. The baroreflex system plays a major role in the autonomic, and ultimately blood pressure and heart rate, adjustments that accompany acute cardiovascular stressors on a daily basis. Therefore, impaired baroreflex function (i.e., decreased sensitivity or gain) can lead to altered neural cardiovascular responses. Since PD affects parasympathetic and sympathetic branches of the autonomic nervous system and both are orchestrated by the baroreflex system, understanding of this crucial mechanism in PD is necessary. In the present review, we summarize the potential altered central and peripheral mechanisms affecting the feedback-controlled loops that comprise the reflex arc in patients with PD. Major factors including arterial stiffness, reduced number of C1 and activation of non-C1 neurons, presence of central α-synuclein aggregation, cardiac sympathetic denervation, attenuated muscle sympathetic nerve activity, and lower norepinephrine release could compromise baroreflex function in PD. Results from patients with PD and from animal models of PD provide the reader with a clearer picture of baroreflex function in this clinical condition. By doing so, our intent is to stimulate future studies to evaluate several unanswered questions in this research area.

Autonomic dysfunction in Parkinson disease and animal models

Parkinson disease has traditionally been classified as a movement disorder, despite patients' accounts of diverse symptoms stemming from impairments in numerous body systems. Today, Parkinson disease is increasingly recognized by clinicians and scientists as a complex neurodegenerative disorder featuring both motor and nonmotor manifestations concomitant with pathology throughout all major branches of the nervous system. Dysfunction of the autonomic nervous system, or dysautonomia, is a common feature of Parkinson disease. It produces signs and symptoms that severely affect patients' quality of life, such as blood pressure dysregulation, hyperhidrosis, and constipation. Treatment options for dysautonomia are limited to symptom alleviation because the cause of these symptoms and Parkinson disease overall are still unknown. Animal models provide a platform to interrogate mechanisms of Parkinson disease-related autonomic nervous system dysfunction and test novel treatment strategies. Several animal models of Parkinson disease are available, each with different effects on the autonomic nervous system. This review critically analyses key dysautonomia signs and symptoms and associated pathology in Parkinson disease patients and relevant findings in animal models. We focus on the cardiovascular system, adrenal medulla, skin/thermoregulation, bladder, pupils, and gastrointestinal tract, to assess the contribution of animal models to the understanding of Parkinson disease autonomic dysfunction.

Dysautonomias in Parkinson's disease: cardiovascular changes and autonomic modulation in conscious rats after infusion of bilateral 6-OHDA in substantia nigra

It is important to elucidate the mechanism of dysautonomias in patients with Parkinson's disease; therefore, this study aimed to investigate the cardiovascular and autonomic changes that occur in an animal model of Parkinsonism. Adult male Wistar rats were anesthetized before bilateral microinfusions of 6-hydroxydopamine (6-OHDA) into the substantia nigra. The sham group underwent the same surgical procedure but received vehicle. After 7 days, the mean arterial pressure (MAP) and heart rate (HR) were measured, and various drugs were injected into conscious rats through cannulas previously implanted in the femoral artery and vein. Spectral analyses of systolic arterial pressure (SAP) and pulse interval (PI) were conducted with the CardioSeries software as the spontaneous baroreflex gain and effectivity. The animals were subjected to α-, β-adrenergic, or muscarinic receptor antagonism. For confirmation of the lesion, the levels of dopamine in the striatum were quantified by high-performance liquid chromatography. Animals that underwent 6-OHDA microinfusion had lower MAP and HR compared with those in the sham group. Spectral analysis of SAP showed that 6-OHDA animals exhibited a decrease in the sympathetic component. The PI values did not differ between groups. After the administration of muscarinic and β-adrenergic antagonists, the cardiovascular measures did not differ between the groups. However, upon administration of the α-adrenergic antagonist, the 6-OHDA animals exhibited a lower decrease in the MAP. We report cardiovascular impairments in 6-OHDA animals, possibly due to decreased sympathetic activity. Determination of the origin of these changes (central or peripheral) requires further investigation.

Ketanserin improves cardiac performance after myocardial infarction in spontaneously hypertensive rats partially through restoration of baroreflex function

AIM

Baroreflex dysfunction is associated with a higher rate of sudden death after myocardial infarction (MI). Ketanserin enhances baroreflex function in rats. The present work was designed to examine whether ketanserin improves the post-MI cardiac function and to explore the possible mechanism involved.

METHODS

Spontaneously hypertensive rats (SHR) were treated with ketanserin (0.3 mg·kg(-1)·d(-1)). Two weeks later, blood pressure and baroreflex function were measured, followed by a ligation of the left coronary artery. The expressions of vesicular acetylcholine transporter (VAChT) and α7 nicotinic acetylcholine receptor (α7-nAChR) in ischemic myocardium, angiogenesis, cardiac function, and left ventricular (LV) remodeling were evaluated subsequently.

RESULTS

Ketanserin significantly improved baroreflex sensitivity (0.62±0.21 vs 0.34±0.12 ms/mmHg, P<0.01) and vagal tonic activity (heart rate changes in response to atropine, 54.8±16.2 vs 37.6±13.4 bpm, P<0.01) without affecting the blood pressure or basic heart rate in SHR. Treatment of SHR with ketanserin prominently improved cardiac function and alleviated LV remodeling, as reflected by increases in the ejection fraction, fractional shortening, and LV systolic pressure as well as decreases in LV internal diameter and LV relative weight. The capillary density, vascular endothelial growth factor expression, and blood flow in the ischemic myocardium were significantly higher in the ketanserin-treated group. In addition, ketanserin markedly increased the expression of VAChT and α7-nAChR in ischemic myocardium.

CONCLUSION

Ketanserin improved post-MI cardiac function and angiogenesis in ischemic myocardium. The findings provide a mechanistic basis for restoring baroreflex function using ketanserin in the treatment of MI.

Baroreflex deficiency hampers angiogenesis after myocardial infarction via acetylcholine-α7-nicotinic ACh receptor in rats

AIMS

Angiogenesis is critical for re-establishing blood supply to ischaemic myocardium after myocardial infarction (MI). Human studies have associated arterial baroreflex (ABR) deficiency with higher rate of sudden death after MI. The present work was designed to examine whether ABR deficiency affects angiogenesis in MI rats.

METHODS AND RESULTS

Baroreflex sensitivity (BRS) was determined in conscious rats at 1 month after occlusion of the left anterior descending coronary artery. The survival time was significantly shorter in Sprague-Dawley rats with BRS <0.60 ms/mmHg vs. those with BRS ≥0.60 ms/mmHg. Sinoaortic denervation destroyed ABR, and decreased capillary density, regional blood flow and vascular endothelial growth factor (VEGF) concentration after MI. Ketanserin (0.6 mg/kg/day) enhanced BRS, and increased capillary density, regional blood flow, and VEGF. Sinoaortic denervation also reduced the expression of vesicular acetylcholine (ACh) transporter and α7-nicotinic ACh receptor (α7-nAChR). Angiogenesis after MI was significantly attenuated in α7-nAChR knockout mice. In contrast, increase in endogenous ACh with cholinesterase inhibitor pyridostigmine (30 mg/kg/day) increased angiogenesis after MI. In cultured cardiac microvascular endothelial cells, ACh stimulated the expression of VEGF, phosphorylation of VEGF receptor 2, and tube formation in a manner dependent upon α7-nAChR.

CONCLUSION

Our results demonstrated that ABR deficiency could attenuate angiogenesis in ischaemic myocardium. These findings provide further mechanistic basis for enhancing baroreflex function in the treatment of MI.

Buspirone improves 6-hydroxydopamine-induced catalepsy through stimulation of nigral 5-HT(1A) receptors in rats

Receptors for 5-HT(1A) are widely distributed throughout the basal ganglia, and their activation results in an inhibition of dopamine (DA) release. This study aimed to investigate the effect of buspirone, as a partial agonist of 5-HT(1A) receptors, on 6-hydroxydopamine (6-OHDA)-induced catalepsy in male Wistar rats. Catalepsy was induced by unilateral infusion of 6-OH-DA (6 microg/2 microl/rat) into the central region of the substantia nigra pars compacta (SNc) and assayed by the bar-test method 60, 120 and 180 min after drug administration. The results demonstrated that intraperitoneal (ip) injection of buspirone at doses of 5, 7.5 and 10 mg/kg decreased catalepsy compared with the control group. In addition, intra-SNc injection of 8-hydroxy-2-[di-n-propylamino]tetralin (8-OH-DPAT; 10 microg/rat), a 5-HT(1A) receptor agonist, decreased 6-OHDA-induced catalepsy. The effects of buspirone (7.5 mg/kg, ip) and 8-OH-DPAT (10 microg/rat, intra-SNc) were abolished by 1-(2-methoxyphenyl)-4-[4-(2-phthalimido) butyl]piperazine hydrobromide (NAN-190; 10 microg/rat, intra-SNc), a 5-HT(1A) receptor antagonist. Our study indicates that buspirone improves catalepsy in a 6-OHDA-induced animal model of Parkinson's disease through activation of nigral 5-HT(1A) receptors. However, further investigations should be undertaken to clarify the exact mechanism of interaction between 5-HT(1A) and DA receptors.

Association of dopamine DA-D2 receptor in rotenone-induced cytotoxicity in PC12 cells

The investigations were aimed to study the possible association of dopamine DA-D2 receptor in rotenone-induced cytotoxicity in PC12 cells, one among the most studied cell line in neurotoxicity studies. PC12 cells were subjected to receive an exposure of rotenone (10-6 to 10-4 M) for 24 and 48 hours. Cytotoxicity studies were carried out using standard end points including, (3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) (MTT), lactate dehydrogenase (LDH) release and neutral red uptake (NRU). Cells were found to be vulnerable to rotenone in dose-dependent manner. In general, 10-4 and 10-5 M concentrations were found to be cytotoxic, whereas 10-6 M and lower concentrations used have shown nonsignificant effect on cell viability. Further, studies were extended to study the rotenone-induced alterations in cellular glutathione (GSH) level and dopamine DA-D2 receptor expression. Significant (p < 0.001) chronological depletion in GSH levels were recorded following rotenone exposure. Expression of dopamine DA-D2 receptor was also found to be effected significantly (p < 0.001) at 24 hours of rotenone exposure (10-4 and 10-5). However, no further depletion in the expression of dopamine DA-D2 receptor could be recorded with extended exposure period, that is, 48 hours. Rotenone at 10-6 M and lower concentrations was found to be ineffective in PC12 cells. Data suggest the vulnerability of PC12 cells against experimental exposure of rotenone, which possibly routed through dopamine DA-D2 receptor and oxidative stress machinery.