Hyperinnervation of mesenteric arteries in spontaneously hypertensive rats by sympathetic but not primary afferent axons

Elevated Vascular Sympathetic Neurotransmission and Remodelling Is a Common Feature in a Rat Model of Foetal Programming of Hypertension and SHR

Hypertension is of unknown aetiology, with sympathetic nervous system hyperactivation being one of the possible contributors. Hypertension may have a developmental origin, owing to the exposure to adverse factors during the intrauterine period. Our hypothesis is that sympathetic hyperinnervation may be implicated in hypertension of developmental origins, being this is a common feature with essential hypertension. Two-animal models were used: spontaneously hypertensive rats (SHR-model of essential hypertension) and offspring from dams exposed to undernutrition (MUN-model of developmental hypertension), with their respective controls. In adult males, we assessed systolic blood pressure (SBP), diastolic blood pressure (DBP), heart rate (HR), sympathetic nerve function (3H-tritium release), sympathetic innervation (immunohistochemistry) and vascular remodelling (histology). MUN showed higher SBP/DBP, but not HR, while SHR exhibited higher SBP/DBP/HR. Regarding the mesenteric arteries, MUN and SHR showed reduced lumen, increased media and adventitial thickness and increased wall/lumen and connective tissue compared to respective controls. Regarding sympathetic nerve activation, MUN and SHR showed higher tritium release compared to controls. Total tritium tissue/tyrosine hydroxylase detection was higher in SHR and MUN adventitia arteries compared to respective controls. In conclusion, sympathetic hyperinnervation may be one of the contributors to vascular remodelling and hypertension in rats exposed to undernutrition during intrauterine life, which is a common feature with spontaneous hypertension.

Sympathetic and Sensory-Motor Nerves in Peripheral Small Arteries

Small arteries, which play important roles in controlling blood flow, blood pressure, and capillary pressure, are under nervous influence. Their innervation is predominantly sympathetic and sensory motor in nature, and while some arteries are densely innervated, others are only sparsely so. Innervation of small arteries is a key mechanism in regulating vascular resistance. In the second half of the previous century, the physiology and pharmacology of this innervation were very actively investigated. In the past 10-20 yr, the activity in this field was more limited. With this review we highlight what has been learned during recent years with respect to development of small arteries and their innervation, some aspects of excitation-release coupling, interaction between sympathetic and sensory-motor nerves, cross talk between endothelium and vascular nerves, and some aspects of their role in vascular inflammation and hypertension. We also highlight what remains to be investigated to further increase our understanding of this fundamental aspect of vascular physiology.

[The functions of arterial sympathetic innervation: from development to pathology]

Arterial sympathetic innervation (ASI) is a complex biological process requiring a fine axonal guidance by arteries. Its physiological impact has remained unknown for decades but recently started to be better understood and recognized. ASI is a key element of the adaptive response of the cardiovascular system to challenging situations (exposure to cold, exercise…) as ASI controls the diameter of resistance arteries, thus blood supply to organs and systemic arterial blood pressure via arterial tone modulation. Defaults in ASI can lead to diseases, acting as a main cause or as an aggravating factor. Its impact is actively studied in cardiovascular diseases representing major public health issues, like hypertension, but ASI could also play a role in aging and many more pathological processes including cancer.

Expression and localization of the AT1 and AT2 angiotensin II receptors and α1A and α1D adrenergic receptors in aorta of hypertensive and diabetic rats

Hypertension and diabetes are multifactorial diseases that frequently coexist and exacerbate each another. During the development of diabetes, the impairment of noradrenergic and renin-angiotensin systems has been reported in the response mediated by α₁-AR and AT₁ receptors. Although their participation in the development of cardiovascular complications is still controversial, some studies have found increased or diminished response to the vasoconstrictive effect of noradrenaline or angiotensin II in a time-dependent manner of diabetes. Thus, the aim of this work was to investigate the possible changes in the expression or localization of α₁-AR (α_1A and α_1D) and angiotensin II receptors (AT₁ and AT₂) in aorta of rats after 4 weeks of the onset of diabetes. In order to be able to examine the expression of these receptors, immunofluorescence procedure was performed in tunica intima and tunica media of histological sections of aorta. Fluorescence was detected by a confocal microscopy. Our results showed that the receptors are expressed in both tunics, where adrenergic receptors have a higher density in tunica intima and tunica media of SHR compared with WKY; meanwhile, the expression of angiotensin II receptors is not modified in both groups of rats. On the other hand, the results showed that diabetes produced an increase or a decrease in the expression of receptors that is not associated to a specific type of receptor, vascular region, or strain of rat. In conclusion, diabetes and hypertension modify the expression of the receptors in tunica intima and tunica media of aorta in a different way.

Lack of endogenous adenosine tonus on sympathetic neurotransmission in spontaneously hypertensive rat mesenteric artery

Background

Increased sympathetic activity has been implicated in hypertension. Adenosine has been shown to play a role in blood flow regulation. In the present study, the endogenous adenosine neuromodulatory role, in mesenteric arteries from normotensive and spontaneously hypertensive rats, was investigated.

Methods and Results

The role of endogenous adenosine in sympathetic neurotransmission was studied using electrically-evoked [³H]-noradrenaline release experiments. Purine content was determined by HPLC with fluorescence detection. Localization of adenosine A₁ or A_2A receptors in adventitia of mesenteric arteries was investigated by Laser Scanning Confocal Microscopy. Results indicate a higher electrically-evoked noradrenaline release from hypertensive mesenteric arteries. The tonic inhibitory modulation of noradrenaline release is mediated by adenosine A₁ receptors and is lacking in arteries from hypertensive animals, despite their purine levels being higher comparatively to those determined in normotensive ones. Tonic facilitatory adenosine A_2A receptor-mediated effects were absent in arteries from both strains. Immunohistochemistry revealed an adenosine A₁ receptors redistribution from sympathetic fibers to Schwann cells, in adventitia of hypertensive mesenteric arteries which can explain, at least in part, the absence of effects observed for these receptors.

Conclusion

Data highlight the role of purines in hypertension revealing that an increase in sympathetic activity in hypertensive arteries is occurring due to a higher noradrenaline/ATP release from sympathetic nerves and the loss of endogenous adenosine inhibitory tonus. The observed nerve-to-glial redistribution of inhibitory adenosine A₁ receptors in hypertensive arteries may explain the latter effect.

Autonomic nerves and perivascular fat: interactive mechanisms

The evidence describing the autonomic innervation of body fat is reviewed with a particular focus on the role of the sympathetic neurotransmitters. In compiling the evidence, a strong case emerges for the interaction between autonomic nerves and perivascular adipose tissue (PVAT). Adipocytes have been shown to express receptors for neurotransmitters released from nearby sympathetic varicosities such as adrenoceptors (ARs), purinoceptors and receptors for neuropeptide Y (NPY). Noradrenaline can modulate both lipolysis (via α2- and β3-ARs) and lipogenesis (via α1- and β3-ARs). ATP can inhibit lipolysis (via P1 purinoceptors) or stimulate lipolysis (via P2y purinoceptors). NPY, which can be produced by adipocytes and sympathetic nerves, inhibits lipolysis. Thus the sympathetic triad of transmitters can influence adipocyte free fatty acid (FFA) content. Substance P (SP) released from sensory nerves has also been shown to promote lipolysis. Therefore, we propose a mechanism whereby sympathetic neurotransmission can simultaneously activate smooth muscle cells in the tunica media to cause vasoconstriction and alter FFA content and release from adjacent adipocytes in PVAT. The released FFA can influence endothelial function. Adipocytes also release a range of vasoactive substances, both relaxing and contractile factors, including adiponectin and reactive oxygen species. The action of adipokines (such as adiponectin) and reactive oxygen species (ROS) on cells of the vascular adventitia and nerves has yet to be fully elucidated. We hypothesise a strong link between PVAT and autonomic fibres and suggest that this poorly understood relationship is extremely important for normal vascular function and warrants a detailed study.

Impaired inhibitory function of presynaptic A1-adenosine receptors in SHR mesenteric arteries

In hypertension, vascular reactivity alterations have been attributed to numerous factors, including higher sympathetic innervation/adenosine. This study examined the modulation of adenosine receptors on vascular sympathetic nerves and their putative contribution to higher noradrenaline spillover in hypertension. We assessed adenosine receptors distribution in the adventitia through confocal microscopy, histomorphometry, and their regulatory function on electrically-evoked [(3)H]-noradrenaline overflow, using selective agonists/antagonists. We found that: i) A1-adenosine receptor agonist (CPA: 100 nM) inhibited tritium overflow to a lower extent in SHR (25% ± 3%, n = 14) compared to WKY (38% ± 3%, n = 14) mesenteric arteries; ii) A2A-adenosine receptor agonist (CGS 21680: 100 nM) induced a slight increase of tritium overflow that was similar in SHR (22% ± 8%, n = 8) and WKY (24% ± 5%, n = 8) mesenteric arteries; iii) A2B- and A3-adenosine receptors did not alter tritium overflow in either strain; iv) all adenosine receptors were present on mesenteric artery sympathetic nerves and/or some adventitial cells of both strains; and v) A1-adenosine receptor staining fractional area was lower in SHR than in WKY mesenteric arteries. We conclude that there is an impaired inhibitory function of vascular presynaptic A1-adenosine receptors in SHR, likely related to a reduced presence of these receptors on sympathetic innervation, which might lead to higher levels of noradrenaline in the synaptic cleft and contribute to hypertension in this strain.

Sympathetic axonopathies and hyperinnervation in the small intestine smooth muscle of aged Fischer 344 rats

It is well documented that the intrinsic enteric nervous system of the gastrointestinal (GI) tract sustains neuronal losses and reorganizes as it ages. In contrast, age-related remodeling of the extrinsic sympathetic projections to the wall of the gut is poorly characterized. The present experiment, therefore, surveyed the sympathetic projections to the aged small intestine for axonopathies. Furthermore, the experiment evaluated the specific prediction that catecholaminergic inputs undergo hyperplastic changes. Jejunal tissue was collected from 3-, 8-, 16-, and 24-month-old male Fischer 344 rats, prepared as whole mounts consisting of the muscularis, and processed immunohistochemically for tyrosine hydroxylase, the enzymatic marker for norepinephrine, and either the protein CD163 or the protein MHCII, both phenotypical markers for macrophages. Four distinctive sympathetic axonopathy profiles occurred in the small intestine of the aged rat: (1) swollen and dystrophic terminals, (2) tangled axons, (3) discrete hyperinnervated loci in the smooth muscle wall, including at the bases of Peyer's patches, and (4) ectopic hyperplastic or hyperinnervating axons in the serosa/subserosal layers. In many cases, the axonopathies occurred at localized and limited foci, involving only a few axon terminals, in a pattern consistent with incidences of focal ischemic, vascular, or traumatic insult. The present observations underscore the complexity of the processes of aging on the neural circuitry of the gut, with age-related GI functional impairments likely reflecting a constellation of adjustments that range from selective neuronal losses, through accumulation of cellular debris, to hyperplasias and hyperinnervation of sympathetic inputs.

Influence of pressure on adenosine triphosphate function as a sympathetic neurotransmitter in small mesenteric arteries from the spontaneously hypertensive rat

OBJECTIVES

Enhanced sympathetic neurotransmission contributes to hypertension in the spontaneously hypertensive rat (SHR). We recently reported a method for studying sympathetic neurotransmission in pressurized small arteries, demonstrating a major role of adenosine triphosphate (ATP) as a sympathetic neurotransmitter under these physiological conditions. We have now used this methodology to assess the role of ATP as a sympathetic neurotransmitter in small mesenteric arteries isolated from SHRs.

METHODS

Small arteries were mounted in a suction electrode, cannulated and pressurized to either 30 or 90 mmHg. Nerve-evoked alterations in membrane potential were assessed using sharp microelectrodes. Neurally evoked vasoconstrictor responses were measured in the absence and presence of the α1-adrenoceptor antagonist, tamsulosin (0.1 μmol/l), or the P2 purinoceptor antagonist suramin (0.1 mmol/l).

RESULTS

At 30 mmHg the P2X-receptor-mediated excitatory junctional potential (EJP) was larger in arteries from SHRs (7.9 ± 0.9 mV) than Wistar-Kyoto (WKY) rats (3.2 ± 0.4 mV, P < 0.05). Increasing pressure increased the amplitude of the EJP, which again, was larger in SHRs. At 90 mmHg, activation of the perivascular nerves produced a larger vasoconstriction in arteries isolated from SHRs compared with WKY rats. The vasoconstrictor response in SHRs was abolished by either suramin or tamsulosin.

CONCLUSION

These data provide electrophysiological evidence for enhanced purinergic function in the SHR and show that ATP is fundamentally important in contributing to the vasoconstriction produced after activation of the perivascular nerves in pressurized arteries from the SHR. This involves a synergistic interaction with noradrenaline to causes enhanced mesenteric arterial vasoconstriction, which may contribute to the hypertension in this model.

Angiotensinergic innervation of the kidney: present knowledge and its significance

Intrarenal neurotransmission implies the co-release of neuropeptides at the neuro-effector junction with direct influence on parameters of kidney function. The presence of an angiotensin (Ang) II-containing phenotype in catecholaminergic postganglionic and sensory fibers of the kidney, based on immunocytological investigations, has only recently been reported. These angiotensinergic fibers display a distinct morphology and intrarenal distribution, suggesting anatomical and functional subspecialization linked to neuronal Ang II-expression. This review discusses the present knowledge concerning these fibers, and their significance for renal physiology and the pathogenesis of hypertension in light of established mechanisms. The data suggest a new role of Ang II as a co-transmitter stimulating renal target cells or modulating nerve traffic from or to the kidney. Neuronal Ang II is likely to be an independent source of intrarenal Ang II. Further physiological experimentation will have to explore the role of the angiotensinergic renal innervation and integrate it into existing concepts.

Paracrine control of vascular innervation in health and disease

Proper vascular regulation is of paramount importance for the control of blood flow to tissues. In particular, the regulation of peripheral resistance arteries is essential for several physiological processes, including control of blood pressure, thermoregulation and increase of blood flow to central nervous system and heart under stress conditions such as hypoxia. Arterial tone is regulated by the periarterial autonomic nervous plexus, as well as by endothelium-dependent, myogenic and humoral mechanisms. Underscoring the importance of proper vascular regulation, defects in these processes can lead to diseases such as hypertension, orthostatic hypotension, Raynaud's phenomenon, defective thermoregulation, hand-foot syndrome, migraine and congestive heart failure. Here, we review the molecular mechanisms controlling the development of the periarterial nerve plexus, retrograde and localized signalling at neuro-effector junctions, the molecular and cellular mechanisms of vascular regulation and adult plasticity and maintenance of periarterial innervation. We particularly highlight a newly discovered role for vascular endothelial growth factor in the structural and functional maintenance of arterial neuro-effector junctions. Finally, we discuss how defects in neuronal vascular regulation can lead to disease.

Eph/ephrin interactions modulate vascular sympathetic innervation

Ephs and ephrins are membrane-bound proteins that interact to modulate axon growth and neuronal function. We tested the hypothesis that eph/ephrin interactions affected the growth and function of vascular sympathetic innervation. Using RT-PCR analyses, we detected both classes of ephs (A and B) and both classes of ephrins (A and B) in sympathetic ganglia from neonatal and adult rats. Both classes of ephs (A and B) and both classes of ephrins (A and B) bound to the cell bodies and neurites of dissociated postganglionic sympathetic neurons. Messenger RNAs encoding for both classes of ephs (A and B) and both classes of ephrins (A and B) were also detected in sympathetically innervated arteries from neonatal and adult rats. These data suggest that ephrins/ephs on nerve fibers of postganglionic sympathetic neurons could interact with ephs/ephrins on cells in innervated arteries. We found that ephA4 reduced reinnervation of denervated femoral arteries. Reinnervation in the presence of ephA4-Fc (38.9±6.6%) was significantly less than that in the presence of IgG-Fc (62±10%; n=5; p<0.05; one-tailed unpaired t-test). These data indicate that eph/ephrin interactions modulated the growth of vascular sympathetic innervation. We also found that ephA4 increased basal release of norepinephrine from nerve terminals of isolated tail arteries. These data indicate that eph/ephrin interactions affect the growth and function of vascular sympathetic innervation.

Neuropeptide Y2 receptors are involved in enhanced neurogenic vasoconstriction in spontaneously hypertensive rats

1. The present study addressed the role of neuropeptide (NPY) Y2 receptors in neurogenic contraction of mesenteric resistance arteries from female spontaneously hypertensive rats (SHR). Arteries were suspended in microvascular myographs, electrical field stimulation (EFS) was performed, and protein evaluated by Western blotting and immunohistochemistry. 2. In vasopressin-activated endothelium-intact arteries, NPY and fragments with selectivity for Y1 receptors, [Leu31,Pro34]NPY, Y2 receptors, NPY(13-36), and rat pancreatic polypeptide evoked more pronounced contractions in segments from SHR than in Wistar Kyoto (WKY) arteries, even in the presence of the Y1 receptor antagonist, BIBP3226 (0.3 microM, (R)-N(2)-(diphenacetyl)-N-[(4-hydroxyphenyl)methyl]D-arginineamide). 3. In the presence of prazosin and during vasopressin activation, EFS-evoked contractions were larger in arteries from SHR compared to WKY. EFS contractions were enhanced by the Y2 receptor selective antagonist BIIE0246TF (0.5 microM, (S)-N2-[[1-[2-[4-[(R,S)-5,11-dihydro-6(6h)-oxodibenz[b,e]azepin-11-y1]-1-piperazinyl]-2-oxoethyl]cyclo-pentyl-N-[2-[1,2-dihydro-3,5 (4H)-dioxo-1,2-diphenyl-3H-1,2,4-triazol-4-yl]ethyl]-argininamide), reduced by BIBP3226, and abolished by the combination of BIBP3226 and BIIE0246TF. 4. Immunoblotting showed NPY Y1 and Y2 receptor expression to be similar in arteries from WKY and SHR, although a specific Y2 receptor band at 80 kDa was detected only in arteries from WKY. 5. Immunoreaction for NPY was enhanced in arteries from SHR. In contrast to arteries from WKY, BIIE0246TF increased NPY immunoreactivity in EFS-stimulated arteries from SHR. 6. The present results suggest that postjunctional neuropeptide Y1 and Y2 receptors contribute to neurogenic contraction of mesenteric small arteries. Moreover, both enhanced NPY content and altered neuropeptide Y1 and Y2 receptor activation apparently contribute to the enhanced neurogenic contraction of arteries from SHR.

Enhanced neurally evoked responses and inhibition of norepinephrine reuptake in rat mesenteric arteries after spinal transection

In patients with high thoracic spinal lesions that remove most of the central drive to splanchnic preganglionic neurons, visceral or nociceptive stimuli below the lesion can provoke large increases in blood pressure (autonomic dysreflexia). We have examined the effects of T4 spinal transection on isometric contractions of mesenteric arteries isolated from spinalized rats. Nerve-evoked contractions involved synergistic roles for norepinephrine and ATP. At 7 wk after spinal transection, responses to perivascular stimulation at 1-5 Hz were enhanced fivefold, whereas the alpha1-adrenoceptor antagonist prazosin (10 nM) produced a twofold larger reduction in contraction (to 20 pulses at 10 Hz) than in unoperated controls. In contrast, the reduction in nerve-evoked contractions by the P2-purinoceptor antagonist suramin (0.1 mM) and the responses to the P2-purinoceptor agonist alpha,beta-methylene ATP or to high K+ concentration did not greatly differ between groups, indicating that arteries from spinalized rats were not generally hyperreactive. Sensitivity to the alpha1-adrenoceptor agonist phenylephrine was enhanced in arteries from spinalized rats, and the difference from controls was abolished by the norepinephrine uptake blocker desmethylimipramine. Sensitivity to the alpha1-adrenoceptor agonist methoxamine, which is not a substrate for the neuronal norepinephrine transporter, was similar among the groups. Thus the increased neurally evoked response after spinal transection appeared to be due to a reduction in neuronal uptake of released norepinephrine, a mechanism that did not explain the enhanced response of tail arteries after spinal transection that we previously reported. The findings provide further support for potentiated neurovascular responses contributing to the genesis of autonomic dysreflexia.