Comparison of density of sympathetic varicosities and their closeness to smooth muscle cells in rabbit middle cerebral and ear arteries and their branches

Targeted Radionuclide Therapy for Patients with Metastatic Pheochromocytoma and Paraganglioma: From Low-Specific-Activity to High-Specific-Activity Iodine-131 Metaiodobenzylguanidine

Low-specific-activity iodine-131–radiolabeled metaiodobenzylguanidine (I-131-MIBG) was introduced last century as a potential systemic therapy for patients with malignant pheochromocytomas and paragangliomas. Collective information derived from mainly retrospective studies has suggested that 30–40% of patients with these tumors benefit from this treatment. A low index of radioactivity, lack of therapeutic standardization, and toxicity associated with intermediate to high activities (absorbed radiation doses) has prevented the implementation of I-131-MIBG’s in clinical practice. High-specific-activity, carrier-free I-131-MIBG has been developed over the past two decades as a novel therapy for patients with metastatic pheochromocytomas and paragangliomas that express the norepinephrine transporter. This drug allows for a high level of radioactivity, and as yet is not associated with cardiovascular toxicity. In a pivotal phase two clinical trial, more than 90% of patients achieved partial responses and disease stabilization with the improvement of hypertension. Furthermore, many patients exhibited long-term persistent antineoplastic effects. Currently, the high-specific-activity I-131-MIBG is the only approved therapy in the US for patients with metastatic pheochromocytomas and paragangliomas. This review will discuss the historical development of high-specific-activity I-131-MIBG, its benefits and adverse events, and future directions for clinical practice applicability and trial development.

Similar pathological effects of sympathectomy and hypercholesterolemia on arterial smooth muscle cells and fibroblasts

In a previous study, we showed that after sympathectomy, the femoral (FA) but not the basilar (BA) artery from non-pathological rabbits manifests migration of adventitial fibroblasts (FBs) into the media and loss of medial smooth muscle cells (SMCs). The aim of the present study was to verify whether similar behaviour of arteries occurred in the pathological context of atherosclerosis. Thus, similar experiments were conducted on hypercholesterolemic rabbits, which were chemically sympathectomized with 6-hydroxydopamine (n=4) or treated with vehicle for control (n=5). Cross-sections of BA and FA were immunolabelled for five markers of phenotypic modulation of vascular SMCs and FBs: vimentin, desmin, alpha-smooth muscle actin, beta-isoform of actin, and h-caldesmon and examined using a confocal microscope. Also, 3D images were constructed and morphometric analysis performed using image analysis software. Both intact and sympathectomized BA and FA developed atherosclerotic plaques, but the thickening of the intima was more advanced in sympathectomized animals, as judged by increased plaque frequency and by the phenotypic modulation of SMCs in the intima. Our results show that in the media of FAs hypercholesterolemia induces changes similar to those observed in sympathectomized rabbits in non-pathological conditions, i.e., migration of adventitial FBs to the media and loss of medial SMCs. These latter changes, which can be ascribed to pathological events, were accentuated after sympathectomy in the hypercholesterolemic rabbits. The present study reveals that pathological events, including migration and phenotypic modulation of vascular FBs and loss of SMCs, may be under the influence of sympathetic nerves.

Differing influence of sympathectomy on smooth muscle cells and fibroblasts in cerebral and peripheral muscular arteries

In the present study, we examined the effect of sympathectomy on the distribution and the relative expression of cytoskeletal proteins used as markers of phenotypic modulation of vascular smooth muscle cells (SMCs) and myofibroblasts (MFBs) in rabbit femoral (FA) and basilar (BA) arteries. Adult rabbits were treated either with repeated 6-hydroxydopamine (6-OHDA) for sympathectomy or with vehicle for control. Cross sections taken from sympathectomized and control arteries 79 days later were immunolabelled for vimentin, desmin, alpha-smooth muscle actin (alpha-SM actin), beta-isoform of actin and h-caldesmon. The distribution of these proteins and the intensity of fluorescent labelled SMCs were examined under a confocal microscope. In the sympathectomized BA, there was no change for desmin, vimentin and h-caldesmon expression, but the expression of both alpha-SM actin and the beta-isoform was significantly higher (+19% and +30%, respectively). In the sympathectomized FA, the expression of the alpha- and beta-isoforms of actin remained unchanged, whereas those of desmin and vimentin were significantly higher (+35% and 17%, respectively) and h-caldesmon expression was lowered by 13%. In contrast to intact FAs, the external layers of sympathectomized FAs revealed migration of fibroblasts from the adventitia and death of SMCs. These results strongly suggest that sympathetic nerves intervene in the cytoskeletal protein remodelling through phenotypic modulation of both SMCs and MFBs during post-natal development, and in pathologies involving similar phenomena, such as atherosclerosis.

Responses of muscle spindles in feline dorsal neck muscles to electrical stimulation of the cervical sympathetic nerve

Previous studies performed in jaw muscles of rabbits and rats have demonstrated that sympathetic outflow may affect the activity of muscle spindle afferents (MSAs). The resulting impairment of MSA information has been suggested to be involved in the genesis and spread of chronic muscle pain. The present study was designed to investigate sympathetic influences on muscle spindles in feline trapezius and splenius muscles (TrSp), as these muscles are commonly affected by chronic pain in humans. Experiments were carried out in cats anesthetized with alpha-chloralose. The effect of electrical stimulation (10 Hz for 90 s or 3 Hz for 5 min) of the peripheral stump of the cervical sympathetic nerve (CSN) was investigated on the discharge of TrSp MSAs (units classified as Ia-like and II-like) and on their responses to sinusoidal stretching of these muscles. In some of the experiments, the local microcirculation of the muscles was monitored by laser Doppler flowmetry. In total, 46 MSAs were recorded. Stimulation of the CSN at 10 Hz powerfully depressed the mean discharge rate of the majority of the tested MSAs (73%) and also affected the sensitivity of MSAs to sinusoidal changes of muscle length, which were evaluated in terms of amplitude and phase of the sinusoidal fitting of unitary activity. The amplitude was significantly reduced in Ia-like units and variably affected in II-like units, while in general the phase was affected little and not changed significantly in either group. The discharge of a smaller percentage of tested units was also modulated by 3-Hz CSN stimulation. Blockade of the neuromuscular junctions by pancuronium did not induce any changes in MSA responses to CSN stimulation, showing that these responses were not secondary to changes in extrafusal or fusimotor activity. Further data showed that the sympathetically induced modulation of MSA discharge was not secondary to the concomitant reduction of muscle blood flow induced by the stimulation. Hence, changes in sympathetic outflow can modulate the afferent signals from muscle spindles through an action exerted directly on the spindles, independent of changes in blood flow. It is suggested that such an action may be one of the mechanisms mediating the onset of chronic muscle pain in these muscles in humans.

Evaluation of functional response of cerebral arteries by a new morphometric technique

The contractile response of the rabbit basilar artery under four conditions was determined: (1) response in a resting condition without exclusion of the sympathetic nervous system (control group I); (2) response in a resting condition with alpha-adrenoceptor blockade (group II); (3) response to subarachnoid haemorrhage (SAH) (group III); and (4) response to SAH with alpha-adrenoceptor blockade (group IV). It was also ascertained whether it was possible to measure contractile response using a new morphometric method. Vessels were prepared by intracardial perfusion fixation, stained by haematoxylin and eosin, and the length of the intimal corrugations were measured by computer image analysis. Two procedures were followed in order to express the intensity of intimal corrugation, indicating the contractibility of the basilar arteries: (1) the corrugation coefficient (CC) of the basilar artery intima was estimated by dividing the precisely measured length of the intimal corrugations by the length of the measured vessel wall section of the vessel cross-sections (obtained histologically); (2) the lumen reduction coefficient (LRC) of the basilar artery was determined by dividing the "ideal" luminal area (calculated from the total length of the intimal circumference) by the real luminal cross-section area. The results of CC measurements revealed the smoothest intima (mean CC = 1.146, P = 0.00) and the least reduction of lumen (mean LRC = 0.26, P = 0.000) in group II (rabbits without SAH but with alpha-blocker phenoxybenzamine), and in group IV (SAH group of rabbits with alpha-blocker phenoxybenzamine) where the mean CC was 1.141 (P = 0.001) and the mean LRC was 0.33 (P = 0.002) in comparison with the SAH-only group III, pointing out the effectiveness of alpha-blockade even against SAH vasospastic stimuli. Control group I (without SAH and without treatment) showed a greater degree of corrugation in the intima and an increased reduction in the lumina than in groups II and IV, but still significantly less than in group III (mean CC = 1.197, P = 0.001, and mean LRC = 0.40, P = 0.028), thus demonstrating a certain resting tone of the basilar arteries (in an ideal situation, without any tone at all, the CC and LRC would be equal one). The highest degree of intimal corrugation and the greatest lumina reduction were discovered in the SAH-only group III (mean CC = 1.374 and mean LRC = 0.60). The differences among groups I, II and IV were insignificant. The results of this study suggest four conclusions: (i) the possibility of evaluating the functional response of rabbit cerebral arteries using this new morphometric technique; (ii) the adrenergic influence on resting tone of these arteries; (iii) the likely preventive role of an alpha-blockade on post-SAH vasospasm of basilar arteries in rabbits; and (iv) good comparability of the results of CC and LRC measurements with the angiographically estimated vessel diameters of other similar studies.

Specialised sympathetic neuroeffector associations in immature rat iris arterioles

Sympathetic nerve-mediated vasoconstriction in iris arterioles of mature rats occurs via the activation of alpha(1B)-adrenoceptors alone, while in immature rat iris arterioles, vasoconstriction occurs via activation of both alpha1- and alpha2-adrenoceptors. In mature rats the vast majority of sympathetic varicosities form close neuroeffector junctions. Serial section electron microscopy of 14 d iris arterioles has been used to determine whether restriction in physiological receptor types with age may result from the establishment of these close neuroeffector junctions. Ninety varicosities which lay within 4 microm of arteriolar smooth muscle were followed for their entire length. Varicosities rarely contained dense cored vesicles even after treatment with 5-hydroxydopamine. 47 % of varicosities formed close associations with muscle cells and 88 % formed close associations with muscle cells or melanocytes. Varicosities in bundles were as likely as single varicosities to form close associations with vascular smooth muscle cells, although the distribution of synaptic vesicles in single varicosities did not show the asymmetric accumulation towards the smooth muscle cells seen in the varicosities in bundles which were frequently clustered together. We conclude that restriction of physiological receptor types during development does not appear to correlate with the establishment of close neuroeffector junctions, although changes in presynaptic structures may contribute to the refinement of postsynaptic responses.

Effect of generalised sympathetic activation by cold pressor test on cerebral haemodynamics in healthy humans

There is no general agreement regarding several aspects of the role of the sympathetic system on cerebral haemodynamics such as extent of effectiveness, operational range and site of action. This study was planned to identify the effect of a generalised sympathetic activation on the cerebral haemodynamics in healthy humans before it is masked by secondary corrections, metabolic or myogenic in nature. A total of 35 healthy volunteers aged 20-35 underwent a 5 min lasting cold pressor test (CPT) performed on their left hand. The cerebral blood flow (CBF) velocity in the middle cerebral arteries and arterial blood pressure were recorded with transcranial Doppler sonography and with a non-invasive finger-cuff method, respectively. The ratio of arterial blood pressure to mean blood velocity (ABP/Vm) and Pulsatility Index (PI) were calculated throughout each trial. CPT induced an increase in mean ABP (range 2-54 mmHg depending on the subject) and only a slight, though significant, increase in blood velocity in the middle cerebral artery (+2.4 and +4.4% on ipsi- and contralateral side, respectively). During CPT, the ratio ABP/Vm increased and PI decreased in all subjects on both sides. These changes began simultaneously with the increase in blood pressure. The increase in ABP/Vm ratio is attributed to an increase in the cerebrovascular resistance, while the concomitant reduction in PI is interpreted as due to the reduction in the compliance of the middle cerebral artery. The results suggest that generalised increases in the sympathetic discharge, causing increases in ABP, can prevent concomitant increases in CBF by acting on both small resistance and large compliant vessels. This effect is also present when a slight increase in blood pressure occurs, which suggests a moderate increase in the sympathetic discharge, i.e. when ABP remains far below the upper limit of CBF autoregulation.

Parasympathetic varicosity proliferation and synaptogenesis in rat eyelid smooth muscle after sympathectomy

Parasympathetic innervation to eyelid smooth muscle inhibits sympathetic neurotransmission pre-junctionally without appreciable direct post-junctional effects. However, 5 weeks after sympathectomy, parasympathetic stimulation elicits substantial cholinergically mediated contractions. This study examined ultrastructural changes accompanying the conversion to parasympathetic excitation. In intact muscles, 64+/-9 nerve varicosities were encountered per 104 micron2. Most were close to muscle cells and not fully enclosed by supporting cells. Axo-axonal synapses were observed occasionally. Two days following sympathectomy, varicosity numbers were reduced by 97% and, relative to controls, remaining varicosities were farther from muscle cells and more frequently fully enclosed by supporting cells, but contained greater numbers of small spherical and large dense vesicles. By 6 weeks post-sympathectomy, numbers of varicosities per unit muscle volume increased to 14% of controls. These varicosities differed from those at 2 days in being closer to smooth muscle cells, less frequently enclosed, and having fewer small vesicles. These findings indicate that intact eyelid smooth muscle varicosities are predominantly sympathetic, but a small number of parasympathetic varicosities are present, some of which may form pre-junctional synapses with sympathetic nerves. Between 2 days and 6 weeks post-sympathectomy, varicosities increased in number and established appositions with smooth muscle cells. This suggests that parasympathetic nerves are capable of re-innervating an atypical smooth muscle target after sympathectomy, and that parasympathetic synaptogenesis is likely to contribute to conversion from pre-junctional inhibition to post-junctional excitation after sympathectomy.

Astroglial and vascular interactions of noradrenaline terminals in the rat cerebral cortex

Noradrenaline (NA) has been shown to influence astrocytic and vascular functions related to brain homeostasis, metabolism, local blood flow, and blood-brain barrier permeability. In the current study, we investigate the possible associations that exist between NA-immunoreactive nerve terminals and astrocytes and intraparenchymal blood vessels in the rat frontoparietal cortex, both at the light and electron microscopic levels. As a second step, we sought to determine whether the NA innervation around intracortical microvessels arises from peripheral or central structures by means of injections of N-(2-chloroethyl-N-ethyl-2-bromobenzylamine) (DSP-4), a neurotoxin that specifically destroys NA neurons from the locus ceruleus. At the light microscopic level, 6.8% of all NA-immunoreactive nerve terminals in the frontoparietal cortex were associated with vascular walls, and this perivascular noradrenergic input, together with that of the cerebral cortex, almost completely disappeared after DSP-4 administration. When analyzed at the ultrastructural level in control rats, NA terminals in the neuropil had a mean surface area of 0.53 +/- 0.03 micron2 and were rarely junctional (synaptic incidence close to 7%). Perivascular terminals (located within a 3-micron perimeter from the vessel basal lamina) counted at the electron microscopic level represented 8.8% of the total NA terminals in the cortical tissue. They were smaller (0.29 +/- 0.01 micron2, P < 0.05) than their neuronal counterparts and were located, on average, 1.34 +/- 0.08 microns away from intracortical blood vessels, which consisted mostly of capillaries (65%). None of the perivascular NA terminals engaged in junctional contacts with surrounding neuronal or vascular elements. The primary targets of both neuronal and perivascular NA nerve terminals consisted of dendrites, nerve terminals, astrocytes, and axons, whereas in the immediate vicinity (0.25 micron or less) of the microvessels, astrocytic processes represented the major target. The results of the current study show that penetrating arteries and intracortical microvessels receive a central NA input, albeit parasynaptic in its interaction, originating from the locus ceruleus. Particularly, they point to frequent appositions between both neuronal and perivascular NA terminals and astroglial cells and their processes. Such NA neuronal-glial and neuronal-glial-vascular associations could be of significance in the regulation of local metabolic and vascular functions under normal and pathologic situations.

Serotonin in the regulation of brain microcirculation

Manipulation of brainstem serotonin (5-HT) raphe neurons induces significant alterations in local cerebral metabolism and perfusion. The vascular consequences of intracerebrally released 5-HT point to a major vasoconstrictor role, resulting in cerebral blood flow (CBF) decreases in several brain regions such as the neocortex. However, vasodilatations, as well as changes in blood-brain barrier (BBB) permeability, which are blocked by 5-HT receptor antagonists also can be observed. A lack of relationship between the changes in flow and metabolism indicates uncoupling between the two variables and is suggestive of a direct neurogenic control by brain intrinsic 5-HT neurons on the microvascular bed. In line with these functional data are the close associations that exist between 5-HT neurons and the microarterioles, capillaries and perivascular astrocytes of various regions but more intimately and/or more frequently so in those where CBF is altered significantly following manipulation of 5-HT neurons. The ability of the microvascular bed to respond directly to intracerebrally released 5-HT is underscored by the expression of distinct 5-HT receptors in the various cellular compartments of the microvascular bed. Thus, it appears that while some 5-HT-mediated microvascular functions involve directly the blood vessel wall, others would be relayed through the perivascular astrocyte. The strategic localization of perivascular astrocytes and the different 5-HT receptors that they harbor strongly emphasize their putative pivotal role in transmitting information between 5-HT neurons and microvessels. It is concluded that the cerebral circulation has full capacity to adequately and locally adapt brain perfusion to changes in central 5-HT neurotransmission either directly or indirectly via the neuronal-astrocytic-vascular tripartite functional unit. Dysfunctions in these neurovascular interactions might result in perfusion deficits and might be involved in specific pathological conditions.