Extraneuronal effects of 6-hydroxydopamine

Summary The effects of various concentrations of 6-hydroxydopamine (6OHDA) on rat adrenocortical cells in tissue culture were studied with phase contrast and electron microscopy. With 40 mg/l of 6-OHDA the first signs of alteration as revealed by microcinematography appeared in isolated cortical cells as early as 15 min after addition of the drug. There was a cessation of movement of cell organelles and an immobilisation of membrane undulations followed by the development of dark inclusion bodies. The cells underwent increasing shrinkage and collapsed by 11/2 h. Chromaffin cells were not affected until 45 min after exposure to the drug and neurons were the most resistant population. However 61/2 h after application of the drug most cells in the culture were dead. 6-OHDA applied in different doses and to adrenal expiants did not alter the sequence of events. Ultrastructurally cortex cells underwent damage along two lines: they either showed lytic changes or developed various types of dense bodies before reaching the lytic stage.Treatment of cortical cells with 40 mg/l 5-or 6-OHDA followed by exposure to buffered 2% glyoxylic acid and heat did not produce a fluorescence within the cells. Microspectrofluorimetry on amine models with noradrenaline, 5- and 6-OHDA revealed that neither 5-nor 6-OHDA are capable to form a fluorophore with glyoxylic acid.Dedicated to Professor Berta Scharrer in honor of her 70th birthdaySupported by a grant from Deutsche Forschungsgemeinschaft (Un 34/3) and a Research Fellowship of the University of Melbourne to K.U., and a Research Fellowship and Grant-in-Aid from the Life Insurance Medical Research Fund of Australia and New Zealand to J.H.C.     

Effect of 6-hydroxydopamine on dark adaptation in the rat retina

6-Hydroxydopamine administered intravitreously to light-adapted rats appears to prevent the recovery of retinal sensitivity during subsequent dark adaptation through its interaction with the photochemical mechanisms.     

Effect of 6-hydroxydopamine on diet-induced hyperlipidemia and atherosclerosis in the rat.

The effect of 6-hydroxydopamine (6-OH-dopamine) and of a cholesterol rich diet on the plasma and aortic cholesterol of female rats were studied. Both the diet and the 6-OH-dopamine produced an important increase in plasmatic and aortic cholesterol. A synergistic effect of these two treatments was observed on the plasma but not on the aortic cholesterol. The mechanism by which 6-OH-dopamine produces hypercholesterolemia and a increase in aortic cholesterol remains to be explained.     

Effect of 6-hydroxydopamine on neuropeptides in the rat female genitourinary tract

The occurrence and distribution of neuropeptide Y has been determined in the rat female genitourinary tract by radioimmunoassay and chromatographic analysis. Within the bladder, higher concentrations of neuropeptide Y were found in the trigone (48.8±5.2 pmol/g) than in the dome (36.0±2.1 pmol/g). In the genital tract, highest concentrations were identified in the vagina (41.4±2.1 pmol/g). Treatment of rats with 6-hydroxydopamine resulted in significant depletion of neuropeptide Y concentrations in both parts of the bladder, together with vagina, uterine horn and fallopian tube. No change was observed in the cervix, uterine body and ovary. Concentrations of vasoactive intestinal polypeptide were unaffected by treatment with 6-hydroxydopamine except in the area of the cervix where concentrations rose from 64.1±5.7 pmol/g to 133.6±15.1 pmol/g (p<0.05). There was a generalised, but statistically insignificant rise in substance P concentrations.     

The effects of 6-hydroxydopamine on the appearance of granulated vesicles in glomus cells of the rat carotid body

The glomus cells of the rat carotid body reveal an intense fluorescence after exposure to paraformaldehyde vapor and contain catecholamines. After initial fixation in glutaraldehyde, many granulated vesicles are seen in the glomus cells. After initial fixation in osmium tetroxide, most of the vesicles are depleted of their dense interiors and granulated vesicles occur infrequently. Administration of 6-hydroxydopamine followed by initial fixation in osmium tetroxide leads to the reappearance of dense interiors in virtually all vesicles. 6-Hydroxydopamine apparently is taken up by the membrane pump of the glomus cell and is incorporated into the amine storage granules, thereby displacing the endogenous monoamines. Osmium tetroxide does not dissolve the 6-hydroxydopamine from the vesicles, as it apparently does for the normal vesicular contents. The 6-hydroxydopamine does not fluoresce, hence 6-hydroxydopamine administration results in a decreased intensity of formaldehyde induced fluorescence in the glomus cells. Administration of reserpine after 6-hydroxydopamine treatment (and subsequent initial fixation in osmium tetroxide) depletes the previously restored dense material from the vesicles of the glomus cells. 6-Hydroxydopamine acts like a monoamine in that it is taken up by the glomus cell, incorporated into the vesicles, and can be depleted from the vesicles by reserpine.     

Comparative effects of 6-hydroxy-dopamine and of reserpine on quabain toxicity in the rat

The toxic effects of ouabain were compared in normal, and in 6-OH-DA and reserpine pretreated rats. Reserpine significantly reduced the lethality of intravenous ouabain. This effect does not seem related to depletion of peripheral catecholamines, since 6-OH-DA did not reduce mortality significantly. Since reserpine, but not 6-OH-DA, does cross the blood brain barrier it is suggested that the protective effect of reserpine might be related to its action on the central nervous system.     

Effects of 6-hydroxydopamine at birth on the development of hypertension in the rat

New-born rats treated daily with 6-hydroxydopamine (150 μg/g for 14 days after birth) produced a widespread and permanent destruction of both peripheral and central sympathetic neurones. The development of genetic hypertension was almost completely prevented by this treatment with 6-hydroxydopamine. However, the development of deoxycorticosterone/NaCl hypertension, although retarded, was not prevented by this treatment. The results point to a central involvement of noradrenergic neurones in the development of hypertension in the rat.     

Electrophysiological effects of ouabain in 6-hydroxydopamine pretreated dogs

Chemical sympathectomy and bilateral vagotomy were used to evaluate the contribution of each division of the autonomic nervous system in the electrophysiological actions of ouabain. Intact and chemically sympathectomized dogs were given successive and cumulative doses of ouabain until toxicity became manifest (ventricular extrasystoles and (or) ventricular tachycardia). An additional group of normal and sympathectomized animals was also submitted to bilateral vagotomy in the presence of a therapeutic dose of ouabain. Sinus cycle length, AH interval of the His bundle electrogram, atrioventricular junctional effective and functional refractory periods were increased by ouabain at therapeutic doses. These effects were no different in sympathectomized dogs than in intact dogs, indicating the absence of any significant contribution of efferent sympathetic neural activity. However, our results suggested that vagal enhancement was the main mechanism whereby ouabain produced sinus bradycardia and depression of atrioventricular conduction. Sympathectomy with 6-OHDA did not modify nor abolish ouabain toxicity. However, toxic doses were significantly higher in sympathectomized animals than in normal animals. Considering that increasing heart rate by cardiac pacing or vagotomy significantly lowered toxic doses of ouabain in both intact and sympathectomized dogs, it is possible that sympathectomy could influence ouabain toxicity by altering heart rate alone.     

Modulation of apomorphine-induced rotations in unilaterally 6-hydroxydopamine lesioned rats by cholinergic agonists and antagonists

In the present study, we examined the effects of the agonists and antagonists of cholinergic receptors on central dopaminergic function using the 6-hydroxydopamine model of dopamine receptor supersensitivity. Unilateral lesioning of the substantia nigra with 6-hydroxydopamine was carried out in Wistar rats. Two weeks after surgery, the rats were tested for the presence of dopaminergic supersensitivity by their response to the dopamine receptor agonist, apomorphine. Apomorphine-induced rotations were significantly reinforced by the muscarinic receptor antagonist, atropine. In contrast to atropine, the muscarinic receptor agonist oxotremorine attenuated apomorphine's effects. Acute treatment of nicotine significantly reduced apomorphine-induced rotations. However, when increasing doses of nicotine were given for nine days, the rotations of the nicotine-dependent rats were significantly enhanced. So the fact that both muscarinic and nicotinic cholinergic activity could modulate apomorphine-induced rotations was readily apparent in these experiments.     

Differences in the effects of 6-hydroxydopamine on rat learning with intraventricular and local administration into nuclei of the brain catecholaminergic system

Correlation of the effects obtained with different routes of 6-hydroxydopamine (6-OHDA) administration to the rat brain on the training has disclosed that the drug injection to the lateral ventricles of the brain markedly interferes with the animals' learning both on food and painful reinforcement. Administration of 6-OHDA to the area where noradrenergic neurons have accumulated (A6) is followed by the embarrassment of active avoidance behavior and facilitation of the training on food reinforcement. Local administration of 6-OHDA to the area where dopaminergic neurons have accumulated (A9) decreases the reliability of the formed habits whatever the emotional sign of the reinforcing stimulant.     

Changes in electrical activity of myometrium during intrauterine distribution of rat blastocysts and after prazosin administration

In the early pregnant rat, electrical activity of the myometrium consisted of regular bursts of spike potential, which appeared well propagated on Day 2 of pregnancy. During Day 3, there was a gradual disappearance of propagated activity. Concomitantly, there was a 7-fold increase (P less than 0.001) of uterine progesterone concentrations. At this stage, mean duration of bursts was 15.2 +/- 0.9 sec and intervals of complete quiescence between bursts were 84.2 +/- 7.0 sec. At 10:00 h on Day 4, there were peaks in the uterine concentrations of oestradiol and progesterone, +36% and +654%, respectively, compared with values on Day 2 (P less than 0.05). Between 10:00 and 20:00 h on Day 4, EMG activity exhibited a rapid and transient rise: bursts were of longer duration at the utero-tubal end of the horn (+60%, P less than 0.05) with an increased amplitude of spike potentials (+67% and +90% respectively at the tubal and cervical ends of the uterus, P less than 0.05). The administration of prazosin depressed EMG activity reversibly in a dose-dependent manner with maximal inhibition at about 2-3 h later. It is concluded that the changes observed during EMG recordings are relevant to the intrauterine distribution of blastocysts and related to changes in the steroidal environment and/or to catecholamine effects via alpha 1-adrenoceptors.     

Fluorescent histochemical studies on the effects of 6-hydroxydopamine on adrenaline-containing nerves in the toad

Summary Fluorescence histochemistry has been used to study the effects of 6-hydroxydopamine (6-OHDA) (100 mg/kg injected into the dorsal lymph sac) on adrenaline-containing nerves in the large intestine, mesentery, lung, bladder and heart atria of the toad Bufo marinus. A gradual decrease both in fluorescence intensity and in number of detectable fibres during the first 4 hours after 6-OHDA was accompanied by a build-up of fluorescence in the nonterminal regions. These phenomena have been discussed in relation to the time course of the degeneration produced by 6-OHDA in noradrenergic nerves of higher vertebrates. Almost complete chemical sympathectomy was seen after one day, and it was not till 13 days that regenerating nerve fibres were seen in any organ. In the large intestine, however, re-innervation was slower, being first noted after 39 days. The time course of regeneration has been compared with that following sympathectomy in various mammalian organs.     

Extraneuronal effects of 6-hydroxydopamine and extraneuronal uptake of noradrenaline

Summary 6-hydroxydopamine (6-OHDA) was shown to cause ultrastructural changes in adrenocortical cells of lizards and rats. These changes comprised the formation of dense bodies with lamellar and crystalloid patterns, a decrease in the number of mitochondria and structural alterations of mitochondria. Alterations in adrenocortical cells of lizards and rats differed in both qualitative and quantitative aspects. Adrenomedullary cells were not affected as a rule. Only in young animals did 6-OHDA cause deposits of an electrondense material in medullary cells.An attempt was made to obtain information on amine uptake into cortical cells using the Falck-Hillarp technique to analyse the in-vivo and in-vitro uptake of noradrenaline (NA) into the adrenal cortex in adult rats. Extraneuronal uptake into heart and spleen was studied as well. Our results suggest that NA is taken up into cortical cells, particularly into nuclei, after exposure to 10^-4 gm/ml in-vitro indicating that uptake of 6-OHDA is also likely. Investigations using labelled 6-OHDA are required for further elucidating its extraneuronal uptake.Supported by a grant from Deutsche Forschungsgemeinschaft (Un 34/3) and a Research Fellowship of the University of Melbourne to K.U.