Christoph Grote

Iron accumulation in the substantia nigra in rats visualized by ultrasound

In recent studies, we have found a marked increase in substantia nigra (SN) echogenicity in patients with Parkinsons disease (PD) using transcranial ultrasound. Because a substantial body of evidence has accumulated indicating a selective elevation of iron in the SN from patients with PD, we set out to test the hypothesis that trace metals like iron could lead to the observed increase of SN echogenicity in PD. Rat brains were scanned after stereotactic injection of iron in different concentrations into the SN and after injecting ferritin, zinc and 6-OHDA alone, and after the addition of desferrioxamine. The amount of iron in the SN was measured spectroscopically. For iron, and partly for 6-OHDA, in different concentrations, a dose-dependent increase of SN echogenicity could be visualized, corresponding to an increase of iron measured by spectroscopy. No increase of echogenicity was visualized after the injection of ferritin and the addition of desferrioxamine to 6-OHDA, though an increase of iron was measured by spectroscopy. Therefore, we conclude that iron not bound to these proteins may lead to an increase of echogenicity of the SN.

Differential induction of c-fos expression in brain nuclei by noxious and non-noxious colonic distension: role of afferent C-fibers and 5-HT3 receptors.

Experimental animal models have been established to gain insight into the pathogenesis and the mechanisms of visceral hyperalgesia in the irritable bowel syndrome (IBS). However, data about the mechanisms and pathways involved in the induction of neuronal activity in forebrain and midbrain structures by a physiological GI stimulus, like colonic distension (CD), in the range from non-noxious to noxious intensities are scarce. Thus, the effect of proximal CD with non-noxious (10 mmHg) and noxious (40 and 70 mmHg) stimulus intensities on neuronal activity in brain nuclei, as assessed by c-fos expression, was established. In additional studies, the role of vagal and non-vagal afferent sensory C-fibers and 5-HT(3) receptors in the mediation of visceral nociception was investigated in this experimental model at noxious colonic distension (70 mmHg). At CD, the number of c-Fos like immunoreactivity (c-FLI)-positive neurons increased pressure-dependently in the nucleus of the solitary tract (NTS), rostral ventrolateral medulla (RVLM), nucleus cuneiformis (NC), periaqueductal gray (PAG), and the amygdala (AM). In the dorsomedial (DMH) and ventromedial nucleus (VMH) of the hypothalamus, as well as in the thalamus (TH), neuronal activity was also increased after CD, but independently of stimulus intensities. A decrease of the CD-induced c-fos expression after sensory vagal denervation by perivagal capsaicin treatment was only observed in brainstem nuclei (NTS and RVLM). In all other activated brain nuclei examined, the CD-related induction of c-fos expression was diminished only after systemic neonatal capsaicin treatment. In the NTS and RVLM, a trend of decrease of c-fos expression was also observed after systemic neonatal capsaicin treatment. In order to assess the role of the 5-HT(3) receptor in CD-induced neuronal activation of brain nuclei, animals were pretreated with the 5-HT(3) receptor antagonist granisetron (1250 microg/kg, i.p. within 18 h before CD). Pretreatment with granisetron significantly reduced the number of c-FLI-positive cells/section in the NTS by 40%, but had no significant effect on the CD-induced c-fos expression in other brain areas. The data suggest that distinct afferent pathways and transmitters are involved in the transmission of nociceptive information from the colon to the brain nuclei activated by proximal colonic distension. Activation of NTS neurons at such a condition seems to be partially mediated via capsaicin-sensitive vagal afferents and 5-HT(3) receptors. In contrast, activation of brain nuclei in the di- and telencephalon by nociceptive mechanical stimulation of the proximal colon, as assessed by c-fos expression, is partially mediated by capsaicin-sensitive, non-vagal afferents, and independent of neurotransmission via 5-HT(3) receptors. The modulation of CD-induced c-fos expression exclusively in the NTS by granisetron points to a role of 5-HT(3) receptor antagonists in the modulation of vago-vagal sensomotoric reflexes rather than an influence on forebrain nuclei involved in nociception.

Intranigral injected iron progressively reduces striatal dopamine metabolism

SummaryThe striatal dopamine metabolism of the rat was followed 1, 3, and 6 weeks after unilateral intranigral iron (III) (50- and 1.5 μg) application. For both concentrations a progredient decrease of extraneuronal 3.4-dihydroxyphenylacetic acid (DOPA) levels was observed in the ipsilateral striatum.

Excitatory stimulation of neurons in the arcuate nucleus inhibits gastric acid secretion via vagal pathways in anesthetized rats.

It is well established that autonomic control of gastrointestinal function is modulated by central autonomic neurotransmission. In this context it has been shown that gastrointestinal motility and secretion can be modulated by exogenous neuropeptides microinjected into the paraventricular nucleus of the hypothalamus (PVN). Furthermore, there is considerable evidence suggesting that neurons projecting from the arcuate nucleus (Arc) to the PVN may be the source of endogenous neuropeptide release in the PVN. This poses the question whether stimulation of neurons in the arcuate nucleus, e.g. by an excitatory amino acid, alters gastrointestinal function. In the present study, we investigated the effect of an excitatory amino acid, kainate, microinjected into the arcuate nucleus on gastric acid secretion in urethane-anesthetized rats. Kainate (140 pmol/rat) bilaterally microinjected into the Arc induced an significant inhibition of pentagastrin (PG) stimulated (16 mg/kg per h) gastric acid secretion throughout an observation period of 120 min after microinjection. Microinjection of kainate into hypothalamic areas outside the arcuate nucleus did not modify gastric secretion. Bilateral cervical vagotomy blocked the effect of kainate injected into the Arc on PG-stimulated gastric acid secretion. These data show that gastric secretory function can be modulated by stimulation of neuronal activity in the Arc via efferent vagal pathways. The results suggest that the arcuate nucleus is a forebrain area involved in the CNS regulation of gastrointestinal function.

Involvement of CCK in the paraventricular nucleus of the hypothalamus in the CNS regulation of colonic motility.

The effects of cholecystokinin octapeptide (CCK8), the CCK-A receptor antagonist, MK-329, and the CCK-B receptor antagonist, L-365, 260, microinfused into the paraventricular nucleus of hypothalamus (PVN) on colonic motor function was investigated in awake rats, chronically implanted with a microinjection cannula into the PVN and a catheter into the proximal colon. In fasted rats, bilateral microinfusion of CCK8 at doses of 1.5 and 3.0 μg/rat into the PVN stimulated colonic transit, as shown by a significant increase in the geometric center by 47 and 54%, respectively. This effect of CCK8 was site-specific to the PVN, since microinjection of the peptide into sites outside of but adjacent to PVN had no effect. In non-fasted rats, L-365,260 bilaterally microinjected into the PVN at a dose of 1.5 μg/rat inhibited propulsive colonic motor function; colonic transit time significantly increased by 73% in comparison to the control condition. Microinfusion of the CCK-A antagonist into in the PVN did not affect colonic transit. These results show that the PVN is a responsive site for the central CCK8-induced modulation of colonic motility. The data suggest, that endogenous CCK in the paraventricular nucleus of the hypothalamus unfolds a stimulatory effect on colonic transit through action on CCK-B receptors.

‘TaClo’, A Chloral-Derived Mammalian Alkaloid with Neurotoxic Properties

Since the discovery that the synthetic compound 1-methyl-4-phenyl-l,2,3,6-tetra-hydropyridine (MPTP, 5) induces parkinsonian-like symptoms in humans, monkeys, and mice, particular attention has been focused on the possible role of environmental toxic agents in causing damage to the central nervous system (Collins et al., 1986).

Effect of lazaroid pretreatment on dopamine-induced impairment of the rat nigrostriatal system.

Summary. Oxidative stress induced by enhanced catecholamine metabolism may subsequently cause damages to the nervous system. We used in vivo-pulse voltammetry to study an enhanced brain dopamine (metabolism) induced either by intranigral dopamine (DA) injection or reduction of cerebral blood flow. One week after intranigral injection of 10 μg DA or unilateral occlusion of one carotid the DA activity in the ipsilateral striatum was decreased as compared to the contralateral side. Three weeks after DA application and carotid clamping the DA activity was restored to normal. The significant reduction of 3,4-dihydroxyphenylacetic acid (DOPAC) after one week was attenuated by pretreatment with the lazaroid U-74389G, injected 20 min before surgery. The results are in accordance with the view that radical mechanisms play a crucial role in the impairment of the nigrostriatal system induced by oligemia.

Progressive Reduction of the Rat Nigrostriatal Dopamine Metabolism Induced by Iron-III and TaClo

Morbus Parkinson is a progressive illness, and until now, no medication is able to stop this progression. Therfore models are needed, showing signs of a progressive reduction of the striatal dopaminergic function. In this study dopaminergic function was followed bilaterally in the striatum using in-vivo-voltammetry with carbon-monofibre-electrodes to measure extracellular DOPAC concentrations. Dopaminergic neurotoxins were applied one time stereotactically into the right substantia nigra pars compacta. Striatal voltammetry of citrate-buffered iron-Ill (1.5μg/2μl) into the right substantia nigra of the rat led to a significant reduction of the ipsilateral striatal voltammetric DOPAC signal one week after injection. Three and 6 weeks after injection a further significant reduction of the voltammetric signal was observed. Also the MPTP-like neurotoxin TaClo (10μg/2μl) reduced the ipsilateral voltammetric DOPAC signal one week after unilateral injection. The 3- and 6-week values were further reduced and a significant reduction is obtained between the one- and 6 week, and 3 and 6 week values. In conclusion, the dopaminergic neurotoxins iron-III and TaClo, when infused unilaterally into the substantia nigra of the rat, lead to a progressive unilateral reduction of the striatal dopamine metabolism, indicating that these neurotoxins may be used for the search for relevant neuroprotective strategies.