Ann Marie Janson

Evidence for neurogenesis in the adult mammalian substantia nigra

New neurons are generated from stem cells in a few regions of the adult mammalian brain. Here we provide evidence for the generation of dopaminergic projection neurons of the type that are lost in Parkinsons disease from stem cells in the adult rodent brain and show that the rate of neurogenesis is increased after a lesion. The number of new neurons generated under physiological conditions in substantia nigra pars compacta was found to be several orders of magnitude smaller than in the granular cell layer of the dentate gyrus of the hippocampus. However, if the rate of neuronal turnover is constant, the entire population of dopaminergic neurons in substantia nigra could be replaced during the lifespan of a mouse. These data indicate that neurogenesis in the adult brain is more widespread than previously thought and may have implications for our understanding of the pathogenesis and treatment of neurodegenerative disorders such as Parkinsons disease.

Relationship among nigrostriatal denervation, parkinsonism, and dyskinesias in the MPTP primate model

Presynaptic denervation is likely to play an important role in the pathophysiology of dyskinesias that develop after levodopa administration to patients with Parkinsons disease. In this study, the thresholds of nigrostriatal damage necessary for the occurrence of parkinsonism and levodopa‐induced involuntary movements were compared in squirrel monkeys lesioned with 1‐methyl‐4‐phenyl‐1,2,3,6‐tetrahydropyridine (MPTP). Animals treated with a regimen of MPTP that caused parkinsonism displayed ≥95% striatal dopamine depletion, 90% reduction of striatal dopamine uptake sites, and 70% nigral neuronal loss. Levodopa administration ameliorated the parkinsonian signs of these monkeys but also induced dyskinesias. A separate group of animals was treated with a milder MPTP regimen that caused 60%–70% striatal dopamine depletion, a 50% decrease in dopamine transporter, and 40% loss of dopaminergic nigral neurons. While these monkeys displayed no behavioral signs of parkinsonism, they all became dyskinetic after levodopa administration. The priming effect of levodopa, that is, the recurrence of dyskinesias in animals previously exposed to the drug, was compared in severely versus mildly lesioned monkeys. When severely injured parkinsonian animals underwent a second cycle of levodopa treatment, they immediately and consistently developed involuntary movements. In contrast, the recurrence of dyskinesias in primed monkeys with a partial nigrostriatal lesion required several levodopa administrations and remained relatively sporadic. The data indicate that moderate nigrostriatal damage which does not induce clinical parkinsonism predisposes to levodopa‐induced dyskinesias. Once dyskinesias have been induced, the severity of denervation may enhance the sensitivity to subsequent levodopa exposures.

Protective actions of human recombinant basic fibroblast growth factor on MPTP-lesioned nigrostriatal dopamine neurons after intraventricular infusion

Basic fibroblast growth factor (bFGF, FGF-2) is a trophic factor for neurons and astrocytes and has recently been demonstrated in the vast majority of dopamine (DA) neurons of the ventral midbrain of the rat. Potential neuroprotective actions of FGF-2 in the l-methyl-4-phenyl-l,2,3,6-tetrahydropyridine (MPTP) model have also been reported. The actions of the FGF-2 have now been further analyzed in a combined morphological and behavioural analysis in the MPTP model of the adult black mouse, using a continuous human recombinant FGF-2 (hrFGF-2) intraventricular (i.v.t.) administration in a heparin-containing (10 IU heparin/ml) mock cerebrospinal fluid (CSF) solution. Tyrosine hydroxylase (TH) immunocytochemistry in combination with computer assisted microdensitometry demonstrated a counteraction of the MPTP-induced disappearance of neostriatal TH-immunoreactive (ir) nerve terminals following the FGF-2 treatment. Unbiased estimates of the total number of nigral TH ir neurons, using stereological methods involving the optical disector (Olympus), showed that the MPTP-induced reduction in the number of nigral TH ir nerve cell bodies counterstained with cresyl violet (CV; by 56%) was partially counteracted by the FGF-2 treatment (by 26%). The behavioral analysis demonstrated an almost full recovery of the MPTP-induced reduction of the locomotor activity after FGF-2 treatment. This action was maintained also 1 week after cessation of treatment. The hrFGF-2 produced an astroglial reaction as determined in the lateral neostriatum and in the substantia nigra (SN) far from the site of the infusion, indicating that the growth factor may have reached these regions by diffusion to activate the astroglia. Immunocytochemistry revealed FGF-2 immunoreactivity (IR) in the nuclei of the astroglia cell population in the dorsomedial striatum and the microdensitometric and morphometric evaluation demonstrated an increase in the number, but not in the intensity, of these profiles on the cannulated side, suggesting the possibility that hrFGF-2 stimulates FGF-2 synthesis in astroglial cells with low endogenous FGF-2 IR. These results indicate that hrFGF-2, directly and/or indirectly via astroglia, upon i.v.t. infusion exerts trophic effects on the nigrostriatal DA system and may increase survival of nigrostriatal DA nerve cells exposed to the MPTP neurotoxin.

Chronic nicotine treatment counteracts the disappearance of tyrosine-hydroxylase-immunoreactive nerve cell bodies, dendrites and terminals in the mesostriatal dopamine system of the male rat after partial hemitransection.

Male Sprague-Dawley rats were partially hemitransected at the mesodiencephalic junction and treated with nicotine (nicotine hydrogen (+)-tartrate) using Alzet minipumps implanted subcutaneously. Nicotine was delivered for 2 weeks in a dose of 0.125 mg/kg/h resulting in a serum nicotine level of 50.0 +/- 5.1 ng/ml. Three other groups of rats were analyzed: hemitransected rats receiving saline treatment and sham-operated animals receiving nicotine and saline, respectively. The effects of hemitransection and nicotine rostrally as well as caudally to the lesion were evaluated with image analysis of tyrosine hydroxylase (TH)-immunoreactive (IR) nerve cell body and dendrite profiles in the rostral and caudal substantia nigra and of TH-IR nerve terminal profiles in the striatum. Adjacent sections were taken to Nissl staining. [3H]Nicotine binding in the midbrain and forebrain was studied by means of receptor autoradiography on partially hemitransected rats receiving no treatment. Catecholamine (CA) levels in the frontal cortex were measured using high-performance liquid chromatography (HPLC). Striatal dopamine (DA) function was analyzed studying apomorphine-induced (1.0 mg/kg) ipsilateral rotational behavior. The spontaneous behavior of the rats was evaluated with a hole board. Furthermore, body temperature and body weight were measured. The results demonstrated a lesion-induced disappearance of TH-IR cell body and dendrite profiles in the substantia nigra and of TH-IR nerve terminal profiles in the striatum. Similar findings were seen after Nissl staining. A significant counteraction of this disappearance was found in the nicotine-treated animals. On the lesioned animals a marked reduction of [3H]nicotine binding in the striatum and the substantia nigra was found. In the functional experiments an enhancement of the apomorphine-induced ipsilateral rotational behavior was demonstrated. The degree of rotation was positively correlated with the serum nicotine level. The study on spontaneous activity in the hole board showed a slower restoration of total activity in the hemitransected nicotine-treated rats. All these results are compatible with the hypothesis that the protective action of nicotine on the mesostriatal DA system may be due to a desensitization of excitatory nicotine cholinoceptors located on the nigral DA nerve cells, leading to a reduction of firing rate and reduced energy demands. Such an action of nicotine could be of importance for a possible anti-parkinsonian effect.

Aging of the nigrostriatal system in the squirrel monkey.

Increasing incidence of Parkinsons disease with advancing age suggests that age‐related processes predispose the nigrostriatal dopaminergic system to neurodegeneration. Several hypotheses concerning the effects of aging on nigrostriatal neurons were assessed in this study using a non‐human primate model. First, we examined the possibility that the total number of dopaminergic neurons decline in the substantia nigra as a function of age. Stereological counting based on both tyrosine hydroxylase immunoreactivity (TH‐ir) and neuromelanin (NM) content revealed no difference in cell number between young, middle‐aged and old squirrel monkeys. We then determined whether advancing age changed the relative proportion of neurons characterized by 1) TH‐ir in the absence of NM, 2) the presence of both TH‐ir and NM, or 3) NM without TH‐ir. Indeed, a progressive age‐related depletion of TH only cells was paralleled by an increase in NM only neurons. The possibility that these changes could underlie a functional impairment of the nigrostriatal system was supported by striatal dopamine measurements showing a decrease in older monkeys. Finally, we tested the hypotheses that aging may enhance cell vulnerability to injury and that different dopaminergic subpopulations display varying degrees of susceptibility. When monkeys were exposed to the neurotoxicant 1‐methyl‐4‐phenyl‐1,2,3,6‐tetrahydropyridine, cell loss was markedly more pronounced in older animals, and the ranking of vulnerability was TH only < TH/NM < NM only cells. The data indicate that, even in the absence of an overall neuronal loss, changes in the characteristics of dopaminergic cells reflect functional deficits and increased vulnerability to injury with age. NM content appears to be an important marker of these age‐related effects. J. Comp. Neurol. 471:387–395, 2004.

Differential effects of acute and chronic nicotine treatment on MPTP-(1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) induced degeneration of nigrostriatal dopamine neurons in the black mouse

SummaryEvidence exists for a negative correlation between Parkinsons disease and smoking. The present and previous studies indicate that nicotine treatment can markedly alter the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced neurotoxicity in the black mouse based on biochemical determinations of dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) levels in neostriatum and substantia nigra 2 weeks after MPTP injection. Acute intermittent treatment with (−)nicotine starting 10 min before the MPTP injection partly protected against MPTP-induced neurotoxicity in the neostriatum and substantia nigra. Also, a partial protection was observed in the substantia nigra when (−)nicotine was given together with MPTP in an acute intermittent treatment schedule. Conversely, chronic infusion of (−)nicotine via minipumps produced a dose-related enhancement of MPTP-induced DA neurotoxicity in the neostriatum. It is suggested that the protective activity of nicotine in the MPTP model is related to a blockade of MPP + uptake into the DA cells via increased DA release. Conversely, the nicotine enhancement of MPTP-induced DA toxicity is suggested to be caused by a failure of the nicotinic cholinoceptors to desensitize to the chronic (−)nicotine exposure, leading to increased chronic influx of Na+ and Ca2+ ions via the ion channels of the nicotinic cholinoceptors located on the DA neurons with associated increased Ca ion toxicity and increased energy demands.

Effects of chronic imipramine treatment on glucocorticoid receptor immunoreactivity in various regions of the rat brain. Evidence for selective increases of glucocorticoid receptor immunoreactivity in the locus coeruleus and in 5-hydroxytryptamine nerve cell groups of the rostral ventromedial medulla.

Glucocorticoid receptor (GR) immunoreactivity (IR) was analyzed semi-automatically in the forebrain and in the lower brain stem of male rats treated for two weeks with imipramine (10 μmol/kg). Serum corticosterone and aldosterone levels were determined by means of radioimmunoassay procedures. The microdensitometric analysis demonstrated a selective increase in the GR IR in the nerve cell nuclei of the locus coeruleus (A6), of the ventral part of the reticular gigantocellular nucleus (B3L) and of the nucleus raphae magnus (B3M), whereas a small reduction of GR IR was found in the nucleus raphe obscurus (B2). In the morphometric analysis significant increases in the mean profile area of nuclear GR IR, which may be secondary to the increase in GR IR, were observed in the B3M. The serum corticosterone and aldosterone levels were not found to be significantly altered. The selective changes of GR IR may reflect the presence of an altered number of GR in these nerve cell groups and/ or an altered translocation of GR to the nuclei. It is of substantial interest that these changes were observed in the presence of unchanged serum levels of corticosterone and aldosterone. It seems possible that adaptive changes in monoamine synapses induced by the chronic imipramine treatment may be responsible for the changes in GR IR found in the noradrenaline (NA) and 5-hydroxytryptamine (5-HT) cell bodies, respectively. The present results open up the possibility that chronic imipramine treatment may help to maintain the glucocorticoid receptor function in the locus coeruleus and in the 5-HT cell groups of the rostral ventromedial medulla of depressed patients.

Inhibitory effects of the psychoactive drug modafinil on γ-aminobutyric acid outflow from the cerebral cortex of the awake freely moving guinea-pig

SummaryThe effects of modafmil on acetylcholine and GABA outflow from the cerebral cortex of awake freely moving guinea pigs provided with an epidural cup were studied. In the dose range of 3–30 mg/kg s. c. modafmil produced a dose dependent significant inhibition of GABA outflow without influencing cortical acetylcholine release. Methysergide (2 mg/kg, i.p.) and ketanserin (0.5 mg/kg, i. p.) but not prazosin (0.14 mg/kg, i. p.) counteracted the inhibitory action of modafinil on cortical GABA outflow. Modafinil both acutely and chronically in the same dose range increased striatal 5-HIAA levels and 5-HT utilization in the rat (acute) and mouse (chronic). The action on cortical GABA release may be dependent on activity at 5-HT2 receptors, since the action of modafmil in this respect is blocked by the non-selective 5-HT antagonist methysergide and the 5-HT2 antagonist ketanserin. The involvement of 5-HT mechanisms in the inhibitory action of modafmil on cortical GABA release is also suggested by the findings that 5-HT metabolism may become increased by modafmil at least in the striatum. The reduction of cortical GABA outflow via 5-HTZ receptors by modafmil is probably related to some of its actions on the central nervous system including behavioural effects.

Nimodipine promotes regeneration and functional recovery after intracranial facial nerve crush

The calcium flow inhibitor, nimodipine, has been shown to promote motor neuron survival in the facial nucleus after intracranial facial nerve transection. However, it has not been known whether the neuroprotective effects primarily involve survival of nerve cell bodies or outgrowth and/or myelination of nerve fibers. Here, we studied the effects of nimodipine in a different injury model in which the facial nerve was unilaterally crushed intracranially. This lesion caused complete anterograde degeneration and partial retrograde degeneration that were studied with a combination of several stereological methods. Nimodipine did not attenuate the modest lesion‐induced neuronal loss (13%) but accelerated the time course of functional recovery and axonal regrowth, inducing increased numbers and sizes of myelinated axons in the facial nerve. It is interesting to note that nimodipine also enlarged the axons and the myelin sheaths in the nonlesioned facial nerve, which points to the possibility of using this substance for new clinical applications to promote axonal growth and remyelination. J. Comp. Neurol. 437:106–117, 2001.

Chapter 20 Morphofunctional studies on the neuropeptide Y/adrenaline costoring terminal systems in the dorsal cardiovascular region of the medulla oblongata. Focus on receptor-receptor interactions in cotransmission

Publisher Summary The morphological studies in the past have been performed by means of computer-assisted morphometry and microdensitometry to give an objective representation of the central adrenaline (A) nerve cell groups in the medulla oblongata. The functional and biochemical studies of the A neurons within the medulla oblongata, especially within the dorsal cardiovascular region, have given evidence that the adrenergic neurons have an important vasodepressor function in the central nervous system (CNS). The existence of neuropeptide Y–like immunoreactivity in the A cell groups C1, C2, and C3 in the medulla oblongata has been demonstrated by studies in the past. Based on these observations, the effects of centrally administered neuropeptide Y (NPY) on cardiovascular and respiratory parameters and on pre- and postsynaptic mechanisms in central A nerve terminal networks in the dorsal cardiovascular region of the medulla oblongata of the rat have been analyzed. These studies have shown that NPY given intracutaneously in the α-chloralose anesthetized rat can reduce arterial blood pressure and heart rate. These actions have been observed in the presence of α-adrenergic receptor blockade.