Circling behaviour in the rat following unilateral injections of p-chlorophenylalanine and ethanolamine-O-sulphate into the substantia nigra

The environment, epigenetics and amyloidogenesis

Alzheimer's Disease (AD) is a progressive, irreversible neurodegenerative disease. Despite several genetic mutations (Haass et al., J. Biol. Chem. 269:17741-17748, 1994; Ancolio et al., Proc. Natl. Acad. Sci. USA 96:4119-4124, 1999; Munoz and Feldman, CMAJ 162:65-72, 2000; Gatz et al., Neurobiol. Aging 26:439-447, 2005) found in AD patients, more than 90% of AD cases are sporadic (Bertram and Tanzi, Hum. Mol. Genet. 13:R135-R141, 2004). Therefore, it is plausible that environmental exposure may be an etiologic factor in the pathogenesis of AD. The AD brain is characterized by extracellular beta-amyloid (Abeta) deposition and intracellular hyperphosphorylated tau protein. Our lab has demonstrated that developmental exposure of rodents to the heavy metal lead (Pb) increases APP (amyloid precursor protein) and Abeta production later in the aging brain (Basha et al., J. Neurosci. 25:823-829, 2005a). We also found elevations in the oxidative marker 8-oxo-dG in older animals that had been developmentally exposed to Pb (Bolin et al., FASEB J. 20:788-790, 2006) as well as promotion of amyloidogenic histopathology in primates. These findings indicate that early life experiences contribute to amyloidogenesis in old age perhaps through epigenetic pathways. Here we explore the role of epigenetics as the underlying mechanism that mediates this early exposure-latent pathogenesis with a special emphasis on alterations in the methylation profiles of CpG dinucleotides in the promoters of genes and their influence on both gene transcription and oxidative DNA damage.

Role of the nucleus accumbens and the striatum in the production of turning behaviour in intact rats

Recent knowledge of the mechanisms underlying turning or circling behaviour in intact rats is reviewed. Most interest has been directed towards the striatum because of the classical hypothesis that turning behaviour results from lateral differences in the activity of the bilateral nigrostriatal pathway. However, the assumption that asymmetrical activation of the striatum is a necessary condition for dopamine-dependent turning behaviour has been questioned by several studies showing that unilateral injection of amphetamine or dopamine receptor agonists into the nucleus accumbens, a target of the mesolimbic dopaminergic system, also produces reliable circling away from the side of injection. Apart from discussing differences in stepping patterns of turning and discussing the role of the dopamine D1/D2 receptor interaction, the present survey focuses attention upon the two-component hypothesis, especially in relation to our recent studies in which activities of dopamine D1 and D2 receptors in the striatum and the nucleus accumbens have been manipulated separately in intact rats. It is hypothesized that turning behaviour is produced by asymmetry within nucleus accumbens circuits which involve neuronal connections from the nucleus accumbens to the A9 cell area, which in turn projects to the ventrolateral striatum that determines the direction of turning.

Serotonin in involuntary movement disorders

Several recent studies have emphasized that serotonergic pathways in the CNS are intimately involved in the modulation of motor behavior, and in the pathophysiology of human involuntary movement disorders. These observations are supported by recent reports demonstrating large serotonergic innervation of the striatum and substantia nigra, and a close interaction between the activity of serotonergic neurons with the dopamine system in the striatum and nigra. In the following communication we summarize evidence demonstrating defective serotonergic functions in a number of human movement disorders and discuss their management with serotonergic drugs.

Inhibitory influence of GABA on central serotonergic transmission. Raphé nuclei as the neuroanatomical site of the GABAergic inhibition of cerebral serotonergic neurons

Acute injection of the gamma-aminobutyric acid (GABA) mimetics progabide, aminooxyacetic acid, gamma-acetylenic GABA and dipropylacetamide reduced 5-hydroxytryptophan (5-HTP) accumulation in serotonergic nerve terminal regions (prefrontal cortex, olfactory tubercle, septum, striatum, hypothalamus, hippocampus, substantia nigra, cerebellum and spinal cord) as well as in corresponding cell body areas (raphé dorsalis, medianus, pontis and magnus). This effect was antagonized by bicuculline. The inhibition of serotonin (5-HT) synthesis induced by a single progabide administration was accentuated on repeated treatment in the striatum, prefrontal cortex and cerebellum but was similar to that seen after acute treatment in the other areas. Local infusion of high concentrations of GABA or GABA mimetics into the striatum, septum or substantia nigra failed to modify 5-HTP accumulation in these areas. Cerebral hemitransection antagonized the ability of progabide (1200 mg/kg i.p.) to diminish 5-HTP accumulation in the striatum, hippocampus and prefrontal cortex. Intra-raphé dorsalis infusion of muscimol (0.1-100 ng) or GABA (1-100 micrograms) decreased 5-HT synthesis in the corresponding projection areas (e.g. striatum, substantia nigra, cortex) but not in the hippocampus or cerebellum. Conversely, intra-raphé medianus infusion of these drugs diminished 5-HTP accumulation in the corresponding projection areas (e.g. hippocampus, septum, cortex) but not in the striatum or cerebellum. Intra-raphé dorsalis or medianus injection of GABA antagonists (bicuculline, picrotoxinin, RU-5135) was without effect on cerebral 5-HT synthesis but antagonized the diminution of the amine synthesis observed in corresponding projection areas after intra-raphé dorsalis or medianus infusion of muscimol or GABA. These results suggest that GABA exerts an inhibitory (non-tonic) control over central serotonergic neurons which is mediated via GABA receptors located in the raphé nuclei.

The involvement of 5-hydroxytryptamine in the release of dendritic dopamine from slices of rat substantia nigra

[3H]dopamine (DA) accumulated by slices of rat substantia nigra was released following superfusion with Krebs solution containing 5-hydroxytryptamine (5-HT) in a dose-dependent fashion with maximal release being produced by 1 X 10(-4) M. This 5-HT-induced release of [3H]DA was calcium-dependent and was inhibited by pretreatment with the 5-HT antagonist cinanserin. These results are interpreted as supporting the possibility that 5-HT has a role in regulating dopaminergic activity in the nigro-striatal pathway.

A role for 5-hydroxytryptamine in the GABA-mimetic potentiation of alpha-flupenthixol-induced catalepsy in the rat

1 alpha-Flupenthixol (alpha-FPT)-induced catalepsy in the rat was potentiated by diaminobutyric acid (DABA), an inhibitor of the neuronal high affinity uptake of gamma-aminobutyric acid (GABA). 2 The depletion of 5-hydroxytryptamine (5-HT) with p-chlorophenylalanine (PCPA) abolished the DABA potentiation of alpha-FPT-induced catalepsy; this response was restored with 5-hydroxytryptophan. 3 Potentiation of alpha-FPT-induced catalepsy by clonazepam was significantly reduced by methysergide. Conversely, the potentiation of catalepsy by clomipramine was significantly reduced by picrotoxin. 4 These results are interpreted as evidence supporting a role for 5-HT in modifying the GABA-ergic inhibition of dopaminergic pathways, possibly by regulating the release of GABA.

The effects of muscimol and picrotoxin injections into the cat substantia nigra

The behaviour of cats after unilateral injections of muscimol, picrotoxin and bicuculline into the posterior and lateral parts of the substantia nigra was observed. The antagonism between muscimol and picrotoxin was limited to some effects of the drugs: (1) muscimol-induced sniffing, licking and enhanced locomotor activity was attenuated by picrotoxin in contrast to the muscimol-induced contralateral turning which was even enhanced by picrotoxin; (2) picrotoxin-induced balance disorders as well as characteristic motor disturbances in the hind legs remained unaffected following an additional treatment with muscimol. Furthermore, subcutaneously given apomorphine did not affect the picrotoxin-induced balance and hind leg disorders, although the apomorphine-induced stereotyped behaviour itself was enhanced and suppressed by intranigral administration of muscimol and picrotoxin respectively. The conclusion is reached that there are at least two distinct populations of GABA receptors within the substantia nigra: one group controlling or being controlled by dopaminergic neurons and one group operating fully independently of the former neurons. The similarities to experiments on rats are stressed. The mechanism underlying the muscimol-resistant picrotoxin-induced effects (balance disorders and motor disturbance of the hind legs) is discussed.

Effects of electrolytic and 6-hydroxydopamine lesions of the lateral hypothalamus on rotation evoked by electrical stimulation of the substantia nigra in rats

Contralateral rotation evoked by electrical stimulation of the left substantia nigra was studied in rats before and after electrolytic or 6-hydroxydopamine (6-OHDA) lesions of the lateral hypothalamus. Electrolytic lesions (2 mA DC, 15 sec) which produced mean ipsilateral striatal dopamine depletion of 58% significantly reduced the rotation at 2 h to 14 days postlesion. 6-OHDA (8 microgram in 4 microliter) which produced mean ipsilateral striatal dopamine depletion of 93% significantly increased the rotation at 3 to 14 days postlesion. Haloperidol 0.1 and 0.5 mg/kg i.p. partially reduced rotation in both control and lesioned rats in a dose-related manner. Control and lesioned rats showed no significant differences in haloperidol sensitivity. If stimulus induced rotation were mediated by activation of dopaminergic neurons, one would have expected lesion effects in the present experiments to parallel those on rotation caused by pharmacologically evoked release of dopamine. The lesions effects we obtained on stimulus induced rotation, however, parallel those on rotation evoked by the predominantly directly acting dopamine agonist, apomorphine, rather than those on rotation evoked by the indirect (presynaptic) action of amphetamine. We suggest that contralateral rotation evoked by electrical stimulation of the substantia nigra may reflect direct activation of neurons postsynaptic to the dopaminergic nigrostriatal neurons.

In vivo evidence for GABAergic control of serotonin release in the cat substantia nigra

Push-pull cannulae were used for estimating the release of endogenously synthesized [3H]serotonin in both substantia nigra and caudate nuclei of halothane-anaesthetized cats. The unilateral nigral application of GABA (10-5 M) reduced the local release of [3H]serotonin picrotoxin induced an opposite effect. Both treatments failed to modify [3H]serotonin release in the caudate nuclei or in the contralateral substantia nigra. These results suggest that GABAergic neurons innervating the substantia nigra may regulate nigral serotonin transmission. The possibility that such a regulation could be presynaptic (direct or through other nigral neurotransmitters) or related to a change in the activity of the nigro-raphe projection is discussed.

Immobility test: effects of 5-hydroxytryptaminergic drugs and role of catecholamines in the activity of some antidepressants

Fenfluramine and m-chlorophenylpiperazine, two drugs purported to enhance central 5-hydroxytryptaminergic transmission, and metergoline, a 5-HT antagonist, did not modify the duration of immobility induced in rats made to swim in a restricted space. Nomifensine, desipramine and amineptine, three antidepressants known to block neuronal catecholamine uptake, significantly reduced the duration of immobility. Penfluridol, a dopamine antagonist at central receptors, counteracted the effect of nomifensine and amineptine but not that of desipramine. Propranolol and phenoxybenzamine respectively reduced the effects of desipramine and nomifensine but did not modify amineptine's effect. Metergoline pretreatment did not counteract the effect of any drug. The results indicate that various antidepressants can reduce the duration of immobility in rats by activating dopaminergic and/or noradrenergic mechanisms in the brain. Either alpha- or beta-noradrenergic receptors could contribute to the anti-immobility effects, depending on the drug used. The immobility test appears to be insensitive to drugs activating or reducing 5-HT-ergic mechanisms in the brain.

Rotation induced by intranigral injections of GABA agonists and antagonists: zone-specific effects

Recent investigations of the function of the strionigral pathway have utilized the intranigral injection of gamma-aminobutyric acid (GABA) agonist and antagonist drugs. While the unilateral application of these substances typically produces rotational behavior, the direction of this turning (ipsilateral or contralateral to the injected hemisphere) differs in several reports. The present study determines whether the direction of this drug-induced turning depends upon the locus of migral stimulation. Picrotoxin and bicuculline methiodide were injected into either the pars compacta or the pars reticulata of the substantia nigra at several anterior-posterior levels. Injection of these drugs into the pars compacta resulted in ipsilateral turning while injection into the pars reticulata produced contralateral rotation. Both of these effects were dose-dependent and were elicited by similar threshold doses of picrotoxin. Prior 6-hydroxydopamine treatment abolished the ipsilateral but not the contralateral rotation. In contrast, muscimol injections produced contralateral turning independent of whether they were made into the pars compacta or pars reticulata. However, 6-hydroxydopamine treatment only attenuated the contralateral turning produced by pars compacta injections. These findings provide a histological basis for understanding the different types of turning behavior elicited by the intranigral injection of GABA agonists and antagonists. In addition, they suggest that GABA receptors mediate at least two independent actions in substantia nigra.

Rotational behaviour elicited by 5-HT in the rat: evidence for an inhibitory role of 5-HT in the substantia nigra and corpus striatum

Unilateral intranigral microinjection of 5-HT or Wy 25093, a selective inhibitor of 5-HT uptake, elicited ipsiversive circling and a fall in striatal dopamine (DA) turnover. Conversely intranigral methysergide or unilateral lesions of the raphé-nigral 5-HT pathway produced contraversive turning and/or increased DA turnover. Both types of behaviour were accentuated by nialamide and attenuated by haloperidol. Caudate injections of 5-HT or Wy 25093 provoked contraversive turning which was increased by nialamide and hyoscine, partially diminished by haloperidol and antagonized by eserine. Ipsiversive circling was induced by intracaudate methysergide, which was potentiated by nialamide and eserine, unaffected by haloperidol and depressed by hyoscine. It is proposed that 5-HT normally subserves an inhibitory function both in the substantia nigra and corpus striatum.

Removal of the telencephalon spares turning induced by injection of GABA agonists and antagonists into the substantia nigra

Unilateral injections into the substantia nigra of GABA agonists or the GABA-related agents muscimol (5-50 ng), baclofen (10-50 ng) or B-aminobutyric acid (25-50 ug) induced intensive turning in the direction contralateral to the injected hemisphere. Intranigral injection of the GABA antagonist picrotoxin (100-500 ng) led to ipsiversive turning. Surgical removal of the whole telencephalon (including the neocortex, hippocampus, striatum, septal nuclei and amygdalae) influenced neither direction nor magnitude of the turning responses induced by these various drugs. A GABAergic system in the substantia nigra can, therefore, cause turning independent of telencephalic structures.

Modulation of dopamine-mediated behavioural responses by antidepressants: effects of single and repeated treatment

The study concerned the effects of maprotiline, imipramine, clomipramine and amitriptyline on the stereotyped and turning behaviour induced by apomorphine in rats. At either single or repeated doses of 25 mg/kg i.p. neither maprotiline nor imipramine changed the stereotyped responses induced by apomorphine. Clomipramine showed in contrast an inhibitory effect which increased after 7 daily injections of the drug. Moderate suppression of stereotypies was also observed after repeated administration of amitriptyline. In rats with unilateral 6-OHDA lesions of the s.nigra apomorphine-induced contralateral turning was markedly suppressed (70%) after a single 25 mg/kg i.p. injection of maprotiline. A tolerance to this effect developed after 7 daily injections of the drug. Clomipramine and amitriptyline caused an inhibition of turning which was markedly increased after repeated treatment. These results suggest that antidepressants do not uniformly affect the behavioural responses mediated by dopamine. Clomipramine and amitriptyline appear to possess dopamine receptor blocking properties which may become more pronounced after chronic treatment. In contrast, the dopamine receptor blockade by maprotiline diminished and disappeared under such conditions. Among the drugs investigated imipramine was the one which seemed to have the weakest influence on dopaminergic receptors.

Contralateral turning after unilateral electrolytic lesion of substantia nigra in thalamic rats

Unilateral electrolytic lesions of the substantia nigra of rats induced contralateral turning in intact as well as 'thalamic' rats (devoid of neocortex, hippocampus, striatum, amygdala, septal nuclei, including most of n. accumbens). Therefore, telencephalic structures, including nigrostriatal neurons, are not critically involved in the turning that follows electrolytic nigral lesions.

Serotonin--dopamine interactions in the nigrostriatal system

A study was made of the effects of serotonin uptake inhibition and receptor blockade on the increase in rat striatal homovanillic acid and 3,4-dihydroxyphenylacetic acid and on some behavioural responses induced by haloperidol. The serotonin uptake inhibitors CGP 6085 A (4-(5,6-dimethyl-2-benzofuranyl)-piperidine-HCl), citalopram (Lu 10-171), fluoxetine (Lilly 110140), and clomipramine potentiated the increase in striatal deaminated dopamine metabolites after the neuroleptic. In contrast, the serotonin antagonists methysergide, mianserin and cinanserin antagonized the acceleration of dopamine turnover induced by haloperidol. The catalepsy induced by this neuroleptic was potentiated by CGP 6085 A and citalopram. These 5-HT uptake inhibitors also potentiated the antagonism by haloperidol of apomorphine-induced stereotypies. These results seem to make it worthwhile to test a combination of haloperidol and a serotonin antagonist in schizophrenic patients to see whether the ratio of the therapeutic effect to the extrapyramidal side effects can be improved.

Muscimol and related GABA receptor agonists: the potency of GABAergic drugs in vivo determined after intranigral injection

Contralateral turning behaviour following unilateral intranigral injection of a large series of GABA analogues was investigated. The results indicated that the turning behaviour was induced stereospecifically and was selectively antagonized by the GABA antagonist bicuculline methochloride. The comparative potencies of a series of GABA agonists related to muscimol in general corresponded well to the affinity for 3H-GABA receptor sites and to the depressant action on single neurons using microelectrophoretic administration. However, the GABA agonists trans-aminocrotonic acid and 3-aminopropanesulphonic acid were much weaker than expected from in vitro studies. The GABA-uptake inhibitors nipecotic acid and guvacine showed only weak and short-lasting effects. The GABA-transaminase inhibitor gamma-acetylenic GABA showed delayed effects compared with the agonists which acted immediately. It is proposed that this behavioural effect may be a sensitive and quantitative method for evaluation of GABA agonists in vivo.

Relevance of GABA in the nigrostriatal system

In conclusion, even though the mutual integration of intrastriatal circuits remains an open question, the electrophysiological and neurochemical approaches seem to provide valid support toward understanding the complexity of these systems. Recent findings seem to indicate that, besides a dopaminergic and cholinergic link, interactions between DA and GABA neurons may be operative within the striatum. An understanding of these functional interplays may facilitate future attempts aimed at clarifying the electrophysiological discrepancies concerning the inhibitory or excitatory action of DA on the target cells in this brain structure. Studies on the functional roles of pathways projecting from striatum to substantia nigra have indicated that these may be implicated in the regulation of nigral efferents to other brain nuclei and that they do control striatal DA activity through feedback mechanisms.