Werner J. Schmidt

Rotenone destroys dopaminergic neurons and induces parkinsonian symptoms in rats

Rotenone (an inhibitor of mitochondrial NADH dehydrogenase, a naturally occurring toxin and a commonly used pesticide) appears to reproduce the neurochemical, neuropathological and behavioural feature of Parkinsons disease (PD) in the rat. In this study, rotenone was administrated on a daily basis systemically by intraperitoneal injection of two different doses: 1.5 mg/kg (low dose) and 2.5 mg/kg (moderate dose), over a period of 2 months. This treatment caused depletion of dopamine in the posterior striatum (CPu) and prefrontal cortex and also reduced tyrosine hydroxylase-immunoreactivity in CPu. Behavioural experiments showed dose-dependent catalepsy in the two treatment groups of rats. Data from this study indicate that in rats rotenone is capable of causing degeneration of dopaminergic neurons and induction of parkinsonian symptoms. It is concluded that the causal mechanisms of neuronal degeneration implicate a complex I deficiency in the aetiology of rotenone-induced and perhaps in some cases of sporadic PD.

Aminoadamantanes as NMDA receptor antagonists and antiparkinsonian agents — preclinical studies

Aminoadamantanes such as 1-aminoadamantane (amantadine) and 1-amino-3,5-dimethyladamantane (memantine) are N-methyl-D-aspartate (NMDA) receptor antagonists which show antiparkinsonian-like activity in animal models and in Parkinsons patients. The issue of whether NMDA antagonism plays a role in the symptomatological antiparkinsonian activity of amantadine and memantine is addressed by comparing: behaviourally effective doses, serum/brain levels, and their potency as NMDA receptor antagonists. In the case of memantine, blockade of NMDA receptors is probably the only mechanism responsible for antiparkinsonian activity, whereas for amantadine the situation is clearly far more complex. There are a number of differences between memantine and amantadine both in vitro and in vivo, and although NMDA receptor antagonism certainly participates in the antiparkinsonian activity of amantadine, other effects, some of which are elusive, also play a role. Moreover, it has been suggested that the pathomechanism of Parkinsons disease involves excitotoxic processes and that treatment with NMDA receptor antagonists might also slow the progression of neurodegeneration. If this claim is true, such an effect could be achieved with amantadine and memantine which show neuroprotective activity in animals at therapeutically relevant doses.

6-Hydroxydopamine lesion of the rat prefrontal cortex increases locomotor activity, impairs acquisition of delayed alternation tasks, but does not affect uninterrupted tasks in the radial maze

The role of mesocortical dopamine neurons in locomotion and acquisition of various delayed and uninterrupted maze tasks was investigated in the rat. Dopaminergic terminals of the medial prefrontal cortex were lesioned by stereotaxically guided injections of the selective neurotoxin 6-hydroxydopamine (6-OHDA), while noradrenergic neurons were protected by systemically administered desipramine. 6-OHDA lesions resulted in a selective depletion of dopamine and its metabolite, dihydroxyphenylacetic acid, in the prefrontal cortex but not in subcortical structures. Prefrontal serotonin was not depleted. 6-OHDA-cloned rats performed uninterrupted alternation tasks (spontaneous and reinforced alternation) in the radial maze in the same manner as controls, whereas performance of delayed alternation in the T-maze and the radial maze was impaired in lesioned rats. In addition, locomotor activity during maze performance was increased in lesioned rats. Based on the hypothesis that increased motor activity and impaired delayed alternation performance are due to increased susceptibility to interfering stimuli, we propose tentatively that prefrontal dopamine may function to suppress interference during the delay period of certain cognitive tasks.

Interactions of MK-801 and GYKI 52466 with morphine and amphetamine in place preference conditioning and behavioural sensitization

In an earlier study we showed that co-administration of the NMDA-receptor antagonist MK-801 during conditioning sessions blocks morphine-induced conditioned place preference (CPP). From this result, the question arose of whether this blockade is due to state-dependency effects induced by MK-801. Therefore, in a first experiment, animals were tested in the drugged state under which they had been conditioned. These animals did not show a CPP, thus it can be concluded that MK-801 does not make state-dependent the morphine conditioning. In the same experiment those animals receiving only morphine during conditioning sessions showed a significant CPP when tested in an undrugged state but failed to show CPP when tested after injection of MK-801 (i.e., in the drugged state). These results indicate that MK-801 not only blocks the development of morphine-induced CPP but is also able to block the expression of a conditioned response that has been acquired before. In the same experiment repeated injection of neither morphine nor MK-801 produced sensitized locomotor activity. However, a strong sensitization was observed following repeated injection of morphine plus MK-801. There was also cross-sensitization between morphine plus MK-801 and MK-801 alone but not with morphine alone, and also between morphine and MK-801, but not vice versa. In a second experiment the effects of the AMPA-receptor antagonist GYKI 52466 were examined. It was found that GYKI 52466 did not produce CPP or behavioural sensitization. Finally, in a third experiment, CPP was induced by morphine and amphetamine (animals tested in the drug-free state), and behavioural sensitization was induced by amphetamine. When animals were tested after an injection of GYKI 52466, neither the morphine- nor the amphetamine-conditioned animals showed a CPP. Likewise, challenge of sensitized animals with amphetamine plus GYKI 52466 failed to produce a sensitized response. It can be concluded, that GYKI 52466, like MK-801, can block the expression of a conditioned response, and can also block the expression of sensitized behaviour.

N-Methyl-d-aspartic acid-receptor antagonists block morphine-induced conditioned place preference in rats

We addressed the question of whether (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine (MK-801) and DL-(E)-2-amino-4-methyl-5-phosphono-3-pentenoic acid (CGP37849), a non-competitive and a competitive N-methyl-D-aspartate (NMDA) receptor antagonist, respectively, are able to block morphine-induced conditioned place preference (CPP). MK-801 alone (0.1 mg/kg) produced neither a place preference nor a place aversion, but was able to completely block morphine-induced CPP. CGP37849 alone (10 mg/kg) produced a small but significant CPP, and was able to significantly attenuate morphine-induced CPP. These results cannot be due to simple additive effects of drug actions, but suggest that NMDA receptors play a complex role in the development of morphine CPP.

MK-801-induced stereotypy and its antagonism by neuroleptic drugs

MK-801 [(+)-5-methyl-10,11-dihydroxy-5H-dibenzo-(a,d)cyclo-hepten-5, 10-imine hydrogen maleate], which blocks glutamatergic transmission at the NMDA-receptor-gated ion chanel, induced stereotypies which are similar to those found after intrastriatal injections of AP-5, e.g. sniffing and locomotion. Tests in familiar or unfamiliar environment (non-stressful or stressful situation) did not qualitatively change MK-801-induced effects. Haloperidol (0.1mg/kg, IP) delayed the onset and shortened the duration of MK-801 (0.16; 0.33mg/ kg, IP)-induced stereotypy whereas clozapine (5 mg/kg, SC) potently antagonized it. However, exact quantification of sniffing, measured in an experimental chamber designed for this purpose, revealed an antagonism by both drugs, haloperidol as well as clozapine. Stereotypy is considered to represent an animal model of schizophrenia, and the antagonism of stereotypy with classical (haloperidol) as well as with atypical (clozapine) antipsychotic drugs is in accordance with the glutamate hypothesis of schizophrenia.

Differential behavioural and neurochemical effects of competitive and non-competitive NMDA receptor antagonists in rats.

The behavioural and biochemical effects of the non-competitive N-methyl-D-aspartate (NMDA) receptor antagonists, dizocilpine and memantine, and the competitive NMDA receptor antagonist, CGP 39551, were investigated in rats. Systemic injections of dizocilpine (0.33 mg/kg) increased locomotion and rearing in an open field, whereas memantine (20 mg/kg) increased only locomotor activity. CGP 39551 (10 and 20 mg/kg) did not change open field activity. Dopamine (DA) metabolism--as measured by the ratio of dihydroxyphenylacetic acid/dopamine (DOPAC/DA)--increased in response to dizocilpine in the prefrontal cortex and the nucleus accumbens. Memantine enhanced DOPAC/DA in the prefrontal cortex, the nucleus accumbens and to a lesser degree in the posterior striatum. In contrast to non-competitive NMDA receptor antagonists, CGP 39551 did not increase DA metabolism of subcortical structures and even decreased DOPAC/DA in the prefrontal cortex. These results indicate that competitive and non-competitive NMDA receptor antagonists affect spontaneous locomotion differentially in rats. The biochemical data imply that the stimulant actions non-competitive NMDA receptor antagonists are at least partially due to activation of ascending dopaminergic systems. Potential mechanisms involved in the differential effects of both types of NMDA receptor antagonists are discussed.

Functional heterogeneity of the rat medial prefrontal cortex: effects of discrete subarea-specific lesions on drug-induced conditioned place preference and behavioural sensitization.

While the principal components of the brain reward system, the nucleus accumbens septi and the ventral tegmental area have received much attention, their efferent and afferent structures have not been investigated to the same degree. One major input to this system originates from the medial prefrontal cortex (mPFC) which is not a homogenous structure but can be divided into different subareas that can be distinguished on anatomical and possibly functional grounds. We examined the effects of discrete bilateral quinolinic acid lesions (45 nmol/0.5 μL) of each of the mPFC subareas, the infralimbic (il), prelimbic (pl) and the anterior cingulate (cg) mPFC, on the conditioned place preference (CPP) and psychomotor activation induced by several drugs. Lesions of the il mPFC blocked CPP induced by morphine (10 mg/kg) and CGP37849 [dl‐(E)‐2‐amino‐4‐methyl‐5‐phosphono‐3‐pentic acid, a competitive N‐methyl‐d‐aspartate receptor antagonist; 10 mg/kg]. Lesions of the pl mPFC blocked CPP induced by cocaine (15 mg/kg) and CGP37849, and lesions of the cg mPFC only blocked CGP37849‐induced CPP. Lesions of the whole mPFC blocked morphine‐, cocaine‐ and CGP37849‐induced CPP. None of the lesions affected dl‐amphetamine (4 mg/kg)‐induced CPP. During the conditioning period, none of the lesions affected amphetamine‐induced psychomotor activation and sensitization, whereas both phenomena were attenuated by pl and whole mPFC lesions in the case of cocaine, and by il and whole mPFC lesions in the case of morphine. These results show that the different mPFC subregions have distinct functional roles in the generation of behavioural effects produced by different classes of drugs. This heterogeneity should be taken into account in future studies addressing the role of the mPFC in drug reward and sensitization.

Behavioural pharmacology of glutamate receptors in the basal ganglia.

Glutamate receptors play a major role in the transmitter balance within the basal ganglia (BG). N-methyl-D-aspartate (NMDA) receptor stimulation within the striatum acts behaviourally depressant while intrastriatal as well as systemic administration of NMDA receptor-antagonists have rather stimulatory effects despite the different profiles of non-competitive-, competitive NMDA receptor- and glycine site-antagonists. In animal models of Parkinsons disease all these NMDA receptor antagonists counteract parkinsonian symptoms or act synergistically with L-3,4-dihydroxyphenylalanine (L-DOPA). The strong locomotion-inducing effect of the non-competitive NMDA receptor antagonists is partly, but not fully, mediated by a dopamine (DA) release in the nucleus accumbens. Manipulations at alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors produce poor behavioural effects. These, however, are different or even opposed to NMDA receptor mediated effects. Local infusions of AMPA receptor-antagonists into the BG output nuclei have an anti-parkinsonian effect but systemic injections are ineffective. These drugs even counteract the anti-parkinsonian effect of DA agonists and of non-competitive NMDA receptor antagonists as well as the DA releasing effects of the latter drugs. Only few data on the role of metabotropic receptors exist but the different receptor subtypes with different regional distribution represent a promising target for pharmacological interventions.

Proliferating cells differentiate into neurons in the hippocampal CA1 region of gerbils after global cerebral ischemia

Neurogenesis in the hippocampal dentate area of adult animals is stimulated by neuropathological diseases. We investigated here whether transient global cerebral ischemia in adult gerbils, which causes neurodegeneration selectively in CA1 pyramidal neurons, affects endogenous stem cells in the dentate area as well as CA1 region. The two key findings of this study are that: (1), global ischemia markedly increases neurogenesis in hippocampal subgranular zone from immature neuronal progenitor cells (betaIII-tubulin) to mature neurons (NeuN); and that (2), there is also an appearance of newly born neurons in the degenerated CA1 pyramidal cell layer, as demonstrated by immunofluorescence. These results provide evidence for an increased neurogenesis in the gerbil CA1 pyramidal cell layer four weeks following global cerebral ischemia, which could promote morphological and functional recovery after cerebral ischemia.