Chronic exposure to dietary sterol glucosides is neurotoxic to motor neurons and induces an ALS-PDC phenotype

Epidemiological studies of the Guamanian variants of amyotrophic lateral sclerosis (ALS) and parkinsonism, amyotrophic lateral sclerosis-parkinsonism dementia complex (ALS-PDC), have shown a positive correlation between consumption of washed cycad seed flour and disease occurrence. Previous in vivo studies by our group have shown that the same seed flour induces ALS and PDC phenotypes in out bred adult male mice. In vitro studies using isolated cycad compounds have also demonstrated that several of these are neurotoxic, specifically, a number of water insoluble phytosterol glucosides of which beta-sitosterol beta-D: -glucoside (BSSG) forms the largest fraction. BSSG is neurotoxic to motor neurons and other neuronal populations in culture. The present study shows that an in vitro hybrid motor neuron (NSC-34) culture treated with BSSG undergoes a dose-dependent cell loss. Surviving cells show increased expression of HSP70, decreased cytosolic heavy neurofilament expression, and have various morphological abnormalities. CD-1 mice fed mouse chow pellets containing BSSG for 15 weeks showed motor deficits and motor neuron loss in the lumbar and thoracic spinal cord, along with decreased glutamate transporter labelling, and increased glial fibrillary acid protein reactivity. Other pathological outcomes included increased caspase-3 labelling in the striatum and decreased tyrosine-hydroxylase labelling in the striatum and substantia nigra. C57BL/6 mice fed BSSG-treated pellets for 10 weeks exhibited progressive loss of motor neurons in the lumbar spinal cord that continued to worsen even after the BSSG exposure ended. These results provide further support implicating sterol glucosides as one potential causal factor in the motor neuron pathology previously associated with cycad consumption and ALS-PDC.

A possible role for dopamine D3 receptor stimulation in the induction of neurogenesis in the adult rat substantia nigra

Small molecule neurotransmitters, such as dopamine, have been shown to regulate cell cycles in the developing brain [Spencer GE, Klumperman J, Syed NI (1998) Neurotransmitters and neurodevelopment: Role of dopamine in neurite outgrowth, target selection and specific synapse formation. Perspect Dev Neurobiol 5:451-467; Ohtani N, Goto T, Waeber C, Bhide PG (2003) Dopamine modulates cell cycle in the lateral ganglionic eminence. J Neurosci 23:2840-2850] and may provide an alternative to traditional growth factors for the regulation of neurogenesis. Specifically, the dopamine D3 receptor appears to play an important role in neural development, and shows a persistent expression through adulthood in the proliferative subventricular zone [Diaz J, Ridray S, Mignon V, Griffon N, Schwartz JC, Sokoloff P (1997) Selective expression of dopamine D3 receptor mRNA in proliferative zones during embryonic development of the rat brain. J Neurosci 17:4282-4292]. Furthermore, pharmacological stimulation of D3 receptors promotes proliferation of adult subventricular zone cells, both in vitro [Coronas V, Bantubungi K, Fombonne J, Krantic S, Schiffmann SN, Roger M (2004) Dopamine D3 receptor stimulation promotes the proliferation of cells derived from the post-natal subventricular zone. J Neurochem 91:1292-1301] and in vivo [Van Kampen JM, Hagg T, Robertson HA (2004) Induction of neurogenesis in the adult rat subventricular zone and neostriatum following dopamine D3 receptor stimulation. Eur J Neurosci 19:2377-2387]. In earlier work, we have demonstrated the induction of cell proliferation in the subventricular zone of the adult rat brain accompanied by a dramatic 10-fold induction of neurogenesis in the neighboring neostriatum, following administration of the preferential D3 receptor agonist, 7-hydroxy-N,N-di-n-propyl-2-aminotetralin [Van Kampen JM, Hagg T, Robertson HA (2004) Induction of neurogenesis in the adult rat subventricular zone and neostriatum following dopamine D3 receptor stimulation. Eur J Neurosci 19:2377-2387]. Dopamine D3 receptors have also been found in the substantia nigra [Diaz J, Pilon C, Le Foll B, Gross C, Triller A, Schwartz JC, Sokoloff P (2000) Dopamine D3 receptors expressed by all mesencephalic dopamine neurons. J Neurosci 20:8677-8684], a region of the adult brain shown to exhibit ongoing cytogenesis and neurogenic potential [Lie DC, Dziewczapolski G, Willhoite AR, Kaspar BK, Shults CW, Gage FH (2002) The adult substantia nigra contains progenitor cells with neurogenic potential. J Neurosci 22:6639-6649; Zhao M, Momma S, Delfani K, Carlen M, Cassidy RM, Johansson CB, Brismar H, Shupliakov O, Frisen J, Janson AM (2003) Evidence for neurogenesis in the adult mammalian substantia nigra. Proc Natl Acad Sci U S A 100:7925-7930]. We have found that chronic intraventricular administration of 7-hydroxy-N,N-di-n-propyl-2-aminotetralin triggers a profound induction of cell proliferation in the rat substantia nigra and promotes the adoption of a neuronal phenotype in a proportion of these newly generated cells.

Dopamine D(1A) receptor function in a rodent model of tardive dyskinesia

Tardive dyskinesia develops as a common complication of long-term neuroleptic use. The emergence of such dyskinesias may reflect a shift in the balance of dopamine D(1) and D(2) receptor-mediated activity, with a relative increase in activity in the D(1) receptor-regulated direct striatonigral pathway. In rats, chronic treatment with the antipsychotic fluphenazine triggers a syndrome of vacuous chewing movements, which are attenuated by dopamine D(1) receptor antagonists. A similar syndrome can be seen in drug-naive animals following acute administration of selective dopamine D(1) receptor agonists. However, not all dopamine D(1) receptor agonists elicit these mouth movements. Thus, some investigators have suggested the existence of novel subtypes of the dopamine D(1) receptor. In these studies, we sought to clarify the role of the dopamine D(1A) receptor in vacuous chewing movements induced both by the selective dopamine D(1) receptor agonist SKF 38393, as well as by chronic neuroleptic administration, using in vivo oligonucleotide antisense to dopamine D(1A) receptor messenger RNA. Intrastriatal antisense treatment significantly and selectively attenuated striatal dopamine D(1) receptor binding, accompanied by reductions in SKF 38393- and chronic fluphenazine-induced vacuous chewing movements. These findings suggest that the dopamine D(1A) receptor plays an important role in the expression of vacuous chewing movements in a rodent model of tardive dyskinesia and may contribute to the pathogenesis of the human disorder. This may have important implications for the treatment of tardive dyskinesia in humans.

Dopamine transporter function assessed by antisense knockdown in the rat: protection from dopamine neurotoxicity

The plasma membrane dopamine transporter is located on presynaptic nerve terminals and is responsible for the termination of dopaminergic neurotransmission via dopamine reuptake. The dopamine transporter may also contribute to the pathogenesis of Parkinson disease. Dopamine transporter expression correlates well with susceptibility to neuronal degeneration in 1-methyl-4-phenyl-1,2,3,6 -tetrahydropyridine (MPTP)-induced parkinsonism. Recent studies have implicated the dopamine transporter in the uptake of both this neurotoxin and its metabolite, MPP(+), as well as another experimental neurotoxin, 6-hydroxydopamine. In these studies we examined the role of the dopamine transporter in the neurotoxicity of both MPP(+) and 6-hydroxydopamine in the rat brain using in vivo administration of phosphorothioate antisense oligonucleotides targeting dopamine transporter mRNA. Infusion of dopamine transporter antisense (1 nmol/day, 7 days) into the left substantia nigra pars compacta resulted in reduced (3)H-WIN 35-428 binding in the left striatum and significant levodopa and amphetamine-induced contralateral rotations. Unilateral pretreatment with dopamine transporter antisense prior to bilateral intrastriatal infusion of either MPP(+) or 6-hydroxydopamine resulted in asymmetrical striatal (3)H-WIN 35-428 binding and dopamine content as well as significant apomorphine-induced ipsilateral rotations, suggesting neuroprotection of nigrostriatal neurons on the antisense-treated side. Thus, the dopamine transporter appears to play a critical role in determining susceptibility to the experimental neurotoxins MPP(+) and 6-hydroxydopamine. In light of this, the dopamine transporter may prove useful, both as a marker for susceptibility to Parkinson's disease and as a target for therapeutic intervention.

Effects of oligonucleotide antisense to dopamine D(1A) receptor messenger RNA in a rodent model of levodopa-induced dyskinesia

Dyskinesias are abnormal involuntary movements which develop as a side-effect of long-term treatment with levodopa in patients with Parkinson's disease. The pathophysiology underlying these dyskinesias remains unclear, although, it has been suggested that heightened activity of dopamine D(1) receptor-bearing striatonigral neurons may play a key role. Chronic pulsatile levodopa administration to hemiparkinsonian rats results in sensitization of rotational responses to apomorphine. This sensitization is thought to be analogous to levodopa-induced dyskinesias in humans. In these studies, we further clarify the role of the dopamine D(1A) receptor in this rodent model of levodopa-induced dyskinesias using an in vivo oligonucleotide antisense approach. Hemiparkinsonian rats received twice daily injections of levodopa for three weeks followed by intrastriatal infusion of dopamine D(1A) receptor antisense (7nmol/day, three days), a scrambled missense control sequence, or saline. Those animals treated with antisense displayed significantly fewer apomorphine-induced rotations than saline- or missense-treated controls.By reducing dopamine D(1A) receptor expression, we were able to attenuate sensitization of the response to apomorphine resulting from chronic pulsatile levodopa treatment. Thus, the dopamine D(1A) receptor appears to play a significant role in levodopa-induced dyskinesias and warrants further examination. These findings may have important implications for the development of selective treatment strategies designed to alleviate parkinsonian symptoms, while minimizing motor complications.