Sensorimotor deficits in a unilateral intrastriatal 6-OHDA partial lesion model of Parkinson’s disease in marmoset monkeys

Abstract

Animal studies investigating the efficacy of neurotrophic factors as treatments for Parkinson’s disease (PD) ideally require partial dopamine (DA) lesion models. The intrastriatal 6-hydroxydopamine (6-OHDA) lesion model may be suitable for this purpose. Although this model has been well characterized in rodents, it has not previously been used in monkeys. The goal of the present study was to characterize the behavioral effects of unilateral injections of 6-OHDA in the basal ganglia of common marmoset monkeys (Callithrix jacchus). Cell counts from tyrosine hydroxylase immunochemistry 5 months postlesion revealed DA cell loss in the substantia nigra on the lesioned side to ∼46% of relative to the unlesioned side. 6-OHDA lesioned monkeys showed a variety of behavioral deficits. Apomorphine induced rotation and simple sensorimotor measures (head position bias and PD disability rating score) were most affected by the lesion. The largest deficits were seen at 1 or 2 weeks postsurgery but had recovered by week 10. 6-OHDA lesioned monkeys took longer to complete a more complex sensorimotor staircase task. At 3.5 months postlesion, 6-OHDA monkeys also showed deficits on an object retrieval task designed to measure sensorimotor planning and skilled hand use. α-Methyl-p-tyrosine, a tyrosine hydroxylase inhibitor, reinstated those deficits which had undergone recovery in the lesioned animals and also exacerbated the deficits on the staircase task. This model has potential in assessing treatments for PD aimed at curtailing disease progression such as continuous delivery of neurotrophic factors.

Introduction

Parkinson’s disease (PD) is one of the most common debilitating neurodegenerative diseases, with 1–2 individuals affected for every 1000 Chio et al 1998, Errea et al 1999, MacDonald et al 2000. The disease manifests itself as bradykinesia, rigidity, and resting tremor (Gelb et al., 1999) with patients frequently presenting cognitive deficits Blanchet et al 2000, Owen et al 1997. The neuropathology underlying PD centers primarily on a severe, but subtotal, loss of dopamine (DA) cells in the substantia nigra pars compacta (SNpc) Damier et al 1999, Hirsch et al 1989.

A variety of strategies has been developed in the search for an effective treatment for PD. These strategies can be broadly classified as those attempting to control the symptoms of the disorder and those attempting to halt or reverse disease progression (Fahn, 1997). The mainstay of the former approach includes the use of dopamine replacement drugs such as l-DOPA (Stern and Freese, 1997). More invasive techniques have also been attempted, including brain stimulation (Durif et al., 1999) and intracerebral transplantation of fetal dopaminergic cells Brundin et al 2000, Hagell et al 1999. Unfortunately, such treatments fail to halt the progress of the disease and are not without risk of serious side effects (e.g., l-DOPA induced dyskinesias) (Rascol et al., 2000).

Within the past decade, attention has been directed at methods designed to halt or reverse disease progression. Such strategies include attempts to rescue damaged neurones with the aid of neurotrophic factors, such as glial cell line-derived neurotrophic factor (GDNF) (Bjorklund and Lindvall, 2000). Strategies involving GDNF, in particular, seem promising since GDNF has been shown to be expressed in the developing and adult striatum Kawamoto et al 2000, Nosrat et al 1996. Furthermore, it has been shown to increase survival of DA neurons in vitro and in vivo when given before or after mechanical axotomy Beck et al 1995, Tseng et al 1997 or treatment with neurotoxins targeting DA neurons Gash et al 1996, Kirik et al 2000b, Kordower et al 2000, Winkler et al 1996.

Studies investigating neurotrophic factors such as GDNF pose a particular challenge regarding the appropriate model needed to test its efficacy. In rodents, the in vivo model most commonly used in PD research involves the unilateral injection of 6-hydroxydopamine (6-OHDA) directly into the substantia nigra or the medial forebrain bundle (Schwarting and Huston, 1996). These procedures produce DA depletions in the terminal striatal areas which are almost complete. Miklyaeva et al 1994, Olsson et al 1995. In order to assess the neuroprotective efficacy of neurotrophic factors, however, a partial lesion animal model is required (Bjorklund et al., 1997). Ideally, such a model should parallel the gradual degeneration of DA neurones characteristic of PD where a small number of DA neurons remain at the end stage of the disease Damier et al 1999, Hirsch et al 1989. These remaining groups of neurons are the targets for neuroprotective strategies (Lapchak et al., 1997). Unfortunately, it is difficult to produce standardized partial lesions by making subtotal lesions within the substantia nigra or medial forbrain bundle (Bjorklund et al., 1997). An alternative approach has been developed in rats in which 6-OHDA is injected directly into the terminal areas in the striatum. In the past few years, this intrastriatal 6-OHDA model in rats has been well characterized and been shown to produce suitable partial DA lesions Barneoud et al 2000, Kirik et al 1998, Lee et al 1996, Przedborski et al 1995. In this model there is a gradual degeneration, in which the first stage of rapid cell loss (1 to 4 weeks postinjection) is followed by protracted cell loss continuing for at least 4 months (Sauer and Oertel, 1994). The majority of rat studies showing the neuroprotective and neuroregenerative capacity of GDNF have used this particular partial lesion model Connor et al 2001, Kirik et al 2001, Kirik et al 2000a, Kirik et al 2000b, Mandel et al 1999, Rosenblad et al 1998, Rosenblad et al 2000, Winkler et al 1996. For example, Kirik and colleagues (Kirik et al., 2000b) have shown that administration of a modified adenoassociated viral vector expressing GDNF to rats prior to intrastriatal injection of 6-OHDA was able to protect dopamine cells against the toxic insult and also to result in preservation of function.

Before treatments using viral vectors expressing GDNF progress to clinical use, their safety and efficacy in monkeys should be assessed as an intermediate step between rodent studies and patient trials for a number of reasons. For example, the putamen and caudate nucleus form a continuous structure in rodents whereas they are discrete structures in humans and monkeys. Also, there are immunological and pharmacokinetic differences between primates and rodents.

Although there have been monkey studies in which GDNF has been evaluated, they have made use of the MPTP model of PD Costa et al 2001, Gash et al 1996, Gerhardt et al 1999, Kordower et al 2000, Zhang et al 1997. Since the majority of rodent studies rely on the intrastriatal 6-OHDA model, we thought it was important to investigate the efficacy of GDNF in monkeys based on the same model. This would allow for direct comparison with the majority of rodent studies testing GDNF. However, the 6-OHDA partial model has not yet been characterized in monkeys. The goal of the present study was to characterize the behavioral effects of 6-OHDA lesions within the dopaminergic terminal striatal area in the common marmoset monkey.