The group III metabotropic glutamate (mGlu) receptor agonist, L-AP4, provides functional neuroprotection against a 6-hydroxydopamine-induced nigrostriatal tract lesion in rats.

Paul Austin, Susan Duty
King’s College London, London, United Kingdom

We previously reported that the group III mGlu receptor agonist, L-AP4, inhibits glutamate release in the rat substantia nigra (SN; Broadstock et al., 2004). Since elevated glutamate release in the SN is implicated in nigrostriatal tract degeneration in Parkinson’s disease, we hypothesised that L-AP4 might protect against a 6-hydroxydopamine (6-OHDA)-induced lesion of the nigrostriatal tract in rats.

Under isofluorane general anaesthesia, male Sprague Dawley rats (270-290g) were cannulated 2 mm above the right SNc. One week later, all animals received an injection of 6-OHDA (12 μg in 2.5 μl 0.1% ascorbate) 1mm above the SNc via the indwelling cannula. L-AP4 (10 or 100 nmol in 4 μl phosphate-buffered saline) or vehicle was likewise injected 1h before 6-OHDA and daily for 7 days thereafter. On day 6 post lesion, motor function was assessed. Firstly, the number of adjusted steps made by both the ipsilateral and contralateral paws was monitored as animals were moved along a 70 cm distance in both the forehand and backhand directions. These data were expressed as a percentage of pre-lesion scores obtained on the day before 6-OHDA administration. Secondly, the number of net ipsiversive rotations was measured between 40 and 45 min following injection of amphetamine (5 mg kg-1 i.p.). On day 7 post lesion, animals were perfuse fixed under terminal anaesthesia and the brains removed and processed for tyrosine hydroxylase (TH) immunohistochemistry in sections obtained from mid-striatum. All data are expressed as mean ± s.e. mean; n=6 (vehicle treatment group) or n=7 (L-AP4 treatment groups).

As expected, there was no significant difference in the adjusted step scores of the ipsilateral paw between vehicle and L-AP4 treated animals. However, contralateral paw use did differ between animals treated with vehicle and the lower but not higher dose of L-AP4. In vehicle-treated animals, contralateral paw use was 30.0 ± 11.8 % and 12.7 ± 4.4 % of pre-lesion scores in backhand and forehand directions, respectively. In contrast, in animals treated with 10 nmol L-AP4, equivalent contralateral paw use was 83.1 ± 39.5 % and 74.0 ± 34.7 % of pre-lesion scores, respectively, with the forehand score being significantly increased compared to vehicle-treated animals (P<0.05; 2-way ANOVA with Bonferroni’s post-hoc test). Amphetamine-induced rotations were also significantly reduced in animals treated with 10 nmol L-AP4 compared to vehicle. Post-mortem examination revealed that little TH immunoreactivity remained in the lesioned striatum of the vehicle group (5.4 ± 1.6 % of levels in the intact striatum). Whilst animals treated with 100 nmol L-AP4 showed no preservation of striatal TH, those treated with 10 nmol L-AP4 did show significantly greater levels of TH in the lesioned striatum (28.5 ± 7.7 % of the intact side) compared to the vehicle group (P<0.05; 1-way ANOVA with Student Newman Keuls test).

These data corroborate a recent report that L-AP4 protects against 6-OHDA induced degeneration of the nigrostriatal tract (Vernon et al., 2007) and further demonstrate accompanying functional improvement in the behaviour of these animals. The findings support further investigation of group III mGlu receptors as possible targets for providing neuroprotection in Parkinson’s disease.

Broadstock et al., (2004). http://www.pa2online.org/Vol2Issue4abst011P.html
Vernon et al., (2007). JPET 320: 397-409.