Excess glutamate-mediated excitation of the substantia nigra pars reticulata (SNpr) from overactive subthalamic nucleus (STN) efferents is believed to contribute to the akinetic symptoms of Parkinson’s disease. We have recently reported that injection of the group III metabotropic glutamate (mGlu) receptor agonist, L-SOP into the SNpr can reverse reserpine-induced akinesia in the rat (MacInnes et al., 2004). The mechanism behind this response remains to be established. The aims of this study were to investigate whether a second group III mGlu receptor agonist, L-AP4 also reversed akinesia and whether inhibition of glutamate release in the SNpr might underlie this response.

Under general isoflurane (3%) anaesthesia, male Sprague-Dawley rats (270-320g) were stereotaxically implanted with a 23-gauge guide cannula 2mm above the SNpr. Animals were rendered akinetic 3d later by the injection of reserpine (5 mg kg^-1 s.c.). 18h later, when fully akinetic, animals were placed in observational bowls for a 30 min baseline assessment of locomotor activity. Animals then received a single unilateral injection of L-AP4 (3-300 nmol in 2 µl phosphate buffered saline, pH 7.4) into the SNpr via a 30-gauge needle. Locomotor activity was recorded for a further 30 min and the number of 360^o contraversive rotations quantified as a measure of relief of akinesia. [³H]-D-aspartate release (an index of glutamate release) from freshly prepared rat nigral slices was measured using a two-stimulation KCl (25 mM)-evoked paradigm, as previously described (Chadha et al., 2000). Transmitter release was measured before (S1) and after (S2) the addition of L-SOP (1-1000 µM), L-AP4 (3-100 µM) or vehicle (Krebs’) and the S2/S1 ratio taken as a measure of agonist efficacy. In both studies, the effects of agonists were analysed using a 1-way ANOVA where P<0.05 is significant.

In reserpine-treated rats, L-AP4 produced a dose-dependent reversal of akinesia, which reached significance only at the highest dose tested (300 nmol; 36 ± 5 contraversive rotations 30 min^-1; mean ± s.e.m., n = 6; P<0.001 versus vehicle). L-AP4 (10 and 30 µM) significantly inhibited [³H]-D-aspartate release from nigral slices by 52.6 ± 0.9 % and 59.7 ± 13.3 %, respectively (mean ± s.e.m., n = 8 animals, P<0.01). Similarly, L-SOP significantly inhibited [³H]-D-aspartate release at 100 and 300µM by 64.8 ± 8.7 % and 64 ± 5.4 %, respectively (mean ± s.e.m., n = 6 animals, P<0.05 versus vehicle).

In conclusion, these data support our previous report that activation of group III mGlu receptors in the SNpr can reverse akinesia in the reserpine-treated rat. That both L-AP4 and L-SOP were able to inhibit [³H]-D-aspartate release from nigral slices supports an autoreceptor role for group III mGlu receptors on glutamatergic terminals in this region. We therefore propose that inhibition of glutamate release from overactive glutamatergic STN efferents in the SNpr may underlie the anti-akinetic effects of both L-SOP and L-AP4 in reserpine-treated rats.

Chadha, A. et al., (2000). Br. J. Pharmacol., 130, 1927-1932.
MacInnes, N. et al. (2004). Br. J. Pharmacol., 141 , 15-22.