Cortical regulation of striatal medium spiny neuron dendritic remodeling in parkinsonism: modulation of glutamate release reverses dopamine depletion-induced dendritic spine loss

Abstract

Striatal medium spiny neurons (MSNs) receive glutamatergic afferents from the cerebral cortex and dopaminergic inputs from the substantia nigra (SN). Striatal dopamine loss decreases the number of MSN dendritic spines. This loss of spines has been suggested to reflect the removal of tonic dopamine inhibitory control over corticostriatal glutamatergic drive, with increased glutamate release culminating in MSN spine loss. We tested this hypothesis in two ways. We first determined in vivo if decortication reverses or prevents dopamine depletion-induced spine loss by placing motor cortex lesions 4 weeks after, or at the time of, 6-hydroxydopamine lesions of the SN. Animals were sacrificed 4 weeks after cortical lesions. Motor cortex lesions significantly reversed the loss of MSN spines elicited by dopamine denervation; a similar effect was observed in the prevention experiment. We then determined if modulating glutamate release in organotypic cocultures prevented spine loss. Treatment of the cultures with the mGluR2/3 agonist LY379268 to suppress corticostriatal glutamate release completely blocked spine loss in dopamine-denervated cultures. These studies provide the first evidence to show that MSN spine loss associated with parkinsonism can be reversed and point to suppression of corticostriatal glutamate release as a means of slowing progression in Parkinson's disease.

Figure 1.

MSNs were analyzed in the striatal region that receives afferents from the M1 cortex (black). In addition, we also analyzed MSNs in a region that does not receive a significant density of inputs from the M1 cortex as an internal negative control site (hatched). CC, corpus callosum; STR, striatum. Scale bar, 500 μm.

Figure 2.

Characterization of focal cortical lesions. (A) A representative 150-μm coronal section illustrating the cortical lesion, which in this case spanned layers I–V. (B) The absence of NeuN-positive cells illustrates the loss of cortical neurons in the lesioned area and shows that the underlying tissue is intact. Reconstructions of the largest (black) and smallest (gray) cortical lesions as assessed by the loss of NeuN-like immunoreactive neurons is shown in panel (C). Numbers refer to distance from the bregma skull suture (Paxinos and Watson 2007). Scale bars in panel (A), 200 μm; panel (B), 100 μm; and panel (C), 500 μm.

Figure 3.

Cortical lesions significantly reverse dopamine depletion–induced MSN spine loss. Cortical lesions attenuated spine loss only in the M1-recipient zone (hatched bar) and not in an adjacent territory (stippled bar). Numbers inside each bar indicate the number of animals/group. *P < 0.01 relative to control animals. #P = 0.0008 relative to 6-OHDA-lesioned animals with intact cortex.

Figure 4.

Photomicrographs of representative Golgi-impregnated MSN dendritic segments are shown for (A) control, (B) 6-OHDA-lesioned, (C) cortically-lesioned, and (D) 6-OHDA- plus cortically-lesioned animals. Scale bar, 4 μm.

Figure 5.

Cortical lesions performed at the same time as 6-OHDA lesions of nigrostriatal dopamine neurons attenuate MSN spine loss. Cortical lesions attenuated dopamine depletion–induced spine loss only in the M1-recipient zone (hatched bar) and not in an adjacent region of the striatum that does not receive inputs from the motor cortex (stippled bar). Numbers inside each bar indicate the number of animals/group. *P < 0.01 relative to control animals. #P = 0.06 relative to 6-OHDA-lesioned animals with intact cortex.

Figure 6.

Treatment of slice cultures with the mGlur2/3 agonist LY379268 completely prevented dopamine depletion–induced spine loss. Each symbol represents the mean spine density in a single culture. **P < 0.005 relative to control cultures.

Figure 7.

Photomicrographs of ballistically labeled MSN dendrites. (A) Dendritic segment of MSN from a control culture, (B) MPP⁺-treated dendrite, and (C) LY379268 plus MPP⁺–treated dendrite. Scale bar, 2 μm.

Figure 8.

Treatment of slice cultures with the mGluR2/3 antagonist LY341495 blocks the effects of the mGluR2/3 agonist LY379268. LY341495 significantly decreased MSN spine density in cultures with an intact dopamine innervation. *P < 0.05 compared with control cultures. **P < 0.05 compared with control cultures.