Glutamine synthetase (GS) in astrocytes regulates glutamatergic neurotransmission by maintaining glutamate clearance in the brain. This study determined that GS in astrocytes of the caudate and putamen (CPu) regulates locomotor sensitization after repeated nicotine exposure. Nicotine increased phosphorylated c-Jun N-terminal kinase (pJNK) by stimulating <i>α</i>7 nicotinic acetylcholine receptors in cultured glioma C6 cells and primary astrocytes in a Ca²⁺-dependent manner. Active JNK phosphorylated metabotropic glutamate receptor 1a (mGluR1a) at the carboxyl terminus of glutathione <i>S</i>-transferase-tagged mGluR1a <i>in vitro</i>. Interference with the pJNK-mGluR1a interaction using the inhibitory peptide, Tat-mGluR1a-i (10 μmol/L), decreased the nicotine-induced increase in GS activity in glioma C6 cells and primary astrocytes. Similar results were obtained by bilateral intra-CPu infusion of the inhibitory peptide (2 nmol/side). Inhibition of GS activity by bilateral intra-CPu infusion of methionine sulfoximine (50 nmol/side) decreased the repeated nicotine-induced increase in locomotor activity. These findings suggest that astrocytes in the CPu upregulate locomotor sensitization by activating GS <i>via</i> the pJNK-mGluR1a interaction, which is linked to <i>α</i>7 nicotinic acetylcholine receptors in response to nicotine.