Induction of stereotypy in dopamine-deficient mice requires striatal D1 receptor activation

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Abstract
Motor stereotypies are abnormally repetitive behaviors that can develop with excessive dopaminergic stimulation and are features of some neurologic disorders. To investigate the mechanisms required for the induction of stereotypy, we examined the responses of dopamine-deficient (DD) mice to increasing doses of the dopamine precursor l-DOPA. DD mice lack the ability to synthesize dopamine (DA) specifically in dopaminergic neurons yet exhibit robust hyperlocomotion relative to wild-type (WT) mice when treated with l-DOPA, which restores striatal DA tissue content to ≈10% of WT levels. To further elevate brain DA content in DD mice, we administered the peripheral l-amino acid decarboxylase inhibitor carbidopa along with l-DOPA (C/l-DOPA). When striatal DA levels reached >50% of WT levels, a transition from hyperlocomotion to intense, focused stereotypy was observed that was correlated with an induction of c-fos mRNA in the ventrolateral and central striatum as well as the somatosensory cortex. WT mice were unaffected by C/l-DOPA treatments. A D1, but not a D2, receptor antagonist attenuated both the C/l-DOPA-induced stereotypy and the c-fos induction. Consistent with these results, stereotypy could be induced in DD mice by a D1, but not by a D2, receptor agonist, with neither agonist inducing stereotypy in WT mice. Intrastriatal injection of a D1 receptor antagonist ameliorated the stereotypy and c-fos induction by C/l-DOPA. These results indicate that activation of D1 receptors on a specific population of striatal neurons is required for the induction of stereotypy in DD mice.

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