Preproenkephalin mRNA expression in rat dorsal striatum induced by selective activation of metabotropic glutamate receptor subtype-5.

Group I metabotropic glutamate receptors (mGluR1 and mGluR5 subtypes) are positively coupled to phosphoinositide hydrolysis through G-proteins and are densely expressed in medium-sized projection neurons of striatum. Selective activation of Group I mGluRs upregulates preproenkephalin (PPE) mRNA expression in the rat dorsal striatum. This study investigated the role of one subtype of Group I receptors, mGluR5, in the regulation of PPE mRNA expression in the rat dorsal striatum using quantitative in situ hybridization. Unilateral injection of the mGluR5 selective agonist (RS)-2-Chloro-5-hydroxyphenylglycine (CHPG) into the dorsal striatum (caudoputamen) of chronically cannulated rats at doses of 50 and 200 nmol elevated basal levels of PPE mRNA in the injected dorsal striatum. The induction of PPE mRNA was evident at 1 h, remained at 3 h, and returned to normal level 6 h after CHPG injection. Pretreatment with an mGluR5 selective antagonist 2-methyl-6-(phenylethynyl) pyridine hydrochloride (MPEP) at a dose of 10 mg/kg (i.p.) blocked CHPG-stimulated PPE expression. MPEP also attenuated PPE expression induced by dopamine D(2) receptor blockade with eticlopride (0.5 mg/kg, i.p.). Administration of MPEP alone had no significant effects on basal levels of PPE mRNA in the striatum. The results from the present study demonstrate that glutamatergic tone on mGluR5 possesses the ability to positively regulate PPE gene expression in striatal neurons in vivo. Moreover, activation of mGluR5 participates in the mediation of D(2) antagonist-induced PPE expression. Copyright 2003 Wiley-Liss, Inc.