Acute administration of cocaine reduces metabotropic glutamate receptor 8 protein expression in the rat striatum in vivo

Abstract

Metabotropic glutamate receptors (mGluRs) are densely expressed in the limbic system of the mammalian brain. Increasing evidence suggests a critical role of mGluRs in the pathogenesis of various mental illnesses, including drug abuse and addiction. In this study, we investigated the effect of psychostimulant, cocaine, on protein expression of a specific mGluR subtype, mGluR8, in the rat forebrain in vivo. A rabbit antibody against the extracellular N-terminus of mGluR8 was developed to detect changes in mGluR8 proteins in immunoblot assays. With this antibody, we found that acute systemic injection of cocaine reduced mGluR8 protein levels in the striatum. The reduction of mGluR8 proteins was rapid and transient as it was induced 25min after cocaine injection and returned to the normal level by 6h. No significant change in mGluR8 protein levels in the prefrontal cortex and the hippocampus was observed following cocaine administration. These data demonstrate that protein expression of mGluR8 is subject to the modulation by dopamine stimulation. Acute exposure to cocaine results in a dynamic and region-specific downregulation of mGluR8 expression in the striatum.

Fig. 1

Representative immunoblots depicting the selectivity of a rabbit mGluR8 antibody. Addition of the mGluR8 antibody produced a single strong immunoreactive band in a molecular weight predicted for the size of mGluR8 (~102 kDa) in an immunoblot with striatal samples (left lane). No immunoblot band was displayed when the antibody was pre-absorbed with an immunogen peptide (antibody:immunogen ratio = 1:3 w/w) (right lane).

Fig. 2

Cocaine reduces mGluR8 protein expression in the rat striatum in vivo. Representative immunoblots of mGluR8 and actin are shown above the quantification of immunoblot results. A single dose of cocaine (5 or 20 mg/kg) or saline was injected i.p. and rats were sacrificed 25 min after injection. The quantified data are expressed as means ± SEM (n = 4–6 per group). *p < 0.05 versus saline.

Fig. 3

Cocaine reduces mGluR8 protein expression in the rat striatum in vivo in a time-dependent manner. A single dose of cocaine (20 mg/kg) or saline was injected i.p. and rats were sacrificed at different time points (25, 60, 120, or 360 min) after drug injection. The quantified data are expressed as means ± SEM (n = 4–6 per group). *p < 0.05 versus saline at the same time point.

Fig. 4

Effects of cocaine on mGluR8 protein expression in the rat prefrontal cortex (A) and the hippocampus (B) in vivo. A single dose of cocaine (20 mg/kg) or saline was injected i.p. and rats were sacrificed 25 min after drug injection. The quantified data are expressed as means ± SEM (n = 4–6 per group).