A review on the role of metabotropic glutamate receptors in neuroplasticity following psychostimulant use disorder

Blockade of mGluR5 in nucleus accumbens modulates calcium sensor proteins, facilitates extinction, and attenuates reinstated morphine place preference in rats

Numerous findings confirm that the metabotropic glutamate receptors (mGluRs) are involved in the conditioned place preference (CPP) induced by morphine. Here we focused on the role of mGluR5 in the nucleus accumbens (NAc) as a main site of glutamate action on the rewarding effects of morphine. Firstly, we investigated the effects of intra-NAc administrating mGluR5 antagonist 3-((2-Methyl-1,3-thiazol-4-yl) ethynyl) pyridine hydrochloride (MTEP; 1, 3, and 10 μg/μl saline) on the extinction and the reinstatement phase of morphine CPP. Moreover, to determine the downstream signaling cascades of mGluR5 in morphine CPP, the protein levels of stromal interaction molecules (STIM1 and 2) in the NAc and hippocampus (HPC) were measured by western blotting. The behavioral data indicated that the mGluR5 blockade by MTEP at the high doses of 3 and 10 μg facilitated the extinction of morphine-induced CPP and attenuated the reinstatement to morphine in extinguished rats. Molecular results showed that the morphine led to increased levels of STIM proteins in the HPC and increased the level of STIM1 without affecting STIM2 in the NAc. Furthermore, intra-NAc microinjection of MTEP (10 μg) in the reinstatement phase decreased STIM1 in the NAc and HPC and reduced the STIM2 in the HPC. Collectively, our data show that morphine could facilitate brain reward function in part by increasing glutamate-mediated transmission through activation of mGluR5 and modulation of STIM proteins. Therefore, these results highlight the therapeutic potential of mGluR5 antagonists in morphine use disorder.

Modulation of Kynurenic Acid Production by N-acetylcysteine Prevents Cognitive Impairment in Adulthood Induced by Lead Exposure during Lactation in Mice

Lead (Pb2+) exposure during early life induces cognitive impairment, which was recently associated with an increase in brain kynurenic acid (KYNA), an antagonist of NMDA and alpha-7 nicotinic receptors. It has been described that N-acetylcysteine (NAC) favors an antioxidant environment and inhibits kynurenine aminotransferase II activity (KAT II, the main enzyme of KYNA production), leading to brain KYNA levels decrease and cognitive improvement. This study aimed to investigate whether the NAC modulation of the brain KYNA levels in mice ameliorated Pb2+-induced cognitive impairment. The dams were divided into four groups: Control, Pb2+, NAC, and Pb2++NAC, which were given drinking water or 500 ppm lead acetate in the drinking water ad libitum, from 0 to 23 postnatal days (PNDs). The NAC and Pb2++NAC groups were simultaneously fed NAC (350 mg/day) in their chow from 0 to 23 PNDs. At PND 60, the effect of the treatment with Pb2+ and in combination with NAC on learning and memory performance was evaluated. Immediately after behavioral evaluation, brain tissues were collected to assess the redox environment; KYNA and glutamate levels; and KAT II activity. The NAC treatment prevented the long-term memory deficit exhibited in the Pb2+ group. As expected, Pb2+ group showed redox environment alterations, fluctuations in glutamate levels, and an increase in KYNA levels, which were partially avoided by NAC co-administration. These results confirmed that the excessive KYNA levels induced by Pb2+ were involved in the onset of cognitive impairment and could be successfully prevented by NAC treatment. NAC could be a tool for testing in scenarios in which KYNA levels are associated with the induction of cognitive impairment.