Agmatine Prevents Adaptation of the Hippocampal Glutamate System in Chronic Morphine-Treated Rats

Abstract Chronic exposure to opioids induces adaptation of glutamate neurotransmission, which plays a crucial role in addiction. Our previous studies revealed that agmatine attenuates opioid addiction and prevents the adaptation of glutamate neurotransmission in the nucleus accumbens of chronic morphine-treated rats. The hippocampus is important for drug addiction; however, whether adaptation of glutamate neurotransmission is modulated by agmatine in the hippocampus remains unknown. Here, we found that continuous pretreatment of rats with ascending doses of morphine for 5 days resulted in an increase in the hip- pocampal extracellular glutamate level induced by nalox- one （2 mg/kg, i.p.） precipitation. Agmatine （20 mg/kg, s.c.） administered concurrently with morphine for 5 days attenuated the elevation of extracellular glutamate levels induced by naloxone precipitation. Furthermore, in the hippocampal synaptosome model, agmatine decreased the release and increased the uptake of glutamate in synapto- somes from chronic morphine-treated rats, which might contribute to the reduced elevation of glutamate levels induced by agrnatine. We also found that expression of the hippocampal NR2B subunit, rather than the NR1 subunit, of N-methyl-D-aspartate receptors （NMDARs） was down-regulated after chronic morphine treatment, and agmatine inhibited this reduction. Taken together, agmatine pre- vented the adaptation of the hippocampal glutamate system caused by chronic exposure to morphine, including mod- ulating extracellular glutamate concentration and NMDAR expression, which might be one of the mechanisms underlying the attenuation of opioid addiction by agmatine.