Morphine and MK-801 administration leads to alternative N-methyl-d-aspartate receptor 1 splicing and associated changes in reward seeking behavior and nociception on an operant orofacial assay

Abstract

The NMDA receptor plays a large role in opioid-induced plastic changes in the nervous system. The expression levels of its NR1 subunit are altered dramatically by morphine but no changes in its alternative splicing have been reported. Changes in the splicing of the N1, C1, C2, and C2’ cassettes can alter the pharmacology and regulation of this receptor. Western blots run on brain tissue from rats made tolerant to morphine revealed altered splicing of the N1 cassettes in the accumbens and amygdala, and the C1 cassette in the amygdala and the dorsal hippocampus. After three days of withdrawal C2’-containing NR1 subunits were down-regulated in each of these areas. These were not due to acute doses of morphine and may represent long term alterations in drug-induced neuroplasticity. We also examined the effects of morphine tolerance on an operant orofacial nociception assay which forces an animal to endure an aversive heat stimulus in order to receive a sweet milk reward. Morphine decreased pain sensitivity as expected but also increased motivational reward seeking in this task. NMDAR antagonism potentiated this reward seeking behavior suggesting that instead of attenuating tolerance, MK-801 may actually alter the rewarding and/or motivational properties of morphine. When combined, MK-801 and morphine had an additive effect which led to altered splicing in the accumbens, amygdala, and the dorsal hippocampus. In conclusion, NR1 splicing may play a major role in the cognitive behavioral aspects especially in motivational reward seeking behaviors.

Long-term changes in reward-seeking following morphine withdrawal are associated with altered N-methyl-d-aspartate receptor 1 splice variants in the amygdala

“…No interaction was observed (RM Two-Way, F(5,217)=1.263, p>0.05). Previous studies in our lab demonstrated that morphine was anti-nociceptive at doses of at least 10 mg/kg when tested at the aversive 46°C as well (Anderson et al, 2012). …”

“…We measured the effects of morphine administration and withdrawal on motivational reward-seeking behavior and nociception with a reward/conflict operant task first described by Neubert et al (2005) which is sensitive to morphine (Neubert et al, 2006) and can measure motivation for reward (Anderson et al, 2012). A total of forty rats were fasted for 17 +/− 1hrs (from 5pm the previous night to 9–11am the next day) then placed in a Plexiglas box for ten minute periods and trained to press their faces between two aluminum tubes in order to receive a reward of diluted sweetened condensed milk (2:1 water:milk ratio).…”

“…The time per contact is the facial contact time divided by the number of contacts made. It measures the average time of a bout of drinking which alters with the change in temperature and the motivational state of the rat (Anderson et al, 2012). Eight rats were designated to be controls and thirty-two were placed in the morphine treatment group.…”

“…We reported that N1, C1, and C2′ decreased significantly in the AMY. N1 and C2′ decreased in the NACC as well (Anderson et al, 2012). We set out to examine the hypothesis that these variants are associated with the long-term effects of morphine withdrawal.…”

Phosphorylation of the N-methyl-d-aspartate receptor is increased in the nucleus accumbens during both acute and extended morphine withdrawal

“…No differences were observed for the morphine only group, demonstrating this is a morphine withdrawal only effect. These changes in pNR1/ NR1 during acute withdrawal are not caused by changes in NR1 total protein (Anderson et al, 2012b). These changes are also subregion specific, because we tested samples from the amygdala, hippocampus, and periaqueductal gray and found no changes in pNR1 levels (data not shown).…”