Magnitude Of Place Preference Research Articles

The long-term impact of footshock stress on addiction-related behaviors in rats

We investigated the impact of electric shocks – frequently used to model post-traumatic stress disorder in rodents – on behaviors relevant to drug abuse in rats. Rats exposed to 10 shocks of 3mA over 5min showed a robust conditioned fear 28 days later, which confirms the traumatic nature of shock exposure. A different set of rats was studied in the conditioned place preference paradigm beginning with the 27th post-shock day. 10mg/kg morphine induced a marked place preference in both shocked and non-shocked rats. Although the magnitude of place preference was not affected, extinction was markedly delayed in shocked rats. We also investigated tolerance to the hyperthermic effects of morphine. A low dose (5mg/kg) that was administered 4 weeks after shock exposure robustly increased body temperature in both shocked and non-shocked rats. Repeated injections resulted in a mild tolerance in non-shocked controls; yet, morphine readily increased body temperature in these rats on the 5th day of injections. In contrast, the temperature-heightening effect of morphine was abolished in shocked rats after 2 days. Thus, shock exposure considerably delayed the extinction of place preference induced by, and dramatically accelerated the tolerance to the effects of, morphine. Our study shows that electric shocks durably affect behavior in tests relevant to drug abuse in conjunction with the development of post-traumatic stress disorder-like behavioral dysfunctions.

Buprenorphine antinociception is abolished, but naloxone-sensitive reward is retained, in mu-opioid receptor knockout mice.

Buprenorphine is a relatively nonselective opioid receptor partial agonist that is used in the management of both pain and addiction. To improve understanding of the opioid receptor subtypes important for buprenorphine effects, we now report the results of our investigation on the roles of mu-, delta-, and kappa-opioid receptors in antinociceptive responses and place preferences induced by buprenorphine. Buprenorphine antinociception, assessed by hot-plate and tail-flick tests, was significantly reduced in heterozygous mu-opioid receptor knockout (MOR-KO) mice and abolished in homozygous MOR-KO mice. In contrast, buprenorphine retained its ability to establish a conditioned place preference (CPP) in homozygous MOR-KO, although the magnitude of place preference was reduced as the number of copies of wild-type mu-opioid receptor genes was reduced. The remaining CPP of buprenorphine was abolished by pretreatment with the nonselective opioid antagonist naloxone, but only partially blocked by pretreatment with either the delta-selective opioid antagonist naltrindole or the kappa-selective opioid antagonist norbinaltorphimine. These data, and biochemical confirmation of buprenorphine actions as a partial delta-, mu-, and kappa-agonist, support the ideas that mu-opioid receptors mediate most of analgesic properties of buprenorphine, but that mu- and delta- and/or kappa-opioid receptors are each involved in the rewarding effects of this drug.