Naltrexone antagonism of morphine antinociception in sucrose- and chow-fed rats

Deciphering the molecular basis of the kappa opioid receptor selectivity: A Molecular Dynamics study

Selective antagonists for the kappa opioid receptor (KOP) have the potential to treat opiate and alcohol addiction, as well as depression and other CNS disorders. Over the years, the development of KOP-selective antagonists yielded very few successful compounds. Recently, N-substituted trans-3,4-dimethyl-4-(3-hydroxyphenyl)piperidines have emerged as a novel class of pure opioid receptor antagonists, including the marketed Mu opioid receptor (MOP) peripheral antagonist Alvimopan and the potent KOP antagonist JDTic. However, the selectivity determinants of this class of compounds towards the opioid receptor subtypes are still vague and understudied. In this work, we have performed Molecular Dynamics (MD) simulation to gain insights into the differential binding of this class of compounds into KOP, as exemplified by Alvimopan and JDTic. Our study indicated that the selectivity largely depends on ligands interaction with the selectivity pocket formed by Val108, Thr111, and Val118, supported by two additional polar and hydrophobic contacts with Asp138 and Trp287, respectively. Our results also demonstrate, for the first time, that non-morphinan ligands can still adopt the "message-address model" for KOP efficacy and selectivity by binding to Glu297.

Eating is a protected behavior even in the face of persistent pain in male rats

Feeding is critical for survival. Yet, patients with chronic pain often lose their appetite and eat less. We previously showed that ad libitum fed male rats continue to feed rather than withdraw from a brief noxious stimulus. This study examined the effects of a sustained noxious stimulus on feeding by testing ad libitum fed male rats for five eating behaviors--latency to eat, time taken to eat each chip, pauses and scanning during eating, and the number of chocolate chips eaten--during the hour following a sham injection or an injection of a low (0.5%) or moderate (1.5%) dose of formalin into the hind paw. Sham-injected rats showed no pain-related behaviors, rats injected with 0.5% formalin showed very few pain-related behaviors, and rats injected with 1.5% formalin showed favoring, lifting and licking of the injured paw with a characteristic biphasic time course. Besides taking less time to commence eating during the first phase of formalin pain, rats injected with either dose of formalin did not differ from sham-injected rats on any of the other eating measures. Rats injected with 0.5% formalin showed no pain behaviors during eating, whereas those given 1.5% formalin typically ate while not exhibiting any pain behaviors but occasionally ate while favoring the paw, rarely while lifting the paw, and never while licking the paw. These results show that eating is a protected activity even in the presence of persistent pain in male rats.

Naloxone attenuates incubated sucrose craving in rats

RATIONALE

Cue-induced craving precedes drug relapse and contributes to eating disorders. Opiate antagonists have been demonstrated to be effective at reducing cravings for drugs and food. Craving, as defined as responding for a stimulus previously associated with a reward, increases, or incubates, over forced abstinence in an animal model of relapse.

OBJECTIVES

This paper aims to determine anticraving effects of the opiate antagonist, naloxone, on the incubation of sucrose craving.

METHODS

106 male Long-Evans rats lever pressed for 10% sucrose solution 2 h/day for 10 days. On either day 1 or 30 of forced abstinence, rats responded in extinction for 6 h and then were injected (ip) with either saline or naloxone (0.001, 0.01, 0.1, 1, or 10 mg/kg). The rats then responded for 1 h for presentation of a tone + light cue previously presented with every sucrose delivery during self-administration training.

RESULTS

The rats responded more in extinction and following saline on day 30 vs day 1 (an incubation of craving). Except for a trend for a decrease in responding following 10 mg/kg on day 1, naloxone was primarily effective on day 30. On day 30, naloxone significantly reduced responding at all doses except for 0.1 mg/kg.

CONCLUSIONS

The time-dependent increase in sensitivity to an opiate antagonist is consistent with time-dependent changes in the opiate system following forced abstinence from sucrose. These changes may partly underlie the incubation of sucrose craving. In addition, these findings could be used to support the use of naloxone as an anticraving medication in protracted abstinence.