Single injections with morphine can induce a state of acute opioid dependence in humans and animals, typically measured as precipitated withdrawal when an antagonist such as naloxone is administered 4-24 h after morphine. Repeated treatment with morphine results in a progressive shift in potency of naloxone to produce such acute withdrawal signs. The current study examined alterations in brain reward thresholds after acute and repeated treatment with morphine (5.6 mg/kg) using a discrete-trial current-intensity brain-stimulation reward procedure. Rats with stimulation electrodes aimed at the medial forebrain bundle at the level of the lateral hypothalamus were tested in twice daily sessions separated by 4 h. Separate groups of rats received treatment with morphine immediately after the first daily test session, and one of several doses of naloxone (0.10, 0.33, 1.0 mg/kg) 4 h later and immediately before the second session; these morphine and naloxone treatments were repeated for four consecutive days (Morphine-Repeat NAL). Additional groups examined the independent contribution of repeated morphine or repeated naloxone. One control group (Morphine-Vehicle) received morphine on all four treatment days, but vehicle before the second test session. A second group (Morphine-Single NAL) also received morphine on all four treatment days, but received 1.0 mg/kg only once after the final morphine pretreatment. A final control group received no morphine at all but received the 1.0-mg/kg dose of naloxone four times (Vehicle-Repeat NAL) before the second daily test session. Repeated naloxone alone (Vehicle-Repeat NAL) produced no changes in brain reward thresholds. Repeated morphine alone (Morphine-Vehicle) failed to alter reward thresholds measured 4 h postmorphine, but produced a slight increase in thresholds in the test sessions that occurred before morphine treatment on Days 3 and 4 (and hence 23.5 h after the previous day's morphine injection). This suggested the development of a modest spontaneous withdrawal-induced reward deficit measurable at 23.5 but not 4 h postmorphine. Naloxone dose-dependently increased brain reward thresholds 4 h after a single morphine pretreatment, with a further shift to the left in the naloxone dose-effect function resulting from repeated morphine and naloxone administration (Morphine-Repeat NAL). However, when the highest dose of naloxone was tested only after the final morphine pretreatment (Morphine-Single NAL), its potency was no different than when administered after the first morphine pretreatment. The results indicate that neuroadaptation within brain reward circuitry results in significant reward deficits after a single morphine pretreatment, and this deficit increases rapidly with repeated morphine and naloxone-induced withdrawal experience. (C) 2004 Elsevier Inc. All rights reserved.

Electric potential distribution in nanoscale electroosmosis has been investigated using the nonequilibrium molecular dynamics ( NEMD), whose results are compared with the continuum based Poisson-Boltzmann (PB) theory. If the bin size of the MD simulation is no smaller than a molecular diameter and the focusing region is limited to the diffusion layer, the ionic density profiles on the bins of the MD results agree well with the predictions based on the PB theory for low and moderate bulk ionic concentrations. The PB equation breaks down at high bulk ionic concentrations, which is also consistent with the macroscopic description.

Rationale: Single injections with morphine can induce a state of acute opioid dependence in humans and animals, typically measured as precipitated withdrawal when an antagonist such as naloxone is administered 4-24 h after morphine. Repeated treatment with morphine results in further increases in naloxone potency, and prior work has shown that this progressive shift in naloxone potency requires repeated naloxone experience under some but not all experimental conditions. Objective: The current study sought to further characterize the experimental conditions that support naloxone experience-dependent and experience-independent potentiation of precipitated suppression of operant responding in morphine pretreated rats, and to assess more directly whether conditioning mechanisms may contribute to the former process. Methods: Rats trained on an FR15 schedule for food received a total of five vehicle or morphine injections (5.6 mg/kg SC) at 4, 8, or 22 h prior to an operant session in which a cumulative dose-effect function for naloxone-induced suppression of responding was determined. Separate groups of animals at each interval between morphine and naloxone received cumulative naloxone dosing after all morphine pretreatments (NAL ALL DAYS) or after just the first and last morphine pretreatment (NAL FIRST/LAST). Additional groups of rats at the 4 h MOR-NAL interval received most of their naloxone cumulative dose-effect experience in either the home cage or in the operant context with levers retracted. Results: Vehicle-pretreated (Morphine-Naive) rats showed little change in the naloxone dose-effect function even after five cumulative dose-effect determinations. With a single morphine pretreatment, naloxone potency was increased at 4 or 8 h post-morphine, but not at 22 h. With repeated morphine treatment, all MOR-NAL intervals resulted in significant shifts in naloxone potency across treatment days even when naloxone was administered only after the first and last morphine pretreatment. However, much greater shifts in naloxone potency were observed at 4-h and 8-h intervals when naloxone was administered on all treatment days. At the 22 h MOR-NAL interval, there was no further potentiation in naloxone potency with additional naloxone experience provided on the intermediate days. Finally, when the repeated naloxone experience occurred in the home cage at the 4-h interval, naloxone potency was identical to that seen after limited naloxone experience (NAL FIRST/LAST), and significantly less than naloxone potency in groups receiving repeated naloxone experience in the operant context. Conclusions: The results suggest that conditioned withdrawal mechanisms may play a significant role in the initial development of opioid dependence.