Interdose interval effects on the development of contextual tolerance to nicotine's analgesic effects in rats (Rattus norvegicus).

Learning models of associative and nonassociative drug tolerance predict that the development of contextual tolerance to drug effects is disrupted when the drug is delivered at short interdose intervals (IDIs). The authors examined the impact of 1 long IDI and 2 short IDIs in the development of contextual nicotine tolerance. Associative tolerance was investigated by giving rats (Rattus norvegicus) 10 subcutaneous injections of nicotine at either long (72-hr) IDIs or short (6-hr and 4.5-hr) IDIs. The delivery of nicotine was either explicitly paired or explicitly unpaired with a distinctive context. A 3rd group of rats was exposed to the experimental procedures but received only saline. Associative tolerance to nicotine's analgesic effects was defined as a shift to the right of the dose-response curve (DRC) of rats in the explicitly paired condition with respect to the DRC of rats in the explicitly unpaired condition. Analgesia was assessed with the tail-flick and hot-plate devices. In the tail-flick assessment, associative tolerance was evident in the 72-hr and the 6-hr IDI conditions only. In the hot-plate assessment, associative tolerance was present in the 72-hr IDI condition only. The findings suggest that contextual tolerance to nicotine's analgesic effects are positively related to IDI length and are more readily demonstrated with the tail-flick method than with the hot-plate method. Overall, the results supported the thesis that nicotine tolerances that develop to different IDIs are qualitatively different and may be mediated by different psychological and physiological mechanisms.

Meal patterns and body weight after nicotine in male rats as a function of chow or high-fat diet.

Studies of the effects of nicotine (NIC) on meal patterns in rats often employ chow pellet diets that contain little fat, whereas humans using NIC commonly consume diets relatively rich in fat. The aim of the present study was therefore to compare the impact of NIC administration and NIC cessation on meal pattern in adult male rats offered a standard powdered chow (CHOW: 10.9% fat by calories) diet or a palatable high-fat (HIFAT: 58.3% fat by calories) diet. Computerized meal pattern analyses were conducted for male rats treated for 14 days with injections of either saline or 1.4 mg/kg/day of NIC (as the free base given in 5 equal amounts) during the dark phase and continued for 10 days after NIC cessation. The suppression of daily caloric intake by NIC was larger in HIFAT-NIC rats than in CHOW-NIC rats (p < .01), such that NIC induced a greater suppression of body weight in HIFAT-NIC rats, relative to CHOW-NIC rats (p < 0.02). NIC administration reduced MS in both CHOW and HIFAT rats. CHOW fed rats showed a gradual increase in meal number in response to NIC, whereas HIFAT fed rats showed a significant initial suppression of meal number, which returned to control levels by day 4 of the 14 day NIC treatment period. In addition, NIC increased water intake more in HIFAT fed rats than in CHOW rats. Cessation of NIC resulted in transient increases in daily caloric intake in CHOW and in HIFAT rats. The present study demonstrates that NIC actions on food intake suppression, meal patterns, and weight reduction differ depending on whether the rats are fed low- or high-fat diets.

An inexpensive food cup for use in a commercially available food monitoring system.

The microstructure of feeding in rats can be probed using a variety of protocols that employ videotape-based ratings, pellet feeders, and/or laboratory balances. A recent commercial product (BioDAQ, Research Diets, New Brunswick, NJ) uses a metal food hopper placed on a load cell to monitor daily food pellet consumption. In this system, movements of the hopper during eating and the cessation of hopper movements after eating are combined with momentary hopper weights for subsequent analyses of daily meal patterns. Our laboratory has devised an improved food cup for the BioDAQ system that is easily balanced, minimizes spillage, and is compatible with either powdered chow diets or nonpelleted soft diets (e.g., a high-fat diet). In the present paper, we describe the methods used to fabricate this food cup and present data illustrating its use in meal pattern analyses for rats fed either a ground laboratory chow or a 33% high-fat diet.