This study confirms and extends previous results suggesting that compounds with high DAT affinity can have strong, moderate, weak, or no effectiveness as reinforcers. The mechanisms that may underlie this variation in reinforcing effectiveness of these DAT ligands remain to be established.
Operant responding on a progressive ratio (PR) schedule for intravenous cocaine as well as sucrose reinforcement was examined in female rats throughout the reproductive process. Self-administration sessions began before mating, and continued throughout pregnancy and until lactational Day 8; following parturition, litters were present with dams during operant sessions. Physiological changes associated with the reproductive process dramatically altered PR responding for cocaine, while PR responding for sucrose was relatively stable throughout pregnancy and lactation. Female animals exhibited the highest number of responses/session for cocaine during estrus and the 1st trimester of pregnancy and the lowest responding near parturition, with levels only partially recovering during lactation. Dams selfadministering cocaine exhibited notably different patterns of maternal behavior in the operant chambers than dams responding for sucrose. Thus, cocaine's reinforcing efficacy may be influenced by (a) the changing physiological profile associated with the reproductive process and (b) competition from the reinforcing properties of offspring during lactation.Epidemiologic data indicate a high frequency of cocaine use in some populations of pregnant women, with estimates that 18 -20% (Zuckerman et al., 1989) to 31% (
To investigate the role of temporal coding in the neural processing of taste, trains of electrical pulses of varying frequency were delivered to the nucleus of the solitary tract (NTS) in awake rats. The temporal patterns of these trains mimicked the temporal patterns of electrophysiological responses of single neurons to natural tastes. In Experiment 1, water-deprived rats were first trained to lick water in an experimental chamber. On training days, licking water produced a sucroselike electrical pulse train in the NTS. At the end of these sessions, experimental animals were made ill by an injection of LiCl and subsequently learned to avoid licking when LiCl was paired with NTS stimulation. In Experiment 2, rats refused to lick water when licking produced a quininelike pattern of NTS stimulation but licked enthusiastically when licking produced a pattern of NTS stimulation similar to the natural response to sucrose.
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