PSYCHOLOGY MASTER'S THESIS ABSTRACT

Mark Wesley Wagster
MA-Research
May 1999

Saccharin Ingestion Is Related to Dopeminergic Sensitivity: Mediation of Taste Reward by the Mesolimbic Dopamine System

    This investigation studied the effect of the dopamine (DA) receptor antagonist, haloperidol (a preferential D2 antagonist), and the DA receptor agonist apomorphine (APO), upon consumption of a 0.15% (wt/vol) saccharin solution (S) in groups of male Sprague-Dawley rats with high (H), medium (M), and low (L) S preference.  Mesolimbic dopamine systems are implicated in the reinforcement of addictive drugs, electrical stimulation of the brain, and feeding.  It was assumed that H, M, and L preferences for S reflect respective H, M, and L levels of endogenous DA.  Therefore, it was hypothesized that L rats would exhibit less inhibition of S consumption in response to a DA receptor antagonist than would H rats.  It was also predicted that L rats would exhibit less augmentation of S consumption in response to a DA receptor agonist than would H rats.  Subjects consisted of 47 male Sprague-Dawley rats, 3 months of age, and experiment-naïve.  Water consumption over 48 hr and S consumption over 24 hr provided the baseline preferences, and the basis for group membership.  All doses were given to all subjects, creating a 3 x 2 x 4 (Group x Drug x Dose) design with one between-groups (group preference) and two repeated-measures variables (drug, dosage).  The dependent variables were S intake (m1), avidity, and preference.  H rats showed the greatest differentiation in drug response: APO evoked linear decreases with increasing dose in all measures of S consumption, whereas H response to haloperidol included negligible change in S intake at any dose, and negligible change in avidity at low and intermediate doses.  Respective M and L responses to both drugs were similar.  With increasing dose, M typically displayed a pattern of decrease-increase-decrease, whereas L displayed mostly linear decreases, except for small increases in S avidity and intake at the high dose of APO.  Repeated-measures ANOVAs yielded significant three-way interactions (p < 0.05) for avidity and intake.  Only APO elicited significant within-group simple effects in H avidity and intake and M avidity (p < 0.05).  The results show a main effect of group qualified by an interaction showing a steeper attenuation of S consumption in H vs. L, in response to APO.  This supports the hypothesis that reward sensitivity is directly related to preference for S, although AO’s effect was inhibition rather than augmentation.

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