Drug-induced place conditioning in goldfish : evolutionary implications for neural mechanisms of reward

Grant, Virginia L. (1992) Drug-induced place conditioning in goldfish : evolutionary implications for neural mechanisms of reward. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The underlying concern of this dissertation is with the evolutionary origins of a reward system extensively studied in rats. This system, called here the dopamine reward system, is characterized by three features: a dopamine mechanism, an opioid mechanism, and a functional dependence of the opioid mechanism on the dopamine mechanism. The same system appears to be present in all mammals, and possibly birds, suggesting that the system was also present in the common ancestor from which modern reptiles, birds, and mammals evolved. Thus, it seems possible that the dopamine reward system originated from an even earlier ancestor of vertebrates. To evaluate this possibility, goldfish were studied, because fish were the first modern vertebrates to evolve and, hence, are the most distantly related of all vertebrates to mammals. -- The presence of dopamine and opioid reward mechanisms in goldfish was tested by determining whether drugs with known effects on dopamine and opioid mechanisms have rewarding effects in the place conditioning procedure. The indirect dopamine agonist amphetamine (3.6 and 5.0 mg/kg) reliably produced conditioned place preferences. The direct and relatively selective dopamine agonist, apomorphine, also had rewarding effects, but only at rather low doses (0.4-0.5 mg/kg; doses of 0.25, 1.0, and 2.0 mg/kg were ineffective). These findings provide evidence for a dopamine reward mechanism in goldfish. Two other findings, however, were not consistent with such an interpretation. (1) No rewarding effects were found with cocaine (10 and 20 mg/kg), a psychomotor stimulant similar to amphetamine. (2) The rewarding effects of amphetamine were not blocked by either of the dopamine antagonists haloperidol (0.15 mg/kg) or flupentixol (0.8 mg/kg). This second finding raises doubts about the role of dopamine in the rewarding effects of amphetamine. -- In tests for the presence of an opioid reward mechanism in goldfish, morphine, whether administered intraperitoneally (5 to 30 mg/kg) or intracranially (0.3 μg), consistently failed to produce a rewarding effect. In view of these findings it was not feasible to test for the third feature of the dopamine reward system, namely, the functional dependence of the opioid mechanism on the dopamine mechanism. -- The phylogenetic implications of these findings were discussed.

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