Evidence that haloperidol impairs learning and motivation scores in a probabilistic task by reducing the reward expectation

Activation of midbrain dopamine neurons in response to positive prediction errors and reward predictive cues is proposed to "energize" reward seeking behaviors and approach responses to places where the reward is expected. In the present study, we tested the effect of the D2-dopamine receptor antagonist haloperidol on response latencies to enter two arms of a Y-maze with different reward probabilities. Adult male Wistar rats were trained to explore the Y-maze with sucrose pellets placed 30% of times at the end of one arm and 70% of times at the opposite arm. Therefore, the reward expectation was different among arms, and was updated in the trials when the reward was omitted. After training, rats received 0.05, 0.10, 0.15 mg/kg haloperidol, or saline 30 min before the test session. In the last, but not in the first trials, haloperidol caused a dose-dependent increase in arm choice latency and response latency. Saline, but not haloperidol, treated rats presented significantly longer response latencies for the 30% compared to the 70% reward probability arm. Haloperidol also caused a dose-dependent decrease in the number of entries in the 70% reward probability arm, increased the number of non-responses, and caused a dose-dependent increase in the number of re-entries in the 30% reward probability arm after non-rewarded trials. Control experiments suggested that haloperidol did not cause motor impairment or satiation, but rather impaired learning and motivation scores by reducing the reward expectation.

Early postnatal inhibition of serotonin synthesis results in long-term reductions of perseverative behaviors, but not aggression, in MAO A-deficient mice

Monoamine oxidase (MAO) A, the major enzyme catalyzing the oxidative degradation of serotonin (5-hydroxytryptamine, 5-HT), plays a key role in emotional regulation. In humans and mice, MAO-A deficiency results in high 5-HT levels, antisocial, aggressive, and perseverative behaviors. We previously showed that the elevation in brain 5-HT levels in MAO-A knockout (KO) mice is particularly marked during the first two weeks of postnatal life. Building on this finding, we hypothesized that the reduction of 5-HT levels during these early stages may lead to enduring attenuations of the aggression and other behavioral aberrances observed in MAO-A KO mice. To test this possibility, MAO-A KO mice were treated with daily injections of a 5-HT synthesis blocker, the tryptophan hydroxylase inhibitor p-chloro-phenylalanine (pCPA, 300 mg/kg/day, IP), from postnatal day 1 through 7. As expected, this regimen significantly reduced 5-HT forebrain levels in MAO-A KO pups. These neurochemical changes persisted throughout adulthood, and resulted in significant reductions in marble-burying behavior, as well as increases in spontaneous alternations within a T-maze. Conversely, pCPA-treated MAO-A KO mice did not exhibit significant changes in anxiety-like behaviors in a novel open-field and elevated plus-maze; furthermore, this regimen did not modify their social deficits, aggressive behaviors and impairments in tactile sensitivity. Treatment with pCPA from postnatal day 8 through 14 elicited similar, yet milder, behavioral effects on marble-burying behavior. These results suggest that early developmental enhancements in 5-HT levels have long-term effects on the modulation of behavioral flexibility associated with MAO-A deficiency.